The 280-kDa β-isoform of acetyl-CoA carboxylase (ACCβ) is predominantly expressed in heart and skeletal muscle, whereas the 265-kDa α-isoform (ACCα) is the major ACC in lipogenic tissues. The ACCβ promoter showed myoblast-specific promoter activity and was strongly induced by MyoD in NIH3T3 cells. Serial deletions of the promoter revealed that MyoD acts on the E-boxes located at positions −498 to −403 and on the proximal region including the 5′-untranslated region. Destruction of the E-boxes at positions −498 to −403 by site-directed mutagenesis resulted in a significant decrease of MyoD responsiveness. The "TGAAA" at −32 to −28 and the region around the transcription start site play important roles in basal transcription, probably as a TATA box and an Inr element, respectively. Mutations of another E-box at −14 to −9 and a "GCCTGTCA" sequence at +17 to +24 drastically decreased the MyoD responsiveness. The novel cis-element GCCTGTCA was preferentially bound by MyoD homodimer in EMSA and conferred MyoD responsiveness to a luciferase reporter, which was repressed by the overexpression of E12. This finding is unique since activation via E-boxes is mediated by heterodimers of MyoD and E-proteins. We screened a human skeletal muscle cDNA library to isolate clones expressing proteins that bind to the region around the GCCTGTCA (+8 to +27) sequence, and isolated Myf4 and Myf6 cDNAs. Electrophoretic mobility shift assay showed that recombinant Myf4 and Myf6 bind to this novel cis-element. Moreover, transient expression of Myf6 induced significant activation on the ACCβ promoter or an artificial promoter harboring this novel cis-element. These findings suggest that muscle regulatory factors, such as MyoD, Myf4, and Myf6, contribute to the muscle-specific expression of ACCβ viaE-boxes and the novel cis-element GCCTGTCA.

Joo-Hun Ha

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