McMahon, Cailin E.

The interplay between intracellular and intravascular lipolysis is crucial for maintaining circulating lipid levels and systemic energy homeostasis. Adipose triglyceride lipase (ATGL) and lipoprotein lipase (LPL), the primary triglyceride (TG) lipases responsible for these two spatially separate processes, are highly expressed in adipose tissue. Yet the mechanisms underlying their coordinated regulation remain undetermined. Here, we demonstrate that genetic ablation of G0S2, a specific inhibitory protein of ATGL, completely abolished diet-induced hypertriglyceridemia and significantly attenuated atherogenesis in mice. These effects were attributable to enhanced whole-body TG clearance, not altered hepatic TG secretion. Specifically, G0S2 deletion increased circulating LPL concentration and activity, predominantly through LPL production from white adipose tissue (WAT). Strikingly, transplantation of G0S2-deficient WAT normalized plasma TG levels in mice with hypertriglyceridemia. In conjunction with improved insulin sensitivity and decreased ANGPTL4 expression, the absence of G0S2 enhanced the stability of LPL protein in adipocytes, a phenomenon that could be reversed upon ATGL inhibition. Collectively, these findings highlight the pivotal role of adipocyte G0S2 in regulating both intracellular and intravascular lipolysis, and the possibility of targeting G0S2 as a viable pharmacological approach to reducing levels of circulating TGs.

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 G0S2 and HIG2 are two selective inhibitors of ATGL (also known as PNPLA2), the key enzyme for intracellular lipolysis. Whereas G0S2 regulates triglyceride (TG) mobilization in adipocytes and hepatocytes, HIG2 functions to enhance intracellular TG accumulation under hypoxic conditions. A homologous hydrophobic domain (HD) is shared by G0S2 and HIG2 (also known as HILPDA) for binding to ATGL. However, the determinants of their lipid droplet (LD) localization are unknown. Here, we study how G0S2 and HIG2 are targeted to LDs, and identify both ATGL-independent and -dependent mechanisms. Structural prediction and studies in cells reveal that ATGL-independent localization of G0S2 to both the endoplasmic reticulum (ER) and LDs is mediated by a hairpin structure consisting of two hydrophobic sequences. Positively charged residues in the hinge region play a crucial role in sorting G0S2, which initially localizes to ER, to LDs. Interestingly, the role of these positive charges becomes dispensable when ATGL is co-expressed. In comparison, HIG2, which lacks a similar hairpin structure, is dependent on ATGL for its full LD targeting. Thus, our studies identify specific structural features and mechanisms for mediating accumulation of these two ATGL inhibitors on LDs.