It is well established that mitogens inhibit differentiation of skeletal muscle cells, but the insulin-like growth factors (IGFs), acting through a single receptor, stimulate both proliferation and differentiation of myoblasts. Although the IGF-I mitogenic signaling pathway has been extensively studied in other cell types, little is known about the signaling pathway leading to differentiation in skeletal muscle. By using specific inhibitors of the IGF signal transduction pathway, we have begun to define the signaling intermediates mediating the two responses to IGFs. We found that PD098059, an inhibitor of mitogen-activated protein (MAP) kinase kinase activation, inhibited IGF-stimulated proliferation of L6A1 myoblasts and the events associated with it, such as phosphorylation of the MAP kinases and elevation of c-fos mRNA and cyclin D protein. Surprisingly, PD098059 caused a dramatic enhancement of differentiation, evident both at a morphological (fusion of myoblasts into myotubes) and biochemical level (elevation of myogenin and p21 cyclin-dependent kinase inhibitor expression, as well as creatine kinase activity). In sharp contrast, LY294002, an inhibitor of phosphatidylinositol 3-kinase, and rapamycin, an inhibitor of the activation of p70 S6 kinase (p70 S6k ), completely abolished IGF stimulation of L6A1 differentiation. We found that p70 S6k activity increased substantially during differentiation, and this increase was further enhanced by PD098059. Our results demonstrate that the MAP kinase pathway plays a primary role in the mitogenic response and is inhibitory to the myogenic response in L6A1 myoblasts, while activation of the phosphatidylinositol 3-kinase/ p70 S6k pathway is essential for IGF-stimulated differentiation. Thus, it appears that signaling from the IGF-I receptor utilizes two distinct pathways leading either to proliferation or differentiation.

It is very clear that the GH-IGF axis plays a major role in controlling the growth and differentiation of skeletal muscles, as it does virtually all of the tissues in the animal body. One aspect of this control is unquestioned: circulating GH acts on the liver to stimulate expression of the IGF-I and IGFBP3 genes, substantially increasing the levels of these proteins in the circulation. It also seems that GH stimulates expression of IGF-I genes in skeletal muscle, although there are a number of cases in which skeletal muscle IGF-I expression is elevated in the absence of GH. It is substantially less clear that GH acts directly on skeletal muscle to stimulate its growth; the presence of GH receptor mRNA in skeletal muscle is well established, but most investigators have been unsuccessful in demonstrating any specific binding of GH to skeletal muscle or to myoblasts in culture. It has been equally difficult to show direct actions of GH on cultured muscle cells; the only positive report concludes that the early insulin-like effects of GH can result from direct interactions between GH and isolated muscle cells. The effects of the IGFs on skeletal muscle are much clearer. It is well established by studies in a number of laboratories on a variety of systems that IGFs stimulate many anabolic responses in myoblasts, as they do in other cell types. IGFs have the unusual property of stimulating both proliferation and differentiation of myoblasts, responses that are generally believed to be mutually exclusive; in myoblasts, they are in fact temporally separated. The stimulation of differentiation by IGF-I is (at least in part) a result of substantially increased levels of the mRNA for myogenin, the member of the MyoD family most directly associated with terminal myogenesis. As levels of myogenin mRNA rise, those of myf-5 mRNA (the only other member of the MyoD family expressed significantly in L6 myoblasts) fall dramatically, although myf-5 expression is required for the initial elevation of myogenin. The effects of IGFs are significantly modulated by IGFBPs secreted by myoblasts in serum-free medium, inhibitory IG-FBPs-4 and -6 are expressed and secreted by L6A1 myoblasts, while expression of IGFBP-5 rises dramatically as differentiation proceeds. Other myoblasts also secrete IGFBP-2. Even if exogenous IGFs are not added to the low-serum "differentiation" medium, myoblasts express sufficient amounts of autocrine IGF-II to stimulate myogenesis after a period of time; some myogenic cell lines, (such as Sol 8) are so active in expressing the IGF-II gene that it is not possible to demonstrate effects of exogenous IGFs. This autocrine expression of IGFs is by no means unique to skeletal muscle cells; indeed, it is so widely seen in cells responding to mitogenic stimuli that we suggest that IGFs can be viewed as extracellular second messengers that mediate most, if not all, such actions of agents that stimulate cell proliferation. The component of serum that suppresses IGF-II gene expression under "growth" conditions appears to ...

We have previously shown that the insulin-like growth factors (IGFs) stimulate both proliferation and differentiation of skeletal muscle cells in culture, and that these actions in L6A1 muscle cells may be modulated by three secreted IGF binding proteins (IGFBPs), IGFBP-4, -5, and -6. Since we found that the temporal expression pattern of IGFBP-4 and IGFBP-5 differed dramatically during the transition from proliferating myoblasts to differentiated myotubes, we undertook the current study to examine the effects of purified IGFBP-4 and IGFBP-5 on IGF-stimulated actions in L6A1 muscle cells. As has been shown for other cell types, we found that IGFBP-4 had only inhibitory actions, inhibiting IGF-I and IGF-II-stimulated proliferation and differentiation. In contrast, IGFBP-5 exhibited both inhibitory and stimulatory actions. When added in the presence of 30 ng/ml IGF-I, IGFBP-5 (250 ng/ml) inhibited all markers of the early proliferative response: the tyrosine phosphorylation of the cytoplasmic signaling molecules IRS-1 and Shc, the activation of the MAP kinases, ERK1 and 2, the elevation of c-fos mRNA, the early inhibition of the elevation in myogenin mRNA, and the increase in cell number. In contrast, IGFBP-5 stimulated all aspects of the myogenic response to IGF-I: the later rise in myogenin mRNA, the elevation of creatine kinase activity, and the fusion of myoblasts into myotubes. This dual response to IGFBP-5 was greatest when it was added at a molar ratio of IGFBP-5 to IGF-I of 2:1. In contrast, when IGFBP-5 was added in the presence of IGF-II, it inhibited both proliferation and differentiation. Neither IGFBP had any effect when added in the presence of R3 IGF-I, an analog with substantially reduced affinity for IGFBPs. Our results suggest that the role of IGFBP-4 is mainly to sequester excess IGFs, and thus inhibit all actions. IGFBP-5, however, is capable of eliciting a dual response, possibly due to its unique ability to associate with the cell membrane.

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Our previous work has demonstrated that the insulin-like growth factors (IGFs), acting through a single receptor, stimulate both proliferation and differentiation of L6A1 myoblasts. This unique model system has enabled us to closely examine the switch that regulates these two opposing responses. We have previously shown, using specific inhibitors of the IGF-I signal transduction pathway, that the mitogenic response is mediated by the Ras/Raf/MAP kinase pathway and the myogenic response by the PI 3-kinase/p70s6k pathway (Coolican SA, Samuel DS, Ewton DZ, McWade FJ, Florini JR, J Biol Chem 1997; 272: 6653-62). In that study we found that PD098059, an inhibitor of MEK activation, inhibited the proliferative response, but dramatically enhanced IGF-stimulated differentiation which was associated with elevation of p70s6k activity. Since there have been reports of elevation of Raf-1 activity in PD098059-treated L6 myoblasts, and stimulation of p70s6k activity in cells expressing an activated Raf-1, it was important to determine whether or not Raf-1 elevation plays a role in the myogenic response. To test this, we have transfected L6A1 myoblasts with delta Raf-1:ER, an estradiol-regulated form of oncogenic Raf-1. We found that activation of Raf-1 by estradiol resulted in increased phosphorylation of p42 and p44 MAP kinases and stimulation of proliferation. In contrast, Raf-1 activation inhibited all measured aspects of the myogenic response: myogenin expression, creatine kinase elevation, and fusion of myoblasts to form myotubes. In addition, we found no elevation of p70s6k activity upon Raf-1 activation. These results indicate the following: (1) stimulation of myogenic differentiation by PD098059 treatment is not simply due to the elevation of Raf-1, (2) Raf-1 has a positive role in the MAP kinase pathway and myoblast proliferation, and (3) Raf-1 activation inhibits myogenesis, possibly by forcing cells to remain in the proliferative state.