Abstract
We now report that transforming growth factor beta1 (TGF-beta1), a potent regulatory cytokine of bone remodeling, is a powerful stimulator for gene expression of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in osteoblastic MC3T3-E1 cells. TGF-beta1 transcriptionally stimulated the expression of RARalpha, RARgamma, and RXRalpha genes, but did not do so for RARbeta, RXRbeta, and RXRgamma genes. We also observed that AP-1, a transcriptional factor, plays an important role in the signal pathway for expression of RARalpha, RARgamma, and RXRalpha genes stimulated by TGF-beta1 because stimulation of the expression of these genes in the cytokine-treated cells was markedly inhibited by a mixture of antisense c-fos and c-jun. A gel mobility shift assay demonstrated that TGF-beta1 is able to increase, in a dose-dependent manner, the binding of nuclear proteins to direct repeat 5, a consensus sequence with high affinity for RAR-RXR heterodimers. The mobility shift assay, using specific antibody for each receptor, showed that direct repeat 5-binding proteins may be RAR and RXR isoforms. The stimulated binding to direct repeat 5 was inhibited strongly by H-7, an inhibitor of serine/threonine kinase, and by curcumin, an inhibitor of AP-1. The present study suggests a novel pathway for TGF-beta1 action in osteoblastic cells via stimulation of RAR-RXR transcriptional activity in a ligand-dependent fashion.
Highlights
Bone remodeling is regulated by a network of local and systemic cytokines and hormones [1,2,3,4]
We report that transforming growth factor 1 (TGF-1), a potent regulatory cytokine of bone remodeling, is a powerful stimulator for gene expression of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in osteoblastic MC3T3-E1 cells
We observed that AP-1, a transcriptional factor, plays an important role in the signal pathway for expression of RAR␣, RAR␥, and RXR␣ genes stimulated by TGF-1 because stimulation of the expression of these genes in the cytokine-treated cells was markedly inhibited by a mixture of antisense c-fos and c-jun
Summary
Bone remodeling is regulated by a network of local and systemic cytokines and hormones [1,2,3,4]. TGF-1 is produced by osteoblastic cells in bone tissues, and this factor accumulates abundantly in bone matrix tissues This cytokine may play a central role in the cytokine network of bone remodeling. Many studies [5,6,7,8,9,10,11,12] have demonstrated a functional role for this cytokine in the regulation of the proliferation and differentiation of osteoblastic and osteoclastic cells. We show here that TGF-1 regulates positively, at the transcriptional level, RAR␣, RAR␥, and RXR␣ genes These results suggest that TGF-1 may effect positive regulation of RA in osteoblasts via the stimulation of its nuclear receptors
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