Inositol trisphosphate as a second messenger in signal transduction

Role of Gi proteins in the regulation of blood pressure and vascular remodeling

Heterotrimeric guanine nucleotide regulatory proteins (G-proteins) through the activation of several signaling mechanisms including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol (PI) turnover. regulate a variety of cellular functions, including vascular reactivity, proliferation and hypertrophy of VSMC. Activity of adenylyl cyclase is regulated by two G proteins, stimulatory (Gsα) and inhibitory (Giα). Gsα stimulates adenylyl cyclase activity and increases the levels of cAMP, whereas Giα inhibits the activity of adenylyl cyclase and results in the reduction of cAMP levels. Abnormalities in Giα protein expression and associated adenylyl cyclase\cAMP levels result in the impaired cellular functions and contribute to various pathological states including hypertension. The expression of Giα proteins is enhanced in various tissues including heart, kidney, aorta and vascular smooth muscle cells (VSMC) from genetic (spontaneously hypertensive rats (SHR)) and experimentally - induced hypertensive rats and contribute to the pathogenesis of hypertension. In addition, the enhanced expression of Giα proteins exhibited by VSMC from SHR is also implicated in the hyperproliferation and hypertrophy, the two key players contributing to vascular remodelling in hypertension. The enhanced levels of endogenous vasoactive peptides including angiotensin II (Ang II), endothelin-1 (ET-1) and growth factors contribute to the overexpression of Giα proteins in VSMC from SHR. In addition, enhanced oxidative stress, activation of c-Src, growth factor receptor transactivation and MAP kinase/PI3kinase signaling also contribute to the augmented expression of Giα proteins in VSMC from SHR. This review summarizes the role of Giα proteins, and the underlying molecular mechanisms implicated in the regulation of high blood pressure and vascular remodelling.

Pharmacological tools to investigate inositol polyphosphate kinases - Enzymes of increasing therapeutic relevance

Inositol poly- and pyrophosphates (InsPs and PP-InsPs) are a group of central eukaryotic metabolites and signaling molecules. Due to the diverse cellular functions and widespread diseases InsPs and PP-InsPs are associated with, pharmacological targeting of the kinases involved in their biosynthesis has become a significant research interest in the last decade. In particular, the development of inhibitors for inositol hexakisphosphate kinases (IP6Ks) has leaped forward, while other inositol phosphate kinases have received scant attention. This review summarizes the efforts undertaken so far for discovering potent and selective inhibitors for this diverse group of small molecule kinases. The benefits of pharmacological inhibition are highlighted, given the multiple kinase-independent functions of inositol phosphate kinases. The distinct structural families of InsP and PP-InsP kinases are presented, and we discuss how compound availability for different inositol phosphate kinase families varies drastically. Lead compound discovery and optimization for the inositol kinases would benefit from detailed structural information on the ATP-binding sites of these kinases, as well as reliable biochemical and cellular read-outs to monitor inositol phosphate kinase activity in complex settings. Efforts to further tune well-established inhibitors, while simultaneously reviving tool compound development for the more neglected kinases from this family are indisputably worthwhile, considering the large potential therapeutic benefits.

Natriuretic peptide receptor-C-mediated attenuation of vascular smooth muscle cell hypertrophy involves Gqα/PLCβ1 proteins and ROS-associated signaling

Hypertension is associated with vascular remodeling due to hyperproliferation and hypertrophy of vascular smooth muscle cells (VSMC). Recently, we showed the implication of enhanced expression of Gqα and PLCβ1 proteins in hypertrophy of VSMCs from 16‐week‐old spontaneously hypertensive rats (SHR). The aim of this study was to investigate whether C‐ANP_4‐23, a natriuretic peptide receptor‐C (NPR‐C) ligand that was shown to inhibit vasoactive peptide‐induced enhanced protein synthesis in A10 VSMC could also attenuate hypertrophy of VSMC isolated from rat model of cardiac hypertrophy and to further explore the possible involvement of Gqα/PLCβ1 proteins and ROS‐mediated signaling in this effect. The protein synthesis and cell volume, markers of hypertrophy were significantly enhanced in VSMC from 16‐week‐old SHR compared with age‐matched WKY rats and C‐ANP_4‐23 treatment attenuated both to WKY levels. In addition, C‐ANP_4‐23 treatment also attenuated the enhanced expression of AT1 receptor, Gqα, PLCβ1, Nox4, and p47^phox proteins, the enhanced activation of EGFR, PDGFR, IGF‐1R, enhanced phosphorylation of ERK1/2/AKT and c‐Src in VSMC from SHR. Furthermore, the enhanced levels of superoxide anion and NADPH oxidase activity exhibited by VSMC from SHR were also attenuated to control levels by C‐ANP_4‐23 treatment. These results indicate that C‐ANP_4‐23 via the activation of NPR‐C attenuates VSMC hypertrophy through decreasing the overexpression of Gqα/PLCβ1 proteins, enhanced oxidative stress, increased activation of growth factor receptors, and enhanced phosphorylation of MAPK/AKT signaling pathways. Thus, it can be suggested that C‐ANP_4‐23 may be used as a therapeutic agent for the treatment of vascular complications associated with hypertension and atherosclerosis.

InsP3 Signaling in Apicomplexan Parasites

Background:

Phosphoinositides (PIs) and their derivatives are essential cellular components that form the building blocks for cell membranes and regulate numerous cell functions. Specifically, the ability to generate myo-inositol 1,4,5-trisphosphate (InsP3) via phospholipase C (PLC) dependent hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to InsP3 and diacylglycerol (DAG) initiates intracellular calcium signaling events representing a fundamental signaling mechanism dependent on PIs. InsP3 produced by PI turnover as a second messenger causes intracellular calcium release, especially from endoplasmic reticulum, by binding to the InsP3 receptor (InsP3R). Various PIs and the enzymes, such as phosphatidylinositol synthase and phosphatidylinositol 4-kinase, necessary for their turnover have been characterized in Apicomplexa, a large phylum of mostly commensal organisms that also includes several clinically relevant parasites. However, InsP3Rs have not been identified in genomes of apicomplexans, despite evidence that these parasites produce InsP3 that mediates intracellular Ca2+ signaling.

Conclusion:

Evidence to supporting IP3-dependent signaling cascades in apicomplexans suggests that they may harbor a primitive or non-canonical InsP3R. Understanding these pathways may be informative about early branching eukaryotes, where such signaling pathways also diverge from animal systems, thus identifying potential novel and essential targets for therapeutic intervention.

A Miniaturized Column Chromatography Method for Measuring Receptor-Mediated Inositol Phosphate Accumulation

Inositol phosphates (IPs), such as 1,4,5-inositol-trisphosphate (IP3), comprise a ubiquitous intracellular signaling cascade initiated in response to G protein-coupled receptor-mediated activation of phospholipase C. Classical methods for measuring intracellular accumulation of these molecules include time-consuming high-performance liquid chromatography (HPLC) separation or large-volume, gravity-fed anion-exchange column chromatography. More recent approaches, such as radio-receptor and AlphaScreen™ assays, offer higher throughput. However, these techniques rely on measurement of IP3itself, rather than its accumulation with other downstream IPs, and often suffer from poor signal-to-noise ratios due to the transient nature of IP3. The authors have developed a miniaturized, anion-exchange chromatography method for measuring inositol phosphate accumulation in cells that takes advantage of signal amplification achieved through measuring IP3and downstream IPs. This assay uses centrifugation of 96-well-formatted anion-exchange mini-columns for the isolation of radiolabeled inositol phosphates from cell extracts, followed by low-background dry-scintillation counting. This improved assay method measures receptor-mediated IP accumulation with signal-to-noise and pharmacological values comparable to the classical large-volume, column-based methods. Assay validation data for recombinant muscarinic receptor 1, galanin receptor 2, and rat astrocyte metabotropic glutamate receptor 5 are presented. This miniaturized protocol reduces reagent usage and assay time as compared to large-column methods and is compatible with standard 96-well scintillation counters.

Oxidative stress contributes to the enhanced expression of Gqα/PLCβ1 proteins and hypertrophy of VSMC from SHR: role of growth factor receptor transactivation

We showed previously that vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHRs) exhibit overexpression of Gqα/PLCβ1 proteins, which contribute to increased protein synthesis through the activation of MAP kinase signaling. Because oxidative stress has been shown to be increased in hypertension, the present study was undertaken to examine the role of oxidative stress and underlying mechanisms in enhanced expression of Gqα/PLCβ1 proteins and VSMC hypertrophy. Protein expression was determined by Western blotting, whereas protein synthesis and cell volume, markers for VSMC hypertrophy, were determined by [3H]-leucine incorporation and three-dimensional confocal imaging, respectively. The increased expression of Gqα/PLCβ1 proteins, increased protein synthesis, and augmented cell volume exhibited by VSMCs from SHRs were significantly attenuated by antioxidants N-acetyl-cysteine (NAC), a scavenger of superoxide anion, DPI, an inhibitor of NAD(P)H oxidase. In addition, PP2, AG1024, AG1478, and AG1295, inhibitors of c-Src, insulin-like growth factor receptor (IGFR), epidermal growth factor receptor (EGFR), and platelet-derived growth factor receptor (PDGFR), respectively, also attenuated the enhanced expression of Gqα/PLCβ1 proteins and enhanced protein synthesis in VSMCs from SHRs toward control levels. Furthermore, the levels of IGF-1R and EGFR proteins and not of PDGFR were also enhanced in VSMCs from SHRs, which were attenuated significantly by NAC, DPI, and PP2. In addition, NAC, DPI, and PP2 also attenuated the enhanced phosphorylation of IGF-1R, PDGFR, EGFR, c-Src, and EKR1/2 in VSMCs from SHRs. These data suggest that enhanced oxidative stress in VSMCs from SHRs activates c-Src, which through the transactivation of growth factor receptors and MAPK signaling contributes to enhanced expression of Gqα/PLCβ1 proteins and resultant VSMC hypertrophy.

Enhanced expression of Gqα and PLC-β1 proteins contributes to vascular smooth muscle cell hypertrophy in SHR: role of endogenous angiotensin II and endothelin-1

Vascular Gqα signaling has been shown to contribute to cardiac hypertrophy. In addition, angiotensin II (ANG II) was shown to induce vascular smooth muscle cell (VSMC) hypertrophy through Gqα signaling; however, the studies on the role of Gqα and PLC-β1 proteins in VSMC hypertrophy in animal model are lacking. The present study was therefore undertaken to examine the role of Gqα/PLC-β1 proteins and the signaling pathways in VSMC hypertrophy using spontaneously hypertensive rats (SHR). VSMC from 16-wk-old SHR and not from 12-wk-old SHR exhibited enhanced levels of Gqα/PLC-β1 proteins compared with age-matched Wistar-Kyoto (WKY) rats as determined by Western blotting. However, protein synthesis as determined by [3H]leucine incorporation was significantly enhanced in VSMC from both 12- and 16-wk-old SHR compared with VSMC from age-matched WKY rats. Furthermore, the knockdown of Gqα/PLC-β1 in VSMC from 16-wk-old SHR by antisense and small interfering RNA resulted in attenuation of protein synthesis. In addition, the enhanced expression of Gqα/PLC-β1 proteins, enhanced phosphorylation of ERK1/2, and enhanced protein synthesis in VSMC from SHR were attenuated by the ANG II AT1 and endothelin-1 (ET-1) ETA receptor antagonists losartan and BQ123, respectively, but not by the ETB receptor antagonist BQ788. In addition, PD98059 decreased the enhanced expression of Gqα/PLC-β1 and protein synthesis in VSMC from SHR. These results suggest that the enhanced levels of endogenous ANG II and ET-1 through the activation of AT1 and ETA receptors, respectively, and MAP kinase signaling, enhanced the expression of Gqα/PLC-β1 proteins in VSMC from 16-wk-old SHR and result in VSMC hypertrophy.

Emerging roles of Orai3 in pathophysiology

Calcium (Ca(2+)) is a ubiquitous second messenger that regulates a plethora of physiological functions. Deregulation of calcium homeostasis has been reported in a wide variety of pathological conditions including cardiovascular disorders, cancer and neurodegenerative diseases. One of the most ubiquitous pathways involved in regulated Ca(2+) influx into cells is the store-operated Ca(2+) entry (SOCE) pathway. In 2006, Orai1 was identified as the channel protein that mediates SOCE in immune cells. Orai1 has two mammalian homologs, Orai2 and Orai3. Although Orai1 has been the most widely studied Orai isoform, Orai3 has recently received significant attention. Under native conditions, Orai3 was demonstrated to be an important component of store-independent arachidonate-regulated Ca(2+) (ARC) entry in HEK293 cells, and more recently of a store-independent leukotrieneC4-regulated Ca(2+) (LRC) entry pathway in vascular smooth muscle cells. Recent studies have shown upregulation of Orai3 in estrogen receptor-expressing breast cancers and a critical role for Orai3 in breast cancer development in immune-compromised mice. Orai3 upregulation was also shown to contribute to vascular smooth muscle remodeling and neointimal hyperplasia caused by vascular injury. Furthermore, Orai3 has been shown to contribute to proliferation of effector T-lymphocytes under oxidative stress. In this review, we will discuss the role of Orai3 in reported pathophysiological conditions and will contribute ideas on the potential role of Orai3 in native Ca(2+) signaling pathways and human disease.

Role of vasoactive peptides in high glucose-induced increased expression of Gαq/11 proteins and associated signaling in vascular smooth muscle cells

We have recently shown that A10 vascular smooth muscle cells (VSMCs) exposed to high glucose exhibited enhanced expression of G(alpha)q and PLCbeta proteins. Since high glucose has been reported to increase the levels of vasoactive peptides and oxidative stress, the present study was undertaken to investigate the implication of angiotensin II (Ang II), endothelin (ET)-1, and oxidative stress in the high glucose-induced enhanced expression of G(alpha)q/11 and PLCbeta proteins and associated signaling in A10 VSMCs. The levels of G(alpha)q, G(alpha)11, PLCbeta-1, and PLCbeta-2 proteins, as determined by Western blotting, were significantly higher in A10 VSMCs exposed to high glucose than in control cells. The elevated levels were restored to control values by the antioxidant diphenyleneiodonium (DPI), as well as by the antagonist of Ang II AT1 receptor losartan and the antagonists of ETA and ETB receptors BQ123 and BQ788, respectively. In addition, ET-1-stimulated production of inositol trisphosphate (IP3), which was enhanced by high glucose, was also restored toward control levels by DPI. Furthermore, the enhanced production of superoxide anion (O2-), increased NADPH oxidase activity, and enhanced expression of p22phox and p47phox proteins induced by high glucose were restored to control levels by losartan, BQ123, and BQ788. These results suggest that through increased oxidative stress, high glucose-induced enhanced levels of endogenous Ang II and ET-1 may contribute to the increased levels of G(alpha)q/11 and mediated signaling in A10 VSMCs.

Role of growth factor receptor transactivation in high glucose-induced increased levels of Gq/11alpha and signaling in vascular smooth muscle cells

We have recently shown that high glucose increased the expression of Gq/11alpha, PLCbeta and mediated signaling in A10 vascular smooth muscle cells (VSMC). Since high glucose has been shown to increase growth factor receptor activation, we investigated the role of epidermal growth factor receptor (EGF-R) and platelet-derived growth factor receptor (PDGF-R) transactivation in high glucose-induced enhanced expression of Gq/11alpha and PLCbeta. Pre-treatment of A10 VSMC with high glucose (26 mM) for 3 days, increased the levels of Gqalpha, G11alpha, PLCbeta-1 and PLCbeta-2 proteins which were restored to control levels by AG1478, an inhibitor of EGF-R, AG1295, an inhibitor of PDGF-R and PP2, an inhibitor of c-Src but not by PP3. In addition, endothelin-1 (ET-1)-stimulated production of IP(3) that was enhanced by high glucose was also restored towards control levels by AG1478, AG1295 and PP2. High glucose also increased the phosphorylation of EGF-R and PDGF-R which was abolished by AG1478, AG1295 and PP2. Furthermore, high glucose-induced enhanced levels of Gqalpha, G11alpha and PLCbeta were also attenuated by PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) and wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K). In addition, AG1478 and AG1295, also attenuated high glucose-induced enhanced phosphorylation of ERK1/2 and AKT. Furthermore, high glucose augmented the phosphorylation of c-Src which was attenuated by antioxidant, DPI. These results suggest that oxidative stress through the activation of c-Src and resultant transactivation of growth factor receptor contributes to the high glucose-induced enhanced expression of Gq/11alpha/PLC and -mediated cell signaling through MAPK/PI3K pathway.

High glucose increases the expression of Gq/11alpha and PLC-beta proteins and associated signaling in vascular smooth muscle cells

The levels and activity of protein kinase C and diacylglycerol were shown to be upregulated in diabetes/hyperglycemia; however, studies on the expression of upstream signaling molecules of phosphatidylinositol turnover were lacking. The present study was therefore undertaken to examine whether hyperglycemia/diabetes could also modulate the expression of Gqalpha and phospholipase C-beta (PLC-beta) proteins and associated phosphatidylinositol turnover signaling in aortic vascular smooth muscle cells (VSMCs) and A10 VSMCs exposed to high glucose. Aortic VSMCs from streptozotocin-diabetic rats exhibited an increased expression of Gqalpha and PLC-beta1 proteins (60% and 30%, respectively) compared with control cells as determined by Western blot analysis. The pretreatment of A10 VSMCs with high glucose (26 mM) for 3 days also augmented the levels of Gqalpha, G11alpha, PLC-beta1 and -beta2 proteins by about 50, 35, 30, and 30%, respectively, compared with control cells that were restored to control levels by endothelin-1 (ET-1), ET types A and B (ET(A) and ET(B)) receptors, and angiotensin II type 1 (AT1) receptor antagonists. In addition, ET-1-stimulated inositol triphosphate formation was also significantly higher in VSMCs exposed to high glucose, whereas the basal levels of inositol triphosphate were not different between the two groups. Furthermore, the treatment of A10 VSMCs with angiotensin II and ET-1 also significantly increased the levels of Gq/11alpha and PLC-beta proteins that were restored toward control levels by ET(A)/ET(B) and AT1 receptor antagonists. These results suggest that high glucose augments the expression of Gq/11alpha, PLC-beta, and mediated signaling in VSMCs, which may be attributed to AT1, ET(A), and ET(B) receptors.

Voltage-dependent mechanisms for raising intracellular free calcium concentration: calcium channels

Single-channel recording by means of the patch-clamp technique provides a method for observing the kinetic properties of individual ion channels. Three distinct types of voltage-dependent calcium channels have so far been characterized: T-type channels are responsible for a rapidly inactivating, transient Ca2+ current; L-type channel openings produce a long-lasting Ca2+ current. N-type channels have the kinetic features of neither T- nor L-type channels. In addition to differences in their gating and conductance properties the three types of channels can also be distinguished by their different sensitivities to pharmacological intervention. The respective distribution of the various types of Ca2+ channels may vary from tissue to tissue. For example, more L-type Ca2+ channels may open during excitation in cardiac and smooth muscle cells than in nerve cells, where the other channel types may prevail. This would explain the different sensitivities of these tissues to dihydropyridines. In view of their functional significance it will be of considerable interest to explore in greater detail the respective densities and sensitivities to neurotransmitters and drugs of various Ca2+ channels in different tissues.

Involvement of non-selective Ca2+ channels in the contraction induced by alkalinization of rat anococcygeus muscle cells

Intracellular pH is a modulator of cellular functions such as smooth muscle contraction. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) associated with contraction are brought about by Ca(2+) influx and release from the sarcoplasmic reticulum, and alterations in the intracellular pH can affect both processes. In this work, therefore, we have investigated the Ca(2+) influx pathway that contributes to the contraction induced by the alkalinizing agent NH(4)Cl in the rat anococcygeus smooth muscle. For this purpose, we measured the isometric tension in muscle preparations, and [Ca(2+)](c) was measured on isolated cells loaded with 5 micromol/l FURA2/AM by using the ratio 340/380 nm. NH(4)Cl (10 mmol/l) induced a larger increase in [Ca(2+)](c) (100%) when compared with the [Ca(2+)](c) increase induced by 0.1 micromol/l phenylephrine (57.0+/-12.3% n=4). Incubation of the muscle preparations for 1 min in Ca(2+)-free medium reduced the contractions induced by 10 mmol/l NH(4)Cl to 11.5+/-5.1% (n=5), when compared with the contractions induced in 2.5 mmol/l Ca(2+) solution (100%). After 3 min in Ca(2+) free medium, contractions stimulated with NH(4)Cl were almost abolished (0.6+/-0.4%, n=5). In the same way, incubation with 10 micromol/l 1-[beta-[3[(4-methoxyphenyl)propoxyl]-4-methoxy-phenetyl]-1H-imidazole hydrochloride (SKF96365), a non-selective Ca(2+) channels, reduced the contractions stimulated with NH(4)Cl to 47.6+/-6.7% (n=7). On the other hand, 1 micromol/l verapamil, a voltage-operated Ca(2+) channel blocker and 0.05 micromol/l calphostin C, a protein kinase-C inhibitor, did not alter the contractions induced by NH(4)Cl. On isolated cells, [Ca(2+)](c) was reduced to 72.2+/-1.7% (n=4) by 10 micromol/l SKF96365. Taken together, our results suggest that NH(4)Cl induces contraction of rat anococcygeus smooth muscle cells, as well as [Ca(2+)](c) increase due to Ca(2+) influx through non-selective Ca(2+) channels.

P2Y purinergic receptors regulate the growth of human melanomas

Adenosine 5'-triphosphate is known to function as a potent extracellular messenger producing its effects via a distinct family of cell surface receptors. Different receptor subtypes have been shown to modulate different cellular functions such as proliferation, differentiation and apoptosis. We investigated the functional expression and proliferative action of metabotropic P2Y receptors in human melanoma tissue and cells. Expression of functional P2Y1, P2Y2 and P2Y6 receptor subtypes was established by reverse transcriptase polymerase chain reaction, immunohistochemistry and intracellular calcium measurements using a Fluorometric Imaging Plate Reader. Incubation of A375 melanoma cells with the P2Y1 receptor-selective agonist 2-methylthioadenosine-5-diphosphate caused a decrease in cell number which was dose-dependent, whereas incubation with the P2Y2 receptor agonist uridine triphosphate caused a dose-dependent increase in cell number. The action of extracellular nucleotides on P2Y receptors was shown to mediate the growth of melanomas and the P2Y1 receptor is a putative target for melanoma therapy.

Novel diacylglycerol kinase inhibitor selectively suppressed an U46619-induced enhancement of mouse portal vein contraction under high glucose conditions

1. Diacylglycerol kinase (DG kinase) is a key enzyme in vascular contraction; however, alterations of the regulatory mechanisms in vascular dysfunction are poorly understood. In this study, the effect of a novel DG kinase inhibitor, stemphone, on vascular contraction was investigated. 2. The conventional DG kinase inhibitor, 6-[2-(4-[(4-fluorophenyl)phenyl-methylene]-1-piperidinyl)ethyl]-7-methyl-5H-thiazolo [3,2-alpha] pyrimidine-5-one (R59022) (0.1-30 microm), inhibited thromboxane A(2) analogue 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha) (U46619)-induced sustained contractions in mouse aorta and porcine coronary artery in a dose-dependent manner. Treatment with stemphone did not affect contractions in these tissues. However, stemphone significantly inhibited (>0.3 microm) U46619-induced spontaneous phasic contraction in mouse portal vein. This inhibitory effect was not detected following R59022 treatment in portal vein. Therefore, stemphone demonstrated selectivity in terms of portal vein contraction. 3. Under high glucose (22.2 mm) conditions, U46619-induced contraction was enhanced in these three types of vascular tissue. Inhibitory effects of R59022 were attenuated under these conditions; however, effects of stemphone were observed. These results indicated that stemphone could inhibit portal vein contraction under high glucose conditions, for example, diabetes. These data suggested the possibility that DG kinase may be a target of hyperportal pressure. 4. Total mass of DG was enhanced under high glucose conditions. DG was derived from incorporated glucose via de novo synthesis in the absence of phospholipase C pathway mediation. This enhanced DG under high glucose conditions activated a calcium-independent protein kinase C (PKC). This PKC was associated with calcium-independent DG kinase activation. Treatment with stemphone also inhibited calcium-independent DG kinase. These signal transduction pathways were distinguishable from a DG-PKC pathway under normal glucose conditions. 5. The present investigation suggested that stemphone selectively inhibited overcontraction of portal vein induced by high glucose levels. This phenomenon was attributable to inhibition of calcium-independent DG kinase activation that occurred under high glucose conditions mediated by both DG synthesized from glucose and calcium-independent PKC activation.

Ca2+ oscillations in hepatocytes do not require the modulation of InsP3 3-kinase activity by Ca2+

Receptor-mediated production of inositol 1,4,5-trisphosphate (InsP(3)) initiates Ca(2+) release and is responsible for cytosolic Ca(2+) oscillations. InsP(3) oscillations have also been observed in some cells. One of the enzymes controlling InsP(3) catabolism, the InsP(3) 3-kinase, is stimulated by Ca(2+); this regulation is presumably part of the reason for InsP(3) oscillations that have been observed in some cells. Here, we investigate the possible role of Ca(2+)-activated InsP(3) catabolism on the characteristics of the InsP(3)-induced Ca(2+) oscillations. Numerical simulations show that if it is assumed that the Ca(2+)-independent InsP(3) catabolism is predominant, Ca(2+) oscillations remain qualitatively unchanged although the relative amplitude of the oscillations in InsP(3) concentrations becomes minimal. We tested this prediction in hepatocytes by masking the Ca(2+)-dependent InsP(3) catabolism by 3-kinase through the injection of massive amounts of InsP(3) 5-phosphatase, which is not stimulated by Ca(2+). We find that in such injected hepatocytes, Ca(2+) oscillations generated by modest agonist levels are suppressed, presumably because of the decreased dose in InsP(3), but that at higher doses of agonist, oscillations reappear, with characteristics similar to those of untreated cells at low agonist doses. Altogether, these results suggest that oscillations in InsP(3) concentration due to Ca(2+)-stimulated InsP(3) catabolism do not play a major role for the oscillations in Ca(2+) concentration.

Lecithin and choline in human health and disease

Choline is involved in methyl group metabolism and lipid transport and is a component of a number of important biological compounds including the membrane phospholipids lecithin, sphingomyelin, and plasmalogen; the neurotransmitter acetylcholine; and platelet activating factor. Although a required nutrient for several animal species, choline is not currently designated as essential for humans. However, recent clinical studies show it to be essential for normal liver function. Additionally, a large body of evidence from the fields of molecular and cell biology shows that certain phospholipids play a critical role in generating second messengers for cell membrane signal transduction. This process involves a cascade of reactions that translate an external cell stimulus such as a hormone or growth factor into a change in cell transport, metabolism, growth, function, or gene expression. Disruptions in phospholipid metabolism can interfere with this process and may underlie certain disease states such as cancer and Alzheimer's disease. These recent findings may be appropriate in the consideration of choline as an essential nutrient for humans.

Histamine potentiates N-methyl-D-aspartate responses in acutely isolated hippocampal neurons

N-methyl-D-aspartate (NMDA)-evoked currents were recorded from acutely isolated rat hippocampal neurons, using the whole-cell patch-clamp technique and a rapid perfusion system. Histamine, at concentrations from 0.5 to 100 microM, reversibly enhanced NMDA currents by up to 50%. The effect cannot be ascribed to activation of the known histamine receptors (H1, H2, H3) but is occluded by spermine. These results suggest an interaction of histamine with the polyamine-binding site on the NMDA receptor complex. This modulatory action could allow the histaminergic system to determine time and loci of NMDA receptor-mediated events, such as memory formation according to behavioral state.

Interaction of substance P and its N- and C-terminal fragments with Ca2+: implications for hormone action

In an attempt to understand the role of Ca2+ on the bioactive conformation of peptide hormones, we have examined the interaction between Ca2+ and the neuropeptide substance P. Using CD spectroscopy to monitor conformational changes caused by Ca2+ binding, we found no significant binding of the cation by substance P in water. However, a substantial conformational change occurred in the hormone on Ca2+ addition in trifluoroethanol or an 80:20 (v/v) mixture of acetonitrile and trifluoroethanol. A biphasic binding of Ca2+ was observed in these solvents with saturation at 2 cations per hormone molecule. Mg2+ caused a relatively smaller conformational change in the hormone. A peptide corresponding to residues 1-7 at the N-terminal fragment of substance P showed a weak nonsaturating binding of Ca2+ in the nonpolar solvents whereas the 7-11 C-terminal fragment peptide displayed a binding indicative of an 1:1 Ca2+/peptide complex. Ca2+ binding by the hormone and the 7-11 fragment was also monitored by changes in fluorescence of the phenylalanyl residues. The results support the conclusion drawn from the CD data about a distinct Ca2+ binding site in the C-terminal part of substance P. The Kd values obtained from fluorescence data were 160 microM for Ca2+ and 1 mM for Mg2+ binding by substance P. The hormone and the two peptide fragments were also tested for their effect on the stability of dimyristoyl lecithin vesicles. Substance P and the N-terminal fragment caused no significant leakage of either fluorescent dyes or K+ trapped in the vesicles. Nor did they cause membrane fusion as monitored by the fluorescence quenching method.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of Kupffer cell membrane phospholipid function by n-3 polyunsaturated fatty acids

Dietary n-3 polyunsaturated fatty acids (PUFAs) have been reported to improve clinical outcome in a number of inflammatory diseases including burns and sepsis. One mechanism contributing to the anti-inflammatory effect is the incorporation of n-3 PUFAs into membrane phospholipids which decreases macrophage eicosanoid production. We hypothesize that an additional mechanism for their effects is an alteration of membrane signal transduction that decreases macrophage responsiveness to inflammatory stimuli. Kupffer cells, the fixed macrophages of the liver, were obtained from rats pair fed diets for 6 weeks with 15% of calories supplied as menhaden (high n-3), corn (control), or safflower (high n-6) oils. The effects of the dietary oils on Kupffer cell membrane signal transduction and eicosanoid production were assessed by measuring inositol phospholipid (PI) metabolism, intracellular calcium responses, and prostaglandin E2 (PGE2) production to the inflammatory signals endotoxin (LPS) and platelet activating factor (PAF). The menhaden oil diet resulted in significant incorporation of n-3 PUFAs into total cellular PUFAs compared to corn and safflower oil. (total n-3 PUFAs, 28.1% menhaden vs 2.1% corn vs 1.2% safflower, P less than 0.03). This incorporation altered signal transduction of PAF as both PI turnover (65% +/- 10% of corn oil) and calcium response (0.6-fold vs 5.0-fold for corn oil) were significantly reduced in the menhaden oil group. (P less than 0.05) The menhaden oil diet also reduced significantly PGE2 production in response to PAF and LPS (corn, 348 +/- 23 pg/ml; menhaden, 48 +/- 6 pg/ml, P less than 0.01). We conclude that, in addition to modulating eicosanoid production, n-3 PUFAs can also alter macrophage membrane signal transduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of omega-3 and omega-6 fatty acids in the ether- and ester-linked phosphoglycerides from tissues of the rainbow trout, Salmo gairdneri

1. Data presented here demonstrate that polyunsaturated fatty acids in the phospholipids of rainbow trout tissues are compartmentalized differently than in mammalian tissues. 2. We have determined the distribution of omega-3 (n-3) and omega-6 (n-6) fatty acids in the alkyl-, alk-1-enyl-, and diacyl- subclasses of phosphatidylcholines (PC), phosphatidyl-ethanolamines (PE), phosphatidylinositols (PI), and phosphatidylserines (PS) from gill, kidney and spleen of rainbow trout. 3. Alkyl-linked PC and alk-1-enyl-linked PE were the most abundant ether-containing phospholipids, amounting to 10-15% of each class; no ether-linked PI or PS was detected. 4. C20:4 n-6 was found in high concentrations only in PI; the n-3 fatty acids were found in highest concentration in the ether-linked phospholipids as compared with the diacyl subclasses and C20:5 n-3 was especially prevalent in 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine and C22:6 n-3 was prevalent in PS.

Evidence that ATP-dependent Ca2+ transport in rat parotid microsomal membranes requires charge compensation

ATP-dependent Ca2+ transport was investigated in a rat parotid microsomal-membrane preparation enriched in endoplasmic reticulum. Ca2+ uptake, in KCl medium, was rapid, linear with time up to 20 s, and unaffected by the mitochondrial inhibitors NaN3 and oligomycin. This Ca2+ uptake followed Michaelis-Menten kinetics, and was of high affinity (Km approximately 38 nM) and high capacity (approximately 30 nmol/min per mg of protein). In the presence of oxalate, Ca2+ uptake continued to increase for at least 5 min, reaching an intravesicular accumulation approx. 10 times higher than without oxalate. Ca2+ uptake was dependent on univalent cations in the order K+ = Na+ greater than trimethylammonium+ greater than mannitol and univalent anions in the order Cl- greater than acetate- greater than Br- = gluconate- = NO3- greater than SCN-. Ca2+ uptake was not elevated if membranes were incubated in the presence of a lipophilic anion (NO3-) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Ca2+ transport was altered by changes in the K+-diffusion potential of the membranes. A relatively negative K+-diffusion potential increased the initial rate of Ca2+ accumulation, whereas a relatively positive potential decreased Ca2+ accumulation. In the presence of an outwardly directed K+ gradient, nigericin had no effect on Ca2+ uptake. In aggregate, these studies suggest that the ATP-dependent Ca2+-transport mechanism present in rat parotid microsomal membranes exhibits an electrogenic Ca2+ flux which requires the movement of other ions for charge compensation.