Thiophosphorylation causes Ca2+-independent norepinephrine secretion from permeabilized PC12 cells

Vanadate induces calcium signaling, Ca2+ release-activated Ca2+ channel activation, and gene expression in T lymphocytes and RBL-2H3 mast cells via thiol oxidation

Using ratiometric Ca2+ imaging and patch-clamp measurement of Ca2+ channel activity, we investigated Ca2+ signaling induced by vanadium compounds in Jurkat T lymphocytes and rat basophilic leukemia cells. In the presence of external Ca2+, vanadium compounds produced sustained or oscillatory Ca2+ elevations; in nominally Ca2+-free medium, a transient Ca2+ rise was generated. Vanadate-induced Ca2+ signaling was blocked by heparin, a competitive inhibitor of the 1,4, 5-inositol trisphosphate (IP3) receptor, suggesting that Ca2+ influx is secondary to depletion of IP3-sensitive Ca2+ stores. In Jurkat T cells, vanadate also activated the Ca2+-dependent transcription factor, NF-AT. Intracellular dialysis with vanadate activated Ca2+ influx through Ca2+ release-activated Ca2+ (CRAC) channels with kinetics comparable to those of dialysis with IP3. Neither phosphatase inhibitors nor nonhydrolyzable nucleotide analogues modified CRAC channel activation. The action of vanadate, but not IP3, was prevented by the thiol-reducing agent DTT. In addition, the activation of CRAC channels by vanadate was mimicked by the thiol-oxidizing agent chloramine T. These results suggest that vanadate enhances Ca2+ signaling via thiol oxidation of a proximal element in the signal transduction cascade.

Alpha-granule secretion from alpha-toxin permeabilized, MgATP-exposed platelets is induced independently by H+ and Ca2+

In order to better understand granule release from platelets, we developed an alpha-toxin permeabilized platelet model to study alpha-granule secretion. Secretion of alpha-granules was analyzed by flow cytometry using P-selectin as a marker for alpha-granule release. P-selectin surface expression occurred when platelets were permeabilized in the presence of Ca2+. Responsiveness to Ca2+ was lost 30 min after permeabilization but could be reconstituted with MgATP. Alpha-toxin-permeabilized, MgATP-exposed platelets also degranulated within a pH range of 5.4-5.9 without exposure to and independent of Ca2+. ATP, GTP, CTP, UTP, and ITP supported Ca2+-induced alpha-granule secretion, while H+-induced alpha-granule secretion occurred only with ATP and GTP. Both Ca2+- and H+-induced alpha-granule secretion required ATP hydrolysis. Kinase inhibitors blocked both Ca2+- and H+-induced secretion. These data suggest that alpha-granule secretion in this permeabilized platelet system shares many characteristics with granule secretion studied in other permeabilized cell models. Furthermore, these results show that H+ can trigger alpha-granule release independent of Ca2+.

ATP is required in platelet serotonin exocytosis for protein phosphorylation and priming of secretory vesicles docked on the plasma membrane

Calcium-evoked secretion generally requires the presence of millimolar concentrations of Mg-ATP. We investigated the role of Mg-ATP in the secretion of serotonin from electropermeabilized bovine platelets. The secretion of serotonin was lost within 5 minutes when the Mg-ATP concentration was diluted to less than 0.1 mM, but was maintained when ATP-gamma S (adenosine 5′-O-3-thiotriphosphate) was used instead of ATP. Okadaic acid, a potent inhibitor of protein phosphatase, could also maintain the exocytotic activity even when ATP was diluted. Decrease in the secretory activity was paralleled by a decrease in phosphorylation level of four proteins after dilution of ATP, but the activity was maintained when the thiophosphorylation level of these proteins was maintained. Two of these proteins were digested by a protease, calpain, which has been shown to lead to a loss in the exocytotic activity. Electron microscopic studies showed that calcium did not induce the formation of distinct bridge-like structures between the granule membrane and the plasma membrane in Mg-ATP-diluted cells, previously shown as the structure transiently formed prior to fusion of the two membranes. Anchorage of the secretory dense granules to the plasma membrane and the presence of the amorphous structures between the granules and the plasma membrane were unchanged by dilution of ATP. These results indicate that ATP is not required for the anchorage itself, but is required to prime anchored granules for calcium-triggered secretion. Maintenance of the phosphorylated state of proteins by ATP enables the calcium trigger to form the bridge-like structures preceding membrane fusion events.

Phosphorylation of ATP-citrate lyase by nucleoside diphosphate kinase

Rat liver nucleoside diphosphate kinase (NDPK) and PC12 cell cytosol were used to determine whether NDPK could function as a protein kinase. NDPK was phosphorylated on its catalytic histidine using [gamma-32P]ATP, and the phosphorylated NDPK separated from [gamma-32P]ATP. The addition of phosphorylated NDPK to dialyzed PC12 cell cytosol resulted in the phosphorylation of a protein with a subunit molecular mass of about 120 kDa. This phosphorylation appeared to occur by a direct transfer of a phosphoryl group from the catalytic histidine of NDPK to a histidine on the 120-kDa protein. The 120-kDa protein was partially purified and shown by peptide sequencing to be ATP-citrate lyase. ATP-citrate lyase is the primary source of cytosolic acetyl-CoA. NDPK phosphorylated the histidine at the catalytic site of ATP-citrate lyase. This histidine can also be phosphorylated by ATP, and its phosphorylation is the first step in the conversion of citrate and CoA to oxaloacetate and acetyl-CoA by ATP-citrate lyase. The level of phosphorylation of PC12 cell ATP-citrate lyase by phosphorylated NDPK was comparable with that by ATP. Thus, in addition to its nucleoside diphosphate kinase activity, NDPK can function as a protein kinase.

Involvement of a phosphoprotein on the zymogen granule membrane in the control of regulated exocytosis in the exocrine pancreas

The pancreatic acinar cell is one of a number of cell types in which phosphoproteins are believed to be involved in the control of regulated exocytosis. We have examined the effects of three agents that affect secretion in the acinar cell on the phosphorylation states of proteins on the zymogen granule membrane. We show that Ca2+ and GTP gamma S, which stimulate secretion, also stimulate the phosphorylation of a protein of M(r) 45,000 (p45) on isolated zymogen granules. On the other hand, the protein kinase inhibitor genistein inhibits both secretion and phosphorylation of p45. For all three agents, p45 phosphorylation is affected over concentration ranges identical to those that affect secretion. The stimulatory effect of GTP gamma S and the inhibitory effect of genistein are also seen when the phosphorylation state of p45 on granules within permeabilized cells is examined. Ca2+, however, over the same concentration range, now causes dephosphorylation of p45. Furthermore, the time-course of this effect is similar to that of Ca(2+)-triggered secretion. Phosphorylation of p45 is exclusively on serine, with no detectable phosphorylation on either threonine or tyrosine. We propose that exocytosis in pancreatic acini is controlled at least in part through the phosphorylation/dephosphorylation of p45, with dephosphorylation acting as a trigger for exocytosis.

Guanine nucleotide stimulation of norepinephrine secretion from permeabilized PC12 cells: effects of Mg2+, other nucleotide triphosphates and N-ethylmaleimide

The addition of either Ca2+ or guanosine 5'-O-3-(thiotriphosphate), GTP gamma S, to digitonin-permeabilized rat pheochromocytoma PC12 cells stimulates norepinephrine release. Unlike Ca(2+)-stimulated release, there is a delay between the time of addition of GTP gamma S to digitonin-permeabilized PC12 cells and stimulation of norepinephrine release. Preincubation of the permeabilized cells in the absence of Mg2+ eliminates this lag and increases the initial rate of GTP-gamma S-stimulated norepinephrine secretion. This suggests that the rate of GDP dissociation from the GTP-binding protein responsible for this stimulation is faster in the absence of Mg2+ than in its presence. While an equimolar concentration of GTP gives 50% inhibition of GTP gamma S-stimulated release, 100-fold excesses of ITP, ATP, UTP and CTP gave no inhibition of GTP gamma S-stimulated release. Both the inability of ITP to inhibit GTP gamma S-stimulated secretion and the increase in GTP gamma S-stimulated secretion caused by preincubation in the absence of Mg2+ indicate that some of the properties of the GTP-binding protein responsible for this stimulation are more like those of the low molecular weight GTP-binding proteins rap1 and ras than those of a heterotrimeric G-protein. Low concentrations of N-ethylmaleimide gave more inhibition of GTP gamma S-stimulated release than Ca(2+)-stimulated release which suggests that the mechanisms by which Ca2+ and GTP gamma S stimulate norepinephrine release are at least in part distinct.

Effects of phosphatase inhibitors and a protein phosphatase on norepinephrine secretion by permeabilized bovine chromaffin cells

A protein phosphatase and phosphatase inhibitors were used to examine the role of protein phosphorylation in the regulation of norepinephrine secretion in digitonin-permeabilized bovine chromaffin cells. Addition of okadaic acid, a potent inhibitor of type 1 and type 2A protein phosphatases, or 1-naphthylphosphate, a more general phosphatase inhibitor, to digitonin-permeabilized chromaffin cells caused about a 100% increase in the amount of norepinephrine secreted in the absence of Ca2+ (in 5 mM EGTA) without affecting the amount of norepinephrine secreted in the presence of 10 microM free Ca2+. This stimulation of norepinephrine secretion by protein phosphatase inhibitors suggests that in the absence of Ca2+ there is a slow rate phosphorylation and that this phosphorylation triggers secretion. Addition of an exogenous type 2A protein phosphatase caused almost a 50% decrease in Ca(2+)-dependent norepinephrine secretion. Thus, the amounts of norepinephrine released both in the absence of Ca2+ and in the presence of Ca2+ appear to depend upon the level of protein phosphorylation.

Generation of inositol phosphates, cytosolic Ca2+, and secretion of noradrenaline in PC12 cells treated with glutamate

Glutamate transiently stimulated rat pheochromocytoma PC12 cells and caused an inositol trisphosphate formation and an increase in levels of Ca+ in the cytosol. The rank order of potency of glutamate greater than N-methyl-D-aspartate (NMDA) much greater than kainate = quisqualate is characteristic of an interaction with NMDA receptors. The effect of glutamate on inositol trisphosphate formation disappeared in a low Mg2+ buffer and was not blocked by DL-2-amino-5-phosphonovalerate, an antagonist for NMDA receptors coupled to ion channels. Although glutamate failed to stimulate noradrenaline secretion, glutamate enhanced the effect of bradykinin, but not of Ca ionophore A23187, or KCl. These results suggest the existence of metabotropic glutamate receptors, different from previously reported receptors, in PC12 cells.

Hydrolysis-resistant GTP analogs stimulate catecholamine release from digitonin-permeabilized PC12 cells

The effect of the hydrolysis-resistant GTP analogs, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and guanylyl imidodiphosphate (GMPPNP), on norepinephrine (NE) secretion from digitonin-permeabilized rat pheochromocytoma cells, PC12, was examined. Although secretion in the presence of saturating Ca2+ (10 microM) was not affected by GTP gamma S or GMPPNP, secretion in the absence of Ca2+ was stimulated by these GTP analogs. Secretion induced by saturating concentrations of GTP gamma S or GMPPNP was approximately 80% of that induced by 10 microM Ca2+. Half-maximum stimulation was induced by 30 microM GTP gamma S or GMPPNP. Both Ca2(+)-stimulated and GTP gamma S-stimulated secretion were ATP dependent and inhibited by N-ethylmaleimide. The GTP gamma S-stimulated secretion of NE from permeabilized PC12 cells does not appear to result from either the release of Ca2+ or the activation of protein kinase C. Activation of protein kinase C by pretreatment of intact cells with 12-O-tetradecanoylphorbol 13-acetate caused a 50% increase in both Ca2(+)-stimulated and GTP gamma S-stimulated secretion. Cholera and pertussis toxins did not affect Ca2(+)-stimulated or GTP gamma S-stimulated NE secretion. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) and GTP inhibited GTP gamma S-stimulated secretion but not Ca2(+)-stimulated secretion. The inability of GDP beta S to inhibit Ca2(+)-stimulated secretion indicates that the process affected by GTP gamma S is not an essential step in the Ca2(+)-stimulated pathway.

ATP-dependent regulation of phospholipase C in permeabilized 3T3 cells

Regulation of phospholipase C (PLC) coupled with a G-protein was studied with Swiss 3T3 cells permeabilized by digitonin. In permeabilized cells, activation of phospholipase C required millimolar concentrations of ATP in addition to a G-protein activator, AlF4- or nonhydrolysable GTP analogues. To determine the mechanism of the action of ATP, we examined the effects of ATP analogues. ATP gamma S directly activated phospholipase C in the presence or absence of AlF4-. On the other hand, neither beta,gamma-methylene ATP nor adenyl-5'-yl imidodiphosphate nor ADP beta S could support the AlF4(-)-dependent activation of phospholipase C. The action of ATP gamma S was not through the substrate supply for phospholipase C, because ATP gamma S did not augment the levels of PIP2 or PIP in permeabilized cells. These results suggested the significance of the gamma-phosphate group of ATP and/or phosphorylation by ATP in the activation of phospholipase C by a putative G-protein.