Multiple roles of the mitogen-activated protein kinase kinase/mitogen-activated protein kinase cascade in Xenopus laevis

Mitogen-activated protein kinase (MAPK) was originally identified as a serine/threonine protein kinase that is rapidly activated in response to various growth factors and tumor promoters in mammalian cultured cells. The kinase cascade including MAPK and its direct activator, MAPK kinase (MAPKK), is now believed to transmit various extracellular signals into their intracellular targets in eukaryotic cells. It has been reported that activation of MAPKK and MAPK occurs during the meiotic maturation of oocytes in several species, including Xenopus laevis . Studies with neutralizing antibodies against MAPKK, MAPK phosphatases and constitutively active MAPKK or MAPK have revealed a crucial role of the MAPKK/MAPK cascade in a number of developmental processes in Xenopus oocytes and embryos.     

Stimulation of the mitogen-activated protein kinase cascade and tyrosine phosphorylation of the epidermal growth factor receptor by hepatopoietin

Hepatopoietin (HPO) is a novel human hepatotrophic growth factor, which specifically stimulates proliferation of cultured primary hepatocytes in vitro and liver regeneration after liver partial hepatectomy in vivo. Recently, the identification of the mitogenic effect of HPO on hepatoma cell lines and the existence of HPO-specific receptors indicate that HPO acts via its specific cell surface receptor. However, the molecular mechanism of HPO action is not fully elucidated. In this report, we examined the signal transduction events induced by HPO in hepatoma cell line (HepG2). Our results demonstrated that HPO induces phosphorylation of mitogen-activated protein kinase kinase and mitogen-activated protein kinase (MAPK) in a rapid and transient manner. HPO stimulates tyrosine phosphorylation of epidermal growth factor receptor (EGFR). Furthermore, we observed that both MAPK activation and the mitogenic effect of HPO on HepG2 cells were completely blocked by AG1478, a specific inhibitor of EGFR tyrosine kinase activity. However, the effects of HPO were not antagonized by an EGFR-blocking antibody, mAb528, which blocks the interaction between epidermal growth factor and EGFR, indicating that stimulation of tyrosine phosphorylation of EGFR by HPO was not mediated by epidermal growth factor. In contrast, genistein, a general tyrosine kinase inhibitor, significantly attenuated the tyrosine phosphorylation of EGFR in response to HPO. In conclusion, our results suggest that tyrosine phosphorylation of EGFR may play a critical role in MAPK activation and mitogenic stimulation by HPO.     

Fibroblast growth factor receptor 3 kinase domain mutation increases cortical progenitor proliferation via mitogen-activated protein kinase activation

We have previously shown that mice carrying the K644E kinase domain mutation in fibroblast growth factor receptor 3 (Fgfr3) (EIIa;Fgfr3(+/K644E)) have enlarged brains with increased proliferation and decreased apoptosis of the cortical progenitors. Despite its unique rostral-low caudal-high gradient expression in the cortex, how Fgfr3 temporally and spatially influences progenitor proliferation is unknown. In vivo BrdU labelling now showed that progenitor proliferation was 10-46% higher in the EIIa;Fgfr3(+/K644E) cortex compared with wild type during embryonic day 11.5 (E11.5)-E13.5. The difference in proliferation between the EIIa;Fgfr3(+/K644E) and wild-type cortices was the greatest in the caudal cortex at E12.5 and E13.5. Inhibition of mitogen-activated or extracellular signal-regulated protein kinase (MEK) in vitro at E11.5 reduced the proliferation rate of the EIIa;Fgfr3(+/K644E) cortical progenitors to similar levels observed in the wild type, indicating that the majority of the increase in cell proliferation caused by the Fgfr3 mutation is mitogen-activated protein kinase (MAPK) pathway-dependent at this stage. In addition, elevated levels of Sprouty were observed in the EIIa;Fgfr3(+/K644E) telencephalon at E14.5, indicating the presence of negative feedback that may have suppressed further MAPK activation. We suggest that temporal activation of MAPK is largely responsible for cell proliferation caused by the Fgfr3 mutation during early stages of cortical development.     

Galphaq-dependent activation of mitogen-activated protein kinase kinase 4/c-Jun N-terminal kinase cascade.

G-protein-coupled receptors (GPCRs) typically activate c-Jun N-terminal kinase (JNK) through the G protein betagamma subunit (Gbetagamma), in a manner dependent on Rho family small GTPases, in mammalian cells. Here we show that JNK activation by the prototypic Gq-coupled alpha1B-adrenergic receptor is mediated by the alpha subunit of Gq (Galphaq), not by Gbetagamma, using a transient transfection system in human embryonic kidney cells. JNK activation by the alpha1B-adrenergic receptor/Galphaq was selectively mediated by mitogen-activated protein kinase kinase 4 (MKK4), but not MKK7. Also, MKK4 activation by the alpha1B-adrenergic receptor/Galphaq required c-Src and Rho family small GTPases. Furthermore, activation of the alpha1B-adrenergic receptor stimulated JNK activity through Src family tyrosine kinases and Rho family small GTPases in hamster smooth muscle cells that natively express the alpha1B-adrenergic receptor. Together, these results suggest that the alpha1B-adrenergic receptor/Galphaq may up-regulate JNK activity through a MKK4 pathway dependent on c-Src and Rho family small GTPases in mammalian cells.     

Docking protein FRS2 links the protein tyrosine kinase RET and its oncogenic forms with the mitogen-activated protein kinase signaling cascade

The receptor tyrosine kinase RET functions as the signal transducing receptor for the GDNF (for "glial cell-derived neurotrophic factors") family of ligands. Mutations in the RET gene were implicated in Hirschsprung disease (HSCR), multiple endocrine neoplasia type 2 (MEN 2), and thyroid carcinomas. In this report we demonstrate that the docking protein FRS2 is tyrosine phosphorylated by ligand-stimulated and by constitutively activated oncogenic forms of RET. Complex formation between RET and FRS2 is mediated by binding of the phosphotyrosine-binding domain of FRS2 to pY1062, a residue in RET that also functions as a binding site for Shc. However, overexpression of FRS2 but not Shc potentiates mitogen-activated protein (MAP) kinase activation by RET oncoproteins. We demonstrate that oncogenic RET-PTC proteins are associated with FRS2 constitutively, leading to tyrosine phosphorylation of FRS2, MAP kinase stimulation, and cell proliferation. However, loss-of-function HSCR-associated RET mutants exhibit impaired FRS2 binding and reduced MAP kinase activation. These experiments demonstrate that FRS2 couples both ligand-regulated and oncogenic forms of RET, with the MAP kinase signaling cascade as part of the response of RET under normal biological conditions and pathological conditions, such as MEN 2 and papillary thyroid carcinomas.     

Phosphorylation of paxillin via the ERK mitogen-activated protein kinase cascade in EL4 thymoma cells

Intracellular signals can regulate cell adhesion via several mechanisms in a process referred to as "inside-out" signaling. In phorbol ester-sensitive EL4 thymoma cells, phorbol-12-myristate 13-acetate (PMA) induces activation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases and promotes cell adhesion. In this study, clonal EL4 cell lines with varying abilities to activate ERKs in response to PMA were used to examine signaling events occurring downstream of ERK activation. Paxillin, a multifunctional docking protein involved in cell adhesion, was phosphorylated on serine/threonine residues in response to PMA treatment. This response was correlated with the extent and time course of ERK activation. PMA-induced phosphorylation of paxillin was inhibited by compounds that block the ERK activation pathway in EL4 cells, primary murine thymocytes, and primary murine splenocytes. Paxillin was phosphorylated in vitro by purified active ERK2. Two-dimensional electrophoresis revealed that PMA treatment generated a complex pattern of phosphorylated paxillin species in intact cells, some of which were generated by ERK-mediated phosphorylation in vitro. An ERK pathway inhibitor interfered with PMA-induced adhesion of sensitive EL4 cells to substrate. These findings describe a novel inside-out signaling pathway by which the ERK cascade may regulate events involved in adhesion.     

Activation of anaplastic lymphoma kinase receptor tyrosine kinase induces neuronal differentiation through the mitogen-activated protein kinase pathway

Anaplastic lymphoma kinase (ALK) is a novel neuronal orphan receptor tyrosine kinase that is essentially and transiently expressed in specific regions of the central and peripheral nervous systems, suggesting a role in its normal development and function. To determine whether ALK could play a role in neuronal differentiation, we established a model system that allowed us to mimic the normal activation of this receptor. We expressed, in PC12 cells, a chimeric protein in which the extracellular domain of the receptor was replaced by the mouse IgG 2b Fc domain. The Fc domain induced the dimerization and oligomerization of the chimeric protein leading to receptor phosphorylation and activation, thus mimicking the effect of ligand binding, whereas the wild type ALK remained as a monomeric nonphosphorylated protein. Expression of the chimera, but not that of the wild type ALK or of a kinase inactive form of the chimera, induced the differentiation of PC12 cells. Analysis of the signaling pathways involved in this process pointed to an essential role of the mitogen-activated protein kinase cascade. These results are consistent with a role for ALK in neuronal differentiation.     

白血病抑制因子受体与白血病细胞U937内促分裂原活化蛋白激酶的关系

探讨人白血病细胞系U937白血病抑制因子 (LIF)受体α亚基和另一亚基gp130细胞内区与促分裂原活化蛋白激酶 (MAPK)的关系 ,旨在研究白血病细胞增殖和分化的机制。用基因重组技术将两基因细胞内区互换以构成两嵌合体受体 (190 130 ,130 190 )并分别在U937表达 ,其与野生受体竞争性结合白血病抑制因子 ,用免疫组化和免疫印迹法分析受体细胞内区形成同源性二聚体(190cyt 190cyt,130cyt 130cyt)后的细胞状况和细胞内MAPK的水平。结果表明 ,转染pE190 130后用LIF作用 6h ,U937细胞MAPK表达量增加 ,MAPK形成的二聚体较明显 ,细胞增殖较快 ;而另一嵌合体受体与α亚基形成 190cyt 190cyt时U937细胞MAPK的表达无变化 ,二聚体不明显。说明LIF受体中gp130亚基的细胞内区参与了MAPK的激活及白血病U937细胞增殖信号的传递。     

Megakaryocytic differentiation induced by constitutive activation of mitogen-activated protein kinase kinase.

The K562 erythroleukemia cell line was used to study the molecular mechanisms regulating lineage commitment of hematopoietic stem cells. Phorbol esters, which initiate megakaryocyte differentiation in this cell line, caused a rapid increase in extracellular-signal-regulated kinase (ERK), which remained elevated for 2 h and returned to near-basal levels by 24 h. In the absence of extracellular stimuli, ERK could be activated by expression of constitutively active mutants of mitogen-activated protein (MAP) kinase kinase (MKK), resulting in cell adhesion and spreading, increased cell size, inhibition of cell growth, and induction of the platelet-specific integrin alphaIIb beta3, all hallmarks of megakaryocytic differentiation. In contrast, expression of wild-type MKK had little effect. In addition, constitutively active MKK suppressed the expression of an erythroid marker, alpha-globin, indicating the ability to suppress cellular responses necessary for alternative cell lineages. The MKK inhibitor PD98059 blocked MKK/ERK activation and cellular responses to phorbol ester, demonstrating that activation of MKK is necessary and sufficient to induce a differentiation program along the megakaryocyte lineage. Thus, the MAP kinase cascade, which promotes cell growth and proliferation in many cell types, instead inhibits cell proliferation and initiates lineage-specific differentiation in K562 cells, establishing a model system to investigate the mechanisms by which this signal transduction pathway specifies cell fate and developmental processes.     

Genistein, EGCG, and capsaicin inhibit adipocyte differentiation process via activating AMP-activated protein kinase

Phytochemicals such as soy isoflavone genistein have been reported to possess therapeutic effects for obesity, diabetes, and cardiovascular diseases. In the present study, the molecular basis of selective phytochemicals with emphasis on their ability to control intracellular signaling cascades of AMP-activated kinase (AMPK) responsible for the inhibition of adipogenesis was investigated. Recently, the evolutionarily conserved serine/threonine kinase, AMPK, emerges as a possible target molecule of anti-obesity. Hypothalamic AMPK was found to integrate nutritional and hormonal signals modulating feeding behavior and energy expenditure. We have investigated the effects of genistein, EGCG, and capsaicin on adipocyte differentiation in relation to AMPK activation in 3T3-L1 cells. Genistein (20-200muM) significantly inhibited the process of adipocyte differentiation and led to apoptosis of mature adipocytes. Genistein, EGCG, and capsaicin stimulated the intracellular ROS release, which activated AMPK rapidly. We suggest that AMPK is a novel and critical component of both inhibition of adipocyte differentiation and apoptosis of mature adipocytes by genistein or EGCG or capsaicin further implying AMPK as a prime target of obesity control.     

A mechanistic study of evolvability using the mitogen-activated protein kinase cascade

Evolvability is a function of the way genetic variation interacts with the mechanisms that produce the phenotype. We explore an explicitly mechanistic way of studying the evolvability of phenotypes that are produced by a relatively simple genetic mechanism, the mitogen-activated protein kinase (MAPK) cascade. We developed a quantitative model of MAPK activation that can be used to study the effects of genetic variation on the various components of this signaling cascade. We show how some standard tools of applied mathematics, such as steady-state formulations and nondimensionalization, can be used to elucidate the relative importance of variation in each gene of this mechanism. We also give insights into non-intuitive patterns of dependence and trade-off among the genes. The mechanism produces several different phenotypes (ultrasensitivity to stimulation, switch-like behavior, amount of MAPK-PP delivered, persistence of MAPK-PP activity), each of which is sensitive to different (but partially overlapping) combinations of genes. We show that the mechanism imposes clear limitations on the evolvability of each of the different phenotypes of the pathway, even in the presence of genetic variation in the components of the mechanism. This approach to the study of evolvability is generally applicable and complements the traditional approach through statistical genetics by providing a mechanistic understanding of the genetic interactions that produce the phenotype.     

Modulation of the MAP kinase signaling cascade by Raf kinase inhibitory protein

Proteins like Rafkinase inhibitory protein (RKIP) that serve as modulators of signaling pathways, either by promoting or inhibiting the formation of productive signaling complexes through protein-protein interactions, have been demonstrated to play an increasingly important role in a number of cell types and organisms. These proteins have been implicated in development as well as the progression of cancer. RKIP is a particularly interesting regulator, as it is a highly conserved, ubiquitously expressed protein that has been shown to play a role in growth and differentiation in a number of organisms and can regulate multiple signaling pathways. RKIP is also the first MAP kinase signaling modulator to be identified as playing a role in cancer metastasis, and identification of the mechanism by which it regulates Raf-1 activation provides new targets for theraoeutic intervention.     

The extracellular signal-regulated kinase mitogen-activated protein kinase/ribosomal S6 protein kinase 1 cascade phosphorylates cAMP response element-binding protein to induce MUC5B gene expression via D-prostanoid receptor signaling

Mucus hypersecretion is a prominent feature of respiratory diseases, and MUC5B is a major airway mucin. Mucin gene expression can be affected by inflammatory mediators, including prostaglandin (PG) D(2,) an inflammatory mediator synthesized by hematopoietic PGD synthase (H-PGDS). PGD(2) binds to either D-prostanoid receptor (DP1) or chemoattractant receptor homologous molecule expressed on T-helper type 2 cells (CRTH2). We investigated the mechanisms by which PGD(2) induces MUC5B gene expression in airway epithelial cells. Western blot analysis showed that H-PGDS was highly expressed in nasal polyps. Similar results were obtained for PGD(2) expression. In addition, we could clearly detect the expressions of both H-PGDS and DP1 in nasal epithelial cells but not CRTH2. We demonstrated that PGD(2) increased MUC5B gene expression in normal human nasal epithelial cells as well as in NCI-H292 cells in vitro. S5751, a DP1 antagonist, inhibited PGD(2)-induced MUC5B expression, whereas a CRTH2 antagonist (OC0459) did not. These data suggest that PGD(2) induced MUC5B expression via DP1. Pretreatment with extracellular signal-regulated kinase (ERK) inhibitor (PD98059) blocked both PGD(2)-induced ERK mitogen-activated protein kinase (MAPK) activation and MUC5B expression. Proximity ligation assays showed direct interaction between RSK1 and cAMP response element-binding protein (CREB). Stimulation with PGD(2) caused an increase in intracellular cAMP levels, whereas intracellular Ca(2+) did not have such an effect. PGD(2)-induced MUC5B mRNA levels were regulated by CREB via direct interaction with two cAMP-response element sites (-921/-914 and -900/-893). Finally, we demonstrated that PGD(2) can induce MUC5B overproduction via ERK MAPK/RSK1/CREB signaling and that DP1 receptor may have suppressive effects in controlling MUC5B overproduction in the airway.     

Control of thrombopoietin-induced megakaryocytic differentiation by the mitogen-activated protein kinase pathway.

Thrombopoietin (TPO) is the major regulator of both growth and differentiation of megakaryocytes. We previously showed that both functions can be generated by TPO in the megakaryoblastic cell line UT7, in which murine Mpl was introduced, and are independently controlled by distinct regions of the cytoplasmic domain of Mpl. Particularly, residues 71 to 94 of this domain (deleted in the mutant mpl delta3) were found to be required for megakaryocytic maturation but dispensable for proliferation. We show here that TPO-induced differentiation in UT7 cells is tightly dependent on a strong, long-lasting activation of the mitogen-activated protein kinase (MAPK) pathway. Indeed, (i) in UT7-mpl cells, TPO induced a strong activation of extracellular signal-regulated kinases (ERK) which was persistent until at least 4 days in TPO-containing medium; (ii) a specific MAPK kinase (MEK) inhibitor inhibited TPO-induced megakaryocytic gene expression; (iii) the Mpl mutant mpl delta3, which displayed no maturation activity, transduced only a weak and transient ERK activation in UT7 cells; and (iv) TPO-induced megakaryocytic differentiation in UT7-mpl delta3 cells was partially restored by expression of a constitutively activated mutant of MEK. The capacity of TPO to trigger a strong and prolonged MAPK signal depended on the cell in which Mpl was introduced. In BAF3-mpl cells, TPO triggered a weak and transient ERK activation, similar to that induced in UT7-mpl delta3 cells. In these cells, no difference in MAPK activation was found between normal Mpl and mpl delta3. Thus, depending on the cellular context, several distinct regions of the cytoplasmic domain of Mpl and signaling pathways may contribute to generate quantitative variations in MAPK activation.     

Pleiotropic coupling of G protein-coupled receptors to the mitogen-activated protein kinase cascade. Role of focal adhesions and receptor tyrosine kinases.

G protein-coupled receptors (GPCRs) initiate Ras-dependent activation of the Erk 1/2 mitogen-activated protein kinase cascade by stimulating recruitment of Ras guanine nucleotide exchange factors to the plasma membrane. Both integrin-based focal adhesion complexes and receptor tyrosine kinases have been proposed as scaffolds upon which the GPCR-induced Ras activation complex may assemble. Using specific inhibitors of focal adhesion complex assembly and receptor tyrosine kinase activation, we have determined the relative contribution of each to activation of the Erk 1/2 cascade following stimulation of endogenous GPCRs in three different cell types. The tetrapeptide RGDS, which inhibits integrin dimerization, and cytochalasin D, which depolymerizes the actin cytoskeleton, disrupt the assembly of focal adhesions. In PC12 rat pheochromocytoma cells, both agents block lysophosphatidic acid (LPA)- and bradykinin-stimulated Erk 1/2 phosphorylation, suggesting that intact focal adhesion complexes are required for GPCR-induced mitogen-activated protein kinase activation in these cells. In Rat 1 fibroblasts, Erk 1/2 activation via LPA and thrombin receptors is completely insensitive to both agents. Conversely, the epidermal growth factor receptor-specific tyrphostin AG1478 inhibits GPCR-mediated Erk 1/2 activation in Rat 1 cells but has no effect in PC12 cells. In HEK-293 human embryonic kidney cells, LPA and thrombin receptor-mediated Erk 1/2 activation is partially sensitive to both the RGDS peptide and tyrphostin AG1478, suggesting that both focal adhesion and receptor tyrosine kinase scaffolds are employed in these cells. The dependence of GPCR-mediated Erk 1/2 activation on intact focal adhesions correlates with expression of the calcium-regulated focal adhesion kinase, Pyk2. In all three cell types, GPCR-stimulated Erk 1/2 activation is significantly inhibited by the Src kinase inhibitors, herbimycin A and 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo-D-3,4-pyrimidine (PP1), suggesting that Src family nonreceptor tyrosine kinases represent a point of convergence for signals originating from either scaffold.     

Regulation of vascular endothelial growth factor production by Leydig cells in vitro: the role of protein kinase A and mitogen-activated protein kinase cascade

We previously reported the presence of vascular endothelial growth factor (VEGF) in testicular cells, and high concentrations of VEGF have been measured in semen, although its role in male reproduction remains obscure. In the present study we focus on understanding the mechanism of VEGF production by mouse Leydig cells cultured in vitro. Production of VEGF protein in medium by testicular cells was markedly increased by the addition of hCG in a time- and dose-dependent manner. Gonadotropin-stimulated VEGF production was mediated by cAMP-dependent protein kinase A (PKA), as evidenced by the effect of hCG being mimicked by 8Br-cAMP and being abolished in the presence of a PKA-specific inhibitor, H-89. Protein kinase C was not involved, as evidenced by phorbol 12-myristate 13-acetate having no influence on VEGF production by Leydig cells. In addition to hCG, atrial natriuretic peptide was also able to stimulate VEGF production, suggesting that cGMP is able to cross-activate PKA. A specific Src kinase inhibitor, PP2, could completely block the stimulatory effects of both gonadotropin and 8Br-cAMP on VEGF production by Leydig cells, implying an involvement of the Src kinase pathway. Furthermore, addition of U0126, an inhibitor of MEK 1/2, abolished the increase in VEGF production stimulated by both hCG and 8Br-cAMP. A similar inhibitory effect was observed by the addition of SB203580, a p38 mitogen-activated protein kinase inhibitor. Thus, in conclusion, Leydig cells are able to produce VEGF by a process under gonadotropic control, and PKA plays a key role in this process. Downstream of PKA, it appears that both MEK 1/2 and Src kinase-dependent pathways are involved, although further research will be necessary to determine the precise link between PKA and other kinases involved.     

Activation of p38 mitogen-activated protein kinase promotes epidermal growth factor receptor internalization

Endocytic trafficking plays an important role in the regulation of the epidermal growth factor receptor (EGFR). To address if cellular kinases regulate EGFR internalization, we used anisomycin, a potent activator of kinase cascades in mammalian cells, especially the stress-activated mitogen-activated protein (MAP) kinase subtypes. Here, we report that activation of p38 MAP kinase by anisomycin is sufficient to induce internalization of EGFR. Anisomycin and EGF employ different mechanisms to promote EGFR endocytosis as anisomycin-induced internalization does not require tyrosine kinase activity or ubiquitination of the receptor. In addition, anisomycin treatment did not result in delivery and degradation of EGFR at lysosomes. Incubation with a specific inhibitor of p38, or depletion of endogenous p38 by small interfering RNAs, abolished anisomycin-induced internalization of EGFR while having no effect on transferrin endocytosis, indicating that the effect of p38 activation on EGFR endocytosis is specific. Interestingly, inhibition of p38 activation also abolished endocytosis of EGFR induced by UV radiation. Our results reveal a novel role for p38 in the regulation of EGFR endocytosis and suggest that stimulation of EGFR internalization by p38 might represent a general mechanism to prevent generation of proliferative or anti-apoptotic signals under stress conditions.