Optimization of a mouse model of pancreatic cancer to simulate the human phenotypes of metastasis and cachexia

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) presents with a high mortality rate. Two important features of PDAC contribute to this poor outcome. The first is metastasis which occurs in ~ 80% of PDAC patients. The second is cachexia, which compromises treatment tolerance for patients and reduces their quality of life. Although various mouse models of PDAC exist, recapitulating both metastatic and cachectic features have been challenging.

METHODS

Here, we optimize an orthotopic mouse model of PDAC by altering several conditions, including the subcloning of parental murine PDAC cells, implantation site, number of transplanted cells, and age of recipient mice. We perform spatial profiling to compare primary and metastatic immune microenvironments and RNA sequencing to gain insight into the mechanisms of muscle wasting in PDAC-induced cachexia, comparing non-metastatic to metastatic conditions.

RESULTS

These modifications extend the time course of the disease and concurrently increase the rate of metastasis to approximately 70%. Furthermore, reliable cachexia endpoints are achieved in both PDAC mice with and without metastases, which is reminiscent of patients. We also find that cachectic muscles from PDAC mice with metastasis exhibit a similar transcriptional profile to muscles derived from mice and patients without metastasis.

CONCLUSION

Together, this model is likely to be advantageous in both advancing our understanding of the mechanism of PDAC cachexia, as well as in the evaluation of novel therapeutics.

The impact of inflammation and acute phase activation in cancer cachexia

The development of cachexia in the setting of cancer or other chronic diseases is a significant detriment for patients. Cachexia is associated with a decreased ability to tolerate therapies, reduction in ambulation, reduced quality of life, and increased mortality. Cachexia appears intricately linked to the activation of the acute phase response and is a drain on metabolic resources. Work has begun to focus on the important inflammatory factors associated with the acute phase response and their role in the immune activation of cachexia. Furthermore, data supporting the liver, lung, skeletal muscle, and tumor as all playing a role in activation of the acute phase are emerging. Although the acute phase is increasingly being recognized as being involved in cachexia, work in understanding underlying mechanisms of cachexia associated with the acute phase response remains an active area of investigation and still lack a holistic understanding and a clear causal link. Studies to date are largely correlative in nature, nonetheless suggesting the possibility for a role for various acute phase reactants. Herein, we examine the current literature regarding the acute phase response proteins, the evidence these proteins play in the promotion and exacerbation of cachexia, and current evidence of a therapeutic potential for patients.

Pubertal exposure to dietary advanced glycation end products disrupts ductal morphogenesis and induces atypical hyperplasia in the mammary gland

BACKGROUND

Advanced glycation end products (AGEs) are reactive metabolites intrinsically linked with modern dietary patterns. Processed foods, and those high in sugar, protein and fat, often contain high levels of AGEs. Increased AGE levels are associated with increased breast cancer risk, however their significance has been largely overlooked due to a lack of direct cause-and-effect relationship.

METHODS

To address this knowledge gap, FVB/n mice were fed regular, low AGE, and high AGE diets from 3 weeks of age and mammary glands harvested during puberty (7 weeks) or adulthood (12 weeks and 7 months) to determine the effects upon mammary gland development. At endpoint mammary glands were harvested and assessed histologically (n ≥ 4). Immunohistochemistry and immunofluorescence were used to assess cellular proliferation and stromal fibroblast and macrophage recruitment. The Kruskal-Wallis test were used to compare continuous outcomes among groups. Mammary epithelial cell migration and invasion in response to AGE-mediated fibroblast activation was determined in two-compartment co-culture models. In vitro experiments were performed in triplicate. The nonparametric Wilcoxon rank sum test was used to compare differences between groups.

RESULTS

Histological analysis revealed the high AGE diet delayed ductal elongation, increased primary branching, as well as increased terminal end bud number and size. The high AGE diet also led to increased recruitment and proliferation of stromal cells to abnormal structures that persisted into adulthood. Atypical hyperplasia was observed in the high AGE fed mice. Ex vivo fibroblasts from mice fed dietary-AGEs retain an activated phenotype and promoted epithelial migration and invasion of non-transformed immortalized and tumor-derived mammary epithelial cells. Mechanistically, we found that the receptor for AGE (RAGE) is required for AGE-mediated increases in epithelial cell migration and invasion.

CONCLUSIONS

We observed a disruption in mammary gland development when mice were fed a diet high in AGEs. Further, both epithelial and stromal cell populations were impacted by the high AGE diet in the mammary gland. Educational, interventional, and pharmacological strategies to reduce AGEs associated with diet may be viewed as novel disease preventive and/or therapeutic initiatives during puberty.

Hepatic protein kinase Cbeta deficiency mitigates late-onset obesity

Although aging is associated with progressive adiposity and a decline in liver function, the underlying molecular mechanisms and metabolic interplay are incompletely understood. Here, we demonstrate that aging induces hepatic protein kinase Cbeta (PKCβ) expression, while hepatocyte PKCβ deficiency (PKCβHep-/-) in mice significantly attenuates obesity in aged mice fed a high-fat diet. Compared with control PKCβfl/fl mice, PKCβHep-/- mice showed elevated energy expenditure with augmentation of oxygen consumption and carbon dioxide production which was dependent on β3-adrenergic receptor signaling, thereby favoring negative energy balance. This effect was accompanied by induction of thermogenic genes in brown adipose tissue (BAT) and increased BAT respiratory capacity, as well as a shift to oxidative muscle fiber type with an improved mitochondrial function, thereby enhancing oxidative capacity of thermogenic tissues. Furthermore, in PKCβHep-/- mice, we determined that PKCβ overexpression in the liver mitigated elevated expression of thermogenic genes in BAT. In conclusion, our study thus establishes hepatocyte PKCβ induction as a critical component of pathophysiological energy metabolism by promoting progressive hepatic and extrahepatic metabolic derangements in energy homeostasis, contributing to late-onset obesity. These findings have potential implications for augmenting thermogenesis as a means of combating aging-induced obesity.

Exploring switch II pocket conformation of KRAS(G12D) with mutant-selective monobody inhibitors

The G12D mutation is among the most common KRAS mutations associated with cancer, in particular, pancreatic cancer. Here, we have developed monobodies, small synthetic binding proteins, that are selective to KRAS(G12D) over KRAS(wild type) and other oncogenic KRAS mutations, as well as over the G12D mutation in HRAS and NRAS. Crystallographic studies revealed that, similar to other KRAS mutant-selective inhibitors, the initial monobody bound to the S-II pocket, the groove between switch II and α3 helix, and captured this pocket in the most widely open form reported to date. Unlike other G12D-selective polypeptides reported to date, the monobody used its backbone NH group to directly recognize the side chain of KRAS Asp12, a feature that closely resembles that of a small-molecule inhibitor, MTRX1133. The monobody also directly interacted with H95, a residue not conserved in RAS isoforms. These features rationalize the high selectivity toward the G12D mutant and the KRAS isoform. Structure-guided affinity maturation resulted in monobodies with low nM KD values. Deep mutational scanning of a monobody generated hundreds of functional and nonfunctional single-point mutants, which identified crucial residues for binding and those that contributed to the selectivity toward the GTP- and GDP-bound states. When expressed in cells as genetically encoded reagents, these monobodies engaged selectively with KRAS(G12D) and inhibited KRAS(G12D)-mediated signaling and tumorigenesis. These results further illustrate the plasticity of the S-II pocket, which may be exploited for the design of next-generation KRAS(G12D)-selective inhibitors.

Abstract 3643: PTEN/STAT3 pathway in cancer-associated fibroblasts in pancreatic cancer

This study aims to test the hypothesis that direct interactions and auto-regulatory loop between PTEN and STAT3 in cancer-associated fibroblasts (CAFs) contribute to an immunosuppressive tumor microenvironment in pancreatic ductal carcinoma (PDAC). Pancreatic cancer accounts for almost as many deaths as cases because of its poor prognosis. Many studies have led to the paradigm that stromal elements contribute to pancreatic tumor growth. Cancer-associated fibroblasts (CAFs) are a primary cell type in the desmoplastic stroma in PDAC, and targeting them to improve PDAC treatment is thought to be a promising strategy, however; studies have shown both tumor-promoting and tumor-limiting activities associated with PDAC CAFs. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is one of the most well-described tumor suppressor genes and can regulate essential transcription factors in stromal fibroblasts. On the other hand, STAT3 signaling in pancreatic CAFs contributes to the immunosuppressive and fibrotic stroma observed with disease progression. In this study, we used novel, dual recombinase mouse models of PDAC, with Kras and p53 mutations engineered in the pancreatic epithelium (Flp-based) and Stat3 deleted in a subpopulation of CAFS (Cre). CAFs isolated from these mice were used to probe PTEN-STAT3 interaction in vitro using the Proximity Ligation Assay (PLA) and by ChIP-seq using an RNA seq. Mouse and Human pancreatic tumor samples were analyzed and quantified using multiplex-immunofluorescence and multispectral imaging. In the mouse genetic model, we found PTEN downregulated in ~80% of CAFs, consistent with previous work from our group that demonstrated PTEN loss in 25% of PDAC patient samples. At the same time, pY705-STAT3 was activated in CAFs, and deletion of STAT3 in CAFs correlated with increased survival, decreased M2-like macrophages, and increased T-cell infiltration. STAT3 ablation in CAFS resulted in the re-expression of PTEN in CAFS, implying an autoregulatory loop. PLA on CAFs isolated revealed that IL6 treatment resulted in the transfer of a STAT3-PTEN protein complex into the nucleus ChIP-seq on CAFs with PTEN STAT3 antibodies revealed a significant overlap in PTEN and STAT3 occupation of gene promoters. This result is consistent with in silico analysis on the PTEN and STAT3 ChIP-Seq data in human cell lines. ChIP-Seq data is being interacted with RNA data to identify essential target genes. Human PDAC tissue microarrays are stained with pSTAT3, PTEN, CD3, and CD8.Our study suggests an autoregulatory look between PTEN and STAT3 in PDAC CAFs. PTEN may suppress specific STAT3 target genes through direct interactions with chromatin. Alternatively, PTEN/STAT3 complexes may target CAF genes that suppress tumor growth or activate anti-tumor immune responses. These investigations could identify potential targets in CAFs for restoring the anti-tumor immune responses in PDAC.

Citation Format: Samaneh Saberi, Sudarshana Sharma, Cameron Bumbleburg, Julia E. Lefler, Michael C. Ostrowski. PTEN/STAT3 pathway in cancer-associated fibroblasts in pancreatic cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3643.

Cancer cachexia: involvement of an expanding macroenvironment

Advanced cancers often present with the cachexia syndrome that impacts peripheral tissues, leading to involuntary weight loss and reduced prognosis. The central tissues undergoing depletion are skeletal muscle and adipose, but recent findings reveal an expanding tumor macroenvironment involving organ crosstalks that underlie the cachectic state.

STAT3 in tumor fibroblasts promotes an immunosuppressive microenvironment in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is associated with an incredibly dense stroma, which contributes to its recalcitrance to therapy. Cancer-associated fibroblasts (CAFs) are one of the most abundant cell types within the PDAC stroma and have context-dependent regulation of tumor progression in the tumor microenvironment (TME). Therefore, understanding tumor-promoting pathways in CAFs is essential for developing better stromal targeting therapies. Here, we show that disruption of the STAT3 signaling axis via genetic ablation of Stat3 in stromal fibroblasts in a Kras G12D PDAC mouse model not only slows tumor progression and increases survival, but re-shapes the characteristic immune-suppressive TME by decreasing M2 macrophages (F480+CD206+) and increasing CD8+ T cells. Mechanistically, we show that loss of the tumor suppressor PTEN in pancreatic CAFs leads to an increase in STAT3 phosphorylation. In addition, increased STAT3 phosphorylation in pancreatic CAFs promotes secretion of CXCL1. Inhibition of CXCL1 signaling inhibits M2 polarization in vitro. The results provide a potential mechanism by which CAFs promote an immune-suppressive TME and promote tumor progression in a spontaneous model of PDAC.

Stromal p53 Regulates Breast Cancer Development, the Immune Landscape, and Survival in an Oncogene-Specific Manner

Coevolution of tumor cells and adjacent stromal elements is a key feature during tumor progression; however, the precise regulatory mechanisms during this process remain unknown. Here, we show stromal p53 loss enhances oncogenic KrasG12D, but not ErbB2, driven tumorigenesis in murine mammary epithelia. Stroma-specific p53 deletion increases both epithelial and fibroblast proliferation in mammary glands bearing the KrasG12D oncogene in epithelia, while concurrently increasing DNA damage and/or DNA replication stress and decreasing apoptosis in the tumor cells proper. Normal epithelia was not affected by stromal p53 deletion. Tumors with p53-null stroma had a significant decrease in total, cytotoxic, and regulatory T cells; however, there was a significant increase in myeloid-derived suppressor cells, total macrophages, and M2-polarized tumor-associated macrophages, with no impact on angiogenesis or connective tissue deposition. Stroma-specific p53 deletion reprogrammed gene expression in both fibroblasts and adjacent epithelium, with p53 targets and chemokine receptors/chemokine signaling pathways in fibroblasts and DNA replication, DNA damage repair, and apoptosis in epithelia being the most significantly impacted biological processes. A gene cluster in p53-deficient mouse fibroblasts was negatively associated with patient survival when compared with two independent datasets. In summary, stroma-specific p53 loss promotes mammary tumorigenesis in an oncogene-specific manner, influences the tumor immune landscape, and ultimately impacts patient survival.

IMPLICATIONS

Expression of the p53 tumor suppressor in breast cancer tumor stroma regulates tumorigenesis in an oncogene-specific manner, influences the tumor immune landscape, and ultimately impacts patient survival.

Origin, activation and heterogeneity of fibroblasts associated with pancreas and breast cancers

Pancreas and breast cancers both contain abundant stromal components within the tumor tissues. A prominent cell type within the stroma is cancer-associated fibroblasts (CAFs). CAFs play critical and complex roles establishing the tumor microenvironment to either promote or prevent tumor progression. Recently, complex genetic models and single cell-based techniques have provided emerging insights on the precise functions and cellular heterogeneity of CAFs. The transformation of normal fibroblasts into CAFs is a key event during tumor initiation and progression. Such coordination between tumor cells and fibroblasts plays an important role in cancer development. Reprograming fibroblasts is currently being explored for therapeutic benefits. In this review, we will discuss recent literature shedding light on the tissues of origin, activation mechanisms, and heterogeneity of CAFs comparing pancreas and breast cancers.

PTEN in cancer associated fibroblasts

Decades of research have concluded that disruptions to Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) have profound effects on cancer progression. However, as our understanding of the tumor stroma has evolved, we can appreciate that disruptions to tumor suppressors such as PTEN should not be studied solely in an epithelial context. Inactivation of PTEN in the stroma is associated with worse outcomes in human cancers, therefore, it is important to understand activities regulated downstream of PTEN in stromal compartments. Studies reviewed herein provide evidence for important mechanistic targets downstream of PTEN signaling in cancer-associated fibroblasts (CAFs), a major component of the tumor stroma. We also discuss the potential clinical implications for these findings.

Targeting the KRAS α4-α5 allosteric interface inhibits pancreatic cancer tumorigenesis

RAS is the most frequently mutated oncogene in human cancer with nearly ~20% of cancer patients possessing mutations in one of three RAS genes (K, N or HRAS). However, KRAS is mutated in nearly 90% of pancreatic ductal carcinomas (PDAC). Although pharmacological inhibition of RAS has been challenging, KRAS(G12C)-specific inhibitors have recently entered the clinic. While KRAS(G12C) is frequently expressed in lung cancers, it is rare in PDAC. Thus, more broadly efficacious RAS inhibitors are needed for treating KRAS mutant-driven cancers such as PDAC. A RAS-specific tool biologic, NS1 Monobody, inhibits HRAS- and KRAS-mediated signalling and oncogenic transformation both in vitro and in vivo by targeting the α4-α5 allosteric site of RAS and blocking RAS self-association. Here, we evaluated the efficacy of targeting the α4-α5 interface of KRAS as an approach to inhibit PDAC development using an immunocompetent orthotopic mouse model. Chemically regulated NS1 expression inhibited ERK and AKT activation in KRAS(G12D) mutant KPC PDAC cells and reduced the formation and progression of pancreatic tumours. NS1-expressing tumours were characterized by increased infiltration of CD4 + T helper cells. These results suggest that targeting the #x3B1;4-#x3B1;5 allosteric site of KRAS may represent a viable therapeutic approach for inhibiting KRAS-mutant pancreatic tumours.

Role of hepatic PKCβ in nutritional regulation of hepatic glycogen synthesis

The signaling mechanisms by which dietary fat and cholesterol signals regulate central pathways of glucose homeostasis are not completely understood. By using a hepatocyte-specific PKCβ-deficient (PKCβHep-/-) mouse model, we demonstrated the role of hepatic PKCβ in slowing disposal of glucose overload by suppressing glycogenesis and increasing hepatic glucose output. PKCβHep-/- mice exhibited lower plasma glucose under the fed condition, modestly improved systemic glucose tolerance and mildly suppressed gluconeogenesis, increased hepatic glycogen accumulation and synthesis due to elevated glucokinase expression and activated glycogen synthase (GS), and suppressed glucose-6-phosphatase expression compared with controls. These events were independent of hepatic AKT/GSK-3α/β signaling and were accompanied by increased HNF-4α transactivation, reduced FoxO1 protein abundance, and elevated expression of GS targeting protein phosphatase 1 regulatory subunit 3C in the PKCβHep-/- liver compared with controls. The above data strongly imply that hepatic PKCβ deficiency causes hypoglycemia postprandially by promoting glucose phosphorylation via upregulating glucokinase and subsequently redirecting more glucose-6-phosphate to glycogen via activating GS. In summary, hepatic PKCβ has a unique and essential ability to induce a coordinated response that negatively affects glycogenesis at multiple levels under physiological postprandial conditions, thereby integrating nutritional fat intake with dysregulation of glucose homeostasis.

Defining the Tumor Microenvironment by Integration of Immunohistochemistry and Extracellular Matrix Targeted Imaging Mass Spectrometry

Breast stroma plays a significant role in breast cancer risk and progression yet remains poorly understood. In breast stroma, collagen is the most abundantly expressed protein and its increased deposition and alignment contributes to progression and poor prognosis. Collagen post-translation modifications such as hydroxylated-proline (HYP) control deposition and stromal organization. The clinical relevance of collagen HYP site modifications in cancer processes remains undefined due to technical issues accessing collagen from formalin-fixed, paraffin-embedded (FFPE) tissues. We previously developed a targeted approach for investigating collagen and other extracellular matrix proteins from FFPE tissue. Here, we hypothesized that immunohistochemistry staining for fibroblastic markers would not interfere with targeted detection of collagen stroma peptides and could reveal peptide regulation influenced by specific cell types. Our initial work demonstrated that stromal peptide peak intensities when using MALD-IMS following IHC staining (αSMA, FAP, P4HA3 and PTEN) were comparable to serial sections of nonstained tissue. Analysis of histology-directed IMS using PTEN on breast tissues and TMAs revealed heterogeneous PTEN staining patterns and suggestive roles in stromal protein regulation. This study sets the foundation for investigations of target cell types and their unique contribution to collagen regulation within extracellular matrix niches.

The small G-protein RalA promotes progression and metastasis of triple-negative breast cancer

Background

Breast cancer (BC) is the most common cancer in women and the leading cause of cancer-associated mortality in women. In particular, triple-negative BC (TNBC) has the highest rate of mortality due in large part to the lack of targeted treatment options for this subtype. Thus, there is an urgent need to identify new molecular targets for TNBC treatment. RALA and RALB are small GTPases implicated in growth and metastasis of a variety of cancers, although little is known of their roles in BC.

Methods

The necessity of RALA and RALB for TNBC tumor growth and metastasis were evaluated in vivo using orthotopic and tail-vein models. In vitro, 2D and 3D cell culture methods were used to evaluate the contributions of RALA and RALB during TNBC cell migration, invasion, and viability. The association between TNBC patient outcome and RALA and RALB expression was examined using publicly available gene expression data and patient tissue microarrays. Finally, small molecule inhibition of RALA and RALB was evaluated as a potential treatment strategy for TNBC in cell line and patient-derived xenograft (PDX) models.

Results

Knockout or depletion of RALA inhibited orthotopic primary tumor growth, spontaneous metastasis, and experimental metastasis of TNBC cells in vivo. Conversely, knockout of RALB increased TNBC growth and metastasis. In vitro, RALA and RALB had antagonistic effects on TNBC migration, invasion, and viability with RALA generally supporting and RALB opposing these processes. In BC patient populations, elevated RALA but not RALB expression is significantly associated with poor outcome across all BC subtypes and specifically within TNBC patient cohorts. Immunohistochemical staining for RALA in patient cohorts confirmed the prognostic significance of RALA within the general BC population and the TNBC population specifically. BQU57, a small molecule inhibitor of RALA and RALB, decreased TNBC cell line viability, sensitized cells to paclitaxel in vitro and decreased tumor growth and metastasis in TNBC cell line and PDX models in vivo.

Conclusions

Together, these data demonstrate important but paradoxical roles for RALA and RALB in the pathogenesis of TNBC and advocate further investigation of RALA as a target for the precise treatment of metastatic TNBC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01438-3.

PRMT5-mediated arginine methylation activates AKT kinase to govern tumorigenesis

AKT is involved in a number of key cellular processes including cell proliferation, apoptosis and metabolism. Hyperactivation of AKT is associated with many pathological conditions, particularly cancers. Emerging evidence indicates that arginine methylation is involved in modulating AKT signaling pathway. However, whether and how arginine methylation directly regulates AKT kinase activity remain unknown. Here we report that protein arginine methyltransferase 5 (PRMT5), but not other PRMTs, promotes AKT activation by catalyzing symmetric dimethylation of AKT1 at arginine 391 (R391). Mechanistically, AKT1-R391 methylation cooperates with phosphatidylinositol 3,4,5 trisphosphate (PIP3) to relieve the pleckstrin homology (PH)-in conformation, leading to AKT1 membrane translocation and subsequent activation by phosphoinositide-dependent kinase-1 (PDK1) and the mechanistic target of rapamycin complex 2 (mTORC2). As a result, deficiency in AKT1-R391 methylation significantly suppresses AKT1 kinase activity and tumorigenesis. Lastly, we show that PRMT5 inhibitor synergizes with AKT inhibitor or chemotherapeutic drugs to enhance cell death. Altogether, our study suggests that R391 methylation is an important step for AKT activation and its oncogenic function.

Abstract PR002: STAT3 in cancer-associated fibroblasts promotes an immunosuppressive tumor microenvironment in PDAC

One of the defining characteristics of pancreatic ductal adenocarcinoma (PDAC) is the formation of a dense stroma comprised of cancer associated fibroblasts (CAFs) and immune cell populations. This stroma is immunosuppressive and can act as a physical barrier against common therapeutic treatments. Attempts to therapeutically target the PDAC stroma have yielded contradictory results, suggesting both tumor promoting and tumor limiting roles for CAFs. These studies emphasize the need to understand important transsignaling pathways between CAFs, tumor cells, and the immune microenvironment. IL-6 is a pleiotropic cytokine involved in several physiological functions and its increased expression is strongly associated with poor survival rates in PDAC patients. STAT3 is a major downstream target of IL-6, and its aberrant activation has been implicated in PDAC tumor progression and immune evasion. IL-6 expression and the IL-6/STAT3 signaling axis in PDAC has been characterized in epithelial tumor cells, however its stromal-specific function on PDAC has yet to be elucidated. We hypothesized that the STAT3 signaling axis in pancreatic CAFs contributes to the immunosuppressive and fibrotic phenotype seen with disease progression. Employing CreLoxP technology, the fibroblast specific protein-1 (Fsp-Cre) transgene was used to conditionally delete STAT3 in fibroblasts in the PdxFlp; KrasG12D; p53frt/frt (KPF) PDAC mouse model developed by our lab. Deletion of STAT3 in fibroblasts significantly increased the survival in a cohort of KPF mice compared to those with intact STAT3. Loss of fibroblast STAT3 also resulted in a significant decrease in ECM deposition in the pancreas. In preliminary investigations into the immune microenvironment, we found an increase in overall T cell infiltration and decrease in regulatory T cell populations in the STAT3 deleted cohort. Further, STAT3 deletion resulted in an increase in the ratio of M1 to M2 macrophages within the PDAC stroma, suggesting CAF STAT3 signaling is important to maintaining an immune microenvironment conducive to tumor progression. Ultimately, these preliminary results suggest STAT3 signaling in PDAC CAFs contributes to an overall survival disadvantage, an immunosuppressive tumor microenvironment, and an increase in ECM deposition.

Citation Format: Julia E. Lefler, Katie MarElia, Michael C. Ostrowski. STAT3 in cancer-associated fibroblasts promotes an immunosuppressive tumor microenvironment in PDAC [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr PR002.

Pten regulates collagen fibrillogenesis by fibroblasts through SPARC

Collagen deposition contributes to both high mammographic density and breast cancer progression. Low stromal PTEN expression has been observed in as many as half of breast tumors and is associated with increases in collagen deposition, however the mechanism connecting PTEN loss to increased collagen deposition remains unclear. Here, we demonstrate that Pten knockout in fibroblasts using an Fsp-Cre;Pten^loxP/loxP mouse model increases collagen fiber number and fiber size within the mammary gland. Pten knockout additionally upregulated Sparc transcription in fibroblasts and promoted collagen shuttling out of the cell. Interestingly, SPARC mRNA expression was observed to be significantly elevated in the tumor stroma as compared to the normal breast in several patient cohorts. While SPARC knockdown via shRNA did not affect collagen shuttling, it notably decreased assembly of exogenous collagen. In addition, SPARC knockdown decreased fibronectin assembly and alignment of the extracellular matrix in an in vitro fibroblast-derived matrix model. Overall, these data indicate upregulation of SPARC is a mechanism by which PTEN regulates collagen deposition in the mammary gland stroma.

Hepatocyte-specific PKCβ deficiency protects against high-fat diet-induced nonalcoholic hepatic steatosis

OBJECTIVE

Nonalcoholic hepatic steatosis, also known as fatty liver, is a uniform response of the liver to hyperlipidic-hypercaloric diet intake. However, the post-ingestive signals and mechanistic processes driving hepatic steatosis are not well understood. Emerging data demonstrate that protein kinase C beta (PKCβ), a lipid-sensitive kinase, plays a critical role in energy metabolism and adaptation to environmental and nutritional stimuli. Despite its powerful effect on glucose and lipid metabolism, knowledge of the physiological roles of hepatic PKCβ in energy homeostasis is limited.

METHODS

The floxed-PKCβ and hepatocyte-specific PKCβ-deficient mouse models were generated to study the in vivo role of hepatocyte PKCβ on diet-induced hepatic steatosis, lipid metabolism, and mitochondrial function.

RESULTS

We report that hepatocyte-specific PKCβ deficiency protects mice from development of hepatic steatosis induced by high-fat diet, without affecting body weight gain. This protection is associated with attenuation of SREBP-1c transactivation and improved hepatic mitochondrial respiratory chain. Lipidomic analysis identified significant increases in the critical mitochondrial inner membrane lipid, cardiolipin, in PKCβ-deficient livers compared to control. Moreover, hepatocyte PKCβ deficiency had no significant effect on either hepatic or whole-body insulin sensitivity supporting dissociation between hepatic steatosis and insulin resistance.

CONCLUSIONS

The above data indicate that hepatocyte PKCβ is a key focus of dietary lipid perception and is essential for efficient storage of dietary lipids in liver largely through coordinating energy utilization and lipogenesis during post-prandial period. These results highlight the importance of hepatic PKCβ as a drug target for obesity-associated nonalcoholic hepatic steatosis.

Abstract PO-016: STAT3 in cancer-associated fibroblasts promotes an immunosuppressive tumor microenvironment in PDAC

One of the defining characteristics of pancreatic ductal adenocarcinoma (PDAC) is the formation of a dense stroma comprised of cancer associated fibroblasts (CAFs) and immune cell populations. This stroma is immunosuppressive and can act as a physical barrier against common therapeutic treatments. Attempts to therapeutically target the PDAC stroma have yielded contradictory results, suggesting both tumor promoting and tumor limiting roles for CAFs. These studies emphasize the need to understand important transsignaling pathways between CAFs, tumor cells, and the immune microenvironment. IL-6 is a pleiotropic cytokine involved in several physiological functions and its increased expression is strongly associated with poor survival rates in PDAC patients. STAT3 is a major downstream target of IL-6, and its aberrant activation has been implicated in PDAC tumor progression and immune evasion. IL-6 expression and the IL-6/STAT3 signaling axis in PDAC has been characterized in epithelial tumor cells, however its stromal-specific function on PDAC has yet to be elucidated. We hypothesized that the STAT3 signaling axis in pancreatic CAFs contributes to the immunosuppressive and fibrotic phenotype seen with disease progression. Employing CreLoxP technology, the fibroblast specific protein-1 (Fsp-Cre) transgene was used to conditionally delete STAT3 in fibroblasts in the PdxFlp; KrasG12D; p53 frt/frt (KPF) PDAC mouse model developed by our lab. Deletion of STAT3 in fibroblasts significantly increased the survival in a cohort of KPF mice compared to those with intact STAT3. Loss of fibroblast STAT3 also resulted in a significant decrease in ECM deposition in the pancreas. In preliminary investigations into the immune microenvironment, we found an increase in overall T cell infiltration and decrease in regulatory T cell populations in the STAT3 deleted cohort. Further, STAT3 deletion resulted in an increase in the ratio of M1 to M2 macrophages within the PDAC stroma, suggesting CAF STAT3 signaling is important to maintaining an immune microenvironment conducive to tumor progression. Ultimately, these preliminary results suggest STAT3 signaling in PDAC CAFs contributes to an overall survival disadvantage, an immunosuppressive tumor microenvironment, and an increase in ECM deposition.

Citation Format: Julia E. Lefler, Katie MarElia, Blake E. Hildreth, Katie A. Thies, Maria C. Cuitino, Michael C. Ostrowski. STAT3 in cancer-associated fibroblasts promotes an immunosuppressive tumor microenvironment in PDAC [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PO-016.

Combinatorial ETS1-dependent control of oncogenic NOTCH1 enhancers in T-cell leukemia

Notch activation is highly prevalent among cancers, in particular T-cell acute lymphoblastic leukemia (T-ALL). However, the use of pan-Notch inhibitors to treat cancers has been hampered by adverse effects, particularly intestinal toxicities. To circumvent this barrier in T-ALL, we aimed to inhibit ETS1, a developmentally important T-cell transcription factor previously shown to co-bind Notch response elements. Using complementary genetic approaches in mouse models, we show that ablation of Ets1 leads to strong Notch-mediated suppressive effects on T-cell development and leukemogenesis, but milder intestinal effects than pan-Notch inhibitors. Mechanistically, genome-wide chromatin profiling studies demonstrate that Ets1 inactivation impairs recruitment of multiple Notch-associated factors and Notch-dependent activation of transcriptional elements controlling major Notch-driven oncogenic effector pathways. These results uncover previously unrecognized hierarchical heterogeneity of Notch-controlled genes and points to Ets1-mediated enucleation of Notch-Rbpj transcriptional complexes as a target for developing specific anti-Notch therapies in T-ALL that circumvent the barriers of pan-Notch inhibition.

Stromal Platelet-Derived Growth Factor Receptor-β Signaling Promotes Breast Cancer Metastasis in the Brain

Platelet-derived growth factor receptor-beta (PDGFRβ) is a receptor tyrosine kinase found in cells of mesenchymal origin such as fibroblasts and pericytes. Activation of this receptor is dependent on paracrine ligand induction, and its preferred ligand PDGFB is released by neighboring epithelial and endothelial cells. While expression of both PDGFRβ and PDGFB has been noted in patient breast tumors for decades, how PDGFB-to-PDGFRβ tumor-stroma signaling mediates breast cancer initiation, progression, and metastasis remains unclear. Here we demonstrate this paracrine signaling pathway that mediates both primary tumor growth and metastasis, specifically, metastasis to the brain. Elevated levels of PDGFB accelerated orthotopic tumor growth and intracranial growth of mammary tumor cells, while mesenchymal-specific expression of an activating mutant PDGFRβ (PDGFRβ^D849V) exerted proproliferative signals on adjacent mammary tumor cells. Stromal expression of PDGFRβ^D849V also promoted brain metastases of mammary tumor cells expressing high PDGFB when injected intravenously. In the brain, expression of PDGFRβ^D849V was observed within a subset of astrocytes, and aged mice expressing PDGFRβ^D849V exhibited reactive gliosis. Importantly, the PDGFR-specific inhibitor crenolanib significantly reduced intracranial growth of mammary tumor cells. In a tissue microarray comprised of 363 primary human breast tumors, high PDGFB protein expression was prognostic for brain metastases, but not metastases to other sites. Our results advocate the use of mice expressing PDGFRβ^D849V in their stromal cells as a preclinical model of breast cancer-associated brain metastases and support continued investigation into the clinical prognostic and therapeutic use of PDGFB-to-PDGFRβ signaling in women with breast cancer. SIGNIFICANCE: These studies reveal a previously unknown role for PDGFB-to-PDGFRβ paracrine signaling in the promotion of breast cancer brain metastases and support the prognostic and therapeutic clinical utility of this pathway for patients.See related article by Wyss and colleagues, p. 594.

Abstract P6-06-06: Platelet derived growth factor-b (PDGFB) promotes breast cancer progression

Objectives of the Study: Platelet-derived growth factor (PDGF) mediated signaling is pro-tumorigenic in many types of cancer, including breast cancer. However, the requirement of individual PDGF ligands in mediating breast cancer progression remains unclear. Our evaluation of publicly available breast cancer patient datasets shows that high primary tumor expression of PDGFB correlates with reduced overall survival (OS) and reduced metastasis free survival (MFS). This is in contrast to the lack of prognostic power of PDGFA and PDGFC and the association with improved OS and MFS for PDGFD. Based on our analysis of patient datasets, we set out to test the requirement of PDGFB in mammary tumor growth.

Methods Used: Gain-of-function and loss-of-function experiments were performed wherein PDGFB was either overexpressed or knocked-down in mammary tumor cells. Manipulated tumor cells were injected directly into the mammary fat pads of adult female mice, and tumor growth was monitored over time. PDGFB is produced by tumor cells whereas the corresponding receptor, PDGFRβ, is expressed by mesenchymal cells in the stroma. As another way to mimic oncogenic PDGFB-to-PDGFRβ signaling in the breast, we developed a mouse model of stroma-specific PDGFRβ activation using the Fsp-cre transgene. We evaluated changes mammary gland development upon mesenchymal specific PDGFRβ activation, and performed orthotopic injections with mammary tumor cells in these animals to test the functional role of receptor activation in mammary tumor growth.

Results and Conclusions: We evaluated expression of PDGF ligands in FVB/N murine mammary tumor cell lines and found that the PDGFB is dramatically higher in DB7 tumor cells compared to other syngeneic cell lines. In the high-PDGFB expressing DB7 cells, we used shRNA technology to knockdown the ligand. At the same time, we overexpressed the ligand in an isogenic cell line that expresses low levels of PDGFB. These cells were injected orthotopically into the mammary fat pads of adult female mice, and in both cases, expression of PDGFB dictated tumor growth. There was a significant reduction in tumor growth with shRNA-mediated knockdown of PDGFB whereas overexpression of the ligand accelerated tumor growth.

In our mouse model of mesenchymal-specific PDGFRβ activation, we reveal that activation of the receptor exerts pro-proliferative signals on adjacent mammary epithelial cells and accelerates orthotopic tumor growth of PDGFB-expressing cells. These findings indicate that PDGFB-to-PDGFRβ signaling is a viable therapeutic target for breast cancer. In fact, treatment with the PDGFR inhibitor, imatinib, impedes tumor cell proliferation when mouse mammary fibroblasts are co-injected orthotopically with DB7 cells.

Significance: The requirement of other PDGF ligands in breast cancer remains to be evaluated, but our data support a pro-tumorigenic role for PDGFB in the breast. Importantly, PDGFR inhibitors are being used in clinical trials for several cancer types. Our data advocate for (1) the potential utility of PDGFB as a prognostic biomarker and (2) the pre-clinical evaluation of PDGFR inhibitors in breast cancer models.

Citation Format: Katie A Thies, Anisha M Hammer, Sarah A Steck, Manjusri Das, Raleigh D Kladney, Steven T Sizemore, Michael C Ostrowski, Gina M Sizemore. Platelet derived growth factor-b (PDGFB) promotes breast cancer progression [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-06-06.

PTEN in the Stroma

Although tremendous progress has been made in understanding the functions of Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) in tumor cells, only recently have tumor cell-non-autonomous PTEN actions within the tumor microenvironment (TME) been appreciated. While it is accepted that the TME actively communicates with cancer cells to influence disease progression, our understanding of the genes and pathways responsible is still evolving. Given that inactivation of PTEN in the stroma is correlated with worse outcomes in human cancers, determining the unique functions and mechanisms of PTEN regulation in various TME cell compartments is essential. In this review, the evidence for PTEN function in different TME cell compartments, the mechanisms governing PTEN inactivation, and the downstream pathways regulated by PTEN that are critical for intracellular communication, are covered. The potential clinical implications of these findings as well as the future directions for the study of stromal PTEN are discussed.

Loss of PTEN Accelerates NKX3.1 Degradation to Promote Prostate Cancer Progression

NKX3.1 is the most commonly deleted gene in prostate cancer and a gatekeeper suppressor. NKX3.1 is a growth suppressor, mediator of apoptosis, inducer of antioxidants, and enhancer of DNA repair. PTEN is a ubiquitous tumor suppressor that is often decreased in prostate cancer during tumor progression. Steady-state turnover of NKX3.1 is mediated by DYRK1B phosphorylation at NKX3.1 serine 185 that leads to polyubiquitination and proteasomal degradation. In this study, we show PTEN is an NKX3.1 phosphatase that protects NKX3.1 from degradation. PTEN specifically opposed phosphorylation at NKX3.1(S185) and prolonged NKX3.1 half-life. PTEN and NKX3.1 interacted primarily in the nucleus as loss of PTEN nuclear localization abrogated its ability to bind to and protect NKX3.1 from degradation. The effect of PTEN on NKX3.1 was mediated via rapid enzyme-substrate interaction. An effect of PTEN on Nkx3.1 gene transcription was seen in vitro, but not in vivo. In gene-targeted mice, Nkx3.1 expression significantly diminished shortly after loss of Pten expression in the prostate. Nkx3.1 loss primarily increased prostate epithelial cell proliferation in vivo. In these mice, Nkx3.1 mRNA was not affected by Pten expression. Thus, the prostate cancer suppressors PTEN and NKX3.1 interact and loss of PTEN is responsible, at least in part, for progressive loss of NKX3.1 that occurs during tumor progression. SIGNIFICANCE: PTEN functions as a phosphatase of NKX3.1, a gatekeeper suppressor of prostate cancer.

Modeling Human Cancer-induced Cachexia

Cachexia is a wasting syndrome characterized by pronounced skeletal muscle loss. In cancer, cachexia is associated with increased morbidity and mortality and decreased treatment tolerance. Although advances have been made in understanding the mechanisms of cachexia, translating these advances to the clinic has been challenging. One reason for this shortcoming may be the current animal models, which fail to fully recapitulate the etiology of human cancer-induced tissue wasting. Because pancreatic ductal adenocarcinoma (PDA) presents with a high incidence of cachexia, we engineered a mouse model of PDA that we named KPP. KPP mice, similar to PDA patients, progressively lose skeletal and adipose mass as a consequence of their tumors. In addition, KPP muscles exhibit a similar gene ontology as cachectic patients. We envision that the KPP model will be a useful resource for advancing our mechanistic understanding and ability to treat cancer cachexia.

Two Distinct E2F Transcriptional Modules Drive Cell Cycles and Differentiation

Orchestrating cell-cycle-dependent mRNA oscillations is critical to cell proliferation in multicellular organisms. Even though our understanding of cell-cycle-regulated transcription has improved significantly over the last three decades, the mechanisms remain untested in vivo. Unbiased transcriptomic profiling of G0, G1-S, and S-G2-M sorted cells from FUCCI mouse embryos suggested a central role for E2Fs in the control of cell-cycle-dependent gene expression. The analysis of gene expression and E2F-tagged knockin mice with tissue imaging and deep-learning tools suggested that post-transcriptional mechanisms universally coordinate the nuclear accumulation of E2F activators (E2F3A) and canonical (E2F4) and atypical (E2F8) repressors during the cell cycle in vivo. In summary, we mapped the spatiotemporal expression of sentinel E2F activators and canonical and atypical repressors at the single-cell level in vivo and propose that two distinct E2F modules relay the control of gene expression in cells actively cycling (E2F3A-8-4) and exiting the cycle (E2F3A-4) during mammalian development.

Abstract PD9-11: Platelet derived growth factor receptor-β signaling: A novel therapeutic target for breast cancer associated brain metastasis

PDGFRβ is a receptor tyrosine kinase found in cells of mesenchymal origin such as fibroblasts and pericytes. Activation of this receptor is dependent on paracrine ligand induction, and its preferred ligand, PDGFB, is released by neighboring epithelial and endothelial cells. While expression of both PDGFRβ and PDGFB has been noted in patient breast tumors for decades, how PDGFB-to-PDGFRβ tumor-stromal signaling mediates breast cancer initiation, progression, and metastasis remains unclear. To test this important research question, we developed a mouse model of mesenchymal-specific PDGFRβ hyper-activation. PDGFRβ mutant mammary glands exhibit increased tertiary side-branching and epithelial proliferation confirming a stromal-specific PDGFRβ effect on neighboring epithelium during normal development. To test the effect of hyper-active mesenchymal PDGFRβ on disease progression, experimental tail vein metastasis assays were performed where we observed prominent brain metastases in 50% of the PDGFRβ mutantmice (n=5/10) with no brain lesions seen in controls (n=0/19). There was no difference in the incidence of lung or liver metastases in the mutant mice suggesting a pro-metastatic function for PDGFRβ in the brain metastatic niche. To rule out dysfunction of the blood brain barrier contributing to the observed metastatic spread, we then intracranially injected mammary tumor cells, and as expected based on our metastasis assay, found that larger tumors formed in the brains of PDGFRβ mutant mice versus controls. To our knowledge, these combined findings are the first example where genetic manipulation of the stroma increases breast cancer associated brain metastases (BCBM). Given that these pre-clinical data suggest that primary breast tumors expressing high PDGFB could preferentially metastasize to the brain, we analyzed PDGFB protein expression in a tissue microarray comprised of HER2-positive and triple negative breast cancer (TNBC) primary tumors (total n=425). While high PDGFB did not correlate with site-independent metastatic recurrence, it was prognostic of brain metastasis, mirroring our mouse data. Evaluation of PDGFB in a small cohort of matched primary breast tumors with associated brain (n=5) and lung metastases (n=2) revealed intense PDGFB staining in 100% of the brain metastases, but only 50% of the lung metastases. These findings further suggest that high primary tumor PDGFBexpression defines a subset of breast cancer patients predisposed to brain metastases and that these patients may benefit from therapeutic inhibition of PDGFRβ signaling. To test this pre-clinically, we treated mice harboring intracranial tumors with the PDGFR specific inhibitor crenolanib. Excitingly, crenolanib treatment significantly inhibited the brain tumor burden in these mice. Combined, our findings to date (1) advocate that primary tumor expression of PDGFB is a novel prognostic biomarker for the development of BCBM and (2) support clinical trial evaluation of PDGFR inhibitors for the prevention and treatment of BCBM. Ongoing studies are evaluating how the PDGFRβ-expressing mesenchymal cells within the brain promote a pro-metastatic niche.

Citation Format: Sizemore GM, Hammer AM, Thies KA, Hildreth BE, Russell LO, Sizemore ST, Trimboli AJ, Kladney RD, Steck SA, Das M, Bolyard CM, Pilarski R, Schoenfield L, Otero J, Chakravarti A, Ringel M, Kaur B, Leone G, Ostrowski MC. Platelet derived growth factor receptor-β signaling: A novel therapeutic target for breast cancer associated brain metastasis [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD9-11.

Eomes partners with PU.1 and MITF to Regulate Transcription Factors Critical for osteoclast differentiation

Bone-resorbing osteoclasts (OCs) are derived from myeloid precursors (MPs). Several transcription factors are implicated in OC differentiation and function; however, their hierarchical architecture and interplay are not well known. Analysis for enriched motifs in PU.1 and MITF chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-seq) data from differentiating OCs identified eomesodermin (EOMES) as a potential novel binding partner of PU.1 and MITF at genes critical for OC differentiation and function. We were able to demonstrate using co-immunoprecipitation and sequential ChIP analysis that PU.1, MITF, and EOMES are in the same complex and present as a complex at OC genomic loci. Furthermore, EOMES knockdown in MPs led to osteopetrosis associated with decreased OC differentiation and function both in vitro and in vivo. Although EOMES is associated with embryonic development and other hematopoietic lineages, this is the first study demonstrating the requirement of EOMES in the myeloid compartment.

Stromal PTEN Regulates Extracellular Matrix Organization in the Mammary Gland

The organization of the extracellular matrix has a profound impact on cancer development and progression. The matrix becomes aligned throughout tumor progression, providing “highways” for tumor cell invasion. Aligned matrix is associated with breast density and is a negative prognostic factor in several cancers; however, the underlying mechanisms regulating this reorganization remain poorly understood. Deletion of the tumor suppressor Pten in the stroma was previously shown to promote extracellular matrix expansion and tumor progression. However, it was unknown if PTEN also regulated matrix organization. To address this question, a murine model with fibroblast-specific Pten deletion was used to examine how PTEN regulates matrix remodeling. Using second harmonic generation microscopy, Pten deletion was found to promote collagen alignment parallel to the mammary duct in the normal gland and further remodeling perpendicular to the tumor edge in tumor-bearing mice. Increased alignment was observed with Pten deletion in vitro using fibroblast-derived matrices. PTEN loss was associated with fibroblast activation and increased cellular contractility, as determined by traction force microscopy. Inhibition of contractility abrogated the increased matrix alignment observed with PTEN loss. Murine mammary adenocarcinoma cells cultured on aligned matrices derived from Pten^−/− fibroblasts migrated faster than on matrices from wild-type fibroblasts. Combined, these data demonstrate that PTEN loss in fibroblasts promotes extracellular matrix deposition and alignment independently from cancer cell presence, and this reorganization regulates cancer cell behavior. Importantly, stromal PTEN negatively correlated with collagen alignment and high mammographic density in human breast tissue, suggesting parallel function for PTEN in patients.

Pyruvate kinase M2 regulates homologous recombination-mediated DNA double-strand break repair

Resistance to genotoxic therapies is a primary cause of treatment failure and tumor recurrence. The underlying mechanisms that activate the DNA damage response (DDR) and allow cancer cells to escape the lethal effects of genotoxic therapies remain unclear. Here, we uncover an unexpected mechanism through which pyruvate kinase M2 (PKM2), the highly expressed PK isoform in cancer cells and a master regulator of cancer metabolic reprogramming, integrates with the DDR to directly promote DNA double-strand break (DSB) repair. In response to ionizing radiation and oxidative stress, ATM phosphorylates PKM2 at T328 resulting in its nuclear accumulation. pT328-PKM2 is required and sufficient to promote homologous recombination (HR)-mediated DNA DSB repair through phosphorylation of CtBP-interacting protein (CtIP) on T126 to increase CtIP's recruitment at DSBs and resection of DNA ends. Disruption of the ATM-PKM2-CtIP axis sensitizes cancer cells to a variety of DNA-damaging agents and PARP1 inhibition. Furthermore, increased nuclear pT328-PKM2 level is associated with significantly worse survival in glioblastoma patients. Combined, these data advocate the use of PKM2-targeting strategies as a means to not only disrupt cancer metabolism but also inhibit an important mechanism of resistance to genotoxic therapies.

Disruption of stromal hedgehog signaling initiates RNF5-mediated proteasomal degradation of PTEN and accelerates pancreatic tumor growth

Disrupting paracrine Hedgehog signaling in pancreatic cancer stroma through genetic deletion of fibroblast Smoothened leads to proteasomal degradation of fibroblast PTEN and accelerates tumor growth.

The contribution of the tumor microenvironment to pancreatic ductal adenocarcinoma (PDAC) development is currently unclear. We therefore examined the consequences of disrupting paracrine Hedgehog (HH) signaling in PDAC stroma. Herein, we show that ablation of the key HH signaling gene Smoothened (Smo) in stromal fibroblasts led to increased proliferation of pancreatic tumor cells. Furthermore, Smo deletion resulted in proteasomal degradation of the tumor suppressor PTEN and activation of oncogenic protein kinase B (AKT) in fibroblasts. An unbiased proteomic screen identified RNF5 as a novel E3 ubiquitin ligase responsible for degradation of phosphatase and tensin homolog (PTEN) in Smo-null fibroblasts. Ring Finger Protein 5 (Rnf5) knockdown or pharmacological inhibition of glycogen synthase kinase 3β (GSKβ), the kinase that marks PTEN for ubiquitination, rescued PTEN levels and reversed the oncogenic phenotype, identifying a new node of PTEN regulation. In PDAC patients, low stromal PTEN correlated with reduced overall survival. Mechanistically, PTEN loss decreased hydraulic permeability of the extracellular matrix, which was reversed by hyaluronidase treatment. These results define non-cell autonomous tumor-promoting mechanisms activated by disruption of the HH/PTEN axis and identifies new targets for restoring stromal tumor-suppressive functions.

Stromal PTEN determines mammary epithelial response to radiotherapy

The importance of the tumor-associated stroma in cancer progression is clear. However, it remains uncertain whether early events in the stroma are capable of initiating breast tumorigenesis. Here, we show that in the mammary glands of non-tumor bearing mice, stromal-specific phosphatase and tensin homolog (Pten) deletion invokes radiation-induced genomic instability in neighboring epithelium. In these animals, a single dose of whole-body radiation causes focal mammary lobuloalveolar hyperplasia through paracrine epidermal growth factor receptor (EGFR) activation, and EGFR inhibition abrogates these cellular changes. By analyzing human tissue, we discover that stromal PTEN is lost in a subset of normal breast samples obtained from reduction mammoplasty, and is predictive of recurrence in breast cancer patients. Combined, these data indicate that diagnostic or therapeutic chest radiation may predispose patients with decreased stromal PTEN expression to secondary breast cancer, and that prophylactic EGFR inhibition may reduce this risk.

Abstract 49: Stromal platelet derived growth factor receptor (PDGFRβ) signaling: A novel therapeutic target for breast cancer brain metastasis (BCBM)

Stromal PDGFRβ has emerged as an actionable mediator of breast tumor-stromal communication. As a receptor tyrosine kinase, PDGFRβ is activated by its ligand, PDGFB, which is released by neighboring tumor epithelium and endothelium. However, how PDGF signaling mediates breast cancer initiation, progression, and metastasis remains unclear. To evaluate PDGFRβ in this disease, we developed a mouse model of stromal-specific PDGFRβ activation using the Fsp-cre transgene previously published by our group (PDGFRβ mutant). PDGFRβ mutant mammary glands exhibit increased tertiary side-branching and epithelial proliferation confirming a stromal-specific PDGFRβ effect on neighboring epithelium. To evaluate the functional relevance of PDGFRβ activation on metastatic progression, we performed tail vein injection of PDGFB expressing murine mammary tumor cells and, surprisingly, observed brain metastases in 50% of the PDGFRβ mutant mice while no brain lesions were seen in controls. There was no difference in the incidence of lung, liver or bone metastases. Mammary tumor cells expressing low PDGFB did not exhibit a similar increase in brain metastases in mutant mice. While there is no observable difference in blood brain barrier permeability in the mutant mice, we bypassed this variable by intracranially injecting mammary tumor cells and found that larger tumors formed in the brains of PDGFRβ mutant mice versus controls. To our knowledge, this is the first example where genetic manipulation of the stroma leads to an increased incidence of BCBM. Also, our pre-clinical data suggests that primary breast tumors that express high PDGFB could preferentially metastasize to the brain. To test this in patients, we analyzed PDGFB protein expression in a tissue microarray comprised of HER2-positive and triple negative breast cancer (TNBC) primary tumors. While high PDGFB did not correlate with site-independent metastatic recurrence, it was prognostic of brain metastasis, mirroring our mouse data. Evaluation of PDGFB in a small cohort of matched primary breast tumors with associated brain (n=5) and lung metastases (n=2) revealed intense PDGFB staining in 100% of the brain metastases, but only 50% of the lung metastases. Our findings suggest high primary tumor PDGFB expression defines a subset of breast cancer patients predisposed to brain metastases. These patients may benefit from therapeutic intervention of PDGFRβ signaling. To test this pre-clinically, we treated mice harboring intracranial tumors with the PDGFR specific inhibitor, Crenolanib. Excitingly, Crenolanib treatment significantly inhibited the brain tumor burden in these mice. Combined, our findings (1) advocate that primary tumor expression of PDGFB is a novel prognostic biomarker for the development of BCBM and (2) support clinical trial evaluation of PDGFR inhibitors for the prevention and treatment of BCBM.

Citation Format: Katie A. Thies, Anisha M. Hammer, B. Eason Hildreth, Luke O. Russell, Steven T. Sizemore, Anthony J. Trimboli, Raleigh D. Kladney, Chelsea M. Bolyard, Robert Pilarski, Lynn Schoenfield, Jose Otero, Arnab Chakravarti, Matthew Ringel, Balveen Kaur, Gustavo Leone, Michael C. Ostrowski, Gina M. Sizemore. Stromal platelet derived growth factor receptor (PDGFRβ) signaling: A novel therapeutic target for breast cancer brain metastasis (BCBM) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 49.

Enhancer variants reveal a conserved transcription factor network governed by PU.1 during osteoclast differentiation

Genome-wide association studies (GWASs) have been instrumental in understanding complex phenotypic traits. However, they have rarely been used to understand lineage-specific pathways and functions that contribute to the trait. In this study, by integrating lineage-specific enhancers from mesenchymal and myeloid compartments with bone mineral density loci, we were able to segregate osteoblast- and osteoclast (OC)-specific functions. Specifically, in OCs, a PU.1-dependent transcription factor (TF) network was revealed. Deletion of PU.1 in OCs in mice resulted in severe osteopetrosis. Functional genomic analysis indicated PU.1 and MITF orchestrated a TF network essential for OC differentiation. Several of these TFs were regulated by cooperative binding of PU.1 with BRD4 to form superenhancers. Further, PU.1 is essential for conformational changes in the superenhancer region of Nfatc1. In summary, our study demonstrates that combining GWASs with genome-wide binding studies and model organisms could decipher lineage-specific pathways contributing to complex disease states.

Genetic variation in non-coding regions of DNA could raise osteoporosis risk by affecting osteoclast differentiation. Osteoporosis occurs when the normal process of bone remodeling by osteoblasts and osteoclasts falls out of balance. Genome-wide association studies (GWAS) have identified numerous single nucleotide polymorphisms (SNPs) associated with osteoporosis, but how these affect specific cell types was unclear. Sudarshana Sharma and Michael Ostrowski at the Medical University of South Carolina and colleagues wondered if variations in non-coding ‘enhancer’ regions of DNA, might shed light on the molecular underpinnings of osteoporosis. So, they overlaid SNPs associated with reduced bone mineral density onto enhancers in mesenchymal and myeloid cells—the precursors of osteoblasts and osteoclasts—identifying a transcription factor network in myeloid cells that drives the differentiation of osteoclasts. When this was disrupted in mice, severe defects in osteoclast differentiation and function resulted.

Synthetic Lethality of PARP Inhibition and Ionizing Radiation is p53-dependent

PARP inhibitors (PARPi) are potentially effective therapeutic agents capable of inducing synthetic lethality in tumors with deficiencies in homologous recombination (HR)-mediated DNA repair such as those carrying BRCA1 mutations. However, BRCA mutations are rare, the majority of tumors are proficient in HR repair, and thus most tumors are resistant to PARPi. Previously, we observed that ionizing radiation (IR) initiates cytoplasmic translocation of BRCA1 leading to suppression of HR-mediated DNA repair and induction of synthetic PARPi lethality in wild-type BRCA1 and HR-proficient tumor cells. The tumor suppressor p53 was identified as a key factor that regulates DNA damage-induced BRCA1 cytoplasmic sequestration following IR. However, the role of p53 in IR-induced PARPi sensitization remains unclear. This study elucidates the role of p53 in IR-induced PARPi cytotoxicity in HR-proficient cancer cells and suggests p53 status may help define a patient population that might benefit from this treatment strategy. Sensitization to PARPi following IR was determined in vitro and in vivo utilizing human breast and glioma tumor cells carrying wild-type BRCA1 and p53, and in associated cells in which p53 function was modified by knockdown or mutation. In breast and glioma cells with proficient HR repair, IR-induced BRCA1 cytoplasmic sequestration, HR repair inhibition, and subsequent PARPi sensitization in vitro and in vivo was dependent upon functional p53.Implications: Implications: p53 status determines PARP inhibitor sensitization by ionizing radiation in multiple BRCA1 and HR-proficient tumor types and may predict which patients are most likely to benefit from combination therapy. Mol Cancer Res; 16(7); 1092-102. ©2018 AACR.

Csf1r-mApple Transgene Expression and Ligand Binding In Vivo Reveal Dynamics of CSF1R Expression within the Mononuclear Phagocyte System

CSF1 is the primary growth factor controlling macrophage numbers, but whether expression of the CSF1 receptor differs between discrete populations of mononuclear phagocytes remains unclear. We have generated a Csf1r-mApple transgenic fluorescent reporter mouse that, in combination with lineage tracing, Alexa Fluor 647–labeled CSF1-Fc and CSF1, and a modified ΔCsf1–enhanced cyan fluorescent protein (ECFP) transgene that lacks a 150 bp segment of the distal promoter, we have used to dissect the differentiation and CSF1 responsiveness of mononuclear phagocyte populations in situ. Consistent with previous Csf1r-driven reporter lines, Csf1r-mApple was expressed in blood monocytes and at higher levels in tissue macrophages, and was readily detectable in whole mounts or with multiphoton microscopy. In the liver and peritoneal cavity, uptake of labeled CSF1 largely reflected transgene expression, with greater receptor activity in mature macrophages than monocytes and tissue-specific expression in conventional dendritic cells. However, CSF1 uptake also differed between subsets of monocytes and discrete populations of tissue macrophages, which in macrophages correlated with their level of dependence on CSF1 receptor signaling for survival rather than degree of transgene expression. A double ΔCsf1r-ECFP-Csf1r-mApple transgenic mouse distinguished subpopulations of microglia in the brain, and permitted imaging of interstitial macrophages distinct from alveolar macrophages, and pulmonary monocytes and conventional dendritic cells. The Csf1r-mApple mice and fluorescently labeled CSF1 will be valuable resources for the study of macrophage and CSF1 biology, which are compatible with existing EGFP-based reporter lines.

IL-6 and PD-L1 antibody blockade combination therapy reduces tumour progression in murine models of pancreatic cancer

OBJECTIVE

Limited efficacy of immune checkpoint inhibitors in pancreatic ductal adenocarcinoma (PDAC) has prompted investigation into combination therapy. We hypothesised that interleukin 6 (IL-6) blockade would modulate immunological features of PDAC and enhance the efficacy of anti-programmed death-1-ligand 1 (PD-L1) checkpoint inhibitor therapy.

DESIGN

Transcription profiles and IL-6 secretion from primary patient-derived pancreatic stellate cells (PSCs) were analyzed via Nanostring and immunohistochemistry, respectively. In vivo efficacy and mechanistic studies were conducted with antibodies (Abs) targeting IL-6, PD-L1, CD4 or CD8 in subcutaneous or orthotopic models using Panc02, MT5 or KPC-luc cell lines; and the aggressive, genetically engineered PDAC model (Kras^LSL-G12D, Trp53^LSL-R270H, Pdx1-cre, Brca2^F/F (KPC-Brca2 mice)). Systemic and local changes in immunophenotype were measured by flow cytometry or immunohistochemical analysis.

RESULTS

PSCs (n=12) demonstrated prominent IL-6 expression, which was localised to stroma of tumours. Combined IL-6 and PD-L1 blockade elicited efficacy in mice bearing subcutaneous MT5 (p<0.02) and Panc02 tumours (p=0.046), which was accompanied by increased intratumoural effector T lymphocytes (CD62L^-CD44^-). CD8-depleting but not CD4-depleting Abs abrogated the efficacy of combined IL-6 and PD-L1 blockade in mice bearing Panc02 tumours (p=0.0016). This treatment combination also elicited significant antitumour activity in mice bearing orthotopic KPC-luc tumours and limited tumour progression in KPC-Brca2 mice (p<0.001). Histological analysis revealed increased T-cell infiltration and reduced α-smooth muscle actin cells in tumours from multiple models. Finally, IL-6 and PD-L1 blockade increased overall survival in KPC-Brca2 mice compared with isotype controls (p=0.0012).

CONCLUSIONS

These preclinical results indicate that targeted inhibition of IL-6 may enhance the efficacy of anti-PD-L1 in PDAC.

IL-18 Drives ILC3 Proliferation and Promotes IL-22 Production via NF-κB

Group 3 innate lymphoid cells (ILC3s) are important regulators of the immune system, maintaining homeostasis in the presence of commensal bacteria, but activating immune defenses in response to microbial pathogens. ILC3s are a robust source of IL-22, a cytokine critical for stimulating the antimicrobial response. We sought to identify cytokines that can promote proliferation and induce or maintain IL-22 production by ILC3s and determine a molecular mechanism for this process. We identified IL-18 as a cytokine that cooperates with an ILC3 survival factor, IL-15, to induce proliferation of human ILC3s, as well as induce and maintain IL-22 production. To determine a mechanism of action, we examined the NF-κB pathway, which is activated by IL-18 signaling. We found that the NF-κB complex signaling component, p65, binds to the proximal region of the IL22 promoter and promotes transcriptional activity. Finally, we observed that CD11c+ dendritic cells expressing IL-18 are found in close proximity to ILC3s in human tonsils in situ. Therefore, we identify a new mechanism by which human ILC3s proliferate and produce IL-22, and identify NF-κB as a potential therapeutic target to be considered in pathologic states characterized by overproduction of IL-18 and/or IL-22.

Discovery of Stromal Regulatory Networks that Suppress Ras-Sensitized Epithelial Cell Proliferation

Mesodermal cells signal to neighboring epithelial cells to modulate their proliferation in both normal and disease states. We adapted a Caenorhabditis elegans organogenesis model to enable a genome-wide mesodermal-specific RNAi screen and discovered 39 factors in mesodermal cells that suppress the proliferation of adjacent Ras pathway-sensitized epithelial cells. These candidates encode components of protein complexes and signaling pathways that converge on the control of chromatin dynamics, cytoplasmic polyadenylation, and translation. Stromal fibroblast-specific deletion of mouse orthologs of several candidates resulted in the hyper-proliferation of mammary gland epithelium. Furthermore, a 33-gene signature of human orthologs was selectively enriched in the tumor stroma of breast cancer patients, and depletion of these factors from normal human breast fibroblasts increased proliferation of co-cultured breast cancer cells. This cross-species approach identified unanticipated regulatory networks in mesodermal cells with growth-suppressive function, exposing the conserved and selective nature of mesodermal-epithelial communication in development and cancer.

Stromal ETS2 Regulates Chemokine Production and Immune Cell Recruitment during Acinar-to-Ductal Metaplasia

Preclinical studies have suggested that the pancreatic tumor microenvironment both inhibits and promotes tumor development and growth. Here we establish the role of stromal fibroblasts during acinar-to-ductal metaplasia (ADM), an initiating event in pancreatic cancer formation. The transcription factor V-Ets avian erythroblastosis virus E26 oncogene homolog 2 (ETS2) was elevated in smooth muscle actin–positive fibroblasts in the stroma of pancreatic ductal adenocarcinoma (PDAC) patient tissue samples relative to normal pancreatic controls. LSL-Kras^G12D/+; LSL-Trp53^R172H/+; Pdx-1-Cre (KPC) mice showed that ETS2 expression initially increased in fibroblasts during ADM and remained elevated through progression to PDAC. Conditional ablation of Ets-2 in pancreatic fibroblasts in a Kras^G12D-driven mouse ADM model decreased the amount of ADM events. ADMs from fibroblast Ets-2–deleted animals had reduced epithelial cell proliferation and increased apoptosis. Surprisingly, fibroblast Ets-2 deletion significantly altered immune cell infiltration into the stroma, with an increased CD8+ T-cell population, and decreased presence of regulatory T cells (Tregs), myeloid-derived suppressor cells, and mature macrophages. The mechanism involved ETS2-dependent chemokine ligand production in fibroblasts. ETS2 directly bound to regulatory sequences for Ccl3, Ccl4, Cxcl4, Cxcl5, and Cxcl10, a group of chemokines that act as potent mediators of immune cell recruitment. These results suggest an unappreciated role for ETS2 in fibroblasts in establishing an immune-suppressive microenvironment in response to oncogenic Kras^G12D signaling during the initial stages of tumor development.

Abstract 4337: PDGFR-α induced stiffness abrogates mammary ductal development and enhances tumorigenesis in vivo

Breast cancer is a leading cause of mortality in women worldwide, in part due to the tumor microenvironment which increases tumor heterogeneity and abets tumor growth. Fibroblasts are cells of mesenchymal origin that are an important component of normal and tumor stroma. Genetic alterations in these cells were shown by several groups including our own to cause fibroblast activation and fuel tumor progression giving rise to more aggressive disease. Platelet-Derived Growth Factor Receptor (PDGFR) alpha is a receptor tyrosine kinase that is chiefly expressed in mesenchymal cells such as fibroblasts. Ligand binding (PDGFAA) activates this receptor. PDGFRα signaling plays critical roles in development and aberrant signaling is seen in several types of cancer, such as lung, pancreas, GI and brain. The central goal of this study was to elucidate the role of stromal PDGFRα in breast cancer development and metastasis, where its role remains largely unknown. To address this goal, we developed a genetic mouse model of stromal activation of PDGFRα in the mammary gland by crossing an auto-activating Pdgfra mutant allele with a mesenchymal specific Cre recombinase. We found that stromal PDGFRα activation completely abrogated postnatal mammary gland ductal formation, with significantly reduced terminal end bud formation. PDGFRα activation also led to progressive fibrosis in the mouse mammary fat pad. As early as four weeks of age, mammary collagen (trichrome staining; second harmonic generation) and hyaluronan deposition (Alcian Blue) was greatly increased in vivo. In fact, this increase in collagen and hyaluronan deposition in mutant animals is believed to be responsible for the observed increased in stiffness of mutant mammary tissue (atomic force microscopy {AFM}). pFAK, which can be activated due to mechanical stress, was increased in mammary epithelia of the mutant mice in vivo corroborating the AFM results. Further, when tumor cells were injected into the mammary glands of the PDGFRα mutants, tumors grew faster as compared to controls. Importantly, we found that mRNA expression of PDGFRA correlates with worsened patient outcomes in HER2+ disease, while expression of both the ligand (PDGFA) and the receptor were found to correlate with increased incidence of lung metastases. We further discovered that in HER2+ patients, PDGFRA levels correlate with breast density. Breast density is the third strongest risk factor for breast cancer, and is directly related to collagen deposition and breast stiffness, thus suggesting a novel predictive role of PDGFRA as a molecular readout of stiffness and density. Studies are underway to utilize mouse models of HER2+ breast cancer to study both primary tumor growth and metastases. Taken together, our mouse studies and paralleling human data analyses suggest that the stromal PDGFRα signaling provides a novel theranostic window in breast cancer treatment and prognosis.

Citation Format: Anisha Mathur Hammer, Gina M. Sizemore, Vasudha Shukla, Steven T. Sizemore, Maria Cuitino, Cynthia J. Timmers, Quinn Verfurth, Arnab Chakravarti, Gustavo W. Leone, Samir N. Ghadiali, Michael C. Ostrowski. PDGFR-α induced stiffness abrogates mammary ductal development and enhances tumorigenesis in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4337. doi:10.1158/1538-7445.AM2017-4337

Abstract 1354: The role of RALA in soft tissue sarcoma tumor growth and metastasis

Soft tissue sarcomas (STS) are a diverse collection of cancers of mesenchymal origin arising from the connective and supportive tissues of the body. While localized STS are well managed by surgery and radiation; metastasis, particularly to the lung, is frequent. More than 30% of adult STS patients develop lung metastases and the 5-year survival for these patients is a dismal 16%. Treatment options for metastatic STS are limited, thus there is an urgent unmet need for a better understanding of the key molecular pathways that drive metastatic spread in STS and identification of inhibitors of these pathways for clinical application. Through analysis of gene expression data from metastatic STS patient samples, we identified decreased expression of PPP2R1B as a hallmark of metastatic STS. To directly test its function as a suppressor of tumor growth and metastasis in STS, PPP2R1B was stably over-expressed in HT1080 cells, a model of metastatic STS. PPP2R1B expression almost completely abolished HT1080 tumor growth in nude mice. PPP2R1B is a subunit of the PP2A protein phosphatase complex that negatively regulates numerous cancer signaling pathways. However, the functional consequences of decreased PPP2R1B expression in STS are unknown. A combination of high-throughput and targeted approaches were utilized to identify 37 phosphoproteins that are significantly dephosphorylated following PPP2R1B expression in HT1080 cells. One of these phosphoproteins, the small GTPase RALA, exhibited decreased phosphorylation on Ser194 following PPP2R1B expression. RALA is significantly prognostic of STS metastasis and is elevated in more aggressive STS subtypes relative to less aggressive subtypes and normal tissue. RALA knockdown in HT1080 significantly slowed tumor growth and decreased the incidence of pulmonary metastasis, mirroring PPP2R1B overexpression. Importantly, RALA is an actionable therapeutic target for improved treatment of STS. Aurora A inhibitors indirectly inhibit RALA function by preventing RALA Ser194 phosphorylation by aurora A. We found that RALA expression and activity predicted response of STS cell lines to aurora A inhibition. Excitingly, the aurora A inhibitor alisertib nearly eradicated growth of HT1080 tumors in vivo. Exploration of the biological mechanisms through which RALA regulates STS metastasis identified regulation of vesicular traffic as a likely critical function of RALA in this process. These findings identify PPP2R1B, RALA, and aurora A as members of a key molecular pathway that drives STS progression and advocate the use of treatments targeting this pathway to improve outcome for STS patients with advanced disease.

Citation Format: Steven T. Sizemore, Gina M. Sizemore, Reena Shakya, Peter Amaya, Anisha M. Hammer, Alexander H. Rice, Jeffrey J. Chalmers, Michael C. Ostrowski, Arnab Chakravarti. The role of RALA in soft tissue sarcoma tumor growth and metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1354. doi:10.1158/1538-7445.AM2017-1354

Abstract 3911: Stromal platelet derived growth factor receptor-beta (PDGFRbeta) promotes breast brain metastasis

The platelet derived growth factor (PDGF) pathway is a prime example of tumor-stroma signaling in a number of cancer types. Others have shown that PDGF receptors are expressed in breast fibroblasts and pericytes while PDGF ligands are often expressed in breast cancer cells and tumor-associated endothelium; however, how PDGF signaling mediates breast cancer initiation, progression and metastasis remains unclear. Importantly, our evaluation of publicly available datasets revealed that PDGFB expression correlates with breast cancer patient metastatic recurrence leading to the hypothesis that PDGF-B to PDGFR signaling promotes metastatic progression of breast cancer. Given that PDGF-B preferentially activates PDGFRβ, we established an in vivo system to investigate this pathway during breast cancer progression. We utilized a mesenchymal-specific promoter to drive Cre recombinase and conditionally activate PDGFRβ by way of the endogenous Pdgfrb promoter (hereafter “PDGFRβ mutant”). A murine mammary tumor cell line which expresses high levels of PDGF-B was injected either by tail vein or intracranially to evaluate metastatic seeding and distant tumor growth. Following tail vein injection of tumor cells, we observed 50% incidence of brain metastases in the PDGFRβ mutant mice while no brain lesions were seen in the controls. There was no difference in incidence of lung, liver or bone metastases (other common sites of breast cancer metastasis). Not surprisingly, larger tumors formed in the brains of PDGFRβ mutant mice when cells were injected intracranially. Brains were stained for phospho-PLCγ as a way to confirm activation of PDGFRβ. To our knowledge, this is the first example where genetic manipulation of the stroma leads to an increased incidence of breast brain metastases. Furthermore, this study highlights a role for stromal activation of PDGFRβ in the brain microenvironment and during metastatic progression. For the 20-30% of patients that develop breast cancer brain metastases, the one-year survival rate is sadly less than 20%, and how the brain microenvironment contributes to metastatic seeding and subsequent growth of tumor cells remains poorly understood. To confirm translational relevance, we analyzed a small cohort of matched primary breast tumors and brain metastases for PDGFRβ expression observing strong stromal staining in fibroblasts and pericytes within and around all of the primary tumors similar to previous studies. Importantly, high PDGFRβ expression was found in the perivasculature of all associated brain metastases suggesting a functional role in the establishment or growth at this site. Combined, our findings strongly suggest that high primary tumor expression of PDGF-B/PDGFRβ might define a subset of breast cancer patients predisposed to brain metastases. These patients may benefit from therapeutic targeting of PDGFR signaling as a means to thwart metastatic seeding in the brain.

Citation Format: Katie A. Thies, Anisha M. Hammer, Anthony J. Trimboli, B. Eason Hildreth, Luke O. Russell, Chelsea M. Bolyard, Raleigh D. Kladney, Steven T. Sizemore, Robert Pilarski, Lynn Schoenfield, Jose Otero, Arnab Chakravarti, Balveen Kaur, Gustavo Leone, Michael C. Ostrowski, Gina M. Sizemore. Stromal platelet derived growth factor receptor-beta (PDGFRbeta) promotes breast brain metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3911. doi:10.1158/1538-7445.AM2017-3911

Abstract 2966: Stromal platelet derived growth factor receptor-beta (PDGFRbeta) promotes breast cancer progression

Over the past decade it has become evident that the tumor microenvironment (TME) actively participates in carcinogenesis. Tumor-associated fibroblasts, for example, modulate neighboring tumor epithelium through growth factor secretion to initiate and promote tumor growth. The platelet derived growth factor receptors (PDGFRs), PDGFRalpha and PDGFRbeta, are receptor tyrosine kinases activated by PDGF that may be critical and actionable mediators of breast tumor-stromal communication. PDGFRs are predominately expressed in breast tumor stroma while their cognate ligands are specifically expressed in tumor epithelium and associated endothelium. In some cancers, tumor-derived PDGFs act on the TME to recruit tumor associated fibroblasts; however, this role has not been described in breast cancer. To begin to evaluate a role for PDGFRbeta, we utilized publicly available gene expression data to confirm upregulation in tumor stroma compared to tumor epithelium. Importantly, PDGFRB is increased in tumor stroma compared to normal stroma. To directly test whether stromal PDGFR activation promotes tumor growth, we co-injected murine mammary tumor cells with or without PDGFR-expressing mouse mammary fibroblasts (MMFs) orthotopically in FVB/N mice. MMF inclusion increased tumor cell proliferation as well as associated angiogenesis while systemic treatment with imatinib mesylate, a small molecule inhibitor for PDGFR, restored both proliferation and angiogenesis back to baseline. These findings indicated the importance of PDGFR signaling in tumor initiation leading us to develop a mouse model of stromal-specific PDGFRbeta activation using the Fsp-cre transgene previously published by our group (henceforth referred to as “PDGFRbeta mutant”). PDGFRbeta mutant mammary glands exhibit increased tertiary side-branching and epithelial proliferation confirming a stromal-specific PDGFRbeta effect on neighboring epithelium during development. Further, MMFs isolated from the PDGFRbeta mutant mice exhibit increased motility towards PDGF-B expressing tumor cells in vitro, which implies increased response and recruitment of the mutant MMFs towards an expanding tumor. To test whether PDGFRbeta mutant mice harbor a mammary TME supportive of increased tumor growth, we injected murine mammary tumor cells orthotopically into either control or PDGFRbeta mutant mice finding that the time required to meet early removal criteria (tumor &gt1.2cm3) was shorter in the mutant mice compared to controls. Ongoing studies are evaluating whether systemic PDGFR inhibition will abrogate this observed increase in tumorigenesis. In summary, our data suggest that stromal PDGFRbeta signaling is pro-tumorigenic in breast cancer and that inhibition using well-described PDGFR inhibitors could be a valid therapeutic approach for women whose tumors express increased PDGF-to-PDGFR tumor-stromal signaling.

Citation Format: Gina M. Sizemore, Anisha M. Hammer, Katie A. Thies, Steven T. Sizemore, Anthony J. Trimboli, B. Eason Hildreth, Raleigh D. Kladney, Arnab Chakravarti, Gustavo Leone, Michael C. Ostrowski. Stromal platelet derived growth factor receptor-beta (PDGFRbeta) promotes breast cancer progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2966. doi:10.1158/1538-7445.AM2017-2966

The ETS family of oncogenic transcription factors in solid tumours

Findings over the past decade have identified aberrant activation of the ETS transcription factor family throughout all stages of tumorigenesis. Specifically in solid tumours, gene rearrangement and amplification, feed-forward growth factor signalling loops, formation of gain-of-function co-regulatory complexes and novel cis-acting mutations in ETS target gene promoters can result in increased ETS activity. In turn, pro-oncogenic ETS signalling enhances tumorigenesis through a broad mechanistic toolbox that includes lineage specification and self-renewal, DNA damage and genome instability, epigenetics and metabolism. This Review discusses these different mechanisms of ETS activation and subsequent oncogenic implications, as well as the clinical utility of ETS factors.

Stromal PDGFR-α Activation Enhances Matrix Stiffness, Impedes Mammary Ductal Development, and Accelerates Tumor Growth

The extracellular matrix (ECM) is critical for mammary ductal development and differentiation, but how mammary fibroblasts regulate ECM remodeling remains to be elucidated. Herein, we used a mouse genetic model to activate platelet derived growth factor receptor-alpha (PDGFRα) specifically in the stroma. Hyperactivation of PDGFRα in the mammary stroma severely hindered pubertal mammary ductal morphogenesis, but did not interrupt the lobuloalveolar differentiation program. Increased stromal PDGFRα signaling induced mammary fat pad fibrosis with a corresponding increase in interstitial hyaluronic acid (HA) and collagen deposition. Mammary fibroblasts with PDGFRα hyperactivation also decreased hydraulic permeability of a collagen substrate in an in vitro microfluidic device assay, which was mitigated by inhibition of either PDGFRα or HA. Fibrosis seen in this model significantly increased the overall stiffness of the mammary gland as measured by atomic force microscopy. Further, mammary tumor cells injected orthotopically in the fat pads of mice with stromal activation of PDGFRα grew larger tumors compared to controls. Taken together, our data establish that aberrant stromal PDGFRα signaling disrupts ECM homeostasis during mammary gland development, resulting in increased mammary stiffness and increased potential for tumor growth.

Stromal PTEN inhibits the expansion of mammary epithelial stem cells through Jagged-1

Fibroblasts within the mammary tumor microenvironment are active participants in carcinogenesis mediating both tumor initiation and progression. Our group has previously demonstrated that genetic loss of PTEN in mammary fibroblasts induces an oncogenic secretome that remodels the extracellular milieu accelerating ErbB2-driven mammary tumor progression. While these prior studies highlighted a tumor suppressive role for stromal PTEN, how the adjacent normal epithelium transforms in response to PTEN loss was not previously addressed. To identify these early events, we have evaluated both phenotypic and genetic changes within the pre-neoplastic mammary epithelium of mice with and without stromal PTEN expression. We report that fibroblast-specific PTEN deletion greatly restricts mammary ductal elongation and induces aberrant alveolar side-branching. These mice concomitantly exhibit an expansion of the mammary epithelial stem cell (MaSC) enriched basal/myoepithelial population and an increase in in vitro stem cell activity. Further analysis revealed that NOTCH signaling, specifically through NOTCH3, is diminished in these cells. Mechanistically, JAGGED-1, a transmembrane ligand for the NOTCH receptor, is downregulated in the PTEN-null fibroblasts leading to a loss in the paracrine activation of NOTCH signaling from the surrounding stroma. Reintroduction of JAGGED-1 expression within the PTEN-null fibroblasts was sufficient to abrogate the observed increase in colony forming activity implying a direct role for stromal JAGGED-1 in regulation of mammary stem cell properties. Importantly, breast cancer patients whose tumors express both low stromal JAG1 and low stromal PTEN exhibit a shorter time to recurrence than those whose tumors express low levels of either alone suggesting similar stromal signaling in advanced disease. Combined, these results unveil a novel stromal PTEN-to-JAGGED-1 axis in maintaining the mammary epithelial stem cell niche, and subsequently inhibiting breast cancer initiation and disease progression.