The mitochondrial NAD+ transporter SLC25A51 is a fasting-induced gene affecting SIRT3 functions.

INTRODUCTION: Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism and energy production. NAD+-dependent deacetylase sirtuin 3 (SIRT3) regulates the acetylation levels of mitochondrial proteins that are involved in mitochondrial homeostasis. Fasting up-regulates hepatic SIRT3 activity, which requires mitochondrial NAD+. What is the mechanism, then, to transport more NAD+ into mitochondria to sustain enhanced SIRT3 activity during fasting? OBJECTIVE: SLC25A51 is a recently discovered mitochondrial NAD+ transporter. We tested the hypothesis that, during fasting, increased expression of SLC25A51 is needed for enhanced mitochondrial NAD+ uptake to sustain SIRT3 activity. Because the fasting-fed cycle and circadian rhythm are closely linked, we further tested the hypothesis that SLC25A51 is a circadian regulated gene. METHODS: We examined Slc25a51 expression in the liver of fasted mice, and examined its circadian rhythm in wild-type mice and those with liver-specific deletion of the clock gene BMAL1 (LKO). We suppressed Slc25a51 expression in hepatocytes and the mouse liver using shRNA-mediated knockdown, and then examined mitochondrial NAD+ levels, SIRT3 activities, and acetylation levels of SIRT3 target proteins (IDH2 and ACADL). We measured mitochondrial oxygen consumption rate using Seahorse analysis in hepatocytes with reduced Slc25a51 expression. RESULTS: We found that fasting induced the hepatic expression of Slc25a51, and its expression showed a circadian rhythm-like pattern that was disrupted in LKO mice. Reduced expression of Slc25a51 in hepatocytes decreased mitochondrial NAD+ levels and SIRT3 activity, reflected by increased acetylation of SIRT3 targets. Slc25a51 knockdown reduced the oxygen consumption rate in intact hepatocytes. Mice with reduced Slc25a51 expression in the liver manifested reduced hepatic mitochondrial NAD+ levels, hepatic steatosis and hypertriglyceridemia. CONCLUSIONS: Slc25a51 is a fasting-induced gene that is needed for hepatic SIRT3 functions.

The mitochondrial NAD kinase functions as a major metabolic regulator upon increased energy demand.

OBJECTIVE: The mitochondrial nicotinamide adenine dinucleotide (NAD) kinase (MNADK) mediates de novo mitochondrial NADP biosynthesis by catalyzing the phosphorylation of NAD to yield NADP. In this study, we investigated the function and mechanistic basis by which MNADK regulates metabolic homeostasis. METHODS: Generalized gene set analysis by aggregating human patient genomic databases, metabolic studies with genetically engineered animal models, mitochondrial bioenergetic analysis, as well as gain- and loss- of-function studies were performed to address the functions and mechanistic basis by which MNADK regulates energy metabolism and redox state associated with metabolic disease. RESULTS: Human MNADK common gene variants or decreased expression of the gene are significantly associated with the occurrence of type-2 diabetes, non-alcoholic fatty liver disease (NAFLD), or hepatocellular carcinoma (HCC). Ablation of the MNADK gene in mice led to decreased fat oxidation, coincident with increased respiratory exchange ratio (RER) and decreased energy expenditure upon energy demand triggered by endurance exercise or fasting. On an atherogenic high-fat diet (HFD), MNADK-null mice exhibited hepatic insulin resistance and glucose intolerance, indicating a type-2 diabetes-like phenotype in the absence of MNADK. MNADK deficiency led to a decrease in mitochondrial NADP(H) but an increase in cellular reactive oxygen species (ROS) in mouse livers. Consistently, protein levels of the major metabolic regulators or enzymes were decreased, while their acetylation modifications were increased in the livers of MNADK-null mice. Feeding mice with a HFD caused S-nitrosylation (SNO) modification, a posttranslational modification that represses protein activities, on MNADK protein in the liver. Reconstitution of an SNO-resistant MNADK variant, MNADK-S193, into MNADK-null mice mitigated hepatic steatosis induced by HFD. CONCLUSION: MNADK, the only known mammalian mitochondrial NAD kinase, plays important roles in preserving energy homeostasis to mitigate the risk of metabolic disorders.

Deficiency of the Mitochondrial NAD Kinase Causes Stress-Induced Hepatic Steatosis in Mice.

BACKGROUND & AIMS: The mitochondrial nicotinamide adenine dinucleotide (NAD) kinase (NADK2, also called MNADK) catalyzes phosphorylation of NAD to yield NADP. Little is known about the functions of mitochondrial NADP and MNADK in liver physiology and pathology. We investigated the effects of reduced mitochondrial NADP by deleting MNADK in mice. METHODS: We generated MNADK knockout (KO) mice on a C57BL/6NTac background; mice with a wild-type Mnadk gene were used as controls. Some mice were placed on an atherogenic high-fat diet (16% fat, 41% carbohydrate, and 1.25% cholesterol supplemented with 0.5% sodium cholate) or given methotrexate intraperitoneally. We measured rates of fatty acid oxidation in primary hepatocytes using radiolabeled palmitate and in mice using indirect calorimetry. We measured levels of reactive oxygen species in mouse livers and primary hepatocytes. Metabolomic analyses were used to quantify serum metabolites, such as amino acids and acylcarnitines. RESULTS: The KO mice had metabolic features of MNADK-deficient patients, such as increased serum concentrations of lysine and C10:2 carnitine. When placed on the atherogenic high-fat diet, the KO mice developed features of nonalcoholic fatty liver disease and had increased levels of reactive oxygen species in livers and primary hepatocytes, compared with control mice. During fasting, the KO mice had a defect in fatty acid oxidation. MNADK deficiency reduced the activation of cAMP-responsive element binding protein-hepatocyte specific and peroxisome proliferator-activated receptor alpha, which are transcriptional activators that mediate the fasting response. The activity of mitochondrial sirtuins was reduced in livers of the KO mice. Methotrexate inhibited the catalytic activity of MNADK in hepatocytes and in livers in mice with methotrexate injection. In mice given injections of methotrexate, supplementation of a diet with nicotinamide riboside, an NAD precursor, replenished hepatic NADP and protected the mice from hepatotoxicity, based on markers such as increased level of serum alanine aminotransferase. CONCLUSION: MNADK facilitates fatty acid oxidation, counteracts oxidative damage, maintains mitochondrial sirtuin activity, and prevents metabolic stress-induced non-alcoholic fatty liver disease in mice.

MiR-27b augments bone marrow progenitor cell survival via suppressing the mitochondrial apoptotic pathway in Type 2 diabetes.

Bone marrow-derived progenitor cells (BMPCs) are potential candidates for autologous cell therapy in tissue repair and regeneration because of their high angiogenic potential. However, increased progenitor cell apoptosis in diabetes directly limits their success in the clinic. MicroRNAs are endogenous noncoding RNAs that regulate gene expression at the posttranscriptional level, but their roles in BMPC-mediated angiogenesis are incompletely understood. In the present study, we tested the hypothesis that the proangiogenic miR-27b inhibits BMPC apoptosis in Type 2 diabetes. Bone marrow-derived EPCs from adult male Type 2 diabetic db/db mice and their normal littermates db/+ mice were used. MiR-27b expression (real-time PCR) in EPCs was decreased after 24 h of exposure to methylglyoxal (MGO) or oxidized low-density lipoprotein but not high glucose, advanced glycation end products, the reactive oxygen species generator LY83583, or H2O2 The increase in BMPC apoptosis in the diabetic mice was rescued following transfection with a miR-27b mimic, and the increased apoptosis induced by MGO was also rescued by the miR-27b mimic. p53 protein expression and the Bax/Bcl-2 ratio in EPCs (Western blot analyses) were significantly higher in db/db mice, both of which were suppressed by miR-27b. Furthermore, mitochondrial respiration, as measured by oxygen consumption rate, was enhanced by miR-27b in diabetic BMPCs, with concomitant decrease of mitochondrial Bax/Bcl-2 ratio. The 3' UTR binding assays revealed that both Bax, and its activator RUNX1, were direct targets of miR-27b, suggesting that miR-27b inhibits Bax expression in both direct and indirect manners. miR-27b prevents EPC apoptosis in Type 2 diabetic mice, at least in part, by suppressing p53 and the Bax/Bcl-2 ratio. These findings may provide a mechanistic basis for rescuing BMPC dysfunction in diabetes for successful autologous cell therapy.

A lipasin/Angptl8 monoclonal antibody lowers mouse serum triglycerides involving increased postprandial activity of the cardiac lipoprotein lipase.

Lipasin/Angptl8 is a feeding-induced hepatokine that regulates triglyceride (TAG) metabolism; its therapeutical potential, mechanism of action, and relation to the lipoprotein lipase (LPL), however, remain elusive. We generated five monoclonal lipasin antibodies, among which one lowered the serum TAG level when injected into mice, and the epitope was determined to be EIQVEE. Lipasin-deficient mice exhibited elevated postprandial activity of LPL in the heart and skeletal muscle, but not in white adipose tissue (WAT), suggesting that lipasin suppresses the activity of LPL specifically in cardiac and skeletal muscles. Consistently, mice injected with the effective antibody or with lipasin deficiency had increased postprandial cardiac LPL activity and lower TAG levels only in the fed state. These results suggest that lipasin acts, at least in part, in an endocrine manner. We propose the following model: feeding induces lipasin, activating the lipasin-Angptl3 pathway, which inhibits LPL in cardiac and skeletal muscles to direct circulating TAG to WAT for storage; conversely, fasting induces Angptl4, which inhibits LPL in WAT to direct circulating TAG to cardiac and skeletal muscles for oxidation. This model suggests a general mechanism by which TAG trafficking is coordinated by lipasin, Angptl3 and Angptl4 at different nutritional statuses.

Exome sequencing identifies novel ApoB loss-of-function mutations causing hypobetalipoproteinemia in type 1 diabetes.

AIM: Diabetic patients commonly suffer from disturbances in production and clearance of plasma lipoproteins, known as diabetic dyslipidemia, resulting in an increased risk of coronary heart disease. The study aimed to examine the cause of hypobetalipoproteinemia in two patients with type 1 diabetes. METHODS: The Diabetes Control and Complications Trial (DCCT) is a study demonstrating that intensive blood glucose control delays the onset and progression of type 1 diabetes complications. Hypobetalipoproteinemia was present in two DCCT subjects, IDs 1427 and 1078, whose LDL-C levels were 36 and 28 mg/dL, respectively, and triglyceride levels were 20 and 28 mg/dL, respectively. We performed exome sequencing on genomic DNA from the two patients with hypobetalipoproteinemia. RESULTS: The subjects 1427 and 1078 had heterozygous loss-of-function mutations in the gene apolipoprotein B (ApoB), and these mutations resulted in premature stop codons at amino acid 1333 (ApoB-29) and 3680 (ApoB-81), respectively. Indeed, the plasma ApoB level of subject 1427 (19 mg/dL) was the lowest and that of subject 1078 (26 mg/dL) was the second to the lowest among all the 1,441 DCCT participants. Sequencing genomic DNA of family members showed that probands 1427 and 1078 inherited the mutations from the father and the mother, respectively. CONCLUSIONS: The identification of ApoB loss-of-function mutations in type 1 diabetic patients presents innovative cases to study the interaction between hypobetalipoproteinemia and insulin deficiency.

Elevated circulating lipasin/betatrophin in human type 2 diabetes and obesity.

Lipasin (also known as C19ORF80, RIFL, ANGPTL8 and betatrophin) is a newly discovered circulating factor that regulates lipid metabolism and promotes pancreatic ß-cell proliferation. Whether circulating levels of lipasin in humans are altered in a) type 2 diabetes; b) obesity and c) the postprandial state, however, is unknown. The current study aimed to compare serum lipasin levels in those who were a) non-diabetic (N=15) or diabetic (BMI- and age-matched; N=14); b) lean or obese (N=53 totally) and c) fasting and 2 hours following a defined meal (N=12). Serum lipasin levels were determined by the enzyme-linked immunosorbent assay. Lipasin levels [mean±SEM] were increased by more than two fold (P<0.001) in the diabetic patients (5.56±0.73 ng/mL) as compared to the control subjects (2.19±0.24 ng/mL). Serum lipasin levels were positively correlated with BMI (rho=0.49, P<0.001), and showed a 35% increase 2 hours following a defined meal (P=0.009). Therefore, lipasin/betatrophin is nutritionally-regulated hepatokine that is increased in human type 2 diabetes and obesity.

Lipasin, thermoregulated in brown fat, is a novel but atypical member of the angiopoietin-like protein family.

Hyperlipidemia is a major contributor to cardiovascular diseases. Members of the angiopoietin-like protein family (ANGPTLs) are important determinants of blood lipid levels. Lipasin, a newly identified gene that regulates serum triglycerides, is homologous to ANGPTL3's N-terminal domain, which is sufficient and necessary for blood lipid regulation. Brown fat is critical in mediating energy homeostasis. Thermogenesis is the primary function of brown fat, in which Lipasin and some ANGPTLs are abundant; it is unknown, however, whether these genes are thermoregulated. We therefore comprehensively examined the thermoregulation of Lipasin and ANGPTLs in brown fat. Here we show that Lipasin is a novel but atypical member of the ANGPTL family because it is within the same branch as ANGPTL3 and 4 by phylogenetic analysis. The mRNA levels of Lipasin are dramatically increased in the cold environment (4 °C for 4 h) whereas those of ANGPTL4 and ANGPTL2 are suppressed. Fasting dramatically suppresses Lipasin but increases ANGPTL4. High-fat diet treatment increases Lipasin, but reduces ANGPTL2. The distinct transcriptional regulations of Lipasin, ANGPTL2 and ANGPTL4 in brown fat in response to cold exposure and nutritional stimulation suggest distinct physiological roles for ANGPTL family members in mediating thermogenesis and energy homeostasis.

Core protein machinery for mammalian phosphatidylinositol 3,5-bisphosphate synthesis and turnover that regulates the progression of endosomal transport. Novel Sac phosphatase joins the ArPIKfyve-PIKfyve complex.

Perturbations in phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2)-synthesizing enzymes result in enlarged endocytic organelles from yeast to humans, indicating evolutionarily conserved function of PtdIns(3,5)P2 in endosome-related events. This is reinforced by the structural and functional homology of yeast Vac14 and human Vac14 (ArPIKfyve), which activate yeast and mammalian PtdIns(3,5)P2-producing enzymes, Fab1 and PIKfyve, respectively. In yeast, PtdIns(3,5)P2-specific phosphatase, Fig4, in association with Vac14, turns over PtdIns(3,5)P2, but whether such a mechanism operates in mammalian cells and what the identity of mammalian Fig4 may be are unknown. Here we have identified and characterized Sac3, a Sac domain phosphatase, as the Fig4 mammalian counterpart. Endogenous Sac3, a widespread 97-kDa protein, formed a stable ternary complex with ArPIKfyve and PIKfyve. Concordantly, Sac3 cofractionated and colocalized with ArPIKfyve and PIKfyve. The intrinsic Sac3(WT) phosphatase activity preferably hydrolyzed PtdIns(3,5)P2 in vitro, although the other D5-phosphorylated polyphosphoinositides were also substrates. Ablation of endogenous Sac3 by short interfering RNAs elevated PtdIns(3,5)P2 in (32)P-labeled HEK293 cells. Ectopically expressed Sac3(WT) in COS cells colocalized with and dilated EEA1-positive endosomes, consistent with the PtdIns(3,5)P2 requirement in early endosome dynamics. In vitro reconstitution of carrier vesicle formation from donor early endosomes revealed a gain of function upon Sac3 loss, whereas PIKfyve or ArPIKfyve protein depletion produced a loss of function. These data demonstrate a coupling between the machinery for PtdIns(3,5)P2 synthesis and turnover achieved through a physical assembly of PIKfyve, ArPIKfyve, and Sac3. We suggest that the tight regulation in PtdIns(3,5)P2 homeostasis is mechanistically linked to early endosome dynamics in the course of cargo transport.

Myocardin enhances Smad3-mediated transforming growth factor-beta1 signaling in a CArG box-independent manner: Smad-binding element is an important cis element for SM22alpha transcription in vivo.

Transforming growth factor (TGF)-beta1 is an important cytokine involved in various diseases. However, the molecular mechanism whereby TGF-beta1 signaling modulates the regulatory network for smooth muscle gene transcription remains largely unknown. To address this question, we previously identified a Smad-binding element (SBE) in the SM22alpha promoter as one of the TGF-beta1 response elements. Here, we show that mutation of the SBE reduces the activation potential of a SM22alpha promoter in transgenic mice during embryogenesis. Chromatin immunoprecipitation assays reveal that TGF-beta1 induces Smad3 binding to the SM22alpha promoter in vivo. A multimerized SBE promoter responsive to TGF-beta1 signaling is highly activated by Smad3 but not by the closely related Smad2. Intriguingly, myocardin (Myocd), a known CArG box-dependent serum response factor coactivator, participates in Smad3-mediated TGF-beta1 signaling and synergistically stimulates Smad3-induced SBE promoter activity independent of the CArG box; no such synergy is seen with Smad2. Importantly, Myocd cooperates with Smad3 to activate the wild-type SM22alpha, SM myosin heavy chain, and SMalpha-actin promoters; they also activate the CArG box-mutated SM22alpha promoter as well as the CArG box-independent aortic carboxypeptidase-like protein promoter. Immunopreciptiation assays reveal that Myocd and Smad3 directly interact both in vitro and in vivo. Mutagenesis studies indicate that the C-terminal transactivation domains of Myocd and Smad3 are required for their functional synergy. These results reveal a novel regulatory mechanism whereby Myocd participates in TGF-beta1 signal pathway through direct interaction with Smad3, which binds to the SBEs. This is the first demonstration that Myocd can act as a transcriptional coactivator of the smooth muscle regulatory network in a CArG box-independent manner.