Abstract 3712: 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 cells to evade the immune system by promoting T cell anergy and apoptosis. Pembrolizumab, a PD-L1 inhibitor, has recently been approved for the treatment of recurrent/metastatic HNSCC. 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 in 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 PI3K inhibitors, in combination with interferon-γ. After 72 hours, cells were harvested and flow cytometry was used to measure the expression of PD-L1. Protein expression of various pathway intermediaries was evaluated via western blot to better delineate the mechanism of PD-L1 upregulation. Results: Treatment with selective PI3K inhibitors in combination with interferon-γ in several cell lines significantly increased expression of PD-L1, beyond the increase noted after 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 maintaining STAT1 phosphorylation were HPV+, but a few HPV- cell lines also maintained this phosphorylation with a correlating upregulation in PD-L1 expression. Additionally, mutation in PIK3Ca despite HPV- status was noted to maintain phosphorylation of STAT1 with upregulation of PD-L1 expression. Conclusions: Treatment with PI3K inhibitors in combination with interferon-γ significantly upregulated PD-L1 expression in several cell lines, suggesting a possible synergistic effect. Since PD-L1 expression correlated with maintenance of phosphorylation of STAT1, these results suggest a pivotal link between PIK3CA signaling, STAT1 activity and PD-L1 expression in PIK3CA aberrant HNSCC. Citation Format: Rebecca C. Hoesli, Nicole L. Michmerhuizen, Vivek Nair, Chloe Matovina, Elizabeth Leonard, Matthew E. Spector, Carol R. Bradford, Mark E. Prince, Andrew C. Birkeland, 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 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 3712. doi:10.1158/1538-7445.AM2017-3712