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Low 25-hydroxyvitamin D levels correlate with the prevalence of diabetes; however, the mechanisms remain uncertain. Here, we show that nutritional deprivation-responsive mechanisms regulate vitamin D metabolism. Both fasting and diabetes suppressed hepatic cytochrome P450 (CYP) 2R1, the main vitamin D 25-hydroxylase responsible for the first bioactivation step. Overexpression of coactivator peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α), induced physiologically by fasting and pathologically in diabetes, resulted in dramatic downregulation of CYP2R1 in mouse hepatocytes in an estrogen-related receptor α (ERRα)-dependent manner. However, PGC-1α knockout did not prevent fasting-induced suppression of CYP2R1 in the liver, indicating that additional factors contribute to the CYP2R1 repression. Furthermore, glucocorticoid receptor (GR) activation repressed the liver CYP2R1, suggesting GR involvement in the regulation of CYP2R1. GR antagonist mifepristone partially prevented CYP2R1 repression during fasting, suggesting that glucocorticoids and GR contribute to the CYP2R1 repression during fasting. Moreover, fasting upregulated the vitamin D catabolizing CYP24A1 in the kidney through the PGC-1α-ERRα pathway. Our study uncovers a molecular mechanism for vitamin D deficiency in diabetes and reveals a novel negative feedback mechanism that controls crosstalk between energy homeostasis and the vitamin D pathway., (© 2019 by the American Diabetes Association.)

Overconsumption of fructose and other sugars has been linked to nonalcoholic fatty liver disease (NAFLD); however, the sugar-associated effects that lead to disease are poorly defined. In this issue of the JCI, Zhang and colleagues show that the carbohydrate response element-binding protein (ChREBP) coordinates an adaptive response to a high-fructose diet in mice and that loss of this transcription factor leads to hepatic inflammation and early signs of fibrosis. Intriguingly, ChREBP-dependent effects were due to an exaggerated activation of the proapoptotic arms of the endoplasmic reticulum stress response that is probably secondary to inappropriate derepression of cholesterol biosynthesis. These findings suggest that a previously unknown link exists between ChREBP and the regulation of cholesterol synthesis that affects liver injury.

The unfolded protein response (UPR) is a signaling pathway required to maintain endoplasmic reticulum (ER) homeostasis and hepatic lipid metabolism. Here, we identify an essential role for the inositol-requiring transmembrane kinase/endoribonuclease 1α (IRE1α)-X box binding protein 1 (XBP1) arm of the UPR in regulation of hepatic very low-density lipoprotein (VLDL) assembly and secretion. Hepatocyte-specific deletion of Ire1α reduces lipid partitioning into the ER lumen and impairs the assembly of triglyceride (TG)-rich VLDL but does not affect TG synthesis, de novo lipogenesis, or the synthesis or secretion of apolipoprotein B (apoB). The defect in VLDL assembly is, at least in part, due to decreased microsomal triglyceride-transfer protein (MTP) activity resulting from reduced protein disulfide isomerase (PDI) expression. Collectively, our findings reveal a key role for the IRE1α-XBP1s-PDI axis in linking ER homeostasis with regulation of VLDL production and hepatic lipid homeostasis that may provide a therapeutic target for disorders of lipid metabolism., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Abnormalities in very low density lipoprotein (VLDL) assembly and secretion impact intrahepatic lipid homeostasis, plasma lipoprotein profile, and energy metabolism of distal peripheral tissues. We have evaluated the role of the transcriptional coactivator, the peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), in VLDL assembly and secretion. PGC-1alpha overexpression in HepG2 cells led to diminished rates of triglyceride (TG) synthesis but strongly stimulated VLDL-TG secretion, markedly increasing the efficiency of secretion of newly synthesized TG. PGC-1alpha overexpression increased the rate of secretion of apoB100 and promoted secretion of larger, less dense VLDL particles. PGC-1alpha overexpression in intact mouse liver also stimulated rates of VLDL TG secretion and attenuated hepatic TG accumulation resulting from high fat diet feeding. To determine the molecular mechanisms mediating the effect of PGC-1alpha on VLDL assembly, we evaluated the expression of several candidate mediators known to be involved in VLDL assembly or hepatic lipid homeostasis. Cell death-inducing DFFA-like effector B (CideB) expression was greatly induced by PGC-1alpha, and siRNA against CideB reversed the effects of PGC-1alpha on the secretion of TG and VLDL-sized particles by HepG2 cells, indicating that CideB is a critical mediator of stimulatory effects of PGC-1alpha on VLDL secretion. Collectively, these data suggest that PGC-1alpha plays an important role in partitioning cytoplasmic TG toward the VLDL secretory compartments and promoting VLDL secretion via transcriptional induction of CideB.

Background: Obesity and diabetes mellitus are complex metabolic problems of pandemic proportion, contributing to significant cardiovascular mortality. Recent studies have shown altered mitochondrial function in the hearts of diabetic animals. We hypothesized that regulatory events involved in the control of mitochondrial function are activated in the prediabetic, insulin-resistant stage., Methods and Results: Morphometric analyses demonstrated that cardiac myocyte mitochondrial volume density was increased in insulin-resistant uncoupling protein-diphtheria toxin A (UCP-DTA) transgenic mice, a murine model of metabolic syndrome, compared with littermate controls. Mitochondrial DNA content and expression of genes involved in multiple mitochondrial pathways were also increased in insulin-resistant UCP-DTA hearts. The nuclear receptor, peroxisome proliferator-activated receptor-alpha (PPARalpha), is known to activate metabolic genes in the diabetic heart. Therefore, we evaluated the role of PPARalpha in the observed mitochondrial biogenesis response in the insulin-resistant heart. Insulin-resistant UCP-DTA mice crossed into a PPARalpha-null background did not exhibit evidence of mitochondrial biogenesis or induction of mitochondrial gene expression. Conversely, transgenic mice with cardiac-specific overexpression of PPARalpha exhibited signatures of cardiac mitochondrial biogenesis. A screen for candidate mediators of the PPARalpha-driven mitochondrial biogenic response revealed that expression of PPARgamma coactivator-1alpha (PGC-1alpha), a known regulator of mitochondrial biogenesis, was activated in wild-type UCP-DTA mice but not in PPARalpha-deficient UCP-DTA mice., Conclusions: These results demonstrate that mitochondrial biogenesis occurs early in the development of diabetic cardiac dysfunction through a transcriptional regulatory circuit that involves activation of PGC-1alpha gene expression by the fatty acid-activated nuclear receptor PPARalpha.

Members of the PPARgamma coactivator-1 (PGC-1) family of transcriptional coactivators serve as inducible coregulators of nuclear receptors in the control of cellular energy metabolic pathways. This Review focuses on the biologic and physiologic functions of the PGC-1 coactivators, with particular emphasis on striated muscle, liver, and other organ systems relevant to common diseases such as diabetes and heart failure.

Hypertension and diabetes are common side effects of glucocorticoid treatment. To determine whether peroxisome proliferator-activated receptor-alpha (PPAR-alpha) mediates these sequelae, mice deficient in low-density lipoprotein receptor (Ldlr-/-), with (Ppara+/+) or without (Ppara-/-) PPAR-alpha, were treated chronically with dexamethasone. Ppara+/+, but not Ppara-/-, mice developed hyperglycemia, hyperinsulinemia and hypertension. Similar effects on glucose metabolism were seen in a different model using C57BL/6 mice. Hepatic gluconeogenic gene expression was increased and insulin-mediated suppression of endogenous glucose production was less effective in dexamethasone-treated Ppara+/+ mice. Adenoviral reconstitution of PPAR-alpha in the livers of nondiabetic, normotensive, dexamethasone-treated Ppara-/- mice induced hyperglycemia, hyperinsulinemia and increased gluconeogenic gene expression. It also increased blood pressure, renin activity, sympathetic nervous activity and renal sodium retention. Human hepatocytes treated with dexamethasone and the PPAR-alpha agonist Wy14,643 induced PPARA and gluconeogenic gene expression. These results identify hepatic activation of PPAR-alpha as a mechanism underlying glucocorticoid-induced insulin resistance.

To explore the role of peroxisome proliferator-activated receptor alpha (PPARalpha)-mediated derangements in myocardial metabolism in the pathogenesis of diabetic cardiomyopathy, insulinopenic mice with PPARalpha deficiency (PPARalpha(-/-)) or cardiac-restricted overexpression [myosin heavy chain (MHC)-PPAR] were characterized. Whereas PPARalpha(-/-) mice were protected from the development of diabetes-induced cardiac hypertrophy, the combination of diabetes and the MHC-PPAR genotype resulted in a more severe cardiomyopathic phenotype than either did alone. Cardiomyopathy in diabetic MHC-PPAR mice was accompanied by myocardial long-chain triglyceride accumulation. The cardiomyopathic phenotype was exacerbated in MHC-PPAR mice fed a diet enriched in triglyceride containing long-chain fatty acid, an effect that was reversed by discontinuing the high-fat diet and absent in mice given a medium-chain triglyceride-enriched diet. Reactive oxygen intermediates were identified as candidate mediators of cardiomyopathic effects in MHC-PPAR mice. These results link dysregulation of the PPARalpha gene regulatory pathway to cardiac dysfunction in the diabetic and provide a rationale for serum lipid-lowering strategies in the treatment of diabetic cardiomyopathy.

The tremendous energy demands of the post-natal mammalian heart are fulfilled via dynamic flux through mitochondrial oxidative pathways. The capacity for energy production via fatty acid (FA) beta-oxidation pathway is determined, in part, by the regulated expression of genes encoding FA utilization enzymes and varies in accordance with diverse dietary and physiologic conditions. For example, fasting and diabetes activate the expression of cardiac FA oxidation (FAO). Peroxisome proliferator-activated receptor alpha (PPARalpha) is a ligand-activated transcription factor that is known to control the expression of many genes involved in cellular FA import and oxidation. Cardiac FA utilization rates are reduced in PPARalpha null mice due to diminished expression of genes encoding FAO enzymes. Recent work has shown that the PPARalpha regulatory pathway is deactivated in pathologic cardiac hypertrophy and hypoxia, two circumstances characterized by reduced FAO and increased dependence on glucose as a fuel source. Conversely, the activity of the PPARalpha gene regulatory pathway is increased in the diabetic heart, which relies primarily on FAO for energy production. In fact, evidence is emerging that excessive FA import and oxidation may be a cause of pathologic cardiac remodeling in the diabetic heart. This review summarizes the regulation of cardiac substrate utilization pathways via the PPARalpha complex in the normal and diseased heart.

Recent evidence has defined an important role for PPARalpha in the transcriptional control of cardiac energy metabolism. To investigate the role of PPARalpha in the genesis of the metabolic and functional derangements of diabetic cardiomyopathy, mice with cardiac-restricted overexpression of PPARalpha (MHC-PPAR) were produced and characterized. The expression of PPARalpha target genes involved in cardiac fatty acid uptake and oxidation pathways was increased in MHC-PPAR mice. Surprisingly, the expression of genes involved in glucose transport and utilization was reciprocally repressed in MHC-PPAR hearts. Consistent with the gene expression profile, myocardial fatty acid oxidation rates were increased and glucose uptake and oxidation decreased in MHC-PPAR mice, a metabolic phenotype strikingly similar to that of the diabetic heart. MHC-PPAR hearts exhibited signatures of diabetic cardiomyopathy including ventricular hypertrophy, activation of gene markers of pathologic hypertrophic growth, and transgene expression-dependent alteration in systolic ventricular dysfunction. These results demonstrate that (a) PPARalpha is a critical regulator of myocardial fatty acid uptake and utilization, (b) activation of cardiac PPARalpha regulatory pathways results in a reciprocal repression of glucose uptake and utilization pathways, and (c) derangements in myocardial energy metabolism typical of the diabetic heart can become maladaptive, leading to cardiomyopathy.

Low 25-hydroxyvitamin D levels correlate with the prevalence of diabetes; however, the mechanisms remain uncertain. Here, we show that nutritional deprivation-responsive mechanisms regulate vitamin D metabolism. Both fasting and diabetes suppressed hepatic cytochrome P450 (CYP) 2R1, the main vitamin D 25-hydroxylase responsible for the first bioactivation step. Overexpression of coactivator peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α), induced physiologically by fasting and pathologically in diabetes, resulted in dramatic downregulation of CYP2R1 in mouse hepatocytes in an estrogen-related receptor α (ERRα)-dependent manner. However, PGC-1α knockout did not prevent fasting-induced suppression of CYP2R1 in the liver, indicating that additional factors contribute to the CYP2R1 repression. Furthermore, glucocorticoid receptor (GR) activation repressed the liver CYP2R1, suggesting GR involvement in the regulation of CYP2R1. GR antagonist mifepristone partially prevented CYP2R1 repression during fasting, suggesting that glucocorticoids and GR contribute to the CYP2R1 repression during fasting. Moreover, fasting upregulated the vitamin D catabolizing CYP24A1 in the kidney through the PGC-1α-ERRα pathway. Our study uncovers a molecular mechanism for vitamin D deficiency in diabetes and reveals a novel negative feedback mechanism that controls crosstalk between energy homeostasis and the vitamin D pathway. [ABSTRACT FROM AUTHOR]

Thiazolidinedione (TZD) insulin sensitizers have the potential to effectively treat a number of human diseases, however the currently available agents have dose-limiting side effects that are mediated via activation of the transcription factor PPARγ. We have recently shown PPARγ-independent actions of TZD insulin sensitizers, but the molecular target of these molecules remained to be identified. Here we use a photo-catalyzable drug analog probe and mass spectrometry-based proteomics to identify a previously uncharacterized mitochondrial complex that specifically recognizes TZDs. These studies identify two well-conserved proteins previously known as brain protein 44 (BRP44) and BRP44 Like (BRP44L), which recently have been renamed Mpc2 and Mpc1 to signify their function as a mitochondrial pyruvate carrier complex. Knockdown of Mpc1 or Mpc2 in Drosophila melanogaster or pre-incubation with UK5099, an inhibitor of pyruvate transport, blocks the crosslinking of mitochondrial membranes by the TZD probe. Knockdown of these proteins in Drosophila also led to increased hemolymph glucose and blocked drug action. In isolated brown adipose tissue (BAT) cells, MSDC-0602, a PPARγ-sparing TZD, altered the incorporation of 13C-labeled carbon from glucose into acetyl CoA. These results identify Mpc1 and Mpc2 as components of the mitochondrial target of TZDs (mTOT) and suggest that understanding the modulation of this complex, which appears to regulate pyruvate entry into the mitochondria, may provide a viable target for insulin sensitizing pharmacology. [ABSTRACT FROM AUTHOR]

Abstract: Myocardial energy metabolism is an important determinant of cardiac structure and function. Modulating metabolism is therefore an attractive therapeutic avenue for the treatment of cardiac disease. The peroxisome proliferator-activated receptor family (PPARα, β/δ, γ) of nuclear receptor transcription factors is an important regulator of cardiac metabolism and has been targeted for pharmacologic therapies designed to modulate metabolism. The PPARs control myocardial metabolism by transcriptionally regulating genes encoding enzymes involved in fatty acid and glucose utilization. The expression and activity of the PPARs and their coactivator protein PGC-1α is dynamically regulated in several cardiomyopathic and metabolic diseases. This review will summarize these findings and other recent studies regarding the effects of experimental PPAR activation and deactivation and its potential impact on cardiomyopathic remodeling. [Copyright &y& Elsevier]

Diabetic patients develop a cardiomyopathy that consists of ventricular hypertrophy and diastolic dysfunction. Although the pathogenesis of this condition is poorly understood, previous studies implicated abnormal G-protein activation. In this work, mice with cardiac overexpression of the transcription factor peroxisome proliferator-activated receptor-alpha (PPAR-alpha) were examined as a model of diabetic cardiomyopathy. PPAR-alpha transgenic mice develop spontaneous cardiac hypertrophy, contractile dysfunction, and "fetal" gene induction. We examined the role of abnormal G-protein activation in the pathogenesis of cardiac dysfunction by crossing PPAR-alpha mice with transgenic mice with cardiac-specific overexpression of regulator of G-protein signaling subtype 4 (RGS4), a GTPase activating protein for Gq and Gi. Generation of compound transgenic mice demonstrated that cardiac RGS4 overexpression ameliorated the cardiomyopathic phenotype that occurred as a result of PPAR-alpha overexpression without affecting the metabolic abnormalities seen in these hearts. Next, transgenic mice with increased or decreased cardiac Gq signaling were made diabetic by injection with streptozotocin (STZ). RGS4 transgenic mice were resistant to STZ-induced cardiac fetal gene induction. Transgenic mice with cardiac-specific expression of mutant Galphaq, Galphaq-G188S, that is resistant to RGS protein action were sensitized to the development of STZ-induced cardiac fetal gene induction and bradycardia. These results establish that Gq-mediated signaling plays a critical role in the pathogenesis of diabetic cardiomyopathy. [ABSTRACT FROM AUTHOR]

Lipin 1 is a bifunctional protein that regulates gene transcription and, as a Mg2+-dependent phosphatidic acid phosphatase (PAP), is a key enzyme in the biosynthesis of phospholipids and triacylglycerol. We describe here the functional interaction between lipin 1 and the nuclear factor of activated T cells c4 (NFATc4). Lipin 1 represses NFATc4 transcriptional activity through protein-protein interaction, and lipin 1 is present at the promoters of NFATc4 transcriptional targets in vivo. Catalytically active and inactive lipin 1 can suppress NFATc4 transcriptional activity, and this suppression may involve recruitment of histone deacetylases to target promoters. In fat pads from mice deficient for lipin 1 (fld mice) and in 3T3-L1 adipocytes depleted of lipin 1 there is increased expression of several NFAT target genes including tumor necrosis factor alpha, resistin, FABP4, and PPARγ. Finally, both lipin 1 protein and total PAP activity are decreased with increasing adiposity in the visceral, but not subcutaneous, fat pads of ob/ob mice. These observations place lipin 1 as a potentially important link between triacylglycerol synthesis and adipose tissue inflammation. [ABSTRACT FROM AUTHOR]

Abstract: Peroxisome proliferator-activated receptor alpha (PPAR-α) is a ligand-activated nuclear receptor transcription factor that regulates the fatty acid β-oxidation. An in vitro assay identified the p-methoxy phenyl ureido thiobutyric acid derivative KSM-01 (IC50 =0.28±0.09nM) having a higher affinity to activate PPAR-α than the PPAR-α agonist GW7647 (IC50 =0.46±0.19nM). In this study, we report the synthesis and initial in vivo evaluation of [11C]KSM-01. The radiosynthesis was carried out by first alkylating the corresponding p-phenol precursor with [11C]MeI in DMF using NaOH, followed by deprotection of the t-butyl ester group by TFA, yielding [11C]KSM-01. SUV analysis of dynamic micro PET/CT imaging data showed that [11C]KSM-01 accumulation was ∼2.0-fold greater in cardiac-specific PPAR-α overexpressing transgenic mice compared to wild-type littermates. The post-PET biodistribution studies were consistent with these results and demonstrated 2.5-fold greater radiotracer uptake in the heart of transgenic mice compared to the wild-type littermates. These results demonstrate the potential utility of PPAR-α agonists as PET radiopharmaceuticals. [Copyright &y& Elsevier]

Abstract: Lipin family proteins (lipin 1, 2, and 3) are bifunctional intracellular proteins that regulate metabolism by acting as coregulators of DNA-bound transcription factors and also dephosphorylate phosphatidate to form diacylglycerol [phosphatidate phosphohydrolase activity] in the triglyceride synthesis pathway. Herein, we report that lipin 1 is enriched in heart and that hearts of mice lacking lipin 1 (fld mice) exhibit accumulation of phosphatidate. We also demonstrate that the expression of the gene encoding lipin 1 (Lpin1) is under the control of the estrogen-related receptors (ERRs) and their coactivator the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α). PGC-1α, ERRα, or ERRγ overexpression increased Lpin1 transcription in cultured ventricular myocytes and the ERRs were associated with response elements in the first intron of the Lpin1 gene. Concomitant RNAi-mediated knockdown of ERRα and ERRγ abrogated the induction of lipin 1 expression by PGC-1α overexpression. Consistent with these data, 3-fold overexpression of PGC-1α in intact myocardium of transgenic mice increased cardiac lipin 1 and ERRα/γ expression. Similarly, injection of the β2-adrenergic agonist clenbuterol induced PGC-1α and lipin 1 expression, and the induction in lipin 1 after clenbuterol occurred in a PGC-1α-dependent manner. In contrast, expression of PGC-1α, ERRα, ERRγ, and lipin 1 was down-regulated in failing heart. Cardiac phosphatidic acid phosphohydrolase activity was also diminished, while cardiac phosphatidate content was increased, in failing heart. Collectively, these data suggest that lipin 1 is the principal lipin protein in the myocardium and is regulated in response to physiologic and pathologic stimuli that impact cardiac metabolism. [Copyright &y& Elsevier]