Phosphorylation of IRS4 at YY111/112 promotes LUSC progression via PI3K/AKT pathway activation.

Lung squamous cell carcinoma (LUSC) lacks effective targeted therapies despite frequent hyperactivation of the PI3K/AKT pathway, and the contribution of adaptor proteins to this process remains poorly defined. Here, we identify Insulin Receptor Substrate 4 (IRS4) as a common substrate of Src kinase and the phosphatase RPTPα, and uncover a previously unrecognized, conserved phosphorylation site at YY111/112 within the IRS4 PH domain. Src-mediated phosphorylation at this site is essential for IRS4-driven proliferation, anchorage-independent growth, and invasiveness in LUSC cells. Mechanistically, phosphorylation at YY111/112 induces conformational rearrangements in IRS4 that enhance the recruitment of the p85β SH2 domain to the YXXM motif of IRS4, thereby promoting AKT activation without altering IRS4 protein stability or subcellular distribution. An IRS4-derived cell-permeable phosphopeptide containing pYY111/112 disrupts the Src-IRS4 interaction and significantly suppresses tumor growth in vivo in a YY111/112-dependent manner. Consistently, YY111/112 phosphorylation is markedly elevated in human LUSC tissues and correlates with advanced disease stages. Collectively, these findings define a Src-IRS4-PI3K/AKT signaling axis and identify YY111/112 as a critical regulatory site of IRS4 and a potential therapeutic vulnerability in LUSC.