Inhibition of GTP gamma S-dependent phospholipase D and Rho membrane association by calphostin is independent of protein kinase C catalytic activity

Role of PKC isoforms in the Fc(gamma)R-mediated inhibition of LPS-stimulated IL-12 secretion by macrophages

Ligation of Fc receptors for immunoglobulin G (FcgammaRs) inhibits lipopolysaccharide (LPS)-stimulated secretion of interleukin (IL)-12 by macrophages. FcgammaR activation of protein kinase C (PKC) contributes to several functions of this receptor including phagocytosis, activation of the reduced nicotinamide adenine dinucleotide phosphate oxidase, and secretion of certain cytokines. Therefore, we tested the hypothesis that PKC mediates the FcgammaR inhibition of IL-12 secretion by macrophages. In murine macrophages, FcgammaR ligation augmented LPS-stimulated activation of PKC-alpha and PKC-delta but reduced IL-12p40 secretion. Similarly, activation of PKC with phorbol 12-myristate 13-acetate (PMA) depressed LPS-stimulated IL-12p40 secretion, and depletion of PKC augmented LPS-stimulated IL-12p40 secretion. Antisense down-regulation of PKC-delta increased LPS-stimulated IL-12p40 secretion and fully prevented the effects of FcgammaR ligation or PMA on IL-12p40 secretion. In contrast, down-regulation of PKC-epsilon blocked LPS-stimulated secretion of IL-12p40. Down-regulation of PKC-alpha had no effect on LPS-stimulated IL-12p40 secretion. The results suggest a negative role for PKC-delta and a positive role for PKC-epsilon in the regulation of LPS-stimulated IL-12p40 secretion.

Differential participation of protein kinase C and Rho kinase in α1-adrenoceptor mediated contraction in rat arteries

The major functional α1-adrenoceptor in the rat aorta is of the α1Dsubtype and that in the caudal artery is of the α1Asubtype. In the present study, the participation of protein kinase C (PKC) and Rho kinase (RhoK) in contractile responses to stimulation of the α1-adrenoceptors in these two arteries was investigated. Both the PKC inhibitor Ro-318220 and the RhoK inhibitor Y-27632 significantly blocked contractile responses of the aorta to phenylephrine (PE) and the selective α1A-adrenoceptor agonist A61603. When used in combination, the inhibitors had an additive blocking effect. In the caudal artery, Y-27632 but not Ro-318220 inhibited contractile responses to PE and A61603, and, in combination, the antagonism produced was no greater than that by Y-27632 alone. Contractile responses to direct activation of PKC with phorbol 12,13-dibutyrate were much smaller and levels of CPI-17 (PKC-activated protein phosphatase inhibitor of 17 kDa) were much lower in the caudal artery than the aorta. The results suggest that both PKC and RhoK contribute independently to contractile responses to stimulation of α1D-adrenoceptors in the aorta. However, RhoK, but not PKC, participates in contractile responses to stimulation of α1A-adrenoceptors in the caudal artery. This difference may largely be due to differences between the two arteries in the extent to which PKC participates in contraction.Key words: vascular smooth muscle, α1-adrenoceptors, protein kinase C, rho kinase, phenylephrine.

Endothelin-1 promotes Ca2+ antagonist-insensitive coronary smooth muscle contraction via activation of epsilon-protein kinase C

Certain forms of coronary artery disease do not respond to treatment with Ca2+ channel blockers, and a role for endothelin-1 (ET-1) in Ca2+ antagonist-insensitive forms of coronary vasospasm has been suggested; however, the signaling mechanisms involved are unclear. We tested the hypothesis that a component of ET-1-induced coronary smooth muscle contraction is Ca2+ antagonist-insensitive and involves activation of protein kinase C (PKC). Cell contraction was measured in smooth muscle cells isolated from porcine coronary artery, [Ca2+]i was measured in fura-2 loaded cells, and the cytosolic and particulate fractions were examined for PKC activity and reactivity with isoform-specific PKC antibodies using Western blot analysis. In Hank's solution (1 mmol/L Ca2+), ET-1 (10(-7) mol/L) caused a transient increase in [Ca2+]i (236+/-14 nmol/L) followed by a maintained increase in [Ca2+]i (184+/-8 nmol/L) and 35% cell contraction. The Ca2+ channel blockers verapamil and diltiazem (10(-6) mol/L) abolished the maintained ET-1-induced [Ca2+]i, but only partially inhibited ET-1-induced cell contraction to 18%. The verapamil-insensitive component of ET-1 contraction was inhibited by the PKC inhibitors calphostin C and epsilon-PKCV1-2. ET-1 caused translocation of Ca2+-dependent alpha-PKC and Ca2+-independent epsilon-PKC from the cytosolic to the particulate fraction that was inhibited by calphostin C. Verapamil abolished ET-1-induced translocation of alpha-PKC, but not that of epsilon-PKC. Phorbol 12-myristate 13-acetate (10(-6) mol/L), a direct activator of PKC, caused 22% cell contraction, with no increase in [Ca2+]i, and translocation of epsilon-PKC that was inhibited by calphostin C, but not by verapamil. KCl (51 mmol/L), which stimulates Ca2+ influx, caused 35% cell contraction and increase in [Ca2+]i (291+/-11 nmol/L) that were inhibited by verapamil, but not by calphostin C, and did not cause translocation of alpha- or epsilon-PKC. In Ca2+-free (2 mmol/L EGTA) Hank's solution, ET-1 caused 15% cell contraction, with no increase in [Ca2+]i, and translocation of epsilon-PKC that were inhibited by epsilon-PKC V1-2 inhibitory peptide. Thus, a significant component of ET-1-induced contraction of coronary smooth muscle is Ca2+ antagonist-insensitive and involves activation and translocation of Ca2+-independent epsilon-PKC, and may represent a signaling mechanism of Ca2+ antagonist-resistant forms of coronary vasospasm.

Ca2+ antagonist-insensitive coronary smooth muscle contraction involves activation of ϵ-protein kinase C-dependent pathway

Certain angina and coronary artery disease forms do not respond to Ca2+ channel blockers, and a role for vasoactive eicosanoids such as PGF2α in Ca2+ antagonist-insensitive coronary vasospasm is suggested; however, the signaling mechanisms are unclear. We investigated whether PGF2α-induced coronary smooth muscle contraction is Ca2+ antagonist insensitive and involves activation of a PKC-dependent pathway. We measured contraction in single porcine coronary artery smooth muscle cells and intracellular free Ca2+ concentration ([Ca2+]i) in fura 2-loaded cells and examined cytosolic and particulate fractions for PKC activity and reactivity with isoform-specific PKC antibodies. In Hanks' solution (1 mM Ca2+), PGF2α (10-5 M) caused transient [Ca2+]i increase followed by maintained [Ca2+]i increase and 34% cell contraction. Ca2+ channel blockers verapamil and diltiazem (10-6 M) abolished maintained PGF2α-induced [Ca2+]i increase but only partially inhibited PGF2α-induced cell contraction to 17%. Verapamil-insensitive PGF2α contraction was inhibited by PKC inhibitors GF-109203X, calphostin C, and ϵ-PKC V1-2. PGF2α caused Ca2+-dependent α-PKC and Ca2+-independent ϵ-PKC translocation from cytosolic to particulate fractions that was inhibited by calphostin C. Verapamil abolished PGF2α-induced α-but not ϵ-PKC translocation. PMA (10-6 M), a direct activator of PKC, caused 21% contraction with no significant [Ca2+]i increase and ϵ-PKC translocation that were inhibited by calphostin C but not verapamil. Membrane depolarization by 51 mM KCl, which stimulates Ca2+ influx, caused 36% cell contraction and [Ca2+]i increase that were inhibited by verapamil but not GF-109203X or calphostin C and did not cause α- or ϵ-PKC translocation. Thus a significant component of PGF2α-induced contraction of coronary smooth muscle is Ca2+ antagonist insensitive, involves Ca2+-independent ϵ-PKC activation and translocation, and may represent a signaling mechanism of Ca2+ antagonist-resistant coronary vasospasm.

M1 protein triggers a phosphoinositide cascade for group A Streptococcus invasion of epithelial cells

Invasion of nonphagocytic cells by bacteria provides a favorable niche for persistence and evasion of host defenses and antibiotics. M protein is a major virulence factor because it promotes high-frequency invasion of epithelial cells by group A Streptococcus (GAS) and also renders the bacterium resistant to phagocytosis. In this study, we investigated the role of M1 protein from serotype M1 strain 90-226 in regulating mammalian signal transduction and cytoskeletal rearrangement for bacterial entry. LY294002 and wortmannin, which are inhibitors of phosphatidylinositol 3-kinase (PI 3-K) blocked invasion of epithelial cells by GAS by 75 and 80%, respectively, but failed to inhibit invasion by Salmonella enterica serovar Typhimurium. Also, epithelial cells transiently transfected with dominant negative p85 and p110 genes, the regulatory and catalytic subunits of PI 3-K, respectively, were less able to be invaded by GAS. To separate the influence of other streptococcal virulence factors from M protein, Lactococcus lactis was engineered to express M1 protein on its surface. L. lactis(pLM1) invaded epithelial cells efficiently in vitro, and PI 3-K inhibitors blocked 90% of this invasion. Purified soluble M1 protein stimulated the formation of stress fibers and actin tuffs on epithelial cells. LY294002 and wortmannin inhibited these cellular changes. A phosphoinositide analogue also inhibited the invasion of epithelial cells by GAS. Therefore, M1 protein, either directly or via bound fibronectin, initiates signals that depend on the lipid kinase PI 3-K pathway, which paves the way for cytoskeletal rearrangement that internalize the bacterium.

pp90RSK- and protein kinase C-dependent pathway regulates p42/44MAPK-induced LDL receptor transcription in HepG2 cells

We have previously shown that different extracellular stimuli require signaling through the Raf/MEK/p42/44MAPK cascade to induce LDL receptor expression. The present studies were designed to delineate the molecular mechanisms underlying p42/44MAPK-induced LDL receptor transcription in HepG2-Delta Raf-1:ER cells, a modified HepG2 cell line in which the Raf-1/MEK/p42/44MAPK cascade can be specifically activated by anti-estradiol ICI182,780 in an agonist-specific manner. Using these cells, we show that: a) LDL receptor induction was reduced in reporter constructs containing mutation in either Sp1 or sterol-regulatory element-1 (SRE-1) sites, whereas inactivation of both sites abolished the induction; b) E1A, which inhibits CREB binding protein (CBP), a common activator of SRE-1 binding protein and Sp1, strongly repressed the induction; c) intracellular inhibition of the 90 kDa ribosomal S6 kinase (pp90RSK) cascade reduced LDL receptor induction; d) highly selective protein kinase C (PKC) inhibitors effectively abrogated the induction without affecting activation of pp90RSK; and e) overexpression of PKC beta significantly induced LDL receptor promoter activity. Taken together, these results demonstrate that pp90RSK and PKC beta are downstream effectors and Sp1, SRE-1 binding protein, and CBP are part of the transcriptional complex resulting in induction of LDL receptor expression in response to activation of the Raf/MEK/p42/44MAPK cascade. These findings identify for the first time a role for PKC beta in determining the specificity of p42/44MAPK signaling by participating with pp90RSK in regulating gene expression.

Critical role of diacylglycerol- and phospholipid-regulated protein kinase C epsilon in induction of low-density lipoprotein receptor transcription in response to depletion of cholesterol

Induction of low-density lipoprotein (LDL) receptor transcription in response to depletion of cellular sterols in animal cells is well established. The intracellular signal or signals involved in regulating this process, however, remain unknown. Using a specific inhibitor of protein kinase C (PKC), calphostin C, we show the requirement of this kinase in the induction process in human hepatoma HepG2 cells. Overexpression of PKC epsilon, but not PKC alpha, -gamma, -delta, or -zeta was found to dramatically induce (approximately 18-fold) LDL receptor promoter activity. Interestingly, PKC epsilon-mediated induction was found to be sterol resistant. To further establish that PKC epsilon is involved in the sterol regulation of LDL receptor gene transcription, endogenous PKC epsilon was specifically inhibited by transfection with antisense PKC epsilon phosphorothionate oligonucleotides. Antisense treatment decreased endogenous PKC epsilon protein levels and completely blocked induction of LDL receptor transcription following sterol depletion. PKC epsilon-induced LDL receptor transcription is independent of the extracellular signal-regulated kinase 1 and 2 (p42/44(MAPK)) cascade, because the MEK-1/2 inhibitor, PD98059 did not inhibit, even though it blocked p42/44(MAPK) activation. Finally, photoaffinity labeling studies showed an isoform-specific interaction between PKC epsilon and sterols, suggesting that sterols may directly modulate its function by hampering binding of activators. This was confirmed by PKC activity assays. Altogether, these results define a novel signaling pathway leading to induction of LDL receptor transcription following sterol depletion, and a model is proposed to account for a new function for PKC epsilon as part of a sterol-sensitive signal transduction pathway in hepatic cells.

Protein kinase C alpha controls erythropoietin receptor signaling

Protein kinase C (PKC) is implied in the activation of multiple targets of erythropoietin (Epo) signaling, but its exact role in Epo receptor (EpoR) signal transduction and in the regulation of erythroid proliferation and differentiation remained elusive. We analyzed the effect of PKC inhibitors with distinct modes of action on EpoR signaling in primary human erythroblasts and in a recently established murine erythroid cell line. Active PKC appeared essential for Epo-induced phosphorylation of the Epo receptor itself, STAT5, Gab1, Erk1/2, AKT, and other downstream targets. Under the same conditions, stem cell factor-induced signal transduction was not impaired. LY294002, a specific inhibitor of phosphoinositol 3-kinase, also suppressed Epo-induced signal transduction, which could be partially relieved by activators of PKC. PKC inhibitors or LY294002 did not affect membrane expression of the EpoR, the association of JAK2 with the EpoR, or the in vitro kinase activity of JAK2. The data suggest that PKC controls EpoR signaling instead of being a downstream effector. PKC and phosphoinositol 3-kinase may act in concert to regulate association of the EpoR complex such that it is responsive to ligand stimulation. Reduced PKC-activity inhibited Epo-dependent differentiation, although it did not effect Epo-dependent "renewal divisions" induced in the presence of Epo, stem cell factor, and dexamethasone.

RhoB prenylation is driven by the three carboxyl-terminal amino acids of the protein: evidenced in vivo by an anti-farnesyl cysteine antibody

Protein isoprenylation is a lipid posttranslational modification required for the function of many proteins that share a carboxyl-terminal CAAX motif. The X residue determines which isoprenoid will be added to the cysteine. When X is a methionine or serine, the farnesyl-transferase transfers a farnesyl, and when X is a leucine or isoleucine, the geranygeranyl-transferase I, a geranylgeranyl group. But despite its CKVL motif, RhoB was reported to be both geranylgeranylated and farnesylated. Thus, the determinants of RhoB prenylation appear more complex than initially thought. To determine the role of RhoB CAAX motif, we designed RhoB mutants with modified CAAX sequence expressed in baculovirus-infected insect cells. We demonstrated that RhoB was prenylated as a function of the three terminal amino acids, i.e., RhoB bearing the CAIM motif of lamin B or CLLL motif of Rap1A was farnesylated or geranylgeranylated, respectively. Next, we produced a specific polyclonal antibody against farnesyl cysteine methyl ester allowing prenylation analysis avoiding the metabolic labeling restrictions. We confirmed that the unique modification of the RhoB CAAX box was sufficient to direct the RhoB distinct prenylation in mammalian cells and, inversely, that a RhoA-CKVL chimera could be alternatively prenylated. Moreover, the immunoprecipitation of endogenous RhoB from cells with the anti-farnesyl cysteine antibody suggested that wild-type RhoB is farnesylated in vivo. Taken together, our results demonstrated that the three last carboxyl amino acids are the main determinants for RhoB prenylation and described an anti-farnesyl cysteine antibody as a useful tool for understanding the cellular control of protein farnesylation.

Quantitative high-performance liquid chromatography-based detection method for calphostin C, a naturally occurring perylenequinone with potent antileukemic activity

Calphostin C is a potent inhibitor of protein kinase C and can induce Ca2+-dependent apoptosis in human ALL cells. Further development of calphostin C will require detailed pharmacodynamic studies in preclinical animal models. Therefore, we established a sensitive and accurate high-performance liquid chromatography (HPLC)-based quantitative detection method for the measurement of calphostin C levels in plasma. Extraction of calphostin C from plasma was performed by precipitation of plasma protein using acetonitrile and an aliquot of extracted supernatant was injected onto a Hewlett-Packard HPLC system constituting a 250x4 mm LiChrospher 100, RP-18 (5 microm) in conjunction with a 4x4 mm LiChrospher 100, RP-18 guard column (5 microm). The eluted compounds were detected by diode array detection set at a wavelength of 479 nm. Acetonitrile-water containing 0.1% trifluoroacetic acid and 0.1% triethylamine (70:30, v/v) was used as the mobile phase. The average extraction recovery from plasma was 97.3%. Good linearity (r>0.999) was observed throughout the concentration range of 0.05-40 microM for calphostin C in 50 microl of plasma. Intra- and inter-assay variabilities were less than 6% in plasma. The lowest detection limit of calphostin C in 50 microl plasma was 0.02 microM at a signal-to-noise ratio of approximately 3. The availability of this assay will now permit detailed pharmacodynamic and pharmacokinetic studies of calphostin C in vivo.

Mechanisms of Ca2+ sensitization of force production by noradrenaline in rat mesenteric small arteries

1. Mechanisms of Ca2+ sensitization of force production by noradrenaline were investigated by measuring contractile responses, intracellular Ca2+ concentration ([Ca2+]i) and phosphorylation of the myosin light chain (MLC) in intact and alpha-toxin-permeabilized rat mesenteric small arteries. 2. The effects of noradrenaline were investigated at constant membrane potential by comparing fully depolarized intact arteries in the absence and presence of noradrenaline. Contractile responses to K-PSS (125 mM K+) and NA-K-PSS (K-PSS + 10 microM noradrenaline) were titrated to 30 and 75%, respectively, of control force, by adjusting extracellular Ca2+ ([Ca2+]o). At both force levels, [Ca2+]i was substantially lower with NA-K-PSS than with K-PSS. With K-PSS, the proportion of MLC phosphorylated (approximately 30%) was similar at 30 and 75% of control force; with NA-K-PSS, MLC phosphorylation was greater at the higher force level (40 vs. 34%). 3. In alpha-toxin-permeabilized arteries, the force response to 1 microM Ca2+ was increased by 10 microM noradrenaline, and MLC phosphorylation was increased from 35 to 45%. The protein kinase C (PKC) inhibitor calphostin C (100 nM) abolished the noradrenaline-induced increase in MLC phosphorylation and contractile response, without affecting the contraction in response to Ca2+. Treatment with ATP gamma S in the presence of the MLC kinase inhibitor ML-9 increased the sensitivity to Ca2+ and abolished the response to noradrenaline. 4. The present results show that that in rat mesenteric small arteries noradrenaline-induced Ca2+ sensitization is associated with an increased proportion of phosphorylated MLC. The results are consistent with a decreased MLC phosphatase activity mediated through PKC. Furthermore, while MLC phosphorylation is a requirement for force production, the results show that other factors are also involved in force regulation.

Inhibitory effect on curcumin on mammalian phospholipase D activity

Curcumin, the major yellow pigment of turmeric (Curcuma longa), has strong anti-carcinogenic and anti-inflammatory activities. We examined the effects of curcumin on enzyme activities of the following phospholipases in a cell-free system: G protein-mediated phospholipase D (PLD), phosphatidylinositol-specific phospholipase C, and phospholipase A2 from mouse macrophage-like cell line J774.1 cells, sphingomyelinase from bovine brain, and phosphatidylcholine-phospholipase C from Bacillus cereus. Curcumin inhibited several types of phospholipases, most effectively PLD among those tested. It also inhibited 12-O-tetradecanoylphorbol-13-acetate-induced PLD activation in intact J774.1 cells in a dose-dependent manner. These results suggest that the anti-inflammatory and anti-carcinogenic action of curcumin is partly due to the inhibition of PLD.