Mdm2 Promotes Myogenesis through the Ubiquitination and Degradation of CCAAT/Enhancer-binding Protein β

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Myogenesis is a tightly regulated differentiation process during which precursor cells express in a coordinated fashion the myogenic regulatory factors, while down-regulating the satellite cell marker Pax7. CCAAT/Enhancer-binding protein β (C/EBPβ) is also expressed in satellite cells and acts to maintain the undifferentiated state by stimulating Pax7 expression and by triggering a decrease in MyoD protein expression. Herein, we show that C/EBPβ protein is rapidly down-regulated upon induction of myogenesis and this is not due to changes in Cebpb mRNA expression. Rather, loss of C/EBPβ protein is accompanied by an increase in Mdm2 expression, an E3 ubiquitin ligase. We demonstrate that Mdm2 interacts with, ubiquitinates and targets C/EBPβ for degradation by the 26 S proteasome, leading to increased MyoD expression. Knockdown of Mdm2 expression in myoblasts using a shRNA resulted in high C/EBPβ levels and a blockade of myogenesis, indicating that Mdm2 is necessary for myogenic differentiation. Primary myoblasts expressing the shMdm2 construct were unable to contribute to muscle regeneration when grafted into cardiotoxin-injured muscle. The differentiation defect imposed by loss of Mdm2 could be partially rescued by loss of C/EBPβ, suggesting that the regulation of C/EBPβ turnover is a major role for Mdm2 in myoblasts. Taken together, we provide evidence that Mdm2 regulates entry into myogenesis by targeting C/EBPβ for degradation by the 26 S proteasome.