Presence of a functional inhibitory GTP-binding regulatory component, Gi, linked to adenylate cyclase in adipocytes of ob/ob mice

Oxytocin stimulates lipolysis, prostaglandin E2 synthesis, and leptin secretion in 3T3-L1 adipocytes

A model of oxytocin in the regulation of metabolic status has described one of oxytocin synthesis and release from the neurohypophysis in response to leptin, to suppress further leptin release. In addition, a lipogenic role for oxytocin has been suggested, consistent with an insulinergic action. This model, however, may be incorrect. Oxytocin reduces fat mass in the absence of either leptin or leptin receptor signalling, thereby challenging the interdependence between leptin and oxytocin. An oxytocin induced production of the anti-lipolytic prostaglandin E₂ (PGE₂) might account for this. Media from 3T3-L1 differentiated adipocytes treated with oxytocin (0-50 nmol.L^-1) for 24 hrs were assayed for PGE₂, leptin, adiponectin, and glycerol. Harvested cells were analysed for lipid droplet triglyceride and cytosolic free fatty acid (FFA) by flow cytometry, and for altered expression of lipolytic and lipogenic associated gene ontology transcripts by cDNA array. Both PGE₂ and leptin secretion were significantly increased by oxytocin treatment whilst adiponectin secretion was not. A significant increase in cytosolic FFA was detected following oxytocin treatment, similar to that determined following treatment with isoproterenol (positive control). A significant increase in glycerol release to the culture media confirmed a lipolytic effect. No enrichment of lipolytic and lipogenic associated gene ontology transcripts was determined, but significant overrepresentation of chemosensory olfactory transcripts was. In conclusion, oxytocin stimulates lipolysis in 3T3-L1 adipocytes, mediated by autocrine/paracrine actions of PGE₂ and leptin. To confirm that this response is mediated solely by the oxytocin receptor, further experiments would require those effects being blocked by a specific oxytocin antagonist.

Regulation of adipocyte lipolysis

In adipocytes the hydrolysis of TAG to produce fatty acids and glycerol under fasting conditions or times of elevated energy demands is tightly regulated by neuroendocrine signals, resulting in the activation of lipolytic enzymes. Among the classic regulators of lipolysis, adrenergic stimulation and the insulin-mediated control of lipid mobilisation are the best known. Initially, hormone-sensitive lipase (HSL) was thought to be the rate-limiting enzyme of the first lipolytic step, while we now know that adipocyte TAG lipase is the key enzyme for lipolysis initiation. Pivotal, previously unsuspected components have also been identified at the protective interface of the lipid droplet surface and in the signalling pathways that control lipolysis. Perilipin, comparative gene identification-58 (CGI-58) and other proteins of the lipid droplet surface are currently known to be key regulators of the lipolytic machinery, protecting or exposing the TAG core of the droplet to lipases. The neuroendocrine control of lipolysis is prototypically exerted by catecholaminergic stimulation and insulin-induced suppression, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and perilipin. Interestingly, in recent decades adipose tissue has been shown to secrete a large number of adipokines, which exert direct effects on lipolysis, while adipocytes reportedly express a wide range of receptors for signals involved in lipid mobilisation. Recently recognised mediators of lipolysis include some adipokines, structural membrane proteins, atrial natriuretic peptides, AMP-activated protein kinase and mitogen-activated protein kinase. Lipolysis needs to be reanalysed from the broader perspective of its specific physiological or pathological context since basal or stimulated lipolytic rates occur under diverse conditions and by different mechanisms.

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

The Sir David Cuthbertson Medal Lecture. Hunting for new pieces to the complex puzzle of obesity

Disentangling the neuroendocrine systems that regulate energy homeostasis and adiposity has been a long-standing challenge in pathophysiology, with obesity being an increasingly important public health problem. Adipose tissue is no longer considered a passive bystander in body-weight regulation. It actively secretes a large number of hormones, growth factors, enzymes, cytokines, complement factors and matrix proteins, at the same time as expressing receptors for most of these elements, which influence fuel storage, mobilisation and utilisation at both central and peripheral sites. Thus, an extensive cross talk at a local and systemic level in response to specific external stimuli or metabolic changes underpins the multifunctional characteristics of adipose tissue. In addition to the already-known adipokines, such as IL, TNFalpha, leptin, resistin and adiponectin, more recently attention has been devoted to 'newcomers' to the 'adipose tissue arena', which include aquaporin, caveolin, visfatin, serum amyloid A and vascular endothelial growth factor. While in vitro and in vivo experiments have provided extremely valuable information, the advances in genomics, proteomics and metabolomics are offering a level of information not previously attainable to help unlock the molecular basis of obesity. The potential and power of combining pathophysiological observations with the wealth of information provided by the human genome, knock-out models, transgenesis, DNA microarrays, RNA silencing and other emerging technologies offer a new and unprecedented view of a complex disease, conferring novel insights into old questions by identifying new pieces to the unfinished jigsaw puzzle of obesity.

Leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes

The aim of the present study was to gain insight into the signaling pathway used by leptin to stimulate lipolysis. The lipolytic rate of white adipocytes from sex- and age-matched lean (+/+) and fa/fa rats was determined in the absence or presence of leptin together with a number of agents acting at different levels of the signaling cascade. Leptin did not modify FSK-, dbcAMP-, and IBMX-stimulated lipolysis. Lipolysis can also be maximally stimulated by lowering media adenosine levels with adenosine deaminase (ADA), i.e., in the ligand-free state. Although ADA produced near maximal lipolysis in adipocytes of lean animals, only half of the maximal lipolytic rate (50.9+/-3.2%) was achieved in fat cells from fa/fa rats (P=0.0034). In adipocytes from lean animals preincubated with ADA, leptin caused a concentration-related stimulation of lipolysis (P=0.0001). However, leptin had no effect on the lipolytic activity of adipocytes in the ligand-free state from fa/fa rats. The adenosine A1 receptor agonist CPA effectively inhibited basal lipolysis in both lean and obese adipocytes (P=0.0001 and P=0.0090, respectively). Leptin had no effect on the lipolytic rate of adipocytes isolated from fa/fa rats and preincubated with CPA. When adipocytes were incubated with the A1 receptor antagonist DPCPX, a significant increase in glycerol release was observed in fa/fa fat cells (P=0.009), whereas cells isolated from lean rats showed no differences to ADA-stimulated lipolysis. After pretreatment with PTX, which inactivates receptor-mediated Gi function, adipocytes of obese rats became as responsive to the stimulatory actions of ISO as cells from lean rats (P=0.0090 vs. ISO in fa/fa rats; P=0.2416 vs. lean rats, respectively). PTX treatment of lean cells, however, did not alter their response to this lipolytic agent. It can be concluded that the lipolytic effect of leptin is located at the adenylate cyclase/Gi proteins level and that leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes.

Targeted disruption of the beta 3-adrenergic receptor gene

beta 3-Adrenergic receptors (beta 3-ARs) are expressed predominantly in white and brown adipose tissue, and beta 3-selective agonists are potential anti-obesity drugs. However, the role of beta 3-ARs in normal physiology is unknown. To address this issue, homologous recombination was used to generate mice that lack beta 3-ARs. This was accomplished by direct injection of a DNA-targeting construct into mouse zygotes. Twenty-three transgenic mice were generated, of which two had targeted disruption of the beta 3-AR gene. Mice that were homozygous for the disrupted allele had undetectable levels of intact beta 3-AR mRNA, as assessed by RNase protection assay and Northern blotting, and lacked functional beta 3-ARs, as demonstrated by complete loss of beta 3-agonist (CL 316,243)-induced stimulation of adenylate cyclase activity and lipolysis. beta 3-AR-deficient mice had modestly increased fat stores (females more than males), indicating that beta 3-ARs play a role in regulating energy balance. Importantly, beta 1 but not beta 2-AR mRNA levels up-regulated in white and brown adipose tissue of beta 3-AR-deficient mice (brown more than white), strongly implying that beta 3-ARs mediate physiologically relevant signaling under normal conditions and that "cross-talk" exists between beta 3-ARs and beta 1-AR gene expression. Finally, acute treatment of normal mice with CL 316,243 increased serum levels of free fatty acids (FFAs) (3.2-fold) and insulin (140-fold), increased energy expenditure (2-fold), and reduced food intake (by 45%). These effects were completely absent in beta 3-AR-deficient mice, proving that the actions of CL are mediated exclusively by beta 3-ARs. beta 3-AR-deficient mice should be useful as a means to a better understanding of the physiology and pharmacology of beta 3-ARs.

Pertussis Toxin in the A10 Region Increases Dopamine Synthesis and Metabolism

Inhibitory regulation of dopamine neurons is mediated by dopamine autoreceptor and gamma-aminobutyric acidB receptor opening of potassium channels. Increased potassium conductance by either receptor is G protein dependent. To evaluate the role of G proteins in vivo, pertussis toxin (PTX) was microinjected into the A10 dopamine region and changes in dopamine metabolism and synthesis measured. PTX produced an elevation in dopamine metabolism and synthesis in the A10 region and nucleus accumbens for up to 4 days after injection. By day 7 the levels of the dopamine precursor and metabolites had returned to normal. A less consistent increase was also measured in the A9 dopamine region and the prefrontal cortex. Although dopamine synthesis and metabolism had returned to normal by day 7, the in vitro ADP-ribosylation of G proteins in the A10 region by PTX remained depressed by approximately 50% from day 1 to day 14 after administration, returning to normal by day 30. The data suggest that in vivo ribosylation of G proteins may lead to a short-term attenuation of the tonic inhibitory control of dopamine neurons, which can be compensated for by PTX-insensitive mechanisms.

Cholera and pertussis toxins modify regulation of glucose transport activity in rat adipose cells: evidence for mediation of a cAMP-independent process by G-proteins

Adenylyl cyclase in rat adipose cells is stimulated by ligands for Rs receptors (e.g. isoproterenol) and inhibited by ligands for Ri receptors (e.g. adenosine). In contrast, Rs receptors mediate inhibition and Ri receptors mediate augmentation of insulin-stimulated glucose transport activity by a process independent of changes in cellular cAMP-dependent protein kinase activity [Kuroda M., Honnor R. C., Cushman S. W., Londos C. and Simpson I. A. (1987) J. biol. Chem. 262, 245-253]. The present study examines the possible role of G-proteins in the regulation of insulin-stimulated glucose transport activity by Rs and Ri receptors. First, conditions were established that permit intoxication of isolated rat adipocytes by cholera and pertussis toxins without compromising cell integrity. Effectiveness of toxin treatment was monitored by examining adenylyl cyclase activity in isolated plasma membranes. Secondly, neither toxin interfered with the ability of a maximal concentration insulin to initiate the glucose transport response. Thirdly, pertussis toxin eliminated the augmenting effects of adenosine on insulin-stimulated glucose transport activity, but enhanced the inhibitory effects of isoproterenol. Findings with ligands for other Ri receptors (nicotinic acid and prostaglandin E2) mirrored those with adenosine. Finally, cholera toxin elicited a modest depression of transport activity, and only in the absence of an Ri ligand (e.g. adenosine). Furthermore, in contrast to the enhanced stimulation of adenylyl cyclase by isoproterenol and GTP, cholera toxin eliminated the inhibitory effect of isoproterenol on transport activity. The augmentative effects of adenosine on transport activity were unchanged. Measurements of (-/+cAMP) cAMP-dependent protein kinase activity ratios reinforce the notion that modulation of glucose transport activity is independent of changes in cAMP. We conclude that regulation of glucose transport activity by Rs and Ri receptors is mediated by the G-proteins, Gs and Gi (or other toxin substrates), respectively. Inasmuch as such regulation occurs at the plasma membrane and appears to be cAMP-independent, it is suggested that glucose transporters may be direct targets for receptor: G-protein interactions.

Possible role for G-proteins in behavioral sensitization to cocaine

The role of G-proteins in behavioral sensitization to cocaine was examined by injecting pertussis toxin (PTX) into the A10 dopamine cell group. The capacity of acute cocaine to increase motor activity and dopamine release in the nucleus accumbens was significantly augmented in rats pretreated 14 days earlier with PTX. These data suggest that injection of PTX into the A10 dopamine cell group produces a long-term alteration in mesolimbic dopamine function, and implicates A10 dopamine neurons and G-proteins in the development of behavioral sensitization.

Effect of the genetic background and specific mutation on adenylate cyclase activity in obesity syndromes

Adenylate cyclase activity and its modulation by guanine nucleotides and isoproterenol were assessed in adipocyte membranes of mice with mutations causing different genetic obesity syndromes. The object was to determine whether the defect in inhibitory modulation observed in the obese (ob/ob) mouse was also present in the diabetes (db/db) mouse. The data show that adipocyte adenylate cyclase in both the ob/ob and the db/db mouse is resistant to activation by isoproterenol. The response to guanosine triphosphate (GTP) differed between the two mutants, such that an inhibitory phase was visible in the db/db but not in the ob/ob membranes. Moreover, pertussis toxin attenuated the inhibitory effect of GTP and significantly stimulated cyclase activity in the db/db but not in the ob/ob membranes. The data show that the two mutations affect the expression of adenylate cyclase activity via different mechanisms.