Discriminative insulin antagonism of stimulatory effects of various cAMP analogs on adipocyte lipolysis and hepatocyte glycogenolysis

Insulin resistance in adipocytes: Novel insights into the pathophysiology of metabolic syndrome

BACKGROUND

Insulin resistance in all major target tissues is present in metabolic syndrome (MetS). The resistance in adipocytes is not well described and was presently examined.

METHODS

In this observational study on isolated abdominal white subcutaneous adipocytes from 419 adults, concentration-response effects of insulin on lipolysis inhibition (glycerol release) and lipogenesis stimulation (glucose conversion to total lipids) were determined. Insights into early and late insulin signaling events were obtained through the determination of insulin sensitivity (half maximum effective concentration) and responsiveness (maximum effect), respectively. In a subgroup of 132 subjects, we analyzed the subcutaneous adipose mRNA expression of genes in the canonical insulin signaling pathway using microarray. These results were validated using quantitative real-time polymerase chain reaction in 74 individuals.

RESULTS

While the insulin responsiveness was similar in subjects with or without Mets, the sensitivity to insulin-mediated inhibition of lipolysis and stimulation of lipogenesis was ∼tenfold lower in subjects with MetS (p < 0.0001). When age, sex, adipocyte volume, body mass index and body shape were considered, only the antilipolytic resistance was independently associated with MetS. The mRNA expression of several genes in the canonical insulin signaling pathway were altered in MetS (p < 0.0006 or better) where the mRNA levels of insulin receptor substrate 2 associated with the antilipolytic effect (Rho = 0.34; p = 0.0016).

CONCLUSION

The sensitivities of the antilipolytic and lipogenic effects of insulin are decreased in the MetS but only antilipolysis remains significant after multiple regression analysis. This resistance is localized at initial and receptor-near events in hormone signaling involving insulin receptor substrate 2.

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.

Somatostatin and insulin mediate glucose-inhibited glucagon secretion in the pancreatic α-cell by lowering cAMP

The dysregulation of glucose-inhibited glucagon secretion from the pancreatic islet α-cell is a critical component of diabetes pathology and metabolic disease. We show a previously uncharacterized [Ca(2+)]i-independent mechanism of glucagon suppression in human and murine pancreatic islets whereby cAMP and PKA signaling are decreased. This decrease is driven by the combination of somatostatin, which inhibits adenylyl cyclase production of cAMP via the Gαi subunit of the SSTR2, and insulin, which acts via its receptor to activate phosphodiesterase 3B and degrade cytosolic cAMP. Our data indicate that both somatostatin and insulin signaling are required to suppress cAMP/PKA and glucagon secretion from both human and murine α-cells, and the combination of these two signaling mechanisms is sufficient to reduce glucagon secretion from isolated α-cells as well as islets. Thus, we conclude that somatostatin and insulin together are critical paracrine mediators of glucose-inhibited glucagon secretion and function by lowering cAMP/PKA signaling with increasing glucose.

CD36 level and trafficking are determinants of lipolysis in adipocytes

CD36 has been linked to the etiology of insulin resistance and inflammation. We explored its function in regulating adipose tissue lipolysis, which influences fat accumulation by liver and muscle and overall metabolism. Knockdown of CD36 in differentiated 3T3-L1 adipocytes decreased lipolysis in response to 10 μM of the β-adrenergic agonist isoproterenol (by 42%), 10 μM of the adenyl cyclase activator forskolin (by 32%), and 500 μM of the phosphodiesterase (PDE) inhibitor isobutylmethylxanthine (by 33%). All three treatments in the knockdown adipocytes were associated with significant decreases of cAMP levels and of the hormone-sensitive lipase (HSL) and perilipin phosphorylation. An important role for PDE was supported by the lack of inhibition of the lipolysis induced by the poorly hydrolyzable dibutyryl cAMP analog. An additional contributory mechanism was diminished activation of the Src-ERK1/2 pathway. Regulation of lipolysis and lipolytic signaling by CD36 was reproduced with adipose tissue from CD36(-/-) mice. The importance of surface CD36 in this regulation was suggested by the finding that the plasma membrane-impermeable CD36 inhibitor sulfo-N-succinimidyl oleate (20 μM) decreased lipolysis. Interestingly, isoproterenol induced CD36 internalization, and this process was blocked by HSL inhibition, suggesting feedback regulation of adipocyte lipolysis via CD36 trafficking.

Inhibition of cyclic nucleotide phosphodiesterases by methylxanthines and related compounds

Naturally occurring methylxanthines were the first inhibitors of cyclic nucleotide (cN) phosphodiesterases (PDEs) to be discovered. To improve potency and specificity for inhibition of various PDEs in research and for treatment of diseases, thousands of compounds with related structures have now been synthesized. All known PDE inhibitors contain one or more rings that mimic the purine in the cN substrate and directly compete with cN for access to the catalytic site; this review focuses on inhibitors that contain a nucleus that is closely related to the xanthine ring of theophylline and caffeine and the purine ring of cNs. The specificity and potency of these compounds for blocking PDE action have been improved by appending groups at positions on the rings as well as by modification of the number and distribution of nitrogens and carbons in those rings. Several of these inhibitors are highly selective for particular PDEs; potent and largely selective PDE5 inhibitors are used clinically for treatment of erectile dysfunction [sildenafil (Viagra™), tadalafil (Cialis™) and vardenafil (Levitra™)] and pulmonary hypertension [sildenafil (Revatio™) and tadalafil (Adenocirca)]. Related compounds target other PDEs and show therapeutic promise for a number of maladies.

Implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity

Insulin and leptin play complementary roles in regulating the consumption, uptake, oxidation and storage of nutrients. Chronic consumption of diets that contain a high proportion of calories from saturated fat induces a progressive deterioration in function of both hormones. Certain rat lines and strains of mice are particularly sensitive to the obesogenic and diabetogenic effects of high fat diets, and have been used extensively to study the developmental progression of insulin and leptin resistance in relation to the increasing adiposity that is characteristic of their response to these diets. Some aspects of the diminished efficacy of each hormone are secondary to increased adiposity but a consensus is emerging to support the view that direct effects of dietary components or their metabolites, independent of the resulting obesity, play important roles in development of insulin and leptin resistance. In this minireview, we will examine the implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity, emphasizing potential interactions between pathways that occur among target sites, and exploring how these interactions may influence the progression of obesity and diabetes.

Regulation of adiponectin and leptin secretion and expression by insulin through a PI3K-PDE3B dependent mechanism in rat primary adipocytes

Adiponectin is intimately involved in the regulation of insulin sensitivity, carbohydrate and lipid metabolism, and cardiovascular functions. The circulating concentration of adiponectin is decreased in obesity and Type 2 diabetes. The present study attempts to elucidate the mechanisms underlying the regulation of adiponectin secretion and expression in rat primary adipocytes. The beta-agonist, isoprenaline, decreased adiponectin secretion and expression in a dose-dependent manner in primary adipocytes. Importantly, such an inhibitory effect could be blocked by insulin. The opposing effects of isoprenaline and insulin could be explained by differential regulation of intracellular cAMP levels, since cAMP analogues suppressed adiponectin secretion and expression in a fashion similar to isoprenaline, and insulin blocked the inhibitory effects of the cAMP analogue hydrolysable by PDE (phosphodiesterase). A specific PDE3 inhibitor, milrinone, and PI3K (phosphoinositide 3-kinase) inhibitors abolished the effects of insulin on adiponectin secretion and expression. In the same studies, leptin secretion and expression displayed a similar pattern of regulation to adiponectin. We conclude that insulin and beta-agonists act directly at the adipocytes in opposing fashions to regulate the production of adiponectin and leptin, and that a PI3K-PDE3B-cAMP pathway mediates the effects of insulin to restore beta-agonist/cAMP-suppressed secretion and expression of these two adipokines.

Multisite phosphorylation of adipocyte and hepatocyte phosphodiesterase 3B

Phosphodiesterase 3B (PDE3B) is an important component of insulin and cAMP-dependent signalling pathways. In order to study phosphorylation of PDE3B, we have used an adenoviral system to express recombinant flag-tagged PDE3B in primary rat adipocytes and H4IIE hepatoma cells. Phosphorylation of PDE3B after treatment of cells with insulin, cAMP-increasing agents, or the phosphatase inhibitor, calyculin A was analyzed by two-dimensional tryptic phosphopeptide mapping and mass spectrometry. We found that PDE3B is multisite phosphorylated in adipocytes and H4IIE hepatoma cells in response to all these stimuli. Several sites were identified; serine (S)273, S296, S421, S424/5, S474 and S536 were phosphorylated in adipocyte as well as H4IIE hepatoma cells whereas S277 and S507 were phosphorylated in hepatoma cells only. Several of the sites were phosphorylated by insulin as well as cAMP-increasing hormones indicating integration of the two signalling pathways upstream of PDE3B, maybe at the level of protein kinase B.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

A newly identified 50 kDa protein, which is associated with phosphodiesterase 3B, is phosphorylated by insulin in rat adipocytes

Phosphodiesterase 3B (PDE3B), a major PDE isoform in adipocytes, plays a pivotal role in the anti-lipolytic action of insulin. Insulin phosphorylates and activates PDE3B in a phosphatidylinositol 3-kinase-dependent manner. We identified a new 50 kDa protein that is phosphorylated by insulin and is co-immunoprecipitated with PDE3B by anti-PDE3B antibodies in rat adipocytes. The insulin-induced phosphorylation of the 50 kDa protein was also detected in a cell free system against the N-terminal and the catalytic regions, which are more than 700 amino acids apart recognize the 50 kDa protein, suggesting that it is not a proteolytic product, but an associated protein with PDE3B. Phosphoamino acid analysis indicated that both serine and threonine residues in the 50 kDa protein were phosphorylated, but only serine residues in PDE3B were phosphorylated. Therefore, it appears likely that this is a new protein which is associated with PDE3B.

Effects of the human immunodeficiency virus-protease inhibitor, ritonavir, on basal and catecholamine-stimulated lipolysis

Several of the aspartic acid protease inhibitors used to treat HIV infection increase basal lipolysis in adipocytes, but the cellular mechanisms leading to this augmentation are not well understood. We therefore studied the effects of chronic exposure to the HIV protease inhibitor, ritonavir, on the lipolytic cascade in 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with ritonavir for 14 d (during and after differentiation) enhanced basal, isoproterenol (Iso)-stimulated, and cAMP analog-stimulated lipolysis. Enhancement of lipolysis was observed after Iso at concentrations between 0.1 and 10 mum. Despite a significant decrease in cyclic nucleotide phosphodiesterase (PDE)3B activity and protein levels, there were no changes in Iso-stimulated intracellular cAMP, protein kinase A (PKA) expression, or PKA activity. Ritonavir-augmented lipolysis was also observed under conditions that reversed the effect on PDE3B activity via preincubation with 1 mum (-)-N(6)-(2-phenylisopropyl)adenosine. In ritonavir-treated cells, protein expression of the lipid droplet-protective protein, perilipin, was significantly decreased, whereas there was no change in hormone-sensitive lipase. Activation of ERK1/2 by Iso did not play a role in the augmentation. We conclude that ritonavir decreases PDE3B and perilipin protein expression and affects both basal and catecholamine-stimulated lipolysis in 3T3-L1 adipocytes primarily through actions at sites downstream of PKA.

Milrinone efficiently potentiates insulin secretion induced by orally but not intravenously administered glucose in C57BL6J mice

To study the effect of phosphodiesterase (PDE) 3 inhibition on plasma insulin and glucose levels, the selective PDE 3 inhibitor milrinone (0.25, 1.0, and 2.5 mg/kg) was given orally to anesthetized CL57Bl/6J mice 10 min before a gastric glucose gavage (150 mg/mouse). It was found that milrinone augmented the glucose-mediated increase in plasma insulin at 1.0 and 2.5 mg/kg without, however, any improvement in glucose elimination. In contrast, when given 10 min before intravenous glucose (1 g/kg), milrinone (1 mg/kg) did not affect the insulin response to glucose. The increase in glucagon-like peptide-1 (GLP-1) levels after gastric glucose was not altered by milrinone. However, the PDE3 inhibitor augmented the insulin response to intravenous GLP-1 (2.8 nmol/kg). We therefore conclude that PDE3 inhibition by milrinone augments insulin secretion in vivo in mice after oral but not after intravenous glucose, which may be explained by enhanced response to the cAMP-dependent insulinotropic action of endogenously released GLP-1.

Milrinone, a selective phosphodiesterase 3 inhibitor, stimulates lipolysis, endogenous glucose production, and insulin secretion

In vivo effects of milrinone, a selective phosphodiesterase 3 (PDE-3) inhibitor, on plasma free fatty acids (FFA), glucose, and insulin levels were examined in alert rats. In dose response studies, intravenous injection of 1, 5 or 25 micromol/kg of milrinone provoked an immediate increase in plasma concentrations of FFA and insulin, while glucose levels rose only in response to the 5- and 25-micromol/kg doses. During euglycemic-hyperinsulinemic (approximately 450 pmol/L) clamps, intravenous injection of milrinone (25 micromol/kg) completely inhibited insulin suppression of lipolysis and of endogenous glucose production, while having no effect on insulin-stimulated glucose uptake (ISGU). To explore the reason why ISGU was not affected, we performed reverse-transcriptase polymerase chain reaction (RT-PCR) with RNA from skeletal muscle, fat, and liver. The results showed that PDE-3B mRNA was expressed in adipose tissue and liver, but it was not detected in skeletal muscle. We conclude that PDE-3 plays a major role in the inhibitory action of insulin on lipolysis in fat and on glucose production in liver and, in addition, seems to be involved in insulin secretion in pancreatic beta cells.

Phosphodiesterase 3B gene expression is enhanced in the liver but reduced in the adipose tissue of obese insulin resistant db/db mouse

Phosphodiesterase (PDE) 3B, when activated by insulin, causes a decrease in intracellular cAMP concentration. The activation of this enzyme results in the reduced output of free fatty acids (FFA) from adipocytes, and an increased lipogenesis in liver. We have recently shown that PDE3B gene expression is reduced in adipose tissues of KKAy mice. We intend to further elucidate the regulation of PDE3B in liver as well as adipose tissues in relation to the insulin resistant state. We examined PDE3B gene expression in liver and adipose tissues of obese, insulin-resistant diabetic db/db mice and also checked the effect of an insulin-sensitizing drug, troglitazone, on this gene expression. In the liver of db/db mice, PDE3B mRNA, its corresponding protein, and the associated catalytic activity were all increased by 2.1, 1.9 and 1.6-fold, respectively, over those in db/+ control mice. Histological examination revealed substantial triglyceride storage in the liver of db/db mice. Conversely, in the adipose tissue of db/db mice, PDE3B mRNA, protein, and its associated activity were all decreased by 0.38, 0.33 and 0.36-fold, respectively. Troglitazone, which has no effect on PDE3B in liver, increased the expression of this gene in adipocytes. This increase is associated with a reduction in the elevated levels of serum insulin, glucose, FFA and triglycerides. The reduced PDE3B gene expression in adipose tissues, which results in the elevation of serum FFA, could be the primary event in the development of insulin resistance in db/db mice. The enhanced PDE3B gene expression may correlate with changes in triglyceride storage in the liver of these mice.

Mechanism of intracellular calcium ([Ca2+]i) inhibition of lipolysis in human adipocytes

We investigated the mechanisms responsible for the anti-lipolytic effect of intracellular Ca2+ ([Ca2+]i) in human adipocytes. Increasing [Ca2+]i inhibited lipolysis induced by b-adrenergic receptor activation, A1 adenosine receptor inhibition, adenylate cyclase activation, and phosphodiesterase (PDE) inhibition, as well as by a hydrolyzable cAMP analog, but not by a nonhydrolyzable cAMP analog. This finding indicates that the anti-lipolytic effect of [Ca2+]i may be mediated by the activation of adipocyte PDE. Consistent with this theory, [Ca2+]i inhibition of isoproterenol-stimulated lipolysis was reversed completely by the nonselective PDE inhibitor isobutyl methylxanthine and also by the selective PDE 3B inhibitor cilostamide, but not by selective PDE 1 and 4 inhibitors. In addition, phosphatidylinositol-3 kinase inhibition with wortmannin completely prevented insulin's anti-lipolytic effect but only minimally blocked [Ca2+]i's effect, which suggests that [Ca2+]i and insulin may activate PDE 3B via different mechanisms. In contrast, the antilipolytic effect of [Ca2+]i was not affected by inhibitors of calmodulin, Ca2+/calmodulin-dependent kinase, protein phosphatase 2B, and protein kinase C. Finally, [Ca2+]i inhibited significantly isoproterenol-stimulated increases in cAMP levels and hormone-sensitive lipase phosphorylation in human adipocytes. In conclusion, increasing [Ca2+]i exerts an antilipolytic effect mainly by activation of PDE, leading to a decrease in cAMP and HSL phosphorylation and, consequently, inhibition of lipolysis.

Analysis of the insulin-sensitive phosphodiesterase 3B gene in type 2 diabetes

We screened for mutations in the gene of insulin-sensitive phosphodiesterase 3B (PDE3B), which regulates antilipolytic actions of insulin via reduction of intracellular cyclic AMP levels, in Japanese patients with type 2 diabetes mellitus and lipoatrophic diabetes mellitus using single-stranded conformation polymorphism analysis and Southern analysis and investigated frequencies of variable number of tandem repeats. A silent polymorphism at the Arg463 codon (AGG-->AGA) in exon 4 was identified after examining all 16 exons and exon-intron splicing junctions of the gene. This polymorphism was found in 53 of 100 subjects with type 2 diabetes mellitus, 2 of 5 lipoatrophic diabetic patients and 24 of 50 control subjects, without any significant difference in allele frequency between groups. An EcoRI restriction fragment length polymorphism was identified in patients with type 2 diabetes mellitus and control subjects, again with no differences in occurrence. The allelic distribution of two polymorphic tandem repeats sequences in introns 5 and 12 of the gene did not differ significantly between patients with type 2 diabetes mellitus and control subjects. In conclusion, alterations in the PDE3B gene are unlikely to contribute importantly to the pathogenesis of type 2 diabetes mellitus or lipoatrophic diabetes mellitus in Japan.

Endogenous nitric oxide is implicated in the regulation of lipolysis through antioxidant-related effect

We studied the influence of nitric oxide (NO) endogenously produced by adipocytes in lipolysis regulation. Diphenyliodonium (DPI), a nitric oxide synthase (NOS) inhibitor, was found to completely suppress NO synthesis in intact adipocytes and was thus used in lipolysis experiments. DPI was found to decrease both basal and dibutyryl cAMP (DBcAMP)-stimulated lipolysis. Inhibition of DBcAMP-stimulated lipolysis by DPI was prevented by S-nitroso-N-acetyl-penicillamine (SNAP), a NO donor. This antilipolytic effect of DPI was also prevented by two antioxidants, ascorbate or diethyldithiocarbamic acid (DDC). Preincubation of isolated adipocytes with DPI (30 min) before exposure to DBcAMP almost completely abolished the stimulated lipolysis. Addition of SNAP or antioxidant during DPI preincubation restored the lipolytic response to DBcAMP, whereas no preventive effects were observed when these compounds were added simultaneously to DBcAMP. Exposure of isolated adipocytes to an extracellular generating system of oxygen species (xanthine/xanthine oxidase) or to H(2)O(2) also resulted in an inhibition of the lipolytic response to DBcAMP. H(2)O(2) or DPI decreased cAMP-dependent protein kinase (PKA) activation. The DPI effect on PKA activity was prevented by SNAP, ascorbate, or DDC. These results provide clear evidence that 1) the DPI antilipolytic effect is related to adipocyte NOS inhibition leading to PKA alterations, and 2) endogenous NO is required for the cAMP lipolytic process through antioxidant-related effect.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

Cyclic nucleotide phosphodiesterase 3B is a downstream target of protein kinase B and may be involved in regulation of effects of protein kinase B on thymidine incorporation in FDCP2 cells

Wild-type (F/B), constitutively active (F/B*), and three kinase-inactive (F/Ba-, F/Bb-, F/Bc-) forms of Akt/protein kinase B (PKB) were permanently overexpressed in FDCP2 cells. In the absence of insulin-like growth factor-1 (IGF-1), activities of PKB, cyclic nucleotide phosphodiesterase 3B (PDE3B), and PDE4 were similar in nontransfected FDCP2 cells, mock-transfected (F/V) cells, and F/B and F/B- cells. In F/V cells, IGF-1 increased PKB, PDE3B, and PDE4 activities approximately 2-fold. In F/B cells, IGF-1, in a wortmannin-sensitive manner, increased PKB activity approximately 10-fold and PDE3B phosphorylation and activity ( approximately 4-fold), but increased PDE4 to the same extent as in F/V cells. In F/B* cells, in the absence of IGF-1, PKB activity was markedly increased ( approximately 10-fold) and PDE3B was phosphorylated and activated (3- to 4-fold); wortmannin inhibited these effects. In F/B* cells, IGF-1 had little further effect on PKB and activation/phosphorylation of PDE3B. In F/B- cells, IGF-1 activated PDE4, not PDE3B, suggesting that kinase-inactive PKB behaved as a dominant negative with respect to PDE3B activation. Thymidine incorporation was greater in F/B* cells than in F/V cells and was inhibited to a greater extent by PDE3 inhibitors than by rolipram, a PDE4 inhibitor. In F/B cells, IGF-1-induced phosphorylation of the apoptotic protein BAD was inhibited by the PDE3 inhibitor cilostamide. Activated PKB phosphorylated and activated rPDE3B in vitro. These results suggest that PDE3B, not PDE4, is a target of PKB and that activated PDE3B may regulate cAMP pools that modulate effects of PKB on thymidine incorporation and BAD phosphorylation in FDCP2 cells.

Wortmannin, a PI3-kinase inhibitor: promoting effect on insulin secretion from pancreatic beta cells through a cAMP-dependent pathway

To determine the role of phosphatidylinositol 3-kinase (PI3-kinase) in the regulation of insulin secretion, we examined the effect of wortmannin, a PI3-kinase inhibitor, on insulin secretion using the isolated perfused rat pancreas and freshly isolated islets. In the perfused pancreas, 10(-8) M wortmannin significantly enhanced the insulin secretion induced by the combination of 8.3 mM glucose and 10(-5) M forskolin. In isolated islets, cyclic AMP (cAMP) content was significantly increased by wortmannin in the presence of 3.3 mM, 8.3 mM, and 16.7 mM glucose with or without forskolin. In the presence of 16.7 mM glucose with or without forskolin, wortmannin promoted insulin secretion significantly. On the other hand, in the presence of 8.3 mM glucose with forskolin, wortmannin augmented insulin secretion significantly; although wortmannin tended to promote insulin secretion in the presence of glucose alone, it was not significant. To determine if wortmannin increases cAMP content by promoting cAMP production or by inhibiting cAMP reduction, we examined the effects of wortmannin on 10(-4) M 3-isobutyl-1-methylxantine (IBMX)-induced insulin secretion and cAMP content. In contrast to the effect on forskolin-induced secretion, wortmannin had no effect on IBMX-induced insulin secretion or cAMP content. Moreover, wortmannin had no effect on nonhydrolyzable cAMP analog-induced insulin secretion in the perfusion study. These data indicate that wortmannin induces insulin secretion by inhibiting phosphodiesterase to increase cAMP content, and suggest that PI3-kinase inhibits insulin secretion by activating phosphodiesterase to reduce cAMP content.

The lipolytic stimulation of 3T3-L1 adipocytes promotes the translocation of hormone-sensitive lipase to the surfaces of lipid storage droplets

Hormone-sensitive lipase catalyzes the rate-limiting step in the release of fatty acids from triacylglycerol-rich lipid storage droplets of adipocytes, which contain the body's major energy reserves. Hormonal stimulation of cAMP formation and the activation of cAMP-dependent protein kinase leads to the phosphorylation of hormone-sensitive lipase and a large increase in lipolysis in adipocytes. By contrast, phosphorylation of hormone-sensitive lipase by the kinase in vitro results in a comparatively minor increase in catalytic activity. In this study, we investigate the basis for this discrepancy by using immunofluorescence microscopy to locate hormone-sensitive lipase in lipolytically stimulated and unstimulated 3T3-L1 adipocytes. In unstimulated cells, hormone-sensitive lipase is diffusely distributed throughout the cytosol. Upon stimulation of cells with the beta-adrenergic receptor agonist, isoproterenol, hormone-sensitive lipase translocates from the cytosol to the surfaces of intracellular lipid droplets concomitant with the onset of lipolysis, as measured by the release of glycerol to the culture medium. Both hormone-sensitive lipase translocation and lipolysis are reversed by the incubation of cells with the beta-adrenergic receptor antagonist, propranolol. The treatment of cells with cycloheximide fails to inhibit lipase translocation or lipolysis, indicating that the synthesis of nascent proteins is not required. Cytochalasin D and nocodazole used singly and in combination also failed to have a major effect, thus suggesting that the polymerization of microfilaments and microtubules and the formation of intermediate filament networks is unnecessary. Hormone-sensitive lipase translocation and lipolysis were inhibited by N-ethylmaleimide and a combination of deoxyglucose and sodium azide. We propose that the major consequence of the phosphorylation of hormone-sensitive lipase following the lipolytic stimulation of adipocytes is the translocation of the lipase from the cytosol to the surfaces of lipid storage droplets.

Type I cAMP-dependent protein kinase delays apoptosis in human neutrophils at a site upstream of caspase-3

Current data suggest that apoptosis controls neutrophil numbers in tissues. We analyzed roles for and the sites of action for the cAMP-dependent protein kinases (cAPKs) in apoptosis induced in human neutrophils by in vitro storage, cycloheximide (CHX) exposure, and anti-Fas exposure. Treatment with 8-chlorophenylthio-cAMP (8-CPT-cAMP) prolonged the time required for 50% of the cells to exhibit apoptotic morphology (t50) from 16.3 to 41.8 h (in vitro culture), from 2.4 to 7.8 h (CHX), and from 4.8 to 6.5 h (anti-Fas). CHX +/- 8-CPT-cAMP did not significantly alter resting intracellular calcium levels and H-89, a selective inhibitor of cAPK, had no effect on apoptosis in the absence of the analogue. In contrast, site-selective cAMP analogues that specifically activated the type I cAPK, but not type II cAPK, synergistically attenuated apoptosis. Exposure to 8-CPT-cAMP delayed, in parallel, the activity of caspase-3 (CPP-32beta), whereas mitogen-activated protein kinase kinase (MAPKK) inhibitor, PD98059, had no effect on CHX-induced apoptosis +/- 8-CPT-cAMP. Together these results indicate that type I cAPK activation is necessary and sufficient to mediate cAMP-induced delay in human neutrophil apoptosis induced by several mechanisms and suggest that one of the major sites of cAPK action is upstream of caspase-3 (CPP-32beta) activation.

Phosphorylation and activation of hormone-sensitive adipocyte phosphodiesterase type 3B

Phosphodiesterases (PDEs) include a large group of structurally related enzymes that belong to at least seven related gene families (PDEs 1-7) that differ in their primary structure, affinity for cAMP and cGMP, response to specific effectors, sensitivity to specific inhibitors, and regulatory mechanism. One characteristic of PDE3s involves their phosphorylation and activation in response to insulin as well as to agents that increase cAMP in adipocytes, hepatocytes, and platelets and in response to insulin-like growth factor 1 in pancreatic beta cells. In adipocytes, activation of the membrane-associated PDE3B is the major mechanism whereby insulin antagonizes catecholamine-induced lipolysis. PDE3B activation results in increased degradation of cAMP and, thereby, a lowering of the activity of cAMP-dependent protein kinase (PKA). The reduced activity of PKA leads to a net dephosphorylation and decreased activity of hormone-sensitive lipase and reduced hydrolysis of triglycerides. Activation of the rat adipocyte PDE3B by insulin is associated with phosphorylation of serine-302. The mechanism whereby insulin stimulation leads to phosphorylation/activation of PDE3B is only partly understood. In rat adipocytes, lipolytic hormones and other agents that increase cAMP, including isoproterenol, also induce rapid phosphorylation, presumably catalyzed by PKA, of serine-302 of PDE3B. The phosphorylation is associated with activation of the enzyme, most likely representing "feedback" regulation of cAMP, presumably allowing close coupling of the regulation of steady-state concentrations of both cAMP and PKA and, thereby, control of lipolysis. In the review we describe methods and strategies used in the authors' laboratories to study phosphorylation and activation of PDE3B in adipocytes and in vitro.

Insulin-induced phosphorylation and activation of phosphodiesterase 3B in rat adipocytes: possible role for protein kinase B but not mitogen-activated protein kinase or p70 S6 kinase

Insulin stimulation of adipocytes results in serine phosphorylation/activation of phosphodiesterase 3B (PDE 3B) and activation of a kinase that phosphorylates PDE 3B in vitro, key events in the antilipolytic action of this hormone. We have investigated the role for p70 S6 kinase, mitogen-activated protein kinases (MAP kinases), and protein kinase B (PKB) in the insulin signaling pathway leading to phosphorylation/activation of PDE 3B in adipocytes. Insulin stimulation of adipocytes resulted in increased activity of p70 S6 kinase, which was completely blocked by pretreatment with rapamycin. However, rapamycin had no effect on the insulin-induced phosphorylation/activation of PDE 3B or the activation of the kinase that phosphorylates PDE 3B. Stimulation of adipocytes with insulin or phorbol myristate acetate induced activation of MAP kinases. Pretreatment of adipocytes with the MAP kinase kinase inhibitor PD 98059 was without effect on the insulin-induced activation of PDE 3B. Furthermore, phorbol myristate acetate stimulation did not result in phosphorylation/activation of PDE 3B or activation of the kinase that phosphorylates PDE 3B. Using Mono Q and Superdex chromatography, the kinase that phosphorylates PDE 3B was found to co-elute with PKB, but not with p70 S6 kinase or MAP kinases. Furthermore, both PKB and the kinase that phosphorylates PDE 3B were found to translocate to membranes in response to peroxovanadate stimulation of adipocytes in a wortmannin-sensitive way. Whereas these results suggest that p70 S6 kinase and MAP kinases are not involved in the insulin-induced phosphorylation/activation of PDE 3B in rat adipocytes, they are consistent with PKB being the kinase that phosphorylates PDE 3B.

A novel assay for evaluating glycogenolysis in rat adipocytes and the inability of insulin to antagonize glycogenolysis in this cell type

We report here on a novel procedure for measuring glycogenolysis in rat adipocytes. In this procedure, cells are incubated for 30 min at 37 degrees C with insulin or vanadate, and with [U-14C]glucose to label the glycogen pool with radioactive glucose. The cells are washed and preincubated for an additional 1 h, before being assayed. The extent of glycogenolysis is determined by the decrease in radioactivity in precipitated glycogen, which was quite substantial under experimental conditions facilitating glycogenolysis. From the assay, we determined the following. (a) Glycogenolysis is activated in rat adipocytes in response to lipolytic hormones (i.e. catecholamines and adrenocorticotropic hormone). (b) Other agents and conditions elevating intracellular adenosine 3',5'-monophosphate levels (i.e. cholera toxin, dibutyryladenosine 3',5'-monophosphate, and isobutylmethylxanthine) also activate glycogenolysis. (c) Glycogenolysis (as opposed to lipolysis) is activated at concentrations of adrenocorticotropic hormone or isoproterenol 7-11-fold lower and at adenosine 3',5'-monophosphate concentrations 7-fold lower. (d) Calyculin A, a specific inhibitor of protein phosphatase 1, activates glycogenolysis as well. Calyculin A also activates lipolysis at an equimolar potency. (e) Insulin does not antagonize glycogenolysis in rat adipocytes. In conclusion, the assay allowed us to compare glycogenolysis to lipolysis within the same cell, and to find that the sensitivity to hormones and adenosine 3',5'-monophosphate was about 1 order of magnitude higher for glycogenolysis than for lipolysis. A more striking finding was the inability of insulin to antagonize glycogenolysis in the rat adipose cell, an effect which occurs readily in liver and muscle cells via protein phosphatase 1-activating machinery. This rules out a role for adipose protein phosphatase 1 activation in the mechanism by which insulin antagonizes lipolysis and supports the contention that the insulin effect in lowering adenosine 3',5'-monophosphate levels is the central mechanism by which insulin antagonizes lipolysis.

Attenuation of insulin secretion by insulin-like growth factor 1 is mediated through activation of phosphodiesterase 3B

Both insulin and insulin-like growth factor 1 (IGF-1) are known to reduce glucose-dependent insulin secretion from the beta cells of pancreatic islets. In this paper we show that the mechanism by which IGF-1 mediates this effect is in large part through activation of a specific cyclic nucleotide phosphodiesterase, phosphodiesterase 3B (PDE3B). More specifically, in both isolated pancreatic islets and insulin-secreting HIT-T15 cells, IGF-1 inhibits insulin secretion that has been increased by glucose and glucagonlike peptide 1 (GLP-1). Moreover, IGF-1 decreases cAMP levels in parallel to the reduction of insulin secretion. Insulin secretion stimulated by cAMP analogs that activate protein kinase A and also are substrates for PDE3B is also inhibited by IGF-1. However, IGF-1 does not inhibit insulin secretion stimulated by nonhydrolyzable cAMP analogs. In addition, selective inhibitors of PDE3B completely block the ability of IGF-1 to inhibit insulin secretion. Finally, PDE3B activity measured in vitro after immunoprecipitation from cells treated with IGF-1 is higher than the activity from control cells. Taken together with the fact that pancreatic beta cells express little or no insulin receptor but large amounts of IGF-1 receptor, these data strongly suggest a new regulatory feedback loop model for the control of insulin secretion. In this model, increased insulin secretion in vivo will stimulate IGF-1 synthesis by the liver, and the secreted IGF-1 in turn feedback inhibits insulin secretion from the beta cells through an IGF-1 receptor-mediated pathway. This pathway is likely to be particularly important when levels of both glucose and secretagogues such as GLP-1 are elevated.

Identification of the site in the cGMP-inhibited phosphodiesterase phosphorylated in adipocytes in response to insulin and isoproterenol

Stimulation of rat adipocytes with insulin and isoproterenol results in serine phosphorylation and activation of the adipocyte cGMP-inhibited phosphodiesterase (cGI PDE), events believed to be important in the antilipolytic action of insulin (Degerman, E., Smith, C.J., Tornqvist, H., Vasta, V., Manganiello, V.C., and Belfrage, P. (1990) Proc. Natl. Acad. Sci. U.S.A. 87,533-537). Here we demonstrate, by two-dimensional phosphopeptide mapping, that the major phosphopeptide generated by trypsin, or trypsin followed by Asp-N protease digestion of [32P]cGI PDE phosphorylated in adipocytes in response to isoproterenol and/or insulin, in each case co-migrates with the phosphopeptide released by the same treatment of M297FRRPS(P)LPCISREQ310. This peptide was synthesized based on the deduced sequence of the cloned rat adipocyte cGI PDE and phosphorylated by cAMP-dependent protein kinase (protein kinase A). Radiosequencing of authentic and synthetic tryptic 32P-peptides showed that a single site in cGI PDE (Ser302) was phosphorylated in adipocytes incubated with isoproterenol and/or insulin. The more than additive phosphorylation and activation of cGI PDE in response to the two hormones found in this report and previously (Smith, C.J., Vasta, V., Degerman, E., Belfrage, P., and Manganiello, V.C. (1991) J. Biol. Chem. 266, 13385-13390) is proposed to reflect cross-talk between their respective signal transduction pathways at the level of the cGI PDE serine protein kinase or upstream regulatory component(s).

Differential cellular pattern of gene expression for two distinct cGMP-inhibited cyclic nucleotide phosphodiesterases in developing and mature rat brain

Cyclic GMP-inhibited phosphodiesterases are characterized by sensitivity of cAMP hydrolysis to inhibition by cGMP. This phosphodiesterase family contains at least two different isoforms (PDE3A and PDE3B) encoded by distinct genes and serving tissue-specific roles in regulation of lipolysis, glycogenolysis, myocardial contractility, and smooth muscle relaxation. Our previous work indicated an abundance of these two phosphodiesterase messenger RNAs in the embryonic rat brain, and therefore, to elucidate the potential functions of these enzymes in brain development as well as in mature brain function, the present study mapped cellular patterns of gene expression for these two enzymes from embryonic day 15 to adulthood using in situ hybridization histochemistry. Phosphodiesterase 3B isoform messenger RNA is uniformly expressed in germinal neuroepithelium and mature neurons, with distribution generally reflecting cell density. Phosphodiesterase isoform 3A messenger RNA, in contrast, demonstrates striking spatiotemporal specificity of expression, with three distinct patterns being evident. Firstly, this mRNA is highly abundant in both primary and secondary neuroepithelial germinal zones. Secondly, during early postnatal development PDE3A mRNA is transiently but highly expressed in neurons localized in basal forebrain, deep cerebellar, pontine, interpeduncular and a variety of thalamic, midbrain and brainstem nuclei. Thirdly, PDE3A mRNA is focally expressed in isolated large striatal and hippocampal neurons from the perinatal period without attenuation into adulthood. In summary, two cGMP-inhibited phosphodiesterase isoforms show distinctive patterns of gene expression in brain: PDE3B gene expression is uniform without evidence of system specificity or developmental stage specificity, suggesting that this isoform has a constitutive role in neuroepithelial metabolism, while PDE3B gene expression demonstrates a high level of spatiotemporal heterogeneity, suggesting that this isoform subserves a variety of developmental stage-specific and system-specific functions.

Type III cGMP-inhibited cyclic nucleotide phosphodiesterases (PDE3 gene family)

Seven different but related cyclic nucleotide phosphodiesterase (PDE) gene families have been identified. Type III cGMP-inhibited (cGI) PDEs, the PDE3 gene family, are found in many tissues. cGI PDEs exhibit a high affinity for both cAMP and cGMP, and are selectively and relatively specifically inhibited by certain agents which augment myocardial contractility, promote smooth muscle relaxation and inhibit platelet aggregation. Adipocyte, platelet, and hepatocyte cGI PDE activities are regulated by cAMP-dependent phosphorylation. Insulin-induced phosphorylation/activation of adipocyte and hepatocyte cGI PDEs is thought to be important in acute regulation of triglyceride and glycogen metabolism by insulin. Two distinct cGI PDE subfamilies, products of distinct but related genes, have been identified. They exhibit the domain structure common to PDEs with a carboxyterminal region, conserved catalytic domain and divergent regulatory domain. In their catalytic domains cGI PDEs contain a 44 amino acid insertion not found in other PDE families. The expression of cGIP1 and cGIP2 mRNAs differs in different rat tissues, suggesting distinct functions for the two cGI PDE subfamilies, i.e., cGIP1 in adipose tissue, liver, testis and cGIP2 in myocardium, platelets and smooth muscle.

Desensitization of beta-adrenergic responses in adipocytes involves receptor subtypes and cAMP phosphodiesterase

Acute exposure of isolated adipocytes to isoproterenol induces the desensitization of lipolytic responses to norepinephrine and selective beta 1-, beta 2- and beta 3-adrenoceptor agonists, as well as the adrenocorticotropic hormone 1-24 fragment (ACTH). Forskolin and 8-bromo-cAMP responses are also desensitized. When lipolysis was measured in the presence of OPC 3911 [N-cyclohexyl-N-2-hydroxyethyl-4(6-(1,2-dihydro-2- oxoquinolyloxy))butyramide], a specific inhibitor of the cAMP phosphodiesterase of adipocytes, the desensitization of all lipolytic agents--except the beta 2-adrenoceptor agonist--was abolished. Isoproterenol induced a similar loss (35%) of both membrane beta 1- and beta 2-adrenoceptors and an uncoupling of beta 1-adrenoceptors, but did not modify the weak coupling of control beta 2-adrenoceptors. These data suggest that isoproterenol induced (i) an activation of the cAMP phosphodiesterase, which is solely responsible for the desensitization of norepinephrine response as well as beta 1- and beta 3-adrenoceptor mediated responses and (ii) an additional desensitization of the sole beta 2-adrenergic signaling system which suggests a subtype-selective pattern of regulating processes.

Evidence for the key role of the adipocyte cGMP-inhibited cAMP phosphodiesterase in the antilipolytic action of insulin

Enhancement of cAMP degradation by increased cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) activity is thought to be an important component of the mechanism whereby insulin counteracts catecholamine-induced lipolysis in adipocytes. In this study the selective cGI-PDE inhibitor OPC3911 was used to evaluate this role of cGI-PDE activation in intact rat adipocytes with special reference to changes in cAMP levels measured as cAMP-dependent protein kinase (cAMP-PK) activity ratios. OPC3911 completely blocked (IC50 = 0.3 microM) the maximal inhibitory effect of insulin on noradrenaline-induced lipolysis and the net dephosphorylation of hormone-sensitive lipase and other intracellular target proteins for insulin action, whereas insulin-induced lipogenesis was not changed. The effect of OPC3911 on cAMP-PK activity ratios at different levels of lipolysis achieved by noradrenaline stimulation revealed that the reduction of cAMP-PK caused by 1 nM insulin was completely blocked by 3 microM OPC3911. The effect of OPC3911 was not due to an excessive increase in cellular cAMP resulting in 'supramaximal' lipolysis unresponsive to insulin. These data demonstrate that reduction in cAMP levels by the activation of cGI-PDE may be sufficient to account for the antilipolytic action of insulin.

Essential role of phosphatidylinositol 3-kinase in insulin-induced activation and phosphorylation of the cGMP-inhibited cAMP phosphodiesterase in rat adipocytes. Studies using the selective inhibitor wortmannin

Incubation of rat adipocytes with wortmannin, a potent and selective phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor, completely blocked the antilipolytic action of insulin (IC50 = 100 nM), the insulin-induced activation and phosphorylation of cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) as well as the activation of the insulin-stimulated cGI-PDE kinase (IC50 = 10-30 nM). No direct effects of the inhibitor on the insulin-stimulated cGI-PDE kinase, the cGI-PDE and the hormone-sensitive lipase were observed. These data suggest that activation of PI 3-kinase upstream of the insulin-stimulated cGI-PDE kinase in the antilipolytic insulin signalchain has an essential role for insulin-induced cGI-PDE activation/phosphorylation and anti-lipolysis.

Actions of the novel antidiabetic agent englitazone in rat hepatocytes

We examined effects of a novel antidiabetic agent, racemic englitazone (CP 68,722, Pfizer), on normal rat hepatocytes in vitro. For optimal effects, CP 68,722 must be preincubated for approximately 20 minutes. CP 68,722 inhibited the actions of glucagon on glycogenolysis (measured by monitoring cyclic adenosine monophosphate [cAMP] levels, phosphorylase activation, and glucose output) and gluconeogenesis (from 14C-lactate). Since CP 68,722 was able to attenuate the ability of glucagon to increase cAMP levels, this may account for part of its inhibitory actions on glycogenolysis and gluconeogenesis. The observation that CP 68,722 also inhibits the ability of the cAMP analog, 8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate (8 CPT cAMP), to stimulate phosphorylase a is consistent with an effect of CP 68,722 to activate cAMP-dependent phosphodiesterase. The ability of vasopressin (an agonist known to stimulate glycogenolysis via a Ca(2+)-dependent mechanism) to stimulate phosphorylase a was slightly inhibited by CP 68,722. Another site of action of CP 68,722 was to inhibit hormonal-mediated Ca2+ influx, an effect that would decrease intracellular free calcium ([Ca2+]i), thereby inhibiting the actions of the Ca(2+)-dependent hormones such as alpha 1-adrenergic agonists and vasopressin, agents known to promote glucose output from the liver. In summary, CP 68,722 inhibits glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes by a mechanism that may include activation of cAMP phosphodiesterase and inhibition of Ca2+ influx.

The role of protein phosphorylation in the regulation of cyclic nucleotide phosphodiesterases

The cyclic nucleotide phosphodiesterases constitute a complex superfamily of enzymes responsible for catalyzing the hydrolysis of cyclic nucleotides. Regulation of cyclic nucleotide phosphodiesterases is one of the two major mechanisms by which intracellular cyclic nucleotide levels are controlled. In many cases the fluctuations in cyclic nucleotide levels in response to hormones is due to the hormone responsiveness of the phosphodiesterase. Isozymes of the cGMP-inhibited, cAMP-specific, calmodulin-stimulated and cGMP-binding phosphodiesterases have been demonstrated to be substrates for protein kinases. Here we review the evidence that hormonally responsive phosphorylation acts to regulate cyclic nucleotide phosphodiesterases. In particular, the cGMP-inhibited phosphodiesterases, which can be phosphorylated by at least two different protein kinases, are activated as a result of phosphorylation. In contrast, phosphorylation of the calmodulin-stimulated phosphodiesterases, which coincides with a decreased sensitivity to activation by calmodulin, results in decreased phosphodiesterase activity.

Hormonal activation of the cGMP-inhibited low-Km cyclic AMP phosphodiesterase of rat adipocytes from different sites: influence of ovariectomy

Hormonal activation of the cGMP-inhibited low Km cyclic AMP phosphodiesterase isoenzyme (cGI.PDE) by effectors, acting either through the cAMP-independent (insulin) or through cAMP-dependent (isoproterenol, forskolin ACTH and 8Br-cAMP) mechanisms, were compared in parametrial (PM) and femoral subcutaneous (SC) adipocytes from sham-operated (SHAM) and ovariectomized (OVX) rats. In SHAM rats, the basal cGI.PDE activity was 50% higher in PM than in SC adipocytes. In OVX rats, the cGi.PDE activatory responses to all the effectors tested remained unchanged in SC, but were completely suppressed in PM adipocytes. The mechanism underlying these defective cGI.PDE activatory responses to cAMP-dependent effectors observed in PM adipocytes after OVX seems to involve protein kinase A, since a decreased activation of cGI.PDE by protein kinase A was also found in these cells. Treatment of OVX rats with both estradiol and progesterone reversed the defective cAMP-dependent activation of cGI.PDE, but not the refractoriness of this isoenzyme to insulin activation. Taken together with previous observations from this laboratory on the fat cell adenylate cyclase system (Lacasa et al. (1991) Endocrinology 128, 747-753), these results: (a) demonstrate that the influence of the ovarian status on the key enzymes controlling cAMP metabolism in fat cells depends on the anatomical origin of these cells, and; (b) provide a biochemical explanation to the insensitivity of the SC adipocyte lipolytic system to ovarian hormones.

Cytophotometric analysis of glycogen, protein and DNA of a glycogen-storing rat hepatoma (N13) cell line

This study examines the behavior of glycogen-storing rat hepatoma (N13) in vitro using cytophotometric techniques. A significant increase in glycogen is observed in these cells after 30 min incubation in a buffered solution containing 0.1 mM glucose, that is 80 times lower than the physiological glucose concentration in rat blood. N13 hepatoma cells grow exponentially in culture using RPMI 1640 tissue culture medium supplemented with 10% fetal bovine serum. During the first day in culture these cells store a large amount of glycogen and this increase is also observed in serum-free cultures. In more prolonged cultures the amount of glycogen per cell gradually becomes lower, although the culturing conditions are maintained. Similar variations of protein are also observed during the initial period of culture. DNA distribution does not show significant changes, although in serum-free cultures an increase in the proportion of cells in S and G2/M phases is observed. The addition of glucagon, epinephrine and cyclic AMP derivatives to serum-free cultures does not impede the storage of glycogen. Nevertheless, addition of either 2 mM N6,O2'-dibutyryl cyclic AMP or 0.1 mM 8-(4-chlorophenylthio)-cyclic AMP blocks the cell cycle at G0/G1 and glycogen content does not decrease after the first day in culture. We believe that this cell line offers an appropriated model to study glycogen metabolism and its involvement in the neoplastic process.

Modulation of IFN-mediated Ly-6E antigen induction by cAMP in a T cell lymphoma: opposite effects on the responses to IFN-gamma and IFN-alpha/beta

This study was initiated to examine the role of cyclic nucleotides in the regulation of the expression of the Ly-6E cell surface Ag by IFN. As a model system, we used the YAC T cell lymphoma where this Ag is constitutively absent but is highly inducible by both IFN-gamma and IFN-alpha/beta. Treatment with cAMP or cGMP analogs did not cause Ly-6E expression in the absence of IFN. However, treatment with cAMP analogs, but not with cGMP analogs, markedly altered Ly-6E expression triggered by IFN, both at the mRNA and at the cell surface protein levels. Interestingly, these effects depended on whether Ly-6E induction was mediated by IFN-gamma or IFN-alpha/beta. Thus, the response to IFN-gamma was enhanced up to tenfold, whereas the response to IFN-alpha/beta was suppressed by 90-95%. Similar differential modulations of Ly-6E induction were also exerted by forskolin and cholera toxin, which are known to elevate intracellular cAMP concentration through distinct mechanisms. A YAC cell variant (C10) was isolated, where cAMP analogs or cAMP inducers could not modify Ly-6E induction. Although resistant to the biological effect of cAMP, the C10 mutant exhibited normal IFN-mediated Ly-6E responses. Moreover, in this mutant, Ly-6E induction was still affected by the PKC activator PMA, as in wild-type YAC cells. Altogether, our data demonstrate that cAMP (and cGMP) is not directly involved as second messenger in Ly-6E induction mediated by IFNs. However, a rise of cAMP modulates in an opposite fashion the Ly-6E-inducing actions of IFN-gamma and IFN-alpha/beta, which suggests that the two types of IFN utilize separate biochemical pathways to regulate Ly-6E expression.