Protein kinase C--a family affair

The molecular structure and function of fibrocystin, the key gene product implicated in autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease is an early onset inherited hepatorenal disorder affecting around 1 in 20,000 births with no approved specific therapies. The disease is almost always caused by variations in the polycystic kidney and hepatic disease 1 gene, which encodes fibrocystin (FC), a very large, single-pass transmembrane glycoprotein found in primary cilia, urine and urinary exosomes. By comparison to proteins involved in autosomal dominant PKD, our structural and molecular understanding of FC has lagged far behind such that there are no published experimentally determined structures of any part of the protein. Bioinformatics analyses predict that the ectodomain contains a long chain of immunoglobulin-like plexin-transcription factor domains, a protective antigen 14 domain, a tandem G8-TMEM2 homology region and a sperm protein, enterokinase and agrin domain. Here we review current knowledge on the molecular function of the protein from a structural perspective.

Nociceptor beta II, delta, and epsilon isoforms of PKC differentially mediate paclitaxel-induced spontaneous and evoked pain

As one of the most effective and frequently used chemotherapeutic agents, paclitaxel produces peripheral neuropathy (paclitaxel-induced peripheral neuropathy or PIPN) that negatively affects chemotherapy and persists after cancer therapy. The mechanisms underlying this dose-limiting side effect remain to be fully elucidated. This study aimed to investigate the role of nociceptor protein kinase C (PKC) isoforms in PIPN. Employing multiple complementary approaches, we have identified a subset of PKC isoforms, namely βII, δ, and ϵ, were activated by paclitaxel in the isolated primary afferent sensory neurons. Persistent activation of PKCβII, PKCδ, and PKCϵ was also observed in the dorsal root ganglion neurons after chronic treatment with paclitaxel in a mouse model of PIPN. Isoform-selective inhibitors of PKCβII, PKCδ, and PKCϵ given intrathecally dose-dependently attenuated paclitaxel-induced mechanical allodynia and heat hyperalgesia. Surprisingly, spinal inhibition of PKCβII and PKCδ, but not PKCϵ, blocked the spontaneous pain induced by paclitaxel. These data suggest that a subset of nociceptor PKC isoforms differentially contribute to spontaneous and evoked pain in PIPN, although it is not clear whether PKCϵ in other regions regulates spontaneous pain in PIPN. The findings can potentially offer new selective targets for pharmacological intervention of PIPN.

Regulation of PKC mediated signaling by calcium during visceral leishmaniasis

Calcium is an ubiquitous cellular signaling molecule that controls a variety of cellular processes and is strictly maintained in the cellular compartments by the coordination of various Ca²⁺ pumps and channels. Two such fundamental calcium pumps are plasma membrane calcium ATPase (PMCA) and Sarco/endoplasmic reticulum calcium ATPase (SERCA) which play a pivotal role in maintaining intracellular calcium homeostasis. This intracellular Ca²⁺ homeostasis is often disturbed by the protozoan parasite Leishmania donovani, the causative organism of visceral leishmaniasis. In the present study we have dileneated the involvement of PMCA4 and SERCA3 during leishmaniasis. We have observed that during leishmaniasis, intracellular Ca²⁺ concentration was up-regulated and was further controlled by both PMCA4 and SERCA3. Inhibition of these two Ca²⁺-ATPases resulted in decreased parasite burden within the host macrophages due to enhanced intracellular Ca²⁺. Contrastingly, on the other hand, activation of PMCA4 was found to enhance the parasite burden. Our findings also highlighted the importance of Ca²⁺ in the modulation of cytokine balance during leishmaniasis. These results thus cumulatively suggests that these two Ca²⁺-ATPases play prominent roles during visceral leishmaniasis.

Protein kinase C (PKC) activity regulates functional effects of Kvβ1.3 subunit on KV1.5 channels: identification of a cardiac Kv1.5 channelosome

K(v)1.5 channels are the primary channels contributing to the ultrarapid outward potassium current (I(Kur)). The regulatory K(v)β1.3 subunit converts K(v)1.5 channels from delayed rectifiers with a modest degree of slow inactivation to channels with both fast and slow inactivation components. Previous studies have shown that inhibition of PKC with calphostin C abolishes the fast inactivation induced by K(v)β1.3. In this study, we investigated the mechanisms underlying this phenomenon using electrophysiological, biochemical, and confocal microscopy approaches. To achieve this, we used HEK293 cells (which lack K(v)β subunits) transiently cotransfected with K(v)1.5+K(v)β1.3 and also rat ventricular and atrial tissue to study native α-β subunit interactions. Immunocytochemistry assays demonstrated that these channel subunits colocalize in control conditions and after calphostin C treatment. Moreover, coimmunoprecipitation studies showed that K(v)1.5 and K(v)β1.3 remain associated after PKC inhibition. After knocking down all PKC isoforms by siRNA or inhibiting PKC with calphostin C, K(v)β1.3-induced fast inactivation at +60 mV was abolished. However, depolarization to +100 mV revealed K(v)β1.3-induced inactivation, indicating that PKC inhibition causes a dramatic positive shift of the inactivation curve. Our results demonstrate that calphostin C-mediated abolishment of fast inactivation is not due to the dissociation of K(v)1.5 and K(v)β1.3. Finally, immunoprecipitation and immunocytochemistry experiments revealed an association between K(v)1.5, K(v)β1.3, the receptor for activated C kinase (RACK1), PKCβI, PKCβII, and PKCθ in HEK293 cells. A very similar K(v)1.5 channelosome was found in rat ventricular tissue but not in atrial tissue.

Leukotrienes antagonize the antiproliferative effect of Larrea divaricata Cav. on a lymphoma cell line interfering with cAMP intracellular level and PKC activity

Larrea divaricata exerts a dual effect on the proliferation of BW5147 cells. The aim of this work was to analyze the effect of leukotrienes on the antiproliferative action of the extract in relation to cyclic adenosine monophosphate production and protein kinase C activation. The extracts induced cyclic adenosine monophosphate and inhibited protein kinase C but induced leukotriene B4 production, which exerted contrary effects. These results suggest the finding of an extract free of compounds, which can increase leukotriene B4 level, in order to obtain a more active extract to be used in the treatment of lymphomas, positively modulated by leukotrienes.

Protein kinase C in the human heart: differential regulation of the isoforms in aortic stenosis or dilated cardiomyopathy

OBJECTIVES

Protein kinase C (PKC) is a central enzyme in the regulation of growth and hypertrophy. Little was known on PKC isoform regulation in human heart. Goal of this study was to characterize the isoforms of protein kinase C in human heart, their changes during ontogenesis, and their regulation in myocardial hypertrophy and heart failure.

METHODS

In left ventricular and atrial samples from adults with end-stage dilated cardiomyopathy (DCM), from adults with severe aortic stenosis (AS), from small infants undergoing repair of ventricular septal defects, and from healthy organ donors (CO), activity of protein kinase C and the expression of its isozymes were examined.

RESULTS

In the adult human heart, the isoforms PKC-alpha, PCK-beta, PKC-delta, PKC-epsilon, PKC-lambda/-iota, and PKC-zeta were detected both on protein and on mRNA level. All isozymes are subjected to downregulation during ontogenesis. No evidence, however, exists for an isoform shift from infancy to adulthood. DCM leads to a pronounced upregulation of PKC-beta. Severe left ventricular hypertrophy in AS, however, recruits a distinct isoform pattern, i.e., isoforms PKC-alpha, PKC-delta, PKC-epsilon, PKC-lambda/-iota, and PKC-zeta are upregulated, whereas PKC-beta is not changed under this condition.

CONCLUSION

This work gives evidence for a differential recruitment of human PKC isoforms in various forms of myocardial hypertrophy and heart failure.

Differential effect of bryostatin 1 and phorbol 12-myristate 13-acetate on HOP-92 cell proliferation is mediated by down-regulation of protein kinase Cdelta

Bryostatin 1 is currently in clinical trials as a cancer chemotherapeutic agent. Although bryostatin 1, like phorbol 12-myristate 13-acetate (PMA), is a potent activator of protein kinase C (PKC), it induces only a subset of those responses induced by PMA and antagonizes others. We report that, in the HOP-92 non-small cell lung cancer line, bryostatin 1 induced a biphasic proliferative response, with maximal proliferation at 1 to 10 nmol/L. This biphasic response mirrored a biphasic suppression of the level of PKCdelta protein, with maximal suppression likewise at 1 to 10 nmol/L bryostatin 1. The typical phorbol ester PMA, in contrast to bryostatin 1, had no effect on the level of PKCdelta and modest suppression of cell proliferation, particularly evident at later treatment times. Flow cytometric analysis revealed changes in the fraction of cells in the G0-G1 and S phases corresponding to the effects on proliferation. Cells overexpressing PKCdelta exhibited a lower rate of cell proliferation compared with control untreated cells and showed neither a proliferative response nor a loss of PKCdelta in response to bryostatin 1. Conversely, treatment with PKCdelta small interfering RNA significantly increased the cellular growth compared with controls. We conclude that the differential effect on cellular proliferation induced by bryostatin 1 compared with PMA reflects the differential suppression of PKCdelta.

Bile acids stimulate PKCalpha autophosphorylation and activation: role in the attenuation of prostaglandin E1-induced cAMP production in human dermal fibroblasts

The aim was to identify the specific PKC isoform(s) and their mechanism of activation responsible for the modulation of cAMP production by bile acids in human dermal fibroblasts. Stimulation of fibroblasts with 25-100 microM of chenodeoxycholic acid (CDCA) and ursodeoxycholic acid (UDCA) led to YFP-PKCalpha and YFP-PKCdelta translocation in 30-60 min followed by a transient 24- to 48-h downregulation of the total PKCalpha, PKCdelta, and PKCepsilon protein expression by 30-50%, without affecting that of PKCzeta. Increased plasma membrane translocation of PKCalpha was associated with an increased PKCalpha phosphorylation, whereas increased PKCdelta translocation to the perinuclear domain was associated with an increased accumulation of phospho-PKCdelta Thr505 and Tyr311 in the nucleus. The PKCalpha specificity on the attenuation of cAMP production by CDCA was demonstrated with PKC downregulation or inhibition, as well as PKC isoform dominant-negative mutants. Under these same conditions, neither phosphatidylinositol 3-kinase, p38 MAP kinase, p42/44 MAP kinase, nor PKA inhibitors had any significant effect on the CDCA-induced cAMP production attenuation. CDCA concentrations as low as 10 microM stimulated PKCalpha autophosphorylation in vitro. This bile acid effect required phosphatidylserine and was completely abolished by the presence of Gö6976. CDCA at concentrations less than 50 microM enhanced the PKCalpha activation induced by PMA, whereas greater CDCA concentrations reduced the PMA-induced PKCalpha activation. CDCA alone did not affect PKCalpha activity in vitro. In conclusion, although CDCA and UDCA activate different PKC isoforms, PKCalpha plays a major role in the bile acid-induced inhibition of cAMP synthesis in fibroblasts. This study emphasizes potential consequences of increased systemic bile acid concentrations and cellular bile acid accumulation in extrahepatic tissues during cholestatic liver diseases.

Phorbol ester phorbol-12-myristate-13-acetate promotes anchorage-independent growth and survival of melanomas through MEK-independent activation of ERK1/2

The phorbol ester, phorbol-12-myristate-13-acetate (PMA), an activator of PKCs, is known to stimulate the in vitro growth of monolayer cultures of normal human melanocytes whereas it inhibits the growth of most malignant melanoma cell lines. We examined the effect of PMA on proliferation and survival of melanoma cells grown as multicellular aggregates in suspension (spheroids), and aimed to elucidate downstream targets of PKC signaling. In contrast to monolayer cultures, PMA increased cell proliferation as well as protected melanoma cells from suspension-mediated apoptosis (anoikis). Supporting the importance of PKC in anchorage-independent growth, treatment of anoikis-resistant melanoma cell lines with antisense oligonucleotides against PKC-alpha, or the PKC inhibitor Gö6976, strongly induced anoikis. PMA induced activation of ERK1/2, but this effect was not prevented by the MEK inhibitors PD98059 or by U0126. Whereas PD98059 treatment alone led to marked activation of the pro-apoptotic Bim and Bad proteins and significantly increased anoikis, these effects were clearly reversed by PMA. In conclusion, our results indicate that the protective effect of PMA on anchorage-independent survival of melanoma cells at least partly is mediated by MEK-independent activation of ERK1/2 and inactivation of downstream pro-apoptotic effector proteins.

The V5 Domain of Protein Kinase C Plays a Critical Role in Determining the Isoform-Specific Localization, Translocation, and Biological Function of Protein Kinase C-δ and -ε

The catalytic domain of overexpressed protein kinase C (PKC)-δ mediates phorbol 12-myristate 13-acetate (PMA)-induced differentiation or apoptosis in appropriate model cell lines. To define the portions of the catalytic domain that are critical for these isozyme-specific functions, we constructed reciprocal chimeras, PKC-δ/εV5 and -ε/δV5, by swapping the V5 domains of PKC-δ and -ε. PKC-δ/εV5 failed to mediate PMA-induced differentiation of 32D cells, showing the essential nature of the V5 domain for PKC-δ's functionality. The other chimera, PKC-ε/δV5, endowed inactive PKC-ε with nearly all PKC-δ's apoptotic ability, confirming the importance of PKC-δ in this function. Green fluorescent protein (GFP)-tagged PKC-δV5 and -ε/δV5 in A7r5 cells showed substantial basal nuclear localization, while GFP-tagged PKC-ε and -δ/εV5 showed significantly less, indicating that the V5 region of PKC-δ contains determinants critical to its nuclear distribution. PKC-ε/δV5-GFP showed much slower kinetics of translocation to membranes in response to PMA than parental PKC-ε, implicating the PKC-εV5 domain in membrane targeting. Thus, the V5 domain is critical in several of the isozyme-specific functions of PKC-δ and -ε.

Atypical protein kinase C plays a critical role in protein transport from pre-Golgi intermediates

The small GTPase Rab2 requires atypical protein kinase C iota/lambda (PKCiota/lambda) kinase activity to promote vesicle budding from normal rat kidney cell microsomes (Tisdale, E. J. (2000) Traffic 1, 702-712). The released vesicles lack anterograde-directed cargo but contain coat protein I (COPI) and the recycling protein p53/p58, suggesting that the vesicles traffic in the retrograde pathway. In this study, we have directly characterized the role of PKCiota/lambda in the early secretory pathway. A peptide corresponding to the unique PKCiota/lambda pseudosubstrate domain was introduced into an in vitro assay that efficiently reconstitutes transport of vesicular stomatitis virus glycoprotein from the endoplasmic reticulum to the cis-medial Golgi compartments. This peptide blocked transport in a dose-dependent manner. Moreover, normal rat kidney cells incubated with Rab2 and the pseudosubstrate peptide displayed abundant swollen or dilated vesicles that contained Rab2, PKCiota/lambda, beta-COP, and p53/p58. Because Rab2, beta-COP, and p53/p58 are marker proteins for pre-Golgi intermediates (vesicular tubular clusters,VTCs), most probably the swollen vesicles are derived from VTCs. Similar results were obtained when the assays were supplemented with kinase-dead PKCiota/lambda (W274K). Both the pseudosubstrate peptide and kinase-dead PKCiota/lambda in tandem with Rab2 caused sustained membrane association of PKCiota/lambda, suggesting that reverse translocation was inhibited. Importantly, the inhibitory phenotype of kinase-dead PKCiota/lambda was reversed by PKCiota/lambda wild type. These combined results indicate that PKCiota/lambda is essential for protein transport in the early secretory pathway and suggest that PKCiota/lambda kinase activity is required to promote Rab2-mediated vesicle budding at a VTC subcompartment enriched in recycling cargo.

Inhibition of protein kinase C catalytic activity by additional regions within the human protein kinase Calpha-regulatory domain lying outside of the pseudosubstrate sequence

The N-terminal pseudosubstrate site within the protein kinase Calpha (PKCalpha)-regulatory domain has long been regarded as the major determinant for autoinhibition of catalytic domain activity. Previously, we observed that the PKC-inhibitory capacity of the human PKCalpha-regulatory domain was only reduced partially on removal of the pseudosubstrate sequence [Parissenti, Kirwan, Kim, Colantonio and Schimmer (1998) J. Biol. Chem. 273, 8940-8945]. This finding suggested that one or more additional region(s) contributes to the inhibition of catalytic domain activity. To assess this hypothesis, we first examined the PKC-inhibitory capacity of a smaller fragment of the PKCalpha-regulatory domain consisting of the C1a, C1b and V2 regions [GST-Ralpha(39-177): this protein contained the full regulatory domain of human PKCalpha fused to glutathione S-transferase (GST), but lacked amino acids 1-38 (including the pseudosubstrate sequence) and amino acids 178-270 (including the C2 region)]. GST-Ralpha(39-177) significantly inhibited PKC in a phorbol-independent manner and could not bind the peptide substrate used in our assays. These results suggested that a region within C1/V2 directly inhibits catalytic domain activity. Providing further in vivo support for this hypothesis, we found that expression of N-terminally truncated pseudosubstrate-less bovine PKCalpha holoenzymes in yeast was capable of inhibiting cell growth in a phorbol-dependent manner. This suggested that additional autoinhibitory force(s) remained within the truncated holoenzymes that could be relieved by phorbol ester. Using tandem PCR-mediated mutagenesis, we observed that mutation of amino acids 33-86 within GST-Ralpha(39-177) dramatically reduced its PKC-inhibitory capacity when protamine was used as substrate. Mutagenesis of a broad range of sequences within C2 (amino acids 159-242) also significantly reduced PKC-inhibitory capacity. Taken together, these observations support strongly the existence of multiple regions within the PKCalpha-regulatory domain that play a direct role in the inhibition of catalytic domain activity.

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.

Inhibition of histone acetyltransferase function of p300 by PKCdelta

Protein kinase Cdelta (PKCdelta) is one of the functionally distinct isoforms in PKC family. p300 is a histone acetyltransferase/transcription coactivator. They share certain properties, such as ubiquitous expression, growth and tumor suppression, and ability to enhance differentiation and apoptosis. In this study, we found that PKCdelta but not classical PKC, specifically phosphorylates p300 at serine 89 in vitro and in vivo. This phosphorylation causes inhibition of p300 intrinsic HAT activity. Subsequently, the targeted acetylation of nucleosomal histones is markedly reduced, which causes repression of p300 transcription coactivator function. These findings identify a new signal transduction pathway by which PKCdelta may inhibit cell growth and promote cellular differentiation.

Dopamine activates ERKs in alveolar epithelial cells via Ras-PKC-dependent and Grb2/Sos-independent mechanisms

Recently it has been described that dopamine (DA), via dopaminergic type 2 receptors (D(2)R), activates the mitogen-activated protein kinase extracellular signal-regulated kinase (MAPK/ERK) proteins in alveolar epithelial cells (AEC), which results in the upregulation of Na(+)-K(+)-ATPase. In the present report, we used AEC to investigate the signaling pathway that links DA with ERK activation. Incubation of AEC with DA resulted in rapid and transient stimulation of ERK activity, which was mediated by Ras proteins and the serine/threonine kinase Raf-1. Pretreatment of AEC with Src homology 3 binding peptide, which blocks the interaction between Grb2 and Sos, did not prevent DA activation of ERK. Diacylglycerol (DAG)-dependent protein kinase C (PKC) isoenzymes, involved in the DA-mediated activation of ERK proteins as pretreatment with either bisindolylmaleimide or Ro-31-8220, prevented the phosphorylation of Elk-1, and quinpirole, a D(2)R activator, stimulates the translocation of PKCepsilon. Together, the data suggest that DA activated MAPK/ERK via Ras, Raf-1 kinase, and DAG-dependent PKC isoenzymes, but, importantly and contrary to the classical model, this pathway did not involve the Grb2-Sos complex formation.

Molecular dynamics characterization of the C2 domain of protein kinase Cbeta

Protein kinase C (PKC) isozymes comprise a family of related enzymes that play a central role in many intracellular eukaryotic signaling events. Isozyme specificity is mediated by association of each PKC isozyme with specific anchoring proteins, termed RACKs. The C2 domain of betaPKC contains at least part of the RACK-binding sites. Because the C2 domain contains also a RACK-like sequence (termed pseudo-RACK), it was proposed that this pseudo-RACK site mediates intramolecular interaction with one of the RACK-binding sites in the C2 domain itself, stabilizing the inactive conformation of betaPKC. BetaPKC depends on calcium for its activation, and the C2 domain contains the calcium-binding sites. The x-ray structure of the C2 domain of betaPKC shows that three Ca(2+) ions can be coordinated by two opposing loops at one end of the domain. Starting from this x-ray structure, we have performed molecular dynamics (MD) calculations on the C2 domain of betaPKC bound to three Ca(2+) ions, to two Ca(2+) ions, and in the Ca(2+)-free state, in order to analyze the effect of calcium on the RACK-binding sites and the pseudo-RACK sites, as well as on the loops that constitute the binding site for the Ca(2+) ions. The results show that calcium stabilizes the beta-sandwich structure of the C2 domain and thus affects two of the three RACK-binding sites within the C2 domain. Also, the interactions between the third RACK-binding site and the pseudo-RACK site are not notably modified by the removal of Ca(2+) ions. On that basis, we predict that the pseudo-RACK site within the C2 domain masks a RACK-binding site in another domain of betaPKC, possibly the V5 domain. Finally, the MD modeling shows that two Ca(2+) ions are able to interact with two molecules of O-phospho-l-serine. These data suggest that Ca(2+) ions may be directly involved in PKC binding to phosphatidylserine, an acidic lipid located exclusively on the cytoplasmic face of membranes, that is required for PKC activation.

p32 (gC1qBP) is a general protein kinase C (PKC)-binding protein; interaction and cellular localization of P32-PKC complexes in ray hepatocytes

The aim of this study was to identify cellular proteins that bind protein kinase C (PKC) and may influence its activity and its localization. A 32-kDa PKC-binding protein was purified to homogeneity from the Triton X-100-insoluble fraction obtained from hepatocytes homogenates. The protein was identified by NH(2)-terminal amino acid sequencing as the previously described mature form of p32 (gC1qR). Recombinant p32 was expressed as a glutathione S-transferase fusion protein, affinity-purified, and tested for an in vitro interaction with PKC using an overlay assay approach. All PKC isoforms expressed in rat hepatocytes interacted in vitro with p32, but the binding dependence on PKC activators was different for each one. Whereas PKCdelta only binds to p32 in the presence of PKC activators, PKCzeta and PKCalpha increase their binding when they are in the activated form. Other PKC isoforms such as beta, epsilon, and theta bind equally well to p32 regardless of the presence of PKC activators, and PKCmu binds even better in their absence. It was also found that p32 is not a substrate for any of the PKC isoforms tested, but interestingly, its presence had a stimulatory effect (2-fold for PKCdelta) on PKC activity. We also observed in vivo interaction between PKC and p32 by immunofluorescence and confocal microscopy. A time course of phorbol ester treatment of cultured rat hepatocytes (C9 cells) showed that PKCtheta and p32 are constitutively associated in vivo, whereas PKCdelta activation is required for its association with p32. Our data also showed that phorbol ester treatment induces a transient translocation of p32 from the cytoplasm to the cell nucleus. Together, these findings suggest that p32 may be a regulator of PKC location and function.

Prostaglandin F2alpha-activated protein kinase Calpha phosphorylates myristoylated alanine-rich C kinase substrate protein in bovine luteal cells

Prostaglandin F2alpha (PGF2alpha)-induced secretion of oxytocin by the bovine corpus luteum involves the phosphorylation of a unique protein kinase C (PKC) substrate, myristoylated alanine-rich C kinase substrate (MARCKS) protein. This study was conducted to determine the specific PKC isoform engaged in phosphorylation of MARCKS protein in bovine luteal cells. In experiment 1, dispersed luteal cells recovered from the corpus luteum on d 8 of the estrous cycle were preincubated with [32P] orthophosphate and then exposed to PGF2alpha alone or in combination with PKC inhibitors. Autoradiography and densitometry of Western blots revealed that MARCKS protein was phosphorylated by a conventional PKC (cPKC) isoform. Experiment 2 was conducted to identify the specific cPKC isoform that phosphorylates MARCKS protein in luteal cells. Corpora lutea were removed from control and PGF2alpha-treated heifers on d 8 of the cycle, and PKC isoforms associated with membrane and cytosolic fractions were determined. Treatment with PGF2alpha increased membrane concentrations of PKCalpha within 5 min after treatment (p < 0.005). Collectively, these data suggest that phosphorylation of MARCKS protein coinciding with oxytocin secretion is mediated by PKCalpha.

Adaptor proteins in protein kinase C-mediated signal transduction

Spatial and temporal organization of signal transduction is essential in determining the speed and precision by which signaling events occur. Adaptor proteins are key to organizing signaling enzymes near their select substrates and away from others in order to optimize precision and speed of response. Here, we describe the role of adaptor proteins in determining the specific function of individual protein kinase C (PKC) isozymes. These isozyme-selective proteins were called collectively RACKs (receptors for activated C-kinase). The role of RACKs in PKC-mediated signaling was determined using isozyme-specific inhibitors and activators of the binding of each isozyme to its respective RACK. In addition to anchoring activated PKC isozymes, RACKs anchor other signaling enzymes. RACK1, the anchoring protein for activated betaIIPKC, binds for example, Src tyrosine kinase, integrin, and phosphodiesterase. RACK2, the epsilonPKC-specific RACK, is a coated-vesicle protein and thus is involved in vesicular release and cell-cell communication. Therefore, RACKs are not only adaptors for PKC, but also serve as adaptor proteins for several other signaling enzymes. Because at least some of the proteins that bind to RACKs, including PKC itself, regulate cell growth, modulating their interactions with RACKs may help elucidate signaling pathways leading to carcinogenesis and could result in the identification of novel therapeutic targets.

Mechanisms of action of flavopiridol

Flavopiridol inhibits phosphokinases. Its activity is strongest on cyclin dependent kinases (cdk-1, -2, -4, -6, -7) and less on receptor tyrosine kinases (EGFR), receptor associates tyrosine kinases (pp60 Src) and on signal transducing kinases (PKC and Erk-1). Although the inhibiting activity of flavopiridol is strongest for cdk, the cytotoxic activity of flavopiridol is not limited to cycling cells. Resting cells are also killed. This fact suggests that inhibition of cdks involved in the control of cell cycle is not the only mechanism of action. Inhibition of cdk's with additional functions (i.e. involved in the control of transcription or function of proteins that do not control cell cycle) may contribute to the antitumoral effect. Moreover, direct and indirect inhibition of receptor activation (EGFR) and/or a direct inhibition of kinases (pp60 Src, PKC, Erk-1) involved in the signal transduction pathway could play a role in the antiproliferative activity of flavopiridol. From pharmacokinetic data in patients it can be concluded that the inhibitory activity (IC50) of flavopiridol on these kinases is in the range of concentrations that might be achieved intracellularly after systemic application of non-toxic doses of flavopiridol. However, no in situ data from flavopiridol treated cells have been published yet that prove that by inhibition of EGFR, pp60 Src, PKC and/or Erk-1 (in addition to inhibition of cdk's) flavopiridol is able to induce apoptosis. Thus many questions regarding the detailed mechanism of antitumoral action of flavopiridol are still open. For the design of protocols for future clinical studies this review covers the essential information available on the mechanism of antitumoral activity of flavopiridol. The characteristics of this antitumoral activity include: High rate of apoptosis, especially in leukemic cells; synergy with the antitumoral activity of many cytostatics; independence of its efficacy on pRb, p53 and Bcl-2 expression; lack of interference with the most frequent multidrug resistance proteins (P-glycoprotein and MRP-190); and a strong antiangiogenic activity. Based on these pharmacological data it can be concluded that flavopiridol could be therapeutically active in tumor patients: independent on the genetic status of their tumors or leukemias (i.e. mutations of the pRb and/or p53, amplification of bcl-2); in spite of drug resistance of their tumors induced by first line treatment (and caused by enhanced expression of multidrug resistance proteins); in combination with conventional chemotherapeutics preferentially given prior to flavopiridol; and due to a complex mechanism involving cytotoxicity on cycling and on resting tumor cells, apoptosis and antiangiogenic activity. In consequence, flavopiridol is a highly attractive, new antitumoral compound and deserves further elucidation of its clinical potency.

Therapeutic RNA and DNA enzymes

Recent progress in understanding how gene products interact in the control of cell proliferation has engendered high hopes for the rational design of specific therapeutic strategies. The demonstration that certain RNA and DNA nucleic acids can enzymatically cleave mRNAs has offered the possibility of inactivating abnormal gene expression. In principle, this technology is applicable to any disease where a specific gene product can be linked to the initiation and/or perpetuation of the disease. Here, a brief description of the technology that can be useful for the design of therapeutic DNA and RNA agents capable of inducing apoptosis in cancer cells is presented. Furthermore, such agents can be a valuable tool for probing gene function.

Protein kinase C gene expression in dispersed guinea-pig gastric parietal cells

BACKGROUND AIMS

It has been implicated that protein kinase C (PKC) is involved in gastric acid secretion. The purpose of this study is to examine whether mRNA expression of PKC isoforms is observed in guinea-pig gastric parietal cells, and whether such PKC expression is regulated by agonists that stimulate gastric acid secretion.

METHODS

Expression of PKC mRNA was assessed using isolated guinea-pig gastric parietal cells treated with or without three kinds of agonists by Northern blot analysis.

RESULTS

(1) alpha, gamma and zetaPKC mRNAs were expressed in guinea-pig gastric parietal cells; (2) both carbachol and gastrin increased the level of alpha and gammaPKC mRNAs, but synthesis of zetaPKC mRNA was not affected by these agonists, and (3) histamine did not affect the expression level of alpha, gamma and zetaPKC mRNAs.

CONCLUSION

alpha and gammaPKC isoforms may be involved in the regulation of gastric acid secretion.

Rab2 requires PKC iota/lambda to recruit beta-COP for vesicle formation

The small GTPase Rab2 initiates the recruitment of soluble components necessary for protein sorting and recycling from pre-Golgi intermediates. Our previous studies showed that Rab2 required protein kinase C (PKC) or a PKC-like protein to recruit beta-COP to membrane (Tisdale EJ, Jackson M. Rab2 protein enhances coatomer recruitment to pre-Golgi intermediates. J Biol Chem 1998;273: 17269-17277). We investigated the role of PKC in Rab2 function by first determining the active isoform that associates with membranes used in our assay. Western blot analysis detected three isoforms: PKC alpha, gamma and iota/lambda. A quantitative binding assay was used to measure recruitment of these kinases when incubated with Rab2. Only PKC iota/lambda translocated to membrane in a dose-dependent manner. Microsomes treated with anti-PKC iota/lambda lost the ability to bind beta-COP, suggesting that Rab2 requires PKC iota/lambda for beta-COP recruitment. The recruitment of beta-COP to membranes is not regulated by PKC iota/lambda kinase activity. However, PKC iota/lambda activity was necessary for Rab2-mediated vesicle budding. We found that the addition of either a kinase-deficient PKC iota/lambda mutant or atypical PKC pseudosubstrate peptide to the binding assay drastically reduced vesicle formation. These data suggest that Rab2 causes translocation of PKC iota/lambda to vesicular tubular clusters (VTCs), which promotes the recruitment of COPI to generate retrograde-transport vesicles.

Cytoskeletal reorganization induced by retinoic acid treatment of human endometrial adenocarcinoma (RL95-2) cells is correlated with alterations in protein kinase C-alpha

We have shown previously that treatment of human endometrial adenocarcinoma (RL95-2) cells with either 13-cis or all-trans retinoic acid results in reorganization of actin filaments, indicating reversion to a stationary phenotype. In the present study, we investigated the role of protein kinase C (PKC) in this process. Treatment of cells with PKC inhibitors (staurosporine, bisindolylmaleimide, or G¿6976) resulted in morphological alterations and reorganization of actin filaments similar to retinoic-acid-treated cells. For example, RL95-2 cells treated with staurosporine flattened, exhibited cell surface extensions and some actin filaments. Bisindolylmaleimide-treated cells flattened, and actin filaments reorganized similar to retinoic-acid-treated cells. RL95-2 cells treated with G¿6976, which inhibits only PKC, alpha, beta and gamma, exhibited many cell surface extensions and some actin filament reorganization. We then investigated whether retinoic acid affected the subcellular localization of PKC-alpha. In control cells, PKC-alpha was mainly evident as diffuse cytoplasmic immunostaining, with a small percentage of total PKC-alpha also evident in the plasma membrane. Retinoic acid treatment dramatically altered PKC-alpha localization, since a more distinct cytoplasmic and perinuclear staining pattern was apparent. Western blot analysis confirmed these results, since the amount of cytosolic PKC-alpha increased following retinoic acid treatment. Thus, retinoic-acid-induced endometrial differentiation may be associated with alterations in PKC-alpha localization and signaling.

Protein kinase C-alpha and -beta play antagonistic roles in the differentiation process of THP-1 cells

The roles of protein kinase C (PKC) isoenzymes in the differentiation process of THP-1 cells are investigated. Inhibition of PKC by RO 31-8220 reduces the phagocytosis of latex particles and the release of superoxide, prostaglandin E(2) (PGE(2)), and tumour necrosis factor (TNF)-alpha. The proliferation of THP-1 cells is slightly enhanced by RO 31-8220. Stable transfection of THP-1 cells with asPKC-alpha, and incubation of THP-1 cells with antisense (as) PKC-alpha oligodeoxynucleotides reduces PKC-alpha levels and PKC activity. asPKC-alpha-transfected THP-1 cells show a decreased phagocytosis and a decreased release of superoxide, PGE(2) and TNF-alpha. The proliferation of asPKC-alpha-transfected THP-1 cells is enhanced. Stable transfection of THP-1 cells with asPKC-beta, and incubation of THP-1 cells with asPKC-beta oligodeoxynucleotides, reduces PKC-beta levels and PKC activity. asPKC-beta-transfected THP-1 cells show a decreased phagocytosis, a decreased TNF-alpha release, and a decreased proliferation. However, no difference is measured in the release of superoxide and PGE(2). These results suggest that: (1) PKC-alpha but not PKC-beta is involved in the release of superoxide and PGE(2); (2) TNF-alpha release and the phagocytosis of latex particles are mediated by PKC-alpha, PKC-beta, and other PKC isoenzymes; and (3) PKC-alpha and PKC-beta play antagonistic roles in the differentiation process of THP-1 cells. PKC-alpha promotes the differentiation process of THP-1 cells, PKC-beta retards the differentiation of THP-1 cells into macrophage-like cells.

Design of nuclease resistant protein kinase calpha DNA enzymes with potential therapeutic application

For the therapeutic application of catalytic nucleic acids it is desirable to have small, stable and inexpensive compounds that are active at physiological Mg(2+) concentrations. We have explored the possibility of using the versatile 10-23 DNA catalytic core to suppress the expression of the protein kinase Calpha (PKCalpha) isoform in malignant cells. By introducing either a 3'-3'-inverted thymidine nucleotide or site-specific phosphorothioate modification into a PKCalpha DNA enzyme, we have designed stable catalysts that retained a significant in vitro cleavage activity. In particular, a DNA enzyme containing phosphorothioate analogues in the antisense arms and in the pyrimidine residues of the catalytic core was found to be remarkably stable in 50 % human serum (t(1/2)>90 hours) and inhibited in vitro cell growth by up to 90 % at nanomolar concentrations. The inhibition of PKCalpha gene expression is sequence-specific, as a DNA enzyme with reversed antisense arms was found to be ineffective. Epifluorescence microscopic analysis of cells transfected with a 5' fluorescein isothiocyanate-conjugated DNA enzyme showed that the DNA enzyme molecules are mainly localised in the nuclei. Most of the DNA enzyme-treated cells were killed by apoptosis. The ability of the described PKCalpha DNA enzymes to trigger apoptosis (apoptozymes) in malignant cells illustrates their therapeutic potential. Furthermore, such agents can be a valuable tool for probing gene function.

Characterization of calreticulin as a protein interacting with protein kinase C

A protein kinase C (PKC)-binding protein was purified to homogeneity from the Triton-insoluble fraction from rat hepatocytes homogenates. The protein was identified as the mature calreticulin chain by N-terminal amino acid sequencing and by its immunoreactivity with anti-calreticulin antibody raised against the C-terminal KDEL (single-letter code) sequence. The calculated molecular mass was 46. 6 kDa but the protein migrates in SDS/PAGE as a doublet with apparent molecular masses of 60 and 55 kDa. Studies in vitro with purified calreticulin with the use of an overlay assay approach demonstrated that it binds to activated PKC isoenzymes expressed in rat hepatocytes. Phosphorylation of purified calreticulin with a PKC isoenzyme-specific immune complex kinase assay showed that it is also a very good substrate for all PKC isoforms in vitro. The treatment of intact cells with phorbol ester or with adrenaline (epinephrine) plus propranolol increased calreticulin phosphorylation, which was blocked by the pretreatment of cells with the PKC-specific inhibitor Ro 31-8220. The analysis of calreticulin immunoprecipitates from control or treated cells indicated that PKCalpha, PKCbeta, PKCtheta;, PKCzeta and PKCmu, but not PKCdelta or PKCepsilon, co-immunoprecipitated with calreticulin. Taken together, our results indicate that PKC interacts in vivo with calreticulin and suggest that they can operate in common signalling pathways.

Genomic structure and chromosomal localization of the rat protein kinase Cdelta-gene

Protein kinase Cdelta (PKCdelta) is a widely expressed calcium-independent PKC isozyme that is induced at mRNA and protein levels upon stimulation of different cellular pathways. We found the rat PKCdelta gene to consist of 19 exons and to span approximately 29 kb. The exon-intron junctions follow the GT/AG rule. The 5' untranslated region is nearly 12 kb in length, and the transcription initiation site is surrounded by CG-rich sequences. The 5' flanking region contains putative binding sites for activator protein 1 (AP-1), nuclear factor kappa B (NFkappaB), stimulatory protein-1 (Sp-1) and nerve growth factor induced-C (NGFI-C) transcription factors. The PKCdelta gene is localized at the rat chromosome 19p14. The cloned gene will help to elucidate the role of PKCdelta in growth, differentiation and death of mammalian cells.

Protein kinase C stimulates PtdIns-4,5-P2-phospholipase C activity

The tumour promoter, phorbol ester 12,13-dibutyrate (PDBu), acts on rectal palisadic epithelial cells and mimics the effects of neuroparsin, an antidiuretic neuronal hormone isolated from nervous lobes of the African locust corpora cardiaca. PDBu stimulated Ca2+-dependent phospholipase C (PLC) activity resulting in inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) production, increased cytosolic free calcium (monitored with the probe indo-1) and rectal fluid resorption. A 15-min pre-treatment with polymyxin B (PMXB), a protein kinase C (PKC) inhibitor acting at the phosphatidylserine (PS) binding site, suppressed PDBu stimulatory effects on free calcium entry and fluid resorption but not on phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P2) breakdown. On the contrary, bisindolylmaleimide Ro 32-0432 (which inhibits PKC at its ATP binding site) abolished entirely PDBu-stimulated PLC activity. It was concluded that two PKC are involved in transduction of the antidiuretic signal of neuroparsin. One PKC is PMXB sensitive and stimulates biological response after cytosolic free Ca2+ increase, while another PKC, insensitive to the PKC inhibitor, regulates the processes induced by the former PKC. Since PMXB-insensitive PKC exerts a stimulatory effect on PtdIns-4,5-P2-PLC production, this original mechanism may be considered as a new signalling pathway under control of PKC.

Ribozyme inhibition of the protein kinase C alpha triggers apoptosis in glioma cells

Although protein kinase C has been shown to be involved in a wide range of biological functions, the precise role of each isoform in a specific cell function remains to be clarified. Here we demonstrate that a ribozyme specific for the human protein kinase C alpha (PKC alpha), a classical PKC isoform, induces cell death in glioma cell lines. This cell death was identified as apoptosis by morphologic alterations and endonucleosomal DNA fragmentation. The inhibition of PKC alpha gene expression by the ribozyme resulted in a significant reduction in Bcl-xL gene expression, a protein that inhibits apoptosis and is overexpressed in glioma cells. Taken together, our data suggest that the PKC alpha ribozymes are a potent inducer of apoptosis in glioma cells, which may act through suppressing Bcl-xL gene expression and/or activity. PKC alpha ribozymes may prove useful in the management of malignant gliomas.

Telomerase is controlled by protein kinase Calpha in human breast cancer cells

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in human somatic tissues and becomes activated during tumor progression in most human cancers. To date, little is known about how telomerase is activated and controlled in cancer, although activation is thought to be involved in cancer cell immortalization. Here, we report that human telomerase-associated protein 1 (hTEP1) and the telomerase catalytic subunit (human telomerase reverse transcriptase (hTERT)) are phosphoproteins and that their phosphorylation is a prerequisite for the activation of telomerase in intact human breast cancer cells. Identified by hTEP1 peptide affinity chromatography, protein kinase Calpha mediates the phosphorylation of hTEP1 and hTERT and induces a marked increase in telomerase activity. Thus, phosphorylation of hTEP1 and hTERT by protein kinase Calpha represents an essential step in the generation of a functional telomerase complex in the initiation and maintenance of telomerase activity in human cancer.

The chirality of phosphatidylserine and the activation of protein kinase C

The properties of phosphatidyl-L-serine (L-PS) and phosphatidyl-D-serine (D-PS) were compared. The two forms of PS have similar but nonidentical L to L phase transition temperatures. Mixtures of phosphatidylserine with phosphatidylethanolamine and cholesterol (molar ratio 1:1:2) show polymorphic behavior at higher temperatures and in the presence of Ca2+. Mixtures with L-PS undergo conversion to nonlamellar phases at lower temperatures than do similar mixtures with D-PS. The aggregation of vesicles upon addition of histones is greater for L-PS than for D-PS. With fluorescence digital imaging microscopy we could show differences in the extent of formation of histone-induced domains enriched in PS or in diacylglycerol. The most enriched domains were induced with histone in membranes containing L-PS. The MARCKS peptide showed no differences in domain formation between L-PS and D-PS. The maximal activity of protein kinase C was greater in the presence of L-PS when histone, which could form more enriched domains, was the substrate. However, with a MARCKS peptide substrate, which formed domains of equal enrichment with L-PS and D-PS, the maximal activity of protein kinase C was the same with D-PS and with L-PS. These observations demonstrate that L-PS and D-PS have different physical properties. These differences likely contribute to the greater ability of L-PS to activate protein kinase C.

Staurosporine-induced versus spontaneous squamous metaplasia in pre- and postmenopausal breast tissue

Breast cancers from pre- vs. postmenopausal women display unique characteristics that may be related to differences in epithelial differentiation between these two populations. In addition to lobular development, lactational changes, and involution, breast epithelium can undergo metaplastic alterations, often in association with carcinoma. Because protein kinase C (PKC) regulates differentiation and proliferation in many cell types, we asked whether modulation of PKC activity could define biochemical differences in breast epithelium from pre- vs. postmenopausal women. Organ cultures of normal human breast were treated with PKC agonists and antagonists. Epithelial differentiation was evaluated based on morphologic criteria and the expression of cell-type specific proteins. Staurosporine, a nonspecific but extremely potent inhibitor of PKC, induced squamous metaplasia in eight of eight cases within 2 weeks of treatment. Other inhibitors of PKC, such as calphostin C and tamoxifen, had no effect on epithelial differentiation. Long-term treatment with phorbol esters also did not induce squamous metaplasia. However, stimulation of cAMP levels by forskolin and isobutyl-methyl-xanthene (IMX) rapidly induced squamous metaplasia, as has been previously reported. Surprisingly, squamous metaplasia occurred in 10 of 12 cultures derived from postmenopausal women in the absence of exogenous agents. Untreated cultures derived from premenopausal women never developed this type of epithelium (0 of 11). Therefore, breast epithelium from pre- and postmenopausal women responded differently to in vitro culture. Forskolin/IMX or staurosporine can reproduce these conditions, acting independent of menopausal status. Because staurosporine's action was unique among PKC inhibitors, staurosporine may induce squamous metaplasia of breast epithelium by a PKC-independent mechanism.

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.

In vitro stimulation of protein kinase C by melatonin

It has been shown that melatonin through binding to calmodulin acts both in vitro and in vivo as a potent calmodulin antagonist. It is known that calmodulin antagonists both bind to the hydrophobic domain of Ca2+ activated calmodulin, and inhibit protein kinase C activity. In this work we explored the effects of melatonin on Ca2+ dependent protein kinase C activity in vitro using both a pure commercial rat brain protein kinase C, and a partially purified enzyme from MDCK and N1E-115 cell homogenates. The results showed that melatonin directly activated protein kinase C with a half stimulatory concentration of 1 nM. In addition the hormone augmented by 30% the phorbol ester stimulated protein kinase C activity and increased [3H] PDBu binding to the kinase. In contrast, calmodulin antagonists (500 microM) and protein kinase C inhibitors (100 microM) abolished the enzyme activity. Melatonin analogs tested were ineffective in increasing either protein kinase C activity or [3H] PDBu binding. Moreover, the hormone stimulated protein kinase C autophosphorylation directly and in the presence of phorbol ester and phosphatidylserine. The results show that besides the melatonin binding to calmodulin, the hormone also interacts with protein kinase C only in the presence of Ca2+. They also suggest that the melatonin mechanism of action may involve interactions with other intracellular hydrophobic and Ca2+ dependent proteins.

Purification and isotype analysis of protein kinase C from rat liver nuclei

The properties and subtype composition of protein kinase C present in rat liver nuclei were studied in a Triton-X-100 extract of isolated purified nuclei. The enzyme activity was dependent on both Ca2+ and phosphatidylserine, but the phorbol ester 12-O-tetradecanoylphorbol 13-acetate gave only a partial stimulation. Both histone and myelin basic protein served as substrate. Purification of the Triton-X-100 extract followed by Q-Sepharose chromatography gave a preparation with a specific activity of 70 pmol/mg protein min. Western blotting of this preparation showed only the presence of the delta and zeta subtypes, but not the alpha-subtype, although the latter was present in rat liver homogenates. The beta, gamma and epsilon subtypes were not found in the homogenate nor in the nuclear extract. The specific activity of protein kinase C could be further increased up to 800 pmol/mg protein min after protamine agarose chromatography. Also in this preparation the presence of the delta and zeta subtypes could be established.

Protein kinase C mediates increase of Ca2+ sensitivity for contraction by cholinoceptor partial agonist in ileal longitudinal muscle of guinea pig

1. Experiments were designed to study the roles of protein kinase C in carbachol- and pilocarpine-induced contraction and the increase in cytosolic Ca2+ concentration ([Ca2+]i) in guinea pig ileal longitudinal muscle. 2. The protein kinase C inhibitors, GF 109203X (10 microM), calphostin C (10 microM) and H-7 (10 microM), reduced the maximum of the concentration response curve produced by pilocarpine more effectively than that produced by carbachol. 3. The slopes of the regression lines between [Ca2+]i and tension development for pilocarpine and carbachol in tissues treated with GF 109203X were significantly gentler than those for untreated tissues. 4. The protein kinase C alpha- and beta 1 selective inhibitor Goe 6976 (1 microM) decreased both [Ca2+]i and contraction, but did not affect the slopes of the regression lines for pilocarpine and carbachol. 5. These results suggest that protein kinase C (both n- and/or a-type) plays an important role in the increase of Ca2+ sensitivity of the contractile element, and that pilocarpine mainly activates the protein kinase C-dependent pathways for contractile mechanisms in guinea pig ileal longitudinal muscle.

Nucleolin is a protein kinase C-zeta substrate. Connection between cell surface signaling and nucleus in PC12 cells

We have previously shown that protein kinase C (PKC)-zeta is activated and required for nerve growth factor (NGF)-induced differentiation of rat pheochromocytoma PC12 cells (Wooten, M. W., Zhou, G., Seibenhener, M. L., and Coleman, E. S. (1994) Cell Growth & Diff. 5, 395-403; Coleman, E. S., and Wooten, M. W. (1994) J. Mol. Neurosci. 5, 39-57). Here we report the characterization and identification of a 106-kDa nuclear protein as a specific substrate of PKC-zeta. NGF treatment of PC12 cells resulted in translocation of PKC-zeta and coincident phosphorylation of a protein that was localized within the nucleoplasm of nuclei isolated from PC12 cells. Addition of PKC-zeta pseudosubstrate peptide in vitro or myristoylated peptide in vivo diminished phosphorylation of pp106 in a dose-dependent fashion. Likewise, addition of purified PKC-zeta, but neither PKC-alpha nor delta, to nuclear extracts resulted in an incremental increase in the phosphorylation of pp106. Expression of dominant-negative PKC-zeta inhibited NGF-induced phosphorylation of pp106, by comparison overexpression of PKC-zeta enhanced basal phosphorylation without a noticeable effect upon NGF-induced effects. Amino acid sequence analysis of four peptides derived from purified pp106 revealed that this protein was homologous to nucleolin. Using an in vitro reconstitution system, purified nucleolin was likewise shown to be phosphorylated by purified PKC-zeta. The staining intensity of both enzyme and substrate in the nucleus increased upon treatment with NGF. In vivo labeling with 32Pi and stimulation of PC12 cells with NGF followed by immunoprecipitation with anti-nucleolin antibody corroborated the in vitro approach documenting enhanced phosphorylation of nucleolin by NGF treatment. Taken together, the findings presented herein document that nucleolin is a target of PKC-zeta that serves to relay NGF signals from cell surface to nucleus in PC12 cells.

Bradykinin- and thrombin-induced increases in endothelial permeability occur independently of phospholipase C but require protein kinase C activation

We determined whether activation of phosphatidylinositol-specific phospholipase C (PI-PLC) and a subsequent increase in cytosolic calcium concentration ([Ca2+]i) was an obligatory signaling event mediating the increase in transendothelial permeability induced by bradykinin (BK) and alpha-thrombin (alpha-T). Both BK and alpha-T (each at a concentration range of 0.01-1 microM) caused dose-dependent increases in transendothelial 125I-albumin permeability in cultured bovine pulmonary artery endothelial cell monolayers. Both agonists also produced a rise in inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] by 10 sec that was followed by a prolonged increase in [Ca2+]i. Pretreatment of endothelial cells with the PLC inhibitor, 1-(6-((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1 H-pyrrole-2,5-dion [(U73122) at 10 microM for 15 min], prevented the increases in Ins(1,4,5)P3 and [Ca2+]i induced by both BK and alpha-T. However, inhibition of PLC with U73122 or another PLC inhibitor, neomycin, did not prevent the increase in endothelial permeability induced by either agonist. In contrast, depletion of cellular protein kinase C (PKC) with phorbol-12-myristate 13-acetate (0.01 microM for 20 hr) increased both BK- and alpha-T-induced phosphoinositide turnover but inhibited the agonist-induced increase in permeability. A PKC inhibitor, staurosporine (5 microM) likewise inhibited the BK-induced increase in endothelial cell permeability to albumin. We conclude that increases in endothelial permeability induced by the inflammatory mediators, BK and thrombin, can occur independently of PLC activation and increased [Ca2+]i but that a PKC-dependent pathway is required for the permeability response.

Activating amphiphiles cause a conformational change of the 1,2-diacylglycerol 3-glucosyltransferase from Acholeplasma laidlawii membranes according to proteolytic digestion

1,2-Diacylglycerol 3-glucosyltransferase synthesizes the major nonbilayer-prone lipid monoglucosyldiacylglycerol (MGlcDAG) in the membrane of Acholeplasma laidlawii, which is important for the spontaneous curvature, and is a regulatory site for the lipid surface charge density. A potential connection between activity and a conformational change of this enzyme, governed by essential lipid activators, was studied with purified MGlcDAG synthase in different lipid aggregates. Critical fractions of anionic phospholipids 1, 2-dioleoyl-phosphatidylglycerol (DOPG) and 1,2-dioleoyl-phosphatidylserine (DOPS) were essential for the restoration of enzyme activity, while the zwitterionic 1,2-dioleoyl-phosphatidylcholine (DOPC) and the uncharged diglucosyldiacylglycerol (DGlcDAG) were not. Proteolytic resistance had a very good correlation with the enzyme activity in various lipid-CHAPS mixed micelles. Anionic lipids DOPG and DOPS could protect the exposed MGlcDAG synthase from digestion, whereas DOPC and DGlcDAG could not. Similar features were observed in liposome bilayers. Likewise, the detergent dodecylphosphoglycerol (PGD), with a phosphatidylglycerol-like headgroup, could also stimulate the MGlcDAG synthase activity efficiently with a concomitant protection toward proteolytic digestion. Neither proteolytic resistance nor restored enzyme activity was observed using soluble glycerol 3-phosphate. It is concluded that in addition to critical amounts, both the negatively charged headgroup and hydrophobic chains of the activator amphiphiles, but not a certain aggregate curvature, seem necessary for a proper conformation and the resulting active state of the MGlcDAG synthase.

Protein kinase inhibitors block neurite outgrowth from explants of goldfish retina

A role for protein phosphorylation in the process of neurite outgrowth has been inferred from many studies of the effects of protein kinase inhibitors and activators on cultured neurotumor cells and primary neuronal cells from developing brain or ganglia. Here we re-examine this issue, using a culture system derived from a fully differentiated neuronal system undergoing axonal regeneration--the explanted goldfish retina following optic nerve crush. Of the relatively non-selective protein kinase inhibitors employed, H7, staurosporine and K252a were found to block neurite outgrowth, whereas HA1004 had no effect, a result which appears to rule out a critical role for protein kinase A. The more selective protein kinase C inhibitors, sphingosine, calphostin C and Ro-31-8220 were all inhibitory, as was prolonged treatment with phorbol ester and the protein phosphatase inhibitor okadaic acid. These results are in support of a role for protein kinase C in axonal regrowth.

Co-transfection with protein kinase D confers phorbol-ester-mediated inhibition on glucagon-stimulated cAMP accumulation in COS cells transfected to overexpress glucagon receptors

Glucagon elicited a profound increase in the intracellular cAMP concentration of COS-7 cells which had been transiently transfected with a cDNA encoding the rat glucagon receptor and under conditions where cAMP phosphodiesterase activity was fully inhibited. This was achieved in a dose-dependent fashion with an EC50 of 1.8+/-0.4 nM glucagon. In contrast with previous observations made using hepatocytes [Heyworth, Whetton, Kinsella and Houslay (1984) FEBS Lett. 170, 38-42], treatment of transfected COS-7 cells with PMA did not inhibit the ability of glucagon to increase intracellular cAMP levels. PMA-mediated inhibition was not conferred by treatment with okadaic acid, nor by co-transfecting cells with cDNAs encoding various protein kinase C isoforms (PKC-alpha, PKC-betaII and PKC-epsilon) or with the PMA-activated G-protein-receptor kinases GRK2 and GRK3. In contrast, PMA induced the marked inhibition of glucagon-stimulated cAMP production in COS-7 cells that had been co-transfected with a cDNA encoding protein kinase D (PKD). Such inhibition was not due to an action on the catalytic unit of adenylate cyclase, as forskolin-stimulated cAMP production was unchanged by PMA treatment of COS cells that had been co-transfected with both the glucagon receptor and PKD. PKD transcripts were detected in RNA isolated from hepatocytes but not from COS-7 cells. Transcripts for GRK2 were present in hepatocytes but not in COS cells, whereas transcripts for GRK3 were not found in either cell type. It is suggested that PKD may play a role in the regulation of glucagon-stimulated adenylate cyclase.

Inhibitory effects of bromocriptine on vascular smooth muscle cell proliferation

Bromocriptine (BC), an ergot alkaloid with wide therapeutic use in humans, has been shown to inhibit proliferation of several abnormally hyperproliferative cells in vivo and in vitro. In the present study, direct effects of BC on mitogen-stimulated proliferation of rat vascular smooth muscle cells (VSMC) (A7r5 cells) and human aortic smooth muscle cells (HAOSMC) were examined in vitro. Twenty-four hour proliferative responses of quiescent A7r5 cells and HAOSMC to a variety of mitogens in the presence or absence of BC were determined by quantifying the incorporation of 3H-thymidine into DNA. BC at 1 microM inhibited the responses of A7r5 cells to various concentrations of fetal calf serum (FCS) by 50-70% without affecting the ED50 of FCS (2%). BC dose dependently inhibited the proliferation of A7r5 cells and HAOSMC stimulated by 2% FCS, with 52% inhibition at 1 and 0.1 microM, respectively. BC at 1 microM also completely inhibited the maximal mitogenic responses of A7r5 cells to prolactin, platelet-derived growth factor, insulin-like growth factor, and phorbol mysterate acetate (PMA), and BC at 1 microM completely inhibited the mitogenic response of HAOSMC to PMA. BC is a dopamine D2 agonist, a noradrenergic alpha 2 agonist, and an .alpha 1 antagonist, but the inhibitory effects of BC on A7r5 cell proliferation could not be mimicked by the specific D2 agonists, LY162502 and LY171555; the alpha 2 agonist, clonidine; or the alpha 1 antagonist, WB-4101. Neither dopamine nor the D2 agonist, LY162502, could inhibit HAOSMC proliferation induced by FCS. The PMA-induced stimulation of protein kinase C (PKC), a positive regulator of mitogenesis, could be completely blocked in A7r5 cells and HAOSMC by 1 and 0.1 microM BC, respectively. However, FPCS (2%)-induced activation of PKC in A7r5 cells and HAOSMC could only be blocked by 61 and 19% by BC (1 microM for A7r5 cells and 0.1 microM for HAOSMC), respectively. Given the existing evidence that BC reduces the severity of several other pathological conditions, such as insulin resistance, inflammation, and hyperlipidemia, which potentiate vascular disease, the current findings further suggest that BC use in the treatment of atherosclerosis and/or restenosis deserves further investigation.