LCK facilitates DNA damage repair by stabilizing RAD51 and BRCA1 in the nucleus of chemoresistant ovarian cancer

Poly-ADP Ribose Polymerase (PARP) targeted therapy is clinically approved for the treatment of homologous recombination (HR) repair deficient tumors. The remarkable success of this therapy in the treatment of HR repair deficient cancers has not translated to HR-proficient cancers. Our studies identify the novel role of non-receptor lymphocyte-specific protein tyrosine kinase (LCK) in the regulation of HR repair in endometrioid epithelial ovarian cancer (eEOC) model. We show that DNA damage leads to direct interaction of LCK with the HR repair proteins RAD51 and BRCA1 in a kinase dependent manner RAD51 and BRCA1 stabilization. LCK expression is induced and activated in the nucleus in response to DNA damage insult. Disruption of LCK expression attenuates RAD51, BRCA1, and BRCA2 protein expression by hampering there stability and results in inhibition of HR-mediated DNA repair including suppression of RAD51 foci formation, and augmentation of γH2AX foci formation. In contrast LCK overexpression leads to increased RAD51 and BRCA1 expression with a concomitant increase in HR DNA damage repair. Importantly, attenuation of LCK sensitizes HR-proficient eEOC cells to PARP inhibitor in cells and pre-clinical mouse studies. Collectively, our findings identify a novel therapeutic strategy to expand the utility of PARP targeted therapy in HR proficient ovarian cancer.

CD55 in cancer: Complementing functions in a non-canonical manner

CD55, or decay accelerating factor, is a membrane lipid microdomain-associated, GPI-anchored protein implicated in the shielding of cells from complement-mediated attack via accelerating decay of C3 and C5. Loss of CD55 is associated with a number of pathologies due to hyperactivation of the complement system. CD55 is also implicated in cancer progression thought to be driven via its role in cell shielding mechanisms. We now appreciate that CD55 can signal intracellularly to promote malignant transformation, cancer progression, cell survival, angiogenesis, and inhibition of apoptosis. Outside-in signaling via CD55 is mediated by signaling pathways including JNK, JAK/STAT, MAPK/NF-κB, and LCK. Moreover, CD55 is enriched in the cancer stem cell (CSC) niche of multiple tumors including breast, ovarian, cervical, and can be induced by chemotherapeutics and hypoxic environments. CSCs are implicated in tumor recurrence and chemoresistance. Here, we review the unexpected roles of CD55 in cancer including the roles of canonical and noncanonical pathways that CD55 orchestrates. We will highlight opportunities for therapeutic targeting CD55 and gaps in the field that require more in-depth mechanistic insights.

TMIC-34. INVESTIGATING THE EFFECT OF IRON IN GLIOBLASTOMA CELL POLARIZATION AND MIGRATION

Glioblastoma represents one of the most difficult-to-treat malignancies as evidenced by the poor prognosis associated with a diagnosis. The ability of glioblastoma cells to diffusely infiltrate into healthy brain tissue renders complete surgical resection challenging. Consequently, a large majority of glioblastoma patients end up with recurrent disease despite receiving maximally feasible surgical resection and rigorous chemoradiation. This work examined how modulation of cellular iron levels in T98G and LN229 glioblastoma cells impacted migratory capacity. Treatment of T98G or LN229 glioblastoma cells with iron in the form of ferric ammonium citrate (FAC) resulted in significantly reduced migration as assessed by time-lapse phase contrast imaging and wound healing assays. The iron-induced reduction in migration was able to be rescued by the addition of equimolar concentrations of deferoxamine, an iron chelator. Cellular proliferation in response to the iron treatments was quantified using both optical confluence and nucleic-acid-based proliferation assays and it was found that iron treatment at the concentrations used for the migration assays (0 – 300 µM FAC) did not result in reduced proliferation. Mechanistically probing iron’s impact on cell migration revealed that addition of iron resulted in decreased expression of Cdc42, a Rho GTPase that is essential to determining cellular polarity during migration. Functional cellular polarization assays further confirmed that reduced expression of Cdc42 corresponded to reduced cellular polarization. Bioinformatic analysis of CDC42 transcripts revealed the presence of potential iron-responsive-elements that may drive the iron-induced reduction in Cdc42 expression. This work highlights the importance of iron biology in impacting glioblastoma cell phenotype and potentially glioblastoma patient outcomes.

STEM-04. CONNEXIN 43 DRIVES GLIOBLASTOMA CANCER STEM CELL PHENOTYPES THROUGH A WNK1 SIGNALING AXIS

Tumors represent a highly dynamic system that relies on coordinated signaling to drive growth and adapt to selective pressures, including those induced by anti-cancer therapies. Given the need for tumors to engage in rapid and coordinated cell-cell communication, mechanisms including gap junction intracellular communication would be essential. Surprisingly, the connexin proteins that make up gap junctions have traditionally been considered tumor suppressors. This hypothesis was based on the loss of GJIC in cancer cells and, frequently, lower connexin expression in tumor cells compared to non-neoplastic tissue, including in glioblastoma (GBM), where connexin 43 (Cx43) was shown to be decreased. However, recent data suggest that this tumor-suppressive effect is context dependent and that connexins can function in a tumor-promoting role. We previously found that GBM cancer stem cells (CSCs) rely on gap junctional intercellular communication through connexin 46 (Cx46) for survival. We now identify a subset of GBM patient-derived xenograft (PDX) CSC models that also rely on Cx43 in addition to Cx46. This dependence on Cx43 is independent of Cx43 expression level. Knockdown of Cx43 using shRNA in these PDX models compromises their survival and self-renewal while inducing apoptosis. We leveraged a protein kinase phospho array to identify signaling changes associated with Cx43 knockdown and observed reductions in phosphorylation of WNK lysine-deficient protein kinase 1 (WNK1) in addition to ERK1/2, AKT, and p38MAPK. Immunoblotting validated that WNK1 phosphorylation is lost in the absence of Cx43. Treatment with a WNK1 inhibitor was toxic to CSCs. Current work is focused on interrogating the link among Cx43, WNK1 phosphorylation, and downstream signaling events and the therapeutic window of WNK1 inhibition. This work identifies a potential new signaling axis downstream of Cx43 in glioblastoma CSCs and further strengthens the hypothesis that connexins may act as both tumor suppressors and tumor drivers depending on context.

CSIG-08. MASS CYTOMETRY IDENTIFIES LOCATION-SPECIFIC TUMOR POPULATION ASSOCIATED WITH CLINICAL PROGRESSION IN GLIOBLASTOMA

Recent advances in single cell expression technology have allowed us to appreciate the complexity in tumoral heterogeneity of solid tumors such as glioblastoma (GBM), which remains uniformly fatal despite aggressive therapies. Currently, there is no knowledge about cell identities and functional characteristics of the different tumoral populations. To this end, we created a 28-marker mass cytometry panel composed of signaling markers, including phospho-proteins, and lineage markers that have been associated with GBM or are considered putative cancer stem cell markers. We evaluated lines established from core and edge areas from the same patient and validated to be functionally different with RNA expression and in vivo experiments. We found clusters shared by cells from both regions as well as region-specific ones. In order to better characterize these, we used marker enrichment modeling (MEM) and generated machine-readable labels. These were compared to MEM labels generated after using the same signaling markers to cluster primary samples from newly diagnosed or recurrent patients. Similarly, MEM labels were obtained from patients in a clinical trial treated with capecitabine that had positive or negative responses. Clusters from edge cells were found in higher proportion in recurrent tumors and were correlated with negative responses to chemotherapy. Initial analyses have also been done correlating immune clusters with tumor clusters in the same samples. Edge clusters are characterized by a CD44+ SOX2+ Nestin+ population. Treatment of tumor cells with temozolomide and irradiation results in an increase of this population in vitro. We are currently conducting single cells analyses to characterize this population as well as in vivo studies to validate the tumor-immune interactions. Our studies exploit single cell cytometry to underscore pathways that are preferentially use by different populations and affect immune cells with the goal of establishing therapeutic efforts to extend patient survival.

CNSC-36. VRK1 IS A PARALOG SYNTHETIC LETHAL TARGET IN VRK2-METHYLATED GLIOBLASTOMA

Synthetic lethality — a genetic interaction that results in cell death when two genetic deficiencies co-occur but not when either deficiency occurs alone — can be co-opted for cancer therapeutics. A pair of paralog genes is among the most straightforward synthetic lethal interaction by virtue of their redundant functions. Here we demonstrate a paralog-based synthetic lethality by targeting Vaccinia-Related Kinase 1 (VRK1) in Vaccinia-Related Kinase 2 (VRK2)-methylated glioblastoma (GBM). VRK2 is silenced by promoter methylation in approximately two-thirds of GBM, an aggressive cancer with few available targeted therapies. Genetic knockdown of VRK1 in VRK2-methylated GBM cell lines and patient-derived models was lethal and resulted in decreased activity of the downstream substrate Barrier to Autointegration Factor (BAF), a regulator of post-mitotic nuclear envelope formation. VRK1 knockdown, and thus reduced BAF activity, caused nuclear lobulation, blebbing and micronucleation, which subsequently resulted in G2/M arrest and DNA damage. The VRK1-VRK2 synthetic lethal interaction was dependent on VRK1 kinase activity and was rescued by ectopic VRK2 expression. Knockdown of VRK1 led to robust tumor growth inhibition in VRK2-methylated GBM xenografts. These results indicate that inhibiting VRK1 kinase activity could be a viable therapeutic strategy in VRK2-methylated GBM.

TMIC-51. SEX-SPECIFIC T CELL BEHAVIOR DRIVES DIFFERENTIAL IMMUNE RESPONSES IN GLIOBLASTOMA

Glioblastoma (GBM) is the most common primary malignant brain tumor and shows poor outcomes as a median survival is 12-18 months with current standard-of-care of therapy. GBM exhibits sex differences in incidence and overall survival, with males experiencing a higher incidence and a worse prognosis compared to females. However, the role of immune cells in GBM sex differences outside of myeloid cells remains poorly understood. Using a syngeneic mouse GBM model, we recapitulated the sex differences observed in patients, with shortened survival in male hosts compared to female hosts. These findings were not recapitulated in immuno-deficient mouse strains such as NSG and RAG1KO mice, suggesting a role for immune system, specifically T cells, in GBM sex differences. Flow cytometry analysis of tumor-infiltrating leukocytes revealed that more T cells were found in female tumors, whereas male tumors were enriched in macrophages. Additionally, more T cells in male tumors exhibited high levels of inhibitory receptors, whereas most female T cells were more functional as measured by expression of anti-tumor cytokines such as TNF, IFN-gamma, and granzyme B. A bone marrow chimera model (BMC) revealed that male T cells retained their phenotype in female hosts, whereas female T cell behavior was affected by the male environment. Yet survival analysis on BMC model and adoptive transfer model suggests strong immune cell-intrinsic effect on controlling tumor progression, further supported by in vitro T cell exhaustion model. Lastly, we found that males have more progenitor exhausted T cells, which led to better response to anti-PD1 treatment. Collectively, these results suggest that both cell-intrinsic and cell-extrinsic factors regulate T cell activity in a sex-specific manner, providing insights to develop sex-specific therapeutic approaches.

TMIC-82. LIPOCALIN-2 MEDIATES CELL-INTRINSIC AND CELL-EXTRINSIC FUNCTIONS IN GLIOBLASTOMA

Glioblastoma (GBM) is the most common primary brain tumor with a median survival of 17-20 months. Despite therapeutic treatments including surgery, radiation, chemotherapy, a subset of GBM clones, termed cancer stem cells (CSCs) are radio-resistant and chemo-resistant, leading to mortality of the patient. CSCs also have altered metabolic profiles and have an enhanced capacity to scavenge nutrients from their microenvironment, including iron. Lipocalin-2 (LCN2) functions to sequester iron and is traditionally considered an inflammatory marker through its function of limiting iron for bacterial usage. LCN2 has been described to have both a pro and anti-tumorigenic and has context-dependent functions depending on iron status. LCN2 has been shown to be important in brain metastasis, but its role in GBM is still largely unknown. To investigate how LCN2 plays a role in the GBM microenvironment, we orthotopically implanted syngeneic mouse GBM cells into male and female LCN2 knockout and wild-type mice. Female LCN2 knockout mice succumbed to tumors faster compared to males, revealing another example of sex differences in the tumor microenvironment. To assess a cell-intrinsic function for LNC2, we added recombinant LCN2 to mouse GBM cell lines and saw that proliferation also increased. When we probed for receptor expression in our knockdown cell lines, we saw that the LCN2 canonical receptor, SLC22a17, was upregulated in response. Furthermore, in a subset of slow-cycling cells of CSCs, LCN2 was shown to be up-regulated. Taken together, these data suggest that LCN2 functions in an iron-dependent manner to affect proliferation and sex-specific tumorigenesis. Given the fact that males have more iron than females, it is worth investigating the role of iron in GBM sex difference progression and therapeutic targets.

IMMU-17. LEVERAGING CORE EPIGENETIC PROGRAMMING OF IMMUNOSUPPRESSIVE MYELOID CELLS FOR THERAPEUTIC TARGETING OF GLIOBLASTOMA

Accumulation of various immunosuppressive myeloid cells, including myeloid-derived suppressor cells (MDSCs), facilitates progression and treatment resistance of glioblastoma (GBM). We previous demonstrated that monocytic MDSCs (mMDSCs) but not granulocytic MDSCs (gMDSCs) infiltrate tumors in mouse models and patients, and that nonspecific targeting of mMDSCs with chemotherapies provided therapeutic benefit in preclinical models of GBM. To investigate the differential role and trafficking of mMDSCs versus gMDSCs in GBM, we adoptively transferred these cells into tumor-bearing mice. Mice that received mMDSCs succumbed to disease at an earlier point compared to control mice or mice that received gMDSCs. To delineate the cellular basis of this distinct behavior of MDSC subsets, we performed assay for transposase-accessible chromatin using sequencing (ATAC-seq) and observed that cell adhesion-related genes were significantly enriched in open chromatin regions of mMDSCs as opposed to gMDSCs. Aligned with this epigenetic profile, mMDSCs from blood and tumors had significantly higher surface integrin β1 expression compared to gMDSCs in both mouse models and GBM patients. To evaluate the functional role of these integrins, we pre-treated mMDSCs with anti-integrin β1 prior to adoptive transfer. Blockade of surface integrin β1 interfered with the pro-tumorigenic role of mMDSCs, as the survival span of mice receiving these cells was similar to that of vehicle controls. Further analysis of the ATAC-Seq data revealed that dipeptidyl peptidase-4 (Dpp4), an interacting partner of integrin β1, was more accessible in mMDSCs compared to gMDSC. Consistently, bone marrow-derived mMDSCs in mouse and tumor-infiltrating mMDSCs in GBM patients expressed high levels of Dpp4. Pharmacological inhibition of Dpp4 reduced chemotaxis of mMDSCs in vitro and extended survival duration of mice with tumors. The findings from this study have broad implications across cancer types to modulate immunosuppressive myeloid cells by leveraging these novel insights into their adhesion mechanisms.

EPID-04. IMPACT OF SEX DIFFERENCES IN IRON SUPPLEMENTATION AND OUTCOMES IN ANEMIC GLIOBLASTOMA PATIENTS

We performed a retrospective sex-stratified analysis on 1750 histologically confirmed surgical glioblastoma patients (737 female, 1013 male) diagnosed between January 1, 2000 – January 1, 2020 in the TriNetX Research Network. Among 737 female glioblastoma patients, 140 (18.99%) were classified as anemic (defined as having mean 5-year-post-diagnosis hemoglobin levels < 12 g/dL). Among 1013 male patients, 177 (17.4%) were classified as anemic (mean 5-year-post-diagnosis hemoglobin levels < 13 g/dL). Of the 140 anemic female patients, 30 (21.4%) received iron supplementation whereas 28 (15.8%) of 177 anemic male patients received iron supplementation. Anemic female patients receiving iron supplementation were on average younger than anemic female patients who did not receive supplementation (mean age at diagnosis (SD): 59.08 vs. 64.87, p = 0.037), however no statistically significant differences in presence of chemotherapy administration, radiation administration, or Charlson comorbidity index (CCI) were detected. Anemic male patients receiving iron supplementation had no significant difference in mean age at diagnosis, chemotherapy administration, radiation administration, or CCI compared to anemic male patients not receiving iron supplementation. Kaplan-Meier analysis revealed that iron supplementation in anemic female patients was associated with prolonged overall median survival (536 vs. 361 days, p = 0.03) whereas iron supplementation in anemic male patients was not associated with any significant increase in overall survival (392 vs. 361 days, p = 0.89). Multivariate analysis using a Cox proportional hazards model adjusted for mean 5-year-post-diagnosis hemoglobin levels, age at diagnosis, chemotherapy administration, radiation administration, and CCI revealed that iron supplementation was associated with improved survival in anemic female patients (HR: 0.53, 95% CI: 0.30 – 0.96) but not in anemic male patients (HR: 0.87, 95% CI: 0.54 – 1.43). These results highlight the importance of iron biology in glioblastoma and provide evidence for further investigation into iron supplementation of anemic glioblastoma patients.

TMIC-69. MITOCHONDRIAL TRANSFER FROM ASTROCYTES ENHANCES METABOLISM AND DRIVES PROLIFERATION OF GLIOBLASTOMA

Mitochondrial transfer occurs both in stroke (central nervous system) and inflammatory pain (peripheral nerves). However, its role in glioblastoma (GBM) remains poorly understood. We hypothesized that mitochondrial transfer from non-malignant to GBM cells supports tumor metabolism and growth. Using transgenic mice expressing fluorophore-tagged mitochondria, we found that ~50% of orthotopically-implanted mouse GBM cells acquire mitochondria. Brain-resident cells, especially astrocytes, were the primary mitochondrial donors in vitro and in vivo. Mitochondrial transfer also occurred from immortalized human astrocytes to patient-derived xenograft (PDX) models in vitro at rates of 15-35%. GBM cells that acquired mitochondria expressed higher levels of the ATP-synthase subunit ATP5A and produced more ATP, while metabolomics revealed multiple upregulated pathways in recipient cells. These data point to increased metabolic activity in recipient cells. In vivo, mouse GBM cells that acquired mitochondria were more likely to be in S/G2/M cell cycle phases. We observed a similar effect in PDX that acquired astrocyte mitochondria in vitro, suggesting that transfer drives GBM proliferation. Using sorted mouse and human GBM cells with/without in vitro astrocyte mitochondrial acquisition, we found that mitochondrial transfer promoted in vitro self-renewal and in vivo tumorigenicity, leading to significant reduction in survival and increased penetrance in orthotopic GBM models. Transfer in mouse and human systems was contact-dependent and was abrogated by physical separation of donor and recipient cells by transwell inserts. Pharmacologic inhibition of cytoskeleton and gap junctions did not affect transfer rate, while blocking growth-associated protein 43 (GAP43) function by c-Jun N-terminus kinase inhibition decreased transfer rate by 15-30%, suggesting a potential role of GAP43. Taken together, mitochondrial transfer comprises a fundamental, protumorigenic mechanism of GBM, enhancing metabolic activity and driving tumor cell proliferation. Elucidating the molecular machinery regulating astrocyte mitochondrial transfer and its downstream protumorigenic effects will lead to therapeutic opportunities targeting this understudied tumor microenvironment interaction.

IMMU-32. SPERMIDINE DRIVES GLIOBLASTOMA PROGRESSION VIA SELECTIVE MODULATION OF THE IMMUNE SYSTEM

Glioblastoma (GBM) is incurable despite aggressive standard of care treatments (maximal safe surgical resection, radiation, chemotherapy). GBM therapeutic resistance is due to multiple factors, including tumor heterogeneity and a highly immunosuppressive environment. Naturally occurring polyamines have been identified as a putative therapeutic target in other cancers based on their increased presence and function in normal conditions; they are critical for cell growth and proliferation and cellular functions including autophagy and apoptosis. While polyamines are increased in GBM patients, little is known about their impact on GBM growth. In syngeneic immune competent mouse glioma models (GL261, SB28), mass spectrometry data revealed that spermidine (SPD) – a member of the polyamine family – is increased in tumor tissue as compared to non-neoplastic control brain tissue (sham implanted animals). To test the impact of SPD on tumor growth, we treated mouse glioma models with exogenous SPD and found treatment significantly decreased survival. However, in immunocompromised host mice, no such difference was observed, indicating the mechanism through which SPD is driving GBM progression likely involves immune system alterations. Depletion of myeloid derived suppressor cells in vivo via anti-Gr-1 antibody rescues the decrease in survival caused by exogenous SPD, indicating that SPD drives GBM by affecting immune-suppressive cell subsets, namely MDSCs. To assess if SPD is associated with more aggressive GBM growth in human patients, we are currently analyzing polyamine levels in human GBM samples of long vs short term survivors. We are also exploring the effect of dietary polyamines on GBM via the gut-brain-microbiome axis, as polyamines are enriched in many foods and produced by a subset of commensal gut microbes. Understanding the interactions between polyamines, the tumor microenvironment, and the immune response provide a new mechanism for GBM regulation and identify opportunities to alter the environment in the body to enhance immunotherapeutic efficacy.

TAMI-18. DIFFERENTIAL LIPID METABOLISM IN CANCER MICROENVIRONMENTS LEADS TO A REQUIREMENT FOR FATTY ACID DESATURASES FADS1 AND FADS2 IN GBM CANCER STEM CELL MAINTENANCE

Glioblastoma (GBM) is marked by cellular heterogeneity through microenvironments of a tumor, including metabolic heterogeneity. While altered cellular metabolism in cancer is well-known, how lipid metabolism is altered in different GBM microenvironmental conditions and cancer stem cell (CSC) states within a tumor remains an open question. We developed 3-dimensional GBM organoid models that mimic the transition zone between nutrient-rich cellular tumor and nutrient-poor psuedopalisading/perinecrotic tumor regions and performed spatially defined RNA-sequencing to investigate lipid metabolism. Spatial analysis revealed striking differences in metabolism between diverse cell populations from the same patient, with lipid enrichment in the hypoxic organoid cores and the pseudopalisading regions of patient tumors. This was accompanied by regionally restricted upregulation of lipid droplets and Hypoxia Inducible Lipid Droplet Associated gene expression in organoid cores and in the pseudopalisading regions of clinical GBM tumors. Using targeted lipidomic analysis, we assessed differences in acutely enriched CSC and non-CSCs from patient-derived models to explore the link between stem cell state and lipid metabolism. CSCs have low lipid droplet accumulation compared to non-CSCs in organoids and xenograft tumors, and prospectively sorted lipid-low GBM cells are functionally enriched for stem cell activity. This suggests lipid metabolism may not be simply a product of the microenvironment but also may be a reflection of cellular state. CSCs had decreased levels of major classes of neutral lipids compared to non-CSCs, but had significantly increased polyunsaturated fatty acid production due to increased expression of fatty acid desaturases FADS1 and FADS2. FADS1 and FADS2 expression are both essential to maintain CSC viability and self-renewal. Our data demonstrate that spatially and hierarchically distinct lipid metabolism phenotypes occur clinically in the majority of patients, can be recapitulated in laboratory models, and these altered lipid metabolic pathways may represent therapeutic targets for GBM.

TAMI-54. SEXUAL DIMORPHISM IN IRON ACQUISITION IN GLIOBLASTOMA

Sexual dimorphism in incidence and the clinical outcomes of Glioblastoma (GBM) has been reported, however, our knowledge of contributing biological mechanisms is limited. Iron acquisition is key to robust tumor growth. Upregulation of Transferrin (TF, iron transport protein)/Transferrin receptor (TFR) is critical for found in multiple different cancers, specifically, we have identified H-ferritin (FTH1) as a contributor to iron transport and protection in cancer stem cells. To interrogate brain tumor iron uptake mechanisms,we performed binding studies on homogenized samples of human male and female GBM tissue samples using 125I labeled TF and FTH1. Tumors from males had a ̴ 3.8-fold increased binding of both proteins compared to tumors from females. We interrogated iron uptake in a syngeneic orthotopic mouse model (GL261 cells) using male and female mice. After the tumors were established, radioactive 125I labeled TF and FTH1 proteins were injected retro-orbitally in the mice. After 24 hours, tumors wereremoved, and analyzed for TF and FTH1 uptake. Male tumors showed an increased uptake, of ̴ 3.2-fold, as compared to female tumors. There was no significant difference in TF uptake between male and female tumors nor between tumor and matched non-tumor brain tissue. We next queried role of FTH1 in the context of sexual dimorphism in GBM in a FTH1+/- mouse strain developed in our laboratory. Survival was monitored in the mice which were injected with GL261 cells at 3 months. Male mice that had reduced expression of FTH1 had poorer survival as compared to the male wild type controls whereas wild type and FTH+/- females had no major differences in survival outcomes. In summary, this study demonstrates sexual dimorphism in iron acquisition in GBM and animal models further suggesting a pathophysiological role of iron metabolism in GBM development and its possible role in prognosis.

IMMU-21. DIFFERENTIAL EXPRESSION OF ADHESION MOLECULES DEFINES MYELOID CELL INFILTRATION IN GLIOBLASTOMA AND COMPRISES A THERAPEUTIC TARGET

A potently immunosuppressive tumor microenvironment facilitates progression of glioblastoma (GBM). Immunotherapies have had variable success in improving the outcome of GBM patients, suggesting that there is a need to gain insight into the mechanisms of immunosuppression. Myeloid-derived suppressor cells (MDSCs) associate with poor prognosis and treatment resistance of GBM patients, but the distinct role of individual populations is not well-defined. We previous showed that monocytic MDSCs (mMDSCs) accumulated in tumors of mice and patients, while granulocytic MDSCs (gMDSCs) mainly remained in the circulation. Furthermore, nonspecific targeting of mMDSCs with chemotherapies provided therapeutic benefit in preclinical models of GBM, suggesting that mMDSCs drive disease progression. To investigate the differential function of mMDSCs versus gMDSCs in GBM, we adoptively transferred bone marrow-derived MDSC subsets into tumor-bearing mice. Mice that received mMDSCs succumbed to disease sooner compared to control mice, which was not observed with gMDSC transfer. To determine the basis of this pro-tumorigenic activity of mMDSCs, we performed ATAC-sequencing and comparison of differentially accessible regions indicated that cell adhesion pathways were significantly upregulated in mMDSCs. Aligned with this epigenetic profile, mMDSCs from bone marrow and blood had significantly higher surface integrin β1 and integrin β7 expression compared to gMDSCs. To evaluate the role of integrins in MDSC behavior, we pre-treated mMDSCs with anti-integrin β1 prior to adoptive transfer. Blockade of integrin β1 interfered with the pro-tumorigenic role of mMDSCs compared to isotype controls. Similarly, blockade of integrin β1 and integrin β7 systemically extended the survival duration of tumor-bearing mice. Finally, high expression of integrin β1 and integrin β7 served as a poor prognostic indicator in GBM patients. Our findings indicate that modulation of immunosuppressive myeloid cells by leveraging differences in adhesion mechanisms represents a potential immunotherapeutic option for GBM.

EPID-08. IMPORTANCE OF THE INTERSECTION OF AGE AND SEX TO UNDERSTAND VARIATION IN INCIDENCE AND SURVIVAL FOR PRIMARY MALIGNANT GLIOMAS

BACKGROUND

Gliomas are the most common type of malignant brain and other CNS tumors, accounting for 80.8% of malignant primary brain and CNS tumors. They cause significant morbidity and mortality. This study investigates the intersection between age and sex to better understand variation of incidence and survival for glioma in the United States.

METHODS

Incidence data from 2000-2017 were obtained from the Central Crain Tumor Registry of the United States, which obtains data from the CDC’s National Program of Cancer Registries and NCI’s Surveillance Epidemiology and End Results Program (SEER), and survival data from the CDC’s NPCR Registries. Age-adjusted incidence rates and rate ratios per 100,000 were generated to compare male-to-female incidence by age group. Cox proportional hazard models were performed by age group, generating hazard ratios to assess male-to-female survival differences.

RESULTS

Overall, glioma incidence was higher in males. Male-to-female incidence was lowest in ages 0-9 years (IRR: 1.04, 95% CI:1.01 - 1.07, p=0.003), increasing with age, peaking at 50-59 years (IRR:1.56, 95% CI: 1.53 - 1.59, p< 0.001). Females had worse survival for ages 0-9 (HR:0.93, 95% CI:0.87-0.99), though male survival was worse for all other age groups, with the difference highest in those 20-29 years (HR:1.36, 95% CI:1.28-1.44). Incidence and survival differences by age and sex also varied by histological subtype of glioma.

CONCLUSION

To better understand the variation in glioma incidence and survival, investigating the intersection of age and sex is key. The current work shows that the combined impact of these variables is dependent on glioma subtype. These results contribute to the growing understanding of sex and age differences that impact cancer incidence and survival.

TAMI-42. THE ROLE OF HFE AND IRON IN CELL ADHESION AND MIGRATION IN GLIOBLASTOMA

Glioblastoma (GBM) remains one of the most difficult to treat malignancies facing modern medicine. The strong migratory and invasive capacity of GBM cells allows for diffuse invasion into neighboring healthy brain which presents a significant hurdle for complete surgical resection of these tumors. Unsurprisingly, even after receiving maximal surgical resection, radiation and chemotherapy, the majority of GBM patients end up with recurrent disease. Increased expression levels of the homeostatic iron regulator gene (HFE) in brain tumors such as GBM have been associated with poorer outcomes. In order to better understand how HFE expression impacts the adhesive and migratory capacity of GBM, we utilized syngeneic mouse glioma models (KR158, CT2A) that have been transfected to either over-express or under-express HFE. We observed that knocking down HFE in the KR158 model resulted in significantly decreased migratory capacity as well as decreased adhesion to fibronectin and artificial basement membrane. Likewise, overexpressing HFE in a CT2A model resulted in increased adhesion to fibronectin or artificial basement membrane. Since HFE is known to regulate iron uptake, we studied how modulating the iron status of GBM cells impacted their ability to migrate and adhere. We found that increasing the iron pool of these mouse glioma models by exposure to exogenous iron compounds decreased migratory capacity. To better understand mechanistically how HFE and iron status impacted migration and adhesion, we probed how expression of integrins and their downstream signaling molecules, the Rho GTPases were altered in response to iron. We found that exposure to iron decreased levels of the Rho GTPases Cdc42 and RhoA. Furthermore, cells that overexpressed HFE were found to have increased expression of integrin β1 and integrin α5 suggesting that HFE and iron may impact integrins and their downstream signaling pathways to alter migration of GBM cells.

STEM-14. THE WRAD COMPLEX REPRESENTS A THERAPEUTIC TARGET FOR CANCER STEM CELLS IN GLIOBLASTOMA

Glioblastoma (GBM) progression and resistance to conventional therapies is driven in part by cells within the tumor with stem cell properties including quiescence, self-renewal and drug efflux potential. It is thought that eliminating these cancer stem cells (CSCs) is a key component to successful clinical management of GBM. However, currently, few known molecular mechanisms driving CSCs can be exploited for therapeutic development. Core transcription factors such as SOX2, OLIG2, OCT4 and NANOG maintain the CSC state in GBM. Our laboratory recently uncovered a self-renewal signaling axis involving RBBP5 that is necessary and sufficient for CSC maintenance through driving expression of these core stem cell maintenance transcription factors. RBBP5 is a component of the WRAD complex, which promotes Lys4 methylation of histone H3 to positively regulate transcription. We hypothesized that targeting RBBP5 could be a means to disrupt epigenetic programs that maintain CSCs in stemness transcriptional states. We found that genetic and pharmacologic inhibition of the WRAD complex reduced CSC growth, self-renewal and tumor initiation potential. WRAD inhibitors partially dissembled the WRAD complex and reduced H3K4 trimethylation both globally and at the promoters of key stem cell maintenance transcription factors. Using a CSC reporter system, we demonstrated that WRAD complex inhibition decreased growth of SOX2/OCT4 expressing CSCs in a concentration-dependent manner as quantified by live imaging. Overall, our studies assess the function of the WRAD complex and the effect of WRAD complex inhibitors in preclinical models and specifically on the stem cell state for the first time in GBM. Studying the functions of the WRAD complex in CSCs may improve understanding of GBM pathogenesis and elucidate how CSCs survive despite aggressive chemotherapy and radiation. Our ongoing studies aim to develop brain penetrant inhibitors targeting the WRAD complex as an anti-CSC strategy that could potentially synergize with standard of care treatments.

STEM-11. HYDROGEN SULFIDE FUNCTIONS AS A TUMOR SUPPRESSION IN GLIOBLASTOMA

Glioblastoma (GBM) cancer stem cells (CSCs) respond to a variety of stimuli within their immediate surroundings. However, little is known about the lifestyle factors that alter CSC enrichment and function within the tumor microenvironment (TME). To examine the consequences of diet-induced obesity on the progression of GBM, we interrogated tumor growth using patient-derived and syngeneic GBM models implanted into the brains of mice fed either an obesogenic high-fat diet (HFD) or a low-fat, control diet. HFD consumption resulted in an accelerated disease trajectory, presenting significantly shortened overall survival. HFD reshaped the TME altering the lipid landscape, enhancing the CSC phenotype, stimulating tumor cell proliferation, and protecting from necrotic cell death. Similar results were not observed in metabolically obese, leptin-deficient (ob/ob) mice. We simultaneously identified a potent suppression of the gasotransmitter, hydrogen sulfide (H2S). H2S functions principally through protein S-sulfhydration and regulates multiple programs including bioenergetics, metabolism, and immune response. Inhibition of H2S increased tumor cell proliferation and chemotherapy resistance, whereas treatment with H2S donors reduced tumor cell fitness in vitro and attenuated GBM growth in vivo. Exogenous treatment with H2S donors also rescued the lipid-mediated increase in tumor cell proliferation. As H2S exerts its action though protein S-sulfhydration, we confirmed that HFD-fed mice, which experienced decreased H2S synthesis, presented a severely depleted S-sulfhydrated protein landscape. Loss of this post-translational modification was confirmed in GBM patient tissues compared to age- and sex-matched controls. Taken together, our findings provide evidence that H2S functions as a tumor suppressor in GBM. Our observations highlight a new mechanism for tumor growth dynamics that can be leveraged for new therapeutic strategies focused on boosting H2S. Finally, our findings indicate that lifestyle factors can have pleiotropic effects on GBM progression through concomitant mechanisms involving tumor metabolism, modifications to the TME, and regulation over the CSC phenotype.

CBIO-06. CELL INTRINSIC HFE DRIVES SEX-SPECIFIC GLIOBLASTOMA GROWTH

Iron is an essential element required for a number of cellular processes and can contribute to malignant transformation and tumor expansion. In glioblastoma (GBM), tumor cells have been shown to modulate expression of iron-associated proteins to enhance iron uptake from the surrounding microenvironment, driving proliferation and tumor growth. The homeostatic iron regulatory (HFE) gene encodes a transmembrane glycoprotein that aids in iron homeostasis by modulating iron uptake and release. HFE is upregulated in GBM tumors compared to non-tumor brain and expression of HFE increases with tumor grade. Furthermore, HFE mRNA expression is associated with significantly reduced survival specifically in female patients with GBM. However, it is unclear how HFE impacts sex-specific GBM growth. To interrogate the underlying mechanism of HFE-mediated sex differences, we employed genetic loss and gain of function approaches using syngeneic mouse glioma models. We observed significant alterations in the expression of several iron-associated genes with Hfe knockdown or overexpression, suggesting global disruption of iron homeostasis. We found that knockdown of Hfe decreased cell number and increased apoptosis in vitro and led to a significant impairment of tumor growth in vivo, with a more pronounced effect seen in female mice. Conversely, overexpression of Hfe increased cell number and significantly decreased survival only in female animals. These findings support the hypothesis that Hfe is a critical regulator of cellular iron status and contributes to tumor aggression in a sex-dependent manner. These data also suggest an unexplored link between cell intrinsic iron signaling and sex-specific microenvironmental and immune responses, which is the focus of ongoing studies.

TAMI-51. HORIZONTAL MITOCHONDRIAL TRANSFER FROM THE TUMOR MICROENVIRONMENT TO GLIOBLASTOMA INCREASES TUMORIGENICITY

Communication between glioblastoma (GBM) and its microenvironment facilitates tumor growth and therapeutic resistance, and is facilitated through a variety of mechanisms. Organelle transfer between cells was recently observed, including mitochondria transfer from astrocytes to neurons after ischemic stroke. Given the dependence of GBM on microenvironmental interactions, we hypothesized that mitochondria transfer from tumor microenvironment to GBM cells could occur and affect metabolism and tumorigenicity. We interrogated this in vivo by establishing intracranial GBM tumors in mito::mKate2 mice (with trackable fluorescent mitochondria) using syngeneic GFP-expressing tumor cells (SB28 and GL261 models). We also cultured stromal cell types from mito::mKate2 mice with tumor cells, enabling sorting of tumor cells with and without exogenous mitochondria. Confocal microscopy revealed horizontal transfer of mKate2+ mitochondria from mouse cells to implanted GBM cells in vivo and was confirmed by flow cytometry where 20-40% of GBM cells acquired exogenous mitochondria. Transfer was negligible in wildtype mice transplanted with mito::mKate2 bone marrow cells, suggesting that brain-resident cells were the main donors. In vitro, astrocytes and microglia exhibited 5 to 10-fold higher mitochondrial transfer rate than bone-marrow derived macrophages. Seahorse metabolic profiling revealed that GBM cells with mKate2+ mitochondria had 40% lower respiratory reserve compared to cells without exogenous mitochondria. Median survival of mice implanted with SB28 that acquired mitochondria was significantly shorter and in vivo limiting dilution confirmed the frequency of tumor-initiating cells was 3-fold higher in SB28 cells with exogenous mitochondria. Our data indicate that horizontal mitochondrial transfer from brain-resident glia to mouse GBM tumors alters tumor cell metabolism and increases their tumorigenicity. Ongoing studies are assessing gene expression in GBM cells acquiring exogenous mitochondria; validating findings in human specimens; and screening for transfer inhibitor drugs. Horizontal mitochondrial transfer represents a foundational tumor microenvironment interaction contributing to glioblastoma plasticity, and is likely to inform next-generation treatment strategies.

STEM-15. SerpinB3 DRIVES CANCER STEM CELL SURVIVAL IN GLIOBLASTOMA

Despite therapeutic interventions for glioblastoma (GBM), self-renewing, therapy-resistant populations of cells referred to as cancer stem cells (CSCs) drive recurrence. Previously, we identified the unique expression of junctional adhesion molecule-A (JAM-A) on CSCs and demonstrated that JAM-A is both necessary and sufficient for self-renewal and tumor growth. Moreover, we determined that JAM-A signals via Akt in GBM CSCs to sustain pluripotency transcription factor activity; however, the entire signaling network has yet to be fully elucidated. To further delineate this pathway, we immunoprecipitated JAM-A from patient-derived GBM CSCs and performed mass spectrometry to determine JAM-A binding proteins. This led to the identification of the cysteine protease inhibitor SerpinB3 as a putative JAM-A binding partner. Using in vitro CSC functional assays, we show that SerpinB3 is necessary for CSC maintenance and survival. In an in vivo orthotopic xenograft model, knockdown of SerpinB3 extended survival. Mechanistically, knockdown of SerpinB3 led to decreased expression of TGF-β, Myc, WNT, and Notch signaling, known regulators of the CSC state. Additionally, knockdown of SerpinB3 increases susceptibility to radiation therapy. SerpinB3 is essential for buffering cells against cathepsin-mediated cell death, and we found that elevated lysosomal membrane permeability after radiation leads to cathepsin release into the cytoplasm. As a result, SerpinB3 knockdown cells have a diminished capacity to inhibit cathepsin-driven cell death after radiation. The addition of the cathepsin inhibitor E64D partially rescues the SerpinB3 knockdown, however, SerpinB3 mutants that are unable to inhibit cathepsins fail to do the same. Taken together, our findings, identify a novel GBM CSC-specific survival mechanism involving a previously uninvestigated cysteine protease inhibitor, SerpinB3, and provide a potential target to increase the efficacy of standard of care GBM therapies against therapy-resistant CSCs.

Multimodal single-cell/nucleus RNA sequencing data analysis uncovers molecular networks between disease-associated microglia and astrocytes with implications for drug repurposing in Alzheimer's disease

Because disease-associated microglia (DAM) and disease-associated astrocytes (DAA) are involved in the pathophysiology of Alzheimer's disease (AD), we systematically identified molecular networks between DAM and DAA to uncover novel therapeutic targets for AD. Specifically, we develop a network-based methodology that leverages single-cell/nucleus RNA sequencing data from both transgenic mouse models and AD patient brains, as well as drug-target network, metabolite-enzyme associations, the human protein-protein interactome, and large-scale longitudinal patient data. Through this approach, we find both common and unique gene network regulators between DAM (i.e., PAK1, MAPK14, and CSF1R) and DAA (i.e., NFKB1, FOS, and JUN) that are significantly enriched by neuro-inflammatory pathways and well-known genetic variants (i.e., BIN1). We identify shared immune pathways between DAM and DAA, including Th17 cell differentiation and chemokine signaling. Last, integrative metabolite-enzyme network analyses suggest that fatty acids and amino acids may trigger molecular alterations in DAM and DAA. Combining network-based prediction and retrospective case-control observations with 7.2 million individuals, we identify that usage of fluticasone (an approved glucocorticoid receptor agonist) is significantly associated with a reduced incidence of AD (hazard ratio [HR] = 0.86, 95% confidence interval [CI] 0.83-0.89, P < 1.0 × 10-8). Propensity score-stratified cohort studies reveal that usage of mometasone (a stronger glucocorticoid receptor agonist) is significantly associated with a decreased risk of AD (HR = 0.74, 95% CI 0.68-0.81, P < 1.0 × 10-8) compared to fluticasone after adjusting age, gender, and disease comorbidities. In summary, we present a network-based, multimodal methodology for single-cell/nucleus genomics-informed drug discovery and have identified fluticasone and mometasone as potential treatments in AD.

Development of an arteriolar niche and self-renewal of breast cancer stem cells by lysophosphatidic acid/protein kinase D signaling

Breast cancer stem cells (BCSCs) are essential for cancer growth, metastasis and recurrence. The regulatory mechanisms of BCSC interactions with the vascular niche within the tumor microenvironment (TME) and their self-renewal are currently under extensive investigation. We have demonstrated the existence of an arteriolar niche in the TME of human BC tissues. Intriguingly, BCSCs tend to be enriched within the arteriolar niche in human estrogen receptor positive (ER+) BC and bi-directionally interact with arteriolar endothelial cells (ECs). Mechanistically, this interaction is driven by the lysophosphatidic acid (LPA)/protein kinase D (PKD-1) signaling pathway, which promotes both arteriolar differentiation of ECs and self-renewal of CSCs likely via differential regulation of CD36 transcription. This study indicates that CSCs may enjoy blood perfusion to maintain their stemness features. Targeting the LPA/PKD-1 -CD36 signaling pathway may have therapeutic potential to curb tumor progression by disrupting the arteriolar niche and effectively eliminating CSCs.

STEM-07. IDENTIFICATION OF A NOVEL CSC SIGNALING AXIS CONTAINING JAM-A/SERPINB3/TGF-BETA

Despite therapeutic interventions including surgery, radiation, and chemotherapy, multiple clones of treatment resistant cells repopulate the tumor, resulting in recurrence and a high rate of mortality in glioblastoma. Effective targeting of these cancer stem cells (CSCs) has been limited by our incomplete understanding of the intracellular signaling mechanisms maintaining the CSC state. Our laboratory previously identified junctional adhesion molecule-A (JAM-A) on CSCs and, through functional studies, demonstrated that JAM-A is both necessary and sufficient for self-renewal and tumor growth. We determined that JAM-A signals via Akt in GBM CSCs to sustain pluripotency transcription factor activity; however, the intermediate signaling network is yet to be fully elucidated. Therefore, our goal was to delineate the pathway downstream of JAM-A. To achieve this, we immunoprecipitated JAM-A from GBM CSCs and performed mass spectrometry leading to the identification of the serine/cysteine protease inhibitor SerpinB3. While SerpinB3 has a functional role in lung adenocarcinoma, breast, pancreas, as well as in hepatocellular carcinoma, there is limited information in GBM or CSCs, and its relationship to JAM-A is yet to be elucidated. Using in vitro CSC functional assays, we observed that SerpinB3 is necessary for the maintenance of CSCs. Additionally, in an intracranial implantation model of GBM the knockdown of SerpinB3 extended the survival of mice. Knockdown of SerpinB3 in multiple GBM CSCs models also led to decreased expression of TGF-β, a known regulator of the CSC state. We also observed that SerpinB3 stabilizes JAM-A expression and further studies are investigating the mechanism of this stabilization, identifying the specific interaction sites of JAM-A and Serpin B3, and determining how TGF-β alters the association through positive and negative feedback mechanisms. Taken together, these data highlight the role of SerpinB3 in GBM CSCs and provides a novel target to inhibit the JAM-A-mediated CSC maintenance as a next-generation GBM therapy.

CSIG-16. SEXUAL DIMPORHISM IN IRON ACQUISITION IN GLIOBLASTOMA

Glioblastoma (GBM) is the most aggressive brain cancer. Sex differences in incidence and clinical outcomes have been reported, however, our knowledge of contributing mechanisms is limited. Iron acquisition is key to robust tumor growth. Upregulation of Transferrin (Tf, iron transport protein)/Transferrin receptor (TfR) is found in multiple different cancers. We have identified H-ferritin (FTH1) as involved in iron transport and explore its uptake in GBM in this study. We interrogated iron uptake in a syngeneic orthotopic mouse model (GL261 cells) using male and female mice. After the tumors were established, radioactive 125I labeled Tf and FTH1 proteins were injected retro-orbitally in the mice. After 24 hours, tumors were removed, homogenized and analyzed for Tf and FTH1 uptake. There was a significant difference in Tf uptake into the tumor versus matched non-tumor tissue in both males and females and the uptake in the tumors was 1.5-fold higher in males than females. There was no significant difference in FTH1 uptake between male and female tumors nor between tumor and matched non-tumor brain tissue. Binding analyses were performed on homogenized samples of human male and female GBM tissue samples using 125I labeled Tf and FTH1. Tumors from males had increased binding of both proteins compared to tumors from females. We next queried the TCGA database and found in females, high TfR expression was associated with poor survival but not in males. TCGA database revealed a robust expression of Tim1, a putative receptor for FTH1, but its expression did not relate to survival. In summary, this study demonstrates FTH1 binding to GBMs and sexual dimorphism in iron acquisition via Tf and survival.

BIOM-30. DECREASED B CELL AND ALTERED CD4+ T CELL SUBSET RATIO PREDICT FAVORABLE RESPONSE TO RUXOLINITIB IN HIGH-GRADE GLIOMAS

Altered cellular growth and suppression of anti-tumor immune response collectively drive progression and therapeutic resistance of high-grade gliomas (HGGs), suggesting that successful treatments will likely target these key mechanisms. Janus-associated kinase (JAK)-STAT pathway comprises one such therapeutic node, as STAT3 signaling is upregulated in proliferating glioma cells and immunosuppressive cell types. We performed retrospective analysis on 13 HGG patients treated with the JAK1/2 inhibitor ruxolinitib in combination with radiation or radiation plus temozolomide in a Phase I setting (NCT03514069). Disease progression and treatment response was determined based on the Response Assessment in Neuro-Oncology (RANO) criteria. Peripheral blood mononuclear cells (PBMCs) cells were collected from patients prior to treatment, and 2- 4- and 6-week time points to characterize early immunological changes that are indicative of treatment response with flow cytometry. 6 patients responded favorably, define as progression-free survival &gt;6 months. Investigation of myeloid and lymphoid cell populations revealed no differences in dendritic cell, myeloid-derived suppressor cell and CD8+ T cell subset frequencies in blood. Similarly, expression of checkpoint inhibitors PD-1, TIGIT, CTLA-4 and TIM3 were not altered with ruxolinitib treatment. However, there was a ~2-fold reduction in the frequency of peripheral B cells in favorable responders at 4-week time point, whereas the levels remained stable in non-responders (p&lt;0.05). In addition, while 5 out of 7 non-responders had increased CD4+ T cells by 4-weeks, 5 out of 6 responders had stable or reduced CD4+ T cells. This effect was driven by reduction in immunosuppressive regulatory T cells (Tregs) as the relative ratio of immunostimulatory T helper 1 (Th1)-to-Treg ratio was 3-fold higher in responders compared to non-responders. These results support evaluation of B and CD4+ T cells in the peripheral blood as biomarkers to identify HGG patients likely to benefit from such approach.

IMMU-10. GENOMIC DIFFERENCES UNDERLIE MYELOID-DERIVED SUPPRESSOR CELL SEXUAL DIMORPHISM IN GLIOBLASTOMA

A potently immunosuppressive tumor microenvironment facilitates progression of glioblastoma (GBM). We previously demonstrated that myeloid-derived suppressor cell (MDSC) subsets promote tumorigenesis in a sex-specific manner, contributing to sexual dimorphism in GBM incidence and prognosis. Our findings indicated that proliferating monocytic MDSCs (mMDSCs) accumulate in tumors of male mice and patients, while female tumor-bearing mice had an increase in circulating granulocytic MDSC (gMDSC) frequency, and a high gMDSC gene signature correlated with worse outcome of female patients. However, the mechanisms underlying sexual dimorphism of MDSC heterogeneity remain understudied and can provide insights for improved immunotherapy response. Using syngeneic mouse glioma models and sequencing approaches, we show that expression of Y-chromosome-linked genes correlates with upregulation of multiple RNA transcription-related pathways specifically in male mMDSCs. Consistently, adoptive transfer of male mMDSCs but not gMDSCs worsened GBM outcome in male recipients, while the transfer of sex-matched mMDSCs did not impact survival of female mice. In contrast to this cell-intrinsic regulatory pathway, sex steroids had no impact on MDSC profile, as castration or ovariectomy failed to alter MDSC subset accumulation patterns in GBM-bearing mice. Correspondingly, IL-1β, which we had identified as a female-specific drug target, was highly expressed in female but not male gMDSCs. Single-cell sequencing revealed that circulating but not tumor-infiltrating gMDSCs were the primary source of IL-1β and that its neutralization provided a female-specific survival advantage by reducing circulating gMDSCs. This was accompanied by declines in tumor infiltration of microglia, microglia activation status and tumor cell proliferation. In vitro, IL-1β inhibition reduced viability and expression of activation markers by primary microglia. These findings highlight a peripheral gMDSC-microglia communication axis mediated by IL-1β signaling in females with GBM and indicate that expression differences in MDSC subsets represent opportunities for improved immunotherapy efficacy while accounting for sex as a biological variable.

TAMI-17. RELATIONSHIP BETWEEN IRON METABOLISM, IMMUNE CELL INFILTRATION AND SEX-BASED SURVIVAL DIFFERENCES IN GLIOMAS

Iron plays a central role in cellular metabolism, both in normal cellular functioning and in tumorigenesis. Recent evidence has shown sex-based survival differences in glioblastoma (GBM) may be related to differential expression of metabolism genes. We previously reported the iron regulating gene, HFE, was shown to have a sex-based survival impact in both low-grade gliomas and GBM. We additionally found that females with low HFE expressing tumors have significantly higher survival than males in GBM. To evaluate the relationship between iron gene expression and sex-based survival differences in GBM, we analyzed TCGA GBM gene expression and clinical data. We first analyzed the impact of iron genes on sex-based survival. In addition to HFE, FTL, TFRC, TF, and SLC39A8 (ZIP8), also showed sex-based survival differences. We then compared correlations of HFE and other iron genes to identify whether male and female GBMs differ in iron regulation and metabolism. HFE expression is significantly positively correlated with HMOX1, SLC25A28, SLC11A2, FTH1, HAMP, and TFR2 only in females. Alternatively, HFE expression is negatively correlated with ACO2 (mitochondrial aconitase) in males and ACO1 (cytoplasmic aconitase) in females. We noted that the expression of certain iron genes was highly associated with immune cell infiltration based on sex. TFR2, LRP1, and XIST expression were negatively correlated with low immune cell infiltration in females, but not males. Alternatively, in males, SLC11A2, ACO2, FOXO1, HIF1a, and HAMP genes were negatively correlated with immune infiltration. This suggests that differences in iron regulation between males and females may be contributing to differences in immune function and subsequent survival in GBM. These data suggest that the iron signature of a tumor reflects and possibly drives the metabolic and immune landscape of the tumor microenvironment thereby directly impacting survival differences between male and female GBMs.

CBIO-10. REDUCED IRON EXPORT FUNCTIONS IN A CELL INTRINSIC MANNER TO DRIVE GLIOBLASTOMA GROWTH

Glioblastoma (GBM), the most common primary malignant brain tumor in adults, is characterized by invasive growth and poor prognosis. Iron is a critical regulator of many cellular processes, and GBM tumor cells have been shown to modulate expression of iron-associated proteins to enhance iron uptake from the surrounding microenvironment, driving tumor initiation and growth. While iron uptake has been the central focus of previous investigations, additional mechanisms of iron regulation, such as compensatory iron efflux, have not been explored in the context of GBM. The hemochromatosis (HFE) gene encodes a transmembrane glycoprotein that aids in iron homeostasis by limiting cellular iron release, resulting in a sequestration phenotype. We find that HFE is upregulated in GBM tumors compared to non-tumor brain and that expression of HFE increases with tumor grade. Furthermore, HFE mRNA expression is associated with significantly reduced survival specifically in female patients with GBM. Based on these findings, we hypothesize that GBM tumor cells upregulate HFE expression to augment cellular iron loading and drive proliferation, ultimately leading to reduced survival of female patients. To test this hypothesis, we generated Hfe knockdown and overexpressing mouse glioma cell lines. We observed significant alterations in the expression of several iron handling genes with Hfe knockdown or overexpression, suggesting global disruption of iron homeostasis. Additionally, we show that knockdown of Hfe in these cells increases apoptosis and leads to a significant impairment of tumor growth in vivo. These findings support the hypothesis that Hfe is a critical regulator of cellular iron status and contributes to tumor aggression. Future work will include further exploration of the mechanisms that contribute to these phenotypes as well as interactions with the tumor microenvironment. Elucidating the mechanisms by which iron effulx contributes to GBM may inform the development of next-generation targeted therapies.

TAMI-12. CANCER STEM CELL ENRICHMENT AND METABOLIC SUBSTRATE ADAPTABILITY ARE DRIVEN BY HYDROGEN SULFIDE SUPPRESSION IN GLIOBLASTOMA

Glioblastoma (GBM) remains among the deadliest of human malignancies. The emergence of the cancer stem cell (CSC) phenotype represents a major challenge to disease management and durable treatment response. The extrinsic, environmental, and lifestyle factors that result in CSC enrichment are not well understood. The CSC state endows cells with a fluid metabolic profile, enabling the utilization of multiple nutrient sources. Therefore, to test the impact of diet on CSC enrichment, we evaluated disease progression in tumor-bearing mice fed an obesity-inducing high-fat diet (HFD) versus an energy-balanced, low-fat control diet. HFD consumption resulted in hyper-aggressive disease that was accompanied by CSC enrichment and shortened survival. HFD consumption also drove intracerebral accumulation of saturated fats, which in turn inhibited the production and signaling of the gasotransmitter hydrogen sulfide (H2S). H2S is an endogenously produced bio-active metabolite derived from sulfur amino acid catabolism. It functions principally through protein S-sulfhydration and regulates a variety of programs including mitochondrial bioenergetics and cellular metabolism. Inhibition of H2S synthesis resulted in increased proliferation and chemotherapy resistance, whereas treatment with H2S donors led to cytotoxicity and death of cultured GBM cells. Compared to non-cancerous controls, patient GBM specimens were reduced in overall protein S-sulfhydration, which was primarily lost from proteins regulating cellular metabolism. These findings support the hypothesis that diet-regulated H2S signaling serves to suppress GBM by restricting metabolic adaptability, while its loss triggers CSC enrichment and disease acceleration. Interventions augmenting H2S bioavailability concurrent with GBM standard of care may improve outcomes for GBM patients.

CTIM-12. RANDOMIZED PHASE 2 STUDY OF NIVOLUMAB (NIVO) PLUS EITHER STANDARD OR REDUCED DOSE BEVACIZUMAB (BEV) IN RECURRENT GLIOBLASTOMA (rGBM)

BACKGROUND

Trials with anti-PD1 in rGBM have shown limited efficacy. VEGF is highly upregulated proangiogenic growth factor in GBM contributing to tumor-associated immunosuppression. Preclinical data suggests a potential dose effect of anti-VEGF therapy on immunomodulation. Hence, a combination of anti-PD1 and anti-VEGF may be a promising approach in rGBM.

METHODS

90 patients with first-recurrent GBM were randomized (1:1) to nivolumab (240 mg IV Q2 weeks) and bevacizumab at standard (10 mg/kg; Arm A) or low dose (3 mg/kg; Arm B) IV Q2 weeks. Eligibility also required KPS≥ 70% and dexamethasone ≤ 4 mg/day. Stratification included extent of resection, age, performance status and MGMT methylation status. Single cell RNA sequencing with CITE-seq was used to analyze blood samples from pre- and 8 weeks post-treatment among 8 responders and 8 non-responders.

RESULTS

90 patients were enrolled (May 2018- Jan 2020) and median follow-up is 7.5 months. Characteristics in 2 arms were comparable. Median age was 60.5 years (range 27–86), median KPS was 80. 35 patients were MGMT methylated, 53 unmethylated and 2 indeterminate. Estimated progression free survival (PFS) and median overall survival (OS) in arm A are 6.13 and 10.85 months and 4.59 and 9.61 months in Arm B, respectively. Single cell RNA sequencing with CITE-seq was used to analyze blood samples from 16 patients, baseline and 8 weeks post treatment. Standard dose bevacizumab treated patients had decreased myeloid derived suppressor cells and an inflammatory response gene signature at 8 weeks. Most frequent toxicities included fatigue (52.8%), headache (32.6%), diarrhea (31.5%), proteinuria (25.8) and hypertension (23.6%). Toxicity was comparable between 2 arms, except hypertension was more common in arm A.

CONCLUSIONS

PFS and OS rates appear similar for nivolumab with either standard or low-dose bevacizumab compared to historical benchmarks of bevacizumab monotherapy. Ongoing response evaluation and immunocorrelative data will be presented.

IMMU-08. MODELING UPFRONT GLIOBLASTOMA SURGICAL RESECTION AND STEROID USE REVEALS IMMUNOSUPPRESSIVE CHANGES AND SUGGESTS THAT PERIPHERAL LYMPHOCYTE COUNTS ARE ASSOCIATED WITH TUMOR VOLUME AND PROGNOSIS

Glioblastoma (GBM) and its treatment produces systemic immunosuppression, which is being targeted by immunotherapies. However, it remains unclear how surgical resection and steroids specifically in GBM alter the immune system. To further explore this issue, immunocompetent C57Bl/6 mice were intracranially inoculated with syngeneic glioma cells (GL261 and CT-2A) and growth of tumors was evaluated by MRI. Host immune cell populations were analyzed during surgical resection and steroid administration. Mice with surgically resected tumors had a longer median survival compared to mice subjected to tumor biopsies, and had increased bone marrow sequestration of both CD4 and CD8 T cells with corresponding decreased blood lymphocytes. Furthermore, physiologic doses of dexamethasone administered perioperatively decreased tumor edema, but increased the number and proliferative capacity of both marrow and circulating MDSCs while generating no survival benefit. Independent of therapy or dexamethasone, intracranial tumor volume correlated linearly with decreased CD4 and CD8 T cells in peripheral blood, and increased T cell sequestration within the bone marrow. We validated these parameters in steroid-naïve newly diagnosed GBM patients and observed decreased lymphocytes correlated linearly with increased tumor volume. When initial lymphocyte counts in both steroid-naïve and steroid-administered patients were used in univariate and multivariate models predicting progression-free survival and overall survival, decreased initial lymphocyte counts were an independent predictor of decreased progression free survival and decreased overall survival, with steroid use and initial tumor size falling out of significance during stepwise selection. Taken together, tumor volume is linearly correlated with marrow sequestration of lymphoid cells, but both surgery and steroid administration further suppress active immune responses along lymphoid and myeloid lineages. Furthermore, decreasing peripheral lymphocyte counts at diagnosis of GBM indicate an immune system less able to mount responses to the tumor and portent a worse progression free and overall survival.

TAMI-43. IMPACT OF SEX AND RADIATION ON IRON TRAFFICKING IN BONE MARROW DERIVED MACROPHAGES

The tumor microenvironment in glioblastoma provides cancer cells with favorable conditions to proliferate and invade surrounding tissues. Macrophages comprise a large portion of the glioblastoma tumor microenvironment (TME) both in terms of volume and function. These cells have been reported to influence tumor progression by modulating immune responses, remodeling extracellular matrix, and providing nutrients to cancer cells among numerous other functions. Radiation therapy forms one of the pillars of glioblastoma management along with surgical resection and chemotherapy. Here we investigated the effects of radiation on macrophage iron metabolism. Using mouse bone-marrow-derived macrophages (BMDMs) we performed in-vitro 59Fe radiotracer assays to study how radiation exposure modified iron trafficking in these cells. We found that low dose radiation at 0.25, 0.5, or 2 Gy from a 60Co source stimulated iron release from the BMDMs with maximal release occurring at 0.5 Gy. Moreover, we observed that iron release was dependent on the amount of serum present in culture media with cells cultured in 20% fetal bovine serum (FBS) showing reduced iron release profiles compared to those cultured in 10% or 1% FBS. Since glioblastoma patients exhibit sexually dimorphic survival outcomes, we investigated whether these radiation-induced responses occurred in a sexually dimorphic pattern. At radiation doses of 0.25 Gy we observed that male macrophages tended to release more iron than female macrophages despite no differences in iron uptake between the sexes – raising the question as to whether differential iron trafficking in response to treatment contributes to the poorer survival outcomes observed in males. Our data suggest that delineating how supporting cells such as macrophages respond to glioblastoma treatment regimens may provide insights into addressing mechanisms of treatment resistance and further our understanding of the sexual dimorphism observed in patient outcomes.

Abstract B69: Gut microbiome attenuates epithelial ovarian cancer growth and sensitivity to cisplatin: New opportunities for ovarian cancer treatments

Epithelial ovarian cancer (EOC) is the second most common gynecologic malignancy in the United States, but the leading cause of gynecologic cancer death. Despite many women achieving remission with first-line therapy, which includes cytoreductive surgery and platinum-taxane chemotherapy, up to 80% of patients will recur and require additional treatment. The interval from last platinum chemotherapy to recurrence has important prognostic and therapeutic implications for patients. Patients with platinum-resistant EOC have fewer treatment options, worse prognosis, and their disease is seldom curable. Antibiotic therapy is frequently used during cancer treatments in patients with EOC for both prophylaxis and treatment of infections after surgery or through chemotherapy. While critical for the care of the patient, we now appreciate that the microbiome has many roles in maintenance of human health and is increasingly linked with many disease states including obesity, cardiovascular disease, and cancer. Recent evidence suggests the gut microbiome may modulate responses to cancer treatment, including traditional chemotherapy and immunotherapy. Given that antibiotics may lead to microbiome disruption for care of EOC patients and lack of understanding of the impact of these antibiotics on tumor progression, in preclinical models, we assessed the impact of microbiome disruption on EOC progression and cisplatin sensitivity. We established a necessary role for the microbiome in suppression of tumor growth and cisplatin response in two mouse models of EOC. Following treatment with antibiotics (ampicillin, neomycin, vancomycin, and metronidazole), murine ID8 or ID8-VEGF EOC that were injected intraperitoneally into C57Bl/6J mice exhibited accelerated tumor growth compared to non-antibiotic treated controls. Tumor growth was monitored by ultrasound weekly. ID8 and ID8-VEGF tumors in antibiotic-treated mice exhibited reduced sensitivity to cisplatin compared to non-antibiotic controls. Mice treated with antibiotics had significantly worse survival compared to non-antibiotic controls. We confirmed depletion of the gut microbiome based on dilated cecum and reduced microbial 16S rRNA concentration in stool of antibiotic-treated compared to control mice. The accelerated tumor growth and cisplatin resistance was not dependent on an intact immune system as we replicated the effect in immune-deficient mice. In mechanistic studies we determined that disruption of the microbiome led to increased cancer stem cells that was further augmented by cisplatin treatment. Collectively, these studies indicate an intact microbiome provides a tumor-suppressive microenvironment and enhances sensitivity to cisplatin. Future studies will assess whether individual microbial communities are sufficient to reverse the accelerated tumor growth and resistance to cisplatin observed in antibiotic-treated mice and use of selective antibiotics to target pathogenic bacteria while sparing beneficial microbes.

Citation Format: Laura Chambers, Emily Esakov, Chad Braley, Leila Edelman, Roberto Vargas, Justin Lathia, Chad Michener, Ofer Reizes. Gut microbiome attenuates epithelial ovarian cancer growth and sensitivity to cisplatin: New opportunities for ovarian cancer treatments [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr B69.

Gliomas display distinct sex-based differential methylation patterns based on molecular subtype

Background

Gliomas are the most common type of primary brain tumor and one of many cancers where males are diagnosed with greater frequency than females. However, little is known about the sex-based molecular differences in glioblastomas (GBMs) or lower grade glioma (non-GBM) subtypes. DNA methylation is an epigenetic mechanism involved in regulating gene transcription. In glioma and other cancers, hypermethylation of specific gene promoters downregulates transcription and may have a profound effect on patient outcome. The purpose of this study was to determine if sex-based methylation differences exist in different glioma subtypes.

Methods

Molecular and clinical data from glioma patients were obtained from The Cancer Genome Atlas and grouped according to tumor grade and molecular subtype (IDH1/2 mutation and 1p/19q chromosomal deletion). Sex-specific differentially methylated probes (DMPs) were identified in each subtype and further analyzed to determine if they were part of differentially methylated regions (DMRs) or associated with differentially methylated DNA transcription regulatory binding motifs.

Results

Analysis of methylation data in 4 glioma subtypes revealed unique sets of both sex-specific DMPs and DMRs in each subtype. Motif analysis based on DMP position also identified distinct sex-based sets of DNA-binding motifs that varied according to glioma subtype. Downstream targets of 2 of the GBM-specific transcription binding sites, NFAT5 and KLF6, showed differential gene expression consistent with increased methylation mediating downregulation.

Conclusion

DNA methylation differences between males and females in 4 glioma molecular subtypes suggest an important, sex-specific role for DNA methylation in epigenetic regulation of gliomagenesis.

The Clinical Impact of Cancer Stem Cells

Patients with cancer can go though many stages in their disease, including diagnosis, recurrence, metastasis, and treatment failure. Cancer stem cells (CSCs) are a subgroup of cells within tumors that may explain the mechanism by which tumors recur and progress. CSCs can both self-renew and produce progenitor cells of more differentiated cancer cells as well as heterogeneously demonstrate resistance and the abilities to migrate and metastasize. These "stemness" characteristics are often the result of dysregulation of one or more pathways, which can be detected by various biomarkers. Although there has been considerable laboratory research conducted on CSCs, its relevance to the practicing oncologist may seem questionable. We sought to determine the clinical impact of CSCs on patients. A systematic literature search was conducted to identify analyses containing survival information based on the expression of known CSC biomarkers in any cancer. Overall, 234 survival analyses were identified, of which 82% reported that high expression of CSC biomarker(s) resulted in poor overall survival and/or disease-free survival compared with low or no expression of the biomarker. Elevated stemness biomarker levels were also associated with decreased tumor differentiation, altered TNM stage, and increased metastasis. This analysis would suggest that CSCs have a clinical impact on patients and that practicing oncologists need to start considering incorporating CSC-targeting therapies into their patients' treatment regimens. IMPLICATIONS FOR PRACTICE: Cancer stem cells (CSCs) may occur at any stage of cancer and are implicated in the occurrence of resistance, recurrence, and metastasis. A systematic literature analysis has shown that the presence of CSCs, identified via the upregulation of stemness pathway biomarkers, results in reduced survival across all cancers studied. Several CSC-targeting agents are currently approved, and several others are in clinical trials. Future treatment regimens will likely include CSC-targeting agents to enable the elimination of these holdouts to current therapies.

ATIM-40. CIRCULATING MYELOID-DERIVED SUPPRESSOR CELLS PREDICT FAVORABLE RESPONSE TO IMMUNE CHECKPOINT THERAPY IN A RANDOMIZED TRIAL OF NIVOLUMAB AND BEVACIZUMAB IN RECURRENT GBM

Glioblastoma (GBM) creates an immunosuppressive environment that presents a challenge to efficacy of immunotherapeutic approaches. Results from the CheckMate-143 trial demonstrated responses in 8% of patients with nivolumab, underscoring the need for further insight into the mechanisms and markers of immune suppression and response. Given a limited set of biomarkers predictive of immunotherapy response in GBM, we explored the changes in immune cell populations in nivolumab and bevacizumab-treated GBM patients pre and post-treatment in order to help predict response. In these studies, we utilized traditional and newly developed approaches, including mass cytometry time-of-flight (CyTOF), single-cell RNA sequencing, and 10X Genomics simultaneous cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq). We analyzed patients’ samples in a randomized, phase 2 study of nivolumab and bevacizumab at GBM first recurrence (NCT03452579). Nine patients were identified as responders or non-responders at 8 weeks after therapy initiation. Utilizing peripheral blood samples, we observed a 6.4-fold decrease in immunosuppressive myeloid-derived suppressor cells (MDSCs) between baseline and first imaging follow-up in responders compared to non-responders, with a 4.9-fold decrease in the granulocytic MDSC (G-MDSC) subtype in responders over non-responders. While no significant changes in overall T-cell numbers were noted, expression of PD-1 on CD4+ T cells was significantly elevated at baseline and follow-up in responders as compared to non-responders – signatures which were confirmed by CyTOF. Given these immunophenotypic changes, preliminary results of a detailed investigation of this cohort by CITE-seq indicate that responders had increased IL7R-positive T cells post-treatment, which was not observed in non-responders. These results are currently being validated in an additional 40 patients that have been enrolled. Altogether, differences in immunophenotypes that were specific to responders and non-responders were observed, and characterization of these immune populations may be helpful in identifying GBM patients likely to benefit from immunotherapy.

EPID-01. SEX DIFFERENCE IN EXPRESSION OF IRON-RELATED GENES AND SURVIVAL IN GLIOBLASTOMA PATIENTS

Sex impacts the clinical outcome of glioblastoma (GBM) patients. Iron is a key metabolic driver in glioma biology impacting both the neoplastic cells directly and tumor progression through the immune system. We previously reported the expression of HFE (iron homeostatic) gene has a sexually dimorphic impact on survival in GBM patients. To further interrogate the relationship of iron and sexual dimorphism in GBMs, we expanded our analysis to include the ferrome (profile of genes and proteins involved in iron regulation). These genes were identified using IronChip and then interrogated for sexually dimorphic expression and survival in primary GBM patients using The Cancer Genome Atlas GBM database. Biological pathway analysis indicated that the Toll receptor signaling and immune system signaling pathways are most strongly correlated to sex difference and GBM patient survival. Toll like receptor 4 (TLR4) expression in the bottom quartile is associated with a modest survival benefit for female GBM patients compared to male, but in the top quartile of expression of this gene the survival for males is dramatically greater than that for females (18 months versus 3.6 months). Males expressing top quartile of Dual specificity phosphatases (DUSP1) also known as MKP1 survive 24.2 months whereas females survive 5.8 months. There is no sex effect of DUSP1 in the bottom quartile of expression. These data suggest that top quartile expression of TLR4/DUSP1 are both a benefit for males (extending their survival) and a detriment to females (shortening their survival). Both TLR4 and DUSP1 are richly expressed in macrophages. The data suggest that macrophage infiltration into the tumor could be particularly negatively impactful in females; a finding consistent with our previous reports for the HFE gene. In summary, iron related gene expression is linked to sexual dimorphism in survival and may provide insights on sex specific tumor biology.

TMIC-31. IMPACT OF IRON ON MACROPHAGE IMMUNE PHENOTYPE IN THE GLIOBLASTOMA TUMOR MICROENVIRONMENT

The tumor microenvironment (TME) in glioblastoma presents a significant hurdle to effective immunotherapies as it consists of an immunosuppressive niche that results in inhibition of anti-tumor immunity. Macrophages comprise a large portion of the glioblastoma TME as they are prolific secretors of immunosuppressive cytokines and can comprise of up to 30% of tumor volume. In addition to their crucial role in immune function, macrophages are important players in iron regulation owing to their ability to efficiently sequester and release iron. We studied the impact of iron status in macrophage anti-tumor immune functions and phenotypic plasticity using RAW264.7 macrophages and bone-marrow-derived macrophages (BMDMs). Using RAW264.7 cells, we found that the iron status of macrophages impacts their immune function by modulating expression of the co-stimulation membrane proteins CD80 and CD86 as well as the mannose receptor CD206. Interestingly, the iron-mediated immunomodulation was dependent on the formulation of iron with nanoparticle formulations such as ferumoxytol (Fe-NP) upregulating CD80, CD86, and decreasing CD206 while ferric ammonium citrate (FAC) downregulated CD80, CD86 and upregulated CD206. We analyzed expression of inflammatory cytokines in BMDMs with multiplex cytokine analysis and found that both FAC and Fe-NP increased expression of the chemotactic signals CXCL10, CCL1, CCL3, and CCL4. Additionally, we found that iron status impacts the ability of macrophages to repolarize from an inflammatory, immune-activating phenotype into an immunosuppressive phenotype upon exposure to glioblastoma tumor-conditioned media. We used qRT-PCR to examine gene expression of the tumor immunity-related genes TNFa, IL1B, NOS2, and IL10 and found that iron loaded RAW264.7 macrophages stimulated with lipopolysaccharide resisted repolarization into an immunosuppressive phenotype 24 hours after exposure to glioblastoma tumor-conditioned media. Our results suggest that understanding the link between iron status and immune function in the tumor microenvironment may be an important step in improving therapies against glioblastoma.

STEM-26. ALTERED LIPID METABOLISM MARKS GLIOBLASTOMA STEM AND NON-STEM CELLS IN SEPARATE TUMOR NICHES

Clinical glioblastoma is marked by a strikingly heterogeneous mix of cell types, cellular metabolisms, and cellular microenvironments spread in different spatial locations throughout a tumor. We have created 3-dimensional organoid models that partially mimic the transition zone between nutrient-rich cellular tumor regions and nutrient-poor psuedopallisading and perinecrotic tumor zones. We found a dramatic disparity in lipid droplet presence between these regions with high lipid accumulation in the hypoxic organoid cores of a wide spectrum of patient derived specimens. This is accompanied by regionally restricted upregulation of HILPDA gene expression in the cores of our models, in clinical GBM specimens but not lower grade brain tumors, and localized specifically to pseudopallisading regions of patient tumors. We further show that lipid droplet accumulation overall marks perinecrotic and pseudopallisading regions in clinical GBM, indicating broadly altered lipid metabolism between these distinct cell populations. High lipid droplet accumulation is largely restricted to the non-stem cell populations of GBM organoids and sorted xenograft tumors whereas the stem cells are lipid-poor, suggesting lipid levels may not be simply a product of the microenvironment but also may be a reflection of cell state. We performed global lipidomic analysis on prospectively sorted stem and non-stem cells of multiple patient-derived models and found that GBM stem cells have comparatively decreased levels of neutral lipids, indicating a significant metabolic shift compared to non-stem cells from the same patient. Conversely, GBM stem cells have significantly increased levels of rare specific lipid species, and also display altered phospholipid synthesis and species specific alterations in phospholipid classes. Our findings suggest avenues for therapy by targeting the altered lipid metabolic pathways of these disparate tumor cell populations.

EPID-03. HISTOLOGY-SPECIFIC BRAIN TUMOR INCIDENCE AND SURVIVAL VARIES BY SEX

BACKGROUND

Significant sex differences exist in cancer, and males have higher incidence and lower survival compared to females for most cancers. No large-scale studies have systematically examined sex differences in incidence and survival across all primary brain histologies. We performed a comprehensive investigation of the differences in incidence and survival in patients diagnosed with primary malignant brain and other CNS tumors by specific histologies.

METHODS

Age-adjusted incidence rate ratios (IRR) and 95% confidence intervals (945% CI) were generated from the United States Cancer Statistics (USCS) Public Use Database. Data from the Surveillance, Epidemiology, and End-Results (SEER) program were used to calculate overall survival. Data was restricted to patients with histologically or radiologically confirmed, primary malignant tumors diagnosed between 2001 and 2015. Histological groupings were categorized based on the Central Brain Tumor Registry of the United States (CBTRUS). Cox proportional hazards models were used to calculate hazard ratios (HR) adjusted for age for males as compared to females.

RESULTS

Males exhibited higher incidence than females in all brain histologies except meningioma (IRR=0.83; 95% CI 0.73–0.93) and other neuroepithelial tumors (i.e. polar spongioblastomas and astroblastomas) (IRR=0.48; 95% CI 0.26–0.88). Males experienced better overall survival in germ cell tumors, cysts and heterotopias (HR=0.68; 95% CI 0.49–0.94) compared to females, but were observed to have lower survival in all other histologies. Survival was lowest for males among patients with nerve sheath tumors (HR=2.32; 95% CI 1.31–4.12) and other neoplasms related to the meninges (e.g. chondrosarcomas and chordomas) (HR=2.23; 95% CI 1.24–3.99). Survival in meningioma patients was significantly higher in females (HR=1.49; 95% CI 1.25–1.77). Patients with glioblastomas, had a slightly worse survival outcome in males (HR=1.02, 95% CI 1.00–1.05).

CONCLUSION

Understanding the role of sex differences is critical for addressing sex based inequalities and needs to be taken into account in clinical paradigms.

TMIC-02. JUNCTIONAL ADHESION MOLECULE-A (JAM-A) DEFICIENCY DRIVES SEX-SPECIFIC DIFFERENCES IN GLIOBLASTOMA PROGRESSION VIA DIFFERENTIAL MICROGLIA RESPONSES IN THE TUMOR MICROENVIRONMENT

Despite the male preponderance for developing glioblastoma (GBM) and better survival outcomes in females, current treatment paradigms do not account for biological sex as a biological or clinical variable. Sex-specific molecular alterations that drive tumor cell growth and therapy response have been documented, however, sex-specific extrinsic differences in the tumor microenvironment have yet to be identified. Based on well-established sex-specific gene signatures and functional differences in microglia, we interrogated influences of male and female microglia in driving GBM growth. Specifically, manipulation of JAM-A expression, a tight junction protein on microglia, was exploited as a paradigm for determining effects on in vivo syngeneic GBM mouse models. Male and female JAM-A KO mice that received orthotopic injection of syngeneic GBM cells presented differential overall survival distinct from their wildtype counterparts. Wild-type male mice phenocopied human males, presenting shorter overall survival than females, this trend was reversed in JAM-A KO mice. Compared to the other genotypes, female JAM-A KO mice presented the greatest number of phagocytic, tumor-promoting, activated microglia. RNA-sequencing of tumors from JAM-A KO and WT mice revealed that female JAM-A KO mice had increased expression of Ifi202b (interferon activated gene 202b), a member of the Activity-regulated Inhibitor of Death (AID) gene family that contributes to mitochondrial resistance to cellular stress. Ifi202b has a role in sex-specific inflammatory diseases, which is consistent with our observation. Female KO microglia had enhanced Ifi202b expression, along with the secretion of Ifi202b associated cytokines, including interleukin-6. Treatment of wild-type female microglia with a JAM-A function blocking antibody demonstrated an increase in Ifi202b levels, confirming direct regulation of Ifi202b expression by neutralizing JAM-A. While cell intrinsic, sex-specific differences have been reported in GBM, our findings demonstrate that differences in the GBM tumor microenvironment also drive sexually dimorphic tumor growth.

TMIC-01. THE DYSTROGLYCAN RECEPTOR MAINTAINS GLIOMA STEM CELLS IN THE VASCULAR NICHE

Glioblastomas (GBMs) are malignant central nervous system (CNS) neoplasms with a very poor prognosis. They display cellular hierarchies containing self-renewing tumourigenic glioma stem cells (GSCs) in a complex heterogeneous microenvironment. One proposed GSC niche is the extracellular matrix (ECM)-rich perivascular bed of the tumour. Here, we report that the ECM binding alpha (α) subunit of the dystroglycan (DG) receptor is expressed and functionally glycosylated on GSCs residing in the vascular niche. Glycosylated αDG is also expressed highly on the most aggressive mesenchymal-like GBM tumour tissue. Furthermore, we found that DG acts to maintain a de-differentiated stem cell-like phenotype via tight control of MAPK activation. Antibody-based blockade of αDG induces robust ERK-mediated differentiation leading to reduced GSC potential. DG was shown to be required for tumour initiation, with constitutive loss significantly delaying or preventing tumourigenic potential in-vivo. These findings reveal a central role of the DG receptor not only as a structural element but also as a critical factor in the maintenance of GSCs in the GBM vascular niche.

STEM-03. CONSUMPTION OF A HIGH-FAT DIET INHIBITS THE TUMOR SUPPRESSIVE ACTIVITY OF HYDROGEN SULFIDE, DRIVING CANCER STEM CELL ENRICHMENT AND DISEASE AGGRESSION IN GLIOBLASTOMA

Glioblastoma (GBM) remains among the deadliest of human malignancies. Effective disease management is lacking due in part to the emergence of the cancer stem cell (CSC) phenotype. The tumor cell extrinsic, environmental, and lifestyle factors that result in CSC enrichment are not well understood. Alongside other pathological features, the CSC state endows populations of tumor cells with a fluid metabolic profile that enables utilization of multiple nutrition sources. Therefore, to test the impact of diet on CSC enrichment, we interrogated disease progression in tumor-bearing mice fed either a high-fat diet (HFD), similar to the Western Pattern diet or a control low-fat diet. Compared to controls, HFD-consumption resulted in the presentation of a hyper-aggressive disease phenotype with truncated survival and tumors markedly enriched in tumor-initiating SOX2+ CSCs. To understand the underlying mechanism driving this finding, we examined tumors for the diet-regulated metabolite hydrogen sulfide (H2S). H2S is an endogenously produced bio-active gasotransmitter similar to nitric oxide. It functions principally through protein S-sulfhydration to regulate a variety of cellular programs including mitochondrial function, stress signaling and metabolism. While there is exceedingly limited information on H2S and GBM, its HFD-driven suppression has been reported in other organ systems. We discovered a significant reduction in H2S synthesis resulting from HFD-consumption in the brain of the mouse and a striking decrease in protein S-sulfhydration in human GBM tumor tissue when compared to non-cancerous control brain tissue. We demonstrated that chemical inhibition of H2S synthesis resulted in increased tumor cell viability whereas exposure to chemical H2S donors led to pronounced cell death of cultured mouse and human GBM cells. These data demonstrate for the first time, that H2S serves as a tumor suppressor for GBM. Moreover, the diet-driven suppression of H2S helps explain the hyper-aggressive in vivo phenotype that presents in response to HFD-consumption.

TMIC-53. IMPACT OF HFE EXPRESSION AND SEX ON THE TUMOR IMMUNE MICROENVIRONMENT IN GLIOBLASTOMA

Glioblastoma (GBM) is the most common primary malignant brain tumor in adults. Despite aggressive treatment, the median survival for patients with GBM remains approximately 1 year. Recent evidence demonstrates GBM is a sexually dimorphic disease, and that females have greater overall survival. We previously showed expression levels of HFE, an iron-regulating gene, significantly impacts survival in GBM. Moreover, this effect is sex-specific: females with low HFE expressing tumors have significantly longer survival than males with similar HFE expression. Moreover, HFE expression and survival appeared to correlate with markers of immune activity within the tumors. To further explore the impact of HFE expression on the immune infiltration of GBM, we utilized available TCGA GBM data for analysis by EstimateMe and DeconvuluteMe. These platforms utilize expression data to estimate the magnitude and cellular composition of immune cell infiltrates in tumors, respectively. We sought to correlate these computational measures with HFE expression levels and survival. We confirmed that immune and stromal cell infiltration of tumors is negatively correlated to survival. Consistent with our findings that high HFE levels have a negative impact on survival, high HFE expressing tumors possessed higher levels of both immune and stromal cell infiltration. Additionally, sex of the patient, but not MGMT methylation status, was significantly correlated with immune and stromal infiltration, with males displaying higher levels of infiltration. High HFE tumors were composed of greater numbers of all immune cell types than low HFE tumors. In both HFE groups, monocytes and macrophages comprised the greatest fraction of cells. At low HFE and positive MGMT methylation (where we previously noted the greatest survival differences), we find females have lower monocyte and M2 macrophage markers compared to males with similar features. Collectively, these data suggest that an HFE-immune cell infiltrate axis contributes to sex differences in GBM survival.

IMMU-28. TARGETING IMMUNOSUPPRESSIVE MYELOID DERIVED SUPPRESSOR CELLS VIA MIF/CD74 SIGNALING AXIS TO ATTENUATE GBM GROWTH

The immunosuppressive microenvironment in glioblastoma (GBM) enables persistent tumor growth and evasion from tumoricidal immune cell recognition. Despite a large accumulation of immune cells in the GBM microenvironment, tumor growth continues, and evidence for potent immunosuppression via myeloid derived suppressor cells (MDSCs) is now emerging. In agreement with these observations, we have recently established that increased MDSCs over time correlates with poor prognosis in GBM, making these cells of interest for therapeutic targeting. In seeking to reduce MDSCs in GBM, we previously identified the cytokine macrophage migration inhibitory factor (MIF) as a possible activator of MDSC function in GBM. Here, using a novel in vitro co-culture system to reproducibly and rapidly create GBM-educated MDSCs, we observed that MIF was essential in the generation of MDSCs and that MDSCs generated via this approach express a repertoire of MIF receptors. CD74 was the primary MIF receptor in monocytic MDSCs (M-MDSC), which penetrate the tumor microenvironment in preclinical models and patient samples. A screen of MIF/CD74 interaction inhibitors revealed that MN-166, a clinically relevant blood brain barrier penetrant drug, which is currently fast tracked for FDA approval, reduced MDSC generation and function in vitro. This effect was specific to M-MDSC subsets expressing CD74, and appeared as reduced downstream pERK signaling and MCP-1 secretion. In vivo, MN-166 was able reduce tumor-infiltrating MDSCs, while conferring a significant increase in survival in the syngeneic glioma model GL261. These data provide proof of concept that M-MDSCs can be targeted in the tumor microenvironment via MN-166 to reduce tumor growth and provide a rationale for future clinical assessment of MN-166 to reduce M-MDSCs in the tumor microenvironment. Ongoing studies are assessing the effects of MDSC inhibition in combination with immune activating approaches, in order to inhibit immune suppression while simultaneously activating the immune system.

IMMU-01. MYELOID-DERIVED SUPPRESSOR CELL HETEROGENEITY DRIVES GLIOBLASTOMA PROGRESSION IN A SEX-DEPENDENT MANNER

An immunosuppressive tumor microenvironment is a major factor facilitating glioblastoma (GBM) progression and therapeutic resistance. Immunotherapies have had variable success in improving the outcome of GBM patients, suggesting that there is a need to gain insight into the mechanisms of immunosuppression. Our group previously demonstrated that myeloid-derived suppressor cells (MDSCs) expand in GBM patients and infiltrate tumors, where they suppress the activity of cytotoxic cells. However, the mechanisms by which individual MDSC subsets promote tumorigenesis remain understudied. Using the syngeneic mouse glioma models GL261, CT-2A and SB28, we show that monocytic MDSCs (mMDSCs) are prevalent in tumors and that their frequency is significantly higher in males, who constitute 60% of GBM patients and have a worse prognosis than females. mMDSC abundance was further associated with poor survival, and male mice reached morbidity endpoint earlier. Consistent with preclinical observations, male GBM patient specimens had significantly more IBA+CD204+ immunosuppressive myeloid cells compared to female GBM tissue. In contrast, female tumor-bearing mice had a two-fold increase in circulating granulocytic MDSC (gMDSC) frequency, while this population remained unchanged in males. Female-to-male bone marrow chimeras demonstrated that intrinsic discrepancies in immune cell characteristics drive the sex differences in survival. Consistent with the differential MDSC accumulation pattern, targeting gMDSCs with anti-Ly6G neutralizing antibodies extended the lifespan of female mice without providing a survival advantage to males. However, mMDSCs were protected from the anti-Ly6C depletion strategy due to their systemic and local proliferation, as indicated by ex vivo Ki-67 staining and subsequently confirmed by gene expression analysis. Drug-prediction algorithms using the differential RNA sequencing profiles demonstrated that mMDSCs can be targeted by chemotherapeutics, while immunomodulatory drugs are effective against gMDSCs. Collectively, these findings indicate that MDSC subset variation might represent a therapeutic opportunity for improved therapeutic efficacy of immunotherapies while accounting for sex as a biological variable.

COMP-17. LARGE-SCALE TRANSCRIPTOMIC CHARACTERIZATION OF PRIMARY AND RECURRENT GLIOBLASTOMA IDENTIFIES GENE EXPRESSION SIGNATURE OF TUMOR RESPONSE TO STANDARD THERAPY

A near-universal phenomenon in glioblastoma is disease recurrence following surgical resection and chemoradiotherapy. Development of biomarkers predictive of therapeutic response to better guide care and inform future targeted therapies is crucial. In this work, a total of 84 glioblastoma surgical specimens involving 44 primary tumors and 40 matched samples at time of re-resection, were characterized utilizing RNA-sequencing. Transcriptomic analysis was carried out with the goal of identifying underlying differences between those patients with prolonged response to standard therapy and delayed time to re-resection. We examined individual gene expression, gene coexpression networks, and well-known gene pathways in this dataset that showed consistent association with time to re-resection in both primary and progressed specimens, independent of tumor molecular subtype. Leveraging this large, well-characterized dataset, and using a novel computational methodology based on a seed-gene approach, we identified a predictive gene signature for therapeutic response. Our analyses revealed a striking degree of heterogeneity among gene expression associated with response to standard therapy and time to re-resection, adding to the complexity of signature derivation. The novel signature we obtained for response showed components involving genes such as those in the IGF pathway (IGF2BP2, IGF2BP3) and PDGF-signalling pathway (MYC, FLI1, ARHGAP4, JAK3) predictive of poor response to therapy. Likewise, predictors of positive response to therapy included genes involved in the apoptosis and RAS pathways (RAB4A, CHUK) and DNA replication pathways (SSBP2). In sum, this is among the largest cohorts of well-characterized clinical tumor samples for which there is transcriptomic information from primary and re-resected samples from matched patients. Our results not only highlight an innovative computational method for gene signature derivation in the setting of significant underlying heterogeneity, but also result in a predictive gene signature, offering the potential to give therapy to those who stand to benefit most.