Distinct roles of adipose triglyceride lipase and hormone‐sensitive lipase in the catabolism of triacylglycerol estolides

Abstract

Branched esters of fatty acids and hydroxy fatty acids (FAHFAs) belong to a wide range group of lipids, some of which have anti-inflammatory and anti-diabetic properties. FAHFAs themselves belong to oligomeric FA esters, known as estolides. Depending on structure, they are divided in subfamilies (e.g. palmitic acid esters of hydroxy stearic acid, PAHSAs) including several regioisomers with different position of the branching position (e.g. 9-PAHSA). In human and mice tissue, glycerol-bound FAHFAs in triacylglycerols (TAGs), named TAG estolides, serve as metabolite reservoir of FAHFAs mobilized by lipases upon demand. Here, we studied the role of two major metabolic lipases, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), in TAG estolide and FAHFA degradation. We synthesized a panel of twenty TAG estolide isomers with different FAHFAs groups varying in branching position, chain length, saturation grade, and position on the glycerol backbone. An in silico mass spectra library was developed of all predicted catabolic intermediates. Homogenates of HEK293T cells overexpressing ATGL, or HSL were used for the determination of TAG estolide hydrolase activity of HSL and ATGL in the presence or absence of comparative gene identification-58 (CGI-58). White adipose tissue from ad libitum fed and fasted wild-type and global HSL knock-out mice was collected and TAG estolides were quantified. We found that ATGL alone or coactivated by CGI-58 efficiently liberated FAHFAs from TAG estolides with a preference for more compact substrates where the estolide branching point is located near the glycerol ester bond. ATGL also participated in transesterification and remodeling reactions leading to the formation of TAG estolides with alternative acyl compositions. HSL represented a much more potent estolide bond hydrolase for both TAG estolides and free FAHFAs. FAHFA and TAG estolide accumulation in white adipose tissue of mice lacking HSL argued for a functional role of HSL in estolide catabolism in vivo. Our data show that ATGL and HSL participate in the metabolism of estolides and TAG estolides in distinct manners and are likely to affect the lipokine function of FAHFAs.