Abstract 1205: HER3 is a functional molecular target in HPV-associated head and neck cancer
Authors: Brand Toni Michel , Hartmann Stefan , Bhola Neil , Peyser Noah D. , Li Hua , Zeng Yan , Randall Max V. , Bandyopadhyay Sourav , Grandis Jennifer R. 
Published: July 2016

Abstract
Background: Head and neck squamous cell carcinoma (HNSCC) is an aggressive cancer with a rising incidence of more than 600,000 new cases each year. Human papillomavirus (HPV)16 plays an etiologic role in a growing subset of HNSCCs, where viral expression of the E6 and E7 oncoproteins are necessary for tumor growth and maintenance. Although patients with HPV(+) tumors have a more favorable prognosis compared with HPV(-) patients, there are currently no HPV-selective therapies. Recent studies have identified differential receptor tyrosine kinase (RTK) profiles in HPV(+) vs. HPV(-) tumors. One such RTK, HER3, is overexpressed and highly associated with phosphoinositide 3-kinase (PI3K) in HPV(+) tumors.
Methods: In the current study, HER3 was investigated as a molecular target in HPV(+) HNSCC cell lines and patient-derived xenografts (PDXs). HNSCC dependency on HER3 was evaluated with both anti-HER3 siRNAs and the clinically advanced anti-HER3 monoclonal antibody, KTN3379. Furthermore, the anti-growth effects of combinatorial HER3 and PI3K inhibition was examined in HPV(+) HNSCC models.
Results: HER3 was overexpressed and activated in HPV(+) HNSCC models. Further investigation indicated that HPV(+) HNSCC cells were reliant on HER3 for cellular proliferation and PI3K pathway signaling, whereas little effect was observed in HPV(-) cell lines. Intriguingly, HPV oncoproteins directly regulated HER3, where knockdown of E6 and E7 resulted in loss of total HER3 protein expression and reduced PI3K pathway signaling (p-AKT, p-p70S6K, p-rpS6) in HPV(+) cell lines. Conversely, overexpression of E6 and E7 increased total HER3 protein expression (5-7 fold) and PI3K pathway signaling in HPV(-) cell lines. Since HER3 signals via the PI3K pathway to mediate cellular proliferation and survival, we sought to identify if HER3 can mediate response to PI3K pathway inhibitors. Stimulation of HPV(+) cell lines with neuregulin-1, the cognate ligand of HER3, resulted in complete resistance to the p110α inhibitor, BYL719, and prevented BYL719 induced inhibition of AKT. Neuregulin-1 stimulation of HPV(-) cell lines had no effect on their response to BYL719, and did not rescue AKT activation. Furthermore, treatment of HPV(+) cell lines with BYL719 resulted in enhanced HER3 expression and activation (20-50 fold), indicating that HER3 may mediate resistance to PI3K inhibitors in HPV(+) cells. Finally, dual targeting of HER3 and PI3K synergistically inhibited HPV(+) cell line proliferation (by 50-70%) and resulted in sustained knockdown of PI3K pathway signaling. While HPV(-) cell lines were responsive to PI3K inhibition, HER3 blockade did not provide additional benefit. HPV(+) PDX models are currently being evaluated for response to dual kinase inhibition.
Conclusions: These studies identify HER3 as a potential therapeutic target, and provide a rationale for the clinical evaluation of combined HER3 and PI3K inhibition in HPV(+) HNSCCs.
Citation Format: Toni Michel Brand, Stefan Hartmann, Neil Bhola, Noah D. Peyser, Hua Li, Yan Zeng, Max V. Randall, Sourav Bandyopadhyay, Jennifer R. Grandis. HER3 is a functional molecular target in HPV-associated head and neck cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1205.