The pro-Forms of Insulin-Like Growth Factor I (IGF-I) Are Predominant in Skeletal Muscle and Alter IGF-I Receptor Activation

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Endocrinology (Impact Factor: 4.5). 03/2013; 154(3):1215-24. DOI: 10.1210/en.2012-1992
Source: PubMed


IGF-I is a key regulator of muscle development and growth. The pre-pro-peptide produced by the Igf1gene undergoes several posttranslational processing steps to result in a secreted mature protein, which is thought to be the obligate ligand for the IGF-I receptor (IGF-IR). The goals of this study were to determine what forms of IGF-I exist in skeletal muscle, and whether the mature IGF-I protein was the only form able to activate the IGF-IR. We measured the proportion of IGF-I species in murine skeletal muscle and found that the predominant forms were nonglycosylated pro-IGF-I and glycosylated pro-IGF-I, which retained the C-terminal E peptide extension, instead of mature IGF-I. These forms were validated using samples subjected to viral expression of IGF-I combined with furin and glycosidase digestion. To determine whether the larger molecular weight IGF-I forms were also ligands for the IGF-IR, we generated each specific form through transient transfection of 3T3 cells and used the enriched media to perform kinase receptor activation assays. Compared with mature IGF-I, nonglycosylated pro-IGF-I had similar ability to activate the IGF-IR, whereas glycosylation of pro-IGF-I significantly reduced receptor activation. Thus, it is important to understand not only the quantity, but also the proportion of IGF-I forms produced, to evaluate the true biological activity of this growth factor.

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    • "In contrast to the activity of mature and pro-IGF-I, we found that glycosylated pro-IGF-I was inefficient at receptor activation in vitro [23]. How, then, can we explain the multipronged benefits to muscle mass, strength and regenerative capacity in mice expressing IGF-IA, in which the predominant form that is stored is glycosylated pro-IGF-I [33] [35]? "
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    • "These E-peptides are either cleaved by proteases to release mature IGF or stay attached and together with ‘mature IGF sequence’ to form pro-IGF-I (A, B or C). It has been recently demonstrated that pro-IGF-1A form is as potent as mature IGF-1 to activate IGF-1R and is a predominant form present in muscle (12). Another level of complexity in the IGF-1 activity is glycosylation of IGF-1A isoform. "
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    • "Subtilisin-related proprotein convertases like furin can cleave polypeptides that include this motif, resulting in free mature IGF-I and an E-peptide (Duguay et al., 1995, 1997; Duguay, 1999). Intriguingly, uncleaved pro-IGF-I is detectable in conditioned media and in vivo in serum (Powell et al., 1987; Conover et al., 1989, 1993; Wilson et al., 2001; Barton et al., 2012; Durzynska et al., 2013a). To date, however, it is unclear if pro-IGF-I is bioactive or simply an inactive precursor or source for mature IGF-I and/or E-peptides. "
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