116. Resistance to retinoic acid and nuclear receptors for retinoic acid in human renal cell carcinoma and breast carcinoma ceil lines: possible role as tumor suppressor genes in kidney and breast cancer

Abstract

Transcriptional cross-talk exists between the estrogen receptor (ERα) and retinoic acid receptor (RAR) pathways in human breast cancer cells. We have previously shown that re-expression of ERα in ER-negative cells stimulates the transcriptional and growth inhibitory effects of all-trans-retinoic acid (tRA) by a mechanism that is independent of the ER ligands estradiol and tamoxifen. In this study, we generated cell lines stably expressing ERα-deletion mutants to elucidate the mechanism whereby ERα modulates RAR transcriptional activity. Using RT-PCR and RNAse protection assays, we observed that expression of ERα suppresses basal expression of the RA-responsive gene RARβ2, while allowing it to be strongly induced by tRA. Repression of basal RARβ2 transcription was confirmed by transient expression of the reporter plasmid βRE-tk-CAT, containing the RARβ2 promoter. In the ERα-negative cells, on the other hand, transcription was only weakly induced by RA. We further determined that this effect of ERα on RARβ induction required the N-terminal AF-1-containing region, including the DNA-binding domain, but was independent of the C-terminal ligand-binding domain. Consistent with these results, the ER agonist estradiol and the AF-2 antagonist 4-hydroxytamoxifen had no significant effect on βRARE activity. Conversely, the full ER antagonist ICI 182,780, which blocks ERα AF-1 activity, was able to completely relieve repression of basal βRARE activity. The effect of ERα is specific for RAR-mediated transcription and does not occur on promoters containing typical response elements for the Vitamin D or thyroid hormone receptors. Moreover, the cross-talk between ERα and RAR does not seem to be mediated by sequestration of a number of common co-regulators, suggesting a novel mechanism whereby the N-terminal region of ERα modulates the transcriptional activity of RAR.