Rewiring of EGFR oncogenic program by opposing actions of membrane versus soluble CD109 in HNSCC

The epidermal growth factor receptor (EGFR) expression is often dysregulated in head and neck squamous cell carcinoma (HNSCC), driving cancer cell proliferation, invasion, and metastasis through diverse pathways, thereby contributing to aggressive chemo- and radio-therapy resistance. A GPI-anchored protein, CD109 is upregulated in multiple cancers, including HNSCC. While membrane-anchored CD109 (mCD109) is pro-tumorigenic in SCC via EGFR/STAT3 activation, the role of protease-cleaved soluble CD109 (sCD109) is poorly understood. Our groundbreaking findings demonstrate that sCD109 antagonizes EGFR signaling by directly binding to the EGFR extracellular domain, preventing mCD109-EGFR stabilizing interactions on the cell surface, followed by inhibition of EGFR phosphorylation at Y1068 and downstream signaling cascades (AKT, MAPK, and STAT3) consequently suppressing cancer cell migration, invasion, 3D tumor spheroid formation and angiogenic tube formation. In addition, we found that sCD109 regulates EGFR fates by inhibiting nuclear localization of phosphorylated EGFR and promoting EGFR degradation. Additionally, sCD109 significantly reduces EGF-induced expression of cancer stem cell markers (CD44 and CD133) and embryonic stem cell markers (Nanog and Sox2), suggesting a suppressive role in cancer stemness. Taken together, these results underscore the opposing roles of mCD109 and sCD109: with sCD109 acting as an antagonist by inhibiting mCD109/EGFR-driven oncogenic signaling and phenotypes. Our current findings reveal a complex interplay among mCD109, sCD109, and EGFR, identifying a mechanism for targeting EGFRs degradation in HNSCC, and lay the groundwork for future research on investigating sCD109s modulatory role in preclinical models of HNSCC.