Calcium-independent inhibition of glucose transport in PC-12 and L6 cells by calcium channel antagonists

Photoinduced Reductive [4 + 2] Cycloaddition of Nitroarenes

Direct C-N bond formation from nitroarenes is a valuable synthetic tool for quick access to aniline derivatives. Transformation via nitroso intermediates could be useful due to their unique properties, but the generation of nitrosoarenes in situ from nitroarenes is challenging due to the tendency for various side reactions. Herein we report the photoinduced reductive [4 + 2] cycloaddition of nitroarenes. The method begins with the photoinduced reduction of nitroarenes to give nitrosoarenes by hydrogen atom abstraction (HAA). The generated nitrosoarenes undergo a nitroso Diels-Alder (NDA) reaction. The key to success is the use of N-heterocyclic carbene (NHC) borane, which promotes efficient HAA, enabling the NDA reaction to proceed without the need for transition metals, strong bases, or elevated temperatures. The developed conditions allow high functional group tolerance, enabling late-stage functionalization and further derivatization of biologically active compounds.

Adverse Metabolic Effects of Diltiazem Treatment During Diabetic Cardiomyopathy

Diabetes is a global epidemic disease, which leads to multiorgan dysfunction, including heart disease. Diabetes results from the limited absorption of glucose into insulin-sensitive tissues. The heart is one of the main organs to utilize glucose as an energy substrate. Glucose uptake into striated muscle is regulated by a family of membrane proteins called glucose transporters (GLUTs). Although calcium channel blockers, including diltiazem, are widely prescribed drugs for cardiovascular diseases, including in patients with diabetes, their pharmacological effects on glucose metabolism are somewhat controversial. We hypothesized that diltiazem treatment will exhibit detrimental effects on whole body glucose homeostasis and glucose transport in the striated muscle of patients with diabetes. Healthy and streptozotocin-treated rats were randomly assigned to receive diltiazem treatment or a placebo for 8 weeks. Blood glucose was significantly increased in the untreated diabetic groups, which worsened after diltiazem treatment. Diabetes decreased protein content of both GLUT4 (the predominate insulin-sensitive glucose transporter) and AS160 (Akt Substrate at 160 kDa, the downstream protein in the signaling cascade that regulates GLUT4 trafficking) in striated muscle of diabetic rats, with a more pronounced alteration after diltiazem treatment. We additionally reported that diabetic rodents displayed marked systolic dysfunction, which was not rescued by diltiazem treatment. In conclusion, diltiazem treatment worsened the effects of diabetes-induced hyperglycemia and diabetes-induced alterations in the regulation of glucose transport in striated muscle.

Ion channels in the regulation of autophagy

Autophagy is a cellular process in which the cell degrades and recycles its own constituents. Given the crucial role of autophagy in physiology, deregulation of autophagic machinery is associated with various diseases. Hence, a thorough understanding of autophagy regulatory mechanisms is crucially important for the elaboration of efficient treatments for different diseases. Recently, ion channels, mediating ion fluxes across cellular membranes, have emerged as important regulators of both basal and induced autophagy. However, the mechanisms by which specific ion channels regulate autophagy are still poorly understood, thus underscoring the need for further research in this field. Here we discuss the involvement of major types of ion channels in autophagy regulation.

Verapamil treatment induces cytoprotective autophagy by modulating cellular metabolism

Verapamil, an L-type calcium channel blocker, has been used successfully to treat cardiovascular diseases. Interestingly, we have recently shown that treatment of cancer cells with verapamil causes an effect on autophagy. As autophagy is known to modulate chemotherapy responses, this prompted us to explore the impact of verapamil on autophagy and cell viability in greater detail. We report here that verapamil causes an induction of autophagic flux in a number or tumor cells and immortalized normal cells. Moreover, we found that inhibition of autophagy in COLO 205 cells, via treatment with the chloroquine (CQ) or by CRISPR/Cas9-mediated disruption of the autophagy genes Atg7 and Atg5, causes an upregulation of apoptotic markers in response to verapamil. In search of a mechanism for this effect and because autophagy can often mitigate metabolic stress, we examined the impact of verapamil on cellular metabolism. This revealed that in normal prostate cells, verapamil diminishes glucose and glycolytic intermediate levels leading to adenosine 5'-triphosphate (ATP) depletion. In contrast, in COLO 205 cells it enhances aerobic glycolysis and maintains ATP. Importantly, we found that the autophagic response in these cells is related to the activity of l-lactate dehydrogenase A (LDHA, EC 1.1.1.27), as inhibition of LDHA reduces both basal and verapamil-induced autophagy and consequently decreases cell viability. In summary, these findings not only identify a novel mechanism of cytoprotective autophagy induction but they also highlight the potential of using verapamil together with inhibitors of autophagy for the treatment of malignant disease. ENZYMES: l-lactate dehydrogenase (LDHA, EC 1.1.1.27).

Calcium homeostasis and ER stress in control of autophagy in cancer cells

Autophagy is a basic catabolic process, serving as an internal engine during responses to various cellular stresses. As regards cancer, autophagy may play a tumor suppressive role by preserving cellular integrity during tumor development and by possible contribution to cell death. However, autophagy may also exert oncogenic effects by promoting tumor cell survival and preventing cell death, for example, upon anticancer treatment. The major factors influencing autophagy are Ca²⁺ homeostasis perturbation and starvation. Several Ca²⁺ channels like voltage-gated T- and L-type channels, IP3 receptors, or CRAC are involved in autophagy regulation. Glucose transporters, mainly from GLUT family, which are often upregulated in cancer, are also prominent targets for autophagy induction. Signals from both Ca²⁺ perturbations and glucose transport blockage might be integrated at UPR and ER stress activation. Molecular pathways such as IRE 1-JNK-Bcl-2, PERK-eIF2α-ATF4, or ATF6-XBP 1-ATG are related to autophagy induced through ER stress. Moreover ER molecular chaperones such as GRP78/BiP and transcription factors like CHOP participate in regulation of ER stress-mediated autophagy. Autophagy modulation might be promising in anticancer therapies; however, it is a context-dependent matter whether inhibition or activation of autophagy leads to tumor cell death.

T-type Ca2+ channel blocker, KYS05090 induces autophagy and apoptosis in A549 cells through inhibiting glucose uptake

It has been reported that [3-(1,1'-biphenyl-4-yl)-2-(1-methyl-5-dimethylamino-pentylamino)-3,4-dihydroquinazolin-4-yl]-N-benzylacetamide 2hydrochloride (KYS05090), a selective T-type Ca²⁺ channel blocker, reduces tumor volume and weight in the A549 xenograft model, but the molecular mechanism of cell death has not yet been elucidated. In this study, KYS05090 induced autophagy- and apoptosis-mediated cell death in human lung adenocarcinoma A549 cells. Although KYS05090 decreased intracellular Ca²⁺ levels, it was not directly related with KYS05090-induced cell death. In addition, KYS05090 generated intracellular reactive oxygen species (ROS) and reduced glucose uptake, and catalase and methyl pyruvate prevented KYS05090-induced cell death. These results indicate that KYS05090 can lead to autophagy and apoptosis in A549 cells through ROS generation by inhibiting glucose uptake. Our findings suggest that KYS05090 has potential chemotherapeutic value for the treatment of lung cancer.

Calcium influx through L-type channels attenuates skeletal muscle contraction via inhibition of adenylyl cyclases

Skeletal muscle contraction is triggered by acetylcholine induced release of Ca(2+) from sarcoplasmic reticulum. Although this signaling pathway is independent of extracellular Ca(2+), L-type voltage-gated calcium channel (Cav) blockers have inotropic effects on frog skeletal muscles which occur by an unknown mechanism. Taking into account that skeletal muscle fiber expresses Ca(+2)-sensitive adenylyl cyclase (AC) isoforms and that cAMP is able to increase skeletal muscle contraction force, we investigated the role of Ca(2+) influx on mouse skeletal muscle contraction and the putative crosstalk between extracellular Ca(2+) and intracellular cAMP signaling pathways. The effects of Cav blockers (verapamil and nifedipine) and extracellular Ca(2+) chelator EGTA were evaluated on isometric contractility of mouse diaphragm muscle under direct electrical stimulus (supramaximal voltage, 2 ms, 0.1 Hz). Production of cAMP was evaluated by radiometric assay while Ca(2+) transients were assessed by confocal microscopy using L6 cells loaded with fluo-4/AM. Ca(2+) channel blockers verapamil and nifedipine had positive inotropic effect, which was mimicked by removal of extracellular Ca(+2) with EGTA or Ca(2+)-free Tyrode. While phosphodiesterase inhibitor IBMX potentiates verapamil positive inotropic effect, it was abolished by AC inhibitors SQ22536 and NYK80. Finally, the inotropic effect of verapamil was associated with increased intracellular cAMP content and mobilization of intracellular Ca(2+), indicating that positive inotropic effects of Ca(2+) blockers depend on cAMP formation. Together, our results show that extracellular Ca(2+) modulates skeletal muscle contraction, through inhibition of Ca(2+)-sensitive AC. The cross-talk between extracellular calcium and cAMP-dependent signaling pathways appears to regulate the extent of skeletal muscle contraction responses.

Metabolic flux analysis gives an insight on verapamil induced changes in central metabolism of HL-1 cells

Verapamil has been shown to inhibit glucose transport in several cell types. However, the consequences of this inhibition on central metabolism are not well known. In this study we focused on verapamil induced changes in metabolic fluxes in a murine atrial cell line (HL-1 cells). These cells were adapted to serum free conditions and incubated with 4 μM verapamil and [U-¹³C₅] glutamine. Specific extracellular metabolite uptake/production rates together with mass isotopomer fractions in alanine and glutamate were implemented into a metabolic network model to calculate metabolic flux distributions in the central metabolism. Verapamil decreased specific glucose consumption rate and glycolytic activity by 60%. Although the HL-1 cells show Warburg effect with high lactate production, verapamil treated cells completely stopped lactate production after 24 h while maintaining growth comparable to the untreated cells. Calculated fluxes in TCA cycle reactions as well as NADH/FADH₂ production rates were similar in both treated and untreated cells. This was confirmed by measurement of cell respiration. Reduction of lactate production seems to be the consequence of decreased glucose uptake due to verapamil. In case of tumors, this may have two fold effects; firstly depriving cancer cells of substrate for anaerobic glycolysis on which their growth is dependent; secondly changing pH of the tumor environment, as lactate secretion keeps the pH acidic and facilitates tumor growth. The results shown in this study may partly explain recent observations in which verapamil has been proposed to be a potential anticancer agent. Moreover, in biotechnological production using cell lines, verapamil may be used to reduce glucose uptake and lactate secretion thereby increasing protein production without introduction of genetic modifications and application of more complicated fed-batch processes.

Verapamil inhibits the glucose transport activity of GLUT1

Calcium channel blocker toxicity has been associated with marked hyperglycemia responsive only to high-dose insulin therapy. The exact mechanism(s) of this induced hyperglycemia has not been clearly delineated. The glucose transporter GLUT1 is expressed in a wide variety of cell types and is largely responsible for a basal level of glucose transport. GLUT1 also is activated by cell stress. The specific purpose of this study was to investigate the effects of the calcium channel blocker verapamil on the glucose uptake activity of GLUT1 in L929 fibroblasts cells. Dose-dependent effects of verapamil on glucose uptake were studied using L929 fibroblast cells with 2-deoxyglucose. Verapamil had a dose-dependent inhibitory effect on both basal and stress-activated transport activity of GLUT1. Basal activity was inhibited 50% by 300 μM verapamil, while 150 μM verapamil completely inhibited the activation induced by the stress of glucose deprivation. These effects were reversible and required verapamil to be present during the stress. Alteration of calcium concentrations by addition of 5 mM CaCl₂ or 4 mM EDTA had no effect on verapamil action. This study reveals the unique finding that verapamil has inhibitory effects on the transport activity of GLUT1 independent of its effects on calcium concentrations. The inhibition of GLUT1 may be one of the contributing factors to the hyperglycemia observed in CCB poisoning.

Transcriptome profiling of neuronal model cell PC12 from rat pheochromocytoma

GeneChip microarray is a cutting-edge technology being used to study the expression patterns of genes with in a particular cell type. In this study, the Affymetrix RAE230A platform was used to profile stably expressed mRNA transcripts from PC12 cells at passage 5 and 15. The whole-cell PC12 transcriptome revealed that a total of 7,531 stable transcripts (P < 0.05), corresponding to 6,785 genes, were found to be consistently expressed between passage 5 and 15. The data analysis revealed 3,080 functional proteins, belonging to 13 families, which indicate that about 65% of the proteins expressed in PC12 cells are uncharacterized. By using our custom-built rat neuronal reference genome database, we mapped endogenously expressed stable neuronal transcripts from PC12 cells comprising about 765 genes responsible for neuronal function and disease. These neuronal transcripts were further analyzed to provide a genetic blueprint that can be used by neurobiologist to unravel the complex cellular and molecular mechanisms underlying biological functions and their associated signalling networks for diseases affecting the nervous system.

Verapamil toxicity dysregulates the phosphatidylinositol 3-kinase pathway

OBJECTIVES

Recent animal research and clinical case reports suggest benefit from high-dose insulin therapy (HDIT) for the treatment of calcium channel blocker (CCB) toxicity. One molecular signaling pathway, the phosphatidylinositol 3-kinase (PI3K) pathway, that contributes to CCB toxicity and the efficacy of HDIT, was examined for a role in this phenomenon.

METHODS

A differentiated 3T3-L1 adipocyte model system was utilized to characterize metabolic and molecular signaling events dysregulated in response to acute CCB toxicity. Glucose uptake assays were performed in the presence of representatives of three classes of CCB drugs, and the ability of HDIT to reverse observed inhibition was assessed. Western blot analyses were utilized to probe which insulin-dependent signaling pathway was inhibited by CCB toxicity.

RESULTS

Representative compounds from the dihydropyridine and phenylalkylamine classes of CCBs were more effective at inhibiting glucose uptake in differentiated 3T3-L1 adipocytes than a representative from the benzothiazepine class. Phosphorylation at serine 473 of the Akt protein (P-Akt), a protein representing a common pathway for insulin receptors (IR), insulinlike growth factor receptors (IGFR), and hybrid receptors formed by IR and IGFR subunits, was abolished in the presence of toxic doses of the phenylalkylamine CCB verapamil. Phosphorylation at serine 473 of Akt was rescued in the presence high concentrations of insulin.

CONCLUSIONS

These data suggest that dysregulation of the insulin-dependent PI3K pathway is partially responsible for insulin resistance and the hyperglycemic state observed in response to acute CCB toxicity.

Glucose transport activation in human hematopoietic cells M07e is modulated by cytosolic calcium and calmodulin

The aim of this work was to investigate the role of cytosolic calcium and calmodulin-dependent systems in the activation of glucose uptake in the human megakaryocytic cell line M07e. Glucose uptake was significantly raised by elevation of cytosolic Ca(2+) concentration ([Ca(2+)](c)) with thapsigargin, this effect being additive to the activation induced by cytokines (SCF, GM-CSF and TPO) and hydrogen peroxide. Intracellular Ca(2+) chelation by BAPTA decreased basal and activated glucose uptake in a dose-dependent manner. BAPTA reduced the GLUT1 translocation induced by SCF and H(2)O(2), suggesting a major role for Ca(2+) in GLUT1 intracellular trafficking. In the absence of extracellular Ca(2+), 2-aminoethoxydiphenyl-borate (2-APB) abolished the activation of glucose uptake induced by cytokines and H(2)O(2) suggesting an involvement in GLUT1 regulation in responses related to InsP(3)-induced Ca(2+) release. Under our experimental conditions, all the stimuli inducing glucose uptake activation failed to increase [Ca(2+)](c) suggesting that cytosolic Ca(2+) plays a permissive role in the regulation of GLUT1. The calmodulin antagonist W-7 and the inhibitor of Ca(2+)-calmodulin dependent protein kinase II (CAMK II) KN-62 removed the glucose transport activation by all the tested stimuli. These results suggest that in M07e cells calmodulin and CAMKII are involved in GLUT1 stimulation by cytokines and ROS.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

A role for calcium/calmodulin kinase in insulin stimulated glucose transport

Previous research has shown that the CAMK (calcium/calmodulin dependent protein kinase) inhibitor, KN62, can lead to reductions in insulin stimulated glucose transport. Although controversial, an L-type calcium channel mechanism has also been hypothesized to be involved in insulin stimulated glucose transport. The purpose of this report was to determine if 1) L-type calcium channels and CAMK are involved in a similar signaling pathway in the control of insulin stimulated glucose transport and 2) determine if insulin induces an increase in CAMKII phosphorylation through an L-type calcium channel dependent mechanism. Insulin stimulated glucose transport was significantly (p<0.05) inhibited to a similar extent ( approximately 30%) by both KN62 and nifedipine in rat soleus and epitrochelaris muscles. The new finding of these experiments was that the combined inhibitory effect of these two compounds was not greater than the effect of either inhibitor alone. To more accurately determine the interaction between CAMK and L-type calcium channels, we measured insulin induced changes in CAMKII phosphorylation using Western blot analysis. The novel finding of this set of experiments was that insulin induced an increase in phosphorylated CAMKII ( approximately 40%) in rat soleus muscle that was reversed in the presence of KN62 but not nifedipine. Taken together these results suggest that a CAMK signaling mechanism may be involved in insulin stimulated glucose transport in skeletal muscle through an L-type calcium channel independent mechanism.

Induction of hyperglycemia in mice with atypical antipsychotic drugs that inhibit glucose uptake

Many antipsychotic drugs disturb the regulation of glucose metabolism in patients treated for schizophrenia. The goal of the present studies was to determine if these antipsychotic drugs produce hyperglycemia in mice in relation to their ability to interfere with glucose uptake and utilization. Male C57BL/6 mice were injected with a panel of typical and atypical antipsychotic drugs and blood glucose levels were determined periodically over a 3- to 6-h time interval. The atypical drugs, clozapine, desmethylclozapine, quetiapine, and loxapine, and the original antipsychotic, chlorpromazine, induced significant hyperglycemia in the mice in accordance with their effects on glucose transport. By contrast, haloperidol and sulpiride, which have little effect on glucose uptake, did not induce hyperglycemia. Risperidone produced a modest elevation of blood glucose levels, but only at a low dose of the drug. Cytochalasin B, a specific inhibitor of the glucose transporter (GLUT) protein, produced significant hyperglycemia in the mice. Overall, there was a strong correlation between the ability of a drug to inhibit glucose transport in vitro and its ability to induce hyperglycemia in vivo. Finally, the drugs that produced hyperglycemia in mice have been linked to the development of diabetes in patients.