Insulin inhibition of 5' adenosine monophosphate-activated protein kinase in the heart results in activation of acetyl coenzyme A carboxylase and inhibition of fatty acid oxidation

Insulin-Heart Axis: Bridging Physiology to Insulin Resistance

Insulin signaling is vital for regulating cellular metabolism, growth, and survival pathways, particularly in tissues such as adipose, skeletal muscle, liver, and brain. Its role in the heart, however, is less well-explored. The heart, requiring significant ATP to fuel its contractile machinery, relies on insulin signaling to manage myocardial substrate supply and directly affect cardiac muscle metabolism. This review investigates the insulin-heart axis, focusing on insulin's multifaceted influence on cardiac function, from metabolic regulation to the development of physiological cardiac hypertrophy. A central theme of this review is the pathophysiology of insulin resistance and its profound implications for cardiac health. We discuss the intricate molecular mechanisms by which insulin signaling modulates glucose and fatty acid metabolism in cardiomyocytes, emphasizing its pivotal role in maintaining cardiac energy homeostasis. Insulin resistance disrupts these processes, leading to significant cardiac metabolic disturbances, autonomic dysfunction, subcellular signaling abnormalities, and activation of the renin-angiotensin-aldosterone system. These factors collectively contribute to the progression of diabetic cardiomyopathy and other cardiovascular diseases. Insulin resistance is linked to hypertrophy, fibrosis, diastolic dysfunction, and systolic heart failure, exacerbating the risk of coronary artery disease and heart failure. Understanding the insulin-heart axis is crucial for developing therapeutic strategies to mitigate the cardiovascular complications associated with insulin resistance and diabetes.

Reciprocal regulation of cardiac β-oxidation and pyruvate dehydrogenase by insulin

The heart alters the rate and relative oxidation of fatty acids and glucose based on availability and energetic demand. Insulin plays a crucial role in this process diminishing fatty acid and increasing glucose oxidation when glucose availability increases. Loss of insulin sensitivity and metabolic flexibility can result in cardiovascular disease. It is therefore important to identify mechanisms by which insulin regulates substrate utilization in the heart. Mitochondrial pyruvate dehydrogenase (PDH) is the key regulatory site for the oxidation of glucose for ATP production. Nevertheless, the impact of insulin on PDH activity has not been fully delineated, particularly in the heart. We sought in vivo evidence that insulin stimulates cardiac PDH and that this process is driven by the inhibition of fatty acid oxidation. Mice injected with insulin exhibited dephosphorylation and activation of cardiac PDH. This was accompanied by an increase in the content of malonyl-CoA, an inhibitor of carnitine palmitoyltransferase 1 (CPT1), and, thus, mitochondrial import of fatty acids. Administration of the CPT1 inhibitor oxfenicine was sufficient to activate PDH. Malonyl-CoA is produced by acetyl-CoA carboxylase (ACC). Pharmacologic inhibition or knockout of cardiac ACC diminished insulin-dependent production of malonyl-CoA and activation of PDH. Finally, circulating insulin and cardiac glucose utilization exhibit daily rhythms reflective of nutritional status. We demonstrate that time-of-day-dependent changes in PDH activity are mediated, in part, by ACC-dependent production of malonyl-CoA. Thus, by inhibiting fatty acid oxidation, insulin reciprocally activates PDH. These studies identify potential molecular targets to promote cardiac glucose oxidation and treat heart disease.

CD36 as a gatekeeper of myocardial lipid metabolism and therapeutic target for metabolic disease

The multifunctional membrane glycoprotein CD36 is expressed in different types of cells and plays a key regulatory role in cellular lipid metabolism, especially in cardiac muscle. CD36 facilitates the cellular uptake of long-chain fatty acids, mediates lipid signaling, and regulates storage and oxidation of lipids in various tissues with active lipid metabolism. CD36 deficiency leads to marked impairments in peripheral lipid metabolism, which consequently impact on the cellular utilization of multiple different fuels because of the integrated nature of metabolism. The functional presence of CD36 at the plasma membrane is regulated by its reversible subcellular recycling from and to endosomes and is under the control of mechanical, hormonal, and nutritional factors. Aberrations in this dynamic role of CD36 are causally associated with various metabolic diseases, in particular insulin resistance, diabetic cardiomyopathy, and cardiac hypertrophy. Recent research in cardiac muscle has disclosed the endosomal proton pump vacuolar-type H+-ATPase (v-ATPase) as a key enzyme regulating subcellular CD36 recycling and being the site of interaction between various substrates to determine cellular substrate preference. In addition, evidence is accumulating that interventions targeting CD36 directly or modulating its subcellular recycling are effective for the treatment of metabolic diseases. In conclusion, subcellular CD36 localization is the major adaptive regulator of cellular uptake and metabolism of long-chain fatty acids and appears a suitable target for metabolic modulation therapy to mend failing hearts.

Inhibition of Pyruvate Dehydrogenase in the Heart as an Initiating Event in the Development of Diabetic Cardiomyopathy

Obesity affects a growing fraction of the population and is a risk factor for type 2 diabetes and cardiovascular disease. Even in the absence of hypertension and coronary artery disease, type 2 diabetes can result in a heart disease termed diabetic cardiomyopathy. Diminished glucose oxidation, increased reliance on fatty acid oxidation for energy production, and oxidative stress are believed to play causal roles. However, the progression of metabolic changes and mechanisms by which these changes impact the heart have not been established. Cardiac pyruvate dehydrogenase (PDH), the central regulatory site for glucose oxidation, is rapidly inhibited in mice fed high dietary fat, a model of obesity and diabetes. Increased reliance on fatty acid oxidation for energy production, in turn, enhances mitochondrial pro-oxidant production. Inhibition of PDH may therefore initiate metabolic inflexibility and oxidative stress and precipitate diabetic cardiomyopathy. We discuss evidence from the literature that supports a role for PDH inhibition in loss in energy homeostasis and diastolic function in obese and diabetic humans and in rodent models. Finally, seemingly contradictory findings highlight the complexity of the disease and the need to delineate progressive changes in cardiac metabolism, the impact on myocardial structure and function, and the ability to intercede.

Insulin signaling in the heart

Insulin receptors are highly expressed in the heart and vasculature. Insulin signaling regulates cardiac growth, survival, substrate uptake, utilization, and mitochondrial metabolism. Insulin signaling modulates the cardiac responses to physiological and pathological stressors. Altered insulin signaling in the heart may contribute to the pathophysiology of ventricular remodeling and heart failure progression. Myocardial insulin signaling adapts rapidly to changes in the systemic metabolic milieu. What may initially represent an adaptation to protect the heart from carbotoxicity may contribute to amplifying the risk of heart failure in obesity and diabetes. This review article presents the multiple roles of insulin signaling in cardiac physiology and pathology and discusses the potential therapeutic consequences of modulating myocardial insulin signaling.

Treatment with XMU-MP-1 erases hyperglycaemic memory in hearts of diabetic mice

Hyperglycaemic memory refers to the damages occurred under early hyperglycaemic environment in organs of diabetic patients persisting after intensive glycaemic control. Mammalian sterile 20-like kinase 1 (Mst1) contributes to the development of diabetic cardiomyopathy. Here, we investigated the role of Mst1 in hyperglycaemic memory and test the effect of XMU-MP-1, a Mst1 inhibitor, on hyperglycaemic memory in hearts. Eight weeks after induction of type 1 diabetes by injection with streptozotocin (STZ) in mice, glycaemic control was obtained by means of insulin treatment and maintained for 4 additional weeks. In the diabetic mice, insulin treatment alone did not reduce phosphorylation of Mst1 or improve cardiac function. Treatment with XMU-MP-1 alone immediately after induction of diabetes for 12 weeks did not improve myocardial function in mice. But treatment with XMU-MP-1 for the later 4 weeks relieved myocardial dysfunction when glycaemic control was obtained by insulin treatment simultaneously. Mst1 deficiency and glycaemic control synergistically improved myocardial function and reduced apoptosis in myocardium of diabetic mice. Mechanistically, when Mst1 was deficient or inhibited by XMU-MP-1, AMPK was activated and mitochondrial dysfunction was attenuated. In vitro, treatment with AMPK activator reversed the detrimental effects of Mst1 overexpression in cultured cardiomyocytes. XMU-MP-1 might thus be envisaged as a complement for insulin treatment against diabetic cardiomyopathy.

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

Intermittent fasting promotes adult hippocampal neuronal differentiation by activating GSK-3β in 3xTg-AD mice

Moderate dietary restriction can ameliorate age-related chronic diseases such as Alzheimer's disease (AD) by increasing the expression of neurotrophic factors and promoting neurogenesis in the brain. Glycogen synthase kinase-3β (GSK-3β) signaling is essential for the coordination of progenitor cell proliferation and differentiation during brain development. The mechanisms by which GSK-3β is involved in dietary restriction-induced neurogenesis and cognitive improvement remain unclear. Six-month-old male 3xTg-AD and wild-type mice were fed on alternate days (intermittent fasting, IF) or ad libitum (AL) for 3 months. GSK-3β activity was regulated by bilaterally infusing lentiviral vectors carrying siRNA targeting GSK-3β into the dentate gyrus region of the hippocampus. Intermittent fasting promoted neuronal differentiation and maturation in the dentate gyrus and ameliorated recognized dysfunction in 3xTg-AD mice. These effects were reversed by siRNA targeting GSK-3β. After intermittent fasting, the insulin and protein kinase A signaling pathways were inhibited, while the adenosine monophosphate-activated protein kinase and brain-derived neurotrophic factor pathways were activated. These findings suggest that intermittent fasting can promote neuronal differentiation and maturation in the hippocampus by activating GSK-3β, thus improving learning and memory.

Glycomacropeptide Prevents Iron/Ascorbate-Induced Oxidative Stress, Inflammation and Insulin Sensitivity with an Impact on Lipoprotein Production in Intestinal Caco-2/15 Cells

Background. Metabolic Syndrome (MetS), a major worldwide concern for the public health system, refers to a cluster of key metabolic components, and represents a risk factor for diabetes and cardiovascular diseases. As oxidative stress (OxS) and inflammation are the major triggers of insulin sensitivity (IS), a cardinal MetS feature, the principal aim of the present work is to determine whether glycomacropeptide (GMP), a milk-derived bioactive peptide, exerts beneficial effects on their expression. Methods. Fully differentiated intestinal Caco-2/15 cells are used to evaluate the preventive action of 2 mg/mL GMP against OxS and inflammation induced by the mixture iron-ascorbate (Fe/Asc) (200 μM:2 mM). The potency of GMP of decreasing the production of lipoproteins, including chylomicrons (CM), very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL) is also assessed. Results. The administration of GMP significantly reduces malondialdehyde, a biomarker of lipid peroxidation, and raises superoxide dismutase 2 and glutathione peroxidase via the induction of the nuclear factor erythroid 2–related factor 2, a transcription factor, which orchestrates cellular antioxidant defenses. Similarly, GMP markedly lowers the inflammatory agents tumor necrosis factor-α and cyclooxygenase-2 via abrogation of the nuclear transcription factor-kB. Moreover, GMP-treated cells show a down-regulation of Fe/Asc-induced mitogen activated protein kinase pathway, suggesting greater IS. Finally, GMP decreases the production of CM, VLDL, and LDL. Conclusions. Our results highlight the effectiveness of GMP in attenuating OxS, inflammation and lipoprotein biogenesis, as well as improving IS, the key components of MetS. Further investigation is needed to elucidate the mechanisms mediating the preventive action of GMP.

The Protective Effect of Insulin on Rat Cortical Neurons in Oxidative Stress and Its Dependence on the Modulation of Akt, GSK-3beta, ERK1/2, and AMPK Activities

Insulin is a promising drug for the treatment of diseases associated with brain damage. However, the mechanism of its neuroprotective action is far from being understood. Our aim was to study the insulin-induced protection of cortical neurons in oxidative stress and its mechanism. Immunoblotting, flow cytometry, colorimetric, and fluorometric techniques were used. The insulin neuroprotection was shown to depend on insulin concentration in the nanomolar range. Insulin decreased the reactive oxygen species formation in neurons. The insulin-induced modulation of various protein kinase activities was studied at eight time-points after neuronal exposure to prooxidant (hydrogen peroxide). In prooxidant-exposed neurons, insulin increased the phosphorylation of GSK-3beta at Ser⁹ (thus inactivating it), which resulted from Akt activation. Insulin activated ERK1/2 in neurons 5–30 min after cell exposure to prooxidant. Hydrogen peroxide markedly activated AMPK, while it was for the first time shown that insulin inhibited it in neurons at periods of the most pronounced activation by prooxidant. Insulin normalized Bax/Bcl-2 ratio and mitochondrial membrane potential in neurons in oxidative stress. The inhibitors of the PI3K/Akt and MEK1/2/ERK1/2 signaling pathways and the AMPK activator reduced the neuroprotective effect of insulin. Thus, the protective action of insulin on cortical neurons in oxidative stress appear to be realized to a large extent through activation of Akt and ERK1/2, GSK-3beta inactivation, and inhibition of AMPK activity increased by neuronal exposure to prooxidant.

Metformin in Reproductive Biology

Initially produced in Europe in 1958, metformin is still one of the most widely prescribed drugs to treat type II diabetes and other comorbidities associated with insulin resistance. Metformin has been shown to improve fertility outcomes in females with insulin resistance associated with polycystic ovary syndrome (PCOS) and in obese males with reduced fertility. Metformin treatment reinstates menstrual cyclicity, decreases the incidence of cesareans, and limits the number of premature births. Notably, metformin reduces steroid levels in conditions associated with hyperandrogenism (e.g., PCOS and precocious puberty) in females and improves fertility of adult men with metabolic syndrome through increased testosterone production. While the therapeutical use of metformin is considered to be safe, in the last 10 years some epidemiological studies have described phenotypic differences after prenatal exposure to metformin. The goals of this review are to briefly summarize the current knowledge on metformin focusing on its effects on the female and male reproductive organs, safety concerns, including the potential for modulating fetal imprinting via epigenetics.

Cytosolic carnitine acetyltransferase as a source of cytosolic acetyl-CoA: a possible mechanism for regulation of cardiac energy metabolism

The role of carnitine acetyltransferase (CrAT) in regulating cardiac energy metabolism is poorly understood. CrAT modulates mitochondrial acetyl-CoA/CoA (coenzyme A) ratios, thus regulating pyruvate dehydrogenase activity and glucose oxidation. Here, we propose that cardiac CrAT also provides cytosolic acetyl-CoA for the production of malonyl-CoA, a potent inhibitor of fatty acid oxidation. We show that in the murine cardiomyocyte cytosol, reverse CrAT activity (RCrAT, producing acetyl-CoA) is higher compared with the liver, which primarily uses ATP-citrate lyase to produce cytosolic acetyl-CoA for lipogenesis. The heart displayed a lower RCrAT K_m for CoA compared with the liver. Furthermore, cytosolic RCrAT accounted for 4.6 ± 0.7% of total activity in heart tissue and 12.7 ± 0.2% in H9C2 cells, while highly purified heart cytosolic fractions showed significant CrAT protein levels. To investigate the relationship between CrAT and acetyl-CoA carboxylase (ACC), the cytosolic enzyme catalyzing malonyl-CoA production from acetyl-CoA, we studied ACC2-knockout mouse hearts which showed decreased CrAT protein levels and activity, associated with increased palmitate oxidation and acetyl-CoA/CoA ratio compared with controls. Conversely, feeding mice a high-fat diet for 10 weeks increased cardiac CrAT protein levels and activity, associated with a reduced acetyl-CoA/CoA ratio and glucose oxidation. These data support the presence of a cytosolic CrAT with a low K_m for CoA, favoring the formation of cytosolic acetyl-CoA, providing an additional source to the classical ATP-citrate lyase pathway, and that there is an inverse relation between CrAT and the ratio of acetyl-CoA/CoA as evident in conditions affecting the regulation of cardiac energy metabolism.

Metabolic enzymes dysregulation in heart failure: the prospective therapy

The heart failure accounts for the highest mortality rate all over the world. The development of preventive therapeutic approaches is still in their infancy. Owing to the extremely high energy demand of the heart, the bioenergetics pathways need to respond efficiently based on substrate availability. The metabolic regulation of such heart bioenergetics is mediated by various rate limiting enzymes involved in energy metabolism. Although all the pertinent mechanisms are not clearly understood, the progressive decline in the activity of metabolic enzymes leading to diminished ATP production is known to cause progression of the heart failure. Therefore, metabolic therapy that can maintain the appropriate activities of metabolic enzymes can be a promising approach for the prevention and treatment of the heart failure. The flavonoids that constitute various human dietary ingredients also effectively offer a variety of health benefits. The flavonoids target a variety of metabolic enzymes and facilitate effective management of the equilibrium between production and utilization of energy in the heart. This review discusses the broad impact of metabolic enzymes in the heart functions and explains how the dysregulated enzyme activity causes the heart failure. In addition, the prospects of targeting dysregulated metabolic enzymes by developing flavonoid-based metabolic approaches are discussed.

Insulin prevents aberrant mitochondrial phenotype in sensory neurons of type 1 diabetic rats

Diabetic neuropathy affects approximately 50% of diabetic patients. Down-regulation of mitochondrial gene expression and function has been reported in both human tissues and in dorsal root ganglia (DRG) from animal models of type 1 and type 2 diabetes. We hypothesized that loss of direct insulin signaling in diabetes contributes to loss of mitochondrial function in DRG neurons and to development of neuropathy. Sensory neurons obtained from age-matched adult control or streptozotocin (STZ)-induced type 1 diabetic rats were cultured with or without insulin before determining mitochondrial respiration and expression of mitochondrial respiratory chain and insulin signaling-linked proteins. For in vivo studies age-matched control rats and diabetic rats with or without trace insulin supplementation were maintained for 5months before DRG were analyzed for respiratory chain gene expression and cytochrome c oxidase activity. Insulin (10nM) significantly (P<0.05) increased phosphorylation of Akt and P70S6K by 4-fold and neurite outgrowth by 2-fold in DRG cultures derived from adult control rats. Insulin also augmented the levels of selective mitochondrial respiratory chain proteins and mitochondrial bioenergetics parameters in DRG cultures from control and diabetic rats, with spare respiratory capacity increased by up to 3-fold (P<0.05). Insulin-treated diabetic animals exhibited improved thermal sensitivity in the hind paw and had increased dermal nerve density compared to untreated diabetic rats, despite no effect on blood glucose levels. In DRG of diabetic rats there was suppressed expression of mitochondrial respiratory chain proteins and cytochrome c oxidase activity that was corrected by insulin therapy. Insulin elevates mitochondrial respiratory chain protein expression and function in sensory neurons and this is associated with enhanced neurite outgrowth and protection against indices of neuropathy.

Dietary Salba (Salvia hispanica L) improves the altered metabolic fate of glucose and reduces increased collagen deposition in the heart of insulin-resistant rats

This study reports the effects of dietary Salba (chia) seeds on the mechanisms underlying impaired glucose metabolism in the heart of dyslipemic insulin-resistant rats fed a sucrose-rich diet (SRD). Wistar rats were fed a SRD for 3 months. Afterwards, half the animals continued with the SRD; in the other half's diet chia seeds replaced corn oil (CO) for three months (SRD+chia). In the control group, corn starch replaced sucrose. The replacement of CO by chia seeds in the SRD restored the activities of key enzymes involved in heart glucose metabolism decreasing fatty acid oxidation. Chia seeds normalized insulin stimulated GLUT-4 transporter, the abundance of IRS-1 and pAMPK, changed the profile of fatty acid phospholipids, reduced left-ventricle collagen deposition and normalized hypertension and dyslipidemia. New evidence is provided concerning the effects of dietary chia seeds in improving the altered metabolic fate of glucose in the heart of dyslipemic insulin-resistant rats.

Hernandezine, a novel AMPK activator induces autophagic cell death in drug-resistant cancers

Drug resistance hinder most cancer chemotherapies and leads to disease recurrence and poor survival of patients. Resistance of cancer cells towards apoptosis is the major cause of these symptomatic behaviours. Here, we showed that isoquinoline alkaloids, including liensinine, isoliensinine, dauricine, cepharanthine and hernandezine, putatively induce cytotoxicity against a repertoire of cancer cell lines (HeLa, A549, MCF-7, PC3, HepG2, Hep3B and H1299). Proven by the use of apoptosis-resistant cellular models and autophagic assays, such isoquinoline alkaloid-induced cytotoxic effect involves energy- and autophagy-related gene 7 (Atg7)-dependent autophagy that resulted from direct activation of AMP activated protein kinase (AMPK). Hernandezine possess the highest efficacy in provoking such cell death when compared with other examined compounds. We confirmed that isoquinoline alkaloid is structurally varied from the existing direct AMPK activators. In conclusion, isoquinoline alkaloid is a new class of compound that induce autophagic cell death in drug-resistant fibroblasts or cancers by exhibiting its direct activation on AMPK.

C-peptide protects against hyperglycemic memory and vascular endothelial cell apoptosis

C-peptide exerts protective effects against diabetic complications; however, its role in inhibiting hyperglycemic memory (HGM) has not been elucidated. We investigated the beneficial effect of C-peptide on HGM-induced vascular damage in vitro and in vivo using human umbilical vein endothelial cells and diabetic mice. HGM induced apoptosis by persistent generation of intracellular ROS and sustained formation of ONOO(-) and nitrotyrosine. These HGM-induced intracellular events were normalized by treatment with C-peptide, but not insulin, in endothelial cells. C-peptide also inhibited persistent upregulation of p53 and activation of mitochondrial adaptor p66(shc) after glucose normalization. Further, C-peptide replacement therapy prevented persistent generation of ROS and ONOO(-) in the aorta of diabetic mice whose glucose levels were normalized by the administration of insulin. C-peptide, but not insulin, also prevented HGM-induced endothelial apoptosis in the murine diabetic aorta. This study highlights a promising role for C-peptide in preventing HGM-induced intracellular events and diabetic vascular damage.

Age-related changes in AMPK activation: Role for AMPK phosphatases and inhibitory phosphorylation by upstream signaling pathways

AMP-activated protein kinase (AMPK) is a fundamental regulator of energy metabolism, stress resistance, and cellular proteostasis. AMPK signaling controls an integrated signaling network which is involved in the regulation of healthspan and lifespan e.g. via FoxO, mTOR/ULK1, CRCT-1/CREB, and SIRT1 signaling pathways. Several studies have demonstrated that the activation capacity of AMPK signaling declines with aging, which impairs the maintenance of efficient cellular homeostasis and enhances the aging process. However, it seems that the aging process affects AMPK activation in a context-dependent manner since occasionally, it can also augment AMPK activation, possibly attributable to the type of insult and tissue homeostasis. Three protein phosphatases, PP1, PP2A, and PP2C, inhibit AMPK activation by dephosphorylating the Thr172 residue of AMPKα, required for AMPK activation. In addition, several upstream signaling pathways can phosphorylate Ser/Thr residues in the β/γ interaction domain of the AMPKα subunit that subsequently blocks the activation of AMPK. These inhibitory pathways include the insulin/AKT, cyclic AMP/PKA, and RAS/MEK/ERK pathways. We will examine the evidence whether the efficiency of AMPK responsiveness declines during the aging process. Next, we will review the mechanisms involved in curtailing the activation of AMPK. Finally, we will elucidate the potential age-related changes in the inhibitory regulation of AMPK signaling that might be a part of the aging process.

Berberine improves mesenteric artery insulin sensitivity through up-regulating insulin receptor-mediated signalling in diabetic rats

BACKGROUND AND PURPOSE

Berberine, a small molecule derived from Coptidis rhizome, has been found to be potent at lowering blood glucose and regulating lipid metabolism. Recent clinical studies have shown that berberine reduces blood pressure and increases systemic insulin sensitivity in patients with metabolic syndrome; however, the underlying mechanism is still unclear. Here, we investigated the mechanism by which berberine improves vascular insulin sensitivity in diabetic rats.

EXPERIMENTAL APPROACH

Diabetes was induced in male Sprague–Dawley rats by feeding a high-fat diet and administration of a low dose of streptozotocin. These diabetic rats were treated with berberine (200 mg·kg(−1)·day(−1), gavage) for 4 weeks. Vascular dilation was determined in isolated mesenteric artery rings. Effects of berberine on insulin signalling were also studied in human artery endothelial cells cultured in high glucose (25 mmol·L(−1)) and palmitate (500 μmol·L(−1)).

KEY RESULTS

Berberine treatment for 4 weeks significantly restored the impaired ACh- and insulin-induced vasodilatation of mesenteric arteries from diabetic rats. In isolated mesenteric artery rings, berberine (2.5–10 μmol·L(−1)) elicited dose-dependent vasodilatation and significantly enhanced insulin-induced vasodilatation. Mechanistically, berberine up-regulated phosphorylation of the insulin receptor and its downstream signalling molecules AMPK, Akt and eNOS, and increased cell viability and autophagy in cultured endothelial cells. Moreover, down-regulating insulin receptors with specific siRNA significantly attenuated berberine-induced phosphorylation of AMPK.

CONCLUSIONS AND IMPLICATIONS

Berberine improves diabetic vascular insulin sensitivity and mesenteric vasodilatation by up-regulating insulin receptor-mediated signalling in diabetic rats. These findings suggest berberine has potential as a preventive or adjunctive treatment of diabetic vascular complications.

LINKED ARTICLES

This article is part of a themed section on Chinese Innovation in Cardiovascular Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-23.

Berberine improves mesenteric artery insulin sensitivity through up-regulating insulin receptor-mediated signalling in diabetic rats

BACKGROUND AND PURPOSE

Berberine, a small molecule derived from Coptidis rhizome, has been found to be potent at lowering blood glucose and regulating lipid metabolism. Recent clinical studies have shown that berberine reduces blood pressure and increases systemic insulin sensitivity in patients with metabolic syndrome; however, the underlying mechanism is still unclear. Here, we investigated the mechanism by which berberine improves vascular insulin sensitivity in diabetic rats.

EXPERIMENTAL APPROACH

Diabetes was induced in male Sprague–Dawley rats by feeding a high-fat diet and administration of a low dose of streptozotocin. These diabetic rats were treated with berberine (200 mg·kg(−1)·day(−1), gavage) for 4 weeks. Vascular dilation was determined in isolated mesenteric artery rings. Effects of berberine on insulin signalling were also studied in human artery endothelial cells cultured in high glucose (25 mmol·L(−1)) and palmitate (500 μmol·L(−1)).

KEY RESULTS

Berberine treatment for 4 weeks significantly restored the impaired ACh- and insulin-induced vasodilatation of mesenteric arteries from diabetic rats. In isolated mesenteric artery rings, berberine (2.5–10 μmol·L(−1)) elicited dose-dependent vasodilatation and significantly enhanced insulin-induced vasodilatation. Mechanistically, berberine up-regulated phosphorylation of the insulin receptor and its downstream signalling molecules AMPK, Akt and eNOS, and increased cell viability and autophagy in cultured endothelial cells. Moreover, down-regulating insulin receptors with specific siRNA significantly attenuated berberine-induced phosphorylation of AMPK.

CONCLUSIONS AND IMPLICATIONS

Berberine improves diabetic vascular insulin sensitivity and mesenteric vasodilatation by up-regulating insulin receptor-mediated signalling in diabetic rats. These findings suggest berberine has potential as a preventive or adjunctive treatment of diabetic vascular complications.

LINKED ARTICLES

This article is part of a themed section on Chinese Innovation in Cardiovascular Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-23.

The opposing actions of target of rapamycin and AMP-activated protein kinase in cell growth control

Cell growth is a highly regulated, plastic process. Its control involves balancing positive regulation of anabolic processes with negative regulation of catabolic processes. Although target of rapamycin (TOR) is a major promoter of growth in response to nutrients and growth factors, AMP-activated protein kinase (AMPK) suppresses anabolic processes in response to energy stress. Both TOR and AMPK are conserved throughout eukaryotic evolution. Here, we review the fundamentally important roles of these two kinases in the regulation of cell growth with particular emphasis on their mutually antagonistic signaling.

Insulin down-regulates the expression of ubiquitin E3 ligases partially by inhibiting the activity and expression of AMP-activated protein kinase in L6 myotubes

We conclude that insulin inhibits AMPK through Akt phosphorylation in L6 myotubes, which may serve as a possible signalling pathway for the down-regulation of protein degradation. Besides, decreased expression of AMPK α2 may partially participate in inhibiting the activity of AMPK.

While insulin is an anabolic hormone, AMP-activated protein kinase (AMPK) is not only a key energy regulator, but it can also control substrate metabolism directly by inducing skeletal muscle protein degradation. The hypothesis of the present study was that insulin inhibits AMPK and thus down-regulates the expression of the ubiquitin E3 ligases, muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1) in skeletal muscle cells. Differentiated L6 myotubes were treated with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) and/or compound C to stimulate and/or block AMPK respectively. These treatments were also conducted in the presence or absence of insulin and the cells were analysed by western blot and quantitative real-time PCR. In addition, nuleotide levels were determined using HPLC. The activation of AMPK with AICAR enhanced the mRNA levels of MAFbx and MuRF1. Insulin reduced the phosphorylation and activity AMPK, which was accompanied by reduced MAFbx and MuRF1 mRNA levels. Using a protein kinase B (PKB/Akt) inhibitor, we found that insulin regulates AMPK through the activation of Akt. Furthermore, insulin down-regulated AMPK α2 mRNA. We conclude that insulin inhibits AMPK through Akt phosphorylation in L6 myotubes, which may serve as a possible signalling pathway for the down-regulation of protein degradation. In addition, decreased expression of AMPK α2 may partially participate in inhibiting the activity of AMPK.

Mechanical stretch activates mammalian target of rapamycin and AMP-activated protein kinase pathways in skeletal muscle cells

Cellular protein synthesis is believed to be antagonistically regulated by mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) signaling pathways. In the present study, we examined the relationship between mTOR/p70 S6 kinase (p70S6K) and AMPK in response to mechanical stretch. C2C12 myoblasts were grown on a silicone elastomer chamber to confluence and further cultured in differentiation medium for 4 days to form multinucleated myotubes. Cells were subjected to 15% cyclic uniaxial stretch for 4 h at a frequency of 1 Hz. Phosphorylation of p70S6K at threonine 389 and AMPK at threonine 172 of the catalytic α subunit were concomitantly increased by mechanical stretch. Stimulation of the mTOR pathway by adding leucine and insulin increased the phosphorylation of p70S6K without inactivation of AMPK. In contrast, addition of compound C, a pharmacological inhibitor of AMPK, increased the phosphorylation of p70S6K in stretched cells. Activation of AMPK by the addition of 5-amino-4-imidazolecarboxamide ribonucleoside reduced the phosphorylation of p70S6K in response to mechanical stretch. In conclusion, crosstalk between mTOR and AMPK signaling was not tightly regulated in response to physiological stimuli, such as mechanical stress and/or nutrients. However, pharmacological modulation of AMPK influenced the mTOR/p70S6K signaling pathway.

C-peptide replacement therapy as an emerging strategy for preventing diabetic vasculopathy

Lack of C-peptide, along with insulin, is the main feature of Type 1 diabetes mellitus (DM) and is also observed in progressive β-cell loss in later stage of Type 2 DM. Therapeutic approaches to hyperglycaemic control have been ineffective in preventing diabetic vasculopathy, and alternative therapeutic strategies are necessary to target both hyperglycaemia and diabetic complications. End-stage organ failure in DM seems to develop primarily due to vascular dysfunction and damage, leading to two types of organ-specific diseases, such as micro- and macrovascular complications. Numerous studies in diabetic patients and animals demonstrate that C-peptide treatment alone or in combination with insulin has physiological functions and might be beneficial in preventing diabetic complications. Current evidence suggests that C-peptide replacement therapy might prevent and ameliorate diabetic vasculopathy and organ-specific complications through conservation of vascular function, as well as prevention of endothelial cell death, microvascular permeability, vascular inflammation, and neointima formation. In this review, we describe recent advances on the beneficial role of C-peptide replacement therapy for preventing diabetic complications, such as retinopathy, nephropathy, neuropathy, impaired wound healing, and inflammation, and further discuss potential beneficial effects of combined C-peptide and insulin supplement therapy to control hyperglycaemia and to prevent organ-specific complications.

Metabolic dysfunction in diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is defined as cardiac disease independent of vascular complications during diabetes. The number of new cases of DCM is rising at epidemic rates in proportion to newly diagnosed cases of diabetes mellitus (DM) throughout the world. DCM is a heart failure syndrome found in diabetic patients that is characterized by left ventricular hypertrophy and reduced diastolic function, with or without concurrent systolic dysfunction, occurring in the absence of hypertension and coronary artery disease. DCM and other diabetic complications are caused in part by elevations in blood glucose and lipids, characteristic of DM. Although there are pathological consequences to hyperglycemia and hyperlipidemia, the combination of the two metabolic abnormalities potentiates the severity of diabetic complications. A natural competition exists between glucose and fatty acid metabolism in the heart that is regulated by allosteric and feedback control and transcriptional modulation of key limiting enzymes. Inhibition of these glycolytic enzymes not only controls flux of substrate through the glycolytic pathway, but also leads to the diversion of glycolytic intermediate substrate through pathological pathways, which mediate the onset of diabetic complications. The present review describes the limiting steps involved in the development of these pathological pathways and the factors involved in the regulation of these limiting steps. Additionally, therapeutic options with demonstrated or postulated effects on DCM are described.

Insulin inhibits AMPK activity and phosphorylates AMPK Ser⁴⁸⁵/⁴⁹¹ through Akt in hepatocytes, myotubes and incubated rat skeletal muscle

Recent studies have highlighted the importance of an inhibitory phosphorylation site, Ser(485/491), on the α-subunit of AMP-activated protein kinase (AMPK); however, little is known about the regulation of this site in liver and skeletal muscle. We examined whether the inhibitory effects of insulin on AMPK activity may be mediated through the phosphorylation of this inhibitory Ser(485/491) site in hepatocytes, myotubes and incubated skeletal muscle. HepG2 and C2C12 cells were stimulated with or without insulin for 15-min. Similarly, rat extensor digitorum longus (EDL) muscles were treated +/- insulin for 10-min. Insulin significantly increased Ser(485/491) p-AMPK under all conditions, resulting in a subsequent reduction in AMPK activity, ranging from 40% to 70%, despite no change in p-AMPK Thr(172). Akt inhibition both attenuated the increase in Ser(485/491) p-AMPK caused by insulin, and prevented the decrease in AMPK activity. Similarly, the growth factor IGF-1 stimulated Ser(485/491) AMPK phosphorylation, and this too was blunted by inhibition of Akt. Inhibition of the mTOR pathway with rapamycin, however, had no effect on insulin-stimulated Ser(485/491) p-AMPK. These data suggest that insulin and IGF-1 diminish AMPK activity in hepatocytes and muscle, most likely through Akt activation and the inhibitory phosphorylation of Ser(485/491) on its α-subunit.

Rosiglitazone inhibits expression and secretion of PEDF in adipose tissue and liver of male SD rats via a PPAR-γ independent mechanism

Pigment epithelium-derived factor (PEDF) plays an important role in insulin resistance (IR). The study aims to investigate the effect of rosiglitazone, an insulin sensitizer, on PEDF production and release both in vivo and in vitro. Male SD rats were divided into normal control group, high-fat group, and rosiglitazone group. Hyperinsulinemic euglycemic clamp was performed to evaluate insulin sensitivity. IR models of 3T3-L1 adipocytes and HepG2 cells were established by the hyperinsulinemic method. Glucose uptake was examined to validate IR of adipocytes, and phosphorylation of protein kinase B and glycogen synthesis kinase 3β were examined to validate IR of HepG2 cells. Rosiglitazone, 2-chloro-5-nitro-N-phenylbenzamide (GW9662, an inhibitor of peroxisome proliferator-activated receptor-γ), and compound C (inhibitor of AMP-activated protein kinase [AMPK]) were used for the in vitro intervention. In vivo, the high-fat group showed increased serum PEDF levels, which negatively correlated with insulin sensitivity, whereas the rosiglitazone treatment decreased the serum PEDF and down-regulated PEDF expression in fat and liver of the obese rats, concomitant with significantly enhanced insulin sensitivity. In vitro, the IR cells showed increased PEDF secretion and expression, whereas rosiglitazone lowered PEDF secretion and expression, accompanied with increased insulin sensitivity. Interestingly, combination with 2-chloro-5-nitro-N-phenylbenzamide did not influence the effect of rosiglitazone on PEDF. However, rosiglitazone stimulated AMPK phosphorylation in fat and liver of the obese rats, whereas in vitro, when combined with compound C, the effect of rosiglitazone on PEDF was abrogated. In summary, rosiglitazone inhibits the expression and secretion of PEDF in fat and liver via promoting AMPK phosphorylation rather than peroxisome proliferator-activated receptor-γ, and changes of PEDF induced by rosiglitazone are closely associated with IR improvement.

Integrated quantitative analysis of nitrogen stress response in Chlamydomonas reinhardtii using metabolite and protein profiling

Nitrogen starvation induces a global stress response in microalgae that results in the accumulation of lipids as a potential source of biofuel. Using GC-MS-based metabolite and iTRAQ-labeled protein profiling, we examined and correlated the metabolic and proteomic response of Chlamydomonas reinhardtii under nitrogen stress. Key amino acids and metabolites involved in nitrogen sparing pathways, methyl group transfer reactions, and energy production were decreased in abundance, whereas certain fatty acids, citric acid, methionine, citramalic acid, triethanolamine, nicotianamine, trehalose, and sorbitol were increased in abundance. Proteins involved in nitrogen assimilation, amino acid metabolism, oxidative phosphorylation, glycolysis, TCA cycle, starch, and lipid metabolism were elevated compared with nonstressed cultures. In contrast, the enzymes of the glyoxylate cycle, one carbon metabolism, pentose phosphate pathway, the Calvin cycle, photosynthetic and light harvesting complex, and ribosomes were reduced. A noteworthy observation was that citrate accumulated during nitrogen stress coordinate with alterations in the enzymes that produce or utilize this metabolite, demonstrating the value of comparing protein and metabolite profiles to understand complex patterns of metabolic flow. Thus, the current study provides unique insight into the global metabolic adjustments leading to lipid storage during N starvation for application toward advanced biofuel production technologies.

Cinnamon extract enhances glucose uptake in 3T3-L1 adipocytes and C2C12 myocytes by inducing LKB1-AMP-activated protein kinase signaling

We previously demonstrated that cinnamon extract (CE) ameliorates type 1 diabetes induced by streptozotocin in rats through the up-regulation of glucose transporter 4 (GLUT4) translocation in both muscle and adipose tissues. This present study was aimed at clarifying the detailed mechanism(s) with which CE increases the glucose uptake in vivo and in cell culture systems using 3T3-L1 adipocytes and C2C12 myotubes in vitro. Specific inhibitors of key enzymes in insulin signaling and AMP-activated protein kinase (AMPK) signaling pathways, as well as small interference RNA, were used to examine the role of these kinases in the CE-induced glucose uptake. The results showed that CE stimulated the phosphorylation of AMPK and acetyl-CoA carboxylase. An AMPK inhibitor and LKB1 siRNA blocked the CE-induced glucose uptake. We also found for the first time that insulin suppressed AMPK activation in the adipocyte. To investigate the effect of CE on type 2 diabetes in vivo, we further performed oral glucose tolerance tests and insulin tolerance tests in type 2 diabetes model rats administered with CE. The CE improved glucose tolerance in oral glucose tolerance tests, but not insulin sensitivity in insulin tolerance test. In summary, these results indicate that CE ameliorates type 2 diabetes by inducing GLUT4 translocation via the AMPK signaling pathway. We also found insulin antagonistically regulates the activation of AMPK.

Isolation, structural elucidation, MS profiling, and evaluation of triglyceride accumulation inhibitory effects of benzophenone C-glucosides from leaves of Mangifera indica L

Seventy percent ethanol-water extract from the leaves of Mangifera indica L. (Anacardiaceae) was found to show an inhibitory effect on triglyceride (TG) accumulation in 3T3-L1 cells. From the active fraction, six new benzophenone C-glucosides, foliamangiferosides A(3) (1), A(4) (2), C(4) (3), C(5) (4), C(6) (5), and C(7) (6) together with 11 known benzophenone C-glucosides (7-17) were obtained. In this paper, isolation, structure elucidation (1-6), and MS fragment cleavage pathways of all 17 isolates were studied. 1-6 showed inhibitory effects on TG and free fatty acid accumulation in 3T3-L1 cells at 10 μM.

The effect of 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) on fatty acid oxidation in hepatocytes isolated from neonatal piglets

In the present study, the effect of 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) on long-chain fatty acid oxidation by hepatocytes isolated from suckled neonatal pig liver (a low ketogenic and lipogenic tissue) was tested. Incubation of hepatocytes with AICAR (0.5 mM) in the presence of 1 mM of carnitine and 10 mM of glucose for 1 hour at 37°C had no significant effect on total [1-¹⁴C]-palmitate (0.5 mM) oxidation (¹⁴CO₂ and ¹⁴C-Acid soluble products (ASP)). Consistent with the fatty acid oxidation, carnitine palmitoyltransferase I activity and inhibition of its activity by malonyl-CoA (10 μM) assayed in cell homogenate also remained constant. However, addition of AICAR to the hepatocytes decreased ¹⁴CO₂ production by 18% compared to control (p < 0.06). The reduction of labeled carboxylic carbon accumulated in CO₂ caused a significant difference in distribution of oxidative products between ¹⁴CO₂ and ¹⁴C-ASP (p < 0.03) compared with the control. It was also noticed that acetyl-CoA carboxylase (ACC) was increased by AICAR (p < 0.03), indicating that ACC might drive acetyl-CoA toward fatty acid synthesis pathway and induce an increase in distribution of fatty acid carbon to ¹⁴C-ASP. Addition of insulin to hepatocyte incubations with AICAR did not change the oxidative product distribution between CO₂ and ASP, but further promoted ACC activity. The increased ACC activity was 70% higher than in the control group when citrate was absent in the reaction medium and was 30% higher when citrate was present in the medium. Our results suggest that AICAR may affect the distribution of metabolic products from fatty acid oxidation by changing ACC activity in hepatocyte isolated from suckled neonatal piglets; however, the basis for the increase in ACC activity elicited by AICAR is not apparent.

TG accumulation inhibitory effects of Jinqi formula by AMPK signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Jinqi formula is a traditional Chinese anti-diabetic formula containing three ingredients (Coptidis rhizoma, Astragali rhadix and Lonicerae japonicae Flos).

MATERIALS AND METHODS

The active fractions of Jinqi formula were purified and HPLC analyses were used for quality control. The anti-adipogenic effects of Jinqi formula were analyzed in vitro using 3T3-L1 cells and in vivo with KK-A(y) mice. RT-PCR and Western blot were used to confirm genes and proteins of interest, respectively.

RESULTS

In vitro study showed that Jinqi formula suppressed the accumulation of triglyceride (TG) and free fatty acids (FFA) in mature 3T3-L1 cells by increasing the expression and tyrosine phosphorylation of 5'-AMP-activated protein kinase (AMPK), as well as decreasing the expression of Acetyl CoA Carboxylase (ACC), Fatty Acid Synthase (FAS) and Hormone Sensitive Lipase (HSL). In vivo study demonstrated that Jinqi formula reduced body weight without changing food intake in KK-A(y) mice, and decreased the levels of serum glucose, TG, FFA. In addition, consistent with the in vitro study results, Jinqi formula increased the expression and tyrosine phosphorylation of AMPK in the liver and muscular tissues of the KK-A(y) mice. Furthermore, Jingqi formula suppressed the expression of ACC and HSL and upregulated the expression of IRS-1 in the liver. Whereas in the skeletal muscles, Jingqi formula decreased the expression of ACC and increased the expression of GLUT-4 and IRS-2.

CONCLUSIONS

Jingqi formula inhibits TG accumulation at least in part via the stimulation of AMPK activity in a multi-target manner.

Augmenting muscle diacylglycerol and triacylglycerol content by blocking fatty acid oxidation does not impede insulin sensitivity

A low fat oxidative capacity has been linked to muscle diacylglycerol (DAG) accumulation and insulin resistance. Alternatively, a low fat oxidation rate may stimulate glucose oxidation, thereby enhancing glucose disposal. Here, we investigated whether an ethyl-2-[6-(4-chlorophenoxy)hexyl]-oxirane-2-carboxylate (etomoxir)-induced inhibition of fat oxidation leads to muscle fat storage and insulin resistance. An intervention in healthy male subjects was combined with studies in human primary myotubes. Furthermore, muscle DAG and triacylglycerol (TAG), mitochondrial function, and insulin signaling were examined in etomoxir-treated C57bl6 mice. In humans, etomoxir administration increased glucose oxidation at the expense of fat oxidation. This effect was accompanied by an increased abundance of GLUT4 at the sarcolemma and a lowering of plasma glucose levels, indicative of improved glucose homeostasis. In mice, etomoxir injections resulted in accumulation of muscle TAG and DAG, yet improved insulin-stimulated GLUT4 translocation. Also in human myotubes, insulin signaling was improved by etomoxir, in the presence of increased intramyocellular lipid accumulation. These insulin-sensitizing effects in mice and human myotubes were accompanied by increased phosphorylation of AMP-activated protein kinase (AMPK). Our results show that a reduction in fat oxidation leading to accumulation of muscle DAG does not necessarily lead to insulin resistance, as the reduction in fat oxidation may activate AMPK.

Regulation of NAD(P)H oxidases by AMPK in cardiovascular systems

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are ubiquitously produced in cardiovascular systems. Under physiological conditions, ROS/RNS function as signaling molecules that are essential in maintaining cardiovascular function. Aberrant concentrations of ROS/RNS have been demonstrated in cardiovascular diseases owing to increased production or decreased scavenging, which have been considered common pathways for the initiation and progression of cardiovascular diseases such as atherosclerosis, hypertension, (re)stenosis, and congestive heart failure. NAD(P)H oxidases are primary sources of ROS and can be induced or activated by all known cardiovascular risk factors. Stresses, hormones, vasoactive agents, and cytokines via different signaling cascades control the expression and activity of these enzymes and of their regulatory subunits. But the molecular mechanisms by which NAD(P)H oxidase is regulated in cardiovascular systems remain poorly characterized. Investigations by us and others suggest that adenosine monophosphate-activated protein kinase (AMPK), as an energy sensor and modulator, is highly sensitive to ROS/RNS. We have also obtained convincing evidence that AMPK is a physiological suppressor of NAD(P)H oxidase in multiple cardiovascular cell systems. In this review, we summarize our current understanding of how AMPK functions as a physiological repressor of NAD(P)H oxidase.

APPL1 mediates adiponectin-induced LKB1 cytosolic localization through the PP2A-PKCzeta signaling pathway

We recently found that the adaptor protein containing pleckstrin homology domain, phosphotyrosine binding domain and leucine zipper motif (APPL)1 is essential for mediating adiponectin signal to induce liver kinase B (LKB)1 cytosloic translocation, an essential step for activation of AMP-activated protein kinase (AMPK) in cells. However, the underlying molecular mechanisms remain unknown. Here, we demonstrate that treating C2C12 myotubes with adiponectin promoted APPL1 interaction with protein phosphatase 2A (PP2A) and protein kinase Cζ (PKCζ), leading to the activation of PP2A and subsequent dephosphorylation and inactivation of PKCζ. The adiponectin-induced inactivation of PKCζ results in dephosphorylation of LKB1 at Ser(307) and its subsequent translocation to the cytosol, where it stimulates AMPK activity. Interestingly, we found that metformin also induces LKB1 cytosolic translocation, but the stimulation is independent of APPL1 and the PP2A-PKCζ pathway. Together, our study uncovers a new mechanism underlying adiponectin-stimulated AMPK activation in muscle cells and shed light on potential targets for prevention and treatment of insulin resistance and its associated diseases.

Regulation of AMPK by the ubiquitin proteasome system

The 5'-AMP-activated protein kinase (AMPK) functions as a metabolic fuel gauge that is activated in response to environmental stressors to restore cellular energy balance. In the heart, AMPK coordinates the activation of glucose and fatty acid metabolic pathways to ensure increased production of myocardial ATP when required, such as during cardiac ischemia/reperfusion and hypertrophy, causing an increase in AMPK activity that can be viewed as both protective and maladaptive. While we understand the basic regulation of AMPK activity by kinases, recent studies have introduced the concept that AMPK is regulated by other post-translational modifications, specifically ubiquitination. These studies reported that the ubiquitin ligase cell death-inducing DFFA-like effector a ubiquitinates the β subunit of AMPK to regulate its steady-state protein levels. Other investigators found that AMPK regulatory components, including the AMPK α subunit and AMPK kinases NUAK1 and MARK4, can be ubiquitinated with atypical ubiquitin chains. The USP9X-deubiquitinating enzyme was identified to remove ubiquitination from both NUAK1 and MARK4. Lastly, AMPK activation increases the expression of the ubiquitin ligases MAFBx/Atrogin-1 and MuRF1. These ubiquitin ligases regulate key cardiac transcription factors to control cardiomyocyte mass and remodeling, thus suggesting another mechanism by which AMPK may function in the heart. The relevance of AMPK ubiquitination in cardiac disease has yet to be tested directly, but it likely represents an important mechanism that occurs in common cardiac diseases that may be targeted for therapy.

Glycosylphosphatidylinositol-specific phospholipase D improves glucose tolerance

Insulin regulation of energy metabolism is complex and involves numerous signaling cascades. Insulin has been suggested to stimulate a phospholipase that cleaves glycosylphosphatidylinositols resulting in the generation of an inositol glycan that serves as an insulin mediator. To determine if glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) may play a role in glucose metabolism, we examined the effect of overexpressing GPI-PLD using adenovirus-mediated gene transfer in C57BL/6 mice. Overexpressing GPI-PLD was associated with a decrease in fasting glucose as well as an improvement in glucose tolerance as determined by an intraperitoneal glucose tolerance test. This effect to improve glucose tolerance does not result from an increase in insulin sensitivity, as overexpressing GPI-PLD does not alter the response to insulin. In contrast, the insulin response during the glucose tolerance test in GPI-PLD-overexpressing mice was increased. Overexpressing GPI-PLD in an insulinoma cell line enhanced glucose-stimulated insulin secretion, suggesting that enhanced insulin secretion in vivo may have contributed to the improved glucose tolerance.

D-chiro-inositol glycans in insulin signaling and insulin resistance

Classical actions of insulin involve increased glucose uptake from the bloodstream and its metabolism in peripheral tissues, the most important and relevant effects for human health. However, nonoxidative and oxidative glucose disposal by activation of glycogen synthase (GS) and mitochondrial pyruvate dehydrogenase (PDH) remain incompletely explained by current models for insulin action. Since the discovery of insulin receptor Tyr kinase activity about 25 years ago, the dominant paradigm for intracellular signaling by insulin invokes protein phosphorylation downstream of the receptor and its primary Tyr phosphorylated substrates-the insulin receptor substrate family of proteins. This scheme accounts for most, but not all, intracellular actions of insulin. Essentially forgotten is the previous literature and continuing work on second messengers generated in cells in response to insulin. Treatment and even prevention of diabetes and metabolic syndrome will benefit from a more complete elucidation of cellular-signaling events activated by insulin, to include the actions of second messengers such as glycan molecules that contain D-chiro-inositol (DCI). The metabolism of DCI is associated with insulin sensitivity and resistance, supporting the concept that second messengers have a role in responses to and resistance to insulin.

AMPK inhibition in health and disease

All living organisms depend on dynamic mechanisms that repeatedly reassess the status of amassed energy, in order to adapt energy supply to demand. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals managing energy balance. Control of AMPK activity involves allosteric AMP and ATP regulation, auto-inhibitory features and phosphorylation of its catalytic (alpha) and regulatory (beta and gamma) subunits. AMPK has a prominent role not only as a peripheral sensor but also in the central nervous system as a multifunctional metabolic regulator. AMPK represents an ideal second messenger for reporting cellular energy state. For this reason, activated AMPK acts as a protective response to energy stress in numerous systems. However, AMPK inhibition also actively participates in the control of whole body energy homeostasis. In this review, we discuss recent findings that support the role and function of AMPK inhibition under physiological and pathological states.

Activation of AMP-activated protein kinase signaling pathway by adiponectin and insulin in mouse adipocytes: requirement of acyl-CoA synthetases FATP1 and Acsl1 and association with an elevation in AMP/ATP ratio

Adiponectin activates AMP-activated protein kinase (AMPK) in adipocytes, but the underlying mechanism remains unclear. Here we tested the hypothesis that AMP, generated in activating fatty acids to their CoA derivatives, catalyzed by acyl-CoA synthetases, is involved in AMPK activation by adiponectin. Moreover, in adipocytes, insulin affects the subcellular localization of acyl-CoA synthetase FATP1. Thus, we also tested whether insulin activates AMPK in these cells and, if so, whether it activates through a similar mechanism. We examined these hypotheses by measuring the AMP/ATP ratio and AMPK activation on adiponectin and insulin stimulation and after knocking down acyl-CoA synthetases in adipocytes. We show that adiponectin activation of AMPK is accompanied by an ∼2-fold increase in the cellular AMP/ATP ratio. Moreover, FATP1 and Acsl1, the 2 major acyl-CoA synthetase isoforms in adipocytes, are essential for AMPK activation by adiponectin. We also show that after 40 min. insulin activated AMPK in adipocytes, which was coupled with a 5-fold increase in the cellular AMP/ATP ratio. Knockdown studies show that FATP1 and Acsl1 are required for these processes, as well as for stimulation of long-chain fatty acid uptake by adiponection and insulin. These studies demonstrate that a change in cellular energy state is associated with AMPK activation by both adiponectin and insulin, which requires the activity of FATP1 and Acsl1.

The pivotal role of protein kinase C zeta (PKCzeta) in insulin- and AMP-activated protein kinase (AMPK)-mediated glucose uptake in muscle cells

Insulin and AMP-activated protein kinase (AMPK) signal pathways are involved in the regulation of glucose uptake. The integration of signals between these two pathways to maintain glucose homeostasis remains elusive. In this work, stimulation of insulin and berberine conferred a glucose uptake or surface glucose transporter 4 (GLUT4) translocation that was less than simple summation of their effects in insulin-sensitive muscle cells. Using specific inhibitors to key kinases of both pathways and PKCzeta small interference RNA, protein kinase C zeta (PKCzeta) was found to regulate insulin-stimulated protein kinase B (PKB) activation and inhibit AMPK activity on dorsal cell surface. In the presence of berberine, PKCzeta controlled AMPK activation and AMPK blocked PKB activity in perinuclear region. The inhibition effect of PKCzeta on AMPK activation or the arrestment of PKB activity by AMPK still existed in basal condition. These results suggest that there is antagonistic regulation between insulin and AMPK signal pathways, which is mediated by the switch roles of PKCzeta.

Short-term AMP-regulated protein kinase activation enhances insulin-sensitive fatty acid uptake and increases the effects of insulin on fatty acid oxidation in L6 muscle cells

Evidence shows that exercise increases insulin-sensitive glucose uptake and that exercise-induced AMP-regulated protein kinase (AMPK) activation is a likely candidate to mediate this metabolic adaptation. The purpose of this study was to determine whether repeated AMPK activation can similarly enhance insulin-sensitive fatty acid (FA) metabolism. L6 myotubes were incubated under the following conditions: repeated plus acute 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) treatment (RAA; 1 mmol/L AICAR for 5 h/d for 5 days plus 1 mmol/L AICAR for 60 min on day 6), repeated AICAR (RA; 1 mmol/L AICAR for 5 h/d for five days) or acute AICAR (AA; 1 mmol/L AICAR for 60 min) and were compared with control cells that were not treated with AICAR. On day six, cells from each group were incubated with or without 100 nmol/L insulin. AICAR treatment and insulin stimulation independently increased (P < 0.05) palmitate uptake in all groups. RAA potentiated the insulin-induced increase in palmitate uptake by 97% (P < 0.05) as compared with control cells. RA and AA treatments prevented the insulin-induced decrease in palmitate oxidation, while RAA treatment restored the sensitivity of the cells to insulin action on palmitate oxidation. Total peroxisome proliferator-activated receptor-gamma co-activator-1 alpha, atypical protein kinase C-zeta, cytochrome C and CD36 protein content was increased (P < 0.05) by RA treatment, but unaffected by insulin. These results indicate that repeated AMPK activation induces improvements in insulin-sensitive FA uptake and oxidation and that this occurs partly via changes in the expression of proteins linked to insulin signaling and FA uptake and oxidation capacity.

Deficiency of phosphoinositide 3-kinase enhancer protects mice from diet-induced obesity and insulin resistance

OBJECTIVE

Phosphoinositide 3-kinase enhancer A (PIKE-A) is a proto-oncogene that promotes tumor growth and transformation by enhancing Akt activity. However, the physiological functions of PIKE-A in peripheral tissues are unknown. Here, we describe the effect of PIKE deletion in mice and explore the role of PIKE-A in obesity development.

RESEARCH DESIGN AND METHODS

Whole-body PIKE knockout mice were generated and subjected to high-fat-diet feeding for 20 weeks. The glucose tolerance, tissue-specific insulin sensitivity, adipocyte differentiation, and lipid oxidation status were determined. The molecular mechanism of PIKE in the insulin signaling pathway was also studied.

RESULTS

We show that PIKE-A regulates obesity development by modulating AMP-activated protein kinase (AMPK) phosphorylation. PIKE-A is important for insulin to suppress AMPK phosphorylation. The expression of PIKE-A is markedly increased in adipose tissue of obese mice, whereas depletion of PIKE-A inhibits adipocyte differentiation. PIKE knockout mice exhibit a prominent phenotype of lipoatrophy and are resistant to high-fat diet-induced obesity, liver steatosis, and diabetes. PIKE knockout mice also have augmented lipid oxidation, which is accompanied by enhanced AMPK phosphorylation in both muscle and adipose tissue. Moreover, insulin sensitivity is improved in PIKE-A-deficient muscle and fat, thus protecting the animals from diet-induced diabetes.

CONCLUSIONS

Our results suggest that PIKE-A is implicated in obesity and associated diabetes development by negatively regulating AMPK activity.

SIRT1, AMP-activated protein kinase phosphorylation and downstream kinases in response to a single bout of sprint exercise: influence of glucose ingestion

This study was designed to examine potential in vivo mechanisms of AMP-activated protein kinase (AMPK) phosphorylation inhibition and its downstream signaling consequences during the recovery period after a single bout of sprint exercise. Sprint exercise induces Thr(172)-AMPK phosphorylation and increased PGC-1alpha mRNA, by an unknown mechanism. Muscle biopsies were obtained in 15 young healthy men in response to a 30-s sprint exercise (Wingate test) randomly distributed into two groups: the fasting (n = 7, C) and the glucose group (n = 8, G), who ingested 75 g of glucose 1 h before exercising to inhibit AMPKalpha phosphorylation. Exercise elicited different patterns of Ser(221)-ACCbeta, Ser(473)-Akt and Thr(642)-AS160 phosphorylation, during the recovery period after glucose ingestion. Thirty minutes after the control sprint, Ser(485)-AMPKalpha1/Ser(491)-AMPKalpha2 phosphorylation was reduced by 33% coinciding with increased Thr(172)-AMPKalpha phosphorylation (both, P < 0.05). Glucose abolished the 30-min Thr(172)-AMPKalpha phosphorylation. Ser(221)-ACCbeta phosphorylation was elevated immediately following and 30 min after exercise in C and G, implying a dissociation between Thr(172)-AMPKalpha and Ser(221)-ACCbeta phosphorylation. Two hours after the sprint, PGC-1alpha protein expression remained unchanged while SIRT1 (its upstream deacetylase) was increased. Glucose ingestion abolished the SIRT1 response without any significant effect on PGC-1alpha protein expression. In conclusion, glucose ingestion prior to a sprint exercise profoundly affects Thr(172)-AMPKalpha phosphorylation and its downstream signaling during the recovery period.

Myocardial fatty acid metabolism in health and disease

There is a constant high demand for energy to sustain the continuous contractile activity of the heart, which is met primarily by the beta-oxidation of long-chain fatty acids. The control of fatty acid beta-oxidation is complex and is aimed at ensuring that the supply and oxidation of the fatty acids is sufficient to meet the energy demands of the heart. The metabolism of fatty acids via beta-oxidation is not regulated in isolation; rather, it occurs in response to alterations in contractile work, the presence of competing substrates (i.e., glucose, lactate, ketones, amino acids), changes in hormonal milieu, and limitations in oxygen supply. Alterations in fatty acid metabolism can contribute to cardiac pathology. For instance, the excessive uptake and beta-oxidation of fatty acids in obesity and diabetes can compromise cardiac function. Furthermore, alterations in fatty acid beta-oxidation both during and after ischemia and in the failing heart can also contribute to cardiac pathology. This paper reviews the regulation of myocardial fatty acid beta-oxidation and how alterations in fatty acid beta-oxidation can contribute to heart disease. The implications of inhibiting fatty acid beta-oxidation as a potential novel therapeutic approach for the treatment of various forms of heart disease are also discussed.

Apelin is necessary for the maintenance of insulin sensitivity

The recently discovered peptide apelin is known to be involved in the maintenance of insulin sensitivity. However, questions persist regarding its precise role in the chronic setting. Fasting glucose, insulin, and adiponectin levels were determined on mice with generalized deficiency of apelin (APKO). Additionally, insulin (ITT) and glucose tolerance tests (GTT) were performed. To assess the impact of exogenously delivered apelin on insulin sensitivity, osmotic pumps containing pyroglutamated apelin-13 or saline were implanted in APKO mice for 4 wk. Following the infusion, ITT/GTTs were repeated and the animals euthanized. Soleus muscles were harvested and homogenized in lysis buffer, and insulin-induced Akt phosphorylation was determined by Western blotting. Apelin-13 infusion and ITTs/GTTs were also performed in obese diabetic db/db mice. To probe the underlying mechanism for apelin's effects, apelin-13 was also delivered to cultured C2C12 myotubes. 2-[3H]deoxyglucose uptake and Akt phosphorylation were assessed in the presence of various inhibitors. APKO mice had diminished insulin sensitivity, were hyperinsulinemic, and had decreased adiponectin levels. Soleus lysates had decreased insulin-induced Akt phosphorylation. Administration of apelin to APKO and db/db mice resulted in improved insulin sensitivity. In C2C12 myotubes, apelin increased glucose uptake and Akt phosphorylation. These events were fully abrogated by pertussis toxin, compound C, and siRNA knockdown of AMPKalpha1 but only partially diminished by LY-294002 and not at all by L-NAME. We conclude that apelin is necessary for the maintenance of insulin sensitivity in vivo. Apelin's effects on glucose uptake and Akt phosphorylation are in part mediated by a G(i) and AMPK-dependent pathway.

Autophagy, redox signaling, and ventricular remodeling

Autophagy is a catabolic process through which damaged or long-lived proteins, macromolecules, or organelles are recycled by using lysosomal degradation machinery. Although the occurrence of autophagy in several cardiac diseases including ischemic or dilated cardiomyopathy, heart failure, hypertrophy, and during ischemia/reperfusion injury have been reported, the exact role of autophagy in these diseases is not known. Emerging studies indicate that oxidative stress in cellular system could induce autophagy, and oxidatively modified macromolecules and organelles can be selectively removed by autophagy. Mild oxidative stress-induced autophagy could provide the first line of protection against major damage like apoptosis and necrosis. Cardiac-specific loss of Atg5, an autophagic gene involved in the formation of autophagosome, causes cardiac hypertrophy, left ventricular dilation, and contractile dysfunction. Recently, it was revealed that Atg4, another autophagic gene involved in the formation of autophagosomes, is controlled through redox regulation under the condition of starvation-induced autophagy. In this review, we discuss the function of autophagy in association with oxidative stress and redox signaling in the remodeling of cardiac myocardium. Further research is needed to explore the possibilities of redox regulation of other autophagic genes and the role of redox signaling-mediated autophagy in the heart.

Adenosine Monophosphate-Activated Protein Kinase (AMPK) as a New Target for Antidiabetic Drugs: A Review on Metabolic, Pharmacological and Chemical Considerations

In view of the epidemic nature of type 2 diabetes and the substantial rate of failure of current oral antidiabetic drugs the quest for new therapeutics is intensive. The adenosine monophosphate-activated protein kinase (AMPK) is an important regulatory protein for cellular energy balance and is considered a master switch of glucose and lipid metabolism in various organs, especially in skeletal muscle and liver. In skeletal muscles, AMPK stimulates glucose transport and fatty acid oxidation. In the liver, it augments fatty acid oxidation and decreases glucose output, cholesterol and triglyceride synthesis. These metabolic effects induced by AMPK are associated with lowering blood glucose levels in hyperglycemic individuals. Two classes of oral antihyperglycemic drugs (biguanidines and thiazolidinediones) have been shown to exert some of their therapeutic effects by directly or indirectly activating AMPK. However, side effects and an acquired resistance to these drugs emphasize the need for the development of novel and efficacious AMPK activators. We have recently discovered a new class of hydrophobic D-xylose derivatives that activates AMPK in skeletal muscles in a non insulin-dependent manner. One of these derivatives (2,4;3,5-dibenzylidene-D-xylose-diethyl-dithioacetal) stimulates the rate of hexose transport in skeletal muscle cells by increasing the abundance of glucose transporter-4 (GLUT-4) in the plasma membrane through activation of AMPK. This compound reduces blood glucose levels in diabetic mice and therefore offers a novel strategy of therapeutic intervention strategy in type 2 diabetes. The present review describes various classes of chemically-related compounds that activate AMPK by direct or indirect interactions and discusses their potential for candidate antihyperglycemic drug development.