BUB1 inhibition sensitizes lung cancer cell lines to radiotherapy and chemoradiotherapy

Background

Lung cancer is a major public health concern, with high incidence and mortality. Despite advances in targeted therapy and immunotherapy, microtubule stabilizers (paclitaxel, docetaxel), DNA intercalating platinum drugs (cisplatin) and radiation therapy continue to play a critical role in the management of locally advanced and metastatic lung cancer. Novel molecular targets would provide opportunities for improving the efficacies of radiotherapy and chemotherapy.

Hypothesis

We hypothesize that BUB1 (Ser/Thr kinase) is over-expressed in lung cancers and that its inhibition will sensitize lung cancers to chemoradiation.

Methods

BUB1 inhibitor (BAY1816032) was combined with platinum (cisplatin), microtubule poison (paclitaxel), a PARP inhibitor (olaparib) and radiation in cell proliferation and radiation sensitization assays. Biochemical and molecular assays were used to evaluate their impact on DNA damage signaling and cell death mechanisms.

Results

BUB1 expression assessed by immunostaining of lung tumor microarrays (TMAs) confirmed higher BUB1 expression in NSCLC and SCLC compared to that of normal tissues. BUB1 overexpression in lung cancer tissues correlated directly with expression of TP53 mutations in non-small cell lung cancer (NSCLC). Elevated BUB1 levels correlated with poorer overall survival in NSCLC and small cell lung cancer (SCLC) patients. A BUB1 inhibitor (BAY1816032) synergistically sensitized lung cancer cell lines to paclitaxel and olaparib. Additionally, BAY1816032 enhanced cell killing by radiation in both NSCLC and SCLC. Molecular changes following BUB1 inhibition suggest a shift towards pro-apoptotic and anti-proliferative states, indicated by altered expression of BAX, BCL2, PCNA, and Caspases 9 and 3.

Conclusion

A direct correlation between BUB1 protein expression and overall survival was shown. BUB1 inhibition sensitized both NSCLC and SCLC to various chemotherapies (cisplatin, paclitaxel) and targeted therapy (PARPi). Furthermore, we present the novel finding that BUB1 inhibition sensitized both NSCLC and SCLC to radiotherapy and chemoradiation. Our results demonstrate BUB1 inhibition as a promising strategy to sensitize lung cancers to radiation and chemoradiation therapies.

A phase I clinical trial of oncolytic adenovirus mediated suicide and interleukin-12 gene therapy in patients with recurrent localized prostate adenocarcinoma

In a phase I dose escalation and safety study (NCT02555397), a replication-competent oncolytic adenovirus expressing yCD, TK and hIL-12 (Ad5-yCD/mutTKSR39rep-hIL-12) was administered in 15 subjects with localized recurrent prostate cancer (T1c-T2) at increasing doses (1 × 1010, to 1 × 1012 viral particles) followed by 7-day treatment of 5-fluorocytosine (5-FC) and valganciclovir (vGCV). The primary endpoint was toxicity through day 30 while the secondary and exploratory endpoints were quantitation of IL-12, IFNγ, CXCL10 and peripheral blood mononuclear cells (PBMC). The study maximum tolerated dose (MTD) was not reached indicating 1012 viral particles was safe. Total 115 adverse events were observed, most of which (92%) were grade 1/2 that did not require any treatment. Adenoviral DNA was detected only in two patients. Increase in IL-12, IFNγ, and CXCL10 was observed in 57%, 93%, and 79% patients, respectively. Serum cytokines demonstrated viral dose dependency, especially apparent in the highest-dose cohorts. PBMC analysis revealed immune system activation after gene therapy in cohort 5. The PSA doubling time (PSADT) pre and post treatment has a median of 1.55 years vs 1.18 years. This trial confirmed that replication-competent Ad5-IL-12 adenovirus (Ad5-yCD/mutTKSR39rep-hIL-12) was well tolerated when administered locally to prostate tumors.

Budding uninhibited by benzimidazoles-1 (BUB1) regulates EGFR signaling by reducing EGFR internalization

EGFR signaling initiates upon ligand binding which leads to activation and internalization of the receptor-ligand complex. Here, we evaluated if BUB1 impacted EGFR signaling by regulating EGFR receptor internalization and activation. BUB1 was ablated genomically (siRNA) or biochemically (2OH-BNPP1) in cells. EGF ligand was used to initiate EGFR signaling while disuccinimidyl suberate (DSS) was used for cross linking cellular proteins. EGFR signaling was measured by western immunoblotting and receptor internalization was evaluated by fluorescent microscopy (pEGFR (pY1068) colocalization with early endosome marker EEA1). siRNA mediated BUB1 depletion led to an overall increase in total EGFR levels and more phospho-EGFR (Y845, Y1092, and Y1173) dimers while the amount of total EGFR (non-phospho) dimers remained unchanged. BUB1 inhibitor (BUB1i) decreased EGF mediated EGFR signaling including pEGFR Y845, pAKT S473 and pERK1/2 in a time dependent manner. Additionally, BUB1i also reduced EGF mediated pEGFR (Y845) dimers (asymmetric dimers) without affecting total EGFR dimers (symmetric dimers) indicating that dimerization of inactive EGFR is not affected by BUB1. Furthermore, BUB1i blocked EGF mediated EGFR degradation (increase in EGFR half-life) without impacting half-lives of HER2 or c-MET. BUB1i also reduced co-localization of pEGFR with EEA1 positive endosomes suggesting that BUB1 might modulate EGFR endocytosis. Our data provide evidence that BUB1 protein and its kinase activity may regulate EGFR activation, endocytosis, degradation, and downstream signaling without affecting other members of the receptor tyrosine kinase family.

Oncolytic virus-based suicide gene therapy for cancer treatment: a perspective of the clinical trials conducted at Henry Ford Health

Gene therapy manipulates or modifies a gene that provides a new cellular function to treat or correct a pathological condition, such as cancer. The approach of using gene manipulation to modify patient's cells to improve cancer therapy and potentially find a cure is gaining popularity. Currently, there are 12 gene therapy products approved by US-FDA, EMA and CFDA for cancer management, these include Rexin-G, Gendicine, Oncorine, Provange among other. The Radiation Biology Research group at Henry Ford Health has been actively developing gene therapy approaches for improving clinical outcome in cancer patients. The team was the first to test a replication-competent oncolytic virus armed with a therapeutic gene in humans, to combine this approach with radiation in humans, and to image replication-competent adenoviral gene expression/activity in humans. The adenoviral gene therapy products developed at Henry Ford Health have been evaluated in more than 6 preclinical studies and evaluated in 9 investigator initiated clinical trials treating more than100 patients. Two phase I clinical trials are currently following patients long term and a phase I trial for recurrent glioma was initiated in November 2022. This systematic review provides an overview of gene therapy approaches and products employed for treating cancer patients including the products developed at Henry Ford Health.

Abstract 2816: BUB1 inhibition radiosensitizes triple-negative breast cancer by targeting the DNA-damage repair pathways

Triple-negative breast cancer (TNBC) is the most lethal type of breast cancer (BC). TNBC has a substantial risk of locoregional recurrence and is difficult to cure due to absence of molecular targets or intrinsic heterogeneity. As radiotherapy (RT) is crucial in the treatment of BC, there is a need to identify novel molecular targets for increasing the efficacy of radiation therapy. BUB1 (Budding uninhibited by benzimidazoles 1) is a serine/threonine kinase implicated in chromosomal segregation during mitosis. However, it is still uncertain how BUB1 contributes to radioresistance in TNBC. By performing expression analysis (Oncomine), we observed that BUB1 is overexpressed in BC and even more in TNBC, indicating that this overexpression could be a prognostic factor for TNBC. MTT assay in several TNBC cell lines demonstrated that pharmacological (BAY1816032) or genomic (CRISPR) ablation of BUB1 was cytotoxic. By performing clonogenic survival assays, we observed that BUB1 ablation led to clinically meaningful radiosensitization in TNBC cell lines (SUM159, MDA-MB-231, MDA-MB-468) with radiation enhancement ratios ranging from 1.1 - 1.38. In addition, we showed that inhibition of BUB1 kinase function is essential for the radiosensitization phenotype using siRNAs and CRISPR knockouts by depleting endogenous BUB1 and reintroducing wild-type (WT) or kinase-dead (KD) BUB1. We observed significant increase in tumor doubling time in SUM159 xenografts (mammary fat pads in CB17/SCID mice) when BUB1i was combined with RT (DT=29.6 vs 7.9 days). A reduction in Ki67 was observed in these tumors further confirming the effectiveness of combination treatment. The effect of BUB1 depletion on DNA double strand break repair was assessed using γ-H2AX foci formation assay, which revealed sustained increase in γ-H2AX foci following radiation in BUB1i-treated cells. Further, BLRR assays confirmed that BUB1i reduces non-homologous end joining (NHEJ) efficiency. Biochemical analyses showed that BUB1i increases and stabilizes radiation-induced DNAPKcs phosphorylation (S2056). To evaluate the recruitment of DDR proteins to the chromatin following irradiation (IR), soluble nuclear and the chromatin-enriched fractionation was performed which demonstrated that cells pretreated with BUB1i had increased recruitment of DDR proteins to chromatin suggesting that BUB1 may play a role in the initial recruitment of NHEJ proteins to DSBs. Our findings demonstrate that both genetic and pharmacological inhibition of BUB1 sensitizes TNBC cells to RT and that BUB1 mediated radiosensitization may occur through NHEJ pathway. Our results suggest that BUB1 may represent a novel molecular target for radiosensitization in women with TNBC.

Citation Format: Sushmitha Sriramulu, Shivani Thoidingjam, Pin Li, Stephen L. Brown, Farzan Siddiqui, Benjamin Movsas, Michael Green, Corey Speers, Shyam Nyati. BUB1 inhibition radiosensitizes triple-negative breast cancer by targeting the DNA-damage repair pathways [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 2816.

Abstract 6271: Hormone receptor inhibition as a strategy for radiosensitization of breast cancer

Purpose: Expression of the androgen receptor (AR) has been identified as a driver of tumor growth in triple negative breast cancers (TNBC), and previous work has nominated AR as a target for radiosensitization. In addition, 70-95% of all estrogen receptor (ER) positive (ER+) breast cancers also have coexpression of AR, suggesting extended utility of AR inhibition in the radiosensitization of these AR+, ER+ tumors. Here we assessed the efficacy of AR inhibition in ER+, AR+ breast cancers to better understand the role of AR signaling across breast cancer models. Further, we also investigated the effect of ER inhibition on radiosensitization of ER+ breast cancer models.

Methods: IC50 values were determined for MDV3100 (enzalutamide), ARN-509 (apalutamide), and ODM-201 (darolutamide) in TNBC cell lines (AR+ TNBC: MDA-MB-453, ACC-422, and SUM-185PE, and AR- TNBC: MDA-MB-231) and ER+ breast cancer cell lines (AR+, ER+: ZR-75-1, BT-474, CAMA-1, and AR-, ER+: MCF-7). IC50 values for tamoxifen were determined for ER+ breast cancer cell lines (MCF-7, T47D, ZR-75-1), and ER- (SUM-159) cells. Clonogenic survival assays were performed to assess radiosensitization with ER or AR inhibition with tamoxifen or second generation anti-androgens, respectively, in TNBC and ER+ breast cancer models.

Results: AR inhibition with enzalutamide, apalutamide, and darolutamide showed limited single agent growth inhibition efficacy in AR+ TNBC and AR+, ER+ breast cancer cell lines (IC50 > 10 μM). AR inhibition with enzalutamide did not induce radiosensitivity in vitro. In AR+, ER+ CAMA-1 cells, AR blockade with enzalutamide had a radioprotective effect with enhancement ratios (enhR) of 0.76-0.83. No radiosensitization was observed in BT-474 (enhR: 0.92-1.01) or ZR-75-1 cells (enhR: 0.94-1.00). Radiosensitization was also assessed with anti-androgens apalutamide and darolutamide in AR+ breast cancer models. Inhibition of ER with tamoxifen, however, induced radiosensitization in MCF-7 (enhR: 1.14-1.50) and T47D (enhR: 1.33-1.60) cells. No radiosensitization was observed with tamoxifen in ER- SUM-159 cells.

Conclusion: Although AR is a mediator of radioresistance in AR+ TNBC, AR inhibition does not provide comparable radiosensitization in AR+, ER+ models and may actually confer a radioprotective effect. In contrast, our results demonstrate ER inhibition is an effective radiosensitizing strategy in ER+ breast cancers, independent of AR status. This work highlights the complexities of androgen and estrogen receptor signaling in AR+, ER+ breast tumors and underscores the necessity for understanding context dependent effects when translating into patients with AR+ breast cancer.

Citation Format: Anna R. Michmerhuizen, Amanda Zhang, Rachel Schwartz, Andrea M. Pesch, Benjamin C. Chandler, Cassandra L. Ritter, Meilan Liu, Kari Wilder-Romans, Daniel E. Spratt, Daniel R. Wahl, Shyam Nyati, Lori J. Pierce, Corey Speers. Hormone receptor inhibition as a strategy for radiosensitization of breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6271.

Abstract 6273: TTK inhibition radiosensitizes basal-like breast cancer through impaired homologous recombination

Background: Basal-like breast cancer (BC) has the highest rates of local recurrence despite the use of radiation therapy. Therefore, approaches for radiosensitization are critically needed for patients with this subtype of BC.

Methods: Four independent datasets were used to correlate gene expression with local recurrence (LR) and Kaplan-Meier analysis validated the impact of TTK expression on LR. The METABRIC dataset was used to determine TTK expression in BC subtypes. Clonogenic survival assays were used to determine the radiosensitization of cell lines after TTK inhibition (TTKi). Mouse models were used to assess TTKi in combination with RT in vivo. DNA damage was quantified using γH2AX staining. HR and NHEJ efficiency assays were performed using HR/NHEJ specific reporter systems. HR competency was also assessed using Rad51 foci formation assays. Rescue experiments were performed using wild-type (WT) and kinase-dead (KD) TTK plasmids in combination with siRNA targeting the UTR region of TTK.

Results: Ten genes were found to significantly correlate with early LR (≤3 years) after surgery and radiation across 4 independent datasets (N=896 pts), with TTK, a cell cycle kinase, ranked the highest. Kaplan-Meier survival analysis in multiple cohorts demonstrated that higher than median TTK expression correlates with decrease LR free survival after RT (HR 1.70-2.42, p<0.01 for all 3 cohorts). Subtype association analysis demonstrated that TTK expression was most elevated in basal-like BC. Using inducible shRNA, the combination of TTK knockdown and RT increases radiosensitivity in multiple basal-like BC cell lines (rER 1.21-1.63). Additionally, TTKi using, Bayer 1161909 (B909), enhanced radiosensitivity in multiple cell lines (rER 1.10-2.27). In vivo, TTKi, using shRNA or B909, in combination with RT led to delayed tumor growth and a significant increase in time to tumor tripling (Placebo: 9 days vs. B909+RT: undefined [>35 days], p<0.0001) in both cell line and PDX models. Increased DNA damage was found after combination treatment of TTKi and RT compared to RT alone, indicating that DNA damage repair mechanisms may be compromised by TTKi. The efficiency of the double strand DNA damage repair mechanism, homologous recombination (HR), but not non-homologous end joining (NHEJ), was reduced upon TTKi in HR/NHEJ specific reporter systems. Additionally, Rad51 foci formation was reduced by TTKi after RT compared to RT alone. Reintroduction of WT TTK, after knockdown of endogenous TTK, rescued radioresistance and HR efficiency, however, reintroduction of kinase-dead (KD) TTK was unable to do so in multiple cell lines. WT TTK also rescued Rad51 foci formation after knockdown of endogenous TTK while KD TTK did not.

Conclusion: These data support TTKi as a radiosensitizing strategy for clinical development in basal-like BC patients and that radiosensitization is mediated, at least in part, through impaired HR repair.

Citation Format: Benjamin Chandler, Leah Moubadder, Cassandra Ritter, Meilan Liu, Meleah Cameron, Kari Wilder-Romans, Amanda Zhang, Andrea Pesch, Anna Michmerhuizen, Nicole Hirsh, Marlie Androsiglio, Tanner Ward, Eric Olsen, Yashar Niknafs, Sofia Merajver, Dafydd Thomas, Powel Brown, Theodore Lawrence, Shyam Nyati, Lori Pierce, Arul Chinnaiyan, Corey Speers. TTK inhibition radiosensitizes basal-like breast cancer through impaired homologous recombination [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6273.

TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling

BUB1 (budding uninhibited by benzimidazoles-1) is required for efficient TGF-β signaling, through its role in stabilizing the TGFBR1 and TGFBR2 complex. Here we demonstrate that TGFBR2 phosphorylates BUB1 at Serine-318, which is conserved in primates. S318 phosphorylation abrogates the interaction of BUB1 with TGFBR1 and SMAD2. Using BUB1 truncation domains (1–241, 241–482 and 482–723), we demonstrate that multiple contact points exist between BUB1 and TGF-β signaling components and that these interactions are independent of the BUB1 tetratricopeptide repeat (TPR) domain. Moreover, substitutions in the middle domain (241–482) encompassing S318 reveals that efficient interaction with TGFBR2 occurs only in its dephosphorylated state (241–482 S318A). In contrast, the phospho-mimicking mutant (241–482 S318D) exhibits efficient binding with SMAD2 and its over-expression results in a decrease in TGFBR1-TGFBR2 and TGFBR1-SMAD2 interactions. These findings suggest that TGFBR2 mediated BUB1 phosphorylation at S318 may serve as a switch for the dissociation of the SMAD2-TGFBR complex, and therefore represents a regulatory event for TGF-β signaling. Finally, we provide evidence that the BUB1-TGF-β signaling axis may mediate aggressive phenotypes in a variety of cancers.

TTK inhibition radiosensitizes basal-like breast cancer through impaired homologous recombination

Increased rates of locoregional recurrence are observed in patients with basal-like breast cancer (BC) despite the use of radiation therapy (RT); therefore, approaches that result in radiosensitization of basal-like BC are critically needed. Using patients' tumor gene expression data from 4 independent data sets, we correlated gene expression with recurrence to find genes significantly correlated with early recurrence after RT. The highest-ranked gene, TTK, was most highly expressed in basal-like BC across multiple data sets. Inhibition of TTK by both genetic and pharmacologic methods enhanced radiosensitivity in multiple basal-like cell lines. Radiosensitivity was mediated, at least in part, through persistent DNA damage after treatment with TTK inhibition and RT. Inhibition of TTK impaired homologous recombination (HR) and repair efficiency, but not nonhomologous end-joining, and decreased the formation of Rad51 foci. Reintroduction of wild-type TTK rescued both radioresistance and HR repair efficiency after TTK knockdown; however, reintroduction of kinase-dead TTK did not. In vivo, TTK inhibition combined with RT led to a significant decrease in tumor growth in both heterotopic and orthotopic, including patient-derived xenograft, BC models. These data support the rationale for clinical development of TTK inhibition as a radiosensitizing strategy for patients with basal-like BC, and efforts toward this end are currently underway.

Seviteronel, a Novel CYP17 Lyase Inhibitor and Androgen Receptor Antagonist, Radiosensitizes AR-Positive Triple Negative Breast Cancer Cells

Increased rates of locoregional recurrence (LR) have been observed in triple negative breast cancer (TNBC) despite multimodality therapy, including radiation (RT). Recent data suggest inhibiting the androgen receptor (AR) may be an effective radiosensitizing strategy, and AR is expressed in 15-35% of TNBC tumors. The aim of this study was to determine whether seviteronel (INO-464), a novel CYP17 lyase inhibitor and AR antagonist, is able to radiosensitize AR-positive (AR+) TNBC models. In cell viability assays, seviteronel and enzalutamide exhibited limited effect as a single agent (IC50 > 10 μM). Using clonogenic survival assays, however, AR knockdown and AR inhibition with seviteronel were effective at radiosensitizing cells with radiation enhancement ratios of 1.20-1.89 in models of TNBC with high AR expression. AR-negative (AR-) models, regardless of their estrogen receptor expression, were not radiosensitized with seviteronel treatment at concentrations up to 5 μM. Radiosensitization of AR+ TNBC models was at least partially dependent on impaired dsDNA break repair with significant delays in repair at 6, 16, and 24 h as measured by immunofluorescent staining of γH2AX foci. Similar effects were observed in an in vivo AR+ TNBC xenograft model where there was a significant reduction in tumor volume and a delay to tumor doubling and tripling times in mice treated with seviteronel and radiation. Following combination treatment with seviteronel and radiation, increased binding of AR occurred at DNA damage response genes, including genes involved both in homologous recombination and non-homologous end joining. This trend was not observed with combination treatment of enzalutamide and RT, suggesting that seviteronel may have a different mechanism of radiosensitization compared to other AR inhibitors. Enzalutamide and seviteronel treatment also had different effects on AR and AR target genes as measured by immunoblot and qPCR. These results implicate AR as a mediator of radioresistance in AR+ TNBC models and support the use of seviteronel as a radiosensitizing agent in AR+ TNBC.

A signature predictive of early versus late recurrence after radiation (RT) for breast cancer that may inform the biology of early, aggressive recurrences

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Background: Unmet clinical needs in breast cancer (BC) management include the identification of patients at high risk to fail locally despite standard local therapy and an understanding of the biology of these recurrences. We previously reported a radiation response signature and here extend those studies to identify a signature predictive of timing of recurrence after RT. Methods: 2 independent patient cohorts were used for training (119 pts) and validation (112 pts). All patients received RT after BCS and systemic therapy as appropriate. Spearman’s rank correlation to correlate gene expression to recurrence time was used for feature selection. Significant genes were used to train a linear model which was locked before validation. Cox regression was used for both UVA and MVA. Results: Spearman’s correlation identified 485 genes whose expression was significantly associated with recurrence time (+/-3 yrs). Feature reduction refined the list to 41 genes retained within the signature. In training, the correlation of score to recurrence time was 0.85, p-value < 1.3x10-31; AUC of 0.91. External validation in an independent BC validation set accurately identified patients with early vs. late recurrences (correlation= 0.75, p-value = 0.001, AUC = 0.92, sens.=0.75, spec.= 1.0, PPV = 1.0, NPV = 0.8). Unique associations of breast cancer intrinsic subtype to timing of local recurrence were found. In UVA and MVA the signature remained the most significant factor associated with recurrence. GSEA analysis of the 41 genes retained within the signature identified proliferation and EGFR concepts associated with early recurrences and luminal and ER-signaling pathways associated with late recurrences. Knockdown of genes associated with the early and late recurrences demonstrated novel effects on proliferation and clonogenic survival, respectively. Conclusions: We report a BC gene expression signatures that may be useful in identifying patients unlikely to respond to adjuvant RT and may be used to predict timing of recurrences, with implications for potential treatment intensification and duration of follow-up for women with breast cancer treated with RT.

PARP1 Inhibition Radiosensitizes Models of Inflammatory Breast Cancer to Ionizing Radiation

Sustained locoregional control of disease is a significant issue in patients with inflammatory breast cancer (IBC), with local control rates of 80% or less at 5 years. Given the unsatisfactory outcomes for these patients, there is a clear need for intensification of local therapy, including radiation. Inhibition of the DNA repair protein PARP1 has had little efficacy as a single agent in breast cancer outside of studies restricted to patients with BRCA mutations; however, PARP1 inhibition (PARPi) may lead to the radiosensitization of aggressive tumor types. Thus, this study investigates inhibition of PARP1 as a novel and promising radiosensitization strategy in IBC. In multiple existing IBC models (SUM-149, SUM-190, MDA-IBC-3), PARPi (AZD2281-olaparib and ABT-888-veliparib) had limited single-agent efficacy (IC₅₀ > 10 μmol/L) in proliferation assays. Despite limited single-agent efficacy, submicromolar concentrations of AZD2281 in combination with RT led to significant radiosensitization (rER 1.12-1.76). This effect was partially dependent on BRCA1 mutational status. Radiosensitization was due, at least in part, to delayed resolution of double strand DNA breaks as measured by multiple assays. Using a SUM-190 xenograft model in vivo, the combination of PARPi and RT significantly delays tumor doubling and tripling times compared with PARPi or RT alone with limited toxicity. This study demonstrates that PARPi improves the effectiveness of radiotherapy in IBC models and provides the preclinical rationale for the opening phase II randomized trial of RT ± PARPi in women with IBC (SWOG 1706, NCT03598257).

Abstract 2539: Discovery of degradation pathway for maternal embryonic leucine zipper kinase (MELK): Implications for breast cancer therapy

Background: Despite previous studies suggesting that MELK is not involved in proliferation in cancer, many groups have shown that MELK is a critical mediator of radioresistance and may be an ideal target for radiosensitization. Furthermore, additional studies suggest MELK functions to regulate the cell-cycle and that its mRNA and protein levels are regulated during cell-cycle, although the mechanisms of this regulation are currently unknown. We sought to identify mediators of MELK protein regulation, determine whether it was ubiquitin-ligase mediated, and if so, which E3-ubiquitin ligase machinery degrades MELK during cell-cycle progression.

Methods: Sequence alignments were carried out to identify degron motifs in MELK. Site directed mutagenesis were performed to mutate degrons in MELK. CDC20 and CDH1 were over-expressed to identify the co-activators of the E3-ubiquitin ligase complex. Cells were synchronized by double thymidine block or by nocodazole. CDC27/APC3 and CDC20 antibodies were used to pull-down the Anaphase Promoting Complex/Cyclosome (APC/C) and Mitotic Checkpoint Complex (MCC), respectively. Resulting pellets were probed with antibodies against BubR1/Mad3, Bub3, CDC20, CDC27, MELK, Cyclin B1, CDK1, BUB1, and Mad2. A role for MELK kinase activity on its half-life and formation of APC/C and MCC was evaluated using kinetically inactive (MELK-KD) mutant or OTSSP167. siRNA against MELK was used to confirm a requirement of MELK for formation of APC/C and MCC. The effect of MELK degron mutant on radiosensitization will be evaluated in MELK CRISPR TNBC cell lines.

Results: In this study we provide evidence that MELK has two D-boxes and a KEN box which are largely conserved from C. elegance to H. sapiens. We identified that MELK is degraded by the E3-ubiquitin ligase APC/C with the coactivator being CDC20. Rescue with MG132 confirms that MELK is degraded by a proteosomal-mediated pathway. CO-IP studies show that MELK is required for APC/C and MCC formation and that MELK interacts with CDC20, CDK1, BUB1, and BUBR1. Cycloheximide chase studies show D-Box1 and KEN box mutants of MELK are more stable while D-BOX2 mutants are least stable. The KEN and D-Box mutant MELK show impairment in APC/C and MCC formation, altered Cyclin B1-CDK1 and Cyclin B1-CDC20 interactions. MELK KD and OTSSP prove that kinase activity of MELK plays a role in its stability. We anticipate that the least stable MELK mutant will result in higher radiosensitivity compared to MELK-WT.

Conclusions: MELK is regulated by APC/CCDC20 through its D-Box1, KEN and D-Box2. Degron mutant and MELK KD show altered protein stability which leads to attenuated APC/C and MCC formation. These results provide proof that MELK is a cell-cycle checkpoint kinase and support the rationale for developing clinical strategies to degrade and inhibit MELK in combination with radiation therapy.

Citation Format: Shyam Nyati, Benjamin Chandler, Anna Michmerhuizen, Andrea Pesch, Cassandra Ritter, Leah Moubadder, Meilan Liu, Meleah Cameron, Eric Olsson, Kari Wilder-Romans, Dipankar Ray, Theodore S. Lawrence, Felix Y. Feng, Lori J. Pierce, Corey Speers. Discovery of degradation pathway for maternal embryonic leucine zipper kinase (MELK): Implications for breast cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2539.

Abstract 2926: Nomination and characterization of TTK for radiosensitization in basal-like breast cancers

Background: Increased rates of locoregional recurrence have been observed in basal-like breast cancer despite the use of radiation therapy (RT), therefore approaches that result in radiosensitization of basal-like breast cancer are critically needed. Studies detailing the poor response of basal-like tumors to adjuvant RT underscore the biologic differences and as of yet undefined oncogenic drivers of these particular types of breast cancer.

Methods: 4 independent datasets were used to correlate gene expression with local recurrence (LR) after 3 years. Kaplan-Meier analysis was used to validate the impact of TTK expression on LR. The TCGA and institutional breast cancer dataset were used to determine TTK expression in BC subtypes. TTK RNA and protein levels were measured using qPCR and western blot at baseline and correlated to intrinsic radiosensitivity. Clonogenic survival assays were used to determine the radiosensitization of cell lines after TTK inhibition (TTKi). Mice models were used to assess TTKi in combination with RT in vivo. DNA damage was quantified using γH2AX staining. HR and NHEJ efficiency assays were performed using HR/NHEJ specific reporter systems. HR competency was also assessed using RAD51 foci formation assays.

Results: Ten genes were found to significantly correlate with early LR (≤3 years) across 4 distinct datasets (N=896 pts) (OR of recurrence &gt; 2, p-value &lt;0.000001), with TTK, a cell cycle kinase, ranked the highest. Kaplan-Meier survival analysis in 3 cohorts demonstrated that higher than median TTK expression correlates with decrease LR free survival after RT (HR 1.70-2.42, p&lt;0.01 for all 3 cohorts). Subtype association analysis demonstrated that TTK expression was most elevated in basal-like breast cancer (p&lt;0.0001). Clonogenic survival assays, using inducible shRNA models, show the combination of TTK knockdown and RT increases radiosensitivity in MDA-MB-231 and BT-549 basal-like breast cancer cell lines (rER 1.21-1.63). Additionally, TTKi using, Bayer 1161909 (B909), enhanced radiosensitivity in both cell lines (rER 1.10-1.39). In vivo, TTKi, using shRNA and B909 in combination with RT led to a significant increased time to tumor tripling. Using γH2AX foci formation assays, increased DNA damage was found after combination treatment of TTKi and RT compared to RT alone, indicating that DNA damage repair mechanisms may be compromised by TTKi. We found that the efficiency of the double strand DNA damage repair mechanism, homologous recombination (HR), but not non-homologous end joining (NHEJ), was reduced upon TTKi in HR/NHEJ specific reporter systems. Additionally, RAD51 foci formation was reduced by TTKi after RT compared to RT alone.

Conclusion: These data support TTK inhibition as a rationale radiosensitizing strategy for clinical development in basal-like breast cancer patients and that the mechanism of radiosensitization is, at least in part, through impaired HR repair.

Citation Format: Benjamin C. Chandler, Leah Moubadder, Cassie Ritter, Kari Wilder-Romans, Meleah Cameron, Meilan Liu, Shyam Nyati, Andrea Pesch, Anna Michmerhuizen, Eric Olsen, Yashar Niknafs, Arul Chinnaiyan, Corey Speers. Nomination and characterization of TTK for radiosensitization in basal-like breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2926.

Abstract 3430: TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling

The Ser/Thr kinase BUB1 (budding uninhibited by benzimidazoles-1) is required for efficient signaling by the TGF-β receptor. Recruitment of BUB1 to the activated heteromeric TGF-β-receptor complex including SMAD2 and SMAD3, is required for ligand mediated canonical and non-canonical downstream cascades (Nyati et. al. 2015). In an effort to delineate the structure-function basis for this, we utilized in vitro kinase reactions followed by mass spectrometry to demonstrate that TGFBR2 phosphorylates BUB1 at Serine 318. To elucidate the functional significance of TGFBR2 mediated phosphorylation of BUB1, S318 was substituted with alanine (S318A) or aspartic acid (S318D). Co-immunoprecipitation studies of these phospho-deficient or phospho-mimicking BUB1 mutants with TGFBR1, and SMAD2 revealed that the S318A mutant interacted more efficiently to TGFBR1 and SMAD2 compared to the S318D BUB1. To investigate if the Ser318 residue was involved in mediating BUB1 interaction with TGF-β signaling components, we utilized deletion mutants of BUB1. Our findings reveal that the N-terminal (1-241) and C-terminal (482-723) domains of BUB1 independently interact with SMAD2, SMAD3, TGFBR1, or TGFBR2. The middle domain (241-482) having a S318A mutation interacted efficiently with TGFBR2 while the S318D mutant did not. Additionally, the 241-482 S318D mutant exhibited an increased interaction with SMAD2. Over-expression of 241-482 S318D resulted in a decrease in TGFBR1-TGFBR2 interaction as well as TGFBR1-SMAD2 interaction, suggesting that TGFBR2 mediated BUB1 phosphorylation at S318 maybe the trigger for the dissociation of the activated receptor complex, and therefore a key regulator of TGF-β signaling. Taken together, we show that TGFBR2 phosphorylates BUB1 at Ser318 and propose a model with the following steps: (i) BUB1 is recruited to TGFBR1-TGFBR2 complex in response to ligand, (ii) BUB1 participates in the recruitment of SMAD2/3 to the receptor, (iii) TGFBR2 phosphorylates BUB1 at S318, which triggers the disassembly of the activated complex.

Citation Format: Shyam Nyati, Brandon Gregg, Jiaqi Xu, Grant Young, Lauren Kimmel, Nyati Mukesh, Dipankar Ray, Hongtao Yu, Alnawaz Rehemtulla. TGFBR2 mediated phosphorylation of BUB1 at Ser-318 is required for transforming growth factor-β signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3430.

Abstract 3218: Maternal embryonic leucine zipper kinase (MELK) confers radioresistance in triple-negative breast cancers (TNBC) through a nonhomologous end joining (NHEJ)-mediated pathway

Background: Increased rates of locoregional recurrence have been observed in TNBC despite the use of radiation (RT); therefore, approaches that result in radiosensitizaton in TNBC may improve clinical outcomes. Despite some controversy as to the role of MELK kinase as an oncogene, we previously established its clear role in conferring radioresistance in TNBC. Here we extend those studies to understand the mechanism of conferred radioresistance.

Methods: Clonogenic survival assays were used to quantify the degree of radiosensitivity after MELK inhibition. Mass spectrometry was utilized to identify proteins associated with MELK. NHEJ assays were performed using linearized pEYFP vector and quantitated by FACS. qRT-PCR was used to determine MELK RNA expression. Rad51 and γH2AX foci counting were performed to evaluate homologous recombination (HR). Neutral comet assay was used to quantitate dsDNA break repair. Laser microirradiation studies were performed to determine the location and kinetics of protein shuttle to and from sites of dsDNA breaks, including MELK, Ku70 and Ku80 proteins. Fluorescent microscopy was performed for localization of endogenous MELK protein post RT. To study the effect of MELK kinase activity on Ku70-Ku80 complex formation and recruitment at the DNA damage site, cells were lysed and immunoprecipitation was performed using MELK, Ku70 or Ku80-specific antibodies.

Results: We previously demonstrated that genomic or pharmacologic inhibition of MELK confers significant radiosensitization. Mechanistic studies were undertaken to understand the pathways critical to MELK-mediated radioresistance. When MELK is genomically or pharmacologically inhibited, NHEJ and HR reporter assays demonstrate that this radiosensitization is driven through impaired NHEJ at 4, 16, and 24 hrs after RT and not mediated by HR in multiple TNBC cell lines. Tandem mass spectrometry studies of tagged MELK protein identify Ku70 and Ku80 as direct interactors with MELK protein, which is confirmed by Co-IP. Laser microirradiation studies confirm that MELK, Ku70 and Ku80 co-localize to sites of dsDNA breaks, and that MELK kinase function is required to stabilize the Ku70/Ku80 complex at these sites. Comet assay confirms that rapid dissolution of the Ku70/Ku80 complex when MELK is inhibited or functionally dead is not a result of accelerated dsDNA break repair.

Conclusion: MELK kinase function is critical in conferring radioresistance in TNBC and inhibition of function confers radiosensitivity through an NHEJ-mediated pathway. MELK also stabilizes Ku70/Ku80 proteins at sites of dsDNA breaks and allows for more efficient repair of breaks induced by ionizing RT. These results further support the rationale for developing clinical strategies to inhibit MELK in combination with RT treatment as a novel radiosensitizing strategy in TNBC.

Citation Format: Shyam Nyati, Ben Chandler, Eric Olsen, Leah Moubadder, Meilan Liu, Meleah Cameron, Kari Wilder-Romans, Theodore S. Lawrence, Powel H. Brown, Fellix Y. Fang, Lori J. Pierce, Corey Speers. Maternal embryonic leucine zipper kinase (MELK) confers radioresistance in triple-negative breast cancers (TNBC) through a nonhomologous end joining (NHEJ)-mediated pathway [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 3218.

Abstract 677: TTK: A novel target for radiosensitization in triple-negative breast cancers

Background: Increased rates of locoregional recurrence leading to poorer clinical outcomes have been observed in triple-negative breast cancer (TNBC) despite the use of radiation therapy (RT), therefore approaches that result in radiosensitizaton in TNBC are critically needed. Our previous work identified a group of cell cycle kinases differentially expressed in estrogen receptor negative (ER-) versus estrogen receptor positive (ER+) breast cancer (BC). Here we described the impact of elevated expression of one of these identified kinases, TTK, on radiation response and patient outcomes in TNBC.

Methods: TCGA breast cancer datasets were used to determine TTK expression in the intrinsic subtypes of BC. TTK RNA and protein levels were measured using qPCR and western blot at baseline and after radiation treatment. Clonogenic survival assays were used to determine the radiosensitization in several cells after TTK inhibition. DNA damage was quantified using γH2AX staining in TNBC cell lines. Kaplan-Meier analysis was used to determine the impact of TTK expression on locoregional recurrence (LRR) and overall survival (OS). A Cox proportional hazards model was constructed to identify potential factors of LRR-free survival in univariate (UVA) and multivariable analyses (MVA).

Results: TTK expression is elevated in breast cancer tissue compared to normal breast tissue (Q-VAL: 8.70 E-291) and is most highly expressed in basal-like tumors. TTK is overexpressed in ER- versus ER+ tumors in both breast cancers patient samples (p-value: &lt;0.0001), and cells lines (p-value: &lt;0.0001). TTK expression is significantly correlated with intrinsic radioresistance in a panel of 23 BC cell lines (R=0.58, p-value=0.0035). TTK RNA and protein levels are significantly increased at 12 and 24 hours after RT in TNBC cell lines. Genomic (siRNA knockdown) or pharmacologic (NMS-P715) inhibition of TTK increased radiosensitivity in vitro in 3 different TNBC cell lines. This sensitization is mediated, at least in part, through impaired DNA damage repair. Increased γH2AX foci was found after combination treatment of RT and TTK inhibition compared to only TTK inhibition, or RT. Clinically, breast cancer patients treated with breast conserving surgery and RT whose tumors have higher than median expression of TTK had worse local-recurrence free (LRF) survival and overall survival (HR for local recurrence 1.7 as continuous variable, p-value 0.004) compared to patients with higher than median expression, and this pattern held when considering quartile expression. TTK expression was associated with poorer LRF survival in UVA and in MVA only TTK expression and grade were significantly associated with worse LRF survival in 3 independent datasets.

Conclusion: Our results support the rationale for developing translational clinical trials to investigate TTK inhibition as a novel radiosensitizing target in TNBC.

Citation Format: Benjamin C. Chandler, Leah Moubadder, Cassie Ritter, Yashar Niknafs, Eric Olsen, Meleah Cameron, Meilan Liu, Kari Wilder-Romans, Shyam Nyati, Powel Brown, Arul Chinnaiyan, Corey Speers. TTK: A novel target for radiosensitization in triple-negative breast cancers [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 677.

Quantitative and Dynamic Imaging of ATM Kinase Activity by Bioluminescence Imaging

Ataxia telangiectasia mutated (ATM) is a serine/threonine kinase critical to the cellular DNA damage response, including DNA double strand breaks (DSBs). ATM activation results in the initiation of a complex cascade of events facilitating DNA damage repair, cell cycle checkpoint control, and survival. Traditionally, protein kinases have been analyzed in vitro using biochemical methods (kinase assays using purified proteins or immunological assays) requiring a large number of cells and cell lysis. Genetically encoded biosensors based on optical molecular imaging such as fluorescence or bioluminescence have been developed to enable interrogation of kinase activities in live cells with a high signal to background. We have genetically engineered a hybrid protein whose bioluminescent activity is dependent on the ATM-mediated phosphorylation of a substrate. The engineered protein consists of the split luciferase-based protein complementation pair with a CHK2 (a substrate for ATM kinase activity) target sequence and a phospho-serine/threonine-binding domain, FHA2, derived from yeast Rad53. Phosphorylation of the serine residue within the target sequence by ATM would lead to its interaction with the phospho-serine-binding domain, thereby preventing complementation of the split luciferase pair and loss of reporter activity. Bioluminescence imaging of reporter-expressing cells in cultured plates or as mouse xenografts provides a quantitative surrogate for ATM kinase activity and therefore the cellular DNA damage response in a noninvasive, dynamic fashion.

A novel reporter for real-time, quantitative imaging of AKT-directed K63-poly-ubiquitination in living cells

Post-translational K63-linked poly-ubiquitination of AKT is required for its membrane recruitment and phosphorylation dependent activation in response to growth-factor stimulation. Current assays for target specific poly-ubiquitination involve cumbersome enzymatic preparations and semi-quantitative readouts. We have engineered a reporter that can quantitatively and in a target specific manner report on AKT-directed K63-polyubiquitination (K63UbR) in live cells. The reporter constitutes the AKT-derived poly-ubiquitination substrate peptide, a K63 poly-ubiquitin binding domain (UBD) as well as the split luciferase protein complementation domains. In cells, wherein signaling events upstream of AKT are activated (e.g. either EGFR or IGFR), poly-ubiquitination of the reporter leads to a stearic constraint that prevents luciferase complementation. However, upon inhibition of growth factor receptor signaling, loss of AKT poly-ubiquitination results in a decrease in interaction between the target peptide and the UBD, allowing for reconstitution of the split luciferase domains and therefore increased bioluminescence in a quantitative and dynamic manner. The K63UbR was confirmed to be suitable for high throughput screen (HTS), thus providing an excellent tool for small molecule or siRNA based HTS to discover new inhibitors or identify novel regulators of this key signaling node. Furthermore, the K63UbR platform could be adapted for non-invasive monitoring of additional target specific K63-polyubiquitination events in live cells.

Abstract P1-09-05: Radiosensitization of androgen receptor (AR)-positive triple-negative breast cancer (TNBC) cells using seviteronel (INO-464), a selective CYP17 lyase and AR inhibitor

Background: Increased rates of locoregional recurrence (LR) have been observed in TNBC despite chemotherapy and radiation (RT). A novel radiosensitizer screen nominated the AR as a promising target for treatment of radioresistant breast cancer, including TNBC. We assessed the activity of seviteronel (Sevi), a selective CYP17 lyase and AR inhibitor in Phase 2 clinical development for advanced breast and prostate cancer, as a potential radiosensitizer in AR+ TNBC model.

Methods: Clonogenic survival assays were used to determine the intrinsic RT sensitivity of 21 breast cancer cell (BCC) lines. IC50 values were determined for 130 clinically available compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Gene expression was measured using RNA Seq or qRT-PCR and protein expression was measured using RPPA arrays. AR function was assessed using functional inhibition with Sevi in MDA-MB-453, ACC-422, ACC-460, SUM-185 (all four AR+ TNBC), MDA-MB-231 (AR- TNBC), and T47D (AR- ER+) BCC lines. Double-stranded DNA (dsDNA) break repair was assessed with γH2AX foci counting. In vivo tumor growth was measured with varying control and treatment groups (16-20 tumors/group). Kaplan-Meier analysis was performed to estimate local control. A Cox proportional hazards model and multi-variate analysis (MVA) were used to determine variables associated with LRF survival.

Results: Our novel radiosensitizer screen identified the activity of anti-androgen therapy as a potentially effective strategy for radiosensitization in RT-resistant BCC lines (R2 =0.46, p-value &lt; 0.01) (Speers et al, J Clin Oncol 35, 2017 (suppl; abstr e12102). Heterogeneity in AR expression was identified in human BCC lines and TNBC samples from patients (N=2098). There was a strong correlation between AR RNA expression and protein expression across all BC intrinsic subtypes. AR inhibition using Sevi induced radiation sensitivity in vitro with an enhancement ratio (ER) of 1.24-1.69 in four different AR+ TNBC lines. No such radiosensitization was seen in AR(-) TNBC or ER+, AR(-) BCC lines. Radiosensitization was at least partially dependent on impaired dsDNA break repair with significant delays in dsDNA break repair at 16 and 24 hours in all AR+ TNBC lines examined (p-value &lt; 0.01). AR inhibition with Sevi significantly radiosensitized AR+ TNBC xenografts in mouse models and markedly delayed tumor-volume tripling time (TTT) and tumor growth (MDA-MB-453: median TTT 16.1 days for RT alone vs. not reached after 45 days for Sevi+RT, p-value &lt;0.001). Similar delays were seen in tumor growth, weight, and tumor doubling. Clinically, TNBC patients whose tumors had higher than median expression of AR had higher rates of LR after RT (HR for LR ˜3, p-value &lt;0.01, 2 independent datasets). In MVA, high AR expression was the variable most significantly associated with worse LR survival after RT in TNBC patients, outperforming all other variables (HR of 3.42; p-value &lt; 0.01).

Conclusions: Our results implicate the AR as a mediator of radioresistance in breast cancer and support the rationale for developing Sevi as a novel radiosensitizing agent in AR+ TNBC.

Citation Format: Speers CW, Chandler B, Olsen E, Wilder-Romans K, Moubadder L, Nyati S, Rae J, Hayes DF, Spratt DE, Wahl DR, Eisner J, Feng FY, Pierce LJ. Radiosensitization of androgen receptor (AR)-positive triple-negative breast cancer (TNBC) cells using seviteronel (INO-464), a selective CYP17 lyase and AR inhibitor [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-09-05.

Differential protein stability of EGFR mutants determines responsiveness to tyrosine kinase inhibitors

Non-small cell lung cancer (NSCLC) patients carrying specific EGFR kinase activating mutations (L858R, delE746-A750) respond well to tyrosine kinase inhibitors (TKIs). However, drug resistance develops within a year. In about 50% of such patients, acquired drug resistance is attributed to the enrichment of a constitutively active point mutation within the EGFR kinase domain (T790M). To date, differential drug-binding and altered ATP affinities by EGFR mutants have been shown to be responsible for differential TKI response. As it has been reported that EGFR stability plays a role in the survival of EGFR driven cancers, we hypothesized that differential TKI-induced receptor degradation between the sensitive L858R and delE746-A750 and the resistant T790M may also play a role in drug responsiveness. To explore this, we have utilized an EGFR-null CHO overexpression system as well as NSCLC cell lines expressing various EGFR mutants and determined the effects of erlotinib treatment. We found that erlotinib inhibits EGFR phosphorylation in both TKI sensitive and resistant cells, but the protein half-lives of L858R and delE746-A750 were significantly shorter than L858R/T790M. Third generation EGFR kinase inhibitor (AZD9291) inhibits the growth of L858R/T790M-EGFR driven cells and also induces EGFR degradation. Erlotinib treatment induced polyubiquitination and proteasomal degradation, primarily in a c-CBL-independent manner, in TKI sensitive L858R and delE746-A750 mutants when compared to the L858R/T790M mutant, which correlated with drug sensitivity. These data suggest an additional mechanism of TKI resistance, and we postulate that agents that degrade L858R/T790M-EGFR protein may overcome TKI resistance.

Androgen receptor as a mediator and biomarker of radioresistance in triple-negative breast cancer

Increased rates of locoregional recurrence have been observed in triple-negative breast cancer despite chemotherapy and radiation therapy. Thus, approaches that combine therapies for radiosensitization in triple-negative breast cancer are critically needed. We characterized the radiation therapy response of 21 breast cancer cell lines and paired this radiation response data with high-throughput drug screen data to identify androgen receptor as a top target for radiosensitization. Our radiosensitizer screen nominated bicalutamide as the drug most effective in treating radiation therapy-resistant breast cancer cell lines. We subsequently evaluated the expression of androgen receptor in >2100 human breast tumor samples and 51 breast cancer cell lines and found significant heterogeneity in androgen receptor expression with enrichment at the protein and RNA level in triple-negative breast cancer. There was a strong correlation between androgen receptor RNA and protein expression across all breast cancer subtypes (R² = 0.72, p < 0.01). In patients with triple-negative breast cancer, expression of androgen receptor above the median was associated with increased risk of locoregional recurrence after radiation therapy (hazard ratio for locoregional recurrence 2.9-3.2)) in two independent data sets, but there was no difference in locoregional recurrence in triple-negative breast cancer patients not treated with radiation therapy when stratified by androgen receptor expression. In multivariable analysis, androgen receptor expression was most significantly associated with worse local recurrence-free survival after radiation therapy (hazard ratio of 3.58) suggesting that androgen receptor expression may be a biomarker of radiation response in triple-negative breast cancer. Inhibition of androgen receptor with MDV3100 (enzalutamide) induced radiation sensitivity (enhancement ratios of 1.22-1.60) in androgen receptor-positive triple-negative breast cancer lines, but did not affect androgen receptor-negative triple-negative breast cancer or estrogen-receptor-positive, androgen receptor-negative breast cancer cell lines. androgen receptor inhibition with MDV3100 significantly radiosensitized triple-negative breast cancer xenografts in mouse models and markedly delayed tumor doubling/tripling time and tumor weight. Radiosensitization was at least partially dependent on impaired dsDNA break repair mediated by DNA protein kinase catalytic subunit. Our results implicate androgen receptor as a mediator of radioresistance in breast cancer and identify androgen receptor inhibition as a potentially effective strategy for the treatment of androgen receptor-positive radioresistant tumors.

Abstract 5839: Androgen receptor (AR): A novel target for radiosensitization in triple-negative breast cancers (TNBC)

Purpose: Increased rates of local recurrence (LR) have been observed in TNBC despite chemotherapy and radiation (RT). Thus, approaches that result in radiosensitization in TNBC are critically needed. We characterized the RT response of 21 breast cancer cell (BCC) lines using clonogenic survival assays and paired this with high-throughput drug screen data, identifying AR as a top target for radiosensitization. We demonstrate that AR inhibition confers radiosensitization in vitro and in vivo, identified a biomarker of response, and characterize the mechanism of AR-mediated radiosensitization in TNBC.

Materials/Methods: Clonogenic survival assays determined the intrinsic RT sensitivity of 21 BCC lines. IC50 values were determined for 130 clinical compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Gene and protein expression was measured using RNA Seq and RPPA arrays, respectively, in tumor samples (n=2,061) and BCC lines (n=51). AR function was assessed using gene knockdown (KD) or functional inhibition with anti-androgen drugs. We measured in vivo tumor growth with varying control and treatment groups (16-20 tumors/group). Kaplan-Meier analysis was performed to estimate local control. A Cox proportional hazards model and MVA were used to determine variables associated with LRF survival.

Results: Our unbiased drug radiosensitizer screen nominated bicalutamide as an effective drug in treating RT-resistant BCC lines (R2= 0.46, p-value &lt;0.01). We interrogated the expression of AR in &gt;2000 human breast tumor samples and 51 BCC lines and found heterogeneity in AR expression with strongly correlated expression of protein and RNA levels in TNBC (R2=0.89, p-value &lt;0.001). Inhibition of AR, using both KD and drug (MDV3100) induced RT sensitivity with an enhancement ratio (ER) of 1.35-1.42 in AR+ TNBC lines with no effect on controls. Radiosensitization was at least partially dependent on impaired dsDNA break repair mediated by DNAPKcs. AR inhibition with MDV3100 significantly radiosensitized TNBC xenografts in mouse models and markedly delayed tumor tripling time (TTT) and tumor growth (median TTT 17.4 days for RT alone vs. not reached after 50 days for MDV3100+RT, p-value &lt;0.001). Activated DNAPK was identified as a biomarker of response. Clinically, TNBC patients whose tumors had higher than median expression of AR had higher rates of LR after RT (HR for LR ~3, p-value &lt;0.01, 2 independent datasets). In MVA, high AR expression was the variable most significantly associated with worse LRF survival after RT in TNBC patients, outperforming all other variables (HR of 3.58; p-value &lt; 0.01).

Conclusion: Our results implicate AR as a mediator of radioresistance in breast cancer and support the rationale for developing clinical strategies, including clinical trials, to inhibit AR as a novel radiosensitizing target in TNBC.

Citation Format: Benjamin C. Chandler, Corey W. Speers, Shuang G. Zhao, Meilan Liu, Kari Wilder-Romans, Eric Olsen, Shyam Nyati, Daniel Spratt, Daniel Wahl, Daniel Hayes, Felix Y. Feng, Lori J. Pierce. Androgen receptor (AR): A novel target for radiosensitization in triple-negative breast cancers (TNBC) [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 5839. doi:10.1158/1538-7445.AM2017-5839

Radiosensitization of androgen receptor (AR)-positive triple-negative breast cancer (TNBC) cells using seviteronel (SEVI), a selective CYP17 lyase and AR inhibitor

e12102

Background: Increased rates of locoregional recurrence have been observed in TNBC despite chemotherapy and radiation (RT). A novel radiosensitizer screen nominated the AR as a promising target for treatment of radioresistant breast cancer, including TNBC. We assessed in vitro activity of SEVI (VT-464), a selective CYP17 lyase and AR inhibitor, as a potential radiosensitizer in AR+ TNBC model. Methods: Clonogenic survival assays were used to determine the intrinsic RT sensitivity of 21 breast cancer cell (BCC) lines. IC50 values were determined for 130 clinically available compounds and correlation coefficients were calculated using IC50 values and SF-2Gy. Gene expression was measured using RNA Seq or qRT-PCR and protein expression was measured using RPPA arrays. AR function was assessed using functional inhibition with SEVI in MDA-MB-453, ACC-460, SUM-185 (all three AR+ TNBC), MDA-MB-231 (AR- TNBC), and T47D (AR- ER+) BCC lines. Double-stranded DNA (dsDNA) break repair was assessed with γH2AX foci counting. Results: Our novel radiosensitizer screen identified the activity of bicalutamide, an AR antagonist, in RT-resistant BCC lines (R2 = 0.46, p-value < 0.01). Heterogeneity in AR expression was identified in human BCC lines. There was a strong correlation between AR RNA expression and protein expression across all BC intrinsic subtypes. AR inhibition using SEVI induced radiation sensitivity in vitro with an enhancement ratio (ER) of 1.24-1.62 in three different AR+ TNBC lines. No such radiosensitization was seen in AR(-) TNBC or ER+, AR(-) BCC lines. Radiosensitization was at least partially dependent on impaired dsDNA break repair with significant delays in dsDNA break repair at 16 and 24 hours in all AR+ TNBC lines examined (p-value < 0.01). Conclusions: Our results implicate the AR as a mediator of radioresistance in breast cancer and support the rationale for developing seviteronel as a novel radiosensitizing agent in AR+ TNBC.

A Requirement for ZAK Kinase Activity in Canonical TGF-β Signaling

The sterile alpha motif and leucine zipper containing kinase ZAK (AZK, MLT, MLK7), is a MAPK-kinase kinase (MKKK). Like most MAPKKKs which are known to activate the c-Jun. amino-terminal kinase (JNK) pathway, ZAK has been shown to participate in the transduction of Transforming growth factor-β (TGF-β)-mediated non-canonical signaling. A role for ZAK in SMAD-dependent, canonical TGF-β signaling has not been previously appreciated. Using a combination of functional genomics and biochemical techniques, we demonstrate that ZAK regulates canonical TGFβRI/II signaling in lung and breast cancer cell lines and may serve as a key node in the regulation of TGFBR kinase activity. Remarkably, we demonstrate that siRNA mediated depletion of ZAK strongly inhibited TGF-β dependent SMAD2/3 activation and subsequent promoter activation (SMAD binding element driven luciferase expression; SBE4-Luc). A ZAK specific inhibitor (DHP-2), dose-dependently activated the bioluminescent TGFBR-kinase activity reporter (BTR), blocked TGF-β induced SMAD2/3 phosphorylation and SBE4-Luc activation and cancer cell-invasion. In aggregate, these findings identify a novel role for the ZAK kinase in canonical TGF-β signaling and an invasive cancer cell phenotype thus providing a novel target for TGF-β inhibition.

The kinase activity of the Ser/Thr kinase BUB1 promotes TGF-β signaling

Transforming growth factor-β (TGF-β) signaling regulates cell proliferation and differentiation, which contributes to development and disease. Upon binding TGF-β, the type I receptor (TGFBRI) binds TGFBRII, leading to the activation of the transcription factors SMAD2 and SMAD3. Using an RNA interference screen of the human kinome and a live-cell reporter for TGFBR activity, we identified the kinase BUB1 (budding uninhibited by benzimidazoles-1) as a key mediator of TGF-β signaling. BUB1 interacted with TGFBRI in the presence of TGF-β and promoted the heterodimerization of TGFBRI and TGFBRII. Additionally, BUB1 interacted with TGFBRII, suggesting the formation of a ternary complex. Knocking down BUB1 prevented the recruitment of SMAD3 to the receptor complex, the phosphorylation of SMAD2 and SMAD3 and their interaction with SMAD4, SMAD-dependent transcription, and TGF-β-mediated changes in cellular phenotype including epithelial-mesenchymal transition (EMT), migration, and invasion. Knockdown of BUB1 also impaired noncanonical TGF-β signaling mediated by the kinases AKT and p38 MAPK (mitogen-activated protein kinase). The ability of BUB1 to promote TGF-β signaling depended on the kinase activity of BUB1. A small-molecule inhibitor of the kinase activity of BUB1 (2OH-BNPP1) and a kinase-deficient mutant of BUB1 suppressed TGF-β signaling and formation of the ternary complex in various normal and cancer cell lines. 2OH-BNPP1 administration to mice bearing lung carcinoma xenografts reduced the amount of phosphorylated SMAD2 in tumor tissue. These findings indicated that BUB1 functions as a kinase in the TGF-β pathway in a role beyond its established function in cell cycle regulation and chromosome cohesion.

Abstract 1137: Bub1 is a key regulator of TGF-β signaling

The transforming growth factor-beta (TGF-β) family of cytokines regulates many processes such as immune suppression, angiogenesis, wound healing and epithelial to mesenchymal transition (EMT. Early in tumorigenesis, when epithelial cells retain exquisite growth sensitivity to this ligand, TGF-β signaling elicits a tumor suppressing activity. However, transformed cells become refractory to TGF-β-mediated growth inhibition and acquire a phenotype wherein the intracellular signaling circuitry of the cells is altered, leading to tumorigenic and metastatic effects in response to TGF-β exposure. Although TGF-β activating pathways have been studied, the molecular participants are poorly defined. Here we identify budding uninhibited by benzimidazoles-1 (Bub1) as an integral component of canonical and non-canonical TGF-β signaling pathways, where Bub1 is required for TGFBRI-TGFBRII complex formation and activation. Bub1-depleted cells exhibited reductions in TGF-β dependent Smad2/3 phosphorylation, recruitment of Smad2/3 to the TGFBRI-II complex, PI3K/Akt and p38MAPK activation, Smad binding element driven promoter activity (SBE4-Luc), and invasion and migration. Furthermore, a targeted small molecule inhibitor of Bub1 kinase activity (2OH-BNPP1), as well as an inactive kinase mutant of Bub1, abrogated ligand mediated TGF-β signaling and phenotypic response. These studies demonstrate a role for the Bub1 kinase in mediating TGF-β dependent signaling beyond its established function in cell-cycle regulation and chromosome cohesion and uncover the underlying basis for the pleiotropic cellular response commonly observed upon activation of the pathway.

Citation Format: Shyam Nyati, Katrina Schinske-Sebolt, Sethuramasundaram Pitchiaya, Katerina Chekhovskiy, Areeb Chator, Nauman Chaudhry, Joseph Dosch, Marcian E. Van Dort, Varambally, Kumar-Sinha, Nyati, Ray, Walter, Sooryanarayana Varambally, Chandan Kumar-Sinha, Mukesh K. Nyati, Dipankar Ray, Nils G. Walter, Hongtao Yu, Brian D. Ross, Alnawaz Rehemtulla. Bub1 is a key regulator of TGF-β signaling. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1137. doi:10.1158/1538-7445.AM2014-1137

Phylogenetic analysis of LSU and SSU rDNA group I introns of lichen photobionts associated with the genera Xanthoria and Xanthomendoza (Teloschistaceae, lichenized Ascomycetes)

We studied group I introns in sterile cultures of selected groups of lichen photobionts, focusing on Trebouxia species associated with Xanthoria s. lat. (including Xanthomendoza spp.; lichen-forming ascomycetes). Group I introns were found inserted after position 798 (Escherichia coli numbering) in the large subunit (LSU) rRNA in representatives of the green algal genera Trebouxia and Asterochloris. The 798 intron was found in about 25% of Xanthoria photobionts including several reference strains obtained from algal culture collections. An alignment of LSU-encoded rDNA intron sequences revealed high similarity of these sequences allowing their phylogenetic analysis. The 798 group I intron phylogeny was largely congruent with a phylogeny of the Internal Transcribed Spacer Region (ITS), indicating that the insertion of the intron most likely occurred in the common ancestor of the genera Trebouxia and Asterochloris. The intron was vertically inherited in some taxa, but lost in others. The high sequence similarity of this intron to one found in Chlorella angustoellipsoidea suggests that the 798 intron was either present in the common ancestor of Trebouxiophyceae, or that its present distribution results from more recent horizontal transfers, followed by vertical inheritance and loss. Analysis of another group I intron shared by these photobionts at small subunit (SSU) position 1512 supports the hypothesis of repeated lateral transfers of this intron among some taxa, but loss among others. Our data confirm that the history of group I introns is characterized by repeated horizontal transfers, and suggests that some of these introns have ancient origins within Chlorophyta.

NanoLuc reporter for dual luciferase imaging in living animals

Bioluminescence imaging is widely used for cell-based assays and animal imaging studies in biomedical research and drug development, capitalizing on the high signal to background of this technique. A relatively small number of luciferases are available for imaging studies, substantially limiting the ability to image multiple molecular and cellular events, as done commonly with fluorescence imaging. To advance dual reporter bioluminescence molecular imaging, we tested a recently developed, adenosine triphosphate–independent luciferase enzyme from Oplophorus gracilirostris (NanoLuc [NL]) as a reporter for animal imaging. We demonstrated that NL could be imaged in superficial and deep tissues in living mice, although the detection of NL in deep tissues was limited by emission of predominantly blue light by this enzyme. Changes in bioluminescence from NL over time could be used to quantify tumor growth, and secreted NL was detectable in small volumes of serum. We combined NL and firefly luciferase reporters to quantify two key steps in transforming growth factor β signaling in intact cells and living mice, establishing a novel dual luciferase imaging strategy for quantifying signal transduction and drug targeting. Our results establish NL as a new reporter for bioluminescence imaging studies in intact cells and living mice that will expand imaging of signal transduction in normal physiology, disease, and drug development.

Molecular imaging of TGFβ-induced Smad2/3 phosphorylation reveals a role for receptor tyrosine kinases in modulating TGFβ signaling

PURPOSE

The dual modality of TGFβ, both as a potent tumor suppressor and a stimulator of tumor progression, invasion, and metastasis, make it a critical target for therapeutic intervention in human cancers. The ability to carry out real-time, noninvasive imaging of TGFβ-activated Smad signaling in live cells and animal models would significantly improve our understanding of the regulation of this unique signaling cascade. To advance these efforts, we developed a highly sensitive molecular imaging tool that repetitively, noninvasively, and dynamically reports on TGFBR1 kinase activity.

EXPERIMENTAL DESIGN

The bioluminescent TGFβR1 reporter construct was developed using a split firefly luciferase gene containing a functional sensor of Smad2 phosphorylation, wherein inhibition of TGFβ receptor1 kinase activity leads to an increase in reporter signaling. The reporter was stably transfected into mammalian cells and used to image in vivo and in vitro bioluminescent activity as a surrogate for monitoring TGFBR1 kinase activity.

RESULTS

The reporter was successfully used to monitor direct and indirect inhibition of TGFβ-induced Smad2 and SMAD3 phosphorylation in live cells and tumor xenografts and adapted for high-throughput screening, to identify a role for receptor tyrosine kinase inhibitors as modulators of TGFβ signaling.

CONCLUSION

The reporter is a dynamic, noninvasive imaging modality for monitoring TGFβ-induced Smad2 signaling in live cells and tumor xenografts. It has immense potential for identifying novel effectors of R-Smad phosphorylation, for validating drug-target interaction, and for studying TGFβ signaling in different metastasis models.

A novel kinase inhibitor of FADD phosphorylation chemosensitizes through the inhibition of NF-κB

Fas-associated protein with death domain (FADD) is a cytosolic adapter protein essential for mediating death receptor-induced apoptosis. It has also been implicated in a number of nonapoptotic activities including embryogenesis, cell-cycle progression, cell proliferation, and tumorigenesis. Our recent studies have shown that high levels of phosphorylated FADD (p-FADD) in tumor cells correlate with increased activation of the antiapoptotic transcription factor NF-κB and is a biomarker for aggressive disease and poor clinical outcome. These findings suggest that inhibition of FADD phosphorylation is a viable target for cancer therapy. A high-throughput screen using a cell-based assay for monitoring FADD-kinase activity identified NSC 47147 as a small molecule inhibitor of FADD phosphorylation. The compound was evaluated in live cells and mouse tumors for its efficacy as an inhibitor of FADD-kinase activity through the inhibition of casein kinase 1α. NSC 47147 was shown to decrease levels of p-FADD and NF-κB activity such that combination therapy leads to greater induction of apoptosis and enhanced tumor control than either agent alone. The studies described here show the utility of bioluminescent cell-based assays for the identification of active compounds and the validation of drug-target interaction in a living subject. In addition, the presented results provide proof-of-principle studies as to the validity of targeting FADD-kinase activity as a novel cancer therapy strategy.

Abstract 2311: A novel kinase inhibitor of FADD phosphorylation chemosensitizes through the inhibition of NF-κB

Fas-Associated protein with Death Domain (FADD) was originally reported as a pro-apoptotic adaptor molecule that mediates receptor induced apoptosis. Recent studies have revealed a potential role of FADD in NF-κB activation, embryogenesis, and cell cycle regulation and proliferation. Over-expression of FADD and its phosphorylation have been associated with the transformed phenotype in many cancers and is therefore a potential target for therapeutic intervention. In an effort to identify novel chemical entities that inhibit FADD phosphorylation, we screened an NCI diversity compound collection using a cell based assay recently developed in our laboratory for the detection of FADD-kinase activity (Khan, 2010). The screen resulted in the identification of NSC 47147 as a potent inhibitor of FADD phosphorylation. This compound was shown to be an inhibitor of CKIα, a previously described FADD kinase, with an IC50 = 1.5 µM. We demonstrate that inhibition of FADD phosphorylation by NSC 47147 results in a decrease in NF-κB activity, thereby potentially sensitizing tumor cells to chemotherapy. In support of this, treatment of lung cancer cells with NSC 47147 and cisplatin in culture as well as in a mouse xenograft model resulted in the synergistic induction of apoptosis and enhanced tumor control compared to either agent alone. These results provide evidence for the inhibition of FADD phosphorylation as a viable target for anti-cancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2311. doi:10.1158/1538-7445.AM2011-2311

High-throughput molecular imaging for the identification of FADD kinase inhibitors

Fas-associated protein with death domain (FADD) was originally reported as a proapoptotic adaptor molecule that mediates receptor-induced apoptosis. Recent studies have revealed a potential role of FADD in NF-κB activation, embryogenesis, and cell cycle regulation and proliferation. Overexpression of FADD and its phosphorylation have been associated with the transformed phenotype in many cancers and is therefore a potential target for therapeutic intervention. In an effort to delineate signaling events that lead to FADD phosphorylation and to identify novel compounds that impinge on this pathway, the authors developed a cell-based reporter for FADD kinase activity. The reporter assay, optimized for a high-throughput screen (HTS), measures bioluminescence in response to modulation of FADD kinase activity in live cells. In addition, the potential use of the reporter cell line in the rapid evaluation of pharmacologic properties of HTS hits in mouse models has been demonstrated.

Novel molecular imaging platform for monitoring oncological kinases

Recent advances in oncology have lead to identification of a plethora of alterations in signaling pathways that are critical to oncogenesis and propagation of malignancy. Among the biomarkers identified, dysregulated kinases and associated changes in signaling cascade received the lion's share of scientific attention and have been under extensive investigations with goal of targeting them for anti-cancer therapy. Discovery of new drugs is immensely facilitated by molecular imaging technology which enables non-invasive, real time, dynamic imaging and quantification of kinase activity. Here, we review recent development of novel kinase reporters based on conformation dependent complementation of firefly luciferase to monitor kinase activity. Such reporter system provides unique insights into the pharmacokinetics and pharmacodynamics of drugs that modulate kinase signaling and have a huge potential in drug discovery, validation, and drug-target interactions.

Molecular imaging of glycogen synthase kinase-3beta and casein kinase-1alpha kinases

Glycogen synthase kinase-3beta (GSK3beta) and casein kinase-1alpha (CK1alpha) are multifunctional kinases that play critical roles in the regulation of a number of cellular processes. In spite of their importance, molecular imaging tools for noninvasive and real-time monitoring of their kinase activities have not been devised. Here we report development of the bioluminescent GSK3beta and CK1alpha reporter (BGCR) based on firefly luciferase complementation. Treatment of SW620 cells stably expressing the reporter with inhibitors of GSK3beta (SB415286 and LiCl) or CK1alpha (CKI-7) resulted in dose- and time-dependent increases in BGCR activity that were validated using Western blotting. No increase in bioluminescence was observed in the case of S37A mutant (GSK3beta inhibitors) or S45A mutant (CKI-7), demonstrating the specificity of the reporter. Imaging of mice tumor xenograft generated with BGCR-expressing SW620 cells following treatment with LiCl showed unique oscillations in GSK3beta activity that were corroborated by phosphorylated GSK3beta immunoblotting. Taken together, the BGCR is a novel molecular imaging tool that reveals unique insight into GSK3beta and CK1alpha kinase activities and may provide a powerful tool in experimental therapeutics for rapid optimization of dose and schedule of targeted therapies and for monitoring therapeutic response.

Abstract 1320: Molecular imaging of EMT reveals crosstalk of multiple pathways and provides novel sites for therapeutic intervention of EMT

Ionizing radiation therapy have been also suggested to result in increased incidence of local or distant metastasis which is, at least in part, mediated by TGFß signaling. TGFß is a known potentiator of epithelial to mesenchymal transition (EMT), a complex process by which a tumor cell bolsters its metastatic potential by transmogrifying into a highly motile and invasive cell that is ready to take on the challenge of distant travel and resettlement in a new milieu. Therefore it is important, in order to increase efficacy of ionizing radiation, a combination therapy along with inhibitors of EMT pathway, be utilized in the treatment of cancers. To date there is no clinically relevant EMT inhibitor and therefore to assist in the process of identifying a small molecular weight inhibitor, we developed three distinct non-invasive and quantitative molecular imaging reporters for monitoring EMT. These were based on changes in expression profile of E-cadherin and N-cadherin and complementation of luciferase based TGFβR kinase (BTR) activity. In the lung cancer EMT model studies using TGFβ treated A549 cells we demonstrated a time and dose dependent increase in activity of luciferase reporter driven by N-cadherin promoter and a decrease in the activity of a luciferase reporter driven by E-cadherin promoter. These changes were validated by phenotypic alteration of A549 cells and by western blotting for signaling intermediates of EMT pathway. BTR (bioluminescent TGFβR reporter), was a gain of function sensor of SMAD2/3 phosphorylation wherein inhibition of TGFβR signaling leads to increased reporter activity. Such reporters are extremely useful in high throughput screening (HTS) as cytotoxic agents are eliminated early in the screening process. Treatment of A549 cells stably expressing BTR with SB431542 (TGFβR specific inhibitor) lead to a more than 20 fold increase in BTR activity. All three reporters were adapted for HTS assay and were used to screen a library of known kinase inhibitors. Analyses and validation of hits suggested that the loss of epithelial or gain of mesenchymal phenotypes are independent processes. Further it also highlighted complex cross talks between EMT and different pathways involving mTor, Src, p38 MAPK, Akt, JNK, PI3-K, PKA, PKC, GSK-3β, PDGFRK and EGFR. Efficacy of several lead compounds (Erlotinib, SP600125, SB203580, and SB431542) in combination with radiotherapy is being accessed to increase radiation sensitivity in epithelial cancers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1320.

Abstract 5222: Molecular imaging of GSK-3β and CK1α kinase activities in response to Wnt signaling

The Wnt signaling pathway has been found to be disregulated in many carcinomas of gastrointestinal system. GSK-3β and CK1α act as molecular switch that dictates both on and off states of Wnt/beta-catenin signaling by regulating cellular levels of the Wnt effector molecule β-catenin. In the absence of the Wnt signal β-catenin is initially phosphorylated by CK1α at S45, followed by GSK-3β at S41, T37 and S33 which marks it for proteosomal degradation. Although GSK-3β is one of the key regulators of the Wnt signaling pathway, the ability for direct, non-invasive & real-time measurement of its kinase activity has been unavailable. Here we describe a chimeric, gain of function, split luciferase based novel reporter for monitoring of dual kinase (GSK-3β and CK1α) activity.

The N-terminal β-catenin peptide (aa 29-48) harboring phosphorylation sites for GSK-3β and CK1α was synthesized and used to construct bioluminescence β-catenin reporter (BCR). This reporter when expressed in SW620 colon carcinoma cells and treated with LiCl and small molecular inhibitors SB415286 (GSK-3β) and CKI-7 (CK1α), lead to a dose and time dependent increase in the bioluminescence which was validated by western blotting. Cell lines stably expressing mutant reporters exhibited attenuated response to LiCl & SB415286 (S37A mutant) and to CKI-7 (S45A mutant) confirming specificity of the reporter. To quantify the Wnt dependent change in the activity of the reporters, stable cell lines were generated in HEK293 background which has an intact Wnt signaling pathway. The WT reporter cell line showed over two fold induction with Wnt conditioned media while S37A mutant reporter cell line did not show a significant change in reporter activity corroborating the specificity of the reporters.

In mice bearing tumors expressing the wild-type reporter, treatment with LiCl resulted an initial increase in the bioluminescence activity at 4 h which peaked at 7 h. Subsequent imaging revealed oscillations of the bioluminescence activity with a frequency of 14 h and an amplitude that decreased with time. Our reporters provide a powerful, novel tool for sensitive, real time, dynamic, non-invasive monitoring of GSK-3β and CK1α kinase activities in-vitro and in-vivo and have the potential in executing novel therapeutic regimens targeting GSK-3β/CK1α.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5222.

Role of epidermal growth factor receptor degradation in cisplatin-induced cytotoxicity in head and neck cancer

Cisplatin and its analogues are the most commonly used agents in the treatment of head and neck squamous cell carcinoma. In this study, we investigated a possible role of epidermal growth factor (EGF) receptor (EGFR) phosphorylation and degradation in cisplatin-induced cytotoxicity. Cisplatin treatment led to an increase in initial EGFR phosphorylation at Y1045, the binding site of ubiquitin ligase, Casitas B-lineage lymphoma (c-Cbl), followed by ubiquitination in the relatively cisplatin-sensitive cell lines. However, cisplatin-resistant cell lines underwent minimal EGFR phosphorylation at the Y1045 site and minimal ubiquitination. We found that EGFR degradation in response to cisplatin was highly correlated with cytotoxicity in seven head and neck cancer cell lines. Pretreatment with EGF enhanced cisplatin-induced EGFR degradation and cytotoxicity, whereas erlotinib pretreatment blocked EGFR phosphorylation, degradation, and cisplatin-induced cytotoxicity. Expression of a mutant Y1045F EGFR, which is relatively resistant to c-Cbl-mediated degradation, in Chinese hamster ovary cells and the UMSCC11B human head and neck cancer cell line protected EGFR from cisplatin-induced degradation and enhanced cell survival compared with wild-type (WT) EGFR. Transfection of WT c-Cbl enhanced EGFR degradation and cisplatin-induced cytotoxicity compared with control vector. These results show that cisplatin-induced EGFR phosphorylation and subsequent ubiquitination and degradation is an important determinant of cisplatin sensitivity. Our findings suggest that treatment with an EGFR inhibitor before cisplatin would be antagonistic, as EGFR inhibition would protect EGFR from cisplatin-mediated phosphorylation and subsequent ubiquitination and degradation, which may explain the negative results of several recent clinical trials. Furthermore, they suggest that EGFR degradation is worth exploring as an early biomarker of response and as a target to improve outcome.

Fine structure and phylogeny of green algal photobionts in the microfilamentous genus Psoroglaena (Verrucariaceae, lichen-forming ascomycetes)

According to the literature the microfilamentous thalli of lichen-forming ascomycetes of the genus Psoroglaena are assumed to harbour vivid green "prochlorophyte" cyanobacterial photobionts. As this would be the first report of terrestrial "prochlorophytes" we investigated the fine structure and two molecular markers (SSU rDNA and rbcL) of the photobionts of P. stigonemoides (Orange) Henssen and P. epiphylla Lücking. Both Psoroglaena spp. had unicellular green algal photobionts, representatives of the Trebouxiophyceae. The photobiont of P. stigonemoides is closely related to the non-symbiotic auxenochlorella protothecoides and to a Chlorella endosymbiont of the freshwater polyp Hydra viridis. The putative photobiont of P. epiphylla may be related to Chlorella luteoviridis, C. saccharophila, and a Pseudochlorella isolate. In contrast to other microfilamentous lichens, which derive their shape from filamentous green algae or cyanobacterial colonies overgrown and ensheathed by the fungal partner, Psoroglaena mycobionts position their unicellular photobiont in uni- or multiseriate rows which strongly resemble the situation in filamentous cyanobacterial colonies.