Sex-dependent adipose glucose partitioning by the mitochondrial pyruvate carrier.

Objective: The mitochondrial pyruvate carrier (MPC) occupies a critical node in intermediary metabolism, prompting interest in its utility as a therapeutic target for the treatment of obesity and cardiometabolic disease. Dysregulated nutrient metabolism in adipose tissue is a prominent feature of obesity pathophysiology, yet the functional role of adipose MPC has not been explored. We investigated whether the MPC shapes the adaptation of adipose tissue to dietary stress in female and male mice. Methods: The impact of pharmacological and genetic disruption of the MPC on mitochondrial pathways of triglyceride assembly (lipogenesis and glyceroneogenesis) was assessed in 3T3L1 adipocytes and murine adipose explants, combined with analyses of adipose MPC expression in metabolically compromised humans. Whole-body and adipose-specific glucose metabolism were subsequently investigated in male and female mice lacking adipocyte MPC1 (Mpc1AD-/-) and fed either standard chow, high-fat western style, or high-sucrose lipid restricted diets for 24 weeks, using a combination of radiolabeled tracers and GC/MS metabolomics. Results: Treatment with UK5099 or siMPC1 impaired the synthesis of lipids and glycerol-3-phosphate from pyruvate and blunted triglyceride accumulation in 3T3L1 adipocytes, whilst MPC expression in human adipose tissue was negatively correlated with indices of whole-body and adipose tissue metabolic dysfunction. Mature adipose explants from Mpc1AD-/- mice were intrinsically incapable of incorporating pyruvate into triglycerides. In vivo, MPC deletion restricted the incorporation of circulating glucose into adipose triglycerides, but only in female mice fed a zero fat diet, and this associated with sex-specific reductions in tricarboxylic acid cycle pool sizes and compensatory transcriptional changes in lipogenic and glycerol metabolism pathways. However, whole-body adiposity and metabolic health were preserved in Mpc1AD-/- mice regardless of sex, even under conditions of zero dietary fat. Conclusion: These findings highlight the greater capacity for mitochondrially driven triglyceride assembly in adipose from female versus male mice and expose a reliance upon MPC-gated metabolism for glucose partitioning in female adipose under conditions of dietary lipid restriction.

Limiting Mrs2-dependent mitochondrial Mg2+ uptake induces metabolic programming in prolonged dietary stress.

The most abundant cellular divalent cations, Mg2+ (mM) and Ca2+ (nM-µM), antagonistically regulate divergent metabolic pathways with several orders of magnitude affinity preference, but the physiological significance of this competition remains elusive. In mice consuming a Western diet, genetic ablation of the mitochondrial Mg2+ channel Mrs2 prevents weight gain, enhances mitochondrial activity, decreases fat accumulation in the liver, and causes prominent browning of white adipose. Mrs2 deficiency restrains citrate efflux from the mitochondria, making it unavailable to support de novo lipogenesis. As citrate is an endogenous Mg2+ chelator, this may represent an adaptive response to a perceived deficit of the cation. Transcriptional profiling of liver and white adipose reveals higher expression of genes involved in glycolysis, ß-oxidation, thermogenesis, and HIF-1α-targets, in Mrs2-/- mice that are further enhanced under Western-diet-associated metabolic stress. Thus, lowering mMg2+ promotes metabolism and dampens diet-induced obesity and metabolic syndrome.

Proximal tubular epithelial insulin receptor mediates high-fat diet-induced kidney injury.

The role of insulin receptor (IR) activated by hyperinsulinemia in obesity-induced kidney injury is not well understood. We hypothesized that activation of kidney proximal tubule epithelial IR contributes to obesity-induced kidney injury. We administered normal-fat diet (NFD) or high-fat diet (HFD) to control and kidney proximal tubule IR-knockout (KPTIRKO) mice for 4 months. Renal cortical IR expression was decreased by 60% in male and female KPTIRKO mice. Baseline serum glucose, serum creatinine, and the ratio of urinary albumin to creatinine (ACR) were similar in KPTIRKO mice compared to those of controls. On HFD, weight gain and increase in serum cholesterol were similar in control and KPTIRKO mice; blood glucose did not change. HFD increased the following parameters in the male control mice: renal cortical contents of phosphorylated IR and Akt, matrix proteins, urinary ACR, urinary kidney injury molecule-1-to-creatinine ratio, and systolic blood pressure. Renal cortical generation of hydrogen sulfide was reduced in HFD-fed male control mice. All of these parameters were ameliorated in male KPTIRKO mice. Interestingly, female mice were resistant to HFD-induced kidney injury in both genotypes. We conclude that HFD-induced kidney injury requires renal proximal tubule IR activation in male mice.

Insulin resistance is mechanistically linked to hepatic mitochondrial remodeling in non-alcoholic fatty liver disease.

OBJECTIVE: Insulin resistance and altered hepatic mitochondrial function are central features of type 2 diabetes (T2D) and non-alcoholic fatty liver disease (NAFLD), but the etiological role of these processes in disease progression remains unclear. Here we investigated the molecular links between insulin resistance, mitochondrial remodeling, and hepatic lipid accumulation. METHODS: Hepatic insulin sensitivity, endogenous glucose production, and mitochondrial metabolic fluxes were determined in wild-type, obese (ob/ob) and pioglitazone-treatment obese mice using a combination of radiolabeled tracer and stable isotope NMR approaches. Mechanistic studies of pioglitazone action were performed in isolated primary hepatocytes, whilst molecular hepatic lipid species were profiled using shotgun lipidomics. RESULTS: Livers from obese, insulin-resistant mice displayed augmented mitochondrial content and increased tricarboxylic acid cycle (TCA) cycle and pyruvate dehydrogenase (PDH) activities. Insulin sensitization with pioglitazone mitigated pyruvate-driven TCA cycle activity and PDH activation via both allosteric (intracellular pyruvate availability) and covalent (PDK4 and PDP2) mechanisms that were dependent on PPARγ activity in isolated primary hepatocytes. Improved mitochondrial function following pioglitazone treatment was entirely dissociated from changes in hepatic triglycerides, diacylglycerides, or fatty acids. Instead, we highlight a role for the mitochondrial phospholipid cardiolipin, which underwent pathological remodeling in livers from obese mice that was reversed by insulin sensitization. CONCLUSION: Our findings identify targetable mitochondrial features of T2D and NAFLD and highlight the benefit of insulin sensitization in managing the clinical burden of obesity-associated disease.

Lactate Elicits ER-Mitochondrial Mg2+ Dynamics to Integrate Cellular Metabolism.

Mg2+ is the most abundant divalent cation in metazoans and an essential cofactor for ATP, nucleic acids, and countless metabolic enzymes. To understand how the spatio-temporal dynamics of intracellular Mg2+ (iMg2+) are integrated into cellular signaling, we implemented a comprehensive screen to discover regulators of iMg2+ dynamics. Lactate emerged as an activator of rapid release of Mg2+ from endoplasmic reticulum (ER) stores, which facilitates mitochondrial Mg2+ (mMg2+) uptake in multiple cell types. We demonstrate that this process is remarkably temperature sensitive and mediated through intracellular but not extracellular signals. The ER-mitochondrial Mg2+ dynamics is selectively stimulated by L-lactate. Further, we show that lactate-mediated mMg2+ entry is facilitated by Mrs2, and point mutations in the intermembrane space loop limits mMg2+ uptake. Intriguingly, suppression of mMg2+ surge alleviates inflammation-induced multi-organ failure. Together, these findings reveal that lactate mobilizes iMg2+ and links the mMg2+ transport machinery with major metabolic feedback circuits and mitochondrial bioenergetics.

Regulation of ANGPTL8 in liver and adipose tissue by nutritional and hormonal signals and its effect on glucose homeostasis in mice.

The angiopoietin-like protein (ANGPTL) family represents a promising therapeutic target for dyslipidemia, which is a feature of obesity and type 2 diabetes (T2DM). The aim of the present study was to determine the metabolic role of ANGPTL8 and to investigate its nutritional, hormonal, and molecular regulation in key metabolic tissues. The regulation of Angptl8 gene expression by insulin and glucose was quantified using a combination of in vivo insulin clamp experiments in mice and in vitro experiments in primary and cultured hepatocytes and adipocytes. The role of AMPK signaling was examined, and the transcriptional control of Angptl8 was determined using bioinformatic and luciferase reporter approaches. The metabolism of Angptl8 knockout mice (ANGPTL8-/-) was examined following chow and high-fat diets (HFD). Insulin acutely increased Angptl8 expression in liver and adipose tissue, which involved the CCAAT/enhancer-binding protein (C/EBPß) transcription factor. In insulin clamp experiments, glucose further enhanced Angptl8 expression in the presence of insulin in adipose tissue. The activation of AMPK signaling antagonized the effect of insulin on Angptl8 expression in hepatocytes and adipocytes. The ANGPTL8-/- mice had improved glucose tolerance and displayed reduced fed and fasted plasma triglycerides. However, there was no change in body weight or steatosis in ANGPTL8-/- mice after the HFD. These data show that ANGPTL8 plays important metabolic roles in mice that extend beyond triglyceride metabolism. The finding that insulin, glucose, and AMPK signaling regulate Angptl8 expression may provide important clues about the distinct function of ANGPTL8 in these tissues.