Protein kinase C isozymes and substrates

Tire-Derived Transformation Product 6PPD-Quinone Induces Mortality and Transcriptionally Disrupts Vascular Permeability Pathways in Developing Coho Salmon

Urban stormwater runoff frequently contains the car tire transformation product 6PPD-quinone, which is highly toxic to juvenile and adult coho salmon (Onchorychus kisutch). However, it is currently unclear if embryonic stages are impacted. We addressed this by exposing developing coho salmon embryos starting at the eyed stage to three concentrations of 6PPD-quinone twice weekly until hatch. Impacts on survival and growth were assessed. Further, whole-transcriptome sequencing was performed on recently hatched alevin to address the potential mechanism of 6PPD-quinone-induced toxicity. Acute mortality was not elicited in developing coho salmon embryos at environmentally measured concentrations lethal to juveniles and adults, however, growth was inhibited. Immediately after hatching, coho salmon were sensitive to 6PPD-quinone mortality, implicating a large window of juvenile vulnerability prior to smoltification. Molecularly, 6PPD-quinone induced dose-dependent effects that implicated broad dysregulation of genomic pathways governing cell-cell contacts and endothelial permeability. These pathways are consistent with previous observations of macromolecule accumulation in the brains of coho salmon exposed to 6PPD-quinone, implicating blood-brain barrier disruption as a potential pathway for toxicity. Overall, our data suggests that developing coho salmon exposed to 6PPD-quinone are at risk for adverse health events upon hatching while indicating potential mechanism(s) of action for this highly toxic chemical.

Protein Kinase C (PKC) Isozymes as Diagnostic and Prognostic Biomarkers and Therapeutic Targets for Cancer

Protein kinase C (PKC) is a large family of calcium- and phospholipid-dependent serine/threonine kinases that consists of at least 11 isozymes. Based on their structural characteristics and mode of activation, the PKC family is classified into three subfamilies: conventional or classic (cPKCs; α, βI, βII, and γ), novel or non-classic (nPKCs; δ, ε, η, and θ), and atypical (aPKCs; ζ, ι, and λ) (PKCλ is the mouse homolog of PKCι) PKC isozymes. PKC isozymes play important roles in proliferation, differentiation, survival, migration, invasion, apoptosis, and anticancer drug resistance in cancer cells. Several studies have shown a positive relationship between PKC isozymes and poor disease-free survival, poor survival following anticancer drug treatment, and increased recurrence. Furthermore, a higher level of PKC activation has been reported in cancer tissues compared to that in normal tissues. These data suggest that PKC isozymes represent potential diagnostic and prognostic biomarkers and therapeutic targets for cancer. This review summarizes the current knowledge and discusses the potential of PKC isozymes as biomarkers in the diagnosis, prognosis, and treatment of cancers.

Synthesis of hydrophilic caged DAG-lactones for chemical biology applications

The 6-bromo-7-hydroxy-coumarin-4-ylmethyl (Bhc) group has been used widely in cage chemistry because of its high molar absorptivity and photolytic efficiency. One of the drawbacks of coumarins however is their low aqueous solubility. Aqueous solubility is important in the behavior of caged compounds because hydrophobic caged compounds might be aggregated in physiological conditions and consequently the photocleavage would be impaired. The 8-azacoumarin-4-ylmethyl derivatives with bromine (8-aza-Bhc) or iodine (8-aza-Ihc), which were previously developed in this laboratory, have aqueous solubilities that are higher than those of related coumarins. Here, to improve the hydrophilicity and management of caged diacylglycerol lactones (DAG-lactones), 8-aza-Bhc and 8-aza-Ihc were introduced into the DAG-lactone structure. The synthesized caged compounds showed high hydrophilicity compared with the parent Bhc-caged DAG-lactone, and the 8-aza-Ihc-caged DAG-lactone (2) showed excellent photolytic efficiency, which allows rapid release of the DAG-lactone (1) by brief photoirradiation. The 8-aza-7-hydroxy-6-iodo-coumarin-4-ylmethyl group might be useful for caging of bioactive compounds, especially hydrophobic compounds.

Role of prelimbic cortex PKC and PKMζ in fear memory reconsolidation and persistence following reactivation

The persistence of newly acquired memories is supported by the activity of PKMζ, an atypical isoform of protein kinase C (PKC). Whether the activity of conventional and atypical PKC isoforms contributes to reactivated memories to persist is still unknown. Similarly, whether memory reactivation is a prerequisite for interventions to be able to change memory persistence is scarcely investigated. Based on the above, we examined the role of conventional and atypical PKC isoforms in the prelimbic cortex in reconsolidation and persistence of a reactivated contextual fear memory in male Wistar rats. It is shown that (i) inhibiting the PKC activity with chelerythrine or the PKMζ activity with ZIP impaired the persistence of a reactivated memory for at least 21 days; (ii) ZIP given immediately after memory reactivation affected neither the reconsolidation nor the persistence process. In contrast, when given 1 h later, it impaired the memory persistence; (iii) chelerythrine given immediately after memory reactivation impaired the reconsolidation; (iv) omitting memory reactivation prevented the chelerythrine- and ZIP-induced effects: (v) the ZIP action is independent of the time elapsed between its administration and the initial memory test. The results indicate that prelimbic cortex PKC and PKMζ are involved in memory reconsolidation and persistence.

Protein kinase C and cardiac dysfunction: a review

Heart failure (HF) is a physiological state in which cardiac output is insufficient to meet the needs of the body. It is a clinical syndrome characterized by impaired ability of the left ventricle to either fill or eject blood efficiently. HF is a disease of multiple aetiologies leading to progressive cardiac dysfunction and it is the leading cause of deaths in both developed and developing countries. HF is responsible for about 73,000 deaths in the UK each year. In the USA, HF affects 5.8 million people and 550,000 new cases are diagnosed annually. Cardiac remodelling (CD), which plays an important role in pathogenesis of HF, is viewed as stress response to an index event such as myocardial ischaemia or imposition of mechanical load leading to a series of structural and functional changes in the viable myocardium. Protein kinase C (PKC) isozymes are a family of serine/threonine kinases. PKC is a central enzyme in the regulation of growth, hypertrophy, and mediators of signal transduction pathways. In response to circulating hormones, activation of PKC triggers a multitude of intracellular events influencing multiple physiological processes in the heart, including heart rate, contraction, and relaxation. Recent research implicates PKC activation in the pathophysiology of a number of cardiovascular disease states. Few reports are available that examine PKC in normal and diseased human hearts. This review describes the structure, functions, and distribution of PKCs in the healthy and diseased heart with emphasis on the human heart and, also importantly, their regulation in heart failure.

Design and synthesis of protein kinase C epsilon selective diacylglycerol lactones (DAG-lactones)

DAG-lactones afford a synthetically accessible, high affinity platform for probing structure activity relationships at the C1 regulatory domain of protein kinase C (PKC). Given the central role of PKC isoforms in cellular signaling, along with their differential biological activities, a critical objective is the design of isoform selective ligands. Here, we report the synthesis of a series of DAG-lactones varying in their side chains, with a particular focus on linoleic acid derivatives. We evaluated their selectivity for PKC epsilon versus PKC alpha both under standard lipid conditions (100% phosphatidylserine, PS) as well as in the presence of a nuclear membrane mimetic lipid mixture (NML). We find that selectivity for PKC epsilon versus PKC alpha tended to be enhanced in the presence of the nuclear membrane mimetic lipid mixture and, for our lead compound, report a selectivity of 32-fold.

Localization of protein kinase C isoforms in the optic pathway of mouse embryos and their role in axon routing at the optic chiasm

Protein kinase C (PKC) plays a key role in many receptor-mediated signaling pathways that regulate cell growth and development. However, its roles in guiding axon growth and guidance in developing neural pathways are largely unknown. To investigate possible functions of PKC in the growth and guidance of axons in the optic chiasm, we first determined the localization of major PKC isoforms in the retinofugal pathway of mouse embryos, at the stage when axons navigate through the midline. Results showed that PKC was expressed in isoform specific patterns in the pathway. PKC-α immunoreactivity was detected in the chiasm and the optic tract. PKC-βΙΙ was strong in the optic stalk but was attenuated on axons in the diencephalon. Immunostaining for PKC-ε showed a colocalization in the chiasmatic neurons that express a surface antigen stage specific embryonic antigen-1 (SSEA-1). These chiasmatic neurons straddled the midline of the optic chiasm, and have been shown in earlier studies a role in regulation of axon growth and guidance. Expression levels of PKC-βΙ, -δ and -γ were barely detectable in the pathway. Blocking of PKC signaling with Ro-32-0432, an inhibitor specific for PKC-α and -β at nanomolar concentration, produced a dramatic reduction of ipsilateral axons from both nasal retina and temporal crescent. We conclude from these studies that PKC-α and -βΙΙ are the predominant forms in the developing optic pathway, whereas PKC-ε is the major form in the chiasmatic neurons. Furthermore, PKC-α and -βΙΙ are likely involved in signaling pathways triggered by inhibitory molecules at the midline that guide optic axons to the uncrossed pathway.

Effects of flutamide on [methyl-(3)h]-choline uptake in human prostate cancer-3 cells: a pilot study

BACKGROUND

Positron emission tomography using [methyl-(11)C]-choline is effective in imaging many types of cancer, especially prostate cancer (PC). The antiandrogen flutamide is often used as part of the initial treatment of PC. Data on the effect of flutamide on and methylcholine incorporation into PC-3 cells are lacking in the experimental and literature work.

OBJECTIVES

The aims of this study were to assess whether human PC-3 cells are susceptible to flutamide and whether the drug modulates the uptake of [methyl-(3)H]-choline into these cells.

METHODS

PC-3 cells were treated for 3 days with flutamide (≤100 nmol/L), inhibiting growth by 20% to 70% with control cells included. Two viability tests (cytotoxic analyses), the thiazole blue assay and the trypan blue exclusion method, were used to determine the median inhibitory concentration for flutamide (10 nmol/L). Control and flutamide-treated cells were incubated with [methyl-(3)H]-choline for 10 minutes and then in nonradioactive medium for 10 minutes to simulate the rapid blood clearance of [methyl-(11)C]-choline tracer that occurs within 5 to 20 minutes, and then extracted using organic and aqueous solvents to determine the intracellular distribution of the tracer. Protein assay and flow-cytometry analysis were used to determine protein content and DNA synthesis in both control and treated cells. The uptake of [methyl-(3)H]-choline was normalized to protein content and expressed as mean (SD) dpm/1Jg protein (n = 6).

RESULTS

PC-3 cell proliferation was inhibited with flutamide treatment. After treatment of PC-3 cells with flutamide 10 nmol/L for 3 days, cells accumulated DNA during the S phase. Mean (SD) [methyl-(3)H]-choline uptake was found to be significantly lower with flutamide 10-nmol/L-treated cells compared with control cells (65.95 [0.72] vs 114.21 [0.57] dpm/1Jg protein; P < 0.001); the difference between the 5-nmol/L-treated cells and controls was nonsignificant.

CONCLUSIONS

In this pilot study, flutamide inhibited tumor cell growth and proliferation and decreased (modulated) the uptake of [methyl-(3)H]-choline into androgen receptor-negative PC-3 cells. These results suggest that flutamide might inhibit proliferation by an androgen-independent mechanism.

Cortical PKC inhibition promotes axonal regeneration of the corticospinal tract and forelimb functional recovery after cervical dorsal spinal hemisection in adult rats

Our previous study shows that conventional protein kinases C (cPKCs) are key signaling mediators that are activated by extracellular inhibitory molecules. Inhibition of cPKC by intrathecal infusion of a cPKC inhibitor, GÖ6976, into the site of dorsal hemisection (DH) induces regeneration of lesioned dorsal column sensory, but not corticospinal tract (CST), axons. Here, we investigated whether a direct cortical delivery of GÖ6976 into the soma of corticospinal neurons promotes regeneration of CST and the recovery of forelimb function in rats with cervical spinal cord injuries. We report that cortical delivery of GÖ6976 reduced injury-induced activation of conventional PKCα and PKCβ1 in CST neurons, promoted regeneration of CST axons through and beyond a cervical DH at C4, formed new synapses on target neurons caudal to the injury, and enhanced forelimb functional recovery in adult rats. When combined with lenti-Chondroitinase ABC treatment, cortical administration of GÖ6976 promoted even greater CST axonal regeneration and recovery of forelimb function. Thus, this study has demonstrated a novel strategy that can promote anatomical regeneration of damaged CST axons and partial recovery of forelimb function. Importantly, such an effect is critically dependent on the efficient blockage of injury-induced PKC activation in the soma of layer V CST neurons.

Protein kinase C-α interaction with iHSP70 in mitochondria promotes recovery of mitochondrial function after injury in renal proximal tubular cells

This study determined the role of PKC-α and associated inducible heat shock protein 70 (iHSP70) in the repair of mitochondrial function in renal proximal tubular cells (RPTCs) after oxidant injury. Wild-type PKC-α (wtPKC-α) and an inactive PKC-α [dominant negative dn; PKC-α] mutant were overexpressed in primary cultures of RPTCs, and iHSP70 levels and RPTC regeneration were assessed after treatment with the oxidant tert-butylhydroperoxide (TBHP). TBHP exposure increased ROS production and induced RPTC death, which was prevented by ferrostatin and necrostatin-1 but not by cyclosporin A. Overexpression of wtPKC-α maintained mitochondrial levels of active PKC-α, reduced cell death, and accelerated proliferation without altering ROS production in TBHP-injured RPTCs. In contrast, dnPKC-α blocked proliferation and monolayer regeneration. Coimmunoprecipitation and proteomic analysis demonstrated an association between inactive, but not active, PKC-α and iHSP70 in mitochondria. Mitochondrial iHSP70 levels increased as levels of active PKC-α decreased after injury. Overexpression of dnPKC-α augmented, whereas overexpression of wtPKC-α abrogated, oxidant-induced increases in mitochondrial iHSP70 levels. iHSP70 overexpression (1) maintained mitochondrial levels of phosphorylated PKC-α, (2) improved the recovery of state 3 respiration and ATP content, (3) decreased RPTC death (an effect abrogated by cyclosporine A), and (4) accelerated proliferation after oxidant injury. In contrast, iHSP70 inhibition blocked the recovery of ATP content and exacerbated RPTC death. Inhibition of PKC-α in RPTC overexpressing iHSP70 blocked the protective effects of iHSP70. We conclude that active PKC-α maintains mitochondrial function and decreases cell death after oxidant injury. iHSP70 is recruited to mitochondria in response to PKC-α dephosphorylation and associates with and reactivates inactive PKC-α, which promotes the recovery of mitochondrial function, decreases RPTC death, and improves regeneration.

Unique in vitro and in vivo thrombopoietic activities of ingenol 3,20 dibenzoate, a Ca(++)-independent protein kinase C isoform agonist

Thrombopoiesis following severe bone marrow injury frequently is delayed, thereby resulting in life-threatening thrombocytopenia for which there are limited treatment options. The reasons for these delays in recovery are not well understood. Protein kinase C (PKC) agonists promote megakaryocyte differentiation in leukemia cell lines and primary cells. However, little is known about the megakaryopoietic effects of PKC agonists on primary CD34+ cells grown in culture or in vivo. Here we present evidence that the novel PKC isoform-selective agonist 3,20 ingenol dibenzoate (IDB) potently stimulates early megakaryopoiesis of human CD34+ cells. In contrast, broad spectrum PKC agonists failed to do so. In vivo, a single intraperitoneal injection of IDB selectively increased platelets in mice without affecting hemoglobin or white counts. Finally, IDB strongly mitigated radiation-induced thrombocytopenia, even when administered 24 hours after irradiation. Our data demonstrate that novel PKC isoform agonists such as IDB may represent a unique therapeutic strategy for accelerating the recovery of platelet counts following severe marrow injury.

Down-regulation of PKCζ in renal cell carcinoma and its clinicopathological implications

Background

Metastatic renal cell carcinoma (RCC) is highly resistant to systemic chemotherapy. Unfortunately, nearly all patients die of the metastatic and chemoresistant RCC. Recent studies have shown the atypical PKCζ is an important regulator of tumorigenesis. However, the correlation between PKCζ expression and the clinical outcome in RCC patients is unclear. We examined the level of PKCζ expression in human RCC.

Methods

PKCζ mRNA and protein expressions were examined by real-time polymerase chain reaction (PCR) and immunohistochemistry (IHC) respectively in RCC tissues of 144 patients. Cellular cytotoxicity and proliferation were assessed by MTT.

Results

PKCζ expression was significantly higher in normal than in cancerous tissues (P < 0.0001) by real-time PCR and IHC. Similarly, PKCζ expression was down-regulated in four renal cancer cell lines compared to immortalized benign renal tubular cells. Interestingly, an increase of PKCζ expression was associated with the elevated tumor grade (P = 0.04), but no such association was found in TNM stage (P = 0.13). Tumors with higher PKCζ expression were associated with tumor size (P = 0.048). Expression of higher PKCζ found a poor survival in patients with high tumor grade. Down-regulation of PKCζ showed the significant chemoresistance in RCC cell lines. Inactivation of PKCζ expression enhanced cellular resistance to cisplatin and paclitaxel, and proliferation in HK-2 cells by specific PKCζ siRNA and inhibitor.

Conclusions

PKCζ expression was associated with tumorigenesis and chemoresistance in RCC.

PKCθ synergizes with TLR-dependent TRAF6 signaling pathway to upregulate MUC5AC mucin via CARMA1

CARD-containing MAGUK protein 1 (CARMA1) plays a crucial role in regulating adaptive immune responses upon T-cell receptor (TCR) activation in T cells. Its role in regulating host mucosal innate immune response such as upregulation of mucin remains unknown. Here we show that CARMA1 acts as a key signaling mediator for synergistic upregulation of MUC5AC mucin by bacterium nontypeable Haemophilus influenzae (NTHi) and phorbol ester PMA in respiratory epithelial cells. NTHi-induced TLR-dependent TRAF6-MKK3-p38 MAPK signaling pathway synergizes with PKCθ-MEK-ERK signaling pathway. CARMA1 plays a crucial role in mediating this synergistic effect via TRAF6, thereby resulting in synergistic upregulation of MUC5AC mucin. Thus our study unveils a novel role for CARMA1 in mediating host mucosal innate immune response.

Regulation of ADP-ribosylation factor 4 expression by small leucine zipper protein and involvement in breast cancer cell migration

ADP-ribosylation factor 4 (ARF4) is a member of the Ras superfamily of small guanine nucleotide-binding proteins. ARF4 is known to interact with the epidermal growth factor receptor (EGFR) and mediates the EGF-dependent signal pathway, and has an anti-apoptotic function in human glioblastoma-derived U373MG cells. Although ARF4 plays a role in cancer cells, the molecular mechanism underlying regulation of its expression and its exact functions in breast cancer are unknown. In this study, we investigated the regulatory mechanism of ARF4 expression and its involvement in breast cancer cell migration. Our results show that phorbol 12-myristate 13-acetate (PMA) treatment increases ARF4 expression at both the transcriptional and translational levels. We found that the novel transcription factor small leucine zipper protein (sLZIP) binds directly to the CRE motif of the -43 to -35 region in the ARF4 promoter and regulates PMA-induced ARF4 expression. We also found that PMA-stimulated ARF4 expression increases AP-1 promoter activity, leading to induction of breast cancer cell migration. These results indicate that sLZIP-regulated ARF4 expression in response to PMA is involved in breast cancer cell migration, and sLZIP and ARF4 are potential therapeutic target molecules for treating breast cancer invasion and metastasis.

The expression and role of protein kinase C (PKC) epsilon in clear cell renal cell carcinoma

Protein kinase C epsilon (PKCε), an oncogene overexpressed in several human cancers, is involved in cell proliferation, migration, invasion, and survival. However, its roles in clear cell renal cell carcinoma (RCC) are unclear. This study aimed to investigate the functions of PKCε in RCC, especially in clear cell RCC, to determine the possibility of using it as a therapeutic target. By immunohistochemistry, we found that the expression of PKCε was up-regulated in RCCs and was associated with tumor Fuhrman grade and T stage in clear cell RCCs. Clone formation, wound healing, and Borden assays showed that down-regulating PKCε by RNA interference resulted in inhibition of the growth, migration, and invasion of clear cell RCC cell line 769P and, more importantly, sensitized cells to chemotherapeutic drugs as indicated by enhanced activity of caspase-3 in PKCε siRNA-transfected cells. These results indicate that the overexpression of PKCε is associated with an aggressive phenotype of clear cell RCC and may be a potential therapeutic target for this disease.

Cholinergic signaling inhibits oxalate transport by human intestinal T84 cells

Urolithiasis remains a very common disease in Western countries. Seventy to eighty percent of kidney stones are composed of calcium oxalate, and minor changes in urinary oxalate affect stone risk. Intestinal oxalate secretion mediated by anion exchanger SLC26A6 plays a major constitutive role in limiting net absorption of ingested oxalate, thereby preventing hyperoxaluria and calcium oxalate urolithiasis. Using the relatively selective PKC-δ inhibitor rottlerin, we had previously found that PKC-δ activation inhibits Slc26a6 activity in mouse duodenal tissue. To identify a model system to study physiologic agonists upstream of PKC-δ, we characterized the human intestinal cell line T84. Knockdown studies demonstrated that endogenous SLC26A6 mediates most of the oxalate transport by T84 cells. Cholinergic stimulation with carbachol modulates intestinal ion transport through signaling pathways including PKC activation. We therefore examined whether carbachol affects oxalate transport in T84 cells. We found that carbachol significantly inhibited oxalate transport by T84 cells, an effect blocked by rottlerin. Carbachol also led to significant translocation of PKC-δ from the cytosol to the membrane of T84 cells. Using pharmacological inhibitors, we observed that carbachol inhibits oxalate transport through the M(3) muscarinic receptor and phospholipase C. Utilizing the Src inhibitor PP2 and phosphorylation studies, we found that the observed regulation downstream of PKC-δ is partially mediated by c-Src. Biotinylation studies revealed that carbachol inhibits oxalate transport by reducing SLC26A6 surface expression. We conclude that carbachol negatively regulates oxalate transport by reducing SLC26A6 surface expression in T84 cells through signaling pathways including the M(3) muscarinic receptor, phospholipase C, PKC-δ, and c-Src.

Propofol restores transient receptor potential vanilloid receptor subtype-1 sensitivity via activation of transient receptor potential ankyrin receptor subtype-1 in sensory neurons

BACKGROUND

Cross talk between peripheral nociceptors belonging to the transient receptor potential vanilloid receptor subtype-1 (TRPV1) and ankyrin subtype-1 (TRPA1) family has been demonstrated recently. Moreover, the intravenous anesthetic propofol has directly activates TRPA1 receptors and indirectly restores sensitivity of TRPV1 receptors in dorsal root ganglion (DRG) sensory neurons. Our objective was to determine the extent to which TRPA1 activation is involved in mediating the propofol-induced restoration of TRPV1 sensitivity.

METHODS

Mouse DRG neurons were isolated by enzymatic dissociation and grown for 24 h. F-11 cells were transfected with complementary DNA for both TRPV1 and TRPA1 or TRPV1 only. The intracellular Ca concentration was measured in individual cells via fluorescence microscopy. After TRPV1 desensitization with capsaicin (100 nM), cells were treated with propofol (1, 5, and 10 μM) alone or with propofol in the presence of the TRPA1 antagonist, HC-030031 (0.5 μM), or the TRPA1 agonist, allyl isothiocyanate (AITC; 100 μM); capsaicin was then reapplied.

RESULTS

In DRG neurons that contain both TRPV1 and TRPA1, propofol and AITC restored TRPV1 sensitivity. However, in DRG neurons containing only TRPV1 receptors, exposure to propofol or AITC after desensitization did not restore capsaicin-induced TRPV1 sensitivity. Similarly, in F-11 cells transfected with both TRPV1 and TRPA1, propofol and AITC restored TRPV1 sensitivity. However, in F-11 cells transfected with TRPV1 only, neither propofol nor AITC was capable of restoring TRPV1 sensitivity.

CONCLUSIONS

These data demonstrate that propofol restores TRPV1 sensitivity in primary DRG neurons and in cultured F-11 cells transfected with both the TRPV1 and TRPA1 receptors via a TRPA1-dependent process. Propofol's effects on sensory neurons may be clinically important and may contribute to peripheral sensitization to nociceptive stimuli in traumatized tissue.

Stimulation of alpha7 nicotinic acetylcholine receptor by nicotine attenuates inflammatory response in macrophages and improves survival in experimental model of sepsis through heme oxygenase-1 induction

Activation of nicotinic acetylcholine receptor alpha7 subunit (α7nAChR) by nicotine leads to the improved survival rate in experimental model of sepsis. Previously, we demonstrated that heme oxygenase (HO)-1 inducers or carbon monoxide significantly increased survival of lipopolysaccharide (LPS)-induced and cecal ligation and puncture-induced septic mice by reduction of high mobility group box 1 release, a late mediator of sepsis. However, that activation of α7nAChR by nicotine provides anti-inflammatory action through HO-1 upregulation has not been elucidated. Here we show that HO-1-inducible effect by nicotine was mediated through sequential event-Ca(2+) influx, classical protein kinase C activation, and reactive oxygen species production-which activates phosphoinositol-3-kinase/Akt/Nrf-2 pathway. In addition, HO-1 is required for nicotine-mediated suppression of tumor necrosis factor-α, inducible nitric oxide synthase, and high mobility group box 1 expression induced by LPS in macrophages, as evidenced by the fact that nicotine failed to inhibit production of these mediators when HO-1 was suppressed. Importantly, nicotine-induced survival rate was reduced by inhibition of HO-1 in LPS- and cecal ligation and puncture-treated septic mice. Collectively, these data suggest that activation of α7nAChR by nicotine is critical in the regulation of anti-inflammatory process, which could be mediated through HO-1 expression. Thus, we conclude that activation of α7nAChR by nicotine provides anti-inflammatory action through HO-1 upregulation.

PKC isoforms interact with and phosphorylate DNMT1

Background

DNA methyltransferase 1 (DNMT1) has been shown to be phosphorylated on multiple serine and threonine residues, based on cell type and physiological conditions. Although recent studies have suggested that protein kinase C (PKC) may be involved, the individual contribution of PKC isoforms in their ability to phosphorylate DNMT1 remains unknown. The PKC family consists of at least 12 isoforms that possess distinct differences in structure, substrate requirement, expression and localization.

Results

Here we show that PKCα, βI, βII, δ, γ, η, ζ and μ preferentially phosphorylate the N-terminal domain of human DNMT1. No such phosphorylation of DNMT1 was observed with PKCε. Using PKCζ as a prototype model, we also found that PKC physically interacts with and phosphorylates DNMT1. In vitro phosphorylation assays conducted with recombinant fragments of DNMT1 showed that PKCζ preferentially phosphorylated the N-terminal region of DNMT1. The interaction of PKCζ with DNMT1 was confirmed by GST pull-down and co-immunoprecipitation experiments. Co-localization experiments by fluorescent microscopy further showed that endogenous PKCζ and DNMT1 were present in the same molecular complex. Endogenous PKCζ activity was also detected when DNMT1 was immunoprecipitated from HEK-293 cells. Overexpression of both PKCζ and DNMT1 in HEK-293 cells, but not of either alone, reduced the methylation status of genes distributed across the genome. Moreover, in vitro phosphorylation of DNMT1 by PKCζ reduced its methytransferase activity.

Conclusions

Our results indicate that phosphorylation of human DNMT1 by PKC is isoform-specific and provides the first evidence of cooperation between PKCζ and DNMT1 in the control of the DNA methylation patterns of the genome.

A novel mouse PKCδ splice variant, PKCδIX, inhibits etoposide-induced apoptosis

Protein kinase C (PKC) δ plays an important role in cellular proliferation and apoptosis. The catalytic fragment of PKCδ generated by caspase-dependent cleavage is essential for the initiation of etoposide-induced apoptosis. In this study, we identified a novel mouse PKCδ isoform named PKCδIX (Genebank Accession No. HQ840432). PKCδIX is generated by alternative splicing and is ubiquitously expressed, as seen in its full-length PKCδ. PKCδIX lacks the C1 domain, the caspase 3 cleavage site, and the ATP binding site but preserves an almost intact c-terminal catalytic domain and a nuclear localization signal (NLS). The structural characteristics of PKCδIX provided a possibility that this PKCδ isozyme functions as a novel dominant-negative form for PKCδ due to its lack of the ATP-binding domain that is required for the kinase activity of PKCδ. Indeed, overexpression of PKCδIX significantly inhibited etoposide-induced apoptosis in NIH3T3 cells. In addition, an in vitro kinase assay showed that recombinant PKCδIX protein could competitively inhibit the kinase activity of PKCδ. We conclude that PKCδIX can function as a natural dominant-negative inhibitor of PKCδin vivo.

High glucose induces apoptosis in embryonic neural progenitor cells by a pathway involving protein PKCδ

Diabetic-induced neural tube defects in embryos are caused by apoptosis of neural progenitor cells (NPCs); however, the underlying mechanisms are poorly understood. The present study is aimed to investigate the specific cellular proteins that may be involved in apoptosis of NPCs. We show here that hyperglycemia-induced apoptosis of NPCs was through a PKCδ-dependent mechanism. Tyrosine phosphorylation of PKCδ was required for PKCδ binding to c-Abl in the cytoplasm, and inhibition of c-Abl by STI571 or knock-down of c-Abl by RNAi decreased the phosphorylation of PKCδ. Moreover, translocation of PKCδ and c-Abl complex from the cytoplasm to the nucleus, was blocked by down-regulation of PKCδ or c-Abl. Furthermore, we found that interaction of PKCδ and c-Abl played a crucial role in p53 accumulation in the nucleus, which was linked to the apoptosis of NPCs in response to high glucose.

PMA up-regulates the transcription of Axl by AP-1 transcription factor binding to TRE sequences via the MAPK cascade in leukaemia cells

BACKGROUND

Axl is a receptor tyrosine kinase promoting anti-apoptosis, invasion and mitogenesis, and is highly expressed in different solid cancers. Axl basal transcriptional activity is driven by Sp1/Sp3, and overexpression of MZF-1 (myeloid zinc-finger 1) induces Axl transcription and gene expression. Furthermore, Axl expression is epigenetically controlled by CpG hypermethylation; however, little is known about inducible Axl gene expression and Axl regulation in haematopoetic malignancies.

RESULTS

In the present study, we studied Axl transcriptional regulation under PMA-stimulated conditions in leukaemia cells. Luciferase analysis with sequential 5'-deletion constructs revealed that the -660/-580 region of the Axl promoter is indispensable for induced promoter activity under PMA stimulation. This region includes AP-1 (activator protein 1)/CREB [CRE (cAMP-response-element)-binding protein] motifs, five times partially overlapping TGCGTG repeats and multiple GT repeats. Mutational, supershift and ChIP (chromatin immunoprecipitation) analysis determined that AP-1 family members bind to AP-1 motifs and to the 5 × TGCGTG overlapping repeats, thus transactivating Axl promoter activity. Furthermore, specific inhibitors of PKC (protein kinase C), ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 reduced Axl expression. Additionally, mithramycin treatment abolished constitutive and PMA-induced Axl expression.

CONCLUSIONS

Taken together the results of the present study suggest that PMA-induced Axl gene expression in leukaemia cells is mediated by AP-1 motifs and 5 × TGCGTG repeats within the promoter region -660/-580, and through the PKC/ERK1/2/AP-1 or PKC/p-38/AP-1 signalling axis.

Role of protein kinase C β and vascular endothelial growth factor receptor in malignant pleural mesothelioma: Therapeutic implications and the usefulness of Caenorhabditis elegans model organism

PURPOSE

To examine the role of both protein kinase C (PKC)-β and vascular endothelial growth factor receptor (VEGFR)-2 in malignant pleural mesothelioma (MPM) using respective inhibitors, enzastaurin and KRN633.

MATERIALS AND METHODS

MPM cell lines, control cells, and a variety of archived MPM tumor samples were used to determine the protein expression levels of PKC-β, VEGFR-2, VEGF, and p-AKT. Effects of enzastaurin and KRN633 on phosphorylation status of key signaling molecules and viability of the mesothelioma cells were determined. The common soil nematode, Caenorhabditis elegans, was treated with enzastaurin to determine its suitability to screen for highly potent kinase inhibitors.

RESULTS

PKC-β1, PKC-β2 and VEGFR-2/KDR were overexpressed in MPM cell lines and MPM tumor tissues. Enzastaurin treatment resulted in significant loss in viability of VEGF induced cell proliferation; however, the effect of KRN633 was much less. Enzastaurin also dramatically decreased the phosphorylation of PKC-β, its downstream target p-AKT, and surprisingly, the upstream VEGFR-2. The combination of the two drugs at best was additive and similar results were obtained with respect to cell viability. Treatment of C. elegans with enzastaurin resulted in clear phenotypic changes and the worms were hypermotile with abnormal pattern and shape of eggs, suggesting altered fecundity.

CONCLUSIONS

PKC-β1 and VEGFR-2 are both excellent therapeutic targets in MPM. Enzastaurin was better at killing MPM cells than KRN633 and the combination lacked synergy. In addition, we show here that C. elegans can be used to screen for the next generation inhibitors as treatment with enzastaurin resulted in clear phenotypic changes that could be assayed.

Mechanical stress activates Smad pathway through PKCδ to enhance interleukin-11 gene transcription in osteoblasts

BACKGROUND

Mechanical stress rapidly induces ΔFosB expression in osteoblasts, which binds to interleukin (IL)-11 gene promoter to enhance IL-11 expression, and IL-11 enhances osteoblast differentiation. Because bone morphogenetic proteins (BMPs) also stimulate IL-11 expression in osteoblasts, there is a possibility that BMP-Smad signaling is involved in the enhancement of osteoblast differentiation by mechanical stress. The present study was undertaken to clarify whether mechanical stress affects BMP-Smad signaling, and if so, to elucidate the role of Smad signaling in mechanical stress-induced enhancement of IL-11 gene transcription.

METHODOLOGY/PRINCIPAL FINDINGS

Mechanical loading by fluid shear stress (FSS) induced phosphorylation of BMP-specific receptor-regulated Smads (BR-Smads), Smad1/5, in murine primary osteoblasts (mPOBs). FSS rapidly phosphorylated Y311 of protein kinase C (PKC)δ, and phosphorylated PKCδ interacted with BR-Smads to phosphorylate BR-Smads. Transfection of PKCδ siRNA or Y311F mutant PKCδ abrogated BR-Smads phosphorylation and suppressed IL-11 gene transcription enhanced by FSS. Activated BR-Smads bound to the Smad-binding element (SBE) of IL-11 gene promoter and formed complex with ΔFosB/JunD heterodimer via binding to the C-terminal region of JunD. Site-directed mutagenesis in the SBE and the AP-1 site revealed that both SBE and AP-1 sites were required for full activation of IL-11 gene promoter by FSS.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that PKCδ-BR-Smads pathway plays an important role in the intracellular signaling in response to mechanical stress, and that a cross-talk between PKCδ-BR-Smads and ΔFosB/JunD pathways synergistically stimulates IL-11 gene transcription in response to mechanical stress.

Adhesion of renal carcinoma cells to endothelial cells depends on PKCmu

Background

The formation of metastases includes the separation of tumor cells from the primary tumor, cell migration into subendothelial tissue and cell proliferation in secondary organ. In this process, cell adhesion of tumor cells to the endothelium is an essential requirement for formation of metastases. Protein kinase C (PKC) regulates adhesion and proliferation. To identify a relation between PKC isoforms and tumor progression in renal cell carcinoma (RCC), the influence of PKC isoforms on cell adhesion and proliferation, and possible influences of integrins were analyzed in RCC cells.

Methods

The experiments were performed in the RCC cell lines CCF-RC1 and CCF-RC2 after pre-incubation (16 h) with the PKC inhibitors GF109203X (inhibits PKCα, βI, βII, γ, δ and ε), GÖ6976 (inhibits PKCα, βI and μ), RO31-8220 (inhibits PKCα, βI, βII, γ and ε) and rottlerin (inhibits PKCδ). Cell adhesion was assessed through adherence of RCC cells to an endothelial monolayer. Cell proliferation was analyzed by a BrdU incorporation assay. The expression of β1 integrins was analyzed by flow cytometry.

Results

In CCF-RC1 cells, cell adhesion was significantly reduced by GÖ6976 to 55% and by RO31-8220 to 45% of control. In CCF-RC2 cells, only GÖ6976 induced a significant reduction of cell adhesion to 50% of control levels. Proliferation of both cell lines was reduced by rottlerin to 39% and 45% of control, respectively. The β1 integrin expression on the cell surface of CCF-RC1 and CCR-RC2 cells was decreased by RO31-8220 to 8% and 7% of control, respectively. β2 and β3 integrins were undetectable in both cell lines.

Conclusions

The combination of the PKC inhibitors leads to the assumption that PKCμ influences cell adhesion in CCF-RC1 and CCF-RC2 cells, whereas in CCF-RC1 cells PKCε also seems to be involved in this process. The expression of β1 integrins appears to be regulated in particular by PKCε. Cell proliferation was inhibited by rottlerin, so that PKCδ might be involved in cell proliferation in these cells.

Regulation of adipocyte differentiation by distinct subcellular pools of protein kinase B (PKB/Akt)

The phosphatidylinositol 3-kinase (PI3K)-protein kinase B (PKB)/Akt-PTEN signal transduction pathway orchestrates a variety of fundamental cell processes and its deregulation is implicated in many human diseases. Although the importance of this pathway to many cellular functions is well established, the mechanisms by which it achieves context-specific physiological outcomes in response to a variety of stimuli, using a relatively limited pool of effectors, remain largely unknown. Spatial restriction of signaling events is one means by which cells coordinate specific responses using common molecules. To investigate the subcellular location-specific roles of the major PI3K effector PKB/Akt in various cell processes, we have developed a novel experimental system employing cellular compartment-directed PKB/Akt pseudosubstrate inhibitors. Subcellular location-restricted PKB/Akt inhibition in the 3T3L1 adipocyte differentiation model revealed that nuclear and plasma membrane, but not cytoplasmic, PKB/Akt activity is required for terminal adipocyte differentiation. Nuclear and plasma membrane pools of PKB/Akt were found to contribute to distinct stages of adipocyte differentiation, revealing that PKB/Akt activity impacts multiple points of this program. Our work establishes the use of localized pseudosubstrate PKB/Akt inhibitors as an effective method for the dissection of PKB/Akt signaling.

Infectivity of Leishmania mexicana is associated with differential expression of protein kinase C-like triggered during a cell-cell contact

Mammalian host cell invasion by Leishmania is a complex process in which various parasite and host cell components interact, triggering the activation of signaling cascades in both cells. Little is known regarding PKC biological functions in Leishmania sp. during parasite-macrophage interaction. PKC-like enzyme was first identified in homogenates and membrane fraction of L. mexicana stationary promastigotes by immunoblot. PKC-like enzyme activity was then detected in cell homogenates but also on intact promastigotes showing for the first time the presence of an ecto-PKC dependent on Ca²⁺/phosphatidylserine for activation. This ecto-PKC was activated with phorbol myristate acetate (PMA) and inhibited by RO-32-0432, a selective PKCαβ_Iε bisindolylmaleimide inhibitor. Interestingly, the Leishmania PKC- activity was higher in the infective stationary than in non-infective logarithmic stage. Then, promastigotes at different stages of time proliferation curve were used in order to identify the role of PKC-like during macrophage invasion. After attachment to macrophages, PKC-like is over-expressed in promastigotes at the 6^th culture day but also at the 4^th day of culture corresponding to the maximal infection capacity. An antibody microarray for MAPK and PKC corroborate the Leishmania PKC-like over-expression during contact with macrophages. Pretreatment with RO-32-0432 inhibitor reduced the number of infected macrophages and the parasite burden. These data suggest for the first time a direct link between PKC expression level and infectivity, and provide evidence that PKC-like plays a critical role in attachment and in the internalization steps involved in the invasion process.

PMA induces expression from the herpes simplex virus thymidine kinase promoter via the activation of JNK and ERK in the presence of adenoviral E1A proteins

The herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) promoter contains elements involved in both constitutive and induced expression. We determined that phorbol 12-myristate 13-acetate (PMA) induces the HSV-1 TK promoter in HEK293 cells. However, PMA did not induce expression from the promoter in HeLa cells and did not result in a globally increased gene expression in HEK293 cells. Induction of HSV-1 TK promoter required activation of both of JNK and ERK pathways. However, activation of the two pathways alone was not sufficient for induction of HSV-1 TK promoter. By transiently transfecting into HeLa cells the adenoviral E1A gene, which exists as an integrant in HEK293 genome, we demonstrated that E1A proteins are necessary for induction of HSV-1 TK promoter by PMA. We propose mechanisms by which signaling pathways activated by the tumor-promoter PMA cooperate with the oncogene E1A to stimulate a eukaryotic promoter, namely the HSV-1 TK promoter.

Macrophage differentiation increases expression of the ascorbate transporter (SVCT2)

To determine whether macrophage differentiation involves increased uptake of vitamin C, or ascorbic acid, we assessed the expression and function of its transporter SVCT2 during phorbol ester-induced differentiation of human-derived THP-1 monocytes. Induction of THP-1 monocyte differentiation by phorbol 12-myristate 13-acetate (PMA) markedly increased SVCT2 mRNA, protein, and function. When ascorbate was present during PMA-induced differentiation, the increase in SVCT2 protein expression was inhibited, but differentiation was enhanced. PMA-induced SVCT2 protein expression was blocked by inhibitors of protein kinase C (PKC), with most of the affect due to the PKCbetaI and betaII isoforms. Activation of MEK/ERK was sustained up to 48 h after PMA treatment, and the inhibitors completely blocked PMA-stimulated SVCT2 protein expression, indicating an exclusive role for the classical MAP kinase pathway. However, inhibitors of NF-kappaB activation, NADPH oxidase inhibitors, and several antioxidants also partially prevented SVCT2 induction, suggesting diverse distal routes for control of SVCT2 transcription. Both known promoters for the SVCT2 were involved in these effects. In conclusion, PMA-induced monocyte-macrophage differentiation is enhanced by ascorbate and associated with increased expression and function of the SVCT2 protein through a pathway involving sustained activation of PKCbetaI/II, MAP kinase, NADPH oxidase, and NF-kappaB.

Protein kinases C isozymes are differentially expressed in human breast carcinomas

AIMS

The protein kinase C (PKC) family of enzymes has been implicated in cellular proliferation, differentiation, and apoptosis. However, the distribution of specific PKC isoforms with varying functions in normal and malignant human tissues remains to be determined. The objective of this study was to investigate the expression of certain PKC isoforms (alpha, betaI, betaII, epsilon) in human breast cancer specimens relative to adjacent uninvolved tissue (n=24) and in the normal breast tissue obtained from patients undergoing reduction mammoplasty (n=12).

MAIN METHODS

Western blot analysis using PKC isoform specific antibodies was performed on tissue extracts from breast tumors, adjacent uninvolved tissues, and reduction mammoplasty tissues.

KEY FINDINGS

Mean levels of cytosolic and membrane PKC-alpha, PKC-betaI, and PKC-betaII were significantly higher in the cancer specimens than in the adjacent uninvolved breast tissues (Wilcoxon signed-ranks test; P<0.05 for each, after adjustment for multiple comparisons). There was a notably higher mean level of membrane PKC-betaII isozyme in Her-2 positive and in poorly differentiated tumors. No significant differences were observed when normal tissue adjacent to tumor was compared to breast tissue obtained from reduction mammoplasty specimens.

SIGNIFICANCE

Higher level of PKC-alpha, PKC-betaI, and PKC-betaII in cancer specimens and higher level of PKC-betaII in Her-2 positive tumors require further exploration of the intracellular pathways involving PKC-alpha and -beta isoforms in breast cancer because both could be specific targets for the development of new therapies and for the prevention and treatment of this disease.

Protein kinase C epsilon: an oncogene and emerging tumor biomarker

Members of the protein kinase C (PKC) family have long been studied for their contributions to oncogenesis. Among the ten different isoforms of this family of serine/threonine kinases, protein kinase Cε (PKCε) is one of the best understood for its role as a transforming oncogene. In vitro, overexpression of PKCε has been demonstrated to increase proliferation, motility, and invasion of fibroblasts or immortalized epithelial cells. In addition, xenograft and transgenic animal models have clearly shown that overexpression of PKCε is tumorigenic resulting in metastatic disease. Perhaps most important in implicating the epsilon isoform in oncogenesis, PKCε has been found to be overexpressed in tumor-derived cell lines and histopathological tumor specimens from various organ sites. Combined, this body of work provides substantial evidence implicating PKCε as a transforming oncogene that plays a crucial role in establishing an aggressive metastatic phenotype. Reviewed here is the literature that has led to the current understanding of PKCε as an oncogene. Moreover, this review focuses on the PKCε-mediated signaling network for cell motility and explores the interaction of PKCε with three major PKCε signaling nodes: RhoA/C, Stat3 and Akt. Lastly, the emerging role of PKCε as a tumor biomarker is discussed.

Protein kinase C signaling in the hypothalamic arcuate nucleus regulates sexual receptivity in female rats

Rapid membrane-mediated estradiol signaling regulating sexual receptivity requires the interaction of the estrogen receptor (ER)-alpha and the metabotropic glutamate receptor 1a (mGluR1a). A cell signaling antibody microarray revealed that estradiol activated 42 proteins in the arcuate nucleus of the hypothalamus (ARH). To begin an analysis of various signaling pathways, protein kinase A and protein kinase C (PKC)-theta, whose signaling pathways have been implicated in the estradiol regulation of sexual receptivity, were examined. In the ARH sample, the increase in phospho-protein kinase A could not be confirmed by Western blotting, in either cytosolic or membrane fractions. However, the increase in phosphorylated PKCtheta seen with the pathway array was verified by Western blotting. To study whether rapid estradiol activation of PKC regulates the ARH-medial preoptic nucleus pathway regulating lordosis, mu-opioid receptor (MOR) internalization and lordosis reflex were tested. Blocking PKC in ARH with 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]3-(1H-indol-3-yl) maleimide significantly attenuated estradiol-induced MOR internalization. Furthermore, disruption of PKC signaling within the ARH at the time of estradiol treatment significantly diminished the lordosis reflex. Moreover, blocking PKC prevented MOR internalization when the circuit was activated by the mGluR1a agonist, (RS)-3,5-dihydroxyphenylglycine. Activation of PKC with phorbol 12, 13-dibutyrate induced MOR internalization, indicating that PKC was a critical step for membrane ERalpha-initiated mGluR1a-mediated cell signaling and phorbol 12, 13-dibutyrate significantly facilitated the lordosis reflex. Together these findings indicate that rapid membrane ERalpha-mGluR1a interactions activate PKCtheta cell signaling, which regulates female sexual receptivity.

Regulation of human beta-defensin-2 by Mycobacterium bovis bacillus Calmette-Guérin (BCG): involvement of PKC, JNK, and PI3K in human lung epithelial cell line (A549)

Human beta-defensin (HBD)-2 is an inducible antimicrobial peptide that plays an important role in innate immunity. Induction of this peptide by mycobacteria in epithelial cells has been reported. However, the mechanism(s) by which Mycobacterium bovis bacillus Calmette-Guérin (BCG) triggers gene transcription of HBD-2 remains poorly understood. In the present work we found that treatment of human epithelial cells with Ro32-0432 or Gö6976, two selective inhibitors of protein kinase C (PKC), significantly reduced the effect of M. bovis BCG on induced HBD-2 mRNA expression (65 and 80% inhibition by 10microM Ro32-0432, and 1microM Gö6976 as assessed by real-time PCR, respectively). Moreover, there was increased activation of c-Jun N-terminal kinase (JNK) and phosphatidylinositol-3-kinase (PI3K)/Akt in A549 cells infected with M. bovis BCG, and this JNK and PI3K activation was mediated through PKC. Finally, we found that M. bovis BCG-induced HBD-2 mRNA gene expression in A549 cells was dependent on JNK, and PI3K determined by real-time PCR analysis, which was attenuated by inhibitors of JNK (SP600125 and AG126), and PI3K (wortmannin and Ly294002). These studies are the first to show that M. bovis BCG-induced HBD-2 mRNA expression in A549 cells is regulated at least in part through activation of signaling proteins of PKC, JNK and PI3K.

Protein kinase C isozymes and their selectivity towards ruboxistaurin

Protein kinase C (PKC) isozymes are an important class of enzymes in cell signaling and as drug targets. They are involved in specific pathways and have selectivity towards certain ligands, despite their high sequence similarities. Ruboxistaurin is a specific inhibitor of PKC-beta. To understand the molecular determinants for the selectivity of ruboxistaurin, we derived the three-dimensional structures of the kinase domains of PKC-alpha, -betaI, and -zeta using homology modeling. Several binding orientations of ruboxistaurin in the binding sites of these PKC catalytic domains were analyzed, and a putative alternative binding site for PKC-zeta was identified in its kinase domain. The calculated free energy of binding correlates well with the IC(50) of the inhibitor against each PKC isozyme. A residue-based energy decomposition analysis attributed the binding free energy to several key residues in the catalytic sites of these enzymes, revealing potential protein-ligand interactions responsible for ligand binding. The contiguous binding site revealed in the catalytic domain of PKC-zeta provides avenues for selective drug design. The details of structural nuances for specific inhibition of PKC isozymes are presented in the context of the three-dimensional structures of this important class of enzymes.