RETINOIC ACID-MEDIATED REPRESSION OF HUMAN PAPILLOMAVIRUS-18 TRANSCRIPTION AND DIFFERENT LIGAND REGULATION OF THE RETINOIC ACID RECEPTOR BETA-GENE IN NONTUMORIGENIC AND TUMORIGENIC HELA HYBRID-CELLS

Human papillomavirus type 18 (HPV18) belongs to the group of genital papillomaviruses involved in the development of cervical carcinomas. Since retinoic acid (RA) is a key regulator of epithelial cell differentiation and a growth inhibitor in vitro of HPV18-positive HeLa cervical carcinoma cells, we have used HeLa and HeLa hybrid cells in order to analyse the effects of RA on expression of the HPV18 E6 and E7 oncogenes and of the cellular RA receptor genes RAR-beta and -gamma. We show here that RA down-regulates HPV18 mRNA levels apparently due to transcriptional repression. Transient cotransfection assays indicated that RARs negatively regulate the HPV18 upstream regulatory region and that the central enhancer can confer RA-dependent repression on a heterologous promoter. RA treatment resulted in induction of RAR-beta-mRNA levels in non-tumorigenic HeLa hybrid cells, but not in tumorigenic hybrid segregants nor in HeLa cels. No alterations of the RAR-beta-gene or of the HeLa RAR-beta-promoter could be revealed by Southern and DNA sequence analysis, respectively. As determined by transient transfection assays, however, the RAR-beta-control region was activated by RA more strongly in non-tumorigenic hybrid cells than in HeLa cells, thus indicating differences in trans-acting regulatory factors. Our data suggest that the RARs are potential negative regulators of HPV18 E6 and E7 gene expression, and that dysregulation of the RAR-beta-gene either causatively contributes to or is an indicator of tumorigenicity in HeLa and HeLa hybrid cells.