CASK Positively Regulates Keratinocyte Proliferation and Modulates the EGFR-ERK-p21 Signaling Axis

CASK is a multidomain scaffold protein with diverse cellular functions, but its role in keratinocytes remains unclear. In this study, we established a CASK-silenced human HaCaT keratinocyte model to investigate its biological function. We found that CASK was localized to the plasma membrane, cytosol, and nucleus, and its silencing suppressed keratinocyte proliferation and induced G1-phase arrest. RNA sequencing revealed that CASK depletion attenuates proliferation-associated transcriptional programs. Consistent with this, CASK silencing increased p53 and p21 expression while reducing cyclin B and phosphorylated Rb levels, with elevated p53 and p21 mRNA in shCASK cells. However, CASK silencing-associated p21 upregulation is not majorly dependent on p53 activity. Mechanistically, CASK silencing enhanced serum-induced activation of EGFR and downstream ERK signaling, which were accompanied by increased expression of the EGFR ligands amphiregulin, TGF-α, and epiregulin, indicating augmented autocrine/paracrine EGFR signaling. Pharmacological inhibition studies demonstrated that activation of the EGFR–ERK signaling pathway is essential for p21 upregulation upon CASK silencing. Promoter reporter analyses further revealed that CASK deficiency enhances p21 transcription. Collectively, these findings reveal a previously unrecognized role of CASK in regulating keratinocyte growth and cell-cycle progression. CASK promotes keratinocyte proliferation by restraining autocrine/paracrine EGFR–ERK signaling and suppressing p21 expression, thereby contributing to epidermal homeostasis and growth control.