HPV-16 oncogenes E6 and E7 are mutagenic in normal human oral keratinocytes

The mutation frequency of pS189 shuttle vector plasmids is higher in human oral keratinocytes (NHOK) immortalized with cloned human papillomavirus-16 (HPV-16) genome than in primary normal NHOK (NHOK). To determine whether oncoproteins E6 and E7 of HPV-16 are responsible for the higher mutation frequency of the plasmids, we measured the mutation frequency in NHOK and in NHOK expressing the HPV-16 oncogenes (E6, E7, or E6 plus E7). We also measured the mutation frequency in NHOK expressing the E6 or E7 proteins of the non-oncogenic HPV-6b. The mutation frequency, either background or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced, in NHOK expressing the HPV-16 oncoproteins (E6, E7, or E6 plus E7) was significantly higher than in NHOK. The HPV-16 oncogenes did not alter the nature of the MNNG-induced mutations (G:C-->A:T), but increased the frequency of deletions and insertions with or without MNNG. The background or MNNG-induced mutation frequency in NHOK expressing the HPV-6b E6 or E7 proteins was the same as in NHOK. NHOK and NHOK expressing HPV6b-E6 or E7 were able to arrest the cell cycle and enhance cellular p53, p21(WAF1/CIP1), and Gadd45 levels when exposed to MNNG, whereas NHOK expressing the HPV-16 E6 oncogene did not demonstrate. NHOK expressing HPV-16 E7 were able to enhance cellular p53, p21(WAF1/CIP1), and Gadd45 levels, but failed to arrest cell cycle progression when exposed to MNNG. These data indicate that HPV-16 E6 and E7 oncogenes are mutagenic in human oral keratinocytes and enhance the mutagenic effect of MNNG. However, the E6 and E7 proteins of the 'low risk' HPV-6b did not demonstrate such an ability.