Positive and negative regulation of myogenic differentiation of C2C12 cells by isoforms of the multiple homeodomain zinc finger transcription factor ATBF1

The ATBF1 gene encodes two protein isoforms, the 404-kDa ATBF1-A, possessin g four homeodomains and 23 zinc fingers, and the 306-kDa ATBF1-B, lacking a 920-amino acid N-terminal region of ATBF1-A which contains 5 zinc fingers. ln vitro, ATBF1-A was expressed in proliferating C2C12 myoblasts, but its expression levels decreased upon induction of myogenic differentiation in l ow serum medium. Forced expression of ATBF1-A in C2C12 cells resulted in re pression of MyoD and myogenin expression and elevation of Id3 and cyclin D1 expression, leading to inhibition of myogenic differentiation in low serum . In contrast, transfection of C2C12 cells with the ATBF1-B isoform led to an acceleration of myogenic differentiation, as indicated by an earlier ons et of myosin heavy chain expression and formation of a higher percentage of multinucleated myotubes. The fourth homeodomain of ATBF1-A bound to an AT- rich element adjacent to the El E-box of the muscle regulatory factor 4 pro moter mediating transcriptional repression. The ATBF1-A-specific N-terminal region possesses general transcription repressor activity. These results s uggest that ATBF1-A plays a role in the maintenance of the undifferentiated myoblast state, and its down-regulation is a prerequisite to initiate term inal differentiation of C2C12 cells.