Regulation of rat hepatocyte function by P2Y receptors: focus on control of glycogen phosphorylase and cyclic AMP by 2-methylthioadenosine 5'-diphosphate

ATP Secretion and Metabolism in Regulating Pancreatic Beta Cell Functions and Hepatic Glycolipid Metabolism

Diabetes (DM), especially type 2 diabetes (T2DM) has become one of the major diseases severely threatening public health worldwide. Islet beta cell dysfunctions and peripheral insulin resistance including liver and muscle metabolic disorder play decisive roles in the pathogenesis of T2DM. Particularly, increased hepatic gluconeogenesis due to insulin deficiency or resistance is the central event in the development of fasting hyperglycemia. To maintain or restore the functions of islet beta cells and suppress hepatic gluconeogenesis is crucial for delaying or even stopping the progression of T2DM and diabetic complications. As the key energy outcome of mitochondrial oxidative phosphorylation, adenosine triphosphate (ATP) plays vital roles in the process of almost all the biological activities including metabolic regulation. Cellular adenosine triphosphate participates intracellular energy transfer in all forms of life. Recently, it had also been revealed that ATP can be released by islet beta cells and hepatocytes, and the released ATP and its degraded products including ADP, AMP and adenosine act as important signaling molecules to regulate islet beta cell functions and hepatic glycolipid metabolism via the activation of P2 receptors (ATP receptors). In this review, the latest findings regarding the roles and mechanisms of intracellular and extracellular ATP in regulating islet functions and hepatic glycolipid metabolism would be briefly summarized and discussed.

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

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Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

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P2X receptors do not contribute to the generation of Ca²⁺ oscillations

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Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

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Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

Purinergic Signaling in Liver Pathophysiology

Extracellular nucleosides and nucleotides activate a group of G protein-coupled receptors (GPCRs) known as purinergic receptors, comprising adenosine and P2Y receptors. Furthermore, purinergic P2X ion channels are activated by ATP. These receptors are expressed in liver resident cells and play a critical role in maintaining liver function. In the normal physiology, these receptors regulate hepatic metabolic processes such as insulin responsiveness, glycogen and lipid metabolism, and bile secretion. In disease states, ATP and other nucleotides serve as danger signals and modulate purinergic responses in the cells. Recent studies have demonstrated that purinergic receptors play a significant role in the development of metabolic syndrome associated non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), fibrosis, hepatocellular carcinoma (HCC) and liver inflammation. In this concise review, we dissect the role of purinergic signaling in different liver resident cells involved in maintaining healthy liver function and in the development of the above-mentioned liver pathologies. Moreover, we discuss potential therapeutic strategies for liver diseases by targeting adenosine, P2Y and P2X receptors.

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

Metabolic Functions of G Protein-Coupled Receptors in Hepatocytes-Potential Applications for Diabetes and NAFLD

G protein-coupled receptors (GPCRs) are cell surface receptors that mediate the function of extracellular ligands. Understanding how GPCRs work at the molecular level has important therapeutic implications, as 30–40% of the drugs currently in clinical use mediate therapeutic effects by acting on GPCRs. Like many other cell types, liver function is regulated by GPCRs. More than 50 different GPCRs are predicted to be expressed in the mouse liver. However, knowledge of how GPCRs regulate liver metabolism is limited. A better understanding of the metabolic role of GPCRs in hepatocytes, the dominant constituent cells of the liver, could lead to the development of novel drugs that are clinically useful for the treatment of various metabolic diseases, including type 2 diabetes, nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). In this review, we describe the functions of multiple GPCRs expressed in hepatocytes and their role in metabolic processes.

Purinergic signalling in liver diseases: Pathological functions and therapeutic opportunities

Extracellular nucleotides, including ATP, are essential regulators of liver function and serve as danger signals that trigger inflammation upon injury. Ectonucleotidases, which are expressed by liver-resident cells and recruited immune cells sequentially hydrolyse nucleotides to adenosine. The nucleotide/nucleoside balance orchestrates liver homeostasis, tissue repair, and functional restoration by regulating the crosstalk between liver-resident cells and recruited immune cells. In this review, we discuss our current knowledge on the role of purinergic signals in liver homeostasis, restriction of inflammation, stimulation of liver regeneration, modulation of fibrogenesis, and regulation of carcinogenesis. Moreover, we discuss potential targeted therapeutic strategies for liver diseases based on purinergic signals involving blockade of nucleotide receptors, enhancement of ectonucleoside triphosphate diphosphohydrolase activity, and activation of adenosine receptors.

Important roles of P2Y receptors in the inflammation and cancer of digestive system

Purinergic signaling is important for many biological processes in humans. Purinoceptors P2Y are widely distributed in human digestive system and different subtypes of P2Y receptors mediate different physiological functions from metabolism, proliferation, differentiation to apoptosis etc. The P2Y receptors are essential in many gastrointestinal functions and also involve in the occurrence of some digestive diseases. Since different subtypes of P2Y receptors are present on the same cell of digestive organs, varying subtypes of P2Y receptors may have opposite or synergetic functions on the same cell. Recently, growing lines of evidence strongly suggest the involvement of P2Y receptors in the pathogenesis of several digestive diseases. In this review, we will focus on their important roles in the development of digestive inflammation and cancer. We anticipate that as the special subtypes of P2Y receptors are studied in depth, specific modulators for them will have good potentials to become promising new drugs to treat human digestive diseases in the near future.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Purinergic signaling in liver disease

Adenosine triphosphate (ATP) is essential for the myriad of metabolic processes upon which life is based and is known widely as the universal energy currency unit of intracellular biologic reactions. ATP, adenosine diphosphate, adenosine, as well as other purines and pyrimidines also serve as ubiquitous extracellular mediators which function through the activation of specific receptors (viz. P2 receptors for nucleotides and purinergic P1 receptors for adenosine). Extracellular nucleotides are rapidly converted to nucleosides, such as adenosine, by highly regulated plasma membrane ectonucleotidases that modulate many of the normal biological and metabolic processes in the liver - such as gluconeogenesis and insulin signaling. Under inflammatory conditions, as with ischemia reperfusion, sepsis or metabolic stress, ATP and other nucleotides can also act as 'damage-associated molecular patterns' causing inflammasome activation in innate immune cells and endothelium resulting in tissue damage. The phosphohydrolysis of ATP by ectonucleotidases, such as those of the CD39/ENTPD family, results in the generation of immune suppressive adenosine, which in turn markedly limits inflammatory processes. Experimental studies by others and our group have implicated purinergic signaling in experimental models of hepatic ischemia reperfusion and inflammation, transplant rejection, hepatic regeneration, steatohepatitis, fibrosis and cancer, amongst others. Expression of ectonucleotidases on sinusoidal endothelial, stellate or immune cells allows for homeostatic integration and linking of the control of vascular inflammatory and immune cell reactions in the liver. CD39 expression also identifies hepatic myeloid dendritic cells and efficiently distinguishes T-regulatory-type cells from other resting or activated T cells. Our evolving data strongly indicate that CD39 serves as a key 'molecular switch' and is an integral component of the suppressive machinery of myeloid, dendritic and T cells. Increased understanding of mechanisms of extracellular ATP scavenging and specifically conversion to nucleosides by ectonucleotidases of the CD39 family have also led to novel insights into the exquisite balance of nucleotide P2-receptor and adenosinergic P1-receptor signaling in inflammatory and hepatic diseases. Further, CD39 and other ectonucleotidases exhibit genetic polymorphisms in humans which alter levels of expression/function and are associated with predisposition to inflammatory and immune diseases, diabetes and vascular calcification, amongst other problems. Development of therapeutic strategies targeting purinergic signaling and ectonucleotidases offers promise for the management of disordered inflammation and aberrant immune reactivity.

Hepatic purinergic signaling gene network expression and its relationship with inflammation and oxidative stress biomarkers in blood from peripartal dairy cattle

The liver plays a central role in allowing dairy cattle to make a successful transition into lactation. In liver, as in other tissues, extracellular nucleotides and nucleosides trigger cellular responses through adenosine and ATP receptors. Adenosine triphosphate and certain nucleotides serve as signals that can heighten purinergic receptor activation in several pathologic processes. We evaluated the mRNA expression of genes associated with the purinergic signaling network in liver tissue during the peripartal period. Seven multiparous Holstein cows were dried off at d -50 relative to expected parturition and fed a controlled-energy diet (net energy for lactation=1.24 Mcal/kg of DM) for ad libitum intake during the entire dry period. After calving, all cows were fed a common lactation diet (net energy for lactation=1.65 Mcal/kg of DM) until 30 DIM. Biopsies of liver were harvested at d -10, 7, and 21 for mRNA expression of 9 purinergic receptors, 7 ATP and adenosine transport channels, and 10 enzymes associated with ATP hydrolysis. Blood collected at d -21, -10, 7, 14, and 21 was used to measure concentrations of inflammation and oxidative stress biomarkers. The expression of type 1 purinergic receptors (ADORA2A and ADORA3), several nucleoside hydrolases [ectonucleoside triphosphate diphosphohydrolase 7 (ENTPD7), ectonucleotide pyrophosphatase/phosphodiesterase 2 (ENPP2), ENPP3, and adenosine deaminase (ADA)], and a type 2 purinergic receptor (P2RX7) was downregulated after calving. In contrast, the expression of type 2 purinergic receptors (P2RX4 and PR2Y11), an ATP release channel (gap junction hemichannel GJB1), and an adenosine uptake protein (SLC29A1) followed the opposite response, increasing after calving and remaining elevated through 21 d. Haptoglobin, ceruloplasmin, and reactive oxygen metabolite concentrations increased gradually from d -21 d through at least d 7. The opposite response was observed for albumin, paraoxonase, α-tocopherol, and nitric oxide, which decreased gradually to a nadir at 7 and 14 d. Our results suggest that alterations after calving of the expression of hepatic purinergic signaling genes could be functionally important because in nonruminants, they play roles in bile formation, glucose metabolism, cholesterol uptake, inflammation, and steatosis. The correlation analysis provided evidence of a link between purinergic signaling genes and biomarkers of inflammation and oxidative stress.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

Cholangiocyte primary cilia are chemosensory organelles that detect biliary nucleotides via P2Y12 purinergic receptors

Cholangiocytes, the epithelial cells lining intrahepatic bile ducts, contain primary cilia, which are mechano- and osmosensory organelles detecting changes in bile flow and osmolality and transducing them into intracellular signals. Here, we asked whether cholangiocyte cilia are chemosensory organelles by testing the expression of P2Y purinergic receptors and components of the cAMP signaling cascade in cilia and their involvement in nucleotide-induced cAMP signaling in the cells. We found that P2Y(12) purinergic receptor, adenylyl cyclases (i.e., AC4, AC6, and AC8), and protein kinase A (i.e., PKA RI-beta and PKA RII-alpha regulatory subunits), exchange protein directly activated by cAMP (EPAC) isoform 2, and A-kinase anchoring proteins (i.e., AKAP150) are expressed in cholangiocyte cilia. ADP, an endogenous agonist of P2Y(12) receptors, perfused through the lumen of isolated rat intrahepatic bile ducts or applied to the ciliated apical surface of normal rat cholangiocytes (NRCs) in culture induced a 1.9- and 1.5-fold decrease of forskolin-induced cAMP levels, respectively. In NRCs, the forskolin-induced cAMP increase was also lowered by 1.3-fold in response to ATP-gammaS, a nonhydrolyzed analog of ATP but was not affected by UTP. The ADP-induced changes in cAMP levels in cholangiocytes were abolished by chloral hydrate (a reagent that removes cilia) and by P2Y(12) siRNAs, suggesting that cilia and ciliary P2Y(12) are involved in nucleotide-induced cAMP signaling. In conclusion, cholangiocyte cilia are chemosensory organelles that detect biliary nucleotides through ciliary P2Y(12) receptors and transduce corresponding signals into a cAMP response.

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

Extracellular nucleotides (e.g. ATP, UTP, ADP) are released by activated endothelium, leukocytes and platelets within the injured vasculature and bind specific cell-surface type-2 purinergic (P2) receptors. This process drives vascular inflammation and thrombosis within grafted organs. Importantly, there are also vascular ectonucleotidases i.e. ectoenzymes that hydrolyze extracellular nucleotides in the blood to generate nucleosides (viz. adenosine). Endothelial cell NTPDase1/CD39 has been shown to critically modulate levels of circulating nucleotides. This process tends to limit the activation of platelet and leukocyte expressed P2 receptors and also generates adenosine to reverse inflammatory events. This vascular protective CD39 activity is rapidly inhibited by oxidative reactions, such as is observed with liver ischemia reperfusion injury. In this review, we chiefly address the impact of these signaling cascades following liver transplantation. Interestingly, the hepatic vasculature, hepatocytes and all non-parenchymal cell types express several components co-ordinating the purinergic signaling response. With hepatic and vascular dysfunction, we note heightened P2- expression and alterations in ectonucleotidase expression and function that may predispose to progression of disease. In addition to documented impacts upon the vasculature during engraftment, extracellular nucleotides also have direct influences upon liver function and bile flow (both under physiological and pathological states). We have recently shown that alterations in purinergic signaling mediated by altered CD39 expression have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses. Future clinical applications in transplantation might involve new therapeutic modalities using soluble recombinant forms of CD39, altering expression of this ectonucleotidase by drugs and/or using small molecules to inhibit deleterious P2-mediated signaling while augmenting beneficial adenosine-mediated effects within the transplanted liver.

Co-localization and functional cross-talk between A1 and P2Y1 purine receptors in rat hippocampus

Adenosine and ATP, via their specific P1 and P2 receptors, modulate a wide variety of cellular and tissue functions, playing a neuroprotective or neurodegenerative role in brain damage conditions. Although, in general, adenosine inhibits excitability and ATP functions as an excitatory transmitter in the central nervous system, recent data suggest the existence of a heterodimerization and a functional interaction between P1 and P2 receptors in the brain. In particular, interactions of adenosine A1 and P2Y1 receptors may play important roles in the purinergic signalling cascade. In the present work, we investigated the subcellular localization/co-localization of the receptors and their functional cross-talk at the membrane level in Wistar rat hippocampus. This is a particularly vulnerable brain area, which is sensitive to adenosine- and ATP-mediated control of glutamatergic transmission. The postembedding immunogold electron microscopy technique showed that the two receptors are co-localized at the synaptic membranes and surrounding astroglial membranes of glutamatergic synapses. To investigate the functional cross-talk between the two types of purinergic receptors, we evaluated the reciprocal effects of their activation on their G protein coupling. P2Y1 receptor stimulation impaired the potency of A1 receptor coupling to G protein, whereas the stimulation of A1 receptors increased the functional responsiveness of P2Y1 receptors. The results demonstrated an A1-P2Y1 receptor co-localization at glutamatergic synapses and surrounding astrocytes and a functional interaction between these receptors in hippocampus, suggesting ATP and adenosine can interact in purine-mediated signalling. This may be particularly important during pathological conditions, when large amounts of these mediators are released.

A role for Akt in epidermal growth factor-stimulated cell cycle progression in cultured hepatocytes: generation of a hyperproliferative window after adenoviral expression of constitutively active Akt

Epidermal growth factor (EGF) stimulation of cell cycle progression in cultured primary hepatocytes has previously been reported to be dependent on the mammalian target of rapamycin (mTOR) elements of the phosphoinositide 3-kinase (PI3K) signaling cascade and not the Akt pathway. Here we have established conditions of combined treatment of rat hepatocytes with insulin and EGF that favor cell cycle progression. The resulting cell population expresses albumin and retains receptor regulation of the signaling pathways leading to glycogen phosphorylase activation. We then investigated the hypothesis that the Akt limb of the PI3K pathway plays a central role in this insulin/EGF enhancement of cell cycle progression. The phosphorylation of Akt, central to the PI3K pathway, was increased by both insulin (sustained) and EGF (transient). The stimulation of Akt phosphorylation was inhibited in a concentration-dependent manner by the PI3K inhibitor, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002). Cell cycle progression in these cultures was reduced, but not abolished, by this inhibitor. The mTOR inhibitor, rapamycin, also inhibited entry into S phase. The novel Akt inhibitor A-443654 [(S)-1-(1H-indol-3-ylmethyl)-2-[5-(3-methyl-1H-indazol-5-yl)-pyridin-3-yloxy]-ethylamine] blocked both EGF-stimulated cell cycle progression and phosphorylation of the Akt substrate glycogen synthase kinase-3. Infection of cells with an adenoviral vector expressing a constitutively active form of Akt but not a kinase-dead form increased hepatocyte proliferation probably through enhanced cell cycle progression and reduced apoptosis. These results show that the Akt element of the PI3K cascade is necessary for EGF-stimulated cell cycle progression and provide evidence that the sustained elevation of Akt alone generates a hyperproliferative window in hepatocyte cultures.

Purinergic P2Y2 receptors promote hepatocyte resistance to hypoxia

BACKGROUND/AIMS

ATP stimulation of purinergic P2 receptors (P2YR and P2XR) regulates several hepatic functions. Here we report the involvement of ATP-mediated signals in enhancing hepatocyte tolerance to lethal stress.

METHODS

The protection given by purinergic agonists was investigated in rat hepatocytes exposed to hypoxia.

RESULTS

ATP released after hypotonic stress (200 mOsm/L) as well as P2YR agonists prevented hepatocyte killing by hypoxia with efficiency ranking UTP > ATPgammaS > ADPbetaS, whereas the P2XR agonist, methylene-adenosine-5'-triphosphate, was ineffective. Adenosine-5'-O-3-thiotriphosphate (ATPgammaS; 100 micromol/L) also prevented Na+ -overload in hypoxic cells by inhibiting the Na+/H+ exchanger, without interfering with hypoxic acidosis. ATPgammaS activated Src and promoted a Src-dependent stimulation of both ERK1/2 and p38MAPK. Blocking p38MAPK with SB203580 reverted the protection given by ATPgammaS on both cell viability and Na+ accumulation, whereas ERK1/2 inhibition with PD98058 was ineffective. An increased phosphorylation of ERK1/2 was also evident in untreated hypoxic hepatocytes. PD98058 ameliorated Na+ accumulation and cell death caused by hypoxia. Hepatocyte pre-treatment with ATPgammaS reverted ERK1/2 activation in hypoxic cells. SB203580 blocked the effects of ATPgammaS on both ERK1/2 and Na+/H+ exchanger.

CONCLUSIONS

The activation of p38MAPK by P2Y2R increases hepatocyte resistance to hypoxia by down-modulating ERK1/2-mediated signals that promote Na+ influx through the Na+/H+ exchanger.

Leukotriene and purinergic receptors are involved in the hyperpolarizing effect of glucagon in liver cells

The pancreatic hormone glucagon hyperpolarizes the liver cell membrane. In the present study, we investigated the cellular signalling pathway of glucagon-induced hyperpolarization of liver cells by using the conventional microelectrode method. The membrane potential was recorded in superficial liver cells of superfused mouse liver slices. In the presence of the K+ channel blockers tetraethylammonium (TEA, 1 mmol/l) and Ba2+ (BaCl2, 5 mmol/l) and the blocker of the Na+/K+ ATPase, ouabain (1 mmol/l), no glucagon-induced hyperpolarization was observed confirming previous findings. The hyperpolarizing effect of glucagon was abolished by the leukotriene B4 receptor antagonist CP 195543 (0.1 mmol/l) and the purinergic receptor antagonist PPADS (5 micromol/l). ATPgammaS (10 micromol/l), a non-hydrolyzable ATP analogue, induced a hyperpolarization of the liver cell membrane similar to glucagon. U 73122 (1 micromol/l), a blocker of phospholipase C, prevented both the glucagon- and ATPgammaS-induced hyperpolarization. These findings suggest that glucagon affects the hepatic membrane potential partly by inducing the formation and release of leukotrienes and release of ATP acting on purinergic receptors of the liver cell membrane.

Purine- and pyrimidine-induced responses and P2Y receptor characterization in the hamster proximal urethra

1. Purine and pyrimidine compounds were investigated on hamster proximal urethral circular smooth muscle preparations. In situ hybridization studies were carried out to localize P2Y(1), P2Y(2), P2Y(4) and P2Y(6) mRNA. Protein expression was studied using Western blotting analysis with antibodies against P2Y(1) and P2Y(2) receptors. 2. The hamster urethra relaxed with an agonist potency order of: 2-MeSADP>beta,gamma-meATP=ATP=adenosine=ADP>2-MeSATP>alpha,beta-meATP>TTP>CTP=UTP>GTP=UDP. The high potency of 2-MeSADP is suggestive of an action via P2Y(1) receptors. Although the order is not characteristic for any known single P2Y receptor subtype, it may represent a combination of P2Y receptor subtypes. 4. The selective P2Y(1) receptor antagonist MRS2179 inhibited ATP-, 2-MeSADP-, 2-MeSATP-, beta,gamma-meATP-, and to a lesser degree alpha,beta-meATP-induced responses. 3. Adenosine, but not ATP, was inhibited by the adenosine receptor antagonist 8-phenyltheophylline, indicating that ATP was not acting via adenosine following enzymatic breakdown. 5. Western blotting analysis showed the expression of both P2Y(1) and P2Y(2) receptors, confirming the results obtained with in situ hybridization that showed the expression of both P2Y(1) and P2Y(2), but not P2Y(4) or P2Y(6) mRNA, in smooth muscle layers of the hamster proximal urethra. 6. It is proposed that the relaxant response of the urethra to ATP may be evoked through the activation of the combination of receptors for P2Y(1) and to a lesser extent P2Y(2) receptors, which may mediate a trophic effect in addition. A P2Y subtype responsive to alpha,beta-meATP and P1 receptors may contribute to urethral smooth muscle relaxation.

Regulation of human hepatocytes by P2Y receptors: control of glycogen phosphorylase, Ca2+, and mitogen-activated protein kinases

In the rat both short-term liver function, such as glycogen metabolism, and long-term events such as proliferation after partial hepatectomy, are in part controlled by release of nucleotides such as ATP acting on hepatocyte P2Y(1) and P2Y(2) receptors (members of a family of P2Y receptors for extracellular nucleotides such as ATP and UTP). Here, we have studied P2Y receptor regulation of signaling pathways involved in glycogen phosphorylase activation and proliferation of primary human hepatocytes. Stimulation of cultured hepatocytes with either ATP and UTP, but not UDP or 2-methylthio ADP, led to concentration-dependent increases in cytosolic free Ca(2+) concentration ([Ca(2+)](c); EC(50) for ATP = 3.3 microM, for UTP = 2.3 microM) and [(3)H]inositol (poly)phosphates (EC(50) for ATP = 9.4 microM, for UTP = 15.4 microM). ATP and UTP also stimulated glycogen phosphorylase in human hepatocytes, each with a threshold for activation of less than 1 microM. Application of 2-methylthio ADP up to 100 microM was ineffective. Phosphorylation of both extracellular signal-related kinase and c-Jun N-terminal kinase was stimulated by ATP and UTP, but not by 2-methylthio ADP or UDP, either alone or when costimulated with epidermal growth factor. In conclusion, in human hepatocytes P2Y receptors control both glycogen metabolism and proliferation-associated responses such as increased [Ca(2+)](c) and mitogen-activated protein kinase cascades. Regulation seems to be primarily through P2Y(2) receptors. In contrast with previous studies on rat hepatocytes, there is an absence of responses mediated by P2Y(1) receptors.

The P2Y(1) receptor is involved in the maintenance of glucose homeostasis and in insulin secretion in mice

Pancreatic β cells express several P2 receptors including P2Y₁ and the modulation of insulin secretion by extracellular nucleotides has suggested that these receptors may contribute to the regulation of glucose homeostasis. To determine whether the P2Y₁ receptor is involved in this process, we performed studies in P2Y₁ mice. In baseline conditions, P2Y₁-mice exhibited a 15% increase in glycemia and a 40% increase in insulinemia, associated with a 10% increase in body weight, pointing to a role of the P2Y₁ receptor in the control of glucose metabolism. Dynamic experiments further showed that P2Y₁-mice exhibited a tendency to glucose intolerance. These features were associated with a decrease in the plasma levels of free fatty acid and triglycerides. When fed a lipids and sucrose enriched diet for 15 weeks, the two genotypes no longer displayed any significant differences. To determine whether the P2Y₁ receptor was directly involved in the control of insulin secretion, experiments were carried out in isolated Langerhans islets. In the presence of high concentrations of glucose, insulin secretion was significantly greater in islets from P2Y₁-mice. Altogether, these results show that the P2Y₁ receptor plays a physiological role in the maintenance of glucose homeostasis at least in part by regulating insulin secretion.