Activation-dependent degradation of protein kinase C eta

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.

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.

Stress granules sense metabolic stress at the plasma membrane and potentiate recovery by storing active Pkc1

As the physical barrier between the cell and the outside environment, the plasma membrane is well-positioned to be the first responder to stress. The membrane is also highly vulnerable to many types of perturbation, including heat, force, osmotic pressure, lipid shortage, and starvation. To determine whether the structural changes in the plasma membrane of Saccharomyces cerevisiae brought about by nutrient stress can be communicated to regulatory networks within the cell, we identified proteins that interact with stress granules (SGs), subcellular structures composed of proteins, and nontranslated RNAs that form when cells are stressed. We found that SG proteins interacted with components of eisosomes, which are subcortical membrane structures with a distinct lipid and protein composition. In response to starvation-triggered phosphorylation of eisosome proteins, eisosomes clustered and recruited SG components, including active Pkc1. The absence of eisosomes impaired SG formation, resulting in delayed recovery from nutrient deprivation. Thus, eisosome clustering is an example of interdomain communication in response to stress and identifies a previously unknown mechanism of SG regulation.

Kinase activity-dependent stability of calcium/calmodulin-dependent protein kinase of Lotus japonicus

Accumulation of calcium/calmodulin-dependent protein kinase (CCaMK) in root cell nucleus depends on its kinase activity but not on nuclear symbiotic components crucial for nodulation. Plant calcium/calmodulin-dependent protein kinase (CCaMK) is a key regulator of symbioses with rhizobia and arbuscular mycorrhizal fungi as it decodes symbiotic calcium signals induced by microsymbionts. CCaMK is expressed mainly in root cells and localizes to the nucleus, where microsymbiont-triggered calcium oscillations occur. The molecular mechanisms that control CCaMK localization are unknown. Here, we analyzed the expression and subcellular localization of mutated CCaMK in the roots of Lotus japonicus and found a clear relation between CCaMK kinase activity and its stability. Kinase-defective CCaMK variants showed lower protein levels than the variants with kinase activity. The levels of transcripts driven by the CaMV 35S promoter were similar among the variants, indicating that stability of CCaMK is regulated post-translationally. We also demonstrated that CCaMK localized to the root cell nucleus in several symbiotic mutants, including cyclops, an interaction partner and phosphorylation target of CCaMK. Our results suggest that kinase activity of CCaMK is required not only for the activation of downstream symbiotic components but also for its stability in root cells.

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.

BDNF-TrkB Signaling Coupled to nPKCε and cPKCβI Modulate the Phosphorylation of the Exocytotic Protein Munc18-1 During Synaptic Activity at the Neuromuscular Junction

Munc18-1, a neuron-specific member of the Sec1/Munc18 family, is involved in neurotransmitter release by binding tightly to syntaxin. Munc18-1 is phosphorylated by PKC on Ser-306 and Ser-313 in vitro which reduces the amount of Munc18-1 able to bind syntaxin. We have previously identified that PKC is involved in neurotransmitter release when continuous electrical stimulation imposes a moderate activity on the NMJ and that muscle contraction through TrkB has an important impact on presynaptic PKC isoforms levels, specifically cPKCβI and nPKCε. Therefore, the present study was designed to understand how Munc18-1 phosphorylation is affected by (1) synaptic activity at the neuromuscular junction, (2) nPKCε and cPKCβI isoforms activity, (3) muscle contraction per se, and (4) the BDNF/TrkB signaling in a neuromuscular activity-dependent manner. We performed immunohistochemistry and confocal techniques to evidence the presynaptic location of Munc18-1 in the rat diaphragm muscle. To study synaptic activity, we stimulated the phrenic nerve (1 Hz, 30 min) with or without contraction (abolished by μ-conotoxin GIIIB). Specific inhibitory reagents were used to block nPKCε and cPKCβI activity and to modulate the tropomyosin receptor kinase B (TrkB). Main results obtained from Western blot experiments showed that phosphorylation of Munc18-1 at Ser-313 increases in response to a signaling mechanism initiated by synaptic activity and directly mediated by nPKCε. Otherwise, cPKCβI and TrkB activities work together to prevent this synaptic activity-induced Munc18-1 phosphorylation by a negative regulation of cPKCβI over nPKCε. Therefore, a balance between the activities of these PKC isoforms could be a relevant cue in the regulation of the exocytotic apparatus. The results also demonstrate that muscle contraction prevents the synaptic activity-induced Munc18-1 phosphorylation through a mechanism that opposes the TrkB/cPKCβI/nPKCε signaling.

Transient activation of PKC results in long-lasting detrimental effects on systolic [Ca2+]i in cardiomyocytes by altering actin cytoskeletal dynamics and T-tubule integrity

AIMS

Protein kinase C (PKC) isozymes contribute to the development of heart failure through dysregulation of Ca²⁺ handling properties and disruption of contractile function in cardiomyocytes. However, the mechanisms by which PKC activation leads to Ca²⁺ dysfunction are incompletely understood.

METHODS AND RESULTS

Shortly upon ventricular pressure overload in mice, we detected transient PKC activation that was associated with pulsed actin cytoskeletal rearrangement. In cultured cardiomyocytes, transient activation of PKC promoted long-term deleterious effects on the integrity of the transverse (T)- tubule system, resulting in a significant decrease in the amplitude and increase in the rising kinetics of Ca²⁺ transients. Treatment with a PKCα/β inhibitor restored the synchronization of Ca²⁺ transients and maintained T-tubule integrity in cultured cardiomyocytes. Supporting these data, PKCα/β inhibition protected against T-tubule remodeling and cardiac dysfunction in a mouse model of pressure overload-induced heart failure. Mechanistically, transient activation of PKC resulted in biphasic actin cytoskeletal rearrangement, consistent with in vivo observations in the pressure overloaded mouse model. Transient inhibition of actin polymerization or depolymerization resulted in severe T-tubule damage, recapitulating the T-tubule damage induced by PKC activation. Moreover, inhibition of stretch activated channels (SAC) protected against T-tubule remodeling and E-C coupling dysfunction induced by transient PKC activation and actin cytoskeletal rearrangement.

CONCLUSIONS

These data identify a key mechanistic link between transient PKC activation and long-term Ca²⁺ handling defects through PKC-induced actin cytoskeletal rearrangement and resultant T-tubule damage.

ULK1 ubiquitylation is regulated by phosphorylation on its carboxy terminus

Autophagy is a highly conserved process that acts sequestering cytoplasmic components for their degradation by the lysosomes. It consists of several sequential steps that have to be finely regulated to ensure both its progression and termination. Post-translational modifications (PTMs) play an important role in regulating ATG proteins function in different stages of autophagy. Recently, we demonstrated that, during prolonged starvation, ULK1 protein is specifically ubiquitylated by NEDD4L, and that this regulation is important to protect cells against excessive autophagy. In this Extra view, we show that ULK1 phosphorylation at 3 different sites on the same ULK1 target region for NEDD4L is preparatory for its ubiquitylation and subsequent degradation. This adds to the complexity of ULK1 multi-level regulation by several factors, including kinases, phosphatases and acetylases, with each contributing to autophagy homeostasis.

The tumor promoter-activated protein kinase Cs are a system for regulating filopodia

Different protein kinase C (PKC) isoforms have distinct roles in regulating cell functions. The conventional (α, β, γ) and novel (δ, ɛ, η, θ) classes are targets of phorbol ester tumor promoters, which are surrogates of endogenous second messenger, diacylglycerol. The promoter-stimulated disappearance of filopodia was investigated by use of blocking peptides (BPs) that inhibit PKC maturation and/or docking. Filopodia were partially rescued by a peptide representing PKC ɛ hydrophobic sequence, but also by a myristoylated PKC α/β pseudosubstrate sequence, and an inhibitor of T-cell protein tyrosine phosphatase (TC-PTP). The ability to turn over filopodia was widely distributed among PKC isoforms. PKC α and η hydrophobic sequences enhanced filopodia in cells in the absence of tumor promoter treatment. With transcriptional knockdown of PKC α, the content of PKC ɛ predominated over other isoforms. PKC ɛ could decrease filopodia significantly in promoter-treated cells, and this was attributed to ruffling. The presence of PKC α counteracted the PKC ɛ-mediated enhancement of ruffling. The results showed that there were two mechanisms of filopodia downregulation. One operated in the steady-state and relied on PKC α and η. The other was stimulated by tumor promoters and relied on PKC ɛ. Cycles of protrusion and retraction are characteristic of filopodia and are essential for the cell to orient itself during chemotaxis and haptotaxis. By suppressing filopodia, PKC ɛ can create a long-term "memory" of an environmental signal that may act in nature as a mnemonic device to mark the direction of a repulsive signal.

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.

A functional SUMO-motif in the active site of PIM1 promotes its degradation via RNF4, and stimulates protein kinase activity

The PIM1 serine/threonine protein kinase mediates growth factor and survival signalling, and cooperates potently with c-MYC during tumorigenesis. PIM1 is overexpressed in many human cancers and is a promising target for drug development. PIM1 levels are regulated mainly through cytokine-induced transcription and protein degradation, but mechanisms regulating its activity and levels remain largely unexplored. Here, we show that PIM1 is modified in vitro and in cultured cells by the Small ubiquitin-like modifier (SUMO) on two independent sites: K169, within a consensus SUMOylation motif (IK¹⁶⁹DE¹⁷¹) in the active site of PIM1, and also at a second promiscuous site. Alanine substitution of E171 (within the consensus motif) abolished SUMOylation, significantly increased the half-life of PIM1, and markedly reduced its ubiquitylation. Mechanistically, SUMOylation promoted ubiquitin-mediated degradation of PIM1 via recruitment of the SUMO-targeted ubiquitin ligase, RNF4. Additionally, SUMOylated PIM1 showed enhanced protein kinase activity in vitro. Interestingly, the E171A mutant was active in vitro but displayed altered substrate specificity in cultured cells, consistent with the idea that SUMOylation may govern PIM1 substrate specificity under certain contexts. Taken together, these data demonstrate that the protein kinase activity and levels of PIM1 can be regulated by a covalent post-translational modification.

Muscle Contraction Regulates BDNF/TrkB Signaling to Modulate Synaptic Function through Presynaptic cPKCα and cPKCβI

The neurotrophin brain-derived neurotrophic factor (BDNF) acts via tropomyosin-related kinase B receptor (TrkB) to regulate synapse maintenance and function in the neuromuscular system. The potentiation of acetylcholine (ACh) release by BDNF requires TrkB phosphorylation and Protein Kinase C (PKC) activation. BDNF is secreted in an activity-dependent manner but it is not known if pre- and/or postsynaptic activities enhance BDNF expression in vivo at the neuromuscular junction (NMJ). Here, we investigated whether nerve and muscle cell activities regulate presynaptic conventional PKC (cPKCα and βI) via BDNF/TrkB signaling to modulate synaptic strength at the NMJ. To differentiate the effects of presynaptic activity from that of muscle contraction, we stimulated the phrenic nerve of rat diaphragms (1 Hz, 30 min) with or without contraction (abolished by μ-conotoxin GIIIB). Then, we performed ELISA, Western blotting, qRT-PCR, immunofluorescence and electrophysiological techniques. We found that nerve-induced muscle contraction: (1) increases the levels of mature BDNF protein without affecting pro-BDNF protein or BDNF mRNA levels; (2) downregulates TrkB.T1 without affecting TrkB.FL or p75 neurotrophin receptor (p75) levels; (3) increases presynaptic cPKCα and cPKCβI protein level through TrkB signaling; and (4) enhances phosphorylation of cPKCα and cPKCβI. Furthermore, we demonstrate that cPKCβI, which is exclusively located in the motor nerve terminals, increases activity-induced acetylcholine release. Together, these results show that nerve-induced muscle contraction is a key regulator of BDNF/TrkB signaling pathway, retrogradely activating presynaptic cPKC isoforms (in particular cPKCβI) to modulate synaptic function. These results indicate that a decrease in neuromuscular activity, as occurs in several neuromuscular disorders, could affect the BDNF/TrkB/PKC pathway that links pre- and postsynaptic activity to maintain neuromuscular function.

The multifaceted role of the E3 ubiquitin ligase HOIL-1: beyond linear ubiquitination

Ubiquitination controls and fine-tunes many signaling processes driving immunity, inflammation, and cancer. The E3 ubiquitin ligase HOIL-1 (heme-oxidized IRP2 ubiquitin ligase-1) is increasingly implicated in different signaling pathways and plays a vital role in immune regulation. HOIL-1 co operates with the E3 ubiquitin ligase HOIP (HOIL-1 interacting protein) to modify specific nuclear factor-κB (NF-κB) signaling proteins with linear M1-linked polyubiquitin chains. In addition, through its ability to also add K48-linked polyubiquitin chains to specific substrates, HOIL-1 has been linked with antiviral signaling, iron and xenobiotic metabolism, cell death, and cancer. HOIL-1 deficiency in humans leads to myopathy, amylopectinosis, auto-inflammation, and immunodeficiency associated with an increased frequency of bacterial infections. HOIL-1-deficient mice exhibit amylopectin-like deposits in the myocardium, pathogen-specific immunodeficiency, but minimal signs of hyper-inflammation. This review summarizes current knowledge on the mechanism of action of HOIL-1 and highlights recent advances regarding its role in health and disease.

Protein kinase c-β-dependent activation of NF-κB in stromal cells is indispensable for the survival of chronic lymphocytic leukemia B cells in vivo

Tumor cell survival critically depends on heterotypic communication with benign cells in the microenvironment. Here, we describe a survival signaling pathway activated in stromal cells by contact to B cells from patients with chronic lymphocytic leukemia (CLL). The expression of protein kinase C (PKC)-βII and the subsequent activation of NF-κB in bone marrow stromal cells are prerequisites to support the survival of malignant B cells. PKC-β knockout mice are insusceptible to CLL transplantations, underscoring the in vivo significance of the PKC-βII-NF-κB signaling pathway in the tumor microenvironment. Upregulated stromal PKC-βII in biopsies from patients with CLL, acute lymphoblastic leukemia, and mantle cell lymphoma suggests that this pathway may commonly be activated in a variety of hematological malignancies.

Novel regulation of protein kinase C-η

Protein kinase C (PKC) is the receptor for tumor promoting phorbol esters, which are potent activators of conventional and novel PKCs, but persistent treatment with phorbol esters leads to downregulation of these PKCs. However, PKCη, a novel PKC isozyme, resists downregulation by tumor-promoting phorbol esters, but little is known about how PKCη level is regulated. Phosphorylation and dephosphorylation play an important role in regulating activity and stability of PKCs. In the present study, we have investigated the molecular mechanism of PKCη regulation. Several PKC activators, including phorbol 12,13-dibutyrate, 12-O-tetradecanoylphorbol-13-acetate and indolactam V caused upregulation of PKCη, whereas the general PKC inhibitor Gö 6983, but not the conventional PKC inhibitor Gö 6976 led to the downregulation of PKCη. Upregulation of PKCη was associated with an increase in phosphorylation of PKCη. Silencing of phosphoinositide-dependent kinase-1, which phosphorylates PKCη at the activation loop, failed to prevent PKC activator-induced upregulation of PKCη. Knockdown of PKCε but not PKCα inhibited PKC activator-induced upregulation of PKCη. Thus, our results suggest that the regulation of PKCη is unique and PKCε is required for the PKC activator-induced upregulation of PKCη.

Lys48-linked TAK1 polyubiquitination at lysine-72 downregulates TNFα-induced NF-κB activation via mediating TAK1 degradation

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular processes. TAK1, a member of the MAPKKK family, is essential for TNFα-induced NF-κB activation. Phosphorylation and Lys(63)-linked polyubiquitination (polyUb) of TAK1 are critical for its activation. However, whether TAK1 is regulated by polyubiquitination-mediated protein degradation after its activation remains unknown. Here we report that TNFα induces TAK1 Lys(48) linked polyubiquitination and degradation at the later time course. Furthermore, we provide direct evidence that TAK1 is modified by Lys(48)-linked polyubiquitination at lysine-72 by mass spectrometry. A K72R point mutation on TAK1 abolishes TAK1 Lys(48)-linked polyubiquitination and enhances TAK1/TAB1 co-overexpression-induced NF-κB activation. As expected, TAK1 K72R mutation inhibits TNFα-induced Lys(48)-linked TAK1 polyubiquitination and degradation. TAK1 K72R mutant prolongs TNFα-induced NF-κB activation and enhances TNFα-induced IL-6 gene expression. Our findings demonstrate that TNFα induces Lys(48)-linked polyubiquitination of TAK1 at lysine-72 and this polyubiquitination-mediated TAK1 degradation plays a critical role in the downregulation of TNFα-induced NF-κB activation.

PKCη is a negative regulator of AKT inhibiting the IGF-I induced proliferation

The PI3K-AKT pathway is frequently activated in human cancers, including breast cancer, and its activation appears to be critical for tumor maintenance. Some malignant cells are dependent on activated AKT for their survival; tumors exhibiting elevated AKT activity show sensitivity to its inhibition, providing an Achilles heel for their treatment. Here we show that the PKCη isoform is a negative regulator of the AKT signaling pathway. The IGF-I induced phosphorylation on Ser473 of AKT was inhibited by the PKCη-induced expression in MCF-7 breast adenocarcinoma cancer cells. This was further confirmed in shRNA PKCη-knocked-down MCF-7 cells, demonstrating elevated phosphorylation on AKT Ser473. While PKCη exhibited negative regulation on AKT phosphorylation it did not alter the IGF-I induced ERK phosphorylation. However, it enhanced ERK phosphorylation when stimulated by PDGF. Moreover, its effects on IGF-I/AKT and PDGF/ERK pathways were in correlation with cell proliferation. We further show that both PKCη and IGF-I confer protection against UV-induced apoptosis and cell death having additive effects. Although the protective effect of IGF-I involved activation of AKT, it was not affected by PKCη expression, suggesting that PKCη acts through a different route to increase cell survival. Hence, our studies show that PKCη provides negative control on AKT pathway leading to reduced cell proliferation, and further suggest that its presence/absence in breast cancer cells will affect cell death, which could be of therapeutic value.

The cytotoxicity of anti-CD22 immunotoxin is enhanced by bryostatin 1 in B-cell lymphomas through CD22 upregulation and PKC-βII depletion

BACKGROUND

In spite of potent first-line therapies for chronic lymphocytic leukemia, treatment remains palliative and all patients frequently relapse. Treatment options for these patients are more limited. BL22 is a recombinant protein composed of the variable region of a monoclonal antibody that binds to CD22 and of PE38, a truncated Pseudomonas exotoxin. BL22 is a very potent drug already used in patients with hairy cell leukemia, whereas in chronic lymphocytic leukemia its cytotoxicity is limited by a lower expression of CD22. Here we demonstrate that this limitation can be overcome by pre-activation of chronic lymphocytic leukemia cells with bryostatin 1.

DESIGN AND METHODS

Primary malignant B cells from chronic lymphocytic leukemia and mantle cell lymphoma patients were used in vitro to assess the therapeutic impact of drug combinations using BL22 and bryostatin 1.

RESULTS

We demonstrate that bryostatin 1 sensitizes chronic lymphocytic leukemia cells for the cytotoxic effects of BL22 through activation of protein kinase C and subsequently increased CD22 surface expression. Dose and time response analysis reveals that activation of protein kinase C further activates an autocrine feedback loop degrading protein kinase C-βII protein. Depletion of protein kinase C-βII and upregulation of CD22 persist for several days following pre-stimulation with bryostatin 1. Therefore, our data provide a rationale for the sequential administration of BL22 following bryostatin 1 treatment. In addition to primary chronic lymphocytic leukemia cells, bryostatin 1 also sensitizes diffuse large B-cell lymphoma and mantle cell lymphoma cells to BL22 induced apoptosis.

CONCLUSIONS

Our data suggest that the combination of bryostatin 1 with antibodies directed against CD22 is a potent drug combination for the treatment of low- and high-grade B-cell lymphoma.

Plk4 trans-autophosphorylation regulates centriole number by controlling betaTrCP-mediated degradation

Centrioles are the main constituents of the mammalian centrosome and act as basal bodies for ciliogenesis. Centrosomes organize the cytoplasmic microtubule network during interphase and the mitotic spindle during mitosis, and aberrations in centrosome number have been implicated in chromosomal instability and tumor formation. The centriolar protein Polo-like kinase 4 (Plk4) is a key regulator of centriole biogenesis and is crucial for maintaining constant centriole number, but the mechanisms regulating its activity and expression are only beginning to emerge. Here, we show that human Plk4 is subject to betaTrCP-dependent proteasomal degradation, indicating that this pathway is conserved from Drosophila to human. Unexpectedly, we found that stable overexpression of kinase-dead Plk4 leads to centriole overduplication. This phenotype depends on the presence of endogenous wild-type Plk4. Our data indicate that centriole overduplication results from disruption of Plk4 trans-autophosphorylation by kinase-dead Plk4, which then shields endogenous Plk4 from recognition by betaTrCP. We conclude that active Plk4 promotes its own degradation by catalyzing betaTrCP binding through trans-autophosphorylation (phosphorylation by the other kinase in the dimer) within homodimers.

Protein kinase A and protein kinase C(alpha)/PPP1CC2 play opposing roles in the regulation of phosphatidylinositol 3-kinase activation in bovine sperm

In order to acquire fertilization competence, spermatozoa have to undergo biochemical changes in the female reproductive tract, known as capacitation. Signaling pathways that take place during the capacitation process are much investigated issue. However, the role and regulation of phosphatidylinositol 3-kinase (PI3K) in this process are still not clear. Previously, we reported that short-time activation of protein kinase A (PRKA, PKA) leads to PI3K activation and protein kinase C(alpha)(PRKCA, PKC(alpha)) inhibition. In the present study, we found that during the capacitation PI3K phosphorylation/activation increases. PI3K activation was PRKA dependent, and down-regulated by PRKCA. PRKCA is found to be highly active at the beginning of the capacitation, conditions in which PI3K is not active. Moreover, inhibition of PRKCA causes significant activation of PI3K. Similar activation of PI3K is seen when the phosphatase PPP1 is blocked suggesting that PPP1 regulates PI3K activity. We found that during the capacitation PRKCA and PPP1CC2 (PP1gamma2) form a complex, and the two enzymes were degraded during the capacitation, suggesting that this degradation enables the activation of PI3K. This degradation is mediated by PRKA, indicating that in addition to the direct activation of PI3K by PRKA, this kinase can enhance PI3K phosphorylation indirectly by enhancing the degradation and inactivation of PRKCA and PPP1CC2.

Degradation of activated protein kinases by ubiquitination

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

The life and death of protein kinase C

Protein kinase C (PKC) is a family of kinases that plays diverse roles in many cellular functions, notably proliferation, differentiation, and cell survival. PKC is processed by phosphorylation and regulated by cofactor binding and subcellular localization. Extensive detail is available on the molecular mechanisms that regulate the maturation, activation, and signaling of PKC. However, less information is available on how signaling is terminated both from a global perspective and isozyme-specific differences. To target PKC therapeutically, various ATP-competitive inhibitors have been developed, but this method has problems with specificity. One possible new approach to developing novel, specific therapeutics for PKC would be to target the signaling termination pathways of the enzyme. This review focuses on the new developments in understanding how PKC signaling is terminated and how current drug therapies as well as information obtained from the recent elucidation of various PKC structures and down-regulation pathways could be used to develop novel and specific therapeutics for PKC.

Parvovirus interference with intracellular signalling: mechanism of PKCeta activation in MVM-infected A9 fibroblasts

Autonomous parvoviruses are strongly dependent on the phosphorylation of the major non-structural protein NS1 by members of the protein kinase C (PKC) family. Besides being accompanied with changes in the overall phosphorylation pattern of NS1 and acquiring new modifications at consensus PKC sites, ongoing minute virus of mice (MVM) infections lead to the appearance of new phosphorylated cellular protein species. This prompted us to investigate whether MVM actively interferes with phosphoinositol-dependent kinase (PDK)/PKC signalling. The activity, subcellular localization and phosphorylation status of the protein kinases PDK1, PKCeta and PKClambda were measured in A9 cells in the presence or absence of MVM infection. Parvovirus infection was found to result in activation of both PDK1 and PKCeta, as evidenced by changes in their subcellular distribution and overall (auto)phosphorylation. We show evidence that activation of PKCeta by PDK1 is driven by atypical PKClambda. By modifying the hydrophobic motif of PKCeta, PKClambda appeared to control docking and consecutive phosphorylation of PKCeta's activation-loop by PDK1, a process that was inhibited in vivo in the presence of a dominant-negative PKClambda mutant.

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.

The von Hippel-Lindau tumour-suppressor protein interaction with protein kinase Cdelta

The VHL (von Hippel-Lindau) tumour-suppressor protein forms a multi-protein complex [VCB (pVHL-elongin C-elongin B)-Cul-2 (Cullin-2)] with elongin C, elongin B, Cul-2 and Rbx1, acting as a ubiquitin-ligase (E3) and directing proteasome-dependent degradation of targeted proteins. The alpha-subunit of Hif1alpha (hypoxia-inducible factor 1alpha) is the principal substrate for the VCB-Cul-2 complex; however, other substrates such as aPKC (atypical protein kinase C) have been reported. In the present study, we show with FRET (fluorescence resonance energy transfer) analysis measured by FLIM (fluorescence lifetime imaging microscopy) that PKCdelta and pVHL (VHL protein) interact directly in cells. This occurs through the catalytic domain of PKCdelta (residues 432-508), which appears to interact with two regions of pVHL, residues 113-122 and 130-154. Despite this robust interaction, analysis of the PMA-induced proteasome-dependent degradation of PKCdelta in different RCC (renal cell carcinoma) lines (RCC4, UMRC2 and 786 O) shows that there is no correlation between the degradation of PKCdelta and the presence of active pVHL. Thus, in contrast with aPKC, PKCdelta is not a conventional substrate of the ubiquitin-ligase complex, VCB-Cul-2, and the observed interaction between these two proteins must underlie a distinct signalling output.

Proteasome Inhibition Induces Nuclear Translocation of the Dioxin Receptor Through an Sp1 and Protein Kinase C-Dependent Pathway

The dioxin receptor (AhR), in addition to its role in xenobiotic-induced carcinogenesis, appears to participate in cell proliferation, differentiation and organ homeostasis. Understanding potential mechanisms of activation of this receptor in the absence of exogenous ligands is therefore important to study its contribution to endogenous cellular functions. Using mouse embryo primary fibroblasts, we have previously shown that proteasome inhibition increased AhR transcriptional activity in the absence of xenobiotics. We suggested that proteasome inhibition-dependent AhR activation could involve an increase in the expression of the partner protein dioxin receptor nuclear translocator (ARNT). Since ARNT over-expression induced nuclear translocation of the AhR, and ARNT-deficient cells were unable to translocate this receptor to the nucleus upon proteasome inhibition, we have analyzed the effect of proteasome inhibition on the expression of regulatory proteins controlling ARNT levels. Treatment with the proteasome inhibitor MG132 increased endogenous Sp1 phosphorylation and its DNA-binding activity to the ARNT promoter. Sp1 phosphorylation and binding to the ARNT promoter, ARNT over-expression and AhR nuclear translocation were inhibited by GF109203X, a protein kinase C-specific inhibitor. In addition, MG132 stimulated protein kinase C activity in MEF cells with a pattern similar to that observed for ARNT expression. These data suggest that cellular control of protein kinase C activity, through Sp1 and ARNT, could regulate AhR transcriptional activity in the absence of xenobiotics.

Impaired degradation of PKCalpha by proteasome in a cellular model of Huntington's disease

In order to investigate any effect of mutant huntingtin aggregation on proteasome function and the degradation of proteins involved in the ubiquitin-proteasome pathway, we studied the degradation of PKCalpha in Neuro2a cells expressing either normal or mutant truncated huntingtin (HD 16Q and HD 150Q cells). We were able to show an elevation of polyubiquitinated PKCalpha in HD 150Q cells. PMA treatment of these cells revealed significant delay of PKCalpha degradation in comparison with control HD 16Q cells. Subcellular fractionation showed association of non-degraded PKCalpha with the membrane fraction of HD 150Q cells. Our data suggest an impairment of the degradation of PKCalpha in HD 150Q cells. This impairment is likely to be connected with the sequestration of proteasome on mutant huntingtin aggregates.

Intestinal sugar absorption is regulated by phosphorylation and turnover of protein kinase C betaII mediated by phosphatidylinositol 3-kinase- and mammalian target of rapamycin-dependent pathways

Stimulation of intestinal fructose absorption by phorbol 12-myristate 13-acetate (PMA) results from rapid insertion of GLUT2 into the brush-border membrane and correlates with protein kinase C (PKC) betaII activation. We have therefore investigated the role of phosphatidylinositol 3 (PI3)-kinase and mammalian target of rapamycin in the regulation of fructose absorption by PKC betaII phosphorylation. In isolated jejunal loops, stimulation of fructose absorption by PMA was inhibited by preperfusion with wortmannin or rapamycin, which blocked GLUT2 activation and insertion into the brush-border membrane. Antibodies to the last 18 and last 10 residues of the C-terminal region of PKC betaII recognized several species differentially in Western blots. Extensive cleavage of native enzyme (80/78 kDa) to a catalytic domain product of 49 kDa occurred. PMA and sugars provoked turnover and degradation of PKC betaII by dephosphorylation to a 42-kDa species, which was converted to polyubiquitylated species detected at 180 and 250+ kDa. PMA increased the level of the PKC betaII 49-kDa species, which correlates with the GLUT2 level; wortmannin and rapamycin blocked these effects of PMA. Rapamycin and wortmannin inhibited PKC betaII turnover. PI3-kinase, PDK-1, and protein kinase B were present in the brush-border membrane, where their levels were increased by PMA and blocked by the inhibitors. We conclude that GLUT2-mediated fructose absorption is regulated through PI3-kinase and mammalian target of rapamycin-dependent pathways, which control phosphorylation of PKC betaII and its substrate-induced turnover and ubiquitin-dependent degradation. These findings suggest possible mechanisms for short term control of intestinal sugar absorption by insulin and amino acids.

Recovery of gap junctional intercellular communication after phorbol ester treatment requires proteasomal degradation of protein kinase C

Reversible down-regulation of gap junctional intercellular communication (GJIC) is proposed to be an important cellular mechanism in tumor promotion. Gap junction function is modified by a variety of tumor promoters, including the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Treatment of cells with TPA results in the activation and subsequent depletion of the TPA-responsive protein kinase C (PKC) isoforms. TPA-induced degradation of the PKC isoforms alpha, delta and epsilon was recently shown to occur via the ubiquitin-proteasome pathway. In the present study we investigated the role of the proteasome in the TPA-induced modification of GJIC in IAR20 rat liver epithelial cells. TPA exposure of IAR20 cells induced hyperphosphorylation of gap junction protein connexin43 and inhibition of GJIC. Prolonged TPA treatment induced down-regulation of PKCalpha, delta and epsilon and a reduction in the total PKC activity, which was associated with recovery of GJIC. Co-treatment of IAR20 cells with TPA and the proteasomal inhibitor MG132 suppressed down-regulation of PKCalpha, delta and epsilon and caused prolonged PKC activity. Under these conditions, the recovery of GJIC was blocked. The general PKC inhibitor GF109203X reversed the effect of MG132, indicating that the prolonged TPA-induced inhibition of GJIC caused by MG132 was due to the prolonged PKC activity. These results indicate that proteasomal degradation of PKC is one mechanism by which the recovery of GJIC after TPA treatment is regulated.

Phosphorylation is required for PMA- and cell-cycle-induced degradation of protein kinase Cdelta

Classical and novel protein kinase C (PKC) isoforms are down-regulated as a result of chronic activation by certain tumour promoters and physiological stimuli; however, the mechanisms leading to down-regulation are not fully understood. In the present study, we have studied the PMA ('TPA')-induced degradation of PKCdelta in NIH 3T3 cells under culture conditions where PKCdelta displays cell-cycle-dependent down-regulation. In contrast with previous studies, a hyperphosphorylated form of this PKC isoform, promoted by calyculin A, was rapidly degraded in PMA-treated cells. Similarly, the presence of calyculin A enhanced the down-regulation of PKCdelta observed on G(1)/S-phase progression through the cell cycle. Analysis of phosphorylation-site mutants indicated that the T-loop Thr(505) phosphorylation site was critical for induced degradation.

Proteasome inhibitor enhances growth hormone-binding protein release

We used murine Ba/F3 cells transfected with human growth hormone receptor (hGHR) cDNA to investigate the regulatory mechanisms of human growth hormone-binding protein (hGH-BP) release. The extracellular domain of hGHRs were cleaved and released as hGH-BPs (a soluble form of hGHR). The hGH-BP release was enhanced by phorbol 12,13-dibutyrate (PDBu), and suggested to be mediated by activation of PKC, the same as in human IM-9 cells. Thus, Ba/F3 cells have hGH-BP-releasing pathways similar to those of human cells. The proteasome inhibitors MG-132 and clasto-lactacystin beta-lactone also increased hGH-BP release from Ba/F3-hGHR cells, and MG-132 and PDBu synergistically increased hGH-BP release. The results obtained by using three PKC inhibitors Gö 6976, GF 109203X and Gö 6983 suggest that the enhancement of hGH-BP release by MG-132 and PDBu is mediated by different mechanisms probably involving different PKC isozymes.