Protein kinase C activates the MEK-ERK pathway in a manner independent of Ras and dependent on Raf

Prostaglandin E2 induces fibroblast growth factor 9 via EP3-dependent protein kinase Cdelta and Elk-1 signaling

Fibroblast growth factor 9 (FGF-9) is a potent mitogen that controls the proper development of many tissues and organs. In contrast, aberrant expression of FGF-9 also results in the evolution of many human diseases, such as cancers and endometriosis. Despite its vital function being reported, the cellular and molecular mechanisms responsible for the regulation of FGF-9 expression are mostly unknown. We report here that prostaglandin E2 (PGE2) induces expression of FGF-9, which promotes endometriotic stromal cell proliferation, through the EP3 receptor-activated protein kinase Cdelta (PKCdelta) signaling pathway. Activation of PKCdelta leads to phosphorylation of ERK1/2, and the transcription factor Elk-1 thereby promotes transcription of FGF-9. Two Elk-1 cis-binding sites located at nucleotides -1324 to -1329 and -1046 to -1051 of the human FGF-9 promoter are identified as crucial for mediating PGE2 actions. Collectively, we demonstrate, for the first time, that PGE2 can directly induce FGF-9 expression via a novel signaling pathway involving EP3, PKCdelta, and a member of the ETS domain-containing transcription factor superfamily in primary human endometriotic stromal cells. Our findings may also provide a molecular framework for considering roles for PGE2 in FGF-9-related embryonic development and/or human diseases.

Oncogenic tyrosine kinase NPM/ALK induces activation of the MEK/ERK signaling pathway independently of c-Raf

The mechanisms of cell transformation mediated by the highly oncogenic, chimeric NPM/ALK tyrosine kinase remain only partially understood. Here we report that cell lines and native tissues derived from the NPM/ALK-expressing T-cell lymphoma (ALK+ TCL) display phosphorylation of the extracellular signal-regulated protein kinase (ERK) 1/2 complex. Transfection of BaF3 cells with NPM/ALK induces phosphorylation of EKR1/2 and of its direct activator mitogen-induced extracellular kinase (MEK) 1/2. Depletion of NPM/ALK by small interfering RNA (siRNA) or its inhibition by WHI-154 abrogates the MEK1/2 and ERK1/2 phosphorylation. The NPM/ALK-induced MEK/ERK activation is independent of c-Raf as evidenced by the lack of MEK1/2 and ERK1/2 phosphorylation upon c-Raf inactivation by two different inhibitors, RI and ZM336372, and by its siRNA-mediated depletion. In contrast, ERK1/2 activation is strictly MEK1/2 dependent as shown by suppression of the ERK1/2 phosphorylation by the MEK1/2 inhibitor U0126. The U0126-mediated inhibition of ERK1/2 activation impaired proliferation and viability of the ALK+ TCL cells and expression of antiapoptotic factor Bcl-xL and cell cycle-promoting CDK4 and phospho-RB. Finally, siRNA-mediated depletion of both ERK1 and ERK2 inhibited cell proliferation, whereas depletion of ERK 1 (but not ERK2) markedly increased cell apoptosis. These findings identify MEK/ERK as a new signaling pathway activated by NPM/ALK and indicate that the pathway represents a novel therapeutic target in the ALK-induced malignancies.

P2Y12 receptor signalling towards PKB proceeds through IGF-I receptor cross-talk and requires activation of Src, Pyk2 and Rap1

Previously it was shown that stimulation of the P2Y12 receptor activates PKB signalling in C6 glioma cells [K. Van Kolen and H. Slegers, J. Neurochem. 89, 442.]. In the present study, the mechanisms involved in this response were further elucidated. In cells transfected with the Gbetagamma-scavenger beta-ARK1/GRK2 or Rap1GAPII, stimulation with 2MeSADP failed to enhance PKB phosphorylation demonstrating that the signalling proceeds through Gbetagamma-subunits and Rap1. Moreover, Rap1-GTP pull-down assays revealed that P2Y12 receptor stimulation induced a rapid activation of Rap1. Treatment of cells with the Ca2+ chelator BAPTA-AM and inhibition of Src and PLD2 with PP2 or 1-butanol, respectively, abrogated P2Y12 receptor-mediated activation of Rap1 and PKB. In addition inhibition of PKCzeta decreased basal and 2MeSADP-stimulated phosphorylation of PKB indicating a role for this PKC isoform in PKB signalling. Although the increased PKB phosphorylation was abolished in the presence of the IGF-I receptor tyrosine kinase inhibitor AG 1024, 2MeSADP did not significantly increase receptor phosphorylation. Nevertheless, phosphorylation of a 120 kDa IGF-I receptor-associated protein was observed. The latter protein was identified by MALDI-TOF/TOF-MS as the proline-rich tyrosine kinase 2 (Pyk2) that co-operates with Src in a PLD2-dependent manner. Consistent with the signalling towards Rap1 and PKB, activation of Pyk2 was abrogated by Ca2+ chelation, inhibition of PLD2 and IGF-I receptor tyrosine kinase activity. In conclusion, the data reveal a novel type of cross-talk between P2Y12 and IGF-I receptors that proceeds through Gbetagamma-, Ca2+-and PLD2-dependent activation of the Pyk2/Src pathway resulting in GTP-loading of Rap1 required for an increased PKB phosphorylation.

ER calcium discharge stimulates GDNF gene expression through MAPK-dependent and -independent pathways in rat C6 glioblastoma cells

Glial cell line-derived neurotrophic factor (GDNF), a neurotrophic and differentiation factor, is expressed under several pathophysiological conditions but its regulatory signals have not yet been clarified. Here, we found that endoplasmic reticulum (ER) Ca(2+) discharge by thapsigargin induced GDNF mRNA as well as COX2 and GRP78 expression in rat C6 glioblastoma cells. GDNF mRNA was immediately induced and peaked at 2h by thapsigargin, and the alternative transcript consisting of exon 3 and exon 4 appeared to be most inducible. In spite of intracellular Ca(2+) perturbation, Ca(2+)-dependent PKC was not responsible for this induction. Instead, a PKCdelta-specific inhibitor, rottlerin, suppressed the thapsigargin-induced GDNF mRNA expression. On the other hand, thapsigargin transiently enhanced phosphorylation status of mitogen-activated protein kinase (MAPK) pathway, including extracellular signal-regulated kinase (Erk), p38 MAPK and c-JUN amino-terminal kinase1 (JNK1) simultaneously; whereas specific inhibitors against MEK1 and JNK only reduced the thapsigargin-induced GDNF mRNA expression. In addition, a pan-PKC inhibitor (Ro-31-8220) attenuated the thapsigargin-enhanced phosphorylation levels of Erk1/2 and JNK1, whereas rottlerin did not. Thus, the present study demonstrated that the thapsigargin-stimulated ER Ca(2+) discharge up-regulated GDNF gene expression through both MAPK-dependent and -independent pathways in C6 glioblastoma cells.

Live-cell imaging of endogenous Ras-GTP illustrates predominant Ras activation at the plasma membrane

Ras-GTP imaging studies using the Ras-binding domain (RBD) of the Ras effector c-Raf as a reporter for overexpressed Ras have produced discrepant results about the possible activation of Ras at the Golgi apparatus. We report that RBD oligomerization provides probes for visualization of endogenous Ras-GTP, obviating Ras overexpression and the side effects derived thereof. RBD oligomerization results in tenacious binding to Ras-GTP and interruption of Ras signalling. Trimeric RBD probes fused to green fluorescent protein report agonist-induced endogenous Ras activation at the plasma membrane (PM) of COS-7, PC12 and Jurkat cells, but do not accumulate at the Golgi. PM illumination is exacerbated by Ras overexpression and its sensitivity to dominant-negative RasS17N and pharmacological manipulations matches Ras-GTP formation assessed biochemically. Our data illustrate that endogenous Golgi-located Ras is not under the control of growth factors and argue for the PM as the predominant site of agonist-induced Ras activation.

BASH-novel PKC-Raf-1 pathway of pre-BCR signaling induces kappa gene rearrangement

The pre-B-cell receptor (pre-BCR) is thought to signal transcriptional activation of the immunoglobulin light (L) chain gene locus, proceeding to its V-J rearrangement. The pre-BCR signaling pathway for this process is largely unknown but may involve the adaptor protein BASH (BLNK/SLP-65). Here we report that the pre-B leukemia cell lines established from affected BASH-deficient mice rearrange kappaL-chain gene locus and down-regulate pre-BCR upon PMA treatment or BASH reconstitution. Analyses with specific inhibitors revealed that activation of novel PKC (nPKC) and MEK, but not Ras, is necessary for the rearrangement. Accordingly, retroviral transduction of active PKCeta, PKCepsilon, or Raf-1, but not Ras, induced the kappa gene rearrangement and expression in the pre-B-cell line. Tamoxifen-mediated BASH reconstitution resulted in the translocation of PKCeta to the plasma membrane and kappa chain expression. These data make evident that the Ras-independent BASH-nPKC-Raf-1 pathway of pre-BCR signaling induces the L-chain gene rearrangement and expression.

N-(4-hydroxyphenyl)retinamide-induced apoptosis triggered by reactive oxygen species is mediated by activation of MAPKs in head and neck squamous carcinoma cells

N-(4-hydroxyphenyl)retinamide (4HPR), a synthetic retinoid effective in cancer chemoprevention and therapy, is thought to act via apoptosis induction resulting from increased reactive oxygen species (ROS) generation. As ROS can activate MAP kinases and protein kinase C (PKC), we examined the role of such enzymes in 4HPR-induced apoptosis in HNSCC UMSCC22B cells. 4HPR increased ROS level within 1 h and induced activation of caspase 3 and PARP cleavage within 24 h. Activation of MKK3/6 and MKK4, JNK, p38 and ERK was detected between 6 and 12 h, increased up to 24 h and preceded apoptosis. 4HPR-induced activation of these kinases was abrogated by the antioxidants BHA and vitamin C. SP600125, a JNK inhibitor, suppressed 4HPR-induced c-Jun phosphorylation, cytochrome c release from mitochondria and apoptosis. Suppression of JNK1 and JNK2 using siRNA decreased, whereas overexpression of wild type-JNK1 enhanced 4HPR-induced apoptosis. PD169316, a p38, inhibitor suppressed phosphorylation of Hsp27 and apoptosis. PD98059, an MEK1/2 inhibitor, also suppressed ERK1/2 activation and apoptosis induced by 4HPR. Likewise, PKC inhibitor GF109203X suppressed ERK and p38 phosphorylation and PARP cleavage. These data indicate that 4HPR-induced apoptosis is triggered by ROS increase, leading to the activation of the mitogen-activated protein serine/threonine kinases JNK, p38, PKC and ERK, and subsequent apoptosis.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

PKC-δ mediates activation of ERK1/2 and induction of iNOS by IL-1β in vascular smooth muscle cells

Although the inflammatory cytokine interleukin-1β (IL-β) is an important regulator of gene expression in vascular smooth muscle (VSM), the signal transduction pathways leading to transcriptional activation upon IL-1β stimulation are poorly understood. Recent studies have implicated IL-1β-mediated ERK1/2 activation in the upregulation of type II nitric oxide synthase (iNOS) in VSM. We report that these events are mediated in a phospholipase C (PLC)- and protein kinase C (PKC)-δ-dependent manner utilizing a signaling mechanism independent of p21ras (Ras) and Raf1 activation. Stimulation of rat aortic VSM cells with IL-1β activated PLC-γ and pharmacological inhibition of PLC attenuated IL-1β-induced ERK1/2 activation and subsequent iNOS expression. Stimulation with IL-1β activated PKC-α and -δ, which was blocked using the PLC inhibitor U-73122. Pharmacological studies using isoform-specific PKC inhibitors and adenoviral overexpression of constitutively active PKC-δ indicated that ERK1/2 activation was PKC-α independent and PKC-δ dependent. Similarly, adenoviral overexpression of constitutively activated PKC-δ enhanced iNOS expression. IL-1β stimulation did not induce either Ras or Raf1 activity. The absence of a functional role for Ras and Raf1 related to ERK1/2 activation and iNOS expression was further confirmed by adenoviral overexpression of dominant-negative Ras and treatment with the Raf1 inhibitor GW5074. Taken together, we have outlined a novel transduction pathway implicating PKC-δ as a critical component of the IL-1-dependent activation of ERK in VSM cells.

Protein kinase Cδ-mediated proteasomal degradation of MAP kinase phosphatase-1 contributes to glutamate-induced neuronal cell death

Mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) is a dual-specificity phosphatase that is involved in the regulation of cell survival, differentiation and apoptosis through inactivating MAPKs by dephosphorylation. Here, we provide evidence for a role of MKP-1 in the glutamate-induced cell death of HT22 hippocampal cells and primary mouse cortical neurons. We suggest that, during glutamate-induced oxidative stress, protein kinase C (PKC) δ becomes activated and induces sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) through a mechanism that involves degradation of MKP-1. Glutamate-induced activation of ERK1/2 was blocked by inhibition of PKCδ, confirming that ERK1/2 is regulated by PKCδ. Prolonged exposure to glutamate caused reduction in the protein level of MKP-1, which correlated with the sustained activation of ERK1/2. Furthermore, knockdown of endogenous MKP-1 by small interfering (si)RNA resulted in pronounced enhancement of ERK1/2 phosphorylation accompanied by increased cytotoxicity under glutamate exposure. In glutamate-treated cells, MKP-1 was polyubiquitylated and proteasome inhibitors markedly blocked the degradation of MKP-1. Moreover, inhibition of glutamate-induced PKCδ activation suppressed the downregulation and ubiquitylation of MKP-1. Taken together, these results demonstrate that activation of PKCδ triggers degradation of MKP-1 through the ubiquitin-proteasome pathway, thereby contributing to persistent activation of ERK1/2 under glutamate-induced oxidative toxicity.

Regulatory properties of adenylate cyclases type 5 and 6: A progress report

Adenylate cyclases (AC) type 5 and 6 comprise the calcium-inhibited family of adenylate cyclase isoforms. Here we review recent discoveries in the regulation of AC5 and AC6 with a focus on posttranslational modifications including glycosylation, nitrosylation, and phosphorylation by the cyclic AMP-dependent protein kinase (PKA), protein kinase C (PKC), and Raf1. We also describe novel signaling interactions such as Galpha(q)-mediated potentiation of AC6 activation. Novel regulators of AC5 and AC6, including small molecules and proteins that physically interact with AC5 and AC6 such as snapin, regulator of G protein signaling 2 (RGS2), protein associated with myc (PAM), and caveolin peptides are discussed. We also describe several recent studies that demonstrate the usefulness of transgenic or adenoviral overexpression of AC5 and AC6 in models for disease states such as cardiovascular hypertrophy. The discovery of novel regulatory mechanisms for AC5 and AC6 and their potential role in crucial physiological processes provide new avenues for research into therapeutic interventions targeting the cyclic AMP pathway.

Phosphorylation of the hinge domain of the nuclear hormone receptor LRH-1 stimulates transactivation

The nuclear receptor LRH-1 (NR5A2) functions to regulate expression of a number of genes associated with bile acid homeostasis and other liver functions, but mechanisms that modulate its activity remain unclear. We have found that mitogenic stimuli, including treatment with phorbol myristate (PMA), increase LRH-1 transactivation. This response maps to the hinge and ligand binding domains of LRH-1 and is blocked by the mitogen-activated protein kinase ERK1/2 inhibitor U0126. LRH-1 is a phosphoprotein and hinge domain serine residues at 238 and 243 are required for effective phosphorylation, both in vitro and in cells. Preventing phosphorylation of these residues by mutating both to alanine decreases PMA-dependent LRH-1 transactivation and mimicking phosphorylation by mutation to positively charged aspartate residues increases basal transactivation. Although serine phosphorylation of the hinge of SF-1 (NR5A1), the closest relative of LRH-1, confers a similar response, the specific targets differ in the two closely related orphan receptors. These results define a novel pathway for the modulation of LRH-1 transactivation and identify specific LRH-1 residues as downstream targets of mitogenic stimuli. This pathway may contribute to recently described proliferative functions of LRH-1.

Differential functions of PKC-delta and PKC-zeta in cisplatin response of normal and transformed thyroid cells

We investigated the effects of cisplatin (cisPt) in normal PC Cl3 and in transformed and tumourigenic PC E1Araf cells. cisPt cytotoxicity was higher in PC Cl3 than in PC E1Araf cells. In both cell lines, cisPt provoked the ERK1/2 phosphorylation; this was unaltered by Gö6976, a conventional PKC inhibitor, whilst it was blocked by low doses (0.1 microM) or high doses (10 microM) of GF109203X, an inhibitor of all PKC isozymes, in PC Cl3 and in PC E1Araf cells, respectively. In PC E1Araf, the cisPt-provoked ERK phosphorylation was also blocked by the use of a myristoylated PKC-zeta pseudosubstrate peptide. Conversely, in PC Cl3 the cisPt-provoked ERK phosphorylation was blocked by the use of rottlerin, a PKC-delta inhibitor. Results show that cisPt activates both PKC (the -delta and the -zeta isozymes in PC Cl3 and in PC E1Araf cells, respectively) and ERK in association with prolonged survival of thyroid cell lines.

Adhesion-dependent redistribution of MAP kinase and MEK promotes muscarinic receptor-mediated signaling to the nucleus

The mitogen-activated protein kinases (MAPKs) are activated by extracellular signals, and translocate to the nucleus where they modulate transcription. Integrin-mediated cell adhesion to extracellular matrix (ECM) proteins is required for efficient transmission of MAPK-based signals initiated by growth factors. However, the modulation of G protein-coupled receptor (GPCR) signaling by adhesion is less well understood. In the present study, we assessed the impact of cell adhesion on MAPK activation by muscarinic M3 receptors. The muscarinic agonist carbachol more efficiently promoted stress fiber formation and tyrosine phosphorylation of focal adhesion-associated proteins in M3 receptor-expressing cells adherent to fibronectin or collagen type I, as compared to polylysine. Overall MAPK activation was robust in cells adherent to all three substrata. However, total levels of MAPK and mitogen-activated protein kinase kinase (MEK) in the nucleus were significantly greater in cells adherent to ECM proteins for 2.5 h, and levels of activated MAPK and MEK in the nuclei of these cells were higher following carbachol stimulation, relative to levels in cells adherent to polylysine. MEK inhibitors did not prevent adhesion-dependent translocation of MAPK and MEK to the nucleus, and increased nuclear phospho-MEK levels in carbachol-stimulated cells. The results suggest that adhesion of cells to ECM triggers the redistribution of MAPK and MEK to the nucleus, possibly as a result of the cytoskeletal rearrangements that accompany cell spreading. This may represent a mechanism for priming the nucleus with MEK and MAPK, leading to more rapid and pronounced increases in intranuclear phospho-MAPK upon GPCR stimulation.

The phosphorylation of phospholipase C-gamma1, Raf-1, MEK, and ERK1/2 induced by a conserved retroviral peptide

A synthetic 17-amino acid peptide (CKS-17) homologous to a highly conserved region of human and animal retroviral transmembrane proteins has been found to exhibit suppressive properties for numerous immune functions. It has been shown that CKS-17 causes an imbalance of human types 1 and 2 cytokines and inhibition of the immune responses of lymphocytes, monocytes, and macrophages. CKS-17 induced increased intracellular levels of cAMP, which plays an important role in regulation of cytokine biosynthesis. In this study, using a Jurkat T-cell line and Western blot analysis, CKS-17 induced phosphorylation of PLC-gamma1, Raf-1, MEK and ERK1/2. Using a PLC selective inhibitor U73122 or PLC-gamma1-deficient Jurkat cell line, phosphorylation induced by CKS-17 of ERK1/2, PLC-gamma1, or Raf-1, respectively, were undetectable or significantly reduced. Reintroduction of PLC-gamma1 into the PLC-gamma1-deficient Jurkat cells restored the phosphorylation of ERK1/2 and PLC-gamma1 induced by CKS-17. Further, pretreatment of Jurkat cells with PKC inhibitors blocks the phosphorylation of Raf-1, MEK, and ERK1/2 induced by CKS-17. These results indicate that CKS-17 induces the PLC-gamma1-PKC-Raf-1-MEK-ERK1/2 signaling pathway.

Functions and regulations of fibroblast growth factor signaling during embryonic development

Fibroblast growth factors (FGF) are secreted molecules which function through the activation of specific tyrosine kinases receptors, the FGF receptors that transduce the signal by activating different pathways including the Ras/MAP kinase and the phospholipase-C gamma pathways. FGFs are involved in the regulation of many developmental processes including patterning, morphogenesis, differentiation, cell proliferation or migration. Such a diverse set of activities requires a tight control of the transduction signal which is achieved through the induction of different feedback inhibitors such as the Sproutys, Sef and MAP kinase phosphatase 3 which are responsible for the attenuation of FGF signals, limiting FGF activities in time and space.

Activation of Nuclear Factor κB by Somatostatin Type 2 Receptor in Pancreatic Acinar AR42J Cells Involves Gα14 and Multiple Signaling Components

Medications targeting the somatostatin type 2 receptor (SSTR2) have been employed for pancreatic inflammations and cancers, possibly via the regulation of the transcription factor nuclear factor kappaB (NFkappaB). Here we demonstrate that in tumoral pancreatic acinar AR42J cells, activation of SSTR2 leads to stimulation of the inhibitor kappaB kinase (IKK)/NFkappaB signaling cascade via pertussis toxin-insensitive G proteins in a time- and dose-dependent manner. The inability of G(q/11) and G(12/13) proteins to activate IKK/NFkappaB by SSTR2 in transfected human embryonic kidney 293 cells and the lack of Galpha(16) in AR42J cells suggested a possible role of Galpha(14) in mediating SSTR2-induced responses. This regulatory role of Galpha(14) was further confirmed by the activation of IKK and NFkappaB in human embryonic kidney 293 cells expressing SSTR2 and Galpha(14) upon induction. The stimulatory effect of Gbeta(1)gamma(2) and the abrogation by overexpressing transducin confirmed the participation of Gbetagamma in SSTR2-mediated IKK/NFkappaB activation. By the application of specific inhibitors and dominant negative mutants, phospholipase Cbeta, protein kinase C, and calmodulin-dependent kinase II were shown to be involved in SSTR2-induced responses. Inhibition of c-Src and numerous intermediates, including Ras, Raf-1 kinase, MEK1/2, along with the extracellular signal-regulated kinase cascade attenuated somatostatin-mediated IKK/NFkappaB activation. Although c-Jun N-terminal kinase and p38 mitogen-activated protein kinase (MAPK) were also stimulated by SSTR2, suppression of these two MAPKs was ineffective in altering the somatostatin-mediated responses. Similar results were also obtained using AR42J cells. These data suggest that activation of the IKK/NFkappaB signaling cascade by SSTR2 requires a complicated network consisting of Galpha(14) and multiple intermediates.

Activation of protein kinase C alpha is required for TPA-triggered ERK (MAPK) signaling and growth inhibition of human hepatoma cell HepG2

The signaling mechanisms for most of the antiproliferative processes are not fully understood. We have demonstrated that ERK(MAPK) signaling was involved in the induction of both p15(INK4b)and p16(INK4a) CDK inhibitors and growth inhibition of hepatoma cell HepG2 triggered by the tumor promoter tetradecanoyl phorbol acetate (TPA). In this study, the upstream signal mechanism for TPA-induced ERK(MAPK) activation was investigated. In HepG2 cells only one of the cPKC isozymes, PKCalpha, but not cPKCbetaII, nPKCepsilon or aPKCzeta was activated by TPA as demonstrated by its membrane translocation within 10-30 min and down-regulation at 24 h after TPA treatment. Pretreatment of 0.2-2.0 microM Bisindolylmaleimides, an inhibitor of PKC, attenuated the TPA-induced phosphorylation of ERK, gene expressions of p15(INK4b) and p16(INK4a), and growth inhibition of HepG2 cell in a dose-dependent manner. Consistently, transfection of HepG2 with 1.0-3.0 microM antisense (AS) PKCalpha, but not (AS) PKCbetaII, or nPKCepsilon oligonucleotides (ODN), for 36 h prior to TPA treatment also prevented the TPA-induced molecular and cellular effects described above. Taken together, we concluded that PKCalpha is specifically required for TPA-induced ERK(MAPK) signaling to trigger gene expressions of p15(INK4b) and p16(INK4a) leading to HepG2 growth inhibition.

Inhibition of H-Ras and MAPK is compensated by PKC-dependent pathways in annexin A6 expressing cells

High-density lipoprotein (HDL)-induced activation of the Ras/MAPK pathway can be mediated by protein kinase C (PKC)-dependent and independent pathways. Although both pathways co-exist in cells, we showed that binding of HDL to scavenger receptor BI (SR-BI) in CHO cells activates Ras and MAPK in a PKC-independent manner. We have recently identified that HDL-induced activation of Ras and Raf-1 is reduced in annexin A6 expressing CHO cells (CHOanx6). In the present study we demonstrate that despite the loss of Ras and Raf-1 activity, HDL induces MAPK phosphorylation in CHOanx6 cells. Since annexin A6 is a PKCalpha-binding protein we therefore investigated the possible involvement of PKC in HDL-induced Ras and MAPK activation in CHOanx6 cells. Taken together our findings demonstrate that HDL-induced H-Ras and MAPK activation is PKC-dependent in cells expressing annexin A6 to compensate for the loss of PKC-independent activation of H-Ras and MAPK.

Overexpression of RasN17 fails to neutralize endogenous Ras in MCF7 breast cancer cells

Receptor tyrosine kinases of the ErbB family have been implicated in the onset/progression of a number of neoplasias. In these diseases, ErbB receptor expression may be accompanied by constitutive activation caused by molecular alterations, overexpression, or ligand binding. An important signaling route activated by these receptors that has been linked to the stimulation of cell proliferation is the Ras route. Here we have investigated the action of a mutant Ras form, H-RasN17, on the proliferation of the breast cancer epithelial cell line MCF7 cells. In these cells expression of RasN17 failed to affect serum or ErbB receptor-stimulated proliferation. Analysis of the action of RasN17 indicated that overexpression of this mutant form of Ras did not affect neuregulin or protein kinase C-induced activation of Erk1/2. In addition, RasN17 failed to prevent activation of endogenous N-Ras and H-Ras even though the levels of the latter were much lower than those of the RasN17 form. The failure of RasN17 to prevent endogenous Ras activation did not appear to be due to deficient processing or sorting of the mutated form. These data indicated that the action of RasN17 as a bona fide inhibitor of Ras depends on the cell type and requires detailed analysis of the biochemical and biological properties of RasN17, particularly with respect to the activation of endogenous Ras.

Taurine suppresses platelet-derived growth factor (PDGF) BB-induced PDGF-β receptor phosphorylation by protein tyrosine phosphatase-mediated dephosphorylation in vascular smooth muscle cells

In atherosclerosis, abnormal vascular smooth muscle cell (VSMC) proliferation plays an important role to form fibroproliferative lesions and platelet-derived growth factor (PDGF)-BB is one of the most potent chemoattractants and proliferative factors for VSMCs. Taurine, sulfur-containing beta-amino acid, has been considered to prevent the development of atherosclerosis, although the molecular mechanism remains obscure. Previously, we demonstrated that taurine significantly suppressed PDGF-BB-induced cell proliferation, DNA synthesis, immediate-early gene expressions and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in VSMCs. The present study was aimed at elucidating the precise molecular mechanism of taurine in PDGF-BB signaling pathway. We showed that taurine significantly suppressed PDGF-BB-induced phosphorylation of PDGF-beta receptor and activation of its downstream signaling molecules such as Ras, MAPK/ERK kinase (MEK)1/2 and Akt. Because taurine did not attenuate phorbol 12-myristate 13-acetate (PMA)-induced PDGF-beta receptor-independent ERK1/2 phosphorylation, we further investigated the suppressive mechanism of taurine in PDGF-beta receptor level. Although taurine did not directly affect PDGF receptor autophosphorylation in vitro, taurine promoted PDGF-beta receptor dephosphorylation and restored PDGF-BB-induced suppression of protein tyrosine phosphatase (PTPase) activity. Taken together, we propose that taurine could prevent or delay the progression of atherosclerosis by PTPase-mediated suppression of PDGF-beta receptor phosphorylation, and by decreasing the activation of its downstream signaling molecules in VSMCs.

Activation of ERK during DNA damage-induced apoptosis involves protein kinase Cδ

We have previously shown that protein kinase C (PKC) acts upstream of caspases to regulate cisplatin-induced apoptosis. Since extracellular signal-regulated kinases (ERKs) have also been implicated in DNA damage-induced apoptosis, we have examined if ERK signaling pathway acts downstream of PKC in the regulation of cisplatin-induced apoptosis. PKC activator PDBu induced ERK1/2 phosphorylation which was inhibited by general PKC inhibitor bisindolylmaleimide and Gö 6983 as well as the MEK inhibitor U0126 but not by the PKCdelta inhibitor rottlerin. Cisplatin caused a concentration-dependent activation of ERK1/2 in HeLa cells. The level of ERK2 was decreased in HeLa cells that acquired resistance to cisplatin (HeLa/CP). The MEK inhibitor U0126 inhibited cisplatin-induced ERK activation and attenuated cisplatin-induced cell death. Inhibition of PKCdelta by rottlerin or depletion of PKCdelta by siRNA inhibited cisplatin-induced ERK activation. These results suggest that cisplatin-induced DNA damage results in activation of ERK1/2 via PKCdelta.

Inositol hexaphosphate (IP6) blocks proliferation of human breast cancer cells through a PKCdelta-dependent increase in p27Kip1 and decrease in retinoblastoma protein (pRb) phosphorylation

Inositol hexaphosphate (IP6) is a naturally occurring polyphosphorylated carbohydrate with demonstrated anti-proliferative and anti-cancer activity in mammary cells. We hypothesized that IP6 modulates cell cycle proteins by action on cytoplasmic signaling molecules. The effects of both pharmacological (2 mM) and physiological (100 microM) doses of IP6 on major PKC isoforms (PKCalpha, delta, epsilon, beta and zeta), PI3-K/Akt and ras/Erk1/2 were evaluated. Treatment of MCF-7 human breast cancer cells with 2 mM IP6 for 24 h caused a 3.1-fold increase in the expression of anti-proliferative PKCdelta. Similar results were observed with 100 microM IP6 at only 30-60 min post-treatment. IP6 also caused an increase in PKCdelta activity, shown by its translocation from cytosol to membrane. No changes in expression of PKC alpha, delta, epsilon, beta and zeta were detected. Additionally, IP6 caused a decrease of Erk1/2 and Akt activity. Among cell cycle control proteins, IP6 resulted in increased p27Kip1 protein levels and marked reduction of pRb phosphorylation. Specificity of the IP6 effects on p27Kip1 and pRb in MCF-7 cells (hormone-dependent) were additionally confirmed in highly invasive hormone-independent MDA-MB 231 breast cancer cells. Use of specific pharmaclogical inhibitors of PKC delta, MEK/Erk, and PI3K/Akt pathways indicated that the IP6-mediated effects on PKC delta were responsible for up-regulation of p27Kip, and pRb hypo-phosphorylation. In addition, IP6-induced apoptosis detected in MCF-7 cells appeared also to be PKC delta-dependent. Our data suggest potential usefulness of IP6 as a novel therapeutic modulator of PKC delta and p27Kip1, an important prognostic factor in human breast cancers.

B cell receptor (BCR) cross-talk: IL-4 creates an alternate pathway for BCR-induced ERK activation that is phosphatidylinositol 3-kinase independent

IL-4 has pleiotropic effects on B cells. These effects include alteration of subsequent BCR-triggered responses. To identify a molecular basis for this receptor cross-talk, we examined ERK activation and NF-kappaB induction. We found that treatment with IL-4, but not other cytokines, affected subsequent BCR signaling by creating a new pathway in which the need for PI3K in ERK activation was eliminated. In contrast, the need for PI3K in NF-kappaB induction was not altered. The new pathway for ERK required time to develop, depended on STAT6, and was blocked by inhibition of macromolecular synthesis. As in the classical pathway, BCR-induced ERK activation in the new, PI3K-independent pathway required MEK and was reflected in c-Raf. Thus, IL-4 promotes an alternate pathway through which BCR is coupled to Raf/MEK/ERK that may function to heighten the responsiveness of B cells during times of immunological stress.

Spatial and Functional Heterogeneity of Sphingolipid-rich Membrane Domains

Little is known about the organization of lipids in biomembranes. Lipid rafts are defined as sphingolipid- and cholesterol-rich clusters in the membrane. Details of the lipid distribution of lipid rafts are not well characterized mainly because of a lack of appropriate probes. Ganglioside GM1-specific protein, cholera toxin, has long been the only lipid probe of lipid rafts. Recently it was shown that earthworm toxin, lysenin, specifically recognizes sphingomyelin-rich membrane domains. Binding of lysenin to sphingomyelin is accompanied by the oligomerization of the toxin that leads to pore formation in the target membrane. In this study, we generated a truncated lysenin mutant that does not oligomerize and thus is non-toxic. Using this mutant lysenin, we showed that plasma membrane sphingomyelin-rich domains are spatially distinct from ganglioside GM1-rich membrane domains in Jurkat T cells. Like T cell receptor activation and cross-linking of GM1, cross-linking of sphingomyelin induced calcium influx and ERK phosphorylation in the cell. However, unlike CD3 or GM1, cross-linking of sphingomyelin did not induce significant protein tyrosine phosphorylation. Combination of lysenin and sphingomyelinase treatment suggested the involvement of G-protein-coupled receptor in sphingomyelin-mediated signal transduction. These results thus suggest that the sphingomyelin-rich domain provides a functional signal cascade platform that is distinct from those provided by T cell receptor or GM1. Our study therefore elucidates the spatial and functional heterogeneity of lipid rafts.

Upregulation of PKC-delta contributes to antiestrogen resistance in mammary tumor cells

Acquired resistance to tamoxifen (Tam) in breast cancer patients is a serious therapeutic problem. We have previously reported that protein kinase C-delta (PKC-delta) plays a major role in estrogen (E2)-mediated cell proliferation. To determine if PKC-delta is one of the major alternate signaling pathways that supports cell growth in the presence of Tam, we determined the levels of PKC isoforms in four different models of antiestrogen-resistant cells. Three out of four antiestrogen resistance cell lines (Tam/MCF-7, ICI/MCF-7 and HER-2/MCF-7) expressed significantly high levels of both total and activated PKC-delta levels compared to sensitive cells. Estrogen receptor (ER) alpha content and function are maintained in all the antiestrogen-resistant cell lines. Overexpressing active PKC-delta in Tam-sensitive MCF-7 cells (PKC-delta/MCF-7) led to Tam resistance both in vitro and in vivo. Inhibition of PKC-delta by rottlerin (a relatively specific inhibitor of PKC-delta) or siRNA significantly inhibited estrogen- and Tam-induced growth in antiestrogen-resistant cells. PKC-delta levels are significantly higher in Tam-resistant tumors compared to Tam-sensitive tumors in xenograft model (P<0.05). Taken together, these data suggest that PKC-delta plays a major role in antiestrogen resistance in breast tumor cells and thus provides a new target for treatment.

Activation of a novel PKC isoform synergistically enhances D2L dopamine receptor-mediated sensitization of adenylate cyclase type 6

Despite acutely inhibiting adenylate cyclase, prolonged activation of Galpha(i/o)-coupled receptors leads to a subsequent heterologous sensitization of adenylate cyclase responsiveness. Recently, protein kinase signaling and phosphorylation have been implicated in the sensitization of adenylate cyclase type 6 (AC6). To examine the sensitization specifically of AC6, we constructed human embryonic kidney cells (HEK293) cells stably expressing AC6 and the Galpha(i/o)-coupled D2L dopamine receptor. In contrast to observations in delta-opioid-expressing Chinese hamster ovary (CHO) cells that express endogenous AC6 and AC7, neither protein kinase C (PKC) nor tyrosine kinase inhibitors attenuated D2L receptor-mediated sensitization of AC6. Inhibition of Raf1 modestly inhibited the magnitude of D2L receptor-induced sensitization of AC6; however, activation of PKC robustly enhanced D2L receptor-mediated AC6 sensitization in a Raf1-dependent manner. These data indicate that, although PKC and Raf1 are not required for sensitization, activation of the PKC-Raf1 pathway robustly potentiated D2L receptor-mediated sensitization of AC6.

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.

The survival of IL-6-dependent myeloma cells critically relies on their capability to transit the G1 to S phase interval of the cell cycle

Interleukin-6 (IL-6) has an essential role in the initial progression of myeloma cell tumours. IL-6 triggers proliferation of these cells via the Ras-mitogen-activated protein kinase (MAPK) cascade and is thought to promote their survival via signal transducer and activator of transcription (STAT) pathway-dependent regulation of Bcl-2 family antiapoptotic members. Using IL-6-dependent murine B9 hybridoma/plasmacytoma cells, we here report that exiting the cell cycle G1 phase is a crucial step contributing to maintain viability. We show that (1) drug-mediated reversible G1 arrest triggered apoptosis despite the presence of IL-6; (2) a short IL-6 pulse to G1-arrested cells was sufficient to induce S phase entry and prevent apoptosis; and (3) phorbol ester and related derivatives promoted S phase entry and survival of IL-6-starved cells without up-regulating bcl-XL expression. Furthermore, that the MAPK kinase (MEK) 1/2 inhibitor, U0126, blocked proliferation and induced death of B9 cells indicate that IL-6 may not exert its survival effect primarily through bcl-XL and emphasizes the key role of Ras-MAPK cascade elements in the regulation of myeloma growth/viability.

B cell receptor (BCR) cross-talk: CD40 engagement enhances BCR-induced ERK activation

Bystander B cells may be initially stimulated through CD40, which enhances susceptibility to Fas-mediated apoptosis, before encountering Ag, which produces Fas resistance. A key issue in this process is to what extent CD40 cross-talk might affect subsequent BCR signaling. It has previously been shown that CD40 engagement bypasses or mitigates the need for Bruton's tyrosine kinase in subsequent BCR signaling for NF-kappaB activation. However, the full extent of the effects of CD40 on BCR signaling has not been delineated. In the present study we evaluated the possibility that CD40-mediated cross-talk also affects another principal outcome of BCR signaling: MAPK activation. We found that prior stimulation of primary murine B cells with CD40L markedly enhanced the level of ERK and JNK (but not p38 MAPK) phosphorylation produced by subsequently added anti-Ig Ab, and much, but not all, of this enhancement was independent of PI3K and phospholipase C. CD40L treatment similarly enhanced BCR-induced MAPK kinase (MEK) phosphorylation, and MEK was required for enhancement of ERK. Although BCR-induced c-Raf phosphorylation was also enhanced by prior CD40L treatment, c-Raf was not required for MEK/ERK phosphorylation. These results identify a novel system of receptor cross-talk between CD40 and BCR and indicate that the effects of CD40 engagement on subsequent BCR stimulation spread beyond NF-kappaB to involve the MAPK pathway.

ERK, PKC and PI3K/Akt pathways mediate extracellular ATP and adenosine-induced proliferation of U138-MG human glioma cell line

OBJECTIVE

Extracellular nucleotides and nucleosides induce proliferation in a set of human glioma cell lines. In this study we investigate the signal transduction pathways involved in ATP and adenosine-mediated proliferation in U138-MG human glioma cells.

METHODS

Cell proliferation was accessed through [(3)H]thymidine incorporation, cell counting and flow cytometry. Protein phosphorylation was detected through Western blotting.

RESULTS

ATP or adenosine (100 microM) induced extracellular signal-regulated protein kinase (ERK), Akt and GSK3beta phosphorylation. The increase in [(3)H]thymidine incorporation induced by ATP or adenosine was decreased when cells were incubated with LY 294002 (by +/-90%), GF 109203X (by +/-76%) or PD 098059 (by +/-63%). The increase in cell numbers with ATP or adenosine was less after a 48-hour treatment of cells with ATP or adenosine plus GF 109203X (by +/-66%) or LY 294002 (by +/-83%). Percentage of cells in S phase was decreased in cells treated with LY 294002 plus ATP when compared to ATP- treated cells.

CONCLUSION

Stimulation of purinergic receptors in U138-MG cells leads to cell proliferation mediated by PI3K/Akt, ERK and PKC signaling. It may be clinically important for pharmacological intervention in gliomas to associate purinergic receptor antagonists and signal transduction pathways blockers.

Opposing effects of protein kinase Calpha and protein kinase Cepsilon on collagen expression by human lung fibroblasts are mediated via MEK/ERK and caveolin-1 signaling

The roles of MEK, ERK, the epsilon and alpha isoforms of protein kinase C (PKC), and caveolin-1 in regulating collagen expression were studied in normal lung fibroblasts. Knocking down caveolin-1 gave particularly striking results. A 70% decrease caused a 5-fold increase in MEK/ERK activation and collagen expression. The combined data reveal a branched signaling pathway. In its central portion MEK activates ERK, leading to increased collagen expression. Two branches converge on MEK/ERK. In one, increased PKCepsilon leads to MEK/ERK activation. In another, increased PKCalpha induces caveolin-1 expression, which in turn inhibits MEK/ERK activation and collagen expression. Lung fibroblasts from scleroderma patients with pulmonary fibrosis showed altered signaling. Consistent with their overexpression of collagen, scleroderma lung fibroblasts contain more activated MEK/ERK and less caveolin-1 than normal lung fibroblasts. Because cutaneous fibrosis is the hallmark of scleroderma, we also studied dermal fibroblasts. As in lung, there was more activated MEK/ERK in cells from scleroderma patients than in control cells, and MEK inhibition decreased collagen expression. However, the distinctive levels of PKCepsilon, PKCalpha, and caveolin-1 in lung and dermal fibroblasts from scleroderma patients and control subjects indicate that the links between these signaling proteins and MEK/ERK must function differently in the four cell types. Finally, we confirmed the relevance of these signaling cascades in vivo. The combined results demonstrate that a branched signaling pathway involving MEK, ERK, PKCepsilon, PKCalpha, and caveolin-1 regulates collagen expression in normal lung tissue and is perturbed during fibrosis.

Distinct PKC isoforms mediate cell survival and DNA synthesis in thrombin-induced myofibroblasts

Thrombin activates protease-activated receptor (PAR)-1 and induces a myofibroblast phenotype in normal lung fibroblasts that resembles the phenotype of scleroderma lung fibroblasts. We now demonstrate that PAR-1 expression is dramatically increased in lung tissue from scleroderma patients, where it is associated with inflammatory and fibroproliferative foci. We also observe that thrombin induces resistance to apoptosis in normal lung fibroblasts, and this process is regulated by protein kinase C (PKC)-epsilon but not by PKC-alpha. Overexpression of a constitutively active (c-a) form of PAR-1 or PKC-epsilon significantly inhibits Fas ligand-induced apoptosis in lung fibroblasts, whereas scleroderma lung fibroblasts are resistant to apoptosis de novo. Thrombin translocates p21Cip1/WAF1, a signaling molecule downstream of PKC, from the nucleus to cytoplasm in normal lung fibroblasts mimicking the localization of p21Cip1/WAF1 in scleroderma lung fibroblasts. Overexpression of c-a PKC-alpha or PKC-epsilon results in accumulation of p21Cip1/WAF1 in the cytoplasm. Depletion of PKC-alpha or inhibition of mitogen-activated protein kinase (MAPK) blocks thrombin-induced DNA synthesis in lung fibroblasts. Inhibition of PKC by calphostin or PKC-alpha, but not PKC-epsilon, by antisense oligonucleotides prevents thrombin-induced MAPK phosphorylation and accumulation of G(1) phase regulatory protein cyclin D1, suggesting that PKC-alpha, MAPK, and cyclin D1 mediate lung fibroblast proliferation. These data demonstrate that two distinct PKC isoforms mediate thrombin-induced resistance to apoptosis and proliferation and suggest that p21Cip1/WAF1 promotes both phenomena.

Protein kinase C and epidermal growth factor stimulation of Raf1 potentiates adenylyl cyclase type 6 activation in intact cells

Adenylyl cyclase type 6 (AC6) activity is inhibited by protein kinase C (PKC) in vitro; however, in intact cells, PKC activation does not inhibit the activity of transiently expressed AC6. To investigate the effects of PKC activation on AC6 activity in intact cells, we constructed human embryonic kidney (HEK) 293 cells that stably express wild-type AC6 (AC6-WT) or an AC6 mutant lacking a PKC and cyclic AMP-dependent protein kinase (PKA) phosphorylation site, Ser674 (AC6-S674A). In contrast to in vitro observations, we observed a PKC-mediated enhancement of forskolin- and isoproterenol-stimulated cyclic AMP accumulation in HEK-AC6 cells. Phorbol 12-myristate 13-acetate also potentiated cyclic AMP accumulation in cells expressing endogenous AC6, including Chinese hamster ovary cells and differentiated Cath.a differentiated cells. In HEK-AC6-S674A cells, the potentiation of AC6 stimulation was significantly greater than in cells expressing AC6-WT. The positive effect of PKC activation on AC6 activity seemed to involve Raf1 kinase because the Raf1 inhibitor 3-(3,5-dibromo-4-hydroxybenzylidene-5-iodo-1,3-dihydro-indol-2-one (GW5074) inhibited the PKC potentiation of AC6 activity. Furthermore, the forskolin-stimulated activity of a recombinant AC6 in which the putative Raf1 regulatory sites have been eliminated was not potentiated by activation of PKC. The ability of Raf1 to regulate AC6 may involve a direct interaction because AC6 and a constitutively active Raf1 construct were coimmunoprecipitated. In addition, we report that epidermal growth factor receptor activation also enhances AC6 signaling in a Raf1-dependent manner. These data suggest that Raf1 potentiates drug-stimulated cyclic AMP accumulation in cells expressing AC6 after activation of multiple signaling pathways.

Establishment and characterisation of a human carcinoma cell line with acquired resistance to Aplidin

Aplidin (APL) is a new antitumoral drug from marine origin currently in phase II clinical trials against a wide multiplicity of cancers. As resistance may be, as with other drugs, an important obstacle to the APL therapeutic efficacy, we have established an acquired resistance cellular model by continuous exposure of HeLa cells to the drug. The stably resistant subline generated (HeLa-APL), possessing more than 1000-fold relative resistance to APL than parental cells, did not show crossresistance to a subset of clinically relevant antitumoral agents. In addition, resistance was not related to overexpression of P-glycoprotein or differences in overall drug accumulation. Comparing to parental cells, HeLa-APL cells did not present either significant differences in the growth rate or apparent alterations in the cell cycle distribution. Aplidin induced rapid and persistent phosphorylation of both JNK and p38 MAPKs, resulting in activation of the mitochondrial apoptotic pathway in parental cells, but, notably, in HeLa-APL-resistant cells MAPKs activation only occurred in a slight and transiently manner, failing to activate the above-mentioned apoptotic machinery. These results suggest that sustained activation of JNK and p38 is essential for triggering the apoptotic programme induced by APL and that HeLa-APL cells bypass this apoptotic response by preventing the specific mechanisms that prime and sustain the long-term activation of these signalling cascades. Although far from human tumour physiology in vivo, HeLa-APL cells represent a potentially useful tool in gaining insights into the mode of action of APL, in selecting non-crossresistant APL structural analogues, as well as in investigating and developing methods to prevent resistance to this drug.

High expression of PKC-MAPK pathway mRNAs correlates with glomerular lesions in human diabetic nephropathy

BACKGROUND

Activation of protein kinase C (PKC) is a major signaling pathway for transforming growth factor (TGF)-beta to induce extracellular matrix (ECM) production in diabetic nephropathy (DN). PKC also activates mitogen-activated protein kinase (MAPK), which is called the PKC-MAPK pathway. The PKC-MAPK pathway is probably responsible for PKC-related abnormalities in diabetic glomeruli. To confirm the involvement of this pathway, we determined the localization and expression of mRNAs in glomeruli by in situ hybridization method.

METHODS

In the present study, we examined expression of PKCbeta1, MAPK/ERK kinase (MEK) 1, MEK2, extracellular signal-regulated protein kinase (ERK) 1, ERK2, and TGF-beta1 mRNAs using renal tissue samples from kidneys affected by DN (N= 21) and from normal human kidney (NHK; N= 6). We also performed an immunohistochemical study using anti-phosphorylated MEK1/2 (P-MEK) and ERK1/2 (P-ERK) antibodies. The glomerular severity of DN was classified into three groups according to mesangial expansion: D1 (N= 4), D2 (N= 13), and D3 (N= 4). We analyzed differences and correlations between variables.

RESULTS

In the glomeruli, the number of cells that stained for these mRNAs in DN was significantly higher than in NHK. The expression of PKC-MAPK pathway mRNAs tended to be inversely proportional to the degree of mesangial expansion. The P-MEK and P-ERK signal intensity were parallel to its mRNA expression pattern. Furthermore, there were significant correlations among the P-MEK, P-ERK signal intensity, PKCbeta1 mRNA expression.

CONCLUSION

Our results suggest that high expression of PKC-MAPK pathway mRNAs plays an important role in the development and/or progression of early tissue damage in DN.

Cooperation between PKC-alpha and PKC-epsilon in the regulation of JNK activation in human lung cancer cells

Phorbol esters can induce activation of two mitogen-activated protein kinase (MAPK) pathways, the extracellular signal-regulated kinase (ERK) pathway and the c-Jun N-terminal kinase (JNK) pathway. Unlike ERK activation, JNK activation by phorbol esters is somehow cell-specific. However, the mechanism(s) that contribute to the cell-specific JNK activation remain elusive. In this study, we found that phorbol 12-myristate 13-acetate (PMA) induced JNK activation only in non-small cell lung cancer (NSCLC) cells, but not in small cell lung cancer (SCLC) cells, whereas ERK activation was detected in both cell types. In NSCLC cells, PMA induced JNK activation in a time- and dose-dependent manner. JNK activation was attenuated by protein kinase C (PKC) down-regulation through prolonged pre-treatment with PMA and significantly inhibited by PKC inhibitors Gö6976 and GF109203X. Subcellular localization studies demonstrated that PMA induced translocation of PKC-alpha, -betaII, and -epsilon isoforms, but not PKC-delta, from the cytosol to the membrane. Analysis of various PKC isoforms revealed that PKC-epsilon was exclusively absent in the SCLC cell lines tested. Ectopic expression of PKC-epsilon in SCLC cells restored PMA activation of JNK signaling only in the presence of PKC-alpha, suggesting that PKC-alpha and PKC-epsilon act cooperatively in regulating JNK activation in response to PMA. Furthermore, using dominant negative mutants and pharmacological inhibitors, we define that a putative Rac1/Cdc42/PKC-alpha pathway is convergent with the PKC-epsilon/MEK1/2 pathway in terms of the activation of JNK by PMA.

Differential signalling of purinoceptors in HeLa cells through the extracellular signal-regulated kinase and protein kinase C pathways

We have previously shown that HeLa cells express P2Y2 and P2Y6 receptors endogenously and determined the pathways by which the P2Y2 controls proliferation and Na+/K+ATPase activity. Our objective in this study was to investigate the hypothesis that P2Y6 also controls proliferation and Na+/K+ATPase activity; the pathways used in these actions were partially characterised. We found that P2Y6 activation controlled cell proliferation but not the activity of the Na+/K+ATPase. UDP activation of P2Y6 provoked: (a) an increase in free cytosolic calcium; (b) the activation of protein kinase C-alpha, -beta, -delta, -epsilon, and -zeta but not of PKC-iota and -eta; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2); (d) the expression of c-Fos protein. The P2Y6 induced cell proliferation was blocked by the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD098059, thereby indicating that the ERK pathway mediates the mitogenic signalling of P2Y6. PKC and phosphoinositide 3-kinase (PI3K) inhibitors were tested at two different time points of ERK1/2 phosphorylation (10 and 60 min). The results suggest that novel PKCs and PI3K initiate the response but both conventional and atypical PKCs are required for the maintenance of the UDP-induced phosphorylation of ERK1/2. The induction of c-Fos was greatly diminished by conventional or atypical PKC-zeta inhibition, suggesting that it may be due to PKC-alpha/beta and -zeta activity. These observations demonstrate that UDP acts as a proliferative agent in HeLa cells activating multiple signalling pathways involving conventional, novel, and atypical PKCs, PI3K, and ERK. Of these pathways, conventional and atypical PKCs appear responsible for the induction of c-Fos, while ERK is responsible for cell proliferation and depends upon both novel and atypical PKCs and PI3K activities.

Divergence and complexities in DAG signaling: looking beyond PKC

For many years protein kinase C (PKC) has been the subject of extensive studies as a molecular target for the treatment of cancer and other diseases. To better define the role of PKC isozymes in the control of cell proliferation, survival and transformation, the examination of PKC-mediated signal transduction pathways by isozyme-specific intervention has become essential. However, issues related to the selectivity of activators and inhibitors of PKC isozymes, in addition to convoluted cross-talks between phorbol ester-regulated pathways, have greatly complicated our understanding of PKC-mediated responses. An additional level of complexity is provided by the fact diacylglycerol (DAG) signals can be transduced by phorbol ester receptors other than PKC. These receptors include chimaerins, RasGRPs, MUNC13s, PKD (PKC mu) and DAG kinases beta and gamma. Thus, it is conceivable that some of the effects that were originally attributed to PKC isozymes in response to phorbol esters might be mediated by PKC-independent pathways. A key issue for the design of novel therapeutic strategies that target PKC isozymes is a comprehensive analysis of isozyme-specific signal transduction pathways in different cell types and the development of pharmacological and molecular tools that can distinguish between the various PKC and 'non-PKC' phorbol ester receptors.

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.

The Expression of the Phosphotyrosine Phosphatase DEP-1/PTPη Dictates the Responsivity of Glioma Cells to Somatostatin Inhibition of Cell Proliferation

Here we characterize the intracellular effectors of the antiproliferative activity of somatostatin in glioma cell lines and post-surgical specimens. The responsiveness to somatostatin correlated with the expression of the phosphotyrosine phosphatase DEP-1/PTPeta, identified in C6 and U87MG cells, in which somatostatin inhibited cell growth. The expression of a dominant negative mutant of DEP-1/PTPeta in C6 cells abolished somatostatin effects, confirming the involvement of this phosphotyrosine phosphatase in such effects. Somatostatin treatment increased the activity of DEP-1/PTPeta and inhibited ERK1/2 activation. Conversely, basic fibroblast growth factor-dependent MEK phosphorylation was not affected, suggesting a direct effect on ERK1/2. In vitro experiments showed that PTPeta was able to interact and dephosphorylate ERK1/2 activated by basic fibroblast growth factor. Furthermore, by transfecting PTPeta in the somatostatin-unresponsive, DEP-1/PTPeta-deficient U373MG cells, the somatostatin-dependent control of cell proliferation was recovered. Finally we evaluated the requirement for DEP-1/PTPeta in somatostatin inhibition of cell proliferation in post-surgical specimens derived from different grade human gliomas. Although all of the glioma analyzed expressed somatostatin receptor mRNA, DEP-1/PTPeta expression was limited to 8 of 22 of the tumors. Culturing seven gliomas, a correlation between the expression of DEP-1/PTPeta and the somatostatin antiproliferative effects was identified. In conclusion we propose that the expression and activation of DEP-1/PTPeta is required for somatostatin inhibition of glioma proliferation.

Involvement of protein kinase C-alpha and -epsilon in extracellular Ca2+ signalling mediated by the calcium sensing receptor

The sensing of extracellular Ca(2+) concentration ([Ca(2+)](o)) and modulation of cellular processes associated with acute or sustained changes in [Ca(2+)](o) are cell-type specific and mediated by the calcium sensing receptor (CaR). [Ca(2+)](o) signalling requires protein kinase C (PKC), but the identity and role of PKC isoforms in CaR-mediated responses remain unclear. Here we show that high [Ca(2+)](o) activated PKC-alpha and PKC- in parathyroid cells and in human embryonic kidney (HEK293) cells overexpressing the CaR (HEK-CaR) and that this response correlated with the CaR-dependent activation of mitogen-activated protein kinases ERK1/2. Activation of ERK1/2 by acute high [Ca(2+)](o) required influx of Ca(2+)through Ni(2+)-sensitive Ca(2+)channels and phosphatidylinositol-dependent phospholipase C-beta activity. Inhibition of PKC by co-expression of dominant-negative (DN) mutants of PKC-alpha or - with the CaR attenuated sustained ERK1/2 activation. Overexpression of a PKC phosphorylation site (T888A) mutant CaR in HEK293 cells showed that this site was important for ERK1/2 activation at high [Ca(2+)](o). Activation of ERK1/2 by high [Ca(2+)](o) was not necessary for the [Ca(2+)](o)-regulated secretion of parathyroid hormone (PTH) in dispersed bovine parathyroid cells. These data suggest that the CaR-mediated [Ca(2+)](o) signal leading to regulated PTH secretion that requires diacylglycerol-responsive PKC isoforms is not mediated via the ERK pathway.

Curcumin-induced apoptosis in scleroderma lung fibroblasts: role of protein kinase cepsilon

Scleroderma, a disease involving excessive collagen deposition, can be studied using fibroblasts cultured from affected tissues. We find that curcumin, the active component of the spice turmeric, causes apoptosis in scleroderma lung fibroblasts (SLF), but not in normal lung fibroblasts (NLF). This effect is likely to be linked to the fact that although curcumin induces the expression of the phase 2 detoxification enzymes heme oxygenase 1 and glutathione S-transferase P1 (GST P1) in NLF, SLF are deficient in these enzymes, particularly after curcumin treatment. The sensitivity of cells to curcumin-induced apoptosis and the expression of GST P1 (but not heme oxygenase 1) are regulated by the epsilon isoform of protein kinase C (PKCepsilon). SLF, which contain less PKCepsilon and less GST P1 than NLF, become less sensitive to curcumin-induced apoptosis and express higher levels of GST P1 when transfected with wild-type PKCepsilon, but not with dominant-negative PKCepsilon. Conversely, NLF become sensitive to curcumin-induced apoptosis and express lower levels of GST P1 when PKCepsilon expression or function is inhibited. The subcellular distribution of PKCepsilon also differs in NLF and SLF. PKCepsilon is predominantly nuclear or perinuclear in NLF but is associated with stress fibers in SLF. Just as PKCepsilon levels are lower in SLF than in NLF in vitro, PKCepsilon expression is decreased in fibrotic lung tissue in vivo. In summary, our results suggest that a signaling pathway involving PKCepsilon and phase 2 detoxification enzymes provides protection against curcumin-induced apoptosis in NLF and is defective in SLF. These observations suggest that curcumin may have therapeutic value in treating scleroderma, just as it has already been shown to protect rats from lung fibrosis induced by a variety of agents.

B cell receptor-induced cAMP-response element-binding protein activation in B lymphocytes requires novel protein kinase Cdelta

The cAMP-response element-binding protein (CREB) is activated by phosphorylation on Ser-133 and plays a key role in the proliferative and survival responses of mature B cells to B cell receptor (BCR) signaling. The signal link between the BCR and CREB activation depends on a phorbol ester (phorbol 12-myristate 13-acetate)-sensitive protein kinase C (PKC) activity and not protein kinase A or calmodulin kinase; however, the identity and role of the PKC(s) activity has not been elucidated. We found the novel PKCdelta (nPKCdelta) activator bistratene A is sufficient to induce CREB phosphorylation in murine splenic B cells. The pharmacological inhibitor Gö6976, which targets conventional PKCs and PKCmu, has no effect on CREB phosphorylation, whereas the nPKCdelta inhibitor rottlerin blocks CREB phosphorylation following BCR cross-linking. Bryostatin 1 selectively prevents nPKCdelta depletion by phorbol 12-myristate 13-acetate when coapplied, coincident with protection of BCR-induced CREB phosphorylation. Ectopic expression of a kinase-inactive nPKCdelta blocks BCR-induced CREB phosphorylation in A20 B cells. In addition, BCR-induced CREB phosphorylation is significantly diminished in nPKCdelta-deficient splenic B cells in comparison with wild type mice. Consistent with the essential role for Bruton's tyrosine kinase and phospholipase Cgamma2 in mediating PKC activation, Bruton's tyrosine kinase- and phospholipase Cgamma2-deficient B cells display defective CREB phosphorylation by the BCR. We also found that p90 RSK directly phosphorylates CREB on Ser-133 following BCR cross-linking and is positioned downstream of nPKCdelta. Taken together, these results suggest a model in which BCR engagement leads to the phosphorylation of CREB via a signaling pathway that requires nPKCdelta and p90 RSK in mature B cells.

Involvement of the ERK Signaling Cascade in Protein Kinase C-mediated Cell Cycle Arrest in Intestinal Epithelial Cells

We have reported previously that protein kinase C (PKC) signaling can mediate a program of cell cycle withdrawal in IEC-18 nontransformed intestinal crypt cells, involving rapid disappearance of cyclin D1, increased expression of Cip/Kip cyclin-dependent kinase inhibitors, and activation of the growth suppressor function of pocket proteins. In the current study, we present evidence to support a requisite role for PKC alpha in mediating these effects. Furthermore, analysis of the signaling events linking PKC/PKC alpha activation to changes in the cell cycle regulatory machinery implicate the Ras/Raf/MEK/ERK cascade. PKC/PKC alpha activity promoted GTP loading of Ras, activation of Raf-1, and phosphorylation/activation of ERK. ERK activation was found to be required for critical downstream effects of PKC/PKC alpha activation, including cyclin D1 down-regulation, p21(Waf1/Cip1) induction, and cell cycle arrest. PKC-induced ERK activation was strong and sustained relative to that produced by proliferative signals, and the growth inhibitory effects of PKC agonists were dominant over proliferative events when these opposing stimuli were administered simultaneously. PKC signaling promoted cytoplasmic and nuclear accumulation of ERK activity, whereas growth factor-induced phospho-ERK was localized only in the cytoplasm. Comparison of the effects of PKC agonists that differ in their ability to sustain PKC alpha activation and growth arrest in IEC-18 cells, together with the use of selective kinase inhibitors, indicated that the length of PKC-mediated cell cycle exit is dictated by the magnitude/duration of input signal (i.e. PKC alpha activity) and of activation of the ERK cascade. The extent/duration of phospho-ERK nuclear localization may also be important determinants of the duration of PKC agonist-induced growth arrest in this system. Taken together, the data point to PKC alpha and the Ras/Raf/MEK/ERK cascade as key regulators of cell cycle withdrawal in intestinal epithelial cells.

Regulation of phorbol ester-mediated TRAF1 induction in human colon cancer cells through a PKC/RAF/ERK/NF-κB-dependent pathway

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are cytoplasmic adapter proteins that link a wide variety of cell surface receptors to the apoptotic signaling cascade. The purpose of this study was to delineate the signaling pathways and TRAF1 promoter elements responsible for phorbol ester-mediated TRAF1 induction in human colon cancers. Here, we found that the PKC activators, phorbol 12-myristate 13-acetate (PMA) and bryostatin I, induced TRAF1 mRNA expression; pretreatment with actinomycin D blocked PMA-mediated TRAF1 expression suggesting induction at the transcriptional level. In contrast, expression of other TRAFs (TRAF2, 3 and 4) was minimally altered by PMA. Various PKC isoform-selective inhibitors blocked PMA-mediated TRAF1 mRNA and promoter stimulation; rottlerin, a selective PKCdelta inhibitor, had no effect suggesting that Ca(2+)-dependent PKC isoforms (e.g., PKCalpha and betaI) play a role in TRAF1 regulation. In addition, the MEK/ERK inhibitors, PD98059 and UO126, suppressed PMA-stimulated TRAF1 promoter activity indicating a role for ERK in TRAF1 induction. Moreover, cotransfection of a dominant-negative Raf-1 (Raf-C4) significantly reduced PMA-stimulated TRAF1 promoter activity whereas transfection of dominant-negative Ras or treatment with Ras inhibitors had minimal to no effect on TRAF1 induction suggesting dependence on Raf, but not Ras, activation. Finally, site-specific mutagenesis of functional NF-kappaB sites (particularly the most proximal site) in the TRAF1 promoter significantly decreased PMA-mediated promoter activity. In conclusion, our results demonstrate selective induction of TRAF1 in human colon cancer cells through a Ca(2+)-dependent PKC/Raf-1/ERK/NF-kappaB-dependent pathway.

HSP25 overexpression attenuates oxidative stress-induced apoptosis: roles of ERK1/2 signaling and manganese superoxide dismutase

HSP25 has been shown to induce resistance to radiation and oxidative stress; however, its exact mechanisms remain unclear. In the present study, a high concentration of H2O2 was found to induce DNA fragmentation in L929 mouse fibroblast cells, and HSP25 overexpression attenuated this phenomenon. To elucidate the mechanisms of H2O2-mediated cell death, ERK1/2, p38 MAPK, and JNK1/2 phosphorylation in the cells after treatment with H2O2 were examined. ERK1/2 and JNK1/2 were activated by H2O2; ERK1/2 activation was inhibited in HSP25-overexpressed cells, while JNK1/2 was indifferent. Inhibition of ERK1/2 activation by treatment of the cells with PD98059 or dominant-negative ERK2 transfection blocked H2O2-induced cell death; similarly treated HSP25-overexpressed cells were not at all affected. Moreover, inhibition of JNK1/2 by dominant-negative JNK1 or JNK2 transfection did not affect H2O2-mediated cell death in control cells. Dominant-negative Ras or Raf transfection inhibited H2O2-mediated ERK1/2 activation and cell death in control cells. On the contrary, HSP25-overexpressed cells did not show any differences. Upstream pathways of H2O2-mediated ERK1/2 activation and cell death involved both tyrosine kinase (PDGFbeta receptor and Src) and PKCdelta, while in HSP25-overexpressed cells these kinases did not respond to H2O2 treatment. Since HSP25 overexpression reduced reactive oxygen species (ROS), increased manganese superoxide dismutase (MnSOD) gene expression, and increased enzyme activity, involvement of MnSOD in HSP25-mediated attenuation of H2O2-mediated ERK1/2 activation and cell death was examined. Blockage of MnSOD with antisense oligonucleotides prevented DNA fragmentation and returned the ERK1/2 activation to the control level. Indeed, when MnSOD was overexpressed in L929 cells, similar to in HSP25-overexpressed cells, DNA fragmentation and ERK1/2 activation were reduced. From the above results, we suggest for the first time that reduced oxidative damage by HSP25 was due to MnSOD-mediated downregulation of ERK1/2.

Helicobacter pylori CagA induces Ras-independent morphogenetic response through SHP-2 recruitment and activation

The CagA protein of Helicobacter pylori, which is injected from the bacteria into bacteria-attached gastric epithelial cells, is associated with gastric carcinoma. CagA is tyrosine-phosphorylated by Src family kinases, binds the SH2 domain-containing SHP-2 phosphatase in a tyrosine phosphorylation-dependent manner, and deregulates its enzymatic activity. We established AGS human gastric epithelial cells that inducibly express wild-type or a phosphorylation-resistant CagA, in which tyrosine residues constituting the EPIYA motifs were substituted with alanines. Upon induction, wild-type CagA, but not the mutant CagA, elicited strong elongation of cell shape, termed the "hummingbird" phenotype. Time-lapse video microscopic analysis revealed that the CagA-expressing cells exhibited a marked increase in cell motility with successive rounds of elongation-contraction processes. Inhibition of CagA phosphorylation by an Src kinase inhibitor, PP2, or knockdown of SHP-2 expression by small interference RNA (siRNA) abolished the CagA-mediated hummingbird phenotype. The morphogenetic activity of CagA also required Erk MAPK but was independent of Ras or Grb2. In AGS cells, CagA prolonged duration of Erk activation in response to serum stimulation. Conversely, inhibition of SHP-2 expression by siRNA abolished the sustained Erk activation. Thus, SHP-2 acts as a positive regulator of Erk activity in AGS cells. These results indicate that SHP-2 is involved in the Ras-independent modification of Erk signals that is necessary for the morphogenetic activity of CagA. Our work therefore suggests a key role of SHP-2 in the pathological activity of H. pylori virulence factor CagA.