Ultraviolet B irradiation-induced G2 cell cycle arrest in human keratinocytes by inhibitory phosphorylation of the cdc2 cell cycle kinase.

In response to genotoxic stress, cell cycle progression can be arrested at certain checkpoints which serve to maintain genomic integrity. We have investigated the mechanism of ultraviolet B (UVB) irradiation-induced cell cycle arrest in normal human keratinocytes and in the HaCaT keratinocyte cell line which carries mutant p53 tumour suppressor protein. While only normal keratinocytes showed a delay in G1 following sublethal UVB irradiation both cell types exhibited prolonged G2 arrest attributable to rapid inhibition of cyclin B-associated cdc2 kinase activity. This inhibition coincided with increased tyrosine phosphorylation of cdc2 and was reversed by the cdc25C phosphatase in vitro. The data indicate that UVB-induced G2 arrest in mammalian cells is mediated by inhibitory tyrosine phosphorylation of cdc2 and acts as a defense mechanism against DNA damage irrespective of the cells' p53 status.