Differential response of normal and HPV immortalized ectocervical epithelial cells to B[a]P

Cigarette smoking has been established as a risk factor for the development of cervical cancer. Polycyclic aromatic hydrocarbons such as benzo[a]pyrene (B[a]P), which are present in cigarette smoke, might account for this increased risk. The effects of B[a]P on cell growth, aryl hydrocarbon hydroxylase, DNA adducts and p53 levels was measured in cervical cells. Since 90% of cervical preneoplastic lesions are positive for the human papillomavirus (HPV) we compared the effects of these chemicals in normal ectocervical epithelial cells (ECE) and human papillomavirus 16 (HPV16) immortalized ectocervical epithelial cells (ECE16-1). Exposure of normal ECE and HPV immortalized ECE16-1 cells to B[a]P inhibited cell proliferation. Inhibition occurred at 20-fold lower concentrations in the normal ECE cells compared to ECE16-1 cells. The proliferation of cervical cells which express mutated p53 was unaffected by B[a]P. Neither cervical stromal cells nor endometrial stromal cells were affected by these compounds. The effects of B[a]P on normal ECE cell proliferation correlated with increased terminal differentiation as measured by increased envelope formation. In contrast, B[a]P exposure did not induce envelope formation in immortalized ECE16-1 cells or in cervical tumor cells. Pretreatment of both ECE and ECE16-1 cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin, which induces P450 expression and activity, did not alter B[a]P metabolism in either normal or immortalized cells. Furthermore, equivalent levels of DNA adducts were formed by B[a]P in ECE and ECE16-1 cells. Neither the extent of adduct formation nor the rate of their removal differed in normal and immortalized cervical cells. Therefore, the diminished growth inhibition of the ECE16-1 cells as compared to normal ECE cells by B[a]P is not due to changes in cytochrome P450 of the 1A family metabolism or DNA adduct number. Furthermore, analysis of the p53 levels in both normal and ECE16-1 cells revealed that p53 levels are higher in normal versus immortalized ectocervical cells, and p53 is induced in both cell types following B[a]P treatment. Thus reduced p53 levels in ECE16-1 cells may contribute to a lack of growth suppression following B[a]P treatment. These results demonstrate that HPV16 immortalization diminishes ectocervical epithelial cell responsiveness to toxicant damage (i.e. decreased cell proliferation and increased terminal differentiation). As a result, ECE16-1 cells that sustain genotoxic damage which leads to DNA adduct formation continue to proliferate and may be at increased risk for mutations and further progression towards a fully transformed phenotype.