ATGL and CGI-58 are lipid droplet proteins of the hepatic stellate cell line HSC-T6.

Lipid droplets (LDs) of hepatic stellate cells (HSCs) contain large amounts of vitamin A [in the form of retinyl esters (REs)] as well as other neutral lipids such as TGs. During times of insufficient vitamin A availability, RE stores are mobilized to ensure a constant supply to the body. To date, little is known about the enzymes responsible for the hydrolysis of neutral lipid esters, in particular of REs, in HSCs. In this study, we aimed to identify LD-associated neutral lipid hydrolases by a proteomic approach using the rat stellate cell line HSC-T6. First, we loaded cells with retinol and FAs to promote lipid synthesis and deposition within LDs. Then, LDs were isolated and lipid composition and the LD proteome were analyzed. Among other proteins, we found perilipin 2, adipose TG lipase (ATGL), and comparative gene identification-58 (CGI-58), known and established LD proteins. Bioinformatic search of the LD proteome for α/ß-hydrolase fold-containing proteins revealed no yet uncharacterized neutral lipid hydrolases. In in vitro activity assays, we show that rat (r)ATGL, coactivated by rat (r)CGI-58, efficiently hydrolyzes TGs and REs. These findings suggest that rATGL and rCGI-58 are LD-resident proteins in HSCs and participate in the mobilization of both REs and TGs.

Adipose triacylglycerol lipase deletion alters whole body energy metabolism and impairs exercise performance in mice.

Adipose triacylglycerol lipase (ATGL) and hormone-sensitive lipase (HSL) are essential for efficient lipolysis in adipose tissue and skeletal muscle. Herein, we utilized whole body knockout mice to address the importance of ATGL and HSL for metabolic function and exercise performance. ATGL deletion severely disrupts whole-body substrate partitioning at rest; reducing plasma free fatty acid (FFA) availability (WT: 0.49 +/- 0.06 vs. ATGL(-/-) 0.34 +/- 0.03 mM), which in turn enhances carbohydrate oxidation during fasting (mean RER, WT: 0.86 +/- 0.02, ATGL(-/-) 0.90 +/- 0.01) and is associated with depleted muscle and liver glycogen stores. While plasma FFA was modestly reduced (23%) and whole body carbohydrate metabolism increased in HSL(-/-) mice, resting glycogen storage was not compromised. Studies in isolated muscles revealed that the capacity of ATGL and HSL(-/-) muscle to transport exogenous fatty acids is not compromised and the capacity to oxidize fatty acids is actually increased (3.7- and 1.3-fold above WT for ATGL and HSL). The exercise-induced increase in plasma FFA and glycerol was blunted with ATGL or HSL deletion, demonstrating an impaired capacity for exercise-induced lipolysis in these mice. Carbohydrate oxidation was increased concomitantly during exercise in ATGL(-/-) and HSL(-/-) mice, resulting in more muscle and liver glycogen depletion. Maximal running velocity and endurance capacity were reduced by 42% and 46% in ATGL(-/-) mice, but not in HSL(-/-) mice. The reduction in performance in ATGL(-/-) mice was not due to defective muscle contractile performance. These results demonstrate an essential role for both ATGL and HSL in maintaining adequate FFA supply to sustain normal substrate metabolism at rest and during exercise.