Amino acid- and lipid-induced insulin resistance in rat heart: molecular mechanisms

Diabetes Mellitus and Cardiovascular Diseases: Nutraceutical Interventions Related to Caloric Restriction

Type 2 diabetes (T2DM) and cardiovascular disease (CVD) are closely associated and represent a key public health problem worldwide. An excess of adipose tissue, NAFLD, and gut dysbiosis establish a vicious circle that leads to chronic inflammation and oxidative stress. Caloric restriction (CR) is the most promising nutritional approach capable of improving cardiometabolic health. However, adherence to CR represents a barrier to patients and is the primary cause of therapeutic failure. To overcome this problem, many different nutraceutical strategies have been designed. Based on several data that have shown that CR action is mediated by AMPK/SIRT1 activation, several nutraceutical compounds capable of activating AMPK/SIRT1 signaling have been identified. In this review, we summarize recent data on the possible role of berberine, resveratrol, quercetin, and L-carnitine as CR-related nutrients. Additionally, we discuss the limitations related to the use of these nutrients in the management of T2DM and CVD.

PD-L1 tumor-intrinsic signaling and its therapeutic implication in triple-negative breast cancer

Although the immune checkpoint role of programmed death ligand 1 (PD-L1) has been established and targeted in cancer immunotherapy, the tumor-intrinsic role of PD-L1 is less appreciated in tumor biology and therapeutics development, partly because of the incomplete mechanistic understanding. Here we demonstrate a potentially novel mechanism by which PD-L1 promotes the epithelial-mesenchymal transition (EMT) in triple-negative breast cancer (TNBC) cells by suppressing the destruction of the EMT transcription factor Snail. PD-L1 directly binds to and inhibits the tyrosine phosphatase PTP1B, thus preserving p38-MAPK activity that phosphorylates and inhibits glycogen synthase kinase 3β (GSK3β). Via this mechanism, PD-L1 prevents the GSK3β-mediated phosphorylation, ubiquitination, and degradation of Snail and consequently promotes the EMT and metastatic potential of TNBC. Significantly, PD-L1 antibodies that confine the tumor-intrinsic PD-L1/Snail pathway restricted TNBC progression in immunodeficient mice. More importantly, targeting both tumor-intrinsic and tumor-extrinsic functions of PD-L1 showed strong synergistic tumor suppression effect in an immunocompetent TNBC mouse model. Our findings support that PD-L1 intrinsically facilitates TNBC progression by promoting the EMT, and this potentially novel PD-L1 signaling pathway could be targeted for better clinical management of PD-L1–overexpressing TNBCs.

Mechanisms behind the immediate effects of Roux-en-Y gastric bypass surgery on type 2 diabetes

BACKGROUND

The most common bariatric surgery, Roux-en-Y gastric bypass, leads to glycemia normalization in most patients long before there is any appreciable weight loss. This effect is too large to be attributed purely to caloric restriction, so a number of other mechanisms have been proposed. The most popular hypothesis is enhanced production of an incretin, active glucagon-like peptide-1 (GLP-1), in the lower intestine. We therefore set out to test this hypothesis with a model which is simple enough to be robust and credible.

METHOD

Our method involves (1) setting up a set of time-dependent equations for the concentrations of the most relevant species, (2) considering an "adiabatic" (or quasi-equilibrium) state in which the concentrations are slowly varying compared to reaction rates (and which in the present case is a postprandial state), and (3) solving for the dependent concentrations (of e.g. insulin and glucose) as an independent concentration (of e.g. GLP-1) is varied.

RESULTS

Even in the most favorable scenario, with maximal values for (i) the increase in active GLP-1 concentration and (ii) the effect of GLP-1 on insulin production, enhancement of GLP-1 alone cannot account for the observations. I.e., the largest possible decrease in glucose predicted by the model is smaller than reported decreases, and the model predicts no decrease whatsoever in glucose ×insulin, in contrast to large observed decreases in homeostatic model assessment insulin resistance (HOMA-IR). On the other hand, both effects can be accounted for if the surgery leads to a substantial increase in some substance that opens an alternative insulin-independent pathway for glucose transport into muscle cells, which perhaps uses the same intracellular pool of GLUT-4 that is employed in an established insulin-independent pathway stimulated by muscle contraction during exercise.

CONCLUSIONS

Glycemia normalization following Roux-en-Y gastric bypass is undoubtedly caused by a variety of mechanisms, which may include caloric restriction, enhanced GLP-1, and perhaps others proposed in earlier papers on this subject. However, the present results suggest that another possible mechanism should be added to the list of candidates: enhanced production in the lower intestine of a substance which opens an alternative insulin-independent pathway for glucose transport.

GSK-3: Functional Insights from Cell Biology and Animal Models

Glycogen synthase kinase-3 (GSK-3) is a widely expressed and highly conserved serine/threonine protein kinase encoded in mammals by two genes that generate two related proteins: GSK-3α and GSK-3β. GSK-3 is active in cells under resting conditions and is primarily regulated through inhibition or diversion of its activity. While GSK-3 is one of the few protein kinases that can be inactivated by phosphorylation, the mechanisms of GSK-3 regulation are more varied and not fully understood. Precise control appears to be achieved by a combination of phosphorylation, localization, and sequestration by a number of GSK-3-binding proteins. GSK-3 lies downstream of several major signaling pathways including the phosphatidylinositol 3′ kinase pathway, the Wnt pathway, Hedgehog signaling and Notch. Specific pools of GSK-3, which differ in intracellular localization, binding partner affinity, and relative amount are differentially sensitized to several distinct signaling pathways and these sequestration mechanisms contribute to pathway insulation and signal specificity. Dysregulation of signaling pathways involving GSK-3 is associated with the pathogenesis of numerous neurological and psychiatric disorders and there are data suggesting GSK-3 isoform-selective roles in several of these. Here, we review the current knowledge of GSK-3 regulation and targets and discuss the various animal models that have been employed to dissect the functions of GSK-3 in brain development and function through the use of conventional or conditional knockout mice as well as transgenic mice. These studies have revealed fundamental roles for these protein kinases in memory, behavior, and neuronal fate determination and provide insights into possible therapeutic interventions.

Multiple abnormalities of myocardial insulin signaling in a porcine model of diet-induced obesity

Heightened cardiovascular risk among patients with systemic insulin resistance is not fully explained by the extent of atherosclerosis. It is unknown whether myocardial insulin resistance accompanies systemic insulin resistance and contributes to increased cardiovascular risk. This study utilized a porcine model of diet-induced obesity to determine if myocardial insulin resistance develops in parallel with systemic insulin resistance and investigated potential mechanisms for such changes. Micropigs (n = 16) were assigned to control (low fat, no added sugars) or intervention (25% wt/wt coconut oil and 20% high-fructose corn syrup) diet for 7 mo. Intervention diet resulted in obesity, hypertension, and dyslipidemia. Systemic insulin resistance was manifest by elevated fasting glucose and insulin, abnormal response to intravenous glucose tolerance testing, and blunted skeletal muscle phosphatidylinositol-3-kinase (PI 3-kinase) activation and protein kinase B (Akt) phosphorylation in response to insulin. In myocardium, insulin-stimulated glucose uptake, PI 3-kinase activation, and Akt phosphorylation were also blunted in the intervention diet group. These findings were explained by increased myocardial content of p85alpha (regulatory subunit of PI 3-kinase), diminished association of PI 3-kinase with insulin receptor substrate (IRS)-1 in response to insulin, and increased serine-307 phosphorylation of IRS-1. Thus, in a porcine model of diet-induced obesity that recapitulates many characteristics of insulin-resistant patients, myocardial insulin resistance develops along with systemic insulin resistance and is associated with multiple abnormalities of insulin signaling.

Dietary fish oil reverses lipotoxicity, altered glucose metabolism, and nPKCepsilon translocation in the heart of dyslipemic insulin-resistant rats

The present study analyzes several markers of energy metabolism in the heart muscle of dyslipemic insulin-resistant rats fed a sucrose-rich diet (SRD, 62.5% wt/wt) for 8 months. It also explores the possible beneficial effects of dietary fish oil supplementation on cardiac lipids and glucose metabolism. With this purpose, male Wistar rats were fed an SRD for 6 months. Whereas half of the animals continued with the same diet for up to 8 months, the other half was fed an SRD in which fish oil (7% + 1% corn oil wt/wt) replaced corn oil (8% wt/wt) from months 6 to 8. The results were compared with rats fed a control diet (starch 62.5% wt/wt). The cardiac muscle of SRD-fed rats showed (1) a significant reduction (P < .05) in key enzymes activities and metabolites involved in glucose metabolism, accompanied by a significant (P < .05) increase of lipid storage (triglyceride, long-chain acyl coenzyme A, and diacylglycerol), and (2) a significant increase (P < .05) of nPKCepsilon protein mass expression in the membrane fraction without changes in the cPKCbetaII. Dietary fish oil, which reduces the availability of plasma lipid flux and normalizes glucose homeostasis, was able to reverse heart muscle lipotoxicity. Fish oil benefits key enzymes activities in glucose metabolism and normalizes glycogen and glucose-6-phosphate concentration, and the altered nPKCepsilon protein mass expression translocation in the heart of SRD-fed rats. Our findings suggest that manipulation of dietary fats may play a key role in the management of lipid disorders, offering a protection against the development of cardiovascular diseases.

Amino acids are necessary for the insulin-induced activation of mTOR/S6K1 signaling and protein synthesis in healthy and insulin resistant human skeletal muscle

BACKGROUND

Amino acids (AA) activate the mammalian target of rapamycin (mTOR) signaling pathway but overactivation has a negative feedback effect on insulin signaling which may lead to insulin resistance and type 2 diabetes (T2DM).

PURPOSE

To determine the effect of reduced AA concentrations on mTOR and insulin signaling during increased nutrient and insulin availability.

METHODS

Six control and six T2DM subjects were studied at baseline and following a 5h AA lowering high energy and insulin clamp. Stable isotopic techniques in combination with femoral catheterizations were used to measure AA kinetics across the leg while muscle biopsies were used to measure mTOR and insulin signaling proteins using immunoblotting techniques.

RESULTS

AA concentrations decreased by approximately 30-60% in both groups (p<0.05). Phospho-mTOR, S6K1, eEF2, and eIF2alpha were unchanged in both groups following the clamp (p>0.05). In T2DM subjects, IRS-1 serine phosphorylation was unchanged while phospho-AMPKalpha decreased and phospho-Akt, phospho-AS160 and glucose uptake increased following the clamp (p<0.05). In comparison, AA concentrations were maintained in a separate group during an insulin infusion. In this group, phospho-Akt, mTOR and S6K1 (n=4) increased.

CONCLUSION

Amino acids are necessary for insulin-induced activation of mTOR signaling and protein synthesis in both healthy and insulin resistant skeletal muscle.

AICAR positively regulate glycogen synthase activity and LDL receptor expression through Raf-1/MEK/p42/44MAPK/p90RSK/GSK-3 signaling cascade

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) is a commonly used pharmacological agent to study physiological effects which are similar to those of exercise. However, signal transduction pathways by which AICAR elicits downstream effects in liver are poorly understood. We report here that AICAR not only activated AMPK but also phosphorylated/deactivated glycogen synthase kinase-3 alpha/beta (GSK-3alpha/beta) and dephophorylated/activated glycogen synthase (GS) in a time-dependent manner in human hepatoma HepG2 cells. The signal connection between AICAR and GSK-3 is indirect and involves activation of Raf-1/MEK/p42/44(MAPK)/p90(RSK) signaling cascade as pharmacologic inhibition of MEK significantly reduced phosphorylation/deactivation of GSK-3 and consequent dephosphorylation/activation of GS. Moreover, silencing the expression of p90(RSK), a substrate of p42/44(MAPK), attenuated AICAR-dependent GSK-3 phosphorylation, implicating this kinase as a key mediator of AICAR signaling to GSK-3. Furthermore, consistent with the involvement of Raf-1 kinase cascade, AICAR-induced low-density lipoprotein (LDL) receptor expression in a p42/44(MAPK)-dependent manner. Finally, AICAR requires AMPK-alpha2-dependent and -independent pathways to activate Raf-1 kinase cascade as suppression of AMPKalpha2 activity, and not of AMPKalpha1, partially blocked AICAR-dependent p42/44(MAPK) activation and GSK-3 phosphorylation/deactivation. Collectively, these results highlight Raf-1 signaling cascade as the critical mediator of AICAR action on glucose and lipid metabolism in HepG2 cells.

Inhibition of GSK-3beta as a target for cardioprotection: the importance of timing, location, duration and degree of inhibition

Cardiovascular disease is the major cause of morbidity and mortality in western countries such as the US. Myocardial infarction leads to loss of myocytes and with extremely limited ability to replenish cardiomyocytes, the heart exhibits depressed contractility. This ultimately results in hypertrophy of the remaining viable myocytes, which is the primary predictor for heart failure. Thus, drug therapies which can reduce myocyte cell death and reduce postischaemic dysfunction would be expected to greatly reduce cardiac hypertrophy and subsequent heart failure and death. Inhibition of glycogen synthase kinase (GSK)-3beta has been proposed as a strategy to improve postischaemic cardiomyocyte survival, as inhibition of GSK-3beta has been shown to reduce myocardial cell death following ischaemia and reperfusion. Therapies for inhibiting GSK are feasible as there are a number of newly developed specific inhibitors of GSK available, although most of these drugs have not been tested in long-term animal studies.

CONVERGENCE OF GLUCOSE- AND FATTY ACID-INDUCED ABNORMAL MYOCARDIAL EXCITATION-CONTRACTION COUPLING AND INSULIN SIGNALLING

1. Myocardial insulin resistance and abnormal Ca(2+) regulation are hallmarks of hypertrophic and diabetic hearts, but deprivation of energetic substrates does not tell the whole story. Is there a link between the aetiology of these dysfunctions? 2. Diabetic cardiomyopathy is defined as phenotypic changes in the heart muscle cell independent of associated coronary vascular disease. The cellular consequences of diabetes on excitation-contraction (E-C) coupling and insulin signalling are presented in various models of diabetes in order to set the stage for exploring the pathogenesis of heart disease. 3. Excess glucose or fatty acids can lead to augmented flux through the hexosamine biosynthesis pathway (HBP). The formation of uridine 5 cent-diphosphate-hexosamines has been shown to be involved in abnormal E-C coupling and myocardial insulin resistance. 4. There is growing evidence that O-linked glycosylation (downstream of HBP) may regulate the function of cytosolic and nuclear proteins in a dynamic manner, similar to phosphorylation and perhaps involving reciprocal or synergistic modification of serine/threonine sites. 5. This review focuses on the question of whether there is a role for HBP and dynamic O-linked glycosylation in the development of myocardial insulin resistance and abnormal E-C coupling. The emerging concept that O-linked glycosylation is a regulatory, post-translational modification of cytosolic/nuclear proteins that interacts with phosphorylation in the heart is explored.

Wasting and the substrate-to-energy controlled pathway: a role for insulin resistance and amino acids

Amino acids contained in proteins can be transformed either in glucose precursors or in acetate, the end product of free fatty acid (FFA) oxidation. The dynamics of glucose, FFA, and amino acid competition for entry into the citric acid cycle (tricarboxylic acid [TCA] cycle) are very complex and not fully understood. Conditions where glucose is insufficiently driven to full oxidation are characterized by lowest efficiency in energy production per mole of oxygen consumed. Moreover, acetate provided by oxidation of FFA increases consumption of amino acids as precursors of the oxaloacetate required for condensation with acetate and for maintenance of citrate synthesis. Increased consumption of amino acids in the TCA cycle, if not matched by adequate intake, leads to muscular wasting and cachexia. Therefore, amino acid needs are very complex, and their intake must provide a balanced ratio of glucogenic and ketogenic precursors suitable to trigger entry of glucose to full oxidation and blunt the level of FFA utilization. Optimization of substrate entry into energy production must also be coupled with sufficient availability of amino acids in ratios suitable for maintaining protein synthesis, inhibiting the catabolic drive, and promoting integrity of cellular proteic structures. Alimentary proteins have a content of amino acids that is far from the stoichiometric ratios of essential amino acids required by humans. An amino acid formulation suitable to match energy needs, control carbohydrate and lipid flow into the TCA cycle, and promote protein synthesis in contracting cells is detailed in this article.

Amino acid signalling and the integration of metabolism

It has become clear in recent years that amino acids are not only important as substrates for various metabolic pathways but that they can also activate a nutrient-sensitive, mTOR-mediated, signalling pathway in synergy with insulin. Leucine is the most effective amino acid in this regard. The signalling pathway is antagonised by AMP-activated protein kinase. Amino acid signalling stimulates protein synthesis and inhibits (autophagic) proteolysis. In addition, many amino acids cause an increase in cell volume. Cell swelling per se stimulates synthesis of protein, glycogen, and lipid, in part by further stimulating signalling and in part by unrelated mechanisms. Amino acids also stimulate signalling in beta-cells and stimulate beta-cell growth and proliferation. This results in increased production of insulin, which enhances the anabolic (and anti-catabolic) properties of amino acids. Finally, amino acid-dependent signalling controls the production of leptin by adipocytes, and thus contributes to the regulation of appetite.

Rosiglitazone, an agonist of peroxisome-proliferator-activated receptor gamma (PPARgamma), decreases inhibitory serine phosphorylation of IRS1 in vitro and in vivo

Peroxisome-proliferator-activated receptor gamma agonists such as rosiglitazone, a thiazolidinedione, improve insulin sensitivity in vivo, but the underlying mechanism(s) remains unclear. Phosphorylation of IRS1 (insulin receptor substrate protein 1) on certain serine residues, including S307 and S612 in rodent IRS1 (equivalent to S312 and S616 in human IRS1), has been shown to play a negative role in insulin signalling. In the present study, we investigated whether rosiglitazone improves insulin sensitivity by decreasing IRS1 inhibitory serine phosphorylation. In HEK-293 (human embryonic kidney 293) cells stably expressing recombinant IRS1 and in 3T3L1 adipocytes, rosiglitazone attenuated PMA-induced IRS1 S307/S612 phosphorylation and decreased insulin-stimulated Akt phosphorylation. We observed increased IRS1 S307 phosphorylation and concomitant decrease in insulin signalling as measured by insulin-stimulated IRS1 tyrosine phosphorylation, and Akt threonine phosphorylation in adipose tissues of Zucker obese rats compared with lean control rats. Treatment with rosiglitazone at 30 mg/kg body weight for 24 and 48 h increased insulin signalling and decreased IRS1 S307 phosphorylation concomitantly. Whereas the 48 h treatment reversed hyper-phosphorylation (and activation) of both c-Jun N-terminal kinase and p38 mitogen-activated protein kinase, the 24 h treatments only decreased hyper-phosphorylation of p38 mitogen-activated protein kinase. The treatment of the Zucker obese rats with rosiglitazone also reversed the high circulating levels of non-esterified fatty acids, which have been shown to be correlated with increased IRS1 serine phosphorylation in other animal models. Taken together, these results suggest that IRS1 inhibitory serine phosphorylation is a key component of insulin resistance and its reversal contributes to the insulin sensitizing effects by rosiglitazone.

Amino acid transporters: roles in amino acid sensing and signalling in animal cells

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

Amino acids as regulators and components of nonproteinogenic pathways

Amino acids are not only important precursors for the synthesis of proteins and other N-containing compounds, but also participate in the regulation of major metabolic pathways. Glutamate and aspartate, for example, are components of the malate/aspartate shuttle and their concentrations control the rate of mitochondrial oxidation of glycolytic NADH. Glutamate also controls the rate of urea synthesis, not only as the precursor of ammonia and aspartate, but as substrate for synthesis of N-acetylglutamate, the essential activator of carbamoyl-phosphate synthase. This mechanism allows large variations in urea synthesis at relatively constant ammonia concentrations. Increases in intracellular amino acid concentration increase cell volume. Cell swelling per se has anabolic effects on protein, carbohydrate and lipid metabolism: enhanced synthesis of macromolecules compensates for increases in intracellular osmolarity. Mechanisms responsible for cell swelling-induced changes in pathway fluxes include changes in intracellular ion concentrations and in signal transduction. Specific amino acids (e.g., leucine) stimulate protein synthesis and inhibit (autophagic) protein degradation independent of changes in cell volume because they stimulate mTOR (mammalian target of rapamycin), a protein kinase, which is one of the components of a signal transduction pathway used by insulin. When the cellular energy state is low, stimulation of mTOR by amino acids is prevented by activation of AMP-dependent protein kinase. Amino acid-dependent signaling also promotes insulin production by beta-cells. This further adds to the anabolic properties of amino acids. It is concluded that amino acids are important regulators of major metabolic pathways.

GSK-3: tricks of the trade for a multi-tasking kinase

Glycogen synthase kinase 3 (GSK-3) is a multifunctional serine/threonine kinase found in all eukaryotes. The enzyme is a key regulator of numerous signalling pathways, including cellular responses to Wnt, receptor tyrosine kinases and G-protein-coupled receptors and is involved in a wide range of cellular processes, ranging from glycogen metabolism to cell cycle regulation and proliferation. GSK-3 is unusual in that it is normally active in cells and is primarily regulated through inhibition of its activity. Another peculiarity compared with other protein kinases is its preference for primed substrates, that is, substrates previously phosphorylated by another kinase. Several recent advances have improved our understanding of GSK-3 regulation in multiple pathways. These include the solution of the crystal structure of GSK-3, which has provided insight into GSK-3's penchant for primed substrates and the regulation of GSK-3 by serine phosphorylation, and findings related to the involvement of GSK-3 in the Wnt/beta-catenin and Hedgehog pathways. Finally, since increased GSK-3 activity may be linked to pathology in diseases such as Alzheimer's disease and non-insulin-dependent diabetes mellitus, several new GSK-3 inhibitors, such as the aloisines, the paullones and the maleimides, have been developed. Although they are just starting to be characterized in cell culture experiments, these new inhibitors hold promise as therapeutic agents.