Structure and function of the upstream promoter of the human Mafbx gene: The proximal upstream promoter modulates tissue-specificity

Muscle loss has been linked to increased expression of an ubiquitin ligase termed muscle atrophy F-box (MAFbx), a nuclear protein involved in degradation of MyoD. To gain insights into mechanisms by which the human MAFbx gene is controlled, the structure of its upstream promotor were studied, and its expression in cultured cells was characterized. Expression of MAFbx was found only in eel Is of muscle lineage. A reporter gene control led by 948 bases of human MAFbx upstream promotor displayed similar cell-type selectivity. MAFbx levels were greatly enhanced upon myogenic differentiation of C2C12 myoblasts, and differentiation markedly increased activity of a reporter gene constructed with 400 bp of upstream promotor from the MAFbx gene. The core promotor spanned approximately 160 bases beginning at -241 bp upstream of the first codon, included potential bind ing sites for MyoD and myogenin, and was highly conserved among mouse, rat, and human MAFbx genes. The major transcription start site for the human MAFbx gene was 340 bases upstream of the ATC and was localized the highly conserved region of 140 bp. The findings indicate an important role for the immediate upstream promotor of the human MAFbx gene in mediating its developmental expression and tissue specificity.