Retinoids directly activate the collagen X promoter in prehypertrophic chondrocytes through a distal retinoic acid response element

Abstract

Retinoids are essential for the terminal differentiation of chondrocytes during endochondral bone formation. This maturation process is characterized by increased cell size, expression of a unique extracellular matrix protein, collagen X, and eventually by mineralization of the matrix. Retinoids stimulate chondrocyte maturation in cultured cells and experimental animals, as well as in clinical studies of synthetic retinoids; furthermore, retinoid antagonists prevent chondrocyte maturation in vivo. However, the mechanisms by which retinoids regulate this process are poorly understood. We and others showed previously that retinoic acid (RA) stimulates expression of genes encoding bone morphogenetic proteins (BMPs), suggesting that retinoid effects on chondrocyte maturation may be indirect. However, we now show that RA also directly stimulates transcription of the collagen X gene promoter. We have identified three RA response element (RARE) half-sites in the promoter, located 2,600 nucleotides upstream from the transcription start site. These three half-sites function as two overlapping RAREs that share the middle half-site. Ablation of the middle half-site destroys both elements, abolishing RA receptor (RAR) binding and drastically decreasing RA stimulation of transcription. Ablation of each of the other two half-sites destroys only one RARE, resulting in an intermediate level of RAR binding and transcriptional stimulation. These results, together with our previously published data, indicate that retinoids stimulate collagen X transcription both directly, through activation of RARs, and indirectly, through increased BMP production.

Retinoic Acids Potentiate BMP9-Induced Osteogenic Differentiation of Mesenchymal Progenitor Cells

“…Drissi et al showed that Runx2 stimulation by RA is potentiated by BMP2 signaling through interaction with Smad1 on the collagen X promoter in chondrocytes [52]. Consistent with the above observation, it has been recently reported that retinoids directly activate the collagen X promoter in prehypertrophic chondrocytes through a distal retinoic acid response element [50]. In our studies, we have found that BMP9 and RA act together to promote the expansion of hypertrophic chondrocyte zone in perinatal limb culture.…”

“…There have been several conflicting reports about the role of retinoic acids in osteogenic differentiation [47], [48], [49], [50], [51], [52], [53], [54]. Here, we first tested if 9CRA and ATRA can induce osteogenic differentiation in MPCs.…”

“…Nonetheless, our current understanding of the role of RAs in adult stem cells and tissue-specific progenitors is relatively limited. In fact, it remains controversial if RA signaling promotes or inhibits chondrogenic and/or osteogenic differentiation [47], [48], [49], [50], [51], [52], [53], [54].…”

Integration of signaling pathways regulating chondrocyte differentiation during endochondral bone formation

“…We have shown that RA rapidly stimulates endogenous collagen X gene transcription in cultured embryonic chick chondrocytes, as well as a transfected reporter gene driven by a collagen X promoter (Adams et al, 2003; Cohen et al, 2006; Lassová et al, 2006). We identified a compound RARE in a distal region of the promoter that both binds RARs and is essential for RA stimulation.…”

“…Retinoids are likely to lie downstream of thyroid hormone in the regulatory cascade. In embryonic chick cartilages, RARγ is by far the most abundant retinoid receptor, and it is found almost exclusively in the hypertrophic zone (Koyama et al, 1999; Cohen et al, 2006). Furthermore, In mice only the hypertrophic zone contains both active retinoids and RARs (von Schroeder and Heersch, 1998).…”