Does obesity reduce load-induced muscle hypertrophy?

Obesity contributes to the development of insulin resistance, hyperglycaemia and metabolic syndrome (Kahn & Flier, 2000; Kahn et al. 2006). However, effects of obesity on the control of skeletal muscle mass and hypertrophy are not well understood. Muscle hypertrophy is marked by an increase in protein synthesis and addition of contractile filaments to generate muscle force (Glass, 2003). A vast array of signalling networks in response to external stimuli including insulin, insulin growth factor-1 (IGF-1), steroid hormones and also amino acids mediate hypertrophy of skeletal muscle (Glass, 2003). Though the external signals such as insulin and IGF-1 activate their specific signalling, they share common downstream signalling molecules such as PI3-K and Akt. IGF-1/insulin act on their cell surface specific receptors and initiate respective signalling by activation of PI3K followed downstream signalling via activation of Akt (Cheatham & Kahn, 1995; Stitt et al. 2004). The mechanism of distinct actions of these peptide hormones despite activating common signalling molecule: Akt is beginning to be understood. Reported studies suggest that different isoforms of Akt (Akt1, Akt2 and Akt3) confer signal specificity of insulin in glucose metabolism and IGF-1 in growth. Mutational analyses revealed that Akt1-deficient mice have retarded growth and perinatal mortality, where as Akt2 knockout mice have no growth defects, but have impaired glucose metabolism (Chen et al. 2001; Cho et al. 2001a,b;). On the other hand, Akt3 knockout mice had normal body weights and glucose metabolism, but reduced brain sizes (Garofalo et al. 2003). In general, IGF-1 and also insulin to a lesser extent mediate protein synthesis by activating Akt1. The activated Akt1 increases protein synthesis by activating mammalian target of rapamycin (mTOR). mTOR increases protein synthesis by phosphorylating S6 protein kinase (p-S6K), a positive regulator of protein synthesis. mTOR also inhibits the activity of eIF4E binding protein-1 (4E-BP1), a negative regulator of the protein initiation factor, eukaryotic translation initiation factor 4E (eIF-4E) (Proud & Denton, 1997; von Manteuffel et al. 1997). Glycogen synthase kinase 3β (GSK3β) is another substrate of Akt, whose repression is known to induce hypertrophy, and its activity is regulated negatively by the phosphorylation of serine 9 (p-GSK-3β) (Rommel et al. 2001). In a recent issue of The Journal of Physiology, Sitnick et al. (2009) investigated load-induced hypertrophy and underlying molecular mechanisms in response to high-fat diet. The authors investigated whether a high-fat diet (HFD) would alter the ability of skeletal muscle to respond to functional overload (FO). Further, they determined if underlying signalling mechanisms of hypertrophy are compromised in mice fed a high-fat diet. As expected, mice increased their body size in response to HFD. Measurements of fat (epididymal and retroperitoneal fat pads) and muscle (gastrocnemius and soleus) suggested that increases in body size were primarily due to increased fat composition in the body. The authors also determined that mice fed a high-fat diet were hyperinsulinemic, suggesting development of insulin resistance potentially mediated by increased intra-myocellular triglycerides in muscle. On all the tested days there was a significant increase in the mass of pantaris muscle in response to FO mice compared to control. After 14 and 30 days of FO the authors found a 10 and 16% reduction in the growth of the plantaris muscle in the HFD versus the low-fat diet (LFD) mice, suggesting a moderate decrease in the growth of plantaris muscle in response to HFD. The authors noticed that extended HFD nutrition (30 weeks) had a dramatic effect, showing no difference in the mass of plantarius muscle compared to control mice. These observations suggest that a HFD diet has a negative effect on the load-induced hypertrophy. The authors give interesting directions for the research into whether obesity may be a negative factor for load-induced muscular hypertrophy. On a path leading toward understanding the mechanism underlying this apparent relationship, the authors measured general protein translation and also status of the cell signalling molecules such as Akt, S6K1 and GSK3-β. The association of the ribosomal 40S and 60S subunits resulting in a 80S peak is an indicator of active translation process. The analyses demonstrate that the 80S peak in LFD mice was larger than in HFD mice suggesting that the translation process is greater in LFD mice compared to HFD. The authors further suggest that the activity of signalling molecules such as Akt and S6K1 were attenuated, but not for GSK3 β in FO/HFD mice compared FO/LFD mice. Though the analyses provided in the Sitnick et al. paper provide useful information about the status of the signalling molecules in hypertrophy, the data/analyses are not sufficient to provide accurate mechanistic insights. The potential link between obesity and hypertrophy at a mechanistic level may be delineated by further analyses of other important targets including IGF-1 expression and also using appropriate controls in the analyses. For example, the authors observed increased Akt phosphorylation in HFD control mice with apparent insulin resistance. This may be due to higher load on the plantaris muscle in the case of HFD mice due to an increased body mass. The measurement of IGF-1 levels would have been useful for interpreting information. Upon examining the Western blots of Akt presented in the paper, it is clear that total Akt levels increase in response to FO, irrespective of diet; however, the authors did not analyse if the total Akt levels show differences in response to FO in the two diets (LFD vs. HFD). The analyses are presented as a ratio of Akt Ser-373 to total Akt to indicate activity levels of Akt; but these analyses in the context of robust changes in total Akt levels will only provide limited information regarding Akt activity. The analyses would have been improved by probing for a housing-keeping protein such as GAPDH, and normalizing total Akt levels and also phosphorylated Akt levels to GAPDH and making comparisons between diets. The same suggestion applies to the quantification of the expression of S6K1, p-S6K1, GSK-3β and p-GSK-3β. All the molecules chosen in the study are not unique to either insulin or IGF-1, but common to both. It is not clear whether the HFD mice exhibited compromised hypertrophy due to attenuation of insulin signalling or IGF-1 signalling. In this context, future studies directed toward deciphering the specific proximal pathways such as IGF-1 serum levels/muscle expression of IGF-1, IGF-1/insulin receptor activation and also further study of specific Akt isoforms may begin to provide a molecular basis for the apparent relationship between obesity and hypertrophy. On the other hand, it has to be determined if obesity decreases activity of the mice. In summary, the recent paper by Stinick et al. suggests that diet plays a role in the hypertrophy of skeletal muscle, although the exact mechanisms for this process remain unclear. The authors provide a base upon which further investigations can build to elucidate the complex signalling mechanisms of diet and their influence on load-induced hypertrophy.