C/EBPα and C/EBPβ Have Unique Functions during Early Myogenesis

Abstract

The CCAAT/enhancer binding proteins (C/EBP) comprise a family of transcription factors that regulate many different cellular processes. Little is known of the function of C/EBPα and C/EBPβ during the early stages of myogenesis. Myogenesis occurs in two distinct phases; the embryonic phase and the fetal phase. The embryonic phase gives rise to the primary musculature and serves as a scaffold for subsequent myogenesis. The fetal phase occurs after primary muscle has formed and yields secondary muscle fibers which comprise the bulk of the musculature at birth. Cellular processes can be differentially regulated between embryonic myogenesis and fetal myogenesis. Myoblasts isolated from embryonic day 4 and day 13 of chick development allow examination of embryonic and fetal myogenic cell cultures. We have found that C/EBPα and C/EBPβ are present in both embryonic and fetal myoblasts and myotubes. Overexpression of C/EBPα repressed the promoter activity of the slow myosin heavy chain 2 (MyHC2) gene in embryonic myotubes. However, overexpression of C/EBPβ did not repress slow MyHC2 promoter activity in the same cells. Deletion analysis of the slow MyHC2 promoter-luciferase reporter demonstrated that the repression occurs within an upstream region relative to the transcription start site of the slow MyHC2 gene. Electromobility shift assays determined that C/EBPα can bind to a non-canonical C/EBP binding site located within the upstream region of the transcription start site. These findings suggest that C/EBPα and C/EBPβ have overlapping expression patterns but unique functions within myogenic stages of development.