Involvement of all-trans-retinoic acid in the breakdown of retinoic acid receptors α and γ through proteasomes in MCF-7 human breast cancer cells

Most studies have reported an up-regulation of retinoic acid receptor (RAR) mRNA expression by all-trans retinoic acid (RA). We aimed to study the effect of RA on RAR protein levels in MCF-7 human breast cancer cells. Incubation of these cells with 10^-6 M RA induced a rapid breakdown of both RARα and RARγ in spite of the accumulation of their mRNAs. Proteasome specific inhibitors blocked the RA-induced breakdown of RARs. Furthermore, RA enhanced the formation of the complex between RARα and ubiquitin in a concentration- and time-dependent manner, suggesting the involvement of ubiquitin and proteasome in this reaction. Retinoid X receptor α (RXRα) was also decreased, albeit to a lesser extent, in RA-treated cells. Use of synthetic receptor agonists and antagonists clearly showed that the effect of the retinoid on the breakdown of the retinoid receptors is receptor-ligand agonist-dependent and blunted by the antagonist. An electrophoretic mobility shift assay, using nuclear extracts from RA-treated cells, showed that a reduction in complex formation with hormone response elements correlated with the reduction of RAR and RXR protein. These data suggest that RA induces the breakdown of RARs through a process involving ubiquitination and that this phenomenon causes a reduction in the formation of DNA-receptor complexes.