Ultraviolet irradiation represses PATCHED gene transcription in human epidermal keratinocytes through an activator protein-1-dependent process

Basal cell carcinoma (BCC) is one of the major types of skin cancer arising from keratinocytes. The SONIC HEDGEHOG pathway is deregulated in 100% of sporadic BCCs, as indicated by the overexpression of PATCHED, whose product encodes the receptor of SONIC HEDGEHOG, in 100% of analyzed BCCs. Reverse transcription-PCR analysis revealed that exposure to UVB irradiation, which is a risk factor known to contribute to BCC development, induces a strong and sharp decrease of PATCHED mRNA level both in vitro and ex vivo. Transcription of a reporter gene driven by the 4.4-kb 5'-regulatory region of the human PATCHED gene was shown to be down-regulated after UVB irradiation. Furthermore. overexpression of c-JUN, a member of the activator protein (AP)-1 family. induced repression of the PATCHED promoter. The role of AP-1 in UVB-induced PATCHED repression was confirmed in mouse embryonic fibroblasts knocked out for c-JUN NH2-terminal protein kinase. This study thus provides the first evidence of UV-induced downregulation at the transcriptional level of the BCC-associated tumor suppressor PATCHED relying on activation of the AP-1 oncogenic pathway.