Transcriptional activation of the glucose transporter GLUT1 in ventricular cardiac myocytes by hypertrophic agonists

Cadmium contributes to cardiac metabolic disruption by activating endothelial HIF1A-GLUT1 axis

Cadmium (Cd) is an environmental risk factor of cardiovascular diseases. Researchers have found that Cd exposure causes energy metabolic disorders in the heart decades ago. However, the underlying molecular mechanisms are still elusive. In this study, male C57BL/6 J mice were exposed to cadmium chloride (CdCl2) through drinking water for 4 weeks. We found that exposure to CdCl2 increased glucose uptake and utilization, and disrupted normal metabolisms in the heart. In vitro studies showed that CdCl2 specifically increased endothelial glucose uptake without affecting cardiomyocytic glucose uptake and endothelial fatty acid uptake. The glucose transporter 1 (GLUT1) as well as its transcription factor HIF1A was significantly increased after CdCl2 treatment in endothelial cells. Further investigations found that CdCl2 treatment upregulated HIF1A expression by inhibiting its degradation through ubiquitin-proteasome pathway, thereby promoted its transcriptional activation of SLC2A1. Administration of HIF1A small molecule inhibitor echinomycin and A-485 reversed CdCl2-mediated increase of glucose uptake in endothelial cells. In accordance with this, intravenous injection of echinomycin effectively ameliorated CdCl2-mediated metabolic disruptions in the heart. Our study uncovered the molecular mechanisms of Cd in contributing cardiac metabolic disruption by inhibiting HIF1A degradation and increasing GLUT1 transcriptional expression. Inhibition of HIF1A could be a potential strategy to ameliorate Cd-mediated cardiac metabolic disorders and Cd-related cardiovascular diseases.

Regulation of myocardial glucose metabolism by YAP/TAZ signaling

The heart utilizes glucose and its metabolites as both energy sources and building blocks for cardiac growth and survival under both physiological and pathophysiological conditions. YAP/TAZ, transcriptional co-activators of the Hippo pathway, are key regulators of cell proliferation, survival, and metabolism in many cell types. Increasing lines of evidence suggest that the Hippo-YAP/TAZ signaling pathway is involved in the regulation of both physiological and pathophysiological processes in the heart. In particular, YAP/TAZ play a critical role in mediating aerobic glycolysis, the Warburg effect, in cardiomyocytes. Here, we summarize what is currently known about YAP/TAZ signaling in the heart by focusing on the regulation of glucose metabolism and its functional significance.

Extracellular Vimentin Alters Energy Metabolism And Induces Adipocyte Hypertrophy

BACKGRUOUND

Previous studies have reported that oxidative stress contributes to obesity characterized by adipocyte hypertrophy. However, mechanism has not been studied extensively. In the current study, we evaluated role of extracellular vimentin secreted by oxidized low-density lipoprotein (oxLDL) in energy metabolism in adipocytes.

METHODS

We treated 3T3-L1-derived adipocytes with oxLDL and measured vimentin which was secreted in the media. We evaluated changes in uptake of glucose and free fatty acid, expression of molecules functioning in energy metabolism, synthesis of adenosine triphosphate (ATP) and lactate, markers for endoplasmic reticulum (ER) stress and autophagy in adipocytes treated with recombinant vimentin.

RESULTS

Adipocytes secreted vimentin in response to oxLDL. Microscopic evaluation revealed that vimentin treatment induced increase in adipocyte size and increase in sizes of intracellular lipid droplets with increased intracellular triglyceride. Adipocytes treated with vimentin showed increased uptake of glucose and free fatty acid with increased expression of plasma membrane glucose transporter type 1 (GLUT1), GLUT4, and CD36. Vimentin treatment increased transcription of GLUT1 and hypoxia-inducible factor 1α (Hif-1α) but decreased GLUT4 transcription. Adipose triglyceride lipase (ATGL), peroxisome proliferator-activated receptor γ (PPARγ), sterol regulatory element-binding protein 1 (SREBP1), diacylglycerol O-acyltransferase 1 (DGAT1) and 2 were decreased by vimentin treatment. Markers for ER stress were increased and autophagy was impaired in vimentin-treated adipocytes. No change was observed in synthesis of ATP and lactate in the adipocytes treated with vimentin.

CONCLUSION

We concluded that extracellular vimentin regulates expression of molecules in energy metabolism and promotes adipocyte hypertrophy. Our results show that vimentin functions in the interplay between oxidative stress and metabolism, suggesting a mechanism by which adipocyte hypertrophy is induced in oxidative stress.

Anticancer effects of alpelisib on PIK3CA-mutated canine mammary tumor cell lines

Canine mammary tumors (CMTs) are commonly observed in old and unspayed female dogs. Recently, dogs have been increasingly spaying at a young age to prevent mammary tumors. These CMTs require extensive local excision and exhibit a high probability of metastasis to the regional lymph nodes and lungs during malignancy. However, the molecular and biological mechanisms underlying CMT development have not been fully elucidated, and research in this area is limited. Therefore, in this study, we established new CMT cell lines by isolating cells from tumor tissues and investigated phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), a target for human breast cancer. PIK3CA mutations were observed at a similar loci as in the human PIK3CA gene in half of all canine samples. Furthermore, we investigated whether alpelisib, a PIK3CA inhibitor approved by the U.S. Food and Drug Administration for human breast cancer treatment, along with fulvestrant, is effective for CMT treatment. Alpelisib exerted stronger anticancer effects on cell lines with PIK3CA mutations than on the wild-type cell lines. In conclusion, we established new CMT cell lines with PIK3CA mutations and confirmed the efficacy of alpelisib for CMT treatment in vitro.

Effects of Normobaric Hypoxia and Adrenergic Blockade over 72 h on Cardiac Function in Rats

In rats, acute normobaric hypoxia depressed left ventricular (LV) inotropic function. After 24 h of hypoxic exposure, a slight recovery of LV function occurred. We speculated that prolonged hypoxia (72 h) would induce acclimatization and, hence, recovery of LV function. Moreover, we investigated biomarkers of nitrosative stress and apoptosis as possible causes of hypoxic LV depression. To elucidate the role of hypoxic sympathetic activation, we studied whether adrenergic blockade would further deteriorate the general state of the animals and their cardiac function. Ninety-four rats were exposed over 72 h either to normal room air (N) or to normobaric hypoxia (H). The rodents received infusion (0.1 mL/h) with 0.9% NaCl or with different adrenergic blockers. Despite clear signs of acclimatization to hypoxia, the LV depression continued persistently after 72 h of hypoxia. Immunohistochemical analyses revealed significant increases in markers of nitrosative stress, adenosine triphosphate deficiency and apoptosis in the myocardium, which could provide a possible explanation for the absence of LV function recovery. Adrenergic blockade had a slightly deteriorative effect on the hypoxic LV function compared to the hypoxic group with maintained sympathetic efficacy. These findings show that hypoxic sympathetic activation compensates, at least partially, for the compromised function in hypoxic conditions, therefore emphasizing its importance for hypoxia adaptation.

Hypoxia-Inducible Factor 1-Alpha and Glucose Metabolism during Cardiac Remodeling Progression from Hypertrophy to Heart Failure

In pathological cardiac hypertrophy, the heart is more dependent on glucose than fatty acids. This shift in energy metabolism occurs due to several factors, including the oxygen deficit, which activates hypoxia-inducible factor-1α (HIF-1α), a critical molecule related to glucose metabolism. However, there are gaps regarding the behavior of key proteins in the glycolytic pathway and HIF-1α during the transition from hypertrophy to heart failure (HF). This study assesses the hypothesis that there is an early change and enhancement of HIF-1α and the glycolytic pathway, as well as an association between them during cardiac remodeling. Sham and aortic stenosis Wistar rats were analyzed at 2, 6, and 18 weeks and in HF (n = 10–18). Cardiac structure and function were investigated by echocardiogram. Myocardial glycolysis, the aerobic and anaerobic pathways and glycogen were analyzed by enzymatic assay, Western blot, and enzyme-linked immunosorbent assay (ELISA). The following were observed: increased left ventricular hypertrophy; early diastolic function change and severe systolic and diastolic dysfunction in HF; increased HIF-1α in the 2nd week and in HF; precocious alteration and intensification of glycolysis with a shift to anaerobic metabolism from the 6th week onwards; association between HIF-1α, glycolysis, and the anaerobic pathway. Our hypothesis was confirmed as there was an early change and intensification in glucose metabolism, alteration in HIF-1α, and an association between data during the progression from hypertrophy to heart failure.

Cannabinoid receptor agonist attenuates angiotensin II-induced enlargement and mitochondrial dysfunction in rat atrial cardiomyocytes

Pathological remodeling of atrial tissue renders the atria more prone to arrhythmia upon arrival of electrical triggers. Activation of the renin-angiotensin system is an important factor that contributes to atrial remodeling, which may result in atrial hypertrophy and prolongation of P-wave duration. In addition, atrial cardiomyocytes are electrically coupled via gap junctions, and electrical remodeling of connexins may result in dysfunction of coordinated wave propagation within the atria. Currently, there is a lack of effective therapeutic strategies that target atrial remodeling. We previously proposed that cannabinoid receptors (CBR) may have cardioprotective qualities. CB13 is a dual cannabinoid receptor agonist that activates AMPK signaling in ventricular cardiomyocytes. We reported that CB13 attenuates tachypacing-induced shortening of atrial refractoriness and inhibition of AMPK signaling in the rat atria. Here, we evaluated the effects of CB13 on neonatal atrial rat cardiomyocytes (NRAM) stimulated by angiotensin II (AngII) in terms of atrial myocyte enlargement and mitochondrial function. CB13 inhibited AngII-induced enhancement of atrial myocyte surface area in an AMPK-dependent manner. CB13 also inhibited mitochondrial membrane potential deterioration in the same context. However, AngII and CB13 did not affect mitochondrial permeability transition pore opening. We further demonstrate that CB13 increased Cx43 compared to AngII-treated neonatal rat atrial myocytes. Overall, our results support the notion that CBR activation promotes atrial AMPK activation, and prevents myocyte enlargement (an indicator that suggests pathological hypertrophy), mitochondrial depolarization and Cx43 destabilization. Therefore, peripheral CBR activation should be further tested as a novel treatment strategy in the context of atrial remodeling.

Desmin Knock-Out Cardiomyopathy: A Heart on the Verge of Metabolic Crisis

Desmin mutations cause familial and sporadic cardiomyopathies. In addition to perturbing the contractile apparatus, both desmin deficiency and mutated desmin negatively impact mitochondria. Impaired myocardial metabolism secondary to mitochondrial defects could conceivably exacerbate cardiac contractile dysfunction. We performed metabolic myocardial phenotyping in left ventricular cardiac muscle tissue in desmin knock-out mice. Our analyses revealed decreased mitochondrial number, ultrastructural mitochondrial defects, and impaired mitochondria-related metabolic pathways including fatty acid transport, activation, and catabolism. Glucose transporter 1 and hexokinase-1 expression and hexokinase activity were increased. While mitochondrial creatine kinase expression was reduced, fetal creatine kinase expression was increased. Proteomic analysis revealed reduced expression of proteins involved in electron transport mainly of complexes I and II, oxidative phosphorylation, citrate cycle, beta-oxidation including auxiliary pathways, amino acid catabolism, and redox reactions and oxidative stress. Thus, desmin deficiency elicits a secondary cardiac mitochondriopathy with severely impaired oxidative phosphorylation and fatty and amino acid metabolism. Increased glucose utilization and fetal creatine kinase upregulation likely portray attempts to maintain myocardial energy supply. It may be prudent to avoid medications worsening mitochondrial function and other metabolic stressors. Therapeutic interventions for mitochondriopathies might also improve the metabolic condition in desmin deficient hearts.

Upregulation of Phospholipase C Gene Expression Due to Norepinephrine-Induced Hypertrophic Response

The activation of phospholipase C (PLC) is thought to have a key role in the cardiomyocyte response to several different hypertrophic agents such as norepinephrine, angiotensin II and endothelin-1. PLC activity results in the generation of diacylglycerol and inositol trisphosphate, which are downstream signal transducers for the expression of fetal genes, increased protein synthesis, and subsequent cardiomyocyte growth. In this article, we describe the signal transduction elements that regulate PLC gene expression. The discussion is focused on the norepinephrine- α₁-adrenoceptor signaling pathway and downstream signaling processes that mediate an upregulation of PLC isozyme gene expression. Evidence is also indicated to demonstrate that PLC activities self-regulate the expression of PLC isozymes with the suggestion that PLC activities may be part of a coordinated signaling process for the perpetuation of cardiac hypertrophy. Accordingly, from the information provided, it is plausible that specific PLC isozymes could be targeted for the mitigation of cardiac hypertrophy.

YAP mediates compensatory cardiac hypertrophy through aerobic glycolysis in response to pressure overload

The heart utilizes multiple adaptive mechanisms to maintain pump function. Compensatory cardiac hypertrophy reduces wall stress and oxygen consumption, thereby protecting the heart against acute blood pressure elevation. The nuclear effector of the Hippo pathway, Yes-associated protein 1 (YAP), is activated and mediates compensatory cardiac hypertrophy in response to acute pressure overload (PO). In this study, YAP promoted glycolysis by upregulating glucose transporter 1 (GLUT1), which in turn caused accumulation of intermediates and metabolites of the glycolytic, auxiliary, and anaplerotic pathways during acute PO. Cardiac hypertrophy was inhibited and heart failure was exacerbated in mice with YAP haploinsufficiency in the presence of acute PO. However, normalization of GLUT1 rescued the detrimental phenotype. PO induced the accumulation of glycolytic metabolites, including l-serine, l-aspartate, and malate, in a YAP-dependent manner, thereby promoting cardiac hypertrophy. YAP upregulated the GLUT1 gene through interaction with TEA domain family member 1 (TEAD1) and HIF-1α in cardiomyocytes. Thus, YAP induces compensatory cardiac hypertrophy through activation of the Warburg effect.

GLUT1 overexpression enhances glucose metabolism and promotes neonatal heart regeneration

The mammalian heart switches its main metabolic substrate from glucose to fatty acids shortly after birth. This metabolic switch coincides with the loss of regenerative capacity in the heart. However, it is unknown whether glucose metabolism regulates heart regeneration. Here, we report that glucose metabolism is a determinant of regenerative capacity in the neonatal mammalian heart. Cardiac-specific overexpression of Glut1, the embryonic form of constitutively active glucose transporter, resulted in an increase in glucose uptake and concomitant accumulation of glycogen storage in postnatal heart. Upon cryoinjury, Glut1 transgenic hearts showed higher regenerative capacity with less fibrosis than non-transgenic control hearts. Interestingly, flow cytometry analysis revealed two distinct populations of ventricular cardiomyocytes: Tnnt2-high and Tnnt2-low cardiomyocytes, the latter of which showed significantly higher mitotic activity in response to high intracellular glucose in Glut1 transgenic hearts. Metabolic profiling shows that Glut1-transgenic hearts have a significant increase in the glucose metabolites including nucleotides upon injury. Inhibition of the nucleotide biosynthesis abrogated the regenerative advantage of high intra-cardiomyocyte glucose level, suggesting that the glucose enhances the cardiomyocyte regeneration through the supply of nucleotides. Our data suggest that the increase in glucose metabolism promotes cardiac regeneration in neonatal mouse heart.

Testosterone activates glucose metabolism through AMPK and androgen signaling in cardiomyocyte hypertrophy

Background

Testosterone regulates nutrient and energy balance to maintain protein synthesis and metabolism in cardiomyocytes, but supraphysiological concentrations induce cardiac hypertrophy. Previously, we determined that testosterone increased glucose uptake—via AMP-activated protein kinase (AMPK)—after acute treatment in cardiomyocytes. However, whether elevated glucose uptake is involved in long-term changes of glucose metabolism or is required during cardiomyocyte growth remained unknown. In this study, we hypothesized that glucose uptake and glycolysis increase in testosterone-treated cardiomyocytes through AMPK and androgen receptor (AR).

Methods

Cultured cardiomyocytes were stimulated with 100 nM testosterone for 24 h, and hypertrophy was verified by increased cell size and mRNA levels of β-myosin heavy chain (β-mhc). Glucose uptake was assessed by 2-NBDG. Glycolysis and glycolytic capacity were determined by measuring extracellular acidification rate (ECAR).

Results

Testosterone induced cardiomyocyte hypertrophy that was accompanied by increased glucose uptake, glycolysis enhancement and upregulated mRNA expression of hexokinase 2. In addition, testosterone increased AMPK phosphorylation (Thr172), while inhibition of both AMPK and AR blocked glycolysis and cardiomyocyte hypertrophy induced by testosterone. Moreover, testosterone supplementation in adult male rats by 5 weeks induced cardiac hypertrophy and upregulated β-mhc, Hk2 and Pfk2 mRNA levels.

Conclusion

These results indicate that testosterone stimulates glucose metabolism by activation of AMPK and AR signaling which are critical to induce cardiomyocyte hypertrophy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40659-021-00328-4.

The Warburg Effect 97 Years after Its Discovery

The deregulation of the oxidative metabolism in cancer, as shown by the increased aerobic glycolysis and impaired oxidative phosphorylation (Warburg effect), is coordinated by genetic changes leading to the activation of oncogenes and the loss of oncosuppressor genes. The understanding of the metabolic deregulation of cancer cells is necessary to prevent and cure cancer. In this review, we illustrate and comment the principal metabolic and molecular variations of cancer cells, involved in their anomalous behavior, that include modifications of oxidative metabolism, the activation of oncogenes that promote glycolysis and a decrease of oxygen consumption in cancer cells, the genetic susceptibility to cancer, the molecular correlations involved in the metabolic deregulation in cancer, the defective cancer mitochondria, the relationships between the Warburg effect and tumor therapy, and recent studies that reevaluate the Warburg effect. Taken together, these observations indicate that the Warburg effect is an epiphenomenon of the transformation process essential for the development of malignancy.

Glucose transporters in cardiovascular system in health and disease

Glucose transporters are essential for the heart to sustain its function. Due to its nature as a high energy-consuming organ, the heart needs to catabolize a huge quantity of metabolic substrates. For optimized energy production, the healthy heart constantly switches between various metabolites in accordance with substrate availability and hormonal status. This metabolic flexibility is essential for the maintenance of cardiac function. Glucose is part of the main substrates catabolized by the heart and its use is fine-tuned via complex molecular mechanisms that include the regulation of the glucose transporters GLUTs, mainly GLUT4 and GLUT1. Besides GLUTs, glucose can also be transported by cotransporters of the sodium-glucose cotransporter (SGLT) (SLC5 gene) family, in which SGLT1 and SMIT1 were shown to be expressed in the heart. This SGLT-mediated uptake does not seem to be directly linked to energy production but is rather associated with intracellular signalling triggering important processes such as the production of reactive oxygen species. Glucose transport is markedly affected in cardiac diseases such as cardiac hypertrophy, diabetic cardiomyopathy and heart failure. These alterations are not only fingerprints of these diseases but are involved in their onset and progression. The present review will depict the importance of glucose transport in healthy and diseased heart, as well as proposed therapies targeting glucose transporters.

Molecular and Biochemical Parameters Related to Plasma Mannose Levels in Coronary Artery Disease Among Nondiabetic Patients

Aims: Nondiabetic patients were studied to determine whether modest elevations in plasma mannose may be associated with a greater incidence of coronary artery disease (CAD). Materials and Methods: Plasma insulin, mannose, glucose, hexokinase 1-2, GLUT1-GLUT4 levels, and serum mannose phosphate isomerase enzyme levels were evaluated with respect to subsequent CAD using records from 120 nondiabetic CAD patients and 120 healthy volunteers. CAD was identified from myocardial infarction and new diagnoses of angina. Results: Of 120 nondiabetic CAD patients studied, their plasma GLUT4 and HK1 levels were significantly lower than those of the control group. In addition, a significant increase in plasma mannose levels was found in the patient group compared to the control group. Conclusion: Our findings showed that elevated baseline mannose levels in plasma are associated with an increased risk of CAD over time.

Intronic CRISPR Repair in a Preclinical Model of Noonan Syndrome-Associated Cardiomyopathy

BACKGROUND

Noonan syndrome (NS) is a multisystemic developmental disorder characterized by common, clinically variable symptoms, such as typical facial dysmorphisms, short stature, developmental delay, intellectual disability as well as cardiac hypertrophy. The underlying mechanism is a gain-of-function of the RAS-mitogen-activated protein kinase signaling pathway. However, our understanding of the pathophysiological alterations and mechanisms, especially of the associated cardiomyopathy, remains limited and effective therapeutic options are lacking.

METHODS

Here, we present a family with two siblings displaying an autosomal recessive form of NS with massive hypertrophic cardiomyopathy as clinically the most prevalent symptom caused by biallelic mutations within the leucine zipper-like transcription regulator 1 (LZTR1). We generated induced pluripotent stem cell-derived cardiomyocytes of the affected siblings and investigated the patient-specific cardiomyocytes on the molecular and functional level.

RESULTS

Patients' induced pluripotent stem cell-derived cardiomyocytes recapitulated the hypertrophic phenotype and uncovered a so-far-not-described causal link between LZTR1 dysfunction, RAS-mitogen-activated protein kinase signaling hyperactivity, hypertrophic gene response and cellular hypertrophy. Calcium channel blockade and MEK inhibition could prevent some of the disease characteristics, providing a molecular underpinning for the clinical use of these drugs in patients with NS, but might not be a sustainable therapeutic option. In a proof-of-concept approach, we explored a clinically translatable intronic CRISPR (clustered regularly interspaced short palindromic repeats) repair and demonstrated a rescue of the hypertrophic phenotype.

CONCLUSIONS

Our study revealed the human cardiac pathogenesis in patient-specific induced pluripotent stem cell-derived cardiomyocytes from NS patients carrying biallelic variants in LZTR1 and identified a unique disease-specific proteome signature. In addition, we identified the intronic CRISPR repair as a personalized and in our view clinically translatable therapeutic strategy to treat NS-associated hypertrophic cardiomyopathy.

Pivotal role of membrane substrate transporters on the metabolic alterations in the pressure-overloaded heart

Cardiac pressure overload (PO), such as caused by aortic stenosis and systemic hypertension, commonly results in cardiac hypertrophy and may lead to the development of heart failure. PO-induced heart failure is among the leading causes of death worldwide, but its pathological origin remains poorly understood. Metabolic alterations are proposed to be an important contributor to PO-induced cardiac hypertrophy and failure. While the healthy adult heart mainly uses long-chain fatty acids (FAs) and glucose as substrates for energy metabolism and to a lesser extent alternative substrates, i.e. lactate, ketone bodies, and amino acids (AAs), the pressure-overloaded heart is characterized by a shift in energy metabolism towards a greater reliance on glycolysis and alternative substrates. A key-governing kinetic step of both FA and glucose fluxes is at the level of their substrate-specific membrane transporters. The relative presence of these transporters in the sarcolemma determines the cardiac substrate preference. Whether the cardiac utilization of alternative substrates is also governed by membrane transporters is not yet known. In this review, we discuss current insight into the role of membrane substrate transporters in the metabolic alterations occurring in the pressure-overloaded heart. Given the increasing evidence of a role for alternative substrates in these metabolic alterations, there is an urgent need to disclose the key-governing kinetic steps in their utilization as well. Taken together, membrane substrate transporters emerge as novel targets for metabolic interventions to prevent or treat PO-induced heart failure.

Insulin Signaling Via Progesterone-Regulated Insulin Receptor Substrate 2 is Critical for Human Uterine Decidualization

Decidualization, the process by which fibroblastic human endometrial stromal cells (HESC) differentiate into secretory decidual cells, is a critical event during the establishment of pregnancy. It is dependent on the steroid hormone progesterone acting through the nuclear progesterone receptor (PR). Previously, we identified insulin receptor substrate 2 (IRS2) as a factor that is directly regulated by PR during decidualization. IRS2 is an adaptor protein that functionally links receptor tyrosine kinases, such as insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R), and their downstream effectors. IRS2 expression was induced in HESC during in vitro decidualization and siRNA-mediated downregulation of IRS2 transcripts resulted in attenuation of this process. Further use of siRNAs targeted to IR or IGF1R transcripts showed that downregulation of IR, but not IGF1R, led to impaired decidualization. Loss of IRS2 transcripts in HESC suppressed phosphorylation of both ERK1/2 and AKT, downstream effectors of insulin signaling, which mediate gene expression associated with decidualization and regulate glucose uptake. Indeed, downregulation of IRS2 resulted in reduced expression and membrane localization of the glucose transporters GLUT1 and GLUT4, resulting in lowered glucose uptake during stromal decidualization. Collectively, these data suggest that the PR-regulated expression of IRS2 is necessary for proper insulin signaling for controlling gene expression and glucose utilization, which critically support the decidualization process to facilitate pregnancy. This study provides new insight into the mechanisms by which steroid hormone signaling intersects with insulin signaling in the uterus during decidualization, which has important implications for pregnancy complications associated with insulin resistance and infertility.

The Novel Phosphatidylinositol-3-Kinase (PI3K) Inhibitor Alpelisib Effectively Inhibits Growth of PTEN-Haploinsufficient Lipoma Cells

Germline mutations in the tumor suppressor gene PTEN cause PTEN Hamartoma Tumor Syndrome (PHTS). Pediatric patients with PHTS frequently develop lipomas. Treatment attempts with the mTORC1 inhibitor rapamycin were unable to reverse lipoma growth. Recently, lipomas associated with PIK3CA-related overgrowth syndrome were successfully treated with the novel PI3K inhibitor alpelisib. Here, we tested whether alpelisib has growth-restrictive effects and induces cell death in lipoma cells. We used PTEN-haploinsufficient lipoma cells from three patients and treated them with alpelisib alone or in combination with rapamycin. We tested the effect of alpelisib on viability, proliferation, cell death, induction of senescence, adipocyte differentiation, and signaling at 1-100 µM alpelisib. Alpelisib alone or in combination with rapamycin reduced proliferation in a concentration- and time-dependent manner. No cell death but an induction of senescence was detected after alpelisib incubation for 72 h. Alpelisib treatment led to a reduced phosphorylation of AKT, mTOR, and ribosomal protein S6. Rapamycin treatment alone led to increased AKT phosphorylation. This effect could be reversed by combining rapamycin with alpelisib. Alpelisib reduced the size of lipoma spheroids by attenuating adipocyte differentiation. Since alpelisib was well tolerated in first clinical trials, this drug alone or in combination with rapamycin is a potential new treatment option for PHTS-related adipose tissue overgrowth.

Modulatory Effect of Guinep (Melicoccus bijugatus Jacq) Fruit Pulp Extract on Isoproterenol-Induced Myocardial Damage in Rats. Identification of Major Metabolites Using High Resolution UHPLC Q-Orbitrap Mass Spectrometry

Guinep is traditionally used in the management of cardiovascular ailments. This study aims to evaluate its medicinal constituents and effects in the management of myocardial injury in an experimental isoproterenol (ISO) rat model. Sprague-Dawley rats were randomly assigned to four groups: Group 1 was the control group; Group 2 received M. bijugatus extract (100 mg/Kg; MB) for six weeks; Group 3 was given ISO (85 mg/Kg) i.p. twice during a 24-hour period; and Group 4 was given ISO (85 mg/Kg) i.p. and MB extract (100 mg/Kg) for six weeks. The MB was administered orally by gavage, daily. The blood pressure of conscious animals was measured, while ECG was performed under anesthesia. Blood and serum were collected for biochemical and hematological analysis. The ISO group treated with MB showed a significant decrease (p < 0.001) in (SBP), diastolic (DBP), mean arterial (MAP) and heart rate (HR) compared to the ISO only group. Conversely, MB treated rats that were not induced with ISO displayed a significant decreases (p < 0.001) in SBP, DBP, MAP, and HR. ISO significantly elevated the ST segment (p < 0.001) and shortened the QTc interval (p < 0.05), which were recovered after treatment with 100 mg/Kg of MB. In addition, the results showed a significant decrease (p < 0.001) in the heart to body weight ratio of the ISO group treated with MB compared to the ISO only group. Furthermore, the extract normalized the hematological values depressed by the ISO while significantly elevating the platelet count. UHPLC high-resolution orbitrap mass spectrometry analysis results revealed the presence of several antioxidants like vitamin C and related compounds, phenolic acids, flavonoid, fatty acids (oxylipins), and terpene derivatives. The results of this study indicated that Melicoccus bijugatus did display some cardio-protective effects in relation to myocardial injury.

Phosphorylation or Mutation of the ERK2 Activation Loop Alters Oligonucleotide Binding

The mitogen-activated protein kinase ERK2 is able to elicit a wide range of context-specific responses to distinct stimuli, but the mechanisms underlying this versatility remain in question. Some cellular functions of ERK2 are mediated through regulation of gene expression. In addition to phosphorylating numerous transcriptional regulators, ERK2 is known to associate with chromatin and has been shown to bind oligonucleotides directly. ERK2 is activated by the upstream kinases MEK1/2, which phosphorylate both tyrosine 185 and threonine 183. ERK2 requires phosphorylation on both sites to be fully active. Some additional ERK2 phosphorylation sites have also been reported, including threonine 188. It has been suggested that this phospho form has distinct properties. We detected some ERK2 phosphorylated on T188 in bacterial preparations of ERK2 by mass spectrometry and further demonstrate that phosphomimetic substitution of this ERK2 residue impairs its kinase activity toward well-defined substrates and also affects its DNA binding. We used electrophoretic mobility shift assays with oligonucleotides derived from the insulin gene promoter and other regions to examine effects of phosphorylation and mutations on the binding of ERK2 to DNA. We show that ERK2 can bind oligonucleotides directly. Phosphorylation and mutations alter DNA binding and support the idea that signaling functions may be influenced through an alternate phosphorylation site.

Glucose Transporters in Cardiac Metabolism and Hypertrophy

The heart is adapted to utilize all classes of substrates to meet the high-energy demand, and it tightly regulates its substrate utilization in response to environmental changes. Although fatty acids are known as the predominant fuel for the adult heart at resting stage, the heart switches its substrate preference toward glucose during stress conditions such as ischemia and pathological hypertrophy. Notably, increasing evidence suggests that the loss of metabolic flexibility associated with increased reliance on glucose utilization contribute to the development of cardiac dysfunction. The changes in glucose metabolism in hypertrophied hearts include altered glucose transport and increased glycolysis. Despite the role of glucose as an energy source, changes in other nonenergy producing pathways related to glucose metabolism, such as hexosamine biosynthetic pathway and pentose phosphate pathway, are also observed in the diseased hearts. This article summarizes the current knowledge regarding the regulation of glucose transporter expression and translocation in the heart during physiological and pathological conditions. It also discusses the signaling mechanisms governing glucose uptake in cardiomyocytes, as well as the changes of cardiac glucose metabolism under disease conditions.

Functional O-GlcNAc modifications: implications in molecular regulation and pathophysiology

O-linked β-N-acetylglucosamine (O-GlcNAc) is a regulatory post-translational modification of intracellular proteins. The dynamic and inducible cycling of the modification is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) in response to UDP-GlcNAc levels in the hexosamine biosynthetic pathway (HBP). Due to its reliance on glucose flux and substrate availability, a major focus in the field has been on how O-GlcNAc contributes to metabolic disease. For years this post-translational modification has been known to modify thousands of proteins implicated in various disorders, but direct functional connections have until recently remained elusive. New research is beginning to reveal the specific mechanisms through which O-GlcNAc influences cell dynamics and disease pathology including clear examples of O-GlcNAc modification at a specific site on a given protein altering its biological functions. The following review intends to focus primarily on studies in the last half decade linking O-GlcNAc modification of proteins with chromatin-directed gene regulation, developmental processes, and several metabolically related disorders including Alzheimer's, heart disease and cancer. These studies illustrate the emerging importance of this post-translational modification in biological processes and multiple pathophysiologies.

Cardioprotective effect of ritonavir, an antiviral drug, in isoproterenol induced myocardial necrosis: a new therapeutic implication

Background

Ritonavir is a HIV protease inhibitor. In addition to its antiviral effect, Ritonavir directly inhibits the insulin-regulated glucose transporter GLUT4 and blocks glucose entry into fat and muscle cells. However, the effect of Ritonavir on cardiac GLUT4 inhibition during myocardial necrosis is not investigated. In the present study, we evaluated the role of Ritonavir in isoproterenol-induced myocardial necrosis in vivo and compared the effect with Phlorizin, a nonslective SGLTs inhibitor.

Methods

Isoproterenol (ISO) (150 mg/kg/day, i.p for 2 consecutive days) was administered to mice to cause myocardial necrosis. Phlorizin (400 mg/kg/day i.p twice daily for 2 days) and Ritonavir (10 mg/kg/day i.p twice daily for 2 days) were administered in two different groups of mice before isoproterenol administration.

Results and discussion

Isoproterenol (ISO) (150 mg/kg/day, i.p for 2 consecutive days) administration caused significant (p < 0.05) increase in heart/body weight ratio, and myocardial necrosis as evident by significant (p < 0.05) increase in serum markers i.e. SGOT and CK; and cardiac histopathological changes. Significant (p < 0.05) reduction in myocardial SOD and catalase activities, and GSH level along with a significant (p < 0.05) rise in myocardial TBARS and nitric oxide levels were observed after ISO administration. However, administration of phlorizin, a SGLT1 inhibitor has been found to exhibit partial protection in ISO induced myocardial necrosis, as observed by significant decrease in heart/body weight ratio and myocardial nitric oxide level; significant increase in myocardial SOD and catalase activities along with no histopathological alterations. On the other hand, administration of ritonavir, a nonspecific GLUT inhibitor has been found to exhibit complete protection as observed by normalisation of heart/body weight ratio, serum markers, antioxidant enzymes activities and histopathological alterations. In vitro study with heart homogenate confirmed no antioxidant effect of ritonavir and phlorizin in the absence and presence of isoproterenol.

Conclusions

Our study concluded that ritonavir, a nonspecific GLUT inhibitors showed complete protection in catecholamine induced myocardial necrosis.

O-GlcNAc signaling is essential for NFAT-mediated transcriptional reprogramming during cardiomyocyte hypertrophy

The regulation of cardiomyocyte hypertrophy is a complex interplay among many known and unknown processes. One specific pathway involves the phosphatase calcineurin, which regulates nuclear translocation of the essential cardiac hypertrophy transcription factor, nuclear factor of activated T-cells (NFAT). Although metabolic dysregulation is frequently described during cardiac hypertrophy, limited insights exist regarding various accessory pathways. One metabolically derived signal, beta-O-linked N-acetylglucosamine (O-GlcNAc), has emerged as a highly dynamic posttranslational modification of serine and threonine residues regulating physiological and stress processes. Given the metabolic dysregulation during hypertrophy, we hypothesized that NFAT activation is dependent on O-GlcNAc signaling. Pressure overload-induced hypertrophy (via transverse aortic constriction) in mice or treatment of neonatal rat cardiac myocytes with phenylephrine significantly enhanced global O-GlcNAc signaling. NFAT-luciferase reporter activity revealed O-GlcNAc-dependent NFAT activation during hypertrophy. Reversal of enhanced O-GlcNAc signaling blunted cardiomyocyte NFAT-induced changes during hypertrophy. Taken together, these results demonstrate a critical role of O-GlcNAc signaling in NFAT activation during hypertrophy and provide evidence that O-GlcNAc signaling is coordinated with the onset and progression of cardiac hypertrophy. This represents a potentially significant and novel mechanism of cardiac hypertrophy, which may be of particular interest in future in vivo studies of hypertrophy.

GnRH increases glucose transporter-1 expression and stimulates glucose uptake in the gonadotroph

GnRH is the main regulator of the hypothalamic-pituitary-gonadal (H-P-G) axis. GnRH stimulates the pituitary gonadotroph to synthesize and secrete gonadotrophins (LH and FSH), and this effect of GnRH is dependent on the availability of glucose and other nutrients. Little is known about whether GnRH regulates glucose metabolism in the gonadotroph. This study examined the regulation of glucose transporters (Gluts) by GnRH in the LβT2 gonadotroph cell line. Using real-time PCR analysis, the expression of Glut1, -2, -4, and -8 was detected, but Glut1 mRNA expression level was more abundant than the mRNA expression levels of Glut2, -4, and -8. After the treatment of LβT2 cells with GnRH, Glut1 mRNA expression was markedly induced, but there was no GnRH-induction of Glut2, -4, or -8 mRNA expression in LβT2 cells. The effect of GnRH on Glut1 mRNA expression is partly mediated by ERK activation. GnRH increased GLUT1 protein and stimulated GLUT1 translocation to the cell surface of LβT2 cells. Glucose uptake assays were performed in LβT2 cells and showed that GnRH stimulates glucose uptake in the gonadotroph. Finally, exogenous treatment of mice with GnRH increased the expression of Glut1 but not the expression of Glut2, -4, or -8 in the pituitary. Therefore, regulation of glucose metabolism by GnRH via changes in Gluts expression and subcellular location in the pituitary gonadotroph reveals a novel response of the gonadotroph to GnRH.

A novel cross-talk between endothelin-1 and cyclic AMP signaling pathways in the regulation of GLUT1 transcription in 3T3-L1 adipocytes

We showed previously that chronic exposure to both endothelin-1 (ET-1) and cAMP resulted in a synergistic increase in Glut1 transcription in 3T3-L1 adipocytes via a protein kinase C (PKC)-dependent mechanism. In the present study, we further examined the molecular mechanism involved. Employing transient transfections with Glut1 promoter/enhancer -luciferase reporter and several dominant negative or constitutively active PKC mutants, we identified PKCε as the responsible PKC. Investigation with deletion and mutation mutants of the promoter/enhancer reporter suggested that Sp1, CREB and AP-1 responsive elements on enhancer 2 were involved. Furthermore, chromatin immunoprecipitation and co-immunoprecipitation analysis were applied to characterize the interactions between these transcription factors and their bindings to enhancer 2 in vivo. The results indicate that there are both negative and positive interactions between ET-1 and cAMP signaling pathways. On the one hand, cAMP inhibits ET-1 induced NF-κB activation required for ET-1-stimulated Glut1 transcription; on the other hand, cAMP, via sustained CREB phosphorylation, may activate AP-1 and cooperate with ET-1-activated PKCε to enhance Sp1 expression and consequently to generate a stable enhancer 2-bound Sp1/pCREB/AP-1 complex, which can strongly facilitate Glut1 transcription more than the additive effect of ET-1 and cAMP alone.

Lipocalin-type prostaglandin D(2) synthase stimulates glucose transport via enhanced GLUT4 translocation

Previously, we demonstrated that lipocalin-type prostaglandin D(2) synthase (L-PGDS) knockout mice become glucose intolerant and display signs of diabetic nephropathy and accelerated atherosclerosis. In the current study we sought to explain the link between L-PGDS and glucose tolerance. Using the insulin-sensitive rat skeletal muscle cell line, L6, we showed that L-PGDS could stimulate glucose transport approximately 2-fold as well as enhance insulin-stimulated glucose transport, as measured by 2-deoxy-[(3)H]-glucose uptake. The increased glucose transport was not attributed to increased GLUT4 production but rather the stimulation of GLUT4 translocation to the plasma membrane, a phenomenon that was lost when cells were cultured under hyperglycemic (20 mM) conditions or pretreated with wortmannin. There was however, an increase in GLUT1 expression as well as a 3-fold increase in hexokinase III expression, which was increased to nearly 5-fold in the presence of insulin, in response to L-PGDS at 20 mM glucose. In addition, adipocytes isolated from L-PGDS knockout mice were significantly less sensitive to insulin-stimulated glucose transport than wild-type. We conclude that L-PGDS, via production of prostaglandin D(2), is an important mediator of muscle and adipose glucose transport which is modulated by glycemic conditions and plays a significant role in the glucose intolerance associated with type 2 diabetes.

Functional coupling of angiotensin II type 1 receptor with insulin resistance of energy substrate uptakes in immortalized cardiomyocytes (HL-1 cells)

BACKGROUND AND PURPOSE

Increased angiotensin II levels and insulin resistance coexist at the early stages of cardiomyopathies. To determine whether angiotensin II increases insulin resistance in cardiomyocytes, we studied the effect of angiotensin II on basal and insulin-stimulated transport rate of energy substrates in immortalized cardiomyocytes (HL-1 cells).

EXPERIMENTAL APPROACH

Glucose and palmitic acid uptakes were measured using [(3)H]2-deoxy-D-glucose and [(14)C]palmitic acid, respectively, in cells exposed or not exposed to angiotensin II (100 nM), angiotensin II plus irbesartan or PD123319, type 1 and 2 receptor antagonists, or PD98059, an inhibitor of ERK1/2 activation. Cell viability, DNA, protein synthesis and surface area were evaluated by the MTT test, [(3)H]thymydine, [(3)H]leucine and morphometric analysis, respectively. Type 1 receptor levels were measured by western blot analysis.

KEY RESULTS

Basal uptakes of glucose and palmitic acid by HL-1 cells (0.37+/-0.07 and 7.31+/-0.22 pmol per 10(4)cells per min, respectively) were both stimulated by 100 nM insulin (+91 and +64%, respectively). Cells exposed to angiotensin II remained viable and did not show signs of hypertrophy. In these conditions, the basal palmitic acid uptake of the cells increased (11.41+/-0.46 pmol per 10(4) cells per min) and insulin failed to stimulate the uptake of glucose and fatty acids. Changes in the rate of uptake of energy substrates were prevented or significantly reduced by irbesartan or PD98059.

CONCLUSIONS AND IMPLICATIONS

Angiotensin II is a candidate for increasing insulin resistance in cardiomyocytes. Our results suggest a further mechanism for the cardiovascular protection offered by the angiotensin II type 1 receptor blockers.

Regulation of myocardial glucose transporters GLUT1 and GLUT4 in chronically anemic fetal lambs

Little is known about the chronic adaptations that take place in the fetal heart to allow for increased substrate delivery in response to chronic stress. Because glucose is an important fuel for the fetal cardiomyocytes, we hypothesized that myocardial glucose transporters 1 and 4 (GLUT1 and GLUT4, respectively) are up-regulated in the fetal sheep heart that is chronically stressed by anemia. Fetal sheep at 128 d gestation underwent daily isovolumic hemorrhage and determination of myocardial blood flow, oxygen consumption, and substrate utilization. At the end of 3 or 7 d of anemia, myocardial levels of GLUT1 and GLUT4 mRNA and protein were measured and subcellular localization was determined. Despite stable heart rate and blood pressure, anemia caused a nearly 4-fold increase in right and left ventricular (RV and LV) free wall blood flow. No significant change in myocardial glucose uptake was found and serum insulin levels remained stable. Both 3-d RV and LV and 7-d RV mRNA and protein levels of GLUT1 and GLUT4 were unchanged; 7-d LV GLUT1 and GLUT4 mRNA levels were also stable. However, LV GLUT1 protein levels declined significantly, whereas LV GLUT4 protein levels were increased. In the steady state, GLUT4 protein localized to the sarcolemma membrane. These findings suggest that the glucose transporters are post-transcriptionally regulated in myocardium of chronically anemic fetal sheep with changes that mimic normal postnatal development. Unlike the postnatal heart, localization of GLUT4 to the cell membrane suggests the importance of GLUT4 in basal glucose uptake in the stressed fetal heart.

Developmental regulation and cellular distribution of human cytosolic malate dehydrogenase (MDH1)

Human cyotsolic malate dehydrogenase (MDH1) is important in transporting NADH equivalents across the mitochondrial membrane, controlling tricarboxylic acid (TCA) cycle pool size and providing contractile function. Cellular localization studies indicate that MDH1 mRNA expression has a strong tissue-specific distribution, being expressed primarily in cardiac and skeletal muscle and in the brain, at intermediate levels in the spleen, kidney, intestine, liver, and testes and at low levels in lung and bone marrow. The observed MDH1 localizations reflect the role of NADH in the support of a variety of functions in different organs. These functions are primarily related to aerobic energy production for muscle contraction, neuronal signal transmission, absorption/resorption functions, collagen-supporting functions, phagocytosis of dead cells, and processes related to gas exchange and cell division. During neonatal development, MDH1 is expressed in human embryonic heart as early as the 3rd month and then is over-expressed from the 5th month until the birth. The expression of MDH1 is maintained in the adult heart but is not present in levels as high as in the fetus. Finally, over-expression of MDH1 is found in left ventricular cardiac muscle of dilated cardiomyopathy (DCM) patients when contrasted to the diseased non-DCM and normal heart muscle by in situ hybridization and Western blot. These observations are compatible with the activation of glucose oxidation in relatively hypoxic environments of fetal and hypertrophied myocardium.

Synergistic effect of endothelin-1 and cyclic AMP on glucose transport in 3T3-L1 adipocytes

We have demonstrated previously that chronic exposure to endothlin-1 enhances glucose transport in 3T3-L1 adipocytes via augmented GLUT1 mRNA and protein accumulation. In the present study, we further examined the combined effect of endothelin-1 (ET-1) and cAMP on glucose transport. In cells pretreated with ET-1 and 8-bromo cAMP for 8 h, a synergy between these two agents on glucose uptake was found. Insulin-stimulated glucose transport, on the other hand, was only slightly affected. The synergistic effect of these two agents was suppressed in the presence of cycloheximide and actinomycin D. Immunoblot and Northern blot analyses revealed that GLUT1 protein and mRNA levels were both increased in cells pretreated with both ET-1 and 8-bromo cAMP, greater than the additive effect of each agent alone. Further examination demonstrated that the stability of GLUT1 mRNA was markedly enhanced in the presence of both ET-1 and cAMP. To investigate the transcriptional activation of Glut1 gene, transient transfection of cells with luciferase reporter construct driven by Glut1 promoter was performed. We found that Glut1 transcription was also increased by ET-1 and cAMP in a synergistic fashion. In addition, similar synergy between ET-1 and beta-adrenergic agonists on glucose transport was found. The synergistic action of ET-1 with 8-bromo cAMP to enhance glucose transport was inhibited by GF109203X, a selective protein kinase C (PKC) inhibitor, and was mimicked by 4beta-phorbol 12beta-myristate 13alpha-acetate (PMA), a PKC activator. Furthermore, PMA was found to act synergistically with 8-bromo cAMP to induce Glut1 transcription and ET-1 was shown to activate novel PKCdelta and PKC. Taken together, these results indicate that ET-1 may act with cAMP in a synergistic way to increase glucose transport, probably through enhanced GLUT1 expression via a PKC-dependent mechanism.

Diabetes can alter the signal transduction pathways in the lens of rats

Diabetes is known to affect cataract formation by means of osmotic stress induced by activated aldose reductase in the sorbitol pathway. In addition, alterations in the bioavailability of numerous extralenticular growth factors has been reported and shown to result in various consequences. We have found that the basic fibroblast growth factor (bFGF) accumulates in the vitreous humor of 3- and 8-week diabetic rats. Consequently, the associating signal transduction cascades were severely disrupted, including upregulated phosphorylation of extracellular signal-regulated kinase (ERK) and the common stress-associated mitogen-activated protein kinases p38 and SAPK/JNK. Conversely, under diabetic condition, we observed a dramatic inhibition of phosphatidylinositol-3 kinase activity in lenses obtained from the same animal. Rats treated with the aldose reductase inhibitor AL01576 for the duration of the diabetic condition showed that the diabetes-induced lenticular signaling alterations were normalized, comparable to controls. However, treatment of AL01576 in vitro was ineffective at normalizing the altered constituents in extracted diabetic vitreous after the onset of diabetes. The effect of AL01576 in the high galactose-induced cataract model in vitro was also examined. Administration of AL01576 to lens organ culture normalized the aberrant signaling effects and morphological characteristics associated with in vitro sugar cataract formation. In conclusion, our findings demonstrate diabetes-associated alterations in the lens signal transduction parameters and the effectiveness of AL01576 at normalizing such alterations. The causes for these alterations can be attributed to elevated vitreal bFGF in conjunction with osmotic stress and associated attenuation in redox status of the lens.

Hydroxymethylglutaryl coenzyme A reductase inhibitor simvastatin prevents cardiac hypertrophy induced by pressure overload and inhibits p21ras activation

BACKGROUND

Patients with cardiac hypertrophy are at increased cardiovascular risk. It has been hypothesized that hydroxymethylglutaryl coenzyme A reductase inhibitors may exert beneficial effects other than their cholesterol-lowering actions. The aims of the study were to assess the in vivo effects of simvastatin (SIM) on cardiac hypertrophy and on Ras signaling in rats with ascending aorta banding.

METHODS AND RESULTS

Wistar rats were randomized to receive either treatment with SIM or placebo, and then short-term (group I) and long-term (group II) left ventricular pressure overload was performed by placing a tantalum clip on ascending aorta. At the end of treatment period, left and right ventricular weight, body weight, and tibial length were measured and echocardiographic evaluations were performed. Ras signaling was investigated by analyzing Ras membrane localization and activation, ERK2 phosphorylation, and p27(kip1) and cdk4 levels. In SIM-treated rats, a significant reduction of left ventricular weight/body weight, echocardiographic left ventricular mass, and left ventricular end-diastolic diameter and end-diastolic pressure was found. In rats with pressure overload, SIM treatment significantly reduced Ras membrane targeting, Ras in vivo activation, ERK2 phosphorylation, and the ratio cdk4/p27(kip1).

CONCLUSIONS

HMG CoA inhibitor SIM inhibits in vivo Ras signaling and prevents left ventricular hypertrophy development in aortic-banded animals.

Cholinergic activation of glucose transport in bovine chromaffin cells involves calmodulin and protein kinase Czeta signaling

The aim of the present study was to delineate possible signaling pathways involved in acetylcholine (Ach)-induced glucose transport in chromaffin cells, a widely applied model system for sympathetic neurons. Acute Ach stimulation (10 min) enhanced the rate of glucose transport through activation of both nicotinic and muscarinic receptors. The calmodulin antagonist, W13, and the protein kinase C (PKC) inhibitor, staurosporine, each partially depressed Ach-induced glucose transport, with staurosporine exhibiting the stronger inhibitory effect. Pretreating the cells with phorbol 12-myristate 13-acetate (PMA) to downregulate PKC activity did not affect the nicotine-induced glucose transport, but completely attenuated that activated by muscarine, suggesting that Ach activation of transport involved both diacylglycerol-independent (PKCzeta) and diacylglycerol-dependent PKCs (PKCalpha/PKCepsilon). The PI 3-kinase inhibitor, wortmannin, diminished the Ach response, consistent with activation of the PKCs by the upstream PI 3-kinase-dependent phosphoinositide-dependent kinase, PDK1. Cholinergic activation strongly activated the ERK1/ERK2 cascade and p38 MAP kinase, but only p38 MAP kinase appeared to play a role, however minor, in nicotine-induced glucose uptake. The results are consistent with PKCs being more important than calmodulin in coupling cholinergic activation to glucose transport in chromaffin cells, but additional, yet unidentified, signaling pathways appear to be needed to obtain full activation of glucose transport in response to Ach.

Convergence of signaling pathways on the activation of ERK in B cells

The B cell receptor (BCR) initiates three major signaling pathways: the Ras pathway, which leads to extracellular signal-regulated kinase (ERK) activation; the phospholipase C-gamma pathway, which causes calcium mobilization; and the phosphoinositide 3-kinase (PI 3-kinase) pathway. These combine to induce different biological responses depending on the context of the BCR signal. Both the Ras and PI 3-kinase pathways are important for B cell development and activation. Several model systems show evidence of cross-regulation between these pathways. Here we demonstrate through the use of PI 3-kinase inhibitors and a dominant-negative PI 3-kinase construct that the BCR-induced phosphorylation and activation of ERK is dependent on PI 3-kinase. PI 3-kinase feeds into the Ras signaling cascade at multiple points, both upstream and downstream of Ras. We also show that ERK activation is dependent on phospholipase C-gamma, in keeping with its dependence on calcium mobilization. Last, the activation of PI 3-kinase itself is completely dependent on Ras. We conclude that the PI 3-kinase and Ras signaling cascades are intimately connected in B cells and that the activation of ERK is a signal integration point, since it requires simultaneous input from all three major signaling pathways.

Insulin signaling coordinately regulates cardiac size, metabolism, and contractile protein isoform expression

To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20-30% due to reduced cardiomyocyte size and had persistent expression of the fetal beta-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial beta-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart.

Chronic acarbose-feeding increases GLUT1 protein without changing intestinal glucose absorption function

As alpha-glucosidase inhibitor, the antidiabetic drug acarbose reduces postprandial glucose levels by retarding the intestinal digestion of polysaccharides. However, it is unknown if acarbose also affects the expression of intestinal glucose transporters, especially the Na(+)-glucose cotransporter (SGLT1) and the glucose transporters GLUT1 and GLUT2. To unravel this question, Wistar rats received standard powdered chow either without (control) or with acarbose (40 mg acarbose/100 g chow) for 40 days. While food intake was slightly enhanced by acarbose, the drug had no influence on weight gain or plasma glucose and insulin levels. The acarbose-treatment did not alter the SGLT1 and GLUT2 gene expression in both upper and middle small intestine, whereas GLUT1 protein was increased by 75% in middle small intestine. Despite the territorial change in GLUT1 protein, the intestinal glucose absorption in an acarbose-free perfusion study was unaltered. In conclusion, the chronic use of acarbose did not alter the acarbose-free glucose absorption profile.

The acute and chronic stimulatory effects of endothelin-1 on glucose transport are mediated by distinct pathways in 3T3-L1 adipocytes

We have recently shown that pretreatment with endothelin-1 (ET-1) for 20 min stimulates GLUT4 translocation in a PI3-kinase-dependent manner in 3T3-L1 adipocytes (Imamura, T. et al., J Biol Chem 274:33691-33695). This study presents another pathway by which ET-1 potentiates glucose transport in 3T3-L1 adipocytes. ET-1 treatment (10 nM) leads to approximately 2.5-fold stimulation of 2-deoxyglucose (2-DOG) uptake within 20 min, reaching a maximal effect of approximately 4-fold at approximately 6 h, and recovering almost to basal levels after 24 h. Insulin treatment (3 ng/ml) results in an approximately 5-fold increase in 2-DOG uptake at 1 h, and recovering to basal levels after 24 h. The ETA receptor antagonist, BQ 610, inhibited ET-1 induced glucose uptake both at 20 min and 6 h, whereas the ETB receptor antagonist, BQ 788, was without effect. Interestingly, ET-1 stimulated 2-DOG uptake at 6 h, not at 20 min, was almost completely blocked by the protein-synthesis inhibitor, cycloheximide and the RNA-synthesis inhibitor, actinomycin D, suggesting that the short-term (20 min) and long-term (6 h) effects of ET-1 involve distinct mechanisms. GLUT4 translocation assay showed that 20 min, but not 6 h, exposure to ET-1 led to GLUT4 translocation to the plasma membrane. In contrast, 6 h, but not 20 min, exposure to ET-1 increased expression of the GLUT1 protein, without affecting expression of GLUT4 protein. ET-1 induced 2-DOG uptake and GLUT1 expression at 6 h were completely inhibited by the MEK inhibitor, PD 98059, and partially inhibited by the PI3-kinase inhibitor, LY 294002, and the G alpha i inhibitor, pertussis toxin. The PLC inhibitor, U 73122, was without effect. These findings suggest that ET-1 induced GLUT1 protein expression is primarily mediated via MAPK, and partially via PI3K in 3T3-L1 adipocytes.

Metabolic adaptation of the hypertrophied heart: role of the malate/aspartate and alpha-glycerophosphate shuttles

Activation of the malate/aspartate and alpha -glycerophosphate shuttles (the NADH shuttles) has been identified in glycolytically active newborn myocardium. The goal of this study was to determine if the NADH shuttles and their regulatory genes are activated in hypertrophied myocardium as substrate utilization shifts away from fatty acids and toward glucose and lactate. Capacity of the shuttles was determined in cardiac mitochondria isolated one week, one month, and three months following aortic banding or sham operation. Myocardial steady-state mRNA and protein levels of regulatory enzymes were also measured. Despite a significant increase in left ventricular mass and activation of the atrial natriuretic peptide gene, no change in malate/aspartate nor alpha -glycerophosphate shuttle capacity was found at any of the three time points studied. Reactivation of the genes encoding the regulatory inner mitochondrial membrane proteins was not found in the hypertrophied myocardium, though these genes were down regulated one week following aortic-banding. These results suggest that sufficient malate/aspartate and alpha -glycerophosphate shuttle capacity exists in cardiac mitochondria to accommodate increased shuttle flux as hypertrophied myocardium becomes more glycolytically active.

Altered expression of proteins of metabolic regulation during remodeling of the left ventricle after myocardial infarction

Non-infarcted myocardium after coronary occlusion undergoes progressive morphological and functional changes. The purpose of this study was to determine whether non-infarcted myocardium exhibits (1) alteration of the substrate pattern of myocardial metabolism and (2) concomitant changes in the expression of regulatory proteins of glucose and fatty acid metabolism. Myocardial infarction was induced in rats by ligation of the left coronary artery. One day and eight weeks after coronary occlusion, glucose and palmitate oxidation were measured. Expression of selected proteins of metabolism were determined one day to 12 weeks after infarction. One day after coronary occlusion no difference of glucose and palmitate oxidation was detectable, whereas after eight weeks, glucose oxidation was increased (+84%, P<0.05) and palmitate oxidation did not change significantly (-19%, P=0.07) in infarct-containing hearts, compared with hearts from sham-operated rats. One day after coronary occlusion, myocardial mRNA expression of the glucose transporter GLUT-1 was increased (+86%, P<0.05) and the expression of GLUT-4 was decreased (-28%, P<0.05) in surviving myocardium of infarct-containing hearts. Protein level of GLUT-1 was increased (+81%, P<0.05) and that of GLUT-4 slightly, but not significantly, decreased (-16%, P=NS). mRNA expressions of heart fatty acid binding protein (H-FABP), and of medium chain acyl-CoA dehydrogenase (MCAD), were decreased by 36% (P<0.05) and 35% (P=0. 07), respectively. Eight weeks after acute infarction, the left ventricle was hypertrophied and, at this time-point, there was no difference in the expression of GLUT-1 and GLUT-4 between infarcted and sham-operated hearts. However, myocardial mRNA and protein content of MCAD were decreased by 30% (P<0.01) and 27% (P<0.05), respectively. In summary, in surviving myocardium, glucose oxidation was increased eight weeks after coronary occlusion. Concomitantly, mRNA and protein expression of MCAD were decreased, compatible with a role of altered expression of regulatory proteins of metabolism in post-infarction modification of myocardial metabolism.

Differential regulation of three glucose transporter genes in a renal epithelial cell line

Three hexose transporter genes, the Na(+)/glucose cotransporters SGLT1 and SGLT3 (formerly SAAT1/pSGLT2) and the facilitative transporter GLUT1, are expressed in a renal epithelial cell line with proximal tubule characteristics. A number of studies have demonstrated that SGLT1 expression is coupled to the cellular differentiation state and is also negatively regulated by its substrate glucose. In the present study, we demonstrate that SGLT3 mRNA expression is relatively unaffected by conditions promoting dedifferentiation (reseeding to a subconfluent density, activation of protein kinase C) or differentiation (confluent cell density, activation of protein kinase A) nor was expression sensitive to hyperglycemic glucose levels in the medium. We further demonstrate that protein kinase A and protein kinase C exert opposing effects on GLUT1 and SGLT1 mRNA levels in polarized cell monolayers, indicating that GLUT1 mRNA is also highly regulated in polarized epithelial cells by agents affecting cell differentiation. The relatively constitutive expression of SGLT3 mRNA suggests a novel role for this low-affinity Na(+)/glucose cotransporter, to provide concentrative glucose uptake under hyperglycemic conditions where expression of high-affinity glucose cotransporter SGLT1 mRNA is significantly downregulated.

The Akt-glycogen synthase kinase 3beta pathway regulates transcription of atrial natriuretic factor induced by beta-adrenergic receptor stimulation in cardiac myocytes

We examined the mechanism of atrial natriuretic factor (ANF) transcription by isoproterenol (ISO), an agonist for the beta-adrenergic receptor (betaAR), in cardiac myocytes. ISO only modestly activated members of the mitogen-activated protein kinase family. ISO-induced ANF transcription was not affected by inhibition of mitogen-activated protein kinases, whereas it was significantly inhibited by KN93, an inhibitor of Ca(2+)/calmodulin-dependent kinase (CaM kinase II). Production of 3'-phosphorylated phosphatidylinositides (3 phosphoinositides) was also required for ISO-induced ANF transcription. ISO caused phosphorylation (Ser-473) and activation of Akt through CaM kinase II- and 3 phosphoinositides-dependent mechanisms. Constitutively active Akt increased myocyte surface area, total protein content, and ANF expression, whereas dominant negative Akt blocked ISO-stimulated ANF transcription. ISO caused Ser-9 phosphorylation and decreased activities of GSK3beta. Overexpression of GSK3beta inhibited ANF transcription, which was reversed by ISO. ISO failed to reverse the inhibitory effect of GSK3beta(S9A), an Akt-insensitive mutant. Kinase-inactive GSK3beta increased ANF transcription. Cyclosporin A partially inhibited ISO-stimulated ANF transcription, indicating that calcineurin only partially mediates ANF transcription. These results suggest that both CaM kinase II and 3 phosphoinositides mediate betaAR-induced Akt activation and ANF transcription in cardiac myocytes. Furthermore, betaAR-stimulated ANF transcription is predominantly mediated by activation of Akt and subsequent phosphorylation/inhibition of GSK3beta.

A Raf-induced, MEK-independent signaling pathway regulates atrial natriuretic factor gene expression in cardiac muscle cells

The atrial natriuretic factor (ANF) gene is activated in cardiac myocytes by Ras and its effector Raf. However, MEK, the best-characterized Raf substrate, cannot efficiently activate ANF suggesting that Raf uses a MEK-independent pathway to activate ANF. By manipulating MEK and Raf activities so that they are equally effective at activating ERK, we now demonstrate that Raf activates at least two signaling pathways in cardiac myocytes that regulate the ANF promoter; the MEK-->ERK pathway inhibits ANF gene expression while a Raf-induced, MEK-independent pathway activates expression. This mechanism may provide increased ability to regulate ANF expression in response to hypertrophic stimuli.