Activation of PKCβII through nuclear trafficking guided by βγ subunits of trimeric G protein and 14-3-3ε

AIMS

Conventional members of protein kinase C (PKC) family, including PKCβII, are constitutively phosphorylated on three major motifs and located in the cytosol in a primed state. In response to cellular stimuli, PKCβII is activated through inducible phosphorylation and Mdm2-mediated ubiquitination. In this study, we aimed to identify the activation mechanism of PKCβII, focusing on the signaling cascade that regulate the phosphorylation and ubiquitination.

MATERIALS AND METHODS

Loss-of-function approaches and mutants of PDK1/PKCβII that display different regulatory properties were used to identify the cellular components and processes responsible for endocytosis.

KEY FINDINGS

Phorbol 12-myristate 13-acetate (PMA)-induced phosphorylation and ubiquitination of PKCβII, which are needed for its translocation to the plasma membrane, required the presence of both Gβγ and 14-3-3ε. Gβγ and 14-3-3ε mediated the constitutive phosphorylation of PKCβII by scaffolding PI3K and PDK1 in the cytosol, which is an inactive but required state for the activation of PKCβII by subsequent signals. In response to PMA treatment, the signaling complex translocated to the nucleus with dissociation of PI3K from it. Thereafter, PDK1 stably interacted with 14-3-3ε and was dephosphorylated; PKCβII interacted with Mdm2 along with Gβγ, leading to its ubiquitination at two lysine residues on its C-tail. Finally, PDK1/14-3-3ε and ubiquitinated PKCβII translocated to the plasma membrane.

SIGNIFICANCE

As PKCβII mediates a wide range of cellular functions and plays important roles in the pathogenesis of various diseases, our results will provide clues to understand the pathogenesis of PKCβII-related disorders and facilitate their treatment.

PKCα and PKCδ: Friends and Rivals

PKC comprises a large family of serine/threonine kinases that share a requirement for allosteric activation by lipids. While PKC isoforms have significant homology, functional divergence is evident among subfamilies and between individual PKC isoforms within a subfamily. Here, we highlight these differences by comparing the regulation and function of representative PKC isoforms from the conventional (PKCα) and novel (PKCδ) subfamilies. We discuss how unique structural features of PKCα and PKCδ underlie differences in activation and highlight the similar, divergent, and even opposing biological functions of these kinases. We also consider how PKCα and PKCδ can contribute to pathophysiological conditions and discuss challenges to targeting these kinases therapeutically.

Protein Kinase C Life Cycle: Explained Through Systems Biology Approach

Protein kinase C (PKC) enzymes are a family of kinases that mediate signal transduction originating at the cell surface. Most cell membranes can contain functional PKC enzymes. Aberrations in the PKC life cycle may result in cellular damage and dysfunction. For example, some cancerous cells exhibit alterations in PKC activity. Here, we use a systems biology approach to describe a molecular model of the PKC life cycle. Understanding the PKC life cycle is necessary to identify new drug targets. The PKC life cycle is composed of three key regulatory processes: maturation, activation, and termination. These processes precisely control PKC enzyme levels. This model describes the fate of PKC during de novo synthesis and PKC’s lipid-mediated activation cycle. We utilize a systems biology approach to show the PKC life cycle is controlled by multiple phosphorylation and dephosphorylation events. PKC processing events can be divided into two types: maturation via processing of newly synthesized enzyme and secondary messenger-dependent activation of dormant, but catalytically competent enzyme. Newly synthesized PKC enzyme is constitutively processed through three ordered phosphorylations and stored in the cytosol as a stable, signaling-competent inactive and autoinhibited molecule. Upon extracellular stimulation, diacylglycerol (DAG) and calcium ion (Ca²⁺) generated at the membrane bind PKC. PKC then undergoes cytosol-to-membrane translocation and subsequent activation. Our model shows that, once activated, PKC is prone to dephosphorylation and subsequent degradation. This model also describes the role of HSP70 in stabilization and re-phosphorylation of dephosphorylated PKC, replenishing the PKC pool. Our model shows how the PKC pool responds to different intensities of extracellular stimuli? We show that blocking PHLPP dephosphorylation replenishes the PKC pool in a dose-dependent manner. This model provides a comprehensive understanding of PKC life cycle regulation.

Protein Kinase C-α Is a Gatekeeper of Cryptosporidium Sporozoite Adherence and Invasion

Cryptosporidium infection is a leading cause of diarrhea-associated morbidity and mortality in young children globally. Single nucleotide polymorphisms (SNPs) in the human protein kinase C-α (PRKCA) gene region have been associated with susceptibility to cryptosporidiosis. Here, we examined the role of protein kinase C-α (PKCα) activity in human HCT-8 intestinal epithelial cells during infection with Cryptosporidium parvum sporozoites. To delineate the role of PKCα in infection, we developed a fluorescence-based imaging assay to differentiate adherent from intracellular parasites. We tested pharmacological agonists and antagonists of PKCα and measured the effect on C. parvum sporozoite adherence to and invasion of HCT-8 cells. We demonstrate that both PKCα agonists and antagonists significantly alter parasite adherence and invasion in vitro. We found that HCT-8 cell PKCα is activated by C. parvum infection. Our findings suggest intestinal epithelial cell PKCα as a potential host-directed therapeutic target for cryptosporidiosis and implicate PKCα activity as a mediator of parasite adherence and invasion.

Targeting Protein Kinase C for Cancer Therapy

Simple Summary

The protein kinase C (PKC) family belongs to serine-threonine kinases and consists of several subtypes. Increasing evidence suggests that PKCs are critical players in carcinogenesis. Interestingly, PKCs exert both promotive and suppressive effects on tumor cell growth and metastasis, which have attracted immense attention. Herein, we systematically review the current advances in the structure, regulation and biological functions of PKCs, especially the relationship of PKCs with anti-cancer therapy-induced cell death, including the current knowledge of PKCs function in tumor metabolism and microenvironment. Moreover, we discuss the potential role of PKCs as a target for therapeutic intervention in cancer from basic research and clinical trials.

Abstract

Protein kinase C (PKC) isoforms, a group of serine-threonine kinases, are important regulators in carcinogenesis. Numerous studies have demonstrated that PKC isoforms exert both positive and negative effects on cancer cell demise. In this review, we systematically summarize the current findings on the architecture, activity regulation and biological functions of PKCs, especially their relationship with anti-cancer therapy-induced cell death. Additionally, we elaborate on current knowledge of the effects of PKCs on tumor metabolism and microenvironment, which have gained increasing attention in oncology-related areas. Furthermore, we underscore the basic experimental and clinical implications of PKCs as a target for cancer therapy to evaluate their therapeutic benefits and potential applications.

Genomic and phenotypic analysis of SspH1 identifies a new Salmonella effector, SspH3

Salmonella is a major foodborne pathogen and is responsible for a range of diseases. Not all Salmonella contribute to severe health outcomes as there is a large degree of genetic heterogeneity among the 2600 serovars within the genus. This variability across Salmonella serovars is linked to numerous genetic elements that dictate virulence. While several genetic elements encode virulence factors with well documented contributions to pathogenesis, many genetic elements implicated in Salmonella virulence remain uncharacterized. Many pathogens encode a family of E3 ubiquitin ligases that are delivered into the cells that they infect using a Type 3 Secretion System (T3SS). These effectors, known as NEL-domain E3s, were first characterized in Salmonella. Most Salmonella encode the NEL-effectors sspH2 and slrP, whereas only a subset of Salmonella encode sspH1. SspH1 has been shown to ubiquitinate the mammalian protein kinase PKN1, which has been reported to negatively regulate the pro-survival program Akt. We discovered that SspH1 mediates the degradation of PKN1 during infection of a macrophage cell line but that this degradation does not impact Akt signaling. Genomic analysis of a large collection of Salmonella genomes identified a putative new gene, sspH3, with homology to sspH1. SspH3 is a novel NEL-domain effector.

M1 and M2 mAChRs activate PDK1 and regulate PKC βI and ε and the exocytotic apparatus at the NMJ

Neuromuscular junctions (NMJ) regulate cholinergic exocytosis through the M₁ and M₂ muscarinic acetylcholine autoreceptors (mAChR), involving the crosstalk between receptors and downstream pathways. Protein kinase C (PKC) regulates neurotransmission but how it associates with the mAChRs remains unknown. Here, we investigate whether mAChRs recruit the classical PKCβI and the novel PKCε isoforms and modulate their priming by PDK1, translocation and activity on neurosecretion targets. We show that each M₁ and M₂ mAChR activates the master kinase PDK1 and promotes a particular priming of the presynaptic PKCβI and ε isoforms. M₁ recruits both primed-PKCs to the membrane and promotes Munc18-1, SNAP-25, and MARCKS phosphorylation. In contrast, M₂ downregulates PKCε through a PKA-dependent pathway, which inhibits Munc18-1 synthesis and PKC phosphorylation. In summary, our results discover a co-dependent balance between muscarinic autoreceptors which orchestrates the presynaptic PKC and their action on ACh release SNARE-SM mechanism. Altogether, this molecular signaling explains previous functional studies at the NMJ and guide toward potential therapeutic targets.

TRAF2 Is a Novel Ubiquitin E3 Ligase for the Na,K-ATPase β-Subunit That Drives Alveolar Epithelial Dysfunction in Hypercapnia

Several acute and chronic lung diseases are associated with alveolar hypoventilation leading to accumulation of CO2 (hypercapnia). The β-subunit of the Na,K-ATPase plays a pivotal role in maintaining epithelial integrity by functioning as a cell adhesion molecule and regulating cell surface stability of the catalytic α-subunit of the transporter, thereby, maintaining optimal alveolar fluid balance. Here, we identified the E3 ubiquitin ligase for the Na,K-ATPase β-subunit, which promoted polyubiquitination, subsequent endocytosis and proteasomal degradation of the protein upon exposure of alveolar epithelial cells to elevated CO2 levels, thus impairing alveolar integrity. Ubiquitination of the Na,K-ATPase β-subunit required lysine 5 and 7 and mutating these residues (but not other lysines) prevented trafficking of Na,K-ATPase from the plasma membrane and stabilized the protein upon hypercapnia. Furthermore, ubiquitination of the Na,K-ATPase β-subunit was dependent on prior phosphorylation at serine 11 by protein kinase C (PKC)-ζ. Using a protein microarray, we identified the tumor necrosis factor receptor-associated factor 2 (TRAF2) as the E3 ligase driving ubiquitination of the Na,K-ATPase β-subunit upon hypercapnia. Of note, prevention of Na,K-ATPase β-subunit ubiquitination was necessary and sufficient to restore the formation of cell-cell junctions under hypercapnic conditions. These results suggest that a hypercapnic environment in the lung may lead to persistent epithelial dysfunction in affected patients. As such, the identification of the E3 ligase for the Na,K-ATPase may provide a novel therapeutic target, to be employed in patients with acute or chronic hypercapnic respiratory failure, aiming to restore alveolar epithelial integrity.

PHLPPing the balance: restoration of protein kinase C in cancer

Protein kinase signalling, which transduces external messages to mediate cellular growth and metabolism, is frequently deregulated in human disease, and specifically in cancer. As such, there are 77 kinase inhibitors currently approved for the treatment of human disease by the FDA. Due to their historical association as the receptors for the tumour-promoting phorbol esters, PKC isozymes were initially targeted as oncogenes in cancer. However, a meta-analysis of clinical trials with PKC inhibitors in combination with chemotherapy revealed that these treatments were not advantageous, and instead resulted in poorer outcomes and greater adverse effects. More recent studies suggest that instead of inhibiting PKC, therapies should aim to restore PKC function in cancer: cancer-associated PKC mutations are generally loss-of-function and high PKC protein is protective in many cancers, including most notably KRAS-driven cancers. These recent findings have reframed PKC as having a tumour suppressive function. This review focusses on a potential new mechanism of restoring PKC function in cancer - through targeting of its negative regulator, the Ser/Thr protein phosphatase PHLPP. This phosphatase regulates PKC steady-state levels by regulating the phosphorylation of a key site, the hydrophobic motif, whose phosphorylation is necessary for the stability of the enzyme. We also consider whether the phosphorylation of the potent oncogene KRAS provides a mechanism by which high PKC expression may be protective in KRAS-driven human cancers.

Equivocal, explicit and emergent actions of PKC isoforms in cancer

The maturing mutational landscape of cancer genomes, the development and application of clinical interventions and evolving insights into tumour-associated functions reveal unexpected features of the protein kinase C (PKC) family of serine/threonine protein kinases. These advances include recent work showing gain or loss-of-function mutations relating to driver or bystander roles, how conformational constraints and plasticity impact this class of proteins and how emergent cancer-associated properties may offer opportunities for intervention. The profound impact of the tumour microenvironment, reflected in the efficacy of immune checkpoint interventions, further prompts to incorporate PKC family actions and interventions in this ecosystem, informed by insights into the control of stromal and immune cell functions. Drugging PKC isoforms has offered much promise, but when and how is not obvious.

Rod bipolar cells dysfunction occurs before ganglion cells loss in excitotoxin-damaged mouse retina

Progressive degeneration of retinal ganglion cells (RGCs) will cause a blinding disease. Most of the study is focusing on the RGCs itself. In this study, we demonstrate a decline of the presynaptic rod bipolar cells (RBCs) response precedes RGCs loss and a decrease of protein kinase Cα (PKCα) protein expression in RBCs dendrites, using whole-cell voltage-clamp, electroretinography (ERG) measurements, immunostaining and co-immunoprecipitation. We present evidence showing that N-methyl D-aspartate receptor subtype 2B (NR2B)/protein interacting with C kinase 1 (PICK1)-dependent degradation of PKCα protein in RBCs contributes to RBCs functional loss. Mechanistically, NR2B forms a complex with PKCα and PICK1 to promote the degradation of PKCα in a phosphorylation- and proteasome-dependent manner. Similar deficits in PKCα expression and response sensitivity were observed in acute ocular hypertension and optic never crush models. In conclusion, we find that three separate experimental models of neurodegeneration, often used to specifically target RGCs, disrupt RBCs function prior to the loss of RGCs. Our findings provide useful information for developing new diagnostic tools and treatments for retinal ganglion cells degeneration disease.

nPKCε Mediates SNAP-25 Phosphorylation of Ser-187 in Basal Conditions and After Synaptic Activity at the Neuromuscular Junction

Protein kinase C (PKC) and substrates like SNAP-25 regulate neurotransmission. At the neuromuscular junction (NMJ), PKC promotes neurotransmitter release during synaptic activity. Thirty minutes of muscle contraction enhances presynaptic PKC isoform levels, specifically cPKCβI and nPKCε, through retrograde BDNF/TrkB signaling. This establishes a larger pool of these PKC isoforms ready to promote neuromuscular transmission. The PKC phosphorylation site in SNAP-25 has been mapped to the serine 187 (Ser-187), which is known to enhance calcium-dependent neurotransmitter release in vitro. Here, we localize SNAP-25 at the NMJ and investigate whether cPKCβI and/or nPKCε regulate SNAP-25 phosphorylation. We also investigate whether nerve and muscle cell activities regulate differently SNAP-25 phosphorylation and the involvement of BDNF/TrkB signaling. Our results demonstrate that nPKCε isoform is essential to positively regulate SNAP-25 phosphorylation on Ser-187 and that muscle contraction prevents it. TrkB and cPKCβI do not regulate SNAP-25 protein level or its phosphorylation during neuromuscular activity. The results provide evidence that nerve terminals need both pre- and postsynaptic activities to modulate SNAP-25 phosphorylation and ensure an accurate neurotransmission process.

Synaptic Activity and Muscle Contraction Increases PDK1 and PKCβI Phosphorylation in the Presynaptic Membrane of the Neuromuscular Junction

Conventional protein kinase C βI (cPKCβI) is a conventional protein kinase C (PKC) isoform directly involved in the regulation of neurotransmitter release in the neuromuscular junction (NMJ). It is located exclusively at the nerve terminal and both synaptic activity and muscle contraction modulate its protein levels and phosphorylation. cPKCβI molecular maturation includes a series of phosphorylation steps, the first of which is mediated by phosphoinositide-dependent kinase 1 (PDK1). Here, we sought to localize PDK1 in the NMJ and investigate the hypothesis that synaptic activity and muscle contraction regulate in parallel PDK1 and cPKCβI phosphorylation in the membrane fraction. To differentiate the presynaptic and postsynaptic activities, we abolished muscle contraction with μ-conotoxin GIIIB (μ-CgTx-GIIIB) in some experiments before stimulation of the phrenic nerve (1 Hz, 30 min). Then, we analyzed total and membrane/cytosol fractions of skeletal muscle by Western blotting. Results showed that PDK1 is located exclusively in the nerve terminal of the NMJ. After nerve stimulation with and without coincident muscle contraction, total PDK1 and phosphorylated PDK1 (pPDK1) protein levels remained unaltered. However, synaptic activity specifically enhanced phosphorylation of PDK1 in the membrane, an important subcellular location for PDK1 function. This increase in pPDK1 coincides with a significant increase in the phosphorylation of its substrate cPKCβI also in the membrane fraction. Moreover, muscle contraction maintains PDK1 and pPDK1 but increases cPKCβI protein levels and its phosphorylation. Thus, even though PDK1 activity is maintained, pcPKCβI levels increase in concordance with total cPKCβI. Together, these results indicate that neuromuscular activity could induce the membrane targeting of pPDK1 in the nerve terminal of the NMJ to promote the phosphorylation of the cPKCβI, which is involved in ACh release.

Bortezomib induces neuropathic pain through protein kinase C-mediated activation of presynaptic NMDA receptors in the spinal cord

Chemotherapeutic drugs, including bortezomib, often cause painful peripheral neuropathy, which is a severe dose-limiting adverse effect experienced by many cancer patients. The glutamate N-methyl-d-aspartate receptors (NMDARs) at the spinal cord level are critically involved in the synaptic plasticity associated with neuropathic pain. In this study, we determined whether treatment with bortezomib, a proteasome inhibitor, affects the NMDAR activity of spinal dorsal horn neurons. Systemic treatment with bortezomib in rats did not significantly affect postsynaptic NMDAR currents elicited by puff application of NMDA directly to dorsal horn neurons. Bortezomib treatment markedly increased the baseline frequency of miniature excitatory postsynaptic currents (EPSCs), which was completely normalized by the NMDAR antagonist 2-amino-5-phosphonopentanoic acid (AP5). AP5 also reduced the amplitude of monosynaptic EPSCs evoked by dorsal root stimulation in bortezomib-treated, but not vehicle-treated, rats. Furthermore, inhibition of protein kinase C (PKC) with chelerythrine fully reversed the increased frequency of miniature EPSCs and the amplitude of evoked EPSCs in bortezomib-treated rats. Intrathecal injection of AP5 and chelerythrine both profoundly attenuated mechanical allodynia and hyperalgesia induced by systemic treatment with bortezomib. In addition, treatment with bortezomib induced striking membrane translocation of PKC-βII, PKC-δ, and PKC-ε in the dorsal root ganglion. Our findings indicate that bortezomib treatment potentiates nociceptive input from primary afferent nerves via PKC-mediated tonic activation of presynaptic NMDARs. Targeting presynaptic NMDARs and PKC at the spinal cord level may be an effective strategy for treating chemotherapy-induced neuropathic pain.

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

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

Tetraspanin CD82 Regulates the Spatiotemporal Dynamics of PKCα in Acute Myeloid Leukemia

Patients with acute myeloid leukemia (AML) have increased myeloid cells within their bone marrow that exhibit aberrant signaling. Therefore, therapeutic targets that modulate disrupted signaling cascades are of significant interest. In this study, we demonstrate that the tetraspanin membrane scaffold, CD82, regulates protein kinase c alpha (PKCα)-mediated signaling critical for AML progression. Utilizing a palmitoylation mutant form of CD82 with disrupted membrane organization, we find that the CD82 scaffold controls PKCα expression and activation. Combining single molecule and ensemble imaging measurements, we determine that CD82 stabilizes PKCα activation at the membrane and regulates the size of PKCα membrane clusters. Further evaluation of downstream effector signaling identified robust and sustained activation of ERK1/2 upon CD82 overexpression that results in enhanced AML colony formation. Together, these data propose a mechanism where CD82 membrane organization regulates sustained PKCα signaling that results in an aggressive leukemia phenotype. These observations suggest that the CD82 scaffold may be a potential therapeutic target for attenuating aberrant signal transduction in AML.

Protein Kinase C Activation as a Potential Therapeutic Strategy in Alzheimer's Disease: Is there a Role for Embryonic Lethal Abnormal Vision-like Proteins?

Alzheimer's disease (AD), the most common cause of dementia, is an irreversible and progressive neurodegenerative disorder. It affects predominantly brain areas that are critical for memory and learning and is characterized by two main pathological hallmarks: extracellular amyloid plaques and intracellular neurofibrillary tangles. Protein kinase C (PKC) has been classified as one of the cognitive kinases controlling memory and learning. By regulating several signalling pathways involved in amyloid and tau pathologies, it also plays an inhibitory role in AD pathophysiology. Among downstream targets of PKC are the embryonic lethal abnormal vision (ELAV)-like RNA-binding proteins that modulate the stability and the translation of specific target mRNAs involved in synaptic remodelling linked to cognitive processes. This MiniReview summarizes the current evidence on the role of PKC and ELAV-like proteins in learning and memory, highlighting how their derangement can contribute to AD pathophysiology. This last aspect emphasizes the potential of pharmacological activation of PKC as a promising therapeutic strategy for the treatment of AD.

Protein Kinase Cα (PKCα) Is Resistant to Long Term Desensitization/Down-regulation by Prolonged Diacylglycerol Stimulation

Sustained activation of PKCα is required for long term physiological responses, such as growth arrest and differentiation. However, studies with pharmacological agonists (e.g. phorbol 12-myristate 13-acetate (PMA)) indicate that prolonged stimulation leads to PKCα desensitization via dephosphorylation and/or degradation. The current study analyzed effects of chronic stimulation with the physiological agonist diacylglycerol. Repeated addition of 1,2-dioctanoyl-sn-glycerol (DiC8) resulted in sustained plasma membrane association of PKCα in a pattern comparable with that induced by PMA. However, although PMA potently down-regulated PKCα, prolonged activation by DiC8 failed to engage known desensitization mechanisms, with the enzyme remaining membrane-associated and able to support sustained downstream signaling. DiC8-activated PKCα did not undergo dephosphorylation, ubiquitination, or internalization, early events in PKCα desensitization. Although DiC8 efficiently down-regulated novel PKCs PKCδ and PKCϵ, differences in Ca(2+) sensitivity and diacylglycerol affinity were excluded as mediators of the selective resistance of PKCα. Roles for Hsp/Hsc70 and Hsp90 were also excluded. PMA, but not DiC8, targeted PKCα to detergent-resistant membranes, and disruption of these domains with cholesterol-binding agents demonstrated a role for differential membrane compartmentalization in selective agonist-induced degradation. Chronic DiC8 treatment failed to desensitize PKCα in several cell types and did not affect PKCβI; thus, conventional PKCs appear generally insensitive to desensitization by sustained diacylglycerol stimulation. Consistent with this conclusion, prolonged (several-day) membrane association/activation of PKCα is seen in self-renewing epithelium of the intestine, cervix, and skin. PKCα deficiency affects gene expression, differentiation, and tumorigenesis in these tissues, highlighting the importance of mechanisms that protect PKCα from desensitization in vivo.

Sequential posttranslational modifications regulate PKC degradation

PKC phosphorylation promotes its sumoylation, which in turn inhibits its ubiquitination and ultimately reduces its degradation via the ubiquitin-proteasome pathway. These findings provide a molecular explanation for the activation-induced down-regulation of PKC proteins.

Cross-talk among different types of posttranslational modifications (PTMs) has emerged as an important regulatory mechanism for protein function. Here we elucidate a mechanism that controls PKCα stability via a sequential cascade of PTMs. We demonstrate that PKCα dephosphorylation decreases its sumoylation, which in turn promotes its ubiquitination and ultimately enhances its degradation via the ubiquitin-proteasome pathway. These findings provide a molecular explanation for the activation-induced down-regulation of PKC proteins.

The unique protein kinase Cη: implications for breast cancer (review)

Deregulation of key signal transduction pathways that govern important cellular processes leads to cancer. The development of effective therapeutics for cancer warrants a comprehensive understanding of the signaling pathways that are deregulated in cancer. The protein kinase C (PKC) family has served as an attractive target for cancer therapy for decades owing to its crucial roles in several cellular processes. PKCη is a novel member of the PKC family that plays critical roles in various cellular processes such as growth, proliferation, differentiation and cell death. The regulation of PKCη appears to be unique compared to other PKC isozymes, and there are conflicting reports regarding its role in cancer. This review focuses on the unique aspects of PKCη in terms of its structure, regulation and subcellular distribution and speculates on how these features could account for its distinct functions. We have also discussed the functional implications of PKCη in cancer with particular emphasis on breast cancer.

Atypical protein kinase Cι as a human oncogene and therapeutic target

Protein kinase inhibitors represent a major class of targeted therapeutics that has made a positive impact on treatment of cancer and other disease indications. Among the promising kinase targets for further therapeutic development are members of the Protein Kinase C (PKC) family. The PKCs are central components of many signaling pathways that regulate diverse cellular functions including proliferation, cell cycle, differentiation, survival, cell migration, and polarity. Genetic manipulation of individual PKC isozymes has demonstrated that they often fulfill distinct, nonredundant cellular functions. Participation of PKC members in different intracellular signaling pathways reflects responses to varying extracellular stimuli, intracellular localization, tissue distribution, phosphorylation status, and intermolecular interactions. PKC activity, localization, phosphorylation, and/or expression are often altered in human tumors, and PKC isozymes have been implicated in various aspects of transformation, including uncontrolled proliferation, migration, invasion, metastasis, angiogenesis, and resistance to apoptosis. Despite the strong relationship between PKC isozymes and cancer, to date only atypical PKCiota has been shown to function as a bona fide oncogene, and as such is a particularly attractive therapeutic target for cancer treatment. In this review, we discuss the role of PKCiota in transformation and describe mechanism-based approaches to therapeutically target oncogenic PKCiota signaling in cancer.

IGF-1 receptor antagonism inhibits autophagy

Inhibition of the insulin/insulin-like growth factor signalling pathway increases lifespan and protects against neurodegeneration in model organisms, and has been considered as a potential therapeutic target. This pathway is upstream of mTORC1, a negative regulator of autophagy. Thus, we expected autophagy to be activated by insulin-like growth factor-1 (IGF-1) inhibition, which could account for many of its beneficial effects. Paradoxically, we found that IGF-1 inhibition attenuates autophagosome formation. The reduced amount of autophagosomes present in IGF-1R depleted cells can be, at least in part, explained by a reduced formation of autophagosomal precursors at the plasma membrane. In particular, IGF-1R depletion inhibits mTORC2, which, in turn, reduces the activity of protein kinase C (PKCα/β). This perturbs the actin cytoskeleton dynamics and decreases the rate of clathrin-dependent endocytosis, which impacts autophagosome precursor formation. Finally, with important implications for human diseases, we demonstrate that pharmacological inhibition of the IGF-1R signalling cascade reduces autophagy also in zebrafish and mice models. The novel link we describe here has important consequences for the interpretation of genetic experiments in mammalian systems and for evaluating the potential of targeting the IGF-1R receptor or modulating its signalling through the downstream pathway for therapeutic purposes under clinically relevant conditions, such as neurodegenerative diseases, where autophagy stimulation is considered beneficial.

Heat shock proteins regulate activation-induced proteasomal degradation of the mature phosphorylated form of protein kinase C

Although alterations in stimulus-induced degradation of PKC have been implicated in disease, mechanistic understanding of this process remains limited. Evidence supports the existence of both proteasomal and lysosomal mechanisms of PKC processing. An established pathway involves rate-limiting priming site dephosphorylation of the activated enzyme and proteasomal clearance of the dephosphorylated protein. However, here we show that agonists promote down-regulation of endogenous PKCα with minimal accumulation of a nonphosphorylated species in multiple cell types. Furthermore, proteasome and lysosome inhibitors predominantly protect fully phosphorylated PKCα, pointing to this form as a substrate for degradation. Failure to detect substantive dephosphorylation of activated PKCα was not due to rephosphorylation because inhibition of Hsp70/Hsc70, which is required for re-priming, had only a minor effect on agonist-induced accumulation of nonphosphorylated protein. Thus, PKC degradation can occur in the absence of dephosphorylation. Further analysis revealed novel functions for Hsp70/Hsc70 and Hsp90 in the control of agonist-induced PKCα processing. These chaperones help to maintain phosphorylation of activated PKCα but have opposing effects on degradation of the phosphorylated protein; Hsp90 is protective, whereas Hsp70/Hsc70 activity is required for proteasomal processing of this species. Notably, down-regulation of nonphosphorylated PKCα shows little Hsp70/Hsc70 dependence, arguing that phosphorylated and nonphosphorylated species are differentially targeted for proteasomal degradation. Finally, lysosomal processing of activated PKCα is not regulated by phosphorylation or Hsps. Collectively, these data demonstrate that phosphorylated PKCα is a direct target for agonist-induced proteasomal degradation via an Hsp-regulated mechanism, and highlight the existence of a novel pathway of PKC desensitization in cells.

Upregulation of PKCη by PKCε and PDK1 involves two distinct mechanisms and promotes breast cancer cell survival

BACKGROUND

Protein kinase C (PKC) serves as the receptor for tumor-promoting phorbol esters, which are potent activators of conventional (c) and novel (n) PKCs. We recently showed that these activators induced selective upregulation of PKCη in breast cancer cells. The objective of this study is to understand unique regulation of PKCη and its importance in breast cancer.

METHODS

The levels of PKC isozymes were monitored in breast cancer cells following treatment with inhibitors of kinases, proteasome and proteases by Western blotting. PKCε was introduced by adenoviral delivery. PKCη and PDK1 were depleted by siRNA silencing. Cell growth was determined by the MTT or clonal assay.

RESULTS

The general PKC inhibitors Gö 6983 and bisindolylmaleimide but not cPKC inhibitor Gö 6976 led to substantial PKCη downregulation, which was partly rescued by the introduction of nPKCε. Inhibition of phosphoinositide-dependent kinase-1 (PDK1) by Ly294002 or knockdown of PDK1 also led to downregulation of basal PKCη but had no effect on PKC activator-induced upregulation of PKCη. Proteasome inhibitors blocked PKCη downregulation triggered by PDK1 inhibition/depletion but not by Gö 6983. PKCη level increased in malignant but not in non-tumorigenic or pre-malignant cells in the progressive MCF-10A series associated with activated PDK1, and knockdown of PKCη inhibited breast cancer cell growth and clonogenic survival.

CONCLUSION

Upregulation of PKCη contributes to breast cancer cell growth and targeting either PKCε or PDK1 triggers PKCη downregulation but involves two distinct mechanisms.

GENERAL SIGNIFICANCE

The status of PKCη may serve as a potential biomarker for breast cancer malignancy.

Agonist-induced down-regulation of endogenous protein kinase c α through an endolysosomal mechanism

Protein kinase C (PKC) isozymes undergo down-regulation upon sustained stimulation. Previous studies have pointed to the existence of both proteasome-dependent and -independent pathways of PKCα processing. Here we demonstrate that these down-regulation pathways are engaged in different subcellular compartments; proteasomal degradation occurs mainly at the plasma membrane, whereas non-proteasomal processing occurs in the perinuclear region. Using cholesterol depletion, pharmacological inhibitors, RNA interference, and dominant-negative mutants, we define the mechanisms involved in perinuclear accumulation of PKCα and identify the non-proteasomal mechanism mediating its degradation. We show that intracellular accumulation of PKCα involves at least two clathrin-independent, cholesterol/lipid raft-mediated pathways that do not require ubiquitination of the protein; one is dynamin-dependent and likely involves caveolae, whereas the other is dynamin- and small GTPase-independent. Internalized PKCα traffics through endosomes and is delivered to the lysosome for degradation. Supportive evidence includes (a) detection of the enzyme in EEA1-positive early endosomes, Rab7-positive late endosomes/multivesicular bodies, and LAMP1-positive lysosomes and (b) inhibition of its down-regulation by lysosome-disrupting agents and leupeptin. Only limited dephosphorylation of PKCα occurs during trafficking, with fully mature enzyme being the main target for lysosomal degradation. These studies define a novel and widespread mechanism of desensitization of PKCα signaling that involves endocytic trafficking and lysosome-mediated degradation of the mature, fully phosphorylated protein.

Transcriptional and post-transcriptional regulation of cGMP-dependent protein kinase (PKG-I): pathophysiological significance

The ability of the endothelium to produce nitric oxide, which induces generation of cyclic guanosine monophosphate (cGMP) that activates cGMP-dependent protein kinase (PKG-I), in vascular smooth muscle cells (VSMCs), is essential for the maintenance of vascular homeostasis. Yet, disturbance of this nitric oxide/cGMP/PKG-I pathway has been shown to play an important role in many cardiovascular diseases. In the last two decades, in vitro and in vivo models of vascular injury have shown that PKG-I is suppressed following nitric oxide, cGMP, cytokine, and growth factor stimulation. The molecular basis for these changes in PKG-I expression is still poorly understood, and they are likely to be mediated by a number of processes, including changes in gene transcription, mRNA stability, protein synthesis, or protein degradation. Emerging studies have begun to define mechanisms responsible for changes in PKG-I expression and have identified cis- and trans-acting regulatory elements, with a plausible role being attributed to post-translational control of PKG-I protein levels. This review will focus mainly on recent advances in understanding of the regulation of PKG-I expression in VSMCs, with an emphasis on the physiological and pathological significance of PKG-I down-regulation in VSMCs in certain circumstances.

Bryostatin-1 promotes long-term potentiation via activation of PKCα and PKCε in the hippocampus

Activation of protein kinase C (PKC) by bryostatin-1 affects various functions of the central nervous system. We explored whether bryostatin-1 influenced synaptic plasticity via a process involving PKC. Our purpose was to examine whether bryostatin-1 affected the induction of hippocampal long-term potentiation (LTP) in Schaffer-collateral fibers (CA1 fibers) of the hippocampus, and/or influenced the intracellular Ca(2+) level of hippocampal neurons. We also determined the PKC isoforms involved in these processes. We found that bryostatin-1 strongly facilitated LTP induction, in a dose-dependent manner, upon single-theta burst stimulation (TBS). Further, intracellular Ca(2+) levels also increased with increasing concentration of bryostatin-1. The facilitative effects of bryostatin-1 in terms of LTP induction and enhancement of intracellular Ca(2+) levels were blocked by specific inhibitors of PKCα and PKCε, but not of PKCδ. Our results suggest that bryostatin-1 is involved in neuronal functioning and facilitates induction of LTP via activation of PKCα and/or PKCε.

The 26S proteasome complex: an attractive target for cancer therapy

The 26S proteasome complex engages in an ATP-dependent proteolytic degradation of a variety of oncoproteins, transcription factors, cell cycle specific cyclins, cyclin-dependent kinase inhibitors, ornithine decarboxylase, and other key regulatory cellular proteins. Thus, the proteasome regulates either directly or indirectly many important cellular processes. Altered regulation of these cellular events is linked to the development of cancer. Therefore, the proteasome has become an attractive target for the treatment of numerous cancers. Several proteasome inhibitors that target the proteolytic active sites of the 26S proteasome complex have been developed and tested for anti-tumor activities. These proteasome inhibitors have displayed impressive anti-tumor functions by inducing apoptosis in different tumor types. Further, the proteasome inhibitors have been shown to induce cell cycle arrest, and inhibit angiogenesis, cell-cell adhesion, cell migration, immune and inflammatory responses, and DNA repair response. A number of proteasome inhibitors are now in clinical trials to treat multiple myeloma and solid tumors. Many other proteasome inhibitors with different efficiencies are being developed and tested for anti-tumor activities. Several proteasome inhibitors currently in clinical trials have shown significantly improved anti-tumor activities when combined with other drugs such as histone deacetylase (HDAC) inhibitors, Akt (protein kinase B) inhibitors, DNA damaging agents, Hsp90 (heat shock protein 90) inhibitors, and lenalidomide. The proteasome inhibitor bortezomib is now in the clinic to treat multiple myeloma and mantle cell lymphoma. Here, we discuss the 26S proteasome complex in carcinogenesis and different proteasome inhibitors with their potential therapeutic applications in treatment of numerous cancers.

Active site inhibitors protect protein kinase C from dephosphorylation and stabilize its mature form

Conformational changes acutely control protein kinase C (PKC). We have previously shown that the autoinhibitory pseudosubstrate must be removed from the active site in order for 1) PKC to be phosphorylated by its upstream kinase phosphoinositide-dependent kinase 1 (PDK-1), 2) the mature enzyme to bind and phosphorylate substrates, and 3) the mature enzyme to be dephosphorylated by phosphatases. Here we show an additional level of conformational control; binding of active site inhibitors locks PKC in a conformation in which the priming phosphorylation sites are resistant to dephosphorylation. Using homogeneously pure PKC, we show that the active site inhibitor Gö 6983 prevents the dephosphorylation by pure protein phosphatase 1 (PP1) or the hydrophobic motif phosphatase, pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP). Consistent with results using pure proteins, treatment of cells with the competitive inhibitors Gö 6983 or bisindolylmaleimide I, but not the uncompetitive inhibitor bisindolylmaleimide IV, prevents the dephosphorylation and down-regulation of PKC induced by phorbol esters. Pulse-chase analyses reveal that active site inhibitors do not affect the net rate of priming phosphorylations of PKC; rather, they inhibit the dephosphorylation triggered by phorbol esters. These data provide a molecular explanation for the recent studies showing that active site inhibitors stabilize the phosphorylation state of protein kinases B/Akt and C.

A protein kinase C/protein kinase D pathway protects LNCaP prostate cancer cells from phorbol ester-induced apoptosis by promoting ERK1/2 and NF-{kappa}B activities

Phorbol esters such as phorbol 12-myristate 13-acetate (PMA) induce apoptosis in many tumor cells including the androgen-sensitive LNCaP prostate cancer cells. Although phorbol ester-induced apoptotic pathways have been well characterized, little is known of the pro-survival pathways modulated by these agents. We now provide experimental evidence to indicate that protein kinase D (PKD) promotes survival signals in LNCaP cells in response to PMA treatment. Knockdown of endogenous PKD1 or PKD2 decreased extracellular signal-regulated kinase (ERK) 1/2 and nuclear factor-kappaB (NF-κB)-dependent transcriptional activities and potentiated PMA-induced apoptosis, whereas overexpression of wild-type PKD1 enhanced ERK1/2 activity and suppressed PMA-induced apoptosis. PMA caused rapid activation, followed by progressive downregulation of endogenous PKD1 in a time- and concentration-dependent manner. The downregulation of PKD1 was dependent on the activity of protein kinase C (PKC), but not that of PKD. Selective depletion of endogenous PKC isoforms revealed that both PKCδ and PKCε were required for PKD1 activation and subsequent downregulation. Further analysis showed that the downregulation of PKD1 was mediated by a ubiquitin-proteasome degradation pathway, inhibition of which correlated to increased cell survival. In summary, our data indicate that PKD1 is activated and downregulated by PMA through a PKC-dependent ubiquitin-proteasome degradation pathway, and the activation of PKD1 or PKD2 counteracts PMA-induced apoptosis by promoting downstream ERK1/2 and NF-κB activities in LNCaP prostate cancer cells.

Down-regulating protein kinase C alpha: functional cooperation between the proteasome and the endocytic system

Ubiquitination, proteasome, caveolae and endosomes have been implicated in controlling protein kinase C alpha (PKC alpha) down-regulation. However, the molecular mechanism remained obscure. Here we show that endosomes and proteasome cooperate in phorbol ester 12-O-tetradecanoyl phorbol acetate (TPA)-induced down-regulation of PKC alpha. We show that following TPA treatment and translocation to the plasma membrane, PKC alpha undergoes multimonoubiquitination prior to its degradation by the proteasome. However, to reach the proteasome, PKC alpha must travel through the endocytic system from early to late endosomes. This route requires functional endosomes, whereby endosomal alkalinization, or ablation, abrogates completely PKC alpha degradation maintaining the enzyme at the plasma membrane. This route also depends on synaptotagmin (Syt) II and the Rab7 GTPase, whereby Syt II knock-down or expression of the GDP-locked Rab7 inactive mutant prevents PKC alpha degradation. We further show that proteasome plays a dual role, where an active proteasome is required for deubiquitination of PKC alpha, a step crucial to prevent PKC alpha targeting to the endocytic recycling compartment. Finally, we show that the association with rafts-localized cell surface proteins that internalize in a clathrin-independent fashion is necessary to allow the trafficking of PKC alpha from the plasma membrane to the proteasome, its ultimate degradation station.

Cyclic GMP specifically suppresses Type-Ialpha cGMP-dependent protein kinase expression by ubiquitination

Type I cGMP-dependent protein kinase (PKG-I) mediates nitric oxide (NO) and hormone dependent smooth muscle relaxation and stimulates smooth muscle cell-specific gene expression. Expression of PKG-I in cultured smooth muscle cells depends on culture conditions and is inhibited by inflammatory cytokines such as interleukin-I and tumor necrosis factor-alpha, which are known to stimulate Type II NO synthase (iNOS) expression. We report here that the suppression of PKG-I protein levels in smooth muscle cells is triggered by the ubiquitin/26S proteasome pathway. Incubation of vascular smooth muscle cells with phosphodiesterase-resistant cyclic GMP analogs (e.g., 8-bromo-cGMP) decreases PKG-I protein level in a time- and concentration-dependent manner. To study this process, we tested the effects of 8-Br-cGMP on PKG-I protein level in Cos7 cells, which do not express endogenous type I PKG mRNA. 8-Br-cGMP induced the ubiquitination and down-regulation of PKG-Ialpha, but not PKG-Ibeta. Treatment of cells with the 26S proteasome inhibitor, MG-132, increased ubiquitination of PKG. Blocking PKG-I catalytic activity using the cell-permeant specific PKG-I inhibitor, DT-2, inhibited cGMP-induced PKG-I ubiquitination and down-regulation, suggesting that PKG catalytic activity and autophosphorylation were required for suppression of PKG-I level. Mutation of the known autophosphorylation sites of PKG-Ialpha to alanine uncovered a specific role for autophosphorylation of serine-64 in cGMP-dependent ubiquitination and suppression of PKG-I level. The results suggest that chronic elevation of cGMP, as seen in inflammatory conditions, triggers ubiquitination and degradation of PKG-Ialpha in smooth muscle.

Lifespan regulation of conventional protein kinase C isotypes

Plasma membrane translocation, following allosteric binding of second messengers, initiates the signal transduction process mediated by cPKC [conventional PKC (protein kinase C)] isotypes. Mechanisms regulating the lifespan of the active enzyme such as its phosphorylation, internalization, dephosphorylation and degradation are key elements of the signalling network. The understanding of such mechanisms is essential for the design of therapeutic strategies targeting PKC isoenzymes.

A randomized phase II trial of interleukin-2 in combination with four different doses of bryostatin-1 in patients with renal cell carcinoma

PURPOSE

Bryostatin-1 is a PKC modulator with direct anti-tumor activity and immunomodulatory properties. We combined different doses of Bryostatin-1 with IL-2 to determine effects on clinical response rate and T cell phenotype in patients with advanced kidney cancer.

EXPERIMENTAL DESIGN

IL-2 naïve patients were given 11 x 10(6) IU subcutaneously of IL-2 on days 1-4, 8-11, and 15-18 of every 28-day cycle. Twenty four patients were randomized to treatment cohorts of 5, 15 or 25 mcg/m2 of Bryostatin-1 on days 1, 8 and 15, starting in the second cycle. An additional nine, non-randomized patients were given 35 mcg/m2. Lymphocytes were analyzed for number, activation status, and production of IL-2, IL-4 and IFN-gamma. Response evaluation was performed every 3 cycles.

RESULTS

Common grade 3 toxicities included fatigue (5), nausea/vomiting (5), myopathy (3), dyspnea (3), and syncope (3). Four patients, in the two highest dose cohorts, demonstrated evidence of tumor shrinkage, although there was only 1 objective PR. The median time to progression was 104 days (95% CI 88-120) and the median survival was 452 days (95% CI = 424-480). There was no significant boosting effect of Bryostatin-1 on lymphocytes.

CONCLUSIONS

The addition of Bryostatin-1 to IL-2 was well tolerated, but the overall response rate was low (3.2%), indicating that further studies with this combination are not warranted.

Down-regulation of Pkc1-mediated signaling by the deubiquitinating enzyme Ubp3

Regulated ubiquitination and degradation of signaling proteins have emerged as key mechanisms for modulating the strength and duration of signaling pathways. The reversible nature of the ubiquitination process as well as the large number and diversity of the deubiquitinating enzymes raise the possibility that signaling pathways might be modulated by specific deubiquitinating enzyme(s). Here we provide evidence that in the yeast Saccharomyces cerevisiae, the Pkc1-mediated signaling pathway that controls the cell wall integrity is negatively regulated by the deubiquitinating enzyme Ubp3. Disruption of the UBP3 gene leads to an enhanced activation of the cell wall integrity pathway MAPK Slt2 when cells are challenged with a variety of pathway activation agents such as pheromone and Congo red. The ubp3 deletion mutants accumulate high levels of Pkc1, suggesting potential regulation of Pkc1 by Ubp3. Consistent with this, Pkc1 and Ubp3 interact in vivo, and the stability of Pkc1 is markedly increased in the ubp3 deletion mutants. Moreover, disruption of the PKC1 gene, but not the genes that encode components downstream of Pkc1, completely suppresses other phenotypes displayed by the ubp3 deletion mutants such as hyperactivation of the pheromone-responsive MAPK Fus3 (Wang, Y., and Dohlman, H. G. (2002) J. Biol. Chem. 277, 15766-15772). These findings demonstrate that Ubp3 can regulate Pkc1 by facilitating its destruction and provide the initial evidence that Pkc1 plays a positive role in modulating the parallel pheromone-signaling pathway.

Amplitude control of protein kinase C by RINCK, a novel E3 ubiquitin ligase

Protein kinase C (PKC) isozymes play a central role in cellular signaling. Levels of PKC control the amplitude of agonist-induced signaling and alterations in these levels are associated with disease states, most notably cancer, yet mechanisms that control the turnover of the protein are poorly understood. Here we identify an E3 ligase that catalyzes the ubiquitin-mediated degradation of PKC. Specifically, we identified a RING finger domain-containing protein, RINCK (for RING-finger protein that interacts with C kinase) from a yeast two-hybrid screen using the amino terminus of PKCbeta as bait. RINCK encodes a protein of 581 amino acids that contains a RING finger domain, a B-box, and two coiled-coil regions, the three domains that form the signature motif of the large family of diverse TRIM (tripartite motif) proteins. Co-immunoprecipitation studies using tsA201 cells reveal that RINCK and PKC associate with each other in cells. Studies using fragments of PKCbeta reveal that this interaction is mediated by the C1A domain of PKC. RINCK induces the ubiquitination of PKC both in vitro and in cells. Overexpression of RINCK reduces the levels of PKC in cells, whereas genetic knockdown of endogenous RINCK increases the levels of PKC. This increase was observed for all PKC isozymes examined (including conventional, novel, and atypical). The RINCK-mediated degradation of PKC occurs independently of the classic phorbol ester-mediated down-regulation: genetic depletion of RINCK had no effect on the phorbol ester-mediated down-regulation and, additionally, up-regulated the levels of isozymes that cannot bind phorbol esters. Our data reveal a novel mechanism that provides amplitude control in PKC signaling through ubiquitination catalyzed by RINCK, an E3 ligase that specifically recognizes the C1 domain of PKC isoforms.

Targeting the protein kinase C family: are we there yet?

Protein kinase C (PKC) comprises a family of serine/threonine kinases that are involved in the transduction of signals for cell proliferation, differentiation, apoptosis and angiogenesis. Unsurprisingly, disruption of PKC regulation is implicated in tumorigenesis and drug resistance. PKC function is complex in this context owing to the differing roles of individual isozymes within the cell and across tumour types. Therapeutically targeting PKC isozymes is not new; however, with many of the early PKC inhibitor cytotoxic drug combinations being discarded at the phase II level, and recent phase III studies in non-small-cell lung cancer proving negative, what's going wrong?

Protein kinase C and other diacylglycerol effectors in cancer

Almost three decades after the discovery of protein kinase C (PKC), we still have only a partial understanding of how this family of serine/threonine kinases is involved in tumour promotion. PKC isozymes - effectors of diacylglycerol (DAG) and the main targets of phorbol-ester tumour promoters - have important roles in cell-cycle regulation, cellular survival, malignant transformation and apoptosis. How do PKC isozymes regulate these diverse cellular processes and what are their contributions to carcinogenesis? Moreover, what is the contribution of all phorbol-ester effectors, which include PKCs and small G-protein regulators? We now face the challenge of dissecting the relative contribution of each DAG signal to cancer progression.

Mutual regulation of conventional protein kinase C and a ubiquitin ligase complex

Several isoforms of protein kinase C (PKC) are degraded by the ubiquitin-proteasome pathway after phorbol ester-mediated activation. However, little is known about the ubiquitin ligase (E3) that targets activated PKCs. We recently showed that an E3 complex composed of HOIL-1L and HOIP (LUBAC) generates linear polyubiquitin chains and induces the proteasomal degradation of a model substrate. HOIL-1L has also been characterized as a PKC-binding protein. Here we show that LUBAC preferentially binds activated conventional PKCs and their constitutively active mutants. LUBAC efficiently ubiquitinated activated PKC in vitro, and degradation of activated PKCalpha was delayed in HOIL-1L-deficient cells. Conversely, PKC activation induced cleavage of HOIL-1L and led to downregulation of the ligase activity of LUBAC. These results indicate that LUBAC is an E3 for activated conventional PKC, and that PKC and LUBAC regulate each other for proper PKC signaling.

RBCK1, a protein kinase CbetaI (PKCbetaI)-interacting protein, regulates PKCbeta-dependent function

RBCK1 (RBCC protein interacting with PKC 1) has originally been identified as a protein kinase CbetaI (PKCbetaI)-binding partner by a two-hybrid screen and as one of the gene transcripts that increases during adult cardiac hypertrophy. To address whether RBCK1 and PKCbetaI functions are interconnected, we used cultured neonatal myocytes where we previously found that the activity of PKCbetaI is required for an increase in cell size, also called hypertrophy. In this study, we showed that acute treatment of cardiac myocytes with phenylephrine, a prohypertrophic stimulant, transiently increased the association of RBCK1 with PKCbetaI within 1 min. A prolonged phenylephrine treatment also resulted in an increase of the interaction of the two proteins. Endogenous RBCK1 protein levels increased upon phenylephrine-induced hypertrophy. Further, adenovirus-based RBCK1 overexpression in the absence of phenylephrine increased cardiac cell size. This RBCK1-mediated hypertrophy required PKCbeta activity, since the increase in cell size was inhibited when the RBCK1-expressing cells were treated with PKCbeta-selective antagonists, supporting our previous observation that both PKCbetaI and PKCbetaII are required for hypertrophy. Unexpectedly, RBCK1-induced increased cell size was inhibited by phenylephrine. This effect correlated with a decrease in the level of both PKCbeta isoforms. Most importantly, RNA interference for RBCK1 significantly inhibited the increase in cell size of cardiac myocytes following phenylephrine treatment. Our results suggest that RBCK1 binds PKCbetaI and is a key regulator of PKCbetaI function in cells and that, together with PKCbetaII, the three proteins are essential for developmental hypertrophy of cardiac myocytes.

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

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

Lymphoid signal transduction mechanisms linked to cellular prion protein

The normal cellular isoform of the prion protein (PrPC) is a glycosylphosphatidylinositol-anchored cell surface protein that is expressed widely, including in lymphoid cells. We compared lectin-induced mitogenesis and selected cell signaling pathways in splenocytes from wild-type BALB/c mice and Zrch Prnp0/0 (PrP0/0) mice bred on a BALB/c background for more than 10 generations. 3H-thymidine incorporation induced by concanavalin A (Con A) or phytohemagglutinin (PHA) was significantly reduced in PrP0/0 splenocytes, most prominently early in activation (24 and 48 h). Con A activation in PrP0/0 splenocytes was associated with differences in the phosphorylation (P) patterns of protein kinase C (PKC alpha/beta, but not delta) and the PKC downstream effectors p44/42MAPK (mitogen-activated protein kinase). P-PKC and P-MAPK profiles were similar in wild-type and PrP0/0 splenocytes following PMA treatment, indicating that the ability of these 2 enzymes to be phosphorylated is not impaired in the absence of PrPC. Con A-induced calcium fluxes, monitored by indo-1 fluorescence, were equivalent in PrP0/0 and PrP+/+ splenocytes, suggesting that calcium-dependent mechanisms are not directly implicated in the differential phosphorylation patterns or mitotic responses. Our data indicate that PrP0/0 splenocytes display defects in upstream or downstream mechanism(s) that modulate PKCalpha/beta phosphorylation, which in turn affects its capacity to regulate splenocyte mitosis, consistent with a role for PrPC in immune function.

Protein ubiquitination, degradation and the proteasome in neuro-degenerative disorders: no clear evidence for a significant pathogenetic role of proteasome failure in Alzheimer disease and related disorders

It has recently been proposed that Alzheimer disease (AD) might be initiated by a molecular 'hit' into a regulatory protein, e.g. a cell surface receptor [Schmitt HP. Neuro-modulation, aminergic neuro-disinhibition and neuro-degeneration: draft of a comprehensive theory for Alzheimer disease. Med Hypoth 2005;65:1106-19]. However, other substrates, in particular intra-cellular protein complexes such as the ubiquitin/proteasome system (UPS) could as well serve as a targets for such a 'hit' which might insert a mutation or induce conformational changes resulting in functional failure of protein degradation along the ubiquitin/proteasome proteolytic pathway. It has been claimed that impairment of the large multi-catalytic protease complex, the 20S/26S proteasome, might represent a key factor in the early pathogenesis of neuro-degenerative disorders characterized by the formation of abnormal protein aggregates such as neuronal cytoplasmic or nuclear inclusion bodies and fibrillary deposits. This article aims to review critically whether current information really supports the idea that impairment of the UPS might play a significant role in the early pathogenesis of neuro-degenerative disorders, with special emphasis on AD. The data provided in favour of proteasome impairment were, as a rule, revealed in in vitro experiments which cannot be unequivocally transferred to the in vivo conditions in neuro-degeneration. The author concludes that there is yet no clear evidence of a pivotal role of proteasome failure in the early pathogenesis of AD.

Peroxisome proliferator-activated receptor-beta/delta inhibits epidermal cell proliferation by down-regulation of kinase activity

Recent work has shown that peroxisome proliferator-activated receptor beta (PPARbeta) attenuates cell proliferation and skin carcinogenesis, and this is due in part to regulation of ubiquitin C expression. In these studies, the role of PPARbeta in modulating ubiquitin-dependent protein kinase Calpha (PKCalpha) levels and phosphorylation signaling pathways was evaluated. Intracellular phosphorylation analysis showed that phosphorylated PKCalpha and other kinases were lower in wild-type mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) as compared with PPARbeta-null mouse skin. No differences in expression levels of other PKC isoforms present in skin were observed. Lower ubiquitination of PKCalpha was found in TPA-treated PPARbeta-null skin as compared with wild-type, and inhibition of ubiquitin-dependent proteasome degradation prevented TPA-induced down-regulation of PKCalpha. The activity of PKCalpha and downstream signaling kinases is enhanced, and expression of cyclooxygenase-2 (COX-2) is significantly greater, in PPARbeta-null mouse skin in response to TPA compared with wild-type mouse skin. Inhibition of PKCalpha or COX-2 reduced cell proliferation in TPA-treated PPARbeta-null keratinocytes in a dose-dependent manner, whereas it only slightly influenced cell proliferation in wild-type keratinocytes. Combined, these studies provide strong evidence that PPARbeta attenuates cell proliferation by modulating PKCalpha/Raf1/MEK/ERK activity that may be due in part to reduced ubiquitin-dependent turnover of PKCalpha.

Bryostatin induces protein kinase C modulation, Mcl-1 up-regulation and phosphorylation of Bcl-2 resulting in cellular differentiation and resistance to drug-induced apoptosis in B-cell chronic lymphocytic leukemia cells

Bryostatin, a macrocyclic lactone and protein kinase C (PKC) modulator, has been shown to have differentiation and anti-tumor activity against several leukemia cell lines in vitro. In this study, we demonstrated Bryostatin-induced differentiation in B-cell chronic lymphocytic leukemia (B-CLL) cells, characterized by an increase in cell size and a marked up-regulation of CD11c expression. The specific inhibitors of the extracellular signal-regulated kinase (ERK) and protein kinase C pathways, PD98059 and GF 109203X respectively, each completely blocked Bryostatin-induced differentiation of B-CLL cells, suggesting that activation of the ERK pathway plays a direct role in this process in a PKC-dependent manner. Furthermore, Bryostatin reduced both spontaneous and drug-induced apoptosis with chlorambucil, fludarabine and 2-chloro-2'-deoxyadenosine (2-Cda) in B-CLL cells. This resistance was associated with an early up-regulation of the anti-apoptotic protein Mcl-1 and post-translational phosphorylation of Bcl-2 at serine 70. The anti-apoptotic effects of Bryostatin were abrogated by GF 109203X, and to a lesser extent by the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, LY294002. Interestingly, the ERK inhibitor, PD98059 inhibited Mcl-1 expression but had little effect on Bryostatin-induced survival suggesting that the ERK pathway predominantly affects differentiation. Taken together these results present an explanation for Bryostatin-induced B-CLL cell survival in which modulation of the PKC pathway couples differentiation with an increase in antiapoptotic protein expression and calls into question the rationale for its use in the treatment of B-CLL.

Enhanced retinoid-induced apoptosis of MDA-MB-231 breast cancer cells by PKC inhibitors involves activation of ERK

Retinoids are vitamin A derivatives, which cause growth inhibition, differentiation and/or apoptosis in various cell types, including some breast cancer cells. In general, estrogen receptor (ER)-positive cells are retinoic acid (RA) sensitive, whereas ER-negative cells are resistant. In this report, we show that ER-negative MDA-MB-231 cells are strongly growth inhibited by retinoids in combination with a PKC inhibitor. While neither RA nor GF109203X (GF) has a significant growth inhibitory effect in these cells, RA+GF potently suppress proliferation. We found that RA+GF induce apoptosis, as shown by an increase in fragmented DNA, Annexin-V-positive cells and caspase-3 activation. Apoptosis was also induced by GF in combination with two synthetic retinoids. Expression of phosphorylated as well as total PKC was decreased by GF and this was potentiated by RA. In addition, treatment with GF caused a strong and sustained activation of ERK1/2 and p38-MAPK, as well as a weaker activation of JNK. Importantly, inhibition of ERK but not p38 or JNK suppressed apoptosis induced by RA+GF, indicating that activation of ERK is specifically required. In support of this novel finding, the ability of other PKC inhibitors to cause apoptosis in combination with RA correlates with ability to cause sustained activation of ERK.

Protein Phosphatase 4 Is a Positive Regulator of Hematopoietic Progenitor Kinase 1

Hematopoietic progenitor kinase 1 (HPK1) is a hematopoietic specific mammalian Ste20-like protein kinase and has been implicated in many cellular signaling pathways including T cell receptor (TCR) signaling. However, little is known about the in vivo regulation of HPK1. We present evidence that HPK1 is positively regulated by protein phosphatase 4 (PP4; also called PPX and PPP4), a serine/threonine phosphatase. We found that PP4 interacted with HPK1 and that the proline-rich region of HPK1 was necessary and sufficient for this interaction. We also found that PP4 had phosphatase activity toward HPK1 in vivo and that co-transfection of PP4 with HPK1 resulted in specific kinase activation of HPK1. Moreover, we found that the PP4-induced HPK1 kinase activation was accompanied by an increase in protein expression of HPK1. Pulse-chase analysis showed that PP4 increased the half-life of HPK1. Further studies showed that HPK1 was subject to regulation by ubiquitination and ubiquitin-targeted degradation and that PP4 inhibited HPK1 ubiquitination. In addition, we found that TCR stimulation enhanced the PP4-HPK1 interaction and that wild-type PP4 enhanced, whereas a phosphatase-dead PP4 mutant inhibited, TCR-induced activation of HPK1 in Jurkat T cells. Combined with the observation that PP4 enhanced HPK1-induced JNK activation, our studies identify PP4 as a positive regulator for HPK1 and the HPK1-JNK signaling pathway.