Purinergic receptors and metabolic function

Influence of purinergic signaling on glucose transporters: A possible mechanism against insulin resistance?

Metabolic disorders, such as insulin resistance, affect many people worldwide due to the prevalence of obesity and type 2 diabetes, which are pathologies that impair glycemic metabolism. Glucose is the primary energetic substrate of the body and is essential for cellular function. As the cell membrane is not permeable to glucose molecules, there are two distinct groups of glucose transporters: sodium-glucose-linked transporters (SGLTs) and the glucose transporter (GLUT) family. These transporters facilitate the entry of glucose into the bloodstream or cytoplasm where it functions in the production of adenosine 5 ́-triphosphate (ATP). This nucleotide acts in several cellular mechanisms, such as protein phosphorylation and cellular immune processes. ATP directly and indirectly acts as an agonist for purinergic receptors in high concentrations in the extracellular environment. Composed by P1 and P2 groups, the purinoreceptors cover several cellular mechanisms involving cytokines, tumors, and metabolic signaling pathways. Previous publications have indicated that the purinergic signaling activity in insulin resistance and glucose transporters modulates relevant actions on the deregulations that can affect glycemic homeostasis. Thus, this review focuses on the pharmacological influence of purinergic signaling on the modulation of glucose transporters, aiming for a new way to combat insulin resistance and other metabolic disorders.

Glucose control of glucagon secretion-'There's a brand-new gimmick every year'

Glucagon from the pancreatic α-cells is a major blood glucose-regulating hormone whose most important role is to prevent hypoglycaemia that can be life-threatening due to the brain's strong dependence on glucose as energy source. Lack of blood glucose-lowering insulin after malfunction or autoimmune destruction of the pancreatic β-cells is the recognized cause of diabetes, but recent evidence indicates that diabetic hyperglycaemia would not develop unless lack of insulin was accompanied by hypersecretion of glucagon. Glucagon release has therefore become an increasingly important target in diabetes management. Despite decades of research, an understanding of how glucagon secretion is regulated remains elusive, and fundamentally different mechanisms continue to be proposed. The autonomous nervous system is an important determinant of glucagon release, but it is clear that secretion is also directly regulated within the pancreatic islets. The present review focuses on pancreatic islet mechanisms involved in glucose regulation of glucagon release. It will be argued that α-cell-intrinsic processes are most important for regulation of glucagon release during recovery from hypoglycaemia and that paracrine inhibition by somatostatin from the δ-cells shapes pulsatile glucagon release in hyperglycaemia. The electrically coupled β-cells ultimately determine islet hormone pulsatility by releasing synchronizing factors that affect the α- and δ-cells.

Inhibition of Ca2+ signaling and glucagon secretion in mouse pancreatic alpha-cells by extracellular ATP and purinergic receptors

Glucagon secreted from pancreatic alpha-cells plays a critical role in glycemia, mainly by hepatic glucose mobilization. In diabetic patients, an impaired control of glucagon release can worsen glucose homeostasis. Despite its importance, the mechanisms that regulate its secretion are still poorly understood. Since alpha-cells are particularly sensitive to neural and paracrine factors, in this report we studied the role of purinergic receptors and extracellular ATP, which can be released from nerve terminals and beta-cell secretory granules. Using immunocytochemistry, we identified in alpha-cells the P2 receptor subtype P2Y1, as well as the P1 receptors A1 and A2A. In contrast, only P2Y1 and A1 receptors were localized in beta-cells. To analyze the role of purinergic receptors in alpha-cell function, we studied their participation in Ca2+ signaling. At low glucose concentrations, mouse alpha-cells exhibited the characteristic oscillatory Ca2+ signals that lead to secretion. Application of ATP (1-10 microM) abolished these oscillations or reduced their frequency in alpha-cells within intact islets and isolated in culture. ATPgammaS, a nonhydrolyzable ATP derivative, indicated that the ATP effect was mainly direct rather than through ATP-hydrolytic products. Additionally, adenosine (1-10 microM) was also found to reduce Ca2+ signals. ATP-mediated inhibition of Ca2+ signaling was accompanied by a decrease in glucagon release from intact islets in contrast to the adenosine effect. Using pharmacological agonists, we found that only P2Y1 and A2A were likely involved in the inhibitory effect on Ca2+ signaling. All these findings indicate that extracellular ATP and purinergic stimulation are effective regulators of the alpha-cell function.

P2 receptor agonists stimulate insulin release from human pancreatic islets

Although P2 receptors for adenosine 5'-triphosphate (ATP) and/or adenosine 5'-diphosphate (ADP) have been characterized in mammalian pancreatic beta cells, no evidence for an insulin-secreting effect of P2 receptor agonists has been reported as yet in humans. The present study aimed at investigating whether P2 receptor agonists could stimulate insulin release in human pancreatic islets obtained from brain-dead organ donors. Experiments were performed using different glucose concentrations and insulin was measured by radioimmunoassay. When the glucose concentration (8.3 mmol/L) was slightly stimulating for insulin release, alpha,beta-methylene ATP (200 micromol/L) and ADPbetaS (50 micromol/L) similarly amplified insulin secretion: both compounds induced a threefold increase in insulin response. In the presence of a nonstimulating glucose concentration (3.0 mmol/L), only alpha,beta-methylene ATP could induce a significant 1.4-fold increase in insulin release, ADPbetaS being completely ineffective. These results give evidence that P2 receptor agonists are effective in stimulating insulin release in humans, the effect of the P2Y agonist being essentially glucose dependent.

Apoptosis induced by 2-chloro-adenosine and 2-chloro-2'-deoxy-adenosine in a human astrocytoma cell line: differential mechanisms and possible clinical relevance

We have previously demonstrated that 2-chloro-adenosine (2-CA) can induce apoptosis of rat astroglial cells (Abbracchio et al. [1995] Biochem. Biophys. Res. Commun. 213:908-915). In the present study, we have characterized, for the first time, the effects induced on a human astrocytoma cell line (ADF cells) by both 2-CA and its related analog 2-chloro-2'-deoxy-adenosine (2-CdA, that is employed as anti-cancer agent in chronic lymphoid malignancies). Exposure of these cells to either adenosine analog resulted in time- and concentration-dependent apoptosis. Experiments with pharmacological agents known to interfere with adenosine receptors, its membrane transporter, and intracellular nucleoside kinases showed that: (i) cell death induced by either adenosine analog did not depend on extracellular adenosine receptors, but on a direct intracellular action; however, only in the case of 2-CA, was entry into cells mediated by the specific nitrobenzyl-tioinosine-sensitive transporter; (ii) for both adenosine analogs, induction of apoptosis required the phosphorylation/activation by specific intracellular nucleoside kinases, i.e., adenosine kinase for 2-CA, and deoxycytidine kinase for 2-CdA. In addition, only in the case of 2-CdA, was induction of apoptosis preceded by a block of cells at the G2/M phase of the cell cycle. Finally, at concentrations of either analog that killed about 80-90% of astrocytoma cells, a significantly lower effect on the viability of primary cortical neurons was observed. In conclusion, both adenosine analogs can trigger apoptosis of human astrocytoma cells, albeit with different mechanisms. This effect together with the relative sparing of neuronal cells, may have potential clinical implications for the therapy of tumors of glial origin.

2-thioether 5'-O-(1-thiotriphosphate)adenosine derivatives as new insulin secretagogues acting through P2Y-Receptors

P2-Receptors (P2-Rs) represent significant targets for novel drug development. P2-Rs were identified also on pancreatic B cells and are involved in insulin secretion. Therefore, novel P2Y-R ligands, 2-thioether 5'-O-phosphorothioate adenosine derivatives (2-RS-ATP-alpha-S), were synthesized as potential insulin secretagogues. An efficient synthesis of these nucleotides and a facile method for separation of the chiral products are described. The enzymatic stability of the compounds toward pig pancreas type I ATPDase was evaluated. The rate of hydrolysis of 2-hexylthio-5'-O-(1-thiotriphosphate)adenosine (2-hexylthio-ATP-alpha-S) isomers by ATPDase was 28% of that of ATP. Some 2-thioether 5'-(monophosphorothioate)adenosine derivatives (2-RS-AMP-S) exerted an inhibitory effect on ATPDase. The apparent affinity of the compounds to P2Y(1)-R was determined by measurement of P2Y-R-promoted phospholipase C activity in turkey erythrocyte membranes. 2-RS-ATP-alpha-S derivatives were agonists, stimulating the production of inositol phosphates with K(0.5) values in the nanomolar range. 2-RS-AMP-S derivatives were full agonists, although 2 orders of magnitude less potent. All the compounds were more potent than ATP. The effect on insulin secretion and pancreatic flow rate was evaluated on isolated and perfused rat pancreas. A high increase, up to 500%, in glucose-induced insulin secretion was due to addition of 2-hexylthio-ATP-alpha-S in the nanomolar concentration range, which represents 100-fold enhancement of activity relative to ATP. 2-Hexylthio-AMP-S was 2.5 orders of magnitude less effective.

Evidence for two different types of P2 receptors stimulating insulin secretion from pancreatic B cell

Adenine nucleotides have been shown to stimulate insulin secretion by acting on P2 receptors of the P2Y type. Since there have been some discrepancies in the insulin response of different analogues of ATP and ADP, we investigated whether two different types of P2 receptors exist on pancreatic B cells. The effects of alpha,beta-methylene ATP, which is more specific for the P2X subtype, were studied in vitro in pancreatic islets and isolated perfused pancreas from rats, in comparison with the potent P2Y receptor agonist ADPbetaS. In isolated islets, incubated with a slightly stimulating glucose concentration (8.3 mM), alpha,beta-me ATP (200 microM) and ADPbetaS (50 microM) similarly stimulated insulin secretion; by contrast, under a non stimulating glucose concentration (3 mM), alpha,beta-me ATP was still effective whereas ADPbetaS was not. In the same way, in islets perifused with 3 mM glucose, alpha,beta-me ATP but not ADPbetaS induced a partial but significant reduction in the peak 86Rb efflux induced by the ATP-dependent potassium channel opener diazoxide. In the isolated pancreas, perfused with a non stimulating glucose concentration (4.2 mM), ADPbetaS and alpha,beta-me ATP (5-50 microM), administered for 10 min, induced an immediate, transient and concentration-dependent increase in the insulin secretion; their relative potency was not significantly different. In contrast, with a slightly stimulating glucose concentration (8.3 mM), ADPbetaS was previously shown to be 100 fold more potent than alpha,beta-me ATP. Furthermore, at 4.2 mM glucose a second administration of alpha,beta-me ATP was ineffective. In the same way, ADPbetaS was also able to desensitize its own insulin response. At 3 mM glucose, alpha,beta-me ATP as well as ADPbetaS (50 microM) induced a transient stimulation of insulin secretion and down regulated the action of each other. These results give evidence that pancreatic B cells, in addition to P2Y receptors, which potentiate glucose-induced insulin secretion, are provided with P2X receptors, which transiently stimulate insulin release at low non-stimulating glucose concentration and slightly affect the potassium conductance of the membrane.

Modulation of apoptosis in human lymphocytes by adenosine analogues

We previously described that 2-chloroadenosine (2CA) and 2-chloro-2'-deoxyadenosine (2CdA) induced apoptosis in human peripheral blood mononuclear cells (PBMC). In this study we tested different adenosine analogues on PBMC and we found that the modifications introduced in the 2CA structure prevented the molecule from exerting its apoptotic effect. On the other hand, substitutions on 2CdA are tolerated, although with a significant decrease in activity.

Apoptosis by 2-chloro-2'-deoxy-adenosine and 2-chloro-adenosine in human peripheral blood mononuclear cells

Adenosine has profound effects on immune cells and has been implicated in the intrathymic apoptotic deletion of T-cells during development. In order to characterize adenosine effects on quiescent peripheral blood mononuclear cells (PBMC), we have evaluated the ability of the previously characterized adenosine receptor agonist 2-chloro-adenosine (2CA; Ceruti, Barbieri et al., 1997) and of the antineoplastic drug 2-chloro-2'-deoxy-adenosine (2CdA, cladribine) to trigger apoptosis of PBMC. Apoptosis was assessed by morphological changes, DNA fragmentation by agarose gel electrophoresis and appearance of hypodiploid DNA peak by flow cytometry. 2CA (10 microM) and 2CdA (1 microM) induced apoptosis in human PBMC, which are relatively insensitive to apoptosis. For both agents, the effect was concentration- and time-dependent, although 2CdA induced apoptosis more potently than 2CA. Evaluation of mitochondrial function in parallel samples using the mitochondrial membrane-potential-specific dye JC-1 showed that mitochondrial damage followed the same kinetics as apoptosis, hence an early damage of mitochondria is likely not responsible for adenosine-induced death of PBMC. The effect of 2CA was partially prevented by addition of dipyridamole (DP), a nucleoside transport inhibitor, hence some of the apoptotic effect of this nucleoside is, at least in part, due to intracellular action. Alternatively, DP did not affect 2CdA-induced apoptosis, suggesting that 2CdA may enter cells via a DP-insensitive transporter. 5-Iodotubercidin (5-Itu), a nucleoside kinase inhibitor, was also able to partially prevent the action of 2CA and was not able to affect 2CdA-induced apoptosis, suggesting a different role for phosphorylation in 2CA- vs 2CdA-induced apoptosis. To test the role of P1 receptors, agonists and antagonists selective at various P1 receptor subtypes were used. Data suggest that, for 2CA, apoptosis is partially sustained by activation of the A2A receptor subtype, whereas no role is exerted by P1 receptors in 2CdA-dependent apoptosis. Moreover, in these cells, apoptosis could also be triggered through intense activation of the A3 receptor via selective agonists such as 2-chloro-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA), but this mechanism plays no role in either 2CA- or 2CdA-induced apoptosis. On the whole, our results suggest that 2CA and 2CdA follow different pathways in inducing apoptosis of immune cells. Moreover, our data also suggest that there are at least three different ways by which adenosine derivatives may induce apoptosis of human PBMC: (i) through an A2A-like extracellular membrane receptor; (ii) through entry of nucleosides into cells and direct activation of intracellular events involved in the apoptotic process; or (iii) through activation of the A3 receptor.