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 skelet al 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 di fferentiation of muscle cells, whereas expression of the beta 3 subuni t was markedly down-regulated at both the message and protein levels. Down-regulation of beta 3 subunit expression did not occur when cultur es were treated with 5-bromo-2'-deoxyuridine to inhibit myoblast termi nal differentiation, but did occur in cultures in which fusion was sel ectively inhibited by growth in EGTA-containing medium. These results suggest that the regulation of integrin beta 3 subunit expression is t ightly coupled to the myogenic terminal differentiation program, but i s not simply a consequence of membrane reorganization due to the fusio n process. Several stably transfected lines of mouse C2 myoblasts were derived from cultures transfected with a cDNA encoding the complete h uman beta 3 integrin sequence under the control of the CMV promoter. L ines which constitutively expressed high levels of the human beta 3 in tegrin subunit did not fuse or biochemically differentiate, whereas li nes expressing moderate levels of the beta 3 integrin subunit showed d elayed fusion and differentiation. Lines expressing very low to undete ctable levels of the human beta 3 integrin subunit exhibited unimpaire d fusion and differentiation. Taken together these results suggest (i) that down-regulation of alpha v beta 3 integrin normally occurs as pa rt 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 Pre ss.