A1 adenosine receptors in the human fat cell: tissue distribution and regulation of radioligand binding

Purinergic Receptors in Adipose Tissue As Potential Targets in Metabolic Disorders

Extracellular nucleosides and nucleotides, such as adenosine and adenosine triphosphate (ATP), are involved in many physiological and pathological processes in adipose tissue (AT). It is becoming accepted that, in addition to the well-established sympathetic and hormonal system, purinergic receptors contribute significantly to regulation of adipocyte functions. Several receptor subtypes for both adenosine (P1) and ATP (P2X and P2Y) have been characterized in white adipocytes (WA) and brown adipocytes (BA). The effects mediated by adenosine and ATP on adipocytes are multiple and often differing, depending on specific receptors activated. Using a variety of agonists, antagonists and transgenic animals it has been demonstrated that adenosine and P2 receptors are involved in lipolysis, lipogenesis, adipokines secretion, glucose uptake, adipogenesis, cell proliferation, inflammation, and other processes. Given their central role in regulating many AT functions, purinergic receptors are considered potential therapeutic targets in different pathological conditions, such as obesity and type-2 diabetes. To achieve this goal, specific and potent P1 and P2 receptors activators and inhibitors are being developed and show promising results. However, more insight is needed into the function of P2 receptors in brown and beige adipocytes and their potential role in thermogenesis. This review aims at summarizing current knowledge on the patho-/physiological role of P1, P2X, and P2Y receptors in WA and BA and their potential exploitation for pharmacological intervention. Furthermore, we analyze impact of purinergic signaling in AT - in health and metabolic diseases.

Adaptive Immunity and Metabolic Health: Harmony Becomes Dissonant in Obesity and Aging

Adipose tissue (AT) is the primary energy reservoir organ, and thereby plays a critical role in energy homeostasis and regulation of metabolism. AT expands in response to chronic overnutrition or aging and becomes a major source of inflammation that has marked influence on systemic metabolism. The chronic, sterile inflammation that occurs in the AT during the development of obesity or in aging contributes to onset of devastating diseases such as insulin resistance, diabetes, and cardiovascular pathologies. Numerous studies have shown that inflammation in the visceral AT of humans and animals is a critical trigger for the development of metabolic syndrome. This work underscores the well-supported conclusion that the inflammatory immune response and metabolic pathways in the AT are tightly interwoven by multiple layers of relatively conserved mechanisms. During the development of diet-induced obesity or age-associated adiposity, cells of the innate and the adaptive immune systems infiltrate and proliferate in the AT. Macrophages, which dominate AT-associated immune cells in mouse models of obesity, but are less dominant in obese people, have been studied extensively. However, cells of the adaptive immune system, including T cells and B cells, contribute significantly to AT inflammation, perhaps more in humans than in mice. Lymphocytes regulate recruitment of innate immune cells into AT, and produce cytokines that influence the helpful-to-harmful inflammatory balance that, in turn, regulates organismal metabolism. This review describes inflammation, or more precisely, metabolic inflammation (metaflammation) with an eye toward the AT and the roles lymphocytes play in regulation of systemic metabolism during obesity and aging. © 2017 American Physiological Society. Compr Physiol 7:1307-1337, 2017.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Adenosine A₁ receptors do not play a major role in the regulation of lipogenic gene expression in hepatocytes

Activation of adenosine A₁ receptors was reported to promote fatty acid synthesis in AML-12 cells, by increasing the expression of SREBP-(1c) (sterol regulatory binding protein 1c) and FAS (fatty acid synthase). Since these findings have important therapeutic implications for the discovery of adenosine A₁ receptor agonists, further studies were undertaken to determine the expression and functional relevance of adenosine A₁ receptor in the liver. To that end, we used two classes of distinct adenosine A₁ receptor agonists: CPA (N⁶-cyclopentyl-adenosine), a full agonist and GS-9667 (2-{6-[((1R,2R)-2-hydroxycyclopentyl)-amino]purin-9-yl}(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]-oxolane-3,4-diol), a partial agonist. Treatment of AML-12 cells, HepG2 cells and primary human hepatocytes with either CPA or GS-9667 did not increase the gene expression of SREBP-(1c) or FAS. Furthermore, in AML-12 and HepG2 cells, CPA did not antagonize forskolin-stimulated cAMP production, a characteristic of adenosine A₁ receptor activation, indicating that these cells lack adenosine A₁ receptor function. Consistent with this finding, adenosine A₁ receptor gene expression was found to be very low and adenosine A₁ receptor protein levels were hardly detectable by radioligand binding assays in hepatic cell lines such as AML-12 and HepG2 as well as in both mouse and human liver tissues. Finally, acute treatment with adenosine A₁ receptor agonist GS-9667 had no significant effect on gene expression of both SREBP-(1c) and FAS in livers of Sprague Dawley rats. Taken together, our data suggest that the expression of adenosine A₁ receptor is too low to play a major role in the regulation of lipogenic gene expression in hepatocytes.

Historical perspectives in fat cell biology: the fat cell as a model for the investigation of hormonal and metabolic pathways

For many years, there was little interest in the biochemistry or physiology of adipose tissue. It is now well recognized that adipocytes play an important dynamic role in metabolic regulation. They are able to sense metabolic states via their ability to perceive a large number of nervous and hormonal signals. They are also able to produce hormones, called adipokines, that affect nutrient intake, metabolism and energy expenditure. The report by Rodbell in 1964 that intact fat cells can be obtained by collagenase digestion of adipose tissue revolutionized studies on the hormonal regulation and metabolism of the fat cell. In the context of the advent of systems biology in the field of cell biology, the present seems an appropriate time to look back at the global contribution of the fat cell to cell biology knowledge. This review focuses on the very early approaches that used the fat cell as a tool to discover and understand various cellular mechanisms. Attention essentially focuses on the early investigations revealing the major contribution of mature fat cells and also fat cells originating from adipose cell lines to the discovery of major events related to hormone action (hormone receptors and transduction pathways involved in hormonal signaling) and mechanisms involved in metabolite processing (hexose uptake and uptake, storage, and efflux of fatty acids). Dormant preadipocytes exist in the stroma-vascular fraction of the adipose tissue of rodents and humans; cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Finally, more recent insights into adipocyte secretion, a completely new role with major metabolic impact, are also briefly summarized.

A1 adenosine receptor: role in diabetes and obesity

Adenosine mediates its diverse effects via four subtypes (A(1), A(2A), A(2B) and A(3)) of G-protein-coupled receptors. The A(1) adenosine receptor (A(1)AR) subtype is the most extensively studied and is well characterized in various organ systems. The A(1)ARs are highly expressed in adipose tissue, and endogenous adenosine has been shown to tonically activate adipose tissue A(1)ARs. Activation of the A(1)ARs in adipocytes reduces adenylate cyclase and cAMP content and causes inhibition of lipolysis. The role of A(1)ARs in lipolysis has been well characterized by using several selective A(1)AR agonists as well as A(1)AR knockout mice. However, the contribution of A(1)ARs to the regulation of lipolysis in pathological conditions like insulin resistance, diabetes and dyslipidemia, where free fatty acids (FFA) play an important role, has not been well characterized. Pharmacological agents that reduce the release of FFA from adipose tissue and thus the availability of circulating FFA have the potential to be useful for insulin resistance and hyperlipidemia. Toward this goal, several selective and efficacious agonists of the A(1)ARs are now available, and some have entered early-phase clinical trials; however, none have received regulatory approval yet. Here we review the existing knowledge on the role of A(1)ARs in insulin resistance, diabetes and obesity, and the progress made in the development of A(1)AR agonists as antilipolytic agents, including the challenges associated with this approach.

Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores

Fatty acids (FAs) are essential components of all lipid classes and pivotal substrates for energy production in all vertebrates. Additionally, they act directly or indirectly as signaling molecules and, when bonded to amino acid side chains of peptides, anchor proteins in biological membranes. In vertebrates, FAs are predominantly stored in the form of triacylglycerol (TG) within lipid droplets of white adipose tissue. Lipid droplet-associated TGs are also found in most nonadipose tissues, including liver, cardiac muscle, and skeletal muscle. The mobilization of FAs from all fat depots depends on the activity of TG hydrolases. Currently, three enzymes are known to hydrolyze TG, the well-studied hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL), discovered more than 40 years ago, as well as the relatively recently identified adipose triglyceride lipase (ATGL). The phenotype of HSL- and ATGL-deficient mice, as well as the disease pattern of patients with defective ATGL activity (due to mutation in ATGL or in the enzyme's activator, CGI-58), suggest that the consecutive action of ATGL, HSL, and MGL is responsible for the complete hydrolysis of a TG molecule. The complex regulation of these enzymes by numerous, partially uncharacterized effectors creates the "lipolysome," a complex metabolic network that contributes to the control of lipid and energy homeostasis. This review focuses on the structure, function, and regulation of lipolytic enzymes with a special emphasis on ATGL.

Adenosine A1 receptors regulate lipolysis and lipogenesis in mouse adipose tissue-interactions with insulin

Adenosine acting at adenosine A1 receptors is considered to be one major regulator of adipose tissue physiology. We have examined the role of adenosine and its interactions with insulin in adipose tissue by using A1R knock out (-/-) mice. Removal of endogenous adenosine with adenosine deaminase caused lipolysis in A1R (+/+), but not A1R (-/-) adipocytes. The adenosine analogue, 2-chloroadenosine, inhibited noradrenaline-stimulated lipolysis and cAMP accumulation in A1R (+/+), but not in A1R (-/-) adipocytes. Insulin reduces lipolysis and cAMP via another mechanism than adenosine and acted additively, but not synergistically, with adenosine. Plasma levels of free fatty acids, glycerol and triglycerides were significantly lower in A1R (+/+) than in A1R (-/-) mice after administration of an adenosine analogue. 2-chloroadenosine induced lipogenesis in presence of insulin in A1R (+/+), but not in A1R (-/-) adipocytes. There were no changes in mRNA levels for several genes involved in fat synthesis in adipose tissue between genotypes. Body weight was similar in young A1R (+/+) and A1R (-/-) mice, but old male A1R (-/-) mice were heavier than wild type controls. In conclusion, adenosine inhibits lipolysis via the adenosine A1 receptor and other adenosine receptors play no significant role. Adenosine and insulin mediate additive but not synergistic antilipolytic effects and 2-chloroadenosine stimulates lipogenesis via adenosine A1 receptors. Thus deletion of adenosine A1 receptors should increase lipolysis and decrease lipogenesis, but in fact an increased fat mass was observed, indicating that other actions of adenosine A1 receptors, possibly outside adipose tissue, are also important.

Overexpression of the A1 adenosine receptor in adipose tissue protects mice from obesity-related insulin resistance

In-vitro studies have implicated the A(1) adenosine receptor (A(1)AR) of adipocytes in inhibition of lipolysis, stimulation of lipogenesis and enhancement of the action of insulin on glucose metabolism. To determine whether any of these activities were physiologically relevant in an intact animal, A(1)AR was overexpressed in adipose tissue of transgenic mice. Lower plasma free fatty acid (FFA) levels were observed in the transgenic mice relative to the litter-matched controls, supporting a significant physiological role for adipocyte A(1)AR in the control of lipolysis. However, no differences were observed in body weights or body composition. On a high fat diet, both the transgenic mice and the litter matched controls, male and female, became equally obese. Unlike the control mice, however, the transgenic mice did not develop insulin resistance, as demonstrated by serum glucose and insulin levels and glucose and insulin tolerance tests. These findings demonstrate that adipocyte A(1)AR plays an important physiological role in the control of insulin sensitivity in an intact animal and therefore should be considered to be a potential therapeutic target for the treatment of obesity-related insulin resistance and type 2 diabetes.

Subcutaneous and visceral adipose tissue: their relation to the metabolic syndrome

Methods for assessment, e.g., anthropometric indicators and imaging techniques, of several phenotypes of human obesity, with special reference to abdominal fat content, have been evaluated. The correlation of fat distribution with age, gender, total body fat, energy balance, adipose tissue lipoprotein lipase and lipolytic activity, adipose tissue receptors, and genetic characteristics are discussed. Several secreted or expressed factors in the adipocyte are evaluated in the context of fat tissue localization. The body fat distribution and the metabolic profile in nonobese and obese individuals is discussed relative to lipolysis, antilypolysis and lipogenesis, insulin sensitivity, and glucose, lipid, and protein metabolism. Finally, the endocrine regulation of abdominal visceral fat in comparison with the adipose tissue localized in other areas is presented.

The beta3-adrenergic receptor activates mitogen-activated protein kinase in adipocytes through a Gi-dependent mechanism

Promiscuous coupling between G protein-coupled receptors and multiple species of heterotrimeric G proteins provides a potential mechanism for expanding the diversity of G protein-coupled receptor signaling. We have examined the mechanism and functional consequences of dual Gs/Gi protein coupling of the beta3-adrenergic receptor (beta3AR) in 3T3-F442A adipocytes. The beta3AR selective agonist disodium (R, R)-5-[2[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]propyl]-1, 3-benzodioxole-2,2-dicarboxylate (CL316,243) stimulated a dose-dependent increase in cAMP production in adipocyte plasma membrane preparations, and pretreatment of cells with pertussis toxin resulted in a further 2-fold increase in cAMP production by CL316,243. CL316,243 (5 microM) stimulated the incorporation of 8-azido-[32P]GTP into Galphas (1.57 +/- 0.12; n = 3) and Galphai (1. 68 +/- 0.13; n = 4) in adipocyte plasma membranes, directly demonstrating that beta3AR stimulation results in Gi-GTP exchange. The beta3AR-stimulated increase in 8-azido-[32P]GTP labeling of Galphai was equivalent to that obtained with the A1-adenosine receptor agonist N6-cyclopentyladenosine (1.56 +/- 0.07; n = 4), whereas inclusion of unlabeled GTP (100 microM) eliminated all binding. Stimulation of the beta3AR in 3T3-F442A adipocytes led to a 2-3-fold activation of mitogen-activated protein (MAP) kinase, as measured by extracellular signal-regulated kinase-1 and -2 (ERK1/2) phosphorylation. Pretreatment of cells with pertussis toxin (PTX) eliminated MAP kinase activation by beta3AR, demonstrating that this response required receptor coupling to Gi. Expression of the human beta3AR in HEK-293 cells reconstituted the PTX-sensitive stimulation of MAP kinase, demonstrating that this phenomenon is not exclusive to adipocytes or to the rodent beta3AR. ERK1/2 activation by the beta3AR was insensitive to the cAMP-dependent protein kinase inhibitor H-89 but was abolished by genistein and AG1478. These data indicate that constitutive beta3AR coupling to Gi proteins serves both to restrain Gs-mediated activation of adenylyl cyclase and to initiate additional signal transduction pathways, including the ERK1/2 MAP kinase cascade.

Differential development of adenosine A1 and A2b receptors in the rat duodenum

1. The development of the adenosine A1 and A2b receptors inducing relaxation of the rat duodenum was studied by use of a combination of functional and radioligand binding assays on rats aged between 5 and 30 days and compared with results previously found in adult rat duodenum. 2. 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX) bound with high affinity to a single site in duodenum preparations from rats aged 20, 25 and 30 days. At 10 and 15 days there was no detectable specific binding of [3H]-DPCPX. 3. The affinity (KD) of the binding site for [3H]-DPCPX was similar in membrane preparations from 20, 25 and 30 day old animals (1.58-2.27 nM), but the density (Bmax) of binding sites was found to increase up to 25 days where peak levels (72.0 +/- 9.5 fmol mg-1 protein) were observed and then decline at 30 days (45.5 +/- 2.9 fmol mg-1 protein) to levels commensurate with those previously determined in the adult rat duodenum. 4. In duodenum from 10 day old rats no responses to N6-cyclopentyladenosine (CPA, 1 nM-10 microM) were observed, at 15 days the duodenum responded to the highest concentration of CPA (3 microM) only, and at 20-30 days concentration-related responses were observed, with the potency of CPA increasing with an increase in age. DPCPX (10 nM) abolished the responses to CPA except at the highest concentration of CPA (3 microM) where the response was markedly attenuated, suggesting the presence of an A1, receptor. 5. In rat duodenum from animals of all ages (5-30 days), concentration-related responses to 5'-N-ethylcarboxamidoadenosine (NECA) were observed. The potency of NECA remained constant with an increase in age, whereas the maximum relaxation response increased from 20% at 5 days to 110% at 25 and 30 days. In the presence of 1 microM DPCPX a right-ward shift in the concentration-response curve to NECA was observed at all ages. In the presence of 10 nM DPCPX, the response to NECA was unaffected in the duodenum from animals aged 10 and 15 days. However, in duodenum from animals aged 20-30 days the concentration-response curve to NECA was shifted to the right suggesting that there is an A1 component to the action of NECA at these ages. Schild analysis of the effects of increasing concentrations of DPCPX versus NECA on the duodenum from 25 day old animals generated a slope of 0.62 suggesting that NECA acts at A1 and A2b receptors as in the adult. 6. The A2b-selective analogue, 2-[p-(carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosi ne (CGS 21680) (10 nM-10 microM) was without effect on the carbachol-contracted duodenum from 15 day old rats and the duodenum from 25 day old rats responded to the highest concentration of CGS 21680 only, suggesting that the A2 receptors here, as in the adult, are not of the A2a subtype. The adenosine antagonist, 8-phenyltheophylline (8-PT) (10 microM), abolished the inhibitory effects of NECA (100 nM-100 microM) on 10, 15 and 25 day old rat duodenum indicating that the responses to NECA were not mediated via an adenosine A3 receptor. 7. These results show that adenosine A1 receptors in rat duodenum are present and functionally viable from day 20 onwards and that the density of A1 receptors varies with age, increasing up to day 25 and then declining at day 30 to a density commensurate with that found in the adult. The responses to CPA, mediated via the A1 receptor, increase with age in a similar fashion. In contrast however, the response to NECA was evident from day 5, the earliest age studied, and from days 5-15 NECA acted via the A2b receptor subtype. However, from day 20 onwards NECA acted at a mixed population of A1 and A2b receptors. These results demonstrate the differential development of the A1 and the A2b receptors in the rat duodenum.

Effect of bovine somatotropin and food deprivation on beta-adrenergic and A1 adenosine receptor binding in adipose tissue of lactating cows

Lactating Holstein cows were used to assess the effect of bovine somatotropin (bST; n = 8) and fasting (FAST; n = 4) on ligand binding to beta-adrenergic (BAR) and Type-1 adenosine (A1R) receptors in adipose tissue. Cows received exogenous bST (sometribove; 40 mg/d) or no hormone (control) for 4 d in a single-reversal design with a 7-d interval between treatment periods. Subcutaneous adipose tissue biopsies were taken on day 4 of each treatment. Eight d after the bST regimen, 4 cows were fasted for 3 d and adipose biopsies were taken. Ligand binding was quantified with a postnuclear, total adipose tissue membrane preparation (100,000 x g pellet). Binding to BAR and A1R was assessed with the antagonists [125I]iodocyanopindolol (ICP) and [3H]8-cyclopentyl-1,3-dipropylxanthine (DCPCX), respectively. The binding affinity (Kd) of BAR for ICP was not affected by bST but was enhanced by FAST; maximal binding (Bmax) was increased with bST treatment (P < 0.06) and reduced by FAST (61%, P < 0.01). Kd values for DCPCX binding to A1R were not changed by bST or FAST. bST did not affect Bmax for A1R; however, FAST reduced the Bmax by 38%. Data highlight the differential regulation of BAR and A1R by bST and FAST.

The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine to adenosine A1 receptors in rat smooth muscle preparations

1. The binding of 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX), an antagonist radioligand selective for adenosine A1 receptors, was studied in rat duodenum, colon muscularis mucosae and longitudinal muscle, urinary bladder and vasa deferentia. 2. [3H]-DPCPX bound with high affinity to a single site in all membrane preparations studied with the exception of the rat urinary bladder in which no specific binding was detected. The affinity (Kd) of the binding site for [3H]-DPCPX was similar in all membrane preparations, the colon longitudinal muscle (1.18 +/- 0.47 nM), colon muscularis mucosae (0.84 +/- 0.15 nM), duodenum (1.59 +/- 0.18 nM) and vasa deferentia (0.93 +/- 0.17 nM). The density of [3H]-DPCPX binding sites was similar in the duodenum (38.8 +/- 4 fmol mg-1 protein), muscularis mucosae (43 +/- 3.5 fmol mg-1 protein) and vasa deferentia (43.3 +/- 12.2 fmol mg-1 protein), but in the longitudinal muscle 6-7 fold more binding sites (295 +/- 70 fmol mg-1 protein) were identified. 3. Inhibition studies using DPCPX (0.1-100 nM), N6-cyclopentyladenosine (CPA) (0.1-100 nM), 5'-N-ethylcarboxamidoadenosine (NECA) (2 nM-10 microM) and (R)-N6-phenylisopropyladenosine (R-PIA) (1 nM-1 microM) to displace the binding of [3H]-DPCPX at a concentration around the Kd value (1 nM), demonstrated an order of potency of displacement in all tissues of DPCPX > or = CPA > R-PIA > NECA. This potency order is characteristic of an A1 receptor, indicating that [3H]-DPCPX binds to adenosine A1 receptors in the rat duodenum, colon and vasa deferentia. Two site analysis revealed that the agonists bind to both a high and low affinity state of the receptor.4. The existence of Al binding sites in the rat vasa deferentia, colon muscularis mucosae and duodenum, and their absence in the urinary bladder, is consistent with previous functional studies.However, in contrast to the findings of the [3H]-DPCPX binding assay, no functional response mediated by adenosine Al receptors could be detected by measuring contractile or relaxant responses to CPA in the colon longitudinal muscle. The functional significance of the binding sites in this tissue has therefore yet to be determined.

A1 adenosine receptors can occur manifesting two kinetic components of 8-cyclopentyl-1,3-[3H]dipropylxanthine ([3H]DPCPX) binding

The results described in this paper show, for the first time, that A1 adenosine receptors can have two kinetic components for the binding of the antagonist [3H]DPCPX. At low ionic strength (< or = 42 mmol/l), dissociation of [3H]DPCPX bound to A1 receptors fitted better to a two kinetic components model than to a one kinetic component model. The kinetic constants were consistent with comparable Kd values for the two components of the antagonist binding, and therefore these two components cannot be distinguished by saturation isotherm analysis.

Coupling of inhibitory receptors with Gi-proteins in hamster adipocytes: comparison between adenosine A1 receptor and alpha 2-adrenoceptor

Adenosine A1 receptors and alpha 2-adrenoceptors are both potent inhibitors of adipocyte lipolysis when activated by their agonists. The aim of this work was to compare the coupling of these receptors to the Gi-proteins in hamster adipocytes. The adenosine A1 receptor was characterized with the antagonist [3H]dipropyl-cyclopentyl-xanthine ([3H]DPCPX) and the agonist [3H](-)-phenylisopropyladenosine ([3H]PIA). It was demonstrated by [32P]ADP-ribosylation with pertussis toxin and immunoblotting that Gi1, Gi2 and Gi3 are expressed in hamster adipocytes. Partial ADP-ribosylation of Gi-proteins by pertussis toxin, acting on the intact cells or on the adipocyte membranes, demonstrated that the adenosine A1 receptor was less sensitive to the disappearance of functional Gi-proteins than the alpha 2-adrenoceptor. These results are in accordance with the weak sensitivity of the binding of the agonist [3H]PIA to guanine nucleotides and seem to confirm that the adenosine A1 receptor is strongly and differently coupled than the alpha 2-adrenoceptor to the Gi-proteins in hamster adipocyte membranes.

Distribution of PYY receptors in human fat cells: an antilipolytic system alongside the alpha 2-adrenergic system

The antilipolytic effect of peptide YY (PYY) and neuropeptide Y has recently been shown in human adipocytes. PYY receptors were investigated in three human adipose deposits. A greater number of 125I-labeled PYY binding sites was found in femoral adipocyte membranes (maximal binding = 40 +/- 4 fmol/mg protein; dissociation constant = 0.3 +/- 0.1 nM) when compared with mammary and pericolonic adipose tissue. PYY receptors, like alpha 2-adrenergic sites, were largely expressed in femoral fat cells. Such a distribution was not specific either to inhibitory or to stimulating adenylyl cyclase systems since adenosine A1 and beta-adrenergic receptors were more numerous in pericolonic adipocytes. On isolated adipocytes, PYY (10(-7) M) inhibited lipolysis by 58 +/- 2% in femoral and 14 +/- 4% in pericolonic fat cells; epinephrine had the following similar response: 62 +/- 5 and 26 +/- 8%, respectively. A close relationship between the number of alpha 2-sites and PYY sites and the antilipolytic effects initiated by PYY and an alpha 2-agonist was observed. No significant differences were noted in the amount of Gi proteins in femoral and pericolonic adipocyte membranes.

Regulation of adenylate cyclase in plasma membranes of human intraabdominal and abdominal subcutaneous adipocytes

Fat cells were isolated from human subcutaneous, omental, and mesenteric adipose tissue. Omental fat cells were the smallest (438 pL), subcutaneous cells were intermediate (494 pL), and mesenteric cells were the largest (600 pL). There was no difference in the stimulation of adenylate cyclase by isoproterenol in plasma membranes of adipocytes prepared from the three sites. N6-(phenylisopropyl)adenosine inhibited 7-deacetyl-6-(N-acetylglycyl)forskolin-stimulated adenylate cyclase activity more potently in subcutaneous than in intraabdominal (especially omental) fat cell membranes. Kd values of the adenosine A1 receptors for 1,3-[3H]dipropyl-8-cyclopentylxanthine were similar in the three fat depots, but the receptor number as calculated per milligram protein was lower in omental than in abdominal subcutaneous adipocytes. Differences in adipocyte size cannot explain regional differences in the regulation of adenylate cyclase.