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

Kim, Chan Joo; Um, Soo Jong; Park, Tae Chul; Park, Jong Sup; Lee, Jin Woo; Lee, Jun Mo; Kim, Seung Jo; Namkoong, Sung Eun

Abstract

BACKGROUNDS: Human papillomavirus (HPV) infection is known as the major causative phenomenon in the development of cervical cancer. E6 and E7 proteins of oncogenic HPV types can play critical roles in immortalization and malignant transformation of cervical epithelial cells. From the previous epidemiologic data, long term use of oral contraceptives may be one of the risk factor for cervical cancer. PURPOSE: Investigation of estrogenic and anti-estrogenic effects on the proliferation of cervical cancer cells and gene expression of HPV under the regulation of HPV upstream regulatory region (URR) would help to explain the role of estradiol in HPV-associated pathogenesis of cervical cancer.
METHODS
Cervical cancer cells (HeLa, CaSki and C33A) were cultured in vitro in the presence of 17 beta-estradiol or tamoxifen and the numbers of cells were directly counted to observe the growth stimulatory or suppressive effect of the treatment. The correlation between the growth regulatory effect and HPV E6/E7 gene expression was determined by reverse transcription-polymerase chain reaction (RT-PCR). The estrogenic effect on the promoter activity of HPV URR was further confirmed by transient co-transfection assays, which were conducted in C33A cells using the HPV-18 URR-CAT reporter plasmid. Supplemental effect of estrogen receptor on the URR promoter activity was also evaluated. To analyze the growth suppressive function at the higher concentration of estradiol or tamoxifen in HeLa cells, DNA fragmentation assay was performed.
RESULTS
The proliferation of HeLa and CaSki cells was stimulated by estradiol at the concentration of physiological level (< or =1 X 10-6M), reaching maximal growth at 0.5 X 10-6M. At concentration of 0.1 X 10-6M, tamoxifen also stimulated the proliferation of HeLa and CaSki cells. In contrast to HPV-positive cervical cells, C33A cells were not influenced to cell proliferation by addition of estradiol at the physiological level, indicating that HPV might play role in growth stimulatory effect of estrogen or tamoxifen. Interestingly, the proliferation of HeLa cells was totally suppressed by estradiol and tamoxifen at the higher concentration (5 and 10 X 10-6M), whereas those of CaSki and C33A cellswere not responded and little suppressed at the concentration, respectively. The levels of HPV-18 E6 and E7 mRNA were significantly increased after treatment of 0.5 X 10-6M estradiol as determined by RT-PCR. Furthermore, transient transfection experiments using the URR-CAT reporter plasmid indicated that the increased expression of HPV E6/E7 genes was related with the growth stimulatory effect of estradiol and tamoxifen. In addition, co-transfection of estrogen receptor (ER) leads to an over 4-fold increase in CAT activity after treatment of estradiol or tamoxifen with 0.5 X 10-6M. When estradiol or tamoxifen was treated at the concentration over 5 X 10-6M for 96 hr, a typical DNA ladder, a indicative of apoptosis, was observed in HeLa cells. However, DNA ladder was not detected in C33A cells of which growth was some suppressed under same concentration of estradiol.
CONCLUSION
At the physiological levels, estradiol stimulated the growth of HPV-positive cervical cancer cells and tamoxifen also did at the concentration of 0.1 X 10-6M. The increased expression of HPV E6/E7 at the physiologic levels appeared to be related with the growth stimulation of HPV-positive cervical cancer cells. Growth suppression observed at the higher concentration (5 and 10 X 10-6M) might be a indicative of apoptosis shown by DNA fragmentation assay in HeLa cells. Taken together, these data suggested that the concentration of estradiol (< or =1 X 10-6M) could be a risk-factor in HPV-mediated cerivcal carcinogenesis.