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Protein kinase Cα: disease regulator and therapeutic target
Opinion
Protein kinase Ca: disease regulator and therapeutic target Olga Konopatskaya and Alastair W. Poole Department of Physiology & Pharmacology, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK
Protein kinase Ca (PKCa) is a member of the AGC (which includes PKD, PKG and PKC) family of serine/threonine protein kinases that is widely expressed in mammalian tissues. It is closely related in structure, function and regulation to other members of the protein kinase C family, but has specific functions within the tissues in which it is expressed. There is substantial recent evidence, from gene knockout studies in particular, that PKCa activity regulates cardiac contractility, atherogenesis, cancer and arterial thrombosis. Selective targeting of PKCa therefore has potential therapeutic value in a wide variety of disease states, although will be technically complicated by the ubiquitous expression and multiple functions of the molecule. Introduction Protein kinase C (PKC) is a family of serine/threonine protein kinases that regulate various cellular functions, including adhesion, secretion, proliferation, differentiation and apoptosis. The family is classified into three groups on the basis of the arrangement of their regulatory domains w2[1]. Conventional isoforms (cPKC; a, b, g) contain a diacylglycerol (DAG)/phorbol ester-binding C1 domain and a Ca2+-binding C2 domain. Novel isoforms (nPKC; d, e, h, u) also contain C1 domains, but their C2 domains are unable to bind Ca2+. Atypical isoforms (aPKC; z and i/l) lack a C2 domain and have an atypical C1 domain, and are therefore regulated independently of Ca2+ or DAG. Here we focus on PKCa because of recent evidence, particularly derived from genetic studies, implicating this kinase in several major disease processes [2–4], and because of the multiple approaches that may be adopted to target PKCa pharmacologically [5–7]. PKCa is widely expressed and being a conventional PKC isoform, it may be regulated downstream of the multitude of receptors that couple to activation of phospholipase C, including Gqcoupled GPCRs (G-protein-coupled receptors), growth factor receptors and adhesion receptors. Pharmacological studies have shown PKCa to regulate multiple biological processes, including cell proliferation, apoptosis, differentiation, migration and adhesion. These roles are described and discussed elsewhere [1,8–10] and are outside of the scope of this article. The more recent introduction of PKCa gene knockout (Prkca / ) mice has provided definitive evidence for the function of this kinase in physiological processes including insulin coupling to PI3kinase [11], and immune cell functions such as IgG class switching Corresponding author: Poole, A.W. ([email protected]).
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[12] and down-regulation of the T cell receptor [13], such as prostate and breast cancer, heart failure and pulmonary disease. PKCa gene knockout mice are now also revealing critical roles for this gene in disease processes. In this opinion, we concentrate on the role of PKCa in the three best studied of these: cardiovascular dysfunction, arterial thrombosis and cancer (Fig. 1). We suggest that, through a variety of pharmacological strategies (Box 1), PKCa may emerge as a key target in the future treatment and management of these diseases. PKCa in cardiovascular disease regulation Heart failure It has been established that various PKC isoforms, including PKCa, contribute to impaired left ventricular filling and ejection in heart failure [6]. PKCa is both necessary and sufficient to induce cardiomyocyte hypertrophy - an adaptive mechanism triggered by decreased cardiac output and, if sustained, leading to heart failure and death - by an increase in protein synthesis, proteinDNA ratio, and cell surface area [14]. Further, PKCa antisense treatment reduces phenylephrine-induced increases in a-actin mRNA and atrial natriuretic peptide secretion [15], thereby causing a loss of some markers of pathological hypertrophy. Overexpression of PKCa also increases cardiomyocyte surface area and atrial natriuretic peptide expression, indicating that PKCa activation induces cardiomyocyte hypertrophy [16]. This regulation of cardiac hypertrophy translates into end-stage heart failure, and in animal models of this disease, expression and activity of PKCa are up-regulated [17]. Ablation of myocardial PKCa by targeted gene knockout leads to increased myocardial contractility, whereas transgenic overexpression of PKCa results in ventricular dysfunction and alterations in Ca2+ homeostasis [3]. The mechanism of PKCa-dependent depression of myofilament contractility involves phosphorylation of the cardiac troponins cTnI and/or cTnT functionally important in controlling maximum tension, ATPase activity, and Ca2+ sensitivity of the myofilaments [18]. Atherosclerosis Atherosclerosis is a disease of large and medium-sized vessels that involves multiple cells and processes including endothelial dysfunction, inflammation and proliferation. In the arterial wall the disease is characterized by the emergence in the intima of atherosclerotic fatty streaks, later transforming into plaques, consisting of connective
0165-6147/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2009.10.006 Available online 5 December 2009
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Figure 1. PKCa plays key physiological and pathological roles. The ubiquitously expressed protein kinase PKCa plays roles in multiple cellular processes, including many not detailed in the Figure. Depicted are the three areas discussed principally in the text where PKCa has been shown to play a role in regulating cardiac and vascular function, platelet function and thrombosis, and cellular proliferation and migration in cancers. In brief, PKCa negatively regulates myocardial contractility, positively regulates angiogenesis and monocyte adhesion to endothelial cells in atherosclerosis development, positively regulates platelet aggregation and thrombus formation in arteries and variously regulates tumor progression, depending upon cell type and nature of the cancer.
tissue and some smooth muscle cells (SMCs). It is accompanied by thickening and hardening (sclerosis) of the arterial wall, loss of elasticity due to infiltration of the intima with mononuclear cells, SMCs, and increased endothelial cell depositions. Atherosclerosis designates a special form of arteriosclerosis that is additionally characterized by the occurrence of foam cells, that is, lipid-laden macrophages and SMCs that may rupture and release their contents into the lesional areas. There is substantial evidence that PKCa plays key roles in these processes. Endothelial cell proliferation Pathological angiogenesis has been implicated in a number of vascular diseases, including unstable atherosclerotic plaque development [19]. Although there are some reports to the contrary [20,21], PKCa has generally been shown to be a positive regulator of angiogenesis [22], involving the processes of endothelial cell migration, adhesion and tube formation mediated by vascular endothelial growth factor (VEGF). In vivo, knockdown of PKCa prevents myocardial vessel formation in a murine model of myocardial infarction [23]. There seems to be positive feedback between the two signalling components, PKCa and VEGF, because VEGF expression in endothelial cells depends on PKCa, but in turn VEGF leads to activation of PKCa [24]. Overexpression of a PKCa pseudosubstrate inhibitory peptide also slows the rate of endothelial cell migration [25]. Additionally, PKCa mediates interleukin-1b-induced expression of matrix metalloproteinase-2, a molecule implicated in angiogenesis [26].
Endothelial barrier function Endothelial permeability is tightly regulated by a balance of contractile forces generated by the cytoskeleton and adhesive forces generated by cell–cell and cell–matrix contact [27]. Early studies suggested that PKC mediates the acute rise in endothelial permeability in vitro, via receptormediated (e.g. through thrombin, histamine or bradykinin receptors), or receptor-independent (by oxidants or shear stress) increases in phospholipase C activity [28]. Subsequently, several studies have narrowed this effect specifically to PKCa and its role in endothelial cell contraction and disassembly of VE-cadherin junctions [29,30]. At the molecular level, the mechanism might involve PKCa-dependent regulation of transient receptor potential canonical-1 (TRPC-1) channels [31] and Rho GTPases [32,33]. The increase in pulmonary endothelial cell permeability caused by TNFa is also associated with an increase in PKCa activity [34], and protein phosphatase type 2B (PP2B), which might regulate PKCa activity, is also implicated in regulating the permeability of pulmonary endothelial cells [35]. Lastly, endothelial permeability is altered in diabetes mellitus, and PKCa has been shown to mediate glucoseinduced increases in endothelial cell permeability [36]. Therefore, there is high consensus among data supporting the idea that PKCa positively regulates endothelial permeability through various molecular mechanisms. Oxidative stress and monocyte adhesion Endothelial cell dysfunction elicited by oxidative stress plays a critical role in the pathogenesis of atherosclerosis 9
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Box 1. Approaches to targeting PKCa Several approaches are used to target PKCa therapeutically, as summarized in Figure I. 1. ATP (adenosine triphosphate) binding site The ATP binding site is attractive for drug targeting, although selectivity may be difficult to achieve due to conservation of sequence and structure for this domain within the PKC family. The high concentration of ATP competes also with inhibitors at this site, decreasing their apparent activity. However, this site is still highly attractive for drug development, particularly by structure-based approaches, which are proving useful for PKCu inhibitors [71]. A publicly available crystal structure for PKCa is still absent, and is clearly a requirement for future development. Some bisinolylmaleimides, derivatives of staurosporine, show good selectivity within the PKC family. Ruboxistaurin (LY333531) and enzostaurin (LY317615) are designed to selectively target PKCb, although a recent report suggests that ruboxistaurin may inhibit PKCa equipotently [72]. Both enzastaurin and ruboxistaurin are well tolerated [73,74]. Enzastaurin shows promise in phase I trials for the treatment of advanced cancer [75]. Ruboxistaurin has completed phase III trials for the management of diabetic retinopathy [70,74], principally through targeting PKCb.
3. C1 domain The C1 domain binds diacylglycerol (DAG) and phorbol esters. Bryostatin is a macrocyclic lactone that competes with DAG for binding [77], and simplified analogues have been designed with improved activity [78]. Calphostin C also binds PKCa, and other PKC isoforms, at the C1 domain. The inhibition is irreversible, and requires prior light activation of the molecule [79]. 4. C2 domain The C2 domain binds Ca2+ and mediates interaction with proteins [80], in particular Receptors-for-Activated-C-Kinase (RACKs) [81]. RACK peptides therefore disrupt the localization of PKCs to their substrates, thereby effectively inhibiting their activity. This inhibition has been shown most extensively for PKCd, where KAI-9803 is undergoing clinical trial for cardiac ST segment elevation of cardiogram in patients with myocardial infarction [82]. PKCa interacts with fascin and lamin A, and inhibition of these interactions might disrupt cytoskeleton- and nucleus-dependent signalling [83,84]. 5. Gene therapy
2. Peptide substrate mimetics Peptide substrate mimetics are short peptides that resemble the preferred substrate sequence for PKCa, often mimicking the pseudosubstrate domain, and compete for substrate binding. In addition, a small-molecule inhibitor, chelerythrine, competitively binds to the substrate-binding pocket, although its specificity for PKCa is poor [76].
Although not shown in Figure I, an antisense drug, aprinocarsen (ISIS 3521, LY900003), has been used to reduce expression of PKCa. This drug has undergone phase III clinical trials for the treatment of various human tumors [61] but has not progressed through to the clinic, due to non-achievement of efficacy endpoints and associated toxicity.
Figure I. PKCa domain structure mapped against pharmacological inhibitors. The structure comprises an N-terminal regulatory domain, consisting of the diacylglycerol (DAG)-binding C1 domain and the Ca2+-binding C2 domain, and a C-terminal catalytic domain, consisting of an ATP-binding pocket followed by the substrate-binding domain. Each component of the molecule is a potential target for pharmacological intervention, as indicated by the drugs listed below the domains. Thus, bryostatin and calphostin C target the C1 domain, RACK peptides mimic the C2 domain region of protein-protein interaction, staurosporine and its derivatives target the ATP binding pocket, whilst peptides that mimic the pseudosubstrate domain, and chelerythrine, bind to the substrate-binding domain. Not illustrated here is the use of genetic tools, such as antisense drugs (e.g. aprinocarsen), that can alter the expression of the protein in cells and tissues.
[37]. Low-density lipoproteins (LDLs) may be oxidized when excessive free radicals react with the lipoproteins. In turn, these oxidized LDLs (Ox-LDLs) become deposited in the arterial walls and can lead to plaque development by stimulating macrophage uptake and foam cell formation. A detrimental feedback loop operates in endothelial cells, where superoxides lead to generation of Ox-LDLs, which in turn stimulate further superoxide generation. Fleming et al. [38] have shown that Ox-LDL stimulation of endothelial cells leads to uncoupling of endothelial nitric 10
oxide synthase (eNOS) from its generation of nitric oxide, and a greater accumulation of superoxide free radicals. The signalling pathway in endothelial cells from Ox-LDL to eNOS has been attributed to inactivation and down-regulation of PKCa, which results in diminished phosphorylation of eNOS on Thr-495 [38]. Additionally, PKCa in inflammatory cells might mediate their adhesion to and interaction with endothelial cells. The adhesion of circulating monocytes to endothelial cells contributes importantly to the inflammatory aspects of
Opinion atherogenesis, which describes the development of atherosclerotic plaques. Pre-incubation of monocytes with Go¨6976, the inhibitor of classical PKC isoforms, significantly reduces monocyte adhesion to HUVECs (human umbilical vein endothelial cells) [39]. Another inflammatory mediator, apolipoprotein CIII, a constituent of apolipoprotein B, enhances the adhesion of monocytes to endothelial cells via PKCa-mediated b1-integrin activation [40]. Lastly, endothelial cell PKCa is important in also regulating inflammatory cell adhesion, through expression of the adhesion molecule PECAM-1 [41], and through regulating exocytosis and surface expression of P-selectin, which mediates neutrophil-endothelial cell interaction [42]. In summary, although there is a mixed picture in terms of the role of PKCa in atherogenesis and cardiomyocyte hypertrophy, there is reasonable consensus that, on balance, inhibitors of PKCa are likely to be useful in the management of these diseases. Platelet function and thrombosis Platelets play a central role in mediating atherothrombosis, characterized by an unpredictable atherosclerotic plaque disruption, and are therefore the target of numerous therapies aimed at reducing their activity, particularly in the prevention of coronary artery thrombosis in heart attacks [43]. PKC has been established, largely by pharmacological studies, as a major regulator of multiple platelet activities [44], and it is increasingly clear that the different isozymes of PKC expressed in platelets perform distinct functions. On activation, platelets release a multitude of active substances from secretory granules, essential for platelet pro-aggregatory responses and development of a stable thrombus in arteries. Yoshioka et al. [45] have analyzed the mechanisms governing Ca2+-induced secretion of agranules and dense-core granules in permeabilized human platelets, and identified PKCa as an essential component of the mechanism. Using similar biochemical approaches, the same group demonstrated that PKCa is also involved in the regulation of Ca2+-induced platelet aggregation [46]. Pula et al. [47] have shown that two tyrosine kinases, Syk and Src, physically interact with PKCa, leading to distinct functional consequences. Although they found that PKCa activity was dependent on Syk, Syk activity was not regulated by PKCa; however, Src activity was negatively regulated by PKCa. These results suggest that PKCa is an important factor in the complex interaction with tyrosine kinases for regulation of functional activities in platelets. Additionally, in a recent elegant study reconstructing the signalling pathway regulating platelet integrin aIIbb3 in a heterologous cell system, it was shown that PKCa expression is required for activation of the integrin through the Rap1 pathway [48]. Key molecular functions of platelet activation are therefore mediated by PKCa. On the basis of pharmacological data, the specificity of the role played by PKCa has been a contentious issue, due to the lack of selectivity of the reagents available. We have therefore developed a genetic approach using PKCa knockout (Prkca / ) mice to determine the role of PKCa in regulating platelet function and thrombus formation [4]. With regard to the suggested role of PKCa in regulating secretion of dense- and a-granules [45], we were able to
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confirm definitively that secretion of these granules was substantially diminished in Prkca / platelets. Prkca / platelets showed significantly reduced functionally relevant phosphorylation on Ser95 of synaptosomal associated protein SNAP-23, an essential component of the membrane fusion machinery and an important regulator of vesicle docking and fusion [4]. This observation therefore may potentially explain the observed secretion defect in PKCa-deficient platelets. In addition, Prkca / platelets show a marked knockdown in activation of integrin aIIbb3 in response either to thrombin or collagen-related peptide, paralleling a deficit in their ability to undergo aggregation at submaximal concentrations of agonist. This is also consistent with the impaired ability of Prkca / platelets to form a thrombus in vitro in blood flowing over a collagen-coated surface and in an in vivo laser-induced model of thrombus formation (Fig. 2). The defect in secretion was shown, however, to be the central significant event, because addition of exogenous ADP (adenosine diphosphate), the major constituent released from platelet dense granules, rescued the deficits in responses seen in Prkca / platelets, confirming that granule secretion is likely to be the primary function for PKCa. These findings, together with the fact that Prkca / mice do not demonstrate any evidence of overt bleeding, have revealed that PKCa is a potential drug target for antithrombotic therapy, because selective inhibitors would be expected to exert a major effect on thrombus formation while sparing primary platelet adhesive functions. PKCa in cancer PKCa has long been recognized to have a role in regulating aspects of tumor growth and development [10], although this role is clearly complex and highly tissue-dependent because in some cases it acts as a tumor promoter, and in others it functions as a tumor suppressor. Nonetheless, PKC in general has, for many years, been a drug target for the treatment of cancer. Pharmacological approaches to control of cancer by PKC inhibitors have been discussed elsewhere [7,49] and some of these approaches for PKCa are discussed in Box 1. Its most consistent role is probably regulation of cell motility, and certainly PKCa activation can result in increased cell motility in several in vivo and in vitro cancer models, the effect of which may be reversed on PKCa inhibition[50,51]. However, the predominant overall picture is of a lack of consistency, which can be illustrated by its differing expression patterns in different tumors. Overexpression of PKCa has been demonstrated in tissue samples of prostate, endometrial, high-grade urinary bladder and hepatocellular cancers[52–55], while for haematological malignancies up- or down-regulation of PKCa has been described [56], and in basal cell carcinoma and colon cancers, down-regulation of PKCa has been observed [2,57]. A mixed picture also pertains to breast cancer cells, where it has been studied extensively. Whereas activation or over-expression of PKCa has been shown in breast cancer cells and in breast tumor samples by some [58,59], down-regulation has been demonstrated by others [60]. In terms of cancer therapeutics, PKCa has been a target for the drug aprinocarsen (ISIS 3521; see also Box 1), a 11
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Figure 2. PKCa plays a critical role regulating platelet function and thrombosis in arteries. Platelets are the smallest cellular component of the blood, and flow close to the endothelial lining of arteries continually surveying the vessel for breaches in endothelial integrity. In the main panel platelets are depicted encountering such a breach, exposing subendothelial collagen to which platelets adhere and activate. Critical for the rapid build up of a platelet aggregate, or thrombus, is the release of positive feedback mediators, many of which, including ADP which acts on its receptors P2Y1 and P2Y12, are contained in secretory granules in platelets. The expanded inset figure illustrates that PKCa plays a crucial role mediating signals from the platelet collagen receptor, GPVI, to release of granule contents, including ADP, and also to activation of the major adhesion molecule, integrin aIIbb3. This integrin bridges platelets through binding the plasma protein fibrinogen. Platelet–platelet interaction and subsequent build up of a thrombus in arteries is therefore dependent upon platelet PKCa. Arterial thrombus formation is the major occlusive event underlying vessel blockage in heart disease and thrombotic stroke, and therefore platelet PKCa may represent a novel target for therapeutic intervention.
phosphorothioate antisense oligonucleotide (ASO) that targets the 3’-untranslated region of human PKCa mRNA, leading to decreased expression of the protein [61]. Aprinocarsen has been studied as a single agent, as well as in combination with standard chemotherapeutics, in cancer patients in over 20 trials from phase I to phase III (reviewed in [61,62]). The studies include individuals with non-small cell lung cancer [63], colon [64], ovarian [65], breast [66] and prostate cancers [67] and non-Hodgkin’s lymphoma [68]. Although encouraging results have been observed in phase II trials, disappointingly aprinocarsen has failed to meet efficacy endpoints in phase III trials when used in combination with conventional chemother12
apy for late-stage lung cancer [69,70]. Reasons for this lack of efficacy could be manifold, including lack of prior screening of patients for expression levels of PKCa, dose reduction due to toxicity during the trial and driving of cell proliferation by other related PKC isoforms [69]. Conclusions PKCa plays important roles in several cellular processes and pathologies, involving cancer, cardiovascular disorders, atherogenesis and thrombosis. Its ubiquity and multiple roles, however, present a potential difficulty in its use as a drug target, although the mutual functional redundancy of various PKC isoforms might help to
Opinion overcome possible unwanted effects of specific pharmacological intervention. It is clear, for instance, that thrombus formation is markedly diminished in the Prkca / mouse, whereas haemostasis is normal [4]. This functional redundancy with related PKC isoforms may also obscure other potential roles for PKCa, determined in particular from gene knockout strategies. There are clearly many strategies that can be adopted to target this kinase therapeutically (Box 1), and in this regard a key advance will be the determination of the crystal structure of the catalytic domain of PKCa. It will be important to continue developing strategies for PKCa targeting in the future, because specific drug targeting could have great value in the treatment of multiple disease states. Conflict of interest declaration The authors have declared that no conflict of interest exists. Acknowledgments The authors thank Drs Matthew Jones and Matthew Harper for their advice and critical reading of the manuscript. Work in the authors’ laboratory is supported by grants from the British Heart Foundation (grant nos. RG/05/015 and PG/07/118/24152).
References 1 Parker, P.J. and Murray-Rust, J. (2004) PKC at a glance. J. Cell Sci. 117, 131–132 2 Oster, H. and Leitges, M. (2006) Protein kinase Ca but not PKCz suppresses intestinal tumor formation in ApcMin/+ mice. Cancer Res. 66, 6955–6963 3 Braz, J.C. et al. (2004) PKCa regulates cardiac contractility and propensity toward heart failure. Nat. Med. 10, 248–254 4 Konopatskaya, O. et al. (2009) PKCa regulates platelet granule secretion and thrombus formation in mice. J. Clin. Invest. 119, 399–407 5 Churchill, E. et al. (2008) PKC isozymes in chronic cardiac disease: possible therapeutic targets? Annu. Rev. Pharmacol. Toxicol. 48, 569– 599 6 Palaniyandi, S.S. et al. (2009) Protein kinase C in heart failure: a therapeutic target? Cardiovasc. Res. 82, 229–239 7 Mackay, H.J. and Twelves, C.J. (2007) Targeting the protein kinase C family: are we there yet? Nat. Rev. Cancer 7, 554–562 8 Dempsey, E.C. et al. (2000) Protein kinase C isozymes and the regulation of diverse cell responses. Am. J. Physiol. Lung Cell Mol. Physiol. 279, L429–438 9 Gould, C.M. and Newton, A.C. (2008) The life and death of protein kinase C. Curr. Drug Targets 9, 614–625 10 Nakashima, S. (2002) Protein kinase Ca (PKCa): regulation and biological function. J. Biochem. 132, 669–675 11 Leitges, M. et al. (2002) Knockout of PKCa enhances insulin signaling through PI3K. Mol. Endocrinol. 16, 847–858 12 Pfeifhofer, C. et al. (2006) Defective IgG2a/2b class switching in PKCa-/- mice. J. Immunol. 176, 6004–6011 13 von Essen, M. et al. (2006) Protein kinase C (PKC) a and PKCu are the major PKC isotypes involved in TCR down-regulation. J. Immunol. 176, 7502–7510 14 Vijayan, K. et al. (2004) Protein kinase Ca-induced hypertrophy of neonatal rat ventricular myocytes. Am. J. Physiol. Heart Circ. Physiol. 287, H2777–2789 15 Kerkela, R. et al. (2002) Identification of PKCa isoform-specific effects in cardiac myocytes using antisense phosphorothioate oligonucleotides. Mol. Pharmacol. 62, 1482–1491 16 Braz, J.C. et al. (2002) PKCa regulates the hypertrophic growth of cardiomyocytes through extracellular signal-regulated kinase1/2 (ERK1/2). J. Cell Biol. 156, 905–919 17 Belin, R.J. et al. (2007) Augmented protein kinase Ca-induced myofilament protein phosphorylation contributes to myofilament dysfunction in experimental congestive heart failure. Circ. Res. 101, 195–204
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18 Sumandea, M.P. et al. (2003) Identification of a functionally critical protein kinase C phosphorylation residue of cardiac troponin T. J. Biol. Chem. 278, 35135–35144 19 Holm, P.W. et al. (2009) Atherosclerotic plaque development and instability: a dual role for VEGF. Ann. Med. 41, 257–264 20 Rask-Madsen, C. and King, G.L. (2008) Differential regulation of VEGF signaling by PKCa and PKCe in endothelial cells. Arterioscler Thromb. Vasc. Biol. 28, 919–924 21 Xia, P. et al. (1996) Characterization of vascular endothelial growth factor’s effect on the activation of protein kinase C, its isoforms, and endothelial cell growth. J. Clin. Invest. 98, 2018–2026 22 Goekjian, P.G. and Jirousek, M.R. (2001) Protein kinase C inhibitors as novel anticancer drugs. Expert Opin. Investig. Drugs 10, 2117– 2140 23 Wang, A. et al. (2002) Inhibition of protein kinase Ca prevents endothelial cell migration and vascular tube formation in vitro and myocardial neovascularization in vivo. Circ. Res. 90, 609–616 24 Xu, H. et al. (2008) Protein kinase Ca promotes angiogenic activity of human endothelial cells via induction of vascular endothelial growth factor. Cardiovasc. Res. 78, 349–355 25 Harrington, E.O. et al. (2000) Endothelial proliferation, migration, and differentiation are blunted by conditionally expressed protein kinase C pseudosubstrate peptides. Biochem. Biophys Res. Commun. 271, 499– 508 26 Mountain, D.J. et al. (2007) Interleukin-1beta increases expression and activity of matrix metalloproteinase-2 in cardiac microvascular endothelial cells: role of PKCalpha/beta1 and MAPKs. Am. J. Physiol. Cell Physiol. 292, C867–875 27 Aghajanian, A. et al. (2008) Endothelial cell junctions and the regulation of vascular permeability and leukocyte transmigration. J. Thromb. Haemost. 6, 1453–1460 28 Lum, H. and Malik, A.B. (1994) Regulation of vascular endothelial barrier function. Am. J. Physiol. 267, L223–241 29 Sandoval, R. et al. (2001) Ca(2+) signalling and PKCa activate increased endothelial permeability by disassembly of VE-cadherin junctions. J. Physiol. 533, 433–445 30 Tiruppathi, C. et al. (2002) Role of Ca2+ signaling in the regulation of endothelial permeability. Vascul. Pharmacol. 39, 173–185 31 Ahmmed, G.U. et al. (2004) Protein kinase Ca phosphorylates the TRPC1 channel and regulates store-operated Ca2+ entry in endothelial cells. J. Biol. Chem. 279, 20941–20949 32 Mehta, D. et al. (2001) Protein kinase C-alpha signals rho-guanine nucleotide dissociation inhibitor phosphorylation and rho activation and regulates the endothelial cell barrier function. J. Biol. Chem. 276, 22614–22620 33 Stamatovic, S.M. et al. (2006) Protein kinase Calpha-RhoA cross-talk in CCL2-induced alterations in brain endothelial permeability. J. Biol. Chem. 281, 8379–8388 34 Ferro, T. et al. (2000) Protein kinase C-alpha mediates endothelial barrier dysfunction induced by TNF-alpha. Am. J. Physiol. Lung Cell Mol. Physiol. 278, L1107–1117 35 Lum, H. et al. (2001) Protein phosphatase 2B inhibitor potentiates endothelial PKC activity and barrier dysfunction. Am. J. Physiol. Lung Cell Mol. Physiol. 281, L546–555 36 Hempel, A. et al. (1997) High glucose concentrations increase endothelial cell permeability via activation of protein kinase C alpha. Circ. Res. 81, 363–371 37 Witztum, J.L. and Steinberg, D. (1991) Role of oxidized low density lipoprotein in atherogenesis. J. Clin. Invest. 88, 1785–1792 38 Fleming, I. et al. (2005) Oxidized low-density lipoprotein increases superoxide production by endothelial nitric oxide synthase by inhibiting PKCalpha. Cardiovasc. Res. 65, 897–906 39 Takahashi, K. et al. (2006) SDF-1-induced adhesion of monocytes to vascular endothelium is modulated by azelnidipine via protein kinase C inhibition. Eur. J. Pharmacol. 552, 162–169 40 Kawakami, A. et al. (2007) Apolipoprotein CIII-induced THP-1 cell adhesion to endothelial cells involves pertussis toxin-sensitive G protein- and protein kinase C alpha-mediated nuclear factor-kappaB activation. Arterioscler Thromb. Vasc. Biol. 27, 219–225 41 Gong, N. et al. (2004) Lactosylceramide recruits PKCalpha/epsilon and phospholipase A2 to stimulate PECAM-1 expression in human monocytes and adhesion to endothelial cells. Proc. Natl. Acad. Sci. U. S. A. 101, 6490–6495
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Opinion 42 Fu, J. et al. (2005) Protease-activated receptor-1 activation of endothelial cells induces protein kinase Calpha-dependent phosphorylation of syntaxin 4 and Munc18c: role in signaling pselectin expression. J. Biol. Chem. 280, 3178–3184 43 Meadows, T.A. and Bhatt, D.L. (2007) Clinical aspects of platelet inhibitors and thrombus formation. Circ. Res. 100, 1261–1275 44 Harper, M.T. and Poole, A.W. (2007) Isoform-specific functions of protein kinase C: the platelet paradigm. Biochem. Soc. Trans. 35, 1005–1008 45 Yoshioka, A. et al. (2001) Identification of protein kinase Calpha as an essential, but not sufficient, cytosolic factor for Ca2+-induced alphaand dense-core granule secretion in platelets. J. Biol. Chem. 276, 39379–39385 46 Tabuchi, A. et al. (2003) Direct demonstration of involvement of protein kinase Calpha in the Ca2+-induced platelet aggregation. J. Biol. Chem. 278, 26374–26379 47 Pula, G. et al. (2005) Functional interaction of protein kinase Calpha with the tyrosine kinases Syk and Src in human platelets. J. Biol. Chem. 280, 7194–7205 48 Han, J. et al. (2006) Reconstructing and deconstructing agonistinduced activation of integrin alphaIIbbeta3. Curr. Biol. 16, 1796–1806 49 Fields, A.P. and Murray, N.R. (2008) Protein kinase C isozymes as therapeutic targets for treatment of human cancers. Adv. Enzyme Regul. 48, 166–178 50 Koivunen, J. et al. (2004) Protein kinase C alpha/beta inhibitor Go6976 promotes formation of cell junctions and inhibits invasion of urinary bladder carcinoma cells. Cancer Res. 64, 5693–5701 51 Masur, K. et al. (2001) High PKC alpha and low E-cadherin expression contribute to high migratory activity of colon carcinoma cells. Mol. Biol. Cell 12, 1973–1982 52 Fournier, D.B. et al. (2001) Protein kinase C alpha expression is inversely related to ER status in endometrial carcinoma: possible role in AP-1-mediated proliferation of ER-negative endometrial cancer. Gynecol. Oncol. 81, 366–372 53 Koren, R. et al. (2004) Expression of protein kinase C isoenzymes in benign hyperplasia and carcinoma of prostate. Oncol. Rep. 11, 321–326 54 Langzam, L. et al. (2001) Patterns of protein kinase C isoenzyme expression in transitional cell carcinoma of bladder. Relation to degree of malignancy. Am. J. Clin. Pathol. 116, 377–385 55 Varga, A. et al. (2004) Tumor grade-dependent alterations in the protein kinase C isoform pattern in urinary bladder carcinomas. Eur. Urol. 46, 462–465 56 Lahn, M. et al. (2006) The role of protein kinase C-alpha in hematologic malignancies. Acta Haematol. 115, 1–8 57 Neill, G.W. et al. (2003) Loss of protein kinase Calpha expression may enhance the tumorigenic potential of Gli1 in basal cell carcinoma. Cancer Res. 63, 4692–4697 58 Lahn, M. et al. (2004) Protein kinase C alpha expression in breast and ovarian cancer. Oncology 67, 1–10 59 Tan, M. et al. (2006) Upregulation and activation of PKC alpha by ErbB2 through Src promotes breast cancer cell invasion that can be blocked by combined treatment with PKC alpha and Src inhibitors. Oncogene 25, 3286–3295 60 Kerfoot, C. et al. (2004) Immunohistochemical analysis of advanced human breast carcinomas reveals downregulation of protein kinase C alpha. J. Histochem. Cytochem. 52, 419–422 61 Hanauske, A.R. et al. (2004) The role of protein kinase C-alpha (PKCalpha) in cancer and its modulation by the novel PKC-alpha-specific inhibitor aprinocarsen. Curr. Pharm. Des. 10, 1923–1936 62 Li, K. and Zhang, J. (2001) ISIS-3521. Isis Pharmaceuticals. Curr. Opin. Investig. Drugs 2, 1454–1461 63 Villalona-Calero, M.A. et al. (2004) A phase I/II study of LY900003, an antisense inhibitor of protein kinase C-alpha, in combination with
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cisplatin and gemcitabine in patients with advanced non-small cell lung cancer. Clin. Cancer Res. 10, 6086–6093 Marshall, J.L. et al. (2004) A phase II trial of ISIS 3521 in patients with metastatic colorectal cancer. Clin. Colorectal. Cancer 4, 268–274 Advani, R. et al. (2004) A Phase II trial of aprinocarsen, an antisense oligonucleotide inhibitor of protein kinase C alpha, administered as a 21-day infusion to patients with advanced ovarian carcinoma. Cancer 100, 321–326 Roychowdhury, D. and Lahn, M. (2003) Antisense therapy directed to protein kinase C-alpha (Affinitak, LY900003/ISIS 3521): potential role in breast cancer. Semin Oncol. 30, 30–33 Tolcher, A.W. et al. (2002) A randomized phase II and pharmacokinetic study of the antisense oligonucleotides ISIS 3521 and ISIS 5132 in patients with hormone-refractory prostate cancer. Clin. Cancer Res. 8, 2530–2535 Rao, S. et al. (2004) Phase II study of ISIS 3521, an antisense oligodeoxynucleotide to protein kinase C alpha, in patients with previously treated low-grade non-Hodgkin’s lymphoma. Ann. Oncol. 15, 1413–1418 Paz-Ares, L. et al. (2006) Phase III study of gemcitabine and cisplatin with or without aprinocarsen, a protein kinase C-alpha antisense oligonucleotide, in patients with advanced-stage non-small-cell lung cancer. J. Clin. Oncol. 24, 1428–1434 Roffey, J. et al. (2009) Protein kinase C intervention: the state of play. Curr. Opin. Cell Biol. 21, 268–279 Cole, D.C. et al. (2008) Identification, characterization and initial hitto-lead optimization of a series of 4-arylamino-3-pyridinecarbonitrile as protein kinase C theta (PKCtheta) inhibitors. J. Med. Chem. 51, 5958–5963 Liu, Q. et al. (2009) Protein Kinase C{alpha}, but Not PKC{beta} or PKC{gamma}, Regulates Contractility and Heart Failure Susceptibility. Implications for Ruboxistaurin As a Novel Therapeutic Approach. Circ. Res. 105, 194–200 Chen, Y.B. and LaCasce, A.S. (2008) Enzastaurin. Expert Opin. Investig. Drugs 17, 939–944 Das Evcimen, N. and King, G.L. (2007) The role of protein kinase C activation and the vascular complications of diabetes. Pharmacol. Res. 55, 498–510 Rademaker-Lakhai, J.M. et al. (2007) Phase I pharmacokinetic and pharmacodynamic study of the oral protein kinase C beta-inhibitor enzastaurin in combination with gemcitabine and cisplatin in patients with advanced cancer. Clin. Cancer Res. 13, 4474–4481 Herbert, J.M. et al. (1990) Chelerythrine is a potent and specific inhibitor of protein kinase C. Biochem. Biophys Res. Commun. 172, 993–999 Szallasi, Z. et al. (1994) Differential regulation of protein kinase C isozymes by bryostatin 1 and phorbol 12-myristate 13-acetate in NIH 3T3 fibroblasts. J. Biol. Chem. 269, 2118–2124 Wender, P.A. et al. (2008) Function-oriented synthesis, step economy, and drug design. Acc. Chem. Res. 41, 40–49 Bruns, R.F. et al. (1991) Inhibition of protein kinase C by calphostin C is light-dependent. Biochem. Biophys Res. Commun. 176, 288–293 Poole, A.W. et al. (2004) PKC-interacting proteins: from function to pharmacology. Trends Pharmacol. Sci. 25, 528–535 Churchill, E.N. et al. (2009) Rationally designed peptide regulators of protein kinase C. Trends Endocrinol. Metab. 20, 25–33 Bates, E. et al. (2008) Intracoronary KAI-9803 as an adjunct to primary percutaneous coronary intervention for acute ST-segment elevation myocardial infarction. Circulation 117, 886–896 Martelli, A.M. et al. (2002) Molecular characterization of protein kinase C-alpha binding to lamin A. J. Cell Biochem. 86, 320–330 Tabellini, G. et al. (2002) Binding of elements of protein kinase C-alpha regulatory domain to lamin B1. Cell Signal. 14, 819–827
Protein kinase Ca: disease regulator and therapeutic target Olga Konopatskaya and Alastair W. Poole Department of Physiology & Pharmacology, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK
Protein kinase Ca (PKCa) is a member of the AGC (which includes PKD, PKG and PKC) family of serine/threonine protein kinases that is widely expressed in mammalian tissues. It is closely related in structure, function and regulation to other members of the protein kinase C family, but has specific functions within the tissues in which it is expressed. There is substantial recent evidence, from gene knockout studies in particular, that PKCa activity regulates cardiac contractility, atherogenesis, cancer and arterial thrombosis. Selective targeting of PKCa therefore has potential therapeutic value in a wide variety of disease states, although will be technically complicated by the ubiquitous expression and multiple functions of the molecule. Introduction Protein kinase C (PKC) is a family of serine/threonine protein kinases that regulate various cellular functions, including adhesion, secretion, proliferation, differentiation and apoptosis. The family is classified into three groups on the basis of the arrangement of their regulatory domains w2[1]. Conventional isoforms (cPKC; a, b, g) contain a diacylglycerol (DAG)/phorbol ester-binding C1 domain and a Ca2+-binding C2 domain. Novel isoforms (nPKC; d, e, h, u) also contain C1 domains, but their C2 domains are unable to bind Ca2+. Atypical isoforms (aPKC; z and i/l) lack a C2 domain and have an atypical C1 domain, and are therefore regulated independently of Ca2+ or DAG. Here we focus on PKCa because of recent evidence, particularly derived from genetic studies, implicating this kinase in several major disease processes [2–4], and because of the multiple approaches that may be adopted to target PKCa pharmacologically [5–7]. PKCa is widely expressed and being a conventional PKC isoform, it may be regulated downstream of the multitude of receptors that couple to activation of phospholipase C, including Gqcoupled GPCRs (G-protein-coupled receptors), growth factor receptors and adhesion receptors. Pharmacological studies have shown PKCa to regulate multiple biological processes, including cell proliferation, apoptosis, differentiation, migration and adhesion. These roles are described and discussed elsewhere [1,8–10] and are outside of the scope of this article. The more recent introduction of PKCa gene knockout (Prkca / ) mice has provided definitive evidence for the function of this kinase in physiological processes including insulin coupling to PI3kinase [11], and immune cell functions such as IgG class switching Corresponding author: Poole, A.W. ([email protected]).
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[12] and down-regulation of the T cell receptor [13], such as prostate and breast cancer, heart failure and pulmonary disease. PKCa gene knockout mice are now also revealing critical roles for this gene in disease processes. In this opinion, we concentrate on the role of PKCa in the three best studied of these: cardiovascular dysfunction, arterial thrombosis and cancer (Fig. 1). We suggest that, through a variety of pharmacological strategies (Box 1), PKCa may emerge as a key target in the future treatment and management of these diseases. PKCa in cardiovascular disease regulation Heart failure It has been established that various PKC isoforms, including PKCa, contribute to impaired left ventricular filling and ejection in heart failure [6]. PKCa is both necessary and sufficient to induce cardiomyocyte hypertrophy - an adaptive mechanism triggered by decreased cardiac output and, if sustained, leading to heart failure and death - by an increase in protein synthesis, proteinDNA ratio, and cell surface area [14]. Further, PKCa antisense treatment reduces phenylephrine-induced increases in a-actin mRNA and atrial natriuretic peptide secretion [15], thereby causing a loss of some markers of pathological hypertrophy. Overexpression of PKCa also increases cardiomyocyte surface area and atrial natriuretic peptide expression, indicating that PKCa activation induces cardiomyocyte hypertrophy [16]. This regulation of cardiac hypertrophy translates into end-stage heart failure, and in animal models of this disease, expression and activity of PKCa are up-regulated [17]. Ablation of myocardial PKCa by targeted gene knockout leads to increased myocardial contractility, whereas transgenic overexpression of PKCa results in ventricular dysfunction and alterations in Ca2+ homeostasis [3]. The mechanism of PKCa-dependent depression of myofilament contractility involves phosphorylation of the cardiac troponins cTnI and/or cTnT functionally important in controlling maximum tension, ATPase activity, and Ca2+ sensitivity of the myofilaments [18]. Atherosclerosis Atherosclerosis is a disease of large and medium-sized vessels that involves multiple cells and processes including endothelial dysfunction, inflammation and proliferation. In the arterial wall the disease is characterized by the emergence in the intima of atherosclerotic fatty streaks, later transforming into plaques, consisting of connective
0165-6147/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2009.10.006 Available online 5 December 2009
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Figure 1. PKCa plays key physiological and pathological roles. The ubiquitously expressed protein kinase PKCa plays roles in multiple cellular processes, including many not detailed in the Figure. Depicted are the three areas discussed principally in the text where PKCa has been shown to play a role in regulating cardiac and vascular function, platelet function and thrombosis, and cellular proliferation and migration in cancers. In brief, PKCa negatively regulates myocardial contractility, positively regulates angiogenesis and monocyte adhesion to endothelial cells in atherosclerosis development, positively regulates platelet aggregation and thrombus formation in arteries and variously regulates tumor progression, depending upon cell type and nature of the cancer.
tissue and some smooth muscle cells (SMCs). It is accompanied by thickening and hardening (sclerosis) of the arterial wall, loss of elasticity due to infiltration of the intima with mononuclear cells, SMCs, and increased endothelial cell depositions. Atherosclerosis designates a special form of arteriosclerosis that is additionally characterized by the occurrence of foam cells, that is, lipid-laden macrophages and SMCs that may rupture and release their contents into the lesional areas. There is substantial evidence that PKCa plays key roles in these processes. Endothelial cell proliferation Pathological angiogenesis has been implicated in a number of vascular diseases, including unstable atherosclerotic plaque development [19]. Although there are some reports to the contrary [20,21], PKCa has generally been shown to be a positive regulator of angiogenesis [22], involving the processes of endothelial cell migration, adhesion and tube formation mediated by vascular endothelial growth factor (VEGF). In vivo, knockdown of PKCa prevents myocardial vessel formation in a murine model of myocardial infarction [23]. There seems to be positive feedback between the two signalling components, PKCa and VEGF, because VEGF expression in endothelial cells depends on PKCa, but in turn VEGF leads to activation of PKCa [24]. Overexpression of a PKCa pseudosubstrate inhibitory peptide also slows the rate of endothelial cell migration [25]. Additionally, PKCa mediates interleukin-1b-induced expression of matrix metalloproteinase-2, a molecule implicated in angiogenesis [26].
Endothelial barrier function Endothelial permeability is tightly regulated by a balance of contractile forces generated by the cytoskeleton and adhesive forces generated by cell–cell and cell–matrix contact [27]. Early studies suggested that PKC mediates the acute rise in endothelial permeability in vitro, via receptormediated (e.g. through thrombin, histamine or bradykinin receptors), or receptor-independent (by oxidants or shear stress) increases in phospholipase C activity [28]. Subsequently, several studies have narrowed this effect specifically to PKCa and its role in endothelial cell contraction and disassembly of VE-cadherin junctions [29,30]. At the molecular level, the mechanism might involve PKCa-dependent regulation of transient receptor potential canonical-1 (TRPC-1) channels [31] and Rho GTPases [32,33]. The increase in pulmonary endothelial cell permeability caused by TNFa is also associated with an increase in PKCa activity [34], and protein phosphatase type 2B (PP2B), which might regulate PKCa activity, is also implicated in regulating the permeability of pulmonary endothelial cells [35]. Lastly, endothelial permeability is altered in diabetes mellitus, and PKCa has been shown to mediate glucoseinduced increases in endothelial cell permeability [36]. Therefore, there is high consensus among data supporting the idea that PKCa positively regulates endothelial permeability through various molecular mechanisms. Oxidative stress and monocyte adhesion Endothelial cell dysfunction elicited by oxidative stress plays a critical role in the pathogenesis of atherosclerosis 9
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Box 1. Approaches to targeting PKCa Several approaches are used to target PKCa therapeutically, as summarized in Figure I. 1. ATP (adenosine triphosphate) binding site The ATP binding site is attractive for drug targeting, although selectivity may be difficult to achieve due to conservation of sequence and structure for this domain within the PKC family. The high concentration of ATP competes also with inhibitors at this site, decreasing their apparent activity. However, this site is still highly attractive for drug development, particularly by structure-based approaches, which are proving useful for PKCu inhibitors [71]. A publicly available crystal structure for PKCa is still absent, and is clearly a requirement for future development. Some bisinolylmaleimides, derivatives of staurosporine, show good selectivity within the PKC family. Ruboxistaurin (LY333531) and enzostaurin (LY317615) are designed to selectively target PKCb, although a recent report suggests that ruboxistaurin may inhibit PKCa equipotently [72]. Both enzastaurin and ruboxistaurin are well tolerated [73,74]. Enzastaurin shows promise in phase I trials for the treatment of advanced cancer [75]. Ruboxistaurin has completed phase III trials for the management of diabetic retinopathy [70,74], principally through targeting PKCb.
3. C1 domain The C1 domain binds diacylglycerol (DAG) and phorbol esters. Bryostatin is a macrocyclic lactone that competes with DAG for binding [77], and simplified analogues have been designed with improved activity [78]. Calphostin C also binds PKCa, and other PKC isoforms, at the C1 domain. The inhibition is irreversible, and requires prior light activation of the molecule [79]. 4. C2 domain The C2 domain binds Ca2+ and mediates interaction with proteins [80], in particular Receptors-for-Activated-C-Kinase (RACKs) [81]. RACK peptides therefore disrupt the localization of PKCs to their substrates, thereby effectively inhibiting their activity. This inhibition has been shown most extensively for PKCd, where KAI-9803 is undergoing clinical trial for cardiac ST segment elevation of cardiogram in patients with myocardial infarction [82]. PKCa interacts with fascin and lamin A, and inhibition of these interactions might disrupt cytoskeleton- and nucleus-dependent signalling [83,84]. 5. Gene therapy
2. Peptide substrate mimetics Peptide substrate mimetics are short peptides that resemble the preferred substrate sequence for PKCa, often mimicking the pseudosubstrate domain, and compete for substrate binding. In addition, a small-molecule inhibitor, chelerythrine, competitively binds to the substrate-binding pocket, although its specificity for PKCa is poor [76].
Although not shown in Figure I, an antisense drug, aprinocarsen (ISIS 3521, LY900003), has been used to reduce expression of PKCa. This drug has undergone phase III clinical trials for the treatment of various human tumors [61] but has not progressed through to the clinic, due to non-achievement of efficacy endpoints and associated toxicity.
Figure I. PKCa domain structure mapped against pharmacological inhibitors. The structure comprises an N-terminal regulatory domain, consisting of the diacylglycerol (DAG)-binding C1 domain and the Ca2+-binding C2 domain, and a C-terminal catalytic domain, consisting of an ATP-binding pocket followed by the substrate-binding domain. Each component of the molecule is a potential target for pharmacological intervention, as indicated by the drugs listed below the domains. Thus, bryostatin and calphostin C target the C1 domain, RACK peptides mimic the C2 domain region of protein-protein interaction, staurosporine and its derivatives target the ATP binding pocket, whilst peptides that mimic the pseudosubstrate domain, and chelerythrine, bind to the substrate-binding domain. Not illustrated here is the use of genetic tools, such as antisense drugs (e.g. aprinocarsen), that can alter the expression of the protein in cells and tissues.
[37]. Low-density lipoproteins (LDLs) may be oxidized when excessive free radicals react with the lipoproteins. In turn, these oxidized LDLs (Ox-LDLs) become deposited in the arterial walls and can lead to plaque development by stimulating macrophage uptake and foam cell formation. A detrimental feedback loop operates in endothelial cells, where superoxides lead to generation of Ox-LDLs, which in turn stimulate further superoxide generation. Fleming et al. [38] have shown that Ox-LDL stimulation of endothelial cells leads to uncoupling of endothelial nitric 10
oxide synthase (eNOS) from its generation of nitric oxide, and a greater accumulation of superoxide free radicals. The signalling pathway in endothelial cells from Ox-LDL to eNOS has been attributed to inactivation and down-regulation of PKCa, which results in diminished phosphorylation of eNOS on Thr-495 [38]. Additionally, PKCa in inflammatory cells might mediate their adhesion to and interaction with endothelial cells. The adhesion of circulating monocytes to endothelial cells contributes importantly to the inflammatory aspects of
Opinion atherogenesis, which describes the development of atherosclerotic plaques. Pre-incubation of monocytes with Go¨6976, the inhibitor of classical PKC isoforms, significantly reduces monocyte adhesion to HUVECs (human umbilical vein endothelial cells) [39]. Another inflammatory mediator, apolipoprotein CIII, a constituent of apolipoprotein B, enhances the adhesion of monocytes to endothelial cells via PKCa-mediated b1-integrin activation [40]. Lastly, endothelial cell PKCa is important in also regulating inflammatory cell adhesion, through expression of the adhesion molecule PECAM-1 [41], and through regulating exocytosis and surface expression of P-selectin, which mediates neutrophil-endothelial cell interaction [42]. In summary, although there is a mixed picture in terms of the role of PKCa in atherogenesis and cardiomyocyte hypertrophy, there is reasonable consensus that, on balance, inhibitors of PKCa are likely to be useful in the management of these diseases. Platelet function and thrombosis Platelets play a central role in mediating atherothrombosis, characterized by an unpredictable atherosclerotic plaque disruption, and are therefore the target of numerous therapies aimed at reducing their activity, particularly in the prevention of coronary artery thrombosis in heart attacks [43]. PKC has been established, largely by pharmacological studies, as a major regulator of multiple platelet activities [44], and it is increasingly clear that the different isozymes of PKC expressed in platelets perform distinct functions. On activation, platelets release a multitude of active substances from secretory granules, essential for platelet pro-aggregatory responses and development of a stable thrombus in arteries. Yoshioka et al. [45] have analyzed the mechanisms governing Ca2+-induced secretion of agranules and dense-core granules in permeabilized human platelets, and identified PKCa as an essential component of the mechanism. Using similar biochemical approaches, the same group demonstrated that PKCa is also involved in the regulation of Ca2+-induced platelet aggregation [46]. Pula et al. [47] have shown that two tyrosine kinases, Syk and Src, physically interact with PKCa, leading to distinct functional consequences. Although they found that PKCa activity was dependent on Syk, Syk activity was not regulated by PKCa; however, Src activity was negatively regulated by PKCa. These results suggest that PKCa is an important factor in the complex interaction with tyrosine kinases for regulation of functional activities in platelets. Additionally, in a recent elegant study reconstructing the signalling pathway regulating platelet integrin aIIbb3 in a heterologous cell system, it was shown that PKCa expression is required for activation of the integrin through the Rap1 pathway [48]. Key molecular functions of platelet activation are therefore mediated by PKCa. On the basis of pharmacological data, the specificity of the role played by PKCa has been a contentious issue, due to the lack of selectivity of the reagents available. We have therefore developed a genetic approach using PKCa knockout (Prkca / ) mice to determine the role of PKCa in regulating platelet function and thrombus formation [4]. With regard to the suggested role of PKCa in regulating secretion of dense- and a-granules [45], we were able to
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confirm definitively that secretion of these granules was substantially diminished in Prkca / platelets. Prkca / platelets showed significantly reduced functionally relevant phosphorylation on Ser95 of synaptosomal associated protein SNAP-23, an essential component of the membrane fusion machinery and an important regulator of vesicle docking and fusion [4]. This observation therefore may potentially explain the observed secretion defect in PKCa-deficient platelets. In addition, Prkca / platelets show a marked knockdown in activation of integrin aIIbb3 in response either to thrombin or collagen-related peptide, paralleling a deficit in their ability to undergo aggregation at submaximal concentrations of agonist. This is also consistent with the impaired ability of Prkca / platelets to form a thrombus in vitro in blood flowing over a collagen-coated surface and in an in vivo laser-induced model of thrombus formation (Fig. 2). The defect in secretion was shown, however, to be the central significant event, because addition of exogenous ADP (adenosine diphosphate), the major constituent released from platelet dense granules, rescued the deficits in responses seen in Prkca / platelets, confirming that granule secretion is likely to be the primary function for PKCa. These findings, together with the fact that Prkca / mice do not demonstrate any evidence of overt bleeding, have revealed that PKCa is a potential drug target for antithrombotic therapy, because selective inhibitors would be expected to exert a major effect on thrombus formation while sparing primary platelet adhesive functions. PKCa in cancer PKCa has long been recognized to have a role in regulating aspects of tumor growth and development [10], although this role is clearly complex and highly tissue-dependent because in some cases it acts as a tumor promoter, and in others it functions as a tumor suppressor. Nonetheless, PKC in general has, for many years, been a drug target for the treatment of cancer. Pharmacological approaches to control of cancer by PKC inhibitors have been discussed elsewhere [7,49] and some of these approaches for PKCa are discussed in Box 1. Its most consistent role is probably regulation of cell motility, and certainly PKCa activation can result in increased cell motility in several in vivo and in vitro cancer models, the effect of which may be reversed on PKCa inhibition[50,51]. However, the predominant overall picture is of a lack of consistency, which can be illustrated by its differing expression patterns in different tumors. Overexpression of PKCa has been demonstrated in tissue samples of prostate, endometrial, high-grade urinary bladder and hepatocellular cancers[52–55], while for haematological malignancies up- or down-regulation of PKCa has been described [56], and in basal cell carcinoma and colon cancers, down-regulation of PKCa has been observed [2,57]. A mixed picture also pertains to breast cancer cells, where it has been studied extensively. Whereas activation or over-expression of PKCa has been shown in breast cancer cells and in breast tumor samples by some [58,59], down-regulation has been demonstrated by others [60]. In terms of cancer therapeutics, PKCa has been a target for the drug aprinocarsen (ISIS 3521; see also Box 1), a 11
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Figure 2. PKCa plays a critical role regulating platelet function and thrombosis in arteries. Platelets are the smallest cellular component of the blood, and flow close to the endothelial lining of arteries continually surveying the vessel for breaches in endothelial integrity. In the main panel platelets are depicted encountering such a breach, exposing subendothelial collagen to which platelets adhere and activate. Critical for the rapid build up of a platelet aggregate, or thrombus, is the release of positive feedback mediators, many of which, including ADP which acts on its receptors P2Y1 and P2Y12, are contained in secretory granules in platelets. The expanded inset figure illustrates that PKCa plays a crucial role mediating signals from the platelet collagen receptor, GPVI, to release of granule contents, including ADP, and also to activation of the major adhesion molecule, integrin aIIbb3. This integrin bridges platelets through binding the plasma protein fibrinogen. Platelet–platelet interaction and subsequent build up of a thrombus in arteries is therefore dependent upon platelet PKCa. Arterial thrombus formation is the major occlusive event underlying vessel blockage in heart disease and thrombotic stroke, and therefore platelet PKCa may represent a novel target for therapeutic intervention.
phosphorothioate antisense oligonucleotide (ASO) that targets the 3’-untranslated region of human PKCa mRNA, leading to decreased expression of the protein [61]. Aprinocarsen has been studied as a single agent, as well as in combination with standard chemotherapeutics, in cancer patients in over 20 trials from phase I to phase III (reviewed in [61,62]). The studies include individuals with non-small cell lung cancer [63], colon [64], ovarian [65], breast [66] and prostate cancers [67] and non-Hodgkin’s lymphoma [68]. Although encouraging results have been observed in phase II trials, disappointingly aprinocarsen has failed to meet efficacy endpoints in phase III trials when used in combination with conventional chemother12
apy for late-stage lung cancer [69,70]. Reasons for this lack of efficacy could be manifold, including lack of prior screening of patients for expression levels of PKCa, dose reduction due to toxicity during the trial and driving of cell proliferation by other related PKC isoforms [69]. Conclusions PKCa plays important roles in several cellular processes and pathologies, involving cancer, cardiovascular disorders, atherogenesis and thrombosis. Its ubiquity and multiple roles, however, present a potential difficulty in its use as a drug target, although the mutual functional redundancy of various PKC isoforms might help to
Opinion overcome possible unwanted effects of specific pharmacological intervention. It is clear, for instance, that thrombus formation is markedly diminished in the Prkca / mouse, whereas haemostasis is normal [4]. This functional redundancy with related PKC isoforms may also obscure other potential roles for PKCa, determined in particular from gene knockout strategies. There are clearly many strategies that can be adopted to target this kinase therapeutically (Box 1), and in this regard a key advance will be the determination of the crystal structure of the catalytic domain of PKCa. It will be important to continue developing strategies for PKCa targeting in the future, because specific drug targeting could have great value in the treatment of multiple disease states. Conflict of interest declaration The authors have declared that no conflict of interest exists. Acknowledgments The authors thank Drs Matthew Jones and Matthew Harper for their advice and critical reading of the manuscript. Work in the authors’ laboratory is supported by grants from the British Heart Foundation (grant nos. RG/05/015 and PG/07/118/24152).
References 1 Parker, P.J. and Murray-Rust, J. (2004) PKC at a glance. J. Cell Sci. 117, 131–132 2 Oster, H. and Leitges, M. (2006) Protein kinase Ca but not PKCz suppresses intestinal tumor formation in ApcMin/+ mice. Cancer Res. 66, 6955–6963 3 Braz, J.C. et al. (2004) PKCa regulates cardiac contractility and propensity toward heart failure. Nat. Med. 10, 248–254 4 Konopatskaya, O. et al. (2009) PKCa regulates platelet granule secretion and thrombus formation in mice. J. Clin. Invest. 119, 399–407 5 Churchill, E. et al. (2008) PKC isozymes in chronic cardiac disease: possible therapeutic targets? Annu. Rev. Pharmacol. Toxicol. 48, 569– 599 6 Palaniyandi, S.S. et al. (2009) Protein kinase C in heart failure: a therapeutic target? Cardiovasc. Res. 82, 229–239 7 Mackay, H.J. and Twelves, C.J. (2007) Targeting the protein kinase C family: are we there yet? Nat. Rev. Cancer 7, 554–562 8 Dempsey, E.C. et al. (2000) Protein kinase C isozymes and the regulation of diverse cell responses. Am. J. Physiol. Lung Cell Mol. Physiol. 279, L429–438 9 Gould, C.M. and Newton, A.C. (2008) The life and death of protein kinase C. Curr. Drug Targets 9, 614–625 10 Nakashima, S. (2002) Protein kinase Ca (PKCa): regulation and biological function. J. Biochem. 132, 669–675 11 Leitges, M. et al. (2002) Knockout of PKCa enhances insulin signaling through PI3K. Mol. Endocrinol. 16, 847–858 12 Pfeifhofer, C. et al. (2006) Defective IgG2a/2b class switching in PKCa-/- mice. J. Immunol. 176, 6004–6011 13 von Essen, M. et al. (2006) Protein kinase C (PKC) a and PKCu are the major PKC isotypes involved in TCR down-regulation. J. Immunol. 176, 7502–7510 14 Vijayan, K. et al. (2004) Protein kinase Ca-induced hypertrophy of neonatal rat ventricular myocytes. Am. J. Physiol. Heart Circ. Physiol. 287, H2777–2789 15 Kerkela, R. et al. (2002) Identification of PKCa isoform-specific effects in cardiac myocytes using antisense phosphorothioate oligonucleotides. Mol. Pharmacol. 62, 1482–1491 16 Braz, J.C. et al. (2002) PKCa regulates the hypertrophic growth of cardiomyocytes through extracellular signal-regulated kinase1/2 (ERK1/2). J. Cell Biol. 156, 905–919 17 Belin, R.J. et al. (2007) Augmented protein kinase Ca-induced myofilament protein phosphorylation contributes to myofilament dysfunction in experimental congestive heart failure. Circ. Res. 101, 195–204
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18 Sumandea, M.P. et al. (2003) Identification of a functionally critical protein kinase C phosphorylation residue of cardiac troponin T. J. Biol. Chem. 278, 35135–35144 19 Holm, P.W. et al. (2009) Atherosclerotic plaque development and instability: a dual role for VEGF. Ann. Med. 41, 257–264 20 Rask-Madsen, C. and King, G.L. (2008) Differential regulation of VEGF signaling by PKCa and PKCe in endothelial cells. Arterioscler Thromb. Vasc. Biol. 28, 919–924 21 Xia, P. et al. (1996) Characterization of vascular endothelial growth factor’s effect on the activation of protein kinase C, its isoforms, and endothelial cell growth. J. Clin. Invest. 98, 2018–2026 22 Goekjian, P.G. and Jirousek, M.R. (2001) Protein kinase C inhibitors as novel anticancer drugs. Expert Opin. Investig. Drugs 10, 2117– 2140 23 Wang, A. et al. (2002) Inhibition of protein kinase Ca prevents endothelial cell migration and vascular tube formation in vitro and myocardial neovascularization in vivo. Circ. Res. 90, 609–616 24 Xu, H. et al. (2008) Protein kinase Ca promotes angiogenic activity of human endothelial cells via induction of vascular endothelial growth factor. Cardiovasc. Res. 78, 349–355 25 Harrington, E.O. et al. (2000) Endothelial proliferation, migration, and differentiation are blunted by conditionally expressed protein kinase C pseudosubstrate peptides. Biochem. Biophys Res. Commun. 271, 499– 508 26 Mountain, D.J. et al. (2007) Interleukin-1beta increases expression and activity of matrix metalloproteinase-2 in cardiac microvascular endothelial cells: role of PKCalpha/beta1 and MAPKs. Am. J. Physiol. Cell Physiol. 292, C867–875 27 Aghajanian, A. et al. (2008) Endothelial cell junctions and the regulation of vascular permeability and leukocyte transmigration. J. Thromb. Haemost. 6, 1453–1460 28 Lum, H. and Malik, A.B. (1994) Regulation of vascular endothelial barrier function. Am. J. Physiol. 267, L223–241 29 Sandoval, R. et al. (2001) Ca(2+) signalling and PKCa activate increased endothelial permeability by disassembly of VE-cadherin junctions. J. Physiol. 533, 433–445 30 Tiruppathi, C. et al. (2002) Role of Ca2+ signaling in the regulation of endothelial permeability. Vascul. Pharmacol. 39, 173–185 31 Ahmmed, G.U. et al. (2004) Protein kinase Ca phosphorylates the TRPC1 channel and regulates store-operated Ca2+ entry in endothelial cells. J. Biol. Chem. 279, 20941–20949 32 Mehta, D. et al. (2001) Protein kinase C-alpha signals rho-guanine nucleotide dissociation inhibitor phosphorylation and rho activation and regulates the endothelial cell barrier function. J. Biol. Chem. 276, 22614–22620 33 Stamatovic, S.M. et al. (2006) Protein kinase Calpha-RhoA cross-talk in CCL2-induced alterations in brain endothelial permeability. J. Biol. Chem. 281, 8379–8388 34 Ferro, T. et al. (2000) Protein kinase C-alpha mediates endothelial barrier dysfunction induced by TNF-alpha. Am. J. Physiol. Lung Cell Mol. Physiol. 278, L1107–1117 35 Lum, H. et al. (2001) Protein phosphatase 2B inhibitor potentiates endothelial PKC activity and barrier dysfunction. Am. J. Physiol. Lung Cell Mol. Physiol. 281, L546–555 36 Hempel, A. et al. (1997) High glucose concentrations increase endothelial cell permeability via activation of protein kinase C alpha. Circ. Res. 81, 363–371 37 Witztum, J.L. and Steinberg, D. (1991) Role of oxidized low density lipoprotein in atherogenesis. J. Clin. Invest. 88, 1785–1792 38 Fleming, I. et al. (2005) Oxidized low-density lipoprotein increases superoxide production by endothelial nitric oxide synthase by inhibiting PKCalpha. Cardiovasc. Res. 65, 897–906 39 Takahashi, K. et al. (2006) SDF-1-induced adhesion of monocytes to vascular endothelium is modulated by azelnidipine via protein kinase C inhibition. Eur. J. Pharmacol. 552, 162–169 40 Kawakami, A. et al. (2007) Apolipoprotein CIII-induced THP-1 cell adhesion to endothelial cells involves pertussis toxin-sensitive G protein- and protein kinase C alpha-mediated nuclear factor-kappaB activation. Arterioscler Thromb. Vasc. Biol. 27, 219–225 41 Gong, N. et al. (2004) Lactosylceramide recruits PKCalpha/epsilon and phospholipase A2 to stimulate PECAM-1 expression in human monocytes and adhesion to endothelial cells. Proc. Natl. Acad. Sci. U. S. A. 101, 6490–6495
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Opinion 42 Fu, J. et al. (2005) Protease-activated receptor-1 activation of endothelial cells induces protein kinase Calpha-dependent phosphorylation of syntaxin 4 and Munc18c: role in signaling pselectin expression. J. Biol. Chem. 280, 3178–3184 43 Meadows, T.A. and Bhatt, D.L. (2007) Clinical aspects of platelet inhibitors and thrombus formation. Circ. Res. 100, 1261–1275 44 Harper, M.T. and Poole, A.W. (2007) Isoform-specific functions of protein kinase C: the platelet paradigm. Biochem. Soc. Trans. 35, 1005–1008 45 Yoshioka, A. et al. (2001) Identification of protein kinase Calpha as an essential, but not sufficient, cytosolic factor for Ca2+-induced alphaand dense-core granule secretion in platelets. J. Biol. Chem. 276, 39379–39385 46 Tabuchi, A. et al. (2003) Direct demonstration of involvement of protein kinase Calpha in the Ca2+-induced platelet aggregation. J. Biol. Chem. 278, 26374–26379 47 Pula, G. et al. (2005) Functional interaction of protein kinase Calpha with the tyrosine kinases Syk and Src in human platelets. J. Biol. Chem. 280, 7194–7205 48 Han, J. et al. (2006) Reconstructing and deconstructing agonistinduced activation of integrin alphaIIbbeta3. Curr. Biol. 16, 1796–1806 49 Fields, A.P. and Murray, N.R. (2008) Protein kinase C isozymes as therapeutic targets for treatment of human cancers. Adv. Enzyme Regul. 48, 166–178 50 Koivunen, J. et al. (2004) Protein kinase C alpha/beta inhibitor Go6976 promotes formation of cell junctions and inhibits invasion of urinary bladder carcinoma cells. Cancer Res. 64, 5693–5701 51 Masur, K. et al. (2001) High PKC alpha and low E-cadherin expression contribute to high migratory activity of colon carcinoma cells. Mol. Biol. Cell 12, 1973–1982 52 Fournier, D.B. et al. (2001) Protein kinase C alpha expression is inversely related to ER status in endometrial carcinoma: possible role in AP-1-mediated proliferation of ER-negative endometrial cancer. Gynecol. Oncol. 81, 366–372 53 Koren, R. et al. (2004) Expression of protein kinase C isoenzymes in benign hyperplasia and carcinoma of prostate. Oncol. Rep. 11, 321–326 54 Langzam, L. et al. (2001) Patterns of protein kinase C isoenzyme expression in transitional cell carcinoma of bladder. Relation to degree of malignancy. Am. J. Clin. Pathol. 116, 377–385 55 Varga, A. et al. (2004) Tumor grade-dependent alterations in the protein kinase C isoform pattern in urinary bladder carcinomas. Eur. Urol. 46, 462–465 56 Lahn, M. et al. (2006) The role of protein kinase C-alpha in hematologic malignancies. Acta Haematol. 115, 1–8 57 Neill, G.W. et al. (2003) Loss of protein kinase Calpha expression may enhance the tumorigenic potential of Gli1 in basal cell carcinoma. Cancer Res. 63, 4692–4697 58 Lahn, M. et al. (2004) Protein kinase C alpha expression in breast and ovarian cancer. Oncology 67, 1–10 59 Tan, M. et al. (2006) Upregulation and activation of PKC alpha by ErbB2 through Src promotes breast cancer cell invasion that can be blocked by combined treatment with PKC alpha and Src inhibitors. Oncogene 25, 3286–3295 60 Kerfoot, C. et al. (2004) Immunohistochemical analysis of advanced human breast carcinomas reveals downregulation of protein kinase C alpha. J. Histochem. Cytochem. 52, 419–422 61 Hanauske, A.R. et al. (2004) The role of protein kinase C-alpha (PKCalpha) in cancer and its modulation by the novel PKC-alpha-specific inhibitor aprinocarsen. Curr. Pharm. Des. 10, 1923–1936 62 Li, K. and Zhang, J. (2001) ISIS-3521. Isis Pharmaceuticals. Curr. Opin. Investig. Drugs 2, 1454–1461 63 Villalona-Calero, M.A. et al. (2004) A phase I/II study of LY900003, an antisense inhibitor of protein kinase C-alpha, in combination with
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cisplatin and gemcitabine in patients with advanced non-small cell lung cancer. Clin. Cancer Res. 10, 6086–6093 Marshall, J.L. et al. (2004) A phase II trial of ISIS 3521 in patients with metastatic colorectal cancer. Clin. Colorectal. Cancer 4, 268–274 Advani, R. et al. (2004) A Phase II trial of aprinocarsen, an antisense oligonucleotide inhibitor of protein kinase C alpha, administered as a 21-day infusion to patients with advanced ovarian carcinoma. Cancer 100, 321–326 Roychowdhury, D. and Lahn, M. (2003) Antisense therapy directed to protein kinase C-alpha (Affinitak, LY900003/ISIS 3521): potential role in breast cancer. Semin Oncol. 30, 30–33 Tolcher, A.W. et al. (2002) A randomized phase II and pharmacokinetic study of the antisense oligonucleotides ISIS 3521 and ISIS 5132 in patients with hormone-refractory prostate cancer. Clin. Cancer Res. 8, 2530–2535 Rao, S. et al. (2004) Phase II study of ISIS 3521, an antisense oligodeoxynucleotide to protein kinase C alpha, in patients with previously treated low-grade non-Hodgkin’s lymphoma. Ann. Oncol. 15, 1413–1418 Paz-Ares, L. et al. (2006) Phase III study of gemcitabine and cisplatin with or without aprinocarsen, a protein kinase C-alpha antisense oligonucleotide, in patients with advanced-stage non-small-cell lung cancer. J. Clin. Oncol. 24, 1428–1434 Roffey, J. et al. (2009) Protein kinase C intervention: the state of play. Curr. Opin. Cell Biol. 21, 268–279 Cole, D.C. et al. (2008) Identification, characterization and initial hitto-lead optimization of a series of 4-arylamino-3-pyridinecarbonitrile as protein kinase C theta (PKCtheta) inhibitors. J. Med. Chem. 51, 5958–5963 Liu, Q. et al. (2009) Protein Kinase C{alpha}, but Not PKC{beta} or PKC{gamma}, Regulates Contractility and Heart Failure Susceptibility. Implications for Ruboxistaurin As a Novel Therapeutic Approach. Circ. Res. 105, 194–200 Chen, Y.B. and LaCasce, A.S. (2008) Enzastaurin. Expert Opin. Investig. Drugs 17, 939–944 Das Evcimen, N. and King, G.L. (2007) The role of protein kinase C activation and the vascular complications of diabetes. Pharmacol. Res. 55, 498–510 Rademaker-Lakhai, J.M. et al. (2007) Phase I pharmacokinetic and pharmacodynamic study of the oral protein kinase C beta-inhibitor enzastaurin in combination with gemcitabine and cisplatin in patients with advanced cancer. Clin. Cancer Res. 13, 4474–4481 Herbert, J.M. et al. (1990) Chelerythrine is a potent and specific inhibitor of protein kinase C. Biochem. Biophys Res. Commun. 172, 993–999 Szallasi, Z. et al. (1994) Differential regulation of protein kinase C isozymes by bryostatin 1 and phorbol 12-myristate 13-acetate in NIH 3T3 fibroblasts. J. Biol. Chem. 269, 2118–2124 Wender, P.A. et al. (2008) Function-oriented synthesis, step economy, and drug design. Acc. Chem. Res. 41, 40–49 Bruns, R.F. et al. (1991) Inhibition of protein kinase C by calphostin C is light-dependent. Biochem. Biophys Res. Commun. 176, 288–293 Poole, A.W. et al. (2004) PKC-interacting proteins: from function to pharmacology. Trends Pharmacol. Sci. 25, 528–535 Churchill, E.N. et al. (2009) Rationally designed peptide regulators of protein kinase C. Trends Endocrinol. Metab. 20, 25–33 Bates, E. et al. (2008) Intracoronary KAI-9803 as an adjunct to primary percutaneous coronary intervention for acute ST-segment elevation myocardial infarction. Circulation 117, 886–896 Martelli, A.M. et al. (2002) Molecular characterization of protein kinase C-alpha binding to lamin A. J. Cell Biochem. 86, 320–330 Tabellini, G. et al. (2002) Binding of elements of protein kinase C-alpha regulatory domain to lamin B1. Cell Signal. 14, 819–827