Activation of the epidermal growth factor receptor signal transduction pathway stimulates tyrosine phosphorylation of protein kinase C delta

Epidermal growth factor-induced signaling in breast cancer cells results in selective target gene activation by orphan nuclear receptor estrogen-related receptor alpha

The orphan nuclear hormone receptor estrogen-related receptor alpha (ERRalpha, NR3B1) is a constitutive transcription factor that is structurally and functionally related to the classic estrogen receptors. ERRalpha can recognize both the estrogen response element and its own binding site (ERRE) in either dimeric or monomeric forms. ERRalpha is also a phosphoprotein whose expression in human breast tumors correlates with that of the receptor tyrosine kinase ErbB2, suggesting that its transcriptional activity could be regulated by signaling cascades. Here, we investigated growth factor regulation of ERRalpha function and found that it is phosphorylated in MCF-7 breast cancer cells in response to epidermal growth factor (EGF), an event that enhances its DNA binding. Interestingly, treatment with alkaline phosphatase shifts ERRalpha from a dimeric to a monomeric DNA-binding factor, and only the dimeric form interacts with the coactivator PGC-1alpha. In vitro, the DNA-binding domain of ERRalpha is selectively phosphorylated by protein kinase Cdelta (PKCdelta), which increases its DNA-binding activity, whereas expression of constitutively active PKCdelta enhances TFF1 promoter activity via the ERRE. However, whereas treatment of MCF-7 cells with the phorbol ester phorbol-12-myristate 13-acetate also enhances ERRalpha activation of the TFF1 promoter reporter, it does not affect ERRalpha activity on its own promoter. In agreement, chromatin immunoprecipitation analysis shows that ERRalpha and RNA polymerase II are preferentially recruited to the TFF1 promoter after EGF treatment, whereas recruitment of these factors to its own promoter is not affected. These results reveal a mechanism through which growth factor signaling can selectively activate ERRalpha target genes in breast cancer cells.

A caspase-resistant mutant of PKC-delta protects keratinocytes from UV-induced apoptosis

Keratinocyte apoptosis induced by UV radiation is a major protective mechanism from skin photocarcinogenesis. The induction of apoptosis by UV radiation, as well as a variety of genotoxic stimuli, involves the activation of PKC-delta by caspase-3-mediated cleavage in its hinge region, thus generating a constitutively active catalytic fragment. To determine the role of PKC-delta cleavage in UV apoptosis signaling, we introduced a caspase-resistant PKC-delta mutant (D330A) into human keratinocytes by retrovirus transduction. Overexpression of PKC-delta(D330A) protected keratinocytes from UV-induced apoptosis and enhanced long-term survival. PKC-delta(D330A) partially prevented the release of cytochrome c from the mitochondria and the loss of Mcl-1, a key antiapoptotic protein downregulated during UV apoptosis. Thus, the cleavage and activation of PKC-delta are critical components of UV-induced apoptosis in human keratinocytes, and the inactivation of PKC-delta can promote the survival of keratinocytes exposed to UV radiation.

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.

Roles of Tyrosine Phosphorylation and Cleavage of Protein Kinase Cδ in Its Protective Effect Against Tumor Necrosis Factor-related Apoptosis Inducing Ligand-induced Apoptosis

Protein kinase Cdelta (PKCdelta) regulates cell apoptosis in a cell- and stimulus-specific manner. Here, we studied the role of PKCdelta in the apoptotic effect of TRAIL in glioma cells. We found that transfection of the cells with a PKCdelta kinase-dead mutant (K376R) or with a small interfering RNA targeting the PKCdelta mRNA increased the apoptotic effect of tumor necrosis factor-related apoptosis inducing ligand (TRAIL), whereas overexpression of PKCdelta decreased it. PKCdelta acted downstream of caspase 8 and upstream of cytochrome c release from the mitochondria. TRAIL induced cleavage of PKCdelta within 2-3 h of treatment, which was abolished by caspase 3, 8, and 9 inhibitors. The cleavage of PKCdelta was essential for its protective effect because overexpression of a caspase-resistant mutant (PKCdeltaD327A) did not protect glioma cells from TRAIL-induced apoptosis but rather increased it. TRAIL induced translocation of PKCdelta to the perinuclear region and the endoplasmic reticulum and phosphorylation of PKCdelta on tyrosine 155. Using a PKCdeltaY155F mutant, we found that the phosphorylation of PKCdelta on tyrosine 155 was essential for the cleavage of PKCdelta in response to TRAIL and for its translocation to the endoplasmic reticulum. In addition, phosphorylation of PKCdelta on tyrosine 155 was necessary for the activation of AKT in response to TRAIL. Our results indicate that PKCdelta protects glioma cells from the apoptosis induced by TRAIL and implicate the phosphorylation of PKCdelta on tyrosine 155 and its cleavage as essential factors in the anti-apoptotic effect of PKCdelta.

The C2 domain of PKCdelta is a phosphotyrosine binding domain

In eukaryotic cells, the SH2 and PTB domains mediate protein-protein interactions by recognizing phosphotyrosine residues on target proteins. Here we make the unexpected finding that the C2 domain of PKCdelta directly binds to phosphotyrosine peptides in a sequence-specific manner. We provide evidence that this domain mediates PKCdelta interaction with a Src binding glycoprotein, CDCP1. The crystal structure of the PKCdelta C2 domain in complex with an optimal phosphopeptide reveals a new mode of phosphotyrosine binding in which the phosphotyrosine moiety forms a ring-stacking interaction with a histidine residue of the C2 domain. This is also the first example of a protein Ser/Thr kinase containing a domain that binds phosphotyrosine.

Distinctive activation mechanisms and functions for protein kinase Cdelta

PKCdelta (protein kinase Cdelta) is a serine/threonine kinase that plays a key role in growth regulation and tissue remodelling. Traditional models of PKC activation have focused on lipid cofactors and anchoring proteins that localize the active conformation of PKCdelta to membranes, in close proximity with its target substrates. However, recent studies identify a distinct mode for PKCdelta activation involving tyrosine phosphorylation by Src family kinases. The tyrosine-phosphorylated form of PKCdelta (which accumulates in the soluble fraction of cells exposed to oxidant stress) displays lipid-independent kinase activity and is uniquely positioned to phosphorylate target substrates throughout the cell (not just on lipid membranes). This review summarizes (1) recent progress towards understanding structure-activity relationships for PKCdelta, with a particular focus on the stimuli that induce (and the distinct functional consequences that result from) tyrosine phosphorylation events in PKCdelta's regulatory, hinge and catalytic domains; (2) current concepts regarding the role of tyrosine phosphorylation as a mechanism to regulate PKCdelta localization and actions in mitochondrial and nuclear compartments; and (3) recent literature delineating distinct roles for PKCdelta (relative to other PKC isoforms) in transcriptional regulation, cell cycle progression and programmed cell death (including studies in PKCdelta-/- mice that implicate PKCdelta in immune function and cardiovascular remodelling). Collectively, these studies argue that the conventional model for PKCdelta activation must be broadened to allow for stimulus-specific differences in PKCdelta signalling during growth factor stimulation and oxidant stress.

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.

Phorbol 12-myristate 13-acetate induces epidermal growth factor receptor transactivation via protein kinase Cdelta/c-Src pathways in glioblastoma cells

Both the epidermal growth factor receptor (EGFR) and protein kinase C (PKC) play important roles in glioblastoma invasive growth; however, the interaction between the EGFR and PKC is not well characterized in glioblastomas. Treatment with EGF stimulated global phosphorylation of the EGFR at Tyr(845), Tyr(992), Tyr(1068), and Tyr(1045) in glioblastoma cell lines (U-1242 MG and U-87 MG). Interestingly, phorbol 12-myristate 13-acetate (PMA) stimulated phosphorylation of the EGFR only at Tyr(1068) in the two glioblastoma cell lines. Phosphorylation of the EGFR at Tyr(1068) was not detected in normal human astrocytes treated with the phorbol ester. PMA-induced phosphorylation of the EGFR at Tyr(1068) was blocked by bisindolylmaleimide (BIM), a PKC inhibitor, and rottlerin, a PKCdelta-specific inhibitor. In contrast, Go 6976, an inhibitor of classical PKC isozymes, had no effect on PMA-induced EGFR phosphorylation. Furthermore, gene silencing with PKCdelta small interfering RNA (siRNA), siRNA against c-Src, and mutant c-Src(S12C/S48A) and treatment with a c-Src inhibitor (4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d]pyrimidine) abrogated PMA-induced EGFR phosphorylation at Tyr(1068). PMA induced serine/threonine phosphorylation of Src, which was blocked by both BIM and rottlerin. Inhibition of the EGFR with AG 1478 did not significantly alter PMA-induced EGFR Tyr(1068) phosphorylation, but completely blocked EGF-induced phosphorylation of the EGFR. The effects of PMA on MAPK phosphorylation and glioblastoma cell proliferation were reduced by BIM, rottlerin, the MEK inhibitor U0126, and PKCdelta and c-Src siRNAs. Taken together, our data demonstrate that PMA transactivates the EGFR and increases cell proliferation by activating the PKCdelta/c-Src pathway in glioblastomas.

Involvement of c-Src and protein kinase C delta in the inhibition of Cl(-)/OH- exchange activity in Caco-2 cells by serotonin

Serotonin (5-hydroxytryptamine (5-HT)) is an important neurotransmitter and intercellular messenger regulating various gastrointestinal functions, including electrolyte transport. To date, however, no information is available with respect to its effects on the human intestinal apical anion exchanger Cl(-)/OH- (HCO3-). The present studies were therefore undertaken to examine the direct effects of serotonin on OH- gradient-driven 4,4'-diisothiocyanato-stilbene-2, 2'-disulfonic acid-sensitive 36Cl- uptake utilizing the post-confluent transformed human intestinal epithelial cell line Caco-2. Our results demonstrate that serotonin inhibits Cl(-)/OH- exchange activity in Caco-2 cells via both tyrosine kinase and Ca(2+)-independent protein kinase C delta-mediated pathways involving either 5-HT3 or 5-HT4 receptor subtype. The data consistent with our inference are as follows. (i) The short term treatment of cells with 5-HT (0.1 microM) for 15-60 min significantly decreased Cl(-)/OH- exchange (50-70%, p < 0.05). (ii) The specific agonists for 5-HT3, m-chlorophenylbiguanide, and 5-HT4, 3-(4-allylpiperazin-1-yl)-2-quinoxaline chloronitrile, mimicked the effects of serotonin. (iii) Tropisetron dual inhibitor for both the 5-HT3/4 receptor subtypes significantly blocked the inhibition, whereas specific 5-HT3 (Y-25130) or 5-HT4 receptor (RS39604) antagonist failed to block the inhibitory effects of 5-HT. (iv) The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl ester) had no effect on the serotonin-induced inhibition. (v) The specific protein kinase C (PKC) inhibitors chelerythrine chloride or calphostin C completely blocked the inhibition by 5-HT. (vi) The specific inhibitor for PKC delta, rottlerin, significantly blocked the inhibition by 5-HT. (vii) The specific tyrosine kinase inhibitor, herbimycin, or Src family kinase inhibitor, PP1, abolished the 5-HT-mediated inhibition of Cl(-)/OH- exchange activity. (viii) 5-HT stimulated tyrosine phosphorylation of c-Src kinase and PKC delta.

The interplay between Src family kinases and receptor tyrosine kinases

Src family tyrosine kinases (SFKs) are involved in a diverse array of physiological processes, as highlighted in this review. An overview of how SFKs interact with, and participate in signaling from, receptor tyrosine kinases (RTKs) is discussed. And also, how SFKs are activated by RTKs, and how SFKs, in turn, can activate RTKs, as well as how SFKs can promote signaling from growth factor receptors in a number of ways including participation in signaling pathways required for DNA synthesis, control of receptor turnover, actin cytoskeleton rearrangements and motility, and survival are discussed.

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.

Protein kinase C delta blocks immediate-early gene expression in senescent cells by inactivating serum response factor

Fibroblasts lose the ability to replicate in response to growth factors and become unable to express growth-associated immediate-early genes, including c-fos and egr-1, as they become senescent. The serum response factor (SRF), a major transcriptional activator of immediate-early gene promoters, loses the ability to bind to the serum response element (SRE) and becomes hyperphosphorylated in senescent cells. We identify protein kinase C delta (PKC delta) as the kinase responsible for inactivation of SRF both in vitro and endogenously in senescent cells. This is due to a higher level of PKC delta activity as cells age, production of the PKC delta catalytic fragment, and its nuclear localization in senescent but not in low-passage-number cells. The phosphorylation of T160 of SRF by PKC delta in vitro and in vivo led to loss of SRF DNA binding activity. Both the PKC delta inhibitor rottlerin and ectopic expression of a dominant negative form of PKC delta independently restored SRE-dependent transcription and immediate-early gene expression in senescent cells. Modulation of PKC delta activity in vivo with rottlerin or bistratene A altered senescent- and young-cell morphology, respectively. These observations support the idea that the coordinate transcriptional inhibition of several growth-associated genes by PKC delta contributes to the senescent phenotype.

Bax Activation and Induction of Apoptosis in Human Keratinocytes by the Protein Kinase C δ Catalytic Domain

The constitutively active catalytic domain of protein kinase C (PKC)delta is an apoptotic effector generated by caspase-3 cleavage of full-length PKCdelta in response to a wide variety of apoptotic stimuli, including UV radiation. The PKCdelta catalytic domain induces apoptosis when ectopically expressed, however, the mechanism of apoptosis induction is unclear. We constructed a chimeric protein encoding the PKCdelta catalytic domain fused to a mutated estrogen receptor ligand-binding domain in order to selectively activate the PKCdelta catalytic domain. The enzymatic activity of the PKCdelta catalytic domain fusion protein was induced in human keratinocytes treated with 4-hydroxytamoxifen, and its activation triggered loss of mitochondrial membrane potential and apoptosis. The apoptosis was associated with release of cytochrome c from the mitochondria and caspase activation, and was blocked by caspase inhibitors and the anti-apoptotic proteins Bcl-2, and Bcl-x(L), suggesting a role for mitochondrial pore formation. Consistent with this, the activated PKCdelta catalytic domain triggered the redistribution and activation of Bax, a Bcl-2 family protein that can directly induce cytochrome c release. In summary, despite being an apoptotic effector activated late in the apoptotic cascade, PKCdelta also activates upstream components of the death effector pathway to insure the demise of cells committed 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.

Epidermal Growth Factor Induces Fibronectin Expression in Human Dermal Fibroblasts via Protein Kinase C δ Signaling Pathway

Epidermal growth factor (EGF) and fibronectin are known to play an important role in wound healing. In this study, we demonstrated that EGF upregulates the expression of fibronectin mRNA and protein in human dermal fibroblasts. Actinomycin D, an RNA synthesis inhibitor, significantly blocked basal mRNA expression, but the addition of EGF compensated the blockage. Cycloheximide, a protein synthesis inhibitor, did not block this upregulation by EGF. In addition, the treatment with EGF significantly reduced the degradation rate of fibronectin mRNA. But EGF did not increase fibronectin promoter activity. EGF-mediated induction of fibronectin expression was inhibited by the treatment of fibroblasts with protein kinase C (PKC) inhibitor, Calphostin C and Rottlerin. The transfection of a dominant-negative mutant of PKCdelta into fibroblasts significantly reduced the induction of fibronectin protein expression by EGF. EGF enhanced PKCdelta protein expression and also translocated PKCdelta to the membrane. Rottlerin blocked the EGF-mediated reduction of mRNA degradation rate. These results indicate that EGF-mediated induction of fibronectin expression occurs at the post-transcriptional level and involves PKCdelta signaling pathway.

Differential regulation of alpha6beta4 integrin by PKC isoforms in murine skin keratinocytes

In mammalian epidermis, alpha6beta4 integrin is expressed exclusively on the basal layer localized to the hemidesmosomes, where it interacts extracellularly with the laminin-5 ligand. During differentiation, loss of alpha6beta4 is associated with keratinocyte detachment from the basement membrane and upward migration. The protein kinase C (PKC) family of isoforms participates in regulation of integrin function and is linked to skin differentiation. Exposure of primary murine keratinocytes to PKC activators specifically downregulates alpha6beta4 expression. Utilizing recombinant adenoviruses, we selectively overexpressed skin PKC isoforms in primary keratinocytes. PKCdelta and PKCzeta induced downregulation of alpha6beta4 protein expression, leading to reduced keratinocyte attachment to laminin-5 and enhanced gradual detachment from the underlying matrix. In contrast, PKCalpha upregulated alpha6beta4 protein expression, leading to increased keratinocyte attachment to laminin-5 and to the underlying matrix. Altogether, these results suggest distinct roles for specific PKC isoforms in alpha6beta4 functional regulation during the early stages of skin differentiation.

Stimulus-specific Differences in Protein Kinase Cδ Localization and Activation Mechanisms in Cardiomyocytes

Protein kinase C (PKC) isoforms play key roles in the regulation of cardiac contraction, ischemic preconditioning, and hypertrophy/failure. Models of PKC activation generally focus on lipid cofactor-induced PKC translocation to membranes. This study identifies tyrosine phosphorylation as an additional mechanism that regulates PKC delta actions in cardiomyocytes. Using immunoblot analysis with antibodies to total PKC delta and PKC delta-pY(311), we demonstrate that PKC delta partitions between soluble and particulate fractions (with little Tyr(311) phosphorylation) in resting cardiomyocytes. Phorbol 12-myristate 13-acetate (PMA) promotes PKC delta translocation to membranes and phosphorylation at Tyr(311). H(2)O(2) also increases PKC delta-pY(311) in association with its release from membranes. Both PMA- and H(2)O(2)-dependent increases in PKC delta-pY(311) are mediated by Src family kinases, but they occur via different mechanisms. The H(2)O(2)-dependent increase in PKC delta-pY(311) results from Src activation and increased Src-PKC delta complex formation. The PMA-dependent increase in PKC delta-pY(311) results from a lipid cofactor-induced conformational change that renders PKC delta a better substrate for phosphorylation by precomplexed Src kinases (without Src activation). PKC delta-Y(311) phosphorylation does not grossly alter the kinetics of PMA-dependent PKC delta down-regulation. Rather, tyrosine phosphorylation regulates PKC delta kinase activity. PKC delta is recovered from the soluble fraction of H(2)O(2)-treated cardiomyocytes as a tyrosine-phosphorylated, lipid-independent enzyme with altered substrate specificity. In vitro PKC delta phosphorylation by Src also increases lipid-independent kinase activity. The magnitude of this effect varies, depending upon the substrate, suggesting that tyrosine phosphorylation fine-tunes PKC delta substrate specificity. The stimulus-specific modes for PKC delta signaling identified in this study allow for distinct PKC delta-mediated phosphorylation events and responses during growth factor stimulation and oxidant stress in cardiomyocytes.

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.

Ceramide-induced Apoptosis by Translocation, Phosphorylation, and Activation of Protein Kinase Cδ in the Golgi Complex

Protein kinase C (PKC), a Ca(2+)/phospholipid-dependent protein kinase, is known as a key enzyme in various cellular responses, including apoptosis. However, the functional role of PKC in apoptosis has not been clarified. In this study, we focused on the involvement of PKCdelta in ceramide-induced apoptosis in HeLa cells and examined the importance of spatiotemporal activation of the specific PKC subtype in apoptotic events. Ceramide-induced apoptosis was inhibited by the PKCdelta-specific inhibitor rottlerin and also was blocked by knockdown of endogenous PKCdelta expression using small interfering RNA. Ceramide induced the translocation of PKCdelta to the Golgi complex and the concomitant activation of PKCdelta via phosphorylation of Tyr(311) and Tyr(332) in the hinge region of the enzyme. Unphosphorylatable PKCdelta (mutants Y311F and Y332F) could translocate to the Golgi complex in response to ceramide, suggesting that tyrosine phosphorylation is not necessary for translocation. However, ceramide failed to activate PKCdelta lacking the C1B domain, which did not translocate to the Golgi complex, but could be activated by tyrosine phosphorylation. These findings suggest that ceramide translocates PKCdelta to the Golgi complex and that PKCdelta is activated by tyrosine phosphorylation in the compartment. Furthermore, we utilized species-specific knockdown of PKCdelta by small interfering RNA to study the significance of phosphorylation of Tyr(311) and Tyr(332) in PKCdelta for ceramide-induced apoptosis and found that phosphorylation of Tyr(311) and Tyr(332) is indispensable for ceramide-induced apoptosis. We demonstrate here that the targeting mechanism of PKCdelta, dual regulation of both its activation and translocation to the Golgi complex, is critical for the ceramide-induced apoptotic event.

The synergistic activation of Raf-1 kinase by phorbol myristate acetate and hydrogen peroxide in NIH3T3 cells

We have previously demonstrated that a 33kDa C-terminal fragment of c-Raf-1 underwent a mobility shift in response to hydrogen peroxide (H(2)O(2)) and phorbol myristate acetate (PMA), respectively. In this study, we have demonstrated that H(2)O(2) induced the activation of N-terminal deletion mutant as well as full length Raf-1 kinase. The pharmacological PKC activator PMA also induced a weak increase in Raf-1 kinase activity through PKC-epsilon activation as determined by the transient expression of dominant negative mutants of PKC-epsilon-K436R. Interestingly, H(2)O(2) produced synergistic increase of PMA-stimulated Raf-1 kinase activation after simultaneous treatment of PMA and H(2)O(2). This synergistic activation of Raf-1 kinase was further enhanced by cypermethrin (an inhibitor of protein phosphatase 2B) and dephostatin (tyrosine kinase inhibitor) implying an inhibitory role for these phosphatases in the Raf-1 signaling pathway. Taken together, our data suggest that the synergistic activation of Raf-1 kinase in response to PMA and H(2)O(2) occurs via mechanisms that involve an interaction of Raf-1 kinase and PKC-epsilon, along with a transient phosphorylation of both Raf-1 kinase and PKC.

Induction of cIAP-2 in human colon cancer cells through PKC delta/NF-kappa B

Activation of protein kinase C (PKC) prevents apoptosis in certain cells; however, the mechanisms are largely unknown. Inhibitors of apoptosis (IAP) family members, including NAIP, cIAP-1, cIAP-2, XIAP/hILP, survivin, and BRUCE, block apoptosis by binding and potently inhibiting caspases. Activation of NF-kappa B contributes to cIAP-2 induction; however, the cellular mechanisms regulating cIAP-2 expression have not been entirely defined. In this study, we examined the role of the PKC and NF-kappa B pathways in the regulation of cIAP-2 in human colon cancers. We found that cIAP-2 mRNA levels were markedly increased in human colon cancer cells by treatment with the phorbol ester, phorbol-12-myristate-13-acetate (PMA), or bryostatin 1. Inhibitors of the Ca2+-independent, novel PKC isoforms, but not inhibitors of MAPK, PI3-kinase, or PKA, blocked PMA-stimulated cIAP-2 mRNA expression, suggesting a role of PKC in PMA-mediated cIAP-2 induction. Pretreatment with the PKC delta-selective inhibitor rottlerin or transfection with an antisense PKC delta oligonucleotide inhibited PMA-induced cIAP-2 expression, whereas cotransfection with a PKC delta plasmid induced cIAP-2 promoter activity, which, taken together, identifies a role for PKC delta in cIAP-2 induction. Treatment with the proteasome inhibitor, MG132 or inhibitors of NF-kappa B (e.g. PDTC and gliotoxin), decreased PMA-induced up-regulation of cIAP-2. PMA-induced NF-kappa B activation was blocked by either GF109203x, MG132, PDTC, or gliotoxin. Moreover, overexpression of PKC delta-induced cIAP-2 promoter activity and increased NF-kappa B transactivation, suggesting regulation of cIAP-2 expression by a PKC delta/NF-kappa B pathway. In conclusion, our findings demonstrate a role for a PKC/NF-kappa B-dependent pathway in the regulation of cIAP-2 expression in human colon cancer cells. These data suggest a novel mechanism for the anti-apoptotic function mediated by the PKC delta/NF-kappa B/cIAP-2 pathway in certain cancers.

Role of proteolytic activation of protein kinase Cdelta in oxidative stress-induced apoptosis

Protein kinase Cdelta (PKCdelta), a member of the novel PKC family, is emerging as a redox-sensitive kinase in various cell types. Oxidative stress activates the PKCdelta kinase by translocation, tyrosine phosphorylation, or proteolysis. During proteolysis, caspase-3 cleaves the native PKCdelta (72-74 kDa) into 41-kDa catalytically active and 38-kDa regulatory fragments to persistently activate the kinase. The proteolytic activation of PKCdelta plays a key role in promoting apoptotic cell death in various cell types, including neuronal cells. Attenuation of PKCdelta proteolytic activation by antioxidants suggests that the cellular redox status can influence activation of the proapoptotic kinase. PKCdelta may also amplify apoptotic signaling via positive feedback activation of the caspase cascade. Thus, the dual role of PKCdelta as a mediator and amplifier of apoptosis may be important in the pathogenesis of major neurodegenerative disorders, such as Parkinson's disease, Alzheimer's disease, and Huntington disease.

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.

Genomic structure and promoter analysis of PKC-delta

Protein kinase C-delta (PKC-delta) is a ubiquitously expressed kinase involved in a variety of cellular signaling pathways including cell growth, differentiation, apoptosis, tumor promotion, and carcinogenesis. While signaling pathways downstream of PKC-delta are well studied, the regulation of the gene has not been extensively analyzed. A mouse genomic DNA fragment containing the PKC-delta gene was sequenced by the primer-walking method, and the subsequent DNA sequence data were used as a query to clone Caenorhabditis elegans and human genomic homologs from the publicly available genomic databases. The genomic structures of C. elegans, mouse, rat, and human PKC-delta were analyzed, and the result revealed that PKC-delta genes comprise 12, 18, 19, and 18 exons for C. elegans, mouse, rat, and human, respectively. The translation start methionine resides in the second exon in mouse and human and in the third exon in rat. The first intron between the first exon and the exon with the translation start methionine in mammalian genes represents a very large gap, as long as 17 kb in human, indicating a complexity involved in gene splicing. Overall exon-intron genomic structure is highly conserved among mammals, while significantly diverged in C. elegans. Putative transcription factor binding sites on the 1.7-kb promoter region of the mouse gene suggest that PKC-delta might be involved in spermatogenesis, embryogenesis, development, brain generation, immune response, oxidative environment, and oncogenesis. Studies on the promoter and subsequent biological testing on mouse keratinocytes indicate that tumor necrosis factor (TNF)-alpha increases the expression of PKC-delta, and this correlates with the time of NFkappaB nuclear translocation and activation. This TNF-alpha-mediated upregulation of PKC-delta is repressed in keratinocytes that are preinfected with IkappaB superrepressor adenovirus, suggesting that NFkappaB is involved directly in PKC-delta expression.

Overexpression of the calcium sensing receptor accelerates epidermal differentiation and permeability barrier formation in vivo

The calcium sensing receptor (CaSR) has emerged as an important mediator of a wide range of Ca(2+)-dependent physiological responses (Ca(2+) signaling) in various tissues. To explore the role of CaSR in the epidermis, we utilised the keratin 14 promoter to express CaSR cDNA constitutively in the basal cells of the stratified squamous epithelium of transgenic mice. Analysis of the transgenic mice revealed that a sensitized response to CaSR signaling accelerates the epidermal differentiation program with the precocious formation of the epidermal permeability barrier (EPB) during development and an accelerated hair growth at birth. Our observations indicate that overexpression of CaSR in the undifferentiated basal cells leads to changes in the differentiation program of the transgenic epidermis, including the stimulation of keratins 1 and 6 as well as the overexpression of several markers of terminal differentiation such as filaggrin, loricrin and involucrin. Our data suggest that the observed modifications in the differentiation pathway are a consequence of a CaSR-induced enhancement of Ca(2+) signaling involving cross-talk with other signaling pathways (e.g. EGF and Wnt/Ca(2+)). These studies provide new insights into the role of CaSR in epidermal differentiation including EPB development and hair follicle morphogenesis.

M-CSF induced differentiation of myeloid precursor cells involves activation of PKC-delta and expression of Pkare

Macrophage-colony stimulating factor (M-CSF) regulates proliferation and differentiation of cells belonging to the monocytic lineage. We investigated the mechanisms of M-CSF differentiation signaling in follicular dendritic cell-P1 cells and analyzed the catalytic activation of different protein kinase C (PKC) isoforms. M-CSF induced rapid catalytic activation of PKC-delta and membrane translocation of the tyrosine phosphorylated form of PKC-delta. Mutation of tyrosine 807 in the M-CSF receptor (Fms) abrogates cell differentiation but not a proliferative response to M-CSF, and FmsY807F failed to activate PKC-delta. We also investigated the downstream signaling pathways from PKC-delta. A cyclic adenosine monophosphate-regulated Ser/Thr kinase gene, protein kinase X (PRKX), has been associated with macrophage differentiation in human cells. We found that M-CSF and PKC-delta induced the expression of the PRKX murine homologue: PKA-related gene. Taken together, our results indicate that PKC-delta functions as a critical mediator of M-CSF-induced differentiation signaling.

Interaction of palmitoylcarnitine with protein kinase C in neuroblastoma NB-2a cells

As reported previously [Acta Neurobiol. Exp. 57 (1997) 263], palmitoylcarnitine was observed to promote differentiation of neuroblastoma NB-2a cells with a concomitant inhibition of proliferation and of the phorbol ester stimulated activity of the protein kinase C (PKC). In the present study, palmitoylcarnitine was observed to inhibit phosphorylation of the PKC peptide substrate and to completely diminish binding of phorbol 12-myristate-13-acetate (PMA), although the effect was found to be uncompetitive. The exposure of NB-2a cells to palmitoylcarnitine in the presence of PMA resulted in a dramatic decrease in phosphorylation of the conventional and novel isozymes of PKC, mainly on serine. This effect was observed to be dose dependent. Inhibitors of serine/threonine phosphatases were not influencing the effect of palmitoylcarnitine what can point to an interaction between PKC and palmitoylcarnitine, affecting the process of autophosphorylation. These findings suggest that pamitoylcarnitine could be a natural modulator of PKC activity, thus regulating the process of cell differentiation.

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.

Activation of SAPK/JNK signaling by protein kinase Cdelta in response to DNA damage

The cellular response to genotoxic stress includes activation of protein kinase Cdelta (PKCdelta). The functional role of PKCdelta in the DNA damage response is unknown. The present studies demonstrate that PKCdelta is required in part for induction of the stress-activated protein kinase (SAPK/JNK) in cells treated with 1-beta-d-arabinofuranosylcytosine (araC) and other genotoxic agents. DNA damage-induced SAPK activation was attenuated by (i) treatment with rottlerin, (ii) expression of a kinase-inactive PKCdelta(K-R) mutant, and (iii) down-regulation of PKCdelta by small interfering RNA (siRNA). Coexpression studies demonstrate that PKCdelta activates SAPK by an MKK7-dependent, SEK1-independent mechanism. Previous work has shown that the nuclear Lyn tyrosine kinase activates the MEKK1 --> MKK7 --> SAPK pathway but not through a direct interaction with MEKK1. The present results extend those observations by demonstrating that Lyn activates PKCdelta, and in turn, MEKK1 is activated by a PKCdelta-dependent mechanism. These findings indicate that PKCdelta functions in the activation of SAPK through a Lyn --> PKCdelta --> MEKK1 --> MKK7 --> SAPK signaling cascade in response to DNA damage.

Rottlerin inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes by uncoupling mitochondrial oxidative phosphorylation

There is increasing evidence that protein kinase C (PKC) isoforms modulate insulin-signaling pathways in both positive and negative ways. Recent reports have indicated that the novel PKCdelta mediates some of insulin's actions in muscle and liver cells. Many studies use the specific inhibitor rottlerin to demonstrate the involvement of PKCdelta. In this study, we investigated whether PKCdelta might play a role in 3T3-L1 adipocytes. We found that PKCdelta is highly expressed in mouse adipose tissue and increased on 3T3-L1 adipocyte differentiation, and insulin-stimulated glucose transport is blocked by rottlerin. The phosphorylation state and activity of PKCdelta are not altered by insulin, but the protein translocates to membranes following insulin treatment. In contrast to the results with rottlerin, inhibition of PKCdelta activity or expression has no effect on glucose transport in adipocytes, unlike muscle cells. Lastly, we found that rottlerin lowers adenosine triphosphate levels in 3T3-L1 cells by acting as a mitochondrial uncoupler, and this is responsible for the observed inhibition of glucose transport.

Calcium: a potential central regulator in wound healing in the skin

Calcium has an established role in the normal homeostasis of mammalian skin and serves as a modulator in keratinocyte proliferation and differentiation. Gradients of calcium concentration increasing from 0.5 mM in the basal layer to > 1.4 mM in the stratum granulosum are consistent with migration patterns in response to minor abrasion (normal wear). Dermal fibroblasts require calcium but are approximately 100 times less sensitive than keratinocytes. Normal calcium metabolism in the skin is dependent on cell membrane and cytosolic calcium binding proteins (calmodulin, cadherins, etc.), but their modulation through parathyroid hormone, vitamin D or growth factors in normal or damaged tissue is not well documented. In wound repair, calcium is predominantly involved as Factor IV in the hemostatic phase, but it is expected to be required in epidermal cell migration and regeneration patterns in later stages of healing. Calcium alginate dressings are designed to liberate calcium early in the acute phase to promote hemostasis, but it is presently unclear whether the supplementary calcium influences the intracellular environment at later stages of wound repair, notably during the remodeling phase. Although experimental studies suggest that control of calcium is obligatory in wound management, we know very little as to how calcium in the wound bed is modulated through hormones, vitamin D, or various growth factors. Also, there is limited information as to how calcium released either from dressings, platelets, or from the circulation through the action of parathyroid hormone, growth factors or other modulators influences cell migration and remodeling in skin wounds, although experimental models suggest that management of calcium is essential in wound management.

Dual role of protein kinase C on mitogen-activated protein kinase activation and human keratinocyte proliferation

Subconfluent normal human keratinocytes exhibit autonomous (autocrine growth factor driven) proliferation and express the specific markers for keratinocyte proliferation K#5 and K#14. In keratinocyte autocrine culture, the exogenously added epidermal growth factor (EGF) has no effect on cell proliferation and mitogen-activated protein kinase (MAPK) activity. PD98059 inhibits MAPK pathway and autocrine keratinocyte proliferation. Staurosporine and Gö6976 strongly inhibit autonomous keratinocyte proliferation. In contrast, Gö6983 (which does not inhibit PKC micro ) inhibits only 20% of autocrine keratinocyte proliferation. Staurosporine inhibits MAPK activity, whereas Gö6976 and Gö6983 strongly increase it. We have concluded that MAPK, PKC micro and probably PKCalpha take part in autocrine keratinocyte proliferation. The effect of Gö6976 and Gö6983 on MAPK activity could be explained by the inhibition of PKC-dependent MAPK-phosphatase expression. The effect of staurosporine could be explained by its paradoxical action (activation) on protein kinase C (PKC) in keratinocytes.

Infection of glioma cells with Sindbis virus induces selective activation and tyrosine phosphorylation of protein kinase C delta. Implications for Sindbis virus-induced apoptosis

Sindbis virus (SV) is an alpha virus used as a model for studying the role of apoptosis in virus infection. In this study, we examined the role of protein kinase C (PKC) in the apoptosis induced by SVNI, a virulent strain of SV. Infection of C6 cells with SVNI induced a selective translocation of PKCdelta to the endoplasmic reticulum and its tyrosine phosphorylation. The specific PKCdelta inhibitor rottlerin and a PKCdelta kinase-dead mutant increased the apoptosis induced by SVNI. To examine the role of the tyrosine phosphorylation of PKCdelta in the apoptosis induced by SVNI we used a PKCdelta mutant in which five tyrosine residues were mutated to phenylalanine (PKCdelta5). PKCdelta5-overexpressing cells exhibited increased apoptosis in response to SVNI as compared with control cells and to cells overexpressing PKCdelta. SVNI also increased the cleavage of caspase 3 in cells overexpressing PKCdelta5 but did not induce cleavage of PKCdelta or PKCdelta5. Using single tyrosine mutants, we identified tyrosines 52, 64, and 155 as the phosphorylation sites associated with the apoptosis induced by SVNI. We conclude that PKCdelta exerts an inhibitory effect on the apoptosis induced by SV and that phosphorylation of PKCdelta on specific tyrosines is required for this function.

Protein kinase C delta regulates function of the DF3/MUC1 carcinoma antigen in beta-catenin signaling

The DF3/MUC1 mucin-like glycoprotein is aberrantly overexpressed in most human carcinomas. The MUC1 cytoplasmic domain interacts directly with beta-catenin, a component of the adherens junction of mammalian epithelial cells. The present results demonstrate that MUC1 associates with protein kinase Cdelta (PKCdelta). A TDR sequence adjacent to the beta-catenin binding motif in the MUC1 cytoplasmic domain functions as a site for PKCdelta phosphorylation. We show that phosphorylation of MUC1 by PKCdelta increases binding of MUC1 and beta-catenin in vitro and in vivo. The functional significance of the MUC1-PKCdelta interaction is further supported by the demonstration that mutation of the PKCdelta phosphorylation site abrogates MUC1-mediated decreases in binding of beta-catenin to E-cadherin. We also show that the stimulatory effects of MUC1 on anchorage-independent growth are abrogated by mutation of the PKCdelta phosphorylation site. These findings support a novel role for PKCdelta in regulating the interaction between MUC1 and the beta-catenin signaling pathway.

Calcium-dependent involucrin expression is inversely regulated by protein kinase C (PKC)alpha and PKCdelta

Calcium is an important physiologic regulator of keratinocyte function that may regulate keratinocyte differentiation via modulation of protein kinase C (PKC) activity. PKCalpha and PKCdelta are two PKC isoforms that are expressed at high levels in keratinocytes. In the present study, we examine the effect of PKCdelta and PKCalpha on calcium-dependent keratinocyte differentiation as measured by effects on involucrin (hINV) gene expression. Our studies indicate that calcium increases hINV promoter activity and endogenous hINV gene expression. This response requires PKCdelta, as evidenced by the observation that treatment with dominant-negative PKCdelta inhibits calcium-dependent hINV promoter activity, whereas wild type PKCdelta increases activity. PKCalpha, in contrast, inhibits calcium-dependent hINV promoter activation, a finding that is consistent with the ability of dominant-negative PKCalpha and the PKCalpha inhibitor, Go6976, to increase hINV gene expression. The calcium-dependent regulatory response is mediated by an AP1 transcription factor-binding site located within the hINV promoter distal regulatory region that is also required for PKCdelta-dependent regulation; moreover, both calcium and PKCdelta produce similar, but not identical, changes in AP1 factor expression. A key question is whether calcium directly influences PKC isoform function. Our studies show that calcium does not regulate PKCalpha or delta levels or cause a marked redistribution to membranes. However, tyrosine phosphorylation of PKCdelta is markedly increased following calcium treatment. These findings suggest that PKCalpha and PKCdelta are required for, and modulate, calcium-dependent keratinocyte differentiation in opposing directions.

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.

Nerve growth factor activates kinases that phosphorylate atypical protein kinase C

Activation of atypical protein kinase C by nerve growth factor (NGF) involves phosphorylation. In order to identify kinases that regulate atypical PKC (aPKC), we surveyed PC12 cell lysates for protein kinases that are activated by NGF and which could phosphorylate aPKC. Employing an in-gel kinase assay where aPKC-zeta was copolymerized within the gel matrix as a substrate, three kinases, pp175, pp87 and pp60, were identified as enzymes that phosphorylated aPKC. Phosphorylation of aPKC by these three kinases coincided with NGF-induced activation of the enzyme. Each kinase possessed a unique subcellular distribution pattern and could be activated by either ceramide or H(2)0(2), second messengers that mimic NGF signaling events. Upstream, pp175 and pp60 lie in a ras pathway, whereas pp87 lies in a pathway dependent upon src. Altogether, these findings reveal that the aPKCs are subject to regulation by a novel group of kinases.

Tyrosine phosphorylation of protein kinase Cdelta is essential for its apoptotic effect in response to etoposide

Protein kinase Cdelta (PKCdelta) is involved in the apoptosis of various cells in response to diverse stimuli. In this study, we characterized the role of PKCdelta in the apoptosis of C6 glioma cells in response to etoposide. We found that etoposide induced apoptosis in the C6 cells within 24 to 48 h and arrested the cells in the G(1)/S phase of the cell cycle. Overexpression of PKCdelta increased the apoptotic effect induced by etoposide, whereas the PKCdelta selective inhibitor rottlerin and the PKCdelta dominant-negative mutant K376R reduced this effect compared to control cells. Etoposide-induced tyrosine phosphorylation of PKCdelta and its translocation to the nucleus within 3 h was followed by caspase-dependent cleavage of the enzyme. Using PKC chimeras, we found that both the regulatory and catalytic domains of PKCdelta were necessary for its apoptotic effect. The role of tyrosine phosphorylation of PKCdelta in the effects of etoposide was examined using cells overexpressing a PKCdelta mutant in which five tyrosine residues were mutated to phenylalanine (PKCdelta5). These cells exhibited decreased apoptosis in response to etoposide compared to cells overexpressing PKCdelta. Likewise, activation of caspase 3 and the cleavage of the PKCdelta5 mutant were significantly lower in cells overexpressing PKCdelta5. Using mutants of PKCdelta altered at individual tyrosine residues, we identified tyrosine 64 and tyrosine 187 as important phosphorylation sites in the apoptotic effect induced by etoposide. Our results suggest a role of PKCdelta in the apoptosis induced by etoposide and implicate tyrosine phosphorylation of PKCdelta as an important regulator of this effect.

Antitumor activity of protein kinase C inhibitors and cisplatin in human head and neck squamous cell carcinoma lines

Protein kinase C (PKC) plays a pivotal role in signal transduction involved in the control of cell proliferation, differentiation and apoptosis. Interference with such signaling pathways may result in altered tumor cell response to antineoplastic drugs. We investigated the effects of two selective PKC inhibitors as single agents and in combination with cisplatin in cell lines derived from squamous cell carcinomas of the head and neck (SCCHN). Safingol (Saf) is directed against the regulatory domain, whereas chelerythrine (Che) interacts with the catalytic domain of PKC. In six SCCHN cell lines (UM-SCC 11B, 14A, 14C and 22B, 8029NA, and a 5-fold cisplatin-resistant subline 8029DDP). PKC activities ranged between 1 and 158 IU/1 x 10(7) cells, and they were inversely proportional to the amount of cellular epidermal growth factor receptor. Using the colorimetric MTT assay, PKC inhibitors Saf and Che showed comparable dose-dependent growth inhibition. The 50% inhibitory concentrations (IC50) were between 3.8-8.6 microM for Saf and 8.5-13.6 microM for Che with no relationship to PKC activity or cisplatin sensitivity of the respective cell lines. Combinations of cisplatin (IC50 = 0.4-5.8 microg/ml) and either PKC inhibitor (5 microM Saf, 10 microM Che) led to a significant decrease of cisplatin IC50 values in most cell lines. However, comparison with theoretical additive dose-response curves showed additive rather than synergistic effects for both PKC inhibitors.

Peroxisome proliferator-activated receptor gamma ligands inhibit mitogenic induction of p21(Cip1) by modulating the protein kinase Cdelta pathway in vascular smooth muscle cells

The cyclin-dependent kinase inhibitor p21(Cip1) is up-regulated in response to mitogenic stimulation in various cells. PPARgamma ligands troglitazone (TRO, 10 microm) and rosiglitazone (RSG, 10 microm) attenuated the induction of p21(Cip1) protein by platelet-derived growth factor (PDGF) and insulin without affecting cognate mRNA levels in rat aortic smooth muscle cells (RASMC). The protein kinase Cdelta (PKCdelta) inhibitor rottlerin also blocked the induction of p21(Cip1) protein, whereas the conventional PKC isotype inhibitor Gö 6976 had no effect. Kinetic studies using the protein synthesis inhibitor cycloheximide showed that TRO, RSG, and rottlerin shortened the half-life of p21(Cip1) protein. TRO, RSG, and rottlerin inhibited PDGF-induced expression of p21(Cip1), but they did not affect insulin-induced expression of p21(Cip1). Both ligands inhibited PKCdelta enzymatic activity in PDGF-stimulated RASMC but not in insulin-stimulated cells. Adenovirus-mediated overexpression of PKCdelta rescued the down-regulation of p21(Cip1) expression both by TRO and RSG in PDGF-treated RASMC. These data suggested that the PKCdelta pathway plays a critical role in PDGF-induced expression of p21(Cip1) in RASMC and may be the potential target for PPARgamma ligand effects. Src kinase-dependent tyrosine phosphorylation of PKCdelta was decreased substantially by TRO and RSG. Tyrosine phosphorylation and activation of c-Src in response to PDGF were unaffected by either PPARgamma ligand. Protein-tyrosine-phosphatase inhibitors sodium orthovanadate and dephostatin prevented PPARgamma ligand effects on PKCdelta tyrosine phosphorylation and enzymatic activity. Both inhibitors also reversed PPARgamma ligand effects on p21(Cip1) expression in PDGF-treated RASMC. PPARgamma ligands enhanced protein-tyrosine-phosphatase activity in RASMC, which may be the mechanism for decreased PKCdelta tyrosine phosphorylation and activity. PPARgamma ligands regulate p21(Cip1) at a post-translational level by blocking PKCdelta signaling and accelerating p21(Cip1) turnover.

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.

Rottlerin is a mitochondrial uncoupler that decreases cellular ATP levels and indirectly blocks protein kinase Cdelta tyrosine phosphorylation

Protein kinase Cdelta (PKCdelta) is activated by stimuli that increase its tyrosine phosphorylation, including neurotransmitters that initiate fluid secretion in salivary gland (parotid) epithelial cells. Rottlerin, a compound reported to be a PKCdelta-selective inhibitor, rapidly increased the rate of oxygen consumption (QO2) of parotid acinar cells and PC12 cells. In parotid cells, this was distinct from the effects of the muscarinic receptor ligand carbachol, which promoted a sodium pump-dependent increase in respiration. Rottlerin increased the QO2 of isolated rat liver mitochondria to a level similar to that produced when oxidative phosphorylation was initiated by ADP or when mitochondria were uncoupled by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). The effects of rottlerin on mitochondrial QO2 were neither mimicked nor blocked by the PKC inhibitor GF109203X. Rottlerin was not effective in blocking PKCdelta activity in vitro. Exposure of freshly isolated parotid acinar cells to rottlerin and FCCP reduced cellular ATP levels and reduced stimuli-dependent increases in tyrosine phosphorylation of PKCdelta. Neither rottlerin nor FCCP reduced stimuli-dependent PKCdelta tyrosine phosphorylation in RPG1 cells (a salivary ductal line) or PC12 cells, consistent with their dependence on glycolysis rather than oxidative phosphorylation for energy-dependent processes. These results demonstrate that rottlerin directly uncouples mitochondrial respiration from oxidative phosphorylation. Previous studies using rottlerin should be evaluated cautiously.

Inhibition of protein kinase C delta has negative effect on anchorage-independent growth of BCR-ABL-transformed Rat1 cells

The BCR-ABL oncoprotein transmits transformation signals mainly through pathways involving Ras, Myc and PI3 kinase. Here we report that inhibition of protein kinase C (PKC) delta had negative influence on anchorage-independent growth of Rat1 cells transformed by BCR-ABL. The effect was observed with delta isoform-specific inhibitor rottlerin, but not with Go6976 that inhibits only conventional isoforms. The kinase activity of delta isoform was found to be roughly two-fold higher in BCR-ABL-expressing Rat1 cells than that in mock. Although overexpression of wild type PKC delta did not enhance soft agar colony number by BCR-ABL-transformed Rat1 cells, that of dominant-negative delta isoform reduced it by approximately 40%.

CTP:phosphocholine cytidylyltransferase inhibition by ceramide via PKC-alpha, p38 MAPK, cPLA2, and 5-lipoxygenase

In a companion paper (Vivekananda J, Smith D, and King RJ. Am J Physiol Lung Cell Mol Physiol 281: L98-L107, 2001), we demonstrated that tumor necrosis factor (TNF)-alpha inhibited the activity of CTP:phosphocholine cytidylyltransferase (CT), the rate-limiting enzyme in the de novo synthesis of phosphatidylcholine (PC), and that its actions were likely exerted through a metabolite of sphingomyelin. In this paper, we explore the signaling pathway employed by TNF-alpha using C2 ceramide as a cell-penetrating sphingolipid representative of the metabolites induced by TNF-alpha. We found that in H441 cells, as reported in other cell types, cytosolic phospholipase A2 (cPLA2) is activated by TNF-alpha. We also observed that the inhibiting action of C2 ceramide on CT requires protein kinase C-alpha, p38 mitogen-activated protein kinase, and cPLA2. The actions of C2 ceramide on CT activity can be duplicated by adding 2 microM lysoPC to these cells. Furthermore, we found that the effects of C2 ceramide are dependent on 5-lipoxygenase but that cyclooxygenase II is unimportant. We hypothesize that CT activity is inhibited by the lysoPC generated as a consequence of the activation of cPLA2 by protein kinase C-alpha and p38 mitogen-activated protein kinase. The other product of the activation of cPLA2, arachidonic acid, is a substrate for the synthesis of leukotrienes, which raise intracellular Ca2+ levels and complete the activation of cPLA2.

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.