BMAL1 and CLOCK proteins in regulating UVB‐induced apoptosis and DNA damage responses in human keratinocytes

A diverse array of biological processes are under circadian controls. In mouse skin, ultraviolet ray (UVR)‐induced apoptosis and DNA damage responses are time‐of‐day dependent, which are controlled by core clock proteins. This study investigates the roles of clock proteins in regulating UVB responses in human keratinocytes (HKCs). We found that the messenger RNA expression of brain and muscle ARNT‐like 1 ( BMAL1 ) and circadian locomotor output cycles kaput ( CLOCK ) genes is altered by low doses (5 mJ/cm 2 ) of UVB in the immortalized HaCat HKCs cell line. Although depletion of BMAL1 or CLOCK has no effect on the activation of Rad3‐related protein kinases–checkpoint kinase 1–p53 mediated DNA damage checkpoints, it leads to suppression of UVB‐stimulated apoptotic responses, and downregulation of UVB‐elevated expression of DNA damage marker γ‐H2AX and cell cycle inhibitor p21. Diminished apoptotic responses are also observed in primary HKCs depleted of BMAL1 or CLOCK after UVB irradiation. While CLOCK depletion shows a suppressive effect on UVB‐induced p53 protein accumulation, depletion of either clock gene triggers early keratinocyte differentiation of HKCs at their steady state. These results suggest that UVB‐induced apoptosis and DNA damage responses are controlled by clock proteins, but via different mechanisms in the immortalized human adult low calcium temperature and primary HKCs. Given the implication of UVB in photoaging and photocarcinogenesis, mechanistic elucidation of circadian controls on UVB effects in human skin will be critical and beneficial for prevention and treatment of skin cancers and other skin‐related diseases.