SOX9 determines RUNX2 transactivity by directing intracellular degradation

Abstract

Mesenchymal stem cell differentiation is controlled by the cooperative activity of a network of signaling mechanisms. Among these, RUNX2 and SOX9 are the major transcription factors for osteogenesis and chondrogenesis, respectively. Their expression is overlapped both temporally and spatially during embryogenesis. Here we have demonstrated that RUNX2 and SOX9 physically interact in intact cells and have confirmed that SOX9 can inhibit the transactivation of RUNX2. In addition, RUNX2 exerts reciprocal inhibition on SOX9 transactivity. In analyses of the mechanism by which SOX9 regulated RUNX2 function, we demonstrated that SOX9 induced a dosedependent degradation of RUNX2. Although RUNX2 is normally degraded by the ubiquitin-proteasome pathway, we found that SOX9-mediated degradation was proteasome-independent but phosphorylation-dependent and required the presence of the RUNX2 Cterminal domain, which contains a nuclear matrix targeting sequence (NMTS). Furthermore, SOX9 was able to decrease the level of ubiquitinated RUNX2 and direct RUNX2 to the lysosome for degradation. SOX9 also preferentially directed b-catenin, an intracellular mediator of canonical Wnt signaling, for lysosomal breakdown. Consequently, the mechanisms by which SOX9 regulates RUNX2 function may underlie broader signaling pathways that can influence osteochondrogenesis and mesenchymal fate. ß

CCAAT/Enhancer-binding Protein β Regulates the Repression of Type II Collagen Expression during the Differentiation from Proliferative to Hypertrophic Chondrocytes

“…One possible mechanism is that SOX9 protein is degraded posttranscriptionally. In fact, it was reported that ubiquitination of SOX9 protein determined its transcriptional activity (14,32). A significant decrease of SOX9 nuclear protein resulted in downregulation of Col2a1 (Fig.…”

“…Although SOX9 is indispensable for chondrogenesis, it was also reported to work as a negative regulator of hypertrophic differentiation (9). Several studies have reported that Winglesstype murine mammary tumor virus integration site (10), bone morphogenetic protein 2 (BMP2) (11), parathyroid hormonerelated protein (PTHrP) (12), IL-1␤ (13), and Runt-related transcription factor 2 (RUNX2) (14) can regulate SOX9 expression or activity in arthritic chondrocytes or during chondrocyte differentiation.…”

Twist1 mediates repression of chondrogenesis by β-catenin to promote cranial bone progenitor specification

“…4). Sox9 is the central regulator of chondrogenesis, but it also triggers Runx2 degradation and negatively regulates Runx2 transcription (Cheng and Genever, 2010;Yamashita et al, 2009). Further, Sox9 overexpression is sufficient at high levels to inhibit bone formation in vivo (Eames et al, 2004;Zhou et al, 2006).…”

AP-1 family members act with Sox9 to promote chondrocyte hypertrophy

“…In the absence of Sox9, chondrocytes switch fate generating ectopic osteoblasts (Dy et al, 2012). Runx2, a key transcriptional regulator promoting hypertrophic chondrocyte development, is expressed in mitotic and postmitotic chondrocytes of the growth plate where Runx2 has been proposed to counter Sox9's transactivation of early chondrocyte targets such as Col2a1 (Cheng and Genever, 2010;Zhou et al, 2006). Sox9 has also been reported to interact with Mef2 to activate Col10a1, a matrix-encoding gene specifically expressed by hypertrophic chondrocytes (Dy et al, 2012).…”