THE EFFECT OF ALL-TRANS AND 9-CIS RETINOIC ACID ON THE STEADY-STATE LEVEL OF HPV16 E6 E7 NRNA AND CELL-CYCLE IN CERVICAL-CARCINOMA CELLS/

Retinoids, including natural vitamin A and its analogs, have been clos ely studied as chemopreventive drugs. The mechanism of action of retin oids, however, is not completely understood. Our study evaluated the e ffects of all-trans (high affinity ligand for both RAR and RXR recepto rs) and 9-cis retinoic acid (binds only with RXR receptors) on E6-E7 t ranscription, cell proliferation, cell cycle distribution, and p53 exp ression in CaSki cells, a cell line derived from cervical carcinoma co ntaining 600 copies of the HPV-16 genome. Using quantitative RT-PCR an alysis; we found that CaSki cells treated with all trans retinoic acid (ATRA) for seven days had a remarkably low level of E6-E7 transcriptio n at 10(-5) M to 10(-9) M concentrations. A smaller inhibitory effect was observed on the E6-E7 transcription at a concentration of 10(-5) M with only 9-cis retinoic acid. Flow cytometric analysis revealed that cells treated with both all trans and 9-cis RA showed an increase in the mean percentage (93.5% and 86.1% respectively) of cells in the GI phase as compared to untreated CaSki cells (55%) and normal keratinocy tes (58%). The percentage of cells in the S phase decreased from a mea n percentage of 28 and 26.5 to 5.8 and 5, respectively, after treatmen t with all trans retinoic acid and 9-cis retinoic acid. An increase in the level of immunophenotypic expression of wild type p53 was also no ted after treatment with all trans retinoic acid and 9-cis retinoic ac id. AU.trans and 9-cis retinoic acid may act on highly proliferating t umor cells by initially arresting DNA synthesis and inducing G1 arrest . In addition, they may be inducing a p53 dependent cell cycle arrest and thus suggests that all-trans and 9-cis retinoic acid may have a cy tostatic effect rather than a cytotoxic effect on CaSki cells. The inc reased expression of p53 positive cells and the inhibition of E6/E7 tr anscription after treatment with these retinoids may indicate the pote ntial role of all trans and 9-cis retinoic acid as a cell cycle regula tor and an antiviral chemoprevention agent.