Role of microRNA-dependent PTEN downregulation in the resistance to PI3K inhibitor alpelisib in head & neck squamous cell carcinoma

Head and neck squamous cell carcinomas (HNSCCs) frequently harbor alterations in the PI3K signaling axis and, particularly, in the PIK3CA gene. The promising rationale of using PI3K inhibitors for the treatment of HNSCC has, however, clashed with a spontaneous development of resistance over time, observed both in PDX models and cell lines. Aimed at identifying valuable targets to circumvent acquired resistance to the PI3Kα inhibitor alpelisib in HNSCC herein we performed microRNA profiling in a cohort of HNSCC PDXs, treated or not with alpelisib, including both responding and resistant tumors. We highlighted that microRNAs specifically altered in resistant PDXs include members of miR-17-92 cluster, encoded by MIR17HG. Mechanistically, we observed that hyperactive c-Myc protein is recruited on MIR17HG regulatory regions in alpelisib-resistant cells, causing the sustained expression of miR-17-5p, miR-19b-3p and miR-20a-5p which, in turn, downregulate the expression of tumor suppressor PTEN. Of note, either knocking out (KO) or silencing PTEN confers resistance to treatment with alpelisib in HNSCC cells. By phosphoproteomic profiling we identified a panel of PTEN-dependent phosphorylation events involved in alpelisib resistance, such as p-Plk1-T210. Interestingly, targeting of Plk1 through rigosertib treatment strongly reduced the viability of alpelisib-resistant cells. In aggregate, this study reveals that post-transcriptional non-genetic events contribute to the downregulation of PTEN in HNSCC during treatment with PI3K inhibitors thus favoring the onset of resistance. We propose the blocking of PTEN-dependent targets as a powerful strategy for the treatment of alpelisib-resistant HNSCCs.