Regulation of cell growth and HPV genes by exogenous estrogen in cervical cancer cells

Human papillomavirus (HPV) infection is known as the major cause of the development of cervical cancer. The E6 and E7 proteins of oncogenic HPV can play critical roles in immortalization and malignant transformation of cervical epithelial cells. From the previous epidemiologic data, it has been determined that long-term use of oral contraceptives may be a risk factor for cervical cancer. Investigation of the estrogenic and antiestrogenic effects on the proliferation of cervical cancer cells and the gene expression of HPV would help to explain the role of estrogen in the HPV-associated pathogenesis of cervical cancer. In this study, cervical cancer cells (HeLa, CaSki, and C33A) were cultured in vitro in the presence of 17 beta-estradiol or tamoxifen to observe their regulatory growth effect and HPV E6/E7 gene expression. The estrogenic effect on the promoter activity of HPV URR was further confirmed by transient transfection assay, which was conducted in C33A cells using the HPV-18 URR-CAT reporter plasmid. The supplemental effect of estrogen receptors on URR promoter activity was also evaluated. The proliferation of HeLa and CaSki cells was stimulated by estradiol at physiologic concentration levels (less than or equal to 1 x 10(-6) M). At a low concentration (0.1 x 10(-6) M), tamoxifen also stimulated the proliferation of HeLa and CaSki cells. In contrast to HPV-positive cervical cells, the proliferation of C33A was not influenced by exogenous estradiol or tamoxifen, indicating that HPV might play a role in the hormonal stimulation of cell growth. Interestingly, the proliferation of HeLa was markedly suppressed at high concentrations of estradiol and tamoxifen (5 and 10 x 10(-6) M). The levels of HPV-18 E6 and E7 mRNA were significantly increased by estradiol at a concentration of 0.5 x 10(-6) M. Transient transfection experiments using the HPV URR-CAT reporter plasmid in C33A cells indicated that the expression of HPV E6/E7 genes was increased by the treatment of estradiol and tamoxifen. Co-transfection of estrogen receptors (ER) and URR-CAT leads to a fourfold increase in CAT activity by estradiol or tamoxifen at physiologic concentrations. When estradiol or tamoxifen was administered at high concentrations (5 x 10(-6) M), a DNA ladder, typically indicative of apoptosis, was observed in HeLa cells. In conclusion, estradiol stimulated the growth of HPV-positive cervical cancer cells, as did tamoxifen at low concentrations (0.1 x 10(-6) M). The growth stimulation of HPV-positive cervical cancer cells by estrogen appeared to be related to the increased expression of HPV E6/E7. Growth suppression observed at high concentrations of estradiol and tamoxifen in HeLa cells might be a result of apoptosis. Taken together, these data suggested that exogenous estradiol might be a risk factor in HPV-mediated cervical carcinogenesis.