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

Abstract

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/β-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.

Hepatic stellate cells as key target in liver fibrosis

“…A retinoic acid receptor β2 agonist reduces HSC activation in NAFLD [301]. Perilipin 2, adipose TG lipase (ATGL), and comparative gene identification-58 (CGI-58) are lipid droplet-associated proteins in rat HSC line, HSC-T6 [302]. LXRs balance lipid stores in HSCs through Rab18, a retinoid responsive lipid droplet protein [303].…”

The perilipin family of lipid droplet proteins: Gatekeepers of intracellular lipolysis

“…ATGL has strong triacylglycerol hydrolase activity and weak retinyl ester hydrolase activity (38), but lacks other lipase activities; catalytic activity is enhanced by a co-activator protein, ABHD5 (alpha beta hydrolase domain 5, also called CGI-58), and inhibited by G0S2. ATGL is most highly expressed in adipose tissue and at lower levels in the same tissues as HSL, as well as in several additional tissues.…”

Lipid droplets and steroidogenic cells

“…CGI-58 does not have lipase activity by itself, and upon hormone stimulation acts as an activator of ATGL hydrolytic activity. Most recently, ATGL and CGI-58 were reported to possibly be involved in the mobilization of retinoids in hepatic stellate cells (41, 42). MglI only hydrolyzes monoacylglycerol (43).…”