A randomized clinical trial evaluating the effect of empagliflozin on triglycerides in obese adults: Role of visceral fat

Background

Empagliflozin, a sodium glucose cotransporter 2 inhibitor, is a medication to treat type 2 diabetes. The effect of empagliflozin in persons without diabetes has received less attention. Here we conducted a randomized, double-blind placebo-controlled clinical trial to examine the effect of empagliflozin on plasma triglycerides in obese non-diabetic adults.

Methods

Participants (n = 35; BMI ≥ 30 kg/m²) underwent body composition assessments using MRI, and were randomly assigned to either placebo or empagliflozin (10 mg/d) for three months. At the baseline and post-treatment visit, after an overnight fast, blood was drawn for biochemical analysis. Participants received [U–¹³C₃]glycerol orally followed by multiple blood draws over 3 h to examine glycerol incorporation into triglycerides using NMR spectroscopy.

Results

The changes in blood triglyceride concentration with empagliflozin therapy related to the mass of baseline visceral adipose tissue (VAT; r = 0.53, p = 0.04). Empagliflozin slightly lowered triglycerides in obese subjects with low VAT, but increased triglycerides in the subjects with high VAT. Consistently, empagliflozin effectively suppressed triglyceride synthesis following [U–¹³C₃]glycerol administration in the subjects with low VAT (p < 0.05), but not in the subjects with high VAT. The subjects with high VAT lost body weight after three months of empagliflozin treatment. In all subjects, about 20% of the triglyceride backbone originated from mitochondrial metabolism of glycerol.

Conclusions

The effect of empagliflozin on triglycerides in obese adults differed depending on VAT. Empagliflozin suppressed triglyceride synthesis in the subjects with low VAT, but tended to increase triglycerides in those with high VAT.

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Visceral fat modulates the effect of empagliflozin on triglycerides in obese adults.

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Empagliflozin suppresses triglyceride synthesis in obese adults with low visceral fat.

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Empagliflozin tends to increase triglycerides in obese adults with high visceral fat.

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Empagliflozin induces weight loss in obese adults with high visceral fat.

Phosphoenolpyruvate carboxykinase in cell metabolism: Roles and mechanisms beyond gluconeogenesis

BACKGROUND

Phosphoenolpyruvate carboxykinase (PCK) has been almost exclusively recognized as a critical enzyme in gluconeogenesis, especially in the liver and kidney. Accumulating evidence has shown that the enhanced activity of PCK leads to increased glucose output and exacerbation of diabetes, whereas the defects of PCK result in lethal hypoglycemia. Genetic mutations or polymorphisms are reported to be related to the onset and progression of diabetes in humans.

SCOPE OF REVIEW

Recent studies revealed that the PCK pathway is more complex than just gluconeogenesis, depending on the health or disease condition. Dysregulation of PCK may contribute to the development of obesity, cardiac hypertrophy, stroke, and cancer. Moreover, a regulatory network with multiple layers, from epigenetic regulation, transcription regulation, to posttranscription regulation, precisely tunes the expression of PCK. Deciphering the molecular basis that regulates PCK may pave the way for developing practical strategies to treat metabolic dysfunction.

MAJOR CONCLUSIONS

In this review, we summarize the metabolic and non-metabolic roles of the PCK enzyme in cells, especially beyond gluconeogenesis. We highlight the distinct functions of PCK isoforms (PCK1 and PCK2), depict a detailed network regulating PCK's expression, and discuss its clinical relevance. We also discuss the therapeutic potential targeting PCK and the future direction that is highly in need to better understand PCK-mediated signaling under diverse conditions.

PPARγ-induced upregulation of subcutaneous fat adiponectin secretion, glyceroneogenesis and BCAA oxidation requires mTORC1 activity

The nutrient sensors peroxisome proliferator-activated receptor γ (PPARγ) and mechanistic target of rapamycin complex 1 (mTORC1) closely interact in the regulation of adipocyte lipid storage. The precise mechanisms underlying this interaction and whether this extends to other metabolic processes and the endocrine function of adipocytes are still unknown. We investigated herein the involvement of mTORC1 as a mediator of the actions of the PPARγ ligand rosiglitazone in subcutaneous inguinal white adipose tissue (iWAT) mass, endocrine function, lipidome, transcriptome and branched-chain amino acid (BCAA) metabolism. Mice bearing regulatory associated protein of mTOR (Raptor) deletion and therefore mTORC1 deficiency exclusively in adipocytes and littermate controls were fed a high-fat diet supplemented or not with the PPARγ agonist rosiglitazone (30 mg/kg/day) for 8 weeks and evaluated for iWAT mass, lipidome, transcriptome (Rnaseq), respiration and BCAA metabolism. Adipocyte mTORC1 deficiency not only impaired iWAT adiponectin transcription, synthesis and secretion, PEPCK mRNA levels, triacylglycerol synthesis and BCAA oxidation and mRNA levels of related proteins but also completely blocked the upregulation in these processes induced by pharmacological PPARγ activation with rosiglitazone. Mechanistically, adipocyte mTORC1 deficiency impairs PPARγ transcriptional activity by reducing PPARγ protein content, as well as by downregulating C/EBPα, a co-partner and facilitator of PPARγ. In conclusion, mTORC1 and PPARγ are essential partners involved in the regulation of subcutaneous adipose tissue adiponectin production and secretion and BCAA oxidative metabolism.

Differential regulation of glyceroneogenesis by glucocorticoids in epididymal and retroperitoneal white adipose tissue from rats

PURPOSE

Investigate the glycerol-3-phosphate generation pathways in epididymal (EPI) and retroperitoneal (RETRO) adipose tissues from dexamethasone-treated rats.

METHODS

Rats were treated with dexamethasone for 7 days. Glycerol-3-phosphate generation pathways via glycolysis, glyceroneogenesis and direct phosphorylation of glycerol were evaluated, respectively, by 2-deoxyglucose uptake, phosphoenolpyruvate carboxykinase (PEPCK-C) activity and pyruvate incorporation into triacylglycerol (TAG)-glycerol, and glycerokinase activity and glycerol incorporation into TAG-glycerol.

RESULTS

Dexamethasone treatment markedly decreased the body weight, but increased the weight and lipid content of EPI and RETRO and plasma insulin, glucose, non-esterified fatty acid and TAG levels. EPI and RETRO from dexamethasone-treated rats showed increased rates of de novo fatty acid synthesis (80 and 100%) and basal lipolysis (20%). In EPI, dexamethasone decreased the 2-deoxyglucose uptake (50%), as well as glyceroneogenesis, evidenced by a decrease of PEPCK-C activity (39%) and TAG-glycerol synthesis from pyruvate (66%), but increased the glycerokinase activity (50%) and TAG-glycerol synthesis from glycerol (72%) in this tissue. In spite of a similar reduction in 2-deoxyglucose uptake in RETRO, dexamethasone treatment increased glyceroneogenesis, evidenced by PEPCK activity (96%), and TAG-glycerol synthesis from pyruvate (110%), accompanied by a decrease in glycerokinase activity (50%) and TAG-glycerol synthesis from glycerol (50%). Dexamethasone effects on RETRO were accompanied by a decrease in p-Akt content and by lower insulin effects on the rates of glycerol release in the presence of isoproterenol and on the rates of glucose uptake in isolated adipocytes.

CONCLUSION

Our data demonstrated differential regulation of glyceroneogenesis and direct phosphorylation of glycerol by glucocorticoids in EPI and RETRO from rats.

Early induction of pyruvate dehydrogenase kinase 4 by retinoic acids in adipocytes

SCOPE

Vitamin A and its metabolites, such as retinoic acids (RA), are related to metabolic diseases, in particular insulin resistance and obesity. Here, we studied the roles of 9-cis RA and all-trans RA on the regulation of pyruvate dehydrogenase kinase 4 (PDK4), an enzyme involved in fatty acid reesterification, which is a crucial metabolic pathway in adipose tissue (AT) lipid homeostasis.

METHODS AND RESULTS

9-cis RA and all-trans RA treatment of human and murine AT explants, as well as adipocytes (3T3-F442A cell line) induces PDK4 expression both at the mRNA and the protein level, via a transcriptional mechanism. Using site-directed mutagenesis and chomatin immuno-precipitation, we showed that this activation involves two new RA responsive elements in the Pdk4 promoter, RAREa (DR1: -125/-112) and RAREb (DR1: -86/-73), specific to AT. Furthermore, even though endogeneous Pdk4 gene was upregulated by RA in Fao cells, a rat hepatoma cell line, the induction did not occur through the newly found RAREs.

CONCLUSION

In this study, we showed that adipocyte PDK4 gene is a new target of the vitamin A derived RA and might participate to the reduced fatty acid efflux from the adipocyte, a step that plays an important role in the developement of metabolic diseases.

Dicarbonyl stress in clinical obesity

The glyoxalase system in the cytoplasm of cells provides the primary defence against glycation by methylglyoxal catalysing its metabolism to D-lactate. Methylglyoxal is the precursor of the major quantitative advanced glycation endproducts in physiological systems - arginine-derived hydroimidazolones and deoxyguanosine-derived imidazopurinones. Glyoxalase 1 of the glyoxalase system was linked to anthropometric measurements of obesity in human subjects and to body weight in strains of mice. Recent conference reports described increased weight gain on high fat diet-fed mouse with lifelong deficiency of glyoxalase 1 deficiency, compared to wild-type controls, and decreased weight gain in glyoxalase 1-overexpressing transgenic mice, suggesting a functional role of glyoxalase 1 and dicarbonyl stress in obesity. Increased methylglyoxal, dicarbonyl stress, in white adipose tissue and liver may be a mediator of obesity and insulin resistance and thereby a risk factor for development of type 2 diabetes and non-alcoholic fatty liver disease. Increased methylglyoxal formation from glyceroneogenesis on adipose tissue and liver and decreased glyoxalase 1 activity in obesity likely drives dicarbonyl stress in white adipose tissue increasing the dicarbonyl proteome and related dysfunction. The clinical significance will likely emerge from on-going clinical evaluation of inducers of glyoxalase 1 expression in overweight and obese subjects. Increased transcapillary escape rate of albumin and increased total body interstitial fluid volume in obesity likely makes levels of glycation of plasma protein unreliable indicators of glycation status in obesity as there is a shift of albumin dwell time from plasma to interstitial fluid, which decreases overall glycation for a given glycemic exposure.

Kinetic and functional properties of human mitochondrial phosphoenolpyruvate carboxykinase

The cytosolic form of phosphoenolpyruvate carboxykinase (PCK1) plays a regulatory role in gluconeogenesis and glyceroneogenesis. The role of the mitochondrial isoform (PCK2) remains unclear. We report the partial purification and kinetic and functional characterization of human PCK2. Kinetic properties of the enzyme are very similar to those of the cytosolic enzyme. PCK2 has an absolute requirement for Mn²⁺ ions for activity; Mg²⁺ ions reduce the K_m for Mn²⁺ by about 60 fold. Its specificity constant is 100 fold larger for oxaloacetate than for phosphoenolpyruvate suggesting that oxaloacetate phosphorylation is the favored reaction in vivo. The enzyme possesses weak pyruvate kinase-like activity (k_cat=2.7 s⁻¹). When overexpressed in HEK293T cells it enhances strongly glucose and lipid production showing that it can play, as the cytosolic isoenzyme, an active role in glyceroneogenesis and gluconeogenesis.

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Purification of recombinant human PCK2 has been performed.

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Its kinetic behavior is very similar to that of human PCK1.

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PCK2 overexpression increases gluconeogenesis and glyceroneogenesis in cell cultures.

Citrulline reduces glyceroneogenesis and induces fatty acid release in visceral adipose tissue from overweight rats

SCOPE

High-fat diet (HFD) increases visceral adipose tissue (AT). Our aim was to evaluate whether citrulline (CIT) affected nonesterified fatty acid (NEFA) metabolism in AT from HFD-fed rats.

METHODS AND RESULTS

Rats were fed for 8 weeks with either a control diet (CD) or HFD. Retroperitoneal AT explants were exposed to 2.5 mmol/L CIT for 24 h. We analyzed lipolysis, beta-oxidation, glyceroneogenesis, and the expression of the key associated enzymes. CIT doubled NEFA release selectively in HFD AT. Phosphorylation of hormone-sensitive lipase was upregulated 50 and 100% by CIT in CD and HFD AT, respectively. Under CIT, beta-oxidation increased similarly whatever the diet, whereas glyceroneogenesis, which permits NEFA re-esterification, was downregulated 50 and 80% in CD and HFD AT, respectively. In the latter, the important decrease in re-esterification probably explains the rise of NEFA release. A pretreatment with the nitric oxide synthase inhibitor N ω-nitro-l-arginine methyl ester abolished CIT effects.

CONCLUSION

These results demonstrate direct lipolytic and antiglyceroneogenic effects of CIT on CD and HFD AT. The selective CIT-mediated NEFA release from HFD AT was probably the consequence of the drastic decrease in glyceroneogenesis and nitric oxide was a mediator of CIT effects. These results provide evidence for a direct action of CIT on AT to reduce overweight.

A role for mitochondrial phosphoenolpyruvate carboxykinase (PEPCK-M) in the regulation of hepatic gluconeogenesis

Synthesis of phosphoenolpyruvate (PEP) from oxaloacetate is an absolute requirement for gluconeogenesis from mitochondrial substrates. Generally, this reaction has solely been attributed to the cytosolic isoform of PEPCK (PEPCK-C), although loss of the mitochondrial isoform (PEPCK-M) has never been assessed. Despite catalyzing the same reaction, to date the only significant role reported in mammals for the mitochondrial isoform is as a glucose sensor necessary for insulin secretion. We hypothesized that this nutrient-sensing mitochondrial GTP-dependent pathway contributes importantly to gluconeogenesis. PEPCK-M was acutely silenced in gluconeogenic tissues of rats using antisense oligonucleotides both in vivo and in isolated hepatocytes. Silencing PEPCK-M lowers plasma glucose, insulin, and triglycerides, reduces white adipose, and depletes hepatic glycogen, but raises lactate. There is a switch of gluconeogenic substrate preference to glycerol that quantitatively accounts for a third of glucose production. In contrast to the severe mitochondrial deficiency characteristic of PEPCK-C knock-out livers, hepatocytes from PEPCK-M-deficient livers maintained normal oxidative function. Consistent with its predicted role, gluconeogenesis rates from hepatocytes lacking PEPCK-M are severely reduced for lactate, alanine, and glutamine, but not for pyruvate and glycerol. Thus, PEPCK-M has a direct role in fasted and fed glucose homeostasis, and this mitochondrial GTP-dependent pathway should be reconsidered for its involvement in both normal and diabetic metabolism.

The role of glycerol-3-phosphate dehydrogenase 1 in the progression of fatty liver after acute ethanol administration in mice

Acute ethanol consumption leads to the accumulation of triglycerides (TGs) in hepatocytes. The increase in lipogenesis and reduction of fatty acid oxidation are implicated as the mechanisms underlying ethanol-induced hepatic TG accumulation. Although glycerol-3-phosphate (Gro3P), formed by glycerol kinase (GYK) or glycerol-3-phosphate dehydrogenase 1 (GPD1), is also required for TG synthesis, the roles of GYK and GPD1 have been the subject of some debate. In this study, we examine (1) the expression of genes involved in Gro3P production in the liver of C57BL/6J mice in the context of hepatic TG accumulation after acute ethanol intake, and (2) the role of GPD1 in the progression of ethanol-induced fatty liver using GPD1 null mice. As a result, in C57BL/6J mice, ethanol-induced hepatic TG accumulation began within 2h and was 1.7-fold greater than that observed in the control group after 6h. The up-regulation of GPD1 began 2h after administering ethanol, and significantly increased 6h later with the concomitant escalation in the glycolytic gene expression. The incorporation of (14)C-labelled glucose into TG glycerol moieties increased during the same period. On the other hand, in GPD1 null mice carrying normal GYK activity, no significant increase in hepatic TG level was observed after acute ethanol intake. In conclusion, GPD1 and glycolytic gene expression is up-regulated by ethanol, and GPD1-mediated incorporation of glucose into TG glycerol moieties together with increased lipogenesis, is suggested to play an important role in ethanol-induced hepatic TG accumulation.

Metabolism of glycerol, glucose, and lactate in the citric acid cycle prior to incorporation into hepatic acylglycerols

During hepatic lipogenesis, the glycerol backbone of acylglycerols originates from one of three sources: glucose, glycerol, or substrates passing through the citric acid cycle via glyceroneogenesis. The relative contribution of each substrate source to glycerol in rat liver acylglycerols was determined using (13)C-enriched substrates and NMR. Animals received a fixed mixture of glucose, glycerol, and lactate; one group received [U-(13)C6]glucose, another received [U-(13)C3]glycerol, and the third received [U-(13)C3]lactate. After 3 h, the livers were harvested to extract fats, and the glycerol moiety from hydrolyzed acylglycerols was analyzed by (13)C NMR. In either fed or fasted animals, glucose and glycerol provided the majority of the glycerol backbone carbons, whereas the contribution of lactate was small. In fed animals, glucose contributed >50% of the total newly synthesized glycerol backbone, and 35% of this contribution occurred after glucose had passed through the citric acid cycle. By comparison, the glycerol contribution was ~40%, and of this, 17% of the exogenous glycerol passed first through the cycle. In fasted animals, exogenous glycerol became the major contributor to acylglycerols. The contribution from exogenous lactate did increase in fasted animals, but its overall contribution remained small. The contributions of glucose and glycerol that had passed through the citric acid cycle first increased in fasted animals from 35 to 71% for glucose and from 17 to 24% for glycerol. Thus, a substantial fraction from both substrate sources passed through the cycle prior to incorporation into the glycerol moiety of acylglycerols in the liver.