In situ phosphorylation of human platelet myosin heavy and light chains by protein kinase C

Aurora-B phosphorylates the myosin II heavy chain to promote cytokinesis

Cytokinesis, the final step of mitosis, is mediated by an actomyosin contractile ring, the formation of which is temporally and spatially regulated following anaphase onset. Aurora-B is a member of the chromosomal passenger complex, which regulates various processes during mitosis; it is not understood, however, how Aurora-B is involved in cytokinesis. Here, we show that Aurora-B and myosin-IIB form a complex in vivo during telophase. Aurora-B phosphorylates the myosin-IIB rod domain at threonine 1847 (T¹⁸⁴⁷), abrogating the ability of myosin-IIB monomers to form filaments. Furthermore, phosphorylation of myosin-IIB filaments by Aurora-B also promotes filament disassembly. We show that myosin-IIB possessing a phosphomimetic mutation at T¹⁸⁴⁷ was unable to rescue cytokinesis failure caused by myosin-IIB depletion. Cells expressing a phosphoresistant mutation at T¹⁸⁴⁷ had significantly longer intercellular bridges, implying that Aurora-B-mediated phosphorylation of myosin-IIB is important for abscission. We propose that myosin-IIB is a substrate of Aurora-B and reveal a new mechanism of myosin-IIB regulation by Aurora-B in the late stages of mitosis.

MYH9: Structure, functions and role of non-muscle myosin IIA in human disease

The MYH9 gene encodes the heavy chain of non-muscle myosin IIA, a widely expressed cytoplasmic myosin that participates in a variety of processes requiring the generation of intracellular chemomechanical force and translocation of the actin cytoskeleton. Non-muscle myosin IIA functions are regulated by phosphorylation of its 20 kDa light chain, of the heavy chain, and by interactions with other proteins. Variants of MYH9 cause an autosomal-dominant disorder, termed MYH9-related disease, and may be involved in other conditions, such as chronic kidney disease, non-syndromic deafness, and cancer. This review discusses the structure of the MYH9 gene and its protein, as well as the regulation and physiologic functions of non-muscle myosin IIA with particular reference to embryonic development. Moreover, the review focuses on current knowledge about the role of MYH9 variants in human disease.

Quantitation of myosin regulatory light chain phosphorylation in biological samples with multiple reaction monitoring mass spectrometry

The 20-kDa regulatory light chain of myosin II plays an important role in regulating smooth muscle contractile force. LC20 is phosphorylated canonically by myosin light chain kinase in a Ca²⁺/calmodulin-dependent manner at S19. The diphosphorylation of LC20 at T18 and S19 has been observed in smooth muscle tissues. Given that the phosphorylation of LC20 is positively correlated with tension development, the molar stoichiometry of LC20 phosphorylation is commonly profiled as a measure of smooth muscle contractility. Herein, we describe a novel multiple reaction monitoring (MRM)-mass spectrometry (MS) approach for the quantification of LC20 phosphorylation at T18 and S19. Unique precursor as well as y- and b-ion transitions were identified for unphosphorylated LC20-(TS), monophosphorylated LC20-(TpS) and diphosphorylated LC20-(pTpS) peptides. The MRM-MS assay could accurately define molar phosphorylation stoichiometries of S19 and T18 over a broad range (i.e., 0-2 mol P/mol LC20). Correlations of the results for two quantification techniques indicate that the MRM-MS assay performs equally to Phos-tag SDS-PAGE for the determination of LC20 phosphorylation stoichiometry in arterial tissue samples. The MRM-MS technique provides a robust alternative to antibody-based detection systems for the quantification of LC20 phosphorylation.

Myosin IIA Heavy Chain Phosphorylation Mediates Adhesion Maturation and Protrusion in Three Dimensions

Non-muscle myosin II (NMII) is a conserved force-producing cytoskeletal enzyme with important but poorly understood roles in cell migration. To investigate myosin heavy chain (MHC) phosphorylation roles in 3D migration, we expressed GFP-tagged NMIIA wild-type or mutant constructs in cells depleted of endogenous NMIIA protein. We find that individual mutation or double mutation of Ser-1916 or Ser-1943 to alanine potently blocks recruitment of GFP-NM-IIA filaments to leading edge protrusions in 2D, and this in turn blocks maturation of anterior focal adhesions. When placed in 3D collagen gels, cells expressing wild-type GFP MHC-IIA behave like parental cells, displaying robust and active formation and retraction of protrusions. However, cells depleted of NMIIA or cells expressing the mutant GFP MHC-IIA display severe defects in invasion and in stabilizing protrusions in 3D. These studies reveal an NMIIA-specific role in 3D invasion that requires competence for NMIIA phosphorylation at Ser-1916 and Ser-1943. In sum, these results demonstrate a critical and previously unrecognized role for NMIIA phosphorylation in 3D invasion.

The heavy chain has its day: regulation of myosin-II assembly

Nonmuscle myosin-II is an actin-based motor that converts chemical energy into force and movement, and thus functions as a key regulator of the eukaryotic cytoskeleton. Although it is established that phosphorylation on the regulatory light chain increases the actin-activated MgATPase activity of the motor and promotes myosin-II filament assembly, studies have begun to characterize alternative mechanisms that regulate filament assembly and disassembly. These investigations have revealed that all three nonmuscle myosin-II isoforms are subject to additional regulatory controls, which impact diverse cellular processes. In this review, we discuss current knowledge on mechanisms that regulate the oligomerization state of nonmuscle myosin-II filaments by targeting the myosin heavy chain.

Mesenchymal chemotaxis requires selective inactivation of myosin II at the leading edge via a noncanonical PLCγ/PKCα pathway

Chemotaxis, migration toward soluble chemical cues, is critical for processes such as wound healing and immune surveillance and is exhibited by various cell types, from rapidly migrating leukocytes to slow-moving mesenchymal cells. To study mesenchymal chemotaxis, we observed cell migration in microfluidic chambers that generate stable gradients of platelet-derived growth factor (PDGF). Surprisingly, we found that pathways implicated in amoeboid chemotaxis, such as PI3K and mammalian target of rapamycin signaling, are dispensable for PDGF chemotaxis. Instead, we find that local inactivation of Myosin IIA, through a noncanonical Ser1/2 phosphorylation of the regulatory light chain, is essential. This site is phosphorylated by PKCα, which is activated by an intracellular gradient of diacylglycerol generated by PLCγ. Using a combination of live imaging and gradients of activators/inhibitors in the microfluidic chambers, we demonstrate that this signaling pathway and subsequent inhibition of Myosin II activity at the leading edge are required for mesenchymal chemotaxis.

Critical role for CD38-mediated Ca2+ signaling in thrombin-induced procoagulant activity of mouse platelets and hemostasis

CD38, a multifunctional enzyme that catalyzes the synthesis of intracellular Ca(2+) messengers, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), is known to be expressed on platelets. However, the role of CD38 in platelets remains unclear. Our present results show that treatment of platelets with thrombin results in a rapid and sustained Ca(2+) signal, resulting from a coordinated interplay of Ca(2+)-mobilizing messengers, inositol 1,4,5-trisphosphate, cADPR, and NAADP. By dissecting the signaling pathway using various agents, we delineated that cADPR and NAADP are sequentially produced through CD38 internalization by protein kinase C via myosin heavy chain IIA following phospholipase C activation in thrombin-induced platelets. An inositol 1,4,5-trisphosphate receptor antagonist blocked the thrombin-induced formation of cADPR and NAADP as well as Ca(2+) signals. An indispensable response of platelets relying on cytosolic calcium is the surface exposure of phosphatidylserine (PS), which implicates platelet procoagulant activity. Scrutinizing this parameter reveals that CD38(+/+) platelets fully express PS on the surface when stimulated with thrombin, whereas this response was decreased on CD38(-/-) platelets. Similarly, PS exposure and Ca(2+) signals were attenuated when platelets were incubated with 8-bromo-cADPR, bafilomycin A1, and a PKC inhibitor. Furthermore, in vivo, CD38-deficient mice exhibited longer bleeding times and unstable formation of thrombus than wild type mice. These results demonstrate that CD38 plays an essential role in thrombin-induced procoagulant activity of platelets and hemostasis via Ca(2+) signaling mediated by its products, cADPR and NAADP.

Protein kinase C activation disrupts epithelial apical junctions via ROCK-II dependent stimulation of actomyosin contractility

BACKGROUND

Disruption of epithelial cell-cell adhesions represents an early and important stage in tumor metastasis. This process can be modeled in vitro by exposing cells to chemical tumor promoters, phorbol esters and octylindolactam-V (OI-V), known to activate protein kinase C (PKC). However, molecular events mediating PKC-dependent disruption of epithelial cell-cell contact remain poorly understood. In the present study we investigate mechanisms by which PKC activation induces disassembly of tight junctions (TJs) and adherens junctions (AJs) in a model pancreatic epithelium.

RESULTS

Exposure of HPAF-II human pancreatic adenocarcinoma cell monolayers to either OI-V or 12-O-tetradecanoylphorbol-13-acetate caused rapid disruption and internalization of AJs and TJs. Activity of classical PKC isoenzymes was responsible for the loss of cell-cell contacts which was accompanied by cell rounding, phosphorylation and relocalization of the F-actin motor nonmuscle myosin (NM) II. The OI-V-induced disruption of AJs and TJs was prevented by either pharmacological inhibition of NM II with blebbistatin or by siRNA-mediated downregulation of NM IIA. Furthermore, AJ/TJ disassembly was attenuated by inhibition of Rho-associated kinase (ROCK) II, but was insensitive to blockage of MLCK, calmodulin, ERK1/2, caspases and RhoA GTPase.

CONCLUSION

Our data suggest that stimulation of PKC disrupts epithelial apical junctions via ROCK-II dependent activation of NM II, which increases contractility of perijunctional actin filaments. This mechanism is likely to be important for cancer cell dissociation and tumor metastasis.

Regulation of the function of mammalian myosin and its conformational change

It has been known that the phosphorylation of the regulatory light chain, residing at the head/rod junction of the molecule activates the motor activity of smooth muscle and non-muscle conventional myosin (myosin II), and triggers a large conformational change of the molecule from the inhibited folded conformation to the active extended conformation. Recent structural analysis has revealed the structural basis of the inhibition of the motor function of the two heads in the inhibited conformation. On the other hand, recent studies have revealed that a processive unconventional myosin, myosin V, also shows a large change in the conformation from the folded to an extended form and this explains the activation mechanism of myosin V motor activity. These findings suggest the presence of a common scenario for the regulation of motor protein functions.

The phosphorylation of myosin II at the Ser1 and Ser2 is critical for normal platelet-derived growth factor induced reorganization of myosin filaments

Phosphorylation of the regulatory light chain of myosin II (MLC(20)) at the activation sites promotes both the motor activity and the filament formation of myosin II, thus playing an important role in various cell motile processes. In contrast, the physiological function of phosphorylation of MLC(20) at the inhibitory sites is unknown. Here we report for the first time the function of the inhibitory site phosphorylation in the cells. We successfully produced the antibodies specifically recognizing the phosphorylation sites of MLC(20) at Ser1, and the platelet-derived growth factor (PDGF)-induced change in the phosphorylation at the Ser1 was monitored. The phosphorylation of MLC(20) at the Ser1 significantly increased during the PDGF-induced actin cytoskeletal reorganization. PDGF disassembled the stress fibers, and this was attenuated with the expression of unphosphorylatable MLC(20) at the Ser1/Ser2 phosphorylation sites. The present results suggest that the down-regulation of myosin II activity achieved by the phosphorylation at the Ser1/Ser2 sites plays an important role in the normal reorganization of actomyosin filaments triggered by PDGF receptor stimulation.

Protein kinase Cgamma regulates myosin IIB phosphorylation, cellular localization, and filament assembly

Nonmuscle myosin II is an important component of the cytoskeleton, playing a major role in cell motility and chemotaxis. We have previously demonstrated that, on stimulation with epidermal growth factor (EGF), nonmuscle myosin heavy chain II-B (NMHC-IIB) undergoes a transient phosphorylation correlating with its cellular localization. We also showed that members of the PKC family are involved in this phosphorylation. Here we demonstrate that of the two conventional PKC isoforms expressed by prostate cancer cells, PKCbetaII and PKCgamma, PKCgamma directly phosphorylates NMHC-IIB. Overexpression of wild-type and kinase dead dominant negative PKCgamma result in both altered NMHC-IIB phosphorylation and subcellular localization. We have also mapped the phosphorylation sites of PKCgamma on NMHC-IIB. Conversion of the PKCgamma phosphorylation sites to alanine residues, reduces the EGF-dependent NMHC-IIB phosphorylation. Aspartate substitution of these sites reduces NMHC-IIB localization into cytoskeleton. These results indicate that PKCgamma regulates NMHC-IIB phosphorylation and cellular localization in response to EGF stimulation.

Gene delivery of l-caldesmon protects cytoskeletal cell membrane integrity against adenovirus infection independently of myosin ATPase and actin assembly

The cytoskeleton is critical to the viral life cycle. Agents like cytochalasin inhibit viral infections but cannot be used for antiviral therapy because of their toxicity. We report the efficacy, safety, and mechanisms by which gene delivery of human wild-type low-molecular-weight caldesmon (l-CaD) protects cell membrane integrity from adenovirus infection in a DF-1 cell line, an immortalized avian fibroblast that is null for l-CaD. Transfection with an adenovirus (Ad)-controlled construct mediated a dose-dependent decline in transcellular resistance. In accordance with a computational model of cytoskeletal membrane properties, Ad disturbed cell-cell and cell-matrix adhesion and membrane capacitance. Transfection with the Ad-l-CaD construct attenuated adenovirus-mediated loss in transcellular resistance. Quantitation of vinculin-stained plaques revealed an increase in total focal contact mass in monolayers transfected with the Ad-l-CaD construct. Expression of l-CaD protected transcellular resistance through primary effects on membrane capacitance and independently of actin solubility and effects on prestress, as measured by the decline in isometric tension in response to cytochalasin D. Expression of l-CaD exhibited less Trypan blue cell toxicity than cytochalasin, and, unlike cytochalasin, it did not interfere with wound closure or adversely effect transcellular resistance. These findings demonstrate the gene delivery of wild-type human l-CaD as a potentially efficacious and safe agent that inhibits some of the cytopathic effects of adenovirus.

Distinct contractile systems for electromechanical and pharmacomechanical coupling in smooth muscle

Electromechanical coupling by KCl depolarization of bladder preparations elicits an initial phasic and subsequent tonic contraction. Using a smooth-muscle myosin heavy chain (SM-MyHC) knock-out mouse model we could previously demonstrate, that phasic and tonic contraction of intact neonatal bladder preparations could be elicited through the recruitment of SM-MyHC and non-muscle myosin heavy chains (NM-MyHC), respectively. Inhibition of myosin light chain kinase (MLCK) by ML-7 eliminated the phasic contraction of wild-type (+/+), rather than tonic contraction of neonatal bladder strips prepared from both +/+ and homozygous SM-MyHC knock-out (-/-) mice. Pharmacomechanical coupling upon PDBu-induced activation of protein kinase C of neonatal bladder preparations elicited tonic contraction of both +/+ and -/- murine. We suggest that: i) electromechanical coupling activates both SM-MyHC and NM-MyHC systems via a ML-7 sensitive and insensitive pathway, respectively. ii) Pharmacomechanical coupling recruits part of the NM-MyHC system rather than SM-MyHC.

Phorbol ester-mediated pulmonary artery endothelial barrier dysfunction through regulation of actin cytoskeletal mechanics

The mechanisms of phorbol ester- and thrombin-mediated pulmonary artery endothelial barrier dysfunction were compared. Phorbol ester dibutyrate (PDBU) mediated slow force velocity and less force than thrombin. Taxol did not attenuate PDBU-mediated tension, while it reversed nocodazole-mediated tension. PDBU-mediated tension was not affected by acrylamide; PDBU increased cell stiffness and produced greater declines in transendothelial resistance (TER) than acrylamide. Thus PDBU caused a net increase in tension and did not unload microtubule or intermediate filaments. Microfilament remodeling, determined on the basis of immunocytochemistry and actin solubility, lacked the sensitivity and specificity to predict actin-dependent mechanical properties. Thrombin increased myosin light chain (MLC) kinase site-specific MLC phosphorylation, according to peptide map analysis, whereas PDBU did not increase PKC-specific MLC phosphorylation. The initial PDBU-mediated tension development temporally correlated with PDBU-mediated decline in TER and increased low-molecular-weight caldesmon (l-CaD) phosphorylation. PDBU-mediated tension development and decreases in TER were associated with a temporal loss of endothelial cell-matrix adhesion, based on a numerical model of TER. Although, on the basis of immunocytochemistry, thrombin-mediated tension was associated with actin insolubility, actin reorganization, and gap formation, these changes did not predict thrombin-mediated gap formation, based on TER and time-lapse differential interference contrast microscopy. These data suggest that PDBU may disrupt endothelial barrier function through loss of cell-matrix adhesion through l-CaD-dependent actin contraction.

Myosin II heavy chain isoforms are phosphorylated in an EGF-dependent manner

To explore the involvement and regulation of the nonmuscle myosin II heavy chains isoforms, MHC-A and MHC-B in the chemotaxis of metastatic tumor cells,we analyzed the changes in phosphorylation and cellular localization of these isoforms upon stimulation of prostate tumor cells with epidermal growth factor(EGF). EGF stimulation of prostate tumor cells resulted in transient increases in MHC-A and MHC-B phosphorylation and subcellular localization with quite different kinetics. Furthermore, the kinetics of subcellular localization correlated with the in vivo kinetics of MHC-B phosphorylation but not of MHC-A phosphorylation, suggesting different modes of regulation for these myosin II isoforms. We further showed that protein kinase C (PKC) is involved in the EGF-dependent phosphorylation of MHC-A and MHC-B. To our knowledge, this is the first report demonstrating that MHC phosphorylation might regulate its subcellular localization and that the EGF signal is transmitted to MHC-A and MHC-B via PKC. The correlation between MHC-B phosphorylation and localization in response to EGF stimulation might suggest that MHC-B is the myosin II isoform that is involved in chemotaxis.

Protein kinase C phosphorylates nonmuscle myosin-II heavy chain from Drosophila but regulation of myosin function by this enzyme is not required for viability in flies

Conventional myosins (myosin-IIs) generate forces for cell shape change and cell motility. Myosin heavy chain phosphorylation regulates myosin function in simple eukaryotes and may also be important in metazoans. To investigate this regulation in a complex eukaryote, we purified the Drosophila myosin-II tail expressed in Escherichia coli and showed that it was phosphorylated in vitro by protein kinase C(PKC) at serines 1936 and 1944, which are located in the nonhelical globular tail piece. These sites are close to a conserved serine that is phosphorylated in vertebrate, nonmuscle myosin-IIs. If the two serines are mutagenized to alanine or aspartic acid, phosphorylation no longer occurs. Using a 341 amino acid tail fragment, we show that there is no difference in the salt-dependent assembly of wild-type phosphorylated and mutagenized polypeptides. Thus, the nonmuscle myosin heavy chain in Drosophila, which is encoded by the zipper gene, appears to be similar to rabbit nonmuscle myosin-IIA. In vivo, we generated transgenic flies that expressed the various myosin heavy chain variants in a zipper null or near-null genetic background. Like their wild-type counterparts, such variants are able to completely rescue the lethal phenotype due to severe zipper mutations. These results suggest that while the myosin-II heavy chain can be phosphorylated by PKC, regulation by this enzyme is not required for viability in Drosophila. Conservation during 530-1000 million years of evolution suggests that regulation by heavy chain phosphorylation may contribute to nonmuscle myosin-II function in some real, but minor, way.

Filopodial behavior is dependent on the phosphorylation state of neuronal growth cones

Several lines of evidence suggest that phosphorylation events play an important role in transducing neurite outgrowth signals. Here we tested if such phosphorylation events altered filopodial dynamics on neuronal growth cones and thereby might affect pathfinding decisions. The general protein kinase inhibitor K252a caused an increase in the overall length of filopodia, thereby increasing the action radius of a growth cone. Application of specific kinase inhibitors demonstrated that myosin light chain kinase, Ca/calmodulin-dependent kinase II, and protein kinase A were likely not involved in this filopodial response. Inhibition of protein kinase C (PKC) with calphostin C or cerebroside, however, induced filopodial elongation similar to that seen with K252a. Activation of PKC with the phorbol ester PMA produced the opposite effect, namely filopodial shortening. Consistent with this finding, the protein phosphatase activator C(2)-ceramide resulted in a significant increase in filopodial length, whereas application of the protein phosphatase inhibitor okadaic acid caused the opposite effect, filopodial shortening. Lastly, the tyrosine kinase inhibitor genistein also caused filopodial elongation, and this effect could be negated by the tyrosine phosphatase inhibitor sodium ortho-vanadate. Using the calcium indicator fura-2, we further showed that these drugs did not cause a measurable change in the free intracellular calcium concentration ([Ca(2+)](i)) in growth cones. Taken together, these results suggest that the action radius of a growth cone and its resulting pathfinding abilities could be rapidly altered by contact with extracellular cues, leading to changes in the activity of protein kinases and phosphatases.

Two distinct mechanisms for regulation of nonmuscle myosin assembly via the heavy chain: phosphorylation for MIIB and mts 1 binding for MIIA

In search of the regulation mechanisms for isoform specific myosin assembly, we have used the COOH-terminal fragments of nonmuscle myosin isoforms MIIA and MIIB (MIIA(F46) and MIIB(alpha)(F47)) as a model system. Phosphorylation by protein kinase C (PK C) or casein kinase II (CK II) within or near the nonhelical tail-end domain inhibits assembly of MIIB(alpha)(F47) [Murakami, N., et al. (1998) Biochemistry 37, 1989]. In the study presented here, we mutated the kinase sites to analyze the inhibition mechanisms of MIIB assembly by phosphorylation. Replacement of the CK II or PK C sites with Asp (MIIB(alpha)(F47)-CK-5D or -PK-4D) strongly inhibited the filament assembly, with or without Mg(2+), by significantly increasing the critical concentrations for assembly. Without Mg(2+), MIIB(alpha)(F47)-CK-5D or -PK-4D inhibited the assembly of wild-type (wt) MIIB(alpha)(F47) by either mixing as homofragments or forming heterofragments. With 2.5 mM Mg(2+), MIIB(alpha)(F47)-wt promoted assembly of MIIB(alpha)(F47)-CK-5D and -PK-4D in homofragment mixtures, but not by forming heterofragments. MIIA(F46) coassembled with MIIB(alpha)(F47)-wt and -CK-5D and altered their assembly patterns. In contrast, assembly of MIIB(alpha)(F47)-PK-4D was unchanged by MIIA(F46). A metastasis-associated protein, mts 1, bound in a Ca(2+)-dependent manner to MIIA(F46), but not appreciably to MIIB(alpha)(F47). At 0.15 M NaCl, mts 1-Ca(2+) not only inhibited MIIA(F46) assembly but also disassembled the MIIA(F46) filaments. Mts 1, however, did not affect the assembly of MIIB(alpha)(F47) in MIIA(F46) and MIIB(alpha)(F47) mixtures, indicating that mts 1 is an inhibitor specific to MIIA assembly. Our results suggest strongly that assembly of MIIA and MIIB is regulated by distinct mechanisms via tail-end domains: phosphorylation of MIIB and mts 1 binding to MIIA. These mechanisms may also function to form MIIA or MIIB homofilaments by selectively inhibiting MIIB or MIIA assembly.

Role of the N-terminal region of the regulatory light chain in the dephosphorylation of myosin by myosin light chain phosphatase

Myosin regulatory light chain (RLC) is phosphorylated at various sites at its N-terminal region, and heterotrimeric myosin light chain phosphatase (MLCP) has been assigned as a physiological phosphatase that dephosphorylates myosin in vivo. Specificity of MLCP toward the various phosphorylation sites of RLC was studied, as well as the role of the N-terminal region of RLC in the dephosphorylation of myosin by MLCP. MLCP dephosphorylated phosphoserine 19, phosphothreonine 18, and phosphothreonine 9 efficiently with almost identical rates, whereas it failed to dephosphorylate phosphorylated serine 1/serine 2. Deletion of the N-terminal seven amino acid residues of RLC markedly decreased the dephosphorylation rate of phosphoserine 19 of RLC incorporated in the myosin molecule, whereas this deletion did not significantly affect the dephosphorylation rate of isolated RLC. On the other hand, deletion of only four N-terminal amino acid residues showed no effect on dephosphorylation of phosphoserine 19 of incorporated RLC. The inhibition of dephosphorylation by deletion of the seven N-terminal residues was also found with the catalytic subunit of MLCP. Phosphorylation at serine 1/serine 2 and threonine 9 did not influence the dephosphorylation rate of serine 19 and threonine 18 by MLCP. These results suggest that the N-terminal region of RLC plays an important role in substrate recognition of MLCP.

Regulation of chick dorsal root ganglion growth cone filopodia by protein kinase C

Growth cone filopodia function both as structural and sensory devices during neuronal pathfinding and their presence is important for correct growth cone navigation. It is assumed that a growth cone can adjust the area of the environment it can explore by regulating the length and number of its filopodial sensors, and in several cell types, these parameters are controlled by the intracellular calcium concentration ([Ca(2+)](i)). In the present report, we address the question whether [Ca(2+)](i) is a general regulator of growth cone filopodia, or whether different cell types utilize different second-messenger systems for this purpose. We show that increasing [Ca(2+)](i) in growth cones of chick dorsal root ganglion (DRG) neurons does not affect average filopodial length in this cell type, suggesting that this parameter is not controlled by [Ca(2+)](i) in chick DRG neurons. Further, we demonstrate that the second-messenger protein kinase C (PKC) is involved in the regulation of filopodial length in chick DRG neurons. Activation of PKC with the phorbol ester, phorbol myristate-13-acetate (PMA), caused filopodial shortening, whereas inhibition of PKC with either bisindolylmaleimide I or calphostin C caused a significant elongation of filopodia. Although the pathway through which PKC mediates its effect on growth cone filopodia in chick DRG neurons remains to be identified, our results indicate that filopodial regulation by [Ca(2+)](i), though clearly important in several other neuronal cell types in vitro, appears to be less important in chick DRG neurons. Rather, we find that in chick DRG neurons, filopodial parameters are controlled by PKC.

Heparin rapidly and selectively regulates protein tyrosine phosphorylation in vascular smooth muscle cells

Aberrant vascular smooth muscle cell (VSMC) hyperplasia is the hallmark of atherosclerosis and restenosis seen after vascular surgery. Heparin inhibits VSMC proliferation in animal models and in cell culture. To test our hypothesis that heparin mediates its antiproliferative effect by altering phosphorylation of key mitogenic signaling proteins in VSMC, we examined tyrosine phosphorylation of cellular proteins in quiescent VSMC stimulated with serum in the presence or absence of heparin. Western blot analysis with anti-phosphotyrosine antibodies shows that heparin specifically alters the tyrosine phosphorylation of only two proteins (42 kDa and 200 kDa). The 200 kDa protein (p200) is dephosphorylated within 2.5 min after heparin treatment with an IC50 that closely parallels the IC50 for growth inhibition. Studies using the tyrosine phosphatase inhibitor, sodium orthovanadate, indicate that heparin blocks p200 phosphorylation by inhibiting a kinase. Phosphorylation of p200 is not altered in heparin-resistant cells, supporting a role for p200 in mediating the antiproliferative effect of heparin. Purification and sequence analysis indicate that p200 exhibits very high homology to the heavy chain of nonmuscle myosin IIA. The 42 kDa protein, identified as mitogen activated protein kinase (MAPK), undergoes dephosphorylation within 15 min after heparin treatment, and this effect is also not seen in heparin-resistant cells. The identification of only two heparin-regulated tyrosine phosphoproteins suggests that they may be key mediators of the antiproliferative effect of heparin.

Molecular mechanisms of nonmuscle myosin-II regulation

Myosin II, the conventional two-headed myosin that forms bipolar filaments, is directly involved in regulating cytokinesis, cell motility and cell morphology in nonmuscle cells. To understand the mechanisms by which nonmuscle myosin-II regulates these processes, investigators are now looking at the regulation of this molecule in vertebrate nonmuscle cells. The identification of multiple isoforms of nonmuscle myosin-II, whose activities and regulation differ from that of smooth muscle myosin-II, suggests that, in addition to regulatory light chain phosphorylation, other regulatory mechanisms control vertebrate nonmuscle myosin-II activity.

Adducin is an in vivo substrate for protein kinase C: phosphorylation in the MARCKS-related domain inhibits activity in promoting spectrin-actin complexes and occurs in many cells, including dendritic spines of neurons

Adducin is a heteromeric protein with subunits containing a COOH-terminal myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that caps and preferentially recruits spectrin to the fast-growing ends of actin filaments. The basic MARCKS-related domain, present in alpha, beta, and gamma adducin subunits, binds calmodulin and contains the major phosphorylation site for protein kinase C (PKC). This report presents the first evidence that phosphorylation of the MARCKS-related domain modifies in vitro and in vivo activities of adducin involving actin and spectrin, and we demonstrate that adducin is a prominent in vivo substrate for PKC or other phorbol 12-myristate 13-acetate (PMA)-activated kinases in multiple cell types, including neurons. PKC phosphorylation of native and recombinant adducin inhibited actin capping measured using pyrene-actin polymerization and abolished activity of adducin in recruiting spectrin to ends and sides of actin filaments. A polyclonal antibody specific to the phosphorylated state of the RTPS-serine, which is the major PKC phosphorylation site in the MARCKS-related domain, was used to evaluate phosphorylation of adducin in cells. Reactivity with phosphoadducin antibody in immunoblots increased twofold in rat hippocampal slices, eight- to ninefold in human embryonal kidney (HEK 293) cells, threefold in MDCK cells, and greater than 10-fold in human erythrocytes after treatments with PMA, but not with forskolin. Thus, the RTPS-serine of adducin is an in vivo phosphorylation site for PKC or other PMA-activated kinases but not for cAMP-dependent protein kinase in a variety of cell types. Physiological consequences of the two PKC phosphorylation sites in the MARCKS-related domain were investigated by stably transfecting MDCK cells with either wild-type or PKC-unphosphorylatable S716A/S726A mutant alpha adducin. The mutant alpha adducin was no longer concentrated at the cell membrane at sites of cell-cell contact, and instead it was distributed as a cytoplasmic punctate pattern. Moreover, the cells expressing the mutant alpha adducin exhibited increased levels of cytoplasmic spectrin, which was colocalized with the mutant alpha adducin in a punctate pattern. Immunofluorescence with the phosphoadducin-specific antibody revealed the RTPS-serine phosphorylation of adducin in postsynaptic areas in the developing rat hippocampus. High levels of the phosphoadducin were detected in the dendritic spines of cultured hippocampal neurons. Spectrin also was a component of dendritic spines, although at distinct sites from the ones containing phosphoadducin. These data demonstrate that adducin is a significant in vivo substrate for PKC or other PMA-activated kinases in a variety of cells, and that phosphorylation of adducin occurs in dendritic spines that are believed to respond to external signals by changes in morphology and reorganization of cytoskeletal structures.

Identification of phosphorylated proteins from thrombin-activated human platelets isolated by two-dimensional gel electrophoresis by electrospray ionization-tandem mass spectrometry (ESI-MS/MS) and liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS)

Two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) is a powerful tool to separate complex protein mixtures including whole cell lysates. In combination with immunoblotting techniques or radioactive labeling techniques it is a fast and convenient way to demonstrate the presence of certain proteins or protein modifications. With the development of extremely sensitive analytical techniques such as matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) or electrospray ionization (ESI)-MS, it has become possible to use 2-D gels not only as an analytical but also as a preparative tool. Starting with a number of spots excised from 2-D gels, a protein can be identified using different strategies involving enzymatic cleavage of the protein in the gel matrix, elution of the resulting peptides and analysis of these peptides by mass spectrometry. The obtained peptide mass fingerprint or fragment ion spectra from peptides can be used to screen protein or nucleic acid databases in order to identify the protein. We have used the techniques described above to identify proteins from human platelets which change their phosphorylation state following activation of platelets by thrombin. Platelets were radioactively labeled with [32P]orthophosphate and stimulated. Several protein spots in the observed range of 10-80 kDa and an isoelectric point of 3-10 showed a significant increase or decrease in phosphorylation. We present the results from the investigation of a spot group representing different isoforms and phosphorylation states of myosin light chain.

Signal transduction pathways involved in enterohemorrhagic Escherichia coli-induced alterations in T84 epithelial permeability

Enterohemorrhagic Escherichia coli (EHEC) infection is associated with watery diarrhea and can lead to complications, including hemorrhagic colitis and the hemolytic-uremic syndrome. The mechanisms by which these organisms produce diarrheal disease remain to be elucidated. Changes in T84 epithelial cell electrophysiology were examined following EHEC infection. T84 cell monolayers infected with EHEC O157:H7 displayed a time-dependent decrease in transepithelial resistance. Increases in the transepithelial flux of both [3H]mannitol and 51Cr-EDTA accompanied the EHEC-induced decreases in T84 resistance. Altered barrier function induced by EHEC occurred at the level of the tight junction since immunofluorescent staining of the tight-junction-associated protein ZO-1 was disrupted when examined by confocal microscopy. Decreased resistance induced by EHEC involved a protein kinase C (PKC)-dependent pathway as the highly specific PKC inhibitor, CGP41251, abrogated the EHEC-induced drop in resistance. PKC activity was also increased in T84 cells infected with EHEC. Calmodulin and myosin light chain kinase played a role in EHEC-induced resistance changes as inhibition of these effector molecules partially reversed the effects of EHEC on barrier function. These studies demonstrate that intracellular signal transduction pathways activated following EHEC infection link the increases in T84 epithelial permeability induced by this pathogen.

Analysis of GTP-binding proteins, phosphoproteins, and cytosolic calcium in functional heterogeneous human blood platelet subpopulations

The biochemical basis for the functional heterogeneity of human blood platelets was investigated in terms of protein phosphorylation, cytoplasmic calcium ([Ca2+]i), the ratio of 46 and 50 kDa vasodilator-stimulated protein (VASP), and GTP-binding proteins (G-proteins). Platelets were fractionated by density. Comparing resting low-density platelets (LDP) to high-density platelets (HDP) revealed higher phosphorylation of proteins in the 47, 31, and 24 kDa ranges. A higher phosphorylation of the 20 kDa protein in LDP compared to HDP was related to an enhanced [Ca2+]i, an increased ADP-ribosylation of the inhibitory G-protein (G(i alpha1-3)) and rhoA, and a decreased ADP-ribosylation of the stimulatory G-protein (G(s alpha)). The differences in the ribosylation patterns of the subpopulations were not influenced by thrombin stimulation or exposure to prostaglandin E1 (PGE1). An 18 kDa phosphoprotein was more highly phosphorylated in resting HDP than in LDP. Thrombin exposure caused dephosphorylation of the 18 kDa phosphoprotein in the HDP, but generally increased phosphorylation of both HDP and LDP in the 47, 31, 24, and 20 kDa bands. Preincubation with prostaglandin E1 or sodium nitroprusside diminished the subsequent thrombin-induced increase in phosphorylation, particularly in HDP. In unstimulated HDP, the 50 kDa VASP phospho form was enhanced, whereas in unstimulated LDP the 46 kDa VASP dephospho form was increased. Our findings suggest that the functional heterogeneity of platelets is partly derived from differences in signal transduction mechanisms reflected in varying phosphoprotein patterns and G-protein properties of platelet stimulatory and inhibitory pathways.

A subset of protein kinase C phosphorylation sites on the myosin II regulatory light chain inhibits phosphorylation by myosin light chain kinase

Protein kinase C (PKC) phosphorylates the regulatory light chains of smooth muscle and cytoplasmic myosin II at three known sites: S1, S2, and T9 [Ikebe, M., Hartshorne, D. J., & Elzinga, M. (1987) J. Biol. Chem. 262, 9569-9573]. Phosphorylation at these sites inhibits the actomyosin ATPase and inhibits phosphorylation of S19 on the regulatory light chain by myosin light chain kinase (MLCK) [Nishikawa, M., Sellers, J. R., Adelstein, R. S., & Hidaka, H. (1984) J. Biol. Chem. 259, 8808-8814]. To compare the effects of phosphorylation at a subset of PKC sites on the rate of MLCK phosphorylation, we substituted alanines for the known PKC phosphorylation sites in the Xenopus regulatory light chain (XRLC). PKC phosphorylation of S1A/S2A/T9A revealed secondary phosphorylation sites at T7 and T10, which are accessible both on isolated S1A/S2A/T9A and S1A/S2A/T9A-myosin hybrids. Apparent kinetic constants were determined for MLCK phosphorylation of WT XRLC and XRLC mutants: T9A, S1A/S2A, S1A/S2A/T9A, and T7A/T9A/T10A. PKC prephosphorylation of S1/2 had no effect on the rate of MLCK phosphorylation, while PKC prephosphorylation of T7/9/10 inhibited MLCK phosphorylation due to a 6-fold increase in Km. Our results suggest that phosphorylation of RLC S1/2 as observed in vivo may not be responsible for an inhibition of MLCK phosphorylation.

Phosphorylation of the large subunit of myosin phosphatase and inhibition of phosphatase activity

The partially purified myosin-bound phosphatase had an associated protein kinase that phosphorylated the holoenzyme, primarily on the large (130-kDa) subunit. Phosphorylation of the 130-kDa subunit resulted in inhibition of phosphatase activity. The major site of phosphorylation was threonine 654 of the 130-kDa subunit or threonine 695 of the 133-kDa isoform. Phosphorylation of the large subunit did not dissociate the holoenzyme. Dephosphorylation of the large subunit was achieved by the holoenzyme, and addition of the catalytic subunit of the type 2A enzyme did not increase the rate of dephosphorylation. The associated kinase was inhibited by chelerythrine, with half-maximal inhibition at approximately 5 microM (in 150 microM ATP). The associated kinase phosphorylated two synthetic peptides, one corresponding to the sequence flanking the phosphorylated threonine, i.e. 648-661 of the 130-kDa subunit, and the other to a known protein kinase C substrate, i.e. a modified sequence from the autoinhibitory region of epsilon protein kinase C. The associated kinase was activated by arachidonic and oleic acid and to a lesser extent by myristic acid. The protein kinase that phosphorylated the 130-kDa subunit and resulted in inhibition of myosin phosphatase activity was not identified.

Regulation of endothelial cell gap formation and barrier dysfunction: role of myosin light chain phosphorylation

Endothelial cell (EC) contraction results in intercellular gap formation and loss of the selective vascular barrier to circulating macromolecules. We tested the hypothesis that phosphorylation of regulatory myosin light chains (MLC) by Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) is critical to EC barrier dysfunction elicited by thrombin. Thrombin stimulated a rapid (< 15 sec) increase in [Ca2+]i which preceded maximal MLC phosphorylation (60 sec) with a 6 to 8-fold increase above constitutive levels of phosphorylated MLC. Dramatic cellular shape changes indicative of contraction and gap formation were observed at 5 min with maximal increases in albumin permeability occurring by 10 min. Neither the Ca2+ ionophore, A23187, nor phorbol myristate acetate (PMA), a direct activator of protein kinase C (PKC), alone or in combination, produced MLC phosphorylation. The combination was synergistic, however, in stimulating EC contraction/gap formation and barrier dysfunction (3 to 4-fold increase). Down-regulation or inhibition of PKC activity attenuated thrombin-induced MLC phosphorylation (approximately 40% inhibition) and both thrombin- and PMA-induced albumin clearance (approximately 50% inhibition). Agents which augmented [cAMP]i partially blocked thrombin-induced MLC phosphorylation (approximately 50%) and completely inhibited both thrombin- and PMA-induced EC permeability (100% inhibition). Furthermore, cAMP produced significant reduction in the basal levels of constitutive MLC phosphorylation. Finally, MLCK inhibition (with either ML-7 or KT 5926) or Ca2+/calmodulin antagonism (with either trifluoperazine or W-7) attenuated thrombin-induced MLC phosphorylation and barrier dysfunction. These results suggest a model wherein EC contractile events, gap formation and barrier dysfunction occur via MLCK-dependent and independent mechanisms and are significantly modulated by both PKC and cAMP-dependent protein kinase A activities.

A Xenopus nonmuscle myosin heavy chain isoform is phosphorylated by cyclin-p34cdc2 kinase during meiosis

There are two vertebrate nonmuscle myosin heavy chain (MHC) genes that encode two separate isoforms of the heavy chain, MHC-A and MHC-B. Recent work has identified additional, alternatively spliced isoforms of MHC-B cDNA with inserted sequences of 30 nucleotides (chicken and human) or 48 nucleotides (Xenopus) at a site corresponding to the ATP binding region in the MHC protein (Takahashi, M., Kawamoto, S., and Adelstein, R.S. (1992) J. Biol. Chem. 267, 17864-17871) and Bhatia-Dey, N., Adelstein, R.S., and Dawid, I.B. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 2856-2859). The deduced amino acid sequence of these inserts contains a consensus sequence for phosphorylation by cyclin-p34cdc2 (cdc2) kinase. In cultured Xenopus XTC cells, we have identified two inserted MHC-B isoforms and a non-inserted MHC-A isoform by immunoblotting of cell extracts. When myosin was immunoprecipitated from XTC cells and phosphorylated in vitro with cdc2 kinase, the kinase catalyzed the phosphorylation of both inserted MHC-B isoforms but not MHC-A. Isoelectric focusing of tryptic peptides generated from MHC-B phosphorylated with cdc2 kinase revealed one major phosphopeptide that was purified by reverse-phase high performance liquid chromatography and sequenced. The phosphorylated residue was Ser-214, the cdc2 kinase consensus site within the insert near the ATP binding region. The same site was phosphorylated in intact XTC cells during log phase of growth and in cell-free lysates of Xenopus eggs stabilized in second meiotic metaphase but not interphase. Moreover, Ser-214 phosphorylation was detected during maturation of Xenopus oocytes when the cdc2 kinase-containing maturation-promoting factor was activated, but not in G2 interphase-arrested oocytes. These results demonstrate that MHC-B phosphorylation is tightly regulated by cdc2 kinase during meiotic cell cycles. Furthermore, MHC-A and MHC-B isoforms are differentially phosphorylated at these stages, suggesting that they may serve different functions in these cells.

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

Two potent inhibitors of protein phosphatase type 1 (PP1) and type 2A (PP2A), calyculin A (CAL-A) and okadaic acid (OKA), inhibited human platelet aggregation induced by thrombin, collagen and 9,11-epithio-11,12-methano-thromboxane A2 (STA2). IC50 values of CAL-A and OKA for STA2-induced aggregation were 53 nM and 3.5 microM, respectively. These drugs also inhibited thrombin-induced [14C]serotonin secretion of platelets. CAL-A and OKA elicited phosphorylation of certain proteins with an apparent M(r) (x 10(-3) of 200, 60, 50 and 20 light chain of myosin (MLC). Agonist-induced 47,000 M(r) protein phosphorylation was strongly inhibited by these compounds, whereas phosphorylation of 20,000 M(r) MLC was enhanced. The increase in 50,000 M(r) protein phosphorylation by CAL-A and OKA was observed in the presence of agonists, and the 50,000 M(r) phosphorylation may be involved in the inhibition of platelet activation by these compounds. Subcellular analysis of the phosphatase activity in human platelets showed that MLC phosphatase activity was present mainly (approx. 78%) in the cytosolic fraction. Chromatography of human platelet extract on heparin-Sepharose resolved two peaks of MLC phosphatase activity: PP2A in 0.1 M NaCl eluate and PP1 in 0.5 NaCl eluate. PP2A and PP1 isozymes (PP1 alpha, PP1 gamma and PP1 delta) have also been identified in human platelets, by cross-reactivity with polyclonal antibodies against PP2A and PP1 isozymes, respectively. These results suggest that PP1 and/or PP2A may play an important role in the process of platelet activation by regulating levels of phosphorylation of certain proteins.

Phosphorylation of myosin light chain kinase: a cellular mechanism for Ca2+ desensitization

Phosphorylation of the regulatory light chain of myosin by the Ca2+/calmodulin-dependent myosin light chain kinase plays an important role in smooth muscle contraction, nonmuscle cell shape changes, platelet contraction, secretion, and other cellular processes. Smooth muscle myosin light chain kinase is also phosphorylated, and recent results from experiments designed to satisfy the criteria of Krebs and Beavo for establishing the physiological significance of enzyme phosphorylation have provided insights into the cellular regulation and function of this phosphorylation in smooth muscle. The multifunctional Ca2+/calmodulin-dependent protein kinase II phosphorylates myosin light chain kinase at a regulatory site near the calmodulin-binding domain. This phosphorylation increases the concentration of Ca2+/calmodulin required for activation and hence increases the Ca2+ concentrations required for myosin light chain kinase activity in cells. However, the concentration of cytosolic Ca2+ required to effect myosin light chain kinase phosphorylation is greater than that required for myosin light chain phosphorylation. Phosphorylation of myosin light chain kinase is only one of a number of mechanisms used by the cell to down regulate the Ca2+ signal in smooth muscle. Since both smooth and nonmuscle cells express the same form of myosin light chain kinase, this phosphorylation may play a regulatory role in cellular processes that are dependent on myosin light chain phosphorylation.

Phosphorylation of vertebrate nonmuscle and smooth muscle myosin heavy chains and light chains

In this article we review the various amino acids present in vertebrate nonmuscle and smooth muscle myosin that can undergo phosphorylation. The sites for phosphorylation in the 20 kD myosin light chain include serine-19 and threonine-18 which are substrates for myosin light chain kinase and serine-1 and/or -2 and threonine-9 which are substrates for protein kinase C. The sites in vertebrate smooth muscle and nonmuscle myosin heavy chains that can be phosphorylated by protein kinase C and casein kinase II are also summarized. Original data indicating that treatment of human T-lymphocytes (Jurkat cell line) with phorbol 12-myristate 13-acetate results in phosphorylation of both the 20 kD myosin light chain as well as the 200 kD myosin heavy chain is presented. We identified the amino acids phosphorylated in the human T-lymphocytes myosin light chains as serine-1 or serine-2 and in the myosin heavy chains as serine-1917 by 1-dimensional isoelectric focusing of tryptic phosphopeptides. Untreated T-lymphocytes contain phosphate in the serine-19 residue of the myosin light chain, and in a residue tentatively identified as serine-1944 in the myosin heavy chain.

Expression of four myosin heavy chain isoforms with development in mouse uterus

In smooth muscle tissue, two or three isoforms of myosin heavy chain (MHC) have been reported (SM1, SM2, and/or NM). In mouse uterus tissue, four bands in the region of the MHC's can be resolved on high resolution SDS polyacrylamide gels. Western blots using smooth muscle (SM) MHC-specific and nonmuscle (NM) MHC-specific polyclonal antibodies show the upper two bands in the MHC region are SM isoforms, whereas the lower two bands are NM isoforms. One-dimensional peptide maps of these four bands show each to have a unique pattern of polypeptide fragments following alpha-chymotrypsin digestion. Developmental expression of myosin heavy chains (MHC) in mouse uterus, aorta, bladder, and stomach (6 ages, 10-150 days) was determined using tissue homogenates. In the uterus, both SM MHC's show an increase in relative content with increasing age, whereas the NM MHC's show a decrease. The mouse aorta shows a significant increase in the SM MHC's and a significant decrease in the NM MHC from day 10 to day 30, which is similar to data reported for the rat aorta. Whereas both the bladder and stomach contain relatively small amounts of NM MHC's (approximately 10% or less), these quantities do show decreases with development. The SM1:SM2 ratio for the uterus remains high (3.4 at 150 days) through development; the aorta, bladder, and stomach also start out high, but tend toward 1.0 in the 150-day animals. The presence of four MHC isoforms in the uterus with unique developmental regulation of expression is consistent with hypotheses of unique functional roles for these isoforms.

Inhibition of platelet activation by tyrosine kinase inhibitors

Protein tyrosine kinase (PTK) blockers (tyrphostins) inhibit in a dose-dependent fashion thrombin-induced aggregation and serotonin release with IC50 values in the 10-35 microM concentration range. The inhibition of thrombin-induced aggregation correlates with their potency in inhibiting phosphorylation of proteins on tyrosine residues. Using metabolically 32P-labelled human platelets, it was found that the tyrphostins have no effect on the decrease in [32P]phosphatidylinositol bisphosphate but prevent the replenishment of [32P]polyphosphoinositide. Tyrphostins decreased [32P]phosphatidic acid production induced by thrombin, although never by more than 50%, and only delayed the peak of diacylglycerol, suggesting that phospholipase C was still activated. Tyrphostins inhibited the thrombin-elicited early phosphorylation of p43 and p20, substrates for protein kinase C (PKC) and myosin light chain kinase, respectively, at short times of activation. This inhibition, however, was overcome after 1 min of stimulation with thrombin. Tyrphostin AG213 also inhibited platelet aggregation and tyrosine protein phosphorylation induced by phorbol myristate acetate (PMA), but did not inhibit pleckstrin phosphorylation. These results suggest that thrombin induces the phosphorylation of proteins on tyrosine residues which most probably results in the activation of phosphoinositide kinases. The ability of tyrphostins to inhibit phosphorylation of p43 and p20 when induced by thrombin but not when induced by PMA confirms that PTKs may be involved subsequent to PKC activation.

Calcium channel blocker treatment of tumor cells induces alterations in the cytoskeleton, mobility of the integrin alpha IIb beta 3 and tumor-cell-induced platelet aggregation

Calcium channel blockers of the phenylalkylamine (i.e. verapamil), benzothiazepine (i.e. diltiazem) and dihydropyridine (i.e. nifedipine) classes were evaluated for effects on the tumor cell/platelet interactions using Walker 256 carcinosarcoma cells (W256 cells). When W256 cells were pretreated for 15 min with channel blockers at concentrations of 50-200 microM, macroscopic tumor-cell-induced platelet aggregation was inhibited (order of potency; nifedipine greater than diltiazem much greater than verapamil). However, ultrastructural analysis revealed limited, focal platelet aggregates associated with tumor cell plasma membranes of verapamil- and diltiazem-treated cells. There was no evidence of platelet activation or platelet association with the tumor cell membrane in cells pretreated with nifedipine. Walker 256 cells possess the intergrin alpha IIb beta 3. Tumor cell alpha IIb beta 3 was shown to mediate tumor cell/platelet interactions in vitro [Chopra et al. (1988) Cancer Res. 48:3787]. Patching and capping of surface alpha IIb beta 3 were inhibited by nifedipine greater than diltiazem much greater than verapamil. The degree of inhibition of alpha IIb beta 3 receptor mobility parallels the inhibition of tumor-cell-induced platelet aggregation. W256 cells are characterized by a well-developed microfilament and intermediate filament network and by the absence of a distinct microtubular network. Calcium channel blockers had no effect on the low polymerization level of tubulin. However, they induced rearrangement of microfilament stress fibers. Intermediate filaments were also rearranged but to varying degrees. The order of effectiveness for alteration of intermediate filament organization was nifedipine greater than diltiazem while verapamil was ineffective. We propose that the previously reported inhibition of tumor cell/platelet interaction and tumor cell metastasis by calcium channel blockers [Honn et al. (1984) Clin Exp Metastasis 1:61] is due not only to the effects of the Ca2+ channel blockers on platelets, but also to their effect on the tumor cell cytoskeleton resulting in an inhibition of the mobility and function of the alpha IIb beta 3 receptor.

Differential tissue expression of multiple genes for chicken smooth muscle/nonmuscle myosin regulatory light chains

The cDNA clones for two distinct mRNAs encoding one of the two known isoforms of chicken smooth muscle/nonmuscle myosin regulatory light chain were isolated. The nucleotide sequences of these cDNAs were very similar to each other (99% nucleotide identities) in the 516 bp translated regions and in the first 33 bp of the 3' noncoding regions, whereas the rest of the 3' noncoding regions and the 5' noncoding regions had no significant similarity. Genomic Southern blot analysis showed that these two mRNAs were encoded in two individual genes. Whereas these two genes encoded almost identical polypeptides with only one conservative substitution of amino acid residues, expression of the mRNAs was differentially regulated both at the transcriptional and translational levels in various tissues of the chicken.

Purification of a 20 kDa phosphoprotein from epithelial cells and identification as a myosin light chain. Phosphorylation induced by enteropathogenic Escherichia coli and phorbol ester

Previous studies on the mechanism of enteropathogenic Escherichia coli (EPEC) infection have revealed an increase in the phosphorylation state of a number of proteins in human laryngeal HEp-2 cells. The most prominent was an acidic phosphoprotein(s) of Mr 20-21 kDa. The present study reports: (a) a simple method for purification of phosphorylated 20 kDa protein; (b) identification of the 20 kDa phosphoprotein as myosin light chain; and (c) that the phorbol ester, TPA, also increased the phosphorylation of the 20 kDa myosin light chain. In contrast to the effects of EPEC, TPA stimulation resulted in the dissociation of myosin from the cytoskeleton to the cytosol.

Protein kinase C-induced redistribution of the cytoskeleton and phosphorylation of vimentin in cultured brain macrophages

The phorbol ester 12-O-tetradecanoyl-acetate (TPA) induced prominent and transient changes in the organization of the cytoskeleton in cultured amoeboid microglial cells including redistribution of actin toward the center of the cells and in the subplasmalemmal region, appearance of fine actin filaments, retraction of microtubules (MT), and rearrangement of intermediate filaments (IF) containing vimentin. The possible implication of protein kinase C (PKC) in mediating the effects of TPA was suggested by a parallel shift of PKC activity from the soluble to membrane fractions and phosphorylation of several microglial proteins. The rearrangement of IF closely correlated with increased vimentin phosphorylation, detected by pulse labeling of intact cells. Two monoclonal antivimentin antibodies, B3 and V9, showed different staining patterns. Immunoreactivity with the antibody B3 was more restricted and could be abolished by treatment of fixed, permeabilized cells with alkaline phosphatase, thus suggesting that the antibody reacts with a phosphorylated epitope. Using this antibody, rearrangement of IF involving vimentin phosphorylation was detected within 15 to 60 min of treatment with 50 nM TPA and consisted in the appearance of intense perinuclear fluorescent label. This perinuclear fluorescence persisted up to 24 hr after TPA removal and gradually diminished during the following 2 to 3 days. Immunochemical analysis of nonionic detergent-soluble and -insoluble extracts from untreated and TPA-treated cells revealed no differences in vimentin solubility suggesting that TPA induced vimentin phosphorylation does not result in notable vimentin filament disassembly. However the extent of vimentin degradation was more prominent in TPA-treated cultures indicating a higher sensitivity of vimentin to proteolytic degradation. The data show that PKC-mediated phosphorylation of vimentin results in precise spatial and temporal rearrangement of IF which are not associated with altered vimentin solubility, but possibly changes the mechanical properties and interactions of vimentin filaments.

Expression of myosin regulatory light chains in rat brain: characterization of a novel isoform

We have characterized cDNA clones of mRNAs encoding two distinct isoforms of myosin regulatory light chain expressed in rat brain. One clone, isolated from a cultured astrocyte cDNA library, is derived from a 1200-base mRNA that is expressed at high levels in cultured astrocytes, and at higher levels in the embryonic brain than in the adult brain. The nucleotide sequence of this cDNA is essentially identical to a previously reported cDNA encoding a smooth muscle isoform from rat aorta cells (Taubman et al., J. Cell Biol., 104 (1987) 1505-1515). The second clone hybridized to a 1300-base mRNA that is expressed abundantly in the adult brain and is the predominant species in cultured neuroblasts. Both mRNAs are expressed, to varying extents, in other muscle and nonmuscle tissues. The deduced amino acid sequences of the two isoforms differ in 4 residues out of 171. On the basis of the tissue distribution of their mRNAs and a comparison of identities among the known amino acid sequences of myosin regulatory light chains we suggest that both proteins should be considered as non-muscle isoforms. We conclude that there are at least two isoforms of the myosin regulatory light chain expressed in rat brain and that their expression is under both cell-specific and developmental regulation.

Prostaglandin production in human endometrium following continuous exposure to low-dose levonorgestrel released from a vaginal ring

Arachidonic acid metabolites produced by primary cultures of human endometrial cells derived from biopsies obtained before and after exposure to 20 micrograms/day levonorgestrel for 84 +/- 1 days were analysed by reverse phase HPLC. This revealed a significant increase in PGF1 alpha and an epoxide metabolite upon levonorgestrel stimulation. The proportion of epoxide metabolite, PGF1 alpha and PGE2 were positively correlated with serum levonorgestrel levels while HETES, PGE2 and epoxide were similarly correlated with serum oestradiol. The extent of intermenstrual bleeding during exposure to levonorgestrel was correlated with the proportion of epoxide and HETES products in vitro which is discussed in relation to their physiological function.

Analysis of myosin light chain phosphopeptides in phorbol dibutyrate-contracted artery

The incorporation of [32P]phosphate into the 20 kDa myosin light chain of phorbol dibutyrate-contracted artery was slightly increased as compared to that of resting muscle. Addition of K+ to the 1-h phorbol dibutyrate-contracted artery immediately doubled the force and greatly increased the light chain phosphorylation. Two-dimensional phosphopeptide mapping of light chain from phorbol dibutyrate-contracted muscle showed distinct peptides phosphorylated on serine residues by myosin light chain kinase and protein kinase C. In addition, the peptide phosphorylated on threonine residue by protein kinase C was revealed for the first time in intact muscle. Upon addition of K+, the distribution of phosphopeptides shifted toward the myosin light chain kinase catalyzed pattern.