Lipid metabolism in contracting muscle – HSL takes a back seat

Abstract

All cells of the body store triglycerides, which are composed of three fatty acids conjugated to a glycerol backbone. Triglycerides are located within discrete intracellular organelles called lipid droplets that are populated with surface proteins that regulate diverse functions, including triglyceride metabolism. While adipose tissue stores the most triglyceride, a significant amount accumulates inside muscle cells. There has been a sustained interest in understanding what regulates the breakdown and synthesis of this intramyocellular triglyceride (IMTG), primarily because it is an important metabolic substrate that accounts for ∼25% of energy production during moderate intensity exercise (Romijn et al. 1993). In addition, accumulating evidence suggests that plasma-derived fatty acids first enter the IMTG pool before oxidation (Kanaley et al. 2009), thereby placing IMTG metabolism as a critical control point of lipid flux in the muscle. From a pathological perspective, the excessive accumulation of IMTG and/or aberrant IMTG metabolism is closely associated with impaired insulin sensitivity (Krssak et al. 1999), a major factor leading to type 2 diabetes. IMTG breakdown was long thought to be regulated by the actions of hormone sensitive lipase (HSL), which cleaves fatty acids from triglycerides and diglycerides. This was based on the findings that lysates prepared from skeletal muscle samples were capable of cleaving triglyceride and diglyceride substrates stored in artificial lipid droplets in vitro, and that blocking the actions of HSL with a neutralising antibody in such an assay system significantly reduced lipase activity (Langfort et al. 1999; Watt et al. 2004). However, the understanding of lipolytic regulation was altered dramatically in 2004 with the discovery of adipose triglyceride lipase (ATGL), which was shown to be a major contributor to triglyceride lipolysis in all tissues. Also, work in cell systems has unravelled the complexity of lipolysis that involves translocation of lipases to lipid droplets and interactions between lipases and resident lipid droplet-associated proteins. These developments questioned the appropriateness of using mixed tissue lysates and artificial lipid droplets to examine ‘lipolysis’ and thereby called into question the relevance of earlier studies examining IMTG metabolism in skeletal muscle. In this issue of The Journal of Physiology, Alsted et al (2013) have re-examined the regulation of IMTG breakdown in an elegant series of experiments. The authors assessed IMTG content in isolated rat and mouse soleus muscles that were either rested or subjected to vigorous contractions. Using this platform, the authors have provided three key observations. First, by using careful microscopy approaches they showed that IMTG is decreased in skeletal muscle after just 5 min of contraction and this was associated with a reduction in the lipid droplet size. IMTG was further degraded after 20 min and this was due to a reduction in lipid droplet number. Thus, these data have overcome the limitations of previous studies using tracer methodology or biochemical determination of IMTG to definitively show substantial and indisputable IMTG degradation during contractions. Second, the authors examined the role of HSL on contraction-induced IMTG lipolysis and showed that acute inhibition of HSL using a specific pharmacological blocker had no influence on IMTG breakdown during contractions. Further studies using muscle from HSL null mice showed that the reduction in IMTG content with contractions was only marginally less than that in wild-type mice. Together, these experiments demonstrated a very minor role for HSL in mediating IMTG breakdown during contractions. Third, the authors assessed TG lipase activity in lysates of non-contracting skeletal muscles from ATGL null mice that were also treated with the pharmacological HSL blocker. These studies showed that ATGL and HSL contribute 98% of the TG lipase activity in mouse skeletal muscle and that other TG lipases are therefore inconsequential. This is consistent with the prominent role of ATGL and HSL reported in adipocytes. The work by Alsted et al (2013) has several important implications; most notable is that ATGL appears to be the predominant TG lipase in skeletal muscle both at rest and during contractions. This conclusion is associative and requires robust experimental support – contraction studies in ATGL null mice will address this critical issue. By extension, if ATGL is ‘contraction sensitive’ this suggests notable regulation of protein activity by post-translational modification (e.g. phosphorylation), translocation, protein–protein interactions or a combination thereof. This paves the way for future mechanistic studies to examine ATGL function and the role of ATGL interacting proteins such as CGI-58, G0-S2 and perilipin 5 in muscle. One can envisage that should these ‘contraction sensitive’ mechanisms be elucidated, we may be one step closer to developing strategies aimed at modulating IMTG metabolism. This would be useful for disease states associated with increased IMTG deposition such as neutral lipid storage disorders and where movement is limited such as in morbid obesity, ageing and some muscle dystrophies.