Mitochondrial glycerol-3-phosphate acyltransferase-deficient mice have reduced weight and liver triacylglycerol content and altered glycerolipid fatty acid composition

Absence of hormone-sensitive lipase inhibits obesity and adipogenesis in Lep ob/ob mice

Hormone-sensitive lipase (HSL) plays a crucial role in the hydrolysis of triacylglycerol and cholesteryl ester in various tissues including adipose tissues. To explore the role of HSL in the metabolism of fat and carbohydrate, we have generated mice lacking both leptin and HSL (Lep(ob/ob)/HSL(-/-)) by cross-breeding HSL(-/-) mice with genetically obese Lep(ob/ob) mice. Unexpectedly, Lep(ob/ob)/HSL(-/-) mice ate less food, gained less weight, and had lower adiposity than Lep(ob/ob)/HSL(+/+) mice. Lep(ob/ob)/HSL(-/-) mice had massive accumulation of preadipocytes in white adipose tissues with increased expression of preadipocyte-specific genes (CAAT/enhancer-binding protein beta and adipose differentiation-related protein) and decreased expression of genes characteristic of mature adipocytes (CCAAT/enhancer-binding protein alpha, peroxisome proliferator activator receptor gamma, and adipocyte determination and differentiation factor 1/sterol regulatory element-binding protein-1). Consistent with the reduced food intake, hypothalamic expression of neuropeptide Y and agouti-related peptide was decreased. Since HSL is expressed in hypothalamus, we speculate that defective generation of free fatty acids in the hypothalamus due to the absence of HSL mediates the altered expression of these orexigenic neuropeptides. Thus, deficiency of both leptin and HSL has unmasked novel roles of HSL in adipogenesis as well as in feeding behavior.

Identification of a new glycerol-3-phosphate acyltransferase isoenzyme, mtGPAT2, in mitochondria

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the initial and rate-limiting step of glycerolipid synthesis. Two distinct GPAT isoenzymes had been identified in mammalian tissues, an N-ethylmaleimide (NEM)-sensitive isoform in the endoplasmic reticulum membrane (microsomal GPAT) and an NEM-resistant form in the outer mitochondrial membrane (mtGPAT). Although only mtGPAT has been cloned, the microsomal and mitochondrial GPAT isoforms can be distinguished, because they differ in acyl-CoA substrate preference, sensitivity to inhibition by dihydroxyacetone phosphate and polymixin B, temperature sensitivity, and ability to be activated by acetone. The preponderance of evidence supports a role for mtGPAT in synthesizing the precursors for triacylglycerol synthesis. In mtGPAT(-/-) mice, PCR genotyping and Northern analysis showed successful knockout of mtGPAT; however, we detected a novel NEM-sensitive GPAT activity in mitochondrial fractions and an anti-mtGPAT immunoreactive protein in liver mitochondria, but not in microsomes. Rigorous analysis using two-dimensional gel electrophoresis revealed that the anti-mtGPAT immunoreactive proteins in wild type and mtGPAT(-/-) liver mitochondria have different isoelectric points. These results suggested the presence of a second GPAT in liver mitochondria from mtGPAT(-/-) mice. Characterization of this GPAT activity in liver from mtGPAT null mice showed that, unlike the mtGPAT activity in wild type samples, activity in mtGPAT knockout mitochondria did not prefer palmitoyl-CoA, was sensitive to inactivation by NEM, was inhibited by dihydroxyacetone phosphate and polymixin B, was temperature-sensitive, and was not activated by acetone. We conclude that a novel GPAT (mtGPAT2) with antigenic epitopes similar to those of mtGPAT is detectable in mitochondria from the livers of mtGPAT(-/-) mice.

Regulation of myocardial triacylglycerol synthesis and metabolism

Studies showing a correlation of excess myocardial triacylglycerol stores with apoptosis, fibrosis, and contractile dysfunction indicate that dysregulation of triacylglycerol metabolism may contribute to cardiac disease. This review covers the regulation of heart triacylglycerol accumulation at the critical control points of fatty acid uptake, enzymes of triacylglycerol synthesis, lipolysis, and lipoprotein secretion. These pathways are discussed in the context of the central role myocardial triacylglycerol plays in cardiac energy metabolism and heart disease.

Decreased fatty acid esterification compensates for the reduced lipolytic activity in hormone-sensitive lipase-deficient white adipose tissue

It has been observed previously that hormone-sensitive lipase-deficient (HSL-ko) mice have reduced white adipose tissue (WAT) stores compared to control mice. These findings contradict the expectation that the decreased lipolytic activity in WAT of HSL-ko mice would cause accumulation of triglycerides (TGs) in that tissue. Here we demonstrate that the cellular TG synthesis in HSL-deficient WAT is markedly reduced due to downregulation of the enzymatic activities of glycerophosphate acyltransferase, dihydroxyacetonphosphate acyltransferase, lysophosphatidate acyltransferase, and diacylglycerol acyltransferase. Fatty acid de novo synthesis is also decreased due to reduced cellular glucose uptake, reduced glucose incorporation into adipose tissue lipids, and reduced activities of acetyl:CoA carboxylase and fatty acid synthase. Finally, the activities of phosphoenolpyruvate carboxykinase (PEPCK), acyl:CoA synthetase (ACS), and glucose 6-phosphate dehydrogenase, the enzymes that provide glycerol-3-phosphate, acyl-CoA, and NADPH for TG synthesis, respectively, are decreased in HSL-ko mice. The reduced expression of the peroxisome proliferator-activated receptor gamma (PPAR gamma) target genes PEPCK, ACS, and aP2, as well as reduced mRNA levels of PPAR gamma itself, suggest the involvement of this transcription factor in the downregulation of lipogenesis. Taken together, these results establish that in the absence of HSL, the reduced NEFA production is counteracted by a drastic reduction of NEFA reesterification that provides sufficient quantities of NEFA for release into the circulation. These metabolic adaptations result in decreased fat mass in HSL-ko mice.