Protein kinase C delta specifically associates with phosphatidylinositol 3-kinase following cytokine stimulation

Negative Regulation of Interleukin-12 Production by a Rapamycin-Sensitive Signaling Pathway: A Brief Communication

Interleukin-12 (IL-12), an important cytokine in host defense against microbial pathogens, regulates natural killer and T-cell function(s) including the induction of γ-interferon production. The major cellular sources of IL-12 are monocytes/macrophages. Bacteria, bacterial products, and intracellular parasites are the most efficient inducers of IL-12 production. In the present study we show that a signal transduction pathway sensitive to rapamycin may have an important role in the regulation/suppression of Staphylococcus aureus–induced IL-12 production in vitro. Human peripheral blood mononuclear cells, monocytes, or a human monocytic cell line THP-1 were stimulated with S. aureus Cowan strain 1 (SAC) in the presence or absence of rapamycin and investigated for production of IL-12 protein by enzyme-linked immunosorbent assay and IL-12 p40 mRNA accumulation by RNase protection assay or real-time quantitative polymerase chain reaction. The results show that rapamycin significantly enhances SAC-induced IL-12 p70 protein production and IL-12 p40 mRNA accumulation. Further the results demonstrate that wortmannin enhances SAC-induced IL-12 p40 mRNA accumulation, whereas Ly294002 does not. These data indicate that a rapamycin-sensitive signaling pathway may act as a negative feedback cascade in the regulatory mechanisms of IL-12 production.

Nicotine decreases the activity of glutamate transporter type 3

Nicotine, the main ingredient of tobacco, elicits seizures in animal models and cigarette smoking is regarded as a behavioral risk factor associated with epilepsy or seizures. In the hippocampus, the origin of nicotine-induced seizures, most glutamate uptake could be performed primarily by excitatory amino acid transporter type 3 (EAAT3). An association between temporal lobe epilepsy and EAAT3 downregulation has been reported. Therefore, we hypothesized that nicotine may elicit seizures through the attenuation of EAAT3 activity. We investigated chronic nicotine exposure (72 h) cause reduction of the activity of EAAT3 in a Xenopus oocyte expression system using a two-electrode voltage clamp. The roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) were also determined. Nicotine (0.001-1 μM) resulted in a time- and dose-dependent decrease in EAAT3 activity with maximal inhibition at nicotine concentrations of 0.03 μM or higher and at an exposure time of 72 h. Vmax on the glutamate response was significantly reduced in the nicotine group (0.03 μM for 72 h), but the Km value of EAAT3 for glutamate was not altered. When nicotine-exposed oocytes (0.03 μM for 72 h) were pretreated with phorbol-12-myristate-13-acetate (PMA, a PKC activator), the nicotine-induced reduction in EAAT3 activity was abolished. PKC inhibitors (staurosporine, chelerythrine, and calphostin C) significantly reduced basal EAAT3 activity, but there were no significant differences among the PKC inhibitors, nicotine, and PKC inhibitors+nicotine groups. Similar response patterns were observed among PI3K inhibitors (wortmannin and LY294002), nicotine, and PI3K inhibitors+nicotine. In conclusion, this study suggests that nicotine decreases EAAT3 activity, and that this inhibition seems to be dependent on PKC and PI3K. Our results may provide an additional mechanism for nicotine-induced seizure.

Corticosterone decreases the activity of rat glutamate transporter type 3 expressed in Xenopus oocytes

Glucocorticoids can increase the extracellular concentrations of glutamate, the major excitatory neurotransmitter. We investigated the effects of corticosterone on the activity of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1; also called excitatory amino acid transporter type 3 [EAAT3]), and the roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in regulating these effects. Rat EAAC1 was expressed in Xenopus oocytes by injecting mRNA. L-Glutamate (30 μM)-induced membrane currents were measured using the two-electrode voltage clamp technique. Exposure of these oocytes to corticosterone (0.01-1 μM) for 72 h decreased EAAC1 activity in a dose-dependent fashion, and this inhibition was incubation time-dependent. Corticosterone (0.01 μM for 72 h) significantly decreased the V(max), but not the K(m), of EAAC1 for glutamate. Furthermore, pretreatment of oocytes with staurosporine, a PKC inhibitor, significantly decreased EAAC1 activity (1.00±0.06 to 0.70±0.05 μC; P<0.05). However, no statistical differences were observed between oocytes treated with staurosporine, corticosterone, or corticosterone plus staurosporine. Similar patterns of responses were achieved by chelerythrine or calphostin C, other PKC inhibitors. Phorbol-12-myristate-13-acetate (PMA), a PKC activator, inhibited corticosterone-induced reduction in EAAC1 activity. Pretreating oocytes with wortmannin or LY294002, PI3K inhibitors, also significantly reduced EAAC1 activity, but no difference was observed between oocytes treated with wortmannin, corticosterone, or wortmannin plus corticosterone. The above results suggest that corticosterone exposure reduces EAAC1 activity and this effect is PKC- and PI3K-dependent.

Protein kinase D mediates the synergistic effects of BMP-7 and IGF-I on osteoblastic cell differentiation

We previously showed that exogenous insulin-like growth factor-I (IGF-I) and bone morphogenetic protein-7 (BMP-7) synergistically stimulated osteoblast differentiation in fetal rat calvaria (FRC) cells. We have now shown that BMP-7 alone and the BMP-7 and IGF-I combination synergistically stimulated protein kinase D (PKD) phosphorylation at Ser744/748 and Ser916. Transfection of FRC cells with a constitutively active PKD stimulated marker expression, while transfection with a catalytically inactive PKD did not. Moreover, G&#x00F6;6976, which inhibits protein kinase C (PKC) &#x03B1; and &#x03B2;1, blocked PKD phosphorylation and the synergistic action of the BMP-7 and IGF-I combination on osteoblast differentiation, whereas G&#x00F6;6983, which inhibits PKC&#x03B1;, &#x03B2;, &#x03B3;, &#x03B4;, and &#x03B6;, did not. Our results suggest that the FRC cell differentiation induced by BMP-7 and the BMP-7 and IGF-I combination requires stimulation of PKD activity. Our results are consistent with a novel mechanism in which combined BMP-7 and IGF-I signaling activates upstream novel PKC(s), which then phosphorylates and activates PKD, leading to enhanced osteoblast differentiation.

Ethanol increases the activity of rat excitatory amino acid transporter type 4 expressed in Xenopus oocytes: role of protein kinase C and phosphatidylinositol 3-kinase

BACKGROUND

Glutamate is the major excitatory neurotransmitter in the central nervous system and is critical for essentially all physiological processes, such as learning, memory, central pain transduction, and control of motor function. Excitatory amino acid transporters (EAATs) play a key role in regulating glutamate neurotransmission by uptake of glutamate into cells. EAAT4 is the major EAAT in the cerebellar Purkinje cells. The authors investigated the effects of ethanol on EAAT4 and the mediatory effects of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in this context.

METHODS

Excitatory amino acid transporter 4 was expressed in Xenopus oocytes by injecting EAAT4 mRNA. l-aspartate-induced membrane currents were measured using a two-electrode voltage clamp. Responses were quantified by integrating current traces and are represented in microCoulombs (microC).

RESULTS

Ethanol increased EAAT4 activity in a dose-dependent manner. At ethanol concentrations of 25, 50, 100, and 200 mM, the responses were significantly higher than untreated control values. Ethanol (25 mM) significantly increased the V(max) (1.5 +/- 0.1 microC for control vs. 2.0 +/- 0.1 microC for ethanol, p < 0.05), but did not affect K(m) (2.3 +/- 0.6 microM for control vs. 1.7 +/- 0.7 microM for ethanol, p > 0.05) of EAAT4 for l-aspartate. Preincubation of oocytes with phorbol-12-myristate-13-acetate (PMA, a PKC activator) significantly increased EAAT4 activity. However, combinations of PMA and ethanol versus PMA or ethanol alone did not increase responses further. Two PKC inhibitors, chelerythrine and staurosporine did not reduce basal EAAT4 activity but abolished ethanol-enhanced EAAT4 activity. Pretreatment with wortmannin (a PI3K inhibitor) also abolished ethanol-enhanced EAAT4 activity.

CONCLUSIONS

These results demonstrate that acute ethanol exposure increases EAAT4 activity at clinically relevant concentrations and that PKC and PI3K may mediate this. The effects of ethanol on EAAT4 may play a role in the cerebellar dysfunction caused by ethanol intoxication.

The protein kinase C (PKC) family of proteins in cytokine signaling in hematopoiesis

The members of the protein kinase C (PKC) family of proteins play important roles in signaling for various growth factors, cytokines, and hormones. Extensive work over the years has led to the identification of three major groups of PKC isoforms. These include the classic PKCs (PKCalpha, PKCbeta(I), PKCbeta(II), PKCgamma), the novel PKCs (PKCdelta, PKCepsilon, PKCeta, PKCmu, PKCtheta), and the atypical PKCs (PKCzeta, PKCiota/lambda). All these PKC subtypes have been shown to participate in the generation of signals for important cellular processes and to mediate diverse and, in some cases, opposing biologic responses. There is emerging evidence that these kinases also play key functional roles in the regulation of cell growth, apoptosis, and differentiation of hematopoietic cells. In this review, both the engagement of the various PKC members in cytokine and growth factor signaling and their role in the regulation of hematopoiesis are discussed.

Obestatin-mediated proliferation of human retinal pigment epithelial cells: regulatory mechanisms

In this work, we have evaluated the effect of the new discovered peptide obestatin on cell proliferation in primary cultures of human retinal epithelial cells (hRPE cells). The results showed that this peptide induced, in a dose-dependent manner, cell proliferation by MEK/ERK 1/2 phosphorylation. A sequential analysis of the obestatin transmembrane signaling pathway showed that the ERK 1/2 activity is partially blocked after preincubation of the cells with pertussis toxin (PTX), as well as by wortmannin (an inhibitor of PI3K), claphostin C (an inhibitor of PKC), and PP2 (which inhibits the non receptor tyrosine kinase Src). Upon administration of obestatin, the intracellular levels of phospho-PKCepsilon-, theta-, and micro-isoenzymes rise with different time courses, from which PKCepsilon might be responsible for ERK 1/2 response. Based on the experimental data, a signaling pathway involving the consecutive activation of Gi, PI3K, novel PKC (probably PKCepsilon), and Src for ERK 1/2 activation is proposed. These results incorporate a new mitogenic factor to the group of factors that regulate proliferation of hRPE cells.

Curcumin-induced GADD153 upregulation: Modulation by glutathione

As we reported previously, GADD153 is upregulated in colon cancer cells exposed to curcumin. In the present study, we ascertained the involvement of glutathione and certain sulfhydryl enzymes associated with signal transduction in mediating the effect of curcumin on GADD153. Curcumin-induced GADD153 gene upregulation was attenuated by reduced glutathione (GSH) or N-acetylcysteine (NAC) and potentiated by the glutathione synthesis inhibitor, L-buthionine-(S,R)-sulfoximine (BSO). Additionally, GSH and NAC decreased the intracellular content of curcumin. Conversely, curcumin decreased intracellular glutathione and also increased the formation of reactive oxygen species (ROS) in cells, but either GSH or NAC prevented both of these effects of curcumin. In affecting the thiol redox status, curcumin caused activation of certain sulfhydryl enzymes involved in signal transduction linked to GADD153 expression. Curcumin increased the expression of the phosphorylated forms of PTK, PDK1, and PKC-delta, which was attenuated by either GSH or NAC and potentiated by BSO. Furthermore, selective inhibitors of PI3K and PKC-delta attenuated curcumin-induced GADD153 upregulation. Collectively, these findings suggest that a regulatory thiol redox-sensitive signaling cascade exists in the molecular pathway leading to induction of GADD153 expression as caused by curcumin.

Stimulation by ghrelin of p42/p44 mitogen-activated protein kinase through the GHS-R1a receptor: Role of G-proteins and β-arrestins

Results presented in this study indicate that in human embryonic kidney 293 cells (HEK 293), the ghrelin receptor growth hormone secretagogue receptor type 1a (GHS-R1a) activates the extracellular signal-related kinases 1 and 2 (ERK 1/2) via three pathways. One pathway is mediated by the beta-arrestins 1 and 2, and requires entry of the receptor into a multiprotein complex with the beta-arrestins, Src, Raf-1, and ERK 1/2. A second pathway is G(q/11)-dependent and involves a Ca(2+)-dependent PKC (PKCalpha/beta) and Src. A third pathway is G(i)-dependent and involves phosphoinositide 3-kinase (PI3K), PKCepsilon, and Src. Our current study reveals that G(i/o)- and G(q/11)-proteins are crucially involved in the beta-arrestin-mediated ERK 1/2 activation. These results thus support the view that the beta-arrestins act as both scaffolding proteins and signal transducers in ERK 1/2 activation, as reported for other receptors. The different pathways of ERK 1/2 activation suggest that binding to GHS-R1a activates ERK 1/2 pools at different locations within the cell, and thus probably with different physiological consequences.

Protein kinase C β and δ isoenzymes mediate cholesterol accumulation in PMA-activated macrophages

Previously, we showed that PMA activation of human monocyte-derived macrophages stimulates macropinocytosis (i.e., fluid-phase endocytosis) of LDL and transforms these macrophages into foam cells. The current study aimed to learn which PKC isoenzymes mediate cholesterol accumulation in PMA-activated human macrophages incubated with LDL. Cholesterol accumulation by PMA-activated macrophages incubated with LDL was nearly completely inhibited (>85%) by the pan PKC inhibitors Go6850, Go6983, and RO 32-0432, but only was inhibited about 50% by the classical group PKC inhibitor, Go6976. This indicated that cholesterol accumulation was mediated by both a classical group and some other PKC isoenzyme. PKC beta was determined to be the classical group isoenzyme that mediated PMA-stimulated cholesterol accumulation. A pseudosubstrate myristoylated peptide inhibitor of PKC alpha and beta showed partial inhibition (congruent with 50%) of cholesterol accumulation. However, a small molecule inhibitor of PKC alpha, HBDDE, show minimal inhibition of cholesterol accumulation while a small molecule inhibitor of PKC beta, LY333513, could completely account for the inhibition of cholesterol accumulation by the classical group PKC isoenzyme. Thus, our findings show that beta and some other PKC isoenzyme, most likely delta, mediate cholesterol accumulation when macropinocytosis of LDL is stimulated in PMA-activated human monocyte-derived macrophages.

Involvement of protein kinase C delta in the alteration of mitochondrial mass in human cells under oxidative stress

Alteration of mitochondrial mass of human 143B osteosarcoma cells upon exposure to hydrogen peroxide (H(2)O(2)) was investigated. We found that mitochondrial mass and the intracellular level of H(2)O(2) were increased by exogenous H(2)O(2), which was accompanied with up-regulation of functional PKCdelta. To investigate the role of PKCdelta in H(2)O(2)-induced increase of mitochondrial mass, we treated 143B cells with PKCdelta activator, bistratene A, and PKCdelta inhibitor, rottlerin, respectively. The results show that bistratene A caused an increase of mitochondrial mass and that the H(2)O(2)-induced increase of mitochondrial mass was completely suppressed by rottlerin. Furthermore, we found that activation of PKCdelta by bistratene A increased the intracellular levels of H(2)O(2) and MnSOD protein expression. By contrast, suppression of PKCdelta by rottlerin decreased the intracellular levels of H(2)O(2) and MnSOD protein expression. Moreover, we noted that MnSOD expression was highly correlated with the expression of p53, which was controlled by PKCdelta. Finally, we demonstrated that PKCdelta was overexpressed in skin fibroblasts of patients with MERRF syndrome. Taken together, we conclude that PKCdelta is involved in the regulation of mitochondrial mass and intracellular H(2)O(2) in human cells and may play a key role in the overproliferation of mitochondria in the affected tissues of patients with mitochondrial diseases such as MERRF syndrome.

Inhibition of phosphatidylinositol 3-kinase stimulates activity of the small-conductance K channel in the CCD

We used Western blotting to examine the expression of phosphatidylinositol 3-kinase (PI3K) in the renal cortex and outer medulla and employed the patch-clamp technique to study the effect of PI3K on the ROMK-like small-conductance K (SK) channels in the cortical collecting duct (CCD). Low K intake increased the expression of the 110-kDa alpha-subunit (p110alpha) of PI3K compared with rats on a normal-K diet. Because low K intake increases superoxide levels (2), the possibility that increases in superoxide anions may be responsible for the effect of low K intake on the expression of PI3K is supported by finding that addition of H(2)O(2) stimulates the expression of p110alpha in M1 cells. Inhibition of PI3K with either wortmannin or LY-294002 significantly increased channel activity in the CCD from rats on a K-deficient (KD) diet or on a normal-K diet. The stimulatory effect of wortmannin on ROMK channel activity cannot be mimicked by inhibition of phospholipase C with U-73122. This suggests that the effect of inhibiting PI3K was not the result of increasing the phosphatidylinositol 4,5-bisphosphate level. Moreover, application of the exogenous phosphatidylinositol 3,4,5-trisphosphate analog had no effect on channel activity in excised patches. Because low K intake has been shown to increase the activity of protein tyrosine kinase (PTK), we explored the role of the interaction between PTK and PI3K in the regulation of the SK channel activity. Inhibition of PTK increased SK channel activity in the CCD from rats on a KD diet. However, addition of wortmannin did not further increase ROMK channel activity. Also, the effect of wortmannin was abolished by treatment of CCD with phalloidin. We conclude that PI3K is involved in mediating the effect of low K intake on ROMK channel activity in the CCD and that the effect of PI3K on SK channels requires the involvement of PTK and the cytoskeleton.

Evidence that insulin-like growth factor-1 requires protein kinase C-epsilon, PI3-kinase and mitogen-activated protein kinase pathways to protect human vascular smooth muscle cells from apoptosis

Insulin-like growth factor (IGF)-1 has been implicated in the development of occlusive vascular lesions. Although its role in vascular smooth muscle cell (VSMC) growth and migration are fairly well characterized, anti-apoptotic signals of IGF-1 in human VSMC remain largely unknown. In this study, we examined IGF-1 signals that protect human and rat VSMC from staurosporine (STAU)- and c-myc- induced apoptosis, respectively. Treatment with STAU resulted in apoptotic DNA fragmentation, phosphatidylserine externalization and cell shrinkage, but only occasional VSMC 'blebbing'. STAU-induced death and IGF-1-mediated survival were concentration dependent, while time-lapse video microscopy showed that IGF-1 inhibited c-myc-induced apoptosis by 90%. Pretreatment with mitogen-activated protein kinase/extracellular signal regulated kinase kinase (MEK) inhibitors UO126 and PD098059, or with the phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin, reversed IGF-1-mediated human VSMC survival by 25-27% and 66%, respectively. Translocation studies showed that IGF-1 activated protein kinase C (PKC)-epsilon, but not PKC-alpha or PKC-delta, even in the presence of STAU, while pharmacological PKC inhibition (Ro-318220 or Go6976) implicated PKC-zeta or a novel PKC isozyme in IGF-1-mediated survival. Transient expression of activated PKC-epsilon but not activated PKC-zeta decreased myc-induced apoptosis in rat VSMC. In human VSMC, antisense oligodeoxynucleotides to PKC-epsilon partially reversed IGF-1-induced survival. In addition, IGF-1 elicited a mild but sustained activation of extracellular signal regulated kinase (ERK)1/2 in human VSMC that was abolished after 1 h in the presence of STAU. PKC downregulation reversed both IGF-1- and PMA-induced ERK activity, but platelet-derived growth factor (PDGF)-induced activity was unchanged. These results indicate for the first time that IGF-1 can protect human VSMC via multiple signals, including PKC-epsilon, PI3-K and mitogen-activated protein kinase pathways.

Protein kinase C delta stimulates apoptosis by initiating G1 phase cell cycle progression and S phase arrest

Overexpression of protein kinase C delta (PKCdelta) stimulates apoptosis in a wide variety of cell types through a mechanism that is incompletely understood. PKCdelta-deficient cells are impaired in their response to DNA damage-induced apoptosis, suggesting that PKCdelta is required to mount an appropriate apoptotic response under conditions of stress. The mechanism through which it does so remains elusive. In addition to effects on cell survival, PKCdelta elicits pleiotropic effects on cellular proliferation. We now provide the first evidence that the ability of PKCdelta to stimulate apoptosis is intimately linked to its ability to stimulate G(1) phase cell cycle progression. Using an adenoviral-based expression system to express PKCalpha,-delta, and -epsilon in epithelial cells, we demonstrate that a modest increase in PKCdelta activity selectively stimulates quiescent cells to initiate G(1) phase cell cycle progression. Rather than completing the cell cycle, PKCdelta-infected cells arrest in S phase, an event that triggers caspase-dependent apoptotic cell death. Apoptosis was preceded by the activation of cell cycle checkpoints, culminating in the phosphorylation of Chk-1 and p53. Strikingly, blockade of S phase entry using the phosphatidylinositol 3-kinase inhibitor LY294002 prevented checkpoint activation and apoptosis. In contrast, inhibitors of mitogen-activated protein kinase cascades failed to prevent apoptosis. These findings demonstrate that the biological effects of PKCdelta can be extended to include positive regulation of G(1) phase cell cycle progression. Importantly, they reveal the existence of a novel, cell cycle-dependent mechanism through which PKCdelta stimulates cell death.

PDGF activates K-Cl cotransport through phosphoinositide 3-kinase and protein phosphatase-1 in primary cultures of vascular smooth muscle cells

K-Cl cotransport (K-Cl COT, KCC) is an electroneutrally coupled movement of K and Cl present in most cells. In this work, we studied the pathways of regulation of K-Cl COT by platelet-derived growth factor (PDGF) in primary cultures of vascular smooth muscle cells (VSMCs). Wortmannin and LY 294002 blocked the PDGF-induced K-Cl COT activation, indicating that the phosphoinositide 3-kinase (PI 3-K) pathway is involved. However, PD 98059 had no effect on K-Cl COT activation by PDGF, suggesting that the mitogen-activated protein kinase pathway is not involved under the experimental conditions tested. Involvement of phosphatases was also examined. Sodium orthovanadate, cyclosporin A and okadaic acid had no effect on PDGF-stimulated K-Cl COT. Calyculin A blocked the PDGF-stimulated K-Cl COT by 60%, suggesting that protein phosphatase-1 (PP-1) is a mediator in the PDGF signaling pathway/s. In conclusion, our results indicate that the PDGF-mediated pathways of K-Cl COT regulation involve the signaling molecules PI 3-K and PP-1.

Regulation of interleukin-5-induced beta2-integrin adhesion of human eosinophils by phosphoinositide 3-kinase

We examined the role of phosphoinositide 3-kinase (PI3K) in integrin-mediated eosinophil adhesion. Deltap85, a dominant-negative form of the class IA PI3K adaptor subunit, was fused to an HIV-TAT protein transduction domain (TAT-Deltap85). Recombinant TAT-Deltap85 inhibited interleukin (IL)-5-stimulated phosphorylation of protein kinase B, a downstream target of PI3K. beta(2)-Integrin-dependent adhesion caused by IL-5 to the plated intracellular adhesion molecule-1 surrogate, bovine serum albumin, was inhibited by TAT-Deltap85 in a concentration-dependent manner. Similarly, two PI3K inhibitors, wortmannin and LY294002, blocked eosinophil adhesion to plated bovine serum albumin. By contrast, beta(1)-integrin-mediated eosinophil adhesion to vascular cell adhesion moelcule-1 was not blocked by TAT-Deltap85, wortmannin, or LY294002. Rottlerin, a protein kinase C (PKC)-delta inhibitor, also blocked beta(2)-integrin adhesion of eosinophils caused by IL-5, whereas beta(1) adhesion to vascular cell adhesion molecule-1 was not affected. IL-5 caused translocation of PKCdelta from the cytosol to cell membrane; inhibition of PI3K by wortmannin blocked translocation of PKCdelta. Western blot analysis demonstrated that extracellular signal-regulated kinase phosphorylation, a critical intermediary in adhesion elicited by IL-5, was blocked by inhibition of either PI3K or PKC-delta. These data suggest that extracellular signal-regulated kinase-mediated adhesion of beta(2)-integrin caused by IL-5 is mediated in human eosinophils by a class IA PI3K through activation of a PKCdelta pathway.

Selective regulation by delta-PKC and PI 3-kinase in the assembly of the antiapoptotic TNFR-1 signaling complex in neutrophils

TNF is implicated in the attenuation of neutrophil constitutive apoptosis during sepsis. Antiapoptotic signaling is mediated principally through the TNF receptor-1 (TNFR-1). In adherent neutrophils, when beta-integrin signaling is activated, TNF phosphorylates TNFR-1 and activates prosurvival and antiapoptotic signaling. Previously, we identified the delta-PKC isotype and phosphatidylinositol (PI) 3-kinase as critical regulators of TNF signaling in adherent neutrophils. Both kinases associate with TNFR-1 in response to TNF and are required for TNFR-1 serine phosphorylation, NF-kappaB activation, and inhibition of apoptosis. The purpose of this study was to examine the role of delta-PKC and PI 3-kinase in the assembly of TNFR-1 signaling complex that regulates NF-kappaB activation and antiapoptotic signaling. Coimmunoprecipitation studies established that PI 3-kinase, delta-PKC, and TNFR-1 formed a signal complex in response to TNF. delta-PKC recruitment required both delta-PKC and PI 3-kinase activity, whereas PI 3-kinase recruitment was delta-PKC independent, suggesting that PI 3-kinase acts upstream of delta-PKC. An important regulatory step in control of antiapoptotic signaling is the assembly of the TNFR-1-TNFR-1-associated death domain protein (TRADD)-TNFR-associated factor 2 (TRAF2)-receptor interacting protein (RIP) complex that controls NF-kappaB activation. Inhibition of either delta-PKC or PI 3-kinase decreased TNF-mediated recruitment of RIP and TRAF2 to TNFR-1. In contrast, TRADD recruitment was enhanced. Thus delta-PKC and PI 3-kinase are positive regulators of TNF-mediated association of TRAF2 and RIP with TNFR-1. Conversely, these kinases are negative regulators of TRADD association. These results suggest that delta-PKC and PI 3-kinase regulate TNF antiapoptotic signaling at the level of the TNFR-1 through control of assembly of a TNFR-1-TRADD-RIP-TRAF2 complex.

Leptin inhibits lysophosphatidic acid-induced intracellular calcium rise by a protein kinase C-dependent mechanism

Leptin communicates the status of body energy stores to the central nervous system, regulating appetite, metabolic rate, and neuroendocrine functions. These effects are mediated by leptin binding and activation of the cognate cell surface receptor, a member of type I cytokine receptor family, which lead to the activation of receptor-associated kinases of the Janus family. In this work, we demonstrate that leptin inhibits the l-alpha-lysophosphatidic acid (LPA)-induced intracellular calcium mobilization in a dose-dependent manner in HEK-293 cells stably expressing full-length leptin receptor (OB-Rb). This action appears to be selective, as it was not observed when other signaling families, such as VIP or EGF, were studied. Pretreatment with the phosphatidylinositol 3-kinase (PI3K) inhibitor, wortmannin, reversed the effect of leptin, pointing to PI3K as an intermediate molecule involved in this process. An unspecific protein kinase C (PKC) inhibitor, staurosporine, disrupted the inhibitory action of leptin. Furthermore, intracellular levels of phosphorylated PKCepsilon and PKCdelta rose to a maximum 5 min after leptin administration, suggesting that these atypical PKC isoforms are involved in the observed cross-desensitization. To define the regions of the OB-Rb intracellular domain required for the cross-desensitization, a series of C-terminal deletion mutants were transfected into HEK-293 cells. C-terminal truncation that removed the consensus Box 3 motif of OB-Rb prevented leptin action, indicating that heterologous desensitization over LPA was exerted at the level of this intracellular motif. Our date demonstrate that leptin plays a key role in the regulation of the earliest signaling pathways activated by growth factors, such as LPA, through a signaling pathway involving PKCdelta and PKCepsilon coupled to Box 3 motif of the OB-Rb through PI3K.

Tea polyphenols inhibit acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (a key enzyme in platelet-activating factor biosynthesis) and platelet-activating factor-induced platelet aggregation

BACKGROUND

Tea extracts have antiallergic and anti-inflammatory actions in rats and mice. However the mechanism through which tea polyphenols act in vivo are still incompletely understood. We found inhibitory effects of black tea extracts on an fMLP-induced aggregating response in a rabbit platelet-polymorphonuclear leukocyte (PMN) system.

METHOD

To elucidate whether 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) production in PMNs and/or PAF-stimulated platelet activation were inhibited, the effects of tea polyphenols were investigated on the enzyme activity of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67), PAF biosynthesis in A23187-activated rabbit PMNs, and rabbit platelet aggregation. By comparing the inhibitory effects of 31 galloyl esters and gallic acid, the structure-inhibitory activity relationship was characterized.

RESULTS

Theaflavin and its galloyl esters and pentagalloyl glucose were found to be potent inhibitors of the acetyltransferase (IC(50) = 28-58 microM) and the PAF biosynthesis as well as (-)-epicatechin-3-O-gallate (IC(50) = 72 +/- 13 microM) and (-)-epigallocatechin-3-O-gallate (IC(50) = 46 +/- 6 microM). On the other hand, flavan-3-ols without galloyl group at C-3 and gallic acid did not show significant enzyme inhibition. In addition, theaflavin and its galloyl esters (IC(50) = 32-77 microM) and geranyl gallate, farnesyl gallate and geranylgeranyl gallate (IC(50) = 6.4-7.6 microM) were found to be potent inhibitors of PAF- and TPA-induced rabbit platelet aggregation but not A23187-induced aggregation.

CONCLUSIONS

Theaflavin and its galloyl esters in black tea extract, and isoprenyl gallates were potent inhibitors of PAF synthesis and platelet aggregation and these activities may be relevant to the claimed therapeutic effects of tea extracts.

The phosphatidylinositol 3-kinase/Akt pathway enhances Smad3-stimulated mesangial cell collagen I expression in response to transforming growth factor-beta1

Transforming growth factor (TGF)-beta has been associated with renal glomerular matrix accumulation. We previously showed that Smad3 promotes COL1A2 gene activation by TGF-beta1 in human glomerular mesangial cells. Here, we report that the PI3K/Akt pathway also plays a role in TGF-beta1-increased collagen I expression. TGF-beta1 stimulates the activity of phosphoinositide-dependent kinase (PDK)-1, a downstream target of PI3K, starting at 1 min. Akt, a kinase downstream of PDK-1, is phosphorylated and concentrates in the membrane fraction within 5 min of TGF-beta1 treatment. The PI3K inhibitor LY294002 decreases TGF-beta1-stimulated alpha1(I) and alpha2(I) collagen mRNA expression. Similarly, LY294002 or an Akt dominant negative construct blocks TGF-beta1 induction of COL1A2 promoter activity. However, PI3K stimulation alone is not sufficient to increase collagen I expression, since neither a constitutively active p110 PI3K construct nor PDGF, which induces Akt phosphorylation, is able to stimulate COL1A2 promoter activity or mRNA expression, respectively. LY294002 inhibits stimulation of COL1A2 promoter activity by Smad3. In a Gal4-LUC assay system, blockade of the PI3K pathway significantly decreases TGF-beta1-induced transcriptional activity of Gal4-Smad3. Activity of SBE-LUC, a Smad3/4-responsive construct, is stimulated by over-expression of Smad3 or Smad3D, in which the three C-terminal serine phospho-acceptor residues are mutated. This induction is blocked by LY294002, suggesting that inhibition of the PI3K pathway decreases Smad3 transcriptional activity independently of C-terminal serine phosphorylation. However, TGF-beta1-induced total serine phosphorylation of Smad3 is decreased by LY294002, suggesting that Smad3 is phosphorylated by the PI3K pathway at serine residues other than the direct TGF-beta receptor I target site. Thus, although the PI3K-PDK1-Akt pathway alone is insufficient to stimulate COL1A2 gene transcription, its activation by TGF-beta1 enhances Smad3 transcriptional activity leading to increased collagen I expression in human mesangial cells. This cross-talk between the Smad and PI3K pathways likely contributes to TGF-beta1 induction of glomerular scarring.

Smad3 and PKCdelta mediate TGF-beta1-induced collagen I expression in human mesangial cells

Transforming growth factor (TGF)-beta has been associated with fibrogenesis in clinical studies and animal models. We previously showed that Smad3 promotes COL1A2 gene activation by TGF-beta1 in human mesangial cells. In addition to the Smad pathway, it has been suggested that TGF-beta1 could also activate more classical growth factor signaling. Here, we report that protein kinase C (PKC)delta plays a role in TGF-beta1-stimulated collagen I production. In an in vitro kinase assay, TGF-beta1 treatment specifically increased mesangial cell PKCdelta activity in a time-dependent manner. Translocation to the membrane was detected by immunocytochemistry and immunoblot, suggesting activation of PKCdelta by TGF-beta1. Inhibition of PKCdelta by rottlerin decreased basal and TGF-beta1-stimulated collagen I production, mRNA expression, and COL1A2 promoter activity, whereas blockade of conventional PKCs by Gö 6976 had little or no effect. In a Gal4-LUC assay system, inhibition of PKCdelta abolished TGF-beta1-induced transcriptional activity of Gal4-Smad3 and Gal4-Smad4(266-552). Overexpression of Smad3 or Smad3D, in which the three COOH-terminal serine phosphoacceptor residues have been mutated, increased activity of the SBE-LUC construct, containing four DNA binding sites for Smad3 and Smad4. This induction was blocked by PKCdelta inhibition, suggesting that rottlerin decreased Smad3 transcriptional activity independently of COOH-terminal serine phosphorylation. Blockade of PKCdelta abolished ligand-independent and ligand-dependent stimulation of COL1A2 promoter activity by Smad3. These data indicate that PKCdelta is activated by TGF-beta1 in human mesangial cells. TGF-beta1-stimulated PKCdelta activity positively regulates Smad transcriptional activity and is required for COL1A2 gene transcription. Thus cross talk among multiple signaling pathways likely contributes to the pathogenesis of glomerular matrix accumulation.

Permeabilization in a cerebral endothelial barrier model by pertussis toxin involves the PKC effector pathway and is abolished by elevated levels of cAMP

Respiratory tract infections caused by Bordetella pertussis are occasionally accompanied by severe neurologic disorders and encephalopathies. For these sequelae to occur the integrity of cerebral barriers needs to be compromised. The influence of pertussis toxin, a decisive virulence factor in the pathogenesis of pertussis disease, on barrier integrity was investigated in model systems for blood-liquor (epithelial) and blood-brain (endothelial) barriers. While pertussis toxin did not influence the barrier function in Plexus chorioideus model systems, the integrity of cerebral endothelial monolayers was severely compromised. Cellular intoxication by pertussis toxin proceeds via ADP-ribosylation of alpha-G(i) proteins, which not only interferes with the homeostatic inhibitory regulation of adenylate cyclase stimulation but also results in a modulation of the membrane receptor coupling. Increasing intra-endothelial cAMP levels by employing cholera toxin or forskolin even inhibited the pertussis toxin-induced permeabilization of endothelial barriers. Therefore, pertussis-toxin-induced permeabilization has to be mediated via a cAMP-independent pathway. To investigate potential signalling pathways we employed several well established cellular drugs activating or inhibiting central effectors of signal transduction pathways, such as phosphatidylinositol 3-kinase, adenylate cyclase, phospholipase C, myosin light chain kinase and protein kinase C. Only inhibitors and activators of protein kinase C and phosphatidylinositol 3-kinase affected the pertussis toxin-induced permeability. In summary, we conclude that permeabilization of cerebral endothelial monolayers by pertussis toxin does not depend on elevated cAMP levels and proceeds via the phosphokinase C pathway.

Calcium receptor-induced serotonin secretion by parafollicular cells: role of phosphatidylinositol 3-kinase-dependent signal transduction pathways

Elevation of extracellular Ca2+ (increase[Ca2+]e) stimulates the Ca2+ receptor (CaR) to induce secretion of 5-hydroxytryptamine (5-HT) from the calcium-sensing parafollicular (PF) cells. The CaR has been reported to couple to Galpha(q) with subsequent activation of protein kinase C-gamma (PKCgamma). We have identified a parallel transduction pathway in primary cultures of sheep PF cells by using a combinatorial approach in which we expressed adenoviral-encoded dominant-negative signaling proteins and performed in vitro kinase assays. The role of the CaR was established by expression of a dominant-negative CaR that eliminated calcium-induced 5-HT secretion but not secretion in response to KCl or phorbol esters. The calcium-induced secretion was inhibited by a dominant-negative p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-K). PI3-K activity was also assayed using isoform-specific antibodies. The activity of p85/p110beta (PI3-Kbeta) immunocomplexes was elevated by increase[Ca2+]e and activated by Gbetagamma subunits. In addition, secretion of 5-HT was antagonized by the expression of a minigene encoding a peptide scavenger of Gbetagamma subunits (C-terminal fragment peptide of bovine beta-adrenergic receptor kinase). One target of PI3-K activity is phosphoinositide-dependent kinase-1 (PDK1), which in turn activated PKCzeta. Expression of a dominant-negative PKCzeta in PF cells reduced 5-HT secretion. Together, these observations establish that increase[Ca2+]e evokes 5-HT secretion from PF cells by stimulating both Galpha(q)- and Gbetagamma-signaling pathways downstream of the CaR. The betagamma cascade subsequently activates PI3-Kbeta-dependent signaling that is coupled to PDK1 and the downstream effector PKCzeta, and results in an increase in 5-HT release.

The second phase activation of protein kinase C delta at late G1 is required for DNA synthesis in serum-induced cell cycle progression

BACKGROUND

Cell lines that stably over-express protein kinase C (PKC) delta frequently show a decrease in growth rate and saturation density, leading to the hypothesis that PKC delta has a negative effect on cell proliferation. However, the mode of PKC delta activation, the cell cycle stage requiring PKC delta activity, and the exact role of PKC delta at that stage remains unknown.

RESULTS

Here we show that the treatment of quiescent fibroblasts with serum activates PKC delta at two distinct time points, within 10 min after serum treatment, and for a longer duration between 6 and 10 h. This biphasic activation correlates with the phosphorylation of Thr-505 at the activation loop of PKC delta. Importantly, an inhibitor of PKC delta, rottlerin, suppresses the biphasic activation of PKC delta, and suppression of the second phase of PKC delta activation is sufficient for the suppression of DNA synthesis. Consistent with this, the transient over-expression of PKC delta mutant molecules lacking kinase activity suppresses serum-induced DNA synthesis. These results imply that PKC delta plays a positive role in cell cycle progression. While the over-expression of PKC delta enhances serum-induced DNA synthesis, this was not observed for PKC epsilon. Similar experiments using a series of PKCdelta/ epsilon chimeras showed that the carboxyl-terminal 51 amino acids of PKC delta are responsible for the stimulatory effect. On the other hand, the over-expression of PKC delta suppresses cell entry into M-phase, being consistent with the previous studies based on stable over-expressors.

CONCLUSIONS

We conclude that PKC delta plays a role in the late-G1 phase through the positive regulation of cell-cycle progression, in addition to negative regulation of the entry into M-phase.

PKD: a new protein kinase C-dependent pathway in platelets

Protein kinase D (PKD, also known as PKCmu) is closely related to the protein kinase C superfamily but is differentially regulated and has a distinct catalytic domain that shares homology with Ca(2+)-dependent protein kinases. PKD is highly expressed in hematopoietic cells and undergoes rapid and sustained activation upon stimulation of immune receptors. PKD is regulated through phosphorylation by protein kinase C (PKC). In the present study, we show that PKD is expressed in human platelets and that it is rapidly activated by receptors coupled to heterotrimeric G-proteins or tyrosine kinases. Activation of PKD is mediated downstream of PKC. Strong agonists such as convulxin, which acts on GPVI, and thrombin cause sustained activation of PKC and PKD, whereas the thromboxane mimetic U46619 gives rise to transient activation of PKC and PKD. Activation of PKD by submaximal concentrations of phospholipase C-coupled receptor agonists is potentiated by G(i)-coupled receptors (eg, adenosine diphosphate and epinephrine). This study shows that PKD is rapidly activated by a wide variety of platelet agonists through a PKC-dependent pathway. Activation of PKD enables phosphorylation of a distinct set of substrates to those targeted by PKC in platelets.

Constitutively activated phosphatidylinositol-3 kinase (PI-3K) is involved in the defect of apoptosis in B-CLL: association with protein kinase Cdelta

In the present study we analyzed the role of phophatidylinositol-3 kinase (PI-3K) in B chronic lymphocytic leukemia (B-CLL) cells. PI-3K is activated by many stimuli and is linked to several different signaling pathways. We demonstrated that inhibition of PI-3K by a specific inhibitor, LY294002, induced apoptosis in B-CLL cells in vitro. This effect was specific for the inhibition of PI-3K because inhibition of other signaling pathways such as extracellular signaling-regulated kinase (ERK), p38, or p70S6 kinase did not affect spontaneous apoptosis. Furthermore, PI-3K was constitutively activated in freshly isolated B-CLL cells. Corresponding to enhanced apoptosis, LY294002 down-regulated expression of the antiapoptotic proteins X-linked inhibitor of apoptosis protein (XIAP) and Mcl-1. Next, we investigated which factors downstream of PI-3K were activated in B-CLL cells. We demonstrated that protein kinase B/Akt is expressed in all tested CLL samples but no activation of Akt was detected. In contrast, we observed a constitutive activation of protein kinase Cdelta (PKCdelta) in freshly isolated B-CLL cells. PKCdelta is linked to PI-3K and is phosphorylated at Thr505 in response to PI-3K activation. We further demonstrated that tyrosine phosphorylation and activity of PKCdelta were dependent on PI-3K activity in B-CLL cells. Inhibition of PKCdelta by the specific inhibitor Rottlerin strikingly enhanced apoptosis. In contrast, peripheral blood B cells of healthy donors were resistant to inhibition of PI-3K or PKCdelta. We conclude that activated PI-3K might be important in the pathogenesis of B-CLL, and survival signals might be mediated via PKCdelta. Therefore, inhibition of PI-3K or PKCdelta may be an innovative approach to treat B-CLL.

Inhibition of apical Cl-/OH- exchange activity in Caco-2 cells by phorbol esters is mediated by PKCepsilon

The present studies were undertaken to examine the possible regulation of apical membrane Cl-/OH- exchanger in Caco-2 cells by protein kinase C (PKC). The effect of the phorbol ester phorbol 12-myristate 13-acetate (PMA), an in vitro PKC agonist, on OH- gradient-driven 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl uptake in Caco-2 cells was assessed. The results demonstrated that PMA decreased apical Cl-/OH- exchanger activity via phosphatidylinositol 3-kinase (PI3-kinase)-mediated activation of PKCepsilon. The data consistent with these conclusions are as follows: 1) short-term treatment of cells for 1-2 h with PMA (100 nM) significantly decreased Cl-/OH- exchange activity compared with control (4alpha-PMA); 2) pretreatment of cells with specific PKC inhibitors chelerythrine chloride, calphostin C, and GF-109203X completely blocked the inhibition of Cl-/OH- exchange activity by PMA; 3) specific inhibitors for PKCepsilon (Ro-318220) but not PKCalpha (Go-6976) significantly blocked the PMA-mediated inhibition; 4) specific PI3-kinase inhibitors wortmannin and LY-294002 significantly attenuated the inhibitory effect of PMA; and 5) PI3-kinase activators IRS-1 peptide and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] mimicked the effects of PMA. These findings provide the first evidence for PKCepsilon-mediated inhibition of Cl-/OH- exchange activity in Caco-2 cells and indicate the involvement of the PI3-kinase-mediated pathways in the regulation of Cl- absorption in intestinal epithelial cells.

Rottlerin inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes by uncoupling mitochondrial oxidative phosphorylation

There is increasing evidence that protein kinase C (PKC) isoforms modulate insulin-signaling pathways in both positive and negative ways. Recent reports have indicated that the novel PKCdelta mediates some of insulin's actions in muscle and liver cells. Many studies use the specific inhibitor rottlerin to demonstrate the involvement of PKCdelta. In this study, we investigated whether PKCdelta might play a role in 3T3-L1 adipocytes. We found that PKCdelta is highly expressed in mouse adipose tissue and increased on 3T3-L1 adipocyte differentiation, and insulin-stimulated glucose transport is blocked by rottlerin. The phosphorylation state and activity of PKCdelta are not altered by insulin, but the protein translocates to membranes following insulin treatment. In contrast to the results with rottlerin, inhibition of PKCdelta activity or expression has no effect on glucose transport in adipocytes, unlike muscle cells. Lastly, we found that rottlerin lowers adenosine triphosphate levels in 3T3-L1 cells by acting as a mitochondrial uncoupler, and this is responsible for the observed inhibition of glucose transport.

The elastin peptides-mediated induction of pro-collagenase-1 production by human fibroblasts involves activation of MEK/ERK pathway via PKA- and PI(3)K-dependent signaling

Elastin peptides, such as kappa-elastin (kE), bind to the elastin receptor at the cell surface of human dermal fibroblasts and stimulate collagenase-1 expression at the gene and protein levels. Using specific inhibitors and phosphospecific antibodies, we show here that the binding of elastin peptides to their receptor activates the extracellular signal-regulated kinase (ERK) pathway; this activation is essential for the induction of pro-collagenase-1 production. Moreover, protein kinase A (PKA) and phosphatidylinositol 3-kinase (PI(3)K) signaling were found to participate in ERK activation. Concomitantly, we demonstrate that stimulation by elastin peptides leads to enhanced DNA binding of activator protein-1 (AP-1). Our data indicate that the up-regulation of collagenase-1 following treatment of fibroblasts with elastin peptides results from a cross-talk between PKA, PI(3)K and the ERK signaling pathways and that this regulation is accompanied by activation of AP-1 transcription factors.

Possible participation of intracellular platelet-activating factor in NF-kappaB activation in rat peritoneal macrophages

As we had found previously that thapsigargin, an endomembrane Ca2+-ATPase inhibitor, induces production of intracellular platelet-activating factor (PAF) [Br. J. Pharmacol. 116 (1995) 2141], we decided to investigate the possible roles of intracellular PAF in nuclear factor (NF)-kappaB activation of thapsigargin-stimulated rat peritoneal macrophages. When rat peritoneal macrophages were stimulated with thapsigargin, the level of inhibitory protein of NF-kappaB-alpha (IkappaB-alpha) was decreased and the nuclear translocation of NF-kappaB was increased. The thapsigargin-induced activation of NF-kappaB was inhibited by the PAF synthesis inhibitor SK&F 98625 and the PAF antagonist E6123. Structurally unrelated PAF antagonists such as E5880 and L-652,731 also inhibited the thapsigargin-induced activation of NF-kappaB. Lipopolysaccharide (LPS)-induced activation of NF-kappaB was also suppressed by these drugs. In a culture of rat peritoneal macrophages, exogenously added PAF did not induce degradation of IkappaB-alpha. These findings suggest that the intracellular PAF produced by the stimulation with thapsigargin or LPS is involved in activation of the NF-kappaB pathway.

Involvement of phosphatidylinositol-3-kinase in membrane ruffling induced by P-glycoprotein substrates in multidrug-resistant carcinoma cells

P-glycoprotein (P-gp) is a transmembrane protein that transports a variety of structurally and functionally diverse drugs. We recently found that the interaction of drugs with P-gp promoted invasion and metastasis. In this study, we sought to determine the mechanism by which the interaction of P-gp with its substrates leads to the earliest membrane changes associated with cellular invasion, i.e., membrane ruffling. We focused on the activation of phosphatidylinositol-3-kinase (PI-3-kinase), a lipid kinase that regulates actin cytoskeletal organization and cell movement. Sensitive or multidrug-resistant (MDR) MCF-7 (human breast cancer) or KB (human oral carcinoma) cells were treated with drugs or vehicle, and then were stained with phalloidin-tetramethyl-rhodamine isothiocyanate. Membrane ruffles were visualized using a fluorescence microscope. PI-3-kinase activity was determined by an in vitro immune-complex kinase assay and thin-layer chromatography. Drugs transported by P-gp, vinblastine and trans-flupenthixol, increased membrane ruffling and PI-3-kinase activity in the MDR cell lines, MCF-7/AdrR and KBV-1, which overexpress P-gp. This effect was not seen with mechlorethamine, a drug that is not transported by P-gp, and was not detected in sensitive parental cell lines that do not express P-gp. A similar effect was also observed in the MDR1 transfectant, MCF-7/BC-19. Wortmannin, an inhibitor of PI-3-kinase, blocked the effect of VBL and tFPT on membrane ruffling and the activity of PI-3-kinase in MDR cells. These results indicate that drugs transported by P-gp induce membrane ruffling, an early indicator of cellular motility and metastatic potential, in cancer cells overexpressing P-gp and that this effect may be mediated through activation of PI-3-kinase.

The role of PI 3-kinase in EGF-stimulated jejunal glucose transport

Epidermal growth factor (EGF) rapidly increases jejunal glucose transport. Signal transduction mechanisms mediating EGF-induced alterations in jejunal glucose transport remain to be determined. New Zealand White rabbit (1 kg) jejunal tissue was stripped and mounted in short-circuited Ussing chambers. The transport of tritiated 3-O-methylglucose was measured in the presence of the PKC agonist 1,2-dioctanoyl-sn-glycerol (1,2-DOG) or the inactive analog 1,3-dioctanoyl-sn-glycerol (1,3-DOG). Additional experiments examined the effect of the PKC inhibitor chelerythrine, the PLC inhibitor U73122, the MAPK inhibitor PD 98059, the G-protein inhibitor GDP-betaS, the PI 3-kinase inhibitor LY294002, or the microtubule inhibitor colchicine on EGF-induced jejunal glucose transport. Net jejunal 3-O-methylglucose absorption was significantly increased following specific activation of PKC. A PKC antagonist inhibited the EGF-induced increase in net 3-O-methylglucose transport, while PI 3-kinase inhibition completely blocked the EGF-induced transport increase. Inhibition of PLC, MAPK, G-proteins, and microtubules had no effect on EGF-stimulated increases in jejunal transport. We conclude that the effect of EGF on jejunal glucose transport is mediated at least in part by PKC and PI 3-kinase.

Regulation of human eosinophil NADPH oxidase activity: a central role for PKCdelta

Eosinophils play a primary role in the pathophysiology of asthma. In the lung, the activation state of the infiltrating eosinophils determines the extent of tissue damage. Interleukin-5 (IL-5) and leukotriene B4 (LTB4) are important signaling molecules involved in eosinophil recruitment and activation. However, the physiological processes that regulate these activation events are largely unknown. In this study we have examined the mechanisms of human eosinophil NADPH oxidase regulation by IL-5, LTB4, and phorbol ester (PMA). These stimuli activate a Zn2+-sensitive plasma membrane proton channel, and treatment of eosinophils with Zn2+ blocks superoxide production. We have demonstrated that eosinophil intracellular pH is not altered by IL-5 activation of NADPH oxidase. Additionally, PKCdelta inhibitors block PMA, IL-5 and LTB4 mediated superoxide formation. Interestingly, the PKCdelta-selective inhibitor, rottlerin, does not block proton channel activation by PMA indicating that the oxidase and the proton conductance are regulated at distinct phosphorylation sites. IL-5 and LTB4, but not PMA, stimulated superoxide production is also blocked by inhibitors of PI 3-kinase indicating that activation of this enzyme is an upstream event common to both receptor signaling pathways. Our results indicate that the G-protein-coupled LTB4 receptor and the IL-5 cytokine receptor converge on a common signaling pathway involving PI 3-kinase and PKCdelta to regulate NADPH oxidase activity in human eosinophils.

Physical and functional interactions between protein tyrosine phosphatase alpha, PI 3-kinase, and PKCdelta

The somatostatin analogue, TT-232 inhibits cell proliferation and induces apoptosis in a variety of tumor cells both in vivo and in vitro. While the early transient activation of Erk/MAPK was found to be important for the induction of cell cycle arrest, the signaling pathway leading to the activation of Erk/MAPK had not been fully established. Here we present evidence that activation of the Erk/MAPK pathway by TT-232 involves PI 3-kinase, PKCdelta and the protein tyrosine phosphatase alpha (PTPalpha). We show a physical interaction of PI 3-kinase and PKCdelta with PTPalpha and show that the tyrosine phosphatase plays a role in the activation of MAPK. In this process, PTPalpha Ser-180 and Ser-204 phosphorylation is critical for the induction of phosphatase activity, which is required for dephosphorylation of pp60(c-src). Taken together, we demonstrate the physical and functional association between PI 3-kinase, PKCdelta and PTPalpha in a signaling complex that mediates the antitumor activity of the somatostatin analogue TT-232.

Importance of PKC and PI3Ks in ethanol-induced contraction of cerebral arterial smooth muscle

We investigated the relationships of two potential intracellular signaling pathways, protein kinase C (PKC) and phosphatidylinositol 3-kinases (PI3Ks), to ethanol-induced contractions in cerebral arteries. Ethanol (20-200 mM) induces concentration-dependent constriction in isolated canine basilar arteries that is inhibited in a concentration-dependent manner by pretreatment of these vessels with 10(-9)-10(-3) M Gö-6976 (an antagonist selective for PKC-alpha and PKC-betaI), 10(-10)-10(-4) M bisindolylmaleimide I (a specific antagonist of PKC), and 10(-10)-10(-4) M wortmannin or 10(-8)-10(-2) M LY-294002 (selective antagonists of PI3Ks). Ethanol-induced increases in intracellular Ca(2+) concentration (from approximately 100 to approximately 500 nM) in canine basilar smooth muscle cells are also suppressed markedly (approximately 20-70%) in the presence of a similar concentration range of Gö-6976, bisindolymaleimide I, wortmannin, or LY-294002. This study suggests that activation of PKC isoforms and PI3Ks appears to be an important signaling pathway in ethanol-induced vasoconstriction of cerebral blood vessels.

Bradykinin signalling to MAP kinase: cell-specific connections versus principle mitogenic pathways

Mitogenic signalling pathways from G protein-coupled receptors (GPCRs) to the mitogen-activated protein kinase (MAPK) cascade may involve alpha- or betagamma-subunits of heterotrimeric G proteins, receptor or non-receptor tyrosine kinases, adaptor molecules, phosphoinositide 3-kinases, protein kinase C, and probably other proteins. The majority of models describing the connection of different signalling proteins within a mitogenic pathway are based on experimental data obtained by co- and overexpression of epitope-tagged MAPK together with the respective GPCR and other signalling proteins of interest in transfectable cell lines. Here the link of the bradykinin B2 receptor (B2R) to MAPK in the COS-7 cell expression system is compared with mitogenic signalling pathways of bradykinin in various tumour cell lines. It becomes evident that in natural or tumour cells expressing individual amounts and different isoforms of signalling proteins completely other relations between B2R and MAPK may exist than in COS-7 cells, suggesting a high degree of cellular specificity in mitogenic signalling.

Low [Mg(2+)](o) induces contraction and [Ca(2+)](i) rises in cerebral arteries: roles of ca(2+), PKC, and PI3

Removal of extracellular Ca(2+) concentration ([Ca(2+)](o)) and pretreatment of canine basilar arterial rings with either an antagonist of voltage-gated Ca(2+) channels (verapamil), a selective antagonist of the sarcoplasmic reticulum Ca(2+) pump [thapsigargin (TSG)], caffeine plus a specific antagonist of ryanodine-sensitive Ca(2+) release (ryanodine), or a D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)]- mediated Ca(2+) release antagonist (heparin) markedly attenuates low extracellular Mg(2+) concentration ([Mg(2+)](o))-induced contractions. Low [Mg(2+)](o)-induced contractions are significantly inhibited by pretreatment of the vessels with Gö-6976 [a protein kinase C-alpha (PKC-alpha)- and PKC-betaI-selective antagonist], bisindolylmaleimide I (Bis, a specific antagonist of PKC), and wortmannin or LY-294002 [selective antagonists of phosphatidylinositol-3 kinases (PI3Ks)]. These antagonists were also found to relax arterial contractions induced by low [Mg(2+)](o) in a concentration-dependent manner. The absence of [Ca(2+)](o) and preincubation of the cells with verapamil, TSG, heparin, or caffeine plus ryanodine markedly attenuates the transient and sustained elevations in the intracellular Ca(2+) concentration ([Ca(2+)](i)) induced by low-[Mg(2+)](o) medium. Low [Mg(2+)](o)-produced increases in [Ca(2+)](i) are also suppressed markedly in the presence of Gö-6976, Bis, wortmannin, or LY-294002. The present study suggests that both Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+) release from intracellular stores [both Ins(1,4,5)P(3) sensitive and ryanodine sensitive] play important roles in low-[Mg(2+)](o) medium-induced contractions of isolated canine basilar arteries. Such contractions are clearly associated with activation of PKC isoforms and PI3Ks.

DIK, a novel protein kinase that interacts with protein kinase Cdelta. Cloning, characterization, and gene analysis

A novel serine/threonine kinase, termed DIK, was cloned using the yeast two-hybrid system to screen a cDNA library from the human keratinocyte cell line HaCaT with the catalytic domain of rat protein kinase Cdelta (PKCdelta(cat)) cDNA as bait. The predicted 784-amino acid polypeptide with a calculated molecular mass of 86 kDa contains a catalytic kinase domain and a putative regulatory domain with ankyrin-like repeats and a nuclear localization signal. Expression of DIK at the mRNA and protein level could be demonstrated in several cell lines. The dik gene is located on chromosome 21q22.3 and possesses 8 exons and 7 introns. DIK was synthesized in an in vitro transcription/translation system and expressed as recombinant protein in bacteria, HEK, COS-7, and baculovirus-infected insect cells. In the in vitro system and in cells, but not in bacteria, various post-translationally modified forms of DIK were produced. DIK was shown to exhibit protein kinase activity toward autophosphorylation and substrate phosphorylation. The interaction of PKCdelta(cat) and PKCdelta with DIK was confirmed by coimmunoprecipitation of the proteins from HEK cells transiently transfected with PKCdelta(cat) or PKCdelta and DIK expression constructs.

Functional interaction between RAFT1/FRAP/mTOR and protein kinase cdelta in the regulation of cap-dependent initiation of translation

Hormones and growth factors induce protein translation in part by phosphorylation of the eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1). The rapamycin and FK506-binding protein (FKBP)-target 1 (RAFT1, also known as FRAP) is a mammalian homolog of the Saccharomyces cerevisiae target of rapamycin proteins (mTOR) that regulates 4E-BP1. However, the molecular mechanisms involved in growth factor-initiated phosphorylation of 4E-BP1 are not well understood. Here we demonstrate that protein kinase Cdelta (PKCdelta) associates with RAFT1 and that PKCdelta is required for the phosphorylation and inactivation of 4E-BP1. PKCdelta-mediated phosphorylation of 4E-BP1 is wortmannin resistant but rapamycin sensitive. As shown for serum, phosphorylation of 4E-BP1 by PKCdelta inhibits the interaction between 4E-BP1 and eIF4E and stimulates cap-dependent translation. Moreover, a dominant-negative mutant of PKCdelta inhibits serum-induced phosphorylation of 4E-BP1. These findings demonstrate that PKCdelta associates with RAFT1 and thereby regulates phosphorylation of 4E-BP1 and cap-dependent initiation of protein translation.

Ca(2+)-evoked serotonin secretion by parafollicular cells: roles in signal transduction of phosphatidylinositol 3'-kinase, and the gamma and zeta isoforms of protein kinase C

Parafollicular (PF) cells secrete 5-HT in response to stimulation of a G-protein-coupled Ca(2+) receptor (CaR) by increased extracellular Ca(2+) (upward arrow[Ca(2+)](e)). We tested the hypothesis that protein kinase C (PKC) participates in stimulus-secretion coupling. Immunoblots from membrane and cytosolic fractions of isolated PF cells revealed conventional (alpha, betaI, and gamma), novel (delta and epsilon), and atypical (iota/lambda and zeta) PKCs. Only PKCgamma was found to have been translocated to the membrane fraction when secretion of 5-HT was evoked by upward arrow[Ca(2+)](e) or phorbol esters. Although phorbol downregulation caused PKCgamma to disappear, secretion was only partially inhibited. A similar reduction of upward arrow[Ca(2+)](e)-evoked secretion was produced by inhibitors of conventional and/or novel PKCs (Gö 6976, calphostin C, and pseudoA), and these compounds did not inhibit secretion at all when applied to phorbol-downregulated cells. In contrast, the phorbol downregulation-resistant component of secretion was abolished by pseudoZ, which inhibits the atypical PKCzeta. Stimulation of PF cells with upward arrow[Ca(2+)](e) increased the activity of immunoprecipitated PKCzeta (but not PKCiota/lambda), and the activity of this PKCzeta was inhibited by pseudoZ. PF cells were found to express regulatory (p85) and catalytic (p110alpha and p110beta) subunits of phosphatidylinositol 3'-kinase (PI3'-kinase). upward arrow[Ca(2+)](e) increased the activity of immunoprecipitated PI3'-kinase; moreover, PI3'-kinase inhibitors (wortmannin and LY294002) antagonized secretion. We suggest that PKC isoforms mediate secretion of 5-HT by PF cells in response to stimulation of the CaR. PKC involvement can be accounted for by PKCgamma and an isoform sensitive to inhibition by pseudoZ, probably PKCzeta, which is activated via PI3'-kinase.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Effects of protein kinase C inhibitors on thromboxane production by thrombin-stimulated platelets

The purpose of these studies was to identify a possible role for protein kinase C in thromboxane production. The effects of four putative protein kinase C inhibitors were studied with platelet stimulation by thrombin (0.5-150 nM), Thrombin Quick I (1.5-500 nM) or a thrombin receptor (protease activated receptor-1) agonist peptide (TRAP) (5-120 microM). Thromboxane production was increased by the bisindolylmaleimide derivative, 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF 109203X), unchanged by the inhibitors 12-(2-cyanoethyl)-6,7, 12,13-tetrahydro-13-methyl-5-oxo-5H-indolo (2,3-a) pyrrolo (3, 4-c)-carbazole (Gö 6976) and 5,21:12,17-dimetheno-18H-dibenzo[i, o]pyrrolo[3,4-l][1,8]diazacyclohexadecine-18,20(19H)-dione, 8-[(dimethylamino)methyl]-6,7,8,9,10,11-hexahydro-, monomethanesulfonate (379196), the latter of which is protein kinase C beta-selective, and decreased by 1-[6-[(3-acetyl-2,4, 6-trihydroxy-5-methylphenyl)methyl]-5,7-dihydroxy-2, 2-dimethyl-2H-1-benzopyran-8-yl]-3-phenyl-2-propen-1-one (rottlerin), an inhibitor selective for protein kinase C delta. These results indicate complex regulation of thromboxane synthesis in human platelets including a probable role for protein kinase C delta. The results taken together further suggest that GF 109203X may suppress negative feedback resulting from an unidentified kinase and that the classical protein kinase C isoforms alpha and beta do not have a significant role in regulating thromboxane production by platelets.

A role for phosphatidylinositol 3-kinase in platelet aggregation in response to low, but not high, concentrations of PAF or thrombin

In this study we show that platelet activating factor (PAF) activates PI 3-kinase over a rapid time course that correlates closely with the aggregation response. Tyrosine kinases are involved in this response, since there is increased PI 3-kinase activity associated with tyrosine-phosphorylated proteins. PI 3-kinase inhibitors were used to probe the dependence of PAF-induced aggregation on PI 3-kinase. Both wortmannin and LY-294002 inhibited PAF-induced aggregation that correlated with PI 3-kinase inhibition only when using lower concentrations of PAF giving reversible aggregation (primary phase). Similar results were obtained with human platelets using thrombin or thrombin receptor activating peptide. The same pattern of response was observed when activation of GPIIbIIIa was assessed by flow cytometry, i.e., PI 3-kinase inhibitors blocked integrin activation only when lower concentrations of agonist were used. We suggest that PI 3-kinase is important for reversible (primary) aggregation of platelets in response to PAF or thrombin, perhaps by contributing to the 'inside-out' activation of the platelet integrin GPIIbIIIa, only when submaximal concentrations of agonists are used. The lack of effect of PI 3-kinase inhibitors, when high concentrations of agonist are used, suggests that PI 3-kinase-independent pathways contribute to aggregation under these conditions.

Multiple forms of p55PIK, a regulatory subunit of phosphoinositide 3-kinase, are generated by alternative initiation of translation

A cDNA encoding p55PIK, one of the regulatory subunits of phosphoinositide (phosphatidylinositol) 3-kinase, was cloned from a cDNA library derived from the mouse mammary epithelial cell line C57MG. The cDNA coding for full-length p55PIK was transiently expressed in COS-7 cells. Western blot analysis of p55PIK expression using a specific antibody against p55PIK revealed that multiple protein products with different molecular masses were detected in COS-7 cell extracts. Experiments presented here demonstrate that multiple forms of p55PIK detected in COS-7 cells were produced by alternative initiation of translation. We also show that at least two in-frame start codons (AUG#2 and AUG#5) in p55PIK mRNA are used in COS-7 cells for the initiation of translation of p55PIK into proteins of 54 kDa and 50 kDa respectively. p55PIK mRNA was also alternatively translated into two proteins in PC cells, a mouse teratoma cell line, indicating that the alternative initiation of translation of p55PIK is not restricted to COS-7 cells. Results from immunoprecipitation and Western blot analysis showed that two forms (54 kDa and 50 kDa protein species) of p55PIK were detected in C57MG cells. Interestingly, when C57MG cells were treated with insulin, only p55PIK, but not p50PIK, bound to insulin receptor substrate-1 protein, providing evidence that different forms of p55PIKs may have specific distinct roles in signal transduction pathways.

Role of PI3-kinase in the development of interstitial cells and pacemaking in murine gastrointestinal smooth muscle

1. Development of the pacemaker system in the small intestine depends upon signalling via tyrosine kinase (Kit) receptors. The downstream pathways initiated by Kit in interstitial cells of Cajal (ICC) have not been investigated. Wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY 294002), inhibitors of phosphatidylinositol 3'-kinase (PI3-kinase), were used to test the involvement of this pathway in the development and maintenance of ICC and electrical rhythmicity in the murine small intestine. 2. ICC and electrical slow waves were present in the murine jejunum at birth. ICC and electrical rhythmicity continued to develop in neonates such that adult activity was recorded after 1 week. Development of ICC and rhythmicity were maintained in organ culture. 3. Wortmannin or LY 294002 inhibited the development of slow waves and blocked rhythmicity within 2-4 days. Loss of slow waves was preceded by disappearance of Kit-positive cells from the myenteric (IC-MY) and deep muscular plexus (IC-DMP) regions. Wortmannin or LY 294002 had no acute effect on slow waves. 4. Muscles from older animals (day 10-day 30) developed resistance to wortmannin treatment, but when the exposure to wortmannin was increased to 35 days, damage to ICC networks and electrical dysrhythmias were observed. 5. PI3-kinase appears to be a critical downstream signalling element linking Kit receptors to ICC development and maintenance of phenotype. ICC are more sensitive to Kit or PI3-kinase blockade at birth, but the importance of the PI3-kinase signalling in the maintenance of ICC persists into adulthood. Interference with PI3-kinase signalling in immature or adult animals could result in disruption of ICC and gastrointestinal dysrhythmias.

Abnormalities of protein kinases in neurodegenerative diseases

In neurodegenerative diseases such as ALS and AD there is evidence for abnormal regulation of protein kinases. In these diseases, altered activities and protein levels of several specific kinases suggest that abnormal phosphorylation is present and this aberrant phosphorylation may be involved in the pathogenesis of these diseases. The observation that regulation of the NMDA receptor ion channel is altered in tissue from ALS patients may arise from the abnormal phosphorylation state of the protein kinase regulating NMDA receptor function. Whether the abnormalities of these protein kinases is a primary event leading to altered receptor regulation or vice versa is still poorly understood. The seemingly multiple pathogenic mechanisms of ALS and AD create complexity in assessing a primary cause that may lead to cell death. The mechanisms causing cell death (apoptosis or necrosis) may be overlapping with integrated events among the components interacting and contributing to a final pathway for neuron death. Thus, evidence of impairment in protein kinase signalling in these diseases may be a primary cause, a secondary event, or a compensatory mechanism. To further study this issue, different model systems could be beneficial to obtain a better understanding of these diseases.

Protein kinase C delta

The protein kinase C (PKC) family consists of 11 isoenzymes that, due to structural and enzymatic differences, can be subdivided into three groups: The Ca(2+)-dependent, diacylglycerol (DAG)-activated cPKCs (conventional PKCs: alpha, beta 1, beta 2, gamma); the Ca(2+)-independent, DAG-activated nPKCs (novel PKCs: delta, epsilon, eta, theta, mu), and the Ca(2+)-dependent, DAG non-responsive aPKCs (atypical PKCs: zeta, lambda/iota). PKC mu is a novel PKC, but with some special structural and enzymatic properties.