Growth factor-induced activation of a kinase activity which causes regulatory phosphorylation of p42/microtubule-associated protein kinase

Involvement of the Akt/mTOR pathway on EGF-induced cell transformation

Our previous study demonstrated that phosphatidylinositol 3-kinase (PI3K) is necessary for epidermal growth factor (EGF)-induced cell transformation in mouse epidermal JB6 cells. Akt and the mammalian target of rapamycin (mTOR) are regarded as PI3K downstream effectors. Therefore, in this study, we investigated the role of Akt and mTOR on EGF-induced cell transformation in JB6 cells using rapamycin, a specific mTOR inhibitor, and cells expressing dominant negative mutants of Akt1 (DNM-Akt1). We found that the treatment of cells with rapamycin inhibited EGF-induced cell transformation but only slightly inhibited JB6 cell proliferation at 72 h. Although LY294002, a PI3K inhibitor, attenuated EGF-induced activator protein 1 (AP-1) activation, treatment with rapamycin did not affect AP-1 activity. Treatment with rapamycin inhibited EGF-induced phosphorylation and activation of ribosomal p70 S6 protein kinase (p70 S6K), an mTOR downstream target, but had no effect on phosphorylation and activation of Akt. Rapamycin also had no effect on EGF-induced phosphorylation of extracellular signal-regulated protein kinases (ERKs). We showed that introduction of DNM-Akt1 into JB6 mouse epidermal Cl 41 (JB6 Cl 41) cells inhibits EGF-induced cell transformation without blocking cell proliferation. The expression of DNM-Akt1 also suppressed EGF-induced p70 S6K activation as well as Akt activation. These results indicated an involvement of the Akt/mTOR pathway in EGF-induced cell transformation in JB6 cells.

Multiple members of the mitogen-activated protein kinase family are necessary for PED/PEA-15 anti-apoptotic function

293 kidney embryonic cells feature very low levels of the anti-apoptotic protein PED. In these cells, expression of PED to levels comparable with those occurring in normal adult cells inhibits apoptosis induced by growth factor deprivation and by exposure to H(2)O(2) or anisomycin. In PED-expressing 293 cells (293(PED)), inhibition of apoptosis upon growth factor deprivation was paralleled by decreased phosphorylation of JNK1/2. In 293(PED) cells, decreased apoptosis induced by anisomycin and H(2)O(2) was also accompanied by block of JNK1/2 and p38 phosphorylations, respectively. Impaired activity of these stress kinases by PED correlated with inhibition of stress-induced Cdc-42, MKK4, and MKK6 activation. At variance with JNK1/2 and p38, PED expression increased basal and growth factor-stimulated Ras-Raf-1 co-precipitation and MAPK phosphorylation and activity. Treatment of 293(PED) cells with the MEK inhibitor PD98059 blocked ERK1/2 phosphorylations with no effect on inhibition of JNK1/2 and p38 activities. Complete rescue of JNK and p38 functions in 293(PED) cells by overexpressing JNK1 or p38, respectively, enabled only partial recovery of apoptotic response to growth factor deprivation and anisomycin. However, simultaneous rescue of JNK and p38 activities accompanied by block of ERK1/2 fully restored these responses. Thus, PED controls activity of the ERK, JNK, and p38 subfamilies of MAPKs. PED anti-apoptotic function in the 293 cells requires PED simultaneous activation of ERK1/2 and inhibition of the JNK/p38 signaling systems by PED.

Differential regulation of MAP kinase by contraction and insulin in skeletal muscle: metabolic implications

We have investigated the activation of the extracellular signal-regulated kinases (ERK1 and ERK2) by muscle contraction and insulin in perfused rat skeletal muscle. Both stimuli activated ERK1 and ERK2 by an upstream kinase MAP/ERK kinase (MEK)-dependent mechanism, as the MEK inhibitor PD-98059 inhibited ERK phosphorylation. The presence of the phosphatidylinositol (PI) 3-kinase inhibitors LY-294002 and wortmannin totally eradicated ERK1 and ERK2 phosphorylation in response to insulin but not contraction. Insulin and muscle contraction activated muscle glucose transport, glycogen synthase, and amino acid transport independently of ERK signaling, whereas the PI 3-kinase inhibitors abolished the stimulatory effects of insulin but not those of contraction on these three cellular processes. We conclude that 1) insulin and contraction activate ERK signaling in skeletal muscle; 2) ERK signaling is not necessary for activation of glucose and amino acid transport or glycogen synthase activity by contraction and insulin in skeletal muscle; and 3) insulin-induced activation of MEK, the upstream activator of ERK, is dependent on PI 3-kinase, whereas contraction utilizes a different mechanism.

12-O-tetradecanoylphorbol-13-acetate-induced apoptosis is mediated by tumor necrosis factor alpha in human monocytic U937 cells

12-O-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester that is known as a tumor promoter, induces differentiation of myeloid cells and suppresses their proliferation. We studied the regulation of apoptosis by TPA in human monocytic cell line U937 cells that lack p53. Untreated U937 cells constitutively underwent apoptosis, and TPA enhanced apoptosis in these cells. Further studies showed that TPA increased production of tumor necrosis factor-alpha (TNFalpha) in U937 cells, and exogenously added TNFalpha induced apoptosis. Moreover, the induction of apoptosis by TPA was blocked by anti-TNFalpha antibody. Similar results were obtained in the myeloblastic cell line KY821 cells. We also found that the induction of apoptosis by TPA was increased in cells overexpressed with TNF receptor 1 but not in control cells. Furthermore, TPA failed to induce the production of TNFalpha and apoptosis in cells with either their protein kinase C or mitogen-activated protein kinase pathway blocked. Our results indicate that TPA induces apoptosis, at least in part, through a pathway that requires endogenous production of TNFalpha in U937 cells. Our data also suggest that the induction of apoptosis by TPA occurs through activation of protein kinase C and mitogen-activated protein kinase and TNFalpha is an autocrine-stimulating factor for the induction of apoptosis in these cells.

Expression and activation of RAS and mitogen-activated protein kinases in macrophages treated in vitro with cisplatin: regulation by kinases, phosphatases and Ca2+/calmodulin

Cisplatin (cis-dichlorodiammineplatinum II), a potent antitumour compound, stimulates immune responses by activating monocytes/macrophages and other cells of the immune system. However, the exact mechanism by which cisplatin activates these cells is poorly characterized and attempts are being made to understand this mechanism. Previous studies from this laboratory have shown that Lyn, a protein tyrosine kinase of the src family, and nuclear factor (NF)-kappaB are involved in cisplatin-induced macrophage activation. Recent studies suggest that the RAS and mitogen-activated protein (MAP) kinases function as a connecting link between activated lyn and NF-kB, which raises the possibility of their involvement in cisplatin-induced macrophage activation. Therefore, this study was undertaken to investigate the effect of cisplatin treatment on the expression/activation of RAS (a low molecular weight GTP-binding protein) and MAP kinases in murine peritoneal macrophages. The underlying mechanism of expression/activation of RAS and MAP kinases in cisplatin-treated macrophages was also investigated. Immunoblotting and immune-complex kinase assays revealed that cisplatin treatment of macrophages leads to increased expression/activation of RAS and MAP kinases, with optimal expression/activation at 15 min of treatment. Using a battery of specific inhibitor/modulators of different signalling molecules, this study shows that expression and activation of MAP kinases are two unrelated processes. It was also observed that kinase (protein tyrosine and protein kinase C) inhibitor and Ca2+/calmodulin antagonist inhibit expression/activation of RAS/MAP kinases in macrophages, whereas phosphatases (protein tyrosine and serine/threonine) inhibitor up-regulate these kinases.

The extracellular-signal-regulated protein kinases (Erks) are required for UV-induced AP-1 activation in JB6 cells

Mitogen activated protein (MAP) kinase belongs to a large family of serine/threonine protein kinases, including extracellular-signal-regulated protein kinases (Erks), P38 kinase and c-Jun N-terminal kinases (JNKs). Although previous work has shown that both Erks and JNKs are activated in cells in response to ultraviolet (UV) irradiation, most studies have focused only on the role of JNKs in UV-induced AP-1 activation. Hence, the role of Erks in UV-induced AP-1 activity is not well defined. We here have investigated this issue by using MAP kinase kinase (MEK1) inhibitor PD098059 and a dominant negative Erk2, as well as wild-type Erk2, in a JB6 cell model. PD098059 inhibited UVB- or UVC-induced AP-1 activity and phosphorylation of MEK1 and Erks, but not JNKs, in JB6 Cl 41 cells. Overexpression of wild-type Erk2 in Cl 30.7b cells that contain small amounts of Erks caused a 46.6- or 138.1-fold increase of AP-1 activity by UVB and UVC, respectively; introduction of a dominant negative Erk2 into Cl 41 cells significantly blocked the UV-induced Erks activation as well as the AP-1 activation. In contrast, overexpression of wild-type Erk2 in Cl 30.7b cells and dominant negative Erk2 in Cl 41 cells did not show a marked influence on the phosphorylation of JNKs. These results demonstrate that activation of Erks, in addition to the previously reported JNKs, is required for UV-induced AP-1 activation.

Phosphatidylinositol 3-kinase is required for the regulation of hepatitis B surface antigen production and mitogen-activated protein kinase activation by insulin but not by TPA

Insulin suppresses hepatitis B surface antigen (HBsAg) gene expression and stimulates cell proliferation in human hepatoma Hep3B cells. 12-O-tetradecanoyl phorbol-13-acetate, TPA, has been demonstrated to mimic insulin actions in these cells. We examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the signaling pathways of insulin and TPA towards these two biological phenomena in Hep3B cells. The pre-treatment of 5 microM of wortmannin diminished insulin suppressed HBsAg production and completely abolished insulin stimulated cell proliferation. However, wortmannin had no effect on TPA actions in both HBsAg suppression and cell growth stimulation. We further investigated the effect of wortmannin in mitogen-activated protein kinases (MAPKs) activation induced by insulin or TPA. After the pretreatment of wortmannin, insulin activated MAPKs was completely blocked, but TPA was still capable to activate MAPKs. These results suggest that PI 3-kinase is involved in insulin actions but not in TPA effects, and allow us to dissociate the signaling pathways of insulin and TPA in human hepatoma Hep3B cells.

Effect of phosphotyrosyl-IRS-1 level and insulin receptor tyrosine kinase activity on insulin-stimulated phosphatidylinositol 3, MAP, and S6 kinase activities

The role of tyrosine phosphorylation of the insulin receptor substrate 1 (IRS-1) was studied utilizing parental CHO cells or CHO cells that overexpress IRS-1, the insulin receptor, or both IRS-1 and the insulin receptor. Insulin stimulation of these four cell lines led to progressive levels of IRS-1 tyrosine phosphorylation of one, two, four, and tenfold. Maximal insulin-stimulated IRS-1 associated PtdIns 3'-kinase activit in these cells was 1-, 1.5-, 3-, and 3-fold, while insulin sensitivity, as determined by ED50, was 1-, 2.5-, 10-, and 10-fold. Both sensitivity and maximal response paralleled the increased level of phosphotyrosyl-IRS-1; however, the increased level of phosphotyrosyl-IRS-1 seen in CHO/IR/IRS-1 cells did not further increase these responses. Likewise, maximal insulin-stimulated MAP kinase activity in these cell lines increased in parallel with IRS-1 tyrosine phosphorylation except in the CHO/IR/IRS-1 cell lines with activity levels of one-, five-, nine-, and ninefold. However, insulin sensitivity of the MAP and S6 kinases and maximal insulin-stimulated S6 kinase activity was not changed by a twofold increase in phosphotyrosyl-IRS-1, but an increase was observed with insulin-stimulated receptor autophosphorylation and kinase activity in CHO/IR cells which led to a tenfold increase in insulin receptor autophosphorylation and a fourfold increase in IRS-1 tyrosine phosphorylation. Thus, these three kinase activities may be differentially coupled to the activation of the insulin receptor kinase activity via IRS-1 and other possible cellular substrates.

Map kinase activation in Swiss 3T3 cells stimulated with gastrin-releasing peptide is associated with increased phosphorylation of a 78,000 M(r) protein immunoprecipitated by anti-raf kinase anti-serum

Addition of 10 nM gastrin-releasing peptide (GRP) to Swiss 3T3 fibroblasts induced a transient (1-2 min) tyrosine phosphorylation and activation of mitogen-activated protein kinase (MAP kinase). Increased activity of 42,000 M(r) MAP kinase was detected with immunochemical methods; however, in situ kinase detection on renaturated SDS-polyacrylamide electrophoresis gel revealed activation of both 42,000 and 44,000 M(r) MAP kinase species. Furthermore, stimulation of 32P-labelled cells with 10 nM GRP for 2 min resulted in an increased phosphorylation of a protein with an approximate molecular mass of 78,000 M(r) in anti-raf kinase and anti-MAP kinase kinase immunoprecipitates of cytosolic extracts from 32P-labelled cells. The presented data demonstrated that GRP induces MAP kinase activation in Swiss 3T3 fibroblasts, and furthermore suggest a role for raf kinase in this process.

A 40-kDa myelin basic protein kinase, distinct from erk1 and erk2, is activated in mitotic HeLa cells

Mitotic HeLa cells showed an increased phosphorylation activity towards myelin basic protein compared to cells in G1 or S phases. Further investigation using renaturation gels revealed that, in mitotic cell lysates, a protein with an apparent molecular mass of around 40 kDa phosphorylates myelin basic protein. This kinase is active early in mitosis, but is then downregulated concomitantly with p34cdc2 kinase as mitosis proceeds, its activity decreasing to basal levels by early G1. The molecular mass of the kinase suggested that it might be one of the human homologues of rat erk1 or erk2. However, antibodies raised against C-terminal sequences of erk1 and erk2 failed to immunoprecipitate renaturable kinase activity from mitotic lysates. In addition, in immunoblots erk1 and erk2 failed to show the well established changes in electrophoretic migration that are consequences of their activation. These data indicate that these two mitogen-activated protein (MAP) kinases are not stimulated during HeLa cell mitosis and indicate that the 40-kDa kinase is either a new member of the MAP kinase family or it is a novel mitotic kinase that has not yet been described.

Deciphering the MAP kinase pathway

MAP kinases (MAPK) are serine/threonine kinases which are activated by a dual phosphorylation on threonine and tyrosine residues. Their specific upstream activators, called MAP kinase kinases (MAPKK), constitute a new family of dual-specific threonine/tyrosine kinases, which in turn are activated by upstream MAP kinase kinase kinases (MAPKKK). These three kinase families are successively stimulated in a cascade of activation described in various species such as mammals, frog, fly, worm or yeast. In mammals, the MAP kinase module lies on the signaling pathway triggered by numerous agonists such as growth factors, hormones, lymphokines, tumor promoters, stress factors, etc. Targets of MAP kinase have been characterized in all subcellular compartments. In yeast, genetic epistasis helped to characterize the presence of several MAP kinase modules in the same system. By complementation tests, the relationships existing between phylogenetically distant members of each kinase family have been described. The roles of the MAP kinase cascade have been analyzed by engineering various mutations in the kinases of the module. The MAP kinase cascade has thus been implicated in higher eukaryotes in cell growth, cell fate and differentiation, and in low eukaryotes, in conjugation, osmotic stress, cell wall construct and mitosis.

Transrepression of type II collagen by TGF-beta and FGF is protein kinase C dependent and is mediated through regulatory sequences in the promoter and first intron

Transforming growth factor beta and basic fibroblast growth factor are multipotential factors found in bone and cartilage that may be involved in both the proliferation and differentiation of chondrocytes. It was previously reported that TGF-beta plus FGF caused a modulation of chondrocyte phenotype that included the down-regulation of steady-state level of the collagen II transcript. In this report, the results of nuclear run-off data indicate that repression of transcript initiation from the collagen II gene is the primary mechanism involved in the growth factor induced inhibition. Transient transfection assays with CAT expression vectors containing portions of the collagen II gene show that the TGF-beta/FGF induced transrepression requires a region in the first intron previously reported to have transcriptional enhancer activity and to bind chondrocyte nuclear proteins. In addition, silencer elements in the promoter also appear to play a role. Protein data as well as transient transfection experiments indicate that the activation of protein kinase C is necessary for the growth factor-induced down-regulation of collagen II expression. These studies suggest that a cascade initiating with PKC activation is responsible for modifying transcription factors that interact with regulatory sequences in the collagen II gene. A detailed understanding of the factors involved in cartilage-specific gene regulation in chondrocytes would facilitate development of therapeutic protocols for the repair of degenerated cartilage in diseases such as osteoarthritis.

Phosphorylation of tau protein in tau-transfected 3T3 cells

The tau protein of Alzheimer paired helical filaments (PHFs) is aberrantly phosphorylated, as evidenced by its reactivity with several phosphate-dependent antibodies. We sought to identify whether this unusual phosphorylation state exists in tau expressed by transfected NIH 3T3 fibroblasts. Immunoblot analysis of cell clones transfected with constructs for either the 3-repeat or 4-repeat isoforms of tau revealed two tau bands, with the lower band migrating with unmodified tau in each case. Antibodies T3P and tau-1 were used to probe these bands, as they also react with PHF-tau in a phosphate-dependent manner. The epitopes for both antibodies were phosphorylated in both tau isoforms. Only the upper band was phosphorylated at the T3P site whereas phosphorylation at the tau-1 site was not always associated with a shift of tau mobility on gels. Tau in both bands was soluble, in contrast to PHF-tau, and was competent to bind to exogenously added bovine microtubules. Colchicine treatment of the cells resulted in an inhibition of phosphorylation at both sites, through an unknown mechanism. In conclusion human tau expressed in 3T3 cells was phosphorylated at the T3P and tau-1 sites as is PHF-tau, although no PHFs formed and the phosphorylated tau was competent to bind to microtubules.

Networking with mitogen-activated protein kinases

Mitogen activated protein (MAP) kinases and their target ribosomal protein S6 (RSK) kinases have been recognized as shared components in the intracellular signaling pathways of many diverse cytokines. Recent studies have extended this protein kinase cascade by identifying the major activator of vertebrate MAP kinases as a serine/threonine/tyrosine-protein kinase called MEK, which is related to yeast mating factor-regulated protein kinases encoded by the STE7 and byr1 genes. MEK, in turn, may be activated following its phosphorylation on serine by either of the kinases encoded by proto-oncogenes raf1 or mos, as well as by p78mekk, which is related to the yeast STE11 and byr2 gene products. Isoforms of all of these protein kinases may specifically combine to assemble distinct modules for intracellular signal transmission. However, the fundamental architecture of these protein kinase cascades has been highly conserved during eukaryotic evolution.

The MAP kinase cascade. Discovery of a new signal transduction pathway

Using biochemical techniques similar to those used by Krebs and Fischer in elucidating the cAMP kinase cascade, a protein kinase cascade has been found that represents a new pathway for signal transduction. This pathway is activated in almost all cells that have been examined by many different growth and differentiation factors, suggesting control of different cell responses. At this writing, four tiers of growth factor regulated kinases, each tier represented by more than one enzyme, have been reconstituted in vitro to form the MAP kinase cascade. Preliminary findings suggesting multiple feedback or feedforward regulation of several components in the cascade predict higher complexity than a simple linear pathway.

MsERK1: a mitogen-activated protein kinase from a flowering plant

The induction of proliferation and differentiation in cultured mammalian cells is mediated by a cascade of protein phosphorylations. A key enzyme in this signaling pathway is mitogen-activated protein (MAP) kinase (or ERK, extracellular signal-regulated kinase). We report the recovery of a full-length cDNA clone encoding a MAP kinase from alfalfa. We have named the 44-kD protein encoded by this clone MsERK1. Recombinant MsERK1 (rMsERK1), when overexpressed in Escherichia coli, is recognized by antibodies raised against MAP kinases from rat, Xenopus, and sea star and by anti-phosphotyrosine antibodies. Site-directed mutagenesis of MsERK1 demonstrated that Tyr-215 is either directly or indirectly responsible for recognition of the protein by anti-phosphotyrosine antibodies. Semipurified rMsERK1 phosphorylated itself and a model substrate, myelin basic protein, in vitro, but the Tyr-215 mutant did neither. Genomic DNA gel blot analysis suggested that the gene that encodes MsERK1 is either a member of a small multigene family or a member of a polymorphic allelic series in alfalfa. Because MAP kinase activation has been associated with mitotic stimulation in animal systems, such an enzyme may play a role in the mitogenic induction of symbiotic root nodules on alfalfa by Rhizobium signal molecules.

Activation of the MAP kinase pathway by the protein kinase raf

Both MAP kinases and the protein kinase p74raf-1 are activated by many growth factors in a c-ras-dependent manner and by oncogenic p21ras. We were therefore interested in determining the relationship between MAP kinases and raf. The MAP kinase ERK2 is activated by expression of oncogenically activated raf, independently of cellular ras. Overexpressed p74raf-1 potentiates activation of ERK2 by EGF and TPA. MAP kinase kinase inactivated by phosphatase 2A treatment is phosphorylated and reactivated by incubation with p74raf-1 immunoprecipitated from phorbol ester-treated cells. We conclude that raf protein kinase is upstream of MAP kinases and is either a MAP kinase kinase kinase or a MAP kinase kinase kinase kinase.

Involvement of MAP kinase activators in angiotensin II-induced activation of MAP kinases in cultured vascular smooth muscle cells

In cultured vascular smooth muscle cells (VSMC) angiotensin II (ang II) induces tyrosine and serine/threonine phosphorylation and activation of two mitogen-activated protein (MAP) kinases. When extracts of ang II-stimulated VSMC were fractionated by Mono Q anion-exchange column chromatography, three peaks of the activities which in vitro activate inactive MAP kinases were detected. These MAP kinase activator activities were not detected in extracts of unstimulated VSMC. In vitro activation of MAP kinases by the MAP kinase activators was accompanied by tyrosine and serine/threonine phosphorylation of MAP kinases. These results suggest that the MAP kinase activators are involved in the ang II-induced phosphorylation and activation of MAP kinases in VSMC.

Raf-1 activates MAP kinase-kinase

The normal cellular homologue of the acutely transforming oncogene v-raf is c-raf-1, which encodes a serine/threonine protein kinase that is activated by many extracellular stimuli. The physiological substrates of the protein c-Raf-1 are unknown. The mitogen-activated protein (MAP) kinases Erk1 and 2 are also activated by mitogens through phosphorylation of Erk tyrosine and threonine residues catalysed by a protein kinase of relative molecular mass 50,000, MAP kinase-kinase (MAPK-K). Here we report that MAPK-K as well as Erk1 and 2 are constitutively active in v-raf-transformed cells. MAPK-K partially purified from v-raf-transformed cells or from mitogen-treated cells can be deactivated by phosphatase 2A. c-Raf-1 purified after mitogen stimulation can reactivate the phosphatase 2A-inactivated MAPK-K over 30-fold in vitro. c-Raf-1 reactivation of MAPK-K coincides with the selective phosphorylation at serine/threonine residues of a polypeptide with M(r) 50,000 which coelutes precisely on cation-exchange chromatography with the MAPK-K activatable by c-Raf-1. These results indicate that c-Raf-1 is an immediate upstream activator of MAPK-K in vivo. To our knowledge, MAPK-K is the first physiological substrate of the c-raf-1 protooncogene product to be identified.

Ordered phosphorylation of p42mapk by MAP kinase kinase

Preparation of milligram amounts of [32P]p42mapk, phosphorylated at Tyr185 or diphosphorylated at Tyr185/Thr183, for use as specific protein phosphatase substrates is described. Tyr- but not Thr-phosphorylated p42mapk, accumulates when ATP is limiting. Furthermore, Tyr185-phosphorylated p42mapk exhibits an apparent 10-fold decrease in apparent Km (46.6 +/- 6.6 nM) for MAP kinase kinase compared to that for the dephospho form (approximately 476 nM). We conclude that Tyr185 precedes Thr183 phosphorylation, and that this is prerequisite, dramatically increasing the affinity of p42mapk for MAP kinase kinase.