Myoblast alpha v beta 3 integrin levels are controlled by transcriptional regulation of expression of the beta 3 subunit and down-regulation of beta 3 subunit expression is required for skeletal muscle cell differentiation

The expression of alpha v beta 3 integrin was examined in human skeletal muscle cells grown in vitro. The alpha v and beta 3 subunits showed different patterns of expression during myogenesis. Expression of the alpha v subunit did not change significantly during the growth and differentiation of muscle cells, whereas expression of the beta 3 subunit was markedly down-regulated at both the message and protein levels. Down-regulation of beta 3 subunit expression did not occur when cultures were treated with 5-bromo-2'-deoxyuridine to inhibit myoblast terminal differentiation, but did occur in cultures in which fusion was selectively inhibited by growth in EGTA-containing medium. These results suggest that the regulation of integrin beta 3 subunit expression is tightly coupled to the myogenic terminal differentiation program, but is not simply a consequence of membrane reorganization due to the fusion process. Several stably transfected lines of mouse C2 myoblasts were derived from cultures transfected with a cDNA encoding the complete human beta 3 integrin sequence under the control of the CMV promoter. Lines which constitutively expressed high levels of the human beta 3 integrin subunit did not fuse or biochemically differentiate, whereas lines expressing moderate levels of the beta 3 integrin subunit showed delayed fusion and differentiation. Lines expressing very low to undetectable levels of the human beta 3 integrin subunit exhibited unimpaired fusion and differentiation. Taken together these results suggest (i) that down-regulation of alpha v beta 3 integrin normally occurs as part of the myogenic terminal differentiation program, (ii) that this is mediated by regulation of the expression of the beta 3 subunit, and (iii) that down-regulation of expression of the beta 3 integrin subunit appears essential for myoblast differentiation. (C) 1997 Academic Press.