Gö 6983: A Fast Acting Protein Kinase C Inhibitor that Attenuates Myocardial Ischemia/Reperfusion Injury

A phosphorylation-controlled switch confers cell cycle-dependent protein relocalization

Tools for acute manipulation of protein localization enable elucidation of spatiotemporally defined functions, but their reliance on exogenous triggers can interfere with cell physiology. This limitation is particularly apparent for studying mitosis, whose highly choreographed events are sensitive to perturbations. Here we exploit the serendipitous discovery of a phosphorylation-controlled, cell cycle-dependent localization change of the adaptor protein PLEKHA5 to develop a system for mitosis-specific protein recruitment to the plasma membrane that requires no exogenous stimulus. Mitosis-enabled anchor-away/recruiter system comprises an engineered, 15 kDa module derived from PLEKHA5 capable of recruiting functional protein cargoes to the plasma membrane during mitosis, either through direct fusion or via GFP-GFP nanobody interaction. Applications of the mitosis-enabled anchor-away/recruiter system include both knock sideways to rapidly extract proteins from their native localizations during mitosis and conditional recruitment of lipid-metabolizing enzymes for mitosis-selective editing of plasma membrane lipid content, without the need for exogenous triggers or perturbative synchronization methods.

A phosphorylation-controlled switch confers cell cycle-dependent protein relocalization

Tools for acute manipulation of protein localization enable elucidation of spatiotemporally defined functions, but their reliance on exogenous triggers can interfere with cell physiology. This limitation is particularly apparent for studying mitosis, whose highly choreographed events are sensitive to perturbations. Here we exploit the serendipitous discovery of a phosphorylation-controlled, cell cycle-dependent localization change of the adaptor protein PLEKHA5 to develop a system for mitosis-specific protein recruitment to the plasma membrane that requires no exogenous stimulus. Mitosis-enabled Anchor-away/Recruiter System (MARS) comprises an engineered, 15-kDa module derived from PLEKHA5 capable of recruiting functional protein cargoes to the plasma membrane during mitosis, either through direct fusion or via GFP-GFP nanobody interaction. Applications of MARS include both knock sideways to rapidly extract proteins from their native localizations during mitosis and conditional recruitment of lipid-metabolizing enzymes for mitosis-selective editing of plasma membrane lipid content, without the need for exogenous triggers or perturbative synchronization methods.

Alternating magnetic fields drive stimulation of gene expression via generation of reactive oxygen species

Magnetogenetics represents a method for remote control of cellular function. Previous work suggests that generation of reactive oxygen species (ROS) initiates downstream signaling. Herein, a chemical biology approach was used to elucidate further the mechanism of radio frequency-alternating magnetic field (RF-AMF) stimulation of a TRPV1-ferritin magnetogenetics platform that leads to Ca2+ flux. RF-AMF stimulation of HEK293T cells expressing TRPV1-ferritin resulted in ∼30% and ∼140% increase in intra- and extracellular ROS levels, respectively. Mutations to specific cysteine residues in TRPV1 responsible for ROS sensitivity eliminated RF-AMF driven Ca2+-dependent transcription of secreted embryonic alkaline phosphatase (SEAP). Using a non-tethered (to TRPV1) ferritin also eliminated RF-AMF driven SEAP production, and using specific inhibitors, ROS-activated TRPV1 signaling involves protein kinase C, NADPH oxidase, and the endoplasmic reticulum. These results suggest ferritin-dependent ROS activation of TRPV1 plays a key role in the initiation of magnetogenetics, and provides relevance for potential applications in medicine and biotechnology.

PKCα Isoform Inhibits Insulin Signaling and Aggravates Neuronal Insulin Resistance

Overexpression of PKCα has been linked to inhibit insulin signaling disrupting IRS-1 and Akt phosphorylations in skeletal muscle. PKCα inhibits IRS-1 and Akt phosphorylations, but not required for insulin-stimulated glucose transport in skeletal muscles. Inhibition of PKCα increased whereas in some studies decreased GLUT-4 levels at the plasma membrane in skeletal muscles and adipocytes. Controversial studies have reported opposite expression pattern of PKCα expression in insulin-resistant skeletal muscles. These findings indicate that the role of PKCα on insulin signaling is controversial and could be tissue specific. Evidently, studies are required to decipher the role of PKCα in regulating insulin signaling and preferably in other cellular systems. Utilizing neuronal cells, like Neuro-2a, SHSY-5Y and insulin-resistant diabetic mice brain tissues; we have demonstrated that PKCα inhibits insulin signaling, through IRS-Akt pathway in PP2A-dependent mechanism by an AS160-independent route involving 14-3-3ζ. Inhibition and silencing of PKCα improves insulin sensitivity by increasing GLUT-4 translocation to the plasma membrane and glucose uptake. PKCα regulates GSK3 isoforms in an opposite manner in insulin-sensitive and in insulin-resistant condition. Higher activity of PKCα aggravates insulin-resistant neuronal diabetic condition through GSK3β but not GSK3α. Our results mechanistically explored the contribution of PKCα in regulating neuronal insulin resistance and diabetes, which opens up new avenues in dealing with metabolic disorders and neurodegenerative disorders.

Cisplatin Dependent Secretion of Immunomodulatory High Mobility Group Box 1 (HMGB1) Protein from Lung Cancer Cells

High mobility group box 1 (HMGB1) is secreted from activated immune cells, necrotic cells, and certain cancers. Previous studies have reported that different patterns of post-translational modification, particularly acetylation and oxidation, mediate HMGB1 release and confer distinct extracellular HMGB1 signaling activity. Here we report that cisplatin but not carboplatin induces secretion of HMGB1 from human A549 non-small cell lung cancer (NSCLC) cells. Cisplatin-mediated HMGB1 secretion was dose-dependent and was regulated by nuclear exportin 1 (XPO1) also known as chromosomal maintenance 1 (CRM1) rather than adenosine diphosphate (ADP)-ribosylation, acetylation, or oxidation. HMGB1, as well as lysine acetylation and cysteine disulfide oxidation of secreted HMGB1, were monitored by sensitive and specific assays using immunoprecipitation, stable isotope dilution, differential alkylation, and nano liquid chromatography parallel reaction monitoring/high-resolution mass spectrometry (nano-LC-PRM/HRMS). A major fraction of the HMGB1 secreted by low-dose cisplatin treatment of A549 NSCLC cells was found to be in the fully reduced form. In contrast, mainly oxidized forms of HMGB1 were secreted by dimethyl sulfoxide (DMSO)-mediated apoptosis. These findings suggest that inhibition of XPO1 could potentiate the anti-tumor activity of cisplatin by increasing the nuclear accumulation of HMGB1 protein, an inhibitor of cisplatin DNA-adduct repair. Furthermore, low-dose cisplatin therapy could modulate the immune response in NSCLC through the established chemokine activity of extracellular reduced HMGB1. This could potentially enhance the efficacy of subsequent immunotherapy treatment.

IL-2 can signal via chemokine receptors to promote regulatory T cells' suppressive function

Canonical interleukin-2 (IL-2) signaling via the high-affinity CD25-containing IL-2 receptor-Janus kinase (JAK)1,3-signal transducer and activator of transcription 5 (STAT5) pathway is essential for development and maintenance of CD4+CD25HiFoxp3+ regulatory T cells (Tregs) that support immune homeostasis. Here, we report that IL-2 signaling via an alternative CD25-chemokine receptor pathway promotes the suppressive function of Tregs. Using an antibody against CD25 that biases IL-2 signaling toward this alternative pathway, we establish that this pathway increases the suppressive activity of Tregs and ameliorates murine experimental autoimmune encephalomyelitis (EAE). Furthermore, heparan sulfate, an IL-2-binding element of cell surfaces and extracellular matrix, or an engineered IL-2 immunocytokine can also direct IL-2 signaling toward this alternative pathway. Overall, these data reveal a non-canonical mechanism for IL-2 signaling that promotes suppressive functions of Tregs, further elucidates how IL-2 supports immune homeostasis, and suggests approaches to promote or suppress Treg functions.

X-rays Stimulate Granular Secretions and Activate Protein Kinase C Signaling in Human Platelets

X-rays can induce morphological as well as functional changes in cells. Platelets are anuclear cellular fragments originating from megakaryocytes and are the major regulators in hemostasis and thrombosis. Platelet products are irradiated to avoid medical complications associated with platelet transfusion. So far, gamma, UV, and laser radiation have been used for this purpose. However, scientists are divided about the effects of radiation on platelet quality. The present study was designed to explore the possible effects of X-rays in washed human platelets and understand the molecular mechanism behind them. In the present study, we exposed washed human platelets to 10 or 30 Gy X-rays at 0.25 Gy/min. Flow cytometry, aggregometry, and western blot were performed to investigate the effect of X-rays on platelet degranulation, integrin activation, platelet aggregation, and apoptosis. It was found that X-rays immediately induced granular secretions with no effect on GP IIb/IIIa activation. Not surprisingly, due to granule secretions in irradiated platelets, platelet aggregation was significantly reduced. In contrast to granular secretions and platelet aggregation, X-rays induced mitochondrial transmembrane potential depolarization in a time-dependent manner to induce apoptosis and activated protein kinase C (PKC) signaling. This study revealed and explained the molecular mechanism activated by X-rays in washed human platelets. Here we also introduced Gö 6983, a PKC inhibitor, as an agent that counteracts X-ray-induced changes and maintains the integrity of platelets.

The thiophene α-terthienylmethanol isolated from Tagetes minuta inhibits angiogenesis by targeting protein kinase C isozymes α and β2

Background: Tumor angiogenesis is considered as a crucial pathologic feature of cancer with a key role in multidrug resistance (MDR). Adverse effects of the currently available drugs and the development of resistance to these remain as the hardest obstacles to defeat.

Objetive: This work explores flora from Argentina as a source of new chemical entities with antiangiogenic activity.

Methods: Tube formation assay using bovine aortic endothelial cells (BAECs) was the experiment of choice to assess antiangiogenic activity. The effect of the pure compound in cell invasiveness was investigated through the trans-well migration assay. The inhibitory effect of the pure compound on VEGFR-2 and PKC isozymes α and β2 activation was studied by molecular and massive dynamic simulations. Cytotoxicity on peripheral blood mononuclear cells and erythrocyte cells was evaluated by means of MTT and hemolysis assay, respectively. In silico prediction of pharmacological properties (ADME) and evaluation of drug-likeness features were performed using the SwissADME online tool.

Results: Among the plants screened, T. minuta, showed an outstanding effect with an IC₅₀ of 33.6 ± 3.4 μg/ml. Bio-guided isolation yielded the terthiophene α-terthienylmethanol as its active metabolite. This compound inhibited VEGF-induced tube formation with an IC₅₀ of 2.7 ± 0.4 μM and significantly impaired the invasiveness of bovine aortic endothelial cells (BAECs) as well as of the highly aggressive breast cancer cells, MDA-MB-231, when tested at 10 μM. Direct VEGFR-2 and PKC inhibition were both explored by means of massive molecular dynamics simulations. The results obtained validated the inhibitory effect on protein kinase C (PKC) isozymes α and β2 as the main mechanism underlying its antiangiogenic activity. α-terthienylmethanol showed no evidence of toxicity against peripheral blood mononuclear and erythrocyte cells.

Conclusion: These findings support this thiophene as a promising antiangiogenic phytochemical to fight against several types of cancer mainly those with MDR phenotype.

Presynaptic nigral GPR55 receptors stimulate [3 H]-GABA release through [3 H]-cAMP production and PKA activation and promote motor behavior

Striatal medium-sized spiny neurons express mRNA and protein of GPR55 receptors that stimulate neurotransmitter release; thus, GPR55 could be sent to nigral striatal projections, where it might modulate GABA release and motor behavior. Here we study the presence of GPR55 receptors at striato-nigral terminals, their modulation of GABA release, their signaling pathway, and their effect on motor activity. By double immunohistochemistry, we found the colocation of GPR55 protein and substance P in the dorsal striatum. In slices of the rat substantia nigra, the GPR55 agonists LPI and O-1602 stimulated [³ H]-GABA release induced by high K⁺ depolarization in a dose-dependent manner. The antagonists CID16020046 and cannabidiol prevented agonist stimulation in a dose-dependent way. The effect of GPR55 on nigral [³ H]-GABA release was prevented by lesion of the striatum with kainic acid, which was accompanied by a decrement of GPR55 protein in nigral synaptosomes, indicating the presynaptic location of receptors. The depletion of internal Ca²⁺ stores with thapsigargin did not prevent the effect of LPI on [³ H]-GABA release, but the remotion or chelation of external calcium did. Blockade of Gi, Gs, PLC, PKC, or dopamine D1 receptor signaling proteins did not prevent the effect of GPR55 on release. However, the activation of GPR55 stimulated [³ H]-cAMP accumulation and PKA activity. Intranigral unilateral injection of LPI induces contralateral turning. This turning was prevented by CID16020046, cannabidiol, and bicuculline but not by SCH 23390. Our data indicate that presynaptic GPR55 receptors stimulate [³ H]-GABA release at striato-nigral terminals through [³ H]-cAMP production and stimulate motor behavior. This article is protected by copyright. All rights reserved.

PKC-Mediated Orai1 Channel Phosphorylation Modulates Ca2+ Signaling in HeLa Cells

The overexpression of the Orai1 channel inhibits SOCE when using the Ca²⁺ readdition protocol. However, we found that HeLa cells overexpressing the Orai1 channel displayed enhanced Ca²⁺ entry and a limited ER depletion in response to the combination of ATP and thapsigargin (TG) in the presence of external Ca²⁺. As these effects require the combination of an agonist and TG, we decided to study whether the phosphorylation of Orai1 S27/S30 residues had any role using two different mutants: Orai1-S27/30A (O1-AA, phosphorylation-resistant) and Orai1-S27/30D (O1-DD, phosphomimetic). Both O1-wt and O1-AA supported enhanced Ca²⁺ entry, but this was not the case with O1-E106A (dead-pore mutant), O1-DD, and O1-AA-E106A, while O1-wt, O1-E106A, and O1-DD inhibited the ATP and TG-induced reduction of ER [Ca²⁺], suggesting that the phosphorylation of O1 S27/30 interferes with the IP₃R activity. O1-wt and O1-DD displayed an increased interaction with IP₃R in response to ATP and TG; however, the O1-AA channel decreased this interaction. The expression of mCherry-O1-AA increased the frequency of ATP-induced sinusoidal [Ca²⁺]_i oscillations, while mCherry-O1-wt and mCherry-O1-DD decreased this frequency. These data suggest that the combination of ATP and TG stimulates Ca²⁺ entry, and the phosphorylation of Orai1 S27/30 residues by PKC reduces IP₃R-mediated Ca²⁺ release.

Histamine Sensitization of the Voltage-Gated Sodium Channel Nav1.7 Contributes to Histaminergic Itch in Mice

Itch, a common clinical symptom of many skin diseases, severely impairs the life quality of patients. Nav1.7, a subtype of voltage-gated sodium channels mainly expressed in primary sensory neurons, is responsible for the amplification of threshold currents that trigger action potential (AP) generation. Gain-of-function mutation of Nav1.7 leads to paroxysmal itch, while pharmacological inhibition of Nav1.7 alleviates histamine-dependent itch. However, the crosstalk between histamine and Nav1.7 that leads to itch is unclear. In the present study, we demonstrated that in the dorsal root ganglion (DRG) neurons from histamine-dependent itch model mice induced by compound 48/80, tetrodotoxin-sensitive (TTX-S) but not TTX-resistant Na⁺ currents were activated at more hyperpolarized membrane potentials compared to those on DRG neurons from vehicle-treated mice. Meanwhile, bath application of histamine shifted the activation voltages of TTX-S Na⁺ currents to the hyperpolarized direction, increased the AP frequency, and reduced the current threshold required to elicit APs. Further mechanistic studies demonstrated that selective activation of H1 but not H2 and H4 receptors mimicked histamine effect on TTX-S Na⁺ channels in DRG neurons. The protein kinase C (PKC) inhibitor GO 8963, but not the PKA inhibitor H89, normalized histamine-sensitized TTX-S Na⁺ channels. We also demonstrated that histamine shifted the activation voltages of Na⁺ currents to the hyperpolarized direction in Chinese hamster ovary (CHO) cells expressing Nav1.7. Importantly, selective inhibition of Nav1.7 by PF-05089771 significantly relieved the scratching frequency in a histamine-dependent itch model induced by compound 48/80. Taken together, these data suggest that activation of H1 receptors by histamine sensitizes Nav1.7 channels through the PKC pathway in DRG neurons that contributes to histamine-dependent itch.

Protein Kinase C-Mediated Phosphorylation and α2δ-1 Interdependently Regulate NMDA Receptor Trafficking and Activity

N-methyl-d-aspartate receptors (NMDARs) are important for synaptic plasticity associated with many physiological functions and neurologic disorders. Protein kinase C (PKC) activation increases the phosphorylation and activity of NMDARs, and α2δ-1 is a critical NMDAR-interacting protein and controls synaptic trafficking of NMDARs. In this study, we determined the relative roles of PKC and α2δ-1 in the control of NMDAR activity. We found that α2δ-1 coexpression significantly increased NMDAR activity in HEK293 cells transfected with GluN1/GluN2A or GluN1/GluN2B. PKC activation with phorbol 12-myristate 13-acetate (PMA) increased receptor activity only in cells coexpressing GluN1/GluN2A and α2δ-1. Remarkably, PKC inhibition with Gӧ6983 abolished α2δ-1-coexpression-induced potentiation of NMDAR activity in cells transfected with GluN1/GluN2A or GluN1/GluN2B. Treatment with PMA increased the α2δ-1-GluN1 interaction and promoted α2δ-1 and GluN1 cell surface trafficking. PMA also significantly increased NMDAR activity of spinal dorsal horn neurons and the amount of α2δ-1-bound GluN1 protein complexes in spinal cord synaptosomes in wild-type mice, but not in α2δ-1 knockout mice. Furthermore, inhibiting α2δ-1 with pregabalin or disrupting the α2δ-1-NMDAR interaction with the α2δ-1 C-terminus peptide abolished the potentiating effect of PMA on NMDAR activity. Additionally, using quantitative phosphoproteomics and mutagenesis analyses, we identified S929 on GluN2A and S1413 (S1415 in humans) on GluN2B as the phosphorylation sites responsible for NMDAR potentiation by PKC and α2δ-1. Together, our findings demonstrate the interdependence of α2δ-1 and PKC phosphorylation in regulating NMDAR trafficking and activity. The phosphorylation-dependent, dynamic α2δ-1-NMDAR interaction constitutes an important molecular mechanism of synaptic plasticity.SIGNIFICANCE STATEMENT A major challenge in studies of protein phosphorylation is to define the functional significance of each phosphorylation event and determine how various signaling pathways are coordinated in response to neuronal activity to shape synaptic plasticity. PKC phosphorylates transporters, ion channels, and G-protein-coupled receptors in signal transduction. In this study, we showed that α2δ-1 is indispensable for PKC-activation-induced surface and synaptic trafficking of NMDARs, whereas the α2δ-1-NMDAR interaction is controlled by PKC-induced phosphorylation. Our findings reveal that α2δ-1 mainly functions as a phospho-binding protein in the control of NMDAR trafficking and activity. This information provides new mechanistic insight into the reciprocal roles of PKC-mediated phosphorylation and α2δ-1 in regulating NMDARs and in the therapeutic actions of gabapentinoids.

HDAC6 inhibitors sensitize non-mesenchymal triple-negative breast cancer cells to cysteine deprivation

Triple-negative breast cancer (TNBC) is a highly malignant type of breast cancer and lacks effective therapy. Targeting cysteine-dependence is an emerging strategy to treat the mesenchymal TNBC. However, many TNBC cells are non-mesenchymal and unresponsive to cysteine deprivation. To overcome such resistance, three selective HDAC6 inhibitors (Tubacin, CAY10603, and Tubastatin A), identified by epigenetic compound library screening, can synergize with cysteine deprivation to induce cell death in the non-mesenchymal TNBC. Despite the efficacy of HDAC6 inhibitor, knockout of HDAC6 did not mimic the synthetic lethality induced by its inhibitors, indicating that HDAC6 is not the actual target of HDAC6 inhibitor in this context. Instead, transcriptomic profiling showed that tubacin triggers an extensive gene transcriptional program in combination with erastin, a cysteine transport blocker. Notably, the zinc-related gene response along with an increase of labile zinc was induced in cells by the combination treatment. The disturbance of zinc homeostasis was driven by PKCγ activation, which revealed that the PKCγ signaling pathway is required for HDAC6 inhibitor-mediated synthetic lethality. Overall, our study identifies a novel function of HDAC6 inhibitors that function as potent sensitizers of cysteine deprivation and are capable of abolishing cysteine-independence in non-mesenchymal TNBC.

Contribution of Syndecans to the Cellular Entry of SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel emerging pathogen causing an unprecedented pandemic in 21st century medicine. Due to the significant health and economic burden of the current SARS-CoV-2 outbreak, there is a huge unmet medical need for novel interventions effectively blocking SARS-CoV-2 infection. Unknown details of SARS-CoV-2 cellular biology hamper the development of potent and highly specific SARS-CoV-2 therapeutics. Angiotensin-converting enzyme-2 (ACE2) has been reported to be the primary receptor for SARS-CoV-2 cellular entry. However, emerging scientific evidence suggests the involvement of additional membrane proteins, such as heparan sulfate proteoglycans, in SARS-CoV-2 internalization. Here, we report that syndecans, the evolutionarily conserved family of transmembrane proteoglycans, facilitate the cellular entry of SARS-CoV-2. Among syndecans, the lung abundant syndecan-4 was the most efficient in mediating SARS-CoV-2 uptake. The S1 subunit of the SARS-CoV-2 spike protein plays a dominant role in the virus's interactions with syndecans. Besides the polyanionic heparan sulfate chains, other parts of the syndecan ectodomain, such as the cell-binding domain, also contribute to the interaction with SARS-CoV-2. During virus internalization, syndecans colocalize with ACE2, suggesting a jointly shared internalization pathway. Both ACE2 and syndecan inhibitors exhibited significant efficacy in reducing the cellular entry of SARS-CoV-2, thus supporting the complex nature of internalization. Data obtained on syndecan specific in vitro assays present syndecans as novel cellular targets of SARS-CoV-2 and offer molecularly precise yet simple strategies to overcome the complex nature of SARS-CoV-2 infection.

Tight junction protein ZO-2 modulates the nuclear accumulation of transcription factor TEAD

The presence of tight junction protein zonula occludens 2 (ZO-2) at the nucleus inhibits the transcription of genes regulated by TEAD transcription factor. Here, we analyzed whether the movement of ZO-2 into the nucleus modulates the nuclear concentration of TEAD. In sparse cultures of ZO-2 knockdown Madin–Darby canine kidney cells, nuclear TEAD was diminished, as in parental cells transfected with a ZO-2 construct without nuclear localization signals, indicating that ZO-2 facilitates the entry of TEAD into the nucleus. Inhibition of nPKCδ in parental cells triggers the interaction between ZO-2 and TEAD at the cytoplasm and facilitates TEAD/ZO-2 complex nuclear importation. Using proximity ligation, immunoprecipitation, and pull-down assays, TEAD/ZO-2 interaction was confirmed. Nuclear TEAD is phosphorylated, and its exit in parental cells is enhanced by activation of a ZO-2 nuclear exportation signal by nPKCε, while the nuclear accumulation of ZO-2 triggered by the mutation of ZO-2 nuclear export signals induces no change in TEAD nuclear concentration. In summary, our results indicate that the movements of ZO-2 in and out of the nucleus modulate the intracellular traffic of TEAD through a process regulated by nPKCδ and ε and provide a novel role of ZO-2 as a nuclear translocator of TEAD.

Discovering the Mechanisms of Wikstroelide E as a Potential HIV-Latency-Reversing Agent by Transcriptome Profiling

The discovery of efficient and specific HIV-latency-reversing agents is critical for HIV therapy. Here, we developed wikstroelide E, a daphnane diterpene from the buds of Wikstroemia chamaedaphne, as a potential HIV-latency-reversing agent that is 2500-fold more potent than the drug prostratin. Based on transcriptome analysis, the underlying mechanism was that wikstroelide E regulated the MAPK, PI3K-Akt, JAK-Stat, TNF, and NF-κB signaling pathways. We clearly demonstrated that wikstroelide E reversed latent HIV infection by activating PKC-NF-κB signals, serving as a proxy for verifying the transcriptome data. Strikingly, the Tat protein contributes to the robust activation of latent HIV in wikstroelide-E-treated cells, producing an unexpected latency-reversing effect against latent HIV. This study provides the basis for the potential development of wikstroelide E as an effective HIV-latency-reversing agent.

Potentials of PKC in Cancer Progression and Anticancer Drug Development

PKC is a family of serine-threonine kinases which play crucial roles in the regulation of important signal transduction pathways in mammalian cell-biology. These enzymes are themselves regulated by various molecules that can serve as ligands to the regulatory domains and translocate PKC to membrane for activity. The role of PKC in the modulation of both proliferative and apoptotic signaling in cancer has become a subject of immense interest after it was discovered that PKC regulates a myriad of enzymes and transcription factors involved in carcinogenic signaling. Therefore, PKC has served as an attractive target for the development of newer generation of anti-cancer drugs. The following review discusses the potential of PKC to be regarded as a target for anti-cancer therapy. We also review all the molecules that have been discovered so far to be regulators/activators/inhibitors of PKC and also how far these molecules can be considered as potential candidates for anti-cancer drug development based on PKC.

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.

Spatial alterations of De Novo purine biosynthetic enzymes by Akt-independent PDK1 signaling pathways

A macromolecular complex of the enzymes involved in human de novo purine biosynthesis, the purinosome, has been shown to consist of a core assembly to regulate the metabolic activity of the pathway. However, it remains elusive whether the core assembly itself can be selectively controlled in the cytoplasm without promoting the purinosome. Here, we reveal that pharmacological inhibition of the cytoplasmic activity of 3-phosphoinositide-dependent protein kinase 1 (PDK1) selectively promotes the formation of the core assembly, but not the purinosome, in cancer cells. However, alternative signaling cascades that are associated with the plasma membrane-bound PDK1 activity, including Akt-mediated cascades, regulate neither the core assembly nor the purinosome in our conditions. Along with immunofluorescence microscopy and a knock-down study against PDK1 using small interfering RNAs, we reveal that cytoplasmic PDK1-associated signaling pathways regulate subcellular colocalization of three enzymes that form the core assembly of the purinosome in an Akt-independent manner. Collectively, this study reveals a new mode of compartmentalization of purine biosynthetic enzymes controlled by spatially resolved signaling pathways.

Curcumin potentiates the function of human α7-nicotinic acetylcholine receptors expressed in SH-EP1 cells

Effects of curcumin, a biologically active ingredient of turmeric, were tested on the Ca²⁺ transients induced by the activation of α₇ subunit of the human nicotinic acetylcholine (α₇ nACh) receptor expressed in SH-EP1 cells. Curcumin caused a significant potentiation of choline (1 mM)-induced Ca²⁺ transients with an EC₅₀ value of 133 nM. The potentiating effect of curcumin was not observed in Ca²⁺ transients induced by high K⁺ (60 mM) containing solutions or activation of α₄β₂ nACh receptors and the extent of curcumin potentiation was not altered in the presence of Ca²⁺ channel antagonists nifedipine (1 μM), verapamil (1 μM), ω-conotoxin (1 μM), and bepridil (10 μM). Noticeably the effect of curcumin was not observed when curcumin and choline were co-applied without curcumin pre-incubation. The effect of curcumin on choline-induced Ca²⁺ transients was not reversed by pre-incubation with inhibitors of protein C, A, and CaM kinases. Metabolites of curcumin such as tetrahydrocurcumin, demethylcurcumin, and didemethylcurcumin also caused potentiation of choline-induced Ca²⁺ transients. Notably, specific binding of [¹²⁵I]-bungarotoxin was not altered in the presence of curcumin. Collectively, our results indicate that curcumin allosterically potentiate the function of the α7-nACh receptor expressed in SH-EP1 cells.

Presynaptic Dopamine D2 Receptors Modulate [3H]GABA Release at StriatoPallidal Terminals via Activation of PLC → IP3 → Calcineurin and Inhibition of AC → cAMP → PKA Signaling Cascades

Striatal dopamine D2 receptors activate the PLC → IP3 → Calcineurin-signaling pathway to modulate the neural excitability of En+ Medium-sized Spiny GABAergic neurons (MSN) through the regulation of L-type Ca²⁺ channels. Presynaptic dopaminergic D2 receptors modulate GABA release at striatopallidal terminals through L-type Ca²⁺ channels as well, but their signaling pathway is still undetermined. Since D2 receptors are Gi/o-coupled and negatively modulate adenylyl cyclase (AC), we investigated whether presynaptic D2 receptors modulate GABA release through the same signaling cascade that controls excitability in the striatum or by the inhibition of AC and decreased PKA activity. Activation of D2 receptors stimulated formation of [³H]IP₁ and decreased Forskolin-stimulated [³H]cAMP accumulation in synaptosomes from rat Globus Pallidus. D2 receptor activation with Quinpirole in the presence of L 745,870 decreased, in a dose-dependent manner, K⁺-induced [³H]GABA release in pallidal slices. The effect was prevented by the pharmacological blockade of Gi/o βγ subunit effects with Gallein, PLC with U 73122, IP3 receptor activation with 4-APB, Calcineurin with FK506. In addition, when release was stimulated with Forskolin to activate AC, D2 receptors also decreased K⁺-induced [³H]GABA release, an effect occluded with the effect of the blockade of PKA with H89 or stimulation of release with the cAMP analog 8-Br-cAMP. These data indicate that D2 receptors modulate [³H]GABA release at striatopallidal terminals by activating the PLC → IP3 → Calcineurin-signaling cascade, the same one that modulates excitability in soma. Additionally, D2 receptors inhibit release when AC is active. Both mechanisms appear to converge to regulate the activity of presynaptic L-type Ca²⁺ channels.

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Endogenous Nur77 Is a Specific Indicator of Antigen Receptor Signaling in Human T and B Cells

Distinguishing true Ag-stimulated lymphocytes from bystanders activated by the inflammatory milieu has been difficult. Nur77 is an immediate early gene whose expression is rapidly upregulated by TCR signaling in murine T cells and human thymocytes. Nur77-GFP transgenes serve as specific TCR and BCR signaling reporters in murine transgenic models. In this study, we demonstrate that endogenous Nur77 protein expression can serve as a reporter of TCR and BCR specific signaling in human PBMCs. Nur77 protein amounts were assessed by immunofluorescence and flow cytometry in T and B cells isolated from human PBMCs obtained from healthy donors that had been stimulated by their respective Ag receptors. We demonstrate that endogenous Nur77 is a more specific reporter of Ag-specific signaling events than the commonly used CD69 activation marker in both human T and B cells. This is reflective of the disparity in signaling pathways that regulate the expression of Nur77 and CD69. Assessing endogenous Nur77 protein expression has great potential to identify Ag-activated lymphocytes in human disease.

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.

Inflammatory Eicosanoids Increase Amyloid Precursor Protein Expression via Activation of Multiple Neuronal Receptors

Senile plaques comprised of Aβ peptides are a hallmark of Alzheimer's disease (AD) brain, as are activated glia that release inflammatory molecules, including eicosanoids. Previous studies have demonstrated that amyloid precursor protein (APP) and Aβ levels can be increased through activation of thromboxane A2-prostanoid (TP) receptors on neurons. We demonstrate that TP receptor regulation of APP expression depends on Gαq-signaling and conventional protein kinase C isoforms. Importantly, we discovered that Gαq-linked prostaglandin E2 and leukotriene D4 receptors also regulate APP expression. Prostaglandin E2 and thromboxane A2, as well as total APP levels, were found to be elevated in the brains of aged 5XFAD transgenic mice harboring Aβ plaques and activated glia, suggesting that increased APP expression resulted from eicosanoid binding to Gαq-linked neuronal receptors. Notably, inhibition of eicosanoid synthesis significantly lowered brain APP protein levels in aged 5XFAD mice. These results provide new insights into potential AD therapeutic strategies.

Endothelin-1 impairs coronary arteriolar dilation: Role of p38 kinase-mediated superoxide production from NADPH oxidase

Elevated levels of endothelin-1 (ET-1), a potent vasoactive peptide, are implicated as a risk factor for cardiovascular diseases by exerting vasoconstriction. The aim of this study was to address whether ET-1, at sub-vasomotor concentrations, elicits adverse effects on coronary microvascular function. Porcine coronary arterioles (50-100μm) were isolated, cannulated and pressurized without flow for in vitro study. Diameter changes were recorded using a videomicrometer. Arterioles developed basal tone (60±3μm) and dilated to the endothelium-dependent nitric oxide (NO)-mediated vasodilators serotonin (1nmol/L to 0.1μmol/L) and adenosine (1nmol/L to 10μmol/L). Treating the vessels with a clinically relevant sub-vasomotor concentration of ET-1 (10pmol/L, 60min) significantly attenuated arteriolar dilations to adenosine and serotonin but not to endothelium-independent vasodilator sodium nitroprusside. The arteriolar wall contains ETA receptors and the adverse effect of ET-1 was prevented by ETA receptor antagonist BQ123, the superoxide scavenger Tempol, the NADPH oxidase inhibitors apocynin and VAS2870, the NOX2-based NADPH oxidase inhibitor gp91 ds-tat, or the p38 kinase inhibitor SB203580. However, ETB receptor antagonist BQ788, H2O2 scavenger catalase, scrambled gp91 ds-tat, or inhibitors of xanthine oxidase (allopurinol), PKC (Gö 6983), Rho kinase (Y27632), and c-Jun N-terminal kinase (SP600125) did not protect the vessel. Immunohistochemical staining showed that ET-1 elicited Tempol-, apocynin- and SB203580-sensitive superoxide productions in the arteriolar wall. Our results indicate that exposure of coronary arterioles to a pathophysiological, sub-vasomotor concentration of ET-1 leads to vascular dysfunction by impairing endothelium-dependent NO-mediated dilation via p38 kinase-mediated production of superoxide from NADPH oxidase following ETA receptor activation.

Ceramide-mediated depression in cardiomyocyte contractility through PKC activation and modulation of myofilament protein phosphorylation

Although ceramide accumulation in the heart is considered a major factor in promoting apoptosis and cardiac disorders, including heart failure, lipotoxicity and ischemia-reperfusion injury, little is known about ceramide's role in mediating changes in contractility. In the present study, we measured the functional consequences of acute exposure of isolated field-stimulated adult rat cardiomyocytes to C6-ceramide. Exogenous ceramide treatment depressed the peak amplitude and the maximal velocity of shortening without altering intracellular calcium levels or kinetics. The inactive ceramide analog C6-dihydroceramide had no effect on myocyte shortening or [Ca(2+)]i transients. Experiments testing a potential role for C6-ceramide-mediated effects on activation of protein kinase C (PKC) demonstrated evidence for signaling through the calcium-independent isoform, PKCε. We employed 2-dimensional electrophoresis and anti-phospho-peptide antibodies to test whether treatment of the cardiomyocytes with C6-ceramide altered myocyte shortening via PKC-dependent phosphorylation of myofilament proteins. Compared to controls, myocytes treated with ceramide exhibited increased phosphorylation of myosin binding protein-C (cMyBP-C), specifically at Ser273 and Ser302, and troponin I (cTnI) at sites apart from Ser23/24, which could be attenuated with PKC inhibition. We conclude that the altered myofilament response to calcium resulting from multiple sites of PKC-dependent phosphorylation contributes to contractile dysfunction that is associated with cardiac diseases in which elevations in ceramides are present.

Diabetic cardiomyopathy: mechanisms and new treatment strategies targeting antioxidant signaling pathways

Cardiovascular disease is the primary cause of morbidity and mortality among the diabetic population. Both experimental and clinical evidence suggest that diabetic subjects are predisposed to a distinct cardiomyopathy, independent of concomitant macro- and microvascular disorders. 'Diabetic cardiomyopathy' is characterized by early impairments in diastolic function, accompanied by the development of cardiomyocyte hypertrophy, myocardial fibrosis and cardiomyocyte apoptosis. The pathophysiology underlying diabetes-induced cardiac damage is complex and multifactorial, with elevated oxidative stress as a key contributor. We now review the current evidence of molecular disturbances present in the diabetic heart, and their role in the development of diabetes-induced impairments in myocardial function and structure. Our focus incorporates both the contribution of increased reactive oxygen species production and reduced antioxidant defenses to diabetic cardiomyopathy, together with modulation of protein signaling pathways and the emerging role of protein O-GlcNAcylation and miRNA dysregulation in the progression of diabetic heart disease. Lastly, we discuss both conventional and novel therapeutic approaches for the treatment of left ventricular dysfunction in diabetic patients, from inhibition of the renin-angiotensin-aldosterone-system, through recent evidence favoring supplementation of endogenous antioxidants for the treatment of diabetic cardiomyopathy. Novel therapeutic strategies, such as gene therapy targeting the phosphoinositide 3-kinase PI3K(p110α) signaling pathway, and miRNA dysregulation, are also reviewed. Targeting redox stress and protective protein signaling pathways may represent a future strategy for combating the ever-increasing incidence of heart failure in the diabetic population.

Granzyme D is a novel murine mast cell protease that is highly induced by multiple pathways of mast cell activation

Granzymes are serine proteases known mostly for their role in the induction of apoptosis. Granzymes A and B have been extensively studied, but relatively little is known about granzymes C to G and K to M. T cells, lymphohematopoietic stromal cells, and granulated metrial gland cells express granzyme D, but the function of granzyme D is unknown. Here we show that granzyme D is expressed by murine mast cells and that its level of expression correlates positively with the extent of mast cell maturation. Coculture of mast cells with live, Gram-positive bacteria caused a profound, Toll-like receptor 2 (TLR2)-dependent induction of granzyme D expression. Granzyme D expression was also induced by isolated bacterial cell wall components, including lipopolysaccharide (LPS) and peptidoglycan, and by stem cell factor, IgE receptor cross-linking, and calcium ionophore stimulation. Granzyme D was released into the medium in response to mast cell activation. Granzyme D induction was dependent on protein kinase C and nuclear factor of activated T cells (NFAT). Together, these findings identify granzyme D as a novel murine mast cell protease and implicate granzyme D in settings where mast cells are activated, such as bacterial infection and allergy.

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

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

The effects of modulating eNOS activity and coupling in ischemia/reperfusion (I/R)

The in vivo role of endothelial nitric oxide synthase (eNOS) uncoupling mediating oxidative stress in ischemia/reperfusion (I/R) injury has not been well established. In vitro, eNOS coupling refers to the reduction of molecular oxygen to L-arginine oxidation and generation of L-citrulline and nitric oxide NO synthesis in the presence of an essential cofactor, tetrahydrobiopterin (BH(4)). Whereas uncoupled eNOS refers to that the electron transfer becomes uncoupled to L-arginine oxidation and superoxide is generated when the dihydrobiopterin (BH(2)) to BH(4) ratio is increased. Superoxide is subsequently converted to hydrogen peroxide (H(2)O(2)). We tested the hypothesis that promoting eNOS coupling or attenuating uncoupling after I/R would decrease H(2)O(2)/increase NO release in blood and restore postreperfused cardiac function. We combined BH(4) or BH(2) with eNOS activity enhancer, protein kinase C epsilon (PKC ε) activator, or eNOS activity reducer, PKC ε inhibitor, in isolated rat hearts (ex vivo) and femoral arteries/veins (in vivo) subjected to I(20 min)/R(45 min). When given during reperfusion, PKC ε activator combined with BH(4), not BH(2), significantly restored postreperfused cardiac function and decreased leukocyte infiltration (p < 0.01) while increasing NO (p < 0.05) and reducing H(2)O(2) (p < 0.01) release in femoral I/R veins. These results provide indirect evidence suggesting that PKC ε activator combined with BH(4) enhances coupled eNOS activity, whereas it enhanced uncoupled eNOS activity when combined with BH(2). By contrast, the cardioprotective and anti-oxidative effects of the PKC ε inhibitor were unaffected by BH(4) or BH(2) suggesting that inhibition of eNOS uncoupling during reperfusion following sustained ischemia may be an important mechanism.

Shedding of the Mer tyrosine kinase receptor is mediated by ADAM17 protein through a pathway involving reactive oxygen species, protein kinase Cδ, and p38 mitogen-activated protein kinase (MAPK)

Mer tyrosine kinase (MerTK) is an integral membrane protein that is preferentially expressed by phagocytic cells, where it promotes efferocytosis and inhibits inflammatory signaling. Proteolytic cleavage of MerTK at an unidentified site leads to shedding of its soluble ectodomain (soluble MER; sMER), which can inhibit thrombosis in mice and efferocytosis in vitro. Herein, we show that MerTK is cleaved at proline 485 in murine macrophages. Site-directed deletion of 6 amino acids spanning proline 485 rendered MerTK resistant to proteolysis and suppression of efferocytosis by cleavage-inducing stimuli. LPS is a known inducer of MerTK cleavage, and the intracellular signaling pathways required for this action are unknown. LPS/TLR4-mediated generation of sMER required disintegrin and metalloproteinase ADAM17 and was independent of Myd88, instead requiring TRIF adaptor signaling. LPS-induced cleavage was suppressed by deficiency of NADPH oxidase 2 (Nox2) and PKCδ. The addition of the antioxidant N-acetyl cysteine inhibited PKCδ, and silencing of PKCδ inhibited MAPK p38, which was also required. In a mouse model of endotoxemia, we discovered that LPS induced plasma sMER, and this was suppressed by Adam17 deficiency. Thus, a TRIF-mediated pattern recognition receptor signaling cascade requires NADPH oxidase to activate PKCδ and then p38, culminating in ADAM17-mediated proteolysis of MerTK. These findings link innate pattern recognition receptor signaling to proteolytic inactivation of MerTK and generation of sMER and uncover targets to test how MerTK cleavage affects efferocytosis efficiency and inflammation resolution in vivo.

Differential regulation of Nr4a subfamily nuclear receptors following mast cell activation

The biological function of the Nr4a subfamily of nuclear receptors is only partially understood. Here we show for the fist time that mast cell (MC) activation processes involve the regulation of Nr4a factors. Exposure of murine bone marrow-derived MCs (BMMCs) to live bacteria causes a robust and selective upregulation of all Nr4a members (Nr4a1-Nr4a3). In response to purified LPS, strong upregulation of Nr4a3, but not of Nr4a1 or Nr4a2 was seen. Nr4a3 expression was also induced after the activation of BMMCs by IgE receptor cross-linking. Moreover, Nr4a expression was induced in activated human MCs. As shown by Western blot analysis, Nr4a phosphorylation was induced by IgE receptor cross-linking and calcium ionophore stimulation of BMMCs and LAD2 cells, respectively. By using various inhibitors of signaling pathways, Nr4a3 induction in BMMCs was shown to be strongly dependent on Gö6976-sensitive kinases and partially dependent on the nuclear factor of activated T-cells (NFAT) pathway, while nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) inhibition failed to inhibit Nr4a3 expression in BMMCs. Together, these data reveal selective induction of Nr4a family members in activated MCs and implicate Nr4a family nuclear receptors in the regulation of MC function.

Exercise preconditioning initiates late cardioprotection against isoproterenol-induced myocardial injury in rats independent of protein kinase C

The objective of this study was to investigate the late cardioprotective effect of exercise preconditioning (EP) on isoproterenol (ISO)-induced myocardial injury in rats and the role of protein kinase C (PKC) in EP. Rats were injected with ISO 24 h after running on a treadmill for four periods of 10 min each at 28-30 m/min with intervening periods of rest of 10 min. Nonselective PKC inhibitor chelerythrine (CHE) was injected before EP. The myocardial injury was evaluated quantitatively in terms of the serum cardiac troponin I (cTnI) levels, the myocardial ischemia/hypoxia area, and the integral optical density (IOD) of haematoxylin-basic fuchsin-picric acid (HBFP) staining, and qualitatively in terms of the myocardial ultrastructure. EP markedly attenuated the ISO-induced myocardial ischemia/hypoxia and ultrastructural damage with lower serum cTnI levels. CHE injection before EP did not block the protective effect of EP, displaying a mild myocardial ischemia/hypoxia and well-preserved ultrastructure with even lower serum cTnI levels. The results indicate that EP can exert a late cardioprotection against ISO-induced myocardial injury, and that an injection of the nonselective PKC inhibitor CHE before EP may have a different effect on ISO-induced myocardial injury. Further investigation needs to be conducted to define the role of different PKC isozymes in EP by using isozyme-selective inhibitors.

Protein kinase D1 mediates stimulation of DNA synthesis and proliferation in intestinal epithelial IEC-18 cells and in mouse intestinal crypts

We examined whether protein kinase D1 (PKD1), the founding member of a new protein kinase family, plays a critical role in intestinal epithelial cell proliferation. Our results demonstrate that PKD1 activation is sustained, whereas that of PKD2 is transient in intestinal epithelial IEC-18 stimulated with the G(q)-coupled receptor agonists angiotensin II or vasopressin. PKD1 gene silencing utilizing small interfering RNAs dramatically reduced DNA synthesis and cell proliferation in IEC-18 cells stimulated with G(q)-coupled receptor agonists. To clarify the role of PKD1 in intestinal epithelial cell proliferation in vivo, we generated transgenic mice that express elevated PKD1 protein in the intestinal epithelium. Transgenic PKD1 exhibited constitutive catalytic activity and phosphorylation at the activation loop residues Ser(744) and Ser(748) and on the autophosphorylation site, Ser(916). To examine whether PKD1 expression stimulates intestinal cell proliferation, we determined the rate of crypt cell DNA synthesis by detection of 5-bromo-2-deoxyuridine incorporated into the nuclei of crypt cells of the ileum. Our results demonstrate a significant increase (p < 0.005) in DNA-synthesizing cells in the crypts of two independent lines of PKD1 transgenic mice as compared with non-transgenic littermates. Morphometric analysis showed a significant increase in the length and in the total number of cells per crypt in the transgenic PKD1 mice as compared with the non-transgenic littermates (p < 0.01). Thus, transgenic PKD1 signaling increases the number of cells per crypt by stimulating the rate of crypt cell proliferation. Collectively, our results indicate that PKD1 plays a role in promoting cell proliferation in intestinal epithelial cells both in vitro and in vivo.

Protein Kinases as Drug Development Targets for Heart Disease Therapy

Protein kinases are intimately integrated in different signal transduction pathways for the regulation of cardiac function in both health and disease. Protein kinase A (PKA), Ca²⁺-calmodulin-dependent protein kinase (CaMK), protein kinase C (PKC), phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) are not only involved in the control of subcellular activities for maintaining cardiac function, but also participate in the development of cardiac dysfunction in cardiac hypertrophy, diabetic cardiomyopathy, myocardial infarction, and heart failure. Although all these kinases serve as signal transducing proteins by phosphorylating different sites in cardiomyocytes, some of their effects are cardioprotective whereas others are detrimental. Such opposing effects of each signal transduction pathway seem to depend upon the duration and intensity of stimulus as well as the type of kinase isoform for each kinase. In view of the fact that most of these kinases are activated in heart disease and their inhibition has been shown to improve cardiac function, it is suggested that these kinases form excellent targets for drug development for therapy of heart disease.

Protein kinase D mediates mitogenic signaling by Gq-coupled receptors through protein kinase C-independent regulation of activation loop Ser744 and Ser748 phosphorylation

Rapid protein kinase D (PKD) activation and phosphorylation via protein kinase C (PKC) have been extensively documented in many cell types cells stimulated by multiple stimuli. In contrast, little is known about the role and mechanism(s) of a recently identified sustained phase of PKD activation in response to G protein-coupled receptor agonists. To elucidate the role of biphasic PKD activation, we used Swiss 3T3 cells because PKD expression in these cells potently enhanced duration of ERK activation and DNA synthesis in response to G(q)-coupled receptor agonists. Cell treatment with the preferential PKC inhibitors GF109203X or Gö6983 profoundly inhibited PKD activation induced by bombesin stimulation for <15 min but did not prevent PKD catalytic activation induced by bombesin stimulation for longer times (>60 min). The existence of sequential PKC-dependent and PKC-independent PKD activation was demonstrated in 3T3 cells stimulated with various concentrations of bombesin (0.3-10 nm) or with vasopressin, a different G(q)-coupled receptor agonist. To gain insight into the mechanisms involved, we determined the phosphorylation state of the activation loop residues Ser(744) and Ser(748). Transphosphorylation targeted Ser(744), whereas autophosphorylation was the predominant mechanism for Ser(748) in cells stimulated with G(q)-coupled receptor agonists. We next determined which phase of PKD activation is responsible for promoting enhanced ERK activation and DNA synthesis in response to G(q)-coupled receptor agonists. We show, for the first time, that the PKC-independent phase of PKD activation mediates prolonged ERK signaling and progression to DNA synthesis in response to bombesin or vasopressin through a pathway that requires epidermal growth factor receptor-tyrosine kinase activity. Thus, our results identify a novel mechanism of G(q)-coupled receptor-induced mitogenesis mediated by sustained PKD activation through a PKC-independent pathway.

Quercetin stimulates Na+/K+/2Cl- cotransport via PTK-dependent mechanisms in human airway epithelium

We investigated regulatory mechanisms of Cl(-) secretion playing an essential role in the maintenance of surface fluid in human airway epithelial Calu-3 cells. The present study reports that quercetin (a flavonoid) stimulated bumetanide-sensitive Cl(-) secretion with reduction of apical Cl(-) conductance, suggesting that quercetin stimulates Cl(-) secretion by activating an entry step of Cl(-) across the basolateral membrane through Na(+)/K(+)/2Cl(-) cotransporter (NKCC1). To clarify the mechanism stimulating NKCC1 by quercetin, we verified involvement of protein kinase (PK)A, PKC, protein tyrosine kinase (PTK), and cytosolic Ca(2+)-dependent pathways. A PKA inhibitor (PKI-14-22 amide), a PKC inhibitor (Gö 6983) or a Ca(2+) chelating agent did not affect the quercetin-stimulated Cl(-) secretion. On the other hand, a PTK inhibitor (AG18) significantly diminished the stimulatory action of quercetin on Cl(-) secretion without inhibitory effects on apical Cl(-) conductance, suggesting that a PTK-mediated pathway is involved in the stimulatory action of quercetin. The quercetin action on Cl(-) secretion was suppressed with brefeldin A (BFA, an inhibitor of vesicular transport from ER to Golgi), and the BFA-sensitive Cl(-) secretion was not observed in the presence of an epidermal growth factor receptor (EGFR) kinase inhibitor (AG1478), suggesting that quercetin stimulates Cl(-) secretion by causing the EGFR kinase-mediated translocation of NKCC1 or an NKC1-activating factor to the basolateral membrane in human airway epithelial Calu-3 cells. However, the surface density of NKCC1 was not increased by quercetin, but quercetin elevated the activity of NKCC1. These observations indicate that quercetin stimulates Cl(-) secretion by activating NKCC1 via translocation of an NKCC1-activating factor through an EGFR kinase-dependent pathway.

Sequential protein kinase C (PKC)-dependent and PKC-independent protein kinase D catalytic activation via Gq-coupled receptors: differential regulation of activation loop Ser(744) and Ser(748) phosphorylation

Protein kinase D (PKD) is a serine/threonine protein kinase rapidly activated by G protein-coupled receptor (GPCR) agonists via a protein kinase C (PKC)-dependent pathway. Recently, PKD has been implicated in the regulation of long term cellular activities, but little is known about the mechanism(s) of sustained PKD activation. Here, we show that cell treatment with the preferential PKC inhibitors GF 109203X or Gö 6983 blocked rapid (1-5-min) PKD activation induced by bombesin stimulation, but this inhibition was greatly diminished at later times of bombesin stimulation (e.g. 45 min). These results imply that GPCR-induced PKD activation is mediated by early PKC-dependent and late PKC-independent mechanisms. Western blot analysis with site-specific antibodies that detect the phosphorylated state of the activation loop residues Ser(744) and Ser(748) revealed striking PKC-independent phosphorylation of Ser(748) as well as Ser(744) phosphorylation that remained predominantly but not completely PKC-dependent at later times of bombesin or vasopressin stimulation (20-90 min). To determine the mechanisms involved, we examined activation loop phosphorylation in a set of PKD mutants, including kinase-deficient, constitutively activated, and PKD forms in which the activation loop residues were substituted for alanine. Our results show that PKC-dependent phosphorylation of the activation loop Ser(744) and Ser(748) is the primary mechanism involved in early phase PKD activation, whereas PKD autophosphorylation on Ser(748) is a major mechanism contributing to the late phase of PKD activation occurring in cells stimulated by GPCR agonists. The present studies identify a novel mechanism induced by GPCR activation that leads to late, PKC-independent PKD activation.