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

Abstract

Although the enzymatic activity of isocitrate dehydrogenase 1 (IDH1) was defined decades ago, its functions in vivo are not yet fully understood. Cytosolic IDH1 converts isocitrate to α-ketoglutarate (α-KG), a key metabolite regulating nitrogen homeostasis in catabolic pathways. It was thought that IDH1 might enhance lipid biosynthesis in liver or adipose tissue by generating NADPH, but we show here that lipid contents are relatively unchanged in both IDH1-null mouse liver and IDH1-deficient HepG2 cells generated using the CRISPR-Cas9 system. Instead, we found that IDH1 is critical for liver amino acid (AA) utilization. Body weights of IDH1-null mice fed a high-protein diet (HPD) were abnormally low. After prolonged fasting, IDH1-null mice exhibited decreased blood glucose but elevated blood alanine and glycine compared with wildtype (WT) controls. Similarly, in IDH1-deficient HepG2 cells, glucose consumption was increased, but alanine utilization and levels of intracellular α-KG and glutamate were reduced. In IDH1-deficient primary hepatocytes, gluconeogenesis as well as production of ammonia and urea were decreased. In IDH1-deficient whole livers, expression levels of genes involved in AA metabolism were reduced, whereas those involved in gluconeogenesis were up-regulated. Thus, IDH1 is critical for AA utilization in vivo and its deficiency attenuates gluconeogenesis primarily by impairing α-KG-dependent transamination of glucogenic AAs such as alanine.isocitrate dehydrogenase 1 | α-ketoglutarate | gluconeogenesis | transamination | liver I socitrate dehydrogenases (IDH) convert isocitrate to α-ketoglutarate (α-KG) by reducing NADP + or NAD + . The recent discovery of IDH1 and IDH2 mutations in human cancers (1-4) has elicited new interest in defining IDH1's functions in vivo, which are still poorly understood. 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 (8-11). However, IDH1-null mice at steady-state are healthy and fertile, maintain normal body weight, and show no inflammatory symptoms (12).The liver maintains normal blood glucose levels through glycogenolysis and gluconeogenesis. Gluconeogenesis generates glucose from noncarbohydrate carbon substrates such as pyruvate, lactate, glycerol, and glucogenic amino acids (AAs). During prolonged fasting, the breakdown of skeletal muscle generates limited carbon resources in the form of AAs such as alanine. Hence, both fasting and feeding of a high-protein diet (HPD) can stimulate AA deamination and urea production in the liver (13), where AA carbon skeletons are converted into glucose or lipids. For most AAs, the amino group is transferred to α-KG to generate glutama...

Oncogenic R132 IDH1 Mutations Limit NADPH for De Novo Lipogenesis through (D)2-Hydroxyglutarate Production in Fibrosarcoma Cells

“…IDH1 is not thought to be a major source of NADPH in mammals (Fan et al, 2014). IDH1 −/− mice exhibit phenotypes only in select tissues or in response to specific stresses (i.e., nutrient deprivation) (Ye et al, 2017). In culture, overall IDH-mediated exchange flux is high, and the reverse reaction can support lipogenesis or compartment-specific redox maintenance in cancer cells under conditions of metabolic stress (Jiang et al, 2016; Metallo et al, 2011; Wise et al, 2011).…”

Cancer-associated mutation and beyond: The emerging biology of isocitrate dehydrogenases in human disease

“…In addition, IDH1-deficient mice, when fed a high-protein diet, had notably lower body weight compared to wild-type control littermates and, after prolonged fasting, showed decreased blood glucose but elevated blood alanine and glycine levels. Accordingly, gluconeogenesis, ammonia, and urea production were diminished, suggesting that IDH1 deficiency and associated drop in αKG impeded the αKG-dependent transamination of glucogenic amino acids ( 24 ). While IDH1 deficiency unexpectedly did not affect lipid content, studies of mice harboring liver and adipose tissue–specific expression of an IDH1 transgene revealed the presence of extensive fat pads characterized by adipocyte hypertrophy, accumulation of lipid droplets, and reduced levels of acetyl-CoA and malonyl-CoA, i.e., carbon precursor metabolites required for de novo fatty acid synthesis ( 25 ).…”

Glutamic-Pyruvic Transaminase 1 Facilitates Alternative Fuels for Hepatocellular Carcinoma Growth—A Small Molecule Inhibitor, Berberine

“…In the glucose-alanine cycle, exogenous alanine taken up by hepatocytes is converted to pyruvate via transamination, which is mediated by GPTs [23]. GPT1 is localized in the cytoplasm, while GPT2 is mostly in the mitochondrial matrix [24]. Previous observation of a significant increase of GPT1 protein expression instead of GPT2 upon alanine supplementation led us to hypothesize that GPT1 activation is essential to sustain alternative energy for HCC growth under alanine-rich conditions.…”