Retinoic acid suppresses human papillomavirus type 16 (HPV16)-mediated transformation of human keratinocytes and inhibits the expression of the HPV16 oncogenes.

We have used a model system of normal HKc and HKc immortalized by transfection with HPV16 DNA (HKc/HPV16) to investigate the effect of RA on the growth of HKc/HPV16 and the expression of the HPV16 oncogenes E6 and E7. These studies found that HKc/HPV16 are about 100-fold more sensitive than normal HKc to growth inhibition by RA in both clonal and mass culture growth assays. The precursor to RA, retinol, was also found to be a more potent inhibitor of growth of HKc/HPV16 than normal HKc while beta-carotene did not inhibit growth of either normal HKc or HKc/HPV16. No differences were observed in the rate of uptake of [3H]RA or [3H]retinol between normal HKc and HKc/HPV16. Northern blot analysis of mRNA extracted from HKc/HPV16 cultured in the absence or in the presence of 10(-7) M RA showed that the expression of the HPV16 oncogenes E6 and E7 as well as the early ORFs E2 and E5 is substantially reduced following RA treatment. In addition, protein levels of E6 and E7, as measured by immunofluorescence (E6 and E7) and Western blot (E7) are also decreased by RA treatment of HKc/HPV16. Since E6 and E7 are considered the oncogenes of HPV16, we explored the possibility that RA may interfere with HPV16-mediated immortalization of HKc. The RA treatment (1 nM) of normal HKc, during or immediately following transfection with HPV16 DNA, inhibited immortalization by about 95%. Overall, these results provide a direct biochemical basis for a role of dietary retinoids in the chemoprevention of HPV-induced cancers.