Role of HPV in tumorigenesis of oral keratinocytes

HPV-immortalized human oral keratinocytes can convert to tumorigenic cells by chemical carcinogens while normal human keratinocytes cannot. The different responses of these cells to chemical carcinogens may be due to their different genomic stability. Since the genomic stability of cells is closely associated with cell cycle control, we correlated cell cycle progression with the cellular levels or activities of key G(1) phase cell cycle regulatory proteins, p21(WAF/CIP1), cyclins (A, D1 and E), cdks (cdk2 and cdk4), gadd45 and PCNA proteins in normal and HPV-immortalized oral keratinocytes before and after treatment with a genotoxic agent, actinomycin D. Actinomycin D treatment in normal cells cause (i) an inhibition in the activity of cdk2 kinase, (ii) an increase in the level of WAF1/CIP1 mRNA and p21(WAF1/CIP1), (iii) an increase in the binding of the p21(WAF1/CIP1) protein with the cdk2 protein, (iv) a reduction in the transcription of cyclin A messages, (v) an increase in the p53 inducible gadd45 protein, and (vi) a significant decrease in the synthesis and level of the DNA replication protein, PCNA. However, actinomycin D failed to induce these changes in the HPV-immortalized cells. These results are consistent with the hypothesis that in HPV-immortalized oral keratinocytes, the key cell cycle regulatory components of the G(1) phase are significantly altered thereby making them prone to mutagenic damage when exposed to DNA damaging agents.