Rationally designed peptide regulators of protein kinase C

Comprehensive insights into herbal P-glycoprotein inhibitors and nanoformulations for improving anti-retroviral therapy efficacy

The worldwide HIV cases were 39.0 million (33.1-45.7 million) in 2022. Due to genetic variations, HIV-1 is more easily transmitted than HIV-2 and favours CD4 + T cells and macrophages, producing AIDS. Conventional HIV drug therapy has many drawbacks, including adherence issues leading to resistance, side effects that lower life quality, drug interactions, high costs limiting global access, inability to eliminate viral reservoirs, chronicity requiring lifelong treatment, emerging toxicities, and a focus on managing infections. Conventional dosage forms have bioavailability issues due to intestinal P-glycoprotein (P-gp) efflux, which can reduce anti-retroviral drug efficacy and lead to resistance. Use of phyto-constituents with P-gp regulating actions has great benefits for semi-synthetic modification to create formulations with greater bioavailability and reduced toxicity, which improves drug effectiveness. Lipid-based nanocarriers, solid lipid nanoparticles, nanostructured lipid carriers, polymer-based nanocarriers, and inorganic nanoparticles may inhibit P-gp efflux. Employing potent P-gp inhibitors within nanocarriers as a Trojan horse approach can enhance the intracellular accumulation of anti-retroviral drugs (ARDs), which are substrates for efflux transporters. This technique increases oral bioavailability and offers lower-dose options, boosting HIV patient compliance and lowering costs. Molecular docking of the inhibitor with P-gp may anticipate optimum binding and function, allowing drug efflux to be minimised.

Effect of N-o-nitrobenzylation on conformation and membrane permeability of linear peptides

In this study, we explored the potential of the photoremovable o-nitrobenzyl (oNB) group as a tool to manipulate the membrane permeability and regulate the conformation of linear peptides by means of experimental and computational studies. We found that the introduction of one or more oNB groups markedly increased the permeability and altered the conformation, as compared to the corresponding unmodified peptides. We thoroughly investigated the impact of peptide length, number of oNB group, oNB insertion position, and introduction of N- and C-terminal protecting groups on the passive membrane permeability by means of parallel artificial membrane permeability assay (PAMPA). Photoreaction of peptides containing one or two oNB groups proceeded cleanly in moderate to high yields, releasing the unprotected parent linear peptide. The oNB-modified peptides showed a cis/trans conformational equilibrium, while after photolysis, the unprotected linear peptides showed only the trans-amide conformation. Furthermore, a comprehensive comparison of oNB-modified peptides and N-methylated peptides was conducted, encompassing conformational analysis and physicochemical properties. N-Substituted peptides favored a folded-like structure, which may contribute to the improvement in permeability.

Current Challenges and Future Directions in Peptidomics

The field of peptidomics has been under development since its start more than 20 years ago. In this chapter we provide a personal outlook for future directions in this field. The applications of peptidomics technologies are spreading more and more from classical research of peptide hormones and neuropeptides towards commercial applications in plant and food-science. Many clinical applications have been developed to analyze the complexity of biofluids, which are being addressed with new instrumentation, automization, and data processing. Additionally, the newly developed field of immunopeptidomics is showing promise for cancer therapies. In conclusion, peptidomics will continue delivering important information in classical fields like neuropeptides and peptide hormones, benefiting from improvements in state-of-the-art technologies. Moreover, new directions of research such as immunopeptidomics will further complement classical omics technologies and may become routine clinical procedures. Taken together, discoveries of new substances, networks, and applications of peptides can be expected in different disciplines.

An Update on Protein Kinases as Therapeutic Targets-Part II: Peptides as Allosteric Protein Kinase C Modulators Targeting Protein-Protein Interactions

Human protein kinases are highly-sought-after drug targets, historically harnessed for treating cancer, cardiovascular disease, and an increasing number of autoimmune and inflammatory conditions. Most current treatments involve small molecule protein kinase inhibitors that interact orthosterically with the protein kinase ATP-binding pocket. As a result, these compounds are often poorly selective and highly toxic. Part I of this series reviews the role of PKC isoforms in various human diseases, featuring cancer and cardiovascular disease, as well as translational examples of PKC modulation applied to human health and disease. In the present Part II, we discuss alternative allosteric binding mechanisms for targeting PKC, as well as novel drug platforms, such as modified peptides. A major goal is to design protein kinase modulators with enhanced selectivity and improved pharmacological properties. To this end, we use molecular docking analysis to predict the mechanisms of action for inhibitor-kinase interactions that can facilitate the development of next-generation PKC modulators.

Storming the gate: New approaches for targeting the dynamic tight junction for improved drug delivery

As biologics used in the clinic outpace the number of new small molecule drugs, an important challenge for their efficacy and widespread use has emerged, namely tissue penetrance. Macromolecular drugs - bulky, high-molecular weight, hydrophilic agents - exhibit low permeability across biological barriers. Epithelial and endothelial layers, for example within the gastrointestinal tract or at the blood-brain barrier, present the most significant obstacle to drug transport. Within epithelium, two subcellular structures are responsible for limiting absorption: cell membranes and intercellular tight junctions. Previously considered impenetrable to macromolecular drugs, tight junctions control paracellular flux and dictate drug transport between cells. Recent work, however, has shown tight junctions to be dynamic, anisotropic structures that can be targeted for delivery. This review aims to summarize new approaches for targeting tight junctions, both directly and indirectly, and to highlight how manipulation of tight junction interactions may help usher in a new era of precision drug delivery.

Impact of peptide permeation enhancer on tight junctions opening cellular mechanisms

The myristoylated pentapeptide, L-R5, contains an amino acid sequence of the zeta inhibitory peptide (ZIP) portion (pseudosubstrate) of protein kinase C zeta (PKC ζ). As PKC ζ is involved in the modulation of epithelial tight junctions (TJs) through the phosphorylation of TJ proteins, L-R5 was suggested to interact with the enzyme resulting in the enhancement of paracellular permeability. This study shows that L-R5 does not bind to the enzyme but interacts directly with TJ proteins. We show here that the binding of PKC ζ to occludin and its successive phosphorylation is prevented by L-R5, which leads to TJ disruption and enhanced epithelial permeability. Although L-R5 did not show any in vitro cytotoxicity, a proteomics study revealed that L-R5 interferes with other regulatory pathways, e.g., apoptosis and immune response. We suggest that structural modification of the peptide may increase the specificity TJ protein-peptide interaction.

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Microscale thermophoresis (MST) showed robust results for protein bindings.

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The competitivity of L-R5 peptide for the binding of occludin-PKC zeta was shown.

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Tight junctions proteins expression was decreased due to L-R5 peptide.

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Multiple other mechanisms can be explored to use L-R5 for other therapies.

Recent Advances in Pain Management: Relevant Protein Kinases and Their Inhibitors

The purpose of this review is to underline the protein kinases that have been established, either in fundamental approach or clinical trials, as potential biological targets in pain management. Protein kinases are presented according to their group in the human kinome: TK (Trk, RET, EGFR, JAK, VEGFR, SFK, BCR-Abl), CMGC (p38 MAPK, MEK, ERK, JNK, ASK1, CDK, CLK2, DYRK1A, GSK3, CK2), AGC (PKA, PKB, PKC, PKMζ, PKG, ROCK), CAMK, CK1 and atypical/other protein kinases (IKK, mTOR). Examples of small molecule inhibitors of these biological targets, demonstrating an analgesic effect, are described. Altogether, this review demonstrates the fundamental role that protein kinase inhibitors could play in the development of new pain treatments.

The PKC-ζ pseudosubstrate peptide induces glutamate release from retinal pigment epithelium cells through kinase- independent activation of Best1

AIMS

The retinal pigment epithelium (RPE) is a highly specialized cell monolayer, that plays a key role in the maintenance of photoreceptor function and the blood-retina barrier (BRB). In this study, we found that a myristoylated pseudosubstrate of PKC-ζ (PKCζ PS), considered as a PKC-ζ inhibitor, plays a distinct role in RPE.

MAIN METHODS

We demonstrated that PKCζ PS stimulates the release of Glutamate (Glu) using in vitro³H-Glutamate release experiments. By western blot, kinase assays, and Fluoresence Ca⁺² Concentration Measurements, we determined the cellular mechanisms involved in such release.

KEY FINDINGS

Surprisingly, PKCζ PS has no effect on either phosphorylation of T560, essential for catalytic activity, nor it has an effect on kinase activity. It induces the dose-dependent release of Glu by increasing intracellular Ca⁺² levels. Interestingly, this release was not observed upon stimulation by other non-competitive PKC-ζ inhibitors. We here demonstrated that the PKCζ PS stimulates the release of Glutamate from RPE by activating the Ca²⁺-dependent Cl channel Bestrophin 1 (Best1).

SIGNIFICANCE

These results question PKCζ PS specificity as an inhibitor of this enzyme. Furthermore, the present results underline the relevance of clarifying the molecular mechanisms involved in glutamate release from the retina under conditions derived from excitotoxic stimuli.

Functional proteomic analysis reveals roles for PKCδ in regulation of cell survival and cell death: Implications for cancer pathogenesis and therapy

Protein Kinase C-δ (PKCδ), regulates a broad group of biological functions and disease processes, including well-defined roles in immune function, cell survival and apoptosis. PKCδ primarily regulates apoptosis in normal tissues and non-transformed cells, and genetic disruption of the PRKCD gene in mice is protective in many diseases and tissue damage models. However pro-survival/pro-proliferative functions have also been described in some transformed cells and in mouse models of cancer. Recent evidence suggests that the contribution of PKCδ to specific cancers may depend in part on the oncogenic context of the tumor, consistent with its paradoxical role in cell survival and cell death. Here we will discuss what is currently known about biological functions of PKCδ and potential paradigms for PKCδ function in cancer. To further understand mechanisms of regulation by PKCδ, and to gain insight into the plasticity of PKCδ signaling, we have used functional proteomics to identify pathways that are dependent on PKCδ. Understanding how these distinct functions of PKCδ are regulated will be critical for the logical design of therapeutics to target this pathway.

Proteasome Inhibitor Drugs

Proteasomes are large, multicatalytic protein complexes that cleave cellular proteins into peptides. There are many distinct forms of proteasomes that differ in catalytically active subunits, regulatory subunits, and associated proteins. Proteasome inhibitors are an important class of drugs for the treatment of multiple myeloma and mantle cell lymphoma, and they are being investigated for other diseases. Bortezomib (Velcade) was the first proteasome inhibitor to be approved by the US Food and Drug Administration. Carfilzomib (Kyprolis) and ixazomib (Ninlaro) have recently been approved, and more drugs are in development. While the primary mechanism of action is inhibition of the proteasome, the downstream events that lead to selective cell death are not entirely clear. Proteasome inhibitors have been found to affect protein turnover but at concentrations that are much higher than those achieved clinically, raising the possibility that some of the effects of proteasome inhibitors are mediated by other mechanisms.

Age-Dependent Levels of Protein Kinase Cs in Brain: Reduction of Endogenous Mechanisms of Neuroprotection

Neurodegenerative diseases are among the leading causes of mortality and disability worldwide. However, current therapeutic approaches have failed to reach significant results in their prevention and cure. Protein Kinase Cs (PKCs) are kinases involved in the pathophysiology of neurodegenerative diseases, such as Alzheimer's Disease (AD) and cerebral ischemia. Specifically ε, δ, and γPKC are associated with the endogenous mechanism of protection referred to as ischemic preconditioning (IPC). Existing modulators of PKCs, in particular of εPKC, such as ψεReceptor for Activated C-Kinase (ψεRACK) and Resveratrol, have been proposed as a potential therapeutic strategy for cerebrovascular and cognitive diseases. PKCs change in expression during aging, which likely suggests their association with IPC-induced reduction against ischemia and increase of neuronal loss occurring in senescent brain. This review describes the link between PKCs and cerebrovascular and cognitive disorders, and proposes PKCs modulators as innovative candidates for their treatment. We report original data showing εPKC reduction in levels and activity in the hippocampus of old compared to young rats and a reduction in the levels of δPKC and γPKC in old hippocampus, without a change in their activity. These data, integrated with other findings discussed in this review, demonstrate that PKCs modulators may have potential to restore age-related reduction of endogenous mechanisms of protection against neurodegeneration.

Spinocerebellar ataxia type 14 caused by a nonsense mutation in the PRKCG gene

Spinocerebellar ataxia type 14 (SCA14) is an autosomal dominant neurodegenerative disorder characterized by cerebellar ataxia with myoclonus, dystonia, spasticity, and rigidity. Although missense mutations and a deletion mutation have been found in the protein kinase C gamma (PRKCG) gene encoding protein kinase C γ (PKCγ) in SCA14 families, a nonsense mutation has not been reported. The patho-mechanisms underlying SCA14 remain poorly understood. However, gain-of-function mechanisms and loss-of-function mechanisms, but not dominant negative mechanisms, were reported the patho-mechanism of SCA14. We identified the c.226C>T mutation of PRKCG, which caused the p.R76X in PKCγ by whole-exome sequencing in patients presenting cerebellar atrophy with cognitive and hearing impairment. To investigate the patho-mechanism of our case, we studied aggregation formation, cell death, and PKC inhibitory effect by confocal microscopy, western blotting with cleaved caspase 3, and pSer PKC motif antibodies, respectively. PKCγ(R76X)-GFP have aggregations the same as wild-type (WT) PKCγ-GFP. The PKCγ(R76X)-GFP inhibited PKC phosphorylation activity more than GFP alone. It also induced more apoptosis in COS7 and SH-SY5Y cells compared to WT-PKCγ-GFP and GFP. We first reported SCA14 patients with p.R76X in PKCγ who have cerebellar atrophy with cognitive and hearing impairment. Our results suggest that a dominant negative mechanism due to truncated peptides produced by p.R76X may be at least partially responsible for the cerebellar atrophy.

PKC and PKN in heart disease

The protein kinase C (PKC) and closely related protein kinase N (PKN) families of serine/threonine protein kinases play crucial cellular roles. Both kinases belong to the AGC subfamily of protein kinases that also include the cAMP dependent protein kinase (PKA), protein kinase B (PKB/AKT), protein kinase G (PKG) and the ribosomal protein S6 kinase (S6K). Involvement of PKC family members in heart disease has been well documented over the years, as their activity and levels are mis-regulated in several pathological heart conditions, such as ischemia, diabetic cardiomyopathy, as well as hypertrophic or dilated cardiomyopathy. This review focuses on the regulation of PKCs and PKNs in different pathological heart conditions and on the influences that PKC/PKN activation has on several physiological processes. In addition, we discuss mechanisms by which PKCs and the closely related PKNs are activated and turned-off in hearts, how they regulate cardiac specific downstream targets and pathways, and how their inhibition by small molecules is explored as new therapeutic target to treat cardiomyopathies and heart failure.

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

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

Differential roles of PKC isoforms (PKCs) in GnRH stimulation of MAPK phosphorylation in gonadotrope derived cells

The role of protein kinase C (PKC) isoforms (PKCs) in GnRH-stimulated MAPK [ERK1/2, JNK1/2 and p38) phosphorylation was examined in gonadotrope derived cells. GnRH induced a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2 and p38MAPK. Gonadotropes express conventional PKCα and PKCβII, novel PKCδ, PKCε and PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein (GFP)-PKCs constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs) has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in ERK1/2, JNK1/2 and p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in MAPKs phosphorylation may be explained by persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane. Thus, we have identified the PKCs involved in GnRH stimulated MAPKs phosphorylation in gonadotrope derived cells. Once activated, the MAPKs will mediate the transcription of the gonadotropin subunits and GnRH receptor genes.

Protein kinase C mechanisms that contribute to cardiac remodelling

Protein phosphorylation is a highly-regulated and reversible process that is precisely controlled by the actions of protein kinases and protein phosphatases. Factors that tip the balance of protein phosphorylation lead to changes in a wide range of cellular responses, including cell proliferation, differentiation and survival. The protein kinase C (PKC) family of serine/threonine kinases sits at nodal points in many signal transduction pathways; PKC enzymes have been the focus of considerable attention since they contribute to both normal physiological responses as well as maladaptive pathological responses that drive a wide range of clinical disorders. This review provides a background on the mechanisms that regulate individual PKC isoenzymes followed by a discussion of recent insights into their role in the pathogenesis of diseases such as cancer. We then provide an overview on the role of individual PKC isoenzymes in the regulation of cardiac contractility and pathophysiological growth responses, with a focus on the PKC-dependent mechanisms that regulate pump function and/or contribute to the pathogenesis of heart failure.

Peptides and peptidomimetics as regulators of protein-protein interactions

Protein-protein interactions are essential for almost all intracellular and extracellular biological processes. Regulation of protein-protein interactions is one strategy to regulate cell fate in a highly selective manner. Specifically, peptides are ideal candidates for inhibition of protein-protein interactions because they can mimic a protein surface to effectively compete for binding. Additionally, peptides are synthetically accessible and can be stabilized by chemical modifications. In this review, we survey screening and rational design methods for identifying peptides to inhibit protein-protein interactions, as well as methods for stabilizing peptides to effectively mimic protein surfaces. In addition, we discuss recent applications of peptides to regulate protein-protein interactions for both basic research and therapeutic purposes.

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

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

Differential roles of PKC isoforms (PKCs) and Ca2+ in GnRH and phorbol 12-myristate 13-acetate (PMA) stimulation of p38MAPK phosphorylation in immortalized gonadotrope cells

We examined the role of PKCs and Ca²⁺ in GnRH-stimulated p38MAPK phosphorylation in the gonadotrope derived αT3-1 and LβT2 cell lines. GnRH induced a slow and rapid increase in p38MAPK phosphorylation in αT3-1 and LβT2 cells respectively, while PMA gave a slow response. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs), has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in p38MAPK phosphorylation may be explained by differential localization of the PKCs. Basal, GnRH- and PMA- stimulation of p38MAPK phosphorylation in αT3-1 cells is mediated by Ca²⁺ influx via voltage-gated Ca²⁺ channels and Ca²⁺ mobilization, while in the differentiated LβT2 gonadotrope cells it is mediated only by Ca²⁺ mobilization. p38MAPK resides in the cell membrane and is relocated to the nucleus by GnRH (∼5 min). Thus, we have identified the PKCs and the Ca²⁺ pools involved in GnRH stimulated p38MAPK phosphorylation.

Analysis of the Yeast Peptidome and Comparison with the Human Peptidome

Peptides function as signaling molecules in species as diverse as humans and yeast. Mass spectrometry-based peptidomics techniques provide a relatively unbiased method to assess the peptidome of biological samples. In the present study, we used a quantitative peptidomic technique to characterize the peptidome of the yeast Saccharomyces cerevisiae and compare it to the peptidomes of mammalian cell lines and tissues. Altogether, 297 yeast peptides derived from 75 proteins were identified. The yeast peptides are similar to those of the human peptidome in average size and amino acid composition. Inhibition of proteasome activity with either bortezomib or epoxomicin led to decreased levels of some yeast peptides, suggesting that these peptides are generated by the proteasome. Approximately 30% of the yeast peptides correspond to the N- or C-terminus of the protein; the human peptidome is also highly represented in N- or C-terminal protein fragments. Most yeast and humans peptides are derived from a subset of abundant proteins, many with functions involving cellular metabolism or protein synthesis and folding. Of the 75 yeast proteins that give rise to peptides, 24 have orthologs that give rise to human and/or mouse peptides and for some, the same region of the proteins are found in the human, mouse, and yeast peptidomes. Taken together, these results support the hypothesis that intracellular peptides may have specific and conserved biological functions.

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

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

Selective Phosphorylation Inhibitor of Delta Protein Kinase C-Pyruvate Dehydrogenase Kinase Protein-Protein Interactions: Application for Myocardial Injury in Vivo

Protein kinases regulate numerous cellular processes, including cell growth, metabolism, and cell death. Because the primary sequence and the three-dimensional structure of many kinases are highly similar, the development of selective inhibitors for only one kinase is challenging. Furthermore, many protein kinases are pleiotropic, mediating diverse and sometimes even opposing functions by phosphorylating multiple protein substrates. Here, we set out to develop an inhibitor of a selective protein kinase phosphorylation of only one of its substrates. Focusing on the pleiotropic delta protein kinase C (δPKC), we used a rational approach to identify a distal docking site on δPKC for its substrate, pyruvate dehydrogenase kinase (PDK). We reasoned that an inhibitor of PDK's docking should selectively inhibit the phosphorylation of only PDK without affecting phosphorylation of the other δPKC substrates. Our approach identified a selective inhibitor of PDK docking to δPKC with an in vitro Kd of ∼50 nM and reducing cardiac injury IC50 of ∼5 nM. This inhibitor, which did not affect the phosphorylation of other δPKC substrates even at 1 μM, demonstrated that PDK phosphorylation alone is critical for δPKC-mediated injury by heart attack. The approach we describe is likely applicable for the identification of other substrate-specific kinase inhibitors.

Structural Basis for the Failure of the C1 Domain of Ras Guanine Nucleotide Releasing Protein 2 (RasGRP2) to Bind Phorbol Ester with High Affinity

The C1 domain represents the recognition module for diacylglycerol and phorbol esters in protein kinase C, Ras guanine nucleotide releasing protein (RasGRP), and related proteins. RasGRP2 is exceptional in that its C1 domain has very weak binding affinity (Kd = 2890 ± 240 nm for [(3)H]phorbol 12,13-dibutyrate. We have identified four amino acid residues responsible for this lack of sensitivity. Replacing Asn(7), Ser(8), Ala(19), and Ile(21) with the corresponding residues from RasGRP1/3 (Thr(7), Tyr(8), Gly(19), and Leu(21), respectively) conferred potent binding affinity (Kd = 1.47 ± 0.03 nm) in vitro and membrane translocation in response to phorbol 12-myristate 13-acetate in LNCaP cells. Mutant C1 domains incorporating one to three of the four residues showed intermediate behavior with S8Y making the greatest contribution. Binding activity for diacylglycerol was restored in parallel. The requirement for anionic phospholipid for [(3)H]phorbol 12,13-dibutyrate binding was determined; it decreased in going from the single S8Y mutant to the quadruple mutant. The full-length RasGRP2 protein with the mutated C1 domains also showed strong phorbol ester binding, albeit modestly weaker than that of the C1 domain alone (Kd = 8.2 ± 1.1 nm for the full-length protein containing all four mutations), and displayed translocation in response to phorbol ester. RasGRP2 is a guanyl exchange factor for Rap1. Consistent with the ability of phorbol ester to induce translocation of the full-length RasGRP2 with the mutated C1 domain, phorbol ester enhanced the ability of the mutated RasGRP2 to activate Rap1. Modeling confirmed that the four mutations helped the binding cleft maintain a stable conformation.

Multifunctional roles of PKCδ: Opportunities for targeted therapy in human disease

The serine-threonine protein kinase, protein kinase C-δ (PKCδ), is emerging as a bi-functional regulator of cell death and proliferation. Studies in PKCδ-/- mice have confirmed a pro-apoptotic role for this kinase in response to DNA damage and a tumor promoter role in some oncogenic contexts. In non-transformed cells, inhibition of PKCδ suppresses the release of cytochrome c and caspase activation, indicating a function upstream of apoptotic pathways. Data from PKCδ-/- mice demonstrate a role for PKCδ in the execution of DNA damage-induced and physiologic apoptosis. This has led to the important finding that inhibitors of PKCδ can be used therapeutically to reduce irradiation and chemotherapy-induced toxicity. By contrast, PKCδ is a tumor promoter in mouse models of mammary gland and lung cancer, and increased PKCδ expression is a negative prognostic indicator in Her2+ and other subtypes of human breast cancer. Understanding how these distinct functions of PKCδ are regulated is critical for the design of therapeutics to target this pathway. This review will discuss what is currently known about biological roles of PKCδ and prospects for targeting PKCδ in human disease.

Protein Kinase C γ Interneurons Mediate C-fiber–induced Orofacial Secondary Static Mechanical Allodynia, but Not C-fiber–induced Nociceptive Behavior

Background

Tissue injury enhances pain sensitivity both at the site of tissue damage and in surrounding uninjured skin (secondary hyperalgesia). Secondary hyperalgesia encompasses several pain symptoms including pain to innocuous punctate stimuli or static mechanical allodynia. How injury-induced barrage from C-fiber nociceptors produces secondary static mechanical allodynia has not been elucidated.

Methods

Combining behavioral, immunohistochemical, and Western blot analysis, the authors investigated the cell and molecular mechanisms underlying the secondary static mechanical allodynia in the rat medullary dorsal horn (MDH) using the capsaicin model (n = 4 to 5 per group).

Results

Intradermal injection of capsaicin (25 μg) into the vibrissa pad produces a spontaneous pain and a secondary static mechanical allodynia. This allodynia is associated with the activation of a neuronal network encompassing lamina I–outer lamina III, including interneurons expressing the γ isoform of protein kinase C (PKCγ) within inner lamina II (IIi) of MDH. PKCγ is concomitantly phosphorylated (+351.4 ± 79.2%, mean ± SD; P = 0.0003). Mechanical allodynia and innocuous punctate stimulus–evoked laminae I to III neuronal activation can be replicated after intracisternally applied γ-aminobutyric acid receptor type A (GABAA) antagonist (bicuculline: 0.05 μg) or reactive oxygen species (ROS) donor (tert-butyl hydroperoxide: 50 to 250 ng). Conversely, intracisternal PKCγ antagonist, GABAA receptor agonist, or ROS scavenger prevent capsaicin-induced static mechanical allodynia and neuronal activation.

Conclusions

Sensitization of lamina IIi PKCγ interneurons is required for the manifestation of secondary static mechanical allodynia but not for spontaneous pain. Such sensitization is driven by ROS and GABAAergic disinhibition. ROS released during intense C-fiber nociceptor activation might produce a GABAAergic disinhibition of PKCγ interneurons. Innocuous punctate inputs carried by Aδ low-threshold mechanoreceptors onto PKCγ interneurons can then gain access to the pain transmission circuitry of superficial MDH, producing pain.

Scaffold proteins LACK and TRACK as potential drug targets in kinetoplastid parasites: Development of inhibitors

Parasitic diseases cause ∼500,000 deaths annually and remain a major challenge for therapeutic development. Using a rational design based approach, we developed peptide inhibitors with anti-parasitic activity that were derived from the sequences of parasite scaffold proteins LACK (Leishmania's receptor for activated C-kinase) and TRACK (Trypanosomareceptor for activated C-kinase). We hypothesized that sequences in LACK and TRACK that are conserved in the parasites, but not in the mammalian ortholog, RACK (Receptor for activated C-kinase), may be interaction sites for signaling proteins that are critical for the parasites' viability. One of these peptides exhibited leishmanicidal and trypanocidal activity in culture. Moreover, in infected mice, this peptide was also effective in reducing parasitemia and increasing survival without toxic effects. The identified peptide is a promising new anti-parasitic drug lead, as its unique features may limit toxicity and drug-resistance, thus overcoming central limitations of most anti-parasitic drugs.

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Identified unique short sequences conserved in parasite but not in host orthologue.

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Peptides corresponding to these sequences are active anti-parasitic drug lead.

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Cyclization of the peptides generates drug leads for in vivo proof of concept.

Activation of medullary dorsal horn γ isoform of protein kinase C interneurons is essential to the development of both static and dynamic facial mechanical allodynia

The γ isoform of protein kinase C (PKCγ), which is concentrated in a specific class of interneurons within inner lamina II (IIi ) of the spinal dorsal horn and medullary dorsal horn (MDH), is known to be involved in the development of mechanical allodynia, a widespread and intractable symptom of inflammatory or neuropathic pain. However, although genetic and pharmacological impairment of PKCγ were shown to prevent mechanical allodynia in animal models of pain, after nerve injury or reduced inhibition, the functional consequences of PKCγ activation alone on mechanical sensitivity are still unknown. Using behavioural and anatomical approaches in the rat MDH, we tested whether PKCγ activation in naive animals is sufficient for the establishment of mechanical allodynia. Intracisternal injection of the phorbol ester, 12,13-dibutyrate concomitantly induced static as well as dynamic facial mechanical allodynia. Monitoring neuronal activity within the MDH with phospho-extracellular signal-regulated kinases 1 and 2 immunoreactivity revealed that activation of both lamina I-outer lamina II and IIi -outer lamina III neurons, including lamina IIi PKCγ-expressing interneurons, was associated with the manifestation of mechanical allodynia. Phorbol ester, 12,13-dibutyrate-induced mechanical allodynia and associated neuronal activations were all prevented by inhibiting selectively segmental PKCγ with KIG31-1. Our findings suggest that PKCγ activation, without any other experimental manipulation, is sufficient for the development of static and dynamic mechanical allodynia. Lamina IIi PKCγ interneurons have been shown to be directly activated by low-threshold mechanical inputs carried by myelinated afferents. Thus, the level of PKCγ activation within PKCγ interneurons might gate the transmission of innocuous mechanical inputs to lamina I, nociceptive output neurons, thus turning touch into pain.

Development of a Backbone Cyclic Peptide Library as Potential Antiparasitic Therapeutics Using Microwave Irradiation

Protein-protein interactions (PPIs) are intimately involved in almost all biological processes and are linked to many human diseases. Therefore, there is a major effort to target PPIs in basic research and in the pharmaceutical industry. Protein-protein interfaces are usually large, flat, and often lack pockets, complicating the discovery of small molecules that target such sites. Alternative targeting approaches using antibodies have limitations due to poor oral bioavailability, low cell-permeability, and production inefficiency. Using peptides to target PPI interfaces has several advantages. Peptides have higher conformational flexibility, increased selectivity, and are generally inexpensive. However, peptides have their own limitations including poor stability and inefficiency crossing cell membranes. To overcome such limitations, peptide cyclization can be performed. Cyclization has been demonstrated to improve peptide selectivity, metabolic stability, and bioavailability. However, predicting the bioactive conformation of a cyclic peptide is not trivial. To overcome this challenge, one attractive approach it to screen a focused library to screen in which all backbone cyclic peptides have the same primary sequence, but differ in parameters that influence their conformation, such as ring size and position. We describe a detailed protocol for synthesizing a library of backbone cyclic peptides targeting specific parasite PPIs. Using a rational design approach, we developed peptides derived from the scaffold protein Leishmania receptor for activated C-kinase (LACK). We hypothesized that sequences in LACK that are conserved in parasites, but not in the mammalian host homolog, may represent interaction sites for proteins that are critical for the parasites' viability. The cyclic peptides were synthesized using microwave irradiation to reduce reaction times and increase efficiency. Developing a library of backbone cyclic peptides with different ring sizes facilitates a systematic screen for the most biological active conformation. This method provides a general, fast, and facile way to synthesize cyclic peptides.

δV1-1 Reduces Pulmonary Ischemia Reperfusion-Induced Lung Injury by Inhibiting Necrosis and Mitochondrial Localization of PKCδ and p53

Ischemia-reperfusion (IR)-induced lung injury is one of the major contributing factors of morbidity and mortality after lung transplantation. To determine the IR-induced molecular changes in lung epithelial cells, we developed a cell-culture model that simulates lung preservation and transplantation. Six hours of cold ischemic time (CIT) and reperfusion elicited production of multiple inflammatory cytokines and chemokines and increased expression of endoplasmic reticulum (ER) proteins. Prolonged hypothermic condition (18 h CIT) reduced ER stress protein levels, and induced apoptosis and necrosis (via mechanisms related to mitochondrial permeability transition pore opening). Protein kinase C (PKCδ) was activated during CIT, and its downregulation via small interference (si) (in siRNA) RNA reduced IR-induced cytokine production and apoptotic cell death. δV1-1, a PKCδ peptide inhibitor, reduced translocation of PKCδ and p53 to the mitochondria after 18 h CIT, rescued ER stress protein expression, and converted the major mode of cell death from necrosis to apoptosis. Administration of δV1-1 effectively reduced lung transplantation and IR-induced pulmonary injury in rats. Therefore, inhibition of PKCδ by δV1-1 could be an effective strategy to ameliorate IR-induced lung injury by inhibiting the signaling pathways leading to necrosis.

Reduced Levels of Proteasome Products in a Mouse Striatal Cell Model of Huntington's Disease

Huntington's disease is the result of a long polyglutamine tract in the gene encoding huntingtin protein, which in turn causes a large number of cellular changes and ultimately results in neurodegeneration of striatal neurons. Although many theories have been proposed, the precise mechanism by which the polyglutamine expansion causes cellular changes is not certain. Some evidence supports the hypothesis that the long polyglutamine tract inhibits the proteasome, a multiprotein complex involved in protein degradation. However, other studies report normal proteasome function in cells expressing long polyglutamine tracts. The controversy may be due to the methods used to examine proteasome activity in each of the previous studies. In the present study, we measured proteasome function by examining levels of endogenous peptides that are products of proteasome cleavage. Peptide levels were compared among mouse striatal cell lines expressing either 7 glutamines (STHdhQ7/Q7) or 111 glutamines in the huntingtin protein, either heterozygous (STHdhQ7/Q111) or homozygous (STHdhQ111/Q111). Both of the cell lines expressing huntingtin with 111 glutamines showed a large reduction in nearly all of the peptides detected in the cells, relative to levels of these peptides in cells homozygous for 7 glutamines. Treatment of STHdhQ7/Q7 cells with proteasome inhibitors epoxomicin or bortezomib also caused a large reduction in most of these peptides, suggesting that they are products of proteasome-mediated cleavage of cellular proteins. Taken together, these results support the hypothesis that proteasome function is impaired by the expression of huntingtin protein containing long polyglutamine tracts.

Effective delivery of a rationally designed intracellular peptide drug with gold nanoparticle-peptide hybrids

A novel gold nanoparticle-peptide hybrid strategy was developed to intracellularly deliver a potent PKCδ inhibitor peptide for the treatment of acute lung injury. The gold nanoparticle-peptide hybrids showed good stability with high uptake, and demonstrated in vitro and in vivo efficacy. Our formulation strategy shows great promise in intracellular delivery of peptides.

Protein Kinase C (PKC)ζ Pseudosubstrate Inhibitor Peptide Promiscuously Binds PKC Family Isoforms and Disrupts Conventional PKC Targeting and Translocation

PKMζ is generated via an alternative transcriptional start site in the atypical protein kinase C (PKC)ζ isoform, which removes N-terminal regulatory elements, including the inhibitory pseudosubstrate domain, consequently rendering the kinase constitutively active. Persistent PKMζ activity has been proposed as a molecular mechanism for the long-term maintenance of synaptic plasticity underlying some forms of memory. Many studies supporting a role for PKMζ in synaptic plasticity and memory have relied on the PKCζ pseudosubstrate-derived ζ-inhibitory peptide (ZIP). However, recent studies have demonstrated that ZIP-induced impairments to synaptic plasticity and memory occur even in the absence of PKCζ, suggesting that ZIP exerts its actions via additional cellular targets. In this study, we demonstrated that ZIP interacts with conventional and novel PKC, in addition to atypical PKC isoforms. Moreover, when brain abundance of each PKC isoform and affinity for ZIP are taken into account, the signaling capacity of ZIP-responsive pools of conventional and novel PKCs may match or exceed that for atypical PKCs. Pseudosubstrate-derived peptides, like ZIP, are thought to exert their cellular action primarily by inhibiting PKC catalytic activity; however, the ZIP-sensitive catalytic core of PKC is known to participate in the enzyme's subcellular targeting, suggesting an additional mode of ZIP action. Indeed, we have demonstrated that ZIP potently disrupts PKCα interaction with the PKC-targeting protein A-kinase anchoring protein (AKAP) 79 and interferes with ionomycin-induced translocation of conventional PKC to the plasma membrane. Thus, ZIP exhibits broad-spectrum action toward the PKC family of enzymes, and this action may contribute to its unique ability to impair memory.

A structural atlas of kinases inhibited by clinically approved drugs

The aberrant activation of protein kinases is associated with many human diseases, most notably cancer. Due to this link between kinase deregulation and disease progression, kinases are one of the most targeted protein families for small-molecule inhibition. Within the last 15 years, the U.S. Food and Drug Administration has approved over 20 small-molecule inhibitors of protein kinases for use in the clinic. These inhibitors target the kinase active site and represent the successful hurdling by medicinal chemists of the formidable challenge posed by the high similarity among the active sites of the approximately 500 human kinases. We review the conserved structural features of kinases that are important for inhibitor binding as well as for catalysis. Many clinically approved drugs elicit selectivity by exploiting subtle variation within the kinase active site. We highlight some of the crystallographic studies on the kinase-inhibitor complexes that have provided valuable guidance for the development of these drugs as well as for future drug design efforts.

A new cell-penetrating peptide that blocks the autoinhibitory XIP domain of NCX1 and enhances antiporter activity

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) is a high-capacity ionic transporter that exchanges 3Na(+) ions for 1Ca(2+) ion. The first 20 amino acids of the f-loop, named exchanger inhibitory peptide (XIP(NCX1)), represent an autoinhibitory region involved in the Na(+)-dependent inactivation of the exchanger. Previous research has shown that an exogenous peptide having the same amino acid sequence as the XIP(NCX1) region exerts an inhibitory effect on NCX activity. In this study, we identified another regulatory peptide, named P1, which corresponds to the 562-688aa region of the exchanger. Patch-clamp analysis revealed that P1 increased the activity of the exchanger, whereas the XIP inhibited it. Furthermore, P1 colocalized with NCX1 thus suggesting a direct binding interaction. In addition, site-directed mutagenesis experiments revealed that the binding and the stimulatory effect of P1 requires a functional XIP(NCX1) domain on NCX1 thereby suggesting that P1 increases the exchanger activity by counteracting the action of this autoinhibitory sequence. Taken together, these results open a new strategy for developing peptidomimetic compounds that, by mimicking the functional pharmacophore of P1, might increase NCX1 activity and thus exert a therapeutic action in those diseases in which an increase in NCX1 activity might be helpful.

Overexpression of atypical protein kinase C in HeLa cells facilitates macropinocytosis via Src activation

Atypical protein kinase C (aPKC) is the first recognized kinase oncogene. However, the specific contribution of aPKC to cancer progression is unclear. The pseudosubstrate domain of aPKC is different from the other PKC family members, and therefore a synthetic peptide corresponding to the aPKC pseudosubstrate (aPKC-PS) sequence, which specifically blocks aPKC kinase activity, is a valuable tool to assess the role of aPKC in various cellular processes. Here, we learned that HeLa cells incubated with membrane permeable aPKC-PS peptide displayed dilated heterogeneous vesicles labeled with peptide that were subsequently identified as macropinosomes. A quantitative membrane binding assay revealed that aPKC-PS peptide stimulated aPKC recruitment to membranes and activated Src. Similarly, aPKC overexpression in transfected HeLa cells activated Src and induced macropinosome formation. Src-aPKC interaction was essential; substitution of the proline residues in aPKC that associate with the Src-SH3 binding domain rendered the mutant kinase unable to induce macropinocytosis in transfected cells. We propose that aPKC overexpression is a contributing factor to cell transformation by interacting with and consequently promoting Src activation and constitutive macropinocytosis, which increases uptake of extracellular factors, required for altered cell growth and accelerated cell migration.

A novel intracellular peptide derived from g1/s cyclin d2 induces cell death

Intracellular peptides are constantly produced by the ubiquitin-proteasome system, and many are probably functional. Here, the peptide WELVVLGKL (pep5) from G1/S-specific cyclin D2 showed a 2-fold increase during the S phase of HeLa cell cycle. pep5 (25-100 μm) induced cell death in several tumor cells only when it was fused to a cell-penetrating peptide (pep5-cpp), suggesting its intracellular function. In vivo, pep5-cpp reduced the volume of the rat C6 glioblastoma by almost 50%. The tryptophan at the N terminus of pep5 is essential for its cell death activity, and N terminus acetylation reduced the potency of pep5-cpp. WELVVL is the minimal active sequence of pep5, whereas Leu-Ala substitutions totally abolished pep5 cell death activity. Findings from the initial characterization of the cell death/signaling mechanism of pep5 include caspase 3/7 and 9 activation, inhibition of Akt2 phosphorylation, activation of p38α and -γ, and inhibition of proteasome activity. Further pharmacological analyses suggest that pep5 can trigger cell death by distinctive pathways, which can be blocked by IM-54 or a combination of necrostatin-1 and q-VD-OPh. These data further support the biological and pharmacological potential of intracellular peptides.

CGX1037 is a novel PKC isoform delta selective inhibitor in platelets

Platelets upon activation change their shape, aggregate and secrete alpha and dense granule contents among which ADP acts as a feedback activator. Different Protein Kinase C (PKC) isoforms have specific non-redundant roles in mediating platelet responses including secretion and thrombus formation. Murine platelets lacking specific PKC isoforms have been used to evaluate the isoform specific functions. Novel PKC isoform δ has been shown to play an important role in some pathological processes. Lack of specific inhibitors for PKCδ has restricted analysis of its role in various cells. The current study was carried out to evaluate a novel small molecule PKCδ inhibitor, CGX1037 in platelets. Platelet aggregation, dense granule secretion and western blotting experiments were performed to evaluate CGX1037. In human platelets, CGX1037 inhibited PAR4-mediated phosphorylation on PKD2, a PKCδ-specific substrate. Pre-treatment of human or murine platelets with CGX1037 inhibited PAR4-mediated dense granule secretion whereas it potentiated GPVI-mediated dense granule secretion similar to the responses observed in murine platelets lacking PKCδ· Furthermore, pre-treatment of platelets from PKCδ(-/-) mice with CGX1037 had no significant additive effect on platelet responses suggesting the specificity of CGX1037. Hence, we show that CGX1037 is a selective small molecule inhibitor of PKCδ in platelets.

Protein kinase C and acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a major public health problem and a leading source of morbidity in intensive care units. Lung tissue in patients with ARDS is characterized by inflammation, with exuberant neutrophil infiltration, activation, and degranulation that is thought to initiate tissue injury through the release of proteases and oxygen radicals. Treatment of ARDS is supportive primarily because the underlying pathophysiology is poorly understood. This gap in knowledge must be addressed to identify urgently needed therapies. Recent research efforts in anti-inflammatory drug development have focused on identifying common control points in multiple signaling pathways. The protein kinase C (PKC) serine-threonine kinases are master regulators of proinflammatory signaling hubs, making them attractive therapeutic targets. Pharmacological inhibition of broad-spectrum PKC activity and, more importantly, of specific PKC isoforms (as well as deletion of PKCs in mice) exerts protective effects in various experimental models of lung injury. Furthermore, PKC isoforms have been implicated in inflammatory processes that may be involved in the pathophysiologic changes that result in ARDS, including activation of innate immune and endothelial cells, neutrophil trafficking to the lung, regulation of alveolar epithelial barrier functions, and control of neutrophil proinflammatory and prosurvival signaling. This review focuses on the mechanistic involvement of PKC isoforms in the pathogenesis of ARDS and highlights the potential of developing new therapeutic paradigms based on the selective inhibition (or activation) of specific PKC isoforms.

Protein kinase C pharmacology: refining the toolbox

PKC (protein kinase C) has been in the limelight since the discovery three decades ago that it acts as a major receptor for the tumour-promoting phorbol esters. Phorbol esters, with their potent ability to activate two of the three classes of PKC isoenzymes, have remained the best pharmacological tool for directly modulating PKC activity. However, with the discovery of other phorbol ester-responsive proteins, the advent of various small-molecule and peptide modulators, and the need to distinguish isoenzyme-specific activity, the pharmacology of PKC has become increasingly complex. Not surprisingly, many of the compounds originally touted as direct modulators of PKC have subsequently been shown to hit many other cellular targets and, in some cases, not even directly modulate PKC. The complexities and reversals in PKC pharmacology have led to widespread confusion about the current status of the pharmacological tools available to control PKC activity. In the present review, we aim to clarify the cacophony in the literature regarding the current state of bona fide and discredited cellular PKC modulators, including activators, small-molecule inhibitors and peptides, and also address the use of genetically encoded reporters and of PKC mutants to measure the effects of these drugs on the spatiotemporal dynamics of signalling by specific isoenzymes.

The tail wagging the dog--regulation of lipid metabolism by protein kinase C

Upon their discovery almost 40 years ago, isoforms of the lipid-activated protein kinase C (PKC) family were initially regarded only as downstream effectors of the second messengers calcium and diacylglycerol, undergoing activation upon phospholipid hydrolysis in response to acute stimuli. Subsequently, several isoforms were found to be associated with the inhibitory effects of lipid over-supply on glucose homeostasis, especially the negative cross-talk with insulin signal transduction, observed upon accumulation of diacylglycerol in insulin target tissues. The PKC family has therefore attracted much attention in diabetes and obesity research, because intracellular lipid accumulation is strongly correlated with defective insulin action and the development of type 2 diabetes. Causal roles for various isoforms in the generation of insulin resistance have more recently been confirmed using PKC-deficient mice. However, during characterization of these animals, it became increasingly evident that the enzymes play key roles in the modulation of lipid metabolism itself, and may control the supply of lipids between tissues such as adipose and liver. Molecular studies have also demonstrated roles for PKC isoforms in several aspects of lipid metabolism, such as adipocyte differentiation and hepatic lipogenesis. While the precise mechanisms involved, especially the identities of protein substrates, are still unclear, the emerging picture suggests that the currently held view of the contribution of PKC isoforms to metabolism is an over-simplification. Although PKCs may inhibit insulin signal transduction, these enzymes are not merely downstream effectors of lipid accumulation, but in fact control the fate of fatty acids, thus the tail wags the dog.

Protein kinase C, an elusive therapeutic target?

Protein kinase C (PKC) has been a tantalizing target for drug discovery ever since it was first identified as the receptor for the tumour promoter phorbol ester in 1982. Although initial therapeutic efforts focused on cancer, additional indications--including diabetic complications, heart failure, myocardial infarction, pain and bipolar disorder--were targeted as researchers developed a better understanding of the roles of eight conventional and novel PKC isozymes in health and disease. Unfortunately, both academic and pharmaceutical efforts have yet to result in the approval of a single new drug that specifically targets PKC. Why does PKC remain an elusive drug target? This Review provides a short account of some of the efforts, challenges and opportunities in developing PKC modulators to address unmet clinical needs.

Analysis of peptides secreted from cultured mouse brain tissue

Peptides represent a major class of cell-cell signaling molecules. Most peptidomic studies have focused on peptides present in brain or other tissues. For a peptide to function in intercellular signaling, it must be secreted. The present study was undertaken to identify the major peptides secreted from mouse brain slices that were cultured in oxygenated buffer for 3-4h. Approximately 75% of the peptides identified in extracts of cultured slices matched the previously reported peptide content of heat-inactivated mouse brain tissue, whereas only 2% matched the peptide content of unheated brain tissue; the latter showed a large number of postmortem changes. As found with extracts of heat-inactivated mouse brain, the extracts of cultured brain slices represented secretory pathway peptides as well as peptides derived from intracellular proteins such as those present in the cytosol and mitochondria. A subset of the peptides detected in the extracts of the cultured slices was detected in the culture media. The vast majority of secreted peptides arose from intracellular proteins and not secretory pathway proteins. The peptide RVD-hemopressin, a CB1 cannabinoid receptor agonist, was detected in culture media, which is consistent with a role for RVD-hemopressin as a non-classical neuropeptide. Taken together with previous studies, the present results show that short-term culture of mouse brain slices is an appropriate system to study peptide secretion, especially the non-conventional pathway(s) by which peptides produced from intracellular proteins are secreted. This article is part of a Special Issue entitled: An Updated Secretome.

Canonical transient receptor potential channel 2 (TRPC2) as a major regulator of calcium homeostasis in rat thyroid FRTL-5 cells: importance of protein kinase C δ (PKCδ) and stromal interaction molecule 2 (STIM2)

Mammalian non-selective transient receptor potential cation channels (TRPCs) are important in the regulation of cellular calcium homeostasis. In thyroid cells, including rat thyroid FRTL-5 cells, calcium regulates a multitude of processes. RT-PCR screening of FRTL-5 cells revealed the presence of TRPC2 channels only. Knockdown of TRPC2 using shRNA (shTRPC2) resulted in decreased ATP-evoked calcium peak amplitude and inward current. In calcium-free buffer, there was no difference in the ATP-evoked calcium peak amplitude between control cells and shTRPC2 cells. Store-operated calcium entry was indistinguishable between the two cell lines. Basal calcium entry was enhanced in shTRPC2 cells, whereas the level of PKCβ1 and PKCδ, the activity of sarco/endoplasmic reticulum Ca(2+)-ATPase, and the calcium content in the endoplasmic reticulum were decreased. Stromal interaction molecule (STIM) 2, but not STIM1, was arranged in puncta in resting shTRPC2 cells but not in control cells. Phosphorylation site Orai1 S27A/S30A mutant and non-functional Orai1 R91W attenuated basal calcium entry in shTRPC2 cells. Knockdown of PKCδ with siRNA increased STIM2 punctum formation and enhanced basal calcium entry but decreased sarco/endoplasmic reticulum Ca(2+)-ATPase activity in wild-type cells. Transfection of a truncated, non-conducting mutant of TRPC2 evoked similar results. Thus, TRPC2 functions as a major regulator of calcium homeostasis in rat thyroid cells.

Cardiac actions of protein kinase C isoforms

Protein kinase C (PKC) isoforms have emerged as important regulators of cardiac contraction, hypertrophy, and signaling pathways that influence ischemic/reperfusion injury. This review focuses on newer concepts regarding PKC isoform-specific activation mechanisms and actions that have implications for the development of PKC-targeted therapeutics.

Specifying protein kinase C functions in melanoma

The protein kinase C (PKC) family of serine/threonine protein kinases is a heterogeneous group of enzymes receiving and integrating signals involved in both normal melanocyte biology and melanoma pathology. Alterations in PKC enzyme expression and activation contribute to the malignant phenotype of melanoma in both oncogenic and tumor suppressive roles. Delineating the diverse and often context-dependent functions of PKC enzymes in melanocyte/melanoma biology is key to capitalize on these kinases as drug targets. This review summarizes several of the diverse functions of PKC in melanocyte and melanoma biology with a focus on PKC enzyme regulation and function.

The role of C2 domains in PKC signaling

More than two decades ago, the discovery of the first C2 domain in conventional Protein Kinase Cs (cPKCs) and of its role as a calcium-binding motif began to shed light on the activation mechanism of this family of Serine/Threonine kinases which are involved in several critical signal transduction pathways. In this chapter, we review the current knowledge of the structure and the function of the different C2 domains in PKCs. The C2 domain of cPKCs is a calcium sensor and its calcium-dependent binding to phospholipids is crucial for kinase activation. While the functional role of the cPKC C2 domain is better understood, phylogenetic analysis revealed that the novel C2 domain is more ancient and related to the C2 domain in the fungal PKC family, while the cPKC C2 domain is first associated with PKC in metazoans. The C2 domain of novel PKCs (nPKCs) does not contain a calcium-binding motif but still plays a critical role in nPKCs activation by regulating C1-C2 domain interactions and consequently C2 domain-mediated inhibition in both the nPKCs of the epsilon family and the nPKCs of the delta family. Moreover, the C2 domain of the nPKCs of the delta family was shown to recognize phosphotyrosines in a novel mode different from the ones observed for the Src Homology 2 (SH2) and the phosphotyrosine binding domains (PTB). By binding to phosphotyrosines, the C2 domain regulates the activation of this subclass of PKCs. The C2 domain was also shown to be involved in protein-protein interactions and binding to the receptor for activated C-kinase (RACKs) thus contributing to the subcellular localization of PKCs. In summary, the C2 domain is a critical player that can sense the activated signaling pathway in response to external stimuli to specifically regulate the different conventional and novel PKC isoforms.

The biology of protein kinase C

This review gives a basic introduction to the biology of protein kinase C, one of the first calcium-dependent kinases to be discovered. We review the structure and function of protein kinase C, along with some of the substrates of individual isoforms. We then review strategies for inhibiting PKC in experimental systems and finally discuss the therapeutic potential of targeting PKC. Each aspect is covered in summary, with links to detailed resources where appropriate.