UVB-induced anti-survival and pro-apoptotic effects on HaCaT human keratinocytes via caspase- and PKC-dependent downregulation of PKB, HIAP-1, Mcl-1, XIAP and ER stress.

Evidence suggests that solar ultraviolet B (UVB) radiation inhibits growth and/or induces apoptosis of human skin cells. However, mechanisms underlying the UVB-induced anti-survival and pro-apoptotic effects on human skin cells remain unclear. In this study, we investigated the effect of UVB radiation on survival and apoptosis of HaCaT human keratinocytes and determined possible molecular, cellular and signaling mechanisms including cross-regulation, which are responsible for the UVB's anti-survival and/or pro-apoptotic effects. The results showed that UVB radiation at 400 mJ/cm² for 8 h largely decreased cell survival and induced DNA fragmentation, an index of apoptosis, in HaCaT human keratinocytes. On a mechanistic level, UVB radiation triggered the activation of caspase-9, cleavage of poly(ADP-ribose) polymerase, and downregulation of myeloid cell leukemia-1 (Mcl-1), human inhibitor of apoptosis protein-1 (HIAP-1), X-linked IAP (XIAP), and protein kinase B (PKB), but did not affect the expression of B-cell lymphoma-2 in HaCaT cells. UVB radiation also upregulated the expression of glucose-regulated protein 78 (GRP78), an endoplasmic reticulum (ER) stress marker, in HaCaT cells. Of note, results of pharmacological inhibition studies have demonstrated that pretreatment with z-VAD-fmk, a pan-caspase inhibitor strongly attenuated UVB-induced apoptosis, the activation of caspase-9, downregulation of Mcl-1, XIAP and PKB (but not HIAP-1), and upregulation of GRP78, while pretreatment with GF109203 or GO6983, pan-PKC inhibitors, substantially blocked the UVB-induced reduction of cell survival, activation of caspase-9, downregulation of HIAP-1, XIAP, and PKB (but not Mcl-1), and GRP78 upregulation in HaCaT cells. Collectively, these results demonstrated that UVB has strong anti-survival and pro-apoptotic effects on HaCaT cells and the effects were largely mediated via the activation of caspase-9 and protein kinase Cs, which subsequently downregulated PKB, XIAP, HIAP-1 and Mcl-1, and triggered ER stress.