Insulin-induced formation of macromolecular complexes involved in activation of cyclic nucleotide phosphodiesterase 3B (PDE3B) and its interaction with PKB

The Molecular Brakes of Adipose Tissue Lipolysis

Adaptation to changes in energy availability is pivotal for the survival of animals. Adipose tissue, the body’s largest reservoir of energy and a major source of metabolic fuel, exerts a buffering function for fluctuations in nutrient availability. This functional plasticity ranges from energy storage in the form of triglycerides during periods of excess energy intake to energy mobilization via lipolysis in the form of free fatty acids for other organs during states of energy demands. The subtle balance between energy storage and mobilization is important for whole-body energy homeostasis; its disruption has been implicated as contributing to the development of insulin resistance, type 2 diabetes and cancer cachexia. As a result, adipocyte lipolysis is tightly regulated by complex regulatory mechanisms involving lipases and hormonal and biochemical signals that have opposing effects. In thermogenic brown and brite adipocytes, lipolysis stimulation is the canonical way for the activation of non-shivering thermogenesis. Lipolysis proceeds in an orderly and delicately regulated manner, with stimulation through cell-surface receptors via neurotransmitters, hormones, and autocrine/paracrine factors that activate various intracellular signal transduction pathways and increase kinase activity. The subsequent phosphorylation of perilipins, lipases, and cofactors initiates the translocation of key lipases from the cytoplasm to lipid droplets and enables protein-protein interactions to assemble the lipolytic machinery on the scaffolding perilipins at the surface of lipid droplets. Although activation of lipolysis has been well studied, the feedback fine-tuning is less well appreciated. This review focuses on the molecular brakes of lipolysis and discusses some of the divergent fine-tuning strategies in the negative feedback regulation of lipolysis, including delicate negative feedback loops, intermediary lipid metabolites-mediated allosteric regulation and dynamic protein–protein interactions. As aberrant adipocyte lipolysis is involved in various metabolic diseases and releasing the brakes on lipolysis in thermogenic adipocytes may activate thermogenesis, targeting adipocyte lipolysis is thus of therapeutic interest.

Protein-Protein Interactions of Phosphodiesterases

BACKGROUND

Phosphodiesterases (PDEs) are enzymes that play a key role in terminating cyclic nucleotides signalling by catalysing the hydrolysis of 3', 5'- cyclic adenosine monophosphate (cAMP) and/or 3', 5' cyclic guanosine monophosphate (cGMP), the second messengers within the cell that transport the signals produced by extracellular signalling molecules which are unable to get into the cells. However, PDEs are proteins which do not operate alone but in complexes that made up of a many proteins.

OBJECTIVE

This review highlights some of the general characteristics of PDEs and focuses mainly on the Protein-Protein Interactions (PPIs) of selected PDE enzymes. The objective is to review the role of PPIs in the specific mechanism for activation and thereby regulation of certain biological functions of PDEs.

METHODS

The article discusses some of the PPIs of selected PDEs as reported in recent scientific literature. These interactions are critical for understanding the biological role of the target PDE.

RESULTS

The PPIs have shown that each PDE has a specific mechanism for activation and thereby regulation a certain biological function.

CONCLUSION

Targeting of PDEs to specific regions of the cell is based on the interaction with other proteins where each PDE enzyme binds with specific protein(s) via PPIs.

Phosphodiesterase 3B (PDE3B) antagonizes the anti-angiogenic actions of PKA in human and murine endothelial cells

Recent reports show that protein kinase A (PKA), but not exchange protein activated by cAMP (EPAC), acts in a cell autonomous manner to constitutively reduce the angiogenic sprouting capacity of murine and human endothelial cells. Specificity in the cellular actions of individual cAMP-effectors can be achieved when a cyclic nucleotide phosphodiesterase (PDE) enzyme acts locally to control the "pool" of cAMP that activates the cAMP-effector. Here, we examined whether PDEs coordinate the actions of PKA during endothelial cell sprouting. Inhibiting each of the cAMP-hydrolyzing PDEs expressed in human endothelial cells revealed that phosphodiesterase 3 (PDE3) inhibition with cilostamide reduced angiogenic sprouting in vitro, while inhibitors of PDE2 and PDE4 family enzymes had no such effect. Identifying a critical role for PDE3B in the anti-angiogenic effects of cilostamide, silencing this PDE3 variant, but not PDE3A, markedly impaired sprouting. Importantly, using both in vitro and ex vivo models of angiogenesis, we show the hypo-sprouting phenotype induced by PDE3 inhibition or PDE3B silencing was reversed by PKA inhibition. Examination of the individual cellular events required for sprouting revealed that PDE3B and PKA each regulated angiogenic sprouting by controlling the invasive capacity of endothelial cells, more specifically, by regulating podosome rosette biogenesis and matrix degradation. In support of the idea that PDE3B acts to inhibit angiogenic sprouting by limiting PKA-mediated reductions in active cdc42, the effects of PDE3B and/or PKA on angiogenic sprouting were negated in cells with reduced cdc42 expression or activity. Since PDE3B and PKA were co-localized in a perinuclear region in human ECs, could be co-immunoprecipitated from lysates of these cells, and silencing PDE3B activated the perinuclear pool of PKA in these cells, we conclude that PDE3B-mediated hydrolysis of cAMP acts to limit the anti-angiogenic potential of PKA in ECs.

Hypothalamic insulin and glucagon-like peptide-1 levels in an animal model of depression and their effect on corticotropin-releasing hormone promoter gene activity in a hypothalamic cell line

BACKGROUND

In depression, excessive glucocorticoid action may cause maladaptive brain changes, including in the pathways controlling energy metabolism. Insulin and glucagon-like peptide-1 (GLP-1), besides regulation of glucose homeostasis, also possess neurotrophic properties. Current study was aimed at investigating the influence of prenatal stress (PS) on insulin, GLP-1 and their receptor (IR and GLP-1R) levels in the hypothalamus. GLP-1 and GLP-1R were assayed also in the hippocampus and frontal cortex - brain regions mainly affected in depression. The second objective was to determine the influence of exendin-4 and insulin on CRH promoter gene activity in in vitro conditions.

METHODS

Adult male PS rats were subjected to acute stress and/or received orally glucose. Levels of hormones and their receptors were assayed with ELISA method. In vitro studies were performed on mHypoA-2/12 hypothalamic cell line, stably transfected with CRH promoter coupled with luciferase.

RESULTS

PS has reduced GLP-1 and GLP-1R levels, attenuated glucose-induced increase in insulin concentration and increased the amount of phosphorylated IR in the hypothalamus of animals subjected to additional stress stimuli, and also decreased the GLP-1R level in the hippocampus. In vitro studies demonstrated that insulin is capable of increasing CRH promoter activity in the condition of stimulation of the cAMP/PKA pathway in the applied cellular model.

CONCLUSION

Prenatal stress may act as a preconditioning factor, affecting the concentrations of hormones such as insulin and GLP-1 in the hypothalamus in response to adverse stimuli. The decreased GLP-1R level in the hippocampus could be linked with the disturbances in neuronal plasticity.

Loss of ABHD15 Impairs the Anti-lipolytic Action of Insulin by Altering PDE3B Stability and Contributes to Insulin Resistance

Elevated circulating fatty acids (FAs) contribute to obesity-associated metabolic complications, but the mechanisms by which insulin suppresses lipolysis are poorly understood. We show that α/β-hydrolase domain-containing 15 (ABHD15) is required for the anti-lipolytic action of insulin in white adipose tissue (WAT). Neither insulin nor glucose treatments can suppress FA mobilization in global and conditional Abhd15-knockout (KO) mice. Accordingly, insulin signaling is impaired in Abhd15-KO adipocytes, as indicated by reduced AKT phosphorylation, glucose uptake, and de novo lipogenesis. In vitro data reveal that ABHD15 associates with and stabilizes phosphodiesterase 3B (PDE3B). Accordingly, PDE3B expression is decreased in the WAT of Abhd15-KO mice, mechanistically explaining increased protein kinase A (PKA) activity, hormone-sensitive lipase (HSL) phosphorylation, and undiminished FA release upon insulin signaling. Ultimately, Abhd15-KO mice develop insulin resistance. Notably, ABHD15 expression is decreased in humans with obesity and diabetes compared to humans with obesity and normal glucose tolerance, identifying ABHD15 as a potential therapeutic target to mitigate insulin resistance.

Functions of PDE3 Isoforms in Cardiac Muscle

Isoforms in the PDE3 family of cyclic nucleotide phosphodiesterases have important roles in cyclic nucleotide-mediated signalling in cardiac myocytes. These enzymes are targeted by inhibitors used to increase contractility in patients with heart failure, with a combination of beneficial and adverse effects on clinical outcomes. This review covers relevant aspects of the molecular biology of the isoforms that have been identified in cardiac myocytes; the roles of these enzymes in modulating cAMP-mediated signalling and the processes mediated thereby; and the potential for targeting these enzymes to improve the profile of clinical responses.

Effects of heterologous expression of human cyclic nucleotide phosphodiesterase 3A (hPDE3A) on redox regulation in yeast

Oxidative stress plays a pivotal role in pathogenesis of cardiovascular diseases and diabetes; however, the roles of protein kinase A (PKA) and human phosphodiesterase 3A (hPDE3A) remain unknown. Here, we show that yeast expressing wild-type (WT) hPDE3A or K13R hPDE3A (putative ubiquitinylation site mutant) exhibited resistance or sensitivity to exogenous hydrogen peroxide (H₂O₂), respectively. H₂O₂-stimulated ROS production was markedly increased in yeast expressing K13R hPDE3A (Oxidative stress Sensitive 1, OxiS1), compared with yeast expressing WT hPDE3A (Oxidative stress Resistant 1, OxiR1). In OxiR1, YAP1 and YAP1-dependent antioxidant genes were up-regulated, accompanied by a reduction in thioredoxin peroxidase. In OxiS1, expression of YAP1 and YAP1-dependent genes was impaired, and the thioredoxin system malfunctioned. H₂O₂ increased cyclic adenosine monophosphate (cAMP)-hydrolyzing activity of WT hPDE3A, but not K13R hPDE3A, through PKA-dependent phosphorylation of hPDE3A, which was correlated with its ubiquitinylation. The changes in antioxidant gene expression did not directly correlate with differences in cAMP-PKA signaling. Despite differences in their capacities to hydrolyze cAMP, total cAMP levels among OxiR1, OxiS1, and mock were similar; PKA activity, however, was lower in OxiS1 than in OxiR1 or mock. During exposure to H₂O₂, however, Sch9p activity, a target of Rapamycin complex 1-regulated Rps6 kinase and negative-regulator of PKA, was rapidly reduced in OxiR1, and Tpk1p, a PKA catalytic subunit, was diffusely spread throughout the cytosol, with PKA activation. In OxiS1, Sch9p activity was unchanged during exposure to H₂O₂, consistent with reduced activation of PKA. These results suggest that, during oxidative stress, TOR-Sch9 signaling might regulate PKA activity, and that post-translational modifications of hPDE3A are critical in its regulation of cellular recovery from oxidative stress.

RNA-binding protein CUGBP1 regulates insulin secretion via activation of phosphodiesterase 3B in mice

AIMS/HYPOTHESIS

CUG-binding protein 1 (CUGBP1) is a multifunctional RNA-binding protein that regulates RNA processing at several stages including translation, deadenylation and alternative splicing, as well as RNA stability. Recent studies indicate that CUGBP1 may play a role in metabolic disorders. Our objective was to examine its role in endocrine pancreas function through gain- and loss-of-function experiments and to further decipher the underlying molecular mechanisms.

METHODS

A mouse model in which type 2 diabetes was induced by a high-fat diet (HFD; 60% energy from fat) and mice on a standard chow diet (10% energy from fat) were compared. Pancreas-specific CUGBP1 overexpression and knockdown mice were generated. Different lengths of the phosphodiesterase subtype 3B (PDE3B) 3' untranslated region (UTR) were cloned for luciferase reporter analysis. Purified CUGBP1 protein was used for gel shift experiments.

RESULTS

CUGBP1 is present in rodent islets and in beta cell lines; it is overexpressed in the islets of diabetic mice. Compared with control mice, the plasma insulin level after a glucose load was significantly lower and glucose clearance was greatly delayed in mice with pancreas-specific CUGBP1 overexpression; the opposite results were obtained upon pancreas-specific CUGBP1 knockdown. Glucose- and glucagon-like peptide1 (GLP-1)-stimulated insulin secretion was significantly attenuated in mouse islets upon CUGBP1 overexpression. This was associated with a strong decrease in intracellular cAMP levels, pointing to a potential role for cAMP PDEs. CUGBP1 overexpression had no effect on the mRNA levels of PDE1A, 1C, 2A, 3A, 4A, 4B, 4D, 7A and 8B subtypes, but resulted in increased PDE3B expression. CUGBP1 was found to directly bind to a specific ATTTGTT sequence residing in the 3' UTR of PDE3B and stabilised PDE3B mRNA. In the presence of the PDE3 inhibitor cilostamide, glucose- and GLP-1-stimulated insulin secretion was no longer reduced by CUGBP1 overexpression. Similar to CUGBP1, PDE3B was overexpressed in the islets of diabetic mice.

CONCLUSIONS/INTERPRETATION

We conclude that CUGBP1 is a critical regulator of insulin secretion via activating PDE3B. Repressing this protein might provide a potential strategy for treating type 2 diabetes.

Copper regulates cyclic-AMP-dependent lipolysis

Cell signaling relies extensively on dynamic pools of redox-inactive metal ions such as sodium, potassium, calcium and zinc, but their redox-active transition metal counterparts such as copper and iron have been studied primarily as static enzyme cofactors. Here we report that copper is an endogenous regulator of lipolysis, the breakdown of fat, which is an essential process in maintaining body weight and energy stores. Using a mouse model of genetic copper misregulation, in combination with pharmacological alterations in copper status and imaging studies in a 3T3-L1 white adipocyte model, we found that copper regulates lipolysis at the level of the second messenger, cyclic AMP (cAMP), by altering the activity of the cAMP-degrading phosphodiesterase PDE3B. Biochemical studies of the copper-PDE3B interaction establish copper-dependent inhibition of enzyme activity and identify a key conserved cysteine residue in a PDE3-specific loop that is essential for the observed copper-dependent lipolytic phenotype.

Differential expression of several drug transporter genes in HepG2 and Huh-7 cell lines

Background:

Cell culture techniques have many advantages for investigation of drug transport to target organ like liver. HepG2 and Huh-7 are two cell lines available from hepatoma that can be used as a model for hepatic drug transport. The present study is aimed to analyze the expression level of several drug transporter genes in two hepatoma cell lines, HepG2 and Huh-7 and their response to inhibitors.

Materials and Methods:

This is an in vitro study using HepG2 and Huh-7 cells. The expression level of the following drug transporter genes was quantified: P-glycoprotein/multidrug resistance protein 1, Organic Anionic Transporter Protein 1B1 (OATP1B1) and Organic Cationic Transporter-1 (OCT1). Ribonucleic acid was extracted from the cells using Tripure isolation reagent, then gene expression level of the transporters is quantified using Applied Biosystems quantitative reverse transcriptase polymerase chain reaction. Verapamil (P-glycoprotein inhibitor), nelfinavir (OATP1B1 inhibitor), quinidine (OCT1 inhibitor) were used to differentiate the inhibitory properties of these agents to the transporter expressions in HepG2 and Huh-7 cells.

Results:

Huh-7 shows a higher level of P-glycoprotein, OATP1B1 and OCT1 expressions compared with those of HepG2. Verapamil reduces the expressions of P-glycoprotein in HepG2 and Huh-7; nelfinavir reduces the expression of OATP1B1 in HepG2 and Huh-7; while quinidine reduces the OCT1 gene expressions in HepG2, but not in Huh-7 cells.

Conclusion:

This study indicates that HepG2 might be a more suitable in vitro model than Huh-7 to study drug transport in hepatocytes involving drug transporters.

Novel approaches to targeting PDE3 in cardiovascular disease

Inhibitors of PDE3, a family of dual-specificity cyclic nucleotide phosphodiesterases, are used clinically to increase cardiac contractility by raising intracellular cAMP content in cardiac myocytes and to reduce vascular resistance by increasing intracellular cGMP content in vascular smooth muscle myocytes. When used in the treatment of patients with heart failure, PDE3 inhibitors are effective in the acute setting but increase sudden cardiac death with long-term administration, possibly reflecting pro-apoptotic and pro-hypertrophic consequences of increased cAMP-mediated signaling in cardiac myocytes. cAMP-mediated signaling in cardiac myocytes is highly compartmentalized, and different phosphodiesterases, by controlling cAMP content in functionally discrete intracellular microcompartments, regulate different cAMP-mediated pathways. Four variants/isoforms of PDE3 (PDE3A1, PDE3A2, PDE3A3, and PDE3B) are expressed in cardiac myocytes, and new experimental results have demonstrated that these isoforms, which are differentially localized intracellularly through unique protein-protein interactions, control different physiologic responses. While the catalytic regions of these isoforms may be too similar to allow the catalytic activity of each isoform to be selectively inhibited, targeting their unique protein-protein interactions may allow desired responses to be elicited without the adverse consequences that limit the usefulness of existing PDE3 inhibitors.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

The Role of PDE3B Phosphorylation in the Inhibition of Lipolysis by Insulin

Inhibition of adipocyte lipolysis by insulin is important for whole-body energy homeostasis; its disruption has been implicated as contributing to the development of insulin resistance and type 2 diabetes mellitus. The main target of the antilipolytic action of insulin is believed to be phosphodiesterase 3B (PDE3B), whose phosphorylation by Akt leads to accelerated degradation of the prolipolytic second messenger cyclic AMP (cAMP). To test this hypothesis genetically, brown adipocytes lacking PDE3B were examined for their regulation of lipolysis. In Pde3b knockout (KO) adipocytes, insulin was unable to suppress β-adrenergic receptor-stimulated glycerol release. Reexpressing wild-type PDE3B in KO adipocytes fully rescued the action of insulin against lipolysis. Surprisingly, a mutant form of PDE3B that ablates the major Akt phosphorylation site, murine S273, also restored the ability of insulin to suppress lipolysis. Taken together, these data suggest that phosphorylation of PDE3B by Akt is not required for insulin to suppress adipocyte lipolysis.

The role of mouse Akt2 in insulin-dependent suppression of adipocyte lipolysis in vivo

AIM/HYPOTHESIS

The release of fatty acids from adipocytes, i.e. lipolysis, is maintained under tight control, primarily by the opposing actions of catecholamines and insulin. A widely accepted model is that insulin antagonises catecholamine-dependent lipolysis through phosphorylation and activation of cAMP phosphodiesterase 3B (PDE3B) by the serine-threonine protein kinase Akt (protein kinase B). Recently, this hypothesis has been challenged, as in cultured adipocytes insulin appears, under some conditions, to suppress lipolysis independently of Akt.

METHODS

To address the requirement for Akt2, the predominant isoform expressed in classic insulin target tissues, in the suppression of fatty acid release in vivo, we assessed lipolysis in mice lacking Akt2.

RESULTS

In the fed state and following an oral glucose challenge, Akt2 null mice were glucose intolerant and hyperinsulinaemic, but nonetheless exhibited normal serum NEFA and glycerol levels, suggestive of normal suppression of lipolysis. Furthermore, insulin partially inhibited lipolysis in Akt2 null mice during an insulin tolerance test (ITT) and hyperinsulinaemic-euglycaemic clamp, respectively. In support of these in vivo observations, insulin antagonised catecholamine-induced lipolysis in primary brown fat adipocytes from Akt2-deficient mice.

CONCLUSIONS/INTERPRETATION

These data suggest that suppression of lipolysis by insulin in hyperinsulinaemic states can take place in the absence of Akt2.

Targeted disruption of PDE3B, but not PDE3A, protects murine heart from ischemia/reperfusion injury

Although inhibition of cyclic nucleotide phosphodiesterase type 3 (PDE3) has been reported to protect rodent heart against ischemia/reperfusion (I/R) injury, neither the specific PDE3 isoform involved nor the underlying mechanisms have been identified. Targeted disruption of PDE3 subfamily B (PDE3B), but not of PDE3 subfamily A (PDE3A), protected mouse heart from I/R injury in vivo and in vitro, with reduced infarct size and improved cardiac function. The cardioprotective effect in PDE3B(-/-) heart was reversed by blocking cAMP-dependent PKA and by paxilline, an inhibitor of mitochondrial calcium-activated K channels, the opening of which is potentiated by cAMP/PKA signaling. Compared with WT mitochondria, PDE3B(-/-) mitochondria were enriched in antiapoptotic Bcl-2, produced less reactive oxygen species, and more frequently contacted transverse tubules where PDE3B was localized with caveolin-3. Moreover, a PDE3B(-/-) mitochondrial fraction containing connexin-43 and caveolin-3 was more resistant to Ca(2+)-induced opening of the mitochondrial permeability transition pore. Proteomics analyses indicated that PDE3B(-/-) heart mitochondria fractions were enriched in buoyant ischemia-induced caveolin-3-enriched fractions (ICEFs) containing cardioprotective proteins. Accumulation of proteins into ICEFs was PKA dependent and was achieved by ischemic preconditioning or treatment of WT heart with the PDE3 inhibitor cilostamide. Taken together, these findings indicate that PDE3B deletion confers cardioprotective effects because of cAMP/PKA-induced preconditioning, which is associated with the accumulation of proteins with cardioprotective function in ICEFs. To our knowledge, our study is the first to define a role for PDE3B in cardioprotection against I/R injury and suggests PDE3B as a target for cardiovascular therapies.

Selective insulin resistance in adipocytes

Aside from glucose metabolism, insulin regulates a variety of pathways in peripheral tissues. Under insulin-resistant conditions, it is well known that insulin-stimulated glucose uptake is impaired, and many studies attribute this to a defect in Akt signaling. Here we make use of several insulin resistance models, including insulin-resistant 3T3-L1 adipocytes and fat explants prepared from high fat-fed C57BL/6J and ob/ob mice, to comprehensively distinguish defective from unaffected aspects of insulin signaling and its downstream consequences in adipocytes. Defective regulation of glucose uptake was observed in all models of insulin resistance, whereas other major actions of insulin such as protein synthesis and anti-lipolysis were normal. This defect corresponded to a reduction in the maximum response to insulin. The pattern of change observed for phosphorylation in the Akt pathway was inconsistent with a simple defect at the level of Akt. The only Akt substrate that showed consistently reduced phosphorylation was the RabGAP AS160 that regulates GLUT4 translocation. We conclude that insulin resistance in adipose tissue is highly selective for glucose metabolism and likely involves a defect in one of the components regulating GLUT4 translocation to the cell surface in response to insulin.

QRFP-43 inhibits lipolysis by preventing ligand-induced complex formation between perilipin A, caveolin-1, the catalytic subunit of protein kinase and hormone-sensitive lipase in 3T3-L1 adipocytes

QRFP (RFamide) peptides are neuropeptides involved in food intake and adiposity regulation in rodents. We have previously shown that QRFP-43 (43RFa) and QRFP-26 (26RFa) inhibited isoproterenol (ISO)-induced lipolysis in adipocytes. However, the antilipolytic signaling pathways activated by QRFP peptides have not been investigated. In the present study, 3T3-L1 adipocytes were used to identify the main pathways involved in QRFP-43 decreasing ISO-induced lipolysis. Our results show that QRFP-43 reduced ISO-induced phosphorylation of perilipin A (PLIN) and hormone-sensitive lipase (HSL) on Ser660 by 43 and 25%, respectively, but increased Akt phosphorylation by 44%. However, the inhibition of phosphodiesterase 3B (PDE3B), a regulator of lipolysis activated by Akt, did not reverse the antilipolytic effect of QRFP-43. PDE3B inhibition reversed the decrease of Ser660 HSL phosphorylation associated with QRFP-43 antilipolytic effect. QRFP-43 also prevented PKC activation and ISO-induced Src kinases activation leading to the inhibition of the caveolin-1 (CAV-1) translocation on lipid droplets. Indeed, QRFP-43 attenuated phorbol 12-myristate 13-acetate-induced lipolysis and ISO-induced extracellular signal-regulated and Src kinases by 28, 37 and 48%, respectively. The attenuation of ISO-induced lipolysis by QRFP-43 was associated with a decrease of phosphorylated Ser660 HSL, PKA-catalytic (PKA-c) subunit and CAV-1 translocation on lipid droplets by 37, 50 and 46%, respectively. The decrease in ISO-induced CAV-1 and PKA-c translocation was associated with a reduction of PLIN phosphorylation by 44% in QRFP-43-treated adipocytes. These results suggest that QRFP-43 attenuated ISO-induced lipolysis by preventing the formation of an active complex on lipid droplets and the activation of Src kinases and PKC.

Regulation of sarcoplasmic reticulum Ca2+ ATPase 2 (SERCA2) activity by phosphodiesterase 3A (PDE3A) in human myocardium: phosphorylation-dependent interaction of PDE3A1 with SERCA2

Cyclic nucleotide phosphodiesterase 3A (PDE3) regulates cAMP-mediated signaling in the heart, and PDE3 inhibitors augment contractility in patients with heart failure. Studies in mice showed that PDE3A, not PDE3B, is the subfamily responsible for these inotropic effects and that murine PDE3A1 associates with sarcoplasmic reticulum Ca(2+) ATPase 2 (SERCA2), phospholamban (PLB), and AKAP18 in a multiprotein signalosome in human sarcoplasmic reticulum (SR). Immunohistochemical staining demonstrated that PDE3A co-localizes in Z-bands of human cardiac myocytes with desmin, SERCA2, PLB, and AKAP18. In human SR fractions, cAMP increased PLB phosphorylation and SERCA2 activity; this was potentiated by PDE3 inhibition but not by PDE4 inhibition. During gel filtration chromatography of solubilized SR membranes, PDE3 activity was recovered in distinct high molecular weight (HMW) and low molecular weight (LMW) peaks. HMW peaks contained PDE3A1 and PDE3A2, whereas LMW peaks contained PDE3A1, PDE3A2, and PDE3A3. Western blotting showed that endogenous HMW PDE3A1 was the principal PKA-phosphorylated isoform. Phosphorylation of endogenous PDE3A by rPKAc increased cAMP-hydrolytic activity, correlated with shift of PDE3A from LMW to HMW peaks, and increased co-immunoprecipitation of SERCA2, cav3, PKA regulatory subunit (PKARII), PP2A, and AKAP18 with PDE3A. In experiments with recombinant proteins, phosphorylation of recombinant human PDE3A isoforms by recombinant PKA catalytic subunit increased co-immunoprecipitation with rSERCA2 and rat rAKAP18 (recombinant AKAP18). Deletion of the recombinant human PDE3A1/PDE3A2 N terminus blocked interactions with recombinant SERCA2. Serine-to-alanine substitutions identified Ser-292/Ser-293, a site unique to human PDE3A1, as the principal site regulating its interaction with SERCA2. These results indicate that phosphorylation of human PDE3A1 at a PKA site in its unique N-terminal extension promotes its incorporation into SERCA2/AKAP18 signalosomes, where it regulates a discrete cAMP pool that controls contractility by modulating phosphorylation-dependent protein-protein interactions, PLB phosphorylation, and SERCA2 activity.

Age-related differences in phosphodiesterase activity and effects of chronic phosphodiesterase inhibition in idiopathic dilated cardiomyopathy

BACKGROUND

Despite the application of proven adult heart failure therapies to children with idiopathic dilated cardiomyopathy (IDC), prognosis remains poor. Clinical experience with phosphodiesterase 3 inhibitors (PDE3i) in pediatric patients with IDC, however, demonstrates improved heart failure symptoms without the increased incidence of sudden death seen in adults treated with PDE3i. We sought to determine age-related differences in PDE activity and associated intracellular signaling responsible for the efficacy and relative safety of chronic PDE3i in pediatric heart failure.

METHODS AND RESULTS

cAMP levels, PDE activity, and phospholamban phosphorylation (pPLB) were determined in explanted human left ventricular myocardium (pediatric n=41; adult n=88). Adults and children with IDC (not treated with PDE3i) had lower cAMP and pPLB compared with nonfailing controls. In contrast to their adult counterparts, pediatric IDC patients chronically treated with PDE3i had elevated cAMP (P=0.0403) and pPLB (P=0.0119). In addition, total PDE- and PDE3-specific activities were not altered in pediatric IDC patients on PDE3i, whereas adult IDC patients on PDE3i demonstrated higher total PDE-specific (74.6±13.8 pmol/mg per minute) and PDE3-specific (48.2±15.9 pmol/mg per minute) activities in comparison with those of nonfailing controls (59.5±14.4 and 35.5±12.8 pmol/mg per minute, respectively).

CONCLUSIONS

Elevated cAMP and higher pPLB may contribute to sustained hemodynamic benefits in pediatric IDC patients treated with PDE3i. In contrast, higher total PDE and PDE3 activities in adult IDC patients treated with PDE3i may perpetuate lower myocardial cAMP and pPLB levels, limiting the potential benefits of PDE3i therapy.

Cyclic nucleotide phosphodiesterases: important signaling modulators and therapeutic targets

By catalyzing hydrolysis of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), cyclic nucleotide phosphodiesterases are critical regulators of their intracellular concentrations and their biological effects. As these intracellular second messengers control many cellular homeostatic processes, dysregulation of their signals and signaling pathways initiate or modulate pathophysiological pathways related to various disease states, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication, chronic obstructive pulmonary disease, and psoriasis. Alterations in expression of PDEs and PDE-gene mutations (especially mutations in PDE6, PDE8B, PDE11A, and PDE4) have been implicated in various diseases and cancer pathologies. PDEs also play important role in formation and function of multimolecular signaling/regulatory complexes, called signalosomes. At specific intracellular locations, individual PDEs, together with pathway-specific signaling molecules, regulators, and effectors, are incorporated into specific signalosomes, where they facilitate and regulate compartmentalization of cyclic nucleotide signaling pathways and specific cellular functions. Currently, only a limited number of PDE inhibitors (PDE3, PDE4, PDE5 inhibitors) are used in clinical practice. Future paths to novel drug discovery include the crystal structure-based design approach, which has resulted in generation of more effective family-selective inhibitors, as well as burgeoning development of strategies to alter compartmentalized cyclic nucleotide signaling pathways by selectively targeting individual PDEs and their signalosome partners.

Advances in targeting cyclic nucleotide phosphodiesterases

Cyclic nucleotide phosphodiesterases (PDEs) catalyse the hydrolysis of cyclic AMP and cyclic GMP, thereby regulating the intracellular concentrations of these cyclic nucleotides, their signalling pathways and, consequently, myriad biological responses in health and disease. Currently, a small number of PDE inhibitors are used clinically for treating the pathophysiological dysregulation of cyclic nucleotide signalling in several disorders, including erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, intermittent claudication and chronic obstructive pulmonary disease. However, pharmaceutical interest in PDEs has been reignited by the increasing understanding of the roles of individual PDEs in regulating the subcellular compartmentalization of specific cyclic nucleotide signalling pathways, by the structure-based design of novel specific inhibitors and by the development of more sophisticated strategies to target individual PDE variants.

Selective regulation of cyclic nucleotide phosphodiesterase PDE3A isoforms

Inhibitors of cyclic nucleotide phosphodiesterase (PDE) PDE3A have inotropic actions in human myocardium, but their long-term use increases mortality in patients with heart failure. Two isoforms in cardiac myocytes, PDE3A1 and PDE3A2, have identical amino acid sequences except for a unique N-terminal extension in PDE3A1. We expressed FLAG-tagged PDE3A1 and PDE3A2 in HEK293 cells and examined their regulation by PKA- and PKC-mediated phosphorylation. PDE3A1, which is localized to intracellular membranes, and PDE3A2, which is cytosolic, were phosphorylated at different sites within their common sequence. Exposure to isoproterenol led to phosphorylation of PDE3A1 at the 14-3-3-binding site S312, whereas exposure to PMA led to phosphorylation of PDE3A2 at an alternative 14-3-3-binding site, S428. PDE3A2 activity was stimulated by phosphorylation at S428, whereas PDE3A1 activity was not affected by phosphorylation at either site. Phosphorylation of PDE3A1 by PKA and of PDE3A2 by PKC led to shifts in elution on gel-filtration chromatography consistent with increased interactions with other proteins, and 2D electrophoresis of coimmunoprecipitated proteins revealed that the two isoforms have distinct protein interactomes. A similar pattern of differential phosphorylation of endogenous PDE3A1 and PDE3A2 at S312 and S428 is observed in human myocardium. The selective phosphorylation of PDE3A1 and PDE3A2 at alternative sites through different signaling pathways, along with the different functional consequences of phosphorylation for each isoform, suggest they are likely to have distinct roles in cyclic nucleotide-mediated signaling in human myocardium, and raise the possibility that isoform-selective inhibition may allow inotropic responses without an increase in mortality.

Impaired hypothalamic insulin signaling in CUMS rats: restored by icariin and fluoxetine through inhibiting CRF system

Epidemiological evidence demonstrates the neuroendocrine link between stress, depression and diabetes. This study observed glucose intolerance of rats exposed to chronic unpredictable mild stress (CUMS) in oral glucose tolerance test (OGTT). CUMS procedure significantly up-regulated corticotropin-releasing factor (CRF)-related peptide urocortin 2 expression and elevated cAMP production, resulting in over-expression of suppressor of cytokine signaling 3 (SOCS3) in hypothalamic arcuate nucleus (ARC) of rats. Furthermore, SOCS3 activation blocked insulin signaling pathway through the suppression of insulin receptor substrate 2 (IRS2) phosphotyrosine and phosphatidylinositol-3-kinase (PI3-K) activation in hypothalamic ARC of CUMS rats after high-level of insulin stimulation. These data indicated that CUMS procedure induced the hyperactivity of CRF system, and subsequently produced conditional loss of insulin signaling in hypothalamic ARC of rats. More importantly, icariin and fluoxetine with the ability to restrain CRF system hyperactivity improved insulin signaling in hypothalamic ARC of CUMS rats, which were consistent with the enhancement of glucose tolerance in OGTT, showing anti-diabetic efficacy. Although effective in OGTT, anti-diabetic drug pioglitazone failed to restore hypothalamic ARC CRF system hyperactivity, paralleling with its inability to ameliorate the loss of insulin signaling and depression-like behavior in CUMS rats. These observations support the hypothesis that signal cross-talk between hypothalamic CRF system and insulin may be impaired in depression with glucose intolerance and suggest that icarrin and fluoxetine aiming at CRF system may have great potential in the prevention and treatment of depression with comorbid diabetes.

Inhibition of phosphoinositide 3-kinase potentiates relaxation of porcine coronary arteries induced by nitroglycerin by decreasing phosphodiesterase type 5 activity

BACKGROUND

Vessel tension can be modulated by phosphoinositide 3-kinase (PI3K) acting on l-type calcium channel, rho kinase and phosphodiesterase (PDE) type 3 in smooth muscle cells. Inhibition of PI3K could increase the relaxation of porcine coronary arteries to nitroglycerin independent of this pathway, and the aim of the present study was therefore to determine the underlying mechanisms.

METHODS AND RESULTS

Isolated porcine coronary arteries were dissected from the heart and cut into rings in ice-cold modified Krebs-Ringer bicarbonate buffer. The response of these vessels was studied by using the organ chamber technique; the content of cyclic guanosine monophosphate (cGMP) was determined by using enzyme-linked immunosorbent assay kit; and PI3K and Akt activity were determined by measuring the phosphorylation level of their downstream signaling molecule on Western blot. Inhibition of PI3K with 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002) potentiated the relaxation of porcine coronary arteries to nitroglycerin and nitric oxide (NO), but not to 8-bromo-guanosine 3'5'-cyclic monophosphate, isoproterenol or (R)-(+)-trans-4-(1-Aminoethyl)-N-(4-Pyridyl)cyclohexanecarboxamide dihydrochloride monohydrate (Y27632). Increased relaxation induced by LY294002 was eliminated by Akt1/2 kinase inhibitor (Akt-I: 1,3-dihydro-1-(1-((4-(6-phenyl-1H-imidazo(4,5-g)quinoxalin-7-yl)phenyl)methyl)-4-piperidinyl)-2H-benzimidazol-2-one trifluoroacetate salt hydrate) or zaprinast, but was not affected by 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one, nifedipine or milrinone. Inhibition of Akt caused similar effects as LY294002. Incubation with LY294002 or Akt-I decreased the activity of PI3K and Akt but augmented the elevation of cGMP caused by NO. Enhanced cGMP elevation induced by LY294002 or Akt-I was also eliminated by zaprinast.

CONCLUSIONS

PI3K-Akt signaling may affect vascular tone through a stimulatory effect on PDE type 5.

From PDE3B to the regulation of energy homeostasis

The incidence of obesity in the developed world is increasing at an alarming rate. Concurrent with the increase in the incidence of obesity is an increase in the incidence of type 2 diabetes. Cyclic AMP (cAMP) and cGMP are key second messengers in all cells; for example, when it comes to processes of relevance for the regulation of energy metabolism, cAMP is a key mediator in the regulation of lipolysis, glycogenolysis, gluconeogenesis and pancreatic β cell insulin secretion. PDE3B, one of several enzymes which hydrolyze cAMP and cGMP, is expressed in cells of importance for the regulation of energy homeostasis, including adipocytes, hepatocytes, hypothalamic cells and β cells. It has been shown, using PDE3 inhibitors and gene targeting approaches in cells and animals, that altered levels of PDE3B result in a number of changes in the regulation of glucose and lipid metabolism and in overall energy homeostasis. This article highlights the complexity involved in the regulation of PDE3B by hormones, and in the regulation of downstream metabolic effects by PDE3B in several interacting tissues.

Insulin contributes to fine-tuning of the pancreatic beta-cell response to glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion from pancreatic β-cells in a glucose-dependent manner. However, factors other than glucose that regulate the β-cell response to GLP-1 remain poorly understood. In this study, we examined the possible involvement of insulin and receptor tyrosine kinase signaling in regulation of the GLP-1 responsiveness of β-cells. Pretreatment of β-cells with HNMPA, an insulin receptor inhibitor, and AG1478, an epidermal growth factor receptor inhibitor, further increased the cAMP level and Erk phosphorylation in the presence of exendin-4 (exe-4), a GLP-1 agonist. When β-cells were exposed to a high concentration of glucose (25 mM), which stimulates insulin secretion, exe-4-induced cAMP formation declined gradually as exposure time was increased. This decreased cAMP formation was not observed in the presence of HNMPA. HNMPA was able to further increase the exe-4-induced insulin secretion when β-cells were exposed to high glucose for 18 h. Treatment of β-cells with insulin significantly decreased exe-4-induced cAMP formation in a dose-dependent manner. Lowering the phospho-Akt level by HNMPA or LY294002, a PI3K inhibitor, further augmented exe-4-induced cAMP formation and Erk phosphorylation. These results suggest that insulin contributes to fine-tuning of the β-cell response to GLP-1.

Acylated and unacylated ghrelin attenuate isoproterenol-induced lipolysis in isolated rat visceral adipocytes through activation of phosphoinositide 3-kinase γ and phosphodiesterase 3B

The acylated peptide ghrelin (AG) and its endogenous non-acylated isoform (UAG) protect cardiomyocytes, pancreatic β-cells, and preadipocytes from apoptosis, and induce preadipocytes differentiation into adipocytes. These events are mediated by AG and UAG binding to a still unidentified receptor, which determines the activation of phosphoinositide 3-kinase (PI3K), protein kinase B (AKT), and mitogen-activated protein kinase (MAPK) ERK1/2. AG and UAG also possess antilipolytic activity in vitro, but the underlying mechanism remains unknown. Thus, the objective of the current study was to characterize the molecular events involved in AG/UAG receptor signaling cascade. We treated rat primary visceral adipocytes with isoproterenol (ISO) and forskolin (FSK) to stimulate lipolysis, simultaneously incubating them with or without AG or UAG. Both peptides blocked ISO- and FSK-induced lipolysis. By direct measurement of cAMP intracellular content, we demonstrated that AG/UAG effect was associated to a reduction of ISO-induced cAMP accumulation. Moreover, the cAMP analog 8Br-cAMP abolished AG/UAG effect. As AG and UAG were ineffective against lipolysis induced by db-cAMP, another poorly hydrolyzable cAMP analog, phosphodiesterase (PDE) involvement was hypothesized. Indeed, cilostamide, a specific PDE3B inhibitor, blocked AG/UAG effect on ISO-induced lipolysis. Furthermore, the PI3K inhibitor wortmannin and AKT inhibitor 1,3-dihydro-1-(1-((4-(6-phenyl-1H-imidazo(4,5-g)quinoxalin-7-yl)phenyl)methyl)-4piperidinyl)-2H-benzimidazol-2-one trifluoroacetate also blocked AG/UAG action, suggesting a role in PDE3B activation. In particular, PI3K isoenzyme gamma (PI3Kγ) selective inhibition through the compound AS605240 prevented AG/UAG effect on ISO-stimulated lipolysis, hampering AKT phosphorylation on Ser(473). Taken together, these data demonstrate for the first time that AG/UAG attenuation of ISO-induced lipolysis involves PI3Kγ/AKT and PDE3B.

Regulation of cAMP by phosphodiesterases in erythrocytes

The erythrocyte, a cell responsible for carrying and delivering oxygen in the body, has often been regarded as simply a vehicle for the circulation of hemoglobin. However, it has become evident that this cell also participates in the regulation of vascular caliber in the microcirculation via release of the potent vasodilator, adenosine triphosphate (ATP). The regulated release of ATP from erythrocytes occurs via a defined signaling pathway and requires increases in cyclic 3',5'- adenosine monophosphate (cAMP). It is well recognized that cAMP is a critical second messenger in diverse signaling pathways. In all cells increases in cAMP are localized and regulated by the activity of phosphodiesterases (PDEs). In erythrocytes activation of either beta adrenergic receptors (beta(2)AR) or the prostacyclin receptor (IPR) results in increases in cAMP and ATP release. Receptor-mediated increases in cAMP are tightly regulated by distinct PDEs associated with each signaling pathway as shown by the finding that selective inhibitors of the PDEs localized to each pathway potentiate both increases in cAMP and ATP release. Here we review the profile of PDEs identified in erythrocytes, their association with specific signaling pathways and their role in the regulation of ATP release from these cells. Understanding the contribution of PDEs to the control of ATP release from erythrocytes identifies this cell as a potential target for the development of drugs for the treatment of vascular disease.

Insulin regulates adipocyte lipolysis via an Akt-independent signaling pathway

After a meal, insulin suppresses lipolysis through the activation of its downstream kinase, Akt, resulting in the inhibition of protein kinase A (PKA), the main positive effector of lipolysis. During insulin resistance, this process is ineffective, leading to a characteristic dyslipidemia and the worsening of impaired insulin action and obesity. Here, we describe a noncanonical Akt-independent, phosphoinositide-3 kinase (PI3K)-dependent pathway that regulates adipocyte lipolysis using restricted subcellular signaling. This pathway selectively alters the PKA phosphorylation of its major lipid droplet-associated substrate, perilipin. In contrast, the phosphorylation of another PKA substrate, hormone-sensitive lipase (HSL), remains Akt dependent. Furthermore, insulin regulates total PKA activity in an Akt-dependent manner. These findings indicate that localized changes in insulin action are responsible for the differential phosphorylation of PKA substrates. Thus, we identify a pathway by which insulin regulates lipolysis through the spatially compartmentalized modulation of PKA.

Differential regulation of adipocyte PDE3B in distinct membrane compartments by insulin and the beta3-adrenergic receptor agonist CL316243: effects of caveolin-1 knockdown on formation/maintenance of macromolecular signalling complexes

In adipocytes, PDE3B (phosphodiesterase 3B) is an important regulatory effector in signalling pathways controlled by insulin and cAMP-increasing hormones. Stimulation of 3T3-L1 adipocytes with insulin or the beta3-adrenergic receptor agonist CL316243 (termed CL) indicated that insulin preferentially phosphorylated/activated PDE3B associated with internal membranes (endoplasmic reticulum/Golgi), whereas CL preferentially phosphorylated/activated PDE3B associated with caveolae. siRNA (small interfering RNA)-mediated KD (knockdown) of CAV-1 (caveolin-1) in 3T3-L1 adipocytes resulted in down-regulation of expression of membrane-associated PDE3B. Insulin-induced activation of PDE3B was reduced, whereas CL-mediated activation was almost totally abolished. Similar results were obtained in adipocytes from Cav-1-deficient mice. siRNA-mediated KD of CAV-1 in 3T3-L1 adipocytes also resulted in inhibition of CL-stimulated phosphorylation of HSL (hormone-sensitive lipase) and perilipin A, and of lipolysis. Superose 6 gel-filtration chromatography of solubilized membrane proteins from adipocytes stimulated with insulin or CL demonstrated the reversible assembly of distinct macromolecular complexes that contained 32P-phosphorylated PDE3B and signalling molecules thought to be involved in its activation. Insulin- and CL-induced macromolecular complexes were enriched in cholesterol, and contained certain common signalling proteins [14-3-3, PP2A (protein phosphatase 2A) and cav-1]. The complexes present in insulin-stimulated cells contained tyrosine-phosphorylated IRS-1 (insulin receptor substrate 1) and its downstream signalling proteins, whereas CL-activated complexes contained beta3-adrenergic receptor, PKA-RII [PKA (cAMP-dependent protein kinase)-regulatory subunit] and HSL. Insulin- and CL-mediated macromolecular complex formation was significantly inhibited by CAV-1 KD. These results suggest that cav-1 acts as a molecular chaperone or scaffolding molecule in cholesterol-rich lipid rafts that may be necessary for the proper stabilization and activation of PDE3B in response to CL and insulin.

Protein kinase B activity is required for the effects of insulin on lipid metabolism in adipocytes

Protein kinase B (PKB) is known to mediate a number of biological responses to insulin and growth factors, its role in glucose uptake being one of the most extensively studied. In this work, we have employed a recently described allosteric inhibitor of PKB, Akti, to clarify the role of PKB in lipid metabolism in adipocytes-a subject that has received less attention. Pretreatment of primary rat and 3T3L1 adipocytes with Akti resulted in dose-dependent inhibition of PKB phosphorylation and activation in response to insulin, without affecting upstream insulin signaling [insulin receptor (IR), insulin receptor substrate (IRS)] or the insulin-induced phosphoinositide 3-kinase (PI3K)-dependent activation of the ERK/p90 ribosomal kinase (RSK) pathway. PKB activity was required for the insulin-induced activation of phosphodiesterase 3B (PDE3B) and for the antilipolytic action of insulin. Moreover, inhibition of PKB activity resulted in a reduction in de novo lipid synthesis and in the ability of insulin to stimulate this process. The regulation of the rate-limiting lipogenic enzyme acetyl-CoA carboxylase (ACC) by insulin through dephosphorylation of S79, which is a target for AMP-activated protein kinase (AMPK), was dependent on the presence of active PKB. Finally, AMPK was shown to be phosphorylated by PKB on S485 in response to insulin, and this was associated with a reduction in AMPK activity. In summary, we propose that PKB is required for the positive effects of insulin on lipid storage and that regulation of PDE3B and AMPK by PKB is important for these effects.

The biological activity of structurally defined inositol glycans

BACKGROUND: The inositol glycans (IGs) are glycolipid-derived carbohydrates produced by insulin-sensitive cells in response to insulin treatment. IGs exhibit an array of insulin-like activities including stimulation of lipogenesis, glucose transport and glycogen synthesis, suggesting that they may be involved in insulin signal transduction. However, because the natural IGs are structurally heterogeneous and difficult to purify to homogeneity, an understanding of the relationship between structure and biological activity has relied principally on synthetic IGs of defined structure. DISCUSSION: This article briefly describes what is known about the role of IGs in signal transduction and reviews the specific biological activities of the structurally defined IGs synthesized and tested to date. CONCLUSION: A pharmacophore for IG activity begins to emerge from the reviewed data and the structural elements necessary for activity are summarized.

Interaction of phosphodiesterase 3A with brefeldin A-inhibited guanine nucleotide-exchange proteins BIG1 and BIG2 and effect on ARF1 activity

ADP-ribosylation factors (ARFs) have crucial roles in vesicular trafficking. Brefeldin A-inhibited guanine nucleotide-exchange proteins (BIG)1 and BIG2 catalyze the activation of class I ARFs by accelerating replacement of bound GDP with GTP. Several additional and differing actions of BIG1 and BIG2 have been described. These include the presence in BIG2 of 3 A kinase-anchoring protein (AKAP) domains, one of which is identical in BIG1. Proteins that contain AKAP sequences act as scaffolds for the assembly of PKA with other enzymes, substrates, and regulators in complexes that constitute molecular machines for the reception, transduction, and integration of signals from cAMP or other sources, which are initiated, propagated, and transmitted by chemical, electrical, or mechanical means. Specific depletion of HeLa cell PDE3A with small interfering RNA significantly decreased membrane-associated BIG1 and BIG2, which by confocal immunofluorescence microscopy were widely dispersed from an initial perinuclear Golgi concentration. Concurrently, activated ARF1-GTP was significantly decreased. Selective inhibition of PDE3A by 1-h incubation of cells with cilostamide similarly decreased membrane-associated BIG1. We suggest that decreasing PDE3A allowed cAMP to accumulate in microdomains where its enzymatic activity limited cAMP concentration. There, cAMP-activated PKA phosphorylated BIG1 and BIG2 (AKAPs for assembly of PKA, PDE3A, and other molecules), which decreased their GEP activity and thereby amounts of activated ARF1-GTP. Thus, PDE3A in these BIG1 and BIG2 AKAP complexes may contribute to the regulation of ARF function via limitation of cAMP effects with spatial and temporal specificity.

Phosphodiesterase 3B is localized in caveolae and smooth ER in mouse hepatocytes and is important in the regulation of glucose and lipid metabolism

Cyclic nucleotide phosphodiesterases (PDEs) are important regulators of signal transduction processes mediated by cAMP and cGMP. One PDE family member, PDE3B, plays an important role in the regulation of a variety of metabolic processes such as lipolysis and insulin secretion. In this study, the cellular localization and the role of PDE3B in the regulation of triglyceride, cholesterol and glucose metabolism in hepatocytes were investigated. PDE3B was identified in caveolae, specific regions in the plasma membrane, and smooth endoplasmic reticulum. In caveolin-1 knock out mice, which lack caveolae, the amount of PDE3B protein and activity were reduced indicating a role of caveolin-1/caveolae in the stabilization of enzyme protein. Hepatocytes from PDE3B knock out mice displayed increased glucose, triglyceride and cholesterol levels, which was associated with increased expression of gluconeogenic and lipogenic genes/enzymes including, phosphoenolpyruvate carboxykinase, peroxisome proliferator-activated receptor γ, sterol regulatory element-binding protein 1c and hydroxyl-3-methylglutaryl coenzyme A reductase. In conclusion, hepatocyte PDE3B is localized in caveolae and smooth endoplasmic reticulum and plays important roles in the regulation of glucose, triglyceride and cholesterol metabolism. Dysregulation of PDE3B could have a role in the development of fatty liver, a condition highly relevant in the context of type 2 diabetes.

Protein kinase C-mediated phosphorylation and activation of PDE3A regulate cAMP levels in human platelets

The elevation of [cAMP](i) is an important mechanism of platelet inhibition and is regulated by the opposing activity of adenylyl cyclase and phosphodiesterase (PDE). In this study, we demonstrate that a variety of platelet agonists, including thrombin, significantly enhance the activity of PDE3A in a phosphorylation-dependent manner. Stimulation of platelets with the PAR-1 agonist SFLLRN resulted in rapid and transient phosphorylation of PDE3A on Ser(312), Ser(428), Ser(438), Ser(465), and Ser(492), in parallel with the PKC (protein kinase C) substrate, pleckstrin. Furthermore, phosphorylation and activation of PDE3A required the activation of PKC, but not of PI3K/PKB, mTOR/p70S6K, or ERK/RSK. Activation of PKC by phorbol esters also resulted in phosphorylation of the same PDE3A sites in a PKC-dependent, PKB-independent manner. This was further supported by the finding that IGF-1, which strongly activates PI3K/PKB, but not PKC, did not regulate PDE3A. Platelet activation also led to a PKC-dependent association between PDE3A and 14-3-3 proteins. In contrast, cAMP-elevating agents such as PGE(1) and forskolin-induced phosphorylation of Ser(312) and increased PDE3A activity, but did not stimulate 14-3-3 binding. Finally, complete antagonism of PGE(1)-evoked cAMP accumulation by thrombin required both G(i) and PKC activation. Together, these results demonstrate that platelet activation stimulates PKC-dependent phosphorylation of PDE3A on Ser(312), Ser(428), Ser(438), Ser(465), and Ser(492) leading to a subsequent increase in cAMP hydrolysis and 14-3-3 binding.

Phosphodiesterase inhibition in heart failure

Drugs that inhibit cyclic nucleotide phosphodiesterase activity act to increase intracellular cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) content. In total, 11 families of these enzymes-which differ with respect to affinity for cAMP and cGMP, cellular expression, intracellular localization, and mechanisms of regulation-have been identified. Inhibitors of enzymes in the PDE3 family of cyclic nucleotide phosphodiesterases raise intracellular cAMP content in cardiac and vascular smooth muscle, with inotropic and, to a lesser extent, vasodilatory actions. These drugs have been used for many years in the treatment of patients with heart failure, but their long-term use has generally been shown to increase mortality through mechanisms that remain unclear. More recently, inhibitors of PDE5 cyclic nucleotide phosphodiesterases have been used as cGMP-raising agents in vascular smooth muscle. With respect to cardiovascular disease, there is evidence that these drugs are more efficacious in the pulmonary than in the systemic vasculature, for which reason they are used principally in patients with pulmonary hypertension. Effects attributable to inhibition of myocardial PDE5 activity are less well characterized. New information indicating that enzymes from the PDE1 family of cyclic nucleotide phosphodiesterases constitute the majority of cAMP- and cGMP-hydrolytic activity in human myocardium raises questions as to their role in regulating these signaling pathways in heart failure.

Coordinated regulation of esterification and lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in rat adipocytes

Inhibition of lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in isolated rat adipocytes has previously been shown to rely on the degradation of cyclic adenosine monophosphate by the phosphodiesterase, Gce1, and the 5'-nucleotidase, CD73. These glycosylphosphatidylinositol (GPI)-anchored proteins are translocated from plasma membrane lipid rafts to intracellular lipid droplets upon H2O2-induced activation of a GPI-specific phospholipase C (GPI-PLC) in response to palmitate and glimepiride in intact adipocytes and, as demonstrated here, in cell-free systems as well. The same agents are also known to stimulate the incorporation of fatty acids into triacylglycerol. Here the involvement of H2O2 production, GPI-PLC activation and translocation of Gce1 and CD73 in the agent-induced esterification and accompanying lipid droplet formation was tested in rat adipocytes using relevant inhibitors. The results demonstrate that upregulation of the esterification and accumulation of triacylglycerol by glimepiride depends on the sequential H2O2-induced GPI-PLC activation and GPI-protein translocation as does inhibition of lipolysis. In contrast, stimulation of the esterification and triacylglycerol accumulation by palmitate relies on insulin-independent tyrosine phosphorylation and thus differs from its anti-lipolytic mechanism. As expected, insulin regulates lipid metabolism via typical insulin signalling independent of H2O2 production, GPI-PLC activation and GPI-protein translocation, albeit these processes are moderately stimulated by insulin. In conclusion, triacylglycerol and lipid droplet formation in response to glimepiride and H2O2 may involve the hydrolysis of cyclic adenosine monophosphate by lipid droplet-associated Gce1 and CD73 which may regulate lipid droplet-associated triacylglycerol-synthesizing and hydrolyzing enzymes in coordinated and inverse fashion.

Action mechanism of metallo-allixin complexes as antidiabetic agents

The metabolic syndrome is a group of factors associated with an increased risk of atherosclerosis and diabetes. Diabetes mellitus (DM) is classified into 2 major types - type 1 DM and type 2 DM - characterized by chronic hyperglycemia resulting from defects in insulin secretion and insulin action, respectively. Several synthetic pharmaceuticals have been developed and clinically used for treating DM; however, these pharmaceuticals continue to cause side effects. Recently, we proposed that oxovanadium(IV) (vanadyl) and zinc(II) (zinc) complexes are potent antidiabetic agents for both type 1 and type 2 DM therapy. This article reviews the vanadyl- and zinc-allixin and their related complexes that are being currently developed as novel types of antidiabetic metal complexes, focusing on their action mechanism in terms of regulation of the insulin signaling pathway and inhibition of lipolysis signaling in adipocyte cells.