Expression, protein stability and transcriptional activity of retinoic acid receptors are affected by microtubules interfering agents and all-trans-retinoic acid in primary rat hepatocytes.

Cellular signaling by glucocorticoid receptor and aryl hydrocarbon receptor is restricted by microtubules interfering agents (MIAs). This leads to down-regulation of drug metabolizing enzymes and drug interactions. Here we investigated the effects of all-trans-retinoic acid (ATRA) and MIAs, i.e. colchicine, nocodazole and taxol on the regulation of retinoic acid receptor (RAR) genes in primary cultures of rat hepatocytes. ATRA (1microM) down-regulated RARalpha and RARgamma mRNAs (decrease 23% and 41%, respectively) whereas it up-regulated RARbeta mRNA (4.3-fold induction). All MIAs diminished the expression of RARs in dose-dependent manner; the potency of MIAs increased in order NOC<COL<TAX and the extent of inhibition increased in order RARalpha<RARgamma<RARbeta. The levels of RARalpha protein were decreased by both MIAs and ATRA. The effects of ATRA were reversed by proteasome inhibitor MG-132, implying ligand-dependent RARalpha degradation. In contrast, the effects of MIAs were proteasome-independent and decrease in RARalpha protein content was due to RARalpha gene down-regulation. We monitored transcriptional activity of RARalpha. For this purpose, we measured catalytic activity of trans-glutaminase-target gene of RARalpha. trans-Glutaminase activity was increased by ATRA (1.23-fold increase) and decreased by colchicine (decrease 51%). Co-treatment with proteasome inhibitor MG-132 partly reversed inhibitory effect of colchicine, and it further augmented the increase of trans-glutaminase activity by ATRA. We have also observed decrease of RARalpha protein level and inhibition of RARs mRNAs expression in HeLa cells by MIAs. In conclusion, our data indicate that microtubules play the role in regulation of RARs activity and expression. Our data are the first report on the effects of ATRA and MIAs on RARs regulation in quiescent cells.