Involvement of all-trans-retinoic acid in the breakdown of retinoic acid receptors alpha and gamma through proteasomes in MCF-7 human breast cancer cells
T. Tanaka et al., Involvement of all-trans-retinoic acid in the breakdown of retinoic acid receptors alpha and gamma through proteasomes in MCF-7 human breast cancer cells, BIOCH PHARM, 61(11), 2001, pp. 1347-1355
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 eff
ect of RA on RAR protein levels in MCF-7 human breast cancer cells. Incubat
ion of these cells with 10(-6) M RA induced a rapid breakdown of both RAR a
lpha and RAR gamma 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 RARa: and ubiquitin in a c
oncentration and time-dependent manner, suggesting the involvement of ubiqu
itin and proteasome in this reaction. Retinoid X receptor cu (RXR alpha) wa
s also decreased, albeit to a lesser extent, in RA-treated cells. Use of sy
nthetic 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 mobili
ty shift assay, using nuclear extracts from RA-treated cells, showed that a
reduction in complex formation with hormone response elements correlated w
ith the reduction of RAR and RXR protein. These data suggest that RA induce
s the breakdown of RARs through a process involving ubiquitination and that
this phenomenon causes a reduction in the formation of DNA-receptor comple
xes. (C) 2001 Elsevier Science Inc. All rights reserved.