TRANSCRIPTIONAL REGULATION BY TRANSFORMING GROWTH-FACTOR-BETA OF THE EXPRESSION OF RETINOIC ACID AND RETINOID-X RECEPTOR GENES IN OSTEOBLASTIC CELLS IS MEDIATED THROUGH AP-1

We now report that transforming growth factor pi (TGF-PI), a potent re gulatory cytokine of bone remodeling, is a powerful stimulator for gen e expression of retinoic acid receptors (RARs) and retinoid X receptor s (RXRs) in osteoblastic MC3T3-E1 cells. TGF-beta 1 transcriptionally stimulated the expression of RAR alpha, RAR gamma, and RXR alpha genes , but did not do so for RAR beta, RXR beta, and RXR gamma genes. We al so observed that AP-1, a transcriptional factor, plays an important ro le in the signal pathway for expression of RAR alpha, RAR gamma, and R XR alpha genes stimulated by TGF-beta 1 because stimulation of the exp ression of these genes in the cytokine-treated cells was markedly inhi bited by a mixture of antisense c-fos and c-jun. A gel mobility shift assay demonstrated that TGF-beta 1 is able to increase, in a dose-depe ndent manner, the binding of nuclear proteins to direct repeat 5, a co nsensus sequence with high affinity for RAR-RXR heterodimers. The mobi lity shift assay, using specific antibody for each receptor, showed th at direct repeat B-binding proteins may be RAR and RXR isoforms. The s timulated binding to direct repeat 5 was inhibited strongly by H-7, an inhibitor of serine/threonine kinase, and by curcumin, an inhibitor o f AP-1, The present study suggests a novel pathway for TGF-beta 1 acti on in osteoblastic cells via stimulation of RAR-RXR transcriptional ac tivity in a ligand-dependent fashion.