Genome-wide CRISPR screening reveals a PKA-driven resistance mechanism to metformin for oral cancer prevention that can be exploited by combination with NSAIDs

Head and neck squamous cell carcinoma (HNSCC) is among the ten most common cancers worldwide and is associated with high morbidity and poor survival. Diminished HNSCC outcomes are often related to delayed diagnosis and treatment of occult progression of premalignant lesions, underscoring the need for effective and low risk chemoprevention strategies. In this regard, metformin has shown promising clinical activity for HNSCC prevention. Here, we performed a genome-wide CRISPR/Cas9 screen of metformin-treated HNSCC cells and identified activation of PKA signaling as the top resistance pathway. We show that metformin mediates PKA activation in HNSCC cells, and that PKA inhibition (PKAi) when combined with metformin treatment synergistically inhibits HNSCC growth. We found that metformin-induced PKA activation is mediated by a prostaglandin E2 (PGE2) autocrine loop, which can be blocked using cyclooxygenase-2 (COX2) inhibitors. Importantly, COX2 inhibition using non-steroidal anti-inflammatory drugs (NSAIDs) combined with metformin treatment synergistically inhibits of HNSCC cell growth and prevents progression of oral premalignant lesions (OPLs) into invasive HNSCC in a model of tobacco driven oral carcinogenesis. Together, these findings demonstrate that metformin and NSAID combination therapy may represent a promising therapeutic strategy for HNSCC chemoprevention.