Time-dependent effects of Prkce deletion on glucose homeostasis and hepatic lipid metabolism on dietary lipid oversupply in mice

Liver Androgen Receptor Knockout Improved High-fat Diet Induced Glucose Dysregulation in Female Mice But Not Male Mice

Previous research has indicated that liver androgen receptors may play a role in modulating disease. This study aims to investigate the pathophysiology of high-fat diet (HFD) induced dysglycemia in male and female liver androgen receptor knockout (LivARKO) mice. We performed metabolic tests on LivARKO female and male mice fed a HFD or a control diet (from Research Diets Inc.) during months 1 or 2 after starting the diet. Additionally, we performed Western blot and quantitative real-time PCR analysis on the livers of the mice to examine intermediates in the insulin signaling pathway. LivARKO-HFD female mice displayed no difference in glucose tolerance compared to female LivARKO-Control (Con) mice, whereas in wild-type female mice, HFD impaired glucose tolerance (IGT). Our data suggests that starting at 1 month, LivARKO may be protecting female mice from HFD-induced metabolic dysfunction. LivARKO-HFD female mice displayed significantly worse insulin sensitivity at 15 minutes compared to LivARKO-Con female mice, but, strangely, LivARKO-HFD female mice had significantly better insulin sensitivity at 60 and 90 minutes compared to LivARKO-Con female mice. Despite protecting against IGT, LivARKO did not protect against HFD-induced hyperinsulinemia in female mice. In contrast to females, male LivARKO-HFD mice displayed impaired glucose tolerance compared to male LivARKO-Con mice. Thus, LivARKO is not protective against HFD-induced glucose metabolic dysfunction in male mice. Lastly, LivARKO-HFD female mice maintained hepatic insulin sensitivity whereas LivARKO-HFD male mice displayed hepatic insulin resistance. These findings suggest that LivARKO delayed the onset of HFD-induced dysglycemia in female mice.

Lipid Metabolism in Metabolic-Associated Steatotic Liver Disease (MASLD)

Metabolic-associated steatotic liver disease (MASLD) is a cluster of pathological conditions primarily developed due to the accumulation of ectopic fat in the hepatocytes. During the severe form of the disease, i.e., metabolic-associated steatohepatitis (MASH), accumulated lipids promote lipotoxicity, resulting in cellular inflammation, oxidative stress, and hepatocellular ballooning. If left untreated, the advanced form of the disease progresses to fibrosis of the tissue, resulting in irreversible hepatic cirrhosis or the development of hepatocellular carcinoma. Although numerous mechanisms have been identified as significant contributors to the development and advancement of MASLD, altered lipid metabolism continues to stand out as a major factor contributing to the disease. This paper briefly discusses the dysregulation in lipid metabolism during various stages of MASLD.

The ketogenic diet prevents steatosis and insulin resistance by reducing lipogenesis, diacylglycerol accummulation, and PKC activity in male rat liver

KEY POINTS

The accumulation of diacylglycerol (DAG), ceramides and inflammation are key factors that cause insulin resistance and nonalcoholic fatty liver (NAFLD). This study provides evidence that a ketogenic diet (KD) rich in fat and devoid of carbohydrate reduced DAG content and preserved insulin signalling in the liver. The KD shifted metabolism away from lipogenesis by enhancing genes involved in mitochondrial biogenesis and fatty acid oxidations in the liver. The KD also promoted the production of beneficial very long-chain ceramides instead of potentially harmful long-chain ceramides. Through multiple mechanisms, the KD exerted anti-steatogenic and insulin-sensitizing effects in the liver, which supports the use of this dietary intervention to treat NAFLD.

ABSTRACT

Obesity-associated insulin resistance plays a major role in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). The accumulation of diacylglycerol (DAG), ceramides and inflammation are key factors that cause NAFLD. In recent years, the ketogenic diet (KD) has emerged as an effective non-pharmacological intervention for the treatment of NAFLD and other obesity-related metabolic disorders. What remains undetermined is how the KD affects DAG and ceramides content and insulin sensitivity in the liver. Thus, this research was designed to assess these variables, as well as glucose and fat metabolism and markers of inflammation in livers of rats exposed for 8 weeks to one of the following diets: standard chow (SC), obesogenic high-fat, sucrose-enriched diet (HFS), or a KD. Despite having a higher fat content than the HFS diet, the KD did not cause steatosis and preserved hepatic insulin signalling. The KD reduced DAG content and protein kinase C epsilon (PKCε) activity, but markedly increased liver ceramides content. However, whereas the KD increased ceramide synthase 2 (CerS2) expression, it suppressed CerS6 expression, an effect that promoted the production of beneficial very long-chain ceramides instead of harmful long-chain ceramides. The KD also enhanced the liver expression of key genes involved in mitochondrial biogenesis and fatty acid oxidation (Pgc-1α and Fgf21), suppressed inflammatory genes (Tnfα, Nf-kb, Tlr4, and Il6), and shifted substrate away from de-novo lipogenesis. Thus, through multiple mechanisms the KD exerted anti-steatogenic and insulin-sensitizing effects in the liver, which supports the use of this dietary intervention to treat NAFLD. Abstract figure legend This article is protected by copyright. All rights reserved.

Targeting lipophagy as a potential therapeutic strategy for nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is becoming an increasingly serious disease worldwide. Unfortunately, no specific drug has been approved to treat NAFLD. Accumulating evidence suggests that lipotoxicity, which is induced by an excess of intracellular triacylglycerols (TAGs), is a potential mechanism underlying the ill-defined progression of NAFLD. Under physiological conditions, a balance is maintained between TAGs and free fatty acids (FFAs) in the liver. TAGs are catabolized to FFAs through neutral lipolysis and/or lipophagy, while FFAs can be anabolized to TAGs through an esterification reaction. However, in the livers of patients with NAFLD, lipophagy appears to fail. Reversing this abnormal state through several lipophagic molecules (mTORC1, AMPK, PLIN, etc.) facilitates NAFLD amelioration; therefore, restoring failed lipophagy may be a highly efficient therapeutic strategy for NAFLD. Here, we outline the lipophagy phases with the relevant important proteins and discuss the roles of lipophagy in the progression of NAFLD. Additionally, the potential candidate drugs with therapeutic value targeting these proteins are discussed to show novel strategies for future treatment of NAFLD.

Insulin Resistance: From Mechanisms to Therapeutic Strategies

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

Concerted regulation of non-alcoholic fatty liver disease progression by microRNAs in apolipoprotein E-deficient mice

The prevalence of non-alcoholic fatty liver disease (NAFLD) is constantly increasing, and altered expression of microRNAs (miRNAs) fosters the development and progression of many pathologies, including NAFLD. Therefore, we explored the role of new miRNAs involved in the molecular mechanisms that trigger NAFLD progression and evaluated them as biomarkers for diagnosis. As a NAFLD model, we used apolipoprotein E-deficient mice administered a high-fat diet for 8 or 18 weeks. We demonstrated that insulin resistance and decreased lipogenesis and autophagy observed after 18 weeks on the diet are related to a concerted regulation carried out by miR-26b-5p, miR-34a-5p, miR-149-5p and miR-375-3p. We also propose circulating let-7d-5p and miR-146b-5p as potential biomarkers of early stages of NAFLD. Finally, we confirmed that circulating miR-34a-5p and miR-375-3p are elevated in the late stages of NAFLD and that miR-27b-3p and miR-122-5p are increased with disease progression. Our results reveal a synergistic regulation of key processes in NAFLD development and progression by miRNAs. Further investigation is needed to unravel the roles of these miRNAs for developing new strategies for NAFLD treatment.

This article has an associated First Person interview with the joint first authors of the paper.

Summary:Apoe^−/− mice administered a high-fat diet represent a model of non-alcoholic fatty liver disease, revealing the synergistic regulation of key processes in disease progression by miRNAs and indicating some miRNAs as biomarkers for diagnosis.

Dietary carbohydrates and fats in nonalcoholic fatty liver disease

The global prevalence of nonalcoholic fatty liver disease (NAFLD) has dramatically increased in parallel with the epidemic of obesity. Controversy has emerged around dietary guidelines recommending low-fat-high-carbohydrate diets and the roles of dietary macronutrients in the pathogenesis of metabolic disease. In this Review, the topical questions of whether and how dietary fats and carbohydrates, including free sugars, differentially influence the accumulation of liver fat (specifically, intrahepatic triglyceride (IHTG) content) are addressed. Focusing on evidence from humans, we examine data from stable isotope studies elucidating how macronutrients regulate IHTG synthesis and disposal, alter pools of bioactive lipids and influence insulin sensitivity. In addition, we review cross-sectional studies on dietary habits of patients with NAFLD and randomized controlled trials on the effects of altering dietary macronutrients on IHTG. Perhaps surprisingly, evidence to date shows no differential effects between free sugars, with both glucose and fructose increasing IHTG in the context of excess energy. Moreover, saturated fat raises IHTG more than polyunsaturated or monounsaturated fats, with adverse effects on insulin sensitivity, which are probably mediated in part by increased ceramide synthesis. Taken together, the data support the use of diets that have a reduced content of free sugars, refined carbohydrates and saturated fat in the treatment of NAFLD.

Mechanisms and disease consequences of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the leading chronic liver disease worldwide. Its more advanced subtype, nonalcoholic steatohepatitis (NASH), connotes progressive liver injury that can lead to cirrhosis and hepatocellular carcinoma. Here we provide an in-depth discussion of the underlying pathogenetic mechanisms that lead to progressive liver injury, including the metabolic origins of NAFLD, the effect of NAFLD on hepatic glucose and lipid metabolism, bile acid toxicity, macrophage dysfunction, and hepatic stellate cell activation, and consider the role of genetic, epigenetic, and environmental factors that promote fibrosis progression and risk of hepatocellular carcinoma in NASH.

The interplay between host cellular and gut microbial metabolism in NAFLD development and prevention

Metabolism regulation centred on insulin resistance is increasingly important in nonalcoholic fatty liver disease (NAFLD). This review focuses on the interactions between the host cellular and gut microbial metabolism during the development of NAFLD. The cellular metabolism of essential nutrients, such as glucose, lipids and amino acids, is reconstructed with inflammation, immune mechanisms and oxidative stress, and these alterations modify the intestinal, hepatic and systemic environments, and regulate the composition and activity of gut microbes. Microbial metabolites, such as short-chain fatty acids, secondary bile acids, protein fermentation products, choline and ethanol and bacterial toxicants, such as lipopolysaccharides, peptidoglycans and bacterial DNA, play vital roles in NAFLD. The microbe-metabolite relationship is crucial for the modulation of intestinal microbial composition and metabolic activity. The intestinal microbiota and their metabolites participate in epithelial cell metabolism via a series of cell receptors and signalling pathways and remodel the metabolism of various cells in the liver via the gut-liver axis. Microbial metabolic manipulation is a promising strategy for NAFLD prevention, but larger-sampled clinical trials are required for future application.

A Membrane-Bound Diacylglycerol Species Induces PKCϵ-Mediated Hepatic Insulin Resistance

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance (HIR); however, the key lipid species and molecular mechanisms linking these conditions are widely debated. We developed a subcellular fractionation method to quantify diacylglycerol (DAG) stereoisomers and ceramides in the endoplasmic reticulum (ER), mitochondria, plasma membrane (PM), lipid droplets, and cytosol. Acute knockdown (KD) of diacylglycerol acyltransferase-2 in liver induced HIR in rats. This was due to PM sn-1,2-DAG accumulation, which promoted PKCϵ activation and insulin receptor kinase (IRK)-T1160 phosphorylation, resulting in decreased IRK-Y1162 phosphorylation. Liver PM sn-1,2-DAG content and IRK-T1160 phosphorylation were also higher in humans with HIR. In rats, liver-specific PKCϵ KD ameliorated high-fat diet-induced HIR by lowering IRK-T1160 phosphorylation, while liver-specific overexpression of constitutively active PKCϵ-induced HIR by promoting IRK-T1160 phosphorylation. These data identify PM sn-1,2-DAGs as the key pool of lipids that activate PKCϵ and that hepatic PKCϵ is both necessary and sufficient in mediating HIR.

Diacylglycerol-evoked activation of PKC and PKD isoforms in regulation of glucose and lipid metabolism: a review

Protein kinase C (PKC) and Protein kinase D (PKD) isoforms can sense diacylglycerol (DAG) generated in the different cellular compartments in various physiological processes. DAG accumulates in multiple organs of the obese subjects, which leads to the disruption of metabolic homeostasis and the development of diabetes as well as associated diseases. Multiple studies proved that aberrant activation of PKCs and PKDs contributes to the development of metabolic diseases. DAG-sensing PKC and PKD isoforms play a crucial role in the regulation of metabolic homeostasis and therefore might serve as targets for the treatment of metabolic disorders such as obesity and diabetes.

Cardio-Metabolic Effects of High-Fat Diets and Their Underlying Mechanisms-A Narrative Review

The majority of the epidemiological evidence over the past few decades has linked high intake of fats, especially saturated fats, to increased risk of diabetes and cardiovascular disease. However, findings of some recent studies (e.g., the PURE study) have contested this association. High saturated fat diets (HFD) have been widely used in rodent research to study the mechanism of insulin resistance and metabolic syndrome. Two separate but somewhat overlapping models-the diacylglycerol (DAG) model and the ceramide model-have emerged to explain the development of insulin resistance. Studies have shown that lipid deposition in tissues such as muscle and liver inhibit insulin signaling via the toxic molecules DAG and ceramide. DAGs activate protein kinase C that inhibit insulin-PI3K-Akt signaling by phosphorylating serine residues on insulin receptor substrate (IRS). Ceramides are sphingolipids with variable acyl group chain length and activate protein phosphatase 2A that dephosphorylates Akt to block insulin signaling. In adipose tissue, obesity leads to infiltration of macrophages that secrete pro-inflammatory cytokines that inhibit insulin signaling by phosphorylating serine residues of IRS proteins. For cardiovascular disease, studies in humans in the 1950s and 1960s linked high saturated fat intake with atherosclerosis and coronary artery disease. More recently, trials involving Mediterranean diet (e.g., PREDIMED study) have indicated that healthy monounsaturated fats are more effective in preventing cardiovascular mortality and coronary artery disease than are low-fat, low-cholesterol diets. Antioxidant and anti-inflammatory effects of Mediterranean diets are potential mediators of these benefits.

Emerging Pharmacological Targets for the Treatment of Nonalcoholic Fatty Liver Disease, Insulin Resistance, and Type 2 Diabetes

Type 2 diabetes (T2D) is characterized by persistent hyperglycemia despite hyperinsulinemia, affects more than 400 million people worldwide, and is a major cause of morbidity and mortality. Insulin resistance, of which ectopic lipid accumulation in the liver [nonalcoholic fatty liver disease (NAFLD)] and skeletal muscle is the root cause, plays a major role in the development of T2D. Although lifestyle interventions and weight loss are highly effective at reversing NAFLD and T2D, weight loss is difficult to sustain, and newer approaches aimed at treating the root cause of T2D are urgently needed. In this review, we highlight emerging pharmacological strategies aimed at improving insulin sensitivity and T2D by altering hepatic energy balance or inhibiting key enzymes involved in hepatic lipid synthesis. We also summarize recent research suggesting that liver-targeted mitochondrial uncoupling may be an attractive therapeutic approach to treat NAFLD, nonalcoholic steatohepatitis, and T2D.

Deconstructing the Role of PKC Epsilon in Glucose Homeostasis

The failure of insulin to suppress glucose production by the liver is a key aspect of the insulin resistance seen in type 2 diabetes. Lipid-activated protein kinase C epsilon has long been identified as an important mediator of diet-induced glucose intolerance and hepatic insulin resistance and the current view emphasizes a mechanism involving phosphorylation of the insulin receptor by the kinase to inhibit downstream insulin action. However, the significance of this direct effect in the liver has now been challenged by tissue-specific deletion of PKCε, which demonstrated a more prominent role for the kinase in adipose tissue to promote glucose intolerance. New insights regarding the role of PKCε therefore contribute to the understanding of indirect effects on hepatic glucose metabolism.

Pathogenesis of Insulin Resistance and Atherogenic Dyslipidemia in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease in the developed world, with a global prevalence of around 25%. NAFLD is considered to be the hepatic manifestation of metabolic syndrome and is strongly associated with obesity, insulin resistance and dyslipidemia. Insulin resistance plays a pivotal role in the development of NAFLD-related dyslipidemia, which ultimately increases the risk of premature cardiovascular diseases, a leading cause of morbidity and mortality in patients with NAFLD. Insulin affects hepatic glucose and lipid metabolism by hepatic or extrahepatic pathways. Aside from insulin resistance, several other factors also contribute to the pathogenesis of atherogenic dyslipidemia in patients with NAFLD. These include diet composition, gut microbiota and genetic factors, to name a few. The identification of potentially modifiable risk factors of NAFLD is of importance, so as to target those who may benefit from lifestyle changes and to help develop targeted therapies that decrease the risk of cardiovascular diseases in patients with NAFLD.

Regulation of Glucose Production in the Pathogenesis of Type 2 Diabetes

PURPOSE OF REVIEW

Increased glucose production associated with hepatic insulin resistance contributes to the development of hyperglycemia in T2D. The molecular mechanisms accounting for increased glucose production remain controversial. Our aims were to review recent literature concerning molecular mechanisms regulating glucose production and to discuss these mechanisms in the context of physiological experiments and observations in humans and large animal models.

RECENT FINDINGS

Genetic intervention studies in rodents demonstrate that insulin can control hepatic glucose production through both direct effects on the liver, and through indirect effects to inhibit adipose tissue lipolysis and limit gluconeogenic substrate delivery. However, recent experiments in canine models indicate that the direct effects of insulin on the liver are dominant over the indirect effects to regulate glucose production. Recent molecular studies have also identified insulin-independent mechanisms by which hepatocytes sense intrahepatic carbohydrate levels to regulate carbohydrate disposal. Dysregulation of hepatic carbohydrate sensing systems may participate in increased glucose production in the development of diabetes.

Hepatic miR‑215 target Rictor and modulation of hepatic insulin signalling in rats

Increasing evidence has suggested that hepatic lipid accumulation is associated with hepatic insulin resistance; however, the underlying mechanism is yet to be determined. It was demonstrated that the levels of microRNA-215 (miR-215) expression in the liver of rats fed a high-fat diet were significantly increased compared with rats on a control diet. Additionally, it was revealed via luciferase assays and western blotting that miR-215 targets rapamycin-insensitive companion of mammalian target of rapamycin (Rictor), an important protein in the hepatic insulin signalling pathway. Following overexpression of miR-215 in the H4IIE rat hepatocarcinoma cell line, it was reported that the intracellular insulin signalling pathway was inhibited; conversely, inhibition of miR-215 expression induced this pathway. Furthermore, it was demonstrated via reverse transcription-quantitative polymerase chain reaction analysis that free fatty acids promoted the expression of miR-215. The present study provided a novel mechanistic insight into the association between nonalcoholic fatty liver and hepatic insulin resistance.

Protein Kinase C Epsilon Deletion in Adipose Tissue, but Not in Liver, Improves Glucose Tolerance

Protein kinase C epsilon (PKCɛ) activation in the liver is proposed to inhibit insulin action through phosphorylation of the insulin receptor. Here, however, we demonstrated that global, but not liver-specific, deletion of PKCɛ in mice protected against diet-induced glucose intolerance and insulin resistance. Furthermore, PKCɛ-dependent alterations in insulin receptor phosphorylation were not detected. Adipose-tissue-specific knockout mice did exhibit improved glucose tolerance, but phosphoproteomics revealed no PKCɛ-dependent effect on the activation of insulin signaling pathways. Altered phosphorylation of adipocyte proteins associated with cell junctions and endosomes was associated with changes in hepatic expression of several genes linked to glucose homeostasis and lipid metabolism. The primary effect of PKCɛ on glucose homeostasis is, therefore, not exerted directly in the liver as currently posited, and PKCɛ activation in this tissue should be interpreted with caution. However, PKCɛ activity in adipose tissue modulates glucose tolerance and is involved in crosstalk with the liver.

Lipometabolism and Glycometabolism in Liver Diseases

The liver is the main metabolic organ in the body especially in lipometabolism and glycometabolism. Carbohydrates and fats disorders can result in insulin resistance in the liver. Metabolic imbalance can even lead to life-threatening conditions. Therefore, it is essential to maintain the normal metabolic function of the liver. When the liver is in a pathological state, liver metabolism homeostasis is damaged, and metabolic disorders will further aggravate liver disease. Consequently, it is essential to determine the relationship between liver diseases and metabolic disorders. Here we review a lot of evidence that liver diseases are closely related to lipometabolism and glycometabolism. Although the disorder of the liver metabolism is caused by different liver diseases, the break of metabolic balance is determined by changes in the state of the liver. We discuss the relationship between liver disease and metabolic changes, outline the process of how metabolic changes are regulated by liver diseases, and describe the role which metabolic changes play in the process and prognosis of liver disease.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Anarchy in the UPR: A Ca2+-insensitive PKC inhibits SERCA activity to promote ER stress

Nonalcoholic fatty liver disease (NAFLD) is highly prevalent in Western countries, and is linked to the development of liver cancer and Type 2 diabetes (T2D). It is strongly associated with obesity, but the dysregulation of liver lipid storage is not fully understood. Fatty acid oversupply to hepatocytes can establish a vicious cycle involving diminished protein folding, endoplasmic reticulum (ER) stress, insulin resistance and further lipogenesis. This commentary discusses the recent findings of Lai et al. published in Bioscience Reports, that implicate protein kinase C delta (PKCδ) activation by fatty acids in the inhibition of the SERCA Ca2+ pump, resulting in reduced ER Ca2+ loading and protein misfolding. PKCδ therefore represents a target for the treatment of both steatosis and insulin resistance, key to the prevention of NAFLD and T2D.

Characterisation of peptide interactions that regulate PKCε activation

Targeting the interaction between PKC isoforms and their anchoring proteins can specifically regulate kinase activity. εV1-2 and pseudoεRACK peptides, derived from the PKCε C2 domain, modulate its association with receptor for activated C-kinase 2 (RACK2) and thus its function. Details of these interactions remain obscure, and we therefore investigated binding of these peptides using biophysical techniques. Surface plasmon resonance (SPR) indicated that the inhibitory εV1-2 peptide bound to RACK2, and inhibited PKCε binding as expected. In contrast, SPR and NMR demonstrated that the activating pseudoεRACK peptide and related sequences did not bind to PKCε, indicating that their mechanisms of action do not involve binding to the kinase as previously proposed. Our results clarify which interactions could be targeted in developing new therapeutics that inhibit PKCε-RACK2 interaction.

Nonalcoholic Fatty Liver Disease as a Nexus of Metabolic and Hepatic Diseases

NAFLD is closely linked with hepatic insulin resistance. Accumulation of hepatic diacylglycerol activates PKC-ε, impairing insulin receptor activation and insulin-stimulated glycogen synthesis. Peripheral insulin resistance indirectly influences hepatic glucose and lipid metabolism by increasing flux of substrates that promote lipogenesis (glucose and fatty acids) and gluconeogenesis (glycerol and fatty acid-derived acetyl-CoA, an allosteric activator of pyruvate carboxylase). Weight loss with diet or bariatric surgery effectively treats NAFLD, but drugs specifically approved for NAFLD are not available. Some new pharmacological strategies act broadly to alter energy balance or influence pathways that contribute to NAFLD (e.g., agonists for PPAR γ, PPAR α/δ, FXR and analogs for FGF-21, and GLP-1). Others specifically inhibit key enzymes involved in lipid synthesis (e.g., mitochondrial pyruvate carrier, acetyl-CoA carboxylase, stearoyl-CoA desaturase, and monoacyl- and diacyl-glycerol transferases). Finally, a novel class of liver-targeted mitochondrial uncoupling agents increases hepatocellular energy expenditure, reversing the metabolic and hepatic complications of NAFLD.

Protein Kinase C Epsilon Is a Key Regulator of Mitochondrial Redox Homeostasis in Acute Myeloid Leukemia

Purpose: The intracellular redox environment of acute myeloid leukemia (AML) cells is often highly oxidized compared to healthy hematopoietic progenitors and this is purported to contribute to disease pathogenesis. However, the redox regulators that allow AML cell survival in this oxidized environment remain largely unknown.Experimental Design: Utilizing several chemical and genetically-encoded redox sensing probes across multiple human and mouse models of AML, we evaluated the role of the serine/threonine kinase PKC-epsilon (PKCε) in intracellular redox biology, cell survival and disease progression.Results: We show that RNA interference-mediated inhibition of PKCε significantly reduces patient-derived AML cell survival as well as disease onset in a genetically engineered mouse model (GEMM) of AML driven by MLL-AF9. We also show that PKCε inhibition induces multiple reactive oxygen species (ROS) and that neutralization of mitochondrial ROS with chemical antioxidants or co-expression of the mitochondrial ROS-buffering enzymes SOD2 and CAT, mitigates the anti-leukemia effects of PKCε inhibition. Moreover, direct inhibition of SOD2 increases mitochondrial ROS and significantly impedes AML progression in vivo Furthermore, we report that PKCε over-expression protects AML cells from otherwise-lethal doses of mitochondrial ROS-inducing agents. Proteomic analysis reveals that PKCε may control mitochondrial ROS by controlling the expression of regulatory proteins of redox homeostasis, electron transport chain flux, as well as outer mitochondrial membrane potential and transport.Conclusions: This study uncovers a previously unrecognized role for PKCε in supporting AML cell survival and disease progression by regulating mitochondrial ROS biology and positions mitochondrial redox regulators as potential therapeutic targets in AML. Clin Cancer Res; 24(3); 608-18. ©2017 AACR.

Roles of Diacylglycerols and Ceramides in Hepatic Insulin Resistance

Although ample evidence links hepatic lipid accumulation with hepatic insulin resistance, the mechanistic basis of this association is incompletely understood and controversial. Diacylglycerols (DAGs) and ceramides have emerged as the two best-studied putative mediators of lipid-induced hepatic insulin resistance. Both lipids were first associated with insulin resistance in skeletal muscle and were subsequently hypothesized to mediate insulin resistance in the liver. However, the putative roles for DAGs and ceramides in hepatic insulin resistance have proved more complex than originally imagined, with various genetic and pharmacologic manipulations yielding a vast and occasionally contradictory trove of data to sort. In this review we examine the state of this field, turning a critical eye toward both DAGs and ceramides as putative mediators of lipid-induced hepatic insulin resistance.

Hypothalamic Insulin Resistance in Obesity: Effects on Glucose Homeostasis

The central link between obesity and type 2 diabetes is the development of insulin resistance. To date, it is still not clear whether hyperinsulinemia causes insulin resistance, which underlies the pathogenesis of obesity-associated type 2 diabetes, owing to the sophisticated regulatory mechanisms that exist in the periphery and in the brain. In recent years, accumulating evidence has demonstrated the existence of insulin resistance within the hypothalamus. In this review, we have integrated the recent discoveries surrounding both central and peripheral insulin resistance to provide a comprehensive overview of insulin resistance in obesity and the regulation of systemic glucose homeostasis. In particular, this review will discuss how hyperinsulinemia and hyperleptinemia in obesity impair insulin sensitivity in tissues such as the liver, skeletal muscle, adipose tissue, and the brain. In addition, this review highlights insulin transport into the brain, signaling pathways associated with hypothalamic insulin receptor expression in the regulation of hepatic glucose production, and finally the perturbation of systemic glucose homeostasis as a consequence of central insulin resistance. We also suggest future approaches to overcome both central and peripheral insulin resistance to treat obesity and type 2 diabetes.

Insulin receptor Thr1160 phosphorylation mediates lipid-induced hepatic insulin resistance

Nonalcoholic fatty liver disease (NAFLD) is a risk factor for type 2 diabetes (T2D), but whether NAFLD plays a causal role in the pathogenesis of T2D is uncertain. One proposed mechanism linking NAFLD to hepatic insulin resistance involves diacylglycerol-mediated (DAG-mediated) activation of protein kinase C-ε (PKCε) and the consequent inhibition of insulin receptor (INSR) kinase activity. However, the molecular mechanism underlying PKCε inhibition of INSR kinase activity is unknown. Here, we used mass spectrometry to identify the phosphorylation site Thr1160 as a PKCε substrate in the functionally critical INSR kinase activation loop. We hypothesized that Thr1160 phosphorylation impairs INSR kinase activity by destabilizing the active configuration of the INSR kinase, and our results confirmed this prediction by demonstrating severely impaired INSR kinase activity in phosphomimetic T1160E mutants. Conversely, the INSR T1160A mutant was not inhibited by PKCε in vitro. Furthermore, mice with a threonine-to-alanine mutation at the homologous residue Thr1150 (InsrT1150A mice) were protected from high fat diet-induced hepatic insulin resistance. InsrT1150A mice also displayed increased insulin signaling, suppression of hepatic glucose production, and increased hepatic glycogen synthesis compared with WT controls during hyperinsulinemic clamp studies. These data reveal a critical pathophysiological role for INSR Thr1160 phosphorylation and provide further mechanistic links between PKCε and INSR in mediating NAFLD-induced hepatic insulin resistance.

XBP1s Is an Anti-lipogenic Protein

Endoplasmic reticulum (ER) stress has been shown to contribute to various metabolic diseases, including non-alcoholic fatty liver disease and type 2 diabetes. Reduction of ER stress by treatment with chemical chaperones or overexpression of ER chaperone proteins alleviates hepatic steatosis. Nonetheless, X-box binding protein 1s (XBP1s), a key transcription factor that reduces ER stress, has been proposed as a lipogenic transcription factor. In this report, we document that XBP1s leads to suppression of lipogenic gene expression and reduction of hepatic triglyceride and diacylglycerol content in livers of diet-induced obese and genetically obese and insulin-resistant ob/ob mice. Furthermore, we also show that PKCϵ activity, which correlates with fatty liver and which causes insulin resistance, was significantly reduced in diet-induced obese mice. Finally, we have shown that XBP1s reduces the hepatic fatty acid synthesis rate and enhances macrolipophagy, an initiating step in lipolysis. Our results reveal that XBP1s reduces hepatic lipogenic gene expression and improves hepatosteatosis in mouse models of obesity and insulin resistance, which leads us to conclude that XBP1s has anti-lipogenic properties in the liver.

Flavin-Containing Monooxygenase 3 Reduces Endoplasmic Reticulum Stress in Lipid-Treated Hepatocytes

Isoforms of flavin-containing monooxygenase (FMO) are involved in xenobiotic metabolism but have also been implicated in the regulation of glucose and lipid homeostasis and in the development of atherosclerosis. However, we have recently shown that improved insulin action is associated with increased FMO expression in livers of protein kinase C-deficient mice. Here, we investigated whether FMO3 expression affected insulin signaling, glucose metabolism, and endoplasmic reticulum (ER) stress in hepatocytes. HepG2 and IHH hepatocytes were transfected with FMO3 cDNA for overexpression, or small interfering RNA for knockdown. Cells were treated with palmitate to induce insulin resistance and insulin signaling, phosphoenolpyruvate carboxykinase (PEPCK) gene expression and ER stress markers were examined by immunoblotting and RT-PCR. Glycogen synthesis was measured using [(14)C]glucose. Palmitate treatment reduced insulin signaling at the level of Akt phosphorylation and glycogen synthesis, which were little affected by FMO3 overexpression. However, the fatty acid also increased the levels of several ER stress markers and activation of caspase 3, which were counteracted by FMO3 overexpression and exacerbated by FMO3 knockdown. Although FMO3 expression did not reverse lipid effects on protein thiol redox in hepatocytes, it did prevent up-regulation of the gluconeogenic enzyme PEPCK by pharmacological ER stress inducers or by palmitate. ER stress and PEPCK levels were also reduced in livers of fat-fed protein kinase Cδ-deficient mice. Our data indicate that FMO3 can contribute to the regulation of glucose metabolism in the liver by reducing lipid-induced ER stress and the expression of PEPCK, independently of insulin signal transduction.

The pathogenesis of insulin resistance: integrating signaling pathways and substrate flux

Insulin resistance arises when the nutrient storage pathways evolved to maximize efficient energy utilization are exposed to chronic energy surplus. Ectopic lipid accumulation in liver and skeletal muscle triggers pathways that impair insulin signaling, leading to reduced muscle glucose uptake and decreased hepatic glycogen synthesis. Muscle insulin resistance, due to ectopic lipid, precedes liver insulin resistance and diverts ingested glucose to the liver, resulting in increased hepatic de novo lipogenesis and hyperlipidemia. Subsequent macrophage infiltration into white adipose tissue (WAT) leads to increased lipolysis, which further increases hepatic triglyceride synthesis and hyperlipidemia due to increased fatty acid esterification. Macrophage-induced WAT lipolysis also stimulates hepatic gluconeogenesis, promoting fasting and postprandial hyperglycemia through increased fatty acid delivery to the liver, which results in increased hepatic acetyl-CoA content, a potent activator of pyruvate carboxylase, and increased glycerol conversion to glucose. These substrate-regulated processes are mostly independent of insulin signaling in the liver but are dependent on insulin signaling in WAT, which becomes defective with inflammation. Therapies that decrease ectopic lipid storage and diminish macrophage-induced WAT lipolysis will reverse the root causes of type 2 diabetes.

Disparate metabolic response to fructose feeding between different mouse strains

Diets enriched in fructose (FR) increase lipogenesis in the liver, leading to hepatic lipid accumulation and the development of insulin resistance. Previously, we have shown that in contrast to other mouse strains, BALB/c mice are resistant to high fat diet-induced metabolic deterioration, potentially due to a lack of ectopic lipid accumulation in the liver. In this study we have compared the metabolic response of BALB/c and C57BL/6 (BL6) mice to a fructose-enriched diet. Both strains of mice increased adiposity in response to FR-feeding, while only BL6 mice displayed elevated hepatic triglyceride (TAG) accumulation and glucose intolerance. The lack of hepatic TAG accumulation in BALB/c mice appeared to be linked to an altered balance between lipogenic and lipolytic pathways, while the protection from fructose-induced glucose intolerance in this strain was likely related to low levels of ER stress, a slight elevation in insulin levels and an altered profile of diacylglycerol species in the liver. Collectively these findings highlight the multifactorial nature of metabolic defects that develop in response to changes in the intake of specific nutrients and the divergent response of different mouse strains to dietary challenges.

Proteomic analysis of livers from fat-fed mice deficient in either PKCδ or PKCε identifies Htatip2 as a regulator of lipid metabolism

Insulin resistance contributes to the development of Type 2 diabetes, and is associated with lipid oversupply. Deletion of isoforms of the lipid-activated protein kinase C (PKC) family, PKCδ or PKCε, improves insulin action in fat-fed mice, but differentially affects hepatic lipid metabolism. To investigate the mechanisms involved, we employed an in vivo adaptation of SILAC to examine the effects of a fat diet together with deletion of PKCδ or PKCε on the expression of liver proteins. We identified a total of 3359 and 3488 proteins from the PKCδ and PKCε knockout study groups, respectively, and showed that several enzymes of lipid metabolism were affected by the fat diet. In fat-fed mice, 23 proteins showed changes upon PKCδ deletion while 19 proteins were affected by PKCε deletion. Enzymes of retinol metabolism were affected by the absence of either PKC. Pathway analysis indicated that monosaccharide metabolism was affected only upon PKCδ deletion, while isoprenoid biosynthesis was affected in a PKCε-specific manner. Certain proteins were regulated inversely, including HIV-1 tat interactive protein 2 (Htatip2). Overexpression or knockdown of Htatip2 in hepatocytes affected fatty acid storage and oxidation, consistent with a novel role in mediating the differential effects of PKC isoforms on lipid metabolism. All MS data have been deposited in the ProteomeXchange with identifier PXD000971 (http://proteomecentral.proteomexchange.org/dataset/PXD000971).

The role of hepatic lipids in hepatic insulin resistance and type 2 diabetes

Non-alcoholic fatty liver disease and its downstream sequelae, hepatic insulin resistance and type 2 diabetes, are rapidly growing epidemics, which lead to increased morbidity and mortality rates, and soaring health-care costs. Developing interventions requires a comprehensive understanding of the mechanisms by which excess hepatic lipid develops and causes hepatic insulin resistance and type 2 diabetes. Proposed mechanisms implicate various lipid species, inflammatory signalling and other cellular modifications. Studies in mice and humans have elucidated a key role for hepatic diacylglycerol activation of protein kinase Cε in triggering hepatic insulin resistance. Therapeutic approaches based on this mechanism could alleviate the related epidemics of non-alcoholic fatty liver disease and type 2 diabetes.

Molecular Mechanisms of Insulin Resistance Development

Insulin resistance (IR) is a phenomenon associated with an impaired ability of insulin to stimulate glucose uptake by target cells and to reduce the blood glucose level. A response increase in insulin secretion by the pancreas and hyperinsulinemia are compensatory reactions of the body. The development of IR leads to the inability of target cells to respond to insulin that results in developing type 2 diabetes mellitus (T2DM) and metabolic syndrome. For this reason, the metabolic syndrome is defined in practice as a combination of IR with one or more pathologies such as T2DM, arterial hypertension, dyslipidemia, abdominal obesity, non-alcoholic fatty liver disease, and some others. However, a combination of high blood glucose and insulin levels always serves as its physiological criterion. IR should be considered as a systemic failure of the endocrine regulation in the body. Physiological causes of IR are diverse. The main ones are nutritional overload and accumulation of certain lipids and their metabolites in cells, low physical activity, chronic inflammation and stress of various nature, including oxidative and endoplasmic reticulum stress (impairment of damaged protein degradation in the cell). Recent studies have demonstrated that these physiological mechanisms likely act through a single intracellular scenario. This is the impairment of signal transduction from the insulin receptor to its targets via the negative feedback mechanism in intracellular insulin-dependent signaling cascades. This review describes the physiological and intracellular mechanisms of insulin action and focuses on their abnormalities upon IR development. Finally, feasible trends in early molecular diagnosis and therapy of IR are discussed.

The tail wagging the dog--regulation of lipid metabolism by protein kinase C

Upon their discovery almost 40 years ago, isoforms of the lipid-activated protein kinase C (PKC) family were initially regarded only as downstream effectors of the second messengers calcium and diacylglycerol, undergoing activation upon phospholipid hydrolysis in response to acute stimuli. Subsequently, several isoforms were found to be associated with the inhibitory effects of lipid over-supply on glucose homeostasis, especially the negative cross-talk with insulin signal transduction, observed upon accumulation of diacylglycerol in insulin target tissues. The PKC family has therefore attracted much attention in diabetes and obesity research, because intracellular lipid accumulation is strongly correlated with defective insulin action and the development of type 2 diabetes. Causal roles for various isoforms in the generation of insulin resistance have more recently been confirmed using PKC-deficient mice. However, during characterization of these animals, it became increasingly evident that the enzymes play key roles in the modulation of lipid metabolism itself, and may control the supply of lipids between tissues such as adipose and liver. Molecular studies have also demonstrated roles for PKC isoforms in several aspects of lipid metabolism, such as adipocyte differentiation and hepatic lipogenesis. While the precise mechanisms involved, especially the identities of protein substrates, are still unclear, the emerging picture suggests that the currently held view of the contribution of PKC isoforms to metabolism is an over-simplification. Although PKCs may inhibit insulin signal transduction, these enzymes are not merely downstream effectors of lipid accumulation, but in fact control the fate of fatty acids, thus the tail wags the dog.

Protein kinase Cε modulates insulin receptor localization and trafficking in mouse embryonic fibroblasts

We have previously shown that deletion of protein kinase C epsilon (PKCε) in mice results in protection against glucose intolerance caused by a high fat diet. This was in part due to reduced insulin uptake by hepatocytes and insulin clearance, which enhanced insulin availability. Here we employed mouse embryonic fibroblasts (MEFs) derived from wildtype (WT) and PKCε-deficient (PKCε^−/−) mice to examine this mechanistically. PKCε^−/− MEFs exhibited reduced insulin uptake which was associated with decreased insulin receptor phosphorylation, while downstream signalling through IRS-1 and Akt was unaffected. Cellular fractionation demonstrated that PKCε deletion changed the localization of the insulin receptor, a greater proportion of which co-fractionated with flotillin-1, a marker of membrane microdomains. Insulin stimulation resulted in redistribution of the receptor in WT cells, while this was markedly reduced in PKCε^−/− cells. These alterations in insulin receptor trafficking were associated with reduced expression of CEACAM1, a receptor substrate previously shown to modulate insulin clearance. Virally-mediated reconstitution of PKCε in MEFs increased CEACAM1 expression and partly restored the sensitivity of the receptor to insulin-stimulated redistribution. These data indicate that PKCε can affect insulin uptake in MEFs through promotion of receptor-mediated endocytosis, and that this may be mediated by regulation of CEACAM1 expression.

Deletion of protein kinase Cε in mice has limited effects on liver metabolite levels but alters fasting ketogenesis and gluconeogenesis

AIMS/HYPOTHESIS

Protein kinase Cε (PKCε) is emerging as a key mediator of lipid-induced insulin resistance in liver and hepatic lipid metabolism itself. We investigated whether PKCε plays a role in other metabolic processes, to further examine its suitability as a therapeutic target.

METHODS

We measured amino acid, organic acid and sugar levels by liquid and gas chromatography-mass spectrometry of liver extracts from chow and fat-fed wild-type (WT) and PKCε-deficient (Prkce(-/-)) mice. Fed and fasting glucose, ketone and fatty acid levels were measured in blood. Triacylglycerol levels and gluconeogenic and ketogenic enzyme expression were measured in liver. The effect of fasting on epididymal fat pad mass was also determined.

RESULTS

Metabolomic analysis indicated that the short-term high-fat diet affected over 20 compounds, including a 50% reduction in the glucogenic amino acid alanine. Prkce deletion resulted only in a reduction of 4-hydroxyproline and aspartate and an increase in glutamate. However, upon fasting, Prkce(-/-) mice were better able to maintain blood glucose levels and also exhibited lower levels of the ketone β-hydroxybutyrate compared with WT mice. Upon fasting, Prkce deletion also resulted in lower liver and plasma lipids and a smaller reduction in fat pad mass.

CONCLUSIONS/INTERPRETATION

Metabolomic analysis provided new insights into the effects of a high-fat diet on liver metabolite levels. Glucose homeostasis under fasting conditions is improved in Prkce(-/-) mice, which, in turn, may reduce the mobilisation of lipid from adipose tissue, reducing the availability of ketogenic substrate in the liver. Together with the protection against fat-diet-induced glucose intolerance previously observed in the fed state, these findings indicate PKCε as a unique therapeutic target for the improvement of glucose homeostasis.

Lipid-induced endoplasmic reticulum stress in liver cells results in two distinct outcomes: adaptation with enhanced insulin signaling or insulin resistance

Chronically elevated fatty acids contribute to insulin resistance through poorly defined mechanisms. Endoplasmic reticulum (ER) stress and the subsequent unfolded protein response (UPR) have been implicated in lipid-induced insulin resistance. However, the UPR is also a fundamental mechanism required for cell adaptation and survival. We aimed to distinguish the adaptive and deleterious effects of lipid-induced ER stress on hepatic insulin action. Exposure of human hepatoma HepG2 cells or mouse primary hepatocytes to the saturated fatty acid palmitate enhanced ER stress in a dose-dependent manner. Strikingly, exposure of HepG2 cells to prolonged mild ER stress activation induced by low levels of thapsigargin, tunicamycin, or palmitate augmented insulin-stimulated Akt phosphorylation. This chronic mild ER stress subsequently attenuated the acute stress response to high-level palmitate challenge. In contrast, exposure of HepG2 cells or hepatocytes to severe ER stress induced by high levels of palmitate was associated with reduced insulin-stimulated Akt phosphorylation and glycogen synthesis, as well as increased expression of glucose-6-phosphatase. Attenuation of ER stress using chemical chaperones (trimethylamine N-oxide or tauroursodeoxycholic acid) partially protected against the lipid-induced changes in insulin signaling. These findings in liver cells suggest that mild ER stress associated with chronic low-level palmitate exposure induces an adaptive UPR that enhances insulin signaling and protects against the effects of high-level palmitate. However, in the absence of chronic adaptation, severe ER stress induced by high-level palmitate exposure induces deleterious UPR signaling that contributes to insulin resistance and metabolic dysregulation.

Mechanisms for insulin resistance: common threads and missing links

Insulin resistance is a complex metabolic disorder that defies explanation by a single etiological pathway. Accumulation of ectopic lipid metabolites, activation of the unfolded protein response (UPR) pathway, and innate immune pathways have all been implicated in the pathogenesis of insulin resistance. However, these pathways are also closely linked to changes in fatty acid uptake, lipogenesis, and energy expenditure that can impact ectopic lipid deposition. Ultimately, these cellular changes may converge to promote the accumulation of specific lipid metabolites (diacylglycerols and/or ceramides) in liver and skeletal muscle, a common final pathway leading to impaired insulin signaling and insulin resistance.

Localisation and composition of skeletal muscle diacylglycerol predicts insulin resistance in humans

AIMS/HYPOTHESIS

We sought to evaluate if the cellular localisation and molecular species of diacylglycerol (DAG) were related to insulin sensitivity in human skeletal muscle.

METHODS

Healthy sedentary obese controls (Ob; n = 6; mean±SEM age 39.5 ± 2.3 years; mean ± SEM BMI 33.3 ± 1.4 kg/m(2)), individuals with type 2 diabetes (T2D; n = 6; age 44 ± 1.8 years; BMI 30.1 ± 2.3 kg/m(2)), and lean endurance-trained athletes (Ath; n = 10; age 35.4 ± 3.1 years; BMI 23.3 ± 0.8 kg/m(2)) were studied. Insulin sensitivity was determined using an IVGTT. Muscle biopsy specimens were taken after an overnight fast, fractionated using ultracentrifugation, and DAG species measured using liquid chromatography/MS/MS.

RESULTS

Total muscle DAG concentration was higher in the Ob (mean ± SEM 13.3 ± 1.0 pmol/μg protein) and T2D (15.2 ± 1.0 pmol/μg protein) groups than the Ath group (10.0 ± 0.78 pmol/μg protein, p = 0.002). The majority (76-86%) DAG was localised in the membrane fraction for all groups, but was lowest in the Ath group (Ob, 86.2 ± 0.98%; T2D, 84.2 ± 1.2%; Ath, 75.9 ± 2.7%; p = 0.008). There were no differences in cytoplasmic DAG species (p > 0.12). Membrane DAG species C18:0/C20:4, Di-C16:0 and Di-C18:0 were significantly more abundant in the T2D group. Cytosolic DAG species were negatively related to activation of protein kinase C (PKC)ε but not PKCθ, whereas membrane DAG species were positively related to activation of PKCε, but not PKCθ. Only total membrane DAG (r = -0.624, p = 0.003) and Di-C18:0 (r = -0.595, p = 0.004) correlated with insulin sensitivity. Disaturated DAG species were significantly lower in the Ath group (p = 0.001), and significantly related to insulin sensitivity (r = -0.642, p = 0.002).

CONCLUSIONS/INTERPRETATION

These data indicate that both cellular localisation and composition of DAG influence the relationship to insulin sensitivity. Our results suggest that only saturated DAG in skeletal muscle membranes are related to insulin resistance in humans.

Identification of BMP and activin membrane-bound inhibitor (BAMBI) as a potent negative regulator of adipogenesis and modulator of autocrine/paracrine adipogenic factors

Adipose tissue dysfunction underpins the association of obesity with type 2 diabetes. Adipogenesis is required for the maintenance of adipose tissue function. It involves the commitment and subsequent differentiation of preadipocytes and is coordinated by autocrine, paracrine, and endocrine factors. We previously reported that fibroblast growth factor-1 (FGF-1) primes primary human preadipocytes and Simpson Golabi Behmel syndrome (SGBS) preadipocytes and increases adipogenesis through a cascade involving extracellular signal-related kinase 1/2 (ERK1/2). Here, we aimed to use the FGF-1 system to identify novel adipogenic regulators. Expression profiling revealed bone morphogenetic protein (BMP) and activin membrane-bound inhibitor (BAMBI) as a putative FGF-1 effector. BAMBI is a transmembrane protein and modulator of paracrine factors that regulate adipogenesis, including transforming growth factor (TGF) superfamily members (TGF-β and BMP) and Wnt. Functional investigations established BAMBI as a negative regulator of adipogenesis and modulator of the anti- and proadipogenic effects of Wnt3a, TGF-β1, and BMP-4. Further studies showed that BAMBI expression levels are decreased in a mouse model of diet-induced obesity. Collectively, these findings establish BAMBI as a novel, negative regulator of adipogenesis that can act as a nexus to integrate multiple paracrine signals to coordinate adipogenesis. Alterations in BAMBI may play a role in the (patho)physiology of obesity, and manipulation of BAMBI may present a novel therapeutic approach to improve adipose tissue function.