VITAMIN-D INTERFERES WITH TRANSACTIVATION OF THE GROWTH-HORMONE GENE BY THYROID-HORMONE AND RETINOIC ACID

The thyroid hormone, retinoic acid (RA) and vitamin D regulate gene ex pression by binding to similar receptors which act as ligand-inducible transcription factors, Incubation of pituitary GH4C1 cells with nanom olar concentrations of vitamin D markedly reduces the response of the rat growth hormone mRNA to thyroid hormone triiodothyronine (T3) and R A. The stimulation of growth hormone gene expression by both ligands i s mediated by a common hormone response element (TRE(GH)) present in t he 5'-flanking region of the gene, and the inhibition caused by vitami n D is due to transcriptional interference of the vitamin D receptor o n this DNA element. No inhibition of the basal promoter activity by th e vitamin was observed, The response to T3 and RA of a heterologous pr omoter containing this element, the palindromic T3- and RA-responsive sequence TRE(PAL), or a direct repeat of the same motif is also inhibi ted by vitamin D. In contrast, vitamin D strongly induces the activity of constructs containing a vitamin D response element, and neither T3 nor RA reduces vitamin D-mediated transactivation, Transfection with an expression vector for the retinoid X receptor alpha (RXR alpha) inc reases transactivation by T3 and RA but does not abolish the inhibitio n caused by the vitamin, Gel retardation experiments show that the vit amin D receptor (VDR) as a heterodimer with RXR weakly binds to the T3 - and RA-responsive elements, Additionally, VDR displaces binding of T 3 and RA receptors in a dose-dependent manner, Our data suggest the fo rmation of TR-VDR and RAR-VDR heterodimers which have low affinities f or these response elements and can inhibit binding of more active hete rodimers with RXR. The fact that the same response element mediates op posite effects of at least four different nuclear receptors provides a greater complexity and flexibility of the transcriptional responses t o their ligands.