Accumulation of retinoid X receptor-alpha in uterine leiomyomas is associated with a delayed ligand-dependent proteasome-mediated degradation and an alteration of its transcriptional activity.

An alteration of the retinoid pathway can influence the development of uterine leiomyomas in animal models, and retinoids have shown efficacy in inhibiting the growth of this benign tumor both in vitro and in vivo. However, the underlying mechanisms and biological implications are unclear. The present study was based on the demonstration of an accumulation of full-length retinoid X receptor alpha (RXRalpha) in leiomyomas that was not associated with a modification of its gene expression. This accumulation was shown to increase the transcription of the RXR-responsive gene cellular retinoic acid binding protein II (CRABP-II) and to be linked to the cellular redistribution of the receptor and to its retarded degradation via the ubiquitin/proteasome pathway. Accordingly, treatment with a specific proteasome inhibitor but not with protease inhibitors strongly inhibited the degradation of full-length RXRalpha in cells deriving from both myometrium and leiomyoma, but the formation of RXRalpha/ubiquitin conjugates was differentially regulated between the two cell types. Moreover, full-length RXRalpha accumulated in leiomyomas was abnormally phosphorylated at serine/threonine residues relative to myometrial tissue. The ligand to RXRalpha, 9-cis-retinoic acid, induced the receptor breakdown in smooth muscle cells deriving from both normal and tumor tissue, whereas a MAPK-specific inhibitor was able to reduce RXRalpha levels only in leiomyoma cells. These results suggest that switching of the ubiquitin/proteasome-dependent degradation of RXRalpha by phosphorylation in leiomyomas may be responsible for the accumulation of the receptor and the consequent dysregulation of retinoic acid target genes. The ability of retinoids to modify this molecular alteration may be the rationale for their use in the treatment of leiomyomas.