Cellular Localization and Associations of the Major Lipolytic Proteins in Human Skeletal Muscle at Rest and during Exercise

Abstract

Lipolysis involves the sequential breakdown of fatty acids from triacylglycerol and is increased during energy stress such as exercise. Adipose triglyceride lipase (ATGL) is a key regulator of skeletal muscle lipolysis and perilipin (PLIN) 5 is postulated to be an important regulator of ATGL action of muscle lipolysis. Hence, we hypothesized that non-genomic regulation such as cellular localization and the interaction of these key proteins modulate muscle lipolysis during exercise. PLIN5, ATGL and CGI-58 were highly (>60%) colocated with Oil Red O (ORO) stained lipid droplets. PLIN5 was significantly colocated with ATGL, mitochondria and CGI-58, indicating a close association between the key lipolytic effectors in resting skeletal muscle. The colocation of the lipolytic proteins, their independent association with ORO and the PLIN5/ORO colocation were not altered after 60 min of moderate intensity exercise. Further experiments in cultured human myocytes showed that PLIN5 colocation with ORO or mitochondria is unaffected by pharmacological activation of lipolytic pathways. Together, these data suggest that the major lipolytic proteins are highly expressed at the lipid droplet and colocate in resting skeletal muscle, that their localization and interactions appear to remain unchanged during prolonged exercise, and, accordingly, that other post-translational mechanisms are likely regulators of skeletal muscle lipolysis.

The effect of diet and exercise on lipid droplet dynamics in human muscle tissue

“…Studies using cell and rodent models have revealed that the absence of PLIN2 (Bosma et al, 2012a) or PLIN5 (Mason et al, 2014) results in a marked reduction in LD number and size. However, under these conditions, the most likely explanation for the loss of LDs is not the compromised incorporation of fatty acids in TAG, but rather uncontrolled and/or continuous lipolysis.…”

“…One study showed that 60 min of cycling at 60% of VȮ 2,max did not alter the interaction of PLIN5, ATGL and its co-activator with LDs (Mason et al, 2014). Another study showed that LDs coated with PLIN5 were depleted during exercise, whereas LDs without PLIN5 were not .…”

Exercise and Regulation of Lipid Metabolism

“…While investigation of whether exercise induces alterations in these protein-protein interactions is in its infancy, several studies are yielding exciting insight into mechanisms responsible for the exercise-induced increase in ATGL activity. [47][48][49][50][51][52][53] HSL has greater specificity for DAGs (10-fold) and cholesteryl (5-fold) than TGs or monoacylglycerides. 17 Moreover, Hsl-deficient mice exhibit DAG accrual suggesting this enzyme has a primary role in DAG hydrolysis in vivo.…”

Piecing together the puzzle of perilipin proteins and skeletal muscle lipolysis

“…However, activation of PKA in cells with ectopic expression of PLIN5 and ATGL resulted in increased lipolysis (Wang et al 2011a). In skeletal muscle, PLIN5 is highly colocalized with ATGL and CGI-58 (Mason et al 2014a), and it is now known that PLIN5 interacts with both ATGL and CGI-58; however, this interaction remains unchanged in the face of lipolytic challenges (i.e., contraction and adrenergic stimulation) (MacPherson et al 2013a(MacPherson et al , 2013b. Further, in whole skeletal muscle, PLIN5 is serine phosphorylated; however, this phosphorylation state and interactions with CGI-58 and ATGL remain unchanged following either contractile or epinephrine stimulation (MacPherson et al 2013b).…”