Liu, Jun

Adipose tissue plays a central role in the regulation of plasma triglyceride (TG) and fatty acid (FA) levels, whose elevation is associated with atherosclerotic cardiovascular disease (ASCVD). Circulating TGs are hydrolyzed by lipoprotein lipase (LPL), releasing FAs that are taken up by adipocytes. Conversely, lipolytic action of adipose tissue triglyceride lipase (ATGL) mobilizes intracellular TG storage, supplying FAs via circulation for utilization by other tissues. However, how intracellular lipolysis influences intravascular lipolysis and atherosclerotic development remains largely unknown. Herein, the current study aimed to explore the role of G0S2, a selective protein inhibitor of ATGL, in this particular context. When treated with a Western diet and antisense oligonucleotides (ASOs) targeting LDLR, mice with a global ablation of G0S2 exhibited a significantly reduced lesion formation in aorta and brachiocephalic artery than their wild type (WT) littermates. Despite of little to no impact on hypercholesterolemia, total plasma and lipoprotein-specific TG content were normalized in G0S2 -/- mice. While it did not elicit any effects on intestinal lipid adsorption and hepatic TG secretion, the G0S2 ablation resulted in considerable elevation of LPL activity and protein levels in both plasma and adipose tissue. Accordingly, these G0S2 -/- mice showed a greatly improved TG clearance upon an oral lipid challenge. Interestingly, transplantation of G0S2 -/- adipose tissue increased circulating LPL in the WD-treated WT mice, which subsequently displayed a profound reduction of plasma TG levels as well as alleviation of hepatic steatosis. By studying adipose tissue explants and primary adipocytes, we observed that the G0S2 ablation led to markedly improved insulin sensitivity, resulting in increased FOXO1 phosphorylation and decreased expression of the LPL inhibitor ANGPTL4. Together, our data indicate that in the absence of G0S2, upregulation of adipose tissue LPL plays an important role in the clearance of circulating TGs and alleviation of diet-induced hypertriglyceridemia and atherosclerosis. We thus conclude that adipocyte G0S2 acts as a critical switch of both intracellular and vascular lipolysis.

ABSTRACT G 0 /G 1 switch gene 2 (G0S2) is a specific inhibitor of adipose triglyceride lipase (ATGL), the rate‐limiting enzyme for intracellular lipolysis. Recent studies show that G0S2 plays a critical role in promoting triacylglycerol (TG) accumulation in the liver, and its encoding gene is a direct target of a major lipogenic transcription factor liver X receptor (LXR)α. Here we sought to investigate a lipolysis‐independent role of G0S2 in hepatic triglyceride synthesis. Knockdown of G0S2 decreased hepatic TG content in mice with ATGL ablation. Conversely, overexpression of G0S2 promoted fatty acid incorporation into TGs and diacylglycerols in both wild‐type and ATGL‐deficient hepatocytes. Biochemical characterization showed that G0S2 mediates phosphatidic acid synthesis from lysophosphatidic acid (LPA) and acyl‐coenzyme A. In response to a high‐sucrose lipogenic diet, G0S2 is up‐regulated via LXRα and required for the increased TG accumulation in liver. Furthermore, deletion of a distinct 4‐aa motif necessary for the LPA‐specific acyltransferase (LPAAT) activity impaired G0S2's ability to mediate TG synthesis both in vitro and in vivo . These studies identify G0S2 as a dual‐function regulator of lipid metabolism as well as a novel mechanism whereby hepatic TG storage is promoted in response to lipogenic stimulation. In addition to its role as a lipolytic inhibitor, G0S2 is capable of directly promoting TG synthesis by acting as a lipid‐synthesizing enzyme.—Zhang, X., Xie, X., Heckmann, B. L., Saarinen, A. M., Gu, H., Zechner, R., Liu, J. Identification of an intrinsic lysophosphatidic acid acyltransferase activity in the lipolytic inhibitor G 0 /G 1 switch gene 2 (G0S2). FASEB J. 33, 6655–6666 (2019). www.fasebj.org

Adipose triglyceride lipase (ATGL), the rate-limiting enzyme for triacylglycerol (TG) hydrolysis, has long been known to be a phosphoprotein. However, the potential phosphorylation events that are involved in the regulation of ATGL function remain incompletely defined. Here, using a combinatorial proteomics approach, we obtained evidence that at least eight different sites of ATGL can be phosphorylated in adipocytes. Among them, Thr372 resides within the hydrophobic region known to mediate lipid droplet (LD) targeting. Although it had no impact on the TG hydrolase activity, substitution of phosphorylation-mimic Asp for Thr372 eliminated LD localization and LD-degrading capacity of ATGL expressed in HeLa cells. In contrast, mutation of Thr372 to Ala gave a protein that bound LDs and functioned the same as the wild-type protein. In nonstimulated adipocytes, the Asp mutation led to decreased LD association and basal lipolytic activity of ATGL, whereas the Ala mutation produced opposite effects. Moreover, the LD translocation of ATGL upon β-adrenergic stimulation was also compromised by the Asp mutation. In accord with these findings, the Ala mutation promoted and the Asp mutation attenuated the capacity of ATGL to mediate lipolysis in adipocytes under both basal and stimulated conditions. Collectively, these studies identified Thr372 as a novel phosphorylation site that may play a critical role in determining subcellular distribution as well as lipolytic action of ATGL.

Given the strong link between visceral adiposity and (hepatic) insulin resistance as well as liver steatosis, it is crucial to characterize obesity-associated alterations in adipocyte function, particularly in fat depots drained to the liver. Yet these adipose tissues are not easily accessible in humans, and the most frequently studied depot in rodents is the perigonadal, which is drained systemically. In the present study, we aimed to study alterations in lipolysis between mesenteric and perigonadal adipocytes in mice. Basal free fatty acid and glycerol release was significantly lower in perigonadal compared with mesenteric adipocytes isolated from chow-fed C57BL/6J mice. However, this difference completely vanished in high-fat diet-fed mice. Consistently, protein levels of the G0/G1switch gene 2 (G0S2), which were previously found to be inversely related to basal lipolysis, were significantly lower in mesenteric compared with perigonadal fat of chow-fed mice. Similarly, perilipin was differently expressed between the two depots. In addition, adipocyte-specific overexpression of G0S2 led to significantly decreased basal lipolysis in mesenteric adipose tissue of chow-fed mice. In conclusion, lipolysis is differently regulated between perigonadal and mesenteric adipocytes, and these depot-specific differences might be explained by altered regulation of G0S2 and/or perilipin.