Effect of UV-irradiation on cell cycle, viability and the expression of p53, gadd153 and gadd45 genes in normal and HPV-immortalized human oral keratinocytes.

We previously demonstrated neoplastic conversion of HPV-immortalized human oral keratinocytes by exposing cells to chemical carcinogens, but failed to transform normal human oral keratinocytes with same chemical carcinogens in vitro. Though the reason for different responses of normal and HPV-immortalized oral keratinocytes to chemical carcinogens remains speculative, the difference may be due to the capacity of normal cells and incapacity of HPV-immortalized cells for repairing damaged DNA induced by carcinogens. Since (1) the repair of damaged DNA takes place in G1/G2 phases of cell cycle, (2) wild type p53 plays major role in the induction of transient G1 and/or G2 arrests, and (3) the expression of gadd45 and gadd153 is also associated with the cell cycle arrest and DNA damage, we investigated transient cell cycle arrest and the expression of p53, gadd45 and gadd153 in normal human oral keratinocytes, HPV-immortalized oral keratinocytes, and an oral cancer cell line expressing mutant p53 after exposing cells to UV light. Normal cells demonstrated transient G1 arrest after exposure to UV light, but other tested cells did not. While UV-irradiation significantly increased the level of intranuclear wild type p53 protein in normal cells, it did not alter p53 protein levels in HPV-immortalized and oral cancer cells. The level of gadd45 transcripts was enhanced in all tested cells, but normal cells demonstrated higher increase in the level of gadd45 after UV-exposure compared to other tested cells. The level of gadd153 gene transcripts was only increased in normal oral keratinocytes after UV-irradiation. These data indicate that UV-induced transient G1 arrest in normal oral keratinocytes may be associated with both enhanced levels of intranuclear wild type p53 protein and gadd45 and gadd153 transcripts.