Retinoid X Receptor Acts as a Hormone Receptor in Vivo to Induce a Key Metabolic Enzyme for 1,25-Dihydroxyvitamin D3

Abstract

We demonstrate here that RNA levels of 25-hydroxyvitamin D 3 -24-hydroxylase (24-(OH)ase), a key catabolic enzyme for 1,25-dihydroxyvitamin D 3 , are increased by a highly selective retinoid X receptor (RXR) ligand, LG100268, in mice within hours. Correspondingly, upon LG100268 treatment, kidney 24-(OH)ase enzymatic activity increases 5-10-fold. The endogenous retinoid hormones, all-trans-retinoic acid and 9-cis-retinoic acid, and the synthetic retinoic acid receptor-selective compound, TTNPB, also stimulate 24-(OH)ase. Additionally, we show that LG100268 stimulates transcription of a luciferase reporter plasmid driven by 24-(OH)ase promoter sequences in the presence of RXR in CV-1 cell cotransactivation assays. This first demonstration of a gene that is regulated in the intact animal through an RXR-mediated pathway confirms earlier hypotheses that RXR is a bona fide hormone receptor. Regulation of a key gene in the vitamin D signaling pathway by a retinoid transducer may provide a molecular basis for some of the documented biological effects of vitamin A on bone and vitamin D metabolism. Retinoic acid receptors (RARs)1 have been shown to act as hormone receptors by virtue of the fact that they bind all-transretinoic acid (tRA) with high affinity and activate target genes in the presence of tRA in cell-based cotransactivation assays and in vivo (reviewed in Ref. 1). Retinoid X receptors (RXRs) are also thought to act as hormone receptors (reviewed in Ref. 1) since they bind 9-cis-retinoic acid (9cRA) with high affinity (2, 3) and have been shown to stimulate transcription from distinct elements known as RXR-responsive elements in promoter sequences of the apolipoprotein A1 (apoA1) and cellular retinol-binding protein II (CRBPII) genes in cell-based cotransactivation assays in response to 9cRA (2, 3), tRA (4, 5), or RXR-selective ligands (6, 7). However, these genes have not been shown to be regulated by retinoids in vivo. While another gene product (growth hormone) has been shown to increase in cultured cells treated with RXR-and RAR-selective ligands (8), there are currently no known biological target genes of RXR in the intact animal.RXRs are thought to form homodimers upon interaction with ligand (1) to activate putative RXR-selective genes, such as CRBPII and apoA1, through RXR-responsive elements in the promoters of those genes (2-7). RXR is also known to participate in heterodimers with a number of other intracellular receptors, including the vitamin D receptor (VDR), RAR, or thyroid hormone receptor to activate a variety of target genes, which are stimulated by 1,25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 ), tRA, or thyroid hormone, respectively (1). RXR is thought to act as a silent partner in these interactions, although the effects of liganding of RXR under these circumstances have not been thoroughly explored. Additionally, recent studies describe instances of stimulation of gene transcription by hormone-occupied RXR interacting with a supposedly unoccupied orphan receptor (9, 10).To study the consequen...

Inhibition of lung carcinogenesis by 1α,25‐dihydroxyvitamin D3 and 9‐cis retinoic acid in the A/J mouse model: Evidence of retinoid mitigation of vitamin D toxicity

“…Allegretto et al were able to show that a synthetic rexinoid (RXR-selective agonist) could stimulate transcription from CYP24 promoter sequences in vitro when cotransfected with RXRg or RARg. 54 While interestingly, this does not adequately explain the retinoid antagonism of vitamin D effects observed in this study or in the recent work by Rohde and DeLuca. 49 Further research needs to be done to confirm results and hypotheses generated from in vitro work with retinoids and vitamin D, and to test their validity in complex living systems.…”

“…The retinoid modulation of vitamin D metabolism observed by Trechsel and Fleisch was in a kidney cell culture model 53 and, to our knowledge, has not been confirmed in vivo. The subtle changes in CYP24 expression (3.1-fold induction with 10 mg/kg 9cRA and 2.1-fold induction with 1 mg/ kg 9cRA) reported by Allegretto et al were measured using Northern blotting analysis in the kidneys of BALB/c strain mice, 54 while our study utilizes the more sensitive tool of real-time PCR analysis of mRNA expression in A/J mice. Allegretto et al were able to show that a synthetic rexinoid (RXR-selective agonist) could stimulate transcription from CYP24 promoter sequences in vitro when cotransfected with RXRg or RARg.…”

Retinoic acid cross-talk with calcitriol activity in Atlantic salmon (Salmo salar)

“…In our study, a persistently elevated plasma RA level in Atlantic salmon decreased plasma CTR levels by at least 80%. All-trans RA and 9-cis RA are potent stimulators of Cyp24 expression in vivo in mice (Allegretto et al 1995). A similar mechanism in salmon could explain the decrease in plasma CTR levels at day 5 and 7 in RA-treated fish.…”

“…RA was found to stimulate Vdr expression in both tumour and non-tumour bone cells of rats (Petkovich et al 1986) and mouse (Suzuki et al 1993). RA also increases the expression of the VD catabolising enzyme 25(OH)D 3 -24-hydroxylase (Cyp24a1; Allegretto et al 1995), and synergistic effects were shown between RA and CTR in their effect on rat renal CYP24A1 activity (Reinhardt et al 1999). Frankel et al (1986) found that chronically VA-overdosed rats had increased osteoclast activity, reduced osteoid formation and reduced circulating levels of 25(OH)D 3 , and that VD-overdosed animals lowered their serum Ca and serum 25(OH)D 3 after a moderate dose of VA.…”

The Vitamin D Response Element-binding Protein

“…In some experiments transfected yeast (Saccharomyces cerevisiae) extract enriched in human VDR (28) and human RXR (29) were added to the reaction mixture, either in the presence or absence of primate cell nuclear extract. The double-strand oligonucleotides used as specific competitors in EMSAs were prepared by annealing complementary sequence.…”