WOX1 is essential for UVB irradiation-induced apoptosis and down-regulated via translational blockade in UVB-induced cutaneous squamous cell carcinoma in vivo

Purpose: We investigated the role of candidate tumor suppressor and proapoptotic WOX1 (also named WWOX, FOR, or WWOXv1) in UVB-induced apoptosis and formation of cutaneous squamous cell carcinomas (SCC). Experimental Design: Expression of WOX1 and family proteins (WWOX) in human primary cutaneous SCCs was examined by immunohistochemistry, in situ hybridization, and reverse transcription-PCR. UVB irradiation - induced WOX1 activation (Tyr³³ phosphorylation and nuclear translocation), apoptosis, and cutaneous SCC formation were examined both in vitro and in vivo. Results: Dp-regulation of human WOX1, isoform WOX2, and Tyr³³ phosphorylation occurred during normal keratinocyte differentiation before cornification and death. Interestingly, significant reduction of these proteins and Tyr³³ phosphorylation was observed in nonmetastatic and metastatic cutaneous SCCs (P < 0.001), but without down-regulation of WWOX mRNA (P > 0.05 versus normal controls), indicating a translational blockade of WWOX mRNA to protein. During acute exposure of hairless mice to UVB, WOX1 was up-regulated and activated in epidermal cells in 24 hours. In parallel with the clinical findings in humans, chronic UVB-treated mice developed cutaneous SCCs in 3 months, with significant reduction of WOX1 and Tyr³³ phosphorylation and, again, without down-regulation of WWOX mRNA. Human SCC-25 and HaCaT cells were transfected with small interfering RNA - targeting WOX1 and shown to resist UVB-induced WOX1 expression, activation, and apoptosis. Conclusions: WOX1 is essential for UVB-induced apoptosis and likely to be involved in the terminal differentiation of normal keratinocytes. During UVB-induced cutaneous SCC, epidermal cells have apparently prevented the apoptotic pressure from overexpressed WOX1 by shutting down the translation machinery for WWOX mRNA.