Evidence for receptor-specific regulation of the metabolism of the second messenger, inositol trisphosphate, in human endothelial cells

d-myo-Inositol 1-phosphate as a surrogate of d-myo-inositol 1,4,5-tris phosphate to monitor G protein-coupled receptor activation

Phospholipase C beta (PLC-beta)-coupled G protein-coupled receptor (GPCR) activities traditionally are assessed by measuring Ca2+ triggered by D-myo-inositol 1,4,5-trisphosphate (IP3), a PLC-beta hydrolysis product, or by measuring the production of inositol phosphate using cumbersome radioactive assays. A specific detection of IP3 production was also established using IP3 binding proteins. The short lifetime of IP3 makes this detection very challenging in measuring GPCR responses. Indeed, this IP3 rapidly enters the metabolic inositol phosphate cascade. It has been known for decades that lithium chloride (LiCl) leads to D-myo-inositol 1-phosphate accumulation on GPCR activation by inhibiting inositol monophosphatase, the final enzyme of the IP3 metabolic cascade. We show here that IP1 can be used as a surrogate of IP3 to monitor GPCR activation. We developed a novel homogeneous time-resolved fluorescence (HTRF) assay that correlates perfectly with existing methods and is easily amenable to high-throughput screening. The IP-One assay was validated on various GPCR models. It has the advantage over the traditional Ca2+ assay of allowing the measurement of inverse agonist activity as well as the analysis of PLC-beta activity in any nontransfected primary cultures. Finally, the high assay specificity for D-myo-inositol 1 monophosphate (IP1(1)) opens new possibilities in developing selective assays to study the functional roles of the various isoforms of inositol phosphates.

Protein tyrosine kinases regulate agonist-stimulated prostacyclin release but not von Willebrand factor secretion from human umbilical vein endothelial cells

The rapid synthesis and release of prostacyclin (PGI2) and the exocytotic secretion of von Willebrand Factor (vWF) elicited by activation of G-protein-coupled receptors on endothelium occur via signaling mechanisms which are incompletely defined. Activation of protein tyrosine kinases (PTKs) and modulation of the tyrosine-phosphorylation state of endogenous proteins have been implicated in several cellular processes including arachidonate release and exocytosis. In the present study we have examined the regulatory role of PTKs in agonist-stimulated release of PGI2 and vWF from human umbilical vein endothelial cells (HUVECs) using two chemically and mechanistically dissimilar PTK inhibitors (genistein and ST271). Genistein, but not the less active analogue daidzein, dose-dependently attenuated PGI2 release in response to thrombin and histamine (IC50 approx. 20 microM), and to the thrombin-receptor-activating peptide. A more potent inhibition of thrombin- and histamine-induced PGI2 synthesis was observed in cells exposed to ST271. In contrast, neither genistein nor ST271 modulated agonist-drive vWF secretion. At concentrations that abolished PGI2 release, genistein blocked thrombin- or histamine-evoked tyrosine phosphorylation of a 42 kDa protein. Ca2+ ionophore-induced PGI2 generation, but not vWF secretion, was also inhibited by both genistein and ST271, suggesting that these agents modulate PGI2 synthesis by acting at, or distal to, agonist-induced changes in intracellular CA2+ ([Ca2+]i). In fura-2-loaded HUVECs genistein partially reduced the histamine-induced peak [Ca2+]i but had no effect on the thrombin response. Ca(2+)-induced PGI2 release from electrically permeabilized HUVECs was abolished in the presence of ST271 or genistein, but not diadzein. The generation of PGI2 in response to exogenous arachidonic acid was not modulated by genistein or ST271, suggesting that PTK inhibitors do not directly inhibit cyclo-oxygenase activity. Taken together, these results suggest that PTKs regulate PGI2 synthesis and release in HUVECs by modulating, directly or indirectly, a CA(2+)-sensitive step upstream of cyclo-oxygenase.

Interactions between inositol trisphosphate and Ca2+ dependent Ca2+ release mechanisms on the endoplasmic reticulum of permeabilised bovine aortic endothelial cells

In this paper we describe data from cultured bovine aortic endothelial (BAE) cells demonstrating a Ca2+ induced Ca2+ release (CICR) process which appears to have pharmacological properties different from CICR mechanisms in other cell types. CICR was measured in saponin permeabilised cells in which the internal stores had been preloaded with 45Ca2+. Step increases in the free Ca2+ concentration of the bathing solution, from 10 nM up to 10 microM were found to increase 45Ca2+ loss. This process was completely inhibited by ruthenium red. Caffeine induced a small release of 45Ca2+ and the response to a subsequent stimulation with a Ca2+ step was reduced. In intact cells, ryanodine activated small oscillations in intracellular Ca2+ in the presence, but not the absence, of external Ca2+. However, in permeabilised cells, ryanodine had no effect on either basal efflux or the increased efflux of 45Ca2+ seen following a step increase in free Ca2+. These data suggest the operation of a ruthenium red sensitive but ryanodine insensitive CICR mechanism on the endoplasmic reticulum (ER) which may also be modulated by caffeine. An IP3 dependent 45Ca2+ release was also observed. In the presence of ruthenium red, the IP3 induced 45Ca2+ release was reduced suggesting that CICR may operate to amplify the magnitude of the IP3 response. The Ca2+ dependence of the IP3 induced release was also measured. Co-operativity between IP3 and Ca2+ could not be detected between 100-300 nM Ca2+. The results suggest that the regulation of IP3 induced Ca2+ release may be different in BAE cells, and point to the operation of a 'novel' CICR process and to complex interactions between Ca2+ release systems in BAE cells.

Biphasic formation of inositol phosphates in opsonized zymosan-stimulated human neutrophils

Stimulation by serum-opsonized zymosan (SOZ) typically causes a biphasic rise in the cytosolic free Ca2+ concentration ([Ca2+]i) of human neutrophils. It consists of an initial slow Ca2+ release from internal pools lasting for 60 s, followed by a rapid but sustained influx of Ca2+. It was the aim of this study to elucidate the underlying mechanism of this atypical Ca2+ response. For this reason we analysed the production of inositol phosphates (InsPs) in myo-[3H]inositol labelled cells. Stimulation by SOZ within 10 s transiently elevated inositol trisphosphate (InsP3) by 1.50-fold. This response was followed by a second, more sustained 1.55-fold rise in InsP3 by 90 s. A similar, biphasic pattern of inositol tetrakisphosphate (InsP4) formation with 1.15- and 1.35-fold increases, respectively, was observed. The SOZ-induced formation of InsP3 was unaffected by the removal of extracellular Ca2+ by 1.4 mM EGTA. In contrast, the early accumulation of InsP4 was stronger and more prolonged and no second rise over the baseline level was seen in the absence of extracellular Ca2+. Under these conditions, the sudden exposure of Fura-2 AM loaded, SOZ-stimulated neutrophils to extracellular Ca2+ at a time point where InsP4 was the predominant InsP resulted in a marked increase in [Ca2+]i. Recalcification at a time point when InsP3 was the major InsP had no effect on [Ca2+]i. These findings suggest that in SOZ-stimulated neutrophils (1) the transient, first accumulation of InsP3 mediates the slow Ca2+ release from internal pools, and (2) the second, more pronounced formation of InsP4 triggers the Ca2+ influx.(ABSTRACT TRUNCATED AT 400 WORDS)

Coexpression of P2Y and P2U receptors on aortic endothelial cells. Comparison of cell localization and signaling pathways

Depending on the vascular bed considered, the actions of ATP on the endothelium are mediated by either P2Y or P2U receptors. The two types of receptors seem to coexist on bovine aortic endothelial cells, where they are both coupled to phospholipase C. In this study, we have investigated whether they are truly coexpressed on the same cells and whether their signaling pathways diverge beyond phospholipase C activation. Measurements of [Ca2+]i in single cells showed that almost all bovine aortic endothelial cells are responsive to both 2-methylthio-ATP (2MeSATP), an agonist of P2Y receptors, and UTP, an agonist of P2U receptors. UTP stimulated the release of prostacyclin from freshly isolated bovine aortic endothelial cells, even when they were exposed to cycloheximide at the time of their collection: this indicates that P2U receptors must already be expressed on endothelial cells in situ and do not appear during cell culture. The time course of inositol phosphate (InsP) accumulation and the relative proportion of Ins(1,4,5)P3, Ins(1,3,4,5)P4, and Ins(1,3,4)P3 were similar in cells stimulated by 2MeSATP or UTP. UTP and 2MeSATP both stimulated the hydrolysis of phosphatidylcholine by phospholipase D, as reflected by the release of [3H]choline from prelabeled cells. The responses to both agents were blocked after downregulation of protein kinase C, resulting from a prolonged exposure to phorbol 12-myristate 13-acetate: this blockade occurred at a step distal to phospholipase C activation. A single difference between the two pathways has been identified: the effect of 2MeSATP on InsP3 was significantly more inhibited after a short exposure to phorbol 12-myristate 13-acetate than that of UTP.(ABSTRACT TRUNCATED AT 250 WORDS)