The protein kinase C ?-specific inhibitor LY379196 blocks TPA-induced monocytic differentiation of HL60 cells the protein kinase C ?-specific inhibitor LY379196 blocks TPA-induced monocytic differentiation of HL60 cells

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Protein kinase C (PKC) isoforms in cancer, tumor promotion and tumor suppression

The AGC family of serine/threonine kinases (PKA, PKG, PKC) includes more than 60 members that are critical regulators of numerous cellular functions, including cell cycle and differentiation, morphogenesis, and cell survival and death. Mutation and/or dysregulation of AGC kinases can lead to malignant cell transformation and contribute to the pathogenesis of many human diseases. Members of one subgroup of AGC kinases, the protein kinase C (PKC), have been singled out as critical players in carcinogenesis, following their identification as the intracellular receptors of phorbol esters, which exhibit tumor-promoting activities. This observation attracted the attention of researchers worldwide and led to intense investigations on the role of PKC in cell transformation and the potential use of PKC as therapeutic drug targets in cancer diseases. Studies demonstrated that many cancers had altered expression and/or mutation of specific PKC genes. However, the causal relationships between the changes in PKC gene expression and/or mutation and the direct cause of cancer remain elusive. Independent studies in normal cells demonstrated that activation of PKC is essential for the induction of cell activation and proliferation, differentiation, motility, and survival. Based on these observations and the general assumption that PKC isoforms play a positive role in cell transformation and/or cancer progression, many PKC inhibitors have entered clinical trials but the numerous attempts to target PKC in cancer has so far yielded only very limited success. More recent studies demonstrated that PKC function as tumor suppressors, and suggested that future clinical efforts should focus on restoring, rather than inhibiting, PKC activity. The present manuscript provides some historical perspectives on the tumor promoting function of PKC, reviewing some of the observations linking PKC to cancer progression, and discusses the role of PKC in the pathogenesis of cancer diseases and its potential usage as a therapeutic target.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

Regulation of monocyte differentiation by specific signaling modules and associated transcription factor networks

Monocyte/macrophages are important players in orchestrating the immune response as well as connecting innate and adaptive immunity. Myelopoiesis and monopoiesis are characterized by the interplay between expansion of stem/progenitor cells and progression towards further developed (myelo)monocytic phenotypes. In response to a variety of differentiation-inducing stimuli, various prominent signaling pathways are activated. Subsequently, specific transcription factors are induced, regulating cell proliferation and maturation. This review article focuses on the integration of signaling modules and transcriptional networks involved in the determination of monocytic differentiation.

The PKCbeta/HuR/VEGF pathway in diabetic retinopathy

We investigated whether the diabetes-related PKCbeta activation affects VEGF expression through the mRNA-stabilizing human embryonic lethal abnormal vision (ELAV) protein, HuR, in the retina of streptozotocin (STZ)-induced diabetic rats. Diabetes was induced in rats by STZ injection. Retinal tissues were processed to detect PKCbetaI, PKCbetaII, VEGF and HuR contents, as well as HuR phosphorylation. Immunoprecipitation coupled to RT-PCR was employed to evaluate HuR binding to VEGF mRNA in RiboNucleoProteic (RNP) complexes. Statistical analysis was performed by ANOVA followed by an appropriate post hoc comparison test. Following experimental diabetes PKCbetaI and PKCbetaII levels were increased compared to sham; there was also a PKC-mediated phosphorylation/activation of HuR. These effects were blunted by the in vivo co-administration of a selective PKCbeta inhibitor. A specific binding between the HuR protein and the VEGF mRNA was also detected. The PKCbeta/HuR activation was accompanied by enhanced VEGF protein expression that was, again, blunted by the PKCbeta inhibitor. These findings first demonstrate the activation, in the retina, of the PKCbeta/HuR/VEGF pathway following experimental diabetes and disclose a new potential pharmacological target to counteract pathologies implicating VEGF deregulation, such as diabetic retinopathy.

Role of host cell polarity and leading edge properties in Pseudomonas type III secretion

Type III secretion (T3S) functions in establishing infections in a large number of Gram-negative bacteria, yet little is known about how host cell properties might function in this process. We used the opportunistic pathogen Pseudomonas aeruginosa and the ability to alter host cell sensitivity to Pseudomonas T3S to explore this problem. HT-29 epithelial cells were used to study cellular changes associated with loss of T3S sensitivity, which could be induced by treatment with methyl-beta-cyclodextrin or perfringolysin O. HL-60 promyelocytic cells are innately resistant to Pseudomonas T3S and were used to study cellular changes occurring in response to induction of T3S sensitivity, which occurred following treatment with phorbol esters. Using both cell models, a positive correlation was observed between eukaryotic cell adherence to tissue culture wells and T3S sensitivity. In examining the type of adhesion process linked to T3S sensitivity in HT-29 cells, a hierarchical order of protein involvement was identified that paralleled the architecture of leading edge (LE) focal complexes. Conversely, in HL-60 cells, induction of T3S sensitivity coincided with the onset of LE properties and the development of actin-rich projections associated with polarized cell migration. When LE architecture was examined by immunofluorescent staining for actin, Rac1, IQ-motif-containing GTPase-activating protein 1 (IQGAP1) and phosphatidylinositol 3 kinase (PI3 kinase), intact LE structure was found to closely correlate with host cell sensitivity to P. aeruginosa T3S. Our model for host cell involvement in Pseudomonas T3S proposes that cortical actin polymerization at the LE alters membrane properties to favour T3S translocon function and the establishment of infections, which is consistent with Pseudomonas infections targeting wounded epithelial barriers undergoing cell migration.

Regulation of plasminogen activators in human thyroid follicular cells and their relationship to differentiated function

Human thyroid cells in culture take up and organify (125)I when cultured in TSH (acting through cAMP) and insulin. They also secrete urokinase (uPA) and tissue-type (tPA) plasminogen activators (5-100 IU/10(6)cells/day). TSH and insulin both decreased secreted PA activity (PAA), uPA and tPA protein and their mRNAs. Autocrine fibroblast growth factor increased secreted PAA and inhibited thyroid cell (125)I uptake. Epidermal growth factor (EGF) and the protein kinase C (PKC) activator, TPA significantly increased PAA and inhibited thyroid differentiated function, (TPA > EGF). For TPA, effects were rapid, increased PAA secretion and decreased (125)I uptake being seen at 4 h whereas for EGF, a 24 h incubation was required. qRT-PCR showed significantly increased mRNA expression of uPA with lesser effects on tPA. Aprotinin, which inhibits PAA, increased (125)I uptake but did not abrogate the effects of TPA and EGF. The MEKK inhibitor, PD98059 partially reversed the effects of EGF and TPA on PAA, and largely reversed the effects of EGF but not TPA on differentiated function. PKC inhibitors bisindoylmaleimide 1, and the specific PKCbeta inhibitor, LY379196 completely reversed the effects of TPA on (125)I uptake and PAA whereas EGF effects were unaffected. TPA inhibited follicle formation and this effect was blocked by LY379196 but not PD98059. We conclude that in thyroid cells, MAPK activation inversely correlates with (125)I uptake and directly correlates with PA expression, in contrast to the effects of cAMP. TPA effects on iodide metabolism, dissolution of follicles and uPA synthesis are mediated predominantly through PKCbeta whereas EGF exerts its effects through MAPK but not PKCbeta.

MEK-ERK is involved in SUMO-1 foci formation on apoptosis

Small ubiquitin-related modifier (SUMO) modification appears to regulate the activity, intracellular localization, and stability of the targeted proteins. To explore the relationship among sumoylation, antitumor reagent, and apoptosis, we treated green fluorescence protein (GFP)-SUMO-1-overexpressed K562 cells (K562/GFP-SUMO-1) with mitoxantrone (MIT) as an antitumor reagent. By the treatment with MIT, GFP-SUMO-1 formed foci in nuclei. While by the treatment with a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), GFP-SUMO-1 located homogeneously in nuclei. When K562/GFP-SUMO-1 cells were treated with TPA plus MIT, GFP-SUMO-1 foci became larger and apoptosis was induced more than with MIT alone. The apoptosis induced by TPA plus MIT was prevented by blockage of GFP-SUMO-1 foci by small interfering RNA (siRNA) against SUMO-1. The formation of GFP-SUMO-1 foci was reduced by a MEK inhibitor U0126 or a nuclear export inhibitor leptomycin B, and endogenous SUMO-1 foci were reduced in K562 cells expressing the dominant-negative MEK1 mutant. These results suggest that the formation of SUMO-1 foci is regulated by the MEK-ERK pathway and may induce apoptosis.

Dysregulated intracellular signaling impairs CTGF-stimulated responses in human mesangial cells exposed to high extracellular glucose

High ambient glucose activates intracellular signaling pathways to induce the expression of extracellular matrix and cytokines such as connective tissue growth factor (CTGF). Cell responses to CTGF in already glucose-stressed cells may act to transform the mesangial cell phenotype leading to the development of glomerulosclerosis. We analyzed cell signaling downstream of CTGF in high glucose-stressed mesangial cells to model signaling in the diabetic milieu. The addition of CTGF to primary human mesangial cells activates cell migration which is associated with a PKC-zeta-GSK3beta signaling axis. In high ambient glucose basal PKC-zeta and GSK3beta phosphorylation levels are selectively increased and CTGF-stimulated PKC-zeta and GSK3beta phosphorylation was impaired. These effects were not induced by osmotic changes. CTGF-driven profibrotic cell signaling as determined by p42/44 MAPK and Akt phosphorylation was unaffected by high glucose. Nonresponsiveness of the PKC-zeta-GSK3beta signaling axis suppressed effective remodeling of the microtubule network necessary to support cell migration. However, interestingly the cells remain plastic: modulation of glucose-induced PKC-beta activity in human mesangial cells reversed some of the pathological effects of glucose damage in these cells. We show that inhibition of PKC-beta with LY379196 and PKC-beta siRNA reduced basal PKC-zeta and GSK3beta phosphorylation in human mesangial cells exposed to high glucose. CTGF stimulation under these conditions again resulted in PKC-zeta phosphorylation and human mesangial cell migration. Regulation of PKC-zeta by PKC-beta in this instance may establish PKC-zeta as a target for constraining the progression of mesangial cell dysfunction in the pathogenesis of diabetic nephropathy.

1Alpha,25-dihydroxyvitamin D3-mediated stimulation of steroid sulphatase activity in myeloid leukaemic cell lines requires VDRnuc-mediated activation of the RAS/RAF/ERK-MAP kinase signalling pathway

1Alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) stimulates the activity of steroid sulphatase (STS) in myeloid cells [Hughes et al., 2001, 2005]. This was attenuated by inhibitors of phospholipase D (PLD) (n-butanol, 2,3-diphosphoglyceric acid, C(2)-ceramide) and phosphatidate phosphohydrolase (PAP) (propranolol and chlorpromazine), but was unaffected by inhibitors of phospholipase C. The 1alpha,25(OH)(2)D(3)-induced STS activity was also attenuated by inhibitors of protein kinase Calpha and protein kinase Cdelta (Go 6976, HBDDE and rottlerin), but not by an inhibitor of protein kinase Cbeta (LY379196). Additionally, 1alpha,25(OH)(2)D(3)-induced STS activity was attenuated by inhibitors of RAS (manumycin A), RAF (GW5074), MEK (PD098059 and U1026) and JNK (SP600125), but not p38 (PD169316). 1alpha,25(OH)(2)D(3) produced a rapid and long lasting stimulation of the ERK-MAP kinase signalling cascade in HL60 myeloid leukaemic cells. This 'non-genomic' effect of 1alpha,25(OH)(2)D(3) blocked by pharmacological antagonists of nuclear vitamin D receptors (VDR(nuc)) and does not appear to require hetero-dimerisation with the retinoid-X receptor (RXR). Inhibitors of the Src tyrosine kinase (PP1), RAS (manumycin A), RAS-RAF interactions (sulindac sulphide and RAS inhibitory peptide), RAF (GW5074 or chloroquine), and protein kinase Calpha (HBDDE) abrogated the 1alpha,25(OH)(2)D(3)-stimulated increase in ERK-MAP kinase activity. Taken together, these results show that 1alpha,25(OH)(2)D(3)/VDR(nuc) activation of the RAS/RAF/ERK-MAP kinase signalling pathway plays an important role in augmenting STS activity in human myeloid leukaemic cell lines.

Retinoid-mediated stimulation of steroid sulfatase activity in myeloid leukemic cell lines requires RARalpha and RXR and involves the phosphoinositide 3-kinase and ERK-MAP kinase pathways

All-trans retinoic acid and 9-cis-retinoic acid stimulate the activity of steroid sulfatase in HL60 acute myeloid leukemia cells in a concentration- and time-dependent manner. Neither of these 'natural retinoids' augmented steroid sulfatase activity in a HL60 sub-line that expresses a dominant-negative retinoic acid receptor alpha (RARalpha). Experiments with synthetic RAR and RXR agonists and antagonists suggest that RARalpha/RXR heterodimers play a role in the retinoid-stimulated increase in steroid sulfatase activity. The retinoid-driven increase in steroid sulfatase activity was attenuated by inhibition of phospholipase D (PLD), but not by inhibitors of phospholipase C. Experiments with inhibitors of protein kinase C (PKC) show that PKCalpha and PKCdelta play an important role in modulating the retinoid-stimulation of steroid sulfatase activity in HL60 cells. Furthermore, we show that pharmacological inhibition of the RAF-1 and ERK MAP kinases blocked the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells and, by contrast, inhibition of the p38-MAP kinase or JNK-MAP kinase had no effect. Pharmacological inhibitors of the phosphatidylinositol 3-kinase, Akt, and PDK-1 also abrogated the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells. These results show that crosstalk between the retinoid-stimulated genomic and non-genomic pathways is necessary to increase steroid sulfatase activity in HL60 cells.

Central Role of PKCβ in Neointimal Expansion Triggered by Acute Arterial Injury

We tested the hypothesis that PKCβ contributes to vascular smooth muscle cell (SMC) migration and proliferation; processes central to the pathogenesis of restenosis consequent to vascular injury. Homozygous PKCβ null (−/−) mice or wild-type mice fed the PKCβ inhibitor, ruboxistaurin, displayed significantly decreased neointimal expansion in response to acute femoral artery endothelial denudation injury compared with controls. In vivo and in vitro analyses demonstrated that PKCβII is critically linked to SMC activation, at least in part via regulation of ERK1/2 MAP kinase and early growth response-1. These data highlight novel roles for PKCβ in the SMC response to acute arterial injury and suggest that blockade of PKCβ may represent a therapeutic strategy to limit restenosis.

Reduction of high glucose and phorbol-myristate-acetate-induced endothelial cell permeability by protein kinase C inhibitors LY379196 and hypocrellin A

Endothelial barrier dysfunction plays a pivotal role in the pathogenesis of diabetic vascular complications. Although recent studies have established a link between protein kinase C (PKC) pathway and hyperglycaemic-induced vascular permeability, it is unclear which PKC isoforms involve increased endothelial cell permeability. In the present study, we investigated whether high glucose induced endothelial hyperpermeability via distinct PKC isoforms in human umbilical vein endothelial cells (HUVECs) and whether increased endothelial permeability could be substantially reversed by PKC inhibitors LY379196 and hypocrellin A (HA). High glucose (20 mM) and phorbol-myristate-acetate (PMA)-induced endothelial hyperpermeability was almost abolished by 150 nM HA and partially reduced by 30 nM PKC beta inhibitor (LY379196). LY379196 and HA inhibited the membrane fraction of PKC activity in a dose-dependent manner. Western blot analysis revealed high-glucose-induced overexpression of PKC alpha and PKC beta2 in the membrane fraction of HUVECs. LY379196 (30 and 150 nM) selectively inhibited PKC beta2 with no significant effect on PKC alpha expression. HA (150 nM) significantly reduced PKC alpha expression with no inhibitory effect on PKC beta2. At higher concentrations (300 nM), both LY379196 and HA were no longer selective for PKC beta or alpha, respectively. This study showed that both PKC alpha and beta2 contributed to endothelial hyperpermeability. Since reduction of endothelial hyperpermeability was greater with inhibition of PKC alpha rather than PKC beta2, we conclude that PKC alpha may be a major isoform involved in endothelial permeability in HUVECs, and that PKC alpha-mediated endothelial permeability was significantly reversed by the PKC inhibitor HA.

Activation of protein kinase C delta by all-trans-retinoic acid

All-trans-retinoic acid (RA) is a potent inhibitor of leukemia cell proliferation and induces differentiation of acute promyelocytic leukemia cells in vitro and in vivo. For RA to induce its biological effects in target cells, binding to specific retinoic acid nuclear receptors is required. The resulting complexes bind to RA-responsive elements (RAREs) in the promoters of RA-inducible genes to initiate gene transcription and to generate protein products that mediate the biological effects of RA. In this report, we provide evidence that a member of the protein kinase C (PKC) family of proteins, PKC delta, is activated during RA treatment of the NB-4 and HL-60 acute myeloid leukemia cell lines as well as the MCF-7 breast cancer cell line. Such RA-dependent phosphorylation was also observed in primary acute promyelocytic leukemia cells and resulted in activation of the kinase domain of PKC delta. In studies aimed at understanding the functional relevance of PKC delta in the induction of RA responses, we found that pharmacological inhibition of PKC delta (but not of other PKC isoforms) diminished RA-dependent gene transcription via RAREs. On the other hand, overexpression of a constitutively active form of the kinase strongly enhanced RA-dependent gene transcription via RAREs. Gel shift assays and chromatin immunoprecipitation studies demonstrated that PKC delta associated with retinoic acid receptor-alpha and was present in an RA-inducible protein complex that bound to RAREs. Pharmacological inhibition of PKC delta activity abrogated the induction of cell differentiation and growth inhibition of NB-4 blast cells, demonstrating that its function is required for such effects. Altogether, our data provide strong evidence that PKC delta is activated in an RA-dependent manner and plays a critical role in the generation of the biological effects of RA in malignant cells.

Resistance to phorbol 12-myristate 13-acetate-induced cell growth arrest in an HL60 cell line chronically exposed to a glutathione S-transferase pi inhibitor

Glutathione S-transferase pi (GSTpi; EC 2.5.1.18) has been shown recently to be a regulator of mitogen-activated protein kinases (MAPK). We have developed, by chronic exposure of HL60 cells to increasing concentrations of a peptidomimetic GSTpi inhibitor TLK199, a 10-fold resistant cell line (HL60/TLK199). Among the cellular adaptations observed in this cell line was an increase in extracellular signal-regulated kinase (ERK) activity without modification of basal expression levels. Phorbol 12-myristate 13-acetate (PMA) induced monocyte/macrophage cytodifferentiation in both HL60 wild-type (WT) and HL60/TLK199 cells. In contrast, PMA induced a pronounced cell growth inhibition and G(0)/G(1) cell cycle arrest in HL60 WT cells, while this differentiating agent had only a mild effect on cell growth without G(0)/G(1) cell cycle arrest in HL60/TLK199. This effect was associated with a rapid and sustained activation of ERK (up to 6hr) in HL60 WT cells but only a transient induction of these kinases (between 30 and 60min) in HL60/TLK199. Furthermore, treatment of both cell lines with PMA in combination with the protein tyrosine phosphatase inhibitors sodium orthovanadate (OV) or 3,4-dephostatin (DPN) circumvented the resistance to cell growth arrest and potentiated differentiation in HL60/TLK199 but had no effect on HL60 WT cells. The circumvention of the resistance to PMA was associated with a sustained activation of ERK. These data suggest that chronic exposure of HL60 cells to TLK199 alters cellular ERK activation by PMA, which may contribute to the differential response of the WT and resistant cells to PMA.

Activation of PKC but not of ERK is required for vitamin E-succinate-induced apoptosis of HL-60 cells

Vitamin E-succinate (VES) induced HL-60 human leukemia cells to undergo apoptosis. Treatment with VES induced membrane translocation of Fas; cleavages of caspase-3, PARP, and lamin B; hypophosphorylation of retinoblastoma protein; and increase of p21(WAF1) protein level. During the induction of apoptosis, activity of PKC was gradually increased with downregulation of VES-induced ERK activity and accompanied by activation of caspase-3. Inhibition of PKC by GF109203X blocked VES-mediated membrane translocation of PKC-alpha and cleavage of caspase-3 cascade, resulting in prevention of VES-induced apoptosis. On the contrary, PKC activation by cotreatment with LPC or thapsigargin and VES synergistically increased VES-mediated apoptosis. However, inhibition of ERK activity by PD98059 showed no significant effect on VES-induced PKC activity and apoptosis. Taken together, our data suggest that VES induces activation of PKC and PKC-dependent hypophosphorylation of retinoblastoma protein, which results in induction of apoptosis, and that VES-induced early activation of ERK and ERK-dependent induction of p21(WAF1) are not required for apoptosis.

The activation of PI 3-K and PKC zeta in PMA-induced differentiation of HL-60 cells

The human myelocytic leukemia cell line HL-60 is a useful model for the study of cellular differentiation. Phorbol 12-myristate 13-acetate (PMA) induces the monocyte/macrophage-like differentiation of HL-60 cells and results in growth arrest, increasing adherence. In PMA-induced differentiation of HL-60 cells, phosphoinositide 3-kinase (PI 3-K) activity was measured as phosphatidylinositol3P recovery from phosphatidylinositol by in vitro kinase assay. PI 3-K activity was increased in HL-60 cells that were stimulated by 20 nM PMA and the activity was inhibited by pretreatment with 20 microM LY294002, a specific inhibitor of PI 3-K. Members of the protein kinase C (PKC) family have been suggested to be one of the downstream targets of PI 3-K. PKC zeta is one of the atypical PKCs, non-diacylglycerol-responsive PKCs, and the activity was measured by the ability of phosphorylation onto myelin basic protein. PMA also induced the activation of PKC zeta during monocytic differentiation of HL-60 cells, and LY294002-pretreated cells failed to induce PKC zeta activation. The activity of PI 3-K is essential for PKC zeta activation, and LY294002 blocks both monocytic differentiation of HL-60 cells and activation of PKC zeta during PMA-induced cell differentiation. This implies that activated PI 3-K subsequently stimulates the PKC zeta in the process of PMA-induced monocytic differentiation.

Protein kinase C-delta regulates thrombin-induced ICAM-1 gene expression in endothelial cells via activation of p38 mitogen-activated protein kinase

The procoagulant thrombin promotes the adhesion of polymorphonuclear leukocytes to endothelial cells by a mechanism involving expression of intercellular adhesion molecule 1 (ICAM-1) via an NF-kappaB-dependent pathway. We now provide evidence that protein kinase C-delta (PKC-delta) and the p38 mitogen-activated protein (MAP) kinase pathway play a critical role in the mechanism of thrombin-induced ICAM-1 gene expression in endothelial cells. We observed the phosphorylation of PKC-delta and p38 MAP kinase within 1 min after thrombin challenge of human umbilical vein endothelial cells. Pretreatment of these cells with the PKC-delta inhibitor rottlerin prevented the thrombin-induced phosphorylation of p38 MAP kinase, suggesting that p38 MAP kinase signals downstream of PKC-delta. Inhibition of PKC-delta or p38 MAP kinase by pharmacological and genetic approaches markedly decreased the thrombin-induced NF-kappaB activity and resultant ICAM-1 expression. The effects of PKC-delta inhibition were secondary to inhibition of IKKbeta activation and of subsequent NF-kappaB binding to the ICAM-1 promoter. The effects of p38 MAP kinase inhibition occurred downstream of IkappaBalpha degradation without affecting the DNA binding function of nuclear NF-kappaB. Thus, PKC-delta signals thrombin-induced ICAM-1 gene transcription by a dual mechanism involving activation of IKKbeta, which mediates NF-kappaB binding to the ICAM-1 promoter, and p38 MAP kinase, which enhances transactivation potential of the bound NF-kappaB p65 (RelA).

Protein phosphatase 2A activates the HIV-2 promoter through enhancer elements that include the pets site

Human immunodeficiency virus type 2 (HIV-2) gene expression is regulated by upstream promoter elements, including the peri-Ets (pets) site, which mediate enhancer stimulation following treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). We previously showed that the oncoprotein DEK binds to the pets site in a site-specific manner. In this report, we show that binding to the HIV-2 pets site is modulated by treatment of U937 monocytic cells with TPA, an activator of protein kinase C. TPA treatment resulted in a reduction in the levels of DEK and the formation of a faster migrating pets complex in gel shift assays. We show further that the actions of TPA on pets binding can be duplicated by phosphatase treatment of nuclear proteins and is blocked with okadaic acid, a protein phospatase-2A (PP2A) inhibitor. Finally, we demonstrate that ectopic expression of the catalytic domain of PP2A can activate the HIV-2 enhancer/promoter alone or in synergy with TPA, an effect mediated in part through the pets site. These results suggest that, through an interaction with the protein kinase C pathway, PP2A is strongly involved in regulating HIV-2 enhancer-mediated transcription. This is a consequence of its effects on DEK expression and binding to the pets site, as well as its effects on other promoter elements. These findings have implications not only for HIV-2 transcription but also for multiple cellular processes involving DEK or PP2A.

Phosphorylation of connexin43 and inhibition of gap junctional communication in 12-O-tetradecanoylphorbol-13-acetate-exposed R6 fibroblasts: minor role of protein kinase C beta I and mu

12-O:-tetradecanoylphorbol-13-acetate (TPA) inhibits gap junctional communication in many cell culture systems, but TPA-induced phosphorylation of the gap junction protein connexin43 (Cx43) varies much between systems. We have here studied whether these responses and their sensitivities can be correlated with total protein kinase C (PKC) enzyme activity and if specific PKC isoenzymes are involved. Rat R6 fibroblasts transfected with the cDNA sequence encoding PKC beta I (R6-PKC3) had a total PKC activity 7- to 16-fold higher than the corresponding control cells (R6-C1), depending on the selection pressure (G418 concentration). Still, R6-PKC3 cells were no more sensitive than R6-C1 cells to TPA-induced down-regulation of communication, except at the highest selection pressure (500 micrograms/ml G418). Thus, total PKC activity does not indicate absolute sensitivity of a cell system to TPA-induced suppression of communication, but within a certain cell system increasing PKC activity may enhance the sensitivity to TPA in this respect. The results also suggest that PKC beta I is of minor importance for TPA-induced regulation of communication. Experiments with the Lilly compound 379196, a PKC beta-specific inhibitor, further supported this conclusion. Except for PKC beta I in R6-PKC3 cells, both cell lines contained the TPA-responsive PKC isoenzymes alpha, delta, epsilon and mu. Long-term treatment with TPA caused strong down-regulation of PKC alpha, delta and epsilon, but little down-regulation of PKC mu. Concurrently, the cells became refractory to repeated exposure to TPA, indicating that PKC mu is of minor importance. Experiments with the general PKC inhibitor GF109203X and the PKC alpha (and beta/gamma) inhibitor Gö6976 suggested that both classical (alpha) and novel PKCs (delta and epsilon) might be involved in TPA-induced suppression of intercellular communication, while phosphorylation of Cx43 may mainly be mediated by PKC alpha in the present systems.

Brief rapid pacing depresses contractile function via Ca(2+)/PKC-dependent signaling in cat ventricular myocytes

The purpose of this study is to determine the effects of brief rapid pacing (RP; approximately 200-240 beats/min for approximately 5 min) on contractile function in ventricular myocytes. RP was followed by a sustained inhibition of peak systolic cell shortening (-44 +/- 4%) that was not due to changes in diastolic cell length, membrane voltage, or L-type Ca(2+) current (I(Ca,L)). During RP, baseline and peak intracellular Ca(2+) concentration ([Ca(2+)](i)) increased markedly. After RP, Ca(2+) transients were similar to control. The effects of RP on cell shortening were not prevented by 1 microM calpain inhibitor I, 25 microM L-N(5)-(1-iminoethyl)-orthinthine, or 100 microM N(G)-monomethyl-L-arginine. However, RP-induced inhibition of cell shortening was prevented by lowering extracellular [Ca(2+)] (0.5 mM) during RP or exposure to chelerythrine (2-4 microM), a protein kinase C (PKC) inhibitor, or LY379196 (30 nM), a selective inhibitor of PKC-beta. Exposure to phorbol ester (200 nM phorbol 12-myristate 13-acetate) inhibited cell shortening (-46 +/- 7%). Western blots indicated that cat myocytes express PKC-alpha, -delta, and -epsilon as well as PKC-beta. These findings suggest that brief RP of ventricular myocytes depresses contractility at the myofilament level via Ca(2+)/PKC-dependent signaling. These findings may provide insight into the mechanisms of contractile dysfunction that follow paroxysmal tachyarrhythmias.