CD9 downregulation activates EGFR/ERK/WAVE2 pathway to remodel F-actin and promote proliferation and migration of cholesteatoma epithelial cells.

Although overproliferation and high migration of keratinocytes are the key pathological features of middle ear cholesteatoma (MEC), the underlying mechanism remains unclear. We found low CD9 and high ADAM17 expression in MEC specimens, alone with their colocalization, and elevated EGF levels. To investigate the role of CD9 in MEC, we created stable CD9-knockdown keratinocyte cell lines. Results showed enhanced proliferation and migration in CD9-knockdown cells, which were inhibited upon restoring CD9. Exogenous EGF reversed the inhibition. Increased ADAM17 sheddase activity in CD9-knockdown cells promoted the release of EGF, activating the EGFR/ERK/WAVE2 pathway. The colocalization of p-ERK and WAVE2 activated the Arp2/3 complex, promoting F-actin polymerization and pseudopodia formation. Similar findings were observed in MEC specimens. Overall, we confirmed that the reduction of CD9 promotes the proliferation and migration of MEC keratinocytes through F-actin remodeling. Therefore, EGFR or Arp2/3 inhibitors could have therapeutic potential in MEC.