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Mol. Cell. Biol., 01 1996, 318-327, Vol 16, No. 1
P Garcia-Villalba, AM Jimenez-Lara and A Aranda
The thyroid hormone, retinoic acid (RA), and vitamin D regulate gene
expression by binding to similar receptors which act as ligand- inducible
transcription factors. Incubation of pituitary GH4C1 cells with nanomolar
concentrations of vitamin D markedly reduces the response of the rat growth
hormone mRNA to thyroid hormone triiodothyronine (T3) and RA. The
stimulation of growth hormone gene expression by both ligands is mediated
by a common hormone response element (TREGH) present in the 5'-flanking
region of the gene, and the inhibition caused by vitamin D is due to
transcriptional interference of the vitamin D receptor on this DNA element.
No inhibition of the basal promoter activity by the vitamin was observed.
The response to T3 and RA of a heterologous promoter containing this
element, the palindromic T3- and RA-responsive sequence TREPAL, or a direct
repeat of the same motif is also inhibited 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) increases transactivation by T3 and RA but does
not abolish the inhibition caused by the vitamin. Gel retardation
experiments show that the vitamin D receptor (VDR) as a heterodimer with
RXR weakly binds to the T3- and RA-responsive elements. Additionally, VDR
displaces binding of T3 and RA receptors in a dose-dependent manner. Our
data suggest the formation of TR-VDR and RAR-VDR heterodimers with RXR. The
fact that the same response element mediates opposite effects of at least
four different nuclear receptors provides a greater complexity and
flexibility of the transcriptional responses to their ligands.
Copyright © 1996, American Society for Microbiology
Vitamin D interferes with transactivation of the growth hormone gene by thyroid hormone and retinoic acid
Instituto de Investigaciones Biomedicas, Consejo Superior de Investigaciones Cientificas, Madrid, Spain.
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