Abstract 29: Mechanistic link between phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) activity and PDL1 expression in head and neck squamous cell carcinoma (HNSCC)

Introduction: HNSCC is an immunosuppressive disease, with multiple functional and quantitative defects contributing to immune evasion and tumor escape. One of the identified areas for therapy development is the Programmed Death-1 (PD-1)/Programmed Death Ligand 1 (PD-L1) pathway, as this pathway has been hypothesized to allow cancer cell evasion of the immune system by promoting T cell anergy and apoptosis. Pembrolizumab, a PD-L1 inhibitor, has recently been approved for treatment in recurrent/metastatic head and neck squamous cell carcinoma. As regulation of PD-L1 expression could play an important role in the effectiveness of therapy, we further explored the regulation of PD-L1 expression by HNSCC cells. Specifically, we targeted our investigation by evaluating the effects on expression of one of the most frequently mutated genes in HNSCC, PiK3CA. We evaluated this with and without interferon-γ which has previously been shown to affect PD-L1 expression, possibly through activation of the STAT1 pathway. Methods: HPV+ and HPV- HNSCC cell lines were grown in cell culture and treated with selective and non-selective PiK3Ca inhibitors, in combination with interferon-γ. After 72 hours, cells were harvested and flow cytometry was used to measure the expression of PD-L1 expression. Protein expression of various pathway intermediaries were evaluated via Western blot to better delineate the mechanism of PD-L1 upregulation. Results: Treatment with selective PiK3Ca inhibitors in combination with interferon-γ in several cell lines was noted to statistically significantly increase expression of PD-L1, beyond treatment with interferon-γ alone. Maintenance of STAT1 phosphorylation correlated with upregulation of PD-L1 expression, while total STAT1 expression remained stable. The majority of the cell lines which were able to maintain STAT1 phosphorylation were HPV+, but a few HPV- cell lines were noted to also maintain this phosphorylation with a correlating upregulation in PDL1 expression. Additionally, mutation in PiK3Ca despite HPV-negative status was noted to maintain phosphorylation of STAT1 with upregulation of PD-L1 expression. Conclusions: Pik3Ca inhibition in combination with interferon-γ was noted to significantly upregulate PD-L1 expression in several cell lines, suggesting a possible synergistic effect of PiK3Ca inhibition with interferon-γ. This was noted to correlate with maintenance of phosphorylation of STAT1, suggesting a pivotal link between PIK3CA signaling, STAT1 activity and PDL1 expression in PIK3CA aberrant HNSCC. Citation Format: Rebecca C. Hoesli, Nicole L. Michmerhuizen, Vivek Nair, Chloe Matovina, Elizabeth Leonard, Mark EP Prince, Matthew E. Spector, J. Chad Brenner. Mechanistic link between phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) activity and PDL1 expression in head and neck squamous cell carcinoma (HNSCC) [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 29.