Hypoxia-inducible lipid droplet-associated protein inhibits adipose triglyceride lipase

Abstract

Elaborate control mechanisms of intracellular triacylglycerol (TAG) breakdown are critically involved in the maintenance of energy homeostasis. Hypoxia-inducible lipid droplet-associated protein (HILPDA)/hypoxia-inducible gene-2 (Hig-2) has been shown to affect intracellular TAG levels, yet, the underlying molecular mechanisms are unclear. Here, we show that HILPDA inhibits adipose triglyceride lipase (ATGL), the enzyme catalyzing the first step of intracellular TAG hydrolysis. HILPDA shares structural similarity with G0/G1 switch gene 2 (G0S2), an established inhibitor of ATGL. HILPDA inhibits ATGL activity in a dose-dependent manner with an IC50 value of ∼2 μM. ATGL inhibition depends on the direct physical interaction of both proteins and involves the N-terminal hydrophobic region of HILPDA and the N-terminal patatin domain-containing segment of ATGL. Finally, confocal microscopy combined with Förster resonance energy transfer-fluorescence lifetime imaging microscopy analysis indicated that HILPDA and ATGL colocalize and physically interact intracellularly. These findings provide a rational biochemical explanation for the tissue-specific increased TAG accumulation in HILPDA-overexpressing transgenic mouse models.

Of mice and men: The physiological role of adipose triglyceride lipase (ATGL)

“…As an antipode for CGI-58, G0/G1 switch gene 2 ( G0S2 ) potently inhibits ATGL activity by direct protein-protein interaction and through impeding substrate accessibility [ 46 ]. Recently, it has been shown that ATGL is profoundly inhibited by a small protein called hypoxia-inducible LD-associated protein ( HILPDA , also known as hypoxia inducible gene 2, HIG2) which directly binds to the patatin domain of ATGL [ 47 , 48 ]. Another player in the team of ATGL regulators is pigment epithelium-derived factor ( PEDF ).…”

“…In humans, ATGL is also activated by CGI-58 [ 39 , 51 ] and inhibited by G0S2 [ 52 ] and HILPDA [ 48 ] ( Fig. 1 ).…”

The Lipolysome—A Highly Complex and Dynamic Protein Network Orchestrating Cytoplasmic Triacylglycerol Degradation

“…HILPDA directly binds and inhibits ATGL in vitro [75,77]. The contact region between HILPDA and ATGL was mapped to the hydrophobic N-terminus of HILPDA and the patatin domain of ATGL [77]. Consistently, deleting the amino acid region Leu7 to Gly11 in HILPDA completely abolishes its binding to ATGL [75].…”

“…HILPDA is structurally similar to G0S2, is ubiquitously expressed upon hypoxia, and localizes to the LD [74][75][76]. HILPDA directly binds and inhibits ATGL in vitro [75,77]. The contact region between HILPDA and ATGL was mapped to the hydrophobic N-terminus of HILPDA and the patatin domain of ATGL [77].…”

“…Kinetic studies determining the IC 50 values revealed that HILPDA is, by two orders of magnitude, less potent as an ATGL inhibitor than G0S2 [77], raising doubts as to whether it is a direct regulator of lipolysis [78]. While HILPDA overexpression or gene deletion affected cellular TAG content in various cancer cells and hepatocytes [74][75][76], no clear impact on TAG hydrolysis was observed in adipocytes [77].…”