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

P2 receptors in the kidney

Our understanding of the actions of extracellular ATP in controlling kidney function via stimulation of P2 receptors is still at an early stage. Recently, several groups, including our own, have begun to address this subject: in this brief review, we discuss some of these effects and speculate on likely function of extracellular nucleotides in the kidney.

Extracellular ATP increases [CA(2+)](i) in distal tubule cells. I. Evidence for a P2Y2 purinoceptor

Experiments were performed to characterize the P2 purinoceptor subtype responsible for cytoplasmic calcium mobilization in cells from the initial part of rabbit distal convoluted tubule (DCT). Free calcium concentration was measured in a DCT cell line (DC1) with the probe fura 2. Both ATP and UTP increased cytosolic Ca(2+) concentration ([Ca(2+)](i); EC(50) 3 and 6 microM, respectively). The order of potency for nucleotide analogs was ATP = UTP > adenosine 5'-O-[thiotriphosphate] >> ADP > UDP, which is consistent with the pharmacology of the P2Y2 receptor subtype. The increased [Ca(2+)](i) responses to ATP and UTP were strongly inhibited by suramin. Pretreatment of cells with pertussis toxin (PTX) attenuated the action of both nucleotides. Inhibition of phospholipase C with U-73122 totally blocked the [Ca(2+)](i) response to ATP. Thus ATP- and UTP-stimulated [Ca(2+)](i) mobilization in DC1 cells appears to be mediated via the activation of P2Y2 purinoceptors coupled to a G protein mechanism that is partially sensitive to PTX. Calcium flux measurements showed that lanthanum- and nifedipine-sensitive calcium channels are involved in the [Ca(2+)](i) response to ATP.

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.

Cellular release of and response to ATP as key determinants of the set-point of signal transduction pathways

The determinants of "basal" activity of signaling pathways regulating cellular responses are poorly defined. One possibility is that cells release factors to establish the set-point of such pathways. Here we show that treatment of Madin-Darby canine kidney cells with the nucleotidase apyrase decreases basal arachidonic acid release and cAMP production 30-40% and that inhibitors of P2Y receptor action also affect basal and forskolin-stimulated cAMP accumulation. Changing medium prominently increases extracellular levels of ATP in Madin-Darby canine kidney, COS-7, and HEK-293 cells. Mechanical stimulation of ATP release likely occurs in virtually every experimental protocol with cultured cells, implicating such release and P2Y receptor activation as critical in establishing the set-point for signal transduction pathways.

Apical endocytosis of ricin in MDCK cells is regulated by the cyclooxygenase pathway

Addition of arachidonic acid or stimulation of arachidonic acid production by secretory phospholipase A2 selectively upregulated apical endocytosis of ricin in MDCK cells without affecting basolateral endocytosis. Electron microscopic studies revealed that MDCK cells treated with secretory phospholipase A2 and incubated with horseradish peroxidase had an increased number of normal appearing peroxidase-labeled endosomes and no sign of membrane ruffling. Moreover, inhibition of basal arachidonic acid release, either by decreasing the cytosolic phospholipase A(2) activity or the diacylglycerol lipase activity, reduced the rate of apical endocytosis. Furthermore, indomethacin, an inhibitor of the cyclooxygenase pathway, counteracted the stimulation of endocytosis seen with both secretory phospholipase A2 and arachidonic acid, suggesting that formation of eicosanoids such as prostaglandins could be essential for the regulation. This idea was supported by the finding that prostaglandin E2, the predominant prostaglandin formed in kidney, also upregulated ricin uptake. The regulatory effect of the cyclooxygenase pathway on apical endocytosis of ricin was found to be independent of protein kinases A and C, which are known to selectively control apical clathrin-independent endocytosis in polarized cells.

Nucleotides regulate NaCl transport in mIMCD-K2 cells via P2X and P2Y purinergic receptors

Extracellular nucleotides regulate NaCl transport in some epithelia. However, the effects of nucleotide agonists on NaCl transport in the renal inner medullary collecting duct (IMCD) are not known. The objective of this study was to determine whether ATP and related nucleotides regulate NaCl transport across mouse IMCD cell line (mIMCD-K2) epithelial monolayers and, if so, via what purinergic receptor subtypes. ATP and UTP inhibited Na(+) absorption [measured via Na(+) short-circuit current (I(Na)(sc))] and stimulated Cl(-) secretion [measured via Cl(-) short-circuit current (I(Cl)(sc))]. Using selective P2 agonists, we report that P2X and P2Y purinoceptors regulate I(Na)(sc) and I(Cl)(sc). By RT-PCR, two P2X receptor channels (P2X(3), P2X(4)) and two P2Y G protein-coupled receptors (P2Y(1), P2Y(2)) were identified. Functional localization of P2 purinoceptors suggest that I(Cl)(sc) is stimulated by apical membrane-resident P2Y purinoceptors and P2X receptor channels, whereas I(Na)(sc) is inhibited by apical membrane-resident P2Y purinoceptors and P2X receptor channels. Together, we conclude that nucleotide agonists inhibit I(Na)(sc) across mIMCD-K2 monolayers through interactions with P2X and P2Y purinoceptors expressed on the apical plasma membrane, whereas extracellular nucleotides stimulate I(Cl)(sc) through interactions with P2X and P2Y purinoceptors expressed on the apical plasma membrane.

Signaling in human osteoblasts by extracellular nucleotides. Their weak induction of the c-fos proto-oncogene via Ca2+ mobilization is strongly potentiated by a parathyroid hormone/cAMP-dependent protein kinase pathway independently of mitogen-activated protein kinase

Extracellular nucleotides acting through specific P2 receptors activate intracellular signaling cascades. Consistent with the expression of G protein-coupled P2Y receptors in skeletal tissue, the human osteosarcoma cell line SaOS-2 and primary osteoblasts express P2Y1 and P2Y2 receptors, respectively. Their activation by nucleotide agonists (ADP and ATP for P2Y1; ATP and UTP for P2Y2) elevates [Ca2+]i and moderately induces expression of the c-fos proto-oncogene. A synergistic effect on c-fos induction is observed by combining ATP and parathyroid hormone, a key bone cell regulator. Parathyroid hormone elevates intracellular cAMP levels and correspondingly activates a stably integrated reporter gene driven by the Ca2+/cAMP-responsive element of the human c-fos promoter. Nucleotides have little effect on either cAMP levels or this reporter, instead activating luciferase controlled by the full c-fos promoter. This induction is reproduced by a stably integrated serum response element reporter independently of mitogen-activated protein kinase activation and ternary complex factor phosphorylation. This novel example of synergy between the cAMP-dependent protein kinase/CaCRE signaling module and a non-mitogen-activated protein kinase/ternary complex factor pathway that targets the serum response element shows that extracellular ATP, via P2Y receptors, can potentiate strong responses to ubiquitous growth and differentiative factors.

Inhibition of phospholipase A2-mediated arachidonic acid release by cyclic AMP defines a negative feedback loop for P2Y receptor activation in Madin-Darby canine kidney D1 cells

In Madin-Darby canine kidney D1 cells extracellular nucleotides activate P2Y receptors that couple to several signal transduction pathways, including stimulation of multiple phospholipases and adenylyl cyclase. For one class of P2Y receptors, P2Y2 receptors, this stimulation of adenylyl cyclase and increase in cAMP occurs via the conversion of phospholipase A2 (PLA2)-generated arachidonic acid (AA) to prostaglandins (e.g. PGE2). These prostaglandins then stimulate adenylyl cyclase activity, presumably via activation of prostanoid receptors. In the current study we show that agents that increase cellular cAMP levels (including PGE2, forskolin, and the beta-adrenergic agonist isoproterenol) can inhibit P2Y receptor-promoted AA release. The protein kinase A (PKA) inhibitor H89 blocks this effect, suggesting that this feedback inhibition occurs via activation of PKA. Studies with PGE2 indicate that inhibition of AA release is attributable to inhibition of mitogen-activated protein kinase activity and in turn of P2Y receptor stimulated PLA2 activity. Although cAMP/PKA-mediated inhibition occurs for P2Y receptor-promoted AA release, we did not find such inhibition for epinephrine (alpha1-adrenergic) or bradykinin-mediated AA release. Taken together, these results indicate that negative feedback regulation via cAMP/PKA-mediated inhibition of mitogen-activated protein kinase occurs for some, but not all, classes of receptors that promote PLA2 activation and AA release. We speculate that receptor-selective feedback inhibition occurs because PLA2 activation by different receptors in Madin-Darby canine kidney D1 cells involves the utilization of different signaling components that are differentially sensitive to increases in cAMP or, alternatively, because of compartmentation of signaling components.