HPV-16 ONCOGENES E6 AND E7 ARE MUTAGENIC IN NORMAL HUMAN ORAL KERATINOCYTES

The mutation frequency of pS189 shuttle vector plasmids is higher in h uman oral keratinocytes (NHOK) immortalized with cloned human papillom avirus-16 (HPV-16) genome than in primary normal NHOK OVHOK), To deter mine whether oncoproteins E6 and E7 of HPV-16 are responsible for the higher mutation frequency of the plasmids, we measured the mutation fr equency in NHOK and in NHOK expressing the HPV-16 oncogenes (E6, E7, o r E6 plus E7). We also measured the mutation frequency in NHOK express ing the E6 or E7 proteins of the non-oncogenic HPV-6b, The mutation fr equency, either background or N-methyl-M-nitro-N-nitrosoguanidine (MNN G)-induced, in NHOK expressing the HPV-16 oncoproteins (E6, E7, or E6 plus E7) was significantly higher than in NHOK, The HPV-16 oncogenes d id not alter the nature of the MNNG-induced mutations (G:C-->A:T), but increased the frequency of deletions and insertions with or without M NNG, The background or MNNG-induced mutation frequency in NHOK express ing the HPV-6b E6 or E7 proteins was the same as in NHOK, NHOK and NHO K expressing HPV6b-E6 or E7 were able to arrest the cell cycle and enh ance cellular p53, p21(WAF1/ClP1) and Gadd45 levels when exposed to MN NG, whereas NHOK expressing the HPV-16 E6 oncogene did not demonstrate , NHOK expressing HPV-16 E7 were able to enhance cellular p53, p21(WAF 1/ClP1), and Gadd45 levels, but failed to arrest cell cycle progressio n when exposed to MNNG, These data indicate that HPV-16 E6 and E7 onco genes are mutagenic in human oral keratinocytes and enhance the mutage nic effect of MNNG, However, the E6 and E7 proteins of the 'low risk' HPV-6b did not demonstrate such an ability.