IGF-I isoforms and ageing skeletal muscle: an 'unresponsive' hypertrophy agent?

Muscle biologists have recognized the ability of growth factors to induce skeletal muscle hypertrophy in adult mammals. Although, the molecular mechanisms by which these growth factors induce skeletal muscle hypertrophy remain unclear, the evidence that growth factors increase muscle growth is obvious. One growth factor implicated in the hypertrophy process is insulin-like growth factor (IGF-I). IGF-I is a complex gene that is regulated by multiple promoters and is capable of producing at least four different mature IGF-I precursor proteins (i.e. isoforms). The two isoforms which appear most relevant to hypertrophy are: IGF-IEa (termed muscle IGF-I), which is similar to the IGF-I produced by the liver, and IGF-IEb (termed mechano-growth factor), which only appears to be produced by damaged or loaded skeletal muscle (Hameed et al. 2003, published in this issue of The Journal of Physiology). Although, the mechanistic roles of these different isoforms of IGF-I in muscle hypertrophy are complex and not well understood, these forms of IGF-I are an integral component of skeletal muscle hypertrophy. Skeletal muscle IGF-I mRNA and protein expression increase during the early phases of mechanical loading (Adams & Haddad, 1996), indicating that the liver is not the only source of IGF-I. As the animal matures, there is decreased production of growth hormone and subsequently a decline in circulating IGF-I released by the liver. Since adult mammals retain the ability to increase muscle mass it is unlikely that increases in systemic IGF-I are necessary for skeletal muscle hypertrophy and instead rely on paracrine/autocrine production of IGF-I. For example, hypophysectomized rats retain the ability to increase muscle mass in response to increased mechanical loading (Goldberg, 1967). However, the ability of skeletal muscle to respond to IGF-I regardless of the form or source of IGF-I was recently emphasized by Fernandez et al. (2002), in that transgenic mice containing a dominant negative IGF-I receptor lacked the ability to increase muscle mass. Skeletal muscle hypertrophy is regulated at least by three major molecular processes: increased satellite cell activity, gene transcription, and protein translation, with each of these processes contributing differently to muscle hypertrophy. Interestingly, IGF-I can influence the activity of all of these mechanisms. For example, IGF-I increases satellite cell proliferation (Chakravarthy et al. 2000), skeletal α-actin mRNA expression (Coleman et al. 1995), and protein synthesis (Vary et al. 2000). Therefore, based upon the molecular and cellular mechanisms that IGF-I influences, it is likely that IGF-I significantly contributes to hypertrophy. Clearly, one could speculate that IGF-I may work as a possible clinical treatment in humans for various muscle wasting conditions such as sarcopenia or muscular dystrophy, and in fact research in animals has shown promising starts towards this hypothesis. As mammals age they do not retain the ability to increase muscle mass after immobilization-induced atrophy (Chakravarthy et al. 2000). The inability of muscle from aged animals to grow with a return to normal loading led investigators to hypothesize that an intrinsic defect exists in aged muscle and prevents growth. Considering the complexity of the three major processes that influence muscle hypertrophy, then one could conceive that a defect in any portion of these mechanisms could prevent muscle hypertrophy in aged animals. However, some processes do not appear to be limiting, in that satellite cells isolated from aged animals still maintain their in vitro proliferative capacity when stimulated with exogenous IGF-I (Chakravarthy et al. 2000). Therefore, the proliferative ability of satellite cells still exists and is not a limiting factor in the muscle hypertrophy process in 30-month-old rats. However, it is equally possible that, although the proliferative capacity of satellites cells is not reduced, the stimulus for increased proliferation is limiting. In addition, IGF-I not only affects satellite cell proliferation, it also profoundly influences signalling proteins that play integral roles in other cellular process. One intriguing protein, glycogen synthase kinase β (GSK-3β) has been implicated as a negative regulator of gene transcription and protein translation. Vyas et al. (2002) found that IGF-I induced myotube hypertrophy and simultaneously inhibited GSK-3β activity. Further, Vyas et al. (2002) found that pharmacological inhibition of GSK-3β induced significant myotube hypertrophy. These data suggest that GSK-3β is an important role player in myotube hypertrophy and a downstream target of IGF-I signalling. Therefore, if IGF-I availability is limiting during increased muscle loading then the activation of any these cellular processes could be altered, thereby reducing muscle growth. One current dilemma with IGF-I biology is that the IGF-I protein exists in multiple isoforms, and therefore it remains unclear if all the forms of IGF-I have similar effects on skeletal muscle. To date most data collected concerning the role of IGF-I in skeletal muscle have focused on the IGF-IEa isoform. However, Hameed et al. (2003) using real-time PCR found no significant increases after resistance exercise training in IGF-IEa expression in either the young or old subjects. Interestingly, mRNA expression of the IGF-IEb form (i.e. mechano-growth factor) increased after resistance exercise in young adult humans, but after a similar resistance training protocol in elderly humans there was no change in IGF-IEb mRNA expression. These data provide indication that a ‘missing’ growth factor may be IGF-I, but more specifically the Eb isoform of IGF-I. These data further indicate that the non-responsiveness of these endogenous forms of IGF-I may be an underlying reason that skeletal muscle in aged mammals does not respond to increases in mechanical loading. Also, the findings indicate that autocrine/paracrine production of IGF-I is an important source of the growth factor for muscle hypertrophy. It will be necessary to determine the molecular role of these different isoforms of IGF-I in muscle hypertrophy. More specifically, it will be of interest to see if all isoforms of IGF-I activate satellite cell proliferation, gene transcription, and/or protein translation to similar extents and whether they operate though the same or distinct cellular signalling pathways? Importantly, the elegant work of Hameed et al. (2003) will further our understanding of muscle hypertrophy in aged mammals by providing researchers with a possible defect in IGF-IEb expression after resistance training.