Calyculin A and okadiac acid inhibit human platelet aggregation by blocking protein phosphatases types 1 and 2A

Opposing Roles for the α Isoform of the Catalytic Subunit of Protein Phosphatase 1 in Inside-Out and Outside-In Integrin Signaling in Murine Platelets

Platelet activation during hemostasis and thrombosis is facilitated by agonist-induced inside-out and integrin αIIbβ3-initiated outside-in signaling via protein kinases and phosphatases. Pharmacological inhibitor studies suggest that the serine/threonine protein phosphatase 1 (PP1) promotes platelet activation. However, since phosphatase inhibitors block all the isoforms of the catalytic subunit of PP1 (PP1c), the role of specific PP1c isoform in platelet signaling remains unclear. Here, we employed a platelet-specific PP1cα-/- mice to explore the contribution of a major PP1 isoform in platelet functions. Loss of PP1cα moderately decreased activation of integrin αIIbβ3, binding of soluble fibrinogen, and aggregation to low-dose thrombin, ADP, and collagen. In contrast, PP1cα-/- platelets displayed increased adhesion to immobilized fibrinogen, fibrin clot retraction, and thrombus formation on immobilized collagen. Mechanistically, post-fibrinogen engagement potentiated p38 mitogen-activated protein kinase (MAPK) activation in PP1cα-/- platelets and the p38 inhibitor blocked the increased integrin-mediated outside-in signaling function. Tail bleeding time and light-dye injury-induced microvascular thrombosis in the cremaster venules and arterioles were not altered in PP1cα-/- mice. Thus, PP1cα displays pleiotropic signaling in platelets as it amplifies agonist-induced signaling and attenuates integrin-mediated signaling with no impact on hemostasis and thrombosis.

Tau Protein in Lung Smooth Muscle Cells

Tau, a microtubule-associated protein, plays a critical role in the pathophysiology of neurons. However, whether tau protein is expressed in smooth muscle cells is unknown. Thus, we tested the hypothesis that tau protein is expressed in the primary cultures of smooth muscle cells. Here, we report that tau protein is expressed and constitutively phosphorylated at threonine 181 in various smooth muscle cell types, including human pulmonary artery smooth muscle cells, bronchial airway smooth muscle cells, and cerebral artery smooth muscle cells. Immunofluorescence staining revealed that phosphorylated tau at threonine 181 is more organized in the cell than is total tau protein. A protein phosphatase inhibitor, calyculin A, induced the formation of higher molecular weight species of phosphorylated tau, as visualized by Western blotting, indicating the occurrence of tau aggregation. Immunofluorescence analysis also showed that calyculin A caused the aggregation of phosphorylated tau and disrupted the cytoskeletal organization. These results demonstrate the existence of tau protein in smooth muscle cells, and that smooth muscle tau is susceptible to protein phosphorylation and aggregation. Lung smooth muscle tau may therefore play an important role in pulmonary pathophysiology.

The Cell Cycle Checkpoint System MAST(L)-ENSA/ARPP19-PP2A is Targeted by cAMP/PKA and cGMP/PKG in Anucleate Human Platelets

The cell cycle is controlled by microtubule-associated serine/threonine kinase-like (MASTL), which phosphorylates the cAMP-regulated phosphoproteins 19 (ARPP19) at S62 and 19e/α-endosulfine (ENSA) at S67and converts them into protein phosphatase 2A (PP2A) inhibitors. Based on initial proteomic data, we hypothesized that the MASTL-ENSA/ARPP19-PP2A pathway, unknown until now in platelets, is regulated and functional in these anucleate cells. We detected ENSA, ARPP19 and various PP2A subunits (including seven different PP2A B-subunits) in proteomic studies of human platelets. ENSA-S109/ARPP19–S104 were efficiently phosphorylated in platelets treated with cAMP- (iloprost) and cGMP-elevating (NO donors/riociguat) agents. ENSA-S67/ARPP19-S62 phosphorylations increased following PP2A inhibition by okadaic acid (OA) in intact and lysed platelets indicating the presence of MASTL or a related protein kinase in human platelets. These data were validated with recombinant ENSA/ARPP19 and phospho-mutants using recombinant MASTL, protein kinase A and G. Both ARPP19 phosphorylation sites S62/S104 were dephosphorylated by platelet PP2A, but only S62-phosphorylated ARPP19 acted as PP2A inhibitor. Low-dose OA treatment of platelets caused PP2A inhibition, diminished thrombin-stimulated platelet aggregation and increased phosphorylation of distinct sites of VASP, Akt, p38 and ERK1/2 MAP kinases. In summary, our data establish the entire MASTL(like)–ENSA/ARPP19–PP2A pathway in human platelets and important interactions with the PKA, MAPK and PI3K/Akt systems.

The Phosphatase Inhibitor Calyculin-A Impairs Clot Retraction, Platelet Activation, and Thrombin Generation

The aim of this study was to investigate the effect of the serine/threonine protein phosphatase inhibitor, calyculin-A (CLA), on clot formation and on the procoagulant activity of human platelets. Platelet-rich plasma (PRP) samples were preincubated with buffer or CLA and subsequently platelets were activated by the protease-activated receptor 1 (PAR-1) activator, thrombin receptor activating peptide (TRAP). Clot retraction was detected by observing clot morphology up to 1 hour, phosphatidylserine- (PS-) expression was studied by flow cytometry, and thrombin generation was measured by a fluorimetric assay. For the intracellular Ca²⁺ assay, platelets were loaded with calcium-indicator dyes and the measurements were carried out using a ratiometric method with real-time confocal microscopy. CLA preincubation inhibited clot retraction, PS-expression, and thrombin formation. TRAP activation elicited Ca²⁺ response and PS-expression in a subset of platelets. The activated PRP displayed significantly faster and enhanced thrombin generation compared to nonactivated samples. CLA pretreatment abrogated PS-exposure and clot retraction also in TRAP-activated samples. As a consequence of the inhibitory effect on calcium elevation and PS-expression, CLA significantly downregulated thrombin generation in PRP. Our results show that CLA pretreatment may be a useful tool to investigate platelet activation mechanisms that contribute to clot formation and thrombin generation.

A Novel Interaction of the Catalytic Subunit of Protein Phosphatase 2A with the Adaptor Protein CIN85 Suppresses Phosphatase Activity and Facilitates Platelet Outside-in αIIbβ3 Integrin Signaling

The transduction of signals generated by protein kinases and phosphatases are critical for the ability of integrin αIIbβ3 to support stable platelet adhesion and thrombus formation. Unlike kinases, it remains unclear how serine/threonine phosphatases engage the signaling networks that are initiated following integrin ligation. Because protein-protein interactions form the backbone of signal transduction, we searched for proteins that interact with the catalytic subunit of protein phosphatase 2A (PP2Ac). In a yeast two-hybrid study, we identified a novel interaction between PP2Ac and an adaptor protein CIN85 (Cbl-interacting protein of 85 kDa). Truncation and alanine mutagenesis studies revealed that PP2Ac binds to the P3 block ((396)PAIPPKKPRP(405)) of the proline-rich region in CIN85. The interaction of purified PP2Ac with CIN85 suppressed phosphatase activity. Human embryonal kidney 293 αIIbβ3 cells overexpressing a CIN85 P3 mutant, which cannot support PP2Ac binding, displayed decreased adhesion to immobilized fibrinogen. Platelets contain the ∼85 kDa CIN85 protein along with the PP2Ac-CIN85 complex. A myristylated cell-permeable peptide derived from residues 395-407 of CIN85 protein (P3 peptide) disrupted the platelet PP2Ac-CIN85 complex and decreased αIIbβ3 signaling dependent functions such as platelet spreading on fibrinogen and thrombin-mediated fibrin clot retraction. In a phospho-profiling study P3 peptide treated platelets also displayed decreased phosphorylation of several signaling proteins including Src and GSK3β. Taken together, these data support a role for the novel PP2Ac-CIN85 complex in supporting integrin-dependent platelet function by dampening the phosphatase activity.

Dissociation of SHP-1 from spinophilin during platelet activation exposes an inhibitory binding site for protein phosphatase-1 (PP1)

We have recently shown that a critical regulatory node in the platelet signaling network lies immediately downstream of platelet receptors for thrombin and TxA₂. This node is comprised of a scaffold protein (spinophilin, SPL), a protein tyrosine phosphatase (SHP-1), and either of the two members of the Regulators of G protein Signaling family predominantly expressed in platelets (RGS10 or RGS18). The SPL/RGS/SHP-1 complex is present in resting platelets, dissociating when thrombin or TxA₂, but not ADP or collagen, activate SHP-1 and release RGS10 and RGS18 to dampen signaling. Here we demonstrate an additional regulatory role for spinophilin, showing that dissociation of SHP-1 from spinophilin is followed by an increase in the binding of spinophilin to PP1, a serine/threonine phosphatase whose binding site maps to a region close to the SHP-1 binding site. The increase in PP1 binding to spinophilin is limited to platelet agonists that cause dissociation of the complex and is selective for the α and γ isoforms of PP1. Studies in cell culture show that SHP-1 and PP1 can compete for binding to spinophilin and that binding inhibits PP1 activity since over-expression of wild type spinophilin, but not spinophilin with a disabled PP1 binding site, causes an increase in the phosphorylation of myosin light chain, a well-characterized PP1 substrate. Collectively, these results indicate that in addition to regulating RGS protein availability in resting platelets, spinophilin can serve as a time-dependent, agonist- and isoform-selective regulator of PP1, inhibiting its activity when decay of the SPL/RGS/SHP-1 complex releases SHP-1 from spinophilin, exposing a binding site for PP1.

Distinct roles for the α , β and γ1 isoforms of protein phosphatase 1 in the outside-in αIIbβ3 integrin signalling-dependent functions

Although protein kinases and phosphatases participate in integrin αIIbβ3 signalling, whether integrin functions are regulated by the catalytic subunit of protein phosphatase 1(PP1c)isoforms are unclear. We show that siRNA mediated knockdown of all PP1c isoforms(α, β and γ1)in 293 αIIbβ3 cells decreased adhesion to immobilised fibrinogen and fibrin clot retraction. Selective knockdown of only PP1cγ1 did not alter adhesion or clot retraction, while depletion of PP1cβ decreased both functions. Unexpectedly, knockdown of PP1cα enhanced αIIbβ3 adhesion to fibrinogen and clot retraction. Protein interaction studies revealed that all PP1c isoforms can interact with the integrin αIIb subunit. Phospho-profiling studies revealed an enhanced activation of mitogen-activated protein kinase (MAPK) p38 in the PP1cα depleted cells. Enhanced adhesive phenotype displayed by the PP1cα-depleted 293 αIIbβ3 cells was blocked by pharmacological inhibition of p38. Conversely, the decreased adhesion of PP1cα overexpressing cells was rescued by the expression of constitutively active p38α or p38γ. Thus, PP1c isoforms have distinct contribution to the outside-in αIIbβ3 signalling-dependent functions in 293 αIIbβ3 cells. Moreover, PP1cα negatively regulates integrin function by suppressing the p38 pathway.

The catalytic subunit of protein phosphatase 1 gamma regulates thrombin-induced murine platelet alpha(IIb)beta(3) function

Background

Hemostasis and thrombosis are regulated by agonist-induced activation of platelet integrin α_IIbβ₃. Integrin activation, in turn is mediated by cellular signaling via protein kinases and protein phosphatases. Although the catalytic subunit of protein phosphatase 1 (PP1c) interacts with α_IIbβ₃, the role of PP1c in platelet reactivity is unclear.

Methodology/Principal Findings

Using γ isoform of PP1c deficient mice (PP1cγ^−/−), we show that the platelets have moderately decreased soluble fibrinogen binding and aggregation to low concentrations of thrombin or protease-activated receptor 4 (PAR4)-activating peptide but not to adenosine diphosphate (ADP), collagen or collagen-related peptide (CRP). Thrombin-stimulated PP1cγ^−/− platelets showed decreased α_IIbβ₃ activation despite comparable levels of α_IIbβ₃, PAR3, PAR4 expression and normal granule secretion. Functions regulated by outside-in integrin α_IIbβ₃ signaling like adhesion to immobilized fibrinogen and clot retraction were not altered in PP1cγ^−/− platelets. Thrombus formation induced by a light/dye injury in the cremaster muscle venules was significantly delayed in PP1cγ^−/− mice. Phosphorylation of glycogen synthase kinase (GSK3)β-serine 9 that promotes platelet function, was reduced in thrombin-stimulated PP1cγ^−/− platelets by an AKT independent mechanism. Inhibition of GSK3β partially abolished the difference in fibrinogen binding between thrombin-stimulated wild type and PP1cγ^−/− platelets.

Conclusions/Significance

These studies illustrate a role for PP1cγ in maintaining GSK3β-serine9 phosphorylation downstream of thrombin signaling and promoting thrombus formation via fibrinogen binding and platelet aggregation.

Protein phosphatase 2A negatively regulates integrin alpha(IIb)beta(3) signaling

Integrin alpha(IIb)beta(3) activation is critical for platelet physiology and is controlled by signal transduction through kinases and phosphatases. Compared with kinases, a role for phosphatases in platelet integrin alpha(IIb)beta(3) signaling is less understood. We report that the catalytic subunit of protein phosphatase 2A (PP2Ac) associates constitutively with the integrin alpha(IIb)beta(3) in resting platelets and in human embryonal kidney 293 cells expressing alpha(IIb)beta(3). The membrane proximal KVGFFKR sequence within the cytoplasmic domain of integrin alpha(IIb) is sufficient to support a direct interaction with PP2Ac. Fibrinogen binding to alpha(IIb)beta(3) during platelet adhesion decreased integrin-associated PP2A activity and increased the phosphorylation of a PP2A substrate, vasodilator associated phosphoprotein. Overexpression of PP2Ac(alpha) in 293 cells decreased alpha(IIb)beta(3)-mediated adhesion to immobilized fibrinogen. Conversely, small interference RNA mediated knockdown of endogenous PP2Ac(alpha) expression in 293 cells, enhanced extracellular signal-regulated kinase (ERK1/2) and p38 activation, and accelerated alpha(IIb)beta(3) adhesion to fibrinogen and von Willebrand factor. Inhibition of ERK1/2, but not p38 activation, abolished the increased adhesiveness of PP2Ac (alpha)-depleted 293 cells to fibrinogen. Furthermore, knockdown of PP2A(calpha) expression in bone marrow-derived murine megakaryocytes increased soluble fibrinogen binding induced by protease-activated receptor 4-activating peptide. These studies demonstrate that PP2Ac (alpha) can negatively regulate integrin alpha(IIb)beta(3) signaling by suppressing the ERK1/2 signaling pathway.

Role of myosin regulatory light chain and Rac1 in the migration of polyamine-depleted intestinal epithelial cells

We have previously shown that polyamine depletion decreased migration, Rac activation, and protein serine threonine phosphatase 2A activity. We have also shown that polyamine depletion increased cortical F-actin and decreased lamellipodia and stress fibers. In this study, we used staurosporine (STS), a potent, cell-permeable, and broad-spectrum serine/threonine kinase inhibitor, and studied migration. STS concentrations above 100 nM induced apoptosis. However, in polyamine-depleted cells, a lower concentration of STS (5 nM) increased attachment, spreading, Rac1 activation, and, subsequently, migration without causing apoptosis. STS-induced migration was completely prevented by a Rac1 inhibitor (NSC-23766) and dominant negative Rac1. These results imply that STS restores migration in polyamine-depleted cells through Rac1. The most important finding in this study was that polyamine depletion increased the association of phosphorylated myosin regulatory light chain (pThr(18)/Ser(19)-MRLC) at the cell periphery, which colocalized with thick cortical F-actin. Localization of pThr(18)- and pSer(19)-MRLC was found with stress fibers and nuclei, respectively. STS decreased the phosphorylation of cellular and peripheral pThr(18)-MRLC without any effect on nuclear pSer(19)-MRLC, dissolved thick cortical F-actin, and increased lamellipodia and stress fiber formation in polyamine-depleted cells. In control and polyamine-depleted cells, focal adhesion kinase (FAK) colocalized with stress fibers and the actin cortex, respectively. STS reorganized FAK, paxillin, and the cytoskeleton. These results suggest that polyamine depletion prevents the dephosphorylation of MRLC and thereby prevents the dynamic reorganization of the actin cytoskeleton and decreases lamellipodia formation resulting in the inhibition of migration.

Protein kinase C-catalyzed phosphorylation of an inhibitory phosphoprotein of myosin phosphatase is involved in human platelet secretion

Protein kinase C (PKC)-potentiated inhibitory phosphoprotein of myosin phosphatase (CPI) was detected in human platelets. Like smooth muscle CPI-17, in vitro phosphorylation of platelet CPI by PKC inhibited the activity of myosin phosphatase containing the PP1delta catalytic subunit and the 130-kd myosin-binding subunit (MBS). Treatment of intact platelets with thrombin or the stable thromboxane A(2) analog STA(2) resulted in increased phosphorylation of both CPI and MBS at Thr-696, whereas phorbol myristate acetate (PMA) and the Ca(++) ionophore ionomycin only induced CPI phosphorylation. PMA induced slow adenosine triphosphate (ATP) secretion of fura 2-loaded platelets with no change in cytosolic Ca(++). The PMA-induced increase in CPI phosphorylation preceded phosphorylation of 20-kd myosin light chain (MLC(20)) at Ser-19 and ATP secretion. The PKC inhibitor, GF109203X, inhibited PMA-induced phosphorylation of CPI and MLC(20) with similar IC(50) values. These findings suggest that the activation of PKC by PMA induces MLC(20) phosphorylation by inhibiting myosin phosphatase through phosphorylation of CPI. STA(2)-induced MLC(20) phosphorylation was also diminished but not abolished by GF109203X, even at high concentrations that completely inhibited STA(2)-induced CPI phosphorylation. A combination of the Rho-kinase inhibitor Y-27632 and GF109203X led to a further decrease in STA(2)-induced MLC(20) phosphorylation, mainly because of a significant inhibition of MBS phosphorylation at Thr-696. Inhibition of STA(2)-induced ATP release by Y-27632, GF109203X, or both appeared to correlate with the extent of MLC(20) phosphorylation. Thus, CPI phosphorylation by PKC may participate in inhibiting myosin phosphatase, in addition to the Rho-kinase-mediated regulation of myosin phosphatase, during agonist-induced platelet secretion. (Blood. 2001;97:3798-3805)

Phosphorylation sites in the integrin beta3 cytoplasmic domain in intact platelets

Protein seryl/threonyl phosphatase inhibitors such as calyculin A block inside-out and outside-in platelet signaling. Our studies demonstrate that the addition of calyculin A blocks platelet adhesion and spreading on fibrinogen, responses that depend on integrin alphaIIb beta3 signaling. We hypothesized that this reflects a change in alphaIIb beta3 structure caused by a specific state of phosphorylation. We show that addition of calyculin A leads to increased phosphorylation of the beta3 subunit, and phosphoamino acid analysis reveals that only threonine residues become phosphorylated; sequence analysis by Edman degradation established that threonine 753 became stoichiometrically phosphorylated during inhibition of platelet phosphatases by calyculin A. This region of beta3 is linked to outside-in signaling such as platelet spreading responses. The effect of calyculin A on platelet adhesion and spreading and on the phosphorylation of T-753 in beta3 is reversed by the calcium ionophore A23187, demonstrating that these effects of calyculin A are not generally toxic ones. We propose that phosphorylation of beta3 on threonine 753, a region of beta3 linked to outside-in signaling, may be a mechanism by which integrin alphaIIb beta3 function is regulated.

Regulation of the L-type Ca2+ channel current in rat pinealocytes: role of basal phosphorylation

In the present study, the role of phosphoprotein phosphatase in the regulation of L-type Ca2+ channel currents in rat pinealocytes was investigated using the whole-cell version of the patch-clamp technique. The effects of three phosphatase inhibitors, calyculin A, tautomycin, and okadaic acid, were compared. Although all three inhibitors were effective in inhibiting the L-type Ca2+ channel current, calyculin A was more potent than either tautomycin or okadaic acid, suggesting the involvement of phosphoprotein phosphatase-1. To determine the kinase involved in the regulation of these channels, cells were pretreated with H7 (a nonspecific kinase inhibitor), H89 (a specific inhibitor of cyclic AMP-dependent kinase), KT5823 (a specific inhibitor of cyclic GMP-dependent kinase), or calphostin C (a specific inhibitor of protein kinase C). Pretreatment with either H7 or calphostin C decreased the inhibitory effect of calyculin A on the L-type Ca2+ channel current. In contrast, pretreatment with H89 or KT5823 had no effect on the inhibition caused by calyculin A. Based on these observations, we conclude that basal phosphatase activity, probably phosphoprotein phosphatase-1, plays an important role in the regulation of L-type Ca2+ channel currents in rat pinealocytes by counteracting protein kinase C-mediated phosphorylation.

Inhibitory effect of calyculin A, a Ser/Thr protein phosphatase type I inhibitor, on renin secretion

We have recently shown that several putative selective inhibitors of Ca2+-calmodulin-dependent myosin light chain kinase (MLCK), such as ML-9 [1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine], reversibly stimulate renin secretion [C. S. Park, S.-H. Chang, H. S. Lee, S.-H. Kim, J. W. Chang, and C. D. Hong. Am. J. Physiol. 271 (Cell Physiol. 40): C242-C247, 1996]. We hypothesized that Ca2+ inhibits renin secretion, via phosphorylation of 20-kDa myosin light chain (MLC20), by activating MLCK. In the present studies, we have investigated the types of protein phosphatase (PP) involved in the control of renin secretion through inhibition of MLC dephosphorylation using inhibitors of various types of serine/threonine-specific protein phosphatases. Cyclosporin A, a putative inhibitor of PP type 2 (calcineurin), was without effect. Calyculin A and okadaic acid, putative selective inhibitors of both PP type 1 (PP1) and type 2A (PP2A), significantly inhibited renin secretion under control conditions. Calyculin A had inhibitory effects at least 10-fold more potent than okadaic acid, suggesting that PP1, rather than PP2A, is involved in the control of renin secretion. Furthermore, calyculin A blocked the reversal of renin secretion preinhibited by raised intracellular Ca2+ concentrations in a concentration-dependent manner. Calyculin A (10(-6) M) significantly inhibited renin secretion stimulated by lowering intracellular Ca2+ concentrations and blocked the stimulatory effect of ML-9 on renin secretion. Taking all of these results into consideration, we hypothesize that dephosphorylation of MLC20 by Ca2+-independent PP1 stimulates renin secretion, whereas phosphorylation of MLC20 by Ca2+-calmodulin-dependent MLCK inhibits it. This hypothesized regulatory model of renin secretion predicts that the rate of renin secretion at a given time is determined by the ratio of phosphorylated to dephosphorylated MLC20, which is, in turn, determined by the dynamic balance between activity of MLCK and MLC phosphatase.

Identification and localization of myosin phosphatase in human platelets

Type 1 (PP1) and type 2A (PP2A) phosphatase activity was measured in three subcellular fractions of human platelets. About 80% of the activity was in the high-speed supernatant. Western blots showed that the catalytic subunit of PP1 (PP1c), including alpha- and delta-isoforms, was present in each fraction, but the level of the catalytic subunit of PP2A was very low in the low-speed pellet (cytoskeletal fraction). Various antibodies detected a subunit similar to the 130 kDa subunit (M130) of myosin phosphatase (MP) of smooth muscle in the low- and the high-speed pellets of human platelets. PP1c and associated proteins were isolated by microcystin-Sepharose. Many proteins were separated from each fraction, including myosin, actin and PP1c. M130 was separated only from the low-speed and the high-speed pellets. Kinase activities were detected in the unbound fractions, and fractions from the low- and high-speed pellets phosphorylated M130 and myosin respectively. Treatment of platelets with calyculin A increased the phosphorylation level of many proteins, including myosin heavy- and light-chains, and caused association of cytoskeletal proteins with the low-speed pellet. No marked change in the distribution of PP1c and M130 was detected. These results suggest that the MP in human platelets is composed of PP1c plus a subunit similar to M130 of the smooth muscle phosphatase.

Regulation of Myosin Phosphatase Through Phosphorylation of the Myosin-Binding Subunit in Platelet Activation

Human platelets were found to contain myosin phosphatase consisting of a 38-kD catalytic subunit of protein phosphatase type 1δ, a 130-kD myosin-binding subunit (MBS) and a 20-kD subunit, all of which cross-reacted with antibodies against these subunits of smooth muscle myosin phosphatase. Anti-MBS antibody coimmunoprecipitated RhoA and Rho-kinase of human platelets. Platelets MBS is a substrate for Rho-kinase and phosphorylation of MBS decreases the activity of myosin phosphatase. Treatment of intact platelets with 9,11-epithio-11,12-methano-thromboxane A2 led to a dramatic increase in phosphorylation of MBS and a significant decrease in the activity of myosin phosphatase. These findings suggest a putative mechanism for agonist-induced regulation of myosin phosphatase activity in platelets.

Platelet Adhesion to Collagen Under Flow Causes Dissociation of a Phosphoprotein Complex of Heat-Shock Proteins and Protein Phosphatase 1

Phosphorylation/dephosphorylation events in human blood platelets were investigated during their adhesion to collagen under flow conditions. Using 32P-labeled platelets and one-dimensional gel electrophoresis, we found that adhesion to collagen mediated primarily by the α2β1 integrin resulted in a strong dephosphorylation of several protein bands. Neither adhesion to polylysine nor thrombin-induced aggregation caused similar protein dephosphorylation. In addition, treatment with okadaic acid (OA), an inhibitor of serine/threonine protein phosphatases type 1 (PP1) and 2A (PP2A), caused significant inhibition of adhesion, suggesting that adhesion is regulated by OA-sensitive phosphatases. Recent studies indicate that phosphatases may be associated with the heat-shock proteins. Immunoprecipitations with antibodies against either the heat-shock cognate protein 70 (hsc70) or heat-shock protein 90 (hsp90) showed the presence of a phosphoprotein complex in 32P-labeled, resting human platelets. Antibody probing of this complex detected hsc70, hsp90, two isoforms of the catalytic subunit of PP1, PP1Cα and PP1Cδ, as well as the M regulatory subunit of PP1 (PP1M). OA, at concentrations that markedly blocked platelet adhesion to collagen, caused hyperphosphorylation of the hsc70 complex. In platelets adhering to collagen, hsc70 was completely dephosphorylated and hsp90, PP1α, and PP1M were dissociated from the complex, suggesting involvement of heat-shock proteins and protein phosphatases in platelet adhesion.

Platelet Adhesion to Collagen Under Flow Causes Dissociation of a Phosphoprotein Complex of Heat-Shock Proteins and Protein Phosphatase 1

Phosphorylation/dephosphorylation events in human blood platelets were investigated during their adhesion to collagen under flow conditions. Using 32P-labeled platelets and one-dimensional gel electrophoresis, we found that adhesion to collagen mediated primarily by the α2β1 integrin resulted in a strong dephosphorylation of several protein bands. Neither adhesion to polylysine nor thrombin-induced aggregation caused similar protein dephosphorylation. In addition, treatment with okadaic acid (OA), an inhibitor of serine/threonine protein phosphatases type 1 (PP1) and 2A (PP2A), caused significant inhibition of adhesion, suggesting that adhesion is regulated by OA-sensitive phosphatases. Recent studies indicate that phosphatases may be associated with the heat-shock proteins. Immunoprecipitations with antibodies against either the heat-shock cognate protein 70 (hsc70) or heat-shock protein 90 (hsp90) showed the presence of a phosphoprotein complex in 32P-labeled, resting human platelets. Antibody probing of this complex detected hsc70, hsp90, two isoforms of the catalytic subunit of PP1, PP1Cα and PP1Cδ, as well as the M regulatory subunit of PP1 (PP1M). OA, at concentrations that markedly blocked platelet adhesion to collagen, caused hyperphosphorylation of the hsc70 complex. In platelets adhering to collagen, hsc70 was completely dephosphorylated and hsp90, PP1α, and PP1M were dissociated from the complex, suggesting involvement of heat-shock proteins and protein phosphatases in platelet adhesion.

Shear stress-induced myosin association with cytoskeleton and phosphorylation in human platelets

Upon platelet activation by a high shear stress (108 dyne/cm2), actin and actin-binding protein increased rapidly into the Triton-insoluble cytoskeleton, whereas the association of myosin increased gradually. The amounts of cytoskeleton-associated myosin depended on the extent of aggregation. Preceding the maximal aggregation and ATP secretion, the 20 kDa light chain of myosin (MLC) is rapidly phosphorylated to approx. 45% of 20 kDa MLC and is then dephosphorylated. Cytoskeletal association of myosin and phosphorylation of 20 kDa MLC was inhibited by OP-41483, a prostaglandin I2 analog, which inhibited the full aggregation response to shear stress. Exposure to high shear stress resulted in an increased association of myosin light chain kinase and protein phosphatase types 1 and 2A with the cytoskeleton, while the cytoskeletal association of protein kinase C was not evident. These results indicate that 20 kDa MLC phosphorylation is involved in shear stress-induced platelet activation, and that cytoskeletal association of protein phosphatases may regulate the phosphorylation level of cytoskeletal elements such as myosin together with myosin light chain kinase.

Total inhibition of phospholipase C and phosphatidylinositol 3-kinase by okadaic acid in thrombin-stimulated platelets

The strong inhibition of thrombin-induced platelet functions induced by okadaic acid is not correlated with the partial modification of pleckstrin phosphorylation, which remains still phosphorylated two min after stimulation, indicating that protein kinase C is not affected by okadaic acid. We then investigated the effect of okadaic acid on platelet lipid metabolism. Our data indicate that inhibition indeed strongly affects phosphatidic acid as well as phosphatidylinositol 3,4-bisphosphate synthesis at low concentrations of okadaic acid, and phosphatidylinositol 4,5-bisphosphate at higher concentrations. Since thrombin-induced tyrosine phosphorylations were completely inhibited in the presence of okadaic acid, as a consequence, phosphatidylinositol 3-kinase was no longer detected in antiphosphotyrosine immunoprecipitates, thus explaining the absence of phosphatidylinositol, 3,4-bisphosphate synthesis. Finally, okadaic acid inhibited thrombin-induced fibrinogen binding, indicating that serine/threonine phosphatases may affect the inside-out signalling which regulates the alpha 11bb3 integrin, downstream protein kinase C activation.

Phosphatase inhibitors potentiate adrenergic-stimulated cAMP and cGMP production in rat pinealocytes

The role of phosphoprotein phosphatase in the regulation of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) accumulation in rat pinealocytes was investigated using the three phosphatase inhibitors calyculin A, tautomycin, and okadaic acid. Calyculin A (0.1 microM) was found to enhance the isoproterenol- and norepinephrine-stimulated cAMP accumulation six- and threefold, respectively, whereas tautomycin and okadaic acid were less effective. The effect of calyculin A was rapid (within 5 min) and persisted in the presence of phosphodiesterase inhibition. However, in contrast to protein kinase C activation or intracellular calcium elevation, the phosphatase inhibitors were less effective in potentiating the cAMP response stimulated by forskolin or cholera toxin, and their effects were not blocked by calphostin C or N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide. The adrenergic-stimulated cGMP response was also less sensitive to the phosphatase inhibition. Therefore, our results suggest that 1) the adrenergic-stimulated cAMP signal is subjected to the tonic inhibition by phosphoprotein phosphatase; 2) phosphatase inhibitors enhance cAMP synthesis through their actions at the receptor level; and 3) the cAMP signal is more sensitive to the regulation by phosphorylation than cGMP in rat pinealocytes.