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

Tyrosine hydroxylase dephosphorylation by protein phosphatase 2A in bovine adrenal chromaffin cells

This study was undertaken to characterise the protein phosphatases in bovine adrenal chromaffin cells acting on tyrosine hydroxylase. Cells were pre-labelled with 32Pi and permeabilized with digitonin. The extent of dephosphorylation of Ser-8, Ser-19, Ser-31 and Ser-40 on tyrosine hydroxylase was found to be 30%, 38%, 37% and 71% respectively over 5 min. For Ser-19, Ser-31 and Ser-40 the dephosphorylation was entirely due to protein phosphatase 2A, as the dephosphorylation could be completely blocked by microcystin, but not by the protein phosphatase I inhibitory peptide. Permeabilization did not change the distribution of protein phosphatase 2A or tyrosine hydroxylase, or the activity of PP2A, from that occurring in intact cells. The dephosphorylation of Ser-8 was not altered by any inhibitor, suggesting the involvement of other protein phosphatases. The method developed here can be used to determine the protein phosphatases acting on substrates in conditions closely approximating those in situ, including the endogenous state of substrate phosphorylation and phosphatase location.

Annexin II tetramer: structure and function

The annexins are a family of proteins that bind acidic phospholipids in the presence of Ca2+. The interaction of these proteins with biological membranes has led to the suggestion that these proteins may play a role in membrane trafficking events such as exocytosis, endocytosis and cell-cell adhesion. One member of the annexin family, annexin II, has been shown to exist as a monomer, heterodimer or heterotetramer. The ability of annexin II tetramer to bridge secretory granules to plasma membrane has suggested that this protein may play a role in Ca(2+)-dependent exocytosis. Annexin II tetramer has also been demonstrated on the extracellular face of some metastatic cells where it mediates the binding of certain metastatic cells to normal cells. Annexin II tetramer is a major cellular substrate of protein kinase C and pp60src. Phosphorylation of annexin II tetramer is a negative modulator of protein function.

Polarized exocytosis in MDCK cells is regulated by phosphorylation

Protein phosphorylation and dephosphorylation systems modulate many cellular activities and have recently been implicated in the in vitro transport of newly synthesized proteins. Here we show that polarized transport from the Golgi to the plasma membrane in intact MDCK cells is regulated by phosphorylation-dephosphorylation. Transport is inhibited by the phosphatase inhibitor okadaic acid and is stimulated by the kinase inhibitor staurosporine. Stimulation of apical transport exceeds stimulation of basolateral transport by up to 5-fold. We also find that the G protein activator aluminum fluoride, which stimulates transport to the surface at low fluoride concentrations as previously reported, inhibits transport at higher concentrations. In the nonpolarized fibroblast cell line CV-1, neither staurosporine nor aluminum fluoride stimulates transport to the cell surface. Our results suggest that the phosphorylation-dephosphorylation system, like the G protein, may be involved in the specialized sorting process characteristic of polarized cells. We show some evidence that these two mechanisms of regulation may act through common intermediates.

Protein phosphatase inhibitors induce the release of serotonin from rat basophilic leukemia cells (RBL-2H3)

Effects of okadaic acid (OA) and calyculin-A (CL-A), selective inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A), on the release of serotonin from the rat basophilic leukemia cell line (RBL-2H3) were investigated. Both OA and CL-A induced the long-lasting release of serotonin in an extracellular Ca(2+)-independent manner. CL-A did not increase intracellular Ca2+ concentration in the fura-2-loaded cells. CL-A was 100-fold more potent than OA in inducing the release, suggesting that PP1 is a dominant protein phosphatase in regulating RBL-2H3 cells. The CL-A-induced release of serotonin was completely inhibited by the nonselective protein kinase inhibitors, staurosporine and K-252a. CL-A induced phosphorylation of several cellular proteins in RBL-2H3 cells, which could be inhibited by staurosporine. These findings suggest that the release of serotonin is subject to tonic, Ca(2+)-independent, inhibition by PP1 in RBL-2H3 cells.

Inhibition of serine/threonine protein phosphatases promotes opening of voltage-activated L-type Ca2+ channels in insulin-secreting cells

The biological activity of many proteins, including voltage-sensitive ion channels, is controlled by their state of phosphorylation. Ca2+ influx through voltage-activated L-type Ca2+ channels serves as the major stimulatory signal in insulin-secreting cells. We have now investigated the extent to which Ca2+ handling in clonal insulin-secreting RiNm5F cells was affected by okadaic acid, an inhibitor of various serine/threonine protein phosphatases. Whole-cell patch-clamp experiments showed that okadaic acid generated an increase in membrane current, suggesting that it promotes Ca2+ influx through L-type voltage-gated Ca2+ channels probably by modifying their phosphorylation state. Okadaic acid was found to provoke a transient rise in the cytoplasmic free Ca2+ concentration ([Ca2+]i) but had no further effect on the K(+)-induced increase. The Ca2+ transient induced by okadaic acid was dependent on the presence of extracellular Ca2+ and was abolished by D600, a blocker of voltage-activated L-type Ca2+ channels. Concomitant with the rise in [Ca2+]i, okadaic acid induced insulin secretion, a phenomenon that was also dependent on extracellular Ca2+. It is proposed that hyperphosphorylation of voltage-activated L-type Ca2+ channels in insulin-secreting cells lowers the threshold potential for their activation.

pp60c-src enhances the acetylcholine receptor-dependent catecholamine release in vaccinia virus-infected bovine adrenal chromaffin cells

Secretion of catecholamines by adrenal chromaffin cells is a highly regulated process that involves serine/threonine and tyrosine phosphorylations. The nonreceptor tyrosine kinase pp60c-src is expressed at high levels and localized to plasma membranes and secretory vesicle membranes in these cells, suggesting an interaction of this enzyme with components of the secretory process. To test the hypothesis that pp60c-src is involved in exocytosis, we transiently expressed exogenous c-src cDNA using a vaccinia virus vector in primary cultures of bovine adrenomedullary chromaffin cells. Chromaffin cells infected with a c-src recombinant virus restored the diminished secretory activity accompanying infection by wild type virus alone or a control recombinant virus. The level of enhanced catecholamine release correlated directly with the time and level of exogenous c-src expression. These results could not be attributed to differences in cytopathic effects of wild type versus recombinant viruses as assessed by cell viability assays, nor to differences in norepinephrine uptake or basal release, suggesting that pp60c-src is involved in stimulus-secretion coupling in infected cells. Surprisingly, exogenous expression of an enzymatically inactive mutant c-src also restored catecholamine release, indicating that regions of the introduced c-src protein other than the kinase domain may affect catecholamine release. Secretory activity was elevated by both forms of c-src in response to either nicotine or carbachol (which activate the nicotinic and the nicotinic/muscarinic receptors, respectively). In contrast, release of catecholamines upon membrane depolarization (as elicited by 55 mM K+) or by treatment with the calcium ionophore A23187 was unaffected by either vaccinia infection or increased levels of pp60c-src. These results suggest that pp60c-src affects secretory processes in vaccinia-infected cells that are activated through ligand-gated, but not voltage-gated, ion channels.

Characterization of serine/threonine protein phosphatases in RINm5F insulinoma cells

This study investigates the occurrence and regulation of serine/threonine protein phosphatases (PPases) in insulin-secreting RINm5F insulinoma cells. PPases types 1 and 2A were identified in crude RINm5F cell homogenates by both enzymatic assay and Western blot analysis. We then characterized and compared the inhibitory actions of several compounds isolated from cyanobacteria, marine dinoflagellates and marine sponges, (viz. okadaic acid, microcystin-LR, calyculin-A and nodularin) cation-independent PPase activities in RINm5F cell homogenates. It was found that okadaic acid was the least potent inhibitor (IC50 approximately 10(-9) M, IC100 approximately 10(-6) M), while the other compounds exhibited IC50 values of approximately 5 x 10(-10) M and IC100 approximately 5 x 10(-9) M. The findings indicate that the inhibitory substances employed in this study may be used pharmacologically to investigate the role of serine/threonine PPases in RINm5F cell insulin secretion, a process that is likely to be regulated to a major extent by protein phosphorylation.

Evidence against a role for alkaline phosphatase in the dephosphorylation of plasma membrane proteins: hypophosphatasia fibroblast study

A major impasse to understanding the physiologic role(s) of alkaline phosphatase (ALP) is uncertainty as to its natural substrates. Various in vitro studies have led other investigators to suggest that ALP functions as a plasma membrane phosphoprotein phosphatase, consistent with our demonstration of ecto-topography of ALP in a variety of cell types. Thus, we compared the phosphorylation of plasma membrane proteins from control fibroblasts to those from profoundly ALP-deficient fibroblasts of hypophosphatasia patients. Fibroblasts from 3 controls and 3 hypophosphatasia patients (ALP activity < 4% of control) were biosynthetically labeled with 32Pi for 2 h. 32P incorporation into total trichloroacetic acid (TCA)-precipitable material was not significantly different in control and patient cells. Plasma membranes were prepared from these cells by hypotonic shock, solubilized, and subjected to two-dimensional (2-D) gel electrophoretic separation. Video densitometric analysis of silver-stained 2-D gels failed to reveal any consistent difference in the protein profile between patient vs. control fibroblasts (i.e., unique species, altered pls, or increased abundance). Autoradiography of individual 2-D gels demonstrated 63 plasma membrane phosphoproteins with molecular weights ranging from 15 to 152 kDa and predominantly acidic pls. Although several of these phosphoproteins appeared to have had donor-specific labeling, none was unique or especially abundant in the hypophosphatasia group. Thus, in ALP-deficient fibroblasts, normal incorporation of 32P into total cellular protein and into all identifiable plasma membrane phosphoproteins indicates that ALP does not modulate the phosphorylation of plasma membrane proteins.

Protein phosphorylation at a postreceptor site can block desensitization and induce potentiation of secretion in chromaffin cells

Desensitization or habituation to repeated or prolonged stimulation is a common property of secretory cells. Phosphorylation of receptors mediates some desensitization processes, but the relationship of phosphorylation to desensitization at postreceptor sites is not well understood. We have tested the effect of protein phosphorylation on desensitization in bovine chromaffin cells. To increase protein phosphorylation, we have used the protein phosphatase inhibitor okadaic acid at 12.5 nM, 100 microM 8-bromo-cyclic AMP to activate protein kinase A, and 10 nM phorbol 12,13-dibutyrate to activate protein kinase C. During repeated 6-s stimulation at 5-min intervals, catecholamine secretion from control cells decreases. Cells exposed to 8-bromo-cyclic AMP or okadaic acid alone show slightly decreased rates of desensitization. In cells pretreated with phorbol 12,13-dibutyrate, desensitization is blocked. Okadaic acid-treated cells stimulated in the presence of 8-bromo-cyclic AMP show potentiation of secretion with repeated stimulation. The protein kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) increases the desensitization rate. Because these phenomena are observed during secretion evoked with elevated K+ as well as by a nicotinic agonist, the effect of phosphorylation is at a postreceptor site. In contrast to desensitization to the repeated stimulations, desensitization to prolonged stimulation with high K+ is not altered by the above protocols in chromaffin cells.

Effects of okadaic acid on insulin secretion from rat islets of Langerhans

We have studied the effects of the phosphatase inhibitor, okadaic acid, on insulin secretion and protein phosphorylation in intact and electrically permeabilized pancreatic islets. Okadaic acid inhibited glucose-induced insulin secretion from intact islets, although this effect was probably non-specific since similar effects were obtained using 1-nor-okadaone, a virtually inactive analogue of okadaic acid. In permeabilized islets, okadaic acid enhanced basal and cyclic-AMP-induced insulin secretion and protein phosphorylation. These results indicate that protein phosphatases may play a role in the regulation of insulin release.

Effects of okadaic acid indicate a role for dephosphorylation in pancreatic stimulus-secretion coupling

Okadaic acid completely inhibits phosphatase 2A at nanomolar concentrations, while complete inhibition of type 1 phosphatases occurs at 1 microM. Phosphatase 2B is significantly inhibited only at concentrations > 1 microM. In rat pancreatic acini, 1 microM okadaic acid shifted the cholecystokinin (CCK) dose-response curve for stimulating amylase release to the right without reducing maximal secretion. At 3 microM, okadaic acid inhibited maximal CCK-induced amylase release to 78 +/- 7% of control, whereas the inactive analogue 1-Nor-okadaone had no effect. Three lines of evidence indicate that this inhibition by okadaic acid occurs at a late step in stimulus-secretion coupling: 1) intracellular Ca2+ signaling in response to agonist stimulation was not appreciably altered by okadaic acid; 2) stimulation with phorbol ester plus thapsigargin (thus by-passing receptor activation), which gave 85 +/- 4% of maximal CCK-induced amylase release, was inhibited 66 +/- 4% by 3 microM okadaic acid; and 3) Ca(2+)-induced amylase secretion in streptolysin O-permeabilized cells was also reduced by 85 +/- 7%. Two-dimensional polyacrylamide gel electrophoresis of 32P-labeled acini and autoradiography demonstrated that okadaic acid dose dependently increased overall protein phosphorylation. Correspondingly, okadaic acid also led to an inhibition of CCK-induced dephosphorylation. These results show that okadaic acid inhibits pancreatic acinar secretion at a step after generation of intracellular messengers and indicate a role for protein dephosphorylation in stimulus-secretion coupling.

Anti-(14-3-3 protein) antibody inhibits stimulation of noradrenaline (norepinephrine) secretion by chromaffin-cell cytosolic proteins

Incubation of digitonin-permeabilized bovine chromaffin cells in the absence of Ca2+ results in a loss of both cytosolic proteins and Ca(2+)-dependent secretion. Addition of these leaked proteins prevents this loss of secretory activity. We have purified a protein from an extract of bovine adrenal medulla which can partially prevent this loss of Ca(2+)-dependent secretion. Antibody against this protein inhibited the ability of leaked chromaffin-cell proteins to prevent the loss of Ca(2+)-dependent secretion. Sequence analysis showed it to have sequence identity with bovine brain 14-3-3 protein. These results demonstrate that 14-3-3 protein makes a significant contribution to the ability of leaked chromaffin-cell proteins to maintain secretory activity.