Decrease in intramuscular lipid droplets and translocation of HSL in response to muscle contraction and epinephrine

Abstract

A better understanding of skeletal muscle lipid metabolism is needed to identify the molecular mechanisms relating intramuscular triglyceride (IMTG) to muscle metabolism and insulin sensitivity. An increasing number of proteins have been reported to be associated with intracellular triglyceride (TG), among them the PAT family members: perilipin, ADRP (for adipocyte differentiation-related protein), and TIP47 (for tail-interacting protein of 47 kDa). Hormone-sensitive lipase (HSL) is thought to be the major enzyme responsible for IMTG hydrolysis in skeletal muscle. In adipocytes, regulation of HSL by intracellular redistribution has been demonstrated. The existence of such regulatory mechanisms in skeletal muscle has long been hypothesized but has never been demonstrated. The aim of this study was to characterize the PAT family proteins associated with IMTG and to investigate the effect of epinephrine stimulation or muscle contraction on skeletal muscle TG content and HSL intracellular distribution. Rat soleus muscles were either incubated with epinephrine or electrically stimulated for 15 min. Single muscle fibers were used for morphological analysis by confocal and transmission electron microscopy. We show a decrease in IMTG in response to both lipolytic stimuli. Furthermore, we identify two PAT family proteins, ADRP and TIP47, associated with IMTG. Finally, we demonstrate HSL translocation to IMTG and ADRP after stimulation with epinephrine or contraction. Interest in skeletal muscle lipid metabolism has increased exponentially during the last decades. One of the reasons is the repeatedly reported inverse relation between intramuscular triglyceride (IMTG) concentrations and insulin sensitivity (1-5). Paradoxically, endurance training is well known to increase insulin sensitivity, whereas it has been shown to increase IMTG content (6, 7). Thus, it seems that IMTG content may not directly determine insulin sensitivity. A better understanding of skeletal muscle lipid metabolism is required to identify the molecular mechanisms involved in the effect of IMTG content on muscle metabolism and insulin sensitivity.For many years, it has been debated whether IMTGs are used during exercise, but today this question remains unresolved. The most likely explanation for the discrepancy between previous studies is that, in most of them, chemical methods to measure triglyceride (TG) were used. Such methods cannot distinguish between IMTG and TG contained in adipocytes surrounding muscle fibers. Illustrating the problem, Donsmark et al. (8) showed that the TG content of a single adipocyte (z0.3 nmol) corresponds to z5% of the TG content of a 2 mg soleus muscle sample, but the adipocyte volume makes up only 0.01% of the sample. Therefore, decreases in IMTG may have been undetected, being masked by contaminating TG from adipocytes (9).IMTG is stored within intracellular lipid droplets (LDs). LDs were initially considered to be relatively static

OXPAT/PAT-1 Is a PPAR-Induced Lipid Droplet Protein That Promotes Fatty Acid Utilization

“…The adipophilin staining identified these structures as lipid droplets. This punctate lipid droplet staining pattern is similar to that reported for TIP47 and adipophilin in rat soleus muscle (37). OXPAT-A ectopic expression drives increased rates of TAG accumulation and LCFA oxidation.…”

Adipose triglyceride lipase plays a key role in the supply of the working muscle with fatty acids

“…So far, most of the published experimental work on exercise-induced lipolysis was focused on the role of HSL in this process. A number of studies demonstrate that this enzyme is involved in the mobilization of FAs in muscle and adipose tissue during exercise (14)(15)(16)(17). Moreover, in a very recent study, HSL was shown to be important for normal mobilization of FAs during exercise ( 18 ).…”

Storing Up Trouble: Does Accumulation of Intramyocellular Triglyceride Protect Skeletal Muscle From Insulin Resistance?

“…Other lipid dropletassociated proteins from the PAT (perilipin/adipocyte differentiationrelated protein (ADRP)/tail-interacting protein of 47 kDa (TIP-47)) family are expressed in skeletal muscle and include ADRP and TIP-47. 15 Immunoinhibition studies indicate that HSL accounts for approximately 60% of total TG lipase activity in skeletal muscle at rest and almost all activity during contractions and adrenergic stimulation. 14,16,17 Adrenaline is a key activator of HSL and lipolysis.…”

“…12 Morphological analysis of single muscle fibres reveals that HSL translocates to lipid droplets within myocytes in response to adrenaline and contraction and that ADRP and TIP-47 are localized to lipid droplets. 15 Association of HSL with ADRP may be essential for lipolytic control in skeletal muscle given the absence of perilipin A in this tissue. The role of PAT proteins in skeletal muscle requires further investigation.…”