We have identified a novel steroid hormone response element in the avian  3 integrin promoter. This sequence, comprising three hexameric direct repeat halfsites separated by nine and three nucleotides binds vitamin D receptor (VDR)-retinoid X receptor (RXR) and retinoic acid receptor (RAR)-RXR heterodimers. VDR-RXR binds direct repeats separated by three base pairs, and RAR-RXR recognizes half-sites separated by nine bases, whereas the central half-site interacts with both heterodimers. Retinoic acid and 1,25-dihydroxyvitamin D 3 activate both a genomic fragment including the transcriptional start site and an oligonucleotide containing the three repeats, linked to a heterologous promoter. Co-addition of the steroids produces neither synergy nor an additive effect; rather the result equals that for retinoic acid alone. Scatchard analysis demonstrates that RAR-RXR has greater affinity than VDR-RXR for the composite element. Based on these findings we propose a model in which there is specific, polarity-defined binding of VDR-RXR and RAR-RXR to three half-sites, which form two overlapping steroid response elements, with the central half-site common to both. Our results identify a novel mechanism by which one steroid hormone can modulate the activity of a second, by competing for a shared half-site in a composite response element.Integrins are membrane receptors recognizing specific motifs in matrix proteins and/or cell-associated molecules (1, 2). These heterodimeric molecules play an important role in cell attachment and signal transduction, being responsible for changes in a number of intracellular parameters, all of which share the ability to alter cytoskeletal formation and cell function, (reviewed in Refs. 3-5).The integrin ␣ v  3 plays a central role in osteoclast function. Proteins such as echistatin or kistrin, containing the ␣ v  3 binding motif, Arg-Gly-Asp (RGD), block bone resorption in vivo and in vitro (6 -11), and we have recently identified an RGD peptide mimetic that arrests bone resorption and osteoporosis in the oophorectomized rat.1 Furthermore, we find a ␣ v  3 -blocking monoclonal antibody blunts the capacity of chicken osteoclasts to resorb bone and attach to osteopontin (13), an RGD-containing bone matrix protein (2,14).Little is known concerning regulation of this integrin during formation of osteoclasts from their bone marrow-derived precursors. We demonstrated recently that treatment of avian osteoclast precursors with the osteoclastogenic hormone 1,25-dihydroxyvitamin D 3 (D 3 ) 2 leads to increased expression of ␣ v  3 on the precursor cells (15).Steroid hormones modulate gene expression via intracellular receptors binding as dimers to cognate response elements, comprising direct or inverted repeats or palindromes (16 -19). In the case of the vitamin D 3 and retinoic acid receptors (VDR and RAR), the transactivating complex also typically contains the retinoid X receptor (RXR) (20, 21), although complexes comprising other combinations of receptors, including VDR-VDR, VDR-RAR, and V...
1,25-Dihydroxyvitamin D3 (1,25(OH)2D3)transactivates the avian beta 3 integrin gene whose promoter contains at least two vitamin D response elements, one of which is in close proximity to a candidate AP1 site (TGACTCA). Since fos/jun and steroid hormones interact to regulate gene expression, we asked whether phorbol-12-myristate-13-acetate (PMA), which stimulates binding of fos/jun to AP1 sites, transactivates the avian beta 3 integrin gene and, if so, does the phorbol ester modulate 1,25(OH)2D3 induction of the gene. We find the candidate AP1 sequence comigrates with the consensus AP1 sequence on electromobility shift assay when incubated with recombinant c-jun protein. Furthermore, PMA prompts expression of beta 3 integrin mRNA in the avian monocytic line, HD11. The increase in message reflects transactivation of the beta 3 gene and is mirrored by plasma membrane appearance of the integrin heterodimer alpha v beta 3. Moreover, attesting to the functional significance of PMA-enhanced alpha v beta 3 expression, cells treated with concentrations of the phorbol ester that induce the beta 3 gene, spread extensively on plastic, an event blocked by an anti-alpha v antibody and a peptide mimetic known to inhibit alpha v beta 3-mediated cell attachment. Interestingly, co-addition of 1,25(OH)2D3 and PMA prompts greater expression of alpha v beta 3 than when the cells are exposed to either agent alone and PMA enhances 1,25(OH)2D3-induced beta 3 integrin mRNA expression. Thus, PMA and 1,25(OH)2D3 impact on the avian beta 3 integrin gene independently and in combination.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.