EXPRESSION OF THE G(S) PROTEIN ALPHA-SUBUNIT DISRUPTS THE NORMAL PROGRAM OF DIFFERENTIATION IN CULTURED MURINE MYOGENIC CELLS

The manner in which growth factors acting at the cell surface regulate activity of myogenic basic-helix-loop-helix proteins in the nucleus a nd thus control the fate of committed skeletal myoblasts remains poorl y understood. In this study, we report that immunoreactive G(s) protei n alpha-subunits (G(s alpha)) localize to nuclei of proliferating C2C1 2 myoblasts but not to nuclei of differentiated postmitotic C2C12 myot ubes. To explore the biological significance of this observation, we p laced a cDNA encoding G(s alpha) in an expression vector under the con trol of a steroid-inducible promoter and isolated colonies of stably t ransfected C2C12 myoblasts. Dexamethasone-induced expression of activa ted G(s alpha) markedly delayed differentiation in comparison with uni nduced stably transfected cells, which differentiated normally in mito gen-depleted media. Northern blot analysis showed that impaired differ entiation was associated with delayed up-regulation of MyoD and myogen in and delayed down-regulation of Id, a dominant negative inhibitor of differentiation. Similar impairment of differentiation could not be r eproduced in wild-type C2C12 cells by increasing intracellular cAMP ei ther with forskolin or treatment with a cell-permeable cAMP analog. Ho wever, treatment of myoblasts with cholera toxin markedly inhibited my ogenic differentiation. Taken together, these findings suggest a novel role for G(s alpha) in modulating myogenic differentiation.