Mechano-growth factor peptide, the COOH terminus of unprocessed insulin-like growth factor 1, has no apparent effect on myoblasts or primary muscle stem cells.

A splice form of IGF-1, IGF-1Eb, is upregulated after exercise or injury. Physiological responses have been ascribed to the 24-amino acid COOH-terminal peptide that is cleaved from the NH3-terminal 70-amino acid mature IGF-1 protein. This COOH-terminal peptide was termed "mechano-growth factor" (MGF). Activities claimed for the MGF peptide included enhancing muscle satellite cell proliferation and delaying myoblast fusion. As such, MGF could represent a promising strategy to improve muscle regeneration. Thus, at our two pharmaceutical companies, we attempted to reproduce the claimed effect of MGF peptides on human and mouse muscle myoblast proliferation and differentiation in vitro. Concentrations of peptide up to 500 ng/ml failed to increase the proliferation of C2C12 cells or primary human skeletal muscle myoblasts. In contrast, all cell types exhibited a proliferative response to mature IGF-1 or full-length IGF-1Eb. MGF also failed to inhibit the differentiation of myoblasts into myotubes. To address whether the response to MGF was lost in these tissue culture lines, we measured proliferation and differentiation of primary mouse skeletal muscle stem cells exposed to MGF. This, too, failed to demonstrate a significant effect. Finally, we tested whether MGF could alter a separate documented in vitro effect of the peptide, activation of p-ERK, but not p-Akt, in cardiac myocytes. Although a robust response to IGF-1 was observed, there were no demonstrated activating responses from the native or a stabilized MGF peptide. These results call in to question whether there is a physiological role for MGF.

ATP citrate lyase improves mitochondrial function in skeletal muscle.

Mitochondrial dysfunction is associated with skeletal muscle pathology, including cachexia, sarcopenia, and the muscular dystrophies. ATP citrate lyase (ACL) is a cytosolic enzyme that catalyzes mitochondria-derived citrate into oxaloacetate and acetyl-CoA. Here we report that activation of ACL in skeletal muscle results in improved mitochondrial function. IGF1 induces activation of ACL in an AKT-dependent fashion. This results in an increase in cardiolipin, thus increasing critical mitochondrial complexes and supercomplex activity, and a resultant increase in oxygen consumption and cellular ATP levels. Conversely, knockdown of ACL in myotubes not only reduces mitochondrial complex I, IV, and V activity but also blocks IGF1-induced increases in oxygen consumption. In vivo, ACL activity is associated with increased ATP. Activation of this IGF1/ACL/cardiolipin pathway combines anabolic signaling with induction of mechanisms needed to provide required ATP.

Gαi2 signaling promotes skeletal muscle hypertrophy, myoblast differentiation, and muscle regeneration.

Skeletal muscle atrophy results in loss of strength and an increased risk of mortality. We found that lysophosphatidic acid, which activates a G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor, stimulated skeletal muscle hypertrophy through activation of Gα(i2). Expression of a constitutively active mutant of Gα(i2) stimulated myotube growth and differentiation, effects that required the transcription factor NFAT (nuclear factor of activated T cells) and protein kinase C. In addition, expression of the constitutively active Gα(i2) mutant inhibited atrophy caused by the cachectic cytokine TNFα (tumor necrosis factor-α) by blocking an increase in the abundance of the mRNA encoding the E3 ubiquitin ligase MuRF1 (muscle ring finger 1). Gα(i2) activation also enhanced muscle regeneration and caused a switch to oxidative fibers. Our study thus identifies a pathway that promotes skeletal muscle hypertrophy and differentiation and demonstrates that Gα(i2)-induced signaling can act as a counterbalance to MuRF1-mediated atrophy, indicating that receptors that act through Gα(i2) might represent potential targets for preventing skeletal muscle wasting.