Effects of antagonists of protein phosphatases on superoxide release by neutrophils.

Neutrophils stimulated with 4 beta-phorbol 12-myristate 13-acetate (PMA) release large quantities of superoxide (O2-) and exhibit phosphorylation of two proteins with molecular masses of 47(p47) and 49 kDa (p49). Addition of inhibitors of protein kinases (e.g. 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7)) to these cells after stimulation with PMA results in the loss of 32P from these proteins and a rapid cessation of O2- release (e.g. Heyworth, P. G., and Badwey, J. A. (1990) Biochim. Biophys. Acta 1052, 299-305). In this paper we report that antagonists of type 1 and 2A protein phosphatases (okadaic acid, calyculin A) prevented both the loss of 32P from p47 and the termination of O2- release in stimulated neutrophils treated with H-7. Calyculin A also caused a remarkable hyperphosphorylation of a number of proteins in neutrophils and increased O2- release from these cells in response to a suboptimal amount of PMA. Enzymes present in both the soluble and particulate fractions of neutrophils catalyzed the near complete dephosphorylation of 32P-labeled p47 and p49 bound to Immobilon-P membranes. Dephosphorylation of these blotted phosphoproteins occurred at physiological rates and was inhibited by okadaic acid and calyculin A. These data strongly suggest that p47 undergoes a continual cycle of phosphorylation and dephosphorylation throughout the period of O2- release when PMA is the stimulus. Moreover, we show that antagonists of type 1 and 2A protein phosphatases block dephosphorylation of p47 both in vivo and in vitro, indicating that these enzymes may modulate O2- release under certain circumstances.     

Novel priming compounds of cystathionine metabolites on superoxide generation in human neutrophils

Human peripheral blood polymorphonuclear leukocytes were preincubated with cystathionine and cystathionine metabolites found in the urine of patients with cystathioninuria. Among the cystathionine metabolites, cystathionine ketimine and N-acetyl-S-(3-oxo-3-carboxy-n-propyl) cysteine (NAc-OCPC) significantly enhanced the N-formylmethionylleucylphenylalanine (fMLP)-induced superoxide generation, but cystathionine, NAc-cystathionine, and cyclothionine did not enhance the superoxide generation. Cystathionine ketimine and NAc-OCPC also enhanced superoxide generation induced by opsonized zymosan (OZ) but not that induced by arachidonic acid (AA) and phorbol 12-myristate 13-acetate (PMA). Superoxide generation induced by cystathionine ketimine and NAc-OCPC was inhibited by genistein, an inhibitor of tyrosine kinase, and was enhanced by 1-(5-isoquinoline sulfonyl)-2-methylpiperazine (H-7), an inhibitor of protein kinase C. Cystathionine ketimine and NAc-OCPC markedly also increased phosphorylation of 45-kDa protein in human neutrophils and the phosphorylation depended on the concentrations of cystathionine ketimine and NAc-OCPC. The phosphorylation of 45-kDa protein induced by cystathionine ketimine and NAc-OCPC was inhibited by genistein and herbimycin A, inhibitors of tyrosine kinase, but was not inhibited by H-7 and staurosporine, inhibitors of protein kinase C. Cystathionine metabolites and l-cystathionine sulfoxides were separated into two diastereoisomers, CS-I and CS-II. CS-I enhanced the superoxide generation induced by AA and PMA but not that induced by fMLP and OZ. In contrast, CS-II enhanced the superoxide generation induced by fMLP and OZ, but not that induced by AA and PMA.     

Pattern of protein phosphorylation in aortic endothelial cells. Modulation by adenine nucleotides and bradykinin

In bovine aortic endothelial cells, ATP (10-100 microM) and bradykinin (0.1-1.0 microM) enhanced the phosphorylation of two major protein substrates with apparent molecular masses of 95 and 28 kDa. The action of ATP involved P2y purinoceptors. The kinetics were distinct for the two phosphopeptides. The phosphorylation of the 95-kDa protein was rapid (within 30 s) but transient (maintained for only 2 min). This time course agrees with that observed for the increase of the cytosolic Ca2+ level induced by ATP in these cells. Ionophore A23187 (greater than or equal to 100 nM) induced this phosphorylation for a longer period (5-10 min), whereas phorbol 12-myristate 13-acetate (PMA) was completely inactive. The enhancement of the 28-kDa protein phosphorylation was detectable after a 5-min lag and was maintained for at least 20 min. PMA (50 nM) stimulated weakly the phosphorylation of the 28-kDa protein, whereas A23187 (100-300 nM) was even more effective than ATP and bradykinin. The 95-kDa phosphoprotein seems to be related to a 100-kDa substrate of calmodulin-dependent protein kinase III recently identified as elongation factor-2. The 28-kDa protein, which was resolved as three variants in bidimensional gel electrophoresis, appears very similar to a slightly heavier phosphoprotein from thrombin-stimulated human platelets. In addition, bidimensional electrophoresis allowed the detection of at least 10 substrates (from 18 to 46 kDa) whose phosphorylation was enhanced equally well by ATP, bradykinin, and A23187 and only partially by PMA. In conclusion, protein phosphorylation induced by ATP and bradykinin in aortic endothelial cells seems to be catalyzed mostly by Ca2+-dependent kinases, distinct from protein kinase C.     

Cytosolic and stored calcium antagonistically control tyrosine phosphorylation of specific platelet proteins.

Depletion of intracellular calcium stores appears to increase plasma membrane permeability for calcium by an as yet obscure mechanism. We found that the Ca2+ ionophore, A23187, and thrombin elevate cytosolic calcium ([Ca2+]i) equally and cause tyrosine phosphorylation of a 130-kDa protein and to a lesser extent 80- and 60-kDa proteins. Chelation of [Ca2+]i by 1,2-bis(2-aminophenoxyethane)-N,N,N',N'-tetraacetic acid/acetomethoxy ester decreased thrombin-induced tyrosine phosphorylation responses. These results suggested that [Ca2+]i elevation promotes tyrosine phosphorylation. Tyrosine phosphorylation persisted in the presence or absence of extracellular calcium after thrombin stimulation but subsided rapidly after A23187 addition if extracellular calcium was present. When Ca2+/ATPase activity, which is apparently required to maintain calcium stores, is inhibited by low temperature, tyrosine phosphorylation of the 130-kDa protein occurs. Rewarming platelets reverses tyrosine phosphorylation only if extracellular calcium is present. Thapsigargin, a calcium ATPase inhibitor, also induces tyrosine phosphorylation of the 130-kDa protein and prevents dephosphorylation of this protein when added prior to rewarming. These observations suggest that homeostatic levels of calcium in storage compartments favor tyrosine dephosphorylation of specific proteins. Thus the levels of [Ca2+]i and stored calcium appear to control tyrosine phosphorylation antagonistically. Tyrosine phosphorylation may play a role in regulating calcium channel function.     

Purified protein kinase C phosphorylates a 47-kDa protein in control neutrophil cytoplasts but not in neutrophil cytoplasts from patients with the autosomal form of chronic granulomatous disease

The neutrophil activators phorbol 12-myristate 13-acetate (PMA), formyl-methionyl-leucyl-phenylalanine, serum-treated zymosan, and IgG-coated latex cause an increase in protein phosphorylation in human neutrophil cytoplasts, concomitantly with an increase in oxygen consumption. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, phosphorylation was apparent in many proteins, must abundantly in 42-, 47-, 50-, 60-, and 80-kDa proteins. In neutrophil cytoplasts from autosomal chronic granulomatous disease (CGD) patients that were stimulated with PMA, the phosphorylation of a 47-kDa protein is absent. The localization of this protein in PMA-activated control cytoplasts is mainly in the cytosol and, to a lower and more variable extent, in the membrane. After addition of purified protein kinase C to lysates of nonstimulated control cytoplasts, phosphorylation occurred at the 47-kDa level in both the cytosol and the membrane fraction. With lysates of autosomal CGD cytoplasts, in vitro phosphorylation of the 47-kDa protein was completely absent. After separation of cytoplast proteins on a sodium dodecyl sulfate-polyacrylamide gel and excision of the 47-kDa protein(s), phosphorylation of the isolated 47-kDa band was observed in the presence of purified protein kinase C. This reaction was again absent when autosomal CGD cytoplasts were used as starting material. Our studies have identified the 47-kDa protein in neutrophil cytoplasts as a true substrate for protein kinase C and indicate that the defect in phosphorylation at the 47-kDa level in autosomal CGD cytoplasts is due to a defective protein.     

Mechanisms of the interleukin 5-induced differentiation of B cells

Interleukin 5 (IL5), a lymphokine produced by T cells, induces differentiation of B cell chronic leukemia BCL1-B20 cells into IgM-producing cells accompanied with growth arrest. To elucidate the intracellular mechanisms, the roles of Ca2+ mobilization and protein phosphorylation in the activation of the cells were examined. F(ab')2 fragment of anti-immunoglobulin (anti-Ig), which cross-links membrane-bound Ig, and calcium ionophore A23187 caused a rapid increase in the intracellular free calcium concentration [( Ca2+]i), whereas these stimulants did not give rise to differentiation of the cells. In contrast, treatment with IL5 did not affect either [Ca2+]i or the rates of Ca2+ uptake from the outside and release from the inside of the cells. Analysis by two-dimensional gel electrophoresis revealed that the in vitro phosphorylation of acidic 80-, 60-, and 45-kDa proteins was induced upon stimulation with IL5. Treatment with IL5 also caused a marked decrease in the in vitro phosphorylation of an acidic 100-kDa protein which was highly phosphorylated in the unstimulated state. Addition of phorbol 12-myristate 13-acetate (PMA) to the culture inhibited IL5-mediated differentiative responses. Therefore, these results suggest that Ca2+ mobilization is not involved but activities of stimulatory and inhibitory kinases may be involved in the IL5-mediated differentiation process.     

Protein tyrosine phosphorylation in rabbit peritoneal neutrophils.

Protein tyrosine phosphorylation in rabbit peritoneal neutrophils was examined by immunoblotting with antibodies specific for phosphotyrosine. Stimulation of the neutrophils with chemotactic factor fMet-Leu-Phe (10 nM) caused rapid increases of tyrosine phosphorylation of several proteins with apparent molecular masses of (Group A) 54-58 kDa and 100-125 kDa and (Group B) 36-41 kDa. Stimulation of Group A proteins was observed by fMet-Leu-Phe (10 nM, maximum at 20 s) and A23187 (1 microM, 1 min). Stimulation of Group B proteins was observed by fMet-Leu-Phe (ED50 0.15 nM, 1 min), leukotriene B4 (ED50 0.15 nM, 1 min), phorbol 12-myristate 13-acetate (PMA) (ED50 25 ng/ml, 10 min) and partially by ionophore A23187 (1 microM, 1 min). Pretreatment of the cell with the protein kinase inhibitor H-7 (25 microM, 5 min) and PMA (0.1 microgram/ml, 3 min) partially inhibited the fMet-Leu-Phe effect. However, pretreatment of the cells with quin 2/AM (20 microM, 10 min) completely inhibited the fMet-Leu-Phe effect. The results indicate that rapid regulation of tyrosine phosphorylation is an early event occurring in stimulated neutrophils. Furthermore the effect of fMet-Leu-Phe on tyrosine phosphorylation may require Ca2+ mobilization and may partially require the activity of H-7-sensitive protein kinases.     

"Numatrin," a nuclear matrix protein associated with induction of proliferation in B lymphocytes

To detect nuclear proteins that might be involved in induction of cellular mitogenesis, we examined the effect of various mitogens on early changes in synthesis of nuclear proteins in murine B lymphocytes. Using two-dimensional gel electrophoresis, we found that activation of B cells by mitogens (anti-immunoglobulin antibody, lipopolysaccharide, phorbol 12-myristate 13-acetate (PMA)/A23187) was associated with a rapid and prominent (5-20-fold) increase in the synthesis of a 40-kDa/pI 5.0 nuclear protein, here termed numatrin. Numatrin was found to be absent from the cytosol (soluble fraction) of resting as well as activated B cells and was markedly resistant to DNase/RNase digestion and 2 N NaCl extraction, indicating that this protein is tightly bound to the nuclear matrix. Kinetic studies showed that the increase in synthesis of numatrin was detected 60-120 min following mitogen activation, reached a peak at 16 h, and declined to almost control level by 48 h, correlating with the peak of cellular DNA synthesis. The increase in synthesis of numatrin in normal B cells was found to be associated exclusively with cellular commitment for mitogenesis because activation of B cells by stimuli such as B cell stimulating factor 1, PMA alone, and calcium ionophore A23187, which do not stimulate an increase in DNA synthesis, also failed to induce an increase in the synthesis of numatrin. Inhibition of anti-Ig-induced proliferation (by PMA pretreatment) was associated with a 63% inhibition in the synthesis of numatrin. Addition of 8-mercaptoguanosine to these PMA-treated cells was associated with restoration of the increase in synthesis of numatrin, concomitant with induction of proliferation. Elevated synthesis of numatrin was also detected in the malignant B lymphoma cells: Raji, BAL-17, and WEHI-231. Taken collectively, these results suggest that numatrin, a tightly bound nuclear matrix protein, is a growth-regulated protein which might have an important role in regulation of cellular mitogenesis in normal and malignant B lymphocytes.     

Possible involvement of optimally phosphorylated L-plastin in activation of superoxide-generating NADPH oxidase

The involvement of protein phosphatases in the activation of superoxide (O2-)- generating enzyme in human neutrophils was examined using calyculin A, an inhibitor of protein phosphatase type 1 and 2A. Calyculin A inhibited the phorbol myristate acetate (PMA)- and opsonized zymosan (OZ)-activated O2- generation by human neutrophils. This inhibitory effect of calyculin A on PMA-activated O2- generation was reversed by the addition of KT5926, a specific inhibitor of myosin light chain kinase and Ca2+/calmodulin-dependent protein kinase II. These results suggest that the addition of calyculin A may cause hyperphosphorylation of some protein(s) that plays a crucial role in the PMA-dependent activation of O2- generating enzyme, and that this protein hyperphosphorylation may be evoked by a KT5926-sensitive kinase or its downstream kinase. Whereas two-dimensional analysis involving 32P revealed that calyculin A caused the hyperphosphorylation of many proteins, KT5926 mainly reduced the calyculin A-induced hyperphosphorylation of a 67 kDa protein in activated neutrophils, suggesting that the hyperphosphorylation of the 67 kDa protein might inhibit the PMA-dependent activation of NADPH oxidase. The 67 kDa cytosolic protein was moderately phosphorylated on the addition of PMA. On the other hand, in the absence of calyculin A, KT5926 inhibited both PMA-induced O2- generation and phosphorylation of the 67 kDa protein. Amino acid sequence analysis of peptides derived from the 67 kDa protein revealed that the 67 kDa protein was identical to L-plastin, an actin-bundling protein. We conclude that optimally phosphorylated L-plastin may play some crucial role in the activation of NADPH oxidase.     

Translocation of pleckstrin requires its phosphorylation and newly formed ligands

Pleckstrin is the major substrate of protein kinase C (PKC) in platelets. We sought to determine whether pleckstrin phosphorylation is sufficient to target the soluble protein to binding sites. Permeabilization of platelets by streptolysin O (SLO) was used to separate bound and soluble pleckstrin. Platelets were incubated with phorbol 12-myristate 13-acetate (PMA) and/or guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in the presence of [gamma-(32)P]ATP and SLO. PMA stimulated pleckstrin phosphorylation, but this pleckstrin diffused from permeabilized platelets. Addition of GTP[S] with PMA caused up to 40-50% of pleckstrin to be retained within platelets and enhanced secretion of platelet 5-hydroxytryptamine. PKC alpha pseudosubstrate peptide inhibited pleckstrin phosphorylation, the binding of pleckstrin and secretion. After extraction of permeabilized platelets containing bound pleckstrin with Triton X-100, the protein was solubilized. Thus, phosphorylated pleckstrin was retained in platelets only after activation of GTP-binding proteins that stimulate the formation of membrane-bound pleckstrin ligands. Translocation of pleckstrin may facilitate the associated secretion.     

The effects of platelet secretion inhibitors on protein phosphorylation

Protein phosphorylation was investigated in human platelets after stimulation to secretion by thrombin. After stimulation by thrombin at 4 degrees C (in which secretion is inhibited), phosphorylations of the 80, 56, and 38 kDa polypeptides and dephosphorylation of the 67 kDa phosphopeptide eventually occurred. The phosphorylations of the 27 and 20 kDa polypeptides remained inhibited until the temperature was increased to 37 degree C, which also resulted in secretion. Various stimulants and inhibitors of platelet function were used to characterize individual protein phosphorylations. The divalent-cation ionophore, A23187, induced the phosphorylations (or dephosphorylation) of the same proteins as thrombin with the exception of the 80 kDa protein, which remained incompletely phosphorylated. The intracellular calcium antagonist, TMB-8, inhibited thrombin-stimulated secretion and phosphorylation of all the polypeptides except the 80 kDa protein. The dephosphorylation of the 67 kDa phosphoprotein was not affected by TMB-8. Incubation of platelets with prostaglandin E1 and isobutylmethylxanthine inhibited thrombin-stimulated secretion and the phosphorylation of the 38 and 20 kDa protein and increased the phosphorylation of the 67 and 27 kDa phosphoproteins. These observations may be used to correlate protein phosphorylation with secretion, suggesting a possible sequence of intracellular events that mediate thrombin-stimulated secretion.     

Phosphorylation of nonmuscle myosin heavy chain IIA on Ser1917 is mediated by protein kinase C beta II and coincides with the onset of stimulated degranulation of RBL-2H3 mast cells

Dynamic remodeling of the actinomyosin cytoskeleton is integral to many biological processes. It is regulated, in part, by myosin phosphorylation. Nonmuscle myosin H chain IIA is phosphorylated by protein kinase C (PKC) on Ser(1917). Our aim was to determine the PKC isoform specificity of this phosphorylation event and to evaluate its potential role in regulated secretion. Using an Ab against the phosphorylated form of Ser(1917), we show that this site is not phosphorylated in unstimulated RBL-2H3 mast cells. The physiological stimulus, Ag, or the pharmacological activators, PMA plus A23187, induced Ser(1917) phosphorylation with a time course coincident with the onset of granule mediator secretion. Dephosphorylation at this site occurred as Ag-stimulated secretion declined from its peak, but dephosphorylation was delayed in cells activated with PMA plus A23187. Phosphate incorporation was also enhanced by PMA alone and by inhibition of protein phosphatase 2A. G?6976, an inhibitor of conventional PKC isoforms, abolished secretion and Ser(1917) phosphorylation with similar dose dependencies consistent with involvement of either PKCalpha or PKCbeta. Phorbol ester-stimulated Ser(1917) phosphorylation was reconstituted in HEK-293 cells (which lack endogenous PKCbeta) by overexpression of both wild-type and constitutively active PKCbetaII but not the corresponding PKCbetaI or PKCalpha constructs. A similar selectivity for PKCbetaII overexpression was also observed in MIN6 insulinoma cells infected with recombinant PKC wild-type adenoviruses. Our results implicate PKC-dependent phosphorylation of myosin H chain IIA in the regulation of secretion in mast cells and suggest that Ser(1917) phosphorylation might be a marker of PKCbetaII activation in diverse cell types.     

Failure of 1-oleoyl-2-acetylglycerol to mimic the cell-differentiating action of 12-O-tetradecanoylphorbol 13-acetate in HL-60 cells

Treatment of human promyelocytic leukemia cells (HL-60 cells) with 12-O-tetradecanoylphorbol 13-acetate (TPA) results in terminal differentiation of the cells to macrophage-like cells. Treatment of the cells with TPA induced marked enhancement of the phosphorylation of 28- and 67-kDa proteins and a decrease in that of a 75-kDa protein. When the cells were treated with diacylglycerol, i.e. 50 micrograms/ml 1-oleoyl-2-acetylglycerol (OAG), similar changes in the phosphorylation of 28-, 67-, and 75-kDa proteins were likewise observed, indicating that OAG actually stimulates protein kinase C in intact HL-60 cells. OAG (1-100 micrograms/ml), which we used, activated partially purified mouse brain protein kinase C in a concentration-dependent manner. Treatment of HL-60 cells with 10 nM TPA for 48 h caused an increase by about 8-fold in cellular acid phosphatase activity. Although a significant increase in acid phosphatase activity was induced by OAG, the effect was scant compared to that of TPA (less than 7% that of TPA). After 48-h exposure to 10 nM TPA, about 95% of the HL-60 cells adhered to culture dishes. On the contrary, treatment of the cells either with OAG (2-100 micrograms/ml) or phospholipase C failed to induce HL-60 cell adhesion. Ca2+ ionophore A23187 failed to act synergistically with OAG. In addition, hourly or bi-hourly cumulative addition of OAG for 24 h also proved ineffective to induce HL-60 cell adhesion. Our present results do not imply that protein kinase C activation is nonessential for TPA-induced HL-60 cell differentiation, but do demonstrate that protein kinase C activation is not the sole event sufficient to induce HL-60 cell differentiation by means of this agent.     

Activation of protein kinase C induces rapid internalization and subsequent degradation of muscarinic acetylcholine receptors in neuroblastoma cells

The tumor-promoting phorbol ester 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), which activates protein kinase C, acted synergistically with A23187 to decrease muscarinic acetylcholine receptor (mAChR) number in neuroblastoma cells (clone N1E-115) as determined by a filter binding assay using [3H]quinuclidinyl benzilate in membrane homogenates. After a 6-h incubation, 10(-7) M PMA and 3 X 10(-7) M A23187 reduced mAChR number 30-40%, compared to the 40-50% reduction observed after treatment with 10(-3) M carbachol, a muscarinic agonist. Incubation with 3 X 10(-7) M A23187 and 10(-7) M 4 alpha-phorbol 12,13-didecanoate, an inactive phorbol ester, did not alter mAChR number. The addition of PMA and A23187 to cultures incubated with 10(-3) M carbachol caused only a modest 6% further reduction in mAChR number as compared to incubation with carbachol alone. The kinetics of the decrease in mAChR number produced by PMA/A23187 were similar to those seen after carbachol treatment. Recovery of mAChR number after treatment with either carbachol or PMA/A23187 was blocked by treatment with the protein synthesis inhibitor cycloheximide. Intact cell binding studies employing [3H]N-methylscopolamine showed that treatment with either PMA/A23187 or carbachol caused a rapid (within 15 min) loss of receptors from the cell surface prior to the decrease in total mAChR number. PMA (10(-7) M), but not 4 alpha-phorbol 12,13-didecanoate, promoted the translocation of protein kinase C activity from the cytosol to the membrane. Incubation with carbachol increased membrane-associated protein kinase C activity within 5 min with an EC50 of 3 X 10(-6) M. This increase persisted for at least 60 min in the continued presence of carbachol and was blocked by simultaneous incubation with atropine. These results suggest that activation of protein kinase C may be involved in the regulation of mAChR number in response to agonist.     

A novel calcium-dependent phosphorylation of a ribosome-associated protein

The degree of phosphorylation of ribosome-associated proteins of intact GH3 pituitary tumor cells before and after Ca2+ depletion was examined. A 26-kDa ribosome-associated protein was found to dephosphorylate upon Ca2+ depletion of the cells and to re-phosphorylate upon restoration of the cation. This protein was distinct from eukaryotic initiation factor 4E on the basis of subcellular distribution, antigenicity, and susceptibility to dephosphorylation in Ca2+-depleted cells. Temporal correlation was observed between the phosphorylation/dephosphorylation state of the 26-kDa protein, and the stimulation/inhibition of protein synthesis by Ca2+ repletion/depletion. Phosphorylation of the 26-kDa protein was rapidly abolished in Ca2+-repleted cells by the ionophore, A23187, which elevates cytosolic Ca2+ while reducing sequestered cation. Dephosphorylation of the protein and inhibition of protein synthesis occurred at comparable times after addition of ionophore. The novelty of a Ca2+-dependent phosphorylation supported by sequestered rather than cytosolic free Ca2+ is discussed.     

Expression and phosphorylation of a MARCKS-like protein in gastric chief cells: Further evidence for modulation of pepsinogen secretion by interaction of Ca2+/calmodulin with protein kinase C

In gastric chief cells, agents that activate protein kinase C (PKC) stimulate pepsinogen secretion and phosphorylation of an acidic 72-kDa protein. The isoelectric point and molecular mass of this protein are similar to those for a common PKC substrate; the MARCKS (for Myristoylated Alanine-Rich C Kinase Substrate) protein. We examined expression and phosphorylation of the MARCKS-like protein in a nearly homogeneous suspension of chief cells from guinea pig stomach. Western blotting of fractions from chief cell lysates with a specific MARCKS antibody resulted in staining of a myristoylated 72-kDa protein (pp72), associated predominantly with the membrane fraction. Using permeabilized chief cells. we examined the effect of PKC activation (with the phorbol ester PMA), in the presence of basal (100 nM) or elevated cellular calcium (1 μM), on pepsinogen secretion and phosphorylation of the 72-kDa MARCKS-like protein. Secretion was increased 2.3-, 2.6-, and 4.5-fold by incubation with 100 nM PMA, 1 μM calcium, and PMA plus calcium, respectively. A PKC inhibitor (1 μM CGP 41 251) abolished PMA-induced secretion, but did not alter calcium-induced secretion. This indicates that calcium-induced secretion is independent of PKC activation. Chief cell proteins were labeled with ³²P-orthophosphate and phosphorylation of pp72 was detected by autoradiography of 2-dimensional polyacrylamide gels. In the presence of basal calcium PMA (100 nM) caused a > two-fold increase in phosphorylation of pp72. Without PMA, calcium did not alter phosphorylation of pp72. However, 1 μM calcium caused an approx. 50% attenuation of PMA-induced phosphorylation of pp72. Experiments with a MARCKS “phosphorylation/calmodulin binding domain peptide” indicated that calcium/calmodulin inhibits phosphorylation of pp72 by binding to the phosphorylation/calmodulin binding domain and not by inhibiting PKC activity. These observations support the hypothesis that, in gastric chief cells, interplay between calcium/calmodulin binding and phosphorylation of a common domain on the 72-kDa MARCKS-like protein plays a role in modulating pepsinogen secretion. J. Cell. Biochem. 64:514–523. © 1997 Wiley-Liss, Inc.     

Cofilin undergoes rapid dephosphorylation in stimulated neutrophils and translocates to ruffled membranes enriched in products of the NADPH oxidase complex. Evidence for a novel cycle of phosphorylation and dephosphorylation

 Neutrophils contain a 21-kDa phosphoprotein that undergoes rapid dephosphorylation upon stimulation of these cells with the chemoattractant N-fMet-Leu-Phe (fMLP), activators of protein kinase C [e.g., 4β-phorbol 12-myristate 13-acetate (PMA)] or the calcium ionophore A23187. This phosphoprotein was identified as the non-muscle form of cofilin by peptide sequencing and immunoblotting with specific antibodies. Evidence is presented that in neutrophils cofilin is regulated by a continual cycle of phosphorylation and dephosphorylation, and that the phosphatase undergoes activation during cell stimulation. Experiments with a wide variety of antagonists further suggested that the protein kinase that participates in these reactions may be a novel enzyme. The kinetics of cofilin dephosphorylation in neutrophils stimulated with fMLP or PMA were very similar to those observed for superoxide (O₂ ^–) release. Immunofluorescent studies revealed that cofilin was present thouroughout the cytosol of resting neutrophils and underwent rapid translocation to the F-actin-rich, ruffled membranes of stimulated cells. Cytochemical analysis further revealed that the ruffled membranes also contained large amounts of hydrogen peroxide (H₂O₂), a product of the O₂ ^–/H₂O₂-generating activity of stimulated neutrophils (NADPH oxidase). Cofilin is therefore well placed to participate in the continual polymerization and depolymerization of F-actin that is thought to give rise to the oscillatory pattern of H₂O₂ production observed under certain conditions. Accepted: 22 April 1997     

Control of endogenous phosphorylation of the major cAMP-dependent protein kinase substrate in adipocytes by insulin and beta-adrenergic stimulation

In isolated, 32Pi-loaded, rat adipocytes, we have examined phosphorylation of the major cAMP-dependent protein kinase (A-kinase) substrate, a protein that appears to be associated with the lipid storage droplet and migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a 65-67-kDa doublet. In control cells, a strong phosphorylation signal is detected as the (+/- cAMP) A-kinase activity ratio ranges from approximately 0.1 to approximately 0.3-0.4 with increasing isoproterenol concentrations. By contrast, insulin-treated cells exhibiting A-kinase activity ratios over the range of 0.1-0.25 contain less 32P in the 65-67-kDa protein than control cells exhibiting identical A-kinase activity ratios. At higher activity ratios (greater than 0.3), this reduction in phosphorylation of the 65-67-kDa protein by insulin disappears. It is concluded that insulin stimulates a phosphatase activity that acts on the 65-67-kDa protein. Insulin actions aside, these studies reveal two interesting phenomena. 1) Whereas elevated, steady-state A-kinase activities are established rapidly (1-2 min) upon isoproterenol stimulation, phosphorylation of the 65-67-kDa substrate proceeds through a burst, followed by a decline to a steady-state level by 10-12 min. An "adaptation" mechanism, providing for a constant response to a constant stimulus, may underlie this lack of parallelism between the time course of phosphorylation and A-kinase activity. 2) Removal of [32Pi] orthophosphate immediately before isoproterenol stimulation leads to a rapid (t approximately 10 min) loss in labeling of the 65-67-kDa protein, whereas the phosphorylation state of other phosphoproteins are not changed. These data suggest that elevation of A-kinase activity leads to a rapid exchange of external Pi with an ATP pool that is used by A-kinase.     

Insulin-like Growth Factor-I Induces Phosphorylation in Leishmania {Leishmania) mexicana Promastigotes and Amastigotes

Protein phosphorylation controls major steps of proliferation and differentiation in eukaryotic cells. However there are few studies done in protozoa particularly when being triggered by external stimuli. In this paper we have examined the tyrosine- and serine/threonine-phosphorylated proteins in both promastigote and amastigote-like forms of Leishmania (Leishmania) mexicana stimulated with insulin-like growth factor (IGF)-I. Stimulation with IGF-I induces major tyrosine phosphorylation of a 185-kDa protein in promastigotes and 60- and 40-kDa proteins in amastigotes. Analysis of total phosphorylation revealed additional sets of phosphorylated proteins: a 110-kDa protein band in promastigotes and two other proteins of 120 and 95 kDa in the amastigote-like forms. To further analyze the IGF-I-mediated response we compared it with the phosphorylation pattern obtained with a known inducer of protein kinase C, phorbol myristate acetate. This analysis showed overlapping phosphorylation of most of the proteins but mainly of the 185- and 110-kDa proteins in the promastigotes and the 95-. 60- and 40-kDa proteins in the amastigote-like forms. We thus conclude that there are phosphorylation-dependem pathways in Leishmania parasites induced by IGF-I that are stage-specific.     

Differential mechanisms of translocation of protein kinase C to plasma membranes in activated human neutrophils

Three classes of activators of human neutrophils that induce the intracellular translocation of protein kinase C from the cytosol to the particulate fraction were compared for their effects on the properties of the particulate (membrane-bound) enzyme. In cells stimulated with 10 ng/ml of phorbol-12-myristate-13-acetate (PMA) the particulate enzyme is almost fully active in the absence of added Ca2+ or phospholipids and this activity is not released by the Ca2+-chelator EDTA. In contrast, binding of protein kinase C to the particulate fraction in cells treated with the chemotactic factor f-Met-Leu-Phe (fMLF) or with the ionophore A-23187 plus Ca2+ is observed only when the cells are lysed in the presence of 1 mM Ca2+. With these stimuli the particulate enzyme retains a nearly absolute requirement for Ca2+ and phospholipids. Thus only the full intercalation of protein kinase C caused by PMA, which is resistant to removal by chelators stabilizes an active form of protein kinase C in the neutrophil membrane. In confirmation of this conclusion, in isolated plasma membranes loaded with partially purified protein kinase C by incubation with 5 microM Ca2+ further incubation with PMA, but not with fMLF, caused a significant fraction of the bound PKC to become resistant to removal by chelators, and to be nearly fully active in the absence of added activators.