Positive and negative controls by protein kinases of sodium-dependent Ca2+ efflux from cultured bovine adrenal chromaffin cells stimulated by lysophosphatidic acid

Regulation of sodium-calcium exchanger activity by creatine kinase

It has been shown that in rat heart NCX1 exists in a macromolecular -complex including PKA, PKA-anchoring protein, PKC, and phosphatases PP1 and PP2A. In addition, several lines of evidence suggest that the interactions of the exchanger with other molecules are closely associated with its function in regulation of [Ca(2+)](i). NCX contains a large intracellular loop (NCXIL) that is responsible for regulating NCX activity. We used the yeast two-hybrid method to screen a human heart cDNA library and found that the C-terminal region of sarcomeric mitochondrial creatine kinase (sMiCK) interacted with NCX1IL. Among the four creatine kinase (CK) isozymes, both sMiCK and the muscle-type cytosolic creatine kinase (CKM) co-immunoprecipitated with NCX1. Both sMiCK and CKM were able to produce a recovery in the decreased NCX1 activity that was lost under energy-compromised conditions. This regulation is mediated through a putative PKC phosphorylation site of sMiCK and CKM. The catalytic activity of sMiCK and CKM is not required for their regulation of NCX1 activity. Our results suggest a novel mechanism for the regulation of NCX1 activity and a novel role for CK.

Regulation of sodium-calcium exchanger activity by creatine kinase under energy-compromised conditions

Na(+)/Ca(2+) exchanger (NCX) is one of the major mechanisms for removing Ca(2+) from the cytosol especially in cardiac myocytes and neurons, where their physiological activities are triggered by an influx of Ca(2+). NCX contains a large intracellular loop (NCXIL) that is responsible for regulating NCX activity. Recent evidence has shown that proteins, including kinases and phosphatases, associate with NCX1IL to form a NCX1 macromolecular complex. To search for the molecules that interact with NCX1IL and regulate NCX1 activity, we used the yeast two-hybrid method to screen a human heart cDNA library and found that the C-terminal region of sarcomeric mitochondrial creatine kinase (sMiCK) interacted with NCX1IL. Moreover, both sMiCK and the muscle-type creatine kinase (CKM) coimmunoprecipitated with NCX1 using lysates of cardiacmyocytes and HEK293T cells that transiently expressed NCX1 and various creatine kinases. Both sMiCK and CKM were able to produce a recovery in the decreased NCX1 activity that was lost under energy-compromised conditions. This regulation is mediated through a putative PKC phosphorylation site of sMiCK and CKM. The autophosphorylation and the catalytic activity of sMiCK and CKM are not required for their regulation of NCX1 activity. Our results suggest a novel mechanism for the regulation of NCX1 activity.

Mechanisms of the lysophosphatidic acid-induced increase in [Ca(2+)](i) in skeletal muscle cells

Although lysophosphatidic acid (LPA) is known to increase intracellularfree calcium concentration ([Ca²⁺]_i) in different cell types, the effect of LPA on the skeletal muscle cells is not known. The present study was therefore undertaken to examine the effect of LPA on the [Ca²⁺]_i in C2C12 cells. LPA induced a concentration and time dependent increase in [Ca²⁺]_i, which was inhibited by VPC12249, VPC 32183 and dioctanoyl glycerol pyrophosphate, LPA1/3 receptor antagonists. Pertussis toxin, a Gⁱ protein inhibitor, also inhibited the LPA-induced increase in [Ca²⁺]_i. Inhibition of tyrosine kinase activities with tyrphostin A9 and genistein also prevented the increase in [Ca²⁺]_i due to LPA. Likewise, wortmannin and LY 294002, phosphatidylinositol 3-kinase (PI3-K) inhibitors, inhibited [Ca²⁺]_i response to LPA. The LPA effect was also attenuated by ethylene glycolbis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), an extracellular Ca²⁺ chelator, Ni²⁺ and KB-R7943, inhibitors of the Na⁺-Ca²⁺ exchanger; the receptor operated Ca²⁺ channel (ROC) blockers, 2-aminoethoxydiphenyl borate and SK&F 96365. However, the L-type Ca²⁺ channel blockers, verapamil and diltiazem; the store operated Ca²⁺ channel blockers, La³⁺ and Gd³⁺; a sarcoplasmic reticulum calcium pump inhibitor, thapsigargin; an inositol trisphosphate receptor antagonist, xestospongin and a phospholipase C inhibitor, U73122, did not prevent the increase [Ca²⁺]_i due to LPA. Our data suggest that the LPA-induced increase in [Ca²⁺]_i might occur through Gⁱ-protein coupled LPA_1/3 receptors that may be linked to tyrosine kinase and PI3-K, and may also involve the Na⁺-Ca²⁺ exchanger as well as the ROC. In addition, LPA stimulated C2C12 cell proliferation via PI3-K. Thus, LPA may be an important phospholipid in the regulation of [Ca²⁺]_i and growth of skeletal muscle cells.

Lysophospholipids elevate [Ca2+]i and trigger exocytosis in bovine chromaffin cells

Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are responsible for many physiological functions, including angiogenesis, neuronal survival, and immunity. However, little is known about their effects in modulating the stimulus-secretion coupling in bovine chromaffin cells. The result of PCR showed that at least two receptors (S1P(3) and LPA(1)) were expressed in bovine chromaffin cells. The elevation of [Ca(2+)](i) by S1P was fast and sustaining; but the elevation by LPA was slow and transient. The EC(50) for S1P and LPA in elevating the [Ca(2+)](i) were 0.55+/-0.01 and 0.54+/-0.40microM, respectively. This elevation could be totally blocked by thapsigargin, 2-APB, and U73122. Pertussis toxin pretreatment inhibited about half of the elevation in [Ca(2+)](i) suggesting the involvement of G(i) and other G-proteins. Repetitive [Ca(2+)](i) elevations elicited by S1P, but not LPA, were inhibited by ryanodine. S1P was more effective than LPA in triggering exocytosis as measured by the changes in membrane capacitance. The whole-cell Ca(2+) current was inhibited by both lysophospholipids but Na(+) current was inhibited by S1P only. These results suggest the differential effects of LPA and S1P in releasing Ca(2+) from the intracellular Ca(2+) stores and modulating the stimulus-secretion coupling in bovine chromaffin cells.

Mechanisms of lysophosphatidic acid-induced increase in intracellular calcium in vascular smooth muscle cells

Although lysophosphatidic acid (LPA) is known to cause an increase in intracellular Ca2+ concentration ([Ca2+]i) in vascular smooth muscle cells (VSMCs), the mechanisms of [Ca2+]i mobilization by LPA are not fully understood. In the present study, the effect of LPA on [Ca2+]i mobilization in cultured A10 VSMCs was examined by Fura-2 fluorescence technique. The expression of LPA receptors was studied by immunostaining. LPA was observed to increase [Ca2+]i in a concentration-dependent manner; this increase was dependent on the concentration of extracellular Ca2+. Both sarcolemmal (SL) Na(+)-Ca2+ exchange inhibitors (amiloride, Ni2+ and KB-R7943) and Na(+)-H+ exchange inhibitor (MIA) as well as SL store-operated Ca2+ channel (SOC) antagonists (SK&F 96365, tyrphostin A9 and gadolinium), unlike SL Ca2+ channel antagonists (verapamil and diltiazem), inhibited the LPA-induced increase in [Ca2+]i. In addition, sarcoplasmic reticulum (SR) Ca2+ channel blocker (ryanodine), SR Ca2+ channel opener (caffeine), SR Ca2+ pump ATPase inhibitor (thapsigargin) and inositol 1,4,5-trisphosphate (InsP3) receptor antagonists (xestospongin and 2-aminoethoxydiphenyl borate) were found to inhibit the LPA-induced Ca2+ mobilization. Furthermore, phospholipase C (PLC) inhibitor (U 73122) and protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate) attenuated the LPA-induced increase in [Ca2+]i. These results indicate that Ca2+ mobilization by LPA involves extracellular Ca2+ entry through SL Na(+)-Ca2+ exchanger, Na(+)-H+ exchanger and SL SOCs. In addition, ryanodine-sensitive and InsP(3)-sensitive intracellular Ca2+ pools may be associated with the LPA-induced increase in [Ca2+]i. Furthermore, the LPA-induced [Ca2+]i mobilization in VSMCs seems to be due to the activation of both PLC and PKC.

Mechanisms of extracellularly regulated kinases 1/2 activation in adrenal glomerulosa cells by lysophosphatidic acid and epidermal growth factor

The regulation of adrenal function, including aldosterone production from adrenal glomerulosa cells, is dependent on a variety of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). In many cell types, GPCR-mediated MAPK activation is mediated through transactivation of RTKs, in particular the epidermal growth factor (EGF) receptor (EGF-R). However, the extent to which this cross-communication between GPCRs and RTKs is operative in the adrenal glomerulosa has not been defined. Bovine adrenal glomerulosa cells express receptors for lysophosphatidic acid (LPA) and EGF. In cultured bovine adrenal glomerulosa cells, LPA, which is predominantly coupled to Gi and partially to Gq/protein kinase C alpha and epsilon, caused phosphorylation of Src (at Tyr416), proline-rich tyrosine kinase (Pyk2 at Tyr402), EGF-R, protein kinase B/Akt, extracellularly regulated signal kinases 1/2, and their dependent protein, p90 ribosomal S6 kinase. Overexpression of dominant negative mutants of Ras or EGF-R, and selective inhibition of EGF-R kinase with AG1478, significantly reduced LPA-induced ERK1/2 phosphorylation. However, this was not impaired by inhibition of matrix metalloproteinase (MMP) and heparin-binding EGF. LPA-induced ERK1/2 activation occurs predominantly through EGF-R transactivation by Gi/Src and partly through activation of protein kinase C, which acts downstream of EGF-R and Ras. In contrast, LPA-induced phosphorylation of Shc and ERK1/2 in clonal hepatocytes (C9 cells) was primarily mediated through MMP-dependent transactivation of the EGF-R. These observations in adrenal glomerulosa and hepatic cells demonstrate that LPA phosphorylates ERK1/2 through EGF-R transactivation in a MMP-dependent or -independent manner in individual target cells. This reflects the ability of GPCRs expressed in cell lines and neoplastic cells to utilize distinct signaling pathways that can elicit altered responses compared with those of native tissues.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Lysophospholipid receptors in the nervous system

The lysophospholipid mediators, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P), are responsible for cell signaling in diverse pathways including survival, proliferation, motility, and differentiation. Most of this signaling occurs through an eight-member family of G-protein coupled receptors once known as the endothelial differentiation gene (EDG) family. More recently, the EDG receptors have been divided into two subfamilies: the lysophosphatidic acid subfamily, which includes LPA1, (EDG-2/VZG-1), LPA2 (EDG-4), and LPA3 (EDG-7), and the sphingosine-1-phosphate receptor subfamily, which includes S1P1 (EDG-1), S1P2 (EDG-5/H218/AGR16), S1P3 (EDG-3), S1P4 (EDG-6), and S1P5 (EDG-8/NRG-1). The ubiquitous expression of these receptors across species, coupled with their diverse cellular functions, has made lysophospholipid receptors an important focus of signal transduction research. Neuroscientists have recently begun to explore the role of lysophospholipid receptors in a number of cell types; this research has implicated these receptors in the survival, migration, and differentiation of cells in the mammalian nervous system.

Effects of decrease of extracellular sodium in carbachol-evoked catecholamine secretion in isolated adrenal medullae of rats

The effect of extracellular Na(+) deprivation on the carbachol-evoked catecholamine secretion was evaluated in chromaffin cells. Isolated adrenal medullae of male Wistar rats were incubated in solutions with different sodium concentrations (144,0; 75,0; 25,0 and psi mM). Catecholamine secretions inversely increased as a response to fall of extracellular concentration of sodium. The magnitude of response to cholinergic stimulus (carbachol 100 microM) was decreased in low extracellular sodium concentration. Atropine (100 microM) inhibited secretion of catecholamine induced by carbachol in the presence and in the absence of extracellular sodium. Results suggest that in isolated adrenal medullae of rats (1) decrease in concentration of extracellular sodium increases secretion of catecholamines, perhaps by a greater influx of calcium from the extracellular environment through reversal of Na(+) /Ca(2+) exchanger; (2) intensity of catecholamine secretion induced by cholinergic stimulus seems to depend on extracellular sodium.

Electrospray ionization mass spectrometry analysis of lysophospholipids in human ascitic fluids: comparison of the lysophospholipid contents in malignant vs nonmalignant ascitic fluids

Lysophospholipids (lyso-PLs), including various glycerol-based and sphingosine-based lysophospholipids, play important roles in many biochemical, physiological, and pathological processes. The classical methods to analyze these lipids involve gas chromatography and/or high-performance liquid chromatography, which are time-consuming, cumbersome, and sometimes inaccurate due to the incomplete separation of closely related lipid species. We now describe the quantitative analysis of lyso-PLs in ascites samples from patients with ovarian cancer using electrospray ionization spectrometry. Three new classes of lyso-PL molecules are detected: alkyl-LPA, alkenyl-LPA, and methylated lysophosphatidylethanolamine. Importantly, the following lysophospholipid species are significantly increased in ascites from patients with ovarian cancer, compared to patients with nonmalignant diseases (e.g., liver failure): LPA (including acyl-, alkyl-, and alkenyl-LPA species), lysophosphatidylinositol, and sphingosylphosphorylcholine. Lysophosphorylcholine contents are also significantly different among ascitic fluids from the two groups of patients. However, the total phosphate content in ascites samples from patients with ovarian cancer is not significantly different compared to that from patients with nonmalignant disease.

Activation of protein kinase calpha in the lysophosphatidic acid-induced bovine sperm acrosome reaction and phospholipase D1 regulation

Protein kinase C (PKC) has been implicated in the sperm acrosome reaction. In the present study, we demonstrate induction of the acrosome reaction and activation of sperm PKCalpha by lysophosphatidic acid (LPA), which is known to induce signal transduction cascades in many cell types via binding to specific cell-surface receptors. Under conditions by which LPA activates PKCalpha, there is significant stimulation of the acrosome reaction, which is inhibited by PKC inhibitors. Protein kinase Calpha belongs to the Ca(2+)-dependent classical PKC family of isoforms, and indeed we show that its activation depends upon the presence of Ca(2+) in the incubation medium. Protein kinase Calpha is a known regulator of phospholipase D (PLD). We investigated the possible regulatory relationships between PKCalpha and PLD1. Using specific antibodies against PLD1, we demonstrate for the first time its presence in bovine sperm. Furthermore, PLD1 coimmunoprecipitates with PKCalpha and the PKCalpha-PLD1 complex decomposes after treatment of the cells with LPA or 12-O:-tetradecanoyl phorbol-13-acetate, resulting in the translocation of PKCalpha to the plasma membrane and translocation of PLD1 to the particulate fraction. A possible bilateral regulation of PKCalpha and PLD1 activation during the sperm acrosome reaction is suggested.

Stimulatory effect of enkephalins on calcium efflux from bovine adrenal chromaffin cells in culture

The effects of leucine- and methionine-enkephalin, opiate peptides, on Ca2+ efflux from cultured bovine adrenal chromaffin cells were examined. These enkephalins stimulated the efflux of 45Ca2+ from cells in a concentration-dependent manner (10(-8) M-10(-6) M). Leucine-enkephalin did not increase the intracellular free Ca2+ level, 45Ca2+ uptake, catecholamine secretion, cAMP level or cGMP level. The peptide-stimulated 45Ca2+ efflux was not inhibited by incubation in Ca2+-free medium, but was inhibited by incubation in Na+-free medium. These results indicate that enkephalins stimulate extracellular Na+-dependent 45Ca2+ efflux from cultured bovine adrenal chromaffin cells, probably by stimulating membrane Na+/Ca2+ exchange.

Protein kinase C-mediated up-regulation of Na+/Ca2+-exchanger in rat hepatocytes determined by a new Na+/Ca2+-exchanger inhibitor, KB-R7943

The regulatory mechanism of the plasma membrane Na+/Ca2+-exchanger in isolated rat hepatocytes was studied using microspectrofluorometry and 45Ca2+ uptake methods. Exposure of single hepatocytes to low-Na+ solutions induced an increase in the intracellular Ca2+ concentration ([Ca2+]i) which depended on the presence of extracellular Ca2+. 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), a novel selective inhibitor of Na+/Ca2+-exchangers, inhibited the initial rate of [Ca2+]i increase induced by exposure to the low-Na+ solution (IC50 = 2 microM). KB-R7943 also reduced the initial rate of 45Ca2+ uptake (IC50 = 4 microM). The increase in [Ca2+]i induced by exposure to the low-Na+ solution was inhibited by pre-incubation with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7, 50 microM), but not with N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H-8, 60 microM) or a tyrosine kinase inhibitor, genistein (100 microM). Furthermore, taurocholate and phorbol-12,13-dibutyrate, both of which activate protein kinase C, promoted the increase in [Ca2+]i. These [Ca2+]i increases were sensitive to KB-R7943. Our results indicate that the Na+/Ca2+-exchanger is up-regulated via protein kinase C. The activity of Na+/Ca2+-exchangers is not evident under normal physiological conditions, suggesting that the exchanger may be activated under pathophysiological conditions.

Endothelin-1 stimulates sodium-dependent calcium efflux from bovine adrenal chromaffin cells in culture

1. The effect of endothelin (ET)-1 on Ca2+ efflux from cultured bovine adrenal chromaffin cells was examined. ET-1 (10(-7) M) significantly increased intracellular free Ca2+ level ([Ca2+]i), 45Ca2+ uptake and catecholamine secretion in the cells. 2. ET-1 stimulated the efflux of 45Ca2+ from the cells preloaded with 45Ca2+ in a concentration-dependent manner (10(-9)-10(-7) M). This stimulatory effect was inhibited by ET(B) receptor antagonist BQ788, but not by ET(A) receptor antagonist BQ123. Selective ET(B) receptor agonists Suc-[Glu9, Ala11.5]-ET-1 and sarafotoxin S6c (SRTX) also stimulated 45Ca2+ efflux from the cells. 3. ET-1, Suc-[Glu9 Ala11.15]-ET-1 and SRTX increased the level of cyclic GMP in the adrenal chromaffin cells. ET-1 induced an increase in the nitric oxide (NO) level in the cells. The stimulatory effects by which ET- increases NO level and 45Ca2+ efflux were inhibited by NG-monomethyl-L-arginine acetate (L-NMMA), a competitive inhibitor of NO synthase. 4. The 45Ca2+ efflux stimulated by ET-1 was inhibited by deprivation of extracellular Na+, but not by deprivation of Ca2+. 5. These results suggest that ET-1 stimulates an extracellular Na+-dependent Ca2+ efflux through the activation of NO synthase in cultured bovine adrenal chromaffin cells.