Follicle-stimulating hormone (FSH) activates extracellular signal-regulated kinase phosphorylation independently of beta-arrestin- and dynamin-mediated FSH receptor internalization

Background  

The follicle-stimulating hormone receptor (FSH-R) is a seven transmembrane spanning receptor (7TMR) which plays a crucial role in male and female reproduction. Upon FSH stimulation, the FSH-R activates the extracellular signal-regulated kinases (ERK). However, the mechanisms whereby the agonist-stimulated FSH-R activates ERK are poorly understood. In order to activate ERK, some 7 TMRs require beta-arrestin-and dynamin-dependent internalization to occur, whereas some others do not. In the present study, we examined the ability of the FSH-activated FSH-R to induce ERK phosphorylation, in conditions where its beta-arrestin- and dynamin-mediated internalization was impaired.     

MUC1 tyrosine phosphorylation activates the extracellular signal-regulated kinase

MUC1 is a transmembrane glycoprotein expressed on the apical surface of epithelial cells and exhibiting structural features characteristic of receptors for cytokines and growth factors. Its intracellular cytoplasmic tail (CT) contains multiple amino acid sequence motifs that, once phosphorylated, serve as docking sites for SH2 domain-containing proteins mediating signal transduction. Most studies examining MUC1 signaling have focused on cancer cells where MUC1 is overexpressed, aberrantly glycosylated, and constitutively phosphorylated. No studies have determined the signaling pathways activated in response to stimulation of its ectodomain. To better understand the signaling mechanisms of MUC1, we stably transfected HEK293 cells with an expression plasmid encoding a chimeric protein consisting of the extracellular and transmembrane domains of CD8 and the MUC1 CT (CD8/MUC1). Extracellular treatment of HEK293-CD8/MUC1 cells with CD8 antibody induced intracellular Tyr phosphorylation of the MUC1 CT and activated ERK1/2, but not the p38, SAPK/JNK, or ERK5 MAP kinases. Moreover, phosphorylation of ERK1/2 was completely blocked using a CT deletion mutant or a mutant construct in which all Tyr residues in the CT were changed to Phe. These results establish that Tyr phosphorylation of the MUC1 CT is required for activation of a downstream ERK1/2 pathway.     

Extracellular signal-regulated kinase activates topoisomerase IIalpha through a mechanism independent of phosphorylation

The mitogen-activated protein (MAP) kinases, extracellular signal-related kinase 1 (ERK1) and ERK2, regulate cellular responses by mediating extracellular growth signals toward cytoplasmic and nuclear targets. A potential target for ERK is topoisomerase IIalpha, which becomes highly phosphorylated during mitosis and is required for several aspects of nucleic acid metabolism, including chromosome condensation and daughter chromosome separation. In this study, we demonstrated interactions between ERK2 and topoisomerase IIalpha proteins by coimmunoprecipitation from mixtures of purified enzymes and from nuclear extracts. In vitro, diphosphorylated active ERK2 phosphorylated topoisomerase IIalpha and enhanced its specific activity by sevenfold, as measured by DNA relaxation assays, whereas unphosphorylated ERK2 had no effect. However, activation of topoisomerase II was also observed with diphosphorylated inactive mutant ERK2, suggesting a mechanism of activation that depends on the phosphorylation state of ERK2 but not on its kinase activity. Nevertheless, activation of ERK by transient transfection of constitutively active mutant MAP kinase kinase 1 (MKK1) enhanced endogenous topoisomerase II activity by fourfold. Our findings indicate that ERK regulates topoisomerase IIalpha in vitro and in vivo, suggesting a potential target for the MKK/ERK pathway in the modulation of chromatin reorganization events during mitosis and in other phases of the cell cycle.     

Glucose activates mitogen-activated protein kinase (extracellular signal-regulated kinase) through proline-rich tyrosine kinase-2 and the Glut1 glucose transporter

Glucose serves as both a nutrient and regulator of physiological and pathological processes. Presently, we found that glucose and certain sugars rapidly activated extracellular signal-regulated kinase (ERK) by a mechanism that was: (a) independent of glucose uptake/metabolism and protein kinase C but nevertheless cytochalasin B-inhibitable; (b) dependent upon proline-rich tyrosine kinase-2 (PYK2), GRB2, SOS, RAS, RAF, and MEK1; and (c) amplified by overexpression of the Glut1, but not Glut2, Glut3, or Glut4, glucose transporter. This amplifying effect was independent of glucose uptake but dependent on residues 463-468, IASGFR, in the Glut1 C terminus. Accordingly, glucose effects on ERK were amplified by expression of Glut4/Glut1 or Glut2/Glut1 chimeras containing IASGFR but not by Glut1/Glut4 or Glut1/Glut2 chimeras lacking these residues. Also, deletion of Glut1 residues 469-492 was without effect, but mutations involving serine 465 or arginine 468 yielded dominant-negative forms that inhibited glucose-dependent ERK activation. Glucose stimulated the phosphorylation of tyrosine residues 402 and 881 in PYK2 and binding of PYK2 to Myc-Glut1. Our findings suggest that: (a) glucose activates the GRB2/SOS/RAS/RAF/MEK1/ERK pathway by a mechanism that requires PYK2 and residues 463-468, IASGFR, in the Glut1 C terminus and (b) Glut1 serves as a sensor, transducer, and amplifier for glucose signaling to PYK2 and ERK.     

Mechanistic basis for catalytic activation of mitogen-activated protein kinase phosphatase 3 by extracellular signal-regulated kinase

The dual specificity mitogen-activated protein kinase phosphatase MKP3 has been shown to down-regulate mitogenic signaling through dephosphorylation of extracellular signal-regulated kinase (ERK). Camps et al. (Camps, M., Nichols, A., Gillieron, C., Antonsson, B., Muda, M., Chabert, C., Boschert, U., and Arkinstall, S. (1998) Science 280, 1262-1265) had demonstrated that ERK binding to the noncatalytic amino-terminal domain of MKP3 can dramatically activate the phosphatase catalytic domain. The physical basis for this activation has not been established. Here, we provide detailed biochemical evidence that ERK activates MKP3 through the stabilization of the active phosphatase conformation, inducing closure of the catalytic "general acid" loop. In the closed conformation, this loop structure can participate efficiently in general acid/base catalysis, substrate binding, and transition-state stabilization. The pH activity profiles of ERK-activated MKP3 clearly indicated the involvement of general acid catalysis, a hallmark of protein-tyrosine phosphatase catalysis. In contrast, unactivated MKP3 did not display this enzymatic group as critical for the low activity form of the enzyme. Using a combination of Br?nsted analyses, pre-steady-state and steady-state kinetics, we have isolated all catalytic steps in the reaction and have quantified the specific rate enhancement. Through protonation of the leaving group and transition-state stabilization, activated MKP3 catalyzes formation of the phosphoenzyme intermediate approximately 100-fold faster than unactivated enzyme. In addition, ERK-activated MKP3 catalyzes intermediate hydrolysis 5-6-fold more efficiently and binds ligands up to 19-fold more tightly. Consistent with ERK stabilizing the active conformation of MKP3, the chemical chaperone dimethyl sulfoxide was able to mimic this activation. A general protein-tyrosine phosphatase regulatory mechanism involving the flexible general acid loop is discussed.     

Intracerebroventricular administration of ouabain, a Na/K-ATPase inhibitor, activates tyrosine hydroxylase through extracellular signal-regulated kinase in rat striatum

Alteration in dopamine neurotransmission has been reported to be involved in the mania of bipolar disorder. Tyrosine hydroxylase (TH) is the rate-limiting enzyme that is crucial for dopamine biosynthesis, and its activity is tightly regulated by phosphorylation at multiple N-terminal serine residues. Previously, we have reported that intracerebroventricular (ICV) injection of ouabain, a selective Na/K-ATPase inhibitor, induces hyperactivity in rats that mimics manic symptoms related to the activation of extracellular signal-regulated protein kinase1/2 (ERK1/2), which plays crucial roles in the modulation of TH phosphorylation. In this study, we investigated the effects of ICV injection of ouabain on TH phosphorylation in rat striatum and the involvement of ERK1/2 in ouabain-induced TH activation. ICV ouabain induced an acute dose-dependent increase in locomotor activity and in TH phosphorylation in rat striatum. TH phosphorylation at Ser19 was significantly increased with 100, 500, and 1000 μM ouabain, and phosphorylation at Ser31 and Ser40 was significantly increased with 500 and 1000 μM. We also found that ICV pretreatment with U0126, a specific MEK1/2 inhibitor, attenuated the 1000 μM ouabain-induced increase in TH phosphorylation at Ser19, Ser31, and Ser40, as well as the hyperactivity of rats. Moreover, the increased phosphorylation of TH (Ser19, Ser31, and Ser40) was maintained until 8 h after single administration ouabain was accompanied by increased phosphorylation of ERK1/2 (Thr202/Tyr204) and p90RSK (Thr359/Ser363). These findings imply that TH activation of the ERK1/2 signal pathway could play an important role in ouabain-induced hyperactivity of rats, a mania model.     

Integrins regulate microtubule nucleating activity of centrosome through mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase (MEK/ERK) signaling

Microtubule nucleation is an essential step in the formation of the microtubule cytoskeleton. We recently showed that androgen and Src promote microtubule nucleation and γ-tubulin accumulation at the centrosome. Here, we explore the mechanisms by which androgen and Src regulate these processes and ask whether integrins play a role. We perturb integrin function by a tyrosine-to-alanine substitution in membrane-proximal NPIY motif in the integrin β1 tail and show that this mutant substantially decreases microtubule nucleation and γ-tubulin accumulation at the centrosome. Because androgen stimulation promotes the interaction of the androgen receptor with Src, resulting in PI3K/AKT and MEK/ERK signaling, we asked whether these pathways are inhibited by the mutant integrin and whether they regulate microtubule nucleation. Our results indicate that the formation of the androgen receptor-Src complex and the activation of downstream pathways are significantly suppressed when cells are adhered by the mutant integrin. Inhibitor studies indicate that microtubule nucleation requires MEK/ERK but not PI3K/AKT signaling. Importantly, the expression of activated RAF-1 is sufficient to rescue microtubule nucleation inhibited by the mutant integrin by promoting the centrosomal accumulation of γ-tubulin. Our data define a novel paradigm of integrin signaling, where integrins regulate microtubule nucleation by promoting the formation of androgen receptor-Src signaling complexes to activate the MEK/ERK signaling pathway.     

Direct suppression of TCR-mediated activation of extracellular signal-regulated kinase by leukocyte protein tyrosine phosphatase, a tyrosine-specific phosphatase.

Leukocyte protein tyrosine phosphatase (LC-PTP)/hemopoietic PTP is a human cytoplasmic PTP that is predominantly expressed in the hemopoietic cells. Recently, it was reported that hemopoietic PTP inhibited TCR-mediated signal transduction. However, the precise mechanism of the inhibition was not identified. Here we report that extracellular signal-regulated kinase (ERK) is the direct target of LC-PTP. LC-PTP dephosphorylated ERK2 in vitro. Expression of wild-type LC-PTP in 293T cells suppressed the phosphorylation of ERK2 by a mutant MEK1, which was constitutively active regardless of upstream activation signals. No suppression of the phosphorylation was observed by LC-PTPCS, a catalytically inactive mutant. In Jurkat cells, LC-PTP suppressed the ERK and p38 mitogen-activated protein kinase cascades. LC-PTP and LC-PTPCS made complexes with ERK1, ERK2, and p38alpha, but not with the gain-of-function sevenmaker ERK2 mutant (D321N). A small deletion (aa 1-46) in the N-terminal portion of LC-PTP or Arg to Ala substitutions at aa 41 and 42 resulted in the loss of ERK binding activity. These LC-PTP mutants revealed little inhibition of the ERK cascade activated by TCR cross-linking. On the other hand, the wild-type LC-PTP did not suppress the phosphorylation of sevenmaker ERK2 mutant. Thus, the complex formation of LC-PTP with ERK is the essential mechanism for the suppression. Taken collectively, these results indicate that LC-PTP suppresses mitogen-activated protein kinase directly in vivo.     

Thiol antioxidant, N-acetylcysteine, activates extracellular signal-regulated kinase signaling pathway in articular chondrocytes

Reactive oxygen species (ROS) generated during inflammation and aging contribute to the resorption of articular cartilage. Low antioxidant levels are a risk factor for arthritis because they protect cartilage from ROS. N-Acetylcysteine (NAC) is a ROS scavenger and, depending upon the concentration, an anti-inflammatory or prooxidant agent. Mechanisms of action for NAC were studied in primary human and bovine chondrocytes. NAC dose-dependently activated phosphorylation of extracellular signal-regulated kinases-mitogen-acivated protein kinases (ERK-MAPK). ERK activation peaked within 15 min and declined afterward up to 180 min. This activation was inhibited by the MAPKK inhibitor, PD098059. The induction was mimicked by other thiols, l-cysteine, reduced glutathione, and pyrrolidine dithiocarbamate (PDTC) but not by a nonthiol, N-acetylalanine. The total nonphosphorylated ERKs levels remained unaffected by these treatments. Activation of the ERK-MAPK pathway provides a mechanism for the reported promotion of chondrocyte survival by thiol antioxidants.     

Luteolin inhibits pyrogallol-induced apoptosis through the extracellular signal-regulated kinase signaling pathway

Luteolin is an antioxidative, antitumor and anti-inflammatory flavone. It has been shown to reduce endothelial dysfunction, but the mechanism is not clear. We set out to explore the effects of luteolin on apoptosis and its mechanism of action in endothelial cells. The effect of luteolin on pyrogallol-induced superoxide stress and the subsequent apoptosis was studied in the mouse heart capillary endothelial cell line H5V and human umbilical vein endothelial cells, by the use of flow cytometry, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide, Hoechst staining, and western blot. Pyrogallol (0-400 μm) dose-dependently induced reactive oxygen species production, cytotoxicity, an annexin V-fluorescein isothiocyanate increase, mitochondrial transmembrane depolarization and DNA condensation in both H5V and human umbilical vein endothelial cells; these actions were reversed by luteolin (0.78-50 μm) in a concentration-dependent manner. Luteolin suppressed the poly (ADP-ribose) polymerase activation, caspase-8 cleavage and p38 mitogen-activated protein kinase activation triggered by pyrogallol, and stimulated the extracellular signal-regulated kinase signaling pathway to counteract the pyrogallol-induced apoptotic signals. Luteolin is an effective agent for the protection of endothelial cells from superoxide stress-induced apoptosis via the extracellular signal-regulated kinase signaling pathway.     

The mechanism of dephosphorylation of extracellular signal-regulated kinase 2 by mitogen-activated protein kinase phosphatase 3.

The mitogen-activated protein (MAP) kinase phosphatase-3 (MKP3) is a dual specificity phosphatase that specifically inactivates one subfamily of MAP kinases, the extracellular signal-regulated kinases (ERKs). Inactivation of MAP kinases occurs by dephosphorylation of Thr(P) and Tyr(P) in the TXY kinase activation motif. To gain insight into the mechanism of ERK2 inactivation by MKP3, we have carried out an analysis of the MKP3-catalyzed dephosphorylation of the phosphorylated ERK2. We find that ERK2/pTpY dephosphorylation by MKP3 involves an ordered, distributive mechanism in which MKP3 binds the bisphosphorylated ERK2/pTpY, dephosphorylates Tyr(P) first, dissociates and releases the monophosphorylated ERK2/pT, which is then subjected to dephosphorylation by a second MKP3, yielding the fully dephosphorylated ERK2. The bisphosphorylated ERK2 is a highly specific substrate for MKP3 with a k(cat)/K(m) of 3.8 x 10(6) m(-1) s(-1), which is more than 6 orders of magnitude higher than that for small molecule aryl phosphates and an ERK2-derived phosphopeptide encompassing the pTEpY motif. This strikingly high substrate specificity displayed by MKP3 may result from a combination of high affinity binding interactions between the N-terminal domain of MKP3 and ERK2 and specific ERK2-induced allosteric activation of the MKP3 C-terminal phosphatase domain.     

Calcium influx through L-type channels is required for selective activation of extracellular signal-regulated kinase by gonadotropin-releasing hormone.

The hypothalamic decapeptide gonadotropin-releasing hormone stimulates mobilization of two discrete pools of calcium in clonal (alphaT3-1) and primary pituitary gonadotropes. A multidisciplinary approach was implemented to investigate the effects of discrete calcium fluctuations on the signaling pathways linking the gonadotropin-releasing hormone receptor to activation of mitogen-activated protein kinases and immediate early genes. Blockade of calcium influx through nifedipine-sensitive voltage-gated calcium channels reduced buserelin-induced activation of extracellular signal-regulated kinase (ERK) and c-Fos while activation of c-Jun N-terminal kinase and c-Jun was unaffected. Inhibition of buserelin-stimulated ERK activity by nifedipine was also observed in rat pituitary cells in primary culture. Direct activation of alphaT3-1 cell L-type calcium channels with the agonist Bay-K 8644 resulted in phosphorylation of ERK and induction of c-Fos. However, simple voltage-induced channel activation did not produce a sufficient calcium signal, since depolarization with 35 mM KCl failed to induce activation of ERK. Depletion of intracellular calcium stores with thapsigargin did not affect buserelin-induced ERK activation. An inhibitor of protein kinase C decreased calcium influx through nifedipine-sensitive calcium channels and phosphorylation of ERK induced by buserelin. Pharmacological inhibition of protein kinase C did not block Bay-K 8644-induced ERK activation. These observations suggest that calcium influx through L-type channels is required for GnRH-induced activation of ERK and c-Fos and that the influence of calcium lies downstream of protein kinase C.     

Sphingosine kinase 1 is involved in dibutyryl cyclic AMP-induced granulocytic differentiation through the upregulation of extracellular signal-regulated kinase, but not p38 MAP kinase, in HL60 cells

The role of sphingosine kinase (SPHK) in the dibutyryl cyclic AMP (dbcAMP)-induced granulocytic differentiation of HL60 cells was investigated. During differentiation, SPHK activity was increased, as were mRNA and protein levels of SPHK1, but not of SPHK2. Pretreatment of HL60 cells with N,N-dimethylsphingosine (DMS), a potent SPHK inhibitor, completely blocked dbcAMP-induced differentiation. The phosphorylation of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 MAPK was also increased during dbcAMP-induced differentiation. Pretreatment of HL60 cells with the MEK inhibitor, U0126, but not the p38 MAPK inhibitor, SB203580, completely suppressed dbcAMP-induced ERK1/2 activation and granulocytic differentiation, but did not affect the increase in SPHK activity. DMS inhibited dbcAMP-induced ERK1/2 activation, but had little effect on p38 MAPK activation. DMS had no effect on the dbcAMP-induced membrane translocation of protein kinase C (PKC) isozymes, and PKC inhibitors had no significant effect on ERK activation. The overexpression of wild-type SPHK1, but not dominant negative SPHK1, resulted in high basal levels of ERK1/2 phosphorylation and stimulated granulocytic differentiation in HL60 cells. These data show that SPHK1 participates in the dbcAMP-induced differentiation of HL60 cells by activating the MEK/ERK pathway.     

Tyrosine phosphorylation of CD45 phosphotyrosine phosphatase by p50csk kinase creates a binding site for p56lck tyrosine kinase and activates the phosphatase.

Src family protein tyrosine kinases (PTKs) play an essential role in antigen receptor-initiated lymphocyte activation. Their activity is largely regulated by a negative regulatory tyrosine which is a substrate for the activating action of the CD45 phosphotyrosine phosphatase (PTPase) or, conversely, the suppressing action of the cytosolic p50csk PTK. Here we report that CD45 was phosphorylated by p50csk on two tyrosine residues, one of them identified as Tyr-1193. This residue was not phosphorylated by T-cell PTKs p56lck and p59fyn. Tyr-1193 was phosphorylated in intact T cells, and phosphorylation increased upon treatment with PTPase inhibitors, indicating that this tyrosine is a target for a constitutively active PTK. Cotransfection of CD45 and csk into COS-1 cells caused tyrosine phosphorylation of CD45 in the intact cells. Tyrosine-phosphorylated CD45 bound p56lck through the SH2 domain of the kinase. Finally, p50csk-mediated phosphorylation of CD45 caused a severalfold increase in its PTPase activity. Our results show that direct tyrosine phosphorylation of CD45 can affect its activity and association with Src family PTKs and that this phosphorylation could be mediated by p50csk. If this is also true in the intact cells, it adds a new dimension to the physiological function of p50csk in T lymphocytes.     

A Chinese herbal decoction, Danggui Buxue Tang, activates extracellular signal-regulated kinase in cultured T-lymphocytes

Danggui Buxue Tang (DBT) is prepared from Radix Astragali and Radix Angelicae Sinensis. This Chinese herbal decoction has been shown to stimulate the proliferation of T-lymphocytes; however, the action mechanism of this stimulation has not been revealed. In cultured T-lymphocytes, application of DBT markedly induced the cell proliferation, the release of interleukin-2, -6 and -10, as well as the phosphorylation of extracellular signal-regulated kinases (ERK). The pre-treatment of ERK inhibitor blocked the DBT-induced immune responses. In addition, the polysaccharide-enriched fraction of DBT showed marked responses on the cultured T-lymphocytes suggesting the important role of DBT polysaccharide in triggering such immune responses.     

Isocitrate dehydrogenase kinase/phosphatase: aceK alleles that express kinase but not phosphatase activity.

For Escherichia coli, growth on acetate requires the induction of the enzymes of the glyoxylate bypass, isocitrate lyase and malate synthase. The branch point between the glyoxylate bypass and the Krebs cycle is controlled by phosphorylation of isocitrate dehydrogenase (IDH), inhibiting that enzyme's activity and thus forcing isocitrate through the bypass. This phosphorylation cycle is catalyzed by a bifunctional enzyme, IDH kinase/phosphatase, which is encoded by aceK. We have employed random mutagenesis to isolate novel alleles of aceK. These alleles were detected by the loss of ability to complement an aceK null mutation. The products of one class of these alleles retain IDH kinase activity but have suffered reductions in IDH phosphatase activity by factors of 200 to 400. Selective loss of the phosphatase activity also appears to have occurred in vivo, since cells expressing these alleles exhibit phenotypes which are reminiscent of strains lacking IDH; these strains are auxotrophic for glutamate. Assays of cell-free extracts confirmed that this phenotype resulted from nearly quantitative phosphorylation of IDH. The availability of these novel alleles of aceK allowed us to assess the significance of the precise control which is a characteristic of the IDH phosphorylation cycle in vivo. The fractional phosphorylation of IDH was varied by controlled expression of one of the mutant alleles, aceK3, in a wild-type strain. Reduction of IDH activity to 50% of the wild-type level did not adversely affect growth on acetate. However, further reductions inhibited growth, and growth arrest occurred when the IDH activity fell to 15% of the wild-type level. Thus, although wild-type cells maintain a precise effective IDH activity during growth on acetate, this precision is not critical.