Cell cycle genes as targets of retinoid induced ovarian tumor cell growth suppression

We have examined the effect of all-trans-retinoic acid (RA) on cell cycle gene expression in RA sensitive CAOV3 and RA resistant SK-OV3 ovarian carcinoma cell lines. Gene expression was analysed by multiprobe RNAse protection, Western blotting and in vitro kinase assays. No differences were observed between RA sensitive and RA resistant ovarian carcinoma cells in the levels of expression of many cell cycle genes including cyclin A, B and E, cdk 2,4 and 6, E2F-1, E2F-2, E2F-3, E2F-4, E2F-5, DP-1 and DP-2. However, RA sensitive CA-OV3 cells expressed higher levels of p53, p27, p21, and p16 compared to RA resistant SK-OV3 cells. In addition, RA treatment of CA-OV3 cells resulted in a significant decrease in hyperphosphorylated RB and RB-2/p130 and corresponding significant increases in the levels of hypophosphorylated and/or partially phosphorylated RB-2/p130 protein and hypophosphorylated RB. Also, RA treatment increased expression of the cdk inhibitor p27 and decreased activity of cdk 2, cdk 4 and cdk 6. Finally, amounts of p27-cyclin E and RB-2/p130-E2F4 complexes were found to increase in CA-OV3 cells growth arrested by RA. These results suggest that the pocket protein pathways are critical targets for retinoid suppression of ovarian carcinoma cell growth.