CCAAT/Enhancer Binding Protein Beta is Expressed in Satellite Cells and Controls Myogenesis

Abstract

Upon injury, muscle satellite cells become activated and produce skeletal muscle precursors that engage in myogenesis. We demonstrate that the transcription factor CCAAT/ enhancer binding protein beta (C/EBPb) is expressed in the satellite cells of healthy muscle. C/EBPb expression is regulated during myogenesis such that C/EBPb is rapidly and massively downregulated upon induction to differentiate. Furthermore, persistent expression of C/EBPb in myoblasts potently inhibits differentiation at least in part through the inhibition of MyoD protein function and stability. As a consequence, myogenic factor expression, myosin heavy chain expression, and fusogenic activity were reduced in C/EBPboverexpressing cells. Using knockout models, we demonstrate that loss of Cebpb expression in satellite cells results in precocious differentiation of myoblasts in growth conditions and greater cell fusion upon differentiation. In vivo, loss of Cebpb expression in satellite cells resulted in larger muscle fiber cross-sectional area and improved repair after muscle injury. Our results support the notion that C/EBPb inhibits myogenic differentiation and that its levels must be reduced to allow for activation of MyoD target genes and the progression of differentiation.

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

“…In addition to the myogenic regulatory factors, the transcription factor CCAAT/Enhancer-binding protein ␤ (C/EBP␤) 2 is implicated in the regulation of adult myogenesis (6). C/EBP␤ is localized to Pax7ϩ satellite cells in healthy muscle, and its expression is rapidly down-regulated in parallel with Pax7 as differentiation progresses (6).…”

“…C/EBP␤ is localized to Pax7ϩ satellite cells in healthy muscle, and its expression is rapidly down-regulated in parallel with Pax7 as differentiation progresses (6). Persistent C/EBP␤ expression blocks myogenesis, through suppression of MyoD protein expression and stimulation of Pax7 expression, and its loss promotes precocious differentiation even under growth conditions, as well as larger myofibers and enhanced repair after a single injury in vivo (6).…”

“…The shortest isoform (Liver Inhibitory Protein, LIP) lacks activation domains and acts as a dominant negative (13,14). In normal skeletal muscle and SCs from young mice, only the LAP*/LAP isoforms are detected in Pax7ϩ cells, and are decreased with differentiation (6).…”

Protein Inhibitor of Activated STAT 1 (PIAS1) Is Identified as the SUMO E3 Ligase of CCAAT/Enhancer-Binding Protein β (C/EBPβ) during Adipogenesis

“…C/EBP␤, a basic leucine zipper (bZIP) transcriptional factor, is involved in many differentiation processes (9)(10)(11)(12). C/EBP␤ plays a role as a major regulator of mesenchymal stem cell fate by acting as an activator of adipogenesis (9,10) and a repressor of osteoblastogenesis (11) and myogenesis (12).…”

“…C/EBP␤ plays a role as a major regulator of mesenchymal stem cell fate by acting as an activator of adipogenesis (9,10) and a repressor of osteoblastogenesis (11) and myogenesis (12). Our previous investigations have shown that C/EBP␤ is rapidly expressed in the 3T3-L1 preadipocyte differentiation program and is maintained at a high level during the early stage of differentiation (6) and that upon sequential phosphorylation by mitogen-activated protein kinase (MAPK), cyclin-dependent kinase 2 (CDK2), and glycogen synthase kinase 3␤ (GSK3␤) (13,14), C/EBP␤ acquires DNA-binding activity, activating the expression of C/EBP␣ and PPAR␥ as well as cell cycle genes essential for mitotic clonal expansion (MCE), a necessary step for terminal adipocyte differentiation (15)(16)(17).…”