Differential Raf requirement for activation of mitogen-activated protein kinase by growth factors, phorbol esters, and calcium

Role and regulation of MKP-1 in airway inflammation

Mitogen-activated protein kinase (MAPK) phosphatase 1 (MKP-1) is a protein with anti-inflammatory properties and the archetypal member of the dual-specificity phosphatases (DUSPs) family that have emerged over the past decade as playing an instrumental role in the regulation of airway inflammation. Not only does MKP-1 serve a critical role as a negative feedback effector, controlling the extent and duration of pro-inflammatory MAPK signalling in airway cells, upregulation of this endogenous phosphatase has also emerged as being one of the key cellular mechanism responsible for the beneficial actions of clinically-used respiratory medicines, including β₂-agonists, phosphodiesterase inhibitors and corticosteroids. Herein, we review the role and regulation of MKP-1 in the context of airway inflammation. We initially outline the structure and biochemistry of MKP-1 and summarise the multi-layered molecular mechanisms responsible for MKP-1 production more generally. We then focus in on some of the key in vitro studies in cell types relevant to airway disease that explain how MKP-1 can be regulated in airway inflammation at the transcriptional, post-translation and post-translational level. And finally, we address some of the potential challenges with MKP-1 upregulation that need to be explored further to fully exploit the potential of MKP-1 to repress airway inflammation in chronic respiratory disease.

The regulation of NHE₁ and NHE₃ activity by angiotensin II is mediated by the activation of the angiotensin II type I receptor/phospholipase C/calcium/calmodulin pathway in distal nephron cells

Angiotensin II (Ang II), acting via the AT1 receptor, induces an increase in intracellular calcium [Ca(2+)]i that then interacts with calmodulin (CaM). The Ca(2+)/CaM complex directly or indirectly activates sodium hydrogen exchanger 1 (NHE1) and phosphorylates calmodulin kinase II (CaMKII), which then regulates sodium hydrogen exchanger 3 (NHE3) activity. In this study, we investigated the cellular signaling pathways responsible for Ang II-mediated regulation of NHE1 and NHE3 in Madin-Darby canine kidney (MDCK) cells. The NHE1- and NHE3-dependent pHi recovery rates were evaluated by fluorescence microscopy using the fluorescent probe BCECF/AM, messenger RNA was evaluated with the reverse transcription polymerase chain reaction (RT-PCR), and protein expression was evaluated by immunoblot. We demonstrated that treatment with Ang II (1pM or 1 nM) for 30 min induced, via the AT1 but not the AT2 receptor, an equal increase in NHE1 and NHE3 activity that was reduced by the specific inhibitors HOE 694 and S3226, respectively. Ang II (1 nM) did not change the total expression of NHE1, NHE3 or calmodulin, but it induced CaMKII, cRaf-1, Erk1/2 and p90(RSK) phosphorylation. The stimulatory effects of Ang II (1 nM) on NHE1 or NHE3 activity or protein abundance was reduced by ophiobolin-A (CaM inhibitor), KN93 (CaMKII inhibitor) or PD98059 (Mek inhibitor). These results indicate that after 30 min, Ang II treatment may activate G protein-dependent pathways, including the AT1/PLC/Ca(2+)/CaM pathway, which induces CaMKII phosphorylation to stimulate NHE3 and induces cRaf-1/Mek/Erk1/2/p90(RSK) activity to stimulate NHE1.

Phospholipase Cgamma2 mediates RANKL-stimulated lymph node organogenesis and osteoclastogenesis

Phospholipase Cgamma2 (PLCgamma2) is an important signaling effector of multiple receptors in the immune system. Here we show that PLCgamma2-deficient mice displayed impaired lymph node organogenesis but normal splenic structure and Peyer's patches. Receptor activator of NF-kappaB ligand (RANKL) is a tumor necrosis factor family cytokine and is essential for lymph node organogenesis. Importantly, PLCgamma2 deficiency severely impaired RANKL signaling, resulting in marked reduction of RANKL-induced activation of MAPKs, p38 and JNK, but not ERK. The lack of PLCgamma2 markedly diminished RANKL-induced activation of NF-kappaB, AP-1, and NFATc1. Moreover, PLCgamma2 deficiency impaired RANKL-mediated biological function, leading to failure of the PLCgamma2-deficient bone marrow macrophage precursors to differentiate into osteoclasts after RANKL stimulation. Re-introduction of PLCgamma2 but not PLCgamma1 restores RANKL-mediated osteoclast differentiation of PLCgamma2-deficient bone marrow-derived monocyte/macrophage. Taken together, PLCgamma2 is essential for RANK signaling, and its deficiency leads to defective lymph node organogenesis and osteoclast differentiation.

B cell lymphoma 10 is essential for FcepsilonR-mediated degranulation and IL-6 production in mast cells

The adaptor protein B cell lymphoma 10 (Bcl10) plays an essential role in the functions of the AgRs in T and B cells. In this study, we report that Bcl10 also plays an important role in mast cells. Bcl10 is expressed in mast cells. Although Bcl10-deficient mast cells undergo normal development, we demonstrate that Bcl10 is essential for specific functions of FcepsilonR. Although Bcl10-deficient mast cells have normal de novo synthesis and release of the lipid mediator arachidonic acid, the mutant cells possess impaired FcepsilonR-mediated degranulation, indicated by decreased serotonin release, and impaired cytokine production, measured by release of IL-6. In addition, Bcl10-deficient mice display impaired IgE-mediated passive cutaneous anaphylaxis. Moreover, although Bcl10-deficient mast cells have normal FcepsilonR-mediated Ca(2+) flux, activation of PI3K, and activation of the three types of MAPKs (ERKs, JNK, and p38), the mutant cells have markedly diminished FcepsilonR-mediated activation of NF-kappaB and decreased activation of AP-1. Thus, Bcl10 is essential for FcepsilonR-induced activation of AP-1, NF-kappaB, degranulation, and cytokine production in mast cells.

Angiotensin II-Induced MAPK Phosphorylation Mediated by Ras and/or Phospholipase C-Dependent Phosphorylations but Not by Protein Kinase C Phosphorylation in Cultured Rat Vascular Smooth Muscle Cells

Angiotensin II (Ang II) induces a rapid increase in mitogen-activated protein kinase (MAPK) activity through the Ang II type 1 receptor in cultured rat vascular smooth muscle cells (VSMCs). In the present study, we examined the effects of the phospholipase C (PLC) inhibitor U73122, the protein kinase C (PKC) inhibitor GF109203X, and the Ras inhibitor farnesylthiosalicylic acid (FTS) on Ang II-induced activation of p42/p44 MAPKs in cultured VSMCs. Phosphorylation was shown using the Western blot technique with specific phospho-antibodies against MAPK proteins. The PLC inhibitor U73122 abolished the Ang II-induced MAPK activity, while the PKC inhibitor GF109203X only decreased it. There was also an inhibition observed with the Ras inhibitor, FTS on Ang II-induced MAPK activity. These data suggest that Ang II-induced MAPK phosphorylation through the Ang II type 1 receptor could be mediated by Ras and/or PLC-dependent phosphorylations but not by PKC phosphorylation.

Oxidative Stress Induces Protein Kinase C-mediated Activation Loop Phosphorylation and Nuclear Redistribution of Protein Kinase D

Oxidative stress induced by cell treatments with H(2)O(2) activates protein kinase D (PKD) via a protein kinase C (PKC)-dependent signal transduction pathway (Waldron, R. T., and Rozengurt, E. (2000) J. Biol. Chem. 275, 17114-17121). Here we show that oxidative stress induces PKC-dependent activation loop Ser(744) and Ser(748) phosphorylation to mediate dose- and time-dependent activation of PKD, both endogenously expressed in Swiss 3T3 cells and stably overexpressed in Swiss 3T3-GFP.PKD cells. Although oxidative stress induced PKD activation loop phosphorylation and activation with identical kinetics, both were dose-dependently blocked by preincubation of cells with selective inhibitors of PKC (GF109203X and Gö6983) or c-Src (PP2). Inhibition of Src tyrosine kinase activity eliminated oxidative stress-induced direct PKD tyrosine phosphorylation, but only partially attenuated activation loop phosphorylation and activation. Mutation of a putative tyrosine phosphorylation site on PKD, Tyr(469) to phenylalanine, had no effect on its activation by oxidative stress in transfected COS-7 cells. Similarly, a mutant with Tyr(469) replaced by aspartic acid had increased basal activity but was also further activated by oxidative stress. Thus, PKD tyrosine phosphorylation at this site neither produced full activation by itself nor was required for oxidative stress-induced activation mediated by activation loop phosphorylation. In addition to PKD activation, activation loop phosphorylation in response to oxidative stress also redistributed activated PKD to cell nuclei, as revealed by PKD indirect immunofluorescence, imaging of a PKD-green fluorescent protein fusion construct (GFP-PKD), and analysis of nuclear pellets. Cell preincubation with Gö6983 strongly diminished H(2)O(2)-induced nuclear relocalization of GFP-PKD. Taken together, these results indicate that PKC-mediated PKD Ser(744) and Ser(748) phosphorylation induced by oxidative stress integrates PKD activation with redistribution to the nucleus.

IQGAP1 binds ERK2 and modulates its activity

IQGAP1 binds several proteins including actin, calmodulin, E-cadherin, beta-catenin, Cdc42, Rac1, and CLIP-170. The interaction with these targets enables IQGAP1 to participate in many cellular functions varying from regulation of the cytoskeleton to gene transcription. Here we show that extracellular signal-regulated kinase (ERK) 2 binds to IQGAP1. In vitro analysis with purified proteins demonstrated a direct interaction between ERK2 and IQGAP1. Moreover, binding occurred in cells as endogenous ERK2 co-immunoprecipitated with IQGAP1 from human breast epithelial cell lysates. The association between ERK2 and IQGAP1 was independent of epidermal growth factor. The in vivo interaction has functional significance. Manipulation of intracellular IQGAP1 levels significantly reduced growth factor-stimulated ERK1 and ERK2 activity. Similarly, stimulation of ERK1 and ERK2 activity by insulin-like growth factor I was reduced when IQGAP1 levels were changed. In contrast, overexpression of an IQGAP1 construct lacking the ERK2 binding region did not interfere with activation of ERK1 and ERK2 by epidermal growth factor. Our data disclose a previously unidentified communication between IQGAP1 and the ERK pathway and imply that IQGAP1 modulates the Ras/mitogen-activated protein kinase signaling cascade.

Nitric Oxide Inhibition of ERK1/2 Activity in Cells Expressing Neuronal Nitric-oxide Synthase

Neuronal nitric-oxide synthase (nNOS) is a constitutively expressed enzyme responsible for the production of nitric oxide (NO*) from l-arginine and O2. Nitric oxide is an intra- and intercellular messenger that mediates a diversity of signaling pathways in target cells. In the absence of l-arginine, nNOS has been shown to generate superoxide (O2*). Superoxide, either directly or through its self-dismutation to H2O2, is likewise believed to be a cell-signaling agent. Because nNOS can generate NO* and O2*, we examined the activation of cellular signal transduction pathways in nNOS-transfected cells grown in the presence or absence of l-arginine. Spin trapping/EPR spectroscopy confirmed that stimulated nNOS-transfected cells grown in an l-arginine environment secreted NO* into the surrounding milieu. Production of NO* blocked Ca2+ ionophore-induced activation of the ERK1/2 through a mechanism involving inhibition of the Ras G-protein and Raf-1 kinase. In contrast, ERK activation was largely unaffected in nNOS-transfected cells grown in l-arginine-free media. Inhibition of nNOS-generated NO* with the competitive NOS inhibitor, NG-nitro-l-arginine methyl ester, in cells grown in l-arginine restored ERK1/2 activation to levels similar to that found when nNOS was activated in l-arginine-free media. These findings indicate that nNOS can differentially regulate the ERK signal transduction pathway in a manner dependent on the presence of l-arginine and the production of NO*.

Caveolin-1 regulates contractility in differentiated vascular smooth muscle

Caveolin is a principal component of caveolar membranes. In the present study, we utilized a decoy peptide approach to define the degree of involvement of caveolin in PKC-dependent regulation of contractility of differentiated vascular smooth muscle. The primary isoform of caveolin in ferret aorta vascular smooth muscle is caveolin-1. Chemical loading of contractile vascular smooth muscle tissue with a synthetic caveolin-1 scaffolding domain peptide inhibited PKC-dependent increases in contractility induced by a phorbol ester or an alpha agonist. Peptide loading also resulted in a significant inhibition of phorbol ester-induced adducin Ser662 phosphorylation, an intracellular monitor of PKC kinase activity, ERK1/2 activation, and Ser789 phosphorylation of the actin binding protein caldesmon. alpha-Agonist-induced ERK1-1/2 activation was also inhibited by the caveolin-1 peptide. Scrambled peptide-loaded tissues or sham-loaded tissues were unaffected with respect to both contractility and signaling. Depolarization-induced activation of contraction was not affected by caveolin peptide loading. Similar results with respect to contractility and ERK1/2 activation during exposure to the phorbol ester or the alpha-agonist were obtained with the cholesterol-depleting agent methyl-beta-cyclodextrin. These results are consistent with a role for caveolin-1 in the coordination of signaling leading to the regulation of contractility of smooth muscle.

Annexin 1 regulates cell proliferation by disruption of cell morphology and inhibition of cyclin D1 expression through sustained activation of the ERK1/2 MAPK signal

Cellular proliferation is controlled by the integration and coordination of extracellular signals. This study explores the role of the protein annexin 1 (ANXA1) in the regulation of such events. We show that ANXA1 has a cell-type independent, anti-proliferative function through sustained activation of the ERK signaling cascade. Moreover, ANXA1 reduces proliferation by ERK-mediated disruption of the actin cytoskeleton and ablation of cyclin D1 protein expression and not by ERK-mediated induction of the cyclin-dependent kinase, CDK2, inhibitor p21(cip/waf). Finally, ANXA1 regulates the ERK pathway at a proximal location, by SH2 domain-independent association with the adapter protein Grb-2. In summary, overexpression of ANXA1 mediates the disruption of normal cell morphology and inhibits cyclin D1 expression, therefore reducing cell proliferation through proximal modulation of the ERK signal transduction pathway.

Involvement of protein kinase Cδ and extracellular signal-regulated kinase-2 in the suppression of microglial inducible nitric oxide synthase expression by N-[3,4-dimethoxycinnamoyl]-anthranilic acid (tranilast)

Excess nitric oxide (NO) in the brain released by microglial cells contributes to neuronal damage in various pathologies of the central nervous system (CNS) including neurodegenerative diseases and multiple sclerosis. N-[3,4-Dimethoxycinnamoyl]-anthranilic acid (tranilast, TNL) is an anti-allergic compound which suppresses the activation of monocytes. We show that inducible nitric oxide synthase (iNOS) mRNA and protein expression and the release of NO from N9 microglial cells stimulated with the bacterial endotoxin lipopolysaccharide (LPS) are inhibited when the cells are exposed to TNL. TNL fails to modulate LPS-stimulated nuclear factor-kappaB (NF-kappaB) reporter gene activity and phosphorylation of inhibitory kappaB (IkappaB), indicating that NF-kappaB is not involved in the TNL-mediated suppression of LPS-induced iNOS expression. Moreover, TNL inhibits LPS-induced phosphorylation of extracellular signal-regulated kinase 2 (ERK-2). Finally, TNL abolishes translocation of protein kinase Cdelta (PKCdelta) to the nucleus and suppresses the phosphorylation of the PKCdelta substrate, myristoylated alanin-rich C kinase substrate (MARCKS). We conclude that the anti-allergic compound TNL suppresses microglial iNOS induction by LPS via inhibition of a signalling pathway involving PKCdelta and ERK-2.

PMA decreases the proliferation of retinal cells in vitro: the involvement of acetylcholine and BDNF

Protein kinase C (PKC) is involved in several cell events including proliferation, survival and differentiation. The aim of this work was to investigate the role of PKC activation on retinal cells proliferation. We demonstrated that PKC activation by phorbol 12-myristate 13-acetate (PMA), a tumor promoter phorbol ester, is able to decrease retinal cells proliferation. This effect was mediated by M1 receptors and dependent on intracellular Ca(2+) increase, tyrosine kinase activity, phosphatidylinositol 3-kinase activity, polypeptide secretion and activation of TrkB receptors. The effect of PMA was not via activation of mitogen-activated protein (MAP) kinase. Carbamylcholine and brain derived neurotrophic factor were both able to decrease retinal cells proliferation to the same level as PMA did. Our results suggest that PKC activation leads to a decrease in retinal cells proliferation through the release of acetylcholine and brain derived neurotrophic factor in the culture, and activation of M1 and TrkB receptors, respectively.

Phospholipase C gamma 2 is essential for specific functions of Fc epsilon R and Fc gamma R

Phospholipase Cgamma2 (PLCgamma2) plays a critical role in the functions of the B cell receptor in B cells and of the FcRgamma chain-containing collagen receptor in platelets. Here we report that PLCgamma2 is also expressed in mast cells and monocytes/macrophages and is activated by cross-linking of Fc(epsilon)R and Fc(gamma)R. Although PLCgamma2-deficient mice have normal development and numbers of mast cells and monocytes/macrophages, we demonstrate that PLCgamma2 is essential for specific functions of Fc(epsilon)R and Fc(gamma)R. While PLCgamma2-deficient mast cells have normal mitogen-activated protein kinase activation and cytokine production at mRNA levels, the mutant cells have impaired Fc(epsilon)R-mediated Ca(2+) flux and inositol 1,4,5-trisphosphate production, degranulation, and cytokine secretion. As a physiological consequence of the effect of PLCgamma2 deficiency, the mutant mice are resistant to IgE-mediated cutaneous inflammatory skin reaction. Macrophages from PLCgamma2-deficient mice have no detectable Fc(gamma)R-mediated Ca(2+) flux; however, the mutant cells have normal Fc(gamma)R-mediated phagocytosis. Moreover, PLCgamma2 plays a nonredundant role in Fc(gamma)R-mediated inflammatory skin reaction.

Glucagon receptor activates extracellular signal-regulated protein kinase 1/2 via cAMP-dependent protein kinase

We prepared a stable cell line expressing the glucagon receptor to characterize the effect of G(s)-coupled receptor stimulation on extracellular signal-regulated protein kinase 1/2 (ERK1/2) activity. Glucagon treatment of the cell line caused a dose-dependent increase in cAMP concentration, activation of cAMP-dependent protein kinase (PKA), and transient release of intracellular calcium. Glucagon treatment also caused rapid dose-dependent phosphorylation and activation of mitogen-activated protein kinase kinase/ERK kinase (MEK1/2) and ERK1/2. Inhibition of either PKA or MEK1/2 blocked ERK1/2 activation by glucagon. However, no significant activation of several upstream activators of MEK, including Ras, Rap1, and Raf, was observed in response to glucagon treatment. In addition, chelation of intracellular calcium reduced glucagon-mediated ERK1/2 activation. In transient transfection experiments, glucagon receptor mutants that bound glucagon but failed to increase intracellular cAMP and calcium concentrations showed no glucagon-stimulated ERK1/2 phosphorylation. We conclude that glucagon-induced MEK1/2 and ERK1/2 activation is mediated by PKA and that an increase in intracellular calcium concentration is required for maximal ERK activation.

Expression of cell cycle proteins in blood vessels of angiotensin II-infused rats: role of AT(1) receptors

Angiotensin II is an important modulator of cell growth through AT(1) receptors, as demonstrated both in vivo and in vitro. We investigated the role of proteins involved in the cell cycle, including cyclin D1, cyclin-dependent kinase 4 (cdk4), and cyclin-dependent kinase inhibitors p21 and p27 in blood vessels of angiotensin II-infused rats and the effect therein of the AT(1)-receptor antagonist losartan. Male Sprague-Dawley rats were infused for 7 days with angiotensin II (120 ng/kg per minute SC) and/or treated with losartan (10 mg/kg per day orally). DNA synthesis in mesenteric arteries was evaluated by radiolabeled (3)H-thymidine incorporation. The expression of cyclin D1, cdk4, p21, and p27, which play critical roles during the G(1)-phase of the cell cycle process, was examined by Western blot analysis. Tail-cuff systolic blood pressure (mm Hg) was elevated (P<0.01, n=9) in angiotensin II-infused rats (161.3+/-8.2) versus control rats (110.1+/-5.3) and normalized by losartan (104.4+/-3.2). Radiolabeled (3)H-thymidine incorporation (cpm/100 microgram DNA) showed that angiotensin II infusion significantly increased DNA synthesis (152+/-5% versus 102+/-6% of control rats, P<0.05). Expression of cyclin D1 and cdk4 was significantly increased in the angiotensin II group to 213.7+/-8% and 263.6+/-37% of control animals, respectively, whereas expression of p21 and p27 was significantly decreased in the angiotensin II group to 23.2+/-10.4% and 10.3+/-5.3% of control animals, respectively. These effects induced by angiotensin II were normalized in the presence of losartan. Thus, when AT(1) receptors are stimulated in vivo, DNA synthesis is enhanced in blood vessels by activation of cyclin D1 and cdk4. Reduction in cell cycle kinase inhibitors p21 and p27 may contribute to activation of growth induced by in vivo AT(1) receptor stimulation.

Intracellular calcium mobilization induces immediate early gene pip92 via Src and mitogen-activated protein kinase in immortalized hippocampal cells

Regulation of intracellular calcium levels plays a central role in cell survival, proliferation, and differentiation. A cell-permeable, tumor-promoting thapsigargin elevates the intracellular calcium levels by inhibiting endoplasmic reticulum Ca(2+)-ATPase. The Src-tyrosine kinase family is involved in a broad range of cellular responses ranging from cell growth and cytoskeletal rearrangement to differentiation. The immediate early gene pip92 is induced in neuronal cell death as well as cell growth and differentiation. To resolve the molecular mechanism of cell growth by intracellular calcium mobilization, we have examined the effect of thapsigargin and subsequent intracellular calcium influx on pip92 expression in immortalized rat hippocampal H19-7 cells. An increase of intracellular calcium ion levels induced by thapsigargin stimulated the expression of pip92 in H19-7 cells. Transient transfection of the cells with kinase-inactive mitogen-activated protein kinase kinase (MEK) and Src kinase or pretreatment with the chemical MEK inhibitor PD98059 significantly inhibited pip92 expression induced by thapsigargin. When constitutively active v-Src or MEK was overexpressed, the transcriptional activity of the pip92 gene was markedly increased. Dominant inhibitory Raf-1 blocked the transcriptional activity of pip92 induced by thapsigargin. The transcription factor Elk1 is activated during thapsigargin-induced pip92 expression. Taken together, these results suggest that an increase of intracellular calcium ion levels by thapsigargin stimulates the pip92 expression via Raf-MEK-extracellular signal-regulated protein kinase- as well as Src kinase-dependent signaling pathways.

Ras-independent activation of the Raf/MEK/ERK pathway upon calcium-induced differentiation of keratinocytes

MAPKs are crucially involved in the regulation of growth and differentiation of a variety of cells. To elucidate the role of MAPKs in keratinocyte differentiation, activation of ERK, JNK, and p38 in response to stimulation with extracellular calcium was analyzed. We provide evidence that calcium-induced differentiation of keratinocytes is associated with rapid and transient activation of the Raf/MEK/ERK pathway. Stimulation of keratinocytes with extracellular calcium resulted in activation of Raf isozymes and their downstream effector ERK within 10-15 min, but did not increase JNK or p38 activity. Calcium-induced ERK activation differed in kinetics from mitogenic ERK activation by epidermal growth factor and could be modulated by alterations of intracellular calcium levels. Interestingly, calcium stimulation led to down-regulation of Ras activity at the same time that ERK activation was initiated. Expression of a dominant-negative mutant of Ras also did not significantly impair calcium-induced ERK activation, indicating that calcium-mediated ERK activation does not require active Ras. Despite the transient nature of ERK activation, calcium-induced expression of the cyclin-dependent kinase inhibitor p21/Cip1 and the differentiation marker involucrin was sensitive to MEK inhibition, which suggests a role for the Raf/MEK/ERK pathway in early stages of keratinocyte differentiation.

The application of high density microarray for analysis of mitogenic signaling and cell-cycle in the adrenal

Angiotensin II (AII) binds to specific G-protein coupled receptors and is mitogenic in adrenal, liver epithelial, and vascular smooth muscle cells. The H295R human adrenocortical cell line, which expresses AII receptors predominantly of the AT1 subclass, proliferates in response to treatment with AII. The induction and maintenance of cellular proliferation involves a precisely coordinated induction of a variety of genes. As the human genome sequencing projects near completion a variety of high throughput technologies have been developed in order to create dynamic displays of genomic responses. One high throughput method, the gridded cDNA microarray has been developed in which immobilised DNA samples are hybridized on glass slides for the identification of global genomic responses. For this purpose high precision robotic microarrayers have been developed at AECOM. The cyclin D1 gene, which encodes the regulatory subunit of the cyclin D1-dependent kinase (CD1K) required for phosphorylation of the retinoblastoma protein (pRB), was induced by AII in H295R cells. Abundance of the cyclin D1 gene is rate-limiting in G1 phase progression of the cell-cycle in a variety of cell types. AII induced cyclin D1 promoter activity through a c-Fos and c-Jun binding sequence at -954 bp. Theabundance of c-Fos within this complex was increased by AII treatment. Analysis of AII signaling in adrenal cells by cDNA microarray demonstrated an induction of the human homologue of Xenopus XPMC2 (HXPMC2). The cDNA for XPMC2 was previously shown to rescue mitotic catastrophe in mutant S. Pombe defective in cdc2 kinase function. Further studies are required to determine the requirement for cyclin D1 and XPMC2H in AII-induced cell-cycle progression and cellular proliferation in the adrenal.

Growth factor-independent proliferation of erythroid cells infected with Friend spleen focus-forming virus is protein kinase C dependent but does not require Ras-GTP

Interaction of erythropoietin (Epo) with its cell surface receptor activates signal transduction pathways which result in the proliferation and differentiation of erythroid cells. Infection of erythroid cells with the Friend spleen focus-forming virus (SFFV) leads to the interaction of the viral envelope glycoprotein with the Epo receptor and renders these cells Epo independent. We previously reported that SFFV induces Epo independence by constitutively activating components of several Epo signal transduction pathways, including the Jak-Stat and the Raf-1/mitogen-activated protein kinase (MAPK) pathways. To further evaluate the mechanism by which SFFV activates the Raf-1/MAPK pathway, we investigated the effects of SFFV on upstream components of this pathway, and our results indicate that SFFV activates Shc and Grb2 and that this leads to Ras activation. While studies with a dominant-negative Ras indicated that Ras was required for Epo-induced proliferation of normal erythroid cells, the Epo-independent growth of SFFV-infected cells can still occur in the absence of Ras, although at reduced levels. In contrast, protein kinase C (PKC) was shown to be required for the Epo-independent proliferation of SFFV-infected cells. Further studies indicated that PKC, which is thought to be involved in the activation of both Raf-1 and MAPK, was required only for the activation of MAPK, not Raf-1, in SFFV-infected cells. Our results indicate that Ras and PKC define two distinct signals converging on MAPK in both Epo-stimulated and SFFV-infected erythroid cells and that activation of only PKC is sufficient for the Epo-independent proliferation of SFFV-infected cells.

Thapsigargin-induced degranulation of mast cells is dependent on transient activation of phosphatidylinositol-3 kinase

Thapsigargin, which elevates cytosolic calcium levels by inhibiting the sarcoplasmic/endoplasmic reticulum calcium-dependent ATPase, was tested for its ability to degranulate bone marrow-derived mast cells (BMMCs) from src homology 2-containing inositol phosphatase +/+ (SHIP+/+) and SHIP-/- mice. As was found previously with steel factor, thapsigargin stimulated far more degranulation in SHIP-/- than in SHIP+/+ BMMCs, and this was blocked with the phosphatidylinositol-3 (PI-3) kinase inhibitors, LY294002 and wortmannin. In contrast to steel factor, however, this heightened degranulation of SHIP-/- BMMCs was not due to a greater calcium influx into these cells, nor was the thapsigargin-induced calcium influx inhibited by LY294002, suggesting that the heightened thapsigargin-induced degranulation of SHIP-/- BMMCs was due to a PI-3 kinase-regulated step distinct from that regulating calcium entry. An investigation of thapsigargin-stimulated pathways in both cell types revealed that MAPK was heavily but equally phosphorylated. Interestingly, the protein kinase C inhibitor, bisindolylmaleimide (compound 3), totally blocked thapsigargin-induced degranulation in both SHIP+/+ and SHIP-/- BMMCs. As well, thapsigargin stimulated a PI-3 kinase-dependent, transient activation of protein kinase B, and this activation was far greater in SHIP-/- than in SHIP+/+ BMMCs. Consistent with this, thapsigargin was found to be a potent survival factor, following cytokine withdrawal, for both cell types and was more potent with SHIP-/- cells. These studies have both identified an additional PI-3 kinase-dependent step within the mast cell degranulation process, possibly involving 3-phosphoinositide-dependent protein kinase-1 and a diacylglycerol-independent protein kinase C isoform, and shown that the tumor-promoting activity of thapsigargin may be due to its activation of protein kinase B and subsequent promotion of cell survival.

Secretory products from PC-3 and MCF-7 tumor cell lines upregulate osteopontin in MC3T3-E1 cells

Tumor cells frequently have pronounced effects on the skeleton including bone destruction, bone pain, hypercalcemia, and depletion of bone marrow cells. Despite the serious sequelae associated with skeletal metastasis, the mechanisms by which tumor cells alter bone homeostasis remain largely unknown. In this study, we tested the hypothesis that the disruption of bone homeostasis by tumor cells is due in part to the ability of tumor cells to upregulate osteopontin (OPN) mRNA in osteoblasts. Conditioned media were collected from tumor cells that elicit either osteolytic (MCF-7, PC-3) or osteoblastic responses (LNCaP) in animal models and their effects on OPN gene expression were compared using an osteoblast precursor cell line, MC3T3-E1 cells. Secretory products from osteolytic but not osteoblastic tumor cell lines were demonstrated to upregulate OPN in osteoblasts while inhibiting osteoblast proliferation and differentiation. Signal transduction studies revealed that regulation of OPN was dependent on both protein kinase C (PKC) and the mitogen-activated protein (MAP) kinase cascade. These results suggest that the upregulation of OPN may play a key role in the development of osteolytic lesions. Furthermore, these results suggest that drugs that prevent activation of the MAP kinase pathway may be efficacious in the treatment of osteolytic metastases.

The topography and subcellular distribution of mitogen-activated protein kinase kinase1 (MEK1) in adult rat brain and differentiating PC12 cells

Mitogen-activated protein kinase signal transduction pathway involved in the regulation of proliferation and differentiation of various mammalian cells consists of a sequential activation of three protein kinases, Raf, mitogen-activated protein kinase kinase, and mitogen-activated protein kinase. These kinases are highly expressed in brain and play an important role in neuronal signalling. In this study, to further characterize mitogen-activated protein kinase signalling pathway in brain, we have elucidated the topography and subcellular distribution of mitogen-activated protein kinase kinasel in adult rat brain and differentiating PC12 cells. Our immunohistochemical data indicate that mitogen-activated protein kinase kinase1 is widely distributed throughout the brain and expressed prominently in cortex, hippocampus, brainstem, hypothalamus and cerebellum. In these areas of brain mitogen-activated protein kinase kinasel is exclusively neuronal in origin and is localized within perikarya and dendrites. Confocal microscopy data has determined that a portion of mitogen-activated protein kinase kinase1 in rat brain is co-localized with microtubules. This co-localization was observed only within neuritic shaft and cilia of ventricular ependymal cells. In nerve growth factor-induced differentiating PC12 cells, mitogen-activated protein kinase kinase1 displays co-localization with microtubules within proximal regions of neuritic shafts and their junctions with the cell somas. From bovine brain extract, mitogen-activated protein kinase kinasel co-purifies with microtubules. In vitro kinase assay detected mitogen-activated protein kinase kinase1 activity within purified microtubules. These observations indicate that mitogen-activated protein kinase kinase1 is associated with microtubules within some specialized compartments of the brain and microtubule-associated mitogen-activated protein kinase kinase1 is catalytically active.

Platelet-derived growth factor stimulation of mitogen-activated protein kinases and cyclin D1 promoter activity in cultured airway smooth-muscle cells. Role of Ras

We hypothesized that in bovine tracheal myocytes, growth factor treatment induces transcription from the cyclin D1 promoter that is dependent on the activation of both Ras and extracellular signal-related kinase (ERK). We found that platelet-derived growth factor (PDGF) treatment induced substantial activation of ERK2 that was blocked by expression of a dominant-negative Ha-Ras. Further, expression of a constitutively active Ha-Ras induced substantial ERK2 activity, consistent with the notion that Ras is required and sufficient for ERK activation. PDGF treatment induced only modest activation of the Jun amino terminal kinase-1 (JNK1) and p38 mitogen-activated protein kinases (MAPKs). Active Ras induced similar responses, implying that complete activation of the JNK and p38 pathways requires additional or alternative upstream signaling intermediates besides Ras. In contrast, expression of a constitutively active Rac1, an alternative guanosine triphosphatase involved in intracellular signaling, produced a high level of JNK1 activation, suggesting that Rac1 is an important upstream activator of JNK in this system. Active Ras and MAPK/ ERK kinase-1 (MEK1) (the upstream activator of ERK) each induced cyclin D1 promoter activity, whereas active stress-activated protein kinase/ERK kinase-1 (SEK1), an upstream activator of JNK, did not. Finally, the synthetic MEK inhibitor PD98059 blocked Ras-induced cyclin D1 promoter activity. Together, these data suggest that in bovine tracheal myocytes: (1) activation of MAPK by PDGF is dependent on Ras; (2) active Ras is sufficient for ERK activation but is insufficient for maximal activation of JNK or p38; (3) activation of Rac1 is sufficient for maximal JNK activation; and (4) Ras, MEK, and ERK constitute a distinct pathway to cyclin D1 transcriptional activation.

Activation of the 9E3/cCAF chemokine by phorbol esters occurs via multiple signal transduction pathways that converge to MEK1/ERK2 and activate the Elk1 transcription factor

Using primary fibroblasts in culture, we have investigated the signal transduction mechanisms by which phorbol esters, a class of tumor promoters, activate the 9E3 gene and its chemokine product the chicken chemotactic and angiogenic factor. This gene is highly stimulated by phorbol 12,13-dibutyrate (PDBu) via three pathways: (i) a small contribution through protein kinase C (the commonly recognized pathway for these tumor promoters), (ii) a contribution involving tyrosine kinases, and (iii) a larger contribution via pathways that can be interrupted by dexamethasone. All three of these pathways converge into the mitogen-activated protein kinases, MEK1/ERK2. Using a luciferase reporter system, we show that although both the AP-1 and PDRIIkB (a NFkappaB-like factor in chickens) response elements are capable of activation in these normal cells, regions of the 9E3 promoter containing them are unresponsive to PDBu stimulation. In contrast, we show for the first time that activation by PDBu occurs through a segment of the promoter containing Elk1 response elements; deletion and mutation of these elements abrogates 9E3/chicken chemotactic and angiogenic factor expression. Electrophoretic mobility shift assays and functional studies using PathDetect systems show that stimulation of the cells by phorbol esters leads to activation of the Elk1 transcription factor, which binds to its element in the 9E3 promoter.

Partial characterization of a novel mitogen-activated protein kinase/extracellular signal-regulated kinase activator in airway smooth-muscle cells

We demonstrated previously that in bovine tracheal myocytes, pretreatment with either forskolin or histamine significantly reduces both platelet-derived growth factor (PDGF)- and epidermal growth factor- induced Raf-1 activation but fails to inhibit extracellular signal-regulated kinase (ERK) activation substantially, evidence of a Raf-1-independent ERK activation pathway. To identify Raf-1-independent upstream signaling intermediates of mitogen-activated protein kinase/ERK kinase-1 (MEK1), the dual-function kinase required and sufficient for ERK activation in these cells, lysates from forskolin and PDGF-treated bovine tracheal myocytes were resolved using ion exchange chromatography. Kinase activity for MEK1 was assessed by in vitro phosphorylation assay. In all experiments, the major peak of MEK1 phosphorylation activity was detected in fractions 18 through 26 (80 to 160 mM NaCl), with the peak fraction eluting at a NaCl concentration of 140 mM. The ability of these fractions to activate MEK1 was confirmed by examining the phosphorylation of myelin basic protein, a known substrate for ERKs, in the presence of functional MEK1 and ERK1. Fractions containing kinase activity were also probed with antibodies against MEK kinase-1, Raf-1, A-Raf, B-Raf, Mos, and Tpl-2. None of these proteins was detected in fractions containing peak kinase activity, suggesting the presence of a novel PDGF-stimulated, forskolin-insensitive MEK1 kinase. Further separation of fractions holding peak MEK phosphorylation activity by gel filtration suggested an apparent molecular mass of 40 to 45 kD. We conclude that PDGF-induced activation of MEK1 in bovine tracheal myocytes is mediated at least in part by a novel kinase.

The p21-Ras signal transduction pathway and growth regulation in human high-grade gliomas

Deregulated p21-Ras function, as a result of mutation, overexpression or growth factor-induced overactivation, contributes to at least 30% of human cancer. This article reviews the potential role of the p21-Ras family of GTPases in the regulation of growth of high-grade gliomas and describes how targeting this oncoprotein clinically may provide a novel strategy to counteract glioma proliferation. The application of strategies directed at selectively opposing the deregulated signal transduction pathway of high-grade gliomas may be of potential therapeutic benefit and may offer a whole new arsenal of antineoplastic agents to be included in the multimodal treatment of these challenging neoplasms.

Epidermal growth factor and angiotensin II regulation of extracellular signal-regulated protein kinase in rat liver epithelial WB cells

Activation of extracellular signal-regulated protein kinase (ERK) is considered essential for mitogenesis. In the present study, rat liver epithelial WB cells were used to investigate the relative roles of Ca2+, protein kinase C (PKC), and protein tyrosine phosphorylation in mitogenesis and activation of the ERK pathway stimulated by epidermal growth factor (EGF) and angiotensin II (Ang II). The sensitivity of the ERK pathway to Ca2+ was studied by using 1,2-bis (O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) to chelate intracellular Ca2+ and a low extracellular Ca2+ concentration to prevent Ca2+ influx. Agonist-induced PKC activation was diminished by inhibition of PKC by GF-109203X (bisindolylmaleimide) or by down-regulation of PKC by long-term treatment of the cells with phorbol myristate acetate (PMA). Our results show that although activation of PKC was critical for mitogenesis induced by Ang II or EGF, the initial activation of ERK by both agonists in these cells was essentially independent of PKC activation and was insensitive to Ca2+ mobilization. This is in contrast to the findings in some cell types that exhibit a marked dependency on mobilization of Ca2+ and/or PKC activation. On the other hand, an obligatory tyrosine phosphorylation step for activation of ERK was indicated by the use of protein tyrosine kinase inhibitors, which profoundly inhibited the activation of ERK by EGF, Ang II, and PMA. Additional experiments indicated that tyrosine phosphorylation by a cytosolic tyrosine kinase may represent a general mechanism for G-protein coupled receptor mediated ERK activation.

No role for Ca++ or protein kinase C in alpha-1A adrenergic receptor activation of mitogen-activated protein kinase pathways in transfected PC12 cells

We studied the role of Ca++ and protein kinase C (PKC) in alpha-1A adrenergic receptor (AR)-mediated activation of mitogen-activated protein kinase pathways in PC12 cells. In PC12 cells stably transfected with the human alpha-1A AR, norepinephrine (NE) strongly activated both extracellular signal regulated kinases (ERKs) and c-jun-NH2-terminal kinases (JNK). Ten nanomolar thapsigargin (TG) increased cytoplasmic Ca++ at least as much as NE but did not activate ERKs or JNK. Higher concentrations of TG caused a small activation of ERKs but not JNK. Emptying [Ca++]i stores by pretreatment with TG prevented the NE-stimulated increase in [Ca++]i but not ERK or JNK activation. The Ca++ chelator bis(2-aminophenoxy)ethane-N-N-N'-N'-tetraacetate (BAPTA) dose dependently abolished NE-stimulated Ca++ responses but not ERK or JNK activation. NE increased tyrosine phosphorylation of Pyk2, and this response was neither blocked by BAPTA nor mimicked by TG. The phorbol ester tumor promoting agent (TPA) caused a dose-dependent activation of ERKs that was potentiated by 10 nM TG. TPA caused only a small activation of JNK relative to that caused by NE, which was not affected by TG. The potent PKC inhibitor bisindolylmaleimide I dose dependently inhibited ERK and JNK activation by TPA, but not NE. ATP and UTP activated similar mitogen-activated protein kinase responses through endogenous P2Y2 receptors, and these responses were not blocked by BAPTA or bisindolylmaleimide I, suggesting that these results may be generalizable to other Gq/11-coupled receptors. The results suggest that Ca++ release and PKC activation are neither necessary nor sufficient for alpha-1A AR-mediated activation of mitogenic responses in PC12 cells.

Strict regulation of c-Raf kinase levels is required for early organogenesis of the vertebrate inner ear

Regulation of organogenesis involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. Here we have investigated the pattern of expression of c-Raf kinase in the inner ear during early developmental stages and the consequences of manipulating c-Raf levels by misexpression of c-raf viral vectors in organotypic cultures of otic vesicle explants. We found that otic vesicles expressed c-Raf and its level remained constant during embryonic days 2 and 3 (E2-E3). c-Raf activity was increased in response to insulin like growth factor-I (IGF-I) and the activation by IGF-I of the c-Raf kinase pathway was a requirement to turn on cell proliferation in the otic vesicle. Overexpression of c-raf in E2.5 explants increased the proliferative response to low serum and IGF-I and blocked differentiation induced by retinoic acid. The increase in c-Raf levels also prevented nerve growth factor (NGF)-dependent induction of programmed cell death. Consistent with these results, the expression of a dominant negative c-Raf mutant potentiated retinoic acid action and decreased the rate of cell proliferation. We conclude that a strict control of c-Raf levels is essential for the co-ordination of the biological processes that operate simultaneously during early inner ear development.

Activation of ERK by Ca2+ store depletion in rat liver epithelial cells

In rat liver epithelial (WB) cells, Ca2+ pool depletion induced by two independent methods resulted in activation of extracellular signal-regulated protein kinase (ERK). In the first method, Ca2+ pool depletion by thapsigargin increased the activity of ERK, even when rise in cytosolic Ca2+ was blocked with the Ca2+ chelator BAPTA-AM. For the second method, addition of extracellular EGTA at a concentration shown to deplete intracellular Ca2+ pools also increased ERK activity. In each instance, ERK activation, as measured by an immunocomplex kinase assay, was greatly reduced by the tyrosine kinase inhibitor genistein, suggesting that Ca2+ store depletion increased ERK activity through a tyrosine kinase pathway. The intracellular Ca2+-releasing agent thapsigargin increased Fyn activity, which was unaffected by BAPTA-AM pretreatment, suggesting that Fyn activity was unaffected by increased cytosolic free Ca2+. Furthermore, depletion of intracellular Ca2+ with EGTA caused inactivation of protein phosphatase 2A and protein tyrosine phosphatases. ANG II-induced activations of Fyn, Raf-1, and ERK were augmented in cells pretreated with BAPTA-AM, but ANG II-induced expression of the dual-specificity phosphatase mitogen-activated protein kinase phosphatase-1 was blocked by BAPTA-AM pretreatment. Together these results indicate that ERK activity is regulated by the balance of phosphorylation vs. dephosphorylation reactions in intact cells and that the amount of Ca2+ stored in intracellular pools plays an important role in this regulation.

The protein kinase Pak3 positively regulates Raf-1 activity through phosphorylation of serine 338

The pathway involving the signalling protein p21Ras propagates a range of extracellular signals from receptors on the cell membrane to the cytoplasm and nucleus. The Ras proteins regulate many effectors, including members of the Raf family of protein kinases. Ras-dependent activation of Raf-1 at the plasma membrane involves phosphorylation events, protein-protein interactions and structural changes. Phosphorylation of serine residues 338 or 339 in the catalytic domain of Raf-1 regulates its activation in response to Ras, Src and epidermal growth factor. Here we show that the p21-activated protein kinase Pak3 phosphorylates Raf-1 on serine 338 in vitro and in vivo. The p21-activated protein kinases are regulated by the Rho-family GTPases Rac and Cdc42. Our results indicate that signal transduction through Raf-1 depends on both Ras and the activation of the Pak pathway. As guanine-nucleotide-exchange activity on Rac can be stimulated by a Ras-dependent phosphatidylinositol-3-OH kinase, a mechanism could exist through which one Ras effector pathway can be influenced by another.

Calcium channel blockers in cardiac failure

Congestive cardiac failure is an increasingly prevalent syndrome associated with a high morbidity and mortality. The role of calcium channel blockers in the treatment of heart failure is unclear. The potential benefits of these agents derive not only from their vasodilator properties, but also from anti-ischemic effects, beneficial effects on endothelial function and the development of atherosclerosis, and favorable effects on calcium cycling at a molecular level. Pitted against this array of potential benefits are direct negative inotropic effects and the potential for neuroendocrine activation. Treatment with short-acting dihydropyridine agents has not resulted in long-term clinical benefits in patients with cardiac failure. Diltiazem may be beneficial in patients with nonischemic heart failure, and verapamil has a neutral effect in cardiac failure, although it may have a role in combination with ace inhibition. To date, amlodipine has been associated with the most promising results, with evidence of a mortality benefit in nonischemic heart failure. Mibefradil is of no benefit in the management of heart failure, although the trend toward increased mortality in the treatment arm of the Mortality Assessment in Congestive Heart Failure (MACH)-1 trial may have been due to drug interactions. The potential role of calcium blockers in diastolic dysfunction and in combination with ace-inhibition requires further study.

Involvement of guanosine triphosphatases and phospholipase C-gamma2 in extracellular signal-regulated kinase, c-Jun NH2-terminal kinase, and p38 mitogen-activated protein kinase activation by the B cell antigen receptor

Mitogen-activated protein (MAP) kinase family members, including extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase ( JNK), and p38 MAP kinase, have been implicated in coupling the B cell antigen receptor (BCR) to transcriptional responses. However, the mechanisms that lead to the activation of these MAP kinase family members have been poorly elucidated. Here we demonstrate that the BCR-induced ERK activation is reduced by loss of Grb2 or expression of a dominant-negative form of Ras, RasN17, whereas this response is not affected by loss of Shc. The inhibition of the ERK response was also observed in phospholipase C (PLC)-gamma2-deficient DT40 B cells, and expression of RasN17 in the PLC-gamma2-deficient cells completely abrogated the ERK activation. The PLC-gamma2 dependency of ERK activation was most likely due to protein kinase C (PKC) activation rather than calcium mobilization, since loss of inositol 1,4,5-trisphosphate receptors did not affect ERK activation. Similar to cooperation of Ras with PKC activation in ERK response, both PLC-gamma2-dependent signal and GTPase are required for BCR-induced JNK and p38 responses. JNK response is dependent on Rac1 and calcium mobilization, whereas p38 response requires Rac1 and PKC activation.

Mitogen-activated protein kinase is increased in the limbic structures of the rat brain during the early stages of status epilepticus

Systemic administration of pilocarpine (PILO) in adult rat produces acute limbic seizures leading to status epilepticus. Recent studies have shown the activation of mitogen-activated protein kinase (MAPK) cascades during experimentally induced seizures. MAPK activation may be triggered by glutamatergic stimulation and may play a key role in signal transduction pathways. In the present study, immunocytochemistry was used to analyze the spatiotemporal distribution pattern of the MAPK protein and its active form (A-MAPK) following PILO-induced status epilepticus. MAPK and A-MAPK immunoreactivities exhibited different patterns of distribution in the brain of normal and epileptic rats. The saline-treated rats, as well as the animals that received PILO but did not evolve to status epilepticus, showed a weak but selective MAPK immunoreactivity, detected in the hippocampal pyramidal neurons, dentate gyrus, hilus, CA3, CA1, and entorhinal, piriform, and cingulate cortices. A-MAPK immunoreactivity was instead observed only in neurites of the CA3 and hilus and in cells of the entorhinal and piriform cortices. In PILO-treated rats, between 30 and 60 min after status epilepticus there was an increase of the immunoreactivity to both antibodies, which were differently distributed throughout several structures of the limbic system. The immunostaining showed a slight decrease after 5 h of status epilepticus. However, MAPK and A-MAPK immunopositivities decreased markedly after 12 h of status epilepticus, returning almost to the basal expression. These findings are consistent with a spatial and time-dependent MAPK expression in selected limbic structures, and its activation could represent an initial trigger for neuronal modifications that may take part in the mechanism underlying acute epileptogenesis and in long-lasting neuropathological changes of the PILO model of epilepsy.

Selective modulation of MAP kinase in embryonic palate cells

Murine embryonic palate mesenchyme (MEPM) cells are responsive to a number of endogenous factors found in the local embryonic tissue environment. Recently, it was shown that activation of the cyclic AMP (cAMP) or the transforming growth factor beta (TGFbeta) signal transduction pathways modulates the proliferative response of MEPM cells to epidermal growth factor (EGF). Since the mitogen-activated protein kinase (MAPK) cascade is a signal transduction pathway that mediates cellular responsiveness to EGF, we examined the possibility that several signaling pathways which abrogate EGF-stimulated proliferation do so via the p42/p44 MAPK signaling pathway. We demonstrate that EGF stimulates MAPK phosphorylation and activity in MEPM cells maximally at 5 minutes. Tyrosine phosphorylation and activation of MAPK was unaffected by treatment of MEPM cells with TGFbeta or cholera toxin. Similarly, TGFbeta altered neither EGF-induced MAPK tyrosine phosphorylation nor activity. However, the calcium ionophore, A23187, significantly increased MAPK phosphorylation which was further increased in the presence of EGF, although calcium mobilization reduced EGF-induced proliferation. Despite the increase in phosphorylation, we could not demonstrate induction of MAPK activity by A23187. Like EGF, phorbol ester, under conditions which activate PKC isozymes in MEPM cells, increased MAPK phosphorylation and activity but was also growth inhibitory to MEPM cells. The MEK inhibitor, PD098059, only partially abrogated EGF-induced phosphorylation. Likewise, depletion of PKC isozymes partially abrogated EGF-induced MAPK phosphorylation. Inhibition of both MEK and PKC isozymes resulted in a marked decrease in MAPK activity, confirming that EGF uses multiple pathways to stimulate MAPK activity. These data indicate that the MAPK cascade does not mediate signal transduction of several agents that inhibit growth in MEPM cells, and that there is a dissociation of the proliferative response and MAP kinase activation. Furthermore, other signaling pathways known to play significant roles in differentiation of palatal tissue converge with the MAPK cascade and may use this pathway in the regulation of alternative cellular processes.

Regulation of the cell cycle at the G2/M boundary in metastatic melanoma cells by 12-O-tetradecanoyl phorbol-13-acetate (TPA) by blocking p34cdc2 kinase activity

12-O-Tetradecanoyl phorbol-13-acetate (TPA) inhibits the growth of most malignant melanoma cells but stimulates the growth of normal human melanocytes. We previously showed that addition of TPA inhibits the growth of the human metastatic melanoma cell line, Demel, by blocking cells at both the G1/S and G2/M cell cycle transitions (D. L. Coppock et al., 1992, Cell Growth Differ. 3, 485-494). To examine the G2/M transition, we developed a method to synchronize the cells in early S phase using Lovastatin and mevalonate, followed by treatment with hydroxyurea (HU). TPA (30 nM) was effective in blocking cells from entering mitosis and reentering G1 when added up to the end of G2. These cells arrested in G2. Examination of the levels of cyclins A and B1 demonstrated that the levels of these cyclins were not limiting for entrance into M. However, the addition of TPA blocked the increase in p34(cdc2)/cyclin B1 kinase activity. In cells treated with TPA, most p34(cdc2) was found in the slowly migrating forms on Western blots, which contained increased levels of phosphotyrosine. In addition, the level of the cyclin-dependent kinase inhibitor p21(Cip1/Waf1), but not of p27(Kip1), was increased. We examined the expression of protein kinase C (PKC) isoforms in Demel cells using Western blots to understand which types were involved in the G2 arrest. Demel cells expressed the PKC alpha, betaI, betaII, delta, epsilon, iota/lambda, zeta, and mu isozymes. PKC eta and PKC theta were not detected. Addition of TPA did not completely down regulate any PKC isozymes over a 12-h period in these synchronized cells. PKC alpha, betaI, betaII, delta, and epsilon isozymes were translocated to the membrane fraction from the cytosolic fraction when treated with TPA. PKC delta appeared as a doublet and the addition of TPA shifted a majority to the slower migrating form. The level of PKC mu was constant; however, a slow mobility form was observed in TPA-treated cells. This reduced mobility was at least partially due to phosphorylation. Thus, the arrest of growth in G2 appears to be due to the inhibition of the p34(cdc2) kinase activity which is associated with the increased expression of p21(Cip1/Waf1) and increased phosphorylation on tyrosine of p34(cdc2). This arrest, in turn, is associated with a shift of PKC isozymes PKC alpha, PKC betaI, PKC betaII, PKC delta, PKC epsilon, and PKC mu to the membrane fraction which is induced by addition of TPA.

Protein kinase C regulates chondrogenesis of mesenchymes via mitogen-activated protein kinase signaling

A possible regulatory mechanism of protein kinase C (PKC) in the chondrogenesis of chick limb bud mesenchymes has been investigated. Inhibition or down-regulation of PKC resulted in the activation of a mitogen-activated protein kinase subtype Erk-1 and the inhibition of chondrogenesis. On the other hand, inhibition of Erk-1 with PD98059 enhanced chondrogenesis and relieved PKC-induced blockage of chondrogenesis. Erk-1 inhibition, however, did not affect expression and subcellular distribution of PKC isoforms expressed in mesenchymes nor cell proliferation. The results suggest that PKC regulates chondrogenesis by modulating Erk-1 activity. Inhibition or depletion of PKC inhibited proliferation of chondrogenic competent cells, and Erk-1 inhibition did not affect PKC modulation of cell proliferation. However, PKC-induced modulation of expression of cell adhesion molecules involved in precartilage condensation was reversed by the inhibition of Erk-1. Expression of N-cadherin was detected at the early period of chondrogenesis. Inhibition or depletion of PKC induced sustained expression of N-cadherin, and Erk-1 inhibition blocked the effects of PKC modulation. The expression of integrin alpha5 beta1 and fibronectin was found to be increased transiently during chondrogenesis. Depletion or inhibition of PKC caused a continuous increase of the expression of these molecules throughout the culture period, and Erk-1 inhibition abolished the modulating effects of PKC. Because reduction of the examined cell adhesion molecule expression is a prerequisite for the progression of chondrogenesis after cell condensation, our results indicate that PKC regulates chondrogenesis by modulating expression of these molecules via Erk-1 signaling.

The Raf-1 protein mediates insulin-like growth factor-induced proliferation of erythroid progenitor cells

Previous studies from this and other laboratories have shown that insulin-like growth factor-1 (IGF-I) and insulin-like growth factor-2 (IGF-II) support erythroid colony formation in cultures supplemented with serum substitute and recombinant erythropoietin. Subpopulations of IGF-I- and IGF-II-dependent, erythropoietin-independent colony-forming unit-erythroid (CFU-E)-derived colonies and BFU-E-derived colonies were identified under serum-substituted conditions for adult bone-marrow-derived erythroid progenitors which proliferate in the absence and presence of exogenous anti-erythropoietin receptor monoclonal antibody and in serum-substituted medium that was preadsorbed with anti-erythropoietin IgG. To assess whether Raf-1 is required for the formation of IGF-dependent, erythropoietin-independent human erythroid colonies, 5-15 microM sense or antisense oligomer to raf-1 were added to serum-substituted cultures containing either 2 U/ml recombinant human erythropoietin (rHuEpo) alone or 0-1,000 ng/ml IGF-I or IGF-II with/without 2 U/ml rHuEpo. Both erythropoietin-induced and IGF-induced erythroid colony formation were completely blocked by antisense (but not sense) oligomers to raf-1. Purified human CFU-Es were examined for Raf-1 message and protein. Total RNA was extracted, and raf-1 mRNA was detected on Northern blots. Furthermore, a 74 kD protein, corresponding to Raf-1, was also detected in CFU-Es purified from human adult sources. Together, these studies support the hypothesis that the Raf-1 protein mediates both erythropoietin-induced and IGF-induced signal transduction in human erythroid progenitor cells.

Activation and glucagon regulation of mitogen-activated protein kinases (MAPK) by insulin and epidermal growth factor in cultured rat and human hepatocytes

Many hepatocellular activities may be proximally regulated by intracellular signalling proteins including mitogen-activated protein kinases (MAPK). In this study, signalling events from epidermal growth factor (EGF) and insulin were examined in primary cultured human and rat hepatocytes. Using Western immunoblots, rat and human hepatocytes were found to produce a rapid tyrosine phosphorylation of the EGF receptor and MAPK following 0.5-1 min exposure to EGF. Phosphorylation of p42 and p44 MAPK was observed following 2.5 min exposure to EGF. Insulin treatment produced phosphorylation of the insulin receptor beta subunit; she phosphorylation was not observed. MAPK phosphorylation corresponded with a shift in molecular weight and an increase in kinase activity. Insulin-dependent activation of MAPK was unequivocally observed only in human hepatocytes, though a slight activation was detected in rat. Co-treatment with insulin and EGF produced phosphorylation and complete electrophoretic shift in molecular weight of MAPK, with an additive or synergistic increase in enzyme activity in rat but not human hepatocytes; human hepatocyte MAPK was maximally stimulated by EGF alone. Glucagon pretreatment blocked phosphorylation, gel mobility shift and kinase activity of MAPK induced by insulin but only partially blocked EGF-induced MAPK activation in human hepatocytes. Glucagon also reduced the activation of MAPK by EGF in rat hepatocytes. Pre-treatments with forskolin or cyclic AMP analogues diminished in the insulin-, EGF- and insulin plus EGF-dependent activation of MAPK in rat hepatocytes without effecting phosphorylation of receptors or MAPK. These results indicate that although EGF and insulin may both signal through the MAPK/ras/raf/MAPK pathway, the response for MAPK differs between these ligands and between species. Further, in both rat and human, glucagon exerts its effects through a cyclic AMP-dependent mechanism at a level in the insulin and EGF signal transduction pathways downstream of MAPK but promixal to MAPK. The partial inhibition of EGF-induced MAPK phosphorylation by glucagon in human hepatocytes provides further evidence for a raf-1-independent pathway for activation of MAPK.

Effects of propranolol on phosphatidate phosphohydrolase and mitogen-activated protein kinase activities in A7r5 vascular smooth muscle cells

High doses of propranolol inhibit phosphatidate phosphohydrolase (PAP) activity in intact cells, thus blocking metabolism of phosphatidic acid (PA), product of the phospholipase D (PLD) reaction. Vasopressin and phorbol ester activate PLD and ERK (extracellular signal-regulated protein kinase) mitogen-activated protein kinases in A7r5, a rat vascular smooth muscle cell line. Propranolol increased PA levels in intact A7r5 cells and inhibited cytosolic PAP and membrane calcium-independent phospholipase A2 but did not activate PLD or enhance agonist-induced PA accumulation. Incubation of cells with 200 microM propranolol for 10-45 min markedly elevated PA but caused only partial activation of ERKs. Propranolol and other lipophilic amines caused a time- and dose-dependent detachment of cells from their substrate. These results confirm that elevation of PA is not a strong signal for ERK activation and emphasize that caution should be exercised in using propranolol as a PAP inhibitor in intact cells.

The RAF family: an expanding network of post-translational controls and protein-protein interactions

Protein kinase RAF is strategically located in the "Ras-MAP-kinase signal transduction pathway", a principle system which transmits signals from growth factor receptors to the nucleus, resulting in cell proliferation. Growth factor responses are mediated in part by activation of Ras, which in turn activates RAF to phosphorylate MEK, its downstream substrate. MEK activates MAP-kinase to influence nuclear events. It is clear, however, that a network of signals other than those carried by Ras plays a role in RAF regulation. These orthogonal influences are mediated by: serine/threonine kinases, tyrosine kinases, and protein-protein interactions. As a further complication to the RAF network, three isoforms of RAF have been established which have divergent N-terminal regulatory domains. Whereas these divergent regulatory domains implicate isoform-specific functions, no clear evidence or hypothesis for distinct functions for individual isoforms has been presented. Recently, "isoform-specific protein interactions" have been identified among numerous proteins interacting with RAF. These studies may serve to delineate independent functions for RAF isoforms.

Differential dose-dependent effects of epidermal growth factor on gene expression in A431 cells: evidence for a signal transduction pathway that can bypass Raf-1 activation

Epidermal growth factor (EGF), which plays an important role in normal and tumoral cell growth regulation, displays an ambivalent dose-dependent effect on the proliferation of epithelial cells overexpressing EGF receptor. However, the underlying molecular mechanisms remain obscure. In this study we have examined the regulation of amphiregulin (AR) gene expression by growth inhibitory (10(-9) M) and stimulatory (10(-12) M) EGF concentrations in A431 cells. The time course of AR messenger RNA (mRNA) accumulation was different with 10(-12) and 10(-9) M EGF; AR induction by 10(-9) M EGF peaked between 1 and 1.5 h, then decreased to the basal level within 2 h. Conversely, the induction by 10(-12) M EGF was slightly delayed, but persisted for 4 h. The involvement of tyrosine phosphorylation in AR induction by EGF was suggested by the ability of the tyrosine phosphatase inhibitor sodium orthovanadate to prolong AR expression induced by 10(-12) or 10(-9) M EGF. In the presence of the protein phosphatase 2A inhibitor, okadaic acid, 10(-9) M EGF induced a persistent accumulation of AR mRNA. On the contrary, okadaic acid abrogated the stimulation of AR mRNA level induced by a low EGF concentration, suggesting that both EGF concentrations activated distinct regulatory mechanisms. The signaling components involved in the differential activities of EGF in A431 cells were then examined. We previously reported a relationship between the ambivalent activity of EGF and the p42-mitogen-activated protein (MAP) kinase activity. Thus, 10(-12) M EGF induced a sustained MAP kinase activation, whereas 10(-9) M EGF led to a sharp, but transitory, activation. The MAP kinases are activated by MAP kinase kinases (MEK1 and MEK2). Whereas no significant effect of 10(-12) M EGF could be detected, 10(-9) M EGF was shown to activate MEK1 and, to a lesser extent, MEK2. Also, both MAP kinase activation and AR induction by 10(-9) M, but not by 10(-12) M, EGF were inhibited by the MEK1 inhibitor PD98059. Moreover, the involvement of c-Raf-1 in the signaling pathway induced by EGF was verified. A concentration of 10(-9) M EGF induced stimulation of c-Raf-1 kinase activity, whereas 10(-12) M EGF not only failed to activate c-Raf-1, but led to a moderate decrease in its kinase activity. These results demonstrate that in EGF receptor-overexpressing cells, EGF may differently affect gene expression and cell proliferation through distinct mechanisms of regulation.

Regulation of Na+/H+ exchanger gene expression: mitogenic stimulation increases NHE1 promoter activity

We examined factors important in regulation of expression of the Na+/H+ exchanger gene in NIH/3T3 cells. A stable fibroblast cell line was generated that contained a 1.1-kb proximal fragment of the mouse NHE1 promoter. The addition of serum to serum-starved cells resulted in an increase in activity of the NHE1 promoter. The mitogenic agonists insulin, thrombin, and epidermal growth factor also increased transcription from the NHE1 promoter. Phorbol esters also increased NHE1 promoter-directed transcription, whereas the serine/threonine protein kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine inhibited this stimulation. The protein kinase inhibitors GF-109203X, PD-98059, and genistein all stimulated promoter activity. Promoter deletion analysis and gel mobility shift assays showed that a region between 0.9 and 1.1 kb from the start site was involved in mediating the effect of mitogenic stimulation. The results show that a variety of mitogenic factors can activate the NHE1 promoter during cell growth and proliferation.

The role of protein kinase C (PKC) in the evolution and proliferation of malignant gliomas, and the application of PKC inhibition as a novel approach to anti-glioma therapy

The present article reviews the role of the second messenger enzyme protein kinase C (PKC) in the growth regulation of high-grade gliomas, and evaluates the efficacy of therapeutic strategies directed against PKC for blocking the proliferation of these malignancies in in vitro and in vivo models. The translation of such strategies to the treatment of patients with malignant gliomas may provide a novel approach for improving the otherwise grim outlook associated with these neoplasms.

A role for Ca2+/calmodulin-dependent protein kinase II in the mitogen-activated protein kinase signaling cascade of cultured rat aortic vascular smooth muscle cells

Exposure of cultured rat aortic vascular smooth muscle (VSM) cells to the Ca2+ ionophore ionomycin produced an increase in extracellular signal-regulated kinase 1/2 (ERK1/2) activity that was maximal between 2 and 5 minutes but then declined to basal values within 20 minutes of stimulation. Elevation of [Ca2+]i in VSM cells leads to an even more rapid activation of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II); thus, it was postulated that the Ca(2+)-dependent component of ERK1/2 activation was mediated by CaM kinase II. Transient ERK1/2 activation by ionomycin was almost completely abolished by pretreating cells with 30 mumol/L KN-93, a CaM kinase II inhibitor. Treatment of cells with KN-93 did not antagonize the ability of ionomycin to mobilize intracellular Ca2+ but prevented CaM kinase II and ERK1/2 activation with almost identical potencies. Consistent with a role for Ca2+ and calmodulin in intracellular Ca(2+)-induced activation of ERK, cells pretreated with calmodulin inhibitors (W-7 or calmidazolium) exhibited an attenuated ERK response to ionomycin. ERK1/2 activation in response to phorbol esters and platelet-derived growth factor were not significantly affected by KN-93, whereas the response to angiotensin II and thrombin were attenuated by 60% and 40%, respectively. Transient expression of wild-type delta 2 CaM kinase II in COS-7 cells resulted in increased ERK2 activity, whereas coexpression of wild-type and a kinase-negative mutant resulted in a diminution of this response. These data suggest that regulation of cellular responses by Ca(2+)-dependent pathways in VSM cells may be mediated in part by CaM kinase II-dependent activation of ERK1/2.

Adhesion of bovine airway smooth muscle cells activates extracellular signal-regulated kinases

Extracellular signal-regulated kinases (ERKs) phosphorylate and regulate cytoskeletal components of contractile cells and have been implicated in integrin-mediated adhesion. In this study, we examined the contributions of adherence, cell flattening, and cytoskeletal reorganization to adhesion-induced ERK activation in cultured bovine tracheal myocytes. We found, as evidenced by a reduction in electrophoretic mobility, that adhesion to fibronectin induced phosphorylation of both p44ERK1 and p42ERK2. In-gel kinase assays confirmed activation of both p44ERK1 and p42ERK2 in fibronectin-adherent cells, consistent with the notion that ligand-integrin binding is required for adhesion-induced ERK activation. However, ERK activation was maximal 2-4 h after plating, and adherence to either polystyrene or poly-L-lysine also caused ERK activation (fold increase 4 h after plating: fibronectin, 3.75 +/- 0.33; polystyrene, 3.95 +/- 0.78; poly-L-lysine, 2.14 +/- 0.36). Inspection of myocytes following passage onto fibronectin showed near 100% adhesion and cell spreading after 4 h, whereas cells plated onto poly-L-lysine demonstrated adherence but minimal spreading. To test whether the cytoskeletal reorganization accompanying cell spreading is required for adhesion-induced ERK activation, we assessed ERK activity following pretreatment with cytochalasin D, an inhibitor of actin polymerization. Cytochalasin inhibited both cell spreading and ERK activation following adhesion to fibronectin, but had no effect on growth factor-induced ERK activation in adherent cells. We conclude that adhesion-induced ERK activation in bovine tracheal myocytes may occur independently of ligand-integrin binding and is primarily related to the cell spreading that follows adhesion.

Regulation of the ERK subgroup of MAP kinase cascades through G protein-coupled receptors

The extracellularly-responsive kinase (ERK) subfamily of mitogen-activated protein kinases (MAPKs) has been implicated in the regulation of cell growth and differentiation. Activation of ERKs involves a two-step protein kinase cascade lying upstream from ERK, in which the Raf family are the MAPK kinase kinases and the MEK1/MEK2 isoforms are the MAPK kinases. The linear sequence of Raf --> MEK --> ERK constitutes the ERK cascade. Although the ERK cascade is activated through growth factor-regulated receptor protein tyrosine kinases, they are also modulated through G protein-coupled receptors (GPCRs). All four G protein subfamilies (Gq/11 Gi/o, Gs and G12/13) influence the activation state of ERKs. In this review, we describe the ERK cascade and characteristics of its activation through GPCRs. We also discuss the identity of the intervening steps that may couple agonist binding at GPCRs to activation of the ERK cascade.

Gastric inhibitory polypeptide activates MAP kinase through the wortmannin-sensitive and -insensitive pathways

The signal transduction pathways of a cloned human gastric inhibitory polypeptide (GIP) receptor have been investigated in CHO cells stably expressing this receptor. Exposure of GIP receptor expressing cells to GIP significantly increased MAP kinase activity. Time course analysis showed that a rapid and marked increase in MAP kinase activation was detected and that this activation reached maximal levels 10 min after the addition of GIP. Dose-response analysis showed that GIP activated MAP kinase activity in a dose-dependent manner with an ED50 value of 5.9 x 10(-10) M of GIP. Wortmannin, a potent inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), partially inhibited GIP-induced MAP kinase activation, suggesting that GIP activates MAP kinase through two different, wortmannin-sensitive and -insensitive pathways. It has been demonstrated that in CHO cells cAMP attenuates MAP kinase activity by inhibiting Raf-1. Since GIP elevates intracellular cAMP, we examined the effects of cAMP on MAP kinase activation. Interestingly, forskolin, which increased intracellular cAMP levels, significantly inhibited MAP kinase activation by GIP, but did not affect MAP kinase activation by GIP in the presence of wortmannin, suggesting that the wortmannin-sensitive pathway activates an MAP kinase cascade at or above the level of Raf-1 and that the wortmannin-insensitive pathway activates an MAP kinase cascade below the level of Raf-1. These findings demonstrate that the GIP receptor is linked to the MAP kinase cascade via at least two different pathways.