Sustained activation of the mitogen-activated protein kinase pathway. A mechanism underlying receptor tyrosine kinase specificity for matrix metalloproteinase-9 induction and cell migration

Phosphorylation of focal adhesion kinase (pp125(FAK)) is increased in human keratinocytes induced to migrate by extracellular matrices

During the healing process of skin wounds, human keratinocytes migrate across a provisional matrix of the wound bed. The mechanisms by which keratinocytes migrate on connective tissue are not known. In this study, we examined the role of focal adhesion kinase (FAK), an 125 kDa protein that co-localizes with focal adhesions in cells plated on extracellular matrix. We induced human keratinocytes into various states of migration by plating them on extracellular matrices that minimally, moderately, or strongly induce cellular migration, and then examined the expression of FAK at the protein level and its degree of tyrosine phosphorylation using Western immunoblotting and immunoprecipitation. In highly migratory human keratinocytes, we found that three proteins were predominantly tyrosine phosphorylated, one of them being FAK. Tyrosine phosphorylation of FAK tightly correlated with the level of cellular motility but not cell attachment to the matrix. Time course experiments demonstrated that in highly motile keratinocytes, tyrosine phosphorylation of FAK peaked at 12 h, the time when maximal migration on the matrix ensues. In contrast to FAK, the beta1 integrin subunit of human keratinocytes that configures with the alpha2, alpha3, and alpha5 integrin subunits to form integrin receptors for matrix, did not display tyrosine phosphorylation linked to motility. Using anti-sense oligonucleotides to FAK, we demonstrate that FAK is required for human keratinocyte migration, but not for focal adhesion formation.

Sustained activation of the extracellular signal-regulated kinase pathway is required for extracellular calcium stimulation of human osteoblast proliferation

Elevated levels of [Ca(2+)](o) in bone milieu as a result of the resorptive action of osteoclasts are implicated in promoting proliferation and migration of osteoblasts during bone remodeling. However, mitogenic effects of [Ca(2+)](o) have only been shown in some, but not all, clonal osteoblast-like cells, and the molecular mechanisms underlying [Ca(2+)](o)-induced mitogenic signaling are largely unknown. In this study we demonstrated for the first time that [Ca(2+)](o) stimulated proliferation of primary human osteoblasts and selectively activated extracellular signal-regulated kinases (ERKs). Neither p38 mitogen-activated protein (MAP) kinase nor stress-activated protein kinase was activated by [Ca(2+)](o). Treatment of human osteoblasts with a MAP kinase kinase inhibitor, PD98059, impaired both basal and [Ca(2+)](o)-stimulated phosphorylation of ERKs and also reduced both basal and [Ca(2+)](o)-stimulated proliferation. [Ca(2+)](o) treatment resulted in two distinctive phases of ERK activation: an acute phase and a sustained phase. An inhibition time course revealed that it was the sustained phase, not the acute phase, that was critical for [Ca(2+)](o)-stimulated osteoblast proliferation. Our results demonstrate that mitogenic responsiveness to [Ca(2+)](o) is present in primary human osteoblasts and is mediated via prolonged activation of the MAP kinase kinase/ERK signal pathway.

Initiation of cell locomotility is a morphogenetic checkpoint in thyroid epithelial cells regulated by ERK and PI3-kinase signals

Epithelial locomotility is a fundamental determinant of tissue patterning that is subject to strict physiological regulation. The current study sought to identify cellular signals that initiate cell migration in cultured thyroid epithelial cells. Porcine thyroid cells cultured as 3-dimensional follicles convert to 2-dimensional monolayers when deprived of agents that stimulate cAMP/PKA signaling. This morphogenetic event is driven by the activation of cell-on-substrate locomotility, providing a convenient assay for events that regulate the initiation of locomotion. In this system, the extracellular signal regulated kinase (ERK) pathway became activated as follicles converted to monolayer, as demonstrated by immunoblotting for activation-specific phosphorylation and nuclear accumulation of ERK. Inhibition of ERK activation using the drug PD98059 effectively prevented cells from beginning to migrate. PD98059 inhibited cell spreading, actin filament reorganization and the assembly of focal adhesions, cellular events that mediate the initiation of thyroid cell locomotility. Akt (PKB) signaling was also activated during follicle-to-monolayer conversion and the phosphoinositide 3-kinase (PI3-kinase) inhibitor, wortmannin, also blocked the initiation of cell movement. Wortmannin did not, however, block activation of ERK signaling. These findings, therefore, identify the ERK and PI3-kinase signaling pathways as important stimulators of thyroid cell locomotility. These findings are incorporated into a model where the initiation of thyroid cell motility constitutes a morphogenetic checkpoint regulated by coordinated changes in stimulatory (ERK, PI3-kinase) and tonic inhibitory (cAMP/PKA) signaling pathways.

Paclitaxel induces prolonged activation of the Ras/MEK/ERK pathway independently of activating the programmed cell death machinery

Paclitaxel is a widely used chemotherapeutic agent and is known to induce programmed cell death (apoptosis) in a variety of cell types, but the precise underlying mechanisms are poorly understood. To elucidate these mechanisms, we challenged human esophageal squamous cancer cell lines with paclitaxel and investigated its effects upon signal transduction pathways. Physiologically relevant concentrations of paclitaxel (1-1,000 nm) induced apoptosis. All three mitogen-activated protein kinase (MAPK) family members, c-Jun N-terminal kinase (JNK), p38 MAPK, and extracellular signal-regulated kinase (ERK) were activated upon paclitaxel treatment. Interestingly, JNK activation and p38 MAPK activation were delayed and peaked at 48 h, whereas ERK activity was sustained over 72 h. In addition, Ras activation and MAPK/ERK kinase (MEK) phosphorylation were observed in concordance with ERK activation. While ERK activation was completely ablated by MEK inhibitors, immunoprecipitation and Western blot analysis revealed that neither MEK-1 nor MEK-2 was involved, but instead another member of the MEK family may potentially participate. Although pretreatment with a general caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone rescued the cell death, it did not prevent Ras or ERK activation. Furthermore, inhibition of JNK, p38 MAPK, or MEK did not alter PARP cleavage and the cell death induced by paclitaxel. These results in aggregate suggest that the delayed activation of JNK, p38 MAPK, and ERK was not linked to activation of the cell death machinery.

Constitutive ERK1/2 activation in esophagogastric rib bone marrow micrometastatic cells is MEK-independent

In this study, we examined the mitogen-activated protein kinase (MAPK) cascade in micrometastatic cell lines generated from rib bone marrow (RBM) of patients undergoing resection of esophagogastric malignancies. The molecular mechanism(s) involved in esophagogastric MAPK activation have not previously been investigated. Constitutive activation of both ERK1 and -2 isoforms was evident in each of the five RBM cell lines. Elk-1, a transcription factor activated by the ERK1/2 pathway was also found to be constitutively activated. Cell lines generated from metastases of involved lymph nodes (OC2) and ascites (OC1) of patients with esophageal cancer do not display, however, hyperphosphorylation of ERK1/2. Constitutive RBM ERK1/2 activation is protein kinase C and phosphatidylinositol 3-kinase dependent. Surprisingly, constitutive ERK1/2 activation is MEK-independent. Pharmacological inhibition of MEK with two specific inhibitors, PD 98059 and U0126, were both ineffective in blocking ERK activation. Similarly, the use of a dominant negative MEK mutant was without effect. Interestingly, experiments overexpressing two different dominant negative Pak1 mutants significantly reduced RBM ERK1/2 activation, albeit not to the same extent for all cell lines. We also examined the role of three different phosphatases, PAC1, MKP-1, and -2. While RBM ERK1/2 activation was found to be PAC1- and MKP-2-independent, surprisingly, MKP-1 was down-regulated in all five RBM cell lines. In conclusion, we provide evidence for the first time for a MEK-independent constitutive ERK1/2 activation pathway in esophagogastric RBM cell lines. These findings have important implications for drug treatment strategies which currently target MEK in other forms of cancer.

Fibroblast growth factors 1 and 2 differently activate MAP kinase in Xenopus oocytes expressing fibroblast growth factor receptors 1 and 4

The mitogen-activated protein kinase (MAP kinase) signalling cascade activated by fibroblast growth factors (FGF1 and FGF2) was analysed in a model system, Xenopus oocytes, expressing fibroblast growth factor receptors (FGFR1 and FGFR4). Stimulation of FGFR1 by FGF1 or FGF2 and FGFR4 by FGF1 induced a sustained phosphorylation of extracellular signal-regulated protein kinase 2 (ERK2) and meiosis reinitiation. In contrast, FGFR4 stimulation by FGF2 induced an early transient activation of ERK2 and no meiosis reinitiation. FGFR4 transduction cascades were differently activated by FGF1 and FGF2. Early phosphorylation of ERK2 was blocked by the dominant negative form of growth factor-bound protein 2 (Grb2) and Ras, for FGF1-FGFR4 and FGF2-FGFR4. The phosphatidylinositol 3-kinase (PI3 kinase) inhibitors wortmannin and LY294002 only prevented the early ERK2 phosphorylation triggered by FGF2-FGFR4 but not by FGF1-FGFR4. ERK2 phosphorylation triggered by FGFR4 depended on the Grb2/Ras pathway and also involved PI3 kinase in a time-dependent manner.

KiSS-1 represses 92-kDa type IV collagenase expression by down-regulating NF-kappa B binding to the promoter as a consequence of Ikappa Balpha -induced block of p65/p50 nuclear translocation

The 92-kDa type IV collagenase (MMP-9) plays a critical role in tissue remodeling. We undertook a study to determine whether the KiSS-1 gene, previously shown to suppress cancer spread (metastases), negatively regulates MMP-9 expression. Six cell lines positive for MMP-9 mRNA were deficient in KiSS-1 mRNA. One of these cell lines, HT-1080, stably transfected with a KiSS-1 expression construct, demonstrated substantially lower MMP-9 enzyme activity/protein and in vitro invasiveness. The lower MMP-9 enzyme activity reflected reduced steady-state mRNA levels which, in turn, was due to attenuated transcription. Activation of ERKs and JNKs by phorbol 12-myristate 13-acetate and tumor necrosis factor alpha, respectively, leading to increased MMP-9 amounts was not antagonized by KiSS-1 expression, suggesting that MAPK pathways modulating MMP-9 synthesis are not the target of KiSS-1. Although MMP-9 expression is regulated by AP-1, Sp1, and Ets transcription factors, KiSS-1 did not alter the binding of these factors to the MMP-9 promoter. However, NF-kappaB binding to the MMP-9 promoter required for expression of this collagenase was reduced by KiSS-1 expression. Diminished NF-kappaB binding reflected less p50/p65 in the nucleus secondary to increased IkappaBalpha levels in the cytosols of the KiSS-1 transfectants. Thus, KiSS-1 diminishes MMP-9 expression by effecting reduced NF-kappaB binding to the promoter.

Rap1A protein interferes with various MAP kinase activating pathways in skeletal myogenic cells

Constitutive expression of the activated Rap1A protein inhibits differentiation of myogenic C2 cells whereas the inactivated Rap1A protein favours cell differentiation and induces late endocytic compartments clustering. Although the role of Rap1A in MAPK activation has been analysed in various cell types, the signalling pathways activated by Rap1A have not been explored in myogenic cells. In this study, we investigated MAP kinase activity in control C2 myoblasts and in stable C2 cell lines expressing mutated Rap1A proteins. We provide evidence that Rap1A mutants promote ERK activation and that the active protein induces a more sustained activation than the inactive protein. In addition, we established that various pathways mediate transient ERK activation in control cells and in cells expressing the inactivated Rap1A protein. In these cells, ERK are activated by a Raf/MEK-dependent pathway, a P13K/Raf-independent pathway and a third undetermined pathway. In cells expressing the activated Rap1A protein, a PI3K/Raf/MEK-dependent pathway mediates transient ERK activation. However, MAPK activation appears more complex since, according to the state of the myoblasts or the duration of MAPK stimulation, we observed that Rap1A protein could interfere or not with ERK activation.

Sustained ERK phosphorylation is necessary but not sufficient for MMP-9 regulation in endothelial cells: involvement of Ras-dependent and -independent pathways

Endothelial expression of matrix metalloproteinase-9 (MMP-9), which degrades native type IV collagen, was implicated as a prerequisite for angiogenesis. Therefore, the aim of this study was to determine signaling requirements that regulate MMP-9 expression in endothelial cells. Both, primary and permanent human umbilical vein endothelial cells (HUVEC and ECV304, respectively) were stimulated with phorbol 12-myristate 13-acetate (PMA) and the cytokine tumor necrosis factor-(alpha) (TNF(alpha)) to induce MMP-9 expression. While both cell types responded to PMA at the protein, mRNA and promoter level by induction of MMP-9, TNF(alpha) caused this response only in ECV304. Inhibitors specific for mitogen-activated protein/ERK kinase 1/2 (MEK1/2), protein kinase C (PKC), and Ras and co-transfections of wild-type and mutant Raf were used to elucidate the signaling cascades involved. Thus, we could show that the Raf/MEK/ERK cascade is mainly responsible for MMP-9 induction in endothelial cells and that this cascade is regulated independently of PKC and Ras subsequent to TNF(alpha) stimulation and in a PKC-dependent manner as a result of PMA treatment. In addition, PMA triggers a Ras-dependent signal transduction pathway bypassing the phosphorylation of ERK. Finally, we provide evidence that sustained phosphorylation of ERK1/2 is necessary but not sufficient for expression of MMP-9.

Phosphorylation of ADF/cofilin abolishes EGF-induced actin nucleation at the leading edge and subsequent lamellipod extension

In metastatic rat mammary adenocarcinoma cells, cell motility can be induced by epidermal growth factor. One of the early events in this process is the massive generation of actin barbed ends, which elongate to form filaments immediately adjacent to the plasma membrane at the tip of the leading edge. As a result, the membrane moves outward and forms a protrusion. To test the involvement of ADF/cofilin in the stimulus-induced barbed end generation at the leading edge, we inhibited ADF/cofilin's activity in vivo by increasing its phosphorylation level using the kinase domain of LIM-kinase 1 (GFP-K). We report here that expression of GFP-K in rat cells results in the near total phosphorylation of ADF/cofilin, without changing either the G/F-actin ratio or signaling from the EGF receptor in vivo. Phosphorylation of ADF/cofilin is sufficient to completely inhibit the appearance of barbed ends and lamellipod protrusion, even in the continued presence of abundant G-actin. Coexpression of GFP-K, together with an active, nonphosphorylatable mutant of cofilin (S3A cofilin), rescues barbed end formation and lamellipod protrusion, indicating that the effects of kinase expression are caused by the phosphorylation of ADF/cofilin. These results indicate a direct role for ADF/cofilin in the generation of the barbed ends that are required for lamellipod extension in response to EGF stimulation.

Sequential activation of ERK and repression of JNK by scatter factor/hepatocyte growth factor in madin-darby canine kidney epithelial cells

The scattering of Madin-Darby canine kidney (MDCK) epithelial cells by scatter factor/hepatocyte growth factor (SF/HGF) is associated with transcriptional induction of the urokinase gene, which occurs essentially through activation of an EBS/AP1 response element. We have investigated the signal transduction pathways leading to this transcriptional response. We found that SF/HGF induces rapid and sustained phosphorylation of the extracellular signal-regulated kinase (ERK) MAPK while stimulating weakly and then repressing phosphorylation of the JUN N-terminal kinase (JNK) MAPK for several hours. This delayed repression of JNK was preceded by phosphorylation of the MKP2 phosphatase, and both MKP2 induction and JNK dephosphorylation were under the control of MEK, the upstream kinase of ERK. ERK and MKP2 stimulate the EBS/AP1-dependent transcriptional response to SF/HGF, but not JNK, which inhibits this response. We further demonstrated that depending on cell density, the RAS-ERK-MKP2 pathway controls this transrepressing effect of JNK. Together, these data demonstrate that in a sequential manner SF/HGF activates ERK and MKP2, which in turn dephosphorylates JNK. This sequence of events provides a model for efficient cell scattering by SF/HGF at low cell density.

Extracellular regulated kinase (ERK)-dependent regulation of sialomucin complex/rat Muc4 in mammary epithelial cells

Sialomucin complex (SMC, rat Muc4) is a membrane mucin implicated in the protection of epithelia and the metastasis of some tumors. It is a heterodimeric complex, containing a mucin subunit with anti-adhesive activity and a transmembrane subunit with epidermal growth factor-like domains, one of which acts as an intramembrane ligand for ErbB2. Serum, insulin and insulin-like growth factor, but not epidermal growth factor, induce the expression of sialomucin complex in mammary epithelial cells. Induction correlates with sustained, but not transient, activation of extracellular-regulated protein kinase (ERK). MEK inhibitor U0126 blocked the induction, while activated MEK-1 transfected into a rat mammary adenocarcinoma cell line induced a sustained activation of ERK and up-regulated SMC/Muc4 expression. Northern and Western blotting indicated that up-regulation occurred concomitantly at the transcript and protein levels, both of which could be blocked by U0126. These results suggest that expression of SMC/Muc4 in mammary epithelial cells is regulated by selected growth factors through an ERK-dependent pathway at the transcript level.

Transactivation of the EGF receptor mediates IGF-1-stimulated shc phosphorylation and ERK1/2 activation in COS-7 cells

The receptor for insulin-like growth factor 1 (IGF-1) mediates multiple cellular responses, including stimulation of both proliferative and anti-apoptotic pathways. We have examined the role of cross talk between the IGF-1 receptor (IGF-1R) and the epidermal growth factor receptor (EGFR) in mediating responses to IGF-1. In COS-7 cells, IGF-1 stimulation causes tyrosine phosphorylation of the IGF-1R beta subunit, the EGFR, insulin receptor substrate-1 (IRS-1), and the Shc adapter protein. Shc immunoprecipitates performed after IGF-1 stimulation contain coprecipitated EGFR, suggesting that IGF-1R activation induces the assembly of EGFR.Shc complexes. Tyrphostin AG1478, an inhibitor of the EGFR kinase, markedly attenuates IGF-1-stimulated phosphorylation of EGFR, Shc, and ERK1/2 but has no effect on phosphorylation of IGF-1R, IRS-1, and protein kinase B (Akt). Cross talk between IGF-1 and EGF receptors is mediated through an autocrine mechanism involving matrix metalloprotease-dependent release of heparin-binding EGF (HB-EGF), because IGF-1-mediated ERK activation is inhibited both by [Glu(52)]Diphtheria toxin, a specific inhibitor of HB-EGF, and the metalloprotease inhibitor 1,10-phenanthroline. These data demonstrate that IGF-1 stimulation of the IRS-1/PI3K/Akt pathway and the EGFR/Shc/ERK1/2 pathway occurs by distinct mechanisms and suggest that IGF-1-mediated "transactivation" of EGFR accounts for the majority of IGF-1-stimulated Shc phosphorylation and subsequent activation of the ERK cascade.

Exogenous expression of N-cadherin in breast cancer cells induces cell migration, invasion, and metastasis

E- and N-cadherin are calcium-dependent cell adhesion molecules that mediate cell-cell adhesion and also modulate cell migration and tumor invasiveness. The loss of E-cadherin-mediated adhesion has been shown to play an important role in the transition of epithelial tumors from a benign to an invasive state. However, recent evidence indicates that another member of the cadherin family, N-cadherin, is expressed in highly invasive tumor cell lines that lacked E-cadherin expression. These findings have raised the possibility that N-cadherin contributes to the invasive phenotype. To determine whether N-cadherin promotes invasion and metastasis, we transfected a weakly metastatic and E-cadherin-expressing breast cancer cell line, MCF-7, with N-cadherin and analyzed the effects on cell migration, invasion, and metastasis. Transfected cells expressed both E- and N-cadherin and exhibited homotypic cell adhesion from both molecules. In vitro, N-cadherin-expressing cells migrated more efficiently, showed an increased invasion of Matrigel, and adhered more efficiently to monolayers of endothelial cells. All cells produced low levels of the matrix metalloproteinase MMP-9, which was dramatically upregulated by treatment with FGF-2 only in N-cadherin-expressing cells. Migration and invasion of Matrigel were also greatly enhanced by this treatment. When injected into the mammary fat pad of nude mice, N-cadherin-expressing cells, but not control MCF-7 cells, metastasized widely to the liver, pancreas, salivary gland, omentum, lung, lymph nodes, and lumbar spinal muscle. The expression of both E- and N-cadherin was maintained both in the primary tumors and metastatic lesions. These results demonstrate that N-cadherin promotes motility, invasion, and metastasis even in the presence of the normally suppressive E-cadherin. The increase in MMP-9 production by N-cadherin-expressing cells in response to a growth factor may endow them with a greater ability to penetrate matrix protein barriers, while the increase in their adherence to endothelium may improve their ability to enter and exit the vasculature, two properties that may be responsible for metastasis of N-cadherin-expressing cells.

Plasminogen-related growth factor and semaphorin receptors: a gene superfamily controlling invasive growth

Plasminogen-related growth factors (PRGFs), also known as "scatter factors," trigger a unique biological program leading to "invasive growth." This is a result of the integration of apparently independent biological responses including cell proliferation, cell survival, cell motility, invasion of extracellular matrices, and induction of cell polarity. Under physiological conditions, the coordinated execution of the underlying genetic programs leads to the formation of tubular structures by epithelial organs (the so-called branching morphogenesis). PRGF receptors are tyrosine kinases, encoded by a family of oncogenes: MET and RON. They feature unique signal transduction properties as their cytoplasmic tails contain a two-tyrosine multifunctional docking site that binds multiple SH2-containing intracellular signal transducers. Invasive growth results from the concomitant activation of Ras (growth), phosphatidylinositol 3-kinase ("scattering"), and signal transducer and activator of transcription (cell polarity and morphogenesis). We recently identified a new human gene family, encoding large transmembrane proteins, sex/plexins, sharing homologies with Met. These molecules are receptors for semaphorins, involved in axon guidance and cell-cell repulsion, a process reminiscent of scattering and invasive growth. Deregulated activation of PRGF or semaphorin ligands or receptors, by mutation or overexpression, confers to cancer cells invasive and metastatic properties.

Conversion of TNF alpha from antiproliferative to proliferative ligand in mouse intestinal epithelial cells by regulating mitogen-activated protein kinase

The mechanisms regulating the balance between intestinal epithelial cell proliferation and differentiation are essential to maintaining an intact mucosal barrier. Mitogen-activated protein (MAP) kinases appear to be key transducers of extracellular signals in these pathways. The goal of this study was to investigate the regulation of MAP kinase by tumor necrosis factor alpha (TNFalpha) and epidermal growth factor (EGF) in intestinal epithelial cells. The young adult mouse colon cell line was studied for TNFalpha and/or EGF regulation of MAP kinase in the presence or absence of the MAP kinase kinase (MEK1) inhibitor PD 98059. Proliferation was determined by hemocytometry, and activated MAP kinase was identified by Western blot analysis, in vitro kinase assay, and confocal laser immunofluorescent microscopy. TNFalpha stimulated sustained nuclear MAP kinase activity, while EGF stimulated transient cytoplasmic MAP kinase activity. Changing TNFalpha's sustained MAP kinase activation to transient converted TNFalpha from an anti-proliferative to a proliferative ligand. These findings demonstrate that both TNFalpha and EGF activate MAP kinase in intestinal epithelial cells. The kinetics and subcellular distribution of this enzyme activity may be pivotal in the transduction of divergent cellular responses in the intestinal epithelium with implications for altered proliferative signals in inflammatory bowel disease.