The Regulation of Telomerase Reverse Transcriptase (TERT) by CCAAT/Enhancer Binding Protein β (C/EBPβ) During Skeletal Muscle Differentiation

Abstract

Our lab has identified the bZIP transcription factor CCAAT/Enhancer Binding Protein beta (C/EBPβ) as a negative regulator of myogenic differentiation. C/EBPβ is highly expressed in satellite cells and is downregulated during myogenic differentiation, a step that is critical for terminal differentiation, as ectopic C/EBPβ expression blocks this process. Telomerase has been identified as a C/EBPβ target gene in liver and other systems, and has been implicated in the regulation of muscle regenerative responses in models of Duchenne Muscular Dystrophy. Given that C/EBPβ is overexpressed in models of muscle wasting, and high levels of telomerase inhibit differentiation, I hypothesized that C/EBPβ inhibits myogenic differentiation through upregulation of TERT (telomerase reverse transcriptase) expression. I demonstrate that overexpression of C/EBPβ in myoblasts increases mTERT expression under both growth and differentiation conditions. Conversely, loss of C/EBPβ expression in myoblasts using shRNA technology or after isolation of primary myoblasts from conditional knockout mice, results in a downregulation of TERT expression and activity. When TERT was pharmacologically inhibited or knocked down using a shRNA, there was a significant improvement in differentiation and fusion in C2C12 myoblasts overexpressing C/EBPβ as evidenced by an increase in the number of MHC+ fibers and expression of muscle-specific differentiation genes. Interestingly, I found that C/EBPβ and TERT expression were increased in both embryonic and alveolar models of rhabdomyosarcoma. In response to this, a knockdown of C/EBPβ in rhabdomyosarcoma cells decreased TERT expression and activity, and enhanced differentiation but not fusion in a model of embryonic rhabdomyosarcoma. These findings illustrate the novel regulation of TERT in skeletal muscle by C/EBPβ, and reveal C/EBPβ as an attractive therapeutic target for the treatment of muscle diseases such as rhabdomyosarcoma.