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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.