Latrotoxin stimulates secretion in permeabilized cells by regulating an intracellular Ca2+ - and ATP-dependent event: a role for protein kinase C

alpha-Latrotoxin, a component of black widow spider venom, stimulates transmitter release from nerve terminals and intact chromaffin cells and enhances secretion from permeabilized chromaffin cells already maximally stimulated by Ca(2+). In this study we demonstrate that chromaffin cells contain a protein antigenically similar to the cloned Ca(2+)-independent receptor for alpha-latrotoxin. Although this receptor has homology to the secretin family of G-protein-linked receptors, pertussis toxin has no effect on the ability of alpha-latrotoxin to enhance secretion, suggesting that neither G(i) nor G(o) is involved in the response. Furthermore, in the absence of Ca(2+), alpha-latrotoxin does not stimulate polyphosphoinositide-specific phospholipase C. alpha-Latrotoxin specifically enhances ATP-dependent secretion in permeabilized cells. An in situ assay for protein kinase C reveals that alpha-latrotoxin augments the activation of protein kinase C by Ca(2+), and use of protein kinase inhibitors demonstrates that this activation is important for the toxin's enhancing effect. This enhancement of secretion requires Ca(2+) concentrations above 3 microm and is not supported by Ba(2+) or nonhydrolyzable guanine nucleotides, which do not stimulate protein kinase C. We conclude that alpha-latrotoxin stimulates secretion in permeabilized cells by regulating a Ca(2+)- and ATP-dependent event involving protein kinase C.     

Mysterious Ca(2+)-independent muscular contraction: déjà vu

The permeabilized cells and muscle fibres technique allows one to study the functional properties of mitochondria without their isolation, thus preserving all of the contacts with cellular structures, mostly the cytoskeleton, to study the whole mitochondrial population in the cell in their natural surroundings and it is increasingly being used in both experimental and clinical studies. The functional parameters (affinity for ADP in regulation of respiration) of mitochondria in permeabilized myocytes or myocardial fibres are very different from those in isolated mitochondria in vitro. In the present study, we have analysed the data showing the dependence of this parameter upon the muscle contractile state. Most remarkable is the effect of recently described Ca(2+)-independent contraction of permeabilized muscle fibres induced by elevated temperatures (30-37°C). We show that very similar strong spontaneous Ca(2+)-independent contraction can be produced by proteolytic treatment of permeabilized muscle fibres that result in a disorganization of mitochondrial arrangement, leading to a significant increase in affinity for ADP. These data show that Ca(2+)-insensitive contraction may be related to the destruction of cytoskeleton structures by intracellular proteases. Therefore the use of their inhibitors is strongly advised at the permeabilization step with careful washing of fibres or cells afterwards. A possible physiologically relevant relationship between Ca(2+)-regulated ATP-dependent contraction and mitochondrial functional parameters is also discussed.     

Leishmania amazonensis: PKC-like protein kinase modulates the (Na++K+)ATPase activity

The present study aimed to identify the presence of protein kinase C-like (PKC-like) in Leishmania amazonensis and to elucidate its possible role in the modulation of the (Na(+)+K(+))ATPase activity. Immunoblotting experiments using antibody against a consensus sequence (Ac 543-549) of rabbit protein kinase C (PKC) revealed the presence of a protein kinase of 80 kDa in L. amazonensis. Measurements of protein kinase activity showed the presence of both (Ca(2+)-dependent) and (Ca(2+)-independent) protein kinase activity in plasma membrane and cytosol. Phorbol ester (PMA) activation of the Ca(2+)-dependent protein kinase stimulated the (Na(+)+K(+))ATPase activity, while activation of the Ca(2+)-independent protein kinase was inhibitory. Both effects of protein kinase on the (Na(+)+K(+))ATPase of the plasma membrane were lower than that observed in intact cells. PMA induced the translocation of protein kinase from cytosol to plasma membrane, indicating that the maximal effect of protein kinase on the (Na(+)+K(+))ATPase activity depends on the synergistic action of protein kinases from both plasma membrane and cytosol. This is the first demonstration of a protein kinase activated by PMA in L. amazonensis and the first evidence for a possible role in the regulation of the (Na(+)+K(+))ATPase activity in this trypanosomatid. Modulation of the (Na(+)+K(+))ATPase by protein kinase in a trypanosomatid opens up new possibilities to understand the regulation of ion homeostasis in this parasite.     

Proteolytic cleavage and nuclear translocation of fibrocystin is regulated by intracellular Ca2+ and activation of protein kinase C

Fibrocystin, a type I membrane protein of unknown function, is the protein affected in the autosomal recessive form of polycystic kidney disease. Here we show that fibrocystin undergoes regulated proteolysis. Several proteolytic cleavages occur within the predicted ectodomain, whereas at least one cleavage occurs within the cytoplasmic portion. The latter generates a C-terminal intracellular fragment that harbors the nuclear localization signal KRKVSRLAVTGERTATPAPKIPRIT and translocates to the nucleus. Proteolytic cleavage of fibrocystin occurs constitutively in long term cultures of polarized inner medullary collecting duct cells (mIMCD-3). Activation of protein kinase C and release of intracellular Ca2+ are required for proteolysis under these conditions. In short term cultures of human embryonic kidney 293 cells (HEK-293), proteolytic cleavage of fibrocystin can be elicited by stimulation of intracellular Ca2+ release or activation of protein kinase C. These results identify a novel Ca2+-dependent pathway that signals from fibrocystin located in the cell membrane to the nucleus.     

Cell-matrix interaction via CD44 is independently regulated by different metalloproteinases activated in response to extracellular Ca(2+) influx and PKC activation

CD44 is an adhesion molecule that interacts with hyaluronic acid (HA) and undergoes sequential proteolytic cleavages in its ectodomain and intramembranous domain. The ectodomain cleavage is triggered by extracellular Ca(2+) influx or the activation of protein kinase C. Here we show that CD44-mediated cell-matrix adhesion is terminated by two independent ADAM family metalloproteinases, ADAM10 and ADAM17, differentially regulated in response to those stimuli. Ca(2+) influx activates ADAM10 by regulating the association between calmodulin and ADAM10, leading to CD44 ectodomain cleavage. Depletion of ADAM10 strongly inhibits the Ca(2+) influx-induced cell detachment from matrix. On the other hand, phorbol ester stimulation activates ADAM17 through the activation of PKC and small GTPase Rac, inducing proteolysis of CD44. Furthermore, depletion of ADAM10 or ADAM17 markedly suppressed CD44-dependent cancer cell migration on HA, but not on fibronectin. The spatio-temporal regulation of two independent signaling pathways for CD44 cleavage plays a crucial role in cell-matrix interaction and cell migration.     

A novel 145 kd brain cytosolic protein reconstitutes Ca(2+)-regulated secretion in permeable neuroendocrine cells.

The regulated secretory pathway is activated by elevated cytoplasmic Ca2+; however, the components mediating Ca2+ regulation have not been identified. In semi-intact neuroendocrine cells, Ca(2+)-activated secretion is ATP- and cytosol protein-dependent. We have identified a novel brain protein, p145, as a cytosolic factor that reconstitutes Ca(2+)-activated secretion in two neuroendocrine cell types. The protein is a dimer of 145 kd subunits, exhibits Ca(2+)-dependent interaction with a hydrophobic matrix, and binds phospholipid vesicles, suggesting a membrane-associated function. A p145-specific antibody inhibits the reconstitution of Ca(2+)-activated secretion by cytosol, indicating an essential role for p145. The restricted expression of p145 in tissues exhibiting a regulated secretory pathway suggests a key role for this protein in the transduction of Ca2+ signals into vectorial membrane fusion events.     

A tetanus toxin sensitive protein other than VAMP 2 is required for exocytosis in the pancreatic acinar cell

The neurotoxin sensitivity of regulated exocytosis in the pancreatic acinar cell was investigated using streptolysin-O permeabilized pancreatic acini. Treatment of permeabilized acini with botulinum toxin B (BoNT/B) or botulinum toxin D (BoNT/D) had no detectable effect on Ca(2+)-dependent amylase secretion but did result in the complete cleavage of VAMP 2. In comparison, tetanus toxin (TeTx) treatment both significantly inhibited Ca(2+)-dependent amylase secretion and cleaved VAMP 2. These results indicate that regulated exocytosis in the pancreatic acinar cell requires a tetanus toxin sensitive protein(s) other than VAMP 2.     

Transforming growth factor-alpha and beta-amyloid precursor protein share a secretory mechanism

Cleavage and release of membrane protein ectodomains, a regulated process that affects many cell surface proteins, remains largely uncharacterized. To investigate whether cell surface proteins are cleaved through a shared mechanism or through multiple independent mechanisms, we mutagenized Chinese hamster ovary (CHO) cells and selected clones that were unable to cleave membrane-anchored transforming growth factor alpha (TGF-alpha). The defect in TGF-alpha cleavage in these clones is most apparent upon cell treatment with the protein kinase C (PKC) activator PMA, which stimulates TGF-alpha cleavage in wild-type cells. The mutant clones do not have defects in TFG-alpha expression, transport to the cell surface or turnover. Concomitant with the loss of TGF-alpha cleavage, these clones have lost the ability to cleave many structurally unrelated membrane proteins in response to PMA. These proteins include beta-amyloid precursor protein (beta-APP), whose cleavage into a secreted form avoids conversion into the amyloidogenic peptide A beta, and a group of cell surface proteins whose release into the medium is stimulated by PMA in wild type CHO cells but not in mutants. The mutations prevent cleavage by PKC- dependent as well as PKC-independent mechanisms, and thus affect an essential component that functions downstream of these various signaling mechanisms. We propose that regulated cleavage and secretion of membrane protein ectodomains is mediated by a common system whose components respond to multiple activators and act on susceptible proteins of diverse structure and function.     

Protein kinase C heterogeneity in GH4C1 rat pituitary cells. Characterization of a Ca2(+)-independent phorbol ester receptor

Clonal GH4C1 rat pituitary cells are heterogeneous with respect to phorbol dibutyrate receptors (PDBu-R) and protein kinase C (PKC) content. GH cell PDBu-Rs can be separated into two categories based on Ca2(+)-modulation of receptor affinity. Approximately 70% of the cytosolic PDBu-Rs demonstrate Ca2(+)-sensitive receptor affinity and redistribute from the soluble to the particulate fraction in the presence of excess Ca2+. The other 30% of the receptors remain in the cytosol in the presence of excess Ca2+. Their receptor affinity is Ca2(+)-independent. Northern blot hybridization and immunoblot analysis showed that GH4C1 cells express Ca2(+)-independent epsilon-PKC as well as Ca2(+)-dependent alpha- and beta-PKCs. Cell lysis in Ca2+ caused the redistribution of greater than 95% of alpha- and beta-PKC to the particulate fraction, whereas approximately 90% of the epsilon-PKC remained in the cytosol. In contrast, brief treatment of GH cell cultures with PDBu or thyrotropin-releasing hormone caused redistribution of all three isozymes. Prolonged treatment with PDBu down-modulated all three isozymes but at different rates and to different extents. In contrast, prolonged thyrotropin-releasing hormone treatment selectively down-modulated epsilon-PKC. These results demonstrate that GH cells have both Ca2(+)-sensitive and -insensitive PKCs and PDBu-Rs and that both populations are regulated by agonists that control prolactin synthesis and secretion by these cells.     

Proteolytic activation of protein kinase C in the extracts of cells treated for a short time with phorbol ester

A 10 min treatment of human neutrophils with phorbol 12-myristate 13-acetate (PMA) has been reported to induce accumulation of the proteolytically activated Ca2+/phospholipid-independent catalytic fragment of protein kinase C in the cytosol of intact cells [(1986) J. Biol. Chem. 261, 4101-4105]. We investigated the proteolytic conversion of protein kinase C to Ca2+/phospholipid-independent form in the cytosol and membrane fractions of pig neutrophils. The activity of protein kinase C was measured with its specific oligopeptide substrate Ala-Ala-Ala-Ser-Phe-Lys-Ala-Lys-Lys-amide designed previously. In our experiments the short-term treatment of neutrophils with PMA did not induce the accumulation of the proteolytically activated form of protein kinase C in the cytosol of intact cells. However, treatment of cells with PMA enhanced the limited proteolysis of protein kinase C during the preparation of cell extracts.     

Ca2+-dependent activator protein for secretion 1 is critical for constitutive and regulated exocytosis but not for loading of transmitters into dense core vesicles

Although CAPS1 was originally identified as a soluble factor that reconstitutes Ca(2+)-dependent secretion from permeabilized neuroendocrine cells, its exact function in intact mammalian cells remains controversial. Here we investigate the role for CAPS1 by generating stable cell lines in which CAPS1 is strongly down-regulated. In these cells, Ca(2+)-dependent secretion was strongly reduced not only of catecholamine but also of a transfected neuropeptide. These secretion defects were rescued by infusion of CAPS1-containing brain cytosol or by transfection-mediated expression of CAPS1. Whole cell patch clamp recording revealed significant reductions in slow burst and sustained release components of exocytosis in the knockdown cells. Unexpectedly, they also accumulated higher amounts of endogenous and exogenous transmitters, which were attributable to reductions in constitutive secretion. Electron microscopy did not reveal abnormalities in the number or docking of dense core vesicles. Our results indicate that CAPS1 plays critical roles not only in Ca(2+)-dependent, regulated exocytosis but also in constitutive exocytosis downstream of vesicle docking. However, they do not support the role for CAPS1 in loading transmitters into dense core vesicles.     

Decoding of short-lived Ca2+ influx signals into long term substrate phosphorylation through activation of two distinct classes of protein kinase C

In electrically excitable cells, membrane depolarization opens voltage-dependent Ca(2+) channels eliciting Ca(2+) influx, which plays an important role for the activation of protein kinase C (PKC). However, we do not know whether Ca(2+) influx alone can activate PKC. The present study was conducted to investigate the Ca(2+) influx-induced activation mechanisms for two classes of PKC, conventional PKC (cPKC; PKCalpha) and novel PKC (nPKC; PKCtheta), in insulin-secreting cells. We have demonstrated simultaneous translocation of both DsRed-tagged PKCalpha to the plasma membrane and green fluorescent protein (GFP)-tagged myristoylated alanine-rich C kinase substrate to the cytosol as a dual marker of PKC activity in response to depolarization-evoked Ca(2+) influx in the DsRed-tagged PKCalpha and GFP-tagged myristoylated alanine-rich C kinase substrate co-expressing cells. The result indicates that Ca(2+) influx can generate diacylglycerol (DAG), because cPKC is activated by Ca(2+) and DAG. We showed this in three different ways by demonstrating: 1) Ca(2+) influx-induced translocation of GFP-tagged C1 domain of PKCgamma, 2) Ca(2+) influx-induced translocation of GFP-tagged pleckstrin homology domain, and 3) Ca(2+) influx-induced translocation of GFP-tagged PKCtheta, as a marker of DAG production and/or nPKC activity. Thus, Ca(2+) influx alone via voltage-dependent Ca(2+) channels can generate DAG, thereby activating cPKC and nPKC, whose activation is structurally independent of Ca(2+).     

Subcellular localization and calcium and pH requirements for proteolytic processing of the Hendra virus fusion protein

Proteolytic cleavage of the Hendra virus fusion (F) protein results in the formation of disulfide-linked F1 and F2 subunits, with cleavage occurring after residue K109 in the sequence GDVK/L. This unusual cleavage site and efficient propagation of Hendra virus in a furin-deficient cell line indicate that the Hendra F protein is not cleaved by furin, the protease responsible for proteolytic activation of many viral fusion proteins. To identify the subcellular site of Hendra F processing, Vero cells transfected with pCAGGS-Hendra F or pCAGGS-SV5 F were metabolically labeled and chased in the absence and presence of inhibitors of exocytosis. The addition of carbonyl-cyanide-3-chlorophenylhydrazone, monensin, brefeldin A, or NaF-AlCl3 or incubation of cells at 20 degrees C all inhibited processing of the Hendra F protein, suggesting that cleavage of Hendra F occurs either in secretory vesicles budding from the trans-Golgi network or at the cell surface. In contrast to proteolytic cleavage of the simian virus 5 (SV5) F protein by the Ca(2+)-dependent protease furin, proteolytic cleavage of the Hendra F protein was not significantly inhibited by decreases in Ca2+ levels following incubation with EGTA or A23187. However, in the presence of weak amines and H+ V-ATPase inhibitors, known to raise intracellular pH, cleavage of Hendra F protein was inhibited while processing of the SV5 F protein was not significantly affected. The subcellular location, sensitivity to pH changes, and decreased Ca2+ requirement suggest that the protease responsible for cleavage of Hendra F protein differs from proteases previously shown to be involved in the processing of other viral glycoproteins.     

Phosphorylation of brush border myosin I by protein kinase C is regulated by Ca(2+)-stimulated binding of myosin I to phosphatidylserine concerted with calmodulin dissociation.

Brush border myosin I from chicken intestine is phosphorylated in vitro by chicken intestinal epithelial cell protein kinase C. Phosphorylation on serine and threonine to a maximum of 0.93 mol of P/mol of myosin I occurs within an approximately 20 kDa region at the end of the COOH-terminal tail of the 119-kDa heavy chain. The effects of Ca2+ on myosin I phosphorylation by protein kinase C are complex, with up to 4-fold stimulation occurring at 0.5-3 microM Ca2+, and up to 80% inhibition occurring at 3-320 microM Ca2+. Phosphorylation required that brush border myosin I be in its phosphatidylserine vesicle-bound state. Previously unknown Ca2+ stimulation of brush border myosin I binding to phosphatidylserine vesicles was found to coincide with Ca2+ stimulation of phosphorylation. A myosin I proteolytic fragment lacking approximately 20 kDa of its tail retained Ca(2+)-stimulated binding, but showed reduced Ca(2+)-independent binding. Ca(2+)-dependent phosphatidylserine binding is apparently due to the concomitant phosphatidylserine-promoted, Ca(2+)-induced dissociation of up to three of the four calmodulin light chains from myosin I. Four highly basic putative calmodulin-binding sites in the Ca(2+)-dependent phosphatidylserine binding region of the heavy chain were identified based on the similarity in their sequence to the calmodulin- and phosphatidylserine-binding site of neuromodulin. Calmodulin dissociation is now shown to occur in the low micromolar Ca2+ concentration range and may regulate the association of brush border myosin I with membranes and its phosphorylation by protein kinase C.     

The involvement of calpain in the activation of protein kinase C in neutrophils stimulated by phorbol myristic acid

The Ca2+/phospholipid-dependent protein kinase (protein kinase C) of human neutrophils is converted to a proteolytically modified Ca2+/phospholipid-independent form (Inoue, M., Kishimoto, A., Takai, Y.U., and Nishizuka, Y. (1977) J. Biol. Chem. 252, 7610-7616) on incubation with neutrophil membranes in the presence of micromolar concentrations of Ca2+ and an endogenous Ca2+-requiring proteinase (Melloni, E., Pontremoli, S., Michetti, M., Sacco, O., Sparatore, B., Salamino, F., and Horecker, B. L. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 6435-6439). We have now demonstrated the appearance of a similar Ca2+/phospholipid-independent kinase in intact human neutrophils stimulated by phorbol 12-myristate 13-acetate (PMA). The following evidence supports the conclusion that the Ca2+/phospholipid-independent protein kinase recovered from the PMA-treated cells is a proteolytically modified form of the "native" protein kinase C. 1) In cells exposed to PMA, the rate of disappearance of Ca2+/phospholipid-dependent protein kinase C activity is correlated with the rate of appearance of the Ca2+/phospholipid-independent kinase. 2) The chromatographic behavior of the new protein kinase and its molecular size (approximately 65 kDa) are identical to those previously reported for the proteolytically modified form of protein kinase C. 3) The modified protein kinase no longer binds to the cell membrane and is recovered almost entirely in the cytosol fraction. 4) In neutrophils preloaded with inhibitors of the Ca2+-requiring proteinase, stimulation with PMA results in translocation of protein kinase C from the cytosol fraction to the particulate fraction, but the appearance of the soluble, Ca2+/phospholipid-dependent form is prevented. We conclude that binding of protein kinase C to the plasma membrane and its proteolytic conversion are related, but independent, processes both elicited by exposure of neutrophils to the phorbol ester. Proteolytic cleavage of the membrane-bound protein kinase C provides an alternative mechanism for its activation and may account for certain of the cellular responses observed in PMA-stimulated neutrophils.     

Protein kinase A- and C-induced insulin release from Ca2+ -insensitive pools

Insulin secretion is known to depend on an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). However, recent studies have suggested that insulin secretion can also be evoked in a Ca(2+)-independent manner. In the present study we show that treatment of intact mouse islets and RINm5F cells with protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) or protein kinase A (PKA) activator forskolin promoted insulin secretion with no changes of [Ca(2+)](i). Moreover, insulin secretion mediated by PMA or forskolin was maintained even when extracellular or cytosolic Ca(2+) was deprived by treatment of cells with ethylene glycol bis(beta-amino ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis(2-amino phenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxy methyl ester) (BAPTA/AM) in RINm5F cells. The secretagogue actions of PMA and forskolin were blocked by GF109203X and H89, selective inhibitors for PKC and PKA, respectively. PMA treatment caused translocation of PKC-alpha and PKC- epsilon from cytosol to membrane, implying that selectively PKC-alpha and PKC- epsilon isoforms might be important for insulin secretion. Co-treatment with high K(+) and PMA showed a comparable level of insulin secretion to that of PMA alone. In addition, PMA and forskolin evoked insulin secretion in cells where Ca(2+)-dependent insulin secretion was completed. Our data suggest that PKC and PKA can elicit insulin secretion not only in a Ca(2+)-sensitive manner but also in a Ca(2+)-independent manner from separate releasable pools.     

Protein kinase C isotypes and signaling in neutrophils. Differential substrate specificities of a translocatable calcium- and phospholipid-dependent beta-protein kinase C and a phospholipid-dependent protein kinase which is inhibited by long chain fatty acyl coenzyme A

Neutrophils possess a classical Ca2+, phosphatidyl serine (PS) and diglyceride (DG)-dependent protein kinase C (beta-PKC) which was translocatable from cytosol to membrane in response to elevated Ca2+ in the physiologic range or to pretreatment with phorbol myristate acetate (PMA). The translocatable beta-PKC was purified from neutrophil membranes prepared in the presence of Ca2+, eluted with EGTA and subjected to hydroxyapatite chromatography. An 80-kDa protein possessing Ca/DG/PS-dependent histone phosphorylating activity was recognized by a monoclonal antibody to beta-PKC but not to alpha-PKC or gamma-PKC. A cytosolic kinase activity remaining after Ca(2+)-induced translocation of beta-PKC was dependent on PS and DG but did not require Ca2+. This novel Ca(2+)-independent, PS/DG-dependent kinase, termed nPKC, eluted from hydroxyapatite between alpha-PKC and beta-PKC, ran as a 76-kDa band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and was reactive to a polyclonal consensus antibody but not to monoclonal antibodies to alpha-PKC, beta-PKC, or gamma-PKC. Long chain fatty acyl-CoA, but not the corresponding free fatty acids, inhibited nPKC in the 1-10 microM range. The chemotactic peptide fMet-Leu-Phe triggered prompt but transient increases in neutrophil long chain fatty acid acyl-CoA, suggesting that nPKC is regulated by fatty acyl-CoA as well as DG during neutrophil activation. Purified beta-PKC phosphorylated a number of cytosolic proteins in a Ca(2+)-dependent manner, including a major 47-kDa cytosolic protein, which may be implicated in superoxide anion generation. In contrast, nPKC did not phosphorylate the 47-kDa protein, but phosphorylated numerous cytosolic proteins in a Ca(2+)-independent manner, including a 66-kDa protein which was not phosphorylated by beta-PKC. Differences in location, substrate specificity, and cofactor dependence between nPKC and beta-PKC suggest these kinases may play selective roles in the activation sequence of the neutrophil.     

Sustained muscle contraction induced by agonists, growth factors, and Ca(2+) mediated by distinct PKC isozymes

The role of protein kinase C (PKC) in sustained contraction was examined in intestinal circular and longitudinal muscle cells. Initial contraction induced by agonists (CCK-8 and neuromedin C) was abolished by 1) inhibitors of Ca(2+) mobilization (neomycin and dimethyleicosadienoic acid), 2) calmidazolium, and 3) myosin light chain (MLC) kinase (MLCK) inhibitor KT-5926. In contrast, sustained contraction was not affected by these inhibitors but was abolished by 1) the PKC inhibitors chelerythrine and calphostin C, 2) PKC-epsilon antibody, and 3) a pseudosubstrate PKC-epsilon inhibitor. GDPbetaS abolished both initial and sustained contraction, whereas a Galpha(q/11) antibody inhibited only initial contraction, implying that sustained contraction was dependent on activation of a distinct G protein. Sustained contraction induced by epidermal growth factor was inhibited by calphostin C, PKC-alpha,beta,gamma antibody, and a pseudosubstrate PKC-alpha inhibitor. Ca(2+) (0.4 microM) induced an initial contraction in permeabilized muscle cells that was blocked by calmodulin and MLCK inhibitors and a sustained contraction that was blocked by calphostin C and a PKC-alpha,beta,gamma antibody. Thus initial contraction induced by Ca(2+), agonists, and growth factors is mediated by MLCK, whereas sustained contraction is mediated by specific Ca(2+)-dependent and -independent PKC isozymes. G protein-coupled receptors are linked to PKC activation via distinct G proteins.     

Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (S1P) can induce intracellular Ca(2+) mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca(2+)-independent mechanisms of cell contraction have been the focus of numerous studies on Ca(2+) sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca(2+)-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in S1P-induced C2C12 cell contraction. First, we showed that depletion of Ca(2+), by pre-treatment with BAPTA, did not affect S1P-induced myoblastic contractility, whereas it abolished S1P-induced Ca(2+) transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in S1P-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca(2+) and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with G?6976 or rottlerin, specific inhibitors of PKC alpha and PKC delta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca(2+)-independent/Rho-Rho kinase-dependent pathways may exert an important role in S1P-induced myoblastic cell contraction.