P2-purinergic receptors are coupled to two signal transduction systems leading to inhibition of cAMP generation and to production of inositol trisphosphate in rat hepatocytes

Calcium Signaling in the Thyroid: Friend and Foe

Simple Summary

All cells in our body are activated by several different signals. The calcium ion is one of the most versatile signaling molecules, and regulates a multitude of different events in the cells. These range from activation of muscle contraction, to the regulation of cell movement, just to name a few. In normal thyroid cells, calcium signaling is of importance for the normal physiology of the cells. In thyroid pathologies, e.g., thyroid cancer, calcium is important for the regulation of proliferation and invasion, and may also activate gene transcription programs important for cancer cell survival. In this Commentary, we summarize what is known regarding calcium in the normal thyroid, and highlight the importance of calcium signaling in thyroid pathologies.

Abstract

Calcium signaling participates in a vast number of cellular processes, ranging from the regulation of muscle contraction, cell proliferation, and mitochondrial function, to the regulation of the membrane potential in cells. The actions of calcium signaling are, thus, of great physiological significance for the normal functioning of our cells. However, many of the processes that are regulated by calcium, including cell movement and proliferation, are important in the progression of cancer. In the normal thyroid, calcium signaling plays an important role, and evidence is also being gathered showing that calcium signaling participates in the progression of thyroid cancer. This review will summarize what we know in regard to calcium signaling in the normal thyroid as, well as in thyroid cancer.

Protective Effects of Hydrolyzed Nucleoproteins from Salmon Milt against Ethanol-Induced Liver Injury in Rats

Dietary nucleotides play a role in maintaining the immune responses of both animals and humans. Oral administration of nucleic acids from salmon milt have physiological functions in the cellular metabolism, proliferation, differentiation, and apoptosis of human small intestinal epithelial cells. In this study, we examined the effects of DNA-rich nucleic acids prepared from salmon milt (DNSM) on the development of liver fibrosis in an in vivo ethanol-carbon tetrachloride cirrhosis model. Plasma aspartate transaminase and alanine transaminase were significantly less active in the DNSM-treated group than in the ethanol plus carbon tetrachloride (CCl₄)-treated group. Collagen accumulation in the liver and hepatic necrosis were observed histologically in ethanol plus CCl₄-treated rats; however, DNSM-treatment fully protected rats against ethanol plus CCl₄-induced liver fibrosis and necrosis. Furthermore, we examined whether DNSM had a preventive effect against alcohol-induced liver injury by regulating the cytochrome p450 2E1 (CYP2E1)-mediated oxidative stress pathway in an in vivo model. In this model, CYP2E1 activity in ethanol plus CCl₄-treated rats increased significantly, but DNSM-treatment suppressed the enzyme's activity and reduced intracellular thiobarbituric acid reactive substances (TBARS) levels. Furthermore, the hepatocytes treated with 100 mM ethanol induced an increase in cell death and were not restored to the control levels when treated with DNSM, suggesting that digestive products of DNSM are effective for the prevention of alcohol-induced liver injury. Deoxyadenosine suppressed the ethanol-induced increase in cell death and increased the activity of alcohol dehydrogenase. These results suggest that DNSM treatment represents a novel tool for the prevention of alcohol-induced liver injury.

Canonical transient receptor potential channel 2 (TRPC2): old name-new games. Importance in regulating of rat thyroid cell physiology

In addition to the TSH-cyclic AMP signalling pathway, calcium signalling is of crucial importance in thyroid cells. Although the importance of calcium signalling has been thoroughly investigated for several decades, the nature of the calcium channels involved in signalling is unknown. In a recent series of investigations using the well-studied rat thyroid FRTL-5 cell line, we showed that these cells exclusively express the transient receptor potential canonical 2 (TRPC2) channel. Our results suggested that the TRPC2 channel is of significant importance in regulating thyroid cell function. These investigations were the first to show that thyroid cells express a member of the TRPC family of ion channels. In this review, we will describe the importance of the TRPC2 channel in regulating TSH receptor expression, thyroglobulin maturation, intracellular calcium and iodide homeostasis and that the channel also regulates thyroid cell proliferation.

Canonical transient receptor potential channel 2 (TRPC2) as a major regulator of calcium homeostasis in rat thyroid FRTL-5 cells: importance of protein kinase C δ (PKCδ) and stromal interaction molecule 2 (STIM2)

Mammalian non-selective transient receptor potential cation channels (TRPCs) are important in the regulation of cellular calcium homeostasis. In thyroid cells, including rat thyroid FRTL-5 cells, calcium regulates a multitude of processes. RT-PCR screening of FRTL-5 cells revealed the presence of TRPC2 channels only. Knockdown of TRPC2 using shRNA (shTRPC2) resulted in decreased ATP-evoked calcium peak amplitude and inward current. In calcium-free buffer, there was no difference in the ATP-evoked calcium peak amplitude between control cells and shTRPC2 cells. Store-operated calcium entry was indistinguishable between the two cell lines. Basal calcium entry was enhanced in shTRPC2 cells, whereas the level of PKCβ1 and PKCδ, the activity of sarco/endoplasmic reticulum Ca(2+)-ATPase, and the calcium content in the endoplasmic reticulum were decreased. Stromal interaction molecule (STIM) 2, but not STIM1, was arranged in puncta in resting shTRPC2 cells but not in control cells. Phosphorylation site Orai1 S27A/S30A mutant and non-functional Orai1 R91W attenuated basal calcium entry in shTRPC2 cells. Knockdown of PKCδ with siRNA increased STIM2 punctum formation and enhanced basal calcium entry but decreased sarco/endoplasmic reticulum Ca(2+)-ATPase activity in wild-type cells. Transfection of a truncated, non-conducting mutant of TRPC2 evoked similar results. Thus, TRPC2 functions as a major regulator of calcium homeostasis in rat thyroid cells.

P2 Receptor-mediated Inhibition of Vasopressin-stimulated Fluid Transport and cAMP Responses in AQP2-transfected MDCK Cells

We cultured canine kidney (MDCK) cells stably expressing aquaporin-2 (AQP2) on collagen-coated permeable membrane filters and examined the effect of extracellular ATP on arginine vasopressin (AVP)-stimulated fluid transport and cAMP production. Exposure of cell monolayers to basolateral AVP resulted in stimulation of apical to basolateral net fluid transport driven by osmotic gradient which was formed by addition of 500 mM mannitol to basolateral bathing solution. Pre-exposure of the basolateral surface of cell monolayers to ATP (100 microM) for 30 min significantly inhibited the AVP-stimulated net fluid transport. In these cells, AVP-stimulated cAMP production was suppressed as well. Profile of the effects of different nucleotides suggested that the P2Y(2) receptor is involved in the action of ATP. ATP inhibited the effect of isoproterenol as well, but not that of forskolin to stimulate cAMP production. The inhibitory effect of ATP on AVP-stimulated fluid movement was attenuated by a protein kinase C inhibitor, calphostin C or pertussis toxin. These results suggest that prolonged activation of the P2 receptors inhibits AVP-stimulated fluid transport and cAMP responses in AQP2 transfected MDCK cells. Depressed responsiveness of the adenylyl cyclase by PKC-mediated modification of the pertussis-toxin sensitive G(i) protein seems to be the underlyihng mechanism.

Characterization of ionotrophic purinergic receptors in hepatocytes

Ionotrophic purinergic (P2X) receptors function as receptor-gated cation channels, where agonist binding leads to opening of a nonselective cation pore permeable to both Na(+) and Ca(2+). Based on evidence that extracellular adenosine 5'-triphosphate (ATP) stimulates glucose release from liver, these studies evaluate whether P2X receptors are expressed by hepatocytes and contribute to ATP-dependent calcium signaling and glucose release. Studies were performed in isolated hepatocytes from rats and mice and hepatoma cells from humans and rats. Transcripts and protein for both P2X4 and P2X7 were detectable, and immunohistochemistry of intact liver revealed P2X4 in the basolateral and canalicular domains. In whole cell patch clamp studies, exposure to the P2X4/P2X7 receptor agonist 2'3'-O-(4-benzoyl-benzoyl)-adenosine 5'-triphosphate (BzATP; 10 microM) caused a rapid increase in membrane Na(+) conductance. Similarly, with Fluo-3 fluorescence, BzATP induced an increase in intracellular [Ca(2+)]. P2X4 receptors are likely involved because the calcium response to BzATP was inhibited by Cu(2+), and the P2X4 modulators Zn(2+) and ivermectin (0.3-3 microM) each increased intracellular [Ca(2+)]. Exposure to BzATP decreased cellular glycogen content; and P2X4 receptor messenger RNA increased in glycogen-rich liver samples.

CONCLUSION

These studies provide evidence that P2X4 receptors are functionally important in hepatocyte Na(+) and Ca(2+) transport, are regulated by extracellular ATP and divalent cation concentrations, and may constitute a mechanism for autocrine regulation of hepatic glycogen metabolism.

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.

Dual roles of P2 purinergic receptors in insulin-stimulated leptin production and lipolysis in differentiated rat white adipocytes

ATP is co-localized with norepinephrine at the sympathetic nerve terminals and may be released simultaneously upon neuronal stimulation, which results in activation of purinergic receptors. To examine whether leptin synthesis and lipolysis are influenced by P2 purinergic receptor activation, the effects of ATP and other nucleotides on leptin secretion and glycerol release have been investigated in differentiated rat white adipocytes. Firstly, insulin-induced leptin secretion was inhibited by nucleotide treatment with the following efficacy order: 3'-O-(4-benzoyl)benzoyl ATP (BzATP) > ATP >> UTP. Secondly, treatment of adipocytes with ATP increased both intracellular Ca(2+) concentration and cAMP content. Intracellular calcium concentration was increased by ATP and UTP, but not BzATP, an effect attributed to phospholipase C-coupled P2Y(2). On the other hand, cAMP was generated by treatment with BzATP and ATPgammaS, but not UTP, indicating functional expression of adenylyl cyclase-coupled P2Y(11) receptors in white adipocytes. Thirdly, lipolysis was significantly activated by BzATP and ATP, which correlated with the characteristics of the P2Y(11) subtype. Taken together, the data presented here suggest that white adipocytes express at least two different types of P2Y receptors and that activation of P2Y(11) receptor might be involved in inhibition of leptin production and stimulation of lipolysis, suggesting that purinergic transmission can play an important role in white adipocyte physiology.

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.

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

Hepatocyte function is regulated by several P2Y receptor subtypes. Here we report that 2-methylthioadenosine 5'-diphosphate (2-MeSADP), an agonist at P2Y(1), P2Y(12), and P2Y(13) receptors, potently (threshold 30 nM) stimulates glycogen phosphorylase in freshly isolated rat hepatocytes. Antagonism by N(6)-methyl 2'-deoxyadenosine 3',5'-bisphosphate (MRS 2179) confirms that this response is mediated by P2Y(1) receptors. In addition, in these cells, both 2-MeSADP and UTP inhibited glucagon-stimulated cyclic AMP accumulation. This inhibitory effect of 2-MeSADP was not reversed by the P2Y(1) antagonists, adenosine-3'-phosphate-5'-phosphate (A3P5P) or MRS 2179, both in the range 1 to 300 microM, indicating that it was not mediated by P2Y(1) receptors. This contrasts with the increase in cytosolic free Ca(2+) concentration ([Ca(2+)](c)) induced by 2-MeSADP, which has shown to be inhibited by A3P5P. Pertussis toxin abolished the inhibitory effect of both UTP and 2-MeSADP. After culture of cells for 48 h, the ability of 2-MeSADP to inhibit cyclic AMP accumulation was greatly diminished. Reverse transcriptase-polymerase chain reaction analysis revealed that during this culture period, there was a decline in the ability to detect transcripts for P2Y(12) and P2Y(13) receptors, both of which are activated by 2-MeSADP and negatively coupled to adenylyl cyclase. However, in freshly isolated cells, the P2Y(12) and P2Y(13) receptor antagonist, 2-propylthio-beta,gamma-dichloromethylene-d-ATP (AR-C67085) (10 nM to 300 microM) did not alter the ability of 2-MeSADP to inhibit glucagon-stimulated cyclic AMP accumulation. We conclude that 2-MeSADP regulates rat hepatocyte glycogen phosphorylase by acting on P2Y(1) receptors coupled to raised [Ca(2+)](c), and by inhibiting cyclic AMP levels by an unknown G(i)-coupled receptor subtype, distinct from P2Y(1), P2Y(12), or P2Y(13) receptors.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Molecular mechanism of nucleotide-induced primary granule release in human neutrophils: role for the P2Y2 receptor

Nucleotides are released during vascular injury from activated platelets and broken cells, which could stimulate human neutrophils. In this study, we characterized the P2Y receptors and investigated the functional effects of extracellular nucleotides on human neutrophils. Pharmacological characterization using selective agonists and pertussis toxin revealed that human neutrophils express only functional P2Y2 receptors. However, P2Y2 receptor agonists ATP or uridine triphosphate (UTP) caused intracellular Ca2+ increases in isolated human neutrophils with an EC50 of 1 microM but failed to cause release of primary granules from human neutrophils. ATP and UTP were equally potent in causing elastase release from human neutrophils in the presence of exogenous soluble fibrinogen, whereas ADP and UDP were without effect. We investigated whether nucleotides depend on generated arachidonic acid metabolites to cause degranulation. However, phenidone and MK-886, inhibitors of the 5-lipoxygenase pathway, failed to block nucleotide-induced intracellular calcium mobilization and elastase release. ATP and UTP caused activation of p38 MAPK and ERK1/2 in human neutrophils. In addition, the inhibitors of the MAPK pathway, SB-203580 and U-0126, inhibited nucleotide-induced elastase release. We conclude that fibrinogen is required for nucleotide-induced primary granule release from human neutrophils through the P2Y2 receptor without a role for arachidonic acid metabolites. Both ERK1/2 and p38 MAPK play an important role in nucleotide-induced primary granule release from human neutrophils.

ADP stimulation of inositol phosphates in hepatocytes: role of conversion to ATP and stimulation of P2Y2 receptors

1 Accumulation of inositol (poly)phosphates (InsP(x)) has been studied in rat hepatocytes labelled with [(3)H]inositol. Stimulation with ADP resulted in a significant increase in total [(3)H]InsP(x), whereas 2-MeSADP had only a small effect and ADPbetaS was ineffective. UTP and ITP also stimulated substantial increases in [(3)H]InsP(x). 2 The dose-response curve to ADP was largely unaltered by the presence of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P). Similarly, inclusion of MRS 2179, a more selective P2Y(1) antagonist, had no effect on the dose-response curve to ADP. 3 The inclusion of hexokinase in the assay reduced, but did not abolish, the response to ADP. 4 HPLC analysis revealed that ADP in the medium was rapidly converted to AMP and ATP. The inclusion of hexokinase removed ATP, but exacerbated the decline in ADP concentration, leading to increased levels of AMP. 2-MeSADP was stable in the medium and ATP was largely unaffected. 5 The addition of the adenylate kinase inhibitor, diadenosine pentaphosphate (Ap(5)A) significantly reduced the ADP response. HPLC analysis conducted in parallel demonstrated that this treatment inhibited conversion of ADP to ATP and AMP. 6 Inclusion of the P1 antagonist CGS 15943 had no effect on the dose-response curve to ADP. 7 These observations indicate that hepatocytes respond to ADP with an increase in inositol (poly)phosphates following conversion to ATP. P2Y(1) activation in hepatocytes does not appear to be coupled to inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) production.

Mobilization of intracellular calcium in kidney epithelial cells is inhibited by lead

The effect of lead (Pb) on intracellular calcium (Cai) after stimulation with agonists was studied in Madin-Darby canine kidney (MDCK) cells. In response to the agonist ADP, the levels of Cai increased by approximately threefold in MDCK cells bathed in a buffer with calcium (Ca) or in a buffer with nominal Ca. Pb inhibited the response to ADP in MDCK cells bathed in either buffer. The inhibition by Pb was observed after a 5 and 20-min exposure to Pb, but not after 2-min. Very high concentrations of ADP did not reverse the effects of Pb. Concentrations of Pb of 1 microM or more inhibited the response to ADP. Similarly, the response to bradykinin was also inhibited by Pb. Protein kinase C did not play a role since the protein kinase C inhibitor GF 109203X did not reverse the effects of Pb. Interestingly, MDCK cells treated with Pb at concentrations above 1 microM, for periods of 5-20 min, displayed elevated levels of inositol 1,4,5-trisphosphate. In conclusion, Pb inhibits mobilization of Cai after agonist stimulation by a mechanism that is unrelated to the type of agonist used. Evidence is presented suggesting that the inhibition is due to increases in levels of inositol 1,4,5-trisphosphate, which possibly decreases the amount of Cai available for mobilization.

UDP acts as a growth factor for vascular smooth muscle cells by activation of P2Y(6) receptors

Mitogenic effects of the extracellular nucleotides ATP and UTP are mediated by P2Y(1), P2Y(2), and P2Y(4) receptors. However, it has not been possible to examine the highly expressed UDP-sensitive P2Y(6) receptor because of the lack of stable, selective agonists. In rat aorta smooth muscle cells (vascular smooth muscle cells; VSMC), UDP and UTP stimulated (3)H-labeled thymidine incorporation with similar pEC(50) values (5.96 and 5.69). Addition of hexokinase did not reduce the mitogenic effect of UDP. In cells transfected with P2Y receptors the stable pyrimidine agonist uridine 5'-O-(2-thiodiphosphate) (UDPbetaS) was specific for P2Y(6) with no effect on P2Y(1), P2Y(2), or P2Y(4) receptors. UDPbetaS stimulated [(3)H]thymidine and [(3)H]leucine incorporation and increased cell number in VSMC. Flow cytometry demonstrated that UDP stimulated cell cycle progression to both the S and G(2) phases. The intracellular signal pathways were dependent on phospholipase C, possibly protein kinase C-delta, and a tyrosine kinase pathway but independent of G(i) proteins, eicosanoids, and protein kinase A. The half-life of P2Y(6) receptor mRNA was <1 h by competitive RT-PCR. The mitogen-activated protein kinase kinase inhibitor PD-098059 significantly suppressed, whereas ATP and interleukin-1beta upregulated, expression of P2Y(6) receptor mRNA. The results demonstrate that UDP stimulates mitogenesis through activation of P2Y(6) receptors and that the receptor is regulated by factors important in the development of vascular disease.

Beneficial effects of adenosine triphosphate on nutritional status in advanced lung cancer patients: a randomized clinical trial

PURPOSE

In a randomized clinical trial in patients with advanced non-small-cell lung cancer (NSCLC), infusion with adenosine 5'-triphosphate (ATP) inhibited loss of body weight and quality of life. In the present article, the effects of ATP on body composition, energy intake, and energy expenditure as secondary outcome measures in the same patients are reported.

PATIENTS AND METHODS

Patients with NSCLC, stage IIIB or IV, were randomized to receive either 10 intravenous, 30-hour ATP infusions every 2 to 4 weeks or no ATP. Fat mass (FM), fat-free mass (FFM), and arm muscle area were assessed at 4-week intervals for 28 weeks. Food intake, body cell mass (BCM), and resting energy expenditure (REE) were assessed at 8-week intervals for 16 weeks. Between-group differences were tested for statistical significance by repeated-measures analysis of covariance.

RESULTS

Fifty-eight patients were randomized (28 ATP, 30 control). No change in body composition over the 28-week follow-up period was found in the ATP group, whereas, per 4 weeks, the control group lost 0.6 kg of FM (P =.004), 0.5 kg of FFM (P =.02), 1.8% of arm muscle area (P =.02), and 0.6% of BCM/kg body weight (P =.054) and decreased 568 KJ/d in energy intake (P =.0001). Appetite also remained stable in the ATP group but decreased significantly in the control group (P =.0004). No significant differences in REE between the ATP and control groups were observed.

CONCLUSION

The inhibition of weight loss by ATP infusions in patients with advanced NSCLC is attributed to counteracting the loss of both metabolically active and inactive tissues. These effects are partly ascribed to maintenance of energy intake.

Selective inhibition of beta(2)-adrenergic receptor-mediated cAMP generation by activation of the P2Y(2) receptor in mouse pineal gland tumor cells

Rhythmic noradrenergic signaling from the hypothalamic clock in the suprachiasmatic nucleus to the pineal gland causes an increase in intracellular cAMP which regulates the circadian fluctuation of melatonin synthesis. The activation of phospholipase C (PLC)-coupled P2Y(2) receptors upon treatment with ATP and UTP exclusively inhibited the isoproterenol-stimulated cAMP production in mouse pineal gland tumor cells. However, the activation of other PLC-coupled receptors including P2Y(1) and bombesin receptors had little or no effect on the isoproterenol-stimulated cAMP production. Also, ATP did not inhibit cAMP production caused by forskolin, prostaglandin E(2), or the adenosine analog NECA. These results suggest a selective coupling between signalings of P2Y(2) and beta(2)-adrenergic receptors. The binding of [(3)H]CGP12177 to beta(2)-adrenergic receptors was not effected by the presence of ATP or UTP. Ionomycin decreased the isoproterenol-stimulated cAMP production, whereas phorbol 12-myristate 13-acetate slightly potentiated the isoproterenol response. Chelation of intracellular Ca(2+), however, had little effect on the ATP-induced inhibition of cAMP production, while it completely reversed the ionomycin-induced inhibition. Treatment of cells with pertussis toxin almost completely blocked the inhibitory effect of nucleotides. Pertussis toxin also inhibited the nucleotide-induced increase in intracellular Ca(2+) and inositol 1,4,5-trisphosphate production by 30-40%, suggesting that the ATP-mediated inhibition of the cAMP generation and the partial activation of PLC are mediated by pertussis toxin-sensitive G(i)-protein. We conclude that one of the functions of P2Y(2) receptors on the pineal gland is the selective inhibition of beta-adrenergic receptor-mediated signaling pathways via the inhibitory G-proteins.

Exogenous Mg-ATP induces a large inhibition of pyruvate kinase in intact rat hepatocytes

Mg-ATP infusion in vivo has been reported to be beneficial both to organ function and survival rate in various models of shock. Moreover, a large variety of metabolic effects has been shown to occur in several tissues due to purinergic receptor activation. In the present work we studied the effects of exogenous Mg-ATP in rat liver cells perifused with dihydroxyacetone to investigate simultaneously gluconeogenetic and glycolytic pathways. We found a significant effect on oxidative phosphorylation as characterized by a decrease in oxygen consumption rate and in the cellular ATP-to-ADP ratio associated with an increase in lactate-to-pyruvate ratio. In addition, exogenous Mg-ATP induced rapid and reversible inhibition of both gluconeogenesis and glycolysis. The main effect on gluconeogenesis was located at the level of the fructose cycle, whereas the decrease in glycolysis was due to a strong inhibition of pyruvate kinase. Although pyruvate kinase inhibition induced by exogenous Mg-ATP was allosteric when assessed in vitro after enzyme extraction, we found a large decrease in the apparent maximal velocity when kinetics were assessed in vivo in intact perifused hepatocytes. This newly described short-term regulation of pyruvate kinase occurs only in the intact cell and may open new potentials for the pharmacological regulation of pyruvate kinase in vivo.

ATP induces c-fos expression in C6 glioma cells by activation of P(2Y) receptors

BACKGROUND

Extracellular ATP functions in the enteric nervous system as a neurotransmitter, and recent evidence suggests ATP may regulate development through effects on cellular proliferation.

METHODS

The action of ATP at purinoceptors and the role of second messenger pathways in c-fos mRNA expression in C6 glioma cells were investigated using the techniques of Northern and Western blotting.

RESULTS

Treatment of C6 cells with ATP caused a time- and dose-dependent increase in c-fos expression. The rank order of agonist potency was ATP = ADP > gammasATP > alphabetaATP > betagammaATP > AMP = UTP. The ATP-induced c-fos increment was inhibited by three P(2Y) receptor antagonists-suramin, reactive blue, and DIDS-by 99+/-3, 89+/-7, and 61+/-14%, respectively. The ATP-stimulated c-fos expression was attenuated by phospholipase C inhibitor (U73122), protein kinase C (PKC) down-regulation (4alpha-phorbol 12-myristate 13-acetate and chelerythrine), mitogen-activated protein (MAP) kinase inhibition (apigenin), an inhibitor of MAP kinase kinase (PD98059), down-regulation of adenylate cyclase (SQ22536), and inhibition of type II protein kinase A (8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphorothioate), but was not affected by inhibition of type I protein kinase A (8-bromoadenosine-3',5'-cyclic monophosphorothioate) and inhibitors of calmodulin kinase (KN93 and KN62). Phosphorylated MAP kinase was increased in cells exposed to ATP. This effect was suppressed by chelerythrine.

CONCLUSIONS

These studies demonstrate that ATP-induced c-fos mRNA expression is under multifactorial regulation.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Two different P2Y receptors linked to steroidogenesis in bovine adrenocortical cells

Both extracellular adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) induced corticoid production (steroidogenesis) concentration-dependently in bovine adrenocortical cells (BA cells). Pertussis toxin (PTX, approx. 2 microg/ml) partially inhibited (approx. 55% inhibition) extracellular ATP (100 microM)-induced steroidogenesis in BA cells. However, PTX did not inhibit extracellular UTP (100 microM)-induced steroidogenesis. Both ATP- and UTP-induced steroidogeneses were significantly inhibited by suramin (50-200 microM). These effects were inhibited significantly by reactive blue-2 (more than 100 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (more than 100 microM). Both nucleotides (1-100 microM) induced inositol phosphates accumulation and intracellular Ca2+ mobilization, but PTX did not inhibit them. The RT-PCR procedure identified only P2Y2-receptor mRNA in BA cells. These results suggest that extracellular ATP induces steroidogenesis via a unique P2 receptor linked to PTX-sensitive guanine nucleotide-binding protein (G-protein), while extracellular UTP induces steroidogenesis via P2 receptor linked to PTX-insensitive G-protein. Thus, it was concluded that at least two different P2Y-like receptors linking to steroidogenesis exist in BA cells.

Diadenosine polyphosphates and the control of cyclic AMP concentrations in isolated rat liver cells

Extracellular diadenosine polyphosphates (Ap(n)A), through their interactions with appropriate P(2) receptors, influence a diverse range of intracellular activities. In particular, Ap(4)A stimulates alterations in intracellular calcium homeostasis and subsequent activation of glycogen breakdown in isolated liver cells. Here we show that, like ATP, Ap(4)A and other naturally occurring diadenosine polyphosphates attenuate glucagon-stimulated accumulation of cyclic AMP in isolated rat liver cells. The characteristics of Ap(4)A- and ATP-dependent modulation of glucagon-stimulated cyclic AMP accumulation are similar. These results are discussed in the context of the repertoire of intracellular signalling processes modulated by extracellular nucleotides.

Inhibition of hepatocytic autophagy by adenosine, adenosine analogs and AMP

Autophagy, measured in isolated rat hepatocytes as the sequestration of electroinjected [3H]raffinose, was moderately (17%) inhibited by adenosine (0.4 mM) alone, but more strongly (85%) in the presence of the adenosine deaminase inhibitor, 2'-deoxycoformycin (50 microM), suggesting that metabolic deamination of adenosine limited its inhibitory effectiveness. The adenosine analogs, 6-methylmercaptopurine riboside and N6,N6-dimethyladenosine, inhibited autophagy by 89% and 99%, respectively, at 0.5 mM, probably reflecting the adenosine deaminase-resistance of their 6-substitutions. 5-Iodotubercidin (10 microM), an adenosine kinase inhibitor, blocked the conversion of adenosine to AMP and largely abolished the inhibitory effects of both adenosine and its analogs, indicating that AMP/nucleotide formation was required for inhibition of autophagy. Inhibition by adenosine of autophagic protein degradation, measured as the release of [14C]valine from prelabelled protein, was similarly potentiated by deoxycoformycin and prevented by iodotubercidin. Inhibition of autophagy by added AMP, ADP or ATP (0.3-1 mM) was, likewise, potentiated by deoxycoformycin and prevented by iodotubercidin, suggesting dephosphorylation to adenosine and intracellular re-phosphorylation to AMP. Suppression of autophagy by AMP may be regarded as a feedback inhibition of autophagic RNA degradation, or as an aspect of the general down-regulation of energy-requiring processes that occurs under conditions of ATP depletion, when AMP levels are high.

Inhibition of hepatocytic autophagy by adenosine, aminoimidazole-4-carboxamide riboside, and N6-mercaptopurine riboside. Evidence for involvement of amp-activated protein kinase

To examine the role of AMP-activated protein kinase (AMPK; EC 2.7.1. 109) in the regulation of autophagy, rat hepatocytes were incubated with the AMPK proactivators, adenosine, 5-amino-4-imidazole carboxamide riboside (AICAR), or N6-mercaptopurine riboside. Autophagic activity was inhibited by all three nucleosides, AICAR and N6-mercaptopurine riboside being more potent (IC50 = 0.3 mM) than adenosine (IC50 = 1 mM). 2'-Deoxycoformycin, an adenosine deaminase (EC 3.5.4.4) inhibitor, increased the potency of adenosine 5-fold, suggesting that the effectiveness of adenosine as an autophagy inhibitor was curtailed by its intracellular deamination. 5-Iodotubercidin, an adenosine kinase (EC 2.7.1.20) inhibitor, abolished the effects of all three nucleosides, indicating that they needed to be phosphorylated to inhibit autophagy. A 5-iodotubercidin-suppressible phosphorylation of AICAR to 5-aminoimidazole-4-carboxamide riboside monophosphate was confirmed by chromatographic analysis. AICAR, up to 0.4 mM, had no significant effect on intracellular ATP concentrations. Because activated AMPK phosphorylates and inactivates 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.88), the rate-limiting enzyme in cholesterol synthesis, the strong inhibition of hepatocytic cholesterol synthesis by all three nucleosides confirmed their ability to activate AMPK under the conditions used. Lovastatin and simvastatin, inhibitors of HMG-CoA reductase, strongly suppressed cholesterol synthesis while having no effect on autophagic activity, suggesting that AMPK inhibits autophagy independently of its effects on HMG-CoA reductase and cholesterol metabolism.

ATP activates cAMP production via multiple purinergic receptors in MDCK-D1 epithelial cells. Blockade of an autocrine/paracrine pathway to define receptor preference of an agonist

Extracellular nucleotides regulate function in many cell types via activation of multiple P2-purinergic receptor subtypes. However, it has been difficult to define which individual subtypes mediate responses to the physiological agonist ATP. We report a novel means to determine this by exploiting the differential activation of an autocrine/paracrine signaling pathway. We used Madin-Darby canine kidney epithelial cells (MDCK-D1) and assessed the regulation of cAMP formation by nucleotides. We found that ATP, 2-methylthio-ATP (MT-ATP) and UTP increase cAMP production. The cyclooxygenase inhibitor indomethacin completely inhibited UTP-stimulated, did not inhibit MT-ATP-stimulated, and only partially blocked ATP-stimulated cAMP formation. In parallel studies, ATP and UTP but not MT-ATP stimulated prostaglandin production. By pretreating cells with indomethacin to eliminate the P2Y2/prostaglandin component of cAMP formation, we could assess the indomethacin-insensitive P2 receptor component. Under these conditions, ATP displayed a ten-fold lower potency for stimulation of cAMP formation compared with untreated cells. These data indicate that ATP preferentially activates P2Y2 relative to other P2 receptors in MDCK-D1 cells (P2Y1 and P2Y11, as shown by reverse transcriptase polymerase chain reaction) and that P2Y2 receptor activation is the principal means by which ATP increases cAMP formation in these cells. Blockade of autocrine/paracrine signaling can aid in dissecting the contribution of multiple receptor subtypes activated by an agonist.

Specific activation of adenylyl cyclase V by a purinergic agonist

The present study was designed to investigate whether and how the purinergic stimulation of rat ventricular myocytes modulates the cAMP-dependent pathway. Stimulation of cardiomyocytes with ATPgammaS in the presence of the phosphodiesterase inhibitor IBMX increases by 3-fold intracellular cAMP content. In contrast to beta-adrenergic stimulation, the purinergic stimulation of adenylyl cyclase was not inhibited by activation or enhanced by inhibition of a Gi protein. Forskolin did not potentiate the effect of extracellular ATPgammaS on intracellular cAMP content but the effect of isoproterenol did. Like isoproterenol, the purinergic agonist decreased subsequent ADP-ribosylation of a 45 kDa G(alpha s) by cholera toxin. ATPgammaS also increased cAMP content in neonatal rat cardiomyocytes, a cell type that expresses a long form of Gs protein (alpha(s), 52 kDa) in contrast to adult rat cardiomyocytes that express mostly a short form of Gs protein (alpha(s), 45 kDa). Both purinergic and beta-adrenergic agonists increased cAMP in HEK 293 cells expressing type V adenylyl cyclase while cAMP was only increased by beta-adrenergic stimulation of HEK expressing type IV or VI adenylyl cyclases. Thus, we propose that the purinergic and beta-adrenergic stimulations differentially activate adenylyl cyclase isoforms in rat cardiomyocytes and that adenylyl cyclase V is the specific target of the purinergic stimulation.

P2 receptor-mediated inhibition of adenylyl cyclase in PC12 cells

PC12 pheochromocytoma cells have P2 receptors which are coupled to Ca2+ influx and catecholamine release. Previously we reported that ATP stimulated cyclic AMP accumulation at low concentrations up to 100 microM but showed inhibitory effects above this concentration [Yakushi, Y., Watanabe. A.. Murayama, T., Nomura, Y., 1996. Eur. J. Pharmacol. (314) 243-248]. In this study we investigated the characteristics of the inhibitory effects of ATP analogs. In the presence of 10 microM forskolin, an activator of adenylyl cyclase, ATP, adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), 2',3'-O-(4-benzoyl) benzoyl ATP, 2-methylthio ATP and adenosine 5'-O-(2-thiodiphosphate) inhibited cyclic AMP accumulation in a dose-dependent manner from 100 microM. UTP, alphabeta and betagamma-methylene ATP had no or very limited effects. The relative order of ATP analogs suggests that the ATP receptor appears to be P2Y-like. However, suramin, an antagonist of P2X and P2Y receptors, and reactive blue-2, which inhibited betagamma-methylene ATP-induced cyclic AMP accumulation, did not modify the inhibitory effect of ATPgammaS. Treatment with pertussis toxin, which completely abolished the effect of carbachol, had no effect on the action of ATP over 300 microM. The existence of a new type of ATP receptor-mediated inhibition of adenylyl cyclase is proposed in PC12 cells.

Comparison of atypical beta3-adrenoceptor agonists with their respective metabolic activities in rat white adipocytes

The metabolic activities of four types of beta3-adrenoceptor (AR) agonists, BRL35135A, BRL28410, ICI215001 and CL316243, were compared with those of other beta1- and beta2-AR agonists in rat white adipocytes. All the beta3-AR agonists caused cAMP formation, free fatty acid release and 2-deoxyglucose uptake; the maximum activity levels were similar except for ICI215001, which was lower. However, the magnitude of potency and selectivity of these agonists differed. The most potent and selective beta3-agonist was CL316243. Metabolic activities and Northern blotting showed that there were three beta-AR subtypes that were coupled to adenylyl cyclase and contributed to the induction of lipolysis and glucose uptake. The rank order of the amounts of beta-AR subtypes was beta3 >>beta1> beta2. However, the physiological functions of beta-AR subtypes were essentially similar in rat white adipocytes. On the other hand, cAMP accumulation and Northern blotting showed that human adipocytes predominantly contained beta2-AR, with far lower levels of beta1- and beta3-ARs. These findings suggested that the beta3-AR plays an important role in energy metabolism and thermogenesis in which cross talk exists between beta1- and beta3-ARs in rat adipocytes, while beta2-AR is the most important for the lipolysis regulation in human subcutaneous adipocytes.

Adenylyl cyclase is involved in desensitization and recovery of ATP-stimulated Cl- secretion in MDCK cells

We investigated the process of and recovery from desensitization of the P2 receptor-mediated stimulation of Cl- secretion in Madin-Darby canine kidney (MDCK) cell monolayers by assaying the response of short-circuit current (Isc). When the cells were exposed to repeated 3-min challenges of ATP or UTP interspersed with 5-min washes, the response of Isc desensitized rapidly followed by spontaneous recovery. The pattern of inhibition by various channel blockers or enzyme inhibitors revealed that both the initial and recovered responses of Isc have the same ionic and signaling mechanisms. The desensitization and recovery processes were confined to the membrane exposed to the repeated challenges. When added during the desensitized phase, 8-bromoadenosine 3',5'-cyclic monophosphate enhanced the ATP-stimulated Isc response, whereas it did not during the initial or recovered phases. ATP-induced increases of intracellular adenosine 3',5'-cyclic monophosphate showed similar desensitization and recovery in parallel with the changes in the responses of Isc. The desensitization process was attenuated by pretreatment with cholera toxin or pertussis toxin. Taken together, our results suggest that the adenylyl cyclase system plays a role in the desensitization and recovery mechanism of the ATP-stimulated Cl- secretion in MDCK cells.

Nuclear accumulation of G-actin in isolated rat hepatocytes by adenine nucleotides

Extracellular ATP induces bleb formation in isolated rat hepatocytes. We examined the effect of extracellular ATP on the actin cytoskeleton of these hepatocytes. Exposure to 100 microM ATP caused pronounced nuclear accumulation of G-actin. ADP, AMP, adenosine, and dibutyryl-cAMP induced the same effect. Adenosine deaminase could inhibit both ATP- and adenosine-induced nuclear accumulation. The P2-receptor agonists, UTP and 2' & 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate, did not induce this redistribution of G-actin. Phalloidin, which prevents depolymerisation of F-actin filaments to G-actin monomers, inhibited adenosine-induced nuclear accumulation of G-actin. These observations suggest that nuclear accumulation of G-actin is mediated by adenosine receptors.

Extracellular adenosine 5'-triphosphate induces Ca2+ efflux from freshly isolated adult rat cardiomyocytes: possible involvement of Na+/Ca2+ exchange mechanism

In the present study, we examined the effect of extracellular adenosine 5'-triphosphate (ATP) on Ca2+ efflux from freshly isolated adult rat cardiomyocytes. ATP at 1 mM caused a release of 3.6+/-0.08% of the total cellular content. The 45Ca2+ efflux from the cells was also stimulated by adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma s), alpha, beta-methylene-ATP and adenosine 5'-diphosphate (ADP), but not by adenosine 5'-monophosphate (AMP) or adenosine. The effect of ATP was inhibited by a known purinergic P2-receptor antagonist, but not by a P1-receptor antagonist. From these results, it is conceivable that the effect of ATP on Ca2+ efflux from cardiomyocytes is mediated through P2-purinoceptors. It was also observed that ATP caused a rise in [Ca2+]i to almost 200 nM. The ATP-stimulated 45Ca2+ efflux was not affected by removal of extracellular Ca2+, but was dependent on the presence of extracellular Na+. Moreover, ATP caused a 22Na+ influx into the cells of about 2.0-fold over the basal value. These result suggest that ATP stimulates extracellular Na+-dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on plasma membrane P2-purinoceptors which may couple to Na+/Ca2+ exchange.

Betagamma subunits of pertussis toxin-sensitive G proteins mediate A1 adenosine receptor agonist-induced activation of phospholipase C in collaboration with thyrotropin. A novel stimulatory mechanism through the cross-talk of two types of receptors

COS-7 cells were transiently transfected with human thyrotropin receptor and dog A1 adenosine receptor cDNAs. An A1 agonist, N6-(L-2-phenylisopropyl) adenosine (PIA), which is ineffective alone, enhanced the thyrotropin (TSH)-induced inositol phosphate production, reflecting phospholipase C (PLC) activation, but inhibited the TSH-induced cAMP accumulation, reflecting adenylyl cyclase inhibition. These PIA-induced actions were completely inhibited by pertussis toxin (PTX) treatment. Moreover, in the cells expressing a PTX-insensitive mutant of Gi2alpha or Gi3alpha, in which a glycine residue was substituted for a cysteine residue to be ADP-ribosylated by PTX, at the fourth position of the C terminus, PIA effectively exerted both stimulatory and inhibitory effects on the TSH-induced actions although the cells were treated with the toxin. Overexpression of the betagamma subunits of the G proteins enhanced the TSH-induced inositol phosphate production without any significant effect on the cAMP response; under these conditions, PIA did not further increase the elevated inositol phosphate response to TSH. On the contrary, overexpression of a constitutively active mutant of Gi2alpha, in which the guanosine triphosphatase activity is lost, inhibited the TSH-induced cAMP accumulation but hardly affected the inositol phosphate response; under these conditions, PIA never exerted further inhibitory effects on the cAMP response to TSH. In contrast to the case of the TSH-induced inositol phosphate response, the response to a constitutively active G11alpha mutant was not appreciably affected, and that to NaF was rather inhibited by PIA and overexpression of the betagamma subunits. Taken together, these results suggest that a single type of PTX-sensitive G protein mediates the A1 adenosine receptor-linked modulation of two signaling pathways in collaboration with an activated thyrotropin receptor; alpha subunits of the PTX-sensitive G proteins mediate the inhibitory action on adenylyl cyclase, and the betagamma subunits mediate the stimulatory action on PLC. In the case of the latter stimulatory action on PLC, the betagamma subunits may not directly activate PLC. The possible mechanism by which betagamma subunits enhance the TSH-induced PLC activation is discussed.

Involvement of protein kinase C in the UTP-mediated potentiation of cyclic AMP accumulation in mouse J774 macrophages

1. We have investigated the effects of nucleotide analogues on cyclic AMP formation in mouse J774 macrophages and the mechanisms involved. 2. UTP, in the concentration range 0.1-100 microM, induced concentration-dependent potentiation of prostaglandin E1 (PGE1)-induced cyclic AMP formation, but had no effect on basal cyclic AMP formation. UDP showed an equal potency, while 2-methylthio ATP, alpha, beta-methylene ATP and beta,gamma-methylene ATP gave either a slight increase or had no effect at concentrations up to 100 microM. ATP, although 100 fold less effective than UTP, also caused cyclic AMP potentiation, but had no effect on agonist-stimulated or basal cyclic AMP levels. 3. The cyclic AMP potentiation effect of UTP correlated with increased [Ca2+]i and inositol phosphate (IP) formation over the same concentration range. 4. Ionomycin, which evokes an increase in [Ca2+]i without affecting IP formation, did not cause an increase in cyclic AMP content, indicating that UTP-induced cyclic AMP regulation is not due to activation of Ca(2+)-sensitive adenylyl cyclase isoforms. 5. Although reduced, UTP potentiation was seen in cells incubated in a Ca(2+)-free and/or BAPTA-containing medium. Under these conditions, the UTP-increased IP accumulation was similarly reduced. 6. Exposure of cells to phorbol 12-myristate 13-acetate (PMA) also increased PGE1 stimulation of cyclic AMP levels, and the UTP-induced potentiation of cyclic AMP formation was inhibited by either staurosporine or Ro 31-8220. Pretreatment of cells with PMA for 4-24 h resulted in marked attenuation of UTP-stimulated cyclic AMP potentiation. 7. Pretreatment with pertussis toxin (24 h, 100 ng ml-1) did not significantly affect UTP-induced cyclic AMP potentiation and IP formation, although it increased the cyclic AMP response to PGE1. 8. Analysis of J774 cells by Western blotting with antibodies specific for different protein kinase C (PKC) isoforms shows the presence of the beta I, beta II, delta, epsilon, eta, mu, lambda and zeta isoforms. Moreover, UTP significantly increased the level of PKC beta I, beta II, delta, epsilon, mu, lambda and zeta immunoreactivity in the membrane fraction and decreased the cytosolic reactivity of PKC beta II, delta, epsilon and zeta. 9. Immunoblot studies also indicate the presence of type II adenylyl cyclase. 10. These results indicate that PKC is required for the potentiation of adenylyl cyclase activity by macrophage pyrimidinoceptors, which exhibit a higher specificity for UTP and UDP than for ATP.

The 6H1 orphan receptor, claimed to be the p2y5 receptor, does not mediate nucleotide-promoted second messenger responses

An orphan G protein-coupled receptor, termed 6H1, with approximately 30% sequence identity to P2Y receptors has been proposed to be a P2Y receptor (p2y5) based solely on a radioligand binding assay with [35S]dATP alphaS [Webb et al. (1997) Biochem. Biophys. Res. Commun. 219:105-110]. Previous work in our laboratory has shown that [35S]-dATP alphaS is not a general ligand for P2Y receptors, and thus inclusion of the p2y5 receptor in the family of P2Y receptors is questionable. To define unambiguously whether the p2y5 receptor is a P2Y receptor, we have cloned the turkey homologue of the chick p2y5 receptor. Sequence analysis indicated that the turkey receptor contains an additional 32 amino acids at its carboxy terminus compared to the published chick sequence. HA epitope-tagged turkey p2y5 receptors were stably expressed in 1321N1 human astrocytoma cells, and cells shown to express the HA-tagged p2y5 receptor by an intact cell-based ELISA were used to determine whether changes in second messenger levels occurred in response to a series of nucleotides. ATP, ADP, UTP, UDP, dATP alphaS, and A2P4 had no effect on either inositol phosphate or cyclic AMP concentrations in cells expressing the p2y5 receptor. Robust inositol phosphate and cyclic AMP responses occurred to other G protein-coupled receptors expressed in 1321N1 cells, which indicate that these cells contain all of the necessary signaling components to generate these second messenger responses. These data indicate that the 6H1/p2y5 receptor is not a member of the P2Y receptor family of signaling proteins.

Calcium release from intracellular stores evoked by extracellular ATP in a Xenopus renal epithelial cell line

1. The signal transduction mechanism mediating extracellular adenosine 5'-triphosphate (ATP)-induced calcium release in a renal epithelial cell line (A6) was investigated using the whole-cell voltage-clamp technique and fura-2 fluorescence measurement. 2. ATP (10 microM) activated calcium-dependent non-selective cation channels in cells held under voltage clamp. 3. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta S; 0.1-1.0 mM) in the pipette inhibited the ATP-activated calcium-dependent currents. With guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 0.1-1.0 mM) in the pipette, the currents were spontaneously elicited without application of ATP. Pretreatment with pertussis toxin (PTX) affected neither the ATP-activated currents nor the increase in intracellular free calcium concentration ([Ca2+]i) evoked by ATP. 4. Intracellular application of neomycin or heparin inhibited the ATP-activated currents. Inositol 1,4,5-trisphosphate (IP3; 0.1-100 microM) in the internal solution produced currents similar to those due to ATP activation. 5. These results suggest that a PTX-insensitive guanosine 5'-triphosphate (GTP)-binding regulatory protein (G. protein) is involved in extracellular. ATP-induced phosphoinositide turnover and subsequent calcium release from IP3-sensitive stores, which subsequently activates the calcium-dependent channels in A6 cells.

Lack of nucleotide-promoted second messenger signaling responses in 1321N1 cells expressing the proposed P2Y receptor, p2y7

A recently cloned G protein-coupled receptor (named the p2y7 receptor) with relatively low sequence identity to previously cloned P2Y receptors was proposed to be a member of this family of receptors on the basis of both a radioligand binding assay with [35S]dATP alphaS and an inositol phosphate response to ATP in COS-7 cells transiently transfected with receptor cDNA. Previous work in our laboratory has shown that [35S]dATP alphaS is not a general radioligand for the identification of P2Y receptors and that COS-7 cells express an endogenous P2Y receptor (P2Y2) that complicates the analysis of nucleotide-promoted inositol phosphate responses. Thus, data supporting inclusion of the p2y7 receptor in the P2Y family of receptors are equivocal. To determine unambiguously whether the p2y7 receptor is a P2Y receptor subtype, a p2y7 receptor bearing an epitope-tag at its NH2-terminus was expressed in 1321N1 cells and cell surface expression of the receptor was demonstrated by an intact cell-based ELISA. Cells shown to express epitope-tagged p2y7 receptors by ELISA were examined for their second messenger signaling properties in response to a range of nucleotides. ATP, UTP, ADP, UDP, and dATP alphaS had no effect on phospholipase C or adenylyl cyclase activities in cells expressing the p2y7 receptor. Experimental controls utilizing expression of other G protein-coupled receptors showed that 1321N1 cells displayed robust responses for each of these signaling pathways. These data, together with the low sequence identity of the p2y7 receptor to other P2Y receptors, indicate that the p2y7 is not a member of the P2Y family of signaling molecules.

Stimulation of hepatocyte glycerolipid synthesis by iron/ADP is due to ADP rather than oxidative stress

ADP-complexed Fe3+ has been used in various in vitro systems and in intact cells to induce lipid peroxidation. During studies on the effects of oxidative stress on lipid metabolism we observed a significant increase in de novo glycerolipid synthesis in Fe3+/ADP-treated rat hepatocytes as evidenced by increased [U-14C]glycerol incorporation. Here we show that this increase, largely due to enhanced triacylglycerol synthesis, is caused by ADP rather than Fe3+/ADP-induced oxidative stress. Hence, metabolic alterations due to treatment of intact cells by Fe3+/ADP must be interpreted with caution.

Signaling by extracellular nucleotides

ATP and other nucleotides can be released from cells through regulated pathways, or following the loss of plasma membrane integrity. Once outside the cell, these compounds take on new roles as intercellular signaling molecules that elicit a broad spectrum of physiological responses through the activation of numerous cell surface receptor subtypes. This review summarizes recent advances in the molecular characterization of ATP receptors and discusses roles for cloned receptors in established and novel physiological processes.

P2 purinoceptor-mediated stimulation of adenylyl cyclase in PC12 cells

PC12 pheochromocytoma cells have P2 purinoceptors which are activated by ATP and coupled to Ca2+ influx and catecholamine release. Also PC12 cells have adenosine receptors coupled positively to adenylyl cyclase, and cyclic AMP regulates cell functions such as catecholamine release. The effects of ATP and ATP analogs on cyclic AMP accumulation in PC12 cells were investigated in this study. ATP and adenosine 5'-0-(3-thiotriphosphate) stimulated cyclic AMP accumulation at low concentrations up to 300 microM but showed inhibitory effects above this concentration. 2',3'-O-(4-Benzoyl)benzoyl ATP and 2-methylthio ATP showed similar effects, although the responses were very limited. Addition of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) or beta, gamma-methylene ATP, but not alpha, beta-methylene ATP, stimulated cyclic AMP accumulation markedly without causing an inhibitory phase. The effects of ATP, ADP beta S and beta, gamma-methylene ATP were not inhibited by adenosine deaminase or specific antagonists to A1 and A2 adenosine receptors. Neither ADp beta S nor beta, gamma-methylene ATP showed any effect on Ca2+ influx or noradrenaline release. Suramin, a P2 receptors antagonists, had no inhibitory effect against ATP analog-stimulated cyclic AMP accumulation, although reactive blue 2 inhibited the beta, gamma-methylene ATP-stimulated reaction but not that up-regulated by ADP beta S. These findings suggest that the pharmacological characteristics of these ATP receptors coupled to adenylyl cyclase are clearly different from those of ligand-gated ion channels defined by P2X purinoceptors, which have been cloned and shown to be coupled to Ca2+ influx and catecholamine release in PC12 cells. The existence of a new type of P2 purinoceptor-mediating stimulation of adenylyl cyclase is proposed in PC12 cells.

Effects of diadenosine triphosphate and diadenosine tetraphosphate on rat liver cells. Differences and similarities with ADP and ATP

Liver cells possess multiple types of purinoceptors that mediate the effects of extracellular nucleotides. Like ADP and ATP, the dinucleotides diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A) fully activated glycogen phosphorylase, with ED50 values of 0.31 microM and 1.3 microM, respectively. At variance with ATP, neither the dinucleotides nor ADP significantly increased the levels of IP3.Ap4A (and also ADP) moderately increased IP3 (+/- 72%) whereas Ap3A was completely ineffective. Like ATP, Ap3A, Ap4A, and ADP inhibited the cAMP increase after glucagon. Phorbol-12-myristate-13-acetate (PMA) pretreatment of the hepatocytes clearly inhibited the glycogenolytic potency of Ap3A and ADP, but had only a minor effect on the potency of Ap4A or ATP. It is concluded that, depending upon the effect studied (glycogenolytic effect with or without PMA, increasing IP3 potency, or inhibition of cAMP increase), different analogies between the agonists studied emerged, indicating the complexity of the interaction of ATP and its analogues with liver purinoceptors and/or of the transduction mechanism(s) initiated by the different nucleotides.

P2U agonists induce chemotaxis and actin polymerization in human neutrophils and differentiated HL60 cells

Human neutrophils or HL60 cells express P2U receptors and respond to micromolar concentrations of ATP, adenosine 5'-O-(thiotriphosphate) (ATPgammaS), or UTP with immediate increases in intracellular Ca2+ through activation of phosphoinositide phospholipase C (Cowen, D. S., Lazarus, H. M., Shurin, S. B., Stoll, S. E., and Dubyak, G. R. (1989) J. Clin. Invest. 83, 1651-1660). P2U agonists reportedly induce limited enzyme secretion and enhance the respiratory burst in response to chemotactic factors. We demonstrate here that P2U agonists are chemotactic for neutrophils or differentiated HL60 cells. Rhodamine phalloidin staining indicates that ATPgammaS treatment induces actin polymerization and shape changes similar to those seen when these cells are treated with chemotactic peptide fMet-Leu-Phe. Although undifferentiated HL60 cells fail to mount a rise in Ca2+ when challenged with fMet-Leu-Phe, they increase Ca2+ in response to P2U agonists. However, functional expression of phospholipase C-coupled receptors is not sufficient for chemotaxis since HL60 cell migration in response to these agonists or to fMet-Leu-Phe occurs only after exposure to differentiating agents such as BT2cAMP. In addition to the well known G protein-linked receptors for lipid or peptide chemotactic factors, neutrophils apparently also can utilize G protein-linked purino/pyrimidino receptors to recognize nucleotides as chemoattractants. High concentrations of ATP and UTP generated at sites of platelet aggregation and tissue injury could thus be important mediators of inflammation.

Partial characterization of a new nucleotide binding glycoprotein of hepatocyte plasma membrane

Hepatocyte plasma membranes contain a glycosylated 230-kDa Ca(2+) -dependent, Mg(2+)-stimulated ATPase (pgp230), which consists of two subunits, one of 120 kDa and the other of 110 kDa. pgp230 can be enriched by the use of affinity chromatography on Concanavalin A-Sepharose, wheat germ lectin-Sepharose, and 5'-AMP-Sepharose. It has a high-affinity Ca2+ binding site. In the presence of Ca2+, it forms a phosphorylated intermediate by autocatalytic transfer of the terminal phosphate residue from ATP. Maximal Ca(2+)-dependent autophosphorylation is observed at pH 5-6. Photoaffinity labeling using 8-azido-[alpha-32P]ATP or [y-32P]ATP confirms the presence of ATP binding sites. Incubation with [alpha-32P]ATP leads to a rapid but transient labeling of pgp230. Various nucleotides, nucleotide receptor agonists, or antagonists inhibit Ca(2+)-dependent phosphorylation by [y-32P]ATP. The concentrations of half-maximal inhibition range from 10(-7) M to 10(-3) M. The rank order of inhibitory potency is: ATP > alpha,beta-methylene-ATP > CTP = TTP > y-4-amino-phenyl-ATP = 2-methyl-thio-ATP > UTP = GTP > GDP = ADP = beta,y-methylene-ATP = beta, y-methylene-TTP = beta,y-methylene-GTP = adenosine-5'-O-2-thiodiphosphate = CMP = AMP > adenosine > cytidine > guanosine = suramin > Reactive blue 2 > iso-butyl-methyl-xanthine > thymidine > uridine. These data suggest a nucleotide binding capacity of this new hepatocyte membrane glycoprotein. Further investigations should be carried out to reveal its biological function.

Effect of extracellular adenosine 5'-triphosphate on Ca2+ efflux from freshly isolated adult rat cardiomyocytes

The effect of extracellular adenosine 5'-triphosphate (ATP) on Ca2+ efflux from freshly isolated adult rat cardiomyocytes was examined. ATP stimulated the efflux of 45Ca2+ from the cells in a concentration-dependent manner (0.01-1 mM). The 45Ca2+ efflux from the cells was also stimulated by adenosine-5'-O-(3-thiotriphosphate)(ATP-gammas) and alpha,beta-methylene-ATP and adenosine 5'-diphosphate, but not by adenosine 5'-monophosphate and adenosine. The ATP-stimulated 45Ca2+ efflux was not affected by deprivation of the extracellular Ca2+, but was dependent on the presence of extracellular Na+. These results indicate that ATP stimulates extracellular Na+-dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on the plasma membrane P2 purinoceptors which may couple to Na+/Ca2+ exchange.

P2 purinergic receptor agonists enhance cAMP production in Madin-Darby canine kidney epithelial cells via an autocrine/paracrine mechanism

Mechanisms of cross-talk between different classes of signaling molecules are inadequately understood. We have used clonal Madin-Darby canine kidney (MDCK-D1) epithelial cells as a model system to investigate the effects of extracellular nucleotides (e.g. ATP, UTP), which promote increase in activity of several phospholipases, on cAMP production. In contrast to observations in some other cell systems, ATP and UTP, acting via P2 purinergic receptors, stimulated cAMP production in MDCK-D1 cells. At maximally effective concentrations, ATP and UTP were not additive with the beta-adrenergic receptor agonist isoproterenol, but were synergistic with forskolin in increasing cAMP production, indicating that G alpha s is activated by these nucleotides. Additionally, we found that (a) nucleotide-induced increases in cAMP were blocked by indomethacin, a cyclooxygenase inhibitor, (b) arachidonic acid increased cellular cAMP levels in an indomethacin-sensitive fashion, and (c) PGE2, the major metabolite of arachidonic acid, stimulated cAMP formation. Overall, our results suggest a mechanism by which extracellular nucleotides stimulate release of arachidonic acid which is metabolized to PGE2 which, in turn, acts in an autocrine/paracrine fashion via prostaglandin receptors to activate G alpha s and increase cAMP. Based on the ability of extracellular nucleotides to stimulate the formation and release of prostaglandins in MDCK-D1 epithelial and other cells, we hypothesize that receptor-mediated prostaglandin release may be a general mechanism that regulates cAMP formation in many types of cells.

Some P2 purinergic agonists increase cytosolic calcium but not inositol 1,4,5-trisphosphate in isolated rat hepatocytes

Based on the capacity to increase IP3 (inositol 1,4,5-trisphosphate), P2 purinergic agonists can be subdivided into two classes: ATP, ADP, UTP, 2deoxyATP, NAD and GTP significantly increased IP3 levels whereas ADP beta S, 2MeSATP, NADP, alpha, beta MeATP, beta, gamma MeATP and ATP alpha S had only a minor, non-significant effect. Irrespective of their potency to increase IP3, all agonists were full glycogenolytic agonists and they all increased cytosolic calcium. With ATP and NAD, IP3 increasing agonists, and 2MeSATP and ADP beta S, non-IP3 increasing agonists, we found that the initial calcium response appeared to be an 'all or none' phenomenon, small amounts of the agonists being either ineffective or equally effective as high amounts. The minimal amount of an agonist needed to initiate a calcium increase and to promote glycogenolysis was very similar. In the absence of extracellular calcium, both groups of purinergic agonists (tested with ATP and 2MeSATP) were equally able to release calcium from intracellular stores. Cells with emptied intracellular calcium stores rapidly took up extracellular calcium upon treatment with ATP or 2MeSATP, the latter being the most potent. It seems therefore that all nucleotides tested increased cytosolic calcium and activated phosphorylase in a very similar way but some nucleotides had no effect on the levels of IP3.

Pertussis toxin inhibits phospholipase C activation and Ca2+ mobilization by sphingosylphosphorylcholine and galactosylsphingosine in HL60 leukemia cells. Implications of GTP-binding protein-coupled receptors for lysosphingolipids

Extracellular sphingosylphosphorylcholine (SPC) and galactosylsphingosine (psychosine) induced Ca2+ mobilization in a dose-dependent manner in HL60 leukemia cells. The rapid and transient increase in intracellular Ca2+ concentration ([Ca2+]i) elicited by SPC and psychosine at concentrations lower than 30 microM was inhibited by treatment of the cells with pertussis toxin (PTX) and U73122, a phospholipase C inhibitor, as was the case for UTP, a P2-purinergic agonist. The increase in [Ca2+]i induced by these lysosphingolipids was associated with inositol phosphate production, which was also sensitive to PTX and U73122. The inositol phosphate response is not secondary to the increase in [Ca2+]i as evidenced by the observation that thapsigargin and ionomycin, Ca2+ mobilizing agents, never induced inositol phosphate production and, unlike lysosphingolipids, the [Ca2+]i rise by these agents was totally insensitive to PTX and U73122. When HL60 cells were differentiated into neutrophil-like cells by dibutyryl cyclic AMP, inositol phosphate and Ca2+ responses to AlF4- were enhanced, probably reflecting an increase in the amount of Gi2 and Gi3 compared with undifferentiated cells. In the neutrophil-like cells, however, the responses to SPC and psychosine were markedly attenuated. This may exclude the possibility that the lysosphingolipids activate rather directly PTX-sensitive GTP-binding proteins or the phospholipase C itself. Other lysosphingolipids including glucosylsphingosine (glucopsychosine) and sphingosylgalactosyl sulfate (lysosulfatides) at 30 microM or lower concentrations also showed PTX- and U73122-sensitive Ca2+ mobilization and inositol phosphate response in a way similar to SPC and psychosine. However, platelet-activating factor and lysoglycerophospholipids such as lysophosphatidylcholine and lysophosphatidic acid were less effective than these lysosphingolipids in the induction of Ca2+ mobilization. Taken together, the results indicate that a group of lysosphingolipids at appropriate doses induces Ca2+ mobilization through inositol phosphate production by phospholipase C activation. The lysosphingolipids-induced enzyme activation may be mediated by PTX-sensitive GTP-binding protein-coupled receptors, which may be different from previously identified platelet-activating factor receptor or lysophosphatidic acid receptor.

Purinergic agonist and G protein stimulation of phospholipase D in rat liver plasma membranes. Independence from phospholipase C activation

Hormonal regulation of phospholipase D (PLD) was studied in isolated rat liver plasma membranes. Purinergic agents and a submaximal concentration of guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of GTP, synergistically stimulate phosphatidylethanol formation, a measure of PLD activity. The rank order of efficacy for stimulation of PLD activity in the presence of 0.2 microM GTP gamma S was beta, gamma-methylene-ATP > adenosine 5'-0-(3-thiotriphosphate) = ATP = ADP = 2-methylthio-ATP > alpha, beta-methylene-ATP = UTP. This pattern of activation does not conform to the series at known P2 receptors. GTP gamma S stimulated PLD activity in a dose-dependent manner, and the GTP gamma S dose-response curve for phosphatidylethanol formation was shifted to the left by an analog of ATP. Activation of PLD by purinergic agents in the presence of GTP gamma S supports the involvement of a purinergic receptor of the P2 class and a GTP-binding protein. Purinergic agents competitively inhibited [35S]adenosine 5'-0-(3-thiotriphosphate) binding to plasma membranes in the rank order adenosine 5'-0'(3-thiotriphosphate) > ATP > alpha,beta-methylene-ATP = UTP >> beta, gamma-methylene-ATP = ADP. Stimulation of phosphoinositide phospholipase C (PI-PLC) by purinergic agents, as measured by release of radioactivity from endogenously myo[3H]inositol-labeled plasma membranes, occurred in the order alpha, beta-methylene-ATP >> 2-methylthio-ATP. Beta, gamma-methylene-ATP had little effect on PI-PLC activity. Different dose-response relationships for agonist-stimulation of PI-PLC and PLD indicate that activation of PI-PLC is not involved in stimulation of PLD in rat liver plasma membranes, and suggest that purinergic activation of PLD occurs via a pathway involving a G protein and a heretofore uncharacterized P2 receptor.

Ca(2+)-dependent and -independent mechanism of cyclic-AMP reduction: mediation by bradykinin B2 receptors

1. Bradykinin caused a transient reduction of about 25% in the cyclic AMP level in forskolin prestimulated DDT1 MF-2 smooth muscle cells (IC50: 36.4 +/- 4.9 nM) and a pronounced, sustained inhibition (40%) of the isoprenaline-stimulated cyclic AMP level (IC50: 37.5 +/- 1.1 nM). 2. The Ca2+ ionophore, ionomycin, mimicked both the bradykinin-induced transient reduction in the forskolin-stimulated cyclic AMP level and the sustained reduction in the isoprenaline-stimulated cyclic AMP level. 3. The Ca(2+)-dependent effect on cyclic AMP induced by bradykinin was mediated solely by Ca2+ release from internal stores, since inhibition of Ca2+ entry with LaCl3 did not reduce the response to bradykinin. 4. The involvement of calmodulin-dependent enzyme activities, protein kinase C or an inhibitory GTP binding protein in the bradykinin-induced responses was excluded since a calmodulin inhibitor, calmidazolium, a PKC inhibitor, staurosporine and pertussis toxin, respectively did not affect the decline in the cyclic AMP level. 5. Bradykinin enhanced the rate of cyclic AMP breakdown in intact cells, which effect was not mimicked by ionomycin. This suggested a Ca(2+)-independent activation of phosphodiesterase activity by bradykinin in DDT1 MF-2 cells. 6. The bradykinin B1 receptor agonist, desArg9-bradykinin, did not affect cyclic AMP formation in isoprenaline prestimulated cells, while the bradykinin B2 receptor antagonists, Hoe 140 (D-Arg[Hyp3, Thi5, D-Tic7, Oic8]-BK) and D-Arg[Hyp3, Thi5,8, D-Phe7]-BK completely abolished the bradykinin response in both forskolin and isoprenaline prestimulated cells. 7. Bradykinin caused an increase in intracellular Ca2+, which was antagonized by the bradykinin B2 receptor antagonists, Hoe 140 and D-Arg[Hyp3, Thi5,8, D-Phe7]-BK. The bradykinin B2 receptor agonist,desArg9-bradykinin, did not evoke a rise in cytoplasmic Ca2 .8. It is concluded, that stimulation of bradykinin B2 receptors causes a reduction in cellular cyclic AMP in DDT1, MF-2 cells. This decline in cyclic AMP is partly mediated by a Ca2+/calmodulin independent activation of phosphodiesterase activity. The increase in [Ca2+], mediated by bradykinin B2 receptors inhibited forskolin- and isoprenaline-activated adenylyl cyclase differently, most likely by interfering with different components of the adenylyl cyclase signalling pathway.