Residues of the minimal sequence of G0S2 collectively contribute to ATGL inhibition while C-and N-terminal extensions promote binding to ATGL

The protein encoded by the G0/G1 switch gene 2 (G0S2) is a potent inhibitor of adipose triglyceride lipase (ATGL) and thus an important regulator of intracellular lipolysis. Since dysfunction of lipolysis is associated with metabolic diseases including diabetes and obesity, inhibition of ATGL is considered a therapeutic strategy. G0S2 interacts with ATGL's patatin-domain to mediate non-competitive inhibition, however atomic details of the inhibition mechanism are incompletely understood. Sequences of G0S2 from higher organisms show a highly conserved N-terminal part, including a hydrophobic region covering amino acids 27 to 42. We show that predicted G0S2 orthologs from platypus, chicken and Japanese rice-fish are able to inhibit human and mouse ATGL, emphasizing the contribution of conserved amino acid to ATGL inhibition. Our site directed mutagenesis and truncation studies give insights in the protein-protein interaction on a per-residue level. We determine that the minimal sequence required for ATGL inhibition ranges from amino acids 20 to 44. Residues Y27, V28, G30, A34 G37, V39 or L42 within this sequence play a substantial role in ATGL inhibition. Furthermore, we show that unspecific interactions of the N-terminal part (amino acids 20-27) of the minimal sequence facilitate the interaction to ATGL. Our studies also demonstrate that full-length G0S2 shows higher tolerance to specific single amino acid exchanges in the hydrophobic region due to the stronger contributions of unspecific interactions. However, exchanges of more than one amino-acid in the hydrophobic region also result in the loss of function as ATGL inhibitor even in the full-length protein.

Mitotic timing is differentially controlled by A- and B-type cyclins and by CDC6 associated with a bona fide CDK inhibitor Xic1 in Xenopus laevis cell-free extract

The timing of the M-phase is precisely controlled by a CDC6-dependent mechanism inhibiting the mitotic histone H1 kinase. Here, we describe the differential regulation of the dynamics of this mitotic kinase activity by exogenous cyclin A or cyclin B in the Xenopus laevis cycling extracts. We show that the experimental increase in cyclin A modifies only the level of histone H1 kinase activity, while the cyclin B increase modifies two parameters: histone H1 kinase activity and the timing of its full activation, which is accelerated. On the other hand, the cyclin A depletion significantly delays full activation of histone H1 kinase. However, when CDC6 is added to such an extract, it inhibits cyclin B-associated histone H1 kinase, but does not modify the mitotic timing in the absence of cyclin A. Further, we show via p9 co-precipitation with Cyclin-Dependent Kinases (CDKs), that both CDC6 and the bona fide CDK1 inhibitor Xic1 associate with the mitotic CDKs. Finally, we show that the Xic1 temporarily separates from the mitotic CDKs complexes during the peak of histone H1 kinase activity. These data show the differential coordination of the M-phase progression by cyclin A- and cyclin B-dependent CDKs, confirm the critical role of the CDC6-dependent histone H1 kinase inhibition in this process, and show that CDC6 acts differentially through the cyclin B- and cyclin A-associated CDKs. This CDC6- and cyclins-dependent mechanism likely depends on the precisely regulated association of Xic1 with the mitotic CDKs complexes. We postulate that: i. the dissociation of Xic1 from the CDKs complexes allows the maximal activation of CDK1 during the M-phase, ii. the switch between cyclin A- and cyclin B-CDK inhibition upon M-phase initiation may be responsible for the diauxic growth of mitotic histone H1 kinase activity.

The lipid droplet-associated protein ABHD5 protects the heart through proteolysis of HDAC4

Catecholamines stimulate the first step of lipolysis by PKA-dependent release of the lipid droplet-associated protein ABHD5 from perilipin to co-activate the lipase ATGL. Here, we unmask a yet unrecognized proteolytic and cardioprotective function of ABHD5. ABHD5 acts in vivo and in vitro as a serine protease cleaving HDAC4. Through the production of an N-terminal polypeptide of HDAC4 (HDAC4-NT), ABHD5 inhibits MEF2-dependent gene expression and thereby controls glucose handling. ABHD5-deficiency leads to neutral lipid storage disease in mice. Cardiac-specific gene therapy of HDAC4-NT does not protect from intra-cardiomyocyte lipid accumulation but strikingly from heart failure, thereby challenging the concept of lipotoxicity-induced heart failure. ABHD5 levels are reduced in failing human hearts and murine transgenic ABHD5 expression protects from pressure-overload induced heart failure. These findings represent a conceptual advance by connecting lipid with glucose metabolism through HDAC4 proteolysis and enable new translational approaches to treat cardiometabolic disease.

Adipose Triglyceride Lipase Regulation: An Overview

Adipose triglyceride lipase (ATGL) is the key-enzyme for the release of fatty acids (FAs) from triacylglycerol (TG) stores during intracellular lipolysis producing FAs used for energy production. There is growing evidence that the products and intermediates from lipolytic breakdown during the FA mobilization process also have fundamental regulatory functions affecting cell signaling, gene expression, metabolism, cell growth, cell death, and lipotoxicity. Regulation of ATGL is therefore vital for maintaining a defined balance between lipid storage and mobilization. This review addresses the regulation of ATGL activity at the post-translational level with special emphasis on protein-mediated interaction at the site of hydrolytic action, namely to the lipid droplet.

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

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.