Hemopoietic growth factors with the exception of interleukin-4 activate the p38 mitogen-activated protein kinase pathway

M-Ras, a Widely Expressed 29-kD Homologue of p21 Ras: Expression of a Constitutively Active Mutant Results in Factor-Independent Growth of an Interleukin-3–Dependent Cell Line

M-Ras, a recently identified homologue of p21 Ras, is widely expressed, with levels of the 29-kD protein in spleen, thymus, and NIH 3T3 fibroblasts equaling or exceeding those of p21 Ras. A G22V mutant of M-Ras was constitutively active and its expression in an interleukin-3 (IL-3)–dependent mast cell/megakaryocyte cell line resulted in increased survival in the absence of IL-3, increased growth in IL-4, and, at high expression levels, in factor-independent growth. Expression of M-Ras G22V, however, had a negative effect on growth in the presence of IL-3, suggesting that M-Ras has both positive and negative effects on growth. Expression of M-Ras G22V in NIH-3T3 fibroblasts resulted in morphological transformation and growth to higher cell densities. M-Ras G22V induced activation of thec-fos promoter, and bound weakly to the Ras-binding domains of Raf-1 and RalGDS. Expression of a mutant of M-Ras G22V that was no longer membrane-bound partially inhibited (40%) activation of thec-fos promoter by N-Ras Q61K, suggesting that M-Ras shared some, but not all, of the effectors of N-Ras. An S27N mutant of M-Ras, like the analogous H-Ras S17N mutant, was a dominant inhibitor of activation of the c-fos promoter by constitutively active Src Y527F, suggesting that M-Ras and p21 Ras shared guanine nucleotide exchange factors and are likely to be activated in parallel. Moreover, M-Ras was recognized by the monoclonal anti-Ras antibody Y13-259, commonly used to study the function and activity of p21 Ras. Mammalian M-Ras and a Caenorhabditis elegans orthologue exhibit conserved structural features, and these are likely to mediate activation of distinctive signaling paths that function in parallel to those downstream of p21 Ras.

A role for p38(MAPK)/HSP27 pathway in smooth muscle cell migration

Smooth muscle cells are exposed to growth factors and cytokines that contribute to pathological states including airway hyperresponsiveness, atherosclerosis, angiogenesis, smooth muscle hypertrophy, and hyperplasia. A common feature of several of these conditions is migration of smooth muscle beyond the initial boundary of the organ. Signal transduction pathways activated by extracellular signals that instigate migration are mostly undefined in smooth muscles. We measured migration of cultured tracheal myocytes in response to platelet-derived growth factor, interleukin-1beta, and transforming growth factor-beta. Cellular migration was blocked by SB203580, an inhibitor of p38(MAPK). Time course experiments demonstrated increased phosphorylation of p38(MAPK). Activation of p38(MAPK) resulted in the phosphorylation of HSP27 (heat shock protein 27), which may modulate F-actin polymerization. Inhibition of p38(MAPK) activity inhibited phosphorylation of HSP27. Adenovirus-mediated expression of activated mutant MAPK kinase 6b(E), an upstream activator for p38(MAPK), increased cell migration, whereas overexpression of p38alpha MAPK dominant negative mutant and an HSP27 phosphorylation mutant blocked cell migration completely. The results indicate that activation of the p38(MAPK) pathway by growth factors and proinflammatory cytokines regulates smooth muscle cell migration and may contribute to pathological states involving smooth muscle dysfunction.

Requirement of Activation of JNK and p38 for Environmental Stress-Induced Erythroid Differentiation and Apoptosis and of Inhibition of ERK for Apoptosis

C-Jun amino terminal kinase/stress-activated protein kinases (JNK/SAPK) and p38 subgroups of mitogen-activated protein kinases have been suggested to play a critical role in apoptosis, cell growth, and/or differentiation. We found that a short exposure of SKT6 cells, which respond to erythropoietin (Epo) and induce erythroid differentiation, to osmotic or heat shock induced transient activation of JNK/SAPK and p38 and inactivation of ERK and resulted in erythroid differentiation without Epo, whereas long exposure of the cells to these stresses induced prolonged activation/inactivation of the same kinases and caused apoptosis. Inhibition of JNK/SAPK and p38 resulted in inhibition of stress-induced erythroid differentiation and apoptosis. Inhibition of ERK had no effect on stress-induced erythroid differentiation, but stimulated apoptosis. Activation of p38 and/or JNK/SAPK for a short time caused erythroid differentiation without Epo, although its prolonged activation induced apoptosis. Activation of ERK suppressed stress-induced apoptosis. These results indicate that short cellular stresses, inducing transient activation of JNK/SAPK and p38, lead to cell differentiation rather than apoptosis. Furthermore, activation of JNK/SAPK and p38 is required for both cell differentiation and apoptosis, and the duration of their activation may determine the cell fate, cell differentiation, and apoptosis. In contrast, inactivation of ERK is required for stress-induced apoptosis but not cell differentiation.

Stress activated protein kinase p38 is involved in IL-6 induced transcriptional activation of STAT3

The pleiotropic cytokine interleukin-6 (IL-6) induces acute phase protein expression in HepG2 human hepatoma cells and promotes the growth of mouse B9 hybridoma. The signaling cascades leading to these biological functions are only partially known. We analysed the involvement of MAPK homologues in IL-6 transduction pathways and found that interleukin-6 triggered activation of p38 stress-activated protein kinase (p38) but not of jun kinase. p38 activity was required for biological functions including acute phase protein secretion from HepG2 hepatoma and proliferation of B9 hybridoma cells. Using a reporter gene construct containing a 190 bp promoter fragment of the acute phase protein haptoglobin we found that p38 is involved in transcriptional activation of the haptoglobin promoter by STAT3 but not by NF-IL6. Thus, we present evidence for a role of p38 in IL-6 induced functions and a possible cross-talk between this MAPK homologue and the STAT pathway.

p38 mitogen-activated protein kinase activation is required for fibroblast growth factor-2-stimulated cell proliferation but not differentiation

Basic fibroblast growth factor (FGF-2) is a member of a family of polypeptides that have roles in a wide range of biological processes. To determine why different cell types show distinct responses to treatment with FGF-2, the array of FGF receptors present on the surface of a cell which differentiates in response to FGF-2 (PC12 cells) was compared with that present on the surface of a cell that proliferates in response to FGF-2 (Swiss 3T3 fibroblasts). Both cell types express exclusively FGFR1, suggesting that there are cell type-specific FGFR1 signaling pathways. Since mitogen-activated protein kinases function as mediators of cellular responses to a variety of stimuli, the roles of these proteins in FGF-mediated responses were examined. FGF-2 activates extracellular signal-regulated kinases with similar kinetics in both fibroblasts and PC12 cells, and a specific inhibitor of extracellular signal-regulated kinase activation blocks differentiation but has little effect on proliferation. In contrast, while p38 mitogen-activated protein kinase is activated weakly and transiently in PC12 cells treated with FGF-2, a much stronger and sustained activation of this kinase is seen in FGF-2-treated fibroblasts. Furthermore, specific inhibitors of this kinase block proliferation but have no effect on differentiation. This effect on proliferation is specific for FGF-2 since the same concentrations of inhibitors have little or no effect on proliferation induced by serum.

Interleukin-4 Synergizes With Raf-1 to Promote Long-Term Proliferation and Activation of c-jun N-terminal Kinase

This report shows that interleukin-4 (IL-4), which plays a key role in regulating immune responses, fails to support cellular growth. We investigated whether this failure of IL-4 to promote growth was because of its unique inability to activate the Ras/Raf/Erk pathway. Consistent with other reports, expression in Ba/F3, a factor-dependent hematopoietic cell line, of either activated Q61KN-Ras or a hormone-inducible activated Raf-1, resulted in suppression of apoptosis but not in long-term growth. However, in the presence of IL-4, Ba/F3 cells that expressed either Q61KN-Ras or activated Raf-1 grew continuously at a rate comparable with that stimulated by IL-3. Investigation of the biochemical events associated with the stimulation of long-term growth showed that, as expected, the presence of activated Raf-1 resulted in an increased activity of extracellular signal regulated kinase (ERK) mitogen-activated protein kinase (MAPK) but not of c-jun N-terminal kinase/stress-activated protein kinase (JNK). However, surprisingly, if IL-4 was present, cells expressing active Raf-1 exhibited increases in JNK activity. These observations point to a novel mechanism for JNK activation involving synergy between Raf-1 and pathways activated by IL-4 and suggest that in hematopoietic cells proliferation is correlated not only with “mitogen activated” ERK activity, but also with JNK activity.

Interleukin-4 Synergizes With Raf-1 to Promote Long-Term Proliferation and Activation of c-jun N-terminal Kinase

This report shows that interleukin-4 (IL-4), which plays a key role in regulating immune responses, fails to support cellular growth. We investigated whether this failure of IL-4 to promote growth was because of its unique inability to activate the Ras/Raf/Erk pathway. Consistent with other reports, expression in Ba/F3, a factor-dependent hematopoietic cell line, of either activated Q61KN-Ras or a hormone-inducible activated Raf-1, resulted in suppression of apoptosis but not in long-term growth. However, in the presence of IL-4, Ba/F3 cells that expressed either Q61KN-Ras or activated Raf-1 grew continuously at a rate comparable with that stimulated by IL-3. Investigation of the biochemical events associated with the stimulation of long-term growth showed that, as expected, the presence of activated Raf-1 resulted in an increased activity of extracellular signal regulated kinase (ERK) mitogen-activated protein kinase (MAPK) but not of c-jun N-terminal kinase/stress-activated protein kinase (JNK). However, surprisingly, if IL-4 was present, cells expressing active Raf-1 exhibited increases in JNK activity. These observations point to a novel mechanism for JNK activation involving synergy between Raf-1 and pathways activated by IL-4 and suggest that in hematopoietic cells proliferation is correlated not only with “mitogen activated” ERK activity, but also with JNK activity.

Roles of the mitogen-activated protein kinase family in macrophage responses to colony stimulating factor-1 addition and withdrawal

Colony stimulating factor-1 (CSF-1) (or macrophage CSF) is involved in the survival, proliferation, differentiation, and activation of cells of the monocyte/macrophage lineage. Because the mitogen-activated protein kinase family members extracellular signal-regulated kinases (ERKs), p38, and c-Jun N-terminal kinase are widely implicated in such cellular functions, we measured their activity in growing and growth-arrested cultures of bone marrow-derived macrophages (BMM), as well as their stimulation by saturating concentrations of CSF-1. ERK activity was approximately 2-fold higher in cycling BMM compared with growth-arrested BMM; in addition, CSF-1-stimulated BMM DNA synthesis was partially inhibited by PD98059, a specific inhibitor of MEK activation, suggesting a role for a mitogen-activated protein-ERK kinase (MEK)/ERK pathway in the control of DNA synthesis but surprisingly not in the control of cyclin D1 mRNA or c-myc mRNA expression. The suppression of BMM apoptosis by CSF-1, i.e. enhanced survival, was not reversed by PD98059, suggesting that a MEK/ERK pathway is not involved in this process. Using a quantitative kinase assay, it was found that CSF-1 gave a slight increase in BMM p38 activity, supporting prior data that CSF-1 is a relatively weak stimulator of inflammatory cytokine production in monocytes/macrophages. Relatively high concentrations of the p38 inhibitor, SKB202190, suppressed CSF-1-stimulated BMM DNA synthesis. No evidence could be obtained for the involvement of p38 activity in BMM apoptosis following CSF-1 withdrawal. We were not able to show that CSF-1 enhanced BMM JNK-1 activity to a significant extent; again, no role could be found for JNK-1 activity in the BMM apoptosis occurring after CSF-1 removal.

Activation of Hematopoietic Progenitor Kinase-1 by Erythropoietin

Hematopoietic progenitor kinase-1 (HPK1), which is expressed predominantly in hematopoietic cells, was identified as a mammalian Ste20 homologue that, upon transfection, leads to activation of JNK/SAPK in nonhematopoietic cells. The JNK/SAPK pathway is activated by various environmental stresses and proinflammatory and hematopoietic cytokines. Upstream activators of HPK1 currently remain elusive, and its precise role in hematopoiesis has yet to be defined. We therefore examined the possible involvement of HPK1 in erythropoietin (Epo) and environmental stress-induced JNK/SAPK activation in the Epo-dependent FD-EPO cells and Epo-responsive SKT6 cells. We found that Epo, but not environmental stresses, induced rapid and transient activation of HPK1, whereas both induced activation of JNK/SAPK. A screen for HPK1 binding proteins identified the hematopoietic cell-specific protein 1 (HS1) as a potential HPK1 interaction partner. We found HPK1 constitutively associated with HS1 and that HS1 was tyrosine-phosphorylated in response to cellular stresses as well as Epo stimulation. Furthermore, antisense oligonucleotides to HPK1 suppressed Epo-dependent cell growth and Epo-induced erythroid differentiation. We therefore conclude that Epo induces activation of both HPK1 and HS1, whereas cellular stresses activate only HS1, and that the HPK1-JNK/SAPK pathway is involved in Epo-induced growth and differentiation signals.

Thioredoxin negatively regulates p38 MAP kinase activation and IL-6 production by tumor necrosis factor-alpha

We examined the regulatory role of a reduction/oxidation (redox) control protein, thioredoxin (TRX), in tumor necrosis factor-alpha (TNF-alpha)-induced p38 MAP kinase activation and p38 MAP kinase-mediated cytokine expression utilizing TRX-transfected murine L929 cells (TRX14). The results showed that TNF-alpha-induced p38 MAP kinase activation and interleukin-6 (IL-6) production by TRX 14 were less than those by the parental L cells and the control transfected L cells (Neo-1). SB 203580 as the specific inhibitor for p38 MAP kinase activity inhibited TNF-alpha-induced IL-6 production by the parental L cells, indicating that TNF-alpha-activated p38 MAP kinase regulates IL-6 production by the cell lines used in this study. These results showed that overexpression of TRX negatively regulates p38 MAP kinase activation and p38 MAP kinase-mediated IL-6 production by TNF-alpha-stimulated cells, indicating that TRX is critical for p38 MAP kinase activation which regulates cytokine expression.

Cooperation of p38 and extracellular signal-regulated kinase mitogen-activated protein kinase pathways during granulocyte colony-stimulating factor-induced hemopoietic cell proliferation

Hemopoietic cytokines such as interleukin-3 and granulocyte colony-stimulating factor (G-CSF) are potent activators of hemopoietic cell growth and strongly induce activation of extracellular signal-regulated kinase (ERK), c-Jun-N-terminal kinase (JNK), and p38 mitogen-activated protein (MAP) kinases. However, the role of these kinases is unclear. Using specific chemical inhibitors for MEK and p38, we demonstrate here that both ERK and p38 pathways are critically involved in the transduction of a proliferative signal and cooperate in G-CSF-induced cell proliferation. We show that, like ERK and JNK activation, activation of p38 and its downstream substrate MAP kinase-activated protein kinase 2 by interleukin-3 or G-CSF requires Ras activation. We demonstrate that two distinct cytoplasmic regions of the G-CSF receptor are involved in activation of the p38 pathway: a region within the 100 membrane-proximal amino acids is sufficient to induce low levels of p38 and MAP kinase-activated protein kinase 2 activation, whereas the membrane-distal phosphorylation site Tyr763 mediates strong activation of these kinases. The levels of p38 activation correlate closely with those of Ras activation by G-CSF, suggesting that the degree of Ras activation is a critical determinant for the extent of p38 activation by hemopoietic cytokines.

MAP kinases, phosphatidylinositol 3-kinase, and p70 S6 kinase mediate the mitogenic response of human endothelial cells to vascular endothelial growth factor

Although the significance of vascular endothelial growth factor (VEGF) and its receptors in angiogenesis is well established, the signal transduction cascades activated by VEGF and their involvement in mediating the mitogenic response of endothelial cells to VEGF are incompletely characterized. Here we demonstrate that VEGF activates mitogen-activated protein (MAP) kinases, including the extracellular signal-regulated protein kinase (ERK) and p38 MAP kinase, phosphatidylinositol 3-kinase (PI 3-kinase), and p70 S6 kinase in human umbilical vein endothelial cells (HUVEC). The activation of these enzymes was assayed by kinase phosphorylation and by kinase activity towards substrates. Studies with PI 3-kinase inhibitors revealed that activation of p70 S6 kinase was mediated by PI 3-kinase. Selective inhibition of ERK, PI 3-kinase, and p70 S6 kinase with the inhibitors PD098059, LY294002, and rapamycin, respectively, inhibited VEGF-stimulated HUVEC proliferation. In marked contrast, the p38 MAP kinase inhibitor SB203580 not only failed to inhibit but actually enhanced HUVEC proliferation; this effect was associated with the phosphorylation of Rb protein. Rb phosphorylation resulted from a decrease in the level of the cdk inhibitor p27KiP1. These results indicate that the activities of ERK, PI 3-kinase, and p70 S6 kinase are essential for VEGF-induced HUVEC proliferation. p38 MAP kinase suppresses endothelial cell proliferation by regulating cell-cycle progression.

Hyperosmolarity-induced interleukin-8 expression in human bronchial epithelial cells through p38 mitogen-activated protein kinase

The changes in airway osmolarity have been described to contribute to the production of exercise- induced bronchoconstriction (EIB) and the development of the late-phase response (LPR). The mechanism has been investigated; however, the responsiveness of bronchial epithelial cells (BEC) to hyperosmolarity and the intracellular signals leading to cell activation have not been determined. In this study, we examined the effect of hyperosmolar medium on interleukin-8 (IL-8) expression and the role of p38 mitogen-activated protein (MAP) kinase and c-Jun NH2 terminal kinase ( JNK) in human BEC in this response in order to clarify the intracellular signals regulating IL-8 expression in hyperosmolarity-stimulated BEC. The results showed that hyperosmolarity induced IL-8 expression in a concentration dependent manner, p38 MAP kinase phosphorylation and activation, and JNK activation whether NaCl or mannitol was used as the solute. SB 203580 as the specific p38 MAP kinase inhibitor inhibited hyperosmolarity-induced p38 MAP kinase activation and partially inhibited hyperosmolarity-induced IL-8 expression. These results indicate that p38 MAP kinase, at least in part, regulates hyperosmolarity-induced IL-8 expression in BEC. However, other signals such as JNK are possibly also involved. These results provide new evidence on the mechanism responsible for the development of the LPR induced by EIB, and a strategy for treatment with the specific p38 MAP kinase inhibitor.

Anti-apoptotic function of Rac in hematopoietic cells

The small GTP-binding protein Rac plays a pivotal role in the regulation of diverse physiological events including reorganization of the actin cytoskeleton, cell cycle progression, and transformation. Here we show an anti-apoptotic effect of Rac in interleukin-3-dependent murine hematopoietic BaF3 cells. Activated Rac(G12V), when ectopically expressed in BaF3 cells, rendered the cells resistant to apoptosis upon interleukin-3 deprivation, while activated mutants of Rho and Cdc42 displayed no significant anti-apoptotic effect. In contrast to activated Ras, which also supports cell survival in the absence of interleukin-3, Rac required fetal bovine serum for the prevention of cell death. The involvement of phosphatidylinositol 3-kinase downstream of Rac was demonstrated by the inhibition of Rac-induced cell survival by wortmannin and LY294002 and the presence of phosphatidylinositol kinase activity in the Rac immunoprecipitate. Furthermore, the serine/threonine kinase Akt was stimulated by activated Rac and fetal bovine serum in a synergistic manner. Rac-induced Akt activation was mediated by phosphorylation of threonine-308 and serine-473. In addition to the phosphatidylinositol 3-kinase/Akt pathway, the p38 mitogen-activated protein kinase pathway was crucial for Rac-dependent survival, whereas p38 mitogen-activated protein kinase nas not implicated in Ras-induced anti-apoptotic signaling. These findings provide evidence for the involvement of Rac in survival signaling of hematopoietic cells.

Ceramide and Cyclic Adenosine Monophosphate (cAMP) Induce cAMP Response Element Binding Protein Phosphorylation via Distinct Signaling Pathways While Having Opposite Effects on Myeloid Cell Survival

The role of ceramide as a second messenger is a subject of great interest, particularly since it is implicated in signaling in response to inflammatory cytokines. Ceramide induces apoptosis in both cytokine-dependent MC/9 cells and factor-independent U937 cells. Elevation of cyclic adenosine monophosphate (cAMP) levels inhibits apoptosis induced by ceramide and several other treatments. One target of cAMP-mediated signaling is the transcription factor CREB (cAMP response element binding protein), and recently CREB phosphorylation at an activating site has been shown to also be mediated by a cascade involving p38 mitogen-activated protein kinase (MAPK), one of the stress-activated MAP kinases. Because no role for p38 MAPK in apoptosis has been firmly established, we examined the relationship between p38 MAPK and CREB phosphorylation under various conditions. Ceramide, or sphingomyelinase, like tumor necrosis factor- (TNF-) or the hematopoietic growth factor, interleukin-3 (IL-3), was shown to activate p38 MAPK, which in turn activated MAPKAP kinase-2. Each of these treatments led to phosphorylation of CREB (and the related factor ATF-1). A selective p38 MAPK inhibitor, SB203580, blocked TNF-– or ceramide-induced CREB phosphorylation, but had no effect on the induction of apoptosis mediated by these agents. The protective agents cAMP and IL-3 also led to CREB phosphorylation, but this effect was independent of p38 MAPK, even though IL-3 was shown to activate both p38 MAPK and MAPKAP kinase-2. Therefore, the opposing effects on apoptosis observed with cAMP and IL-3, compared with ceramide and TNF-, could not be explained on the basis of phosphorylation of CREB. In addition, because SB203580 had no effect of TNF- or ceramide-induced apoptosis, our results strongly argue against a role for p38 MAPK in the induction of TNF-– or ceramide-induced apoptosis.

Ceramide and Cyclic Adenosine Monophosphate (cAMP) Induce cAMP Response Element Binding Protein Phosphorylation via Distinct Signaling Pathways While Having Opposite Effects on Myeloid Cell Survival

The role of ceramide as a second messenger is a subject of great interest, particularly since it is implicated in signaling in response to inflammatory cytokines. Ceramide induces apoptosis in both cytokine-dependent MC/9 cells and factor-independent U937 cells. Elevation of cyclic adenosine monophosphate (cAMP) levels inhibits apoptosis induced by ceramide and several other treatments. One target of cAMP-mediated signaling is the transcription factor CREB (cAMP response element binding protein), and recently CREB phosphorylation at an activating site has been shown to also be mediated by a cascade involving p38 mitogen-activated protein kinase (MAPK), one of the stress-activated MAP kinases. Because no role for p38 MAPK in apoptosis has been firmly established, we examined the relationship between p38 MAPK and CREB phosphorylation under various conditions. Ceramide, or sphingomyelinase, like tumor necrosis factor- (TNF-) or the hematopoietic growth factor, interleukin-3 (IL-3), was shown to activate p38 MAPK, which in turn activated MAPKAP kinase-2. Each of these treatments led to phosphorylation of CREB (and the related factor ATF-1). A selective p38 MAPK inhibitor, SB203580, blocked TNF-– or ceramide-induced CREB phosphorylation, but had no effect on the induction of apoptosis mediated by these agents. The protective agents cAMP and IL-3 also led to CREB phosphorylation, but this effect was independent of p38 MAPK, even though IL-3 was shown to activate both p38 MAPK and MAPKAP kinase-2. Therefore, the opposing effects on apoptosis observed with cAMP and IL-3, compared with ceramide and TNF-, could not be explained on the basis of phosphorylation of CREB. In addition, because SB203580 had no effect of TNF- or ceramide-induced apoptosis, our results strongly argue against a role for p38 MAPK in the induction of TNF-– or ceramide-induced apoptosis.

Inhibitors of the p38 Mitogen-Activated Kinase Modulate IL-4 Induction of Low Affinity IgE Receptor (CD23) in Human Monocytes

CD23, the low affinity IgE receptor, is up-regulated on the surface of IL-4-treated B cells and monocytes and is immediately proteolytically processed, releasing soluble fragments of CD23. Here, we report that inhibitors of the p38 mitogen-activated kinase (p38 MAPK), SK&F 86002 or the more selective inhibitor, SB 203580, reduce the levels of soluble CD23 formed by IL-4-stimulated human monocytes or the human monocytic cell line, U937. In contrast to compounds such as the metalloprotease inhibitor batimastat ([4-(N-hydroxyamino)-2-(R)-isobutyl-3-(S)-(2-thiophenethiomethyl)succinyl]-(S)-phenylalanine-N-methylamide, sodium salt), p38 MAPK inhibitors do not directly inhibit proteolytic processing of CD23. Further, evaluation of surface intact CD23 (iCD23) by flow cytometry demonstrated that SK&F 86002 and SB 203580 reduced the surface expression of iCD23 in a concentration-dependent fashion, while batimastat increased the surface expression of iCD23. The decrease in surface iCD23 was accompanied by a decrease in total cell-associated CD23 protein levels but not CD23 mRNA. IL-4 induced a late (>4-h) increase in p38 MAPK activity and corresponding activation of its substrate MAPKAPK-2. This activation was blocked by addition of SB 203580 before IL-4 induction, in parallel with the inhibition of CD23 expression. Modulation of CD23 by antibodies has been shown to alleviate the symptoms of murine collagen-induced arthritis, implicating CD23 as an important proinflammatory agent. These data show that in addition to the known cytokine inhibitory actions of SK&F 86002 and SB 203580, they also confer an additional potential anti-inflammatory activity through modulation of CD23 expression.

Stromal Cell-Derived Factor-1α and Stem Cell Factor/kit Ligand Share Signaling Pathways in Hemopoietic Progenitors: A Potential Mechanism for Cooperative Induction of Chemotaxis

Stromal cell-derived factor (SDF-1α), the ligand for CXCR4, is a chemokine that acts as a potent chemoattractant for hemopoietic progenitor cells. Stem cell factor/kit ligand (SCF/KL), an early acting cytokine, has recently been reported to enhance the chemotaxis induced by SDF-1α. However, very little is known about downstream signaling events following these receptor-ligand interactions. To investigate these events, we utilized a model progenitor cell line, CTS, which expresses both the CXCR4 and c-kit receptors. We observed strong Ca2+ mobilization and enhancement of chemotaxis following treatment with SDF-1α or SCF/KL. A combination of these factors enhanced this chemotaxis in CTS cells as well as in CD34+ bone marrow cells. Prior treatment of CTS cells with pertussis toxin inhibited the SDF-1α-induced chemotaxis, suggesting that SDF-1α signaling involves a pertussis-sensitive Gi-coupled protein. SDF-1α treatment resulted in a rapid phosphorylation of the focal adhesion molecules RAFTK (related adhesion focal tyrosine kinase), paxillin, and p130cas, which then declined within minutes. SCF/KL alone or in combination with SDF-1α induced a rapid and sustained effect on phosphorylation of these substrates. SDF-1α treatment resulted in a rapid and robust activation of p44/42 mitogen-activated protein kinase compared with the relatively weak and delayed effect of SCF/KL treatment. Interestingly, a delayed but sustained activation of mitogen-activated protein kinase activation was observed when the factors were used in combination. Such cooperativity in downstream signaling pathways may explain the enhanced chemotaxis of progenitors observed with SDF-1α in combination with SCF/KL.

Activation of p38 MAP Kinase and JNK But Not ERK Is Required for Erythropoietin-Induced Erythroid Differentiation

p38 MAP kinase (p38) and JNK have been described as playing a critical role in the response to a variety of environmental stresses and proinflammatory cytokines. It was recently reported that hematopoietic cytokines activate not only classical MAP kinases (ERK), but also p38 and JNK. However, the physiological function of these kinases in hematopoiesis remains obscure. We found that all MAP kinases examined, ERK1, ERK2, p38, JNK1, and JNK2, were rapidly and transiently activated by erythropoietin (Epo) stimulation in SKT6 cells, which can be induced to differentiate into hemoglobinized cells in response to Epo. Furthermore, p38-specific inhibitor SB203580 but not MEK-specific inhibitor PD98059 significantly suppressed Epo-induced differentiation and antisense oligonucleotides of p38, JNK1, and JNK2, but neither ERK1 nor ERK2 clearly inhibited Epo-induced hemoglobinization. However, in Epo-dependent FD-EPO cells, inhibition of either ERKs, p38, or JNKs suppressed cell growth. Furthermore, forced expression of a gain-of-function MKK6 mutant, which specifically activated p38, induced hemoglobinization of SKT6 cells without Epo. These results indicate that activation of p38 and JNKs but not of ERKs is required for Epo-induced erythroid differentiation of SKT6 cells, whereas all of these kinases are involved in Epo-induced mitogenesis of FD-EPO cells.

© 1998 by The American Society of Hematology.

Activation of p38 MAP Kinase and JNK But Not ERK Is Required for Erythropoietin-Induced Erythroid Differentiation

p38 MAP kinase (p38) and JNK have been described as playing a critical role in the response to a variety of environmental stresses and proinflammatory cytokines. It was recently reported that hematopoietic cytokines activate not only classical MAP kinases (ERK), but also p38 and JNK. However, the physiological function of these kinases in hematopoiesis remains obscure. We found that all MAP kinases examined, ERK1, ERK2, p38, JNK1, and JNK2, were rapidly and transiently activated by erythropoietin (Epo) stimulation in SKT6 cells, which can be induced to differentiate into hemoglobinized cells in response to Epo. Furthermore, p38-specific inhibitor SB203580 but not MEK-specific inhibitor PD98059 significantly suppressed Epo-induced differentiation and antisense oligonucleotides of p38, JNK1, and JNK2, but neither ERK1 nor ERK2 clearly inhibited Epo-induced hemoglobinization. However, in Epo-dependent FD-EPO cells, inhibition of either ERKs, p38, or JNKs suppressed cell growth. Furthermore, forced expression of a gain-of-function MKK6 mutant, which specifically activated p38, induced hemoglobinization of SKT6 cells without Epo. These results indicate that activation of p38 and JNKs but not of ERKs is required for Epo-induced erythroid differentiation of SKT6 cells, whereas all of these kinases are involved in Epo-induced mitogenesis of FD-EPO cells.

© 1998 by The American Society of Hematology.

A role for the p38 mitogen-activated protein kinase/Hsp 27 pathway in cholecystokinin-induced changes in the actin cytoskeleton in rat pancreatic acini

Cholecystokinin (CCK) and other pancreatic secretagogues have recently been shown to activate signaling kinase cascades in pancreatic acinar cells, leading to the activation of extracellular signal-regulated kinases and Jun N-terminal kinases. We now show the presence of a third kinase cascade activating p38 mitogen-activated protein (MAP) kinase in isolated rat pancreatic acini. CCK and osmotic stress induced by sorbitol activated p38 MAP kinase within minutes; their effects were dose-dependent, with maximal activation of 2.8- and 4.4-fold, respectively. The effects of carbachol and bombesin on p38 MAP kinase activity were similar to those of CCK, whereas phorbol ester, epidermal growth factor, and vasoactive intestinal polypeptide stimulated p38 MAP kinase by 2-fold or less. Both CCK and sorbitol also increased the tyrosyl phosphorylation of p38 MAP kinase. Using the specific inhibitor of p38 MAP kinase, SB 203580, we found that p38 MAP kinase activity was required for MAP kinase-activated protein kinase-2 activation in pancreatic acini. SB 203580 reduced the level of basal phosphorylation and blocked the increased phosphorylation of Hsp 27 after stimulation with either CCK or sorbitol. CCK treatment induced an initial rapid decrease in total F-actin content of acini, followed by an increase after 40 min. Preincubation with SB 203580 significantly inhibited these changes in F-actin content. Staining of the actin cytoskeleton with rhodamine-conjugated phalloidin and analysis by confocal fluorescence microscopy showed disruption of the actin cytoskeleton after 10 and 40 min of CCK stimulation. Pretreatment with SB 203580 reduced these changes. These findings demonstrate that the activation of p38 MAP kinase is involved not only in response to stress, but also in physiological signaling by gastrointestinal hormones such as CCK, where activation of Gq-coupled receptors stimulates a cascade in which p38 MAP kinase activates MAP kinase-activated protein kinase-2, resulting in Hsp 27 phosphorylation. Activation of p38 MAP kinase, most likely through phosphorylation of Hsp 27, plays a role in the organization of the actin cytoskeleton in pancreatic acini.

Tenascin-C expression in human epidermal keratinocytes is regulated by inflammatory cytokines and a stress response pathway

Recently we showed that human epidermal keratinocytes express the extracellular matrix protein tenascin-C (TN-C) during wound healing, but not in normal adult skin. To gain further insight into the regulation of epidermal TN-C expression, we tested the effect of various stimuli on TN-C expression by cultured keratinocytes. Our results indicate that IL-4 is a very strong inducer of TN-C protein and mRNA expression in normal keratinocytes. Furthermore, TNFalpha and IFNgamma moderately increased TN-C expression. No other cytokines and growth factors that we tested, including various factors that stimulate TN-C expression in mesenchymal cells, significantly affected TN-C secretion by cultured keratinocytes. The regulation of TN-C expression in keratinocytes is distinct from that of fibronectin, since IL-4 and IFNgamma did not affect fibronectin expression in our experiments, and TNFalpha only slightly increased fibronectin levels. To investigate the role of cellular stress response pathways that can be activated by TNFalpha in the regulation of TN-C expression, we tested the effect of different inhibitors and an activator of these intracellular signalling cascades. The results show that the p38 MAP-kinase pathway is not involved in TNFalpha-induced TN-C expression in cultured keratinocytes. Activation of the JNK/SAPK-1 pathway by the addition of sphingomyelinase resulted in a dose-dependent increase of TN-C expression. TN-C expression by squamous carcinoma cell lines was differentially affected by the cytokines that stimulated TN-C expression in normal keratinocytes: TNFalpha again increased TN-C secretion, but IL-4 and IFNgamma had little effect. We conclude that there are distinct regulation mechanisms for TN-C expression in normal keratinocytes, tumor-derived keratinocytes and mesenchymal cells. The observation that TN-C is abundant in inflamed skin is a strong indication that inflammatory cytokines such as IL-4, TNFalpha and IFNgamma could also be involved in the regulation of epidermal TN-C expression in vivo.

PRAK, a novel protein kinase regulated by the p38 MAP kinase

We have identified and cloned a novel serine/ threonine kinase, p38-regulated/activated protein kinase (PRAK). PRAK is a 471 amino acid protein with 20-30% sequence identity to the known MAP kinase-regulated protein kinases RSK1/2/3, MNK1/2 and MAPKAP-K2/3. PRAK was found to be expressed in all human tissues and cell lines examined. In HeLa cells, PRAK was activated in response to cellular stress and proinflammatory cytokines. PRAK activity was regulated by p38alpha and p38beta both in vitro and in vivo and Thr182 was shown to be the regulatory phosphorylation site. Activated PRAK in turn phosphorylated small heat shock protein 27 (HSP27) at the physiologically relevant sites. An in-gel kinase assay demonstrated that PRAK is a major stress-activated kinase that can phosphorylate small heat shock protein, suggesting a potential role for PRAK in mediating stress-induced HSP27 phosphorylation in vivo.

beta-Amyloid fibrils activate parallel mitogen-activated protein kinase pathways in microglia and THP1 monocytes

The senile plaques of Alzheimer's disease are foci of local inflammatory responses, as evidenced by the presence of acute phase proteins and oxidative damage. Fibrillar forms of beta-amyloid (Abeta), which are the primary constituents of senile plaques, have been shown to activate tyrosine kinase-dependent signal transduction cascades, resulting in inflammatory responses in microglia. However, the downstream signaling pathways mediating Abeta-induced inflammatory events are not well characterized. We report that exposure of primary rat microglia and human THP1 monocytes to fibrillar Abeta results in the tyrosine kinase-dependent activation of two parallel signal transduction cascades involving members of the mitogen-activated protein kinase (MAPK) superfamily. Abeta stimulated the rapid, transient activation of extracellular signal-regulated kinase 1 (ERK1) and ERK2 in microglia and ERK2 in THP1 monocytes. A second superfamily member, p38 MAPK, was also activated with similar kinetics. Scavenger receptor and receptor for advanced glycated end products (RAGE) ligands failed to activate ERK and p38 MAPK in the absence of significant increases in protein tyrosine phosphorylation, demonstrating that scavenger receptors and RAGE are not linked to these pathways. Importantly, the stress-activated protein kinases (SAPKs) were not significantly activated in response to Abeta. Downstream effectors of the MAPK signal transduction cascades include MAPKAP kinases, such as RSK1 and RSK2, as well as transcription factors. Exposure of microglia and THP1 monocytes to Abeta resulted in the activation of RSK1 and RSK2 and phosphorylation of cAMP response element-binding protein at Ser133, providing a mechanism for Abeta-induced changes in gene expression.

PRAK, a novel protein kinase regulated by the p38 MAP kinase

We have identified and cloned a novel serine/ threonine kinase, p38-regulated/activated protein kinase (PRAK). PRAK is a 471 amino acid protein with 20-30% sequence identity to the known MAP kinase-regulated protein kinases RSK1/2/3, MNK1/2 and MAPKAP-K2/3. PRAK was found to be expressed in all human tissues and cell lines examined. In HeLa cells, PRAK was activated in response to cellular stress and proinflammatory cytokines. PRAK activity was regulated by p38alpha and p38beta both in vitro and in vivo and Thr182 was shown to be the regulatory phosphorylation site. Activated PRAK in turn phosphorylated small heat shock protein 27 (HSP27) at the physiologically relevant sites. An in-gel kinase assay demonstrated that PRAK is a major stress-activated kinase that can phosphorylate small heat shock protein, suggesting a potential role for PRAK in mediating stress-induced HSP27 phosphorylation in vivo.

Heat-shock protein-25/27 phosphorylation by the delta isoform of protein kinase C

Small heat-shock proteins (sHSPs) are widely expressed 25-28 kDa proteins whose functions are dynamically regulated by phosphorylation. While recent efforts have clearly delineated a stress-responsive p38 mitogen-activated protein-kinase (MAPK)-dependent kinase pathway culminating in activation of the heat-shock (HSP)-kinases, mitogen-activated protein-kinase-activated protein kinase-2 and -3, not all sHSP phosphorylation events can be explained by the p38 MAPK-dependent pathway. The contribution of protein kinase C (PKC) to sHSP phosphorylation was suggested by early studies but later questioned on the basis of the reported poor ability of purified PKC to phosphorylate sHSP in vitro. The current study re-evaluates the role of PKC in sHSP phosphorylation in the light of the isoform complexity of the PKC family. We evaluated the sHSP phosphorylation status in rat corpora lutea obtained from two stages of pregnancy, mid-pregnancy and late-pregnancy, which express different levels of the novel PKC isoform, PKC-delta. Two-dimensional Western blot analysis showed that HSP-27 was more highly phosphorylated in vivo in corpora lutea of late pregnancy, corresponding to the developmental stage in which PKC-delta is abundant and active. Late-pregnant luteal extracts contained a lipid-sensitive HSP-kinase activity which exactly co-purified with PKC-delta using hydroxyapatite and S-Sepharose column chromatography. To determine whether there might be preferential phosphorylation of sHSP by a particular PKC isoform, purified recombinant PKC isoforms corresponding to those PKC isoforms detected in rat corpora lutea were evaluated for HSP-kinase activity in vitro. Recombinant PKC-delta effectively catalysed the phosphorylation of sHSP in vitro, and PKC-alpha was 30-50% as effective as an HSP-kinase; other PKCs tested (beta1, beta2, epsilon and zeta) were poor HSP-kinases. These results show that select PKC family members can function as direct HSP-kinases in vitro. Moreover, the observation of enhanced luteal HSP-27 phosphorylation in vivo, in late pregnancy, when PKC-delta is abundant and active, suggests that select PKC family members contribute to sHSP phosphorylation events in vivo.

p38 Mitogen-activated protein kinase is a critical component of the redox-sensitive signaling pathways activated by angiotensin II. Role in vascular smooth muscle cell hypertrophy

Angiotensin II induces an oxidant stress-dependent hypertrophy in cultured vascular smooth muscle cells. To investigate the growth-related molecular targets of H2O2, we examined the redox sensitivity of agonist-stimulated activation of the mitogen-activated protein kinase (MAPK) family. We show here that angiotensin II elicits a rapid increase in intracellular H2O2 and a rapid and robust phosphorylation of both p42/44MAPK (16-fold) and p38MAPK (15-fold). However, exogenous H2O2 activates only p38MAPK (14-fold), and diphenylene iodonium, an NADH/NADPH oxidase inhibitor, attenuates angiotensin II-stimulated phosphorylation of p38MAPK, but not p42/44MAPK. Furthermore, in cells stably transfected with human catalase, angiotensin II-induced intracellular H2O2 generation is almost completely blocked, resulting in inhibition of phosphorylation of p38MAPK, but not p42/44MAPK, and a subsequent partial decrease in angiotensin II-induced hypertrophy. Specific inhibition of either the p38MAPK pathway with SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H- imidaz ole) or the p42/44MAPK pathway with PD98059 (2-(2'-amino-3'-methoxyphenyl)oxanaphthalen-4-one) also partially, but significantly, attenuates angiotensin II-induced hypertrophy; however, simultaneous blockade of both pathways has an additive inhibitory effect, indicating that the hypertrophic response to angiotensin II requires parallel, independent activation of both MAPK pathways. These results provide the first evidence that p38MAPK is a critical component of the oxidant stress (H2O2)-sensitive signaling pathways activated by angiotensin II in vascular smooth muscle cells and indicate that it plays a crucial role in vascular hypertrophy.

SEK1 deficiency reveals mitogen-activated protein kinase cascade crossregulation and leads to abnormal hepatogenesis

SEK1 (MKK4/JNKK) is a mitogen-activated protein kinase activator that has been shown to participate in vitro in two stress-activated cascades terminating with the SAPK and p38 kinases. To define the role of SEK1 in vivo, we studied stress-induced signaling in SEK1(-/-) embryonic stem and fibroblast cells and evaluated the phenotype of SEK1(-/-) mouse embryos during development. Studies of SEK1(-/-) embryonic stem cells demonstrated defects in stimulated SAPK phosphorylation but not in the phosphorylation of p38 kinase. In contrast, SEK1(-/-) fibroblasts exhibited defects in both SAPK and p38 phosphorylation, demonstrating that crosstalk exists between the stress-activated cascades. Tumor necrosis factor alpha and interleukin 1 stimulation of both stress-activated cascades are severely affected in the SEK1(-/-) fibroblast cells. SEK1 deficiency leads to embryonic lethality after embryonic day 12.5 and is associated with abnormal liver development. This phenotype is similar to c-jun null mouse embryos and suggests that SEK1 is required for phosphorylation and activation of c-jun during the organo-genesis of the liver.

Bidirectional regulation of p38 kinase and c-Jun N-terminal protein kinase by insulin-like growth factor-I

We have previously shown that insulin-like growth factor I (IGF-I) activation of the IGF-I receptor rescues SH-SY5Y human neuroblastoma cells from high glucose-mediated programmed cell death (PCD). In the current study, we further explored the potential points in the cell death cascade where IGF-I receptor activation may afford neuroprotection. As an initial step, we examined the effects of the PCD stimulus, high glucose, on stress-activated protein kinases, specifically the two mitogen-activated protein kinases p38 kinase and c-Jun N-terminal kinase (JNK). High glucose treatment activated the tyrosine phosphorylation of both p38 kinase and JNK in a dose- and time-dependent fashion. We next examined the effects of IGF-I on JNK and p38 kinase under normoglycemic and hyperglycemic conditions. IGF-I activated p38 kinase alone and had additive effects on glucose-induced p38 kinase phosphorylation. In contrast, IGF-I inhibited glucose activation of JNK phosphorylation and JNK activity. IGF-I also inhibited the glucose-induced nuclear translocation of JNK, but did not effect glucose-induced translocation of p38 kinase. Finally, IGF-I inhibition of JNK phosphorylation was blocked by the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor, PD98059. Collectively, these data imply cross-talk between the mitogen-activated protein kinase pathway and JNK and suggest that IGF-I activation of mitogen-activated protein kinases interferes with JNK activation and protects cells from PCD.

Interferon-gamma expression by Th1 effector T cells mediated by the p38 MAP kinase signaling pathway

Signal transduction via MAP kinase pathways plays a key role in a variety of cellular responses, including growth factor-induced proliferation, differentiation and cell death. In mammalian cells, p38 MAP kinase can be activated by multiple stimuli, such as pro-inflammatory cytokines and environmental stress. Although p38 MAP kinase is implicated in the control of inflammatory responses, the molecular mechanisms remain unclear. Upon activation, CD4+ T cells differentiate into Th2 cells, which potentiate the humoral immune response or pro-inflammatory Th1 cells. Here, we show that pyridinyl imidazole compounds (specific inhibitors of p38 MAP kinase) block the production of interferon-gamma (IFNgamma) by Th1 cells without affecting IL-4 production by Th2 cells. These drugs also inhibit transcription driven by the IFNgamma promoter. In transgenic mice, inhibition of the p38 MAP kinase pathway by the expression of dominant-negative p38 MAP kinase results in selective impairment of Th1 responses. In contrast, activation of the p38 MAP kinase pathway by the expression of constitutivelyactivated MAP kinase kinase 6 in transgenic mice caused increased production of IFNgamma during the differentiation and activation of Th1 cells. Together, these data demonstrate that the p38 MAP kinase is relevant for Th1 cells, not Th2 cells, and that inhibition of p38 MAP kinase represents a possible site of therapeutic intervention in diseases where a predominant Th1 immune response leads to a pathological outcome. Moreover, our study provides an additional mechanism by which the p38 MAP kinase pathway controls inflammatory responses.

Human mitogen-activated protein kinase kinase 7 (MKK7) is a highly conserved c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) activated by environmental stresses and physiological stimuli

We report the cloning of a novel human activator of c-Jun N-terminal kinase (JNK), mitogen-activated protein kinase kinase 7 (MKK7). The mRNA for MKK7 is widely expressed in humans and mice and encodes a 47-kDa protein (419 amino acids), as determined by immunoblotting endogenous MKK7 with an antibody raised against its N terminus. The kinase domain of MKK7 is closely related to a Drosophila JNK kinase dHep (69% identity) and to a newly identified ortholog from Caenorhabditis elegans (54% identity), and was more distantly related to MKK4, MKK3, and MKK6. MKK7 phosphorylated and activated JNK1 but failed to activate p38 MAPK in co-expression studies. In hematopoietic cells, endogenous MKK7 was activated by treatment with the growth factor interleukin-3 (but not interleukin-4), or by ligation of CD40, the B-cell antigen receptor, or the receptor for the Fc fragment of immunoglobulin. MKK7 was also activated when cells were exposed to heat, UV irradiation, anisomycin, hyperosmolarity or the pro-inflammatory cytokine tumor necrosis factor-alpha. Co-expression of constitutively active mutants of RAS, RAC, or CDC42 in HeLa epithelial cells or of RAC or CDC42 in Ba/F3 factor-dependent hematopoietic cells also activated MKK7, suggesting that MKK7 will be involved in many physiological pathways.

Nerve growth factor activates extracellular signal-regulated kinase and p38 mitogen-activated protein kinase pathways to stimulate CREB serine 133 phosphorylation

The mechanisms by which growth factor-induced signals are propagated to the nucleus, leading to the activation of the transcription factor CREB, have been characterized. Nerve growth factor (NGF) was found to activate multiple signaling pathways that mediate the phosphorylation of CREB at the critical regulatory site, serine 133 (Ser-133). NGF activates the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases (MAPKs), which in turn activate the pp90 ribosomal S6 kinase (RSK) family of Ser/Thr kinases, all three members of which were found to catalyze CREB Ser-133 phosphorylation in vitro and in vivo. In addition to the ERK/RSK pathway, we found that NGF activated the p38 MAPK and its downstream effector, MAPK-activated protein kinase 2 (MAPKAP kinase 2), resulting in phosphorylation of CREB at Ser-133. Inhibition of either the ERK/RSK or the p38/MAPKAP kinase 2 pathway only partially blocked NGF-induced CREB Ser-133 phosphorylation, suggesting that either pathway alone is sufficient for coupling the NGF signal to CREB activation. However, inhibition of both the ERK/RSK and the p38/MAPKAP kinase 2 pathways completely abolished NGF-induced CREB Ser-133 phosphorylation. These findings indicate that NGF activates two distinct MAPK pathways, both of which contribute to the phosphorylation of the transcription factor CREB and the activation of immediate-early genes.

Extracellular signal-regulated kinase and p38 subgroups of mitogen-activated protein kinases regulate inducible nitric oxide synthase and tumor necrosis factor-alpha gene expression in endotoxin-stimulated primary glial cultures

Tumor necrosis factor-alpha (TNFalpha) and nitric oxide (NO), the product of inducible NO synthase (iNOS), mediate inflammatory and immune responses in the CNS under a variety of neuropathological situations. They are produced mainly by "activated" astrocytes and microglia, the two immune regulatory cells of the CNS. In this study we have examined the regulation of TNFalpha and iNOS gene expression in endotoxin-stimulated primary glial cultures, focusing on the role of mitogen-activated protein (MAP) kinase cascades. The bacterial lipopolysaccharide (LPS) was able to activate extracellular signal-regulated kinase (ERK) and p38 kinase subgroups of MAP kinases in microglia and astrocytes. ERK activation was sensitive to PD98059, the kinase inhibitor that is specific for ERK kinase. The activity of p38 kinase was inhibited by SB203580, a member of the novel class of cytokine suppressive anti-inflammatory drugs (CSAIDs), as revealed by blocked activation of the downstream kinase, MAP kinase-activated protein kinase-2. The treatment of glial cells with either LPS alone (microglia) or a combination of LPS and interferon-gamma (astrocytes) resulted in an induced production of NO and TNFalpha. The two kinase inhibitors, at micromolar concentrations, individually suppressed and, in combination, almost completely blocked glial production of NO and the expression of iNOS and TNFalpha, as determined by Western blot analysis. Reverse transcriptase-PCR analysis showed changes in iNOS mRNA levels that paralleled iNOS protein and NO while indicating a lack of effect of either of the kinase inhibitors on TNFalpha mRNA expression. The results demonstrate key roles for ERK and p38 MAP kinase cascades in the transcriptional and post-transcriptional regulation of iNOS and TNFalpha gene expression in endotoxin-activated glial cells.

β-Chemokine Receptor CCR5 Signals Via the Novel Tyrosine Kinase RAFTK

Chemokine receptors are coupled to G-proteins and their activation results in prominent changes in cell migration and growth. The downstream signaling pathways that mediate these effects of chemokines are largely uncharacterized. Macrophage inflammatory protein 1β (MIP 1β) binding to its cognate receptor CCR5 resulted in activation of the related adhesion focal tyrosine kinase (RAFTK), with subsequent activation of the cytoskeletal protein paxillin and the downstream transcriptional activators, c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) and p38 mitogen-activated protein (MAP) kinase. Inhibition of RAFTK by a dominant-negative kinase mutant markedly attenuated JNK/SAPK activity. Thus, RAFTK appears to provide a functional “bridge” for the transmission of CCR5 receptor signaling to the cytoskeleton and nucleus, primary sites of chemotaxis and growth regulation.

β-Chemokine Receptor CCR5 Signals Via the Novel Tyrosine Kinase RAFTK

Chemokine receptors are coupled to G-proteins and their activation results in prominent changes in cell migration and growth. The downstream signaling pathways that mediate these effects of chemokines are largely uncharacterized. Macrophage inflammatory protein 1β (MIP 1β) binding to its cognate receptor CCR5 resulted in activation of the related adhesion focal tyrosine kinase (RAFTK), with subsequent activation of the cytoskeletal protein paxillin and the downstream transcriptional activators, c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) and p38 mitogen-activated protein (MAP) kinase. Inhibition of RAFTK by a dominant-negative kinase mutant markedly attenuated JNK/SAPK activity. Thus, RAFTK appears to provide a functional “bridge” for the transmission of CCR5 receptor signaling to the cytoskeleton and nucleus, primary sites of chemotaxis and growth regulation.

An interleukin (IL)-13 receptor lacking the cytoplasmic domain fails to transduce IL-13-induced signals and inhibits responses to IL-4

Interleukin (IL)-13 is a pleiotropic immunoregulatory cytokine that shares many, although not all, of the biological activities of IL-4. The overlapping biological properties of IL-4 and IL-13 appear to be due to the existence of shared components of the receptors, and we and others showed that the IL-4 receptor-alpha is involved in signal transduction paths activated by both. We show here that expression of the IL-13 receptor-alpha in two factor-dependent cell lines, the premyeloid FD5 and the T lymphoid CT4.S, conferred the ability to grow continuously in response to IL-13; to respond to IL-13 with tyrosine phosphorylation of JAK1, Tyk2, IL-4Ralpha, IRS-2, and STAT6; and to respond to IL-4 with tyrosine phosphorylation of Tyk2 in addition to those induced in parental cell lines. Expression of a truncated IL-13 receptor-alpha that lacked the cytoplasmic domain demonstrated that this domain was essential for IL-13-dependent growth and phosphorylation of the above substrates. Expression of this truncated IL-13 receptor also resulted in an inhibition of biochemical and biological responses to IL-4 that was exacerbated by the presence of IL-13. These dominant inhibitory effects indicate that the extracellular domain of the truncated IL-13 receptor competes with gammac for complexes of IL-4 and the IL-4 receptor-alpha, or, when itself bound to IL-13, competes with IL-4 for the IL-4 receptor-alpha.

Activation of c-Jun N-terminal kinase by human granulocyte macrophage-colony stimulating factor in BA/F3 cells

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF) induces various signaling events in hematopoietic cells. We reported that there are at least two distinct pathways of hGM-CSF signals, one for activation of proliferation and the other one for activation of c-fos promoter through the MAPK cascade. Activation of other members of the MAPK family, c-Jun N-terminal kinase (JNK) and p38 MAPK under various cellular stress have also been reported. We found that hGM-CSF activates JNK in BA/F3 cells expressing the hGM-CSF receptor (hGMR) and that activation depends on a membrane proximal region including box1 and requires a more membrane distal region of hGMR beta subunit (beta c). There are 8 known tyrosine (tyr) residues in the cytoplasmic region of beta c. Mutant beta c lacking all the tyr residues hardly activates JNK, thereby indicating that the tyr residue(s) is essential for the activation of JNK. Mutation analyses of each tyr residue indicated that none of the tyr residues seems essential for the activation of JNK, indicating multiple tyr residues play a similar function to transduce signals for this activation.

Activation of the Stress-Activated Protein Kinases by Multiple Hematopoietic Growth Factors With the Exception of Interleukin-4

The stress-activated protein/c-Jun N-terminal kinases (SAPK/JNK) have been shown to be activated by pro-inflammatory cytokines, as well as physical and chemical stresses. We now show that a variety of hematopoietic growth factors, including Steel locus factor (SLF ), granulocyte-macrophage colony-stimulating factor (GM-CSF ), and interleukin-3 (IL-3), all of which promote the growth and survival of various lineages of hematopoietic cells, activate the stress-activated protein kinases in the factor-dependent cell line MC/9. These hematopoietic growth factors activated both 46- and 55-kD isoforms of both SAPKγ and SAPKα. Furthermore, we demonstrate that SAPK activation correlated with the phosphorylation of SAPK/ERK kinase-1 (SEK1) after treatment with SLF or GM-CSF. Interestingly, IL-4, a cytokine with distinctive and important effects on the immune system, was the exception among the hematopoietic growth factors we examined in failing to induce activation of SAPKγ, SAPKα, or SEK1. These findings show that activation of SAPK is involved, not only in responses to stresses, but also in signaling by growth factors that regulate the normal development and function of cells of the immune system.