Growth Hormone and the Insulin-Like Growth Factor System in Myogenesis*

Abstract

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 ...

The Mitogenic and Myogenic Actions of Insulin-like Growth Factors Utilize Distinct Signaling Pathways

“…However, the stimulation of myogenic differentiation by the IGFs has been demonstrated independently in a number of laboratories (reviewed in Ref. 1), and the effect is now well established although not fully explained.…”

“…However, the insulin-like growth factors (IGF-I and IGF-II) 1 are unique among growth factors in that they stimulate both proliferation and differentiation of muscle cells in culture. Recently, it has been shown that IGF actions occur through two phases (reviewed in Ref.…”

“…Recently, it has been shown that IGF actions occur through two phases (reviewed in Ref. 1). This involves an initial mitogenic response in which intracellular mediators of proliferation such as cyclin D mRNA and c-fos mRNA are up-regulated and mediators of the myogenic response such as elevation of myogenin mRNA are suppressed (2), followed by a strong myogenic response characterized by expression of the muscle-specific transcription factor myogenin and fusion of myoblasts into myotubes.…”

Insulin-like growth factor (IGF-I) induces myotube hypertrophy associated with an increase in anaerobic glycolysis in a clonal skeletal-muscle cell model

“…Insulin-like growth factor-I (IGF-I) is therefore unique among growth factors as it stimulates both proliferation and differentiation of skeletal-muscle cells in culture [1][2][3][4][5][6][7]. In normal cell division, most cell lines require IGF-I, i.e.…”

“…Moreover, IGFAbbreviations used : IGF-I, insulin-like growth factor-I ; IRES, internal ribosome-entry site ; GFP, green fluorescent protein ; DMEM, Dulbecco's modified Eagle's medium ; K s , fractional synthesis rate ; ALT, alanine aminotransferase ; LDH, lactate dehydrogenase ; MNF, myocyte nuclear factor ; MRF, myogenic regulatory factor. 1 To whom correspondence should be addressed (e-mail b.graham!victorchang.unsw.edu.au).…”