Mobilization of fatty acids from stored triacylglycerol (TG) in adipose tissue and skeletal muscle [intramyocellular triacylglycerol (IMTG)] requires activity of lipases. Although exercise training increases the lipolytic capacity of skeletal muscle, the expression of hormone-sensitive lipase (HSL) is not changed. Recently, adipose triglyceride lipase (ATGL) was identified as a TG-specific lipase in various rodent tissues. To investigate whether human skeletal muscle ATGL protein is regulated by endurance exercise training, 10 healthy young men completed 8 wk of supervised endurance exercise training. Western blotting analysis on lysates of skeletal muscle biopsy samples revealed that exercise training induced a twofold increase in skeletal muscle ATGL protein content. In contrast to ATGL, expression of comparative gene identification 58 (CGI-58), the activating protein of ATGL, and HSL protein was not significantly changed after the training period. The IMTG concentration was significantly decreased by 28% at termination of the training program compared with before. HSL-phoshorylation at Ser 660 was increased, HSL-Ser 659 phosporylation was unchanged, and HSL-phoshorylation at Ser 565 was decreased altogether, indicating an enhanced basal activity of this lipase. No change was found in the expression of diacylglycerol acyl transferase 1 (DGAT1) after training. Inhibition of HSL with a monospecific, small molecule inhibitor (76-0079) and stimulation of ATGL with CGI-58 revealed that significant ATGL activity is present in human skeletal muscle. These results suggest that ATGL in addition to HSL may be important for human skeletal muscle lipolysis.comparative gene identification 58; hormone sensitive lipase; diacylglycerol acyl transferase 1; intramyocellular triacylglycerol; lipolysis HORMONE-SENSITIVE LIPASE (HSL) has generally been accepted to be the primary lipase responsible for hydrolysis of intramyocellular triacylglycerol (IMTG). This notion is supported by findings in both rat and human skeletal muscle demonstrating that immunoinhibition of HSL with an anti-HSL antibody completely abolished contraction-induced increase in triacylglycerol (TG)-lipase activity (29,40,52). On the other hand, dissociations between HSL activity and net change of IMTG content during skeletal muscle contractions in humans have been observed (40,50,51). Also, in resting human skeletal muscle, it was shown that 40 -80% TG-hydrolase activity was still remaining after immunoinhibition of HSL (40,52). In line with this, recent studies (17) revealed that basal TG-hydrolase (lipolytic) activity was not reduced in the skeletal muscle of HSL knockout mice compared with wild-type controls and that, in the wild-type mice, diacylglycerol (DAG) rather than TG was found to accumulate in skeletal muscle and in adipose tissue. These findings together indicate that TG lipases other than HSL may be of importance in skeletal muscle TG hydrolysis. Recently, a previously unknown TG lipase, named adipose triglyceride lipase (ATGL), was identified (2...