Thrombin-, bradykinin-, and arachidonic acid-induced Ca2+ signaling in Ehrlich ascites tumor cells

Effects of carboxymethylpachymaran on signal molecules in chicken immunocytes

The study was carried out to investigate the immunomodulation mechanism of carboxymethylpachymaran (CMP). Chicken splenic lymphocytes were cultured in medium alone or with CMP at the final concentration of 50mg/L, 100mg/L, 200mg/L or 400mg/L in vitro for 4h, 8h, 12h or 24h, respectively. The supernatants at different culture periods were analyzed for changes in levels of 6-keto-prostaglandin F1α (6-keto-PGF1α), thromboxane B2 (TXB2) and nitric oxide (NO). The cells were collected to determine contents of oxidized glutathione (GSSG), reduced glutathione (GSH), cyclic AMP (cAMP) and cyclic GMP (cGMP). The results showed that CMP increase the values of NO, 6-keto-PGF1α, TXB2, and the ratio of 6-keto-PGF1α to TXB2 in supernatants. The contents of intracellular GSH, cAMP, cGMP and the ratio of cAMP to cGMP were increased in the cells treated with CMP. The results suggested that CMP enhanced immune functions by increasing the contents of GSH and by regulating arachidonic acid signal transduction systems in chicken splenic lymphocytes. The signal pathway of NO-cGMP plays an important role in CMP-induced activation of chicken splenic lymphocytes.

Gintonin, newly identified compounds from ginseng, is novel lysophosphatidic acids-protein complexes and activates G protein-coupled lysophosphatidic acid receptors with high affinity

Recently, we isolated a subset of glycolipoproteins from Panax ginseng, that we designated gintonin, and demonstrated that it induced [Ca2+]i transients in cells via G protein-coupled receptor (GPCR) signaling pathway(s). However, active components responsible for Ca2+ mobilization and the corresponding receptor(s) were unknown. Active component(s) for [Ca2+]i transients of gintonin were analyzed by liquid chromatography-electrospray ionization-tandem mass spectrometry and ion-mobility mass spectrometry, respectively. The corresponding receptor(s)were investigated through gene expression assays. We found that gintonin contains LPA C18:2 and other LPAs. Proteomic analysis showed that ginseng major latex-like protein and ribonuclease-like storage proteins are protein components of gintonin. Gintonin induced [Ca2+]i transients in B103 rat neuroblastoma cells transfected with human LPA receptors with high affinity in order of LPA2 >LPA5 > LPA1 > LPA3 > LPA4. The LPA1/LPA3 receptor antagonist Ki16425 blocked gintonin action in cells expressing LPA1 or LPA3. Mutations of binding sites in the LPA3 receptor attenuated gintonin action. Gintonin acted via pertussis toxin (PTX)-sensitive and -insensitive G protein-phospholipase C (PLC)-inositol 1,4,5-trisphosphate (IP3)-Ca2+ pathways. However, gintonin had no effects on other receptors examined. In human umbilical vein endothelial cells (HUVECs) gintonin stimulated cell proliferation and migration. Gintonin stimulated ERK1/2 phosphorylation. PTX blocked gintonin-mediated migration and ERK1/2 phosphorylation. In PC12 cells gintonin induced morphological changes, which were blocked by Rho kinase inhibitorY-27632. Gintonin contains GPCR ligand LPAs in complexes with ginseng proteins and could be useful in the development of drugs targeting LPA receptors.

Cholinergic agonists activate P2X7 receptors to stimulate protein secretion by the rat lacrimal gland

PURPOSE

To determine the interaction of M3 muscarinic receptors (M3AChR) and P2X(7) receptors to increase intracellular [Ca2+] ([Ca2+]i) and stimulate protein secretion in rat lacrimal gland cells.

METHODS

Exorbital lacrimal glands from male Sprague-Dawley rats were divided into pieces or digested with collagenase to form acinar clumps. [Ca2+]i was measured using an imaging system in acini incubated with fura-2/AM. Adenosine triphosphate (ATP) release was determined using the luciferin-luciferase reaction. Peroxidase secretion, our index for protein secretion, was measured spectrophotometrically. Acini were stimulated with the P2X7 receptor agonist, (benzoylbenzoyl)adenosine 5' triphosphate (BzATP), cholinergic agonist carbachol, or the activator of conventional and novel PKC isoforms, phorbol 12-myristate 13-acetate (PMA).

RESULTS

The increase in [Ca2+]i caused by carbachol and BzATP used simultaneously was less than additive, but the increase in protein secretion was additive. The M3AChR antagonist atropine blocked the BzATP-stimulated increase in [Ca2+]i and peroxidase secretion. The P2X7 antagonist did not alter the carbachol-stimulated increase in [Ca2+]i or peroxidase. PMA- and BzATP-stimulated increases in [Ca2+]i were additive. Neither constitutively active PKCα, dominant-negative PKCα, nor PKCε altered BzATP-stimulated increases in [Ca2+]i. Carbachol increased ATP release from lacrimal gland pieces but not from acini.

CONCLUSIONS

In lacrimal gland cells, the activation of M3AChRs stimulates P2X7 receptors to increase [Ca2+]i and protein secretion. The underlying mechanisms are unknown but could include the release of ATP or intracellular interactions not mediated by PKC isoforms. In addition, M3AChRs use signaling pathways that overlap with those used by P2X7 receptors to increase [Ca2+]i, but they also use signaling pathways not used by P2X7 receptors to stimulate protein secretion.

Direct coupling between arachidonic acid-induced Ca2+ release and Ca2+ entry in HEK293 cells

Arachidonic acid (AA) modulates intracellular Ca2+ signaling via Ca2+ release or/and Ca2+ entry. However, the mechanism underlies either process is unknown; nor is it clear as to whether the two processes are mechanistically linked. By using Fura2/AM, we found that AA induced mobilization of internal Ca2+ store and an increment in Ca2+, Mn2+ and Ba2+ influx in HEK293 cells. The AA-mediated Ca2+ signaling was not due to AA metabolites, and insensitive to capacitative Ca2+ entry inhibitors. Interestingly, isotetrandrine and Gd3+ inhibited both AA-induced Ca2+ release and Ca2+ entry in a concentration-dependent manner without affecting Ca2+ discharge caused by carbachol, caffeine, or thapsigargin. Additionally, similar pattern of inhibition was observed with tetracaine treatment. More importantly, the three compounds exhibited almost equal potent inhibition of AA-initiated Ca2+ release as well as Ca2+ influx. Therefore, this study, for the first time, provides evidence for a direct coupling between AA-mediated Ca2+ release and Ca2+ entry.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Arachidonic acid and docosahexaenoic acid suppress thrombin-evoked Ca2+ response in rat astrocytes by endogenous arachidonic acid liberation

Arachidonic (AA) and docosahexaenoic acid (DHA) are the major polyunsaturated fatty acids (PUFAs) in the brain. However, their influence on intracellular Ca2+ signalling is still widely unknown. In astrocytes, the amplitude of thrombin- induced Ca2+ response was time-dependently diminished by AA and DHA, or by the AA tetraynoic analogue ETYA, but not by eicosapentaenoic acid (EPA). Thrombin-elicited Ca2+ response was reduced (20-30%) by 1-min exposure to AA or DHA. Additionally, 1-min application of AA or DHA together with thrombin in Ca2+-free medium blocked Ca2+ influx, which followed after readdition of extracellular Ca2+. EPA and ETYA, however, were ineffective. Long-term treatment of astrocytes with AA and DHA, but not EPA reduced the amplitude of the thrombin-induced Ca2+ response by up to 80%. AA and DHA caused a comparable decrease in intracellular Ca2+ store content. Only DHA and AA, but not EPA or ETYA, caused liberation of endogenous AA by cytosolic phospholipase A2 (cPLA2). Therefore, we reasoned that the suppression of Ca2+ response to thrombin by AA and DHA could be due to release of endogenous AA. Possible participation of AA metabolites, however, was excluded by the finding that specific inhibitors of the different oxidative metabolic pathways of AA were not able to abrogate the inhibitory AA effect. In addition, thrombin evoked AA release via activation of cPLA2. From our data we propose a novel model of positive/negative-feed-back in which agonist-induced release of AA from membrane phospholipids promotes further AA release and then suppresses agonist-induced Ca2+ responses.

Signaling pathways underlying muscarinic receptor-induced [Ca2+]i oscillations in HEK293 cells

We have investigated the signaling pathways underlying muscarinic receptor-induced calcium oscillations in human embryonic kidney (HEK293) cells. Activation of muscarinic receptors with a maximal concentration of carbachol (100 microm) induced a biphasic rise in cytoplasmic calcium ([Ca2+]i) comprised of release of Ca2+ from intracellular stores and influx of Ca2+ from the extracellular space. A lower concentration of carbachol (5 microm) induced repetitive [Ca2+]i spikes or oscillations, the continuation of which was dependent on extracellular Ca2+. The entry of Ca2+ with 100 microm carbachol and with the sarcoplasmic-endoplasmic reticulum calcium ATPase inhibitor, thapsigargin, was completely blocked by 1 microm Gd3+, as well as 30-100 microm concentrations of the membrane-permeant inositol 1,4,5-trisphosphate receptor inhibitor, 2-aminoethyoxydiphenyl borane (2-APB). Sensitivity to these inhibitors is indicative of capacitative calcium entry. Arachidonic acid, a candidate signal for Ca2+ entry associated with [Ca2+]i oscillations in HEK293 cells, induced entry that was inhibited only by much higher concentrations of Gd3+ and was unaffected by 100 microm 2-APB. Like arachidonic acid-induced entry, the entry associated with [Ca2)]i oscillations was insensitive to inhibition by Gd3+ but was completely blocked by 100 microm 2-APB. These findings indicate that the signaling pathway responsible for the Ca2+) entry driving [Ca2+]i oscillations in HEK293 cells is more complex than originally thought, and may involve neither capacitative calcium entry nor a role for PLA2 and arachidonic acid.