Hormone-sensitive cyclic GMP-inhibited cyclic AMP phosphodiesterase in rat adipocytes. Regulation of insulin- and cAMP-dependent activation by phosphorylation

Phosphodiesterases 2, 3 and 4 can decrease cardiac effects of H2-histamine-receptor activation in isolated atria of transgenic mice

Histamine exerts cAMP-dependent positive inotropic effects (PIE) and positive chronotropic effects (PCE) on isolated left and right atria, respectively, of transgenic mice which overexpress the human H₂-receptor in the heart (=H₂-TG). To determine whether these effects are antagonized by phosphodiesterases (PDEs), contractile studies were done in isolated left and right atrial preparations of H₂-TG. The contractile effects of histamine were tested in the additional presence of the PDE-inhibitorserythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA, 1 μM, PDE2-inhibitor) or cilostamide (1 μM, PDE3-inhibitor), rolipram (10 μM, a PDE4-inhibitor), and their combinations. Cilostamide (1 μM) and EHNA (1 μM), rolipram (1 μM), and EHNA (1 μM) and the combination of rolipram (0.1 μM) and cilostamide (1 μM) each increased the potency of histamine to elevate the force of contraction (FOC) in H₂-TG. Cilostamide (1 μM) and rolipram (10 μM) alone increased and EHNA (1 μM) decreased alone, and their combination increased the potency of histamine to increase the FOC in H₂-TG indicating that PDE3 and PDE4 regulate the inotropic effects of histamine in H₂-TG. The PDE inhibitors (EHNA, cilostamide, rolipram) alone did not alter the potency of histamine to increase the heart beat in H₂-TG whereas a combination of rolipram, cilostamide, and EHNA, or of rolipram and EHNA increased the potency of histamine to act on the beating rate. In summary, the data suggest that the PCE of histamine in H₂-TG atrium involves PDE 2 and 4 activities, whereas the PIE of histamine are diminished by activity of PDE 3 and 4.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Phosphodiesterase PDE2 activity, increased by isoprenaline, does not reduce β-adrenoceptor-mediated chronotropic and inotropic effects in rat heart

Myocardial PDE2 activity increases in terminal human heart failure and after isoprenaline infusion in rat heart. PDE2 inhibitors do not potentiate the murine sinoatrial tachycardia produced by noradrenaline. We investigated whether isoprenaline infusion induces PDE2 to decrease the chronotropic and inotropic effects of catecholamines in rat heart. Sprague-Dawley rats were infused with isoprenaline (2.4 mg kg^-1 day^-1) for 3 days. We used spontaneously beating right atria, paced right ventricular strips and left ventricular papillary muscles. The effects of the PDE2 inhibitors EHNA (10 μM) and Bay 60-7550 (0.1-1 μM) were investigated on the cardiostimulation produced by noradrenaline (ICI118551 50 nM present to block β₂-adrenoceptors) and adrenaline (CGP20712A 300 nM present to block β₁-adrenoceptors). Hydrolysis of cAMP by PDE2 was measured by radioenzyme assay. Bay 60-7550 but not EHNA increased sinoatrial beating. A stable tachycardia elicited by noradrenaline (10 nM) or adrenaline (1 μM) was not increased by the PDE2 inhibitors. Isoprenaline infusion increased the hydrolytic PDE2 activity threefold in left ventricle, reduced the chronotropic and inotropic effects and potency of noradrenaline and abolished the effects of adrenaline. The potency of the catecholamines was not increased by the PDE2 inhibitors. Neither EHNA nor Bay 60-7550 potentiated the effects of the catecholamines. Rat PDE2 decreased basal sinoatrial beating but did not reduce the sinoatrial tachycardia or increases of ventricular force mediated through β₁- and β₂-adrenoceptors. The β-adrenoceptor desensitization induced by the isoprenaline infusion was not reversed by the PDE2 inhibitors despite the increased hydrolysis of cAMP by PDE2.

Rosmarinic acid suppresses adipogenesis, lipolysis in 3T3-L1 adipocytes, lipopolysaccharide-stimulated tumor necrosis factor-α secretion in macrophages, and inflammatory mediators in 3T3-L1 adipocytes

Background: Rosmarinic acid (RA) is a natural phenol carboxylic acid with many promising biological effects. It may be a suitable candidate for improving obesity-related adipose tissue dysfunction.

Objective: We aimed to investigate the therapeutic use of RA as an anti-obesity agent by measuring its effects on adipogenesis, lipolysis, and messenger RNA (mRNA) expression of major adipokines in 3T3-L1 adipocytes; and its effects on lipopolysaccharide (LPS)-induced tumor necrosis factor-α (TNF-α) secretion in macrophages and inflammatory mediators in 3T3-L1 adipocytes incubated with macrophage-conditioned medium (MCM).

Methods: 3T3-L1 preadipocytes were used to explore how RA affects adipogenesis, as well as the involvement of phosphorylated extracellular signal-regulated kinase-1/2 (p-ERK1/2) and mothers against decapentaplegic homolog 3 (p-Smad3). 3T3-L1 preadipocytes were also differentiated into mature adipocytes to explore how RA affects basal and isoproterenol- and forskolin-stimulated lipolysis; and how RA affects key adipokines’ mRNA expression. RAW 264.7 macrophages were stimulated with LPS in the absence or presence of RA to explore RA’s effects on TNF-α secretion. MCM was collected and 3T3-L1 adipocytes were incubated with MCM to explore RA’s effects on interleukin-6 (IL-6), IL-1β, monocyte chemoattractant protein-1 (MCP-1), and RANTES mRNA expression.

Results: During the preadipocyte differentiation process, RA suppressed peroxisome proliferator-activated receptor-γ and CCAAT/enhancer binding protein-α, and activated p-ERK1/2 and p-Smad3; inhibition of adipogenesis by RA was partially restored following treatment with p-ERK1/2 and p-Smad3 inhibitors. In mature adipocytes, RA inhibited basal lipolysis; phosphodiesterase-3 inhibitor reversed this. RA also inhibited isoproterenol- and forskolin-stimulated glycerol and free fatty acid release, and the phosphorylation of hormone-sensitive lipase and perilipin. RA had no effects on leptin, adiponectin, resistin, or visfatin mRNA expression. RA suppressed TNF-α mRNA expression and secretion in LPS-stimulated RAW 264.7 macrophages; and reduced LPS-MCM-induced IL-6, IL-1β, MCP-1, and RANTES mRNA expression in 3T3-L1 adipocytes.

Conclusions: RA exerts inhibitory effects on adipogenesis, lipolysis, and inflammation. RA could be a promising natural product for improving adipose mobilization in obesity.

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.

Urinary polycyclic aromatic hydrocarbons and childhood obesity: NHANES (2001-2006)

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) are known carcinogens and suspected endocrine disruptors. Prenatal exposure to PAHs has been associated with obesity in early childhood.

OBJECTIVE

We examined the association of urinary PAH metabolites with adiposity outcomes [body mass index (BMI) z-score, waist circumference (WC), and rate of obesity] in children and adolescents.

METHODS

We performed whole-sample analyses of 3,189 individuals 6-19 years of age who participated in the 2001-2006 National Health and Nutrition Examination Survey. We performed multivariate linear and logistic regression to analyze the association of BMI z-score, WC, and obesity with concentrations of single urinary PAH compounds and the sum of PAHs. Furthermore, the analyses were stratified by developmental stage [i.e., children (6-11 years) and adolescents (12-19 years)].

RESULTS

BMI z-score, WC, and obesity were positively associated with the molecular mass sum of the PAHs and the total sum of naphthalene metabolites. Most associations increased monotonically with increasing quartiles of exposure among children 6-11 years of age, whereas dose-response trends were less consistent for adolescents (12-19 years of age). Neither total PAHs nor total naphthalene metabolites were associated with overweight in either age group, and there was little evidence of associations between the outcomes and individual PAHs.

CONCLUSIONS

Total urinary PAH metabolites and naphthalene metabolites were associated with higher BMI, WC, and obesity in children 6-11 years of age, with positive but less consistent associations among adolescents.

CITATION

Scinicariello F, Buser MC. 2014. Urinary polycyclic aromatic hydrocarbons and childhood obesity: NHANES (2001-2006). Environ Health Perspect 122:299-303; http://dx.doi.org/10.1289/ehp.1307234.

YC-1, a potent antithrombotic agent, induces lipolysis through the PKA pathway in rat visceral fat cells

This study investigated the effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), a soluble guanylyl cyclase (sGC) activator and potential antithrombotic agent, on lipolysis in isolated visceral fat cells of the rat. Visceral fat cells were isolated from epididymal fat pads of rats and treated with YC-1 at different doses and times. Glycerol release, and intracellular cAMP and cGMP levels were analyzed by specific kits. Moreover, several inhibitors or drugs were used to examine the signal transduction pathways of YC-1-induced lipolysis in adipocytes. Herein we report that YC-1 stimulated glycerol release in dose- and time-dependent manners. Intracellular cAMP and cGMP levels of adipocytes both increased in time-dependent manners, but elevation of the cGMP level was faster and higher than that of the cAMP level after YC-1 treatment. An sGC inhibitor (ODQ) inhibited YC-1-induced glycerol release, indicating the involvement of sGC in YC-1-induced lipolysis. Administration of insulin, an activator of type-3B phosphodiesterase (PDE-3B), attenuated YC-1-induced lipolysis, indicating that elevation of the cAMP level is an important step in the lipolytic effect of YC-1. In addition, YC-1-induced lipolysis was inhibited by a protein kinase A (PKA) inhibitor (KT5720) but not by a PKG inhibitor (KT5823), indicating that YC-1-induced lipolysis occurs through a PKA-dependent pathway. A Western blot analysis showed that extracellular signal-regulated kinase was not phosphorylated by YC-1 treatment. In conclusion, our results suggest that YC-1 might stimulate lipolysis via activation of sGC/cGMP and then activation of the cAMP/PKA signaling cascade in isolated rat visceral adipocytes.

Yin and Yang of hypothalamic insulin and leptin signaling in regulating white adipose tissue metabolism

Fatty acids released from white adipose tissue (WAT) provide important energy substrates during fasting. However, uncontrolled fatty acid release from WAT during non-fasting states causes lipotoxicity and promotes inflammation and insulin resistance, which can lead to and worsen type 2 diabetes (DM2). WAT is also a source for insulin sensitizing fatty acids such as palmitoleate produced during de novo lipogenesis. Insulin and leptin are two major hormonal adiposity signals that control energy homeostasis through signaling in the central nervous system. Both hormones have been implicated to regulate both WAT lipolysis and de novo lipogenesis through the mediobasal hypothalamus (MBH) in an opposing fashion independent of their respective peripheral receptors. Here, we review the current literature on brain leptin and insulin action in regulating WAT metabolism and discuss potential mechanisms and neuro-anatomical substrates that could explain the opposing effects of central leptin and insulin. Finally, we discuss the role of impaired hypothalamic control of WAT metabolism in the pathogenesis of insulin resistance, metabolic inflexibility and type 2 diabetes.

Mammalian Cyclic Nucleotide Phosphodiesterases: Molecular Mechanisms and Physiological Functions

The superfamily of cyclic nucleotide (cN) phosphodiesterases (PDEs) is comprised of 11 families of enzymes. PDEs break down cAMP and/or cGMP and are major determinants of cellular cN levels and, consequently, the actions of cN-signaling pathways. PDEs exhibit a range of catalytic efficiencies for breakdown of cAMP and/or cGMP and are regulated by myriad processes including phosphorylation, cN binding to allosteric GAF domains, changes in expression levels, interaction with regulatory or anchoring proteins, and reversible translocation among subcellular compartments. Selective PDE inhibitors are currently in clinical use for treatment of erectile dysfunction, pulmonary hypertension, intermittent claudication, and chronic pulmonary obstructive disease; many new inhibitors are being developed for treatment of these and other maladies. Recently reported x-ray crystallographic structures have defined features that provide for specificity for cAMP or cGMP in PDE catalytic sites or their GAF domains, as well as mechanisms involved in catalysis, oligomerization, autoinhibition, and interactions with inhibitors. In addition, major advances have been made in understanding the physiological impact and the biochemical basis for selective localization and/or recruitment of specific PDE isoenzymes to particular subcellular compartments. The many recent advances in understanding PDE structures, functions, and physiological actions are discussed in this review.

Endocrine disrupting chemicals and the developmental programming of adipogenesis and obesity

Obesity and related disorders are a burgeoning public health epidemic, particularly in the U.S. Currently 34% of the U.S. population is clinically obese (BMI > 30) and 68% are overweight (BMI > 25), more than double the worldwide average and 10-fold higher than Japan and South Korea. Obesity occurs when energy intake exceeds energy expenditure; however, individuals vary widely in their propensity to gain weight and accrue fat mass, even at identical levels of excess caloric input. Clinical, epidemiological, and biological studies show that obesity is largely programmed during early life, including the intrauterine period. The environmental obesogen hypothesis holds that prenatal or early life exposure to certain endocrine disrupting chemicals can predispose exposed individuals to increased fat mass and obesity. Obesogen exposure can alter the epigenome of multipotent stromal stem cells, biasing them toward the adipocyte lineage at the expense of bone. Hence, humans exposed to obesogens during early life might have an altered stem cell compartment, which is preprogrammed toward an adipogenic fate. This results in a higher steady state number of adipocytes and potentially a lifelong struggle to maintain a healthy weight, which can be exacerbated by societal influences that promote poor diet and inadequate exercise. This review focuses on the developmental origins of the adipocyte, the relationship between adipocyte number and obesity, and how obesogenic chemicals may interfere with the highly efficient homeostatic mechanisms regulating adipocyte number and energy balance.

Brain insulin controls adipose tissue lipolysis and lipogenesis

White adipose tissue (WAT) dysfunction plays a key role in the pathogenesis of type 2 diabetes (DM2). Unrestrained WAT lipolysis results in increased fatty acid release, leading to insulin resistance and lipotoxicity, while impaired de novo lipogenesis in WAT decreases the synthesis of insulin-sensitizing fatty acid species like palmitoleate. Here, we show that insulin infused into the mediobasal hypothalamus (MBH) of Sprague-Dawley rats increases WAT lipogenic protein expression, inactivates hormone-sensitive lipase (Hsl), and suppresses lipolysis. Conversely, mice that lack the neuronal insulin receptor exhibit unrestrained lipolysis and decreased de novo lipogenesis in WAT. Thus, brain and, in particular, hypothalamic insulin action play a pivotal role in WAT functionality.

17beta-estradiol supplementation attenuates ovariectomy-induced increases in ATGL signaling and reduced perilipin expression in visceral adipose tissue

Adipocytes from post-menopausal females have higher basal lipolytic rates than pre-menopausal females, which contributes to increased risk of developing dyslipidemia following menopause. The purpose of this study was to delineate cellular mechanisms affecting adipose tissue function in the ovariectomized (OVX) mouse and also determine if physical activity or estrogen supplementation alter any detected changes. Female C57/Bl6 mice were placed into SHAM, OVX sedentary (OVX), OVX exercise (OVX-Ex), and OVX sedentary + 17beta-estradiol (OVX + E(2)) groups. Visceral fat mass, glycerol, and NEFA levels were significantly higher in OVX mice compared to SHAM animals, but were not elevated in the E(2)-treated animals. Voluntary running failed to change circulating levels of glycerol or NEFA in OVX mice, but did partially attenuate the increase in visceral fat mass. Adipose triglyceride lipase (ATGL) protein content was significantly elevated in visceral fat from OVX and OVX-Ex groups compared to SHAM, while ATGL-CGI-58 interaction was significantly higher in OVX than SHAM and OVX + E(2) mice. No significant differences in HSL phosphorylation were detected between groups, however, ERK1/2 phosphorylation was significantly elevated in the OVX mice. To determine if ERK1/2 function was critical for the increased glycerol levels, visceral fat was treated with MEK inhibitor PD98059, with no differences in glycerol release detected. Perilipin protein content was decreased significantly in OVX and OVX-Ex mice compared to SHAM. Thus, these data suggest that increased ATGL signaling and reduced perilipin protein content may contribute to increased NEFA and glycerol levels in OVX mice, which are attenuated with E(2) treatment, but not by exercise.

Phosphodiesterases do not limit beta1-adrenoceptor-mediated sinoatrial tachycardia: evidence with PDE3 and PDE4 in rabbits and PDE1-5 in rats

The mammalian heart expresses at least five phosphodiesterases (PDE1-5). Catecholamines produce surges of inotropically relevant cAMP through beta(1)-adrenoceptor stimulation. cAMP is mainly hydrolysed by PDE3 and/or PDE4 thereby blunting contractility. Basal sinoatrial beating rate in mouse, rat, piglet and rabbit sinoatrial cells is reduced by PDE3 and/or PDE4 through hydrolysis of cAMP. However, in rodents, the tachycardia elicited by catecholamines through production of cAMP by beta-adrenoceptor activation is not controlled by PDE3 and PDE4, despite a blunting effect of PDE3 or/and PDE4 on basal sinoatrial beating, but it is unknown whether PDE3 limits catecholamine-evoked tachycardia in the rabbit. Since rabbit sinoatrial cells are an important model for pacemaker research, we investigated whether the positive chronotropic effects of (-)-noradrenaline on spontaneously beating right atria of the rabbit are potentiated by inhibition of PDE3 with cilostamide (300 nM). We also studied the sinoatrial effects of the PDE4 inhibitor rolipram (10 microM) and its influence on the responses to (-)-noradrenaline. For comparison, we investigated the influence of cilostamide and rolipram on the positive inotropic responses to (-)-noradrenaline on rabbit left atria and right ventricular papillary muscles. Cilostamide and concurrent cilostamide + rolipram, but not rolipram alone, increased sinoatrial rate by 15% and 31% of the effect of (-)-isoprenaline (200 microM) but the PDE inhibitors did not significantly change the chronotropic potency of (-)-noradrenaline. In contrast in papillary muscle, the positive inotropic effects of (-)-noradrenaline were potentiated 2.4-, 2.6- and 44-fold by cilostamide, rolipram and concurrent cilostamide + rolipram, respectively. In left atrium, the positive inotropic effects of (-)-noradrenaline were marginally potentiated by cilostamide, as well as potentiated 2.7- and 32-fold by rolipram and by concurrent cilostamide and rolipram respectively. To compare the influence of PDE1-5 on basal sinoatrial rate and (-)-noradrenaline-evoked tachycardia, we investigated on rat right atria the effects of selective inhibitors. The PDE4 inhibitor rolipram and non-selective inhibitor isobutyl-methylxanthine caused tachycardia with -logEC(50)s of 7.2 and 5.0 and E(max) of 18% and 102% of (-)-isoprenaline, respectively. Rolipram did not change the chronotropic potency of (-)-noradrenaline. At high concentrations (10-30 microM), the PDE1, PDE3 and PDE5 inhibitors 8-methoxymethyl-3-isobutyl-1-methylxanthine, cilostamide and sildenafil, respectively, caused marginal tachycardia but did not significantly change the chronotropic potency of (-)-noradrenaline. The PDE2-selective inhibitor erythro-9-[2-hydroxy-3-nonyl]adenine caused marginal bradycardia at 30 microM and tended to reduce the chronotropic potency of (-)-noradrenaline. Rabbit PDE3 reduces basal sinoatrial rate. Although PDE4 only marginally reduces rate, under conditions of PDE3 inhibition, it further reduces sinoatrial rate. Both PDE3 and PDE4 control atrial and ventricular positive inotropic effects of (-)-noradrenaline. In contrast, neither PDE3 nor PDE4 limit the sinoatrial tachycardia induced by (-)-noradrenaline. In the rat, only PDE4, but not PDE1, PDE2, PDE3 and PDE5, reduces basal sinoatrial rate. None of the five rat PDEs limits the (-)-noradrenaline-evoked tachycardia. Taken together, these results confirm and expand evidence for our proposal that the cAMP-compartment modulating basal sinoatrial rate, controlled by PDE3 and/or PDE4, is different from the PDE-resistant cAMP compartment involved in beta(1)-adrenoceptor-mediated sinoatrial tachycardia.

Inotropy and L-type Ca2+ current, activated by beta1- and beta2-adrenoceptors, are differently controlled by phosphodiesterases 3 and 4 in rat heart

BACKGROUND AND PURPOSE

beta(1)- and beta(2)-adrenoceptors coexist in rat heart but beta(2)-adrenoceptor-mediated inotropic effects are hardly detectable, possibly due to phosphodiesterase (PDE) activity. We investigated the influence of the PDE3 inhibitor cilostamide (300 nmol x L(-1)) and the PDE4 inhibitor rolipram (1 micromol x L(-1)) on the effects of (-)-catecholamines.

EXPERIMENTAL APPROACH

Cardiostimulation evoked by (-)-noradrenaline (ICI118551 present) and (-)-adrenaline (CGP20712A present) through beta(1)- and beta(2)-adrenoceptors, respectively, was compared on sinoatrial beating rate, left atrial and ventricular contractile force in isolated tissues from Wistar rats. L-type Ca(2+)-current (I(Ca-L)) was assessed with whole-cell patch clamp.

KEY RESULTS

Rolipram caused sinoatrial tachycardia. Cilostamide and rolipram did not enhance chronotropic potencies of (-)-noradrenaline and (-)-adrenaline. Rolipram but not cilostamide potentiated atrial and ventricular inotropic effects of (-)-noradrenaline. Cilostamide potentiated the ventricular effects of (-)-adrenaline but not of (-)-noradrenaline. Concurrent cilostamide + rolipram uncovered left atrial effects of (-)-adrenaline. Both rolipram and cilostamide augmented the (-)-noradrenaline (1 micromol x L(-1)) evoked increase in I(Ca-L). (-)-Adrenaline (10 micromol x L(-1)) increased I(Ca-L) only in the presence of cilostamide but not rolipram.

CONCLUSIONS AND IMPLICATIONS

PDE4 blunts the beta(1)-adrenoceptor-mediated inotropic effects. PDE4 reduces basal sinoatrial rate in a compartment distinct from compartments controlled by beta(1)- and beta(2)-adrenoceptors. PDE3 and PDE4 jointly prevent left atrial beta(2)-adrenoceptor-mediated inotropy. Both PDE3 and PDE4 reduce I(Ca-L) responses through beta(1)-adrenoceptors but the PDE3 component is unrelated to inotropy. PDE3 blunts both ventricular inotropic and I(Ca-L) responses through beta(2)-adrenoceptors.

Phosphodiesterases PDE3 and PDE4 jointly control the inotropic effects but not chronotropic effects of (-)-CGP12177 despite PDE4-evoked sinoatrial bradycardia in rat atrium

Acting through a low-affinity site of the beta(1)-adrenoceptor (beta(1L)AR), CGP12177 causes sinoatrial tachycardia and positive inotropic effects in left atrium but not in the ventricle of the rat. However, inhibition of either PDE3 or PDE4 also uncovers positive inotropic effects of CGP12177 in ventricle, but whether these phosphodiesterases also control the atrial agonist effects of CGP12177 was unknown. We, therefore, investigated the effects of the PDE3-selective inhibitor cilostamide (300 nM) and PDE4 inhibitor rolipram (1 microM) on the (-)-CGP12177-evoked increases of sinoatrial beating rate and force of paced left atria of the rat. Rolipram (n = 8) increased basal sinoatrial rate by 27 +/- 5 bpm but cilostamide (n = 8) had no effect. The chronotropic potency of (-)-CGP12177 (-logEC(50)M = 7.5) was not changed by rolipram and cilostamide or their combination. (-)-CGP12177 increased left atrial force with intrinsic activity 0.25 compared to (-)-isoprenaline. Rolipram (n = 8) and cilostamide (n = 8) did not change basal force of left atria but concurrent rolipram + cilostamide (n = 8) increased force by 52 +/- 9% of the effect of 200 microM (-)-isoprenaline. Neither rolipram nor cilostamide affected the inotropic potency of (-)-CGP12177 (-logEC(50)M = 7.4) but concurrent rolipram + cilostamide caused potentiation (-logEC(50)M = 8.2) and converted (-)-CGP12177 into a full agonist compared to (-)-isoprenaline. Cyclic AMP appears to maintain sinoatrial rate and PDE4 elicits bradycardia through hydrolysis of cAMP in a compartment distinct from the beta(1L)AR-induced cAMP compartment through which (-)-CGP12177 causes tachycardia. In contrast to the (-)-CGP12177-evoked tachycardia, not controlled by PDE3 and PDE4, these isoenzymes jointly reduce (-)-CGP12177-evoked increases of left atrial contractility through beta(1L)AR.

Phosphodiesterase-4 blunts inotropism and arrhythmias but not sinoatrial tachycardia of (-)-adrenaline mediated through mouse cardiac beta(1)-adrenoceptors

BACKGROUND AND PURPOSE

beta(1) and beta(2)-adrenoceptors coexist in murine heart but beta(2)-adrenoceptor-mediated effects have not been detected in atrial and ventricular tissues, possibly due to marked phosphodiesterase (PDE) activity. We investigated the influence of the PDE3 inhibitor cilostamide and PDE4 inhibitor rolipram on the effects of (-)-adrenaline in three regions of murine heart.

EXPERIMENTAL APPROACH

(-)-Adrenaline-evoked cardiostimulation was compared on sinoatrial beating rate, left atrial and right ventricular contractile force in isolated tissues from 129SvxC57B1/6 cross mice. Ventricular arrhythmic contractions were also assessed.

KEY RESULTS

Both rolipram (1 microM) and cilostamide (300 nM) caused transient sinoatrial tachycardia but neither enhanced the chronotropic potency of (-)-adrenaline. Rolipram potentiated 19-fold (left atrium) and 7-fold (right ventricle) the inotropic effects of (-)-adrenaline. (-)-Adrenaline elicited concentration-dependent ventricular arrhythmias that were potentiated by rolipram. All effects of (-)-adrenaline were antagonized by the beta(1)-adrenoceptor-selective antagonist CGP20712A (300 nM). Cilostamide (300 nM) did not increase the chronotropic and inotropic potencies of (-)-adrenaline, but administered jointly with rolipram in the presence of CGP20712A, uncovered left atrial inotropic effects of (-)-adrenaline that were prevented by the beta(2)-adrenoceptor-selective antagonist ICI118551.

CONCLUSIONS AND IMPLICATIONS

PDE4 blunts the beta(1)-adrenoceptor-mediated effects of (-)-adrenaline in left atrium and right ventricle but not in sinoatrial node. Both PDE3 and PDE4 reduce basal sinoatrial rate in a compartment distinct from the beta(1)-adrenoceptor compartment. PDE3 and PDE4, acting in concert, prevent left atrial beta(2)-adrenoceptor-mediated inotropy. PDE4 partially protects the right ventricle against (-)-adrenaline-evoked arrhythmias.

mRNA expression and antilipolytic role of phosphodiesterase 4 in rat adipocytes in vitro

Adipocyte lipolysis is dependent on an increase in the intracellular concentration of cAMP. Intracellular phosphodiesterases (PDEs) hydrolyze cAMP and limit stimulation of lipolysis. In the present study, the mRNA expression of PDE4 subtypes and the antilipolytic role of PDE4 in rat adipocytes were investigated. Fragments encoding PDE4A (233 bp), PDE4B (786 bp), PDE4C (539 bp), and PDE4D (262 bp) sequences were amplified by RT-PCR. The mRNA expression of PDE4 subtypes (A, B, C, D) determined by real-time quantitative PCR was 7, 18.7, 18.9, and 7.2% relative to PDE3B. Inhibition of PDE4 by rolipram increased basal lipolysis and reversed in part prostaglandin E2 antilipolysis. The combination of PDE3 and PDE4 inhibitors synergistically reversed both prostaglandin E2 and phenylisopropyl adenosine antilipolysis. Stimulation of adipocytes with prostaglandin E2 increased total PDE activity and PDE3 activity measured by hydrolysis of 3[H]cAMP by the particulate fraction of adipocytes. The present study confirmed that mRNAs for all four PDE4 subtypes were expressed in rat adipocytes, with PDE4B and PDE4C predominant. Moreover, PDE4 not only limits the rate of basal lipolysis but also contributes to prostaglandin E2 antilipolysis in rat adipocytes.

Differential effects of phosphodiesterase PDE-3/PDE-4-specific inhibitors on vasoconstriction and cAMP-dependent vasorelaxation following balloon angioplasty

It is known that cAMP and cGMP are important for vasorelaxation, and cyclic nucleotide phosphodiesterases (PDEs) regulate their levels. Balloon angioplasty (BAL) is associated with reduced cAMP and cGMP levels, and inhibition of PDE-3 reduces restenosis. In this study, we found that BAL increased PDE-3 activity, which affected vasoreactivity of rat aortic rings 24-h post-BAL; these were compared with intact (INT) and ex vivo endothelium-denuded rings (RUB) from sham rats. In BAL and RUB rings, vasorelaxant responses to ACh were abolished. The EC(50) for phenylephrine (PE) was 1.8-fold less in RUB than in INT or BAL, whereas the maximal contractile effect of PE was greater in BAL than in INT or RUB. PDE-3 inhibitors reduced the maximal response to PE by >65% in BAL compared with 10-30% in INT and RUB; the reduction of the maximal response to U-46619 was 37% in BAL compared with 8% in INT with no reduction in RUB. PDE-4 inhibitors reduced PE-induced tone by <30% in an endothelium-dependent manner. Vasorelaxant responses to agonists that utilize cAMP were greatly impaired in BAL and RUB rings, and inhibition of PDE-3 enhanced the vasorelaxant responses in BAL or RUB. Inhibition of PDE-4 increased vasorelaxant responses to isoproterenol (ISO) to a much lesser degree. Thus PDE-3 and PDE-4 inhibitors exhibited differential effects on PE-induced tone and vasorelaxant responses to ISO. Inhibition of PDE-3 also produced a greater increase in cAMP in BAL than INT or RUB rings. These results suggest that increased PDE-3 activity after BAL may promote a vasospastic state and that the reduction in cAMP may, possibly, influence vessel remodeling.

Cilostamide potentiates more the positive inotropic effects of (-)-adrenaline through beta(2)-adrenoceptors than the effects of (-)-noradrenaline through beta (1)-adrenoceptors in human atrial myocardium

Activation of both beta(1)- and beta(2)-adrenoceptors increases the contractility of human atrial myocardium through cyclic AMP-dependent pathways. Cyclic AMP is hydrolised by phosphodiesterases, but little is known about which isoenzymes catalyse inotropically relevant cyclic AMP accumulated upon stimulation of beta-adrenoceptor subtypes. We have compared the positive inotropic effects of (-)-noradrenaline and (-)-adrenaline, mediated through beta(1)- and beta(2)-adrenoceptors, respectively, in the absence and presence of the PDE3 inhibitor cilostamide (300 nM) or PDE4 inhibitor rolipram (1 muM) on human atrial trabeculae from non-failing hearts. Cilostamide, but not rolipram, potentiated the effects of both (-)-noradrenaline and (-)-adrenaline. Cilostamide increased the -logEC(50)M of (-)-adrenaline more than of (-)-noradrenaline (P < 0.05), regardless of whether or not the patients had been chronically treated with beta-blockers. The results are consistent with a greater PDE3-catalysed hydrolysis of inotropically relevant cyclic AMP produced through beta(2)-adrenoceptors than beta(1)-adrenoceptors in human atrium.

Functional localization of cAMP signalling in cardiac myocytes

The cAMP pathway is of cardinal importance for heart physiology and pathology. The spatial organization of the various components of the cAMP pathway is thought to allow the segregation of functional responses triggered by the different neuromediators and hormones that use this pathway. PDEs (phosphodiesterases) hydrolyse cAMP (and cGMP) and play a major role in this process by preventing cAMP diffusion to the whole cytosol and inadequate target activation. The development of olfactory cyclic nucleotide-gated channels to directly monitor cAMP beneath the plasma membrane in real time allows us to gain new insights into the molecular mechanisms responsible for cAMP homoeostasis and hormonal specificity in cardiac cells. The present review summarizes the recent results we obtained using this approach in adult rat ventricular myocytes. In particular, the role of PDEs in the maintenance of specific cAMP signals generated by beta-adrenergic receptors and other G(s)-coupled receptors will be discussed.

[The adipose tissue as a regulatory center of the metabolism]

The recent progress in the research about the metabolic properties of the adipose tissue and the discovery of its ability to produce hormones that are very active in pathophysiologic as well as physiologic processes is rebuilding the concepts about its biology. Its involvement in conditions like obesity, type 2 diabetes mellitus, arterial hypertension, arteriosclerosis, dislipidemias and chronic and acute inflammatory processes indicate that the understanding of its functional capacities may contribute to improve the prognosis of those diseases whose prevalence increased in a preoccupying manner. Here we review some functional aspects of adipocytes, such as the metabolism, its influence on energy homeostasis, its endocrine ability and the adipogenesis, i.e., the potential of pre-adipocytes present in adipose tissue stroma to differentiate into new adipocytes and regenerate the tissue. In addition, we are including some studies on the relationship between the adipose tissue and the pineal gland, a new and poorly known, although, as will be seen, very promising aspect of adipocyte physiology together with its possible favorable repercussions to the therapy of the obesity related diseases.

Ginseng extract inhibits lipolysis in rat adipocytes in vitro by activating phosphodiesterase 4

Elevated concentrations of plasma free fatty acids (FFA) may cause insulin resistance. Inhibition of lipolysis reduces FFA availability and improves insulin sensitivity. Ginseng extract (Panax spp., GE) was shown to improve glycemia in Type 2 diabetes. In the present study, the antilipolytic effect of GE in rat adipocytes and the signaling pathway for GE antilipolysis were investigated. Adipocytes were isolated from rat fat tissue by collagenase digestion. The ability of GE to inhibit lipolysis was assessed by measuring glycerol and FFA release into the incubation medium. Phosphatidylinositol 3-kinase (PI3-K) inhibitor and various phosphodiesterase (PDE) inhibitors were applied to investigate the signaling pathway for GE antilipolysis. The present study showed that insulin and GE inhibited lipolysis by 42.4 and 49% compared with basal, respectively (P < 0.05). Unlike insulin, the PI3-K inhibitor wortmannin did not reverse GE antilipolysis, and GE did not affect phosphorylation of protein kinase B (PKB). The nonselective PDE inhibitor enprofylline reversed both insulin and GE antilipolysis. The specific phosphodiesterase 3 (PDE3) inhibitor cilostamide reversed insulin antilipolysis completely, but did not significantly affect GE antilipolysis. The specific phosphodiesterase 4 (PDE4) inhibitor rolipram did not significantly affect insulin antilipolysis, but almost completely reversed GE antilipolysis. Moreover, the combination of PDE3 and PDE4 inhibitors completely reversed GE antilipolysis. None of the ginsenosides (Rb1, Re, Rg1, Rc, Rb2, and Rd) were responsible for GE antilipolysis. The results suggest that ginseng exerts its antilipolytic effect through a signaling pathway different from that of insulin. GE antilipolysis is mediated in part by activating PDE4 in rat adipocytes.

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.

Signalling mechanisms regulating lipolysis

Adipose tissue plays an important role providing energy to other tissues and functioning as an energy reserve organ. The energy supply is produced by triglycerides stored in a large vacuole representing approximately 95% of adipocyte volume. In the fasting period, triglyceride hydrolysis produces glycerol and free fatty acids which are important oxidative fuels for other tissues such as liver, skeletal muscle, kidney and myocardium. Hormone-sensitive lipase (HSL) is the enzyme that hydrolyzes intracellular triacylglycerol and diacylglycerol, and is one of the key molecules controlling lipolysis. Hormones and physiological factors such as dieting, physical exercise and ageing regulate intensively the release of glycerol and free fatty acids from adipocytes. One of the best known mechanisms that activate lipolysis in the adipocyte is the cAMP dependent pathway. cAMP production is modulated by hormone receptors coupled to Gs/Gi family of GTP binding proteins, such as beta-adrenergic receptors, whereas cAMP degradation is controlled by modulation of phosphodiesterase activity, increased by insulin receptor signalling. cAMP activates PKA which activates HSL by promoting its phosphorylation. Hormonal control of lipolysis can also be achieved by receptors coupled G proteins of the Gq family, through molecular mechanisms that involve PKC and MAPK, which are currently under investigation. cGMP and PKG have also been found to activate lipolysis in adipocytes. In this review we have compiled data from literature reporting both the classical and the alternative mechanisms of lipolysis.

Expression, refolding, and purification of recombinant human phosphodiesterase 3B: definition of the N-terminus of the catalytic core

We have developed an expression, refolding, and purification protocol for the catalytic domain of human Phosphodiesterase 3B (PDE3B). High level expression in Escherichia coli has been achieved with yields of up to 20mg/L. The catalytic domain of the enzyme was purified by affinity chromatography utilizing a novel affinity ligand. PDE3B, purified by affinity chromatography, with no single impurity #10878;1% as determined by SDS-PAGE, has a specific activity of 2210+/-442nmol/min/mg and a KM for cAMP of 44+/-4.5nM. Reducing the size of the expressed catalytic domain from residues 387-1112 to residues 654-1086 greatly reduced the aggregation phenomena observed with the affinity purified PDE3B. The definition of the N-terminus of the catalytic core was examined through the generation of several truncation mutants spanning amino acid residues 636-674. Constructs starting at E665 and M674 were fully active and devoid of activity, respectively. A construct starting at D668 had a Vmax reduced by approximately 10-fold relative to the longer constructs, yet the KM was not affected. This indicates the minimal N-terminus of the catalytic core lies between E665 and Y667. Refolding and affinity purification of the 654-1073 catalytic core of PDE3B has been employed to produce large quantities of highly pure enzyme for structural studies.

Negative feedback exerted by cAMP-dependent protein kinase and cAMP phosphodiesterase on subsarcolemmal cAMP signals in intact cardiac myocytes: an in vivo study using adenovirus-mediated expression of CNG channels

Intracardiac cAMP levels are modulated by hormones and neuromediators with specific effects on contractility and metabolism. To understand how the same second messenger conveys different information, mutants of the rat olfactory cyclic nucleotide-gated (CNG) channel alpha-subunit CNGA2, encoded into adenoviruses, were used to monitor cAMP in adult rat ventricular myocytes. CNGA2 was not found in native myocytes but was strongly expressed in infected cells. In whole cell patch-clamp experiments, the forskolin analogue L-858051 (L-85) elicited a non-selective, Mg2+ -sensitive current observed only in infected cells, which was thus identified as the CNG current (ICNG). The beta-adrenergic agonist isoprenaline (ISO) also activated ICNG, although the maximal efficiency was approximately 5 times lower than with L-85. However, ISO and L-85 exerted a similar maximal increase of the L-type Ca2+ current. The use of a CNGA2 mutant with a higher sensitivity for cAMP indicated that this difference is caused by the activation of a localized fraction of CNG channels by ISO. cAMP-dependent protein kinase (PKA) blockade with H89 or PKI, or phosphodiesterase (PDE) inhibition with IBMX, dramatically potentiated ISO- and L-85-stimulated ICNG. A similar potentiation of beta-adrenergic stimulation occurred when PDE4 was blocked, whereas PDE3 inhibition had a smaller effect (by 2-fold). ISO and L-85 increased total PDE3 and PDE4 activities in cardiomyocytes, although this effect was insensitive to H89. However, in the presence of IBMX, H89 had no effect on ISO stimulation of ICNG. This study demonstrates that subsarcolemmal cAMP levels are dynamically regulated by a negative feedback involving PKA stimulation of subsarcolemmal cAMP-PDE.

Inhibition of leptin release by atrial natriuretic peptide (ANP) in human adipocytes

The addition of atrial natriuretic peptide (ANP) to isolated human adipocytes in primary culture from very obese individuals resulted in an inhibition of leptin release after a 24- or 48-hr incubation. There was also an inhibition of leptin release by isoproterenol (ISO) that was partially reversed by insulin, whereas the inhibition due to ANP was unaffected. Similar results were seen with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulphonamide (H-89), which is a cell-permeable inhibitor of protein kinase A. H-89 markedly reduced the effects of ISO on both lipolysis and leptin release without affecting the stimulation of lipolysis or the inhibition of leptin release due to ANP. Inhibition of endogenous nitric oxide formation using N(omega)-nitro-L-arginine resulted in a 20% increase in leptin release over 48 hr, which suggests that the nitric oxide/cyclic GMP pathway might play a small role in the regulation of endogenous leptin release. Similarly, the addition of the nitric oxide donor (Z)-1-[2-aminoethyl)-N-(2-aminoethyl)diazen-1-ium-1,2-diolate (DETA NONOate) at 0.1 or 1 microM to explants of human adipose tissue enhanced lipolysis by 29%. Our data demonstrate that the lipolytic effect of ANP is probably secondary to stimulation of cyclic GMP accumulation in human adipocytes, and this is accompanied by an inhibition of leptin release.

Identification of the insulin-regulated interaction of phosphodiesterase 3B with 14-3-3 beta protein

Phosphodiesterase (PDE)-3B, a major PDE isoform in adipocytes, plays a pivotal role in the antilipolytic action of insulin. Insulin-induced phosphorylation and activation of PDE3B is phosphatidylinositol 3-kinase (PI3-K) and Akt dependent, but the precise mechanism of PDE3B activation is not fully understood. We have identified 14-3-3 beta, a critical scaffolding molecule in signal transduction, as a protein that interacts with PDE3B using the yeast two-hybrid system. The interaction between PDE3B and 14-3-3 beta was then confirmed in vitro. The glutathione S-transferase (GST)-tagged 14-3-3 beta interacts with endogenous PDE3B of rat adipocytes, and this interaction is enhanced when adipocytes are treated with insulin. Coimmunoprecipitation experiments reveal that endogenous PDE3B also associates with endogenous 14-3-3 beta in rat adipocytes, and this interaction is enhanced by insulin. Two different PI3-K inhibitors, wortmannin and Ly294002, block this induction, suggesting that PI3-K is required. Synthetic 15 amino acid peptides of rat PDE3B containing phosphorylated Ser-279 or -302 inhibit this interaction, indicating that the insulin-regulated phosphorylation of these serine residues is involved. Because insulin receptor substrate-1 also associates with 14-3-3, the dimeric 14-3-3 beta could function as a scaffolding protein in the activation of PDE3B by insulin.

Isoforms of cyclic nucleotide phosphodiesterase PDE3A in cardiac myocytes

PDE3A cyclic nucleotide phosphodiesterases regulate cAMP- and cGMP-mediated intracellular signaling in cardiac myocytes. We used antibodies to different regions of PDE3A to demonstrate the presence of three PDE3A isoforms in these cells. These isoforms, whose apparent molecular weights are 136,000, 118,000, and 94,000 ("PDE3A-136," "PDE3A-118," and "PDE3A-94"), are identical save for the deletion of different lengths of N-terminal sequence containing two membrane-association domains and sites for phosphorylation/activation by protein kinase B ("PK-B") and protein kinase A ("PK-A"). PDE3A-136 contains both membrane-association domains and the PK-B and PK-A sites. PDE3A-118 contains only the downstream membrane-association domain and the PK-A sites. PDE3A-94 lacks both membrane localization domains and the PK-B and PK-A sites. The three isoforms are translated from two mRNAs derived from the PDE3A1 gene: PDE3A-136 is translated from PDE3A1 mRNA, whereas PDE3A-118 and PDE3A-94 are translated from PDE3A2 mRNA. Experiments involving in vitro transcription/translation indicate that PDE3A-118 and PDE3A-94 may be translated from different AUGs in PDE3A2 mRNA. These findings suggest that alternative transcriptional and post-transcriptional processing of the PDE3A gene results in the generation of two mRNAs and three protein isoforms in cardiac myocytes that differ with respect to intracellular localization and may be regulated through different signaling pathways.

C(2)-ceramide influences the expression and insulin-mediated regulation of cyclic nucleotide phosphodiesterase 3B and lipolysis in 3T3-L1 adipocytes

Cyclic nucleotide phosphodiesterase (PDE) 3B plays an important role in the antilipolytic action of insulin and, thereby, the release of fatty acids from adipocytes. Increased concentrations of circulating fatty acids as a result of elevated or unrestrained lipolysis cause insulin resistance. The lipolytic action of tumor necrosis factor (TNF)-alpha is thought to be one of the mechanisms by which TNF-alpha induces insulin resistance. Ceramide is the suggested second messenger of TNF-alpha action, and in this study, we used 3T3-L1 adipocytes to investigate the effects of C(2)-ceramide (a short-chain ceramide analog) on the expression and regulation of PDE3B and lipolysis. Incubation of adipocytes with 100 micromol/l C(2)-ceramide (N-acetyl-sphingosine) resulted in a time-dependent decrease of PDE3B activity, accompanied by decreased PDE3B protein expression. C(2)-ceramide, in a time- and dose-dependent manner, stimulated lipolysis, an effect that was blocked by H-89, an inhibitor of protein kinase A. These ceramide effects were prevented by 20 micromol/l troglitazone, an antidiabetic drug. In addition to downregulation of PDE3B, the antilipolytic action of insulin was decreased by ceramide treatment. These results, together with data from other studies on PDE3B and lipolysis in diabetic humans and animals, suggest a novel pathway by which ceramide induces insulin resistance. Furthermore, PDE3B is demonstrated to be a target for troglitazone action in adipocytes.

Inhibition of isoproterenol-induced lipolysis in rat inguinal adipocytes in vitro by physiological melatonin via a receptor-mediated mechanism

Because the pineal hormone melatonin has been implicated in affecting adiposity in rats and fatty acid transport in certain rat tumor models, we tested whether melatonin regulates lipolysis in a normal cell system in vitro. Adipocytes were isolated from the inguinal fat pads (i.e. sc fat) of Sprague Dawley male rats during mid-light phase. Lipolysis was stimulated with isoproterenol (3 microM), and cells were incubated for 4 h in the presence or absence of a physiological circulating concentration of melatonin (1 nM). Lipolysis was measured by determining the amount of glycerol present in the incubation buffer, expressed as nmol glycerol/mg cellular fatty acid. We observed a 20- to 30-fold stimulation of basal lipolysis by isoproterenol, and this stimulation was inhibited 50--70% by melatonin. Melatonin exhibited this effect over a wide range of concentrations tested (100 pM-1 microM) with an IC(50) of approximately 500 pM. The effect by melatonin (1 nM) was completely blocked by pertussis toxin (50 ng/ml), by 8-bromo-cAMP (10 nM), and by the melatonin receptor antagonist S-20928 (1 nM). These results suggest that the antilipolytic effect occurs through one of the G(i) protein-coupled melatonin receptors because we have shown that both the mt(1) (Mel 1a) and MT(2) (Mel 1b) melatonin receptors are expressed in inguinal adipocytes. Melatonin inhibition of lipolysis was not observed in adipocytes isolated from rat epididymal fat pads (i.e. visceral fat), even though these cells also express both the mt(1) and MT(2) receptors. The results indicate that physiological circulating concentrations of melatonin inhibit isoproterenol-induced lipolysis in rat adipocytes via a G protein-coupled melatonin receptor-mediated signal transduction pathway in a site-specific manner.

Tyrosine kinase-mediated serine phosphorylation of adenylyl cyclase

Receptor tyrosine kinase (RTK) activation is associated with modulation of heptahelical receptor-stimulated adenylyl cyclase responses. The mechanisms underlying the RTK-mediated enhancement of adenylyl cyclase function remain unclear. In the present studies, we show that the tyrosine kinase-dependent enhancement of adenylyl cyclase isoform VI function parallels an enhancement in serine phosphorylation of the enzyme. This effect was mediated by both RTK activation, with IGF-1, and by tyrosine phosphatase inhibition, with sodium orthovanadate. This enhancement of adenylyl cyclase function was not attenuated by inhibitors of ERK, PKC, PKA, or PI3 kinase activity but was blunted by inhibition of endogenous p74(raf-1)() activity. To characterize the molecular site of this effect we identified multiple candidate serine residues in and adjacent to the adenylyl cyclase VI C1b catalytic region and performed serine-to-alanine site-directed mutagenesis using adenylyl cyclase VI as a template. Mutation of serine residues 603 and 608 or serine residues 744, 746, 750, and 754 attenuated both the tyrosine kinase-mediated enhancement of enzyme phosphorylation as well as the sensitization of function. Together, these data define a novel tyrosine kinase-mediated mechanism leading to serine phosphorylation of adenylyl cyclase isoform VI and the sensitization of adenylyl cyclase responsiveness.

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.

Phosphorylation of phosphodiesterase-5 by cyclic nucleotide-dependent protein kinase alters its catalytic and allosteric cGMP-binding activities

In addition to its cGMP-selective catalytic site, cGMP-binding cGMP-specific phosphodiesterase (PDE5) contains two allosteric cGMP-binding sites and at least one phosphorylation site (Ser92) on each subunit [Thomas, M.K., Francis, S.H. & Corbin, J.D. (1990) J. Biol. Chem. 265, 14971-14978]. In the present study, prior incubation of recombinant bovine PDE5 with a phosphorylation reaction mixture [cGMP-dependent protein kinase (PKG) or catalytic subunit of cAMP-dependent protein kinase (PKA), MgATP, cGMP, 3-isobutyl-1-methylxanthine], shown earlier to produce Ser92 phosphorylation, caused a 50-70% increase in enzyme activity and also increased the affinity of cGMP binding to the allosteric cGMP-binding sites. Both effects were associated with increases in its phosphate content up to 0.6 mol per PDE5 subunit. Omission of any one of the preincubation components caused loss of stimulation of catalytic activity. Addition of the phosphorylation reaction mixture to a crude bovine lung extract, which contains PDE5, also produced a significant increase in cGMP PDE catalytic activity. The increase in recombinant PDE5 catalytic activity brought about by phosphorylation was time-dependent and was obtained with 0.2-0.5 microM PKG subunit, which is approximately the cellular level of this enzyme in vascular smooth muscle. Significantly greater stimulation was observed using cGMP substrate concentrations below the Km value for PDE5, although stimulation was also seen at high cGMP concentrations. Considerably higher concentration of the catalytic subunit of PKA than of PKG was required for activation. There was no detectable difference between phosphorylated and unphosphorylated PDE5 in median inhibitory concentration for the PDE5 inhibitors, sildenafil, or zaprinast 3-isobutyl-1-methylxanthine. Phosphorylation reduced the cGMP concentration required for half-maximum binding to the allosteric cGMP-binding sites from 0.13 to 0.03 microM. The mechanism by which phosphorylation of PDE5 by PKG could be involved in physiological negative-feedback regulation of cGMP levels is discussed.

Cyclic nucleotide phosphodiesterases (PDE) 3 and 4 in normal, malignant, and HTLV-I transformed human lymphocytes

Intracellular cyclic AMP, determined in part by cyclic nucleotide phosphodiesterases (PDEs), regulates proliferation and immune functions in lymphoid cells. Total PDE, PDE3, and PDE4 activities were measured in phytohemagglutinin (PHA)-activated peripheral blood mononuclear cells (PBMC-PHA), normal natural killer (NK) cells, Jurkat and Kit225-K6 leukemic T-cells, T-cell lines transformed with human T-lymphotropic virus (HTLV)-I (a retrovirus that causes adult T-cell leukemia/lymphoma) and HTLV-II (a nonpathogenic retrovirus), normal B-cells, and B-cells transformed with Epstein-Barr virus (EBV). All cells exhibited PDE3 and PDE4 activities but in different proportions. In EBV-transformed B cells, PDE4 was much higher than PDE3. HTLV-I+ T-cells differed significantly from other T-lymphocyte-derived cells in also having a higher proportion of PDE4 activities, which apparently were not related to selective induction of any one PDE4 mRNA (judged by reverse transcription-polymerase chain reaction) or expression of the HTLV-I regulatory protein Tax. In MJ cells (an HTLV-I+ T-cell line), Jurkat cells, and PBMC-PHA cells, the tyrosine kinase inhibitor herbimycin A strongly inhibited PDE activity. Growth of MJ cells was inhibited by herbimycin A and a protein kinase C (PKC) inhibitor, and was arrested in G1 by rolipram, a specific PDE4 inhibitor. Proliferation of several HTLV-I+ T-cell lines, PBMC-PHA, and Jurkat cells was inhibited differentially by forskolin (which activates adenylyl cyclase), the selective PDE inhibitors cilostamide and rolipram, and the nonselective PDE inhibitors pentoxifylline and isobutyl methylxanthine. These results suggest that PDE4 isoforms may be functionally up-regulated in HTLV-I+ T-cells and may contribute to the virus-induced proliferation, and that PDEs could be therapeutic targets in immune/inflammatory and neoplastic diseases.

Mechanisms regulating adipocyte lipolysis

Mechanisms regulating adipocyte lipolysis are reviewed in three stages. The first stage examines plasma membrane hormone receptors and G-proteins. The primary regulators of adipose tissue lipolysis, the catecholamines, bind to the alpha 2, beta 1, beta 2, and beta 3 adrenergic receptors. The alpha 2 receptor couples with Gi-proteins to inhibit cyclic AMP formation and lipolysis, while the beta receptors couple with Gs-proteins to stimulate cyclic AMP formation and lipolysis. The beta 1 receptor may mediate low level catecholamine stimulation, while the beta 3 receptor, which is activated by higher levels of catecholamines, may deliver a more sustained signal. The second stage examines the regulation of cyclic AMP, the intracellular messenger that activates protein kinase A. Adenylyl cyclase synthesizes cyclic AMP from ATP and is regulated by the G-proteins. Phosphodiesterase 3B hydrolyzes cyclic AMP to AMP and is activated and phosphorylated by both insulin and the catecholamines norepinephrine and epinephrine. The third stage focuses on the rate-limiting enzyme of lipolysis, hormone-sensitive lipase (HSL). This 82 to 88 kDa protein is regulated by reversible phosphorylation. Protein kinase A activates and phosphorylates the enzyme at 2 sites, and 3 phosphatases have been implicated in HSL dephosphorylation. The translocation of HSL from the cytosol to the lipid droplet in response to lipolytic stimulation may be facilitated by a family of lipid-associated droplets called perilipins that are heavily phosphorylated by protein kinase A and dephosphorylated by insulin. As the mechanisms regulating adipocyte lipolysis continue to be uncovered, we look forward to the challenges of integrating these findings with research at the in situ and in vivo levels.

Combined effects of insulin and dexamethasone on cyclic AMP phosphodiesterase 3 and glycogen metabolism in cultured rat hepatocytes

Primary cultures of rat hepatocytes were used to study the combined effects of insulin and dexamethasone on cyclic AMP phosphodiesterase 3 (PDE 3) and glycogen metabolism. PDE activity was measured in extracts obtained by hypotonic shock treatment of the particulate fraction from cultured hepatocytes. PDE 3 was identified by inhibition with ICI 118233, Western blotting, immunoprecipitation of the activity with the use of a new PDE 3B-specific anti-peptide antibody and stimulation of the activity after adding insulin, glucagon and okadaic acid to the culture medium. Specific PDE inhibitors were always used to identify the measured PDE activities. Hypotonic extracts contained 30% PDE 3 and 50% PDE 4. Both PDE types show a nearly constant level during cultivation up to 48 h. Long-term exposure of dexamethasone alone has no effect on PDE 3 activity, whereas, in combination with insulin, the insulin stimulation of PDE 3 activity was found to be increased between 48 and 72 h of cultivation. Additionally, db-cAMP was able to stimulate PDE 3. A possible effect of insulin or db-cAMP on PDE 3B expression could not be found. On the other hand, activation of PDE 3B after 48 h of culturing decreased rapidly after removal of insulin or db-cAMP from the culture medium. Insulin-stimulated incorporation of 14C-glucose into glycogen was inhibited by PDE 3- and PDE 4-specific inhibitors as well as by the unspecific PDE inhibitor IMBX. Inhibitions by PDE 3- and PDE 4-specific inhibitors were found to be additive and reached the same extent as with IMBX. Summarising our results, we can conclude that PDE 3 and PDE 4 effectively control the hepatic glycogen metabolism. Insulin effects on PDE activity and glycogen metabolism require the presence of dexamethasone. Insulin-stimulated PDE seems to play an important role in realising insulin effects on hepatic glycogen metabolism.

Induction of phosphodiesterases 3B, 4A4, 4D1, 4D2, and 4D3 in Jurkat T-cells and in human peripheral blood T-lymphocytes by 8-bromo-cAMP and Gs-coupled receptor agonists. Potential role in beta2-adrenoreceptor desensitization

In this study, a potential mechanism of beta2-adrenoreceptor desensitization has been explored that is based upon the enhanced degradation of cAMP by phosphodiesterase (PDE). Pretreatment of Jurkat T-cells with 8-bromo cAMP (8-Br-cAMP) or prostaglandin E2 increased PDE3 and PDE4 activity in an actinomycin D- and cycloheximide-sensitive manner. This effect was associated with increased expression of HSPDE3B, HSPDE4A4, HSPDE4D1, HSPDE4D2, and HSPDE4D3 mRNA transcripts. Western analysis reproducibly labeled a band of immunoreactivity in vehicle-treated cells that corresponded to HSPDE4A4 (125 kDa). Although the intensity of this band was unchanged in cells treated with 8-Br-cAMP, additional 68-72-kDa proteins (HSPDE4D2, HSPDE4D1) were labeled that were not detected after vehicle. Similar results were obtained with T-lymphocytes exposed to 8-Br-cAMP and fenoterol. However, in those experiments HSPDE4A4 and HSPDE4D1 appeared to be equally expressed in vehicle- and treated cells, whereas HSPDE4D2 (72 kDa) was detected only after 8-Br-cAMP. The up-regulation of PDE activity in Jurkat T-cells abolished the ability of isoproterenol to elevate cAMP, which was partially reversed by the non-selective PDE inhibitor, 3-isobutyl-1-methylxanthine, and by the PDE3 and PDE4 inhibitors, Org 9935 and rolipram, respectively. Collectively, these data suggest that chronic treatment of T-cells with cAMP-elevating agents compromises beta2-adrenoreceptor-mediated cAMP accumulation by increasing the expression of HSPDE3B and HSPDE4D gene products.

Downregulation of right ventricular phosphodiesterase PDE-3A mRNA and protein before the development of canine heart failure

Phosphodiesterase III (PDE-3) inhibitors are inotropes used to treat congestive heart failure (HF). Previous studies showed PDE-3A mRNA levels were reduced in the left ventricle (LV) in dogs subjected to pacing-induced HF. The present study evaluated a time-course for RV-specific changes in PDE-3A mRNAs and proteins after pacing for 3 wk (n = 4) or in HF (4-5 wk; n = 4-6). Total RNA from LV/RV tissues was isolated for Northern analyses; cytosolic and microsomal proteins were prepared for PDE-3A immunoblots. PDE-3A mRNAs (7-8 and 10 kb) were normalized against glyceraldehyde-3-phosphodehydrogenase (GAPDH) or ribosomal 18s with similar results. PDE-3A/GAPDH ratios in 3 wk were unchanged in LV, but significantly (p < 0.05) reduced by 48% in RV vs unpaced controls (n = 8). In contrast, PDE-3A (7-8 kb)/GAPDH ratios were significantly reduced in HF by 50-59% in both ventricles. Consistent with mRNA levels, significant reductions in microsomal 135 kDa (93-96%) and cytosolic 120 kDa PDE-3A (57-69%) were seen in both ventricles in HF or in the RV at 3 wk; an LV-specific reduction (50%) in cytosolic 80 kDa PDE-3A in HF was also detected. In summary, RV-specific downregulation of PDE-3A mRNA/protein(s) at 3 wk suggests that hemodynamic rather than humoral mechanisms are responsible, and provides a molecular basis for the limited efficacy of milrinone in the progression of HF.

Identification of novel phosphorylation sites in hormone-sensitive lipase that are phosphorylated in response to isoproterenol and govern activation properties in vitro

Hormone-sensitive lipase (HSL) is the rate-limiting enzyme in lipolysis. Stimulation of rat adipocytes with isoproterenol results in phosphorylation of HSL and a 50-fold increase in the rate of lipolysis. In this study, we used site-directed mutagenesis and two-dimensional phosphopeptide mapping to show that phosphorylation sites other than the previously identified Ser-563 are phosphorylated in HSL in response to isoproterenol stimulation of 32P-labeled rat adipocytes. Phosphorylation of HSL in adipocytes in response to isoproterenol and in vitro phosphorylation of HSL containing Ser --> Ala mutations in residues 563 and 565 (S563A, S565A) with protein kinase A (PKA), followed by tryptic phosphopeptide mapping resulted in two tryptic phosphopeptides. These tryptic phosphopeptides co-migrated with the phosphopeptides released by the same treatment of F654HPRRSSQGVLHMPLYSSPIVK675 phosphorylated with PKA. Analysis of the phosphorylation site mutants, S659A, S660A, and S659A,S660A disclosed that mutagenesis of both Ser-659 and Ser-660 was necessary to abolish the activation of HSL toward a triolein substrate after phosphorylation with PKA. Mutation of Ser-563 to alanine did not cause significant change of activation compared with wild-type HSL. Hence, our results demonstrate that in addition to the previously identified Ser-563, two other PKA phosphorylation sites, Ser-659 and Ser-660, are present in HSL and, furthermore, that Ser-659 and Ser-660 are the major activity controlling sites in vitro.

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.

Development of decompensated dilated cardiomyopathy is associated with decreased gene expression and activity of the milrinone-sensitive cAMP phosphodiesterase PDE3A

BACKGROUND

Phosphodiesterase III (PDE3) inhibitors are inotropic agents used to treat congestive heart failure (CHF) and are less effective in patients with severe CHF. Little is known about relative changes in PDE3 activity or gene expression during the evolution of cardiomyopathy.

METHODS AND RESULTS

In the present study, we evaluated temporal changes in PDE3A gene expression before and after pacing-induced CHF in nine mongrel dogs. Three weeks of left ventricular (LV) pacing produced LV end-diastolic pressures of 15+/-1.7 mm Hg, whereas overt CHF at 4 to 5 weeks was associated with LV end-diastolic pressures of 24+/-1.7 mm Hg; prepacing values were 6.6+/-0.6 mm Hg. Total RNA isolated from LV tissues was analyzed on Northern blots; 10 unpaced normal hearts served as tissue controls. Signals for PDE3A mRNAs (7, 8, and 10 kb) or PDE4D (7.6 kb) were normalized against glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or ribosomal 18S RNA. Before the onset of CHF, PDE3A/GAPDH ratios were not different between the control and 3-week paced groups. In contrast, all PDE3A/GAPDH ratios were selectively reduced by 52%, and PDE3A/18S was reduced by 70% (P<.05) in CHF; PDE4D/GAPDH (or 18S) was unchanged. LV tissues from four control and four CHF dogs were also processed to isolate cytosolic and microsomal membrane protein for cAMP PDE3 activity assays. CHF was associated with a significant 54% reduction (P<.05) in microsomal but not cytosolic PDE3 activity.

CONCLUSIONS

Selective downregulation of PDE3A may account in part for the ineffectiveness of milrinone in the treatment of severe CHF.

Insulin and cyclic AMP act at different levels on transcription of the L-type pyruvate kinase gene

We previously demonstrated that, in hepatocytes in primary culture, the role of insulin on induction of L-type pyruvate kinase (L-PK) gene expression was mainly to induce glucokinase synthesis, needed for glucose phosphorylation to glucose 6-phosphate. However, we show here that when hepatocytes have been isolated from rats starved for 72 h, glucose and constitutive glucokinase expression was not sufficient to fully stimulate the L-PK promoter, low insulin concentrations being still required. In addition, activation remains sensitive to cAMP inhibition, but cannot be reproduced in the absence of insulin by a competitive cAMP antagonist. We propose that both insulin and cAMP act on expression of the L-PK gene at, at least, two levels: positive and negative regulation of glucokinase gene expression, and more downstream levels.