Cytosolic NADP+-dependent Isocitrate Dehydrogenase Plays a Key Role in Lipid Metabolism

Abstract

NADPH is an essential cofactor for many enzymatic reactions including glutathione metabolism and fat and cholesterol biosynthesis. We have reported recently an important role for mitochondrial NADP ؉ -dependent isocitrate dehydrogenase in cellular defense against oxidative damage by providing NADPH needed for the regeneration of reduced glutathione. However, the role of cytosolic NADP ؉ -dependent isocitrate dehydrogenase (IDPc) is still unclear. We report here for the first time that IDPc plays a critical role in fat and cholesterol biosynthesis. During differentiation of 3T3-L1 adipocytes, both IDPc enzyme activity and its protein content were increased in parallel in a time-dependent manner. Increased expression of IDPc by stable transfection of IDPc cDNA positively correlated with adipogenesis of 3T3-L1 cells, whereas decreased IDPc expression by an antisense IDPc vector retarded adipogenesis. Furthermore, transgenic mice with overexpressed IDPc exhibited fatty liver, hyperlipidemia, and obesity. In the epididymal fat pads of the transgenic mice, the expressions of adipocyte-specific genes including peroxisome proliferator-activated receptor ␥ were markedly elevated. The hepatic and epididymal fat pad contents of acetylCoA and malonyl-CoA in the transgenic mice were significantly lower, whereas the total triglyceride and cholesterol contents were markedly higher in the liver and serum of transgenic mice compared with those measured in wild type mice, suggesting that the consumption rate of those lipogenic precursors needed for fat biosynthesis must be increased by elevated IDPc activity. Taken together, our findings strongly indicate that IDPc would be a major NADPH producer required for fat and cholesterol synthesis.Abnormal lipid metabolism is frequently associated with obesity and hyperlipidemia. In fat and cholesterol biosynthesis, NADPH is an essential cofactor for numerous enzymes. For instance, 3-L-hydroxylacyl-coenzyme A dehydrogenase and enoyl-coenzyme A reductase in fatty acid synthesis and hydroxymethylglutaryl-coenzyme A reductase, the rate-limiting enzyme in cholesterol biosynthesis, require NADPH for their enzyme activities. It has been demonstrated that glucose-6-phosphate dehydrogenase (G6PDH), 1 6-phosphogluconate dehydrogenase, and malic enzyme are considered as the major enzymes producing cytosolic NADPH (1). Nevertheless, the activities of these enzymes were markedly lower than that of cytosolic NADP ϩ -dependent isocitrate dehydrogenase (IDPc) in the rat liver (1, 2). Consistent with this observation, McLean and co-workers (3) reported that certain adaptive changes in the pentose phosphate pathway dehydrogenases did not take place in parallel with fat synthesis in adipose tissue and suggested that a major source of NADPH for fat synthesis could be IDPc. It is worthy of note that IDPc is expressed mainly in lipogenic tissues such as liver and adipocytes, whereas G6PDH and 6-phosphogluconate dehydrogenase are expressed ubiquitously (4, 5). These data indicate that NADPH-producing IDPc may ...

Quantifying Reductive Carboxylation Flux of Glutamine to Lipid in a Brown Adipocyte Cell Line

“…Thus, the CAC enzyme is unlikely to be involved in lipogenesis from glutamine. Likewise, in rapidly growing cells the need for cytosolic NADPH for biosynthesis will favor flux of the cytosolic NADP-dependent enzyme toward ␣-ketoglutarate as shown recently in 3T3-L1 cells (10). This study also found that overexpression of cytosolic NADP-dependent enzyme in mice leads to increased lipogenesis in liver and fat, consistent with a reaction producing cytoso-lic NADPH.…”

“…An alternative to glutaminolysis is the reductive carboxylation pathway where glutamine enters the CAC as ␣-ketoglutarate and is converted to citrate via isocitrate dehydrogenase (IDH) operating in reverse of the CAC direction. Evidence for reductive carboxylation in transformed cells is the finding that [5][6][7][8][9][10][11][12][13][14] C]glutamine labels acetyl-CoA-derived carbons of lipids (6,7). Additionally, reversal of IDH has been detected in perfused liver and heart as evidenced by the mass isotopomer distribution (MID) of [ 13 C]citrate following perfusion with [ 13 C]glutamine (8,9).…”

IDH1 deficiency attenuates gluconeogenesis in mouse liver by impairing amino acid utilization

“…Amino acids can be used for lipogenesis in adipose tissues, which might explain why these tissues need high levels of cytosolic IDH1 and why overexpression of IDH1 promotes lipogenesis in adipose tissues and livers of IDH1 transgenic mice (7). In contrast, in skeletal muscle and heart where cytosolic IDH1 is absent or very low, the mitochondrial α-KG is S4C.…”

“…It was recently reported that IDH1 protects murine hepatocytes from endotoxin-induced oxidative stress by regulating the intracellular NADP + /NADPH ratio (12). Because NADPH is a major cofactor for fatty acid biosynthesis, IDH1 transgenic mice showed markedly higher liver and serum levels of total TG and cholesterol compared with WT mice, suggesting that IDH1 might be a major NADPH producer for lipid biosynthesis (7). Others have found that IDH1 mediates reductive glutamine metabolism for lipogenesis in hypoxic cells (9).…”

“…IDH1 is abundant in liver and was reported to participate in lipid biosynthesis in hepatocyte cytoplasm and peroxisomes (5,6). Mutant mice overexpressing IDH1 in liver and adipose tissue are obese, have fatty livers, and show elevated plasma triglycerides (TG) and cholesterol (7). In tumor cells, IDH1 contributes to de novo lipogenesis by generating acetyl-CoA building blocks via the NADPH-dependent reductive carboxylation of α-KG to isocitrate (8)(9)(10)(11).…”