Tyrosine phosphorylation and stimulation of protein kinase C delta from porcine spleen by src in vitro. Dependence on the activated state of protein kinase C delta

Abl kinase-mediated FUS Tyr526 phosphorylation alters nucleocytoplasmic FUS localization in FTLD-FUS

Nuclear to cytoplasmic mislocalization and aggregation of multiple RNA-binding proteins (RBPs), including FUS, are the main neuropathological features of the majority of cases of amyotrophic lateral sclerosis (ALS) and frontotemporal lobular degeneration (FTLD). In ALS-FUS, these aggregates arise from disease-associated mutations in FUS, whereas in FTLD-FUS, the cytoplasmic inclusions do not contain mutant FUS, suggesting different molecular mechanisms of FUS pathogenesis in FTLD that remain to be investigated. We have previously shown that phosphorylation of the C-terminal Tyr526 of FUS results in increased cytoplasmic retention of FUS due to impaired binding to the nuclear import receptor TNPO1. Inspired by the above notions, in the current study we developed a novel antibody against the C-terminally phosphorylated Tyr526 FUS (FUSp-Y526) that is specifically capable of recognizing phosphorylated cytoplasmic FUS, which is poorly recognized by other commercially available FUS antibodies. Using this FUSp-Y526 antibody, we demonstrated a FUS phosphorylation-specific effect on the cytoplasmic distribution of soluble and insoluble FUSp-Y526 in various cells and confirmed the involvement of the Src kinase family in Tyr526 FUS phosphorylation. In addition, we found that FUSp-Y526 expression pattern correlates with active pSrc/pAbl kinases in specific brain regions of mice, indicating preferential involvement of cAbl in the cytoplasmic mislocalization of FUSp-Y526 in cortical neurons. Finally, the pattern of immunoreactivity of active cAbl kinase and FUSp-Y526 revealed altered cytoplasmic distribution of FUSp-Y526 in cortical neurons of post-mortem frontal cortex tissue from FTLD patients compared with controls. The overlap of FUSp-Y526 and FUS signals was found preferentially in small diffuse inclusions and was absent in mature aggregates, suggesting possible involvement of FUSp-Y526 in the formation of early toxic FUS aggregates in the cytoplasm that are largely undetected by commercially available FUS antibodies. Given the overlapping patterns of cAbl activity and FUSp-Y526 distribution in cortical neurons, and cAbl induced sequestration of FUSp-Y526 into G3BP1 positive granules in stressed cells, we propose that cAbl kinase is actively involved in mediating cytoplasmic mislocalization and promoting toxic aggregation of wild-type FUS in the brains of FTLD patients, as a novel putative underlying mechanism of FTLD-FUS pathophysiology and progression.

Role and Mechanism of PKC-δ for Cardiovascular Disease: Current Status and Perspective

Protein kinase C (PKC) is a protein kinase with important cellular functions. PKC-δ, a member of the novel PKC subfamily, has been well-documented over the years. Activation of PKC-δ plays an important regulatory role in myocardial ischemia/reperfusion (IRI) injury and myocardial fibrosis, and its activity and expression levels can regulate pathological cardiovascular diseases such as atherosclerosis, hypertension, cardiac hypertrophy, and heart failure. This article aims to review the structure and function of PKC-δ, summarize the current research regarding its activation mechanism and its role in cardiovascular disease, and provide novel insight into further research on the role of PKC-δ in cardiovascular diseases.

TIRAP in the Mechanism of Inflammation

The Toll-interleukin-1 Receptor (TIR) domain-containing adaptor protein (TIRAP) represents a key intracellular signalling molecule regulating diverse immune responses. Its capacity to function as an adaptor molecule has been widely investigated in relation to Toll-like Receptor (TLR)-mediated innate immune signalling. Since the discovery of TIRAP in 2001, initial studies were mainly focused on its role as an adaptor protein that couples Myeloid differentiation factor 88 (MyD88) with TLRs, to activate MyD88-dependent TLRs signalling. Subsequent studies delineated TIRAP’s role as a transducer of signalling events through its interaction with non-TLR signalling mediators. Indeed, the ability of TIRAP to interact with an array of intracellular signalling mediators suggests its central role in various immune responses. Therefore, continued studies that elucidate the molecular basis of various TIRAP-protein interactions and how they affect the signalling magnitude, should provide key information on the inflammatory disease mechanisms. This review summarizes the TIRAP recruitment to activated receptors and discusses the mechanism of interactions in relation to the signalling that precede acute and chronic inflammatory diseases. Furthermore, we highlighted the significance of TIRAP-TIR domain containing binding sites for several intracellular inflammatory signalling molecules. Collectively, we discuss the importance of the TIR domain in TIRAP as a key interface involved in protein interactions which could hence serve as a therapeutic target to dampen the extent of acute and chronic inflammatory conditions.

Fyn Kinase-Mediated PKCδ Y311 Phosphorylation Induces Dopaminergic Degeneration in Cell Culture and Animal Models: Implications for the Identification of a New Pharmacological Target for Parkinson's Disease

Oxidative stress, neuroinflammation and apoptosis are some of the key etiological factors responsible for dopamin(DA)ergic degeneration during Parkinson's disease (PD), yet the downstream molecular mechanisms underlying neurodegeneration are largely unknown. Recently, a genome-wide association study revealed the FYN gene to be associated with PD, suggesting that Fyn kinase could be a pharmacological target for PD. In this study, we report that Fyn-mediated PKCδ tyrosine (Y311) phosphorylation is a key event preceding its proteolytic activation in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinsonism. MPP+/MPTP induced Fyn kinase activation in N27 DAergic neuronal cells and the mouse substantia nigra. PKCδ-Y311 phosphorylation by activated Fyn initiates the apoptotic caspase-signaling cascade during DAergic degeneration. Pharmacological attenuation of Fyn activity protected DAergic neurons from MPP+-induced degeneration in primary mesencephalic neuronal cultures. We further employed Fyn wild-type and Fyn knockout (KO) mice to confirm whether Fyn is a valid pharmacological target of DAergic neurodegeneration. Primary mesencephalic neurons from Fyn KO mice were greatly protected from MPP+-induced DAergic cell death, neurite loss and DA reuptake loss. Furthermore, Fyn KO mice were significantly protected from MPTP-induced PKCδ-Y311 phosphorylation, behavioral deficits and nigral DAergic degeneration. This study thus unveils a mechanism by which Fyn regulates PKCδ's pro-apoptotic function and DAergic degeneration. Pharmacological inhibitors directed at Fyn activation could prove to be a novel therapeutic target in the delay or halting of selective DAergic degeneration during PD.

LysoPCs induce Hck- and PKCδ-mediated activation of PKCγ causing p47phox phosphorylation and membrane translocation in neutrophils

Lysophosphatidylcholines (lysoPCs) are effective polymorphonuclear neutrophil (PMN) priming agents implicated in transfusion-related acute lung injury (TRALI). LysoPCs cause ligation of the G2A receptor, cytosolic Ca²⁺ flux, and activation of Hck. We hypothesize that lysoPCs induce Hck-dependent activation of protein kinase C (PKC), resulting in phosphorylation and membrane translocation of 47 kDa phagocyte oxidase protein (p47^phox). PMNs, human or murine, were primed with lysoPCs and were smeared onto slides and examined by digital microscopy or separated into subcellular fractions or whole-cell lysates. Proteins were immunoprecipitated or separated by polyacrylamide gel electrophoresis and immunoblotted for proteins of interest. Wild-type (WT) and PKCγ knockout (KO) mice were used in a 2-event model of TRALI. LysoPCs induced Hck coprecipitation with PKCδ and PKCγ and the PKCδ:PKCγ complex also had a fluorescence resonance energy transfer (FRET)⁺ interaction with lipid rafts and Wiskott-Aldrich syndrome protein family verprolin-homologous protein 2 (WAVE2). PKCγ then coprecipitated with p47^phox Immunoblotting, immunoprecipitation (IP), specific inhibitors, intracellular depletion of PKC isoforms, and PMNs from PKCγ KO mice demonstrated that Hck elicited activation/Tyr phosphorylation (Tyr311 and Tyr525) of PKCδ, which became Thr phosphorylated (Thr507). Activated PKCδ then caused activation of PKCγ, both by Tyr phosphorylation (Τyr514) and Ser phosphorylation, which induced phosphorylation and membrane translocation of p47^phox In PKCγ KO PMNs, lysoPCs induced Hck translocation but did not evidence a FRET⁺ interaction between PKCδ and PKCγ nor prime PMNs. In WT mice, lysoPCs served as the second event in a 2-event in vivo model of TRALI but did not induce TRALI in PKCγ KO mice. We conclude that lysoPCs prime PMNs through Hck-dependent activation of PKCδ, which stimulates PKCγ, resulting in translocation of phosphorylated p47^phox.

Clostridium perfringens phospholipase C induced ROS production and cytotoxicity require PKC, MEK1 and NFκB activation

Clostridium perfringens phospholipase C (CpPLC), also called α-toxin, is the most toxic extracellular enzyme produced by this bacteria and is essential for virulence in gas gangrene. At lytic concentrations, CpPLC causes membrane disruption, whereas at sublytic concentrations this toxin causes oxidative stress and activates the MEK/ERK pathway, which contributes to its cytotoxic and myotoxic effects. In the present work, the role of PKC, ERK 1/2 and NFκB signalling pathways in ROS generation induced by CpPLC and their contribution to CpPLC-induced cytotoxicity was evaluated. The results demonstrate that CpPLC induces ROS production through PKC, MEK/ERK and NFκB pathways, the latter being activated by the MEK/ERK signalling cascade. Inhibition of either of these signalling pathways prevents CpPLC's cytotoxic effect. In addition, it was demonstrated that NFκB inhibition leads to a significant reduction in the myotoxicity induced by intramuscular injection of CpPLC in mice. Understanding the role of these signalling pathways could lead towards developing rational therapeutic strategies aimed to reduce cell death during a clostridialmyonecrosis.

Role of VEGF receptors in normal and psoriatic human keratinocytes: evidence from irradiation with different UV sources

Vascular endothelial growth factor (VEGF) promotes angiogenesis and plays important roles both in physiological and pathological conditions. VEGF receptors (VEGFRs) are high-affinity receptors for VEGF and are originally considered specific to endothelial cells. We previously reported that VEGFRs were also constitutively expressed in normal human keratinocytes and overexpressed in psoriatic epidermis. In addition, UVB can activate VEGFRs in normal keratinocytes, and the activated VEGFR-2 signaling is involved in the pro-survival mechanism. Here, we show that VEGFRs were also upregulated and activated by UVA in normal human keratinocytes via PKC, and interestingly, both the activated VEGFR-1 and VEGFR-2 protected against UVA-induced cell death. As VEGFRs were over-expressed in psoriatic epidermis, we further investigated whether narrowband UVB (NB-UVB) phototherapy or topical halomethasone monohydrate 0.05% cream could affect their expression. Surprisingly, the over-expressed VEGFRs in psoriatic epidermis were significantly attenuated by both treatments. During NB-UVB therapy, VEGFRs declined first in the basal, and then gradually in the upper psoriatic epidermis. VEGFRs were activated in psoriatic epidermis, their activation was enhanced by NB-UVB, but turned undetectable after whole therapy. This process was quite different from that by halomethasone, in which VEGFRs and phospho-VEGFRs decreased in a gradual, homogeneous manner. Our findings further suggest that UV-induced activation of VEGFRs serves as a pro-survival signal for keratinocytes. In addition, VEGFRs may be involved in the pathological process of psoriasis, and UV phototherapy is effective for psoriasis by directly modulating the expression of VEGFRs.

The role of C2 domains in PKC signaling

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

Hck is a key regulator of gene expression in alternatively activated human monocytes

IL-13 is a Th2 cytokine that promotes alternative activation (M2 polarization) in primary human monocytes. Our studies have characterized the functional IL-13 receptor complex and the downstream signaling events in response to IL-13 stimulation in alternatively activated monocytes/macrophages. In this report, we present evidence that IL-13 induces the activation of a Src family tyrosine kinase, which is required for IL-13 induction of M2 gene expression, including 15-lipoxygenase (15-LO). Our data show that Src kinase activity regulates IL-13-induced p38 MAPK tyrosine phosphorylation via the upstream kinases MKK3 or MKK6. Our findings also reveal that the IL-13 receptor-associated tyrosine kinase Jak2 is required for the activation of both Src kinase as well as p38 MAPK. Further, we found that Src tyrosine kinase-mediated activation of p38 MAPK is required for Stat1 and Stat3 serine 727 phosphorylation in alternatively activated monocytes/macrophages. Additional studies identify Hck as the specific Src family member, stimulated by IL-13 and involved in regulating both p38 MAPK activation and p38 MAPK-mediated 15-LO expression. Finally we show that the Hck regulates the expression of other alternative state (M2)-specific genes (Mannose receptor, MAO-A, and CD36) and therefore conclude that Hck acts as a key regulator controlling gene expression in alternatively activated monocytes/macrophages.

Environmental neurotoxic pesticide dieldrin activates a non receptor tyrosine kinase to promote PKCδ-mediated dopaminergic apoptosis in a dopaminergic neuronal cell model

Oxidative stress and apoptosis are two key pathophysiological mechanisms underlying dopaminergic degeneration in Parkinson's disease (PD). Recently, we identified that proteolytic activation of protein kinase C-delta (PKCδ), a member of the novel PKC family, contributes to oxidative stress-induced dopaminergic degeneration and that phosphorylation of tyrosine residue 311 (tyr311) on PKCδ is a key event preceding the PKCδ proteolytic activation during oxidative damage. Herein, we report that a non-receptor tyrosine kinase Fyn is significantly expressed in a dopaminergic neuronal N27 cell model. Exposure of N27 cells to the dopaminergic toxicant dieldrin (60 μM) rapidly activated Fyn kinase, PKCδ-tyr311 phosphorylation and proteolytic cleavage. Fyn kinase activation precedes the caspase-3-mediated proteolytic activation of PKCδ. Pre-treatment with p60-tyrosine-specific kinase inhibitor (TSKI) almost completely attenuated dieldrin-induced phosphorylation of PKCδ-tyr311 and its proteolytic activation. Additionally, TSKI almost completely blocked dieldrin-induced apoptotic cell death. To further confirm Fyn's role in the pro-apoptotic function of PKCδ, we adopted the RNAi approach. siRNA-mediated knockdown of Fyn kinase also effectively attenuated dieldrin-induced phosphorylation of PKCδ-tyr311, caspase-3-mediated PKCδ proteolytic cleavage, and DNA fragmentation, suggesting that Fyn kinase regulates the pro-apoptotic function of PKCδ. Collectively, these results demonstrate for the first time that Fyn kinase is a pro-apoptotic kinase that regulates upstream signaling of the PKCδ-mediated apoptotic cell death pathway in neurotoxicity models of pesticide exposure.

PKC-delta and -eta, MEKK-1, MEK-6, MEK-3, and p38-delta are essential mediators of the response of normal human epidermal keratinocytes to differentiating agents

Previous studies suggest that the novel protein kinase C (PKC) isoforms initiate a mitogen-activated protein kinase (MAPK) signaling cascade that regulates keratinocyte differentiation. However, assigning these functions has relied on treatment with pharmacologic inhibitors and/or manipulating kinase function using overexpression of wild-type or dominant-negative kinases. As these methods are not highly specific, an obligatory regulatory role for individual kinases has not been assigned. In this study, we use small interfering RNA knockdown to study the role of individual PKC isoforms as regulators of keratinocyte differentiation induced by the potent differentiating stimulus, 12-O-tetradecanoylphorbol-13-acetate (TPA). PKC-delta knockdown reduces TPA-activated involucrin promoter activity, nuclear activator protein-1 factor accumulation and binding to DNA, and cell morphology change. Knockdown of PKC downstream targets, including MEKK-1, MEK-6, MEK-3, or p38-delta, indicates that these kinases are required for these responses. Additional studies indicate that knockdown of PKC-eta inhibits TPA-dependent involucrin promoter activation. In contrast, knockdown of PKC-alpha (a classical PKC isoform) or PKC-epsilon (a novel isoform) does not inhibit these TPA-dependent responses. Further studies indicate that PKC-delta is required for calcium and green tea polyphenol-dependent regulation of end responses. These findings are informative as they suggest an essential role for selected PKC and MAPK cascade enzymes in mediating a range of end responses to a range of differentiation stimuli in keratinocytes.

Integrin alphaMbeta2 clustering triggers phosphorylation and activation of protein kinase C delta that regulates transcription factor Foxp1 expression in monocytes

Integrins are type I membrane and heterodimeric (alphabeta) cell adhesion receptors. Intracellular signals triggered by ligand-bound integrins are important for cell growth, differentiation, and migration. Integrin alpha(M)beta(2) plays key roles in myeloid cell adhesion, phagocytosis, and degranulation. In this study, we show that protein kinase C (PKC) delta is involved in alpha(M)beta(2) signaling. In human monocytic U937 cells and peripheral blood monocytes, alpha(M)beta(2) clustering induced PKCdelta translocation to the plasma membrane, followed by Tyr(311) phosphorylation and activation of PKCdelta by the src family kinases Hck and Lyn. Interestingly, alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation was not mediated by the tyrosine kinase Syk, which is a well reported kinase in beta(2) integrin signaling. Analysis of the beta(2) cytoplasmic tail showed that the sequence Asn(727)-Ser(734) is important in alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation. It has been shown that alpha(M)beta(2) clustering regulates the expression the transcription factor Foxp1 that has a role in monocyte differentiation. We show that Foxp1 expression was reduced in monocytes that were allowed to adhere to human microvascular endothelial cells. However, the expression of Foxp1 was not affected in monocytes that were treated with PKCdelta-targeting small interfering RNA, suggesting that PKCdelta regulates Foxp1 expression. These results demonstrate a role of PKCdelta in alpha(M)beta(2)-mediated Foxp1 regulation in monocytes.

Differential regulation of threonine and tyrosine phosphorylations on protein kinase Cdelta by G-protein-mediated pathways in platelets

Phosphorylation of activation loop threonine (Thr(505)) and regulatory domain tyrosine (Tyr(311)) residues are key regulators of PKC (protein kinase C) delta function in platelets. In the present study, we show that G(q) and G(12/13) pathways regulate the Thr(505) and Tyr(311) phosphorylation on PKCdelta in an interdependent manner. DiC8 (1,2-dioctanoylglycerol), a synthetic analogue of DAG (diacylglycerol), caused Thr(505), but not Tyr(311), phosphorylation on PKCdelta, whereas selective activation of G(12/13) pathways by the YFLLRNP peptide failed to cause phosphorylation of either residue. However, simultaneous activation by DiC8 and YFLLRNP resulted in Thr(505) and Tyr(311) phosphorylation on PKCdelta. In addition, we found that the activation of SFKs (Src family tyrosine kinases) is essential for G(12/13)-mediated Tyr(311) phosphorylation of PKCdelta. These results were confirmed using G(q)-deficient mouse platelets. Finally, we investigated whether Thr(505) phosphorylation is required for Tyr(311) phosphorylation. A T505A PKCdelta mutant failed to be phosphorylated at Tyr(311), even upon stimulation of both G(q) and G(12/13) pathways. We conclude that (i) PKCdelta binding to DAG, downstream of G(q) pathways, and its translocation results in Thr(505) phosphorylation, (ii) G(12/13) pathways activate SFKs required for the phosphorylation of Tyr(311) on Thr(505)-phosphorylated PKCdelta, and (iii) Thr(505) phosphorylation is a prerequisite for Tyr(311) phosphorylation on PKCdelta.

Coincident regulation of PKCdelta in human platelets by phosphorylation of Tyr311 and Tyr565 and phospholipase C signalling

PKC (protein kinase C)d plays a complex role in platelets, having effects on both positive and negative signalling functions. It is phosphorylated on tyrosine residues in response to thrombin and collagen, and it has recently been shown that Tyr311 is phosphorylated in response to PAR (protease-activated receptor) 1 and PAR4 receptor activation. In the present study, we show that Tyr311 and Tyr565 are phosphorylated in response to thrombin, and have examined the interplay between phosphorylation and the classical lipid-mediated activation of PKCd. Phosphorylation of both Tyr311 and Tyr565 is dependent on Src kinase and PLC (phospholipase C) activity in response to thrombin. Importantly, direct allosteric activation of PKCd with PMA also induced phosphorylation of Tyr311 and Tyr565, and this was dependent on the activity of Src kinases, but not PLC. Membrane recruitment of PKCd is essential for phosphorylation of this tyrosine residue, but tyrosine phosphorylation is not required for membrane recruitment of PKCd. Both thrombin and PMA induce recruitment of PKCd to the membrane, and for thrombin, this recruitment is a PLC-dependent process. In order to address the functional role of tyrosine residue phosphorylation of PKCd, we demonstrate that phosphorylation can potentiate the activity of the kinase, although phosphorylation does not play a role in membrane recruitment of the kinase. PKCd is therefore regulated in a coincident fashion, PLC-dependent signals recruiting it to the plasma membrane and by phosphorylation on tyrosine residues, potentiating its activity.

CUB domain-containing protein 1 is a novel regulator of anoikis resistance in lung adenocarcinoma

Malignant tumor cells frequently achieve resistance to anoikis, a form of apoptosis induced by detachment from the basement membrane, which results in the anchorage-independent growth of these cells. Although the involvement of Src family kinases (SFKs) in this alteration has been reported, little is known about the signaling pathways involved in the regulation of anoikis under the control of SFKs. In this study, we identified a membrane protein, CUB-domain-containing protein 1 (CDCP1), as an SFK-binding phosphoprotein associated with the anchorage independence of human lung adenocarcinoma. Using RNA interference suppression and overexpression of CDCP1 mutants in lung cancer cells, we found that tyrosine-phosphorylated CDCP1 is required to overcome anoikis in lung cancer cells. An apoptosis-related molecule, protein kinase Cdelta, was found to be phosphorylated by the CDCP1-SFK complex and was essential for anoikis resistance downstream of CDCP1. Loss of CDCP1 also inhibited the metastatic potential of the A549 cells in vivo. Our findings indicate that CDCP1 is a novel target for treating cancer-specific disorders, such as metastasis, by regulating anoikis in lung adenocarcinoma.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

PKCdelta signaling: mechanisms of DNA damage response and apoptosis

The cellular response to genotoxic stress that damages DNA includes cell cycle arrest, activation of DNA repair, and in the event of irreparable damage, induction of apoptosis. However, the signals that determine cell fate, that is, survival or apoptosis, are largely unknown. The delta isoform of protein kinase C (PKCdelta) has been implicated in many important cellular processes, including regulation of apoptotic cell death. The available information supports a model in which certain sensors of DNA lesions activate PKCdelta. This activation is triggered in part by tyrosine phosphorylation of PKCdelta by c-Abl tyrosine kinase. PKCdelta is further proteolytically activated by caspase-3. The cleaved catalytic fragment of PKCdelta translocates to the nucleus and induces apoptosis. Importantly, accumulating data have revealed the nuclear targets for PKCdelta in the induction of apoptosis. A pro-apoptotic function of activated PKCdelta is mediated by at least several downstream effectors known to be associated with the elicitation of apoptosis. Recent findings also demonstrated that PKCdelta is involved in cell cycle-specific activation and induction of apoptotic cell death. Moreover, previous studies have shown that PKCdelta regulates transcription by phosphorylating various transcription factors, including the p53 tumor suppressor that is critical for cell cycle arrest and apoptosis in response to DNA damage. These findings collectively support a pivotal role for PKCdelta in the induction of apoptosis with significant impact. This review is focused on the current views regarding the regulation of cell fate by PKCdelta signaling in response to DNA damage.

Stimulation by ghrelin of p42/p44 mitogen-activated protein kinase through the GHS-R1a receptor: Role of G-proteins and β-arrestins

Results presented in this study indicate that in human embryonic kidney 293 cells (HEK 293), the ghrelin receptor growth hormone secretagogue receptor type 1a (GHS-R1a) activates the extracellular signal-related kinases 1 and 2 (ERK 1/2) via three pathways. One pathway is mediated by the beta-arrestins 1 and 2, and requires entry of the receptor into a multiprotein complex with the beta-arrestins, Src, Raf-1, and ERK 1/2. A second pathway is G(q/11)-dependent and involves a Ca(2+)-dependent PKC (PKCalpha/beta) and Src. A third pathway is G(i)-dependent and involves phosphoinositide 3-kinase (PI3K), PKCepsilon, and Src. Our current study reveals that G(i/o)- and G(q/11)-proteins are crucially involved in the beta-arrestin-mediated ERK 1/2 activation. These results thus support the view that the beta-arrestins act as both scaffolding proteins and signal transducers in ERK 1/2 activation, as reported for other receptors. The different pathways of ERK 1/2 activation suggest that binding to GHS-R1a activates ERK 1/2 pools at different locations within the cell, and thus probably with different physiological consequences.

Modulation of Kv3.1b potassium channel phosphorylation in auditory neurons by conventional and novel protein kinase C isozymes

In fast-spiking neurons such as those in the medial nucleus of the trapezoid body (MNTB) in the auditory brainstem, Kv3.1 potassium channels are required for high frequency firing. The Kv3.1b splice variant of this channel predominates in the mature nervous system and is a substrate for phosphorylation by protein kinase C (PKC) at Ser-503. In resting neurons, basal phosphorylation at this site decreases Kv3.1 current, reducing neuronal ability to follow high frequency stimulation. We used a phospho-specific antibody to determine which PKC isozymes control serine 503 phosphorylation in Kv3.1b-tranfected cells and in auditory neurons in brainstem slices. By using isozyme-specific inhibitors, we found that the novel PKC-delta isozyme, together with the novel PKC-epsilon and conventional PKCs, contributed to the basal phosphorylation of Kv3.1b in MNTB neurons. In contrast, only PKC-epsilon and conventional PKCs mediate increases in phosphorylation produced by pharmacological activation of PKC in MNTB neurons or by metabotropic glutamate receptor activation in Kv3.1/mGluR1-cotransfected cells. We also measured the time course of dephosphorylation and recovery of basal phosphorylation of Kv3.1b following brief high frequency electrical stimulation of the trapezoid body, and we determined that the recovery process is mediated by both novel PKC-delta and PKC-epsilon isozymes and by conventional PKCs. The association between Kv3.1b and PKC isozymes was confirmed by reciprocal coimmunoprecipitation of Kv3.1b with multiple PKC isozymes. Our results suggest that the Kv3.1b channel is regulated by both conventional and novel PKC isozymes and that novel PKC-delta contributes specifically to the maintenance of basal phosphorylation in auditory neurons.

Induction of ornithine decarboxylase in T/C-28a2 chondrocytes by lysophosphatidic acid: signaling pathway and inhibition of cell proliferation

Among several extracellular messengers tested, lysophosphatidic acid (LPA) was able to cause the most marked induction of ornithine decarboxylase (ODC) in serum-starved human T/C-28a2 chondrocytes. LPA also induced the activation/phosphorylation of Src, Akt and p44/42 MAPK, and the translocation of PKC-delta from cytosol to membrane coupled to its tyrosine phosphorylation. Experiments with selective signaling inhibitors indicate that LPA leads to Src activation through Gi protein-coupled receptors. In turn Src can activate PI3K and PKC-delta, and all these signaling proteins are required for ODC induction. In conclusion these results show that chondrocytes may be a novel target for LPA action. However, although LPA is considered a mitogen for several cell types and ODC induction is generally correlated to cell growth, LPA was not able to stimulate chondrocyte growth, but rather exerted an anti-proliferative effect.

Tyrosine phosphorylation regulates the proteolytic activation of protein kinase Cdelta in dopaminergic neuronal cells

Oxidative stress is a key apoptotic stimulus in neuronal cell death and has been implicated in the pathogenesis of many neurodegenerative disorders, including Parkinson disease (PD). Recently, we demonstrated that protein kinase C-delta (PKCdelta) is an oxidative stress-sensitive kinase that can be activated by caspase-3-dependent proteolytic cleavage to induce apoptotic cell death in cell culture models of Parkinson disease (Kaul, S., Kanthasamy, A., Kitazawa, M., Anantharam, V., and Kanthasamy, A. G. (2003) Eur. J. Neurosci. 18, 1387-1401 and Kanthasamy, A. G., Kitazawa, M., Kanthasamy, A., and Anantharam, V. (2003) Antioxid. Redox. Signal. 5, 609-620). Here we showed that the phosphorylation of a tyrosine residue in PKCdelta can regulate the proteolytic activation of the kinase during oxidative stress, which consequently influences the apoptotic cell death in dopaminergic neuronal cells. Exposure of a mesencephalic dopaminergic neuronal cell line (N27 cells) to H(2)O(2)(0-300 microm) induced a dose-dependent increase in cytotoxicity, caspase-3 activation and PKCdelta cleavage. H(2)O(2)-induced proteolytic activation of PKC was delta mediated by the activation of caspase-3. Most interestingly, both the general Src tyrosine kinase inhibitor genistein (25 microm) and the p60(Src) tyrosine-specific kinase inhibitor (TSKI; 5 microm) dramatically inhibited H(2)O(2) and the Parkinsonian toxin 1-methyl-4-phenylpyridinium-induced PKCdelta cleavage, kinase activation, and apoptotic cell death. H(2)O(2) treatment also increased phosphorylation of PKCdelta at tyrosine site 311, which was effectively blocked by co-treatment with TSKI. Furthermore, N27 cells overexpressing a PKCdelta(Y311F) mutant protein exhibited resistance to H(2)O(2)-induced PKCdelta cleavage, caspase activation, and apoptosis. To our knowledge, these data demonstrate for the first time that phosphorylation of Tyr-311 on PKCdelta can regulate the proteolytic activation and proapoptotic function of the kinase in dopaminergic neuronal cells.

PKC-δ-dependent pathways contribute to PDGF-stimulated ERK1/2 activation in vascular smooth muscle

Platelet-derived growth factor (PDGF) is an important regulator of vascular smooth muscle (VSM) cell growth and migration and has been identified as a key mediator of neointima formation resulting from vascular injury. PDGF exerts its effects, in part, through activation of ERK1/2. Previously, we reported that PKC-δ, specifically compared with PKC-α, mediated phorbol ester- and ATP-dependent activation of ERK1/2 in VSM cells. The purpose of this study was to determine whether PKC-δ was involved in PDGF-dependent activation of ERK1/2 in VSM cells. The addition of PDGF resulted in the activation, and Src family kinase-dependent tyrosine phosphorylation, of PKC-δ. Treatment with rottlerin (0.1–10 μM), a selective PKC-δ inhibitor, or adenoviral overexpression of kinase-negative PKC-δ significantly attenuated PDGF-induced activation of ERK1/2. The effects of the PKC-δ inhibitors decreased with increasing concentrations of activator PDGF. Interestingly, treatment with Gö6976 (0.1–3 μM), a selective inhibitor of cPKCs, or adenoviral overexpression of kinase-negative PKC-α also inhibited PDGF-stimulated ERK1/2. Furthermore, inhibition of cPKC activity with Gö6976 or overexpression of kinase-negative PKC-α attenuated PKC-δ activation and tyrosine phosphorylation in response to PDGF. These studies indicate involvement of both PKC-δ and PKC-α isozymes in PDGF-stimulated signaling in VSM and suggest an unexpected role for PKC-α in the regulation of PKC-δ activity.

Molecular mechanism and functional implications of thrombin-mediated tyrosine phosphorylation of PKCdelta in platelets

Thrombin has been known to cause tyrosine phosphorylation of protein kinase C delta (PKCdelta) in platelets, but the molecular mechanisms and function of this tyrosine phosphorylation is not known. In this study, we investigated the signaling pathways used by protease-activated receptors (PARs) to cause tyrosine phosphorylation of PKCdelta and the role of this event in platelet function. PKCdelta was tyrosine phosphorylated by either PAR1 or PAR4 in a concentration- and time-dependent manner in human platelets. In particular, the tyrosine 311 residue was phosphorylated downstream of PAR receptors. Also the tyrosine phosphorylation of PKCdelta did not occur in Galpha(q)-deficient mouse platelets and was inhibited in the presence of a phospholipase C (PLC) inhibitor U73122 and calcium chelator BAPTA (5,5'-dimethyl-bis(o-aminophenoxy)ethane-N, N, N ', N '-tetraacetic acid), suggesting a role for Galpha(q) pathways and calcium in this event. Both PAR1 and PAR4 caused a time-dependent activation of Src (pp60c-src) tyrosine kinase and Src tyrosine kinase inhibitors completely blocked the tyrosine phosphorylation of PKCdelta. Inhibition of tyrosine phosphorylation or the kinase activity of PKCdelta dramatically blocked PAR-mediated thromboxane A2 generation. We conclude that thrombin causes tyrosine phosphorylation of PKCdelta in a calcium- and Src-family kinase-dependent manner in platelets, with functional implications in thromboxane A2 generation.

Src tyrosine kinase regulates insulin-induced activation of protein kinase C (PKC) delta in skeletal muscle

Insulin stimulation of skeletal muscle results in rapid activation of protein kinase Cdelta (PKCdelta), which is associated with its tyrosine phosphorylation and physical association with insulin receptor (IR). The mechanisms underlying tyrosine phosphorylation of PKCdelta have not been determined. In this study, we investigated the possibility that the Src family of nonreceptor tyrosine kinases may be involved upstream insulin signaling. Studies were done on differentiated rat skeletal myotubes in primary culture. Insulin caused an immediate stimulation of Src and induced its physical association with both IR and PKCdelta. Inhibition of Src by treatment with the Src family inhibitor PP2 reduced insulin-stimulated Src-PKCdelta association, PKCdelta tyrosine phosphorylation and PKCdelta activation. PP2 inhibition of Src also decreased insulin-induced IR tyrosine phosphorylation, IR-PKCdelta association and association of Src with both PKCdelta and IR. Finally, inhibition of Src decreased insulin-induced glucose uptake. We conclude that insulin activates Src tyrosine kinase, which regulates PKCdelta activity. Thus, Src tyrosine kinase may play an important role in insulin-induced tyrosine phosphorylation of both IR and PKCdelta. Moreover, both Src and PKCdelta appear to be involved in IR activation and subsequent downstream signaling.

PKCδ plays opposite roles in growth mediated by wild-type Kit and an oncogenic Kit mutant

The Kit receptor tyrosine kinase is critical for normal hematopoiesis. Mutation of the aspartic acid residue encoded by codon 816 of human c-kit or codon 814 of the murine gene results in an oncogenic form of Kit. Here we investigate the role of protein kinase Cδ (PKCδ) in responses mediated by wild-type murine Kit and the D814Y mutant in a murine mast cell-like line. PKCδ is activated after wild-type (WT) Kit binds stem cell factor (SCF), is constitutively active in cells expressing the Kit catalytic domain mutant, and coprecipitates with both forms of Kit. Inhibition of PKCδ had opposite effects on growth mediated by wild-type and mutant Kit. Both rottlerin and a dominant-negative PKCδ construct inhibited the growth of cells expressing mutant Kit, while SCF-induced growth of cells expressing wild-type Kit was not inhibited. Further, overexpression of PKCδ inhibited growth of cells expressing wild-type Kit and enhanced growth of cells expressing the Kit mutant. These data demonstrate that PKCδ contributes to factor-independent growth of cells expressing the D814Y mutant, but negatively regulates SCF-induced growth of cells expressing wild-type Kit. This is the first demonstration that PKCδ has different functions in cells expressing normal versus oncogenic forms of a receptor.

Functional interaction of protein kinase Calpha with the tyrosine kinases Syk and Src in human platelets

There is a high degree of cross-talk between tyrosine phosphorylation and the serine/threonine phosphorylation signaling pathways. Here we show a physical and functional interaction between the classical protein kinase C isoform (cPKC), PKCalpha, and two major nonreceptor tyrosine kinases in platelets, Syk and Src. In the presence of the cPKC-selective inhibitor Go6976, platelet 5-hydroxytryptamine release was abolished in response to co-activation of glycoproteins VI and Ib-IX-V by the snake venom alboaggregin A, whereas platelet aggregation was substantially inhibited. Of the two platelet cPKCs, PKCalpha but not PKCbeta was activated, occurring in an Syk- and phospholipase C-dependent manner. Syk and PKCalpha associate in a stimulation-dependent manner, requiring Syk but not PKC activity. PKCalpha and Syk also co-translocate from the cytosol to the plasma membrane upon platelet activation, in a manner dependent upon the activities of both kinases. Although PKCalpha is phosphorylated on tyrosine downstream of Syk, we provide evidence against phosphorylation of Syk by PKCalpha, consistent with a lack of effect of PKCalpha inhibition on Syk activity. PKCalpha also associates with Src; although in contrast to interaction with Syk, PKCalpha activity is required for the association of these kinases but not the stimulation-induced translocation of Src to the cell membrane. Finally, the activity of Src is negatively regulated by PKC, as shown by potentiation of Src activity in the presence of the PKC inhibitors GF109203X or Go6976. Therefore, there is a complex interplay between PKCalpha, Syk, and Src involving physical interaction, phosphorylation, translocation within the cell, and functional activity regulation.

Apoptotic effect of PP2 a Src tyrosine kinase inhibitor, in murine B cell leukemia

Src is a non-receptor protein tyrosine kinase that transduces signals regulating cell growth and differentiation. We report here that activation of signaling pathway after blockade of tyrosine phosphorylation by PP2 (4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), a potent and selective inhibitor of the Src-family tyrosine kinase, can lead to cell death in murine B cell leukemia, 70Z/3. Death from PP2 occurred by apoptosis as indicated by the induction of caspase activation and annexin V/propidium iodide staining. Interestingly, PP2 was found to be able to enhance the DNA binding activity of nuclear factor kappaB (NF-kappaB) before induction of apoptosis without accompanying by increased phosphorylation of inhibitor of NF-kappaB-alpha (IkappaB-alpha). Additionally, immunoblotting analysis with PP2-treated cell extract demonstrated that, compared to other protein kinase C (PKC) isotypes, the translocation of novel PKC isotypes from the cytosol to membrane fraction was sustained for a longer time. These data suggest that the inhibition of Src-mediated tyrosine phosphorylation by PP2 may tilt the balance between each PKC isotypes, which in turn, activate NF-kappaB transcription factor, leading to apoptosis.

The enigmatic protein kinase Cdelta: complex roles in cell proliferation and survival

Protein kinase Cdelta (PKCdelta) has been implicated both as a tumor suppressor and a positive regulator of cell cycle progression. PKCdelta has also been reported to positively and negatively regulate apoptotic programs. This has led to conflicting hypotheses on the role of PKCdelta in the control of cell proliferation and survival. Surprisingly, PKCdelta mice develop normally and are fertile, indicating that PKCdelta is not critical for normal cell proliferation during development. However, PKCdelta may play important roles in neoplastic cell proliferation. In this review, we have summarized the apparent multifunctional properties of this enigmatic protein with regard to its role in the regulation of cell cycle progression and cell survival. It is proposed that PKCdelta has both tumor suppressor and proliferation capabilities that can be recruited as a backup kinase for both gatekeeper tumor suppression and as an activator of the Ras/Raf/MEK/MAP kinase signaling pathway in cell proliferation.

Activation of Vascular Endothelial Growth Factor Receptor-3 and Its Downstream Signaling Promote Cell Survival under Oxidative Stress

Reactive oxygen species (ROS) mediate cell damage and have been implicated in the pathogenesis of diseases that involve endothelial injury. Cells possess antioxidant systems, including intracellular antioxidants and ROS scavenging enzymes, that control the redox state and prevent cell damage. In addition to intracellular antioxidants, certain growth factor receptors can be activated under oxidative stress and trigger downstream cell survival signaling cascades. Vascular endothelial growth factor receptor-3 (VEGFR-3) is a primary modulator of lymphatic endothelial proliferation and survival. Here, we provide evidence that activation of VEGFR-3 signaling in response to hydrogen peroxide (H(2)O(2)) promotes endothelial cell survival. Treatment with H(2)O(2) induced the tyrosine phosphorylation of VEGFR-3 and its association with the signaling adaptor proteins Shc, growth factor receptor binding protein 2, Sos, p85, SHP-2, and phospholipase C-gamma. Of note, a hereditary lymphoedema-linked mutant of VEGFR-3 was not phosphorylated by H(2)O(2) treatment. Isoforms of protein kinase C (PKC), alpha and delta, were also tyrosine-phosphorylated after H(2)O(2) stimulation. However, only the delta isoform of PKC was required for H(2)O(2)-induced phosphorylation of VEGFR-3. The tyrosine phosphorylation of VEGFR-3 or isoforms of PKC was completely inhibited by treatment with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, a specific inhibitor for Src family kinases, indicating that Src family kinases are upstream of PKC and VEGFR-3. Furthermore, expression of the wild-type but not the lymphoedema-linked mutant form of VEGFR-3 in porcine artery endothelial cells significantly enhanced the activation of Akt after H(2)O(2) stimulation. Consistent with these biochemical changes, we observed that expression and activation of the wild-type but not the mutant form of VEGFR-3 inhibited H(2)O(2)-induced apoptosis. These studies suggest that VEGFR-3 protects against oxidative damage in endothelial cells, and that patients with hereditary lymphoedema may be susceptible to ROS-induced cell damage.

Pervanadate-induced reverse translocation and tyrosine phosphorylation of phorbol ester-stimulated protein kinase C betaII are mediated by Src-family tyrosine kinases in porcine neutrophils

Protein kinase C (PKC), upon activation, translocates from the cytosol to the plasma membrane. Phorbol 12-myristate 13-acetate (PMA), a potent PKC activator, is known to induce irreversible translocation of PKC to the plasma membrane, in contrast to the reversible translocation resulting from physiological stimuli and subsequent rapid return to the cytosol (reverse translocation). However, we have previously shown that tyrosine phosphatase (PTPase) inhibitors induce reverse translocation of PMA-stimulated PKCbetaII in porcine polymorphonuclear leukocytes (PMNs). In the present study, we showed that pervanadate, a potent PTPase inhibitor, also induces tyrosine phosphorylation of PMA-stimulated PKCbetaII in porcine PMNs. Furthermore, PP2, a specific inhibitor of Src-family tyrosine kinases (PTKs), was found to inhibit both pervanadate-induced reverse translocation and tyrosine phosphorylation of PMA-stimulated PKCbetaII, suggesting that these two pervanadate-induced responses are mediated by Src-family PTKs. Our findings provide novel insight into the relationship between the subcellular localization and tyrosine phosphorylation of PKC.

Cholecystokinin-stimulated protein kinase C-delta kinase activation, tyrosine phosphorylation, and translocation are mediated by Src tyrosine kinases in pancreatic acinar cells

Protein kinase C-delta (PKC-delta) is involved in growth, differentiation, tumor suppression, and regulation of other cellular processes. PKC-delta activation causes translocation, tyrosine phosphorylation, and serine-threonine kinase activity. However, little is known about the ability of G protein-coupled receptors to activate these processes or the mediators involved. In the present study, we explored the ability of the neurotransmitter/hormone, CCK, to stimulate these changes in PKC-delta and explored the mechanisms. In rat pancreatic acini under basal conditions, PKC-delta is almost exclusively located in cytosol. CCK and TPA stimulated a rapid PKC-delta translocation to membrane and nuclear fractions, which was transient with CCK. CCK stimulated rapid tyrosine phosphorylation of PKC-delta and increased kinase activity. Using tyrosine kinase (B44) and a tyrosine phosphatase inhibitor (orthovanadate), changes in both CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation were shown to correlate with changes in its kinase activity but not translocation. Both PKC-delta tyrosine phosphorylation and activation occur exclusively in particulate fractions. The Src kinase inhibitors, SU6656 and PP2, but not the inactive related compound, PP3, inhibited CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation and activation. In contrast, PP2 also had a lesser effect on CCK- but not TPA-stimulated PKC-delta translocation. CCK stimulated the association of Src kinases with PKC-delta, demonstrated by co-immunoprecipitation. These results demonstrate that CCKA receptor activation results in rapid translocation, tyrosine phosphorylation, and activation of PKC-delta. Stimulation of PKC-delta translocation precedes tyrosine phosphorylation, which is essential for activation to occur. Activation of Src kinases is essential for the tyrosine phosphorylation and kinase activation to occur and plays a partial role in translocation.

Physical and functional interaction between protein kinase C delta and Fyn tyrosine kinase in human platelets

An increasing number of tyrosine kinases have been shown to associate with isoforms of the protein kinase C (PKC) family. Here, we show evidence for physical and functional interaction between PKCdelta and the Src family kinase Fyn in human platelets activated by alboaggregin-A, a snake venom capable of activating both GPIb-V-IX and GPVI adhesion receptors. This interaction involves phosphorylation of PKCdelta on tyrosine and is specific in that other isoforms of PKC, PKCepsilon and lambda, which also become tyrosine-phosphorylated, do not interact with Fyn. In addition, PKCdelta does not interact with other platelet-expressed tyrosine kinases Syk, Src, or Btk. Stimulation also leads to activation of both Fyn and PKCdelta and to serine phosphorylation of Fyn within a PKC consensus sequence. Alboaggregin-A-dependent activation of Fyn is blocked by bisindolylmaleimide I, suggesting a role for PKC isoforms in regulating Fyn activity. Platelet activation with alboaggregin-A induces translocation of the two kinases from cytoplasm to the plasma membrane of platelets, as observed by confocal immunofluorescence microscopy. Translocation of Fyn and PKCdelta are blocked by PP1 and bisindolylmaleimide I, showing a dependence upon Src and PKC kinase activities. Although PKC activity is required for translocation, it is not required for association between the two kinases, because this was not blocked by bisindolylmaleimide I. Rottlerin, which inhibited PKCdelta activity, did not block translocation of either PKCdelta or Fyn but potentiated platelet aggregation, 5-hydroxytryptamine secretion, and the calcium response induced by alboaggregin-A, indicating that this kinase plays a negative role in the control of these processes.

Cholecystokinin-stimulated tyrosine phosphorylation of PKC-delta in pancreatic acinar cells is regulated bidirectionally by PKC activation

PKC-delta is important in cell growth, apoptosis, and secretion. Recent studies show its stability is regulated by tyrosine phosphorylation (TYR-P), which can be stimulated by a number of agents. Many of these stimuli also activate phospholipase C (PLC) cascades and little is known about the relationship between these cascades and PKC-delta TYR-P. Cholecystokinin (CCK) stimulates PKCs but it is unknown if it causes PKC-delta TYR-P and if so, the relationship between these cascades is unknown. In rat pancreatic acini, CCK-8 stimulated rapid PKC-delta TYR-P by activation of the low affinity CCK(A) receptor state. TPA had a similar effect. BAPTA did not decrease CCK-stimulated PKC-delta TYR-P but instead, increased it. A23187 did not stimulate PKC-delta TYR-P. Wortmannin and LY 294002 did not alter CCK-stimulated PKC-delta TYR-P. GF 109203X, at low concentrations, increased PKC-delta TYR-P stimulated by CCK or TPA and at higher concentrations, inhibited it. The cPKC inhibitors, Gö 6976 and safingol, caused a similar increase in TPA- and CCK-stimulated PKC-delta TYR-P. These results demonstrate that CCK(A) receptor activation causes PKC-delta TYR-P through activation of only one of its two receptor affinity states. This PKC-delta TYR-P is not directly influenced by changes in [Ca(2+)](i); however, the resultant activation of PKC-alpha has an inhibitory effect. Therefore, CCK activates both stimulatory and inhibitory PKC cascades regulating PKC-delta TYR-P and, hence, likely plays an important role in regulating PKC-delta degradation and cellular abundance.

Constitutively activated phosphatidylinositol-3 kinase (PI-3K) is involved in the defect of apoptosis in B-CLL: association with protein kinase Cdelta

In the present study we analyzed the role of phophatidylinositol-3 kinase (PI-3K) in B chronic lymphocytic leukemia (B-CLL) cells. PI-3K is activated by many stimuli and is linked to several different signaling pathways. We demonstrated that inhibition of PI-3K by a specific inhibitor, LY294002, induced apoptosis in B-CLL cells in vitro. This effect was specific for the inhibition of PI-3K because inhibition of other signaling pathways such as extracellular signaling-regulated kinase (ERK), p38, or p70S6 kinase did not affect spontaneous apoptosis. Furthermore, PI-3K was constitutively activated in freshly isolated B-CLL cells. Corresponding to enhanced apoptosis, LY294002 down-regulated expression of the antiapoptotic proteins X-linked inhibitor of apoptosis protein (XIAP) and Mcl-1. Next, we investigated which factors downstream of PI-3K were activated in B-CLL cells. We demonstrated that protein kinase B/Akt is expressed in all tested CLL samples but no activation of Akt was detected. In contrast, we observed a constitutive activation of protein kinase Cdelta (PKCdelta) in freshly isolated B-CLL cells. PKCdelta is linked to PI-3K and is phosphorylated at Thr505 in response to PI-3K activation. We further demonstrated that tyrosine phosphorylation and activity of PKCdelta were dependent on PI-3K activity in B-CLL cells. Inhibition of PKCdelta by the specific inhibitor Rottlerin strikingly enhanced apoptosis. In contrast, peripheral blood B cells of healthy donors were resistant to inhibition of PI-3K or PKCdelta. We conclude that activated PI-3K might be important in the pathogenesis of B-CLL, and survival signals might be mediated via PKCdelta. Therefore, inhibition of PI-3K or PKCdelta may be an innovative approach to treat B-CLL.

Src family kinases phosphorylate protein kinase C delta on tyrosine residues and modify the neoplastic phenotype of skin keratinocytes

Protein kinase C delta (PKC delta) is tyrosine-phosphorylated and catalytically inactive in mouse keratinocytes transformed by a ras oncogene. In several other model systems, Src kinases are upstream regulators of PKC delta. To examine this relationship in epidermal carcinogenesis, v-ras transformed mouse keratinocytes were treated with a selective Src kinase inhibitor (PD 173958). PD 173958 decreased autophosphorylation of Src, Fyn, and Lyn kinases and prevented tyrosine phosphorylation of the Src kinase substrate p120. PD 173958 also prevented PKC delta tyrosine phosphorylation and activated PKC delta as detected by membrane translocation. Expression of keratinocyte differentiation markers increased in PD 173958-treated v-ras-keratinocytes, and fluid-filled domes emerged, indicative of tight junction formation. Antisense PKC delta or bryostatin 1 inhibited dome formation, while overexpression of PKC delta in the presence of PD 173958 enhanced the formation of domes. Plasmids encoding phenylalanine mutants of PKC delta tyrosine residues 64 and 565 induced domes in the absence of PD 173958, while phenylalanine mutants of tyrosine residues 52, 155, and 187 were inactive. Thus, Src kinase mediated post-translational modification of PKC delta on specific tyrosine residues in ras-transformed mouse keratinocytes inactivates PKC delta and contributes to alterations in the differentiated phenotype and tight junction formation associated with neoplasia.

An important role for protein kinase C-delta in human keratinocyte migration on dermal collagen

Migration of human keratinocytes plays a critical role in the re-epithelialization of human skin wounds, the process by which the wound bed is resurfaced and closed by keratinocytes as it forms a new epidermis. While the importance of ECM components and serum factors in the regulation of keratinocytes motility is well established, the intracellular signaling mechanisms remain fragmentary. In this study, we investigated the role of protein kinase Cdelta (PKCdelta) signaling in the promotion of human keratinocyte migration by a collagen matrix and bovine pituitary extract. We found that pharmacological inhibition of the PKCdelta pathway completely blocks migration. Using a lentivirus-based vector system, which offers more than 90% gene transduction efficiency to human keratinocytes, we show that the kinase-defective mutant of PKCdelta (K376R) dramatically inhibits human keratinocyte migration. Furthermore, PKCdelta is activated in migrating human keratinocytes. These observations indicate for the first time that the PKCdelta pathway plays an important role in the control of human keratinocyte migration.

Nerve growth factor stimulates multisite tyrosine phosphorylation and activation of the atypical protein kinase C's via a src kinase pathway

Atypical protein kinase C (PKC) isoforms are required for nerve growth factor (NGF)-initiated differentiation of PC12 cells. In the present study, we report that PKC-iota becomes tyrosine phosphorylated in the membrane coincident with activation posttreatment with nerve growth factor. Tyrosine phosphorylation and activation of PKC-iota were inhibited in a dose-dependent manner by both PP2 and K252a, src and TrkA kinase inhibitors. Purified src was observed to phosphorylate and activate PKC-iota in vitro. In PC12 cells deficient in src kinase activity, both NGF-induced tyrosine phosphorylation and activation of PKC-iota were also diminished. Furthermore, we demonstrate activation of src by NGF along with formation of a signal complex including the TrkA receptor, src, and PKC-iota. Recruitment of PKC-iota into the complex was dependent on the tyrosine phosphorylation state of PKC-iota. The association of src and PKC-iota was constitutive but was enhanced by NGF treatment, with the src homology 3 domain interacting with a PXXP sequence within the regulatory domain of PKC-iota (amino acids 98 to 114). Altogether, these findings support a role for src in regulation of PKC-iota. Tyrosine 256, 271, and 325 were identified as major sites phosphorylated by src in the catalytic domain. Y256F and Y271F mutations did not alter src-induced activation of PKC-iota, whereas the Y325F mutation significantly reduced src-induced activation of PKC-iota. The functional relevance of these mutations was tested by determining the ability of each mutant to support TRAF6 activation of NF-kappaB, with significant impairment by the Y325F PKC-iota mutant. Moreover, when the Y352F mutant was expressed in PC12 cells, NGF's ability to promote survival in serum-free media was reduced. In summary, we have identified a novel mechanism for NGF-induced activation of atypical PKC involving tyrosine phosphorylation by c-Src.

Nerve growth factor stimulates the interaction of ZIP/p62 with atypical protein kinase C and targets endosomal localization: evidence for regulation of nerve growth factor-induced differentiation

Atypical protein kinase Cs zeta and lambda/iota play a functional role in the regulation of NGF-induced differentiation and survival of pheochromocytoma, PC12 cells [Coleman and Wooten, 1994; Wooten et al., 1999]. Here we demonstrate an NGF-dependent interaction of aPKC with its binding protein, ZIP/p62. Although, ZIP/p62 was not a PKC-iota substrate, the formation of a ZIP/p62-aPKC complex in PC12 cells by NGF occurred post activation of PKC-iota and was regulated by the tyrosine phosphorylation state of aPKC. Furthermore, NGF-dependent localization of ZIP/p62 was observed within vesicular structures, identified as late endosomes by colocalization with a Rab7 antibody. Both ZIP/p62 as well as PKC-iota colocalized with Rab7 upon NGF stimulation. Inhibition of the tyrosine phosphorylation state of PKC-iota did not prevent movement of ZIP/p62 to the endosomal compartment. These observations indicate that the subcellular localization of ZIP/p62 does not depend entirely upon activation of aPKC itself. Of functional importance, transfection of an antisense p62 construct into PC12 cells significantly diminished NGF-induced neurite outgrowth. Collectively, these findings demonstrate that ZIP/p62 acts as a shuttling protein involved in routing activated aPKC to an endosomal compartment and is required for mediating NGF's biological properties.

Involvement of protein kinase Cdelta in contact-dependent inhibition of growth in human and murine fibroblasts

There is evidence that protein kinase C delta (PKCdelta) is a tumor suppressor, although its physiological role has not been elucidated so far. Since important anti-proliferative signals are mediated by cell-cell contacts we studied whether PKCdelta is involved in contact-dependent inhibition of growth in human (FH109) and murine (NIH3T3) fibroblasts. Cell-cell contacts were imitated by the addition of glutardialdehyde-fixed cells to sparsely seeded fibroblasts. Downregulation of the PKC isoforms alpha, delta, epsilon, and mu after prolonged treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1 microM) resulted in a significant release from contact-inhibition in FH109 cells. Bryostatin 1 selectively prevented TPA-induced PKCdelta-downregulation and reversed TPA-induced release from contact-inhibition arguing for a role of PKCdelta in contact-inhibition. In accordance, the PKCdelta specific inhibitor Rottlerin (1 microM) totally abolished contact-inhibition. Interestingly, immunofluorescence revealed a rapid translocation of PKCdelta to the nucleus when cultures reached confluence with a peak in early-mid G1 phase. Nuclear translocation of PKCdelta in response to cell-cell contacts could also be demonstrated after subcellular fractionation by Western blotting and by measuring PKCdelta-activity after immunoprecipitation. Transient transfection of NIH3T3 cells with a dominant negative mutant of PKCdelta induced a transformed phenotype. We conclude that PKCdelta is involved in contact-dependent inhibition of growth.

Phosphorylation sites of protein kinase C delta in H2O2-treated cells and its activation by tyrosine kinase in vitro

Protein kinase C delta (PKC delta) is normally activated by diacylglycerol produced from receptor-mediated hydrolysis of inositol phospholipids. On stimulation of cells with H(2)O(2), the enzyme is tyrosine phosphorylated, with a concomitant increase in enzymatic activity. This activation does not appear to accompany its translocation to membranes. In the present study, the tyrosine phosphorylation sites of PKC delta in the H(2)O(2)-treated cells were identified as Tyr-311, Tyr-332, and Tyr-512 by mass spectrometric analysis with the use of the precursor-scan method and by immunoblot analysis with the use of phosphorylation site-specific antibodies. Tyr-311 was the predominant modification site among them. In an in vitro study, phosphorylation at this site by Lck, a non-receptor-type tyrosine kinase, enhanced the basal enzymatic activity and elevated its maximal velocity in the presence of diacylglycerol. The mutation of Tyr-311 to phenylalanine prevented the increase in this maximal activity, but replacement of the other two tyrosine residues did not block such an effect. The results indicate that phosphorylation at Tyr-311 between the regulatory and catalytic domains is a critical step for generation of the active PKC delta in response to H(2)O(2).

Modulation of PKCδ tyrosine phosphorylation and activity in salivary and PC-12 cells by Src kinases

Protein kinase C (PKC) δ becomes tyrosine phosphorylated in rat parotid acinar cells exposed to muscarinic and substance P receptor agonists, which initiate fluid secretion in this salivary cell. Here we examine the signaling components of PKCδ tyrosine phosphorylation and effects of phosphorylation on PKCδ activity. Carbachol- and substance P-promoted increases in PKCδ tyrosine phosphorylation were blocked by inhibiting phospholipase C (PLC) but not by blocking intracellular Ca2+ concentration elevation, suggesting that diacylglycerol, rather than d- myo-inositol 1,4,5-trisphosphate production, positively modulated this phosphorylation. Stimuli-dependent increases in PKCδ activity in parotid and PC-12 cells were blocked in vivo by inhibitors of Src tyrosine kinases. Dephosphorylation of tyrosine residues by PTP1B, a protein tyrosine phosphatase, reduced the enhanced PKCδ activity. Lipid cofactors modified the tyrosine phosphorylation-dependent PKCδ activation. Two PKCδ regulatory sites (Thr-505 and Ser-662) were constitutively phosphorylated in unstimulated parotid cells, and these phosphorylations were not altered by stimuli that increased PKCδ tyrosine phosphorylation. These results demonstrate that PKCδ activity is positively modulated by tyrosine phosphorylation in parotid and PC-12 cells and suggest that PLC-dependent effects of secretagogues on salivary cells involve Src-related kinases.

Subtype-specific translocation of the delta subtype of protein kinase C and its activation by tyrosine phosphorylation induced by ceramide in HeLa cells

We investigated the functional roles of ceramide, an intracellular lipid mediator, in cell signaling pathways by monitoring the intracellular movement of protein kinase C (PKC) subtypes fused to green fluorescent protein (GFP) in HeLa living cells. C(2)-ceramide but not C(2)-dihydroceramide induced translocation of delta PKC-GFP to the Golgi complex, while alpha PKC- and zeta PKC-GFP did not respond to ceramide. The Golgi-associated delta PKC-GFP induced by ceramide was further translocated to the plasma membrane by phorbol ester treatment. Ceramide itself accumulated to the Golgi complex where delta PKC was translocated by ceramide. Gamma interferon also induced the delta PKC-specific translocation from the cytoplasm to the Golgi complex via the activation of Janus kinase and Mg(2+)-dependent neutral sphingomyelinase. Photobleaching studies showed that ceramide does not evoke tight binding of delta PKC-GFP to the Golgi complex but induces the continuous association and dissociation of delta PKC with the Golgi complex. Ceramide inhibited the kinase activity of delta PKC-GFP in the presence of phosphatidylserine and diolein in vitro, while the kinase activity of delta PKC-GFP immunoprecipitated from ceramide-treated cells was increased. The immunoprecipitated delta PKC-GFP was tyrosine phosphorylated after ceramide treatment. Tyrosine kinase inhibitor abolished the ceramide-induced activation and tyrosine phosphorylation of delta PKC-GFP. These results suggested that gamma interferon stimulation followed by ceramide generation through Mg(2+)-dependent sphingomyelinase induced delta PKC-specific translocation to the Golgi complex and that translocation results in delta PKC activation through tyrosine phosphorylation of the enzyme.

Targeting of the c-Abl tyrosine kinase to mitochondria in the necrotic cell death response to oxidative stress

The ubiquitously expressed c-Abl tyrosine kinase is activated in the response of cells to genotoxic and oxidative stress. The present study demonstrates that reactive oxygen species (ROS) induce targeting of c-Abl to mitochondria. We show that ROS-induced localization of c-Abl to mitochondria is dependent on activation of protein kinase C (PKC)delta and the c-Abl kinase function. Targeting of c-Abl to mitochondria is associated with ROS-induced loss of mitochondrial transmembrane potential. The results also demonstrate that c-Abl is necessary for ROS-induced depletion of ATP and the activation of a necrosis-like cell death. These findings indicate that the c-Abl kinase targets to mitochondria in response to oxidative stress and thereby mediates mitochondrial dysfunction and cell death.