Sox9 directly promotes Bapx1 gene expression to repress Runx2 in chondrocytes

Abstract

The transcription factor, Sry-related High Mobility Group (HMG) box containing gene 9 (Sox9), plays a critical role in cartilage development by initiating chondrogenesis and preventing the subsequent maturation process called chondrocyte hypertrophy. This suppression mechanism by Sox9 on late-stage chondrogenesis partially results from the inhibition of Runt-related transcription factor 2 (Runx2), the main activator of hypertrophic chondrocyte differentiation. However, the precise mechanism by which Sox9 regulates late chondrogenesis is poorly understood.In the present study, the transcriptional repressor vertebrate homolog of Drosophila bagpipe (Bapx1) was found to be a direct target of Sox9 for repression of Runx2 expression in chondrocytes. We identified a critical Sox9 responsive region in the Bapx1 promoter via a luciferase reporter assay. Analysis by chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that Sox9 physically bound to this region of the Bapx1 promoter. Consistent with the notion that Bapx1 and Sox9 act as negative regulators of chondrocyte hypertrophy by regulating Runx2 expression, transient knockdown of Sox9 or Bapx1 expression by shRNA in chondrocytes increased Runx2 expression, as well as expression of the late chondrogenesis marker, Col10a1. Furthermore, while over-expression of Sox9 decreased Runx2 and Col10a1 expressions, simultaneous transient knockdown of Bapx1 diminished that Sox9 over-expressing effect.Our findings reveal that the molecular pathway modulated by Bapx1 links two major regulators in chondrogenesis, Sox9 and Runx2, to coordinate skeletal formation.

Runx2 deletion in smooth muscle cells inhibits vascular osteochondrogenesis and calcification but not atherosclerotic lesion formation

“…12,27,36,37 Initial chondrocyte differentiation through Sox9, a master chondrogenic transcriptional factor, is absolutely critical and has been shown to act upstream of Runx2, suggesting possible Runx2-independent mechanisms during early-stage chondrogenesis. 38,39 In a Runx2 global knockout mouse model, isolated chondrocytes proliferated normally but were unable to maintain chondrocyte phenotype in culture. 36 Similarly, in our study, we found that Runx2 was not required for SMC-specific chondrocyte differentiation, further supporting a Runx2-independent mechanism for initial chondrocyte commitment.…”

SOX9 determines RUNX2 transactivity by directing intracellular degradation

“…(54) Interestingly, it was reported recently that SOX9 can repress the expression of RUNX2 in chondrocytes indirectly through the vertebrate homologue of Drosophila bagpipe (Bapx1), a transcriptional repressor of RUNX2. (55) This indicates that SOX9 regulated RUNX2 through a variety of mechanisms both transcriptionally and posttranscriptionally. In addition, we found that RUNX2 inhibited the transactivity of SOX9, which is consistent with the finding that SOX9 is expressed in mature osteoblasts (47) and provides a mechanism whereby the expression levels of SOX9 may be maintained but its transactivity repressed by RUNX2 during the process of osteogenic differentiation in MSCs.…”

The skeleton: a multi-functional complex organ. The growth plate chondrocyte and endochondral ossification

“…PTHrP exerts this effect at least in part through inhibition of expression of the transcription factor RUNX2; RUNX2 induces chondrocytic transcription of hypertrophy-associated genes including col10a1 and promotes hypertrophy (Lefebvre & Smits 2005, Guo et al 2006. The suppression of hypertrophy by PTHrP is probably also mediated by SOX9, which is activated by PTHrP through protein kinase A-dependent phosphorylation and indirectly inhibits RUNX2 expression (Huang et al 2000, Akiyama et al 2002, Yamashita et al 2009). PTHrP stimulates cyclin D1 expression in chondrocytes, and is unable to down-regulate RUNX2 expression in chondrocytes from cyclin D1-null mice, apparently because cyclin D1 contributes to proteasomal degradation of RUNX2 (Beier et al 2001, Zhang et al 2009).…”