Unique substrate specificity and regulatory properties of PKC-epsilon: a rationale for diversity

PKC-ε regulates vesicle delivery and focal exocytosis for efficient IgG-mediated phagocytosis

Protein kinase C (PKC)-ε is required for membrane addition during IgG-mediated phagocytosis, but its role in this process is ill defined. Here, we performed high-resolution imaging, which reveals that PKC-ε exits the Golgi and enters phagosomes on vesicles that then fuse. TNF and PKC-ε colocalize at the Golgi and on vesicles that enter the phagosome. Loss of PKC-ε and TNF delivery upon nocodazole treatment confirmed vesicular transport on microtubules. That TNF+ vesicles were not delivered in macrophages from PKC-ε null mice, or upon dissociation of the Golgi-associated pool of PKC-ε, implies that Golgi-tethered PKC-ε is a driver of Golgi-to-phagosome trafficking. Finally, we established that the regulatory domain of PKC-ε is sufficient for delivery of TNF+ vesicles to the phagosome. These studies reveal a novel role for PKC-ε in focal exocytosis - its regulatory domain drives Golgi-derived vesicles to the phagosome, whereas catalytic activity is required for their fusion. This is one of the first examples of a PKC requirement for vesicular trafficking and describes a novel function for a PKC regulatory domain. This article has an associated First Person interview with the first author of the paper.

Atypical protein kinase Cι as a human oncogene and therapeutic target

Protein kinase inhibitors represent a major class of targeted therapeutics that has made a positive impact on treatment of cancer and other disease indications. Among the promising kinase targets for further therapeutic development are members of the Protein Kinase C (PKC) family. The PKCs are central components of many signaling pathways that regulate diverse cellular functions including proliferation, cell cycle, differentiation, survival, cell migration, and polarity. Genetic manipulation of individual PKC isozymes has demonstrated that they often fulfill distinct, nonredundant cellular functions. Participation of PKC members in different intracellular signaling pathways reflects responses to varying extracellular stimuli, intracellular localization, tissue distribution, phosphorylation status, and intermolecular interactions. PKC activity, localization, phosphorylation, and/or expression are often altered in human tumors, and PKC isozymes have been implicated in various aspects of transformation, including uncontrolled proliferation, migration, invasion, metastasis, angiogenesis, and resistance to apoptosis. Despite the strong relationship between PKC isozymes and cancer, to date only atypical PKCiota has been shown to function as a bona fide oncogene, and as such is a particularly attractive therapeutic target for cancer treatment. In this review, we discuss the role of PKCiota in transformation and describe mechanism-based approaches to therapeutically target oncogenic PKCiota signaling in cancer.

Protein Kinase C Inhibitors as Modulators of Vascular Function and their Application in Vascular Disease

Blood pressure (BP) is regulated by multiple neuronal, hormonal, renal and vascular control mechanisms. Changes in signaling mechanisms in the endothelium, vascular smooth muscle (VSM) and extracellular matrix cause alterations in vascular tone and blood vessel remodeling and may lead to persistent increases in vascular resistance and hypertension (HTN). In VSM, activation of surface receptors by vasoconstrictor stimuli causes an increase in intracellular free Ca(2+) concentration ([Ca(2+)]i), which forms a complex with calmodulin, activates myosin light chain (MLC) kinase and leads to MLC phosphorylation, actin-myosin interaction and VSM contraction. Vasoconstrictor agonists could also increase the production of diacylglycerol which activates protein kinase C (PKC). PKC is a family of Ca(2+)-dependent and Ca(2+)-independent isozymes that have different distributions in various blood vessels, and undergo translocation from the cytosol to the plasma membrane, cytoskeleton or the nucleus during cell activation. In VSM, PKC translocation to the cell surface may trigger a cascade of biochemical events leading to activation of mitogen-activated protein kinase (MAPK) and MAPK kinase (MEK), a pathway that ultimately increases the myofilament force sensitivity to [Ca(2+)]i, and enhances actin-myosin interaction and VSM contraction. PKC translocation to the nucleus may induce transactivation of various genes and promote VSM growth and proliferation. PKC could also affect endothelium-derived relaxing and contracting factors as well as matrix metalloproteinase (MMPs) in the extracellular matrix further affecting vascular reactivity and remodeling. In addition to vasoactive factors, reactive oxygen species, inflammatory cytokines and other metabolic factors could affect PKC activity. Increased PKC expression and activity have been observed in vascular disease and in certain forms of experimental and human HTN. Targeting of vascular PKC using PKC inhibitors may function in concert with antioxidants, MMP inhibitors and cytokine antagonists to reduce VSM hyperactivity in certain forms of HTN that do not respond to Ca(2+) channel blockers.

A cancer-associated mutation in atypical protein kinase Cι occurs in a substrate-specific recruitment motif

Atypical protein kinase Cι (PKCι) has roles in cell growth, cellular polarity, and migration, and its abundance is frequently increased in cancer. We identified a protein interaction surface containing a dibasic motif (RIPR) that bound a distinct subset of PKCι substrates including lethal giant larvae 2 (LLGL2) and myosin X, but not other substrates such as Par3. Further characterization demonstrated that Arg471 in this motif was important for binding to LLGL2, whereas Arg474 was critical for interaction with myosin X, indicating that multiple complexes could be formed through this motif. A somatic mutation of the dibasic motif (R471C) was the most frequent mutation of PKCι in human cancer, and the intact dibasic motif was required for normal polarized epithelial morphogenesis in three-dimensional cysts. Thus, the R471C substitution is a change-of-function mutation acting at this substrate-specific recruitment site to selectively disrupt the polarizing activity of PKCι.

PKC-ε pseudosubstrate and catalytic activity are necessary for membrane delivery during IgG-mediated phagocytosis

In RAW 264.7 cells, PKC-ε regulates FcγR-mediated phagocytosis. BMDM behave similarly; PKC-ε concentrates at phagosomes and internalization are reduced in PKC-ε⁻/⁻ cells. Two questions were asked: what is the role of PKC-ε? and what domains are necessary for PKC-ε concentration? Function was studied using BMDM and frustrated phagocytosis. On IgG surfaces, PKC-ε⁻/⁻ macrophages spread less than WT. Patch-clamping revealed that the spreading defect is a result of the failure of PKC-ε⁻/⁻ macrophages to add membrane. The defect is specific for FcγR ligation and can be reversed by expression of full-length (but not the isolated RD) PKC-ε in PKC-ε⁻/⁻ BMDM. Thus, PKC-ε function in phagocytosis requires translocation to phagosomes and the catalytic domain. The expression of chimeric PKC molecules in RAW cells identified the εPS as necessary for PKC-ε targeting. When placed into (nonlocalizing) PKC-δ, εPS was sufficient for concentration, albeit to a lesser degree than intact PKC-ε. In contrast, translocation of δ(εPSC1B) resembled that of WT PKC-ε. Thus, εPS and εC1B cooperate for optimal phagosome targeting. Finally, cells expressing εK437W were significantly less phagocytic than their PKC-ε-expressing counterparts, blocked at the pseudopod-extension phase. In summary, we have shown that εPS and εC1B are necessary and sufficient for targeting PKC-ε to phagosomes, where its catalytic activity is required for membrane delivery and pseudopod extension.

Sensitisation of TRPV1 in rat sensory neurones by activation of SNSRs

The novel sensory neurone specific receptor (SNSR) family of G-protein coupled receptors are activated by non-opiate fragments of opioid precursor peptides. SNSRs are expressed in nociceptors, and SNSR agonists have been found to cause sensitisation to painful stimuli in vivo. We explored the basis of sensitisation caused by SNSR agonists in sensory neurones by investigating the effect of the SNSR-selective agonist bovine adrenal medulla peptide 8-22 (BAM (8-22)) on gating of the heat and capsaicin-sensitive ion channel TRPV1. Using calcium imaging we found that BAM (8-22) caused sensitisation of the TRPV1 response in approximately 13% of DRG neurones. Sensitisation of TRPV1 in a similar proportion of neurones was observed using whole-cell patch clamping. The PKC-specific inhibitor Ro-31-8220 reduced but did not completely abolish sensitisation, while the protein kinase A inhibitor H-89 was without significant effect. No translocation of the PKC delta, epsilon and zeta isoforms to the cell membrane was observed in response to BAM (8-22). These observations implicate PKC in the sensitisation of TRPV1, but suggest that other pathways are also involved.

Sensitization of transient receptor potential vanilloid 1 by the prokineticin receptor agonist Bv8

Small mammalian proteins called the prokineticins [prokineticin 1 (PK1) and PK2] and two corresponding G-protein-coupled receptors [prokineticin receptor 1 (PKR1) and PKR2] have been identified recently, but the physiological role of the PK/PKR system remains mostly unexplored. Bv8, a protein extracted from frog skin, is a convenient and potent agonist for both PKR1 and PKR2, and injection of Bv8 in vivo causes a potent and long-lasting hyperalgesia. Here, we investigate the cellular basis of hyperalgesia caused by activation of PKRs. Bv8 caused increases in [Ca]i in a population of isolated dorsal root ganglion (DRG) neurons, which we identified as nociceptors, or sensors for painful stimuli, from their responses to capsaicin, bradykinin, mustard oil, or proteases. Bv8 enhanced the inward current carried by the heat and capsaicin receptor, transient receptor potential vanilloid 1 (TRPV1) via a pathway involving activation of protein kinase Cepsilon (PKCepsilon), because Bv8 caused translocation of PKCepsilon to the neuronal membrane and because PKC antagonists reduced both the enhancement of current carried by TRPV1 and behavioral hyperalgesia in rodents. The neuronal population expressing PKRs consisted partly of small peptidergic neurons and partly of neurons expressing the N52 marker for myelinated fibers. Using single-cell reverse transcriptase-PCR, we found that mRNA for PKR1 was mainly expressed in small DRG neurons. Exposure to GDNF (glial cell line-derived neurotrophic factor) induced de novo expression of functional receptors for Bv8 in a nonpeptidergic population of neurons. These results show that prokineticin receptors are expressed in nociceptors and cause heat hyperalgesia by sensitizing TRPV1 through activation of PKCepsilon. The results suggest a role for prokineticins in physiological inflammation and hyperalgesia.

Regulation of skin microvasculature angiogenesis, cell migration, and permeability by a specific inhibitor of PKCalpha

Activation of protein kinase C (PKC) induces phenotypic changes in the morphology of microvascular endothelial cells that affect major functions of the microvasculature. These functions include the first stages of sprouting in angiogenesis, cell migration following wounding, and vascular permeability. The specific isoform(s) of PKC responsible for each of these changes has not been previously identified. In this study, we used two inflammatory agents, IL-1beta and phorbol myristic acetate, to activate PKC isozymes and specific inhibitors of PKCalpha (Gö6976) and PKCbeta (hispidin) to distinguish how each of these isoform(s) controls angiogenesis, wound healing, and permeability. In all cases, only inhibition of PKCalpha inhibited each of these functions when compared to the inhibition of PKCbeta. Additional analysis of the mechanism of action of Gö6976 (RT-PCR, Western blots, and immunohistochemistry) of the changes in the phosphorylated and nonphosphorylated forms of PKCalpha in the cell membrane and cytoplasm confirmed the specificity of PKCalpha inhibition by Gö6976. These studies therefore indicate a specific and a regulatory role of the PKCalpha isoform in three major endothelial cell functions that are important in the maintenance of microvascular homeostasis.

PKC-delta sensitizes Kir3.1/3.2 channels to changes in membrane phospholipid levels after M3 receptor activation in HEK-293 cells

G protein-gated inward rectifier (Kir3) channels are inhibited by activation of G(q/11)-coupled receptors and this has been postulated to involve the signaling molecules protein kinase C (PKC) and/or phosphatidylinositol 4,5-bisphosphate (PIP(2)). Their precise roles in mediating the inhibition of this family of channels remain controversial. We examine here their relative roles in causing inhibition of Kir3.1/3.2 channels stably expressed in human embryonic kidney (HEK)-293 cells after muscarinic M(3) receptor activation. In perforated patch mode, staurosporine prevented the G(q/11)-mediated, M(3) receptor, inhibition of channel activity. Recovery from M(3)-mediated inhibition was wortmannin sensitive. Whole cell currents, where the patch pipette was supplemented with PIP(2), were still irreversibly inhibited by M(3) receptor stimulation. When adenosine A(1) receptors were co-expressed, inclusion of PIP(2) rescued the A(1)-mediated response. Recordings from inside-out patches showed that catalytically active PKC applied directly to the intracellular membrane face inhibited the channels: a reversible effect modulated by okadaic acid. Generation of mutant heteromeric channel Kir3.1S185A/Kir3.2C-S178A, still left the channel susceptible to receptor, pharmacological, and direct kinase-mediated inhibition. Biochemically, labeled phosphate is incorporated into the channel. We suggest that PKC-delta mediates channel inhibition because recombinant PKC-delta inhibited channel activity, M(3)-mediated inhibition of the channel, was counteracted by overexpression of two types of dominant negative PKC-delta constructs, and, by using confocal microscopy, we have demonstrated translocation of green fluorescent protein-tagged PKC-delta to the plasma membrane on M(3) receptor stimulation. Thus Kir3.1/3.2 channels are sensitive to changes in membrane phospholipid levels but this is contingent on the activity of PKC-delta after M(3) receptor activation in HEK-293 cells.

Chronic exposure to ammonia induces isoform-selective alterations in the intracellular distribution and NMDA receptor-mediated translocation of protein kinase C in cerebellar neurons in culture

Hyperammonemia is responsible for most neurological alterations in patients with hepatic encephalopathy by mechanisms that remain unclear. Hyperammonemia alters phosphorylation of neuronal protein kinase C (PKC) substrates and impairs NMDA receptor-associated signal transduction. The aim of this work was to analyse the effects of hyperammonemia on the amount and intracellular distribution of PKC isoforms and on translocation of each isoform induced by NMDA receptor activation in cerebellar neurons. Chronic hyperammonemia alters differentially the intracellular distribution of PKC isoforms. The amount of all isoforms (except PKC zeta) was reduced (17-50%) in the particulate fraction. The contents of alpha, beta1, and epsilon isoforms decreased similarly in cytosol (65-78%) and membranes (66-83%), whereas gamma, delta, and theta; isoforms increased in cytosol but decreased in membranes, and zeta isoform increased in membranes and decreased in cytosol. Chronic hyperammonemia also affects differentially NMDA-induced translocation of PKC isoforms. NMDA-induced translocation of PKC alpha and beta is prevented by ammonia, whereas PKC gamma, delta, epsilon, or theta; translocation is not affected. Inhibition of phospholipase C did not affect PKC alpha translocation but reduced significantly PKC gamma translocation, indicating that NMDA-induced translocation of PKC alpha is mediated by Ca2+, whereas PKC gamma translocation is mediated by diacylglycerol. Chronic hyperammonemia reduces Ca+2-mediated but not diacylglycerol-mediated translocation of PKC isoforms induced by NMDA.

Kinase peptide specificity: improved determination and relevance to protein phosphorylation

Specificity of phosphorylation is critical to signal transduction. Recent emphasis on colocalization of substrate and kinase has eclipsed emphasis on peptide specificity, i.e., kinase preference for particular amino acids surrounding the phosphorylation site. We describe an approach to determining peptide specificity by using positional scanning of biotinylated oriented peptide libraries and insights emerging from those determinations. We accurately determine preference (or disfavor) for residues at a given substrate position (such as P+2) by comparison of in vitro phosphorylation of peptide libraries differing by a single residue at that position. By analysis of all positions near the phosphorylation site, position-specific scoring matrices are generated and used both to understand the basis of specificity and to predict phosphorylation. PKC-delta and -zeta predictions have been validated rigorously by comparisons with measured phosphorylation. The results demonstrate specificity and sensitivity (80-90%) much better than the previous predictive method. These predictions can be accessed at http://mpr.nci.nih.gov. The accuracy of the specificity determination allows identification of an important difference in peptide specificity between these closely related kinases; Ile/Leu at the P-1 position is disfavored by PKC-zeta but not PKC-delta. Our findings and visual representation of peptide specificity highlight the importance of disfavored residues. Finally, analysis of 124 experimentally determined PKC sites from the literature demonstrates a very strong role of peptide specificity in many of those sites. Thus, position-specific scoring matrices generated by this method provide a foundation for quantitative analyses of kinase specificity and improved predictions of previously determined physiologically relevant phosphorylation sites.

Regulatory properties of p105: a novel PKC isoenzyme in mantle tissue from marine mussels

Previous results suggested operative similarities between Apl II from nervous cells of Aplysia californica, epsilonPKC from brain of vertebrates, and p105 from mantle tissue of Mytilus galloprovincialis Lmk., all of them belonging to the nPKC family. The optimal substrate for Apl II and p105 from mussel is protamine sulfate. In contrast, Ca2+ inhibits p105 but does not affect Apl II. As occurs in other epsilonPKC, p105 is autophosphorylable; however, in Apl II, no P-Tyr residues are detected in the most phosphorylated form. The presence of p105 in all the tissues of M. galloprovincialis studied, proves the important, yet unknown, physiological role that this enzyme must play.

Signalling pathways involved in the sensitisation of mouse nociceptive neurones by nerve growth factor

Nerve growth factor (NGF) causes a rapid sensitisation of nociceptive sensory neurones to painful thermal stimuli owing to an action on the heat and capsaicin receptor TRPV1 (formerly known as VR1). We have developed a new technique to study this rapid sensitisation of TRPV1 by monitoring the effects of NGF on the increase in intracellular calcium concentration ([Ca2+]i) following exposure to capsaicin. Brief applications of capsaicin caused a rise in [Ca2+]i, and NGF was found to enhance this rise in 37 % of capsaicin-responsive neurones within 2 min. Pathways responsible for transducing the sensitisation of TRPV1 by TrkA, the NGF receptor, were characterised by observing the effects of inhibitors of key members of NGF-activated second messenger signalling cascades. Specific inhibitors of the ras/MEK (mitogen-activated protein and extracellular signal-regulated kinases) pathway and of phospholipase C did not abolish the NGF-induced sensitisation, but wortmannin, a specific inhibitor of phosphatidylinositol-3-kinase (PI3K), totally abolished the effect of NGF. Pharmacological blockade of protein kinase C (PKC) or calcium-calmodulin-dependent protein kinase II (CaMK II) activation also prevented NGF-induced sensitisation, while blockade of protein kinase A (PKA) was without effect. These data indicate that the crucial early pathway activated by NGF involves PI3K, while PKC and CaMK II are also involved, probably at subsequent stages of the NGF-activated signalling pathway.

PKC-epsilon translocation in enteric neurons and interstitial cells of Cajal in response to muscarinic stimulation

Interstitial cells of Cajal in the deep muscular plexus (ICC-DMP) of the small intestine express excitatory neurotransmitter receptors. We tested whether ICC-DMP are functionally innervated by cholinergic neurons in the murine intestine. Muscles were stimulated by intrinsic nerves and ACh and processed for immunohistochemistry to determine these effects on PKC-epsilon activation. Under control conditions, PKC-epsilon-like immunoreactivy (PKC-epsilon-LI) was only observed in myenteric neurons within the tunica muscularis. Electrical field stimulation or ACh caused translocation of neural PKC-epsilon-LI from the cytosol to a peripheral compartment. After stimulation, PKC-epsilon-LI was found in spindle-shaped cells in the DMP. These cells were identified as ICC-DMP by Kit-LI and vimentin-LI. PKC-epsilon-LI in ICC-DMP and translocation of PKC epsilon-LI in neurons were blocked by tetrodotoxin or atropine, suggesting that these responses were due to activation of muscarinic receptors. Western blots also confirmed translocation of PKC-epsilon-LI. In conclusion, PKC-epsilon translocation is linked to muscarinic receptor activation in ICC-DMP and a subpopulation of myenteric neurons. These studies demonstrate that ICC-DMP are functionally innervated by excitatory motoneurons.

Protein kinase C isoform expression and activity in the mouse heart

The expression of protein kinase C (PKC) isoforms in the developing murine ventricle was studied using Western blotting, assays of PKC activity, and immunoprecipitations. The abundance of two Ca2+-dependent isoforms, PKCalpha and PKCbetaII, as well as two Ca2+-independent isoforms, PKCdelta and PKCepsilon, decreased during postnatal development to <15% of the levels detected at embryonic day 18. The analysis of the subcellular distribution of the four isoforms showed that PKCdelta and PKCepsilon were associated preferentially with the particulate fraction in fetal ventricles, indicating a high intrinsic activation state of these isoforms at this developmental time point. The expression of PKCalpha in cardiomyocytes underwent a developmental change. Although preferentially expressed in neonatal cardiomyocytes, this isoform was downregulated in adult cardiomyocytes. In fast-performance liquid chromatography-purified ventricular extracts, the majority of PKC activity was Ca2+-independent in both fetal and adult ventricles. Immunoprecipitation assays indicated that PKCdelta and PKCepsilon were responsible for the majority of the Ca2+-independent activity. These studies indicate a prominent role for Ca2+-independent PKC isoforms in the mouse heart.

Regulation of a G protein-gated inwardly rectifying K+ channel by a Ca(2+)-independent protein kinase C

1. Members of the Kir3.0 family of inwardly rectifying K(+) channels are expressed in neuronal, atrial and endocrine tissues and play key roles in generating late inhibitory postsynaptic potentials (IPSPs), slowing heart rate and modulating hormone release. They are activated directly by G(betagamma) subunits released in response to G(i/o)-coupled receptor stimulation. However, it is not clear to what extent this process can be dynamically regulated by other cellular signalling systems. In this study we have explored pathways activated by the G(q/11)-coupled M(1) and M(3) muscarinic receptors and their role in the regulation of Kir3.1+3.2A neuronal-type channels stably expressed in the human embryonic kidney cell line HEK293. 2. We describe a novel biphasic pattern of behaviour in which currents are initially stimulated but subsequently profoundly inhibited through activation of M(1) and M(3) receptors. This contrasts with the simple stimulation seen through activation of M(2) and M(4) receptors. 3. Channel stimulation via M(1) but not M(3) receptors was sensitive to pertussis toxin whereas channel inhibition through both M(1) and M(3) receptors was insensitive. In contrast over-expression of the C-terminus of phospholipase Cbeta1 or a G(q/11)-specific regulator of G protein signalling (RGS2) essentially abolished the inhibitory phase. 4. The inhibitory effects of M(1) and M(3) receptor stimulation were mimicked by phorbol esters and a synthetic analogue of diacylglycerol but not by the inactive phorbol ester 4alphaphorbol. Inhibition of the current by a synthetic analogue of diacylglycerol effectively occluded any further inhibition (but not activation) via the M(3) receptor. 5. The receptor-mediated inhibitory phenomena occur with essentially equal magnitude at all intracellular calcium concentrations examined (range, 0-669 nM). 6. The expression of endogenous protein kinase C (PKC) isoforms in HEK293 cells was examined by immunoblotting, and their translocation in response to phorbol ester treatment by cellular extraction. The results indicated the expression and translocation of the novel PKC isoforms PKCdelta and PKCepsilon. 7. We also demonstrate that activation of such a pathway via both receptor-mediated and receptor-independent means profoundly attenuated subsequent channel stimulation by G(i/o)-coupled receptors. 8. Our data support a role for a Ca(2+)-independent PKC isoform in dynamic channel regulation, such that channel activity can be profoundly reduced by M(1) and M(3) muscarinic receptor stimulation.

The 5' UTR of protein kinase C epsilon confers translational regulation in vitro and in vivo

We have examined translational regulation conferred by the 5' untranslated region (UTR) of PKCepsilon on expression of the luciferase reporter gene in vitro, using rabbit reticulocyte lysates and in vivo, in contact-inhibiting mouse Swiss 3T3 fibroblasts and non-contact-inhibiting Swiss 3T6 fibroblasts. In rabbit reticulocyte lysates, the 5' UTR of PKCepsilon significantly represses translation. In 3T3 and 3T6 cells, the 5' UTR of PKCepsilon reduces luciferase activity, but not to the same extent as it does in vitro. In rabbit reticulocyte lysate, the degree of repression mediated by different PKCepsilon 5' UTR-deletion constructs correlates with the free energy (DeltaG) of their predicted secondary structures. However, in cells, secondary structure is not the only determinant of repression; an internal region of the 5' UTR is both necessary and sufficient for repression. Mutation of an upstream AUG (uAUG) motif in this region partially relieves repression. We conclude that the 5' UTR of PKCepsilon can mediate translational regulation and that translation inhibition in vivo involves the uAUG motif. Our findings also suggest that there are factors present in fibroblasts, but not in rabbit reticulocyte lysates that substantially overcome the repressive qualities of the long, structured 5' UTR. Thus, we have identified a potential new level of regulation of PKC in mammalian cells.

Electrical activity regulates AChR gene expression via JNK, PKCzeta and Sp1 in skeletal chick muscle

Electrical activity of myotubes represses nicotinic acetylcholine receptor (AChR) gene expression. This effect is mimicked by okadaic acid and blocked by tetrodotoxin (TTX) or staurosporine in cultured myocytes [Altiok et al., EMBO J. 16 (1997) 717-725]. In this study, we investigated the mechanism of this repression. We show that addition of exogenous phospholipase D (PLD) and C inhibits AChR expression in a manner which parallels that of okadaic acid. Furthermore, okadaic acid caused an increase of the threonine phosphorylation of protein kinase Czeta (PKCzeta) and activator of transcription factor (ATF2) and a decrease of the phosphorylation of Sp1. All these effects were reversed by staurosporine, and TTX also abolished ATF2 phosphorylation. These data reveal a possible involvement of PLD, c-jun N-terminal kinase, PKCzeta and Sp1 in the repression of AChR genes by electrical activity.

Gender-related distinctions in protein kinase C activity in rat vascular smooth muscle

Gender differences in vascular reactivity have been suggested; however, the cellular mechanisms involved are unclear. We tested the hypothesis that the gender differences in vascular reactivity reflect gender-related, possibly estrogen-mediated, distinctions in the expression and activity of specific protein kinase C (PKC) isoforms in vascular smooth muscle. Aortic strips were isolated from intact and gonadectomized male and female Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Isometric contraction was measured in endothelium-denuded aortic strips. PKC activity was measured in the cytosolic and particulate fractions, and the amount of PKC was measured using Western blots and isoform-specific anti-PKC antibodies. In intact male WKY rats, phenylephrine (Phe, 10(-5) M) and phorbol 12,13-dibutyrate (PDBu, 10(-6) M) stimulated contraction to 0.37 +/- 0.02 and 0.42 +/- 0.02 g/mg tissue wt, respectively. The basal particulate/cytosolic PKC activity ratio was 0.86 +/- 0.06, and Western blots revealed alpha-, delta-, and zeta-PKC isoforms. Phe and PDBu increased PKC activity and caused significant translocation of alpha- and delta-PKC from the cytosolic to particulate fraction. In intact female WKY rats, basal PKC activity, the amount of alpha-, delta-, and zeta-PKC, the Phe- and PDBu-induced contraction, and PKC activity and translocation of alpha- and delta-PKC were significantly reduced compared with intact male WKY rats. The basal PKC activity, the amount of alpha-, delta-, and zeta-PKC, the Phe and PDBu contraction, and PKC activity and alpha- and delta-PKC translocation were greater in SHR than WKY rats. The reduction in Phe and PDBu contraction and PKC activity in intact females compared with intact males was greater in SHR ( approximately 30%) than WKY rats ( approximately 20%). Phe and PDBu contraction and PKC activity were not significantly different between castrated males and intact males but were greater in ovariectomized (OVX) females than intact females. Treatment of OVX females or castrated males with 17 beta-estradiol, but not 17 alpha-estradiol, subcutaneous implants caused significant reduction in Phe and PDBu contraction and PKC activity that was greater in SHR than WKY rats. Phe and PDBu contraction and PKC activity in OVX females or castrated males treated with 17 beta-estradiol plus the estrogen receptor antagonist ICI-182,780 were not significantly different from untreated OVX females or castrated males. Thus a gender-related reduction in vascular smooth muscle contraction in female WKY rats with intact gonads compared with males is associated with reduction in the expression and activity of vascular alpha-, delta-, and zeta-PKC. The gender differences in vascular smooth muscle contraction and PKC activity are augmented in the SHR and are possibly mediated by estrogen.

1,25(OH)(2)-vitamin D(3) affects the subcellular distribution of protein kinase C isoenzymes in rat duodenum: influence of aging

We have previously shown that the steroid hormone 1, 25-dihydroxy-vitamin D(3) [1,25(OH)(2)D(3)] stimulates total cell protein kinase C (PKC) activity in rat duodenum, an effect that is severely impaired in old animals. We further examined the role of 1, 25(OH)(2)D(3) on PKC as it relates to aging by measuring hormone-induced changes in subcellular localization of PKC activity and isoenzymes in duodenal mucosae from young (three-month-old) and aged (24-month-old) rats. Short treatment of duodenum with 1, 25(OH)(2)D(3) (0.1 nM, 1 min) increased membrane-associated PKC activity, whereas it decreased the activity in the cytosol of young rats but was without significant effect in aged animals. Furthermore, the ability to translocate was present in young animals after a short treatment with the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA; 100 nM) or dioctanoyl-glycerol (50 microM), whereas the ability was absent in aged rats, suggesting that PKC function was impaired with aging independent of agonist stimulation. The expression of specific PKC isoenzymes and changes in their subcellular distribution after short exposure of the duodenum to the hormone were determined. Western blot analysis of total homogenates using antibodies to various PKC isoforms allowed detection of PKC alpha, beta, and delta. The expression of the straight theta and the zeta isoforms was in addition demonstrated by reverse transcription-polymerase chain reaction. The pattern of isoenzymes present in the duodenum was unaffected by aging. In young rats, 1, 25(OH)(2)D(3) translocates PKC alpha, beta, and delta to the membrane and nucleus; however, no translocation of PKC isoforms was observed in 24-month-old animals in response to the hormone. In summary, in rat duodenum, 1,25(OH)(2)D(3) modulation of PKC activity and isoenzyme subcellular distribution are impaired with aging and may explain age-induced alterations in the intestinal processes under the control of the hormone.

Increased protein kinase C delta in mammary tumor cells: relationship to transformtion and metastatic progression

Relatively little is known about the molecular mechanisms of tumor promotion/progression in mammary carcinogenesis. Increased protein kinase C (PKC) activity is known to promote tumor formation in several tissues; however, its role in mammary carcinogenesis is not yet known. To determine if individual PKCs may selectively regulate properties of mammary tumor cells, we compared PKC isozyme levels in mammary tumor cell lines with low, moderate and high metastatic potential. All three cell lines expressed alpha, delta, epsilon and zeta PKCs; however, PKC delta levels were relatively increased in the highly metastatic cells. To determine if increased PKC delta could contribute to promotion/progression, we overexpressed PKC delta in the low and moderately metastatic cell lines. PKC delta overexpression had no significant effect on growth of adherent cells, but significantly increased anchorage-independent growth. Conversely, expressing the regulatory domain of PKC delta (RD delta), a putative PKC delta inhibitory fragment, inhibited anchorage-independent growth. The efficacy of RD delta as a PKC delta inhibitor was demonstrated by showing that RD delta selectively interfered with PKC delta subcellular location and significantly interfered with phosphorylation of the PKC cytoskeletal substrate, adducin. PKC-dependent phosphorylation of cytoskeletal substrate proteins, such as adducin, provides a mechanistic link between increased PKC delta activity and phenotypic changes in cytoskeletal-dependent processes such as migration and attachment, two processes that are relevant to metastatic potential. The reciprocal growth effects of expressing PKC delta and RD delta as gain and loss of function constructs, respectively, provide strong evidence that PKC delta regulates processes important for anchorage-independent growth in these mammary tumor cells.

Effects of methylselenocysteine on PKC activity, cdk2 phosphorylation and gadd gene expression in synchronized mouse mammary epithelial tumor cells

Methylselenocysteine (MSC), an organic selenium compound is an effective chemopreventive agent against mammary cell growth both in vivo and in vitro but its mechanism of action is still not understood. We have previously demonstrated that MSC is able to inhibit growth in a synchronized TM6 mouse mammary epithelial tumor cell line at 16 h time point followed by apoptosis at 48 h. The decrease in cdk2 kinase activity was coincident with prolonged arrest of cells in S-phase. The present set of experiments showed that cdk2 phosphorylation was reduced by 72% in the MSC-treated cells at 16 h time point. Expression for gadd34, 45 and 153 was elevated 2.5 to 7 fold following MSC treatment only after 16 h time point. In order to investigate a possible upstream target for MSC, we analyzed protein kinase C (PKC) in this model. Total PKC activity was reduced in TM6 cells by MSC (50 microM) within 30 min of treatment, both in cytosolic (55.4 and 77.6%) and membrane (35.2 and 34.1%) fractions for calcium-dependent and independent PKCs, respectively. PMA significantly elevated the PKC activity in membrane fraction (P < 0.01) and MSC inhibited this activation by more than 57%. The effect of MSC was selenium specific as selenomethionine and sulfurmethyl-L-cysteine (SMC) did not alter PKC activity either in cytosolic or membrane fraction. Immunoblot analysis showed that PKC-alpha was translocated to the membrane by PMA and MSC did not alter this translocation. PKC-delta was faintly detectable in membrane fractions of control and MSC-treated cells. MSC treatment slightly reduced levels of PKC-e (in cytosolic and membrane fractions) and PKC-zeta (cytosolic fractions). The data presented herein suggest that PKC is a potential upstream target for MSC that may trigger one or all of the downstream effects; i.e. the decrease of cdk2 kinase activity, decreased DNA synthesis, elevation of gadd gene expression and finally apoptosis.

A novel nociceptor signaling pathway revealed in protein kinase C epsilon mutant mice

There is great interest in discovering new targets for pain therapy since current methods of analgesia are often only partially successful. Although protein kinase C (PKC) enhances nociceptor function, it is not known which PKC isozymes contribute. Here, we show that epinephrine-induced mechanical and thermal hyperalgesia and acetic acid-associated hyperalgesia are markedly attenuated in PKCepsilon mutant mice, but baseline nociceptive thresholds are normal. Moreover, epinephrine-, carrageenan-, and nerve growth factor- (NGF-) induced hyperalgesia in normal rats, and epinephrine-induced enhancement of tetrodotoxin-resistant Na+ current (TTX-R I(Na)) in cultured rat dorsal root ganglion (DRG) neurons, are inhibited by a PKCepsilon-selective inhibitor peptide. Our findings indicate that PKCepsilon regulates nociceptor function and suggest that PKCepsilon inhibitors could prove useful in the treatment of pain.

Characterization of the binding and phosphorylation of cardiac calsequestrin by epsilon protein kinase C

In this study, we report the cloning of the rat cardiac isoform of calsequestrin on the basis of its interaction with an epsilonprotein kinase C-unique sequence (epsilonV1) derived form the epsilonprotein kinase C regulatory domain. Calsequestrin binds activated epsilonprotein kinase C holoenzyme better than the inactive enzyme and nearly three times better than other protein kinase C isozymes. The interaction between epsilonprotein kinase C and calsequestrin is mediated by sequences in both the regulatory and kinase domains of the epsilonprotein kinase C. Finally, we show that calsequestrin is an epsilonprotein kinase C substrate in vitro and protein kinase C phosphorylation of calsequestrin leads to a decreased binding of epsilonprotein kinase C to calsequestrin.

Specific involvement of PKC-epsilon in sensitization of the neuronal response to painful heat

Pain is unique among sensations in that the perceived intensity increases, or sensitizes, during exposure to a strong stimulus. One important mediator of sensitization is bradykinin (BK), a peptide released as a consequence of tissue damage. BK enhances the membrane ionic current activated by heat in nociceptive neurons, using a pathway that involves activation of protein kinase C (PKC). We find that five PKC isoforms are present in sensory neurons but that only PKC-epsilon is translocated to the cell membrane by BK. The heat response is sensitized when constitutively active PKC-epsilon is incorporated into nociceptive neurons. Conversely, BK-induced sensitization is suppressed by a specific peptide inhibitor of PKC-epsilon. We conclude that PKC-epsilon is principally responsible for sensitization of the heat response in nociceptors by bradykinin.

The subcellular distribution of protein kinase Calpha, -epsilon, and -zeta isoforms during cardiac cell differentiation

There is little information on the molecular events that control the subcellular distribution of protein kinase C during cardiac cell differentiation. We examined protein kinase C activity and the subcellular distribution of representatives of the "classical," "novel," and "atypical" protein kinase C's in P19 murine teratoma cells induced to undergo differentiation into cardiac myocytes by the addition of dimethylsulfoxide to the medium (Grepin et al., Development 124, 2387-2395, 1997). Differentiation was assessed by the presence of striated myosin, a morphological marker for cardiac cells. Addition of dimethyl sulfoxide to the medium resulted in the appearance of striated myosin by 10 days postincubation. Immunolocalization and Western blot studies revealed that a significant proportion of protein kinase Calpha, -epsilon, and -zeta were associated with the particulate fraction in P19 cells prior to differentiation. Differentiation into cardiac cells resulted in a translocation of protein kinase C activity from the particulate fraction to cytosol and localization of most of protein kinase Calpha, -epsilon, and -zeta to the cytoplasmic compartment. The total cellular protein kinase C activity was unaltered during differentiation. The translocation of protein kinase C activity during differentiation of P19 cells into cardiac myocytes was associated with a decrease in the levels of cellular 1, 2-diacyl-sn-glycerol. The cellular levels of phosphatidylserine and phosphatidylinositol did not change during differentiation. Addition of 1,2-dioctanoyl-sn-glycerol, a cell-permeant 1, 2-diacyl-sn-glycerol analog, reversed the differentiation-induced switch in the relative distribution of protein kinase C activity and dramatically increased the association of protein kinase Calpha with the particulate fraction. Addition of 1,2-dioctanoyl-sn-glycerol did not reverse the pattern of distribution for protein kinase Cepsilon or -zeta. The results indicate that protein kinase C activity and protein kinase Calpha, -epsilon and -zeta isoforms are redistributed from the particulate to the cytosolic fraction during differentiation of P19 cells into cardiomyocytes. The mechanism for the redistribution of protein kinase Calpha may be related to the reduction in the cellular 1,2-diacyl-sn-glycerol levels that accompany differentiation.

Signal transduction by protein kinase C in mammalian cells

1. The past two decades have witnessed great advances in our understanding of the role of protein kinase C (PKC) in signal transduction. The Ca(2+)-activated, phospholipid-dependent protein kinase discovered by Nishizuka's group in 1977 is now a family of at least 11 isoforms. Protein kinase C isoforms exist in different proportions in a host of mammalian cells and each isoform has a characteristic subcellular distribution in each cell type. 2. Stimulation of a specific PKC isoform often causes redistribution of the isoform from one subcellular compartment to another compartments where it complexes with and phosphorylates a specific protein substrate. 3. The interaction of a specific PKC isoform with its protein substrate may directly activate a specific function of the cell or may trigger a cascade of protein kinases that ultimately stimulates a specific response in differentiated cells or regulates growth and proliferation in undifferentiated cells.

Antisense oligonucleotide to PKC-epsilon alters cAMP-dependent stimulation of CFTR in Calu-3 cells

Protein kinase C (PKC) regulates cystic fibrosis transmembrane conductance regulator (CFTR) channel activity but the PKC signaling mechanism is not yet known. The goal of these studies was to identify PKC isotype(s) required for control of CFTR function. CFTR activity was measured as 36Cl efflux in a Chinese hamster ovary cell line stably expressing wild-type CFTR (CHO-wtCFTR) and in a Calu-3 cell line. Chelerythrine, a PKC inhibitor, delayed increased CFTR activity induced with phorbol 12-myristate 13-acetate or with the cAMP-generating agents (-)-epinephrine or forskolin plus 8-(4-chlorophenylthio)adenosine 3',5'- cyclic monophosphate. Immunoblot analysis of Calu-3 cells revealed that PKC-alpha, -betaII, -delta, -epsilon, and -zeta were expressed in confluent cell cultures. Pretreatment of cell monolayers with Lipofectin plus antisense oligonucleotide to PKC-epsilon for 48 h prevented stimulation of CFTR with (-)-epinephrine, reduced PKC-epsilon activity in unstimulated cells by 52.1%, and decreased PKC-epsilon mass by 76.1% but did not affect hormone-activated protein kinase A activity. Sense oligonucleotide to PKC-epsilon and antisense oligonucleotide to PKC-delta and -zeta did not alter (-)-epinephrine-stimulated CFTR activity. These results demonstrate the selective regulation of CFTR function by constitutively active PKC-epsilon.

The sevenfold way of PKC regulation

Protein kinase C (PKC) is a family of enzymes that are physiologically activated by 1,2-diacylglycerol (DAG) and other lipids. To date, 11 different isozymes, alpha, betaI, betaII, gamma, delta, epsilon, nu, lambda(iota), mu, theta and zeta, have been identified. On the basis of their structure and activators, they can be divided into three groups, two of which are activated by DAG or its surrogate, phorbol 12-myristate 13-acetate (PMA). PKC isozymes are remarkably different in number and prevalence in different cell lines and tissues. When activated, the isozymes bind to membrane phospholipids or to receptors that are located in and anchor the enzymes in a subcellular compartment. Some PKCs may also be activated in their soluble form. These enzymes phosphorylate serine and threonine residues on protein substrates, perhaps the best known of which are the myristoylated, alanine-rich C kinase substrate and nuclear lamins A, B and C. The enzymes clearly play a role in signal transduction, and, because of the importance of PMA as a tumor promoter, they are thought to affect some aspect of cell cycling. How PKC takes part in the regulation of cell transformation, growth, differentiation, ruffling, vesicle trafficking and gene expression, however, is largely unknown.

From amplification to gene in thyroid cancer: a high-resolution mapped bacterial-artificial-chromosome resource for cancer chromosome aberrations guides gene discovery after comparative genome hybridization

Chromosome rearrangements associated with neoplasms provide a rich resource for definition of the pathways of tumorigenesis. The power of comparative genome hybridization (CGH) to identify novel genes depends on the existence of suitable markers, which are lacking throughout most of the genome. We now report a general approach that translates CGH data into higher-resolution genomic-clone data that are then used to define the genes located in aneuploid regions. We used CGH to study 33 thyroid-tumor DNAs and two tumor-cell-line DNAs. The results revealed amplifications of chromosome band 2p21, with less-intense amplification on 2p13, 19q13.1, and 1p36 and with least-intense amplification on 1p34, 1q42, 5q31, 5q33-34, 9q32-34, and 14q32. To define the 2p21 region amplified, a dense array of 373 FISH-mapped chromosome 2 bacterial artificial chromosomes (BACs) was constructed, and 87 of these were hybridized to a tumor-cell line. Four BACs carried genomic DNA that was amplified in these cells. The maximum amplified region was narrowed to 3-6 Mb by multicolor FISH with the flanking BACs, and the minimum amplicon size was defined by a contig of 420 kb. Sequence analysis of the amplified BAC 1D9 revealed a fragment of the gene, encoding protein kinase C epsilon (PKCepsilon), that was then shown to be amplified and rearranged in tumor cells. In summary, CGH combined with a dense mapped resource of BACs and large-scale sequencing has led directly to the definition of PKCepsilon as a previously unmapped candidate gene involved in thyroid tumorigenesis.

The broad specificity of dominant inhibitory protein kinase C mutants infers a common step in phosphorylation

Dominant negative properties are conferred on protein kinase (PK) Calpha by mutation of the phosphorylation site in the activation loop of the kinase domain. To address the universality and/or specificity of such mutations, analogous alterations were introduced in other members of the PKC family and tested for their effects on the function of co-transfected activated PKC. For all three subclasses of the PKC family, mutations of the predicted activation loop phosphorylation sites resulted in dominant negative properties. These properties were not restricted to the cognate PKC isotypes, but were effective across the different subclasses. For example, two PKCzeta mutants (atypical isotype) inhibited both PKCalpha (classical isotype) and PKCepsilon (novel isotype). For all these mutants, inhibition correlated with an ability to prevent the accumulation of phosphorylated PKCalpha, consistent with the expected mode of action. In the case of the PKCalpha mutant, it was shown that inhibition required the full-length mutant protein. The results provide evidence for the involvement of a common step in the phosphorylation of all PKC isotypes.

ZmcPKC70, a protein kinase C-type enzyme from maize. Biochemical characterization, regulation by phorbol 12-myristate 13-acetate and its possible involvement in nitrate reductase gene expression

The crucial enzyme in diacylglycerol-mediated signaling is protein kinase C (PKC). In this paper we provide evidence for the existence and role of PKC in maize. A protein of an apparent molecular mass of 70 kDa was purified. The protein showed kinase activity that was stimulated by phosphatidylserine and oleyl acetyl glycerol (OAG) in the presence of Ca2+. Phorbol 12-myristate 13-acetate (PMA) replaced the requirement of OAG. [3H]PMA binding to the 70-kDa protein was competed by unlabeled PMA and OAG but not by 4alpha-PMA, an inactive analog. The kinase phosphorylates histone H1 at serine residue(s), and this activity was inhibited by H-7 and staurosporine. These properties suggest that the 70-kDa protein is a conventional serine/threonine protein kinase C (cPKC). Polyclonal antibodies raised against the polypeptide precipitate the enzyme activity and immunostained the protein on Western blots. The antibodies also cross-reacted with a protein of expected size from sorghum, rice, and tobacco. A rapid increase in the protein level was observed in maize following PMA treatments. In order to assign a possible role of PKC in gene regulation, the nitrate reductase transcript level was investigated. The transcript level increased by PMA, not by 4alpha-PMA treatments, and the increase was inhibited by H-7 but not by okadaic acid. The data show the existence and possible function of PKC in higher plants.

Effects of gestational methylmercury exposure on immunoreactivity of specific isoforms of PKC and enzyme activity during post-natal development of the rat brain

Protein kinase C (PKC)-mediated phosphorylation has been implicated in neuronal growth and differentiation [R.S. Turner, R.L. Mazzei, G.J. Raynor, P.R. Girard, J.F. Kuo, Proc. Natl. Acad. Sci. U.S.A., 81 (1984) 3143-3147.]. We examined effects of gestational exposure to the neurotoxicant, methylmercury (CH3Hg), on the developmental profile of immunoreactivity (IR) for alpha, beta, gamma and epsilon PKC isoforms and cytosolic PKC activity. Long-Evans dams were dosed on gestational days (GD)6-15 (p.o.) with 0, 1, or 2 mg kg-1 day-1 CH3Hg dissolved in saline. Pups were sacrificed and perfused with buffered paraformaldehyde on post-natal days (PND) 1, 4, 10, 21, 45 and 85. The brains were sectioned sagittally, stained immunohistochemically, and examined throughout the medial to lateral extent. IR in neuronal cell bodies for PKC isoforms alpha, beta, gamma, and epsilon was densest in the olfactory bulb, hippocampus, shell of the inferior colliculus, pons, cerebral, piriform, and cerebellar cortex, whereas axonal staining was prominent in the brainstem, internal capsule, corpus callosum, anterior commissure, fornix and olfactory tract. In controls, the PKC alpha and epsilon IR was highest on PND1-4, decreased dramatically by PND10, and decreased further by PND21. In the neonate, the regional and cellular distributions of alpha and epsilon IR were similar. The PKC gamma IR was greater at post-weaning ages (PND21-85) with the greatest regional density apparent in the hippocampus, cortex, and cerebellum. Only the highest dose of CH3Hg (2 mg kg-1 day-1; GD6-15) produced a persistent decrease in regional alpha and epsilon, but not beta or gamma IR during the post-natal period. These regional and time-dependent changes in PKC isoforms were complemented by the examination of PKC activity in cortex, olfactory bulb, cerebellum and brainstem. Cytosolic PKC activity increased from PND1 to 10 in cortex, olfactory bulb, and cerebellum. On PND21, PKC activity decreased in the cortex and olfactory bulb, but remained high in the cerebellum. By contrast, PKC activity in the brainstem was highest on PND1 and 4 and decreased dramatically by PND21. CH3Hg (2 mg kg-1 day-1) significantly decreased PKC activity on PND1 and 4 in the cortex. The present results characterize the cellular and regional ontogeny of PKC isoenzymes alpha, beta, gamma and epsilon, and indicate that developmental exposure to CH3Hg can alter the ontogeny of specific isoforms and regional PKC activity.

Overexpression of PKCepsilon in R6 fibroblasts causes increased production of active TGFbeta

In previous studies, our laboratory demonstrated that Rat 6 (R6) fibroblasts which stably overproduce high levels of PKCepsilon display abnormalities in growth control that are characteristic of malignant transformation (Cacace et al., 1993, Oncogene, 8:2095-2104). The R6-PKCepsilon overproducing cell lines also exhibited a decreased growth factor requirement. The present study demonstrates that conditioned medium (CM) from two individual clones, R6-PKCepsilon 10 and 30, stimulates DNA synthesis in control R6-C1 cells. Maximal DNA synthesis and morphologic transformation was achieved in control cells when they were treated with medium from R6-PKCepsilon cells grown in the presence of TPA (TPA-CM). Size fractionation of the TPA-CM from PKCepsilon 30 cells revealed that this activity is due to a factor(s) that has an apparent molecular weight in the range of 10-30 kD and is heat and acid stable. This factor, like TGFbeta1, stimulated anchorage-independent growth of NRK cells. Western blot analysis (under nonreducing conditions) of the TPA-CM from R6-PKCepsilon 30 and R6-PKCepsilon 10 cells revealed the presence of the 25 kD active forms of TGFbeta2 and 3. These active forms of TGFbeta were not found in the CM of control R6 cells, or R6 cells that overexpress PKCalpha or PKCbeta1. The addition of a pan-specific TGFbeta antibody to NRK cells treated with the 10-30 kD fraction of TPA-CM from PKCepsilon 30 cells blocked the ability of this material to stimulate thymidine incorporation. Taken together, these studies suggest that the oncogenic activity of PKCepsilon in R6 cells is due, at least in part, to its ability to induce production of the active forms of TGFbeta2 and 3.

1,25(OH)2-vitamin D3 affects the subcellular distribution of protein kinase C isoenzymes in muscle cells

Previous studies have shown the involvement of protein kinase C (PKC) in 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] regulation of DNA synthesis (long-term effect) and Ca2+ channel activity (short-term effect) in cultured myoblasts. Both events mediate stimulation of myoblast cell proliferation and growth by 1,25(OH)2D3. To characterise further the role of PKC in the hormone mode of action in muscle cells, the presence of PKC isoenzymes in chicken embryo myoblasts and changes in their total cell and subcellular levels after treatment (72 h and 5 min) with 1,25(OH)2D3 (1 nM), 12-O-tetradecanoyl phorbol 13-acetate (TPA; 100 nM) and 1,2-dioctanoyl-rac-glycerol (DOG; 50 microM) were investigated. Western blot analysis provided evidence on the expression of PKC alpha, beta and delta isoforms in avian myoblasts. Two immunoreactive bands of 80 kDa (intact molecule) and 50 kDa (catalytic fragment) were detected for each isoenzyme. 1,25(OH)2D3 and DOG, which increased myoblast PKC activity parallel with the stimulation of DNA synthesis and culture growth and the phorbol ester TPA which induced the opposite changes, exerted differential effects on PKC isoenzymes. Long-term (72 h) treatment with 1,25(OH)2D3 and DOG did not change total PKC isoform levels but decreased the 80 kDa species and increased the release of the catalytic fragment of PKC delta and beta, whereas TPA augmented the total amounts of the three PKC isoforms, increasing the band of 80 kDa of PKC beta and delta and the 50 kDa species for PKC alpha. Subcellular distribution studies showed that the 80 kDa molecule is only present in the cytosolic fraction whereas in the particulate fractions the 50 kDa fragments are detected. Increased amounts of the catalytic fragments of PKC beta and delta both in the nucleus and membranes were observed after 72 h treatment with DOG while 1,25(OH)2D3 increases PKC beta in the nucleus and PKC delta in membranes. TPA induced the appearance of the 50 kDa species of PKC alpha in the nuclear and membrane fractions. The phorbol ester also decreased the catalytic fragments of PKC beta and delta in membranes. Increased levels of PKC beta, and to a lesser extent of PKC delta, in membranes and cytosol could be detected after short exposure (5 min) of myoblasts to 1,25(OH)2D3, DOG and TPA. In conclusion, the data indicate the operation in myoblasts of PKC signal transduction pathways mediated by the Ca(2+)-dependent PKCs alpha and beta and the Ca(2+)-independent PKC delta. Moreover, the results suggest that the beta and delta isoforms of PKC could play a role in the regulation of muscle cell metabolism by 1,25(OH)2D3.

The presence of an unusual PKC isozyme profile in rat liver cells

We have previously shown that protein kinase C (PKC) is involved in the mitogenic response of T51B cells to epidermal growth factor. In fact, epidermal growth factor was an excellent mitogen, even after prolonged pretreatment of cells with TPA, suggesting that the PKC isoform implicated in proliferation is not down-regulated by 12-O-tetradecanoyl phorbol-13-acetate (TPA). We have now determined that the PKC isozymes -α, -βI, -δ, -ε, and -ζ are present in T51B cells. All five isoforms are associated with the plasma membrane and the cytoplasm and are either in or around the nucleus. PKC-βI has a slightly different subcellular profile from that of the other isoforms in that it is clearly and strongly associated with the nuclear membrane. Also, a unique and novel pattern is obtained from immunoblots with anti-PKC-βI. PKC-βI is detected as a single band of 70 kDa in the cytosolic fraction and as a doublet of 65 and 77 kDa in the membrane fraction. PKC-α, -δ, and -ε were down-regulated by pretreatment of cells with TPA, while PKC-ζ was unaffected. Of particular interest was the fact that TPA did not down-regulate PKC-βI. In fact, the amount of this isoform associated with the plasma membrane increased. These findings indicate that it is probably PKC-βI that is involved in the mitogenic response of T51B cells to epidermal growth factor. Since PKC-ζ is also not down-regulated by TPA, the possible involvement of this isoform needs to be resolved.Key words: protein kinase C, intracellular localization, cell proliferation, liver.

Ischemic preconditioning induces selective translocation of protein kinase C isoforms epsilon and eta in the heart of conscious rabbits without subcellular redistribution of total protein kinase C activity

Considerable controversy surrounds the role of protein kinase C (PKC) in ischemic preconditioning (PC). Previous studies have used pharmacological agents and/or measured total myocardial PKC activity; however, no information is available regarding the effects of PC on individual isoforms in vivo. We performed a comprehensive evaluation (using Western immunoblotting) of the expression and subcellular distribution of all 11 currently known PKC isoforms in the heart of conscious rabbits subjected to four different ischemic PC protocols known to induce early and/or late PC (one, three, or six cycles of 4-minute coronary occlusion [4'O]/4-minute reperfusion [4'R]; four cycles of 5-minute occlusion [5'O]/10-minute reperfusion [10'R]). Ten PKC isoforms (alpha, beta1/beta2, gamma, delta, epsilon, zeta, eta, iota, lambda, and mu) were found to be expressed in the rabbit heart. Quantitative immunoblotting demonstrated that as a subgroup, conventional PKCs (cPKCs) are more abundant than novel PKCs (nPKCs) (1445 versus 313 pg PKC/microg tissue protein, respectively) and that PKC alpha is the predominant isoform among the cPKCs (alpha, beta1, beta2, and gamma), representing 51% of this subgroup, and PKC epsilon is the most abundant among the nPKCs (delta, epsilon, zeta, and eta), accounting for 62% of this subgroup. None of the ischemic PC protocols examined caused appreciable changes in total PKC activity, in the subcellular distribution of total PKC activity, or in the subcellular distribution of PKC isoforms alpha, beta1/beta2, gamma, delta, zeta, iota, lambda, and mu. In contrast, all PC protocols caused significant translocation of PKC epsilon and PKC eta isoforms from the cytosolic to the particulate fraction. The particulate fraction of PKC epsilon increased in a dose-dependent fashion with the number of occlusion/reperfusion cycles performed, from 35+/-2% in the control group to 43+/-2% after one 4'O/5-minute reperfusion (5'R) cycle (P<.05), 52+/-2% after three cycles (P<.05 versus one cycle), and 66+/-3% after six cycles (P<.05 versus three cycles). The particulate fraction of PKC epsilon also increased, after four 5'O/10'R cycles, to 50+/-3% (P<.05 versus control). In contrast to PKC epsilon, the translocation of PKC eta was independent of the number of occlusion/reperfusion cycles performed. The particulate fraction of PKC eta increased from 67+/-3% in the control group to 84+/-2% after one 4'O/5'R cycle (P<.05), 84+/-2% after three 4'O/4'R cycles (P<.05), 86+/-3% after six 4'O/4'R cycles (P<.05), and 83+/-2% after four 5'O/10'R cycles (P<.05). When expressed as a percentage of control values, the increases in the particulate fraction of isoform epsilon were greater than those of isoform eta. The effects of 4'O without reperfusion were similar to those of one cycle of 4'O/5'R, indicating that 5'R did not attenuate isoform translocation. This is the first study to demonstrate PKC translocation after ischemic PC in vivo. The results indicate that in the conscious rabbit, ischemic PC causes selective translocation of the epsilon and eta isoforms without demonstrable changes in total myocardial PKC activity, implying that measurements of total PKC activity are not sufficiently sensitive to detect the involvement of PKC in PC. The results are consistent with the concept that the epsilon and eta isozymes play an important role in the genesis of ischemic PC in the conscious rabbit.

Involvement of phosphatidylcholine-specific phospholipase C in platelet-derived growth factor-induced activation of the mitogen-activated protein kinase pathway in Rat-1 fibroblasts

The role of phosphatidylcholine (PC) hydrolysis in activation of the mitogen-activated protein kinase (MAPK) pathway by platelet-derived growth factor (PDGF) was studied in Rat-1 fibroblasts. PDGF induced the transient formation of phosphatidic acid, choline, diacylglycerol (DG), and phosphocholine, the respective products of phospholipase D (PLD) and phospholipase C (PC-PLC) activity, with peak levels at 5-10 min. PLD-catalyzed transphosphatidylation (with n-butyl alcohol) diminished DG formation at 5 min but not at later stages of PDGF stimulation. Phorbol ester-induced down-regulation of protein kinase C (PKC) completely blocked PLD activation but not the formation of DG and phosphocholine at 10 min of PDGF stimulation. Collectively, these data indicate that PDGF activates both PLD and PC-PLC. In contrast, epidermal growth factor did not activate PC-PLC in these cells, and it activated PLD only weakly. DG formation by itself, through Bacillus cereus PC-PLC treatment of cells, was sufficient to mimic PDGF in activation of MAPK independent of phorbol ester-sensitive PKC. Since PKC down-regulation blocked PDGF-induced PLD but not MAPK activation, we conclude that PLD is not involved in MAPK signaling. In contrast, MAPK activation by exogenous (bacterial) PLD was not affected by PKC down-regulation, indicating that signals evoked by exogenous PLD differ from endogenous PLD. D609 (2-10 microg/ml), an inhibitor of PC-PLC, blocked PDGF- but not epidermal growth factor-induced MAPK activation. However, D609 should be used with caution since it also affects PLD activity. The results suggest that PC-PLC rather than PLD plays a critical role in the PDGF-activated MAPK pathway.

Protein kinase C isoenzymes in rat and human cardiovascular tissues

1. We have compared the expression of protein kinase C (PKC) activity and immuno-detectable isoenzymes in cytosolic and membrane extracts of rat and human cardiovascular tissues (heart, kidney, aorta, saphenous vein). Experiments were performed in raw extracts and upon combined diethylaminoethylcellulose (DEAE) and phenylsepharose column chromatography. 2. PKC activity that bound to DEAE mostly eluted with 200 mM NaCl. DEAE-purified PKC from all tissues except rat kidney bound almost quantitatively to phenylsepharose and eluted with 0.5-0 M NaCl. 3. Immunoblots with an antibody against classical PKCs and the activator profile for phosphatidylserine, diolein and Ca2+ revealed that the PKC from rat kidney, which did not bind to phenylsepharose, was most probably due to a proteolytically-generated, constitutively active PKC which is not under the control of a regulatory subunit. 4. Studies in the reference tissue, rat brain, demonstrated that all PKC isoenzymes investigated (classical PKCs alpha, beta, gamma, new PKCs delta, epsilon, eta, theta, and atypical PKCs zeta, lambda, iota) have similar DEAE and phenylsepharose chromatography elution profiles. In the functional assay an inhibitor of all known PKC isoenzymes, bisindolylmaleimide, and a specific inhibitor of classical PKCs, Gö 6976, both inhibited PKC from rat brain completely and with high potency indicating that the functional assay preferentially detects classical PKC isoenzymes. 5. Each PKC isoenzyme had a tissue-specific expression profile which was similar in rat and man. The classical PKC alpha, the new PKCs delta and epsilon and all atypical PKCs were detectable in most tissues, whereas the PKC beta and PKC gamma were not detected in any pheripheral tissue; PKC eta and PKC theta were found in some tissues. 6. We conclude that combined DEAE and phenylsepharose chromatography is useful to enrich and detect PKC isoenzymes; no major species differences in tissues-specific expression patterns appear to exist between rat and man.

The co-mitogenic combination of transforming growth factor beta 1 and bombesin protects protein kinase C-delta from late-phase down-regulation, despite synergy in diacylglycerol accumulation

Bombesin induces the down-regulation of protein kinase C-delta (PKC-delta) and PKC-epsilon in Swiss 3T3 cells. Simultaneous addition of transforming growth factor beta 1 (TGF beta 1) selectively blocks PKC-delta down-regulation at mid-S-phase, whereas PKC-epsilon levels continue to decline. Northern blot analysis shows that PKC-epsilon levels could be controlled in part at the level of transcription; PKC-delta mRNA levels remained constant at these later times. Bombesin induces a sustained elevation of some species of diacylglycerol (DAG), consistent with the observed loss of PKC-delta and PKC-epsilon. Interestingly, the combination of bombesin and TGF-beta 1 produces an even greater DAG response. Flow cytometric analysis demonstrates that bombesin induces only 15% of the cells to enter the cell cycle, in contrast to the combination of TGF beta 1 plus bombesin which induces 75-80% of the cells to progress through the cycle. The protection of PKC-delta from down-regulation under conditions of sustained DAG elevation correlates with the mitogenic response and implies that the down-regulation process itself is regulated. Consistent with this, it is demonstrated that bombesin plus TGF beta 1 protects PKC-delta from phorbol ester-induced down-regulation.

Calcium-independent binding to interfacial phorbol esters causes protein kinase C to associate with membranes in the absence of acidic lipids

The mechanism of interaction of phorbol esters with conventional protein kinase Cs was addressed by examining the direct binding of this class of activators to protein kinase C beta II. Binding measurements reveal that the major role of phorbol esters is to increase the affinity of protein kinase C for membranes by several orders of magnitude. The relative increase depends linearly on the mole fraction of phorbol esters in membranes, with the potency illustrated by the finding that 1 mol% phorbol 12-myristate 13-acetate (PMA) increases protein kinase C's membrane association by approximately 4 orders of magnitude. For comparison, diacylglycerol (DG), which also activates protein kinase C by increasing the enzyme's membrane affinity, is 2 orders of magnitude less effective than PMA in altering protein kinase C's membrane affinity. The remarkably high-affinity interaction with phorbol esters allowed us to measure the direct binding of protein kinase C to PMA in neutral membranes and, thus, to evaluate the effect of Ca2+ on the phorbol ester interaction in the absence of Ca2+ effects on the enzyme's interaction with acidic lipids. Changing the Ca2+ concentration over 5 orders of magnitude had no effect on the direct interaction of protein kinase C with PMA immobilized in phosphatidylcholine membranes. Thus, the Ca(2+)-binding site for membrane association and the phorbol ester-binding site do not interact allosterically. Lastly, a method that does not have the limitations of the Scatchard plot for analysis of amphitropic proteins was used to determine the dissociation constant of protein kinase C from phorbol esters: expressed relative to membrane lipids, the dissociation constant is 1.5 x 10(-5) mol %. In summary, our data reveal that (1) the direct binding of protein kinase C to phorbol esters, in the absence of interactions with acidic lipids, provides a major contribution to the free energy change involved in the association of protein kinase C with membranes and (2) this interaction is not regulated by Ca2+.

Protein kinase C epsilon subcellular localization domains and proteolytic degradation sites. A model for protein kinase C conformational changes

Protein kinase C (PCK) epsilon has been found to have unique properties among the PCK isozymes in terms of its membrane association, oncogenic potential, and substrate specificity. Recently we have demonstrated that PKC epsilon localizes to the Golgi network via its zinc finger domain and that both the holoenzyme and its zinc finger region modulate Golgi function. To further characterize the relationship between the domain organization and the subcellular localization of PKC epsilon, a series of NIH 3T3 cell lines were created, each overexpressing a different truncated version of PKC epsilon. The overexpressed proteins each were designed to contain an epsilon-epitope tag peptide at the COOH terminus to allow ready detection with an antibody specific for the tag. The subcellular localization of the recombinant proteins was analyzed by in vivo phorbol ester binding, immunocytochemistry, and cell fractionation followed by immunoblotting. Results revealed several regions of PKC epsilon that contain putative subcellular localization signals. The presence either of the hinge region or of a 33-amino-acid region including the pseudosubstrate sequence in the recombinant proteins resulted in association with the plasma membrane and cytoskeletal components. The catalytic domain was found predominantly in the cytosolic fraction. The accessibility and thus the dominance of these localization signals is likely to be affected by the overall conformation of the recombinant proteins. Regions with putative proteolytic degradation sites also were identified. The susceptibility of the overexpressed proteins to proteolytic degradation was dependent on the protein conformation. Based on these observations, a model depicting the interaction and hierarchy of the suspected localization signals and proteolytic degradation sites is presented.