Discovery and Characterization of ZL-2201, a Potent, Highly Selective, and Orally Bioavailable Small-molecule DNA-PK Inhibitor

DNA-dependent protein kinase (DNA-PK), a driver of the non-homologous end-joining (NHEJ) DNA damage response pathway, plays an instrumental role in repairing double-strand breaks (DSB) induced by DNA-damaging poisons. We evaluate ZL-2201, an orally bioavailable, highly potent, and selective pharmacologic inhibitor of DNA-PK activity, for the treatment of human cancerous malignancies. ZL-2201 demonstrated greater selectivity for DNA-PK and effectively inhibited DNA-PK autophosphorylation in a concentration- and time-dependent manner. Initial data suggested a potential correlation between ataxia-telangiectasia mutated (ATM) deficiency and ZL-2201 sensitivity. More so, ZL-2201 showed strong synergy with topoisomerase II inhibitors independent of ATM status in vitro. In vivo oral administration of ZL-2201 demonstrated dose-dependent antitumor activity in the NCI-H1703 xenograft model and significantly enhanced the activity of approved DNA-damaging agents in A549 and FaDu models. From a phosphoproteomic mass spectrometry screen, we identified and validated that ZL-2201 and PRKDC siRNA decreased Ser108 phosphorylation of MCM2, a key DNA replication factor. Collectively, we have characterized a potent and selective DNA-PK inhibitor with promising monotherapy and combinatory therapeutic potential with approved DNA-damaging agents. More importantly, we identified phospho-MCM2 (Ser108) as a potential proximal biomarker of DNA-PK inhibition that warrants further preclinical and clinical evaluation.

Significance

ZL-2201, a potent and selective DNA-PK inhibitor, can target tumor models in combination with DNA DSB-inducing agents such as radiation or doxorubicin, with potential to improve recurrent therapies in the clinic.

Caveolin-1 and Sox-2 are predictive biomarkers of cetuximab response in head and neck cancer

The epidermal growth factor receptor (EGFR) inhibitor cetuximab is the only FDA-approved oncogene-targeting therapy for head and neck squamous cell carcinoma (HNSCC). Despite variable treatment response, no biomarkers exist to stratify patients for cetuximab therapy in HNSCC. Here, we applied unbiased hierarchical clustering to reverse-phase protein array molecular profiles from patient-derived xenograft (PDX) tumors and revealed 2 PDX clusters defined by protein networks associated with EGFR inhibitor resistance. In vivo validation revealed unbiased clustering to classify PDX tumors according to cetuximab response with 88% accuracy. Next, a support vector machine classifier algorithm identified a minimalist biomarker signature consisting of 8 proteins — caveolin-1, Sox-2, AXL, STING, Brd4, claudin-7, connexin-43, and fibronectin — with expression that strongly predicted cetuximab response in PDXs using either protein or mRNA. A combination of caveolin-1 and Sox-2 protein levels was sufficient to maintain high predictive accuracy, which we validated in tumor samples from patients with HNSCC with known clinical response to cetuximab. These results support further investigation into the combined use of caveolin-1 and Sox-2 as predictive biomarkers for cetuximab response in the clinic.

PD-L1 is upregulated via BRD2 in head and neck squamous cell carcinoma models of acquired cetuximab resistance

BACKGROUND

Tumor models resistant to EGFR tyrosine kinase inhibitors or cisplatin express higher levels of the immune checkpoint molecule PD-L1. We sought to determine whether PD-L1 expression is elevated in head and neck squamous cell carcinoma (HNSCC) models of acquired cetuximab resistance and whether the expression is regulated by bromodomain and extraterminal domain (BET) proteins.

METHODS

Expression of PD-L1 was assessed in HNSCC cell line models of acquired cetuximab resistance. Proteolysis targeting chimera (PROTAC)- and RNAi-mediated targeting were used to assess the role of BET proteins.

RESULTS

Cetuximab-resistant HNSCC cells expressed elevated PD-L1 compared to cetuximab-sensitive controls. Treatment with the BET inhibitor JQ1, the BET PROTAC MZ1, or RNAi-mediated knockdown of BRD2 decreased PD-L1 expression. Knockdown of BRD2 also reduced the elevated levels of PD-L1 seen in a model of acquired cisplatin resistance.

CONCLUSIONS

PD-L1 is significantly elevated in HNSCC models of acquired cetuximab and cisplatin resistance where BRD2 is the primary regulator.

The H1047R PIK3CA oncogene induces a senescence-like state, pleiotropy and acute HSP90 dependency in HER2+ mammary epithelial cells

In pre-clinical models, co-existence of Human Epidermal Growth Factor Receptor-2 (HER2)-amplification and PI3K catalytic subunit (PIK3CA) mutations results in aggressive, anti-HER2 therapy-resistant breast tumors. This is not always reflected in clinical setting. We speculated that the complex interaction between the HER2 and PIK3CA oncogenes is responsible for such inconsistency. We performed series of biochemical, molecular and cellular assays on genetically engineered isogenic mammary epithelial cell lines and breast cancer cells expressing both oncogenes. In vitro observations were validated in xenografts models. We showed that H1047R, one of the most common PIK3CA mutations, is responsible for endowing a senescence-like state in mammary epithelial cells overexpressing HER2. Instead of imposing a permanent growth arrest characteristic of oncogene-induced senescence, the proteome secreted by the mutant cells promotes stem cell enrichment, angiogenesis, epithelial-to-mesenchymal transition, altered immune surveillance and acute vulnerability toward HSP90 inhibition. We inferred that the pleiotropism, as observed here, conferred by the mutated oncogene, depending on the host microenvironment, contributes to conflicting pre-clinical and clinical characteristics of HER2+, mutated PIK3CA-bearing tumor cells. We also came up with a plausible model for evolution of breast tumors from mammary epithelial cells harboring these two molecular lesions.

Interleukin 6 is increased in preclinical HNSCC models of acquired cetuximab resistance, but is not required for maintenance of resistance

The epidermal growth factor receptor inhibitor cetuximab is the only oncogene-targeted agent that has been FDA approved for the treatment of head and neck squamous cell carcinoma (HNSCC). Currently, there are no biomarkers used in the clinic to predict which HNSCC tumors will respond to cetuximab, and even in tumors that regress with treatment, acquired resistance occurs in the majority of cases. Though a number of mechanisms of acquired resistance to cetuximab have been identified in preclinical studies, no therapies targeting these resistance pathways have yet been effectively translated into the clinic. To address this unmet need, we examined the role of the cytokine interleukin 6 (IL-6) in acquired cetuximab resistance in preclinical models of HNSCC. We found that IL-6 secretion was increased in PE/CA-PJ49 cells that had acquired resistance to cetuximab compared to the parental cells from which they were derived. However, addition of exogenous IL-6 to parental cells did not promote cetuximab resistance, and inhibition of the IL-6 pathway did not restore cetuximab sensitivity in the cetuximab-resistant cells. Further examination of the IL-6 pathway revealed that expression of IL6R, which encodes a component of the IL-6 receptor, was decreased in cetuximab-resistant cells compared to parental cells, and that treatment of the cetuximab-resistant cells with exogenous IL-6 did not induce phosphorylation of signal transducer and activator of transcription 3, suggesting that the IL-6 pathway was functionally impaired in the cetuximab-resistant cells. These findings demonstrate that, even if IL-6 is increased in the context of cetuximab resistance, it is not necessarily required for maintenance of the resistant phenotype, and that targeting the IL-6 pathway may not restore sensitivity to cetuximab in cetuximab-refractory HNSCC.

ATR inhibition sensitizes HPV- and HPV+ head and neck squamous cell carcinoma to cisplatin

OBJECTIVES

Cisplatin is commonly used in the treatment of head and neck squamous cell carcinoma (HNSCC), and the repair of cisplatin-induced DNA damage involves activation of the DNA damage response protein ataxia telangiectasia and Rad3-related (ATR). Resistance to cisplatin therapy exacerbates adverse toxicities and is associated with poor outcomes. Since repair of cisplatin-induced DNA damage contributes to resistance, we hypothesized that inhibition of ATR using AZD6738, a well-tolerated and orally-bioavailable inhibitor, would enhance the sensitivity of HNSCC cells and tumors to cisplatin.

MATERIALS AND METHODS

A panel of human papilloma virus-negative (HPV^-) and HPV⁺ HNSCC cell lines were treated with cisplatin in the absence or presence of AZD6738, and effects on cell viability, colony formation, apoptosis signaling, and DNA damage were assessed. The impact of co-treatment with cisplatin plus AZD6738 on the growth of HPV^- and HPV⁺ cell line- and patient-derived xenograft tumors was also examined.

RESULTS

Inhibition of ATR with AZD6738 enhanced cisplatin-induced growth inhibition of HNSCC cell lines and tumors, in association with increased apoptosis signaling and DNA damage. Both HPV^- and HPV⁺ models were sensitized to cisplatin by ATR inhibition.

CONCLUSION

Inhibition of ATR promotes sensitization to cisplatin in preclinical in vitro and in vivo models of HPV^- and HVP⁺ HNSCC, supporting clinical evaluation of this strategy in this disease.

Use of nonsteroidal anti-inflammatory drugs predicts improved patient survival for PIK3CA-altered head and neck cancer

Head and neck cancer patients taking NSAIDs with PIK3CA tumor alterations demonstrate improved survival. Studies in relevant preclinical models implicate signaling via COX2-mediated production of PGE₂ as an underlying mechanism for this survival benefit.

PIK3CA is the most commonly altered oncogene in head and neck squamous cell carcinoma (HNSCC). We evaluated the impact of nonsteroidal anti-inflammatory drugs (NSAIDs) on survival in a PIK3CA-characterized cohort of 266 HNSCC patients and explored the mechanism in relevant preclinical models including patient-derived xenografts. Among subjects with PIK3CA mutations or amplification, regular NSAID use (≥6 mo) conferred markedly prolonged disease-specific survival (DSS; hazard ratio 0.23, P = 0.0032, 95% CI 0.09–0.62) and overall survival (OS; hazard ratio 0.31, P = 0.0043, 95% CI 0.14–0.69) compared with nonregular NSAID users. For PIK3CA-altered HNSCC, predicted 5-yr DSS was 72% for NSAID users and 25% for nonusers; predicted 5-yr OS was 78% for regular NSAID users and 45% for nonregular users. PIK3CA mutation predicted sensitivity to NSAIDs in preclinical models in association with increased systemic PGE₂ production. These findings uncover a biologically plausible rationale to implement NSAID therapy in PIK3CA-altered HNSCC.

Abstract 1972: Targeting the IL-6 signaling pathway overcomes cetuximab resistance in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC), the sixth most common cancer worldwide, has a five-year survival rate of 50%. The last few decades have seen only marginal improvements to the survival rate due to high rates of resistance to currently available treatments. The only oncogene-targeted agent that has been FDA approved for the treatment of HNSCC is cetuximab, a monoclonal antibody directed against the epidermal growth factor receptor (EGFR). The response rate for single-agent cetuximab is below 20%, and both intrinsic and acquired resistance to cetuximab remain major obstacles in the treatment of HNSCC. Identification and targeting of mediators that confer cetuximab resistance are needed to improve patient outcomes in HNSCC and in other cancers in which cetuximab is employed. To address this unmet need, we generated cetuximab-resistant variants of the human HNSCC cell lines PE/CA-PJ49 and FaDu. Because secretion of interleukin 6 (IL-6) has been proposed as a mechanism of cetuximab resistance in HNSCC, we assessed IL-6 levels in these models of acquired resistance. IL6 mRNA expression was increased in PE/CA-PJ49 and FaDu cells that have acquired resistance to cetuximab compared to the parental cells from which they were derived. In addition, cetuximab-resistant PE/CA-PJ49 variants exhibited increased levels of glycoprotein 130 (gp130), a molecule that oligomerizes with IL-6R alpha (IL-6Rα) to form the IL-6 receptor. Since IL-6 is known to promote tumor cell proliferation and survival, we hypothesized that cetuximab-resistant PE/CA-PJ49 and FaDu variants would be sensitive to inhibition of components of the IL-6 signaling pathway. To test this hypothesis, we treated PE/CA-PJ49 parental cells and cetuximab-resistant variants with cetuximab and/or inhibitors of the IL-6 pathway. Colony formation assays revealed that cetuximab-treated parental cells formed fewer colonies than vehicle-treated cells, while cetuximab treatment did not affect the ability of cetuximab-resistant PE/CA-PJ49 variants to form colonies. In contrast, siRNA-mediated knockdown of IL6 or IL6R (encoding IL-6Rα) markedly reduced the colony-forming ability of the cetuximab-resistant variants, but not parental cells. These results suggest that targeted inhibition of IL-6 signaling inhibits proliferation and/or survival in cetuximab-resistant PE/CA-PJ49 cells. Ongoing studies are testing our hypothesis using small-molecule and antibody-based inhibitors of the IL-6 pathway in our cell line models and in cetuximab-resistant patient-derived xenografts. By conducting these experiments, we aim to assess the therapeutic potential of targeting the IL-6 pathway in cetuximab-resistant HNSCC.

Citation Format: Rachel A. O'Keefe, Neil E. Bhola, David S. Lee, Daniel E. Johnson, Jennifer R. Grandis. Targeting the IL-6 signaling pathway overcomes cetuximab resistance in head and neck squamous cell carcinoma [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 1972.

BET Inhibition Overcomes Receptor Tyrosine Kinase-Mediated Cetuximab Resistance in HNSCC

Cetuximab, the FDA-approved anti-EGFR antibody for head and neck squamous cell carcinoma (HNSCC), has displayed limited efficacy due to the emergence of intrinsic and acquired resistance. We and others have demonstrated that cetuximab resistance in HNSCC is driven by alternative receptor tyrosine kinases (RTK), including HER3, MET, and AXL. In an effort to overcome cetuximab resistance and circumvent toxicities associated with the administration of multiple RTK inhibitors, we sought to identify a common molecular target that regulates expression of multiple RTK. Bromodomain-containing protein-4 (BRD4) has been shown to regulate the transcription of various RTK in the context of resistance to PI3K and HER2 inhibition in breast cancer models. We hypothesized that, in HNSCC, targeting BRD4 could overcome cetuximab resistance by depleting alternative RTK expression. We generated independent models of cetuximab resistance in HNSCC cell lines and interrogated their RTK and BRD4 expression profiles. Cetuximab-resistant clones displayed increased expression and activation of several RTK, such as MET and AXL, as well as an increased percentage of BRD4-expressing cells. Both genetic and pharmacologic inhibition of BRD4 abrogated cell viability in models of acquired and intrinsic cetuximab resistance and was associated with a robust decrease in alternative RTK expression by cetuximab. Combined treatment with cetuximab and bromodomain inhibitor JQ1 significantly delayed acquired resistance and RTK upregulation in patient-derived xenograft models of HNSCC. These findings indicate that the combination of cetuximab and bromodomain inhibition may be a promising therapeutic strategy for patients with HNSCC.Significance: Inhibition of bromodomain protein BRD4 represents a potential therapeutic strategy to circumvent the toxicities and financial burden of targeting the multiple receptor tyrosine kinases that drive cetuximab resistance in HNSCC and NSCLC.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/15/4331/F1.large.jpg Cancer Res; 78(15); 4331-43. ©2018 AACR.

Cross-talk Signaling between HER3 and HPV16 E6 and E7 Mediates Resistance to PI3K Inhibitors in Head and Neck Cancer

Human papillomavirus (HPV) type 16 is implicated in approximately 75% of head and neck squamous cell carcinomas (HNSCC) that arise in the oropharynx, where viral expression of the E6 and E7 oncoproteins promote cellular transformation, tumor growth, and maintenance. An important oncogenic signaling pathway activated by E6 and E7 is the PI3K pathway, a key driver of carcinogenesis. The PI3K pathway is also activated by mutation or amplification of PIK3CA in over half of HPV(+) HNSCC. In this study, we investigated the efficacy of PI3K-targeted therapies in HPV(+) HNSCC preclinical models and report that HPV(+) cell line- and patient-derived xenografts are resistant to PI3K inhibitors due to feedback signaling emanating from E6 and E7. Receptor tyrosine kinase profiling indicated that PI3K inhibition led to elevated expression of the HER3 receptor, which in turn increased the abundance of E6 and E7 to promote PI3K inhibitor resistance. Targeting HER3 with siRNA or the mAb CDX-3379 reduced E6 and E7 abundance and enhanced the efficacy of PI3K-targeted therapies. Together, these findings suggest that cross-talk between HER3 and HPV oncoproteins promotes resistance to PI3K inhibitors and that cotargeting HER3 and PI3K may be an effective therapeutic strategy in HPV(+) tumors.Significance: These findings suggest a new therapeutic combination that may improve outcomes in HPV(+) head and neck cancer patients. Cancer Res; 78(9); 2383-95. ©2018 AACR.

Abstract PR04: HER3 crosstalk with HPV16-E6E7 is a feedback resistance mechanism to PI3K-targeted therapies in head and neck cancer

Human papillomavirus (HPV) 16 plays an etiologic role in a growing subset of head and neck squamous cell carcinoma (HNSCC), where viral expression of the E6 and E7 oncoproteins is necessary for tumor growth and maintenance. Although patients with HPV(+) tumors have a more favorable prognosis, there are currently no HPV-selective therapies. Accumulating evidence indicates that HPV oncoproteins can activate the Phosphoinositol-3-Kinase (PI3K) pathway, which contributes to transformation. Furthermore, PI3K is genomically activated by PIK3CA mutation or amplification in a disproportionately high number of HPV(+) tumors as compared to HPV(-) tumors. Based on this knowledge, we investigated the efficacy of PI3K-targeted therapies in preclinical models of HPV(+) HNSCC. Our results indicate that HPV(+) preclinical models were less sensitive to the PI3K inhibitors BYL719, BKM120, and BEZ235 as compared with HPV(-) models. Sensitivity of HPV(+) cell lines to PI3K inhibitors was increased upon knockdown of the E6 and E7 oncoproteins. Reciprocally, overexpression of E6 and E7 in HPV(-) cells rendered them resistant to PI3K-targeted therapies. Proteomic analyses indicated that treatment of HPV(+) cell lines and patient-derived xenografts (PDXs) with the PI3Kα inhibitor BYL719 induced expression of the receptor tyrosine kinase HER3, as well as E6 and E7. HER3 was found to regulate the abundance of E6 and E7 in the HPV(+) models. Targeting HER3 with siRNAs or the monoclonal antibody, KTN3379, blocked the increase in E6 and E7 protein levels following BYL719 treatment, and enhanced the efficacy of PI3K inhibitors in HPV(+) cell line and PDX models. Taken together, these results suggest that crosstalk between HER3 and HPV16-E6E7 can limit the efficacy of PI3K inhibitors, and that co-targeting HER3 and PI3K may be an effective therapeutic strategy in HPV(+) tumors.

This abstract is also being presented as Poster 67.

Citation Format: Toni M. Brand, Stefan Hartmann, Neil E. Bhola, Hua Li, Yan Zeng, Rachel O'Keefe, Max V. Ranall, Sourav Bandyopadhyay, Margaret Soucheray, Danielle L. Swaney, Nevan Krogan, Carolyn Kemp, Umamaheswar Duvvuri, Daniel E. Johnson, Michelle A. Ozbun, Julie E. Bauman, Jennifer R. Grandis. HER3 crosstalk with HPV16-E6E7 is a feedback resistance mechanism to PI3K-targeted therapies in head and neck cancer [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; April 23-25, 2017; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(23_Suppl):Abstract nr PR04.

Association of FGFR1 with ERα Maintains Ligand-Independent ER Transcription and Mediates Resistance to Estrogen Deprivation in ER+ Breast Cancer

Purpose:FGFR1 amplification occurs in approximately 15% of estrogen receptor-positive (ER⁺) human breast cancers. We investigated mechanisms by which FGFR1 amplification confers antiestrogen resistance to ER⁺ breast cancer.Experimental Design: ER⁺ tumors from patients treated with letrozole before surgery were subjected to Ki67 IHC, FGFR1 FISH, and RNA sequencing (RNA-seq). ER⁺/FGFR1-amplified breast cancer cells, and patient-derived xenografts (PDX) were treated with FGFR1 siRNA or the FGFR tyrosine kinase inhibitor lucitanib. Endpoints were cell/xenograft growth, FGFR1/ERα association by coimmunoprecipitation and proximity ligation, ER genomic activity by ChIP sequencing, and gene expression by RT-PCR.Results: ER⁺/FGFR1-amplified tumors in patients treated with letrozole maintained cell proliferation (Ki67). Estrogen deprivation increased total and nuclear FGFR1 and FGF ligands expression in ER⁺/FGFR1-amplified primary tumors and breast cancer cells. In estrogen-free conditions, FGFR1 associated with ERα in tumor cell nuclei and regulated the transcription of ER-dependent genes. This association was inhibited by a kinase-dead FGFR1 mutant and by treatment with lucitanib. ChIP-seq analysis of estrogen-deprived ER⁺/FGFR1-amplified cells showed binding of FGFR1 and ERα to DNA. Treatment with fulvestrant and/or lucitanib reduced FGFR1 and ERα binding to DNA. RNA-seq data from FGFR1-amplified patients' tumors treated with letrozole showed enrichment of estrogen response and E2F target genes. Finally, growth of ER⁺/FGFR1-amplified cells and PDXs was more potently inhibited by fulvestrant and lucitanib combined than each drug alone.Conclusions: These data suggest the ERα pathway remains active in estrogen-deprived ER⁺/FGFR1-amplified breast cancers. Therefore, these tumors are endocrine resistant and should be candidates for treatment with combinations of ER and FGFR antagonists. Clin Cancer Res; 23(20); 6138-50. ©2017 AACR.

Abstract 95: Targeting BRD4 overcomes cetuximab resistance in HNSCC

Background: Nearly 600,000 people are diagnosed with head and neck cancer worldwide and 60% succumb to the disease within 5 years. The epidermal growth factor receptor (EGFR) is a major driver of HNSCC and in 2006 the EGFR monoclonal antibody cetuximab was FDA-approved for HNSCC treatment. However, cetuximab has not conferred significant long-term benefit compared to chemoradiation in patients with poorly differentiated recurrent-metastatic HNSCC. We and others have demonstrated that cetuximab treatment activates alternative receptor tyrosine kinases (RTKs) including Fibroblast Growth Factor (FGF) receptors, Met and Axl in HNSCC. To circumvent administration of multiple RTK inhibitors in combination with cetuximab, we sought to identify a common molecular target that regulates the expression of these RTKs. Bromodomain-containing protein-4 (BRD4) has been shown to regulate the transcription of RTKs in breast cancer models.

Hypothesis: We hypothesized that targeting BRD4 will overcome cetuximab resistance by depleting the expression of alternative RTKs in HNSCC. We further hypothesize that genetic and pharmacological targeting of BRD4 will synergize with cetuximab.

Results: Using a phospho-RTK array, 72-hour cetuximab treatment increased or sustained phosphorylated levels of EGFR, HER2, HER3, MET, and AXL in several HNSCC cell lines. Treatment with the BRD4 inhibitor JQ1 abrogated both phosphorylated and total RTK abundance in the presence of cetuximab. Cetuximab and JQ1 robustly decreased phosphorylated Src and induced the senescence marker p21. Phenotypically, cetuximab and JQ1 significantly decreased survival and increased apoptosis in 6 HNSCC cell line models, while the normal oral keratinocyte cell line NOKsi had approximately 10-fold higher IC50s for the BRD4 inhibitors, JQ1 and I-BET762, relative to HNSCC cell lines. Importantly, two HNSCC models of acquired cetuximab resistance exhibited robust sensitivity to pharmacological (JQ1, IBET-762) and genetic (RNAi) BRD4 targeting strategies. Moreover, exogneous overexpression of different RTKs (HER3, ALK, and ROR1) resulted in cetuximab resistance that was reversed upon BRD4 targeting (RNAi and JQ1). Combination of cetuximab and JQ1 in co-culture experiments with T cells decreased the CD4+/CD25+ Treg population and PD-L1 expression on HNSCC cell lines. Further, preliminary findings indicate that JQ1 treatment prevents outgrowth of cetuximab-treated HNSCC patient-derived xenograft models.

Conclusion: Our findings indicate that targeting BRD4 decreases the activation and expression of multiple RTKs that mediate resistance to the FDA-approved EGFR inhibitor cetuximab. Furthermore, BRD4 abrogates expression of immunosuppressive markers, making it a promising tumor intrinsic and extrinsic therapeutic strategy for HNSCC.

Citation Format: Toni Brand, Yan Zeng, Brandon Leonard, Rachel O' Keefe, Hua Li, Daniel Johnson, Jennifer Grandis, Neil E. Bhola. Targeting BRD4 overcomes cetuximab resistance in HNSCC [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 95. doi:10.1158/1538-7445.AM2017-95

Kinome-Wide RNA Interference Screen Reveals a Role for PDK1 in Acquired Resistance to CDK4/6 Inhibition in ER-Positive Breast Cancer

Acquired resistance to cyclin-dependent kinases 4 and 6 (CDK4/6) small-molecule inhibitors in breast cancer arises through mechanisms that are yet uncharacterized. In this study, we used a kinome-wide siRNA screen to identify kinases that, when downregulated, yield sensitivity to the CDK4/6 inhibitor ribociclib. In this manner, we identified 3-phosphoinositide-dependent protein kinase 1 (PDK1) as a key modifier of ribociclib sensitivity in estrogen receptor-positive MCF-7 breast cancer cells. Pharmacologic inhibition of PDK1 with GSK2334470 in combination with ribociclib or palbociclib, another CDK4/6 inhibitor, synergistically inhibited proliferation and increased apoptosis in a panel of ER-positive breast cancer cell lines. Ribociclib-resistant breast cancer cells selected by chronic drug exposure displayed a relative increase in the levels of PDK1 and activation of the AKT pathway. Analysis of these cells revealed that CDK4/6 inhibition failed to induce cell-cycle arrest or senescence. Mechanistic investigations showed that resistant cells coordinately upregulated expression of cyclins A, E, and D1, activated phospho-CDK2, and phospho-S477/T479 AKT. Treatment with GSK2334470 or the CDK2 inhibitor dinaciclib was sufficient to reverse these events and to restore the sensitivity of ribociclib-resistant cells to CDK4/6 inhibitors. Ribociclib, in combination with GSK2334470 or the PI3Kα inhibitor alpelisib, decreased xenograft tumor growth more potently than each drug alone. Taken together, our results highlight a role for the PI3K-PDK1 signaling pathway in mediating acquired resistance to CDK4/6 inhibitors. Cancer Res; 77(9); 2488-99. ©2017 AACR.

Human Papillomavirus Regulates HER3 Expression in Head and Neck Cancer: Implications for Targeted HER3 Therapy in HPV+ Patients

Purpose: Human papillomavirus (HPV) 16 plays an etiologic role in a growing subset of head and neck squamous cell carcinomas (HNSCC), where viral expression of the E6 and E7 oncoproteins is necessary for tumor growth and maintenance. Although patients with HPV⁺ tumors have a more favorable prognosis, there are currently no HPV-selective therapies. Recent studies identified differential receptor tyrosine kinase (RTK) profiles in HPV⁺ versus HPV^- tumors. One such RTK, HER3, is overexpressed and interacts with phosphoinositide-3-kinase (PI3K) in HPV⁺ tumors. Therefore, we investigated the role of HPV oncoproteins in regulating HER3-mediated signaling and determined whether HER3 could be a molecular target in HPV⁺ HNSCC.Experimental Design: HER3 was investigated as a molecular target in HPV⁺ HNSCC using established cell lines, patient-derived xenografts (PDX), and human tumor specimens. A mechanistic link between HPV and HER3 was examined by augmenting E6 and E7 expression levels in HNSCC cell lines. The dependency of HPV⁺ and HPV^- HNSCC models on HER3 was evaluated with anti-HER3 siRNAs and the clinical stage anti-HER3 monoclonal antibody KTN3379.Results: HER3 was overexpressed in HPV⁺ HNSCC, where it was associated with worse overall survival in patients with pharyngeal cancer. Further investigation indicated that E6 and E7 regulated HER3 protein expression and downstream PI3K pathway signaling. Targeting HER3 with siRNAs or KTN3379 significantly inhibited the growth of HPV⁺ cell lines and PDXs.Conclusions: This study uncovers a direct relationship between HPV infection and HER3 in HNSCC and provides a rationale for the clinical evaluation of targeted HER3 therapy for the treatment of HPV⁺ patients. Clin Cancer Res; 23(12); 3072-83. ©2016 AACR.

HGF/Met Signaling in Head and Neck Cancer: Impact on the Tumor Microenvironment

Studies to date have revealed several major molecular alterations that contribute to head and neck squamous cell carcinoma (HNSCC) initiation, progression, metastatic spread, and therapeutic failure. The EGFR is the only FDA-approved therapeutic target, yet responses to cetuximab have been limited. Activation and cross-talk of cellular receptors and consequent activation of different signaling pathways contribute to limited activity of blockade of a single pathway. The hepatocyte growth factor (HGF) receptor, Met, has been implicated in HNSCC tumorigenesis and EGFR inhibitor resistance. HGF, the sole ligand of Met, is overexpressed in the tumor microenvironment. The role of HGF/Met signaling in proliferation, metastasis, and angiogenesis has been investigated in HNSCC, leading to clinical trials with various Met inhibitors and HGF antibodies. However, the role of the HGF/Met signaling axis in mediating the tumor microenvironment has been relatively understudied in HNSCC. In this review, we discuss the functional roles of Met and HGF in HNSCC with a focus on the tumor microenvironment and the immune system. Clin Cancer Res; 22(16); 4005-13. ©2016 AACR.

Abstract S3-03: Nuclear FGFR1 interaction with estrogen receptor (ER) α is associated with resistance to endocrine therapy in ER+/FGFR1-amplified breast cancer

Background: Estrogen receptor (ER)-positive breast cancers (BC) initially respond to antiestrogens but eventually become hormone-independent and recur. FGFR1 is amplified in ∼10% of ER+ BC and is associated with early recurrence on antiestrogen therapy. Notably, one third of FGFR1-amplified tumors have simultaneous amplification of CCND1, FGF3, FGF4 and FGF19 on chromosome 11q12-14. Herein, we investigated the mechanisms by which FGFR1 amplification confers resistance to antiestrogen therapy in ER+ BC cells.

Results: We performed whole exome sequencing in tumor biopsies from 130 patients with an operable ER+/HER2- BC who had received letrozole for 10-21 days prior to surgery. Tumors were categorized by the natural log (ln) of post-letrozole Ki67 as sensitive (ln ≤1 or ≤2.7% Ki67+ cells; n=68) or resistant (ln ≥2 or ≥7.4%; n=18). We found amplifications in FGFR1 and/or 11q12-14 in 6/11 (55%) resistant tumors compared with 5/34 (15%) in sensitive tumors (p=0.006); all cases were confirmed by FGFR1-fluorescence in situ hydridization (FISH). Resistant tumors with FGFR1 and/or 11q12-14-amplification showed a marked increase in nuclear FGFR1 with letrozole. ER+/FGFR1-amplified CAMA1 and MDA134 cell lines also exhibited co-localization of ER and FGFR1 in the nucleus. Cell proliferation was partially reduced by estrogen deprivation, and FGFR1 siRNA further reduced cell growth in hormone-depleted medium. We generated CAMA1 and MDA134 cells resistant to long-term estrogen deprivation (LTED). These cells exhibited overexpression of FGF3/4/19 and ERα with a concomitant increase in ligand-independent ER transcriptional activity and growth. An ER-FGFR1 interaction was observed in the nucleus and cytosol of CAMA1 parental cells with enhanced interaction in CAMA1 LTED cells. Genetic (with siRNA) and pharmacologic (with lucitinib) inhibition of FGFR1 reduced a) nuclear localization of FGFR1; b) ER transcriptional activity; and c) cell proliferation. Nuclear localization and ER-FGFR1 interaction were disrupted by a kinase-deficient FGFR1. Conversely, addition of FGF3 ligand stimulated ER-FGFR1 interaction and ER transcriptional activity, suggesting FGFR activation can regulate ER function. Inhibition of FGF receptor-specific substrate (FRS2), a principal mediator of FGFR1 signal transduction to the MAPK and PI3K pathways, with siRNA or pharmacologic inhibition of PI3K with buparlisib or MEK with GSK1120212 did not reduce ER transcriptional activity suggesting that, in ER+/FGFR1-amplified cancer cells, ER function is not modulated by FGFR signal transducers. Finally, using chromatin immunoprecipitation (ChIP) we showed that FGFR1 binds directly to estrogen response elements (ERE). This association was reduced with lucitanib. We are currently investigating genes modulated by ER/FGFR1 in ER+ BC and the in vivo anti-tumor efficacy of dual inhibition of FGFR1 and ER in ER+/FGFR1-amplified patient-derived breast cancer xenografts.

Conclusions: These data support a critical role of ER and FGFR1 interaction in endocrine resistance in ER+/FGFR1-amplified breast cancer. Targeting of FGFR1 in combination with antiestrogens may abrogate resistance to endocrine therapy in these tumors and is worthy of clinical investigation.

Citation Format: Formisano L, Young CD, Bhola NE, Bulen B, Estrada VM, Wagle N, Van Allen E, Red Brewer ML, Jansen VM, Guerrero AL, Giltnane JM, Strcker T, Arteaga CL. Nuclear FGFR1 interaction with estrogen receptor (ER) α is associated with resistance to endocrine therapy in ER+/FGFR1-amplified breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr S3-03.

Abstract PD2-06: Inhibition of 3-phosphoinositide dependent protein kinase 1 (PDK1) synergizes with CDK4/6 inhibitors against ER-positive breast cancer

Background: Dysregulation in cell cycle checkpoints is common in cancer. Small molecule inhibitors that target the CDK4/6/cyclinD1 pathway of the cell cycle are in clinical development. Recently the combination of the CDK4/6 inhibitor palbociclib and the aromatase inhibitor letrozole was approved for the treatment of post-menopausal women with ER+/HER2- advanced breast cancer. However, not all patients benefit from CDK4/6 inhibitors and a significant fraction of them eventually progress on these agents, underscoring the need to develop potent therapeutic strategies to circumvent drug resistance.

Methods: We performed a high-throughput RNA interference (RNAi) kinome screen targeting 720 kinases to identify targetable molecules whose inhibition, in combination with the CDK4/6 inhibitor LEE011 (ribociclib), induced synthetic lethality in MCF7 ER+ breast cancer cells. PDK1 RNAi oligonucleotides and the PDK1 inhibitor GSK2334470 in combination with each of the CDK4/6 inhibitors, palbociclib and LEE011, were tested against ER+ breast cancer cells. In vivo anti-tumor efficacy of LEE011 and GSK2334470 was assessed in ovariectomized athymic nude mice bearing MCF7 xenografts.

Results: A siRNA kinome screen identified PDK1 as the top RNA whose downregulation sensitized MCF7 cells to CDK4/6 inhibitors. This was confirmed with independent siRNAs in ER+ MCF7, T47D, HCC1428 and HCC1500 breast cancer cells. Pharmacological inhibition of PDK1 with the ATP-competitive, small molecule inhibitor GSK2334470 in combination with each of the CDK4/6 inhibitors, LEE011 and palbociclib, synergistically inhibited proliferation and increased apoptosis of MCF7 and T47D cells (combination index 0.19-0.89). LEE011-resistant MCF7 and T47D cells were generated by chronic treatment with doses of LEE011 up to 1 µM. Drug-resistant cells displayed increased levels of PDK1, phosphorylated Rb, and phosphorylated S6 ribosomal protein (pS6), an effector of the PDK1 substrate p70S6K, compared to parental drug-sensitive cells. Inhibition of PDK1 with siRNA or GSK2334470 re-sensitized the LEE011-resistant cells to the CDK4/6 inhibitors. Genetic (RNAi) and pharmacological inhibition of PDK1 (with GSK2334470) abrogated pS6 levels whereas inhibition of AKT with the small molecule inhibitor MK2206 did not affect pS6 levels, suggesting PDK1 can induce resistance to CDK4/6 inhibitors via p70S6K/pS6 signaling in an AKT-independent manner. The effects observed in cell lines in culture were recapitulated in vivo using MCF7 xenografts established in ovariectomized nude mice in the absence of estrogen supplementation. Treatment with GSK2334470 and LEE011 induced tumor regressions (8/8 tumors by RECIST criteria) more potently than either drug alone.

Conclusions: These data support a critical role of PDK1 in mediating acquired resistance to CDK4/6 inhibitors in ER+ breast cancer cells. Co-targeting of the PDK1 and CDK4/6 pathways may overcome resistance to CDK4/6 inhibitors and is worthy of further translational and clinical investigation in patients with ER+ breast cancer.

Citation Format: Jansen VM, Bhola NE, Bauer JA, Formisano L, Moore P, Koch J, Arteaga CL. Inhibition of 3-phosphoinositide dependent protein kinase 1 (PDK1) synergizes with CDK4/6 inhibitors against ER-positive breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD2-06.

Treatment of Triple-Negative Breast Cancer with TORC1/2 Inhibitors Sustains a Drug-Resistant and Notch-Dependent Cancer Stem Cell Population

Approximately 30% of triple-negative breast cancers (TNBC) harbor molecular alterations in PI3K/mTOR signaling, but therapeutic inhibition of this pathway has not been effective. We hypothesized that intrinsic resistance to TORC1/2 inhibition is driven by cancer stem cell (CSC)-like populations that could be targeted to enhance the antitumor action of these drugs. Therefore, we investigated the molecular mechanisms by which PI3K/mTOR inhibitors affect the stem-like properties of TNBC cells. Treatment of established TNBC cell lines with a PI3K/mTOR inhibitor or a TORC1/2 inhibitor increased the expression of CSC markers and mammosphere formation. A CSC-specific PCR array revealed that inhibition of TORC1/2 increased FGF1 and Notch1 expression. Notch1 activity was also induced in TNBC cells treated with TORC1/2 inhibitors and associated with increased mitochondrial metabolism and FGFR1 signaling. Notably, genetic and pharmacologic blockade of Notch1 abrogated the increase in CSC markers, mammosphere formation, and in vivo tumor-initiating capacity induced by TORC1/2 inhibition. These results suggest that targeting the FGFR-mitochondrial metabolism-Notch1 axis prevents resistance to TORC1/2 inhibitors by eradicating drug-resistant CSCs in TNBC, and may thus represent an attractive therapeutic strategy to improve drug responsiveness and efficacy.

Abstract 2844: RNA interference kinome-wide screen reveals a role for PDK1 in acquired resistance to CDK4/6 inhibition ER-positive breast cancer

Background: Small molecule inhibitors that target the CDK4/6/cyclinD1 pathway are in clinical development. Clinical trials combining the CDK4/6 inhibitor pallbociclib and the aromatase inhibitor letrozole have demonstrated significantly improved clinical outcomes in patients with ER-positive breast cancer. This combination is likely to be approved for the treatment of patients with this breast cancer subtype. However, as for other targeted therapies, development of resistance to CDK4/6 inhibitors in a significant fraction of patients is anticipated. Therefore, there is a need to develop potent therapeutic strategies to circumvent drug resistance.

Methods: We performed a high-throughput RNAi kinome screen targeting 720 kinases to identify potentially targetable molecules whose inhibition in combination with the CDK4/6 inhibitor LEE011 induced synthetic lethality in MCF7 ER+ breast cancer cells.

Results: The sensitivity index (SI) score, which measures the influence of siRNA-induced gene knockdown on drug sensitivity, was calculated for each siRNA in MCF7 cells treated with LEE011 (IC50, 144 nM). A cutoff of SI >0.15 (2 standard deviations above the mean) was used for hit selection. Individual knockdown of 15 (2.1%) kinases sensitized MCF7 cells to the CDK4/6 inhibitor (p<0.05). 3-phosphoinositide dependent protein kinase 1 (PDK1) was identified as the top hit (SI = 0.32) whose downregulation sensitized MCF7 cells to LEE011. This was confirmed with independent siRNAs in ER+ MCF7, T47D, HCC1428 and HCC1500 breast cancer cell lines. Pharmacological inhibition of PDK1 with the ATP-competitive, small molecule inhibitor GSK2334470 synergistically inhibited proliferation of MCF7 and T47D cells (combination index 0.19-0.89). LEE011-resistant MCF7 and T47D cells were generated by chronic treatment with doses of LEE011 up to 1 μM. These cells displayed increased PDK1 protein expression compared to parental cells. Inhibition of PDK1 with siRNA or GSK2334470 resensitized the LEE011-resistant cells to the CDK4/6 inhibitor. LEE011-resistant MCF7 cells expressed high levels of phosphorylated S6 ribosomal protein (pS6), an effector of the PDK1 substrate p70S6K. Genetic (with siRNA) and pharmacological inhibition of PDK1 (with GSK2334470) abrogated pS6 levels whereas inhibition of AKT with the small molecule inhibitor AZD5363 did not affect pS6 levels, suggesting that PDK1 mediates resistance to CDK4/6 inhibition via p70S6K/pS6 signaling in an AKT-independent manner.

Conclusions: These studies suggest that PDK1 inhibition can overcome resistance to the CDK4/6 inhibitor LEE011 and offer a rationale for further translational and clinical investigation of combinations of CDK4/6 and PDK1 inhibitors in ER+ breast cancer.

Citation Format: Valerie M. Jansen, Neil E. Bhola, Joshua A. Bauer, Carlos L. Arteaga. RNA interference kinome-wide screen reveals a role for PDK1 in acquired resistance to CDK4/6 inhibition ER-positive breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2844. doi:10.1158/1538-7445.AM2015-2844

Abstract 4240: TORC inhibition enriches for a cancer stem cell-like population with FGFR-dependent Notch1 activation

Background. Cancer stem cells (CSCs) are associated with metastatic recurrences and poor prognosis in triple negative breast cancer (TNBC) patients. Genomic and proteomic data suggest that >30% of TNBC harbor PI3K/mTOR pathway aberrations. Furthermore, PI3K/mTOR inhibitors have shown antitumor activity in preclinical TNBC models. However, resistance to PI3K/mTOR inhibitors in breast and ovarian cancer has been recently shown. We hypothesized that resistance to TORC inhibition in TNBC is driven by a CSC-like population and targeting this population can improve antitumor action of TORC inhibitors.

Methods and Results. Treatment of TNBC cells, SUM159 (PIK3CA-H1047L), BT549 (PTEN-del), and MDA468 (PTEN-del), with the PI3K/mTOR inhibitor BEZ235 or the TORC1/2 inhibitor MLN128 reduced growth. However, cells that survived BEZ235 and MLN128 treatment displayed CSC properties as assessed by FACS analysis for CSC markers and mammosphere formation. Using a Stem Cell-specific PCR Array and qRT-PCR, Notch1, Jagged1 and FGF1 mRNA levels were elevated in BEZ235 and MLN128-treated TNBC cells. Transient and long-term treatment with BEZ235 and MLN128 also increased expression of the active Notch intracellular domain (NICD), Notch target genes Hes1, Hey1 and c-Myc, and NICD-luciferase reporter activity. Nanostring analysis of post-chemotherapy primary breast cancers displayed increased expression of CSC markers (ALDH1A1 and CD44), Notch1, and Jagged1. Cells surviving the combination of the chemotherapeutic paclitaxel with BEZ235 or MLN128 displayed increased NICD and CSC-like properties. Similar changes were observed in OVCAR8 ovarian cancer cells treated with MLN128 ± cisplatin. Notch1 siRNA and the γ-secretase inhibitor (GSI) DAPT abrogated BEZ235- and MLN128-mediated enrichment of CSC markers and mammosphere formation. Treatment of SUM159 and MDA468 xenografts established in nude mice with MLN128 and DAPT did not significantly decrease tumor growth compared to MLN128 alone. However, MLN128 and DAPT-treated tumors displayed decreased tumorigenicity compared to MLN128-treated tumors as assessed by an in vivo limiting dilution assay. Furthermore, TNBC cells stably expressing mTOR shRNA exhibited increased NICD levels, CSC markers and tumorigenicity in vivo which was reduced with DAPT. With the increased FGF1 levels observed with BEZ235 and MLN128 treatment, we examined the effect of FRS2 siRNA and the FGFR1 kinase inhibitor lucitanib. Both approaches resulted in a reduction of the CSCs and NICD expression in MLN128-treated cells.

Conclusions. These data suggest that treatment of TNBC harboring PI3K pathway aberrations with TORC1/2 inhibitors results in decreased tumor growth but may spare a drug-resistant CSC population with FGFR-Notch signaling. Thus, combination of Notch and TORC1/2 inhibitors in addition to chemotherapy may effectively decrease primary tumor growth and prevent recurrences in TNBC patients.

Citation Format: Neil E. Bhola, Valerie M. Jansen, Carlos L. Arteaga. TORC inhibition enriches for a cancer stem cell-like population with FGFR-dependent Notch1 activation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4240. doi:10.1158/1538-7445.AM2015-4240

Small molecule inhibitor of the bone morphogenetic protein pathway DMH1 reduces ovarian cancer cell growth

The bone morphogenetic protein (BMP) pathway belonging to the Transforming Growth Factor beta (TGFβ) family of secreted cytokines/growth factors is an important regulator of cancer. BMP ligands have been shown to play both tumor suppressive and promoting roles in human cancers. We have found that BMP ligands are amplified in human ovarian cancers and that BMP receptor expression correlates with poor progression-free-survival (PFS). Furthermore, active BMP signaling has been observed in human ovarian cancer tissue. We also determined that ovarian cancer cell lines have active BMP signaling in a cell autonomous fashion. Inhibition of BMP signaling with a small molecule receptor kinase antagonist is effective at reducing ovarian tumor sphere growth. Furthermore, BMP inhibition can enhance sensitivity to Cisplatin treatment and regulates gene expression involved in platinum resistance in ovarian cancer. Overall, these studies suggest targeting the BMP pathway as a novel source to enhance chemo-sensitivity in ovarian cancer.

Activating PIK3CA Mutations Induce an Epidermal Growth Factor Receptor (EGFR)/Extracellular Signal-regulated Kinase (ERK) Paracrine Signaling Axis in Basal-like Breast Cancer

Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K) have been shown to transform human mammary epithelial cells (MECs). These mutations are present in all breast cancer subtypes, including basal-like breast cancer (BLBC). Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we identified 72 protein expression changes in human basal-like MECs with knock-in E545K or H1047R PIK3CA mutations versus isogenic MECs with wild-type PIK3CA. Several of these were secreted proteins, cell surface receptors or ECM interacting molecules and were required for growth of PIK3CA mutant cells as well as adjacent cells with wild-type PIK3CA. The proteins identified by MS were enriched among human BLBC cell lines and pointed to a PI3K-dependent amphiregulin/EGFR/ERK signaling axis that is activated in BLBC. Proteins induced by PIK3CA mutations correlated with EGFR signaling and reduced relapse-free survival in BLBC. Treatment with EGFR inhibitors reduced growth of PIK3CA mutant BLBC cell lines and murine mammary tumors driven by a PIK3CA mutant transgene, all together suggesting that PIK3CA mutations promote tumor growth in part by inducing protein changes that activate EGFR.

Kinome-wide functional screen identifies role of PLK1 in hormone-independent, ER-positive breast cancer

Estrogen receptor (ER) α-positive breast cancers initially respond to antiestrogens but eventually become estrogen independent and recur. ER(+) breast cancer cells resistant to long-term estrogen deprivation (LTED) exhibit hormone-independent ER transcriptional activity and growth. A kinome-wide siRNA screen using a library targeting 720 kinases identified Polo-like kinase 1 (PLK1) as one of the top genes whose downregulation resulted in inhibition of estrogen-independent ER transcriptional activity and growth of LTED cells. High PLK1 mRNA and protein correlated with a high Ki-67 score in primary ER(+) breast cancers after treatment with the aromatase inhibitor letrozole. RNAi-mediated knockdown of PLK1 inhibited ER expression, estrogen-independent growth, and ER transcription in MCF7 and HCC1428 LTED cells. Pharmacologic inhibition of PLK1 with volasertib, a small-molecule ATP-competitive PLK1 inhibitor, decreased LTED cell growth, ER transcriptional activity, and ER expression. Volasertib in combination with the ER antagonist, fulvestrant, decreased MCF7 xenograft growth in ovariectomized mice more potently than each drug alone. JUNB, a component of the AP-1 complex, was expressed 16-fold higher in MCF7/LTED compared with parental MCF7 cells. Furthermore, JUNB and BCL2L1 (which encodes antiapoptotic BCL-xL) mRNA levels were markedly reduced upon volasertib treatment in MCF7/LTED cells, while they were increased in parental MCF7 cells. Finally, JUNB knockdown decreased ER expression and transcriptional activity in MCF7/LTED cells, suggesting that PLK1 drives ER expression and estrogen-independent growth via JUNB. These data support a critical role of PLK1 in acquired hormone-independent growth of ER(+) human breast cancer and is therefore a promising target in tumors that have escaped estrogen deprivation therapy.

Abstract 1945: TORC inhibitors increase the cancer stem cell (CSC) population and Notch signaling in triple negative breast cancer

Tumor-initiating cells (TICs) or cancer stem cells (CSCs) are resistant to chemotherapy and have been associated with metastatic recurrences and poor patient outcome particularly among patients with triple negative breast cancer (TNBC). Genomic and proteomic data have indicated more than 30% of TNBC patients have PI3K/mTOR pathway lesions making this pathway a promising therapeutic target. Recent publications have demonstrated mechanisms of resistance (JAK2/STAT5 and MYC amplification) to PI3K pathway inhibitors. We hypothesized that resistance to TORC inhibition is due to the survival of a CSC population and that targeting pathways that sustain these cells can provide a significant therapeutic benefit. Treatment of TNBC cell lines with the PI3K/mTOR inhibitor NVP-BEZ235 or the TORC1/2 inhibitor MLN128 resulted in a significant reduction of proliferation in vitro. However, we observed that both BEZ235 and MLN128 enriched for a CSC population as assessed by FACS analysis of cancer stem-like markers and mammosphere formation. This observation was specific to TNBC cell lines since BEZ235 and MLN128 significantly abrogated the CSC population in ER+ (MCF7) and HER2+ (HCC1954) breast cancer cell lines. To determine the mechanisms involved in this CSC enrichment we used a Stem Cell specific PCR Array. We observed an increase in Notch1, FGF1 and ABCG2 mRNA levels in TNBC cells treated with BEZ235 and MLN128. Treatment with these inhibitors also increased the expression of the active Notch intracellular domain, the Notch ligand Jagged1, and the Notch1 target genes Hes1 and Hey1 by qRT-PCR and transcriptional reporter activity. In addition to Hes1 and Hey1, c-myc, another Notch target gene, expression was augmented in 2 of the 3 TNBC cell lines tested. Treament with the γ-secretase inhibitor, DAPT and transfection with Notch1 siRNA oligonucleotides abrogated BEZ235 and INK128-mediated enrichment of CSC populations as measured by FACS analysis and mammosphere formation assays. To determine whether inhibition of either TORC1 or TORC2 enriched for the CSC population, we used RNAi against Rictor (TORC2), Raptor (TORC2) or both. We observed that only the combined knockdown of Rictor and raptor increased the CSC population in TNBC cell lines.

These observations suggest that treatment of TNBC harboring PI3K pathway aberrations with TORC1/2 inhibitors results in an initial reduction of tumor burden but do not eradicate the drug-resistant, slow cycling CSC population driven by Notch signaling. Thus, combination of a Notch inhibitor with TORC1/2 inhibitors and chemotherapy may be an effective therapeutic strategy to decrease primary tumor growth and prevent recurrences in patients with TNBC.

Citation Format: Neil E. Bhola, Valerie Jansen, Carlos Arteaga. TORC inhibitors increase the cancer stem cell (CSC) population and Notch signaling in triple negative breast cancer. [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 1945. doi:10.1158/1538-7445.AM2014-1945

Activation of MAPK pathways due to DUSP4 loss promotes cancer stem cell-like phenotypes in basal-like breast cancer

Basal-like breast cancer (BLBC) is an aggressive disease that lacks a clinically approved targeted therapy. Traditional chemotherapy is effective in BLBC, but it spares the cancer stem cell (CSC)-like population, which is likely to contribute to cancer recurrence after the initial treatment. Dual specificity phosphatase-4 (DUSP4) is a negative regulator of the mitogen-activated protein kinase (MAPK) pathway that is deficient in highly aggressive BLBCs treated with chemotherapy, leading to aberrant MAPK activation and resistance to taxane-induced apoptosis. Herein, we investigated how DUSP4 regulates the MAP-ERK kinase (MEK) and c-jun-NH2-kinase (JNK) pathways in modifying CSC-like behavior. DUSP4 loss increased mammosphere formation and the expression of the CSC-promoting cytokines interleukin (IL)-6 and IL-8. These effects were caused in part by loss of control of the MEK and JNK pathways and involved downstream activation of the ETS-1 and c-JUN transcription factors. Enforced expression of DUSP4 reduced the CD44(+)/CD24(-) population in multiple BLBC cell lines in a MEK-dependent manner, limiting tumor formation of claudin-low SUM159PT cells in mice. Our findings support the evaluation of MEK and JNK pathway inhibitors as therapeutic agents in BLBC to eliminate the CSC population.

TGF-β inhibition enhances chemotherapy action against triple-negative breast cancer

After an initial response to chemotherapy, many patients with triple-negative breast cancer (TNBC) have recurrence of drug-resistant metastatic disease. Studies with TNBC cells suggest that chemotherapy-resistant populations of cancer stem-like cells (CSCs) with self-renewing and tumor-initiating capacities are responsible for these relapses. TGF-β has been shown to increase stem-like properties in human breast cancer cells. We analyzed RNA expression in matched pairs of primary breast cancer biopsies before and after chemotherapy. Biopsies after chemotherapy displayed increased RNA transcripts of genes associated with CSCs and TGF-β signaling. In TNBC cell lines and mouse xenografts, the chemotherapeutic drug paclitaxel increased autocrine TGF-β signaling and IL-8 expression and enriched for CSCs, as indicated by mammosphere formation and CSC markers. The TGF-β type I receptor kinase inhibitor LY2157299, a neutralizing TGF-β type II receptor antibody, and SMAD4 siRNA all blocked paclitaxel-induced IL8 transcription and CSC expansion. Moreover, treatment of TNBC xenografts with LY2157299 prevented reestablishment of tumors after paclitaxel treatment. These data suggest that chemotherapy-induced TGF-β signaling enhances tumor recurrence through IL-8-dependent expansion of CSCs and that TGF-β pathway inhibitors prevent the development of drug-resistant CSCs. These findings support testing a combination of TGF-β inhibitors and anticancer chemotherapy in patients with TNBC.

Trastuzumab-resistant cells rely on a HER2-PI3K-FoxO-survivin axis and are sensitive to PI3K inhibitors

The antibody trastuzumab is approved for treatment of patients with HER2 (ERBB2)-overexpressing breast cancer. A significant fraction of these tumors are either intrinsically resistant or acquire resistance rendering the drug ineffective. The development of resistance has been attributed to failure of the antibody to inhibit phosphoinositide 3-kinase (PI3K), which is activated by the HER2 network. Herein, we examined the effects of PI3K blockade in trastuzumab-resistant breast cancer cell lines. Treatment with the pan-PI3K inhibitor XL147 and trastuzumab reduced proliferation and pAKT levels, triggering apoptosis of trastuzumab-resistant cells. Compared with XL147 alone, the combination exhibited a superior antitumor effect against trastuzumab-resistant tumor xenografts. Furthermore, treatment with XL147 and trastuzumab reduced the cancer stem-cell (CSC) fraction within trastuzumab-resistant cells both in vitro and in vivo. These effects were associated with FoxO-mediated inhibition of transcription of the antiapoptosis gene survivin (BIRC5) and the CSC-associated cytokine interleukin-8. RNA interference-mediated or pharmacologic inhibition of survivin restored sensitivity to trastuzumab in resistant cells. In a cohort of patients with HER2-overexpressing breast cancer treated with trastuzumab, higher pretreatment tumor levels of survivin RNA correlated with poor response to therapy. Together, our results suggest that survivin blockade is required for therapeutic responses to trastuzumab and that by combining trastuzumab and PI3K inhibitors, CSCs can be reduced within HER2(+) tumors, potentially preventing acquired resistance to anti-HER2 therapy.

Profiling of residual breast cancers after neoadjuvant chemotherapy identifies DUSP4 deficiency as a mechanism of drug resistance

Neoadjuvant chemotherapy (NAC) induces a pathological complete response (pCR) in ~30% of patients with breast cancer. However, many patients have residual cancer after chemotherapy, which correlates with a higher risk of metastatic recurrence and poorer outcome than those who achieve a pCR. We hypothesized that molecular profiling of tumors after NAC would identify genes associated with drug resistance. Digital transcript counting was used to profile surgically resected breast cancers after NAC. Low concentrations of dual specificity protein phosphatase 4 (DUSP4), an ERK phosphatase, correlated with high post-NAC tumor cell proliferation and with basal-like breast cancer (BLBC) status. BLBC had higher DUSP4 promoter methylation and gene expression patterns of Ras-ERK pathway activation relative to other breast cancer subtypes. DUSP4 overexpression increased chemotherapy-induced apoptosis, whereas DUSP4 depletion dampened the response to chemotherapy. Reduced DUSP4 expression in primary tumors after NAC was associated with treatment-refractory high Ki-67 scores and shorter recurrence-free survival. Finally, inhibition of mitogen-activated protein kinase kinase (MEK) synergized with docetaxel treatment in BLBC xenografts. Thus, DUSP4 downregulation activates the Ras-ERK pathway in BLBC, resulting in an attenuated response to anti-cancer chemotherapy.

Abstract 3475: Inhibition of the TGFβ pathway inhibits chemotherapy-induced enrichment of breast cancer stem cells in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is the most virulent form of breast cancer and is associated with a worse prognosis compared to hormone receptor- and HER2-positive tumors. The standard treatment of TNBC is cytotoxic chemotherapy. TNBC patients tend to display an initial response to chemotherapy; however, they exhibit higher recurrence rates and overall poor long term survival. Reasons for this high metastatic recurrence rate and mortality are believed to be the existence of a chemo-resistant tumor-initiating population called cancer stem cells (CSCs). CSCs are defined as cells with the ability to self-renew, differentiate into non-tumorigenic cells, and initiate tumors in vivo. CSCs isolated from breast cancer tissue display increased transforming growth factor (TGF) ≤ and TGFβ type 2 receptor (TGFβR2) mRNA expression compared to the non-CSC population. Furthermore, breast CSCs exhibit characteristics of the TGFβ-driven epithelial to mesenchymal transition (EMT). Therefore, we hypothesize that inhibiting the TGFβ pathway can abrogate the breast CSC population and sensitize TNBC to chemotherapy, thus reducing metastatic recurrences and tumor progression. From a cohort of pre and post-chemotherapy treated TNBC patient samples we observed increased TGFβ signaling receptors and ligands, CD44 and ALDH1 levels in post-chemotherapy treated patients. We examined the effect of TGFβ1, the TGFβ receptor 1/2 kinase inhibitor LY2157299, the TGFβR2 receptor neutralizing antibody TR1, and paclitaxel on the CSC population in TNBC cell lines by 1) FACS analysis of CSC markers (ALDH, CD44, and PROCR); and 2) mammosphere formation assays. Paclitaxel treatment resulted in an enrichment of CSCs as measured by FACS and mammosphere formation (p≤0.005) both in vitro and in SUM159 xenografts. Further, treatment with paclitaxel upregulated TGFβR2 and TGFβ1 mRNAs and phosphorylated SMAD2 levels from post-therapy SUM159 xenografts. LY2157299 and TR1 abrogated TGFβ-mediated CSC enrichment and mammosphere formation (p≤0.01). Specific targeting of TGFβR2 and SMAD4 by RNAi decreased the CD44hi/CD24lo population in both cell lines. SMAD4 downmodulation also decreased mammosphere formation and interleukin-8 (IL-8) expression in three TNBC cell lines. Finally, a combination of either LY2157299 or SMAD4 siRNA with paclitaxel decreased CSC marker expression, IL-8, and mammosphere formation compared to each treatment alone (p≤0.006) from bulk and sorted CSC populations. These findings suggest that TGFβ signaling plays a maintenance role in breast CSCs viability. Second, blockade of the TGFβ pathway with genetic or pharmacological inhibitors can ameliorate or prevent the enrichment of drug-resistant CSCs by chemotherapy. These studies provide a rationale for studies of chemotherapy ± TGFβ inhibitors in patients with TNBC using CSC markers as surrogates of clinical response.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3475. doi:1538-7445.AM2012-3475

Antitumor mechanisms of targeting the PDK1 pathway in head and neck cancer

G-protein-coupled receptors (GPCR) activate the epidermal growth factor receptor (EGFR) and mediate EGFR-independent signaling pathways to promote the growth of a variety of cancers, including head and neck squamous cell carcinoma (HNSCC). Identification of the common signaling mechanisms involved in GPCR-induced EGFR-dependent and EGFR-independent processes will facilitate the development of more therapeutic strategies. In this study, we hypothesized that phosphoinositide-dependent kinase 1 (PDK1) contributes to GPCR-EGFR cross-talk and signaling in the absence of EGFR and suggests that inhibition of the PDK1 pathway may be effective in the treatment of HNSCC. The contribution of PDK1 to the EGFR-dependent and EGFR-independent signaling in HNSCC was determined using RNA interference, a kinase-dead mutant, and pharmacologic inhibition. In vivo xenografts studies were also carried out to determine the efficacy of targeting PDK1 alone or in combination with the U.S. Food and Drug Administration-approved EGFR inhibitor cetuximab. PDK1 contributed to both GPCR-induced EGFR activation and cell growth. PDK1 also mediated activation of p70S6K in the absence of EGFR. Blockade of PDK1 with a small molecule inhibitor (AR-12) abrogated HNSCC growth, induced apoptosis, and enhanced the antiproliferative effects of EGFR tyrosine kinase inhibitors in vitro. HNSCC xenografts expressing kinase-dead PDK1 showed increased sensitivity to cetuximab compared with vector-transfected controls. Administration of AR-12 substantially decreased HNSCC tumor growth in vivo. These cumulative results show that PDK1 is a common signaling intermediate in GPCR-EGFR cross-talk and EGFR-independent signaling, and in which targeting the PDK1 pathway may represent a rational therapeutic strategy to enhance clinical responses to EGFR inhibitors in HNSCC.

PD08-03: Inhibition of MEK/ERK- and JNK-Dependent Expression of Interleukin-6 and Interleukin-8 Targets Basal-Like Breast Cancer Stem Cells

Background: Neoadjuvant chemotherapy (NAC) induces a pathological complete response in approximately 30% of triple-negative or basal-like breast cancers (BLBC). However, patients with residual disease often recur after surgery, presumably due to persistent drug-resistant subpopulations with cancer stem cell (CSC)-like properties. Thus, elimination of this CSC compartment in BLBC has the potential to improve survival by reducing post-surgical metastatic recurrences.

Methods: We sampled 49 post-NAC breast cancer samples including 22 BLBCs and quantified RNA for 355 cancer-related transcripts using Nanostring technology. Transcripts associated with a high Ki67 in the residual disease (a biomarker of early recurrence) were identified and bioinformatically examined for an association with drug resistance and a CSC phenotype. Loss of DUSP4, a negative feedback regulator of ERK1/2 and JNK1/2, was highly associated with a high post-NAC Ki67. We examined the role of loss of DUSP4 in promoting a drug-resistant, CSC phenotype.

Results: Low DUSP4 expression in post-NAC tumors was associated with high ERK1/2 activation and BLBC gene expression. siRNA knockdown of DUSP4 enhanced resistance to anti-cancer chemotherapy (docetaxel, camptothecin). Alternatively, forced DUSP4 expression in breast cancer cell lines abrogated the activation of transcription factors downstream of ERK and JNK and sensitized cells to docetaxel-induced apoptosis. In highly metastastic BLBC cell lines, MEK inhibition with AZD6244 and JNK inhibition with SP600125 significantly reduced mammosphere formation and self-renewal. Inhibition of JNK and MEK reduced IL6 and IL8 expression, two cytokines known to expand the CSC compartment. Reconstitution of exogenous IL6 and IL8 after MEK inhibition restored mammosphere formation potential. Chromatin immunoprecipitation demonstrated that the oncogenic transcription factor ETS-1, an ERK1/2 substrate, binds the IL8 promoter in a MEK1/2-dependent manner. We are currently confirming the association of DUSP4 loss with Ki67, ERK activation, and IL6/IL8 expression in a cohort of 113 post-NAC triple negative breast cancers. Conclusions: Our data demonstrate that loss of DUSP4 in BLBC promotes ERK and JNK activation by impairing negative feedback of these pathways. Activation of ERK and JNK drives expression of IL6 and IL8 expression, possibly through ETS-1 activation. Thus, targeting these signaling pathways may eliminate residual cancer stem cells after neoadjuvant chemotherapy and improve cure rates in BLBC.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD08-03.

Targeting GPCR-mediated p70S6K activity may improve head and neck cancer response to cetuximab

PURPOSE

Epidermal growth factor receptor (EGFR) overexpression is correlated with decreased survival in head and neck cancer (HNC) where the addition of EGFR inhibition to standard chemoradiation approaches has improved treatment responses. However, the basis for the limited efficacy of EGFR inhibitors in HNC is incompletely understood. G-protein-coupled receptors (GPCR) have been shown to be overexpressed in HNC where GPCR activation induces HNC growth via both EGFR-dependent and -independent pathways. We hypothesized that targeting GPCR-induced EGFR-independent signaling would improve the efficacy of EGFR inhibition.

EXPERIMENTAL DESIGN

Using a high-throughput phosphoproteome array, we identified proteins that were phosphorylated in HNC cells where EGFR expression was downmodulated by RNA interference (RNAi) in the presence or absence of a GPCR ligand. We confirmed the findings from the array by Western blotting followed by in vitro and in vivo phenotypic assays.

RESULTS

p70S6K phosphorylation was elevated approximately sixfold in EGFR siRNA-transfected cells treated with a GPCR ligand. In addition to RNAi-mediated EGFR downmodulation, GPCR-mediated phosphorylation of p70S6K was modestly increased by EGFR inhibitor cetuximab approved by the Food and Drug Administration. Biopsies from cetuximab-treated patients also displayed increased phospho-p70S6K staining compared with pretreatment biopsies. HNC cells were growth inhibited by both genetic and pharmacologic p70S6K targeting strategies. Furthermore, p70S6K targeting in combination with cetuximab resulted in enhanced antitumor effects in both in vitro and in vivo HNC models.

CONCLUSIONS

These results indicate that increased phosphorylation of p70S6K in cetuximab-treated patients may be due to increased GPCR signaling. Therefore, the addition of p70S6K targeting strategies may improve treatment responses to EGFR inhibition.

Honokiol inhibits epidermal growth factor receptor signaling and enhances the antitumor effects of epidermal growth factor receptor inhibitors

PURPOSE

This study aimed to investigate the utility of honokiol, a naturally occurring compound, in the treatment of head and neck squamous cell carcinoma (HNSCC) as well as its ability to target the epidermal growth factor receptor (EGFR), a critical therapeutic target in HNSCC, and to enhance the effects of other EGFR-targeting therapies.

EXPERIMENTAL DESIGN

Human HNSCC cell lines and the xenograft animal model of HNSCC were used to test the effects of honokiol treatment.

RESULTS

Honokiol was found to inhibit growth in human HNSCC cell lines, with 50% effective concentration (EC(50)) values ranging from 3.3 to 7.4 micromol/L, and to induce apoptosis, as shown through Annexin V staining. These effects were associated with inhibition of EGFR signaling, including downstream inhibition of mitogen-activated protein kinase, Akt, and signal transducer and activator of transcription 3 (STAT3), and expression of STAT3 target genes, Bcl-X(L) and cyclin D1. Furthermore, honokiol enhanced the growth inhibitory and anti-invasion activity of the EGFR-targeting agent erlotinib. Although HNSCC xenograft models did not show significant inhibition of in vivo tumor growth with honokiol treatment alone, the combination of honokiol plus cetuximab, a Food and Drug Administration-approved EGFR inhibitor for this malignancy, significantly enhanced growth inhibition. Finally, HNSCC cells rendered resistant to erlotinib retained sensitivity to the growth inhibitory effects of honokiol.

CONCLUSIONS

These results suggest that honokiol may be an effective therapeutic agent in HNSCC, in which it can augment the effects of EGFR inhibitors and overcome drug resistance.

Crosstalk between G-protein-coupled receptors and epidermal growth factor receptor in cancer

EGFR and its respective ligands are overexpressed in various tumors and this over-expression correlates with poor prognosis in selected cancers. In addition to direct activation by EGFR autocrine ligands, the large family of G-protein-coupled receptors (GPCRs) has been reported to transactivate EGFR via both ligand-dependent and independent mechanisms. GPCRs can induce the cleavage of membrane-bound EGFR-ligand precursors or directly activate the juxtamembrane tyrosine kinase domain of EGFR. Due to the heterogenous expression of GPCRs in tumors, this form of receptor crosstalk may contribute to the modest clinical responses to EGFR-targeted therapies observed to date. Studies, so far, have indicated that the signaling mechanisms involved in transactivation are specifically influenced by the activated GPCR and the tumor type in question. The progression of colon, lung, breast, head and neck, prostate and ovarian cancers have all been reported to be mediated, at least in part, by GPCR-EGFR crosstalk. Increased understanding of the specific signaling pathways involved in EGFR transactivation by GPCR will facilitate the identification of new biomarkers for molecular targeting strategies.

Antitumor mechanisms of combined gastrin-releasing peptide receptor and epidermal growth factor receptor targeting in head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) is characterized by epidermal growth factor receptor (EGFR) overexpression, where EGFR levels correlate with survival. To date, EGFR targeting has shown limited antitumor effects in head and neck cancer when administrated as monotherapy. We previously identified a gastrin-releasing peptide/gastrin-releasing peptide receptor (GRP/GRPR) aurocrine regulatory pathway in HNSCC, where GRP stimulates Src-dependent cleavage of EGFR proligands with subsequent EGFR phosphorylation and mitogen-activated protein kinase (MAPK) activation. To determine whether GRPR targeting can enhance the antitumor efficacy of EGFR inhibition, we investigated the effects of a GRPR antagonist (PD176252) in conjunction with an EGFR tyrosine kinase inhibitor (erlotinib). Combined blockade of GRPR and EGFR pathways significantly inhibited HNSCC, but not immortalized mucosal epithelial cell, proliferation, invasion, and colony formation. In addition, the percentage of apoptotic cells increased upon combined inhibition. The enhanced antitumor efficacy was accompanied by increased expression of cleaved poly(ADP-ribose) polymerase (PARP) and decreased phospho-EGFR, phospho-MAPK, and proliferating cell nuclear antigen (PCNA). Using reverse-phase protein microarray (RPPA), we further detected decreased expression of phospho-c-Jun, phospho-p70S6K, and phospho-p38 with combined targeting. Cumulatively, these results suggest that GRPR targeting can enhance the antitumor effects of EGFR inhibitors in head and neck cancer.

Cross-talk between G Protein–Coupled Receptor and Epidermal Growth Factor Receptor Signaling Pathways Contributes to Growth and Invasion of Head and Neck Squamous Cell Carcinoma

G protein-coupled receptors (GPCR) and the epidermal growth factor receptor (EGFR) are often both overexpressed and contribute to the growth of cancers by activating autocrine pathways. GPCR ligands have been reported to trigger EGFR signaling via receptor cross-talk in cancer cells. Here, we show that GPCR ligands prostaglandin E2 (PGE2) and bradykinin (BK) activate EGFR signaling. Inhibition of EGFR using several strategies, including small-molecule inhibitors and an EGFR-specific antibody, resulted in partial attenuation of signaling downstream of EGFR. PGE2 and BK triggered EGFR signaling by increasing selective autocrine release of transforming growth factor-alpha (TGF-alpha). Inhibition of tumor necrosis factor-alpha-converting enzyme abrogated BK- or PGE2-mediated activation of EGFR signaling. Both PGE2 and BK stimulated head and neck squamous cell carcinoma (HNSCC) invasion via EGFR. Treatment of HNSCC cells with the BK antagonist CU201 resulted in growth inhibition. The combination of CU201 with the EGFR small-molecule inhibitor erlotinib resulted in additive inhibitory effects on HNSCC cell growth in vitro. Inhibition of the PGE2 synthesis pathway with sulindac induced HNSCC cytotoxicity at high doses (EC(50), 620 micromol/L). However, combined inhibition of both EGFR with the tyrosine kinase inhibitor erlotinib and GPCR with sulindac at low doses of 6 and 310 micromol/L, respectively, resulted in synergistic killing of HNSCC tumor cells. Combined blockade of both EGFR and GPCRs may be a rational strategy to treat cancers, including HNSCC that shows cross-talk between GPCR and EGFR signaling pathways.