INFLUENCE OF PDGF-BB ON PROLIFERATION AND TRANSITION THROUGH THE MYOD-MYOGENIN-MEF2A EXPRESSION PROGRAM DURING MYOGENESIS IN MOUSE C2 MYOBLASTS

We have previously demonstrated that PDGF-BB enhances proliferation of C2 myoblasts. This has led us to examine whether the mitogenic influe nce of PDGF-BB in the C2 model correlates with modulation of specific steps associated with myogenic differentiation. C2 myoblasts transitin g through these differentiation specific steps were monitored via immu nocytochemistry. We show that the influence of PDGF on enhancing cell proliferation correlates with a delay in the emergence of cells positi ve for sarcomeric myosin. We further monitored the influence of PDGF-B B on differentiation steps preceding the emergence of myosin+ cells. W e demonstrate that mononucleated C2 cells first express MyoD (MyoD+/my ogenin- cells) and subsequently, myogenin, Cells negative for both Myo D and myogenin (the phenotype preceding the MyoD+ state) were present at all times in culture and comprised the majority, if not all, of the cells which responded mitogenically to PDGF. Additionally, the freque ncy of the MyoD+/myogenin+ cell phenotype was reduced in cultures rece iving PDGF, suggesting that PDGF can modulate the transition of the ce lls into the myogenin+ state. We determined that many of the myogenin cells subsequently become MEF2A+ and this phenomenon is not influence d by PDGF-BB, FGF-2 also enhanced the proliferation of C2 myoblasts an d suppressed the appearance of the myogenin+ cells, but did not influe nce the subsequent transition into the MEF2A+ state. The study raises the possibility that PDGF-BB and FGF-2 might delay the transition of t he C2 cells into the MyoD+/myogenin+ state by depressing a paracrine s ignal that enhances differentiation.