Involvement of protein phosphatase-1 in cytoskeletal organization of cultured endothelial cells

Interrelationship between Protein Phosphatase-2A and Cytoskeletal Architecture during the Endothelial Cell Response to Soluble Products Produced by Human Head and Neck Cancer

Tumor neovascularization is necessary for the progressive development of all solid tumors, including head and neck squamous cell carcinomas (HNSCCs). The angiogenic process includes increased endothelial cell motility. Our prior studies have shown the importance of protein phos-phatase-2A (PP-2A) in restricting endothelial cell motility. Because motility is regulated by the polymerization/depolymerization of the cellular cytoskeleton, the present study defined the interrelationship between PP-2A and the cytoskeleton during endothelial cell responses to HNSCC-derived angiogenic factors. PP-2A was shown to colocalize with microtubules of unstimulated endothelial cells. However, exposure to HNSCC-derived products resulted in a more diffuse distribution of PP-2A staining and a loss of filamentous tubulin. The feasibility of pharmacologically preventing this cytoskeletal disorganization as a means of blocking tumor-induced angiogenesis was tested. This was accomplished by use of 1α,25-dihydroxyvitamin D3[1,25 (OH)2D3] and all- trans-retinoic acid to indirectly stimulate PP-2A activity through their capacity to elevated intracellular levels of the second messenger ceramide. Pretreatment of endothelial cells with either 1,25(OH)2D3or retinoic acid prevented the cytoskeletal disorganization that otherwise occurs in endothelial cells on exposure to HNSCC-derived products. These studies support the feasibility of using elevation of PP-2A to prevent the mor-phogenic component of the angiogenic process that is stimulated by HNSCC-derived factors.

Cytoskeleton reorganization as an alternative mechanism of store-operated calcium entry control in neuroendocrine-differentiated cells

Neuroendocrine differentiation (NED) is a hallmark of advanced androgen-independent prostate cancer, for which no successful therapy exists. NED tumour cells escape apoptotic cell death by alterations of Ca(2+) homeostasis where the store-operated Ca(2+) entry (SOCE) is known to be a key event. We have previously shown that the downregulation of Orai1 protein representing the major molecular component of endogenous SOCE in human prostate cancer cells, and constituting the principal source of Ca(2+) influx used by the cell to trigger apoptosis, contributes to the establishment of an apoptosis-resistant phenotype (Cell Death Dis. 2010 Sep 16;1:e75.). Here, we report for the first time that the decrease of SOCE during NED may be caused by alternative NED-induced mechanism involving cytoskeleton reorganisation. NED induced by androgen deprivation resulted in a decrease of SOCE due to cortical F-actin over-polymerization which inhibits thapsigargin-induced SOCE. The disruption of F-actin polymerization by Cytochalasin D in NED cells restored SOCE, while the induction of F-actin polymerization by jasplakinolide or calyculin A diminished SOCE without changing the expression of key SOCE players: Orai1, STIM1, and TRPC1. Our data suggest that targeting cytoskeleton-induced pathways of malignant cells together with SOCE-involved channels may prove a useful strategy in the treatment of advanced prostate cancer.

Combination of reverse and chemical genetic screens reveals angiogenesis inhibitors and targets

We combined reverse and chemical genetics to identify targets and compounds modulating blood vessel development. Through transcript profiling in mice, we identified 150 potentially druggable microvessel-enriched gene products. Orthologs of 50 of these were knocked down in a reverse genetic screen in zebrafish, demonstrating that 16 were necessary for developmental angiogenesis. In parallel, 1280 pharmacologically active compounds were screened in a human cell-based assay, identifying 28 compounds selectively inhibiting endothelial sprouting. Several links were revealed between the results of the reverse and chemical genetic screens, including the serine/threonine (S/T) phosphatases ppp1ca, ppp1cc, and ppp4c and an inhibitor of this gene family; Endothall. Our results suggest that the combination of reverse and chemical genetic screens, in vertebrates, is an efficient strategy for the identification of drug targets and compounds that modulate complex biological systems, such as angiogenesis.

Role of CPI-17 in the regulation of endothelial cytoskeleton

We have previously shown that myosin light chain (MLC) phosphatase (MLCP) is critically involved in the regulation of agonist-mediated endothelial permeability and cytoskeletal organization (Verin AD, Patterson CE, Day MA, and Garcia JG. Am J Physiol Lung Cell Mol Physiol 269: L99–L108, 1995). The molecular mechanisms of endothelial MLCP regulation, however, are not completely understood. In this study we found that, similar to smooth muscle, lung microvascular endothelial cells expressed specific endogenous inhibitor of MLCP, CPI-17. To elucidate the role of CPI-17 in the regulation of endothelial cytoskeleton, full-length CPI-17 plasmid was transiently transfected into pulmonary artery endothelial cells, where the background of endogenous protein is low. CPI-17 had no effect on cytoskeleton under nonstimulating conditions. However, stimulation of transfected cells with direct PKC activator PMA caused a dramatic increase in F-actin stress fibers, focal adhesions, and MLC phosphorylation compared with untransfected cells. Inflammatory agonist histamine and, to a much lesser extent, thrombin were capable of activating CPI-17. Histamine caused stronger CPI-17 phosphorylation than thrombin. Inhibitory analysis revealed that PKC more significantly contributes to agonist-induced CPI-17 phosphorylation than Rho-kinase. Dominant-negative PKC-α abolished the effect of CPI-17 on actin cytoskeleton, suggesting that the PKC-α isoform is most likely responsible for CPI-17 activation in the endothelium. Depletion of endogenous CPI-17 in lung microvascular endothelial cell significantly attenuated histamine-induced increase in endothelial permeability. Together these data suggest the potential importance of PKC/CPI-17-mediated pathway in histamine-triggered cytoskeletal rearrangements leading to lung microvascular barrier compromise.

Regulation of membrane trafficking and subcellular organization of endocytic compartments revealed with FM1-43 in resting and activated human T cells

FM1-43, a fluorescent styryl dye that penetrates into and stains membranes, was used to investigate kinetics of constitutive endocytosis and to visualize the fate of endocytic organelles in resting and activated human T lymphocytes. The rate of dye accumulation was strongly temperature dependent and approximately 10-fold higher in activated than in resting T cells. Elevation of cytosolic free Ca2+ concentration with thapsigargin or ionomycin further accelerated the rate of FM1-43 accumulation associated with cytosolic actin polymerization. Direct modulation of actin polymerization affected membrane trafficking. Actin condensation beneath the plasma membrane with calyculin A abolished FM1-43 internalization, whereas actin depolymerization with cytochalasin D had no effect. Photoconversion of DAB by FM1-43 revealed altered endocytic compartment targeting associated with T cell activation. Internalized cargo was carried to lysosome-like compartments in resting T cells and to multivesicular bodies (MVB) in activated T cells. Externalization of exosomes from MVB occurred commonly in activated but not in resting T cells. T cell exosomes contained raft-associated CD3 proteins, GM1 glycosphingolipids, and phosphatidylserine at the outer membrane leaflet. The present study demonstrates the utility of FM1-43 as a marker of membrane trafficking in T cells and reveals possible mechanisms of its modulation during T cell activation.

Role of cortical filamentous actin in the melanotrope cell of Xenopus laevis

In secretory cells filamentous actin (f-actin) is mostly present subjacent to the plasma membrane, referred to as cortical actin. While the function of cortical actin in the secretory processes has been extensively studied, little attention has been given to the role of actin in signal transduction and intracellular second messenger dynamics. Analysis with the fluorescent f-actin probe Alexa-phalloidin shows that Xenopus laevis pituitary melanotrope cells possess a thick cortical actin ring. This cell is a good model to study the possible function(s) of f-actin in signal transduction processes. Regulation of the release of alpha-MSH from this cell involves a convergence of various receptor mechanisms to regulate the activity of voltage-operated Ca2+ channels. We have considered three potential functions for the cortical actin ring in the signaling process of the melanotrope: (1) it functions as a barrier for access of secretory granules to the membrane for exocytosis, (2) it is involved in anchoring components of the Ca2+ signalling machinery of the cell, and/or (3) it helps to form a scaffold for components of the signal transduction machinery used by the various neurotransmitters and neuropeptides that regulate the activity of the cell. To test these possibilities we have examined the effect of the f-actin depolymerising toxin latrunculin B on Ca2+ signaling, signal transduction and alpha-MSH secretion in the melanotrope. We show that while the toxin is effective in disrupting the cortical actin ring, this treatment has no effect on either Ca2+ signaling or the signal transduction processes studied. The toxin does induce an increase in alpha-MSH release, indicating that the cortical actin ring acts as a barrier for secretory granule access to the membrane.

Possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation in Ehrlich ascites tumor cells

Osmotic shrinkage of Ehrlich ascites tumor cells (EATC) elicited translocation of myosin II from the cytosol to the cortical region, and swelling elicits concentration of myosin II in the Golgi region. Rho kinase and p38 both appeared to be involved in shrinkage-induced myosin II reorganization. In contrast, the previously reported shrinkage-induced actin polymerization [Pedersen et al. (1999) Exp. Cell Res. 252, 63-74] was independent of Rho kinase, p38, myosin light chain kinase (MLCK), and protein kinase C (PKC), which thus do not exert their effects on the shrinkage-activated transporters via effects on F-actin. The subsequent F-actin depolymerization, however, appeared MLCK- and PKC-dependent, and the initial swelling-induced F-actin depolymerization was MLCK-dependent; both effects were apparently secondary to kinase-mediated effects on cell volume changes. NHE1 in EATC is activated both by osmotic shrinkage and by the serine/threonine phosphatase inhibitor Calyculin A (CL-A). Both stimuli caused Rho kinase-dependent myosin II relocation to the cortical cytoplasm, but in contrast to the shrinkage-induced F-actin polymerization, CL-A treatment elicited a slight F-actin depolymerization. Moreover, Rho kinase inhibition did not significantly affect NHE1 activation, neither by shrinkage nor by CL-A. Implications for the possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation are discussed.

Calyculin A-induced endothelial cell shape changes are independent of [Ca(2+)](i) elevation and may involve actin polymerization

Changes in endothelial cell (EC) shape result in inter-EC gap formation and subsequently regulate transendothelial passage. In this work, we investigated the effects of protein phosphorylation (induced by inhibition of protein phosphatases) on EC shape changes. Treatment of bovine pulmonary artery endothelial cells (BPAEC) with calyculin A (100 nM, an inhibitor of protein Ser/Thr phosphatases 1 and 2A) resulted in cell retraction, surface bleb formation and cell rounding. Trypan blue and electrophysiological experiments suggested that the plasma membrane of these rounded cells maintained functional integrity. Calyculin A-induced morphological changes were strongly inhibited by staurosporine, but not affected by specific inhibitors of the myosin light chain (MLC) kinase, protein kinases A, C and G, and tyrosine kinases. The calyculin A effects were not mimicked by phorbol myristate acetate, dibutyryl cAMP, 8-bromo-cGMP or ionomycin. Cytochalasin B (an inhibitor of actin polymerization) almost completely abolished such shape changes while colchicine (an inhibitor of microtubule polymerization) had no inhibitory effect at all. Ca(2+) imaging experiments showed that the morphological changes were not associated with any global or local cytosolic Ca(2+) concentration ([Ca(2+)](i)) elevation. The results suggest that calyculin A unmasked the basal activities of some protein Ser/Thr kinases other than MLC kinase and protein kinases A, C and G; these unknown kinases might cause BPAEC shape changes by a mechanism involving actin polymerization but not [Ca(2+)](i) elevation.

Stabilization of cortical actin induces internalization of transient receptor potential 3 (Trp3)-associated caveolar Ca2+ signaling complex and loss of Ca2+ influx without disruption of Trp3-inositol trisphosphate receptor association

Ca(2+) influx via plasma membrane Trp3 channels is proposed to be regulated by a reversible interaction with inositol trisphosphate receptor (IP(3)R) in the endoplasmic reticulum. Condensation of the cortical actin layer has been suggested to physically disrupt this interaction and inhibit Trp3-mediated Ca(2+) influx. This study examines the effect of cytoskeletal reorganization on the localization and function of Trp3 and key Ca(2+) signaling proteins. Calyculin-A treatment resulted in formation of condensed actin layer at the plasma membrane; internalization of Trp3, Galpha(q/11), phospholipase Cbeta, and caveolin-1; and attenuation of 1-oleoyl-2-acetyl-sn-glycerol- and ATP-stimulated Sr(2+) influx. Importantly, Trp3 and IP(3)R-3 remained co-localized inside the cell and were co-immunoprecipitated. Jasplakinolide also induced internalization of Trp3 and caveolin-1. Pretreatment of cells with cytochalasin D or staurosporine did not affect Trp3 but prevented calyculin-A-induced effects. Based on these data, we suggest that Trp3 is assembled in a caveolar Ca(2+) signaling complex with IP(3)R, SERCA, Galpha(q/11), phospholipase Cbeta, caveolin-1, and ezrin. Furthermore, our data demonstrate that conditions which stabilize cortical actin induce loss of Trp3 activity due to internalization of the Trp3-signaling complex, not disruption of IP(3)R-Trp3 interaction. This suggests that localization of the Trp3-associated signaling complex, rather than Trp3-IP(3)R coupling, depends on the status of the actin cytoskeleton.

Calyculin-A induces focal adhesion assembly and tyrosine phosphorylation of p125(Fak), p130(Cas), and paxillin in Swiss 3T3 cells

Treatment of intact Swiss 3T3 cells with calyculin-A, an inhibitor of myosin light chain (MLC) phosphatase, induces tyrosine phosphorylation of p125(Fak) in a sharply concentration- and time-dependent manner. Maximal stimulation was 4.2 +/- 2.1-fold (n = 14). The stimulatory effect of calyculin-A was observed at low nanomolar concentrations (<10 nM); at higher concentrations (>10 nM) tyrosine phosphorylation of p125(Fak) was strikingly decreased. Calyculin-A induced tyrosine phosphorylation of p125(Fak) through a protein kinase C- and Ca(2+)-independent pathway. Exposure to either cytochalasin-D or latrunculin-A, which disrupt actin organization by different mechanisms, abolished tyrosine phosphorylation of p125(Fak) in response to calyculin-A. Treatment with high concentrations of platelet-derived growth factor (20 ng/ml) which also disrupt actin stress fibers, completely inhibited tyrosine phosphorylation of p125(Fak) in response to calyculin-A. This agent also induced tyrosine phosphorylation of the focal adhesion-associated proteins p130(Cas) and paxillin. These tyrosine phosphorylation events were associated with a striking increase in the assembly of focal adhesions. The Rho kinase (ROK) inhibitor HA1077 that blocked focal adhesion formation by bombesin, had no effect on the focal adhesion assembly induced by calyculin-A. Thus, calyculin-A induces transient focal adhesion assembly and tyrosine phosphorylation of p125(Fak), p130(Cas), and paxillin, acting downstream of ROK.

An examination of the secretion-like coupling model for the activation of the Ca2+ release-activated Ca2+ current I(CRAC) in RBL-1 cells

One popular model for the activation of store-operated Ca2+ influx is the secretion-like coupling mechanism, in which peripheral endoplasmic reticulum moves to the plasma membrane upon store depletion thereby enabling inositol 1,4,5-trisphosphate (InsP3) receptors on the stores to bind to, and thus activate, store-operated Ca2+ channels. This movement is regulated by the underlying cytoskeleton. We have examined the validity of this mechanism for the activation of I(CRAC), the most widely distributed and best characterised store-operated Ca2+ current, in a model system, the RBL-1 rat basophilic cell line. Stabilisation of the peripheral cytoskeleton, disassembly of actin microfilaments and disaggregation of microtubules all consistently failed to alter the rate or extent of activation of I(CRAC). Rhodamine-phalloidin labelling was used wherever possible, and revealed that the cytoskeleton had been significantly modified by drug treatment. Interference with the cytoskeleton also failed to affect the intracellular calcium signal that occurred when external calcium was re-admitted to cells in which the calcium stores had been previously depleted by exposure to thapsigargin/ionomycin in calcium-free external solution. Application of positive pressure through the patch pipette separated the plasma membrane from underlying structures (cell ballooning). However, I(CRAC) was unaffected irrespective of whether cell ballooning occurred before or after depletion of stores. Pre-treatment with the membrane-permeable InsP3 receptor antagonist 2-APB blocked the activation of I(CRAC). However, intracellular dialysis with 2-APB failed to prevent I(CRAC) from activating, even at higher concentrations than those used extracellularly to achieve full block. Local application of 2-APB, once I(CRAC) had been activated, resulted in a rapid loss of the current at a rate similar to that seen with the rapid channel blocker La3+. Studies with the more conventional InsP3 receptor antagonist heparin revealed that occupation of the intracellular InsP3-sensitive receptors was not necessary for the activation or maintenance of I(CRAC). Similarly, the InsP3 receptor inhibitor caffeine failed to alter the rate or extent of activation of I(CRAC). Exposure to Li+, which reduces InsP3 levels by interfering with inositol monophosphatase, also failed to alter I(CRAC). Caffeine and Li+ did not affect the size of the intracellular Ca2+ signal that arose when external Ca2+ was re-admitted to cells which had been pre-exposed to thapsigargin/ionomycin in Ca2+-free external solution. Our findings demonstrate that the cytoskeleton does not seem to regulate calcium influx and that functional InsP3 receptors are not required for activation of I(CRAC). If the secretion-like coupling model indeed accounts for the activation of I(CRAC) in RBL-1 cells, then it needs to be revised significantly. Possible modifications to the model are discussed.

Lymphocyte migration through monolayers of endothelial cell lines involves VCAM-1 signaling via endothelial cell NADPH oxidase

Lymphocytes migrate from the blood across endothelial cells to reach foreign substances sequestered in peripheral lymphoid organs and inflammatory sites. To study intracellular signaling in endothelial cells during lymphocyte migration, we used murine endothelial cell lines that promote lymphocyte migration and constitutively express VCAM-1. The maximum rate of resting splenic lymphocyte migration across monolayers of the endothelial cells occurred at 0-24 h. This migration was inhibited by anti-VCAM-1 or anti-alpha4 integrin, suggesting that VCAM-1 adhesion was required for migration. To determine whether signals within the endothelial cells were required for migration, irreversible inhibitors of signal transduction molecules were used to pretreat the endothelial cell lines. Inhibitors of NADPH oxidase activity (diphenyleneiodonium and apocynin) blocked migration >65% without affecting adhesion. Because NADPH oxidase catalyzes the production of reactive oxygen species (ROS), we examined whether ROS were required for migration. Scavengers of ROS inhibited migration without affecting adhesion. Furthermore, VCAM-1 ligand binding stimulated NADPH oxidase-dependent production of ROS by the endothelial cells lines and primary endothelial cell cultures. Finally, VCAM-1 ligand binding induced an apocynin-inhibitable actin restructuring in the endothelial cell lines at the location of the lymphocyte or anti-VCAM-1-coated bead, suggesting that an NADPH oxidase-dependent endothelial cell shape change was required for lymphocyte migration. In summary, VCAM-1 signaled the activation of endothelial cell NADPH oxidase, which was required for lymphocyte migration. This suggests that endothelial cells are not only a scaffold for lymphocyte adhesion, but play an active role in promoting lymphocyte migration.

Interrelationship between protein phosphatase-2A and cytoskeletal architecture during the endothelial cell response to soluble products produced by human head and neck cancer

Tumor neovascularization is necessary for the progressive development of all solid tumors, including head and neck squamous cell carcinomas (HNSCCs). The angiogenic process includes increased endothelial cell motility. Our prior studies have shown the importance of protein phosphatase-2A (PP-2A) in restricting endothelial cell motility. Because motility is regulated by the polymerization/depolymerization of the cellular cytoskeleton, the present study defined the interrelationship between PP-2A and the cytoskeleton during endothelial cell responses to HNSCC-derived angiogenic factors. PP-2A was shown to colocalize with microtubules of unstimulated endothelial cells. However, exposure to HNSCC-derived products resulted in a more diffuse distribution of PP-2A staining and a loss of filamentous tubulin. The feasibility of pharmacologically preventing this cytoskeletal disorganization as a means of blocking tumor-induced angiogenesis was tested. This was accomplished by use of 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and all-trans -retinoic acid to indirectly stimulate PP-2A activity through their capacity to elevated intracellular levels of the second messenger ceramide. Pretreatment of endothelial cells with either 1,25(OH)(2)D(3) or retinoic acid prevented the cytoskeletal disorganization that otherwise occurs in endothelial cells on exposure to HNSCC-derived products. These studies support the feasibility of using elevation of PP-2A to prevent the morphogenic component of the angiogenic process that is stimulated by HNSCC-derived factors.

A role for the actin cytoskeleton in the initiation and maintenance of store-mediated calcium entry in human platelets. Evidence for conformational coupling

The nature of the mechanism underlying store-mediated Ca(2+) entry has been investigated in human platelets through a combination of cytoskeletal modifications. Inhibition of actin polymerization by cytochalasin D or latrunculin A had a biphasic time-dependent effect on Ca(2+) entry, showing an initial potentiation followed by inhibition of Ca(2+) entry. Moreover, addition of these agents after induction of store-mediated Ca(2+) entry inhibited the Ca(2+) influx mechanism. Jasplakinolide, which reorganizes actin filaments into a tight cortical layer adjacent to the plasma membrane, prevented activation of store-mediated Ca(2+) entry but did not modify this process after its activation. In addition, jasplakinolide prevented cytochalasin D-induced inhibition of store-mediated Ca(2+) entry. Calyculin A, an inhibitor of protein serine/threonine phosphatases 1 and 2 which activates translocation of existing F-actin to the cell periphery without inducing actin polymerization, also prevented activation of store-mediated Ca(2+) entry. Finally, inhibition of vesicular transport with brefeldin A inhibited activation of store-mediated Ca(2+) entry but did not alter this mechanism once initiated. These data suggest that store-mediated Ca(2+) entry in platelets may be mediated by a reversible trafficking and coupling of the endoplasmic reticulum with the plasma membrane, which shows close parallels to the events mediating secretion.

Protein phosphatases 1 and 2A maintain endothelial cells in a resting state, limiting the motility that is needed for the morphogenic process of angiogenesis

Angiogenesis that is induced by cancers, including those of the head and neck, requires endothelial cells to shift from a nonmotile resting state to an increased level of motility. Using a human microvascular endothelial cell line, this study shows the importance of the serine/threonine protein phosphatases 1 (PP1) and 2A (PP2A) in restricting endothelial cell motility. Treatment of endothelial cells with increasing concentrations of the PP1 and PP2A inhibitor okadaic acid resulted in cell rounding and increased motility, which was accompanied by cytoskeletal disorganization involving a loss of filamentous beta-tubulin and F-actin. These effects occurred at okadaic acid levels that selectively inhibit PP2A and became more prominent with higher levels that inhibit both PP2A and PP1. This study shows the importance of PP1 and PP2A in maintaining cytoskeletal organization, thereby limiting endothelial cell motility, and suggests that pharmacologic approaches to enhance PP1 and PP2A activities may be useful in preventing key events of the angiogenic process.

Store-operated Ca2+ entry: evidence for a secretion-like coupling model

The elusive coupling between endoplasmic reticulum (ER) Ca2+ stores and plasma membrane (PM) "store-operated" Ca2+ entry channels was probed through a novel combination of cytoskeletal modifications. Whereas coupling was unaffected by disassembly of the actin cytoskeleton, in situ redistribution of F-actin into a tight cortical layer subjacent to the PM displaced cortical ER and prevented coupling between ER and PM Ca2+ entry channels, while not affecting inositol 1,4,5-trisphosphate-mediated store release. Importantly, disassembly of the induced cortical actin layer allowed ER to regain access to the PM and reestablish coupling of Ca2+ entry channels to Ca2+ store depletion. Coupling is concluded to be mediated by a physical "secretion-like" mechanism involving close but reversible interactions between the ER and the PM.

Role of Ca2+/calmodulin-dependent phosphatase 2B in thrombin-induced endothelial cell contractile responses

Thrombin-induced Ca2+ mobilization, activation of Ca2+/calmodulin-dependent myosin light chain (MLC) kinase (MLCK), and increased phosphorylation of MLCs precede and are critical to endothelial cell (EC) barrier dysfunction. Net MLC dephosphorylation after thrombin is nearly complete by 60 min and involves type 1 phosphatase (PPase 1) activity. We now report that thrombin does not alter total PPase 1 activity in EC homogenates but rather decreases myosin-associated PPase 1 activity. The PPase 1 inhibitor calyculin fails to prevent thrombin-induced MLC dephosphorylation. However, thrombin significantly increased the activity of Ca2+-dependent PPase 2B in EC homogenates (approximately 1.5- to 2-fold), with PPase 2B activation correlating with phosphorylation of the PPase 2B catalytic subunit. Western immunoblotting revealed PPase 2B to be present in cytoskeletal EC fractions, with specific PPase 2B inhibitors such as cyclosporin (200 nM) and deltamethrin (100 nM to 1 microM) attenuating thrombin-induced cytoskeletal protein dephosphorylation, including EC MLC dephosphorylation. These results suggest a model whereby thrombin-inducible contraction is determined by the phosphorylation status of EC MLC regulated by the balance between EC MLCK, PPase 1 (constitutive), and PPase 2B (inducible) activities.

Phosphorylation of the Cytoplasmic Domain of E-Selectin Is Regulated During Leukocyte-Endothelial Adhesion

E-selectin, a selectin expressed on activated vascular endothelium, supports rolling and stable adhesion of leukocytes at sites of inflammation. Previously, we have reported that leukocyte adhesion to cultured endothelial cells induces association of the cytoplasmic domain of E-selectin with cytoskeletal elements, suggesting that outside-in signaling may occur during E-selectin-mediated adhesion. To investigate this potential signaling function of E-selectin, HUVEC activated with recombinant human IL-1β (10 U/ml, 4 h) were labeled with [32P]orthophosphate, and E-selectin was immunoprecipitated using mAb H18/7. Autoradiography revealed constitutive phosphorylation of E-selectin in these cells and time-dependent dephosphorylation following adhesion of HL-60 cells. Cross-linking of cell surface E-selectin using H18/7 and a polyclonal secondary Ab induced E-selectin dephosphorylation, as did adhesion of beads coated with recombinant P-selectin glycoprotein ligand-1 (PSGL-1), an E-selectin ligand. Using adenoviral vector-mediated transfection in HUVEC of a tail-less E-selectin and phosphoamino acid analysis, we documented phosphorylation occurring exclusively within the cytoplasmic domain and involving serine residues. Additional experiments using a series of cytoplasmic domain mutants of E-selectin expressed in COS-7 cells localized the regions that were constitutively phosphorylated. Preincubation with okadaic acid and sodium vanadate abrogated adhesion-induced dephosphorylation of E-selectin. Thus, E-selectin, which is constitutively phosphorylated in cytokine-activated human endothelial cells, undergoes an enzymatically regulated dephosphorylation following leukocyte adhesion. This process appears to be triggered by multivalent ligand binding and/or cross-linking of cell surface E-selectin. Ligand-dependent regulation of the phosphorylation of E-selectin’s cytoplasmic domain provides additional evidence for a transmembrane signaling function of this molecule during leukocyte-endothelial interactions.