Differential requirements for p21ras and protein kinase C in the regulation of neuronal gene expression by nerve growth factor and neurokines

The role of the leukemia inhibitory factor receptor in neuroprotective signaling

Several neurotropic cytokines relay their signaling through the leukemia inhibitory factor receptor. This 190kDa subunit couples with the 130kDa gp130 subunit to transduce intracellular signaling in neurons and oligodendrocytes that leads to expression of genes associated with neurosurvival. Moreover, activation of this receptor alters the phenotype of immune cells to an anti-inflammatory one. Although cytokines that activate the leukemia inhibitory factor receptor have been studied in the context of neurodegenerative disease, therapeutic targeting of the specific receptor subunit has been understudied in by comparison. This review examines the role of this receptor in the CNS and immune system, and its application in the treatment in stroke and other brain pathologies.

Regulation of neurokine receptor signaling and trafficking

Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) are neurally active cytokines, or neurokines. LIF signals through a receptor consisting of gp130 and the low affinity LIF receptor (LIFR), while the CNTF receptor consists of gp130, LIFR, and the low affinity CNTF receptor (CNTFR). Ser1044 of the LIFR is phosphorylated by Erk1/2 MAP kinase. Stimulation of neural cells with growth factors which strongly activate Erk1/2 decreases LIF-mediated signal transduction due to increased degradation of the LIFR as a consequence of Erk1/2-dependent phosphorylation of the receptor at Ser1044. The gp130 receptor subunit is phosphorylated, at least in part by calmodulin-dependent protein kinase II, at Ser782, which is adjacent to a dileucine internalization motif. Ser782 appears to negatively regulate cytokine receptor expression, as mutagenesis of Ser782 results in increased gp130 expression and cytokine-induced neuropeptide gene transcription. The LIFR and gp130 are transmembrane proteins, while CNTFR is a peripheral membrane protein attached to the cell surface via a glycosylphosphatidylinositol tail. In unstimulated cells, CNTFR but not LIFR and gp130 is localized to detergent-resistant lipid rafts. Stimulation of cells with CNTFR causes translocation of LIFR and gp130 into the lipid rafts, while stimulation with LIF does not induce receptor translocation, raising the possibility that CNTF could induce different patterns of signaling and/or receptor trafficking than caused by LIF. We used a compartmentalized culture system to examine the mechanisms for retrograde signaling by LIF and CNTF from distal neurites to the cell bodies of mouse sympathetic neurons. Stimulation with neurokines of the distal neurites of sympathetic neurons grown in a compartmentalized culture system resulted in the activation and nuclear translocation of the transcription factor Stat3. Retrograde signaling required Jak kinase activity in the cell body but not the distal neurites, and could be blocked by inhibitors of microtubule but not microfilament function. The results are consistent with a signaling endosomes model in which the ctyokine/receptor complex is transported back to the cell body where Stat3 is activated. While both LIF and CNTF mediate retrograde activation of Stat3, the kinetics for retrograde signaling differ for the two neurokines.

Transregulation of leukemia inhibitory [corrected] factor receptor expression and function by growth factors in neuroblastoma cells

The cytokines that signal through the leukemia inhibitory factor (LIF) receptor are members of the neuropoietic cytokine family and have varied and numerous roles in the nervous system. In this report, we have determined the effects of growth factor stimulation on LIF receptor (LIFR) expression and signal transduction in the human neuroblastoma cell line NBFL. We show here that stimulation of NBFL cells with either epidermal growth factor or fibroblast growth factor decreases the level of LIFR in an extracellular signal-regulated kinase (Erk)1/2-dependent manner and that this down-regulation is due to an increase in the apparent rate of lysosomal LIFR degradation. Growth factor-induced decreases in LIFR level inhibit both LIF-stimulated phosphorylation of signal transducers and activators of transcription 3 and LIFR-mediated gene induction. We also show that Ser1044 of LIFR, which we have previously shown to be phosphorylated by Erk1/2, is required for the inhibitory effects of growth factors. Neurons are exposed to varying combinations and concentrations of growth factors and cytokines that influence their growth, development, differentiation, and repair in vivo. These findings demonstrate that LIFR expression and signaling in neuroblastoma cells can be regulated by growth factors that are potent activators of the mitogen-activated protein kinase pathway, and thus illustrate a fundamental mechanism that underlies crosstalk between receptor tyrosine kinase and neuropoietic cytokine signaling pathways.

Differential stimulation-induced receptor localization in lipid rafts for interleukin-6 family cytokines signaling through the gp130/leukemia inhibitory factor receptor complex

Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) are cytokines which signal through receptor complexes that include the receptor subunits glycoprotein 130 (gp130) and the LIF receptor (LIFR), but CNTF also requires the non-signal transducing CNTF receptor (CNTFR) for binding. We show here that in IMR-32 neuronal cells endogenously expressing the receptor subunits for LIF and CNTF, CNTFR, but not gp130 or LIFR, is found in detergent-resistant lipid rafts. In addition, stimulation of these cells with CNTF resulted in a rapid translocation of a portion of gp130 and LIFR into detergent-resistant lipid rafts while an equivalent stimulation with LIF did not. Disruption of lipid rafts by cholesterol depletion of cell membranes blocked the CNTF-induced translocation of LIFR and gp130. Interestingly, while cholesterol-depletion did not inhibit signal transducer and activator of transcription 3 phosphorylation by either CNTF or LIF stimulation, it strongly inhibited both CNTF- and LIF-mediated phosphorylation of extracellular signal-regulated kinases 1 and 2 and Akt. LIF and CNTF generally appear to have redundant effects in cells responsive to both cytokines. Intriguingly, the data presented here suggest a possible mechanism whereby CNTF or other cytokines that signal through CNTFR could generate signals distinct from those elicited by cytokines such as LIF which utilize a LIFR/gp130 heterodimer, via association with or exclusion from lipid rafts.

Multiple promoter elements required for leukemia inhibitory factor-stimulated M2 muscarinic acetylcholine receptor promoter activity

Treatment of neuronal cells with leukemia inhibitory factor (LIF) results in increased M(2) muscarinic acetylcholine receptor promoter activity. We demonstrate here that multiple promoter elements mediate LIF stimulation of M(2) gene transcription. We identify a LIF inducible element (LIE) in the M(2) promoter with high homology to a cytokine-inducible ACTG-containing sequence in the vasoactive intestinal peptide promoter. Mutagenesis of both a STAT (signal transducers and activators of transcription) element and the LIE in the M(2) promoter is required to attenuate stimulation of M(2) promoter activity by LIF completely. Mobility shift assays indicate that a LIF-stimulated complex binds to a 70 base pair M(2) promoter fragment. Furthermore, a STAT element within this fragment can bind to LIF-stimulated nuclear STAT1 homodimers in vitro. Mutagenesis experiments show that cytokine-stimulated activation of M(2) promoter activity requires tyrosine residues on glycoprotein 130 (gp130) that are also required for both STAT1 and STAT3 activation. Dominant negative STAT1 or STAT3 can block LIF-stimulated M(2) promoter activity. Real-time RT-PCR analysis indicates that LIF-stimulated induction of M(2) mRNA is partially dependent on protein synthesis. These results show that regulation of M(2) gene transcription in neuronal cells by LIF occurs through a complex novel mechanism that is dependent on LIE, STAT and de novo protein synthesis.

Leukaemia inhibitory factor or Oncostatin M induction of Swiss 3T3 cells does not require mevalonic acid synthesis nor protein isoprenylation to initiate DNA replication

Leukaemia inhibitory factor (LIF) or Oncostatin M (OSM), both mitogens for Swiss mouse 3T3 cells, triggers initiation of DNA synthesis without the requirement for mevalonic acid. Thus, Lovastatin (LOV), an inhibitor of the hydroxy methylglutaryl CoA (HMGCoA) reductase, does not block LIF or OSM induced DNA replication and cell multiplication. In contrast, increasing concentrations of LOV from 1 to 60 microM block the mitogenic action of PGF(2alpha) by decreasing the number of cells capable of entering S-phase and dividing. This inhibition by LOV can be reversed by addition of mevanolactone (MEV), an analogue of mevalonic acid. Thus, LIF or OSM triggers initiation of DNA replication independently of mevalonic acid synthesis and therefore without the involvement of isoprenylation of various signalling proteins.

Cytokine suppression of dopamine-beta-hydroxylase by extracellular signal-regulated kinase-dependent and -independent pathways

Cholinergic differentiation factors (CDFs) suppress noradrenergic properties and induce cholinergic properties in sympathetic neurons. The CDFs leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) bind to a LIFR.gp130 receptor complex to activate Jak/signal transducers and activators of transcription and Ras/mitogen-activated protein kinases signaling pathways. Little is known about how these differentiation factors suppress noradrenergic properties. We used sympathetic neurons and SK-N-BE(2)M17 neuroblastoma cells to investigate CDF down-regulation of the norepinephrine synthetic enzyme dopamine-beta-hydroxylase (DBH). LIF and CNTF activated extracellular signal-regulated kinases (ERKs) 1 and 2 but not p38 or Jun N-terminal kinases in both cell types. Preventing ERK activation with PD98059 blocked CNTF suppression of DBH protein in sympathetic neurons but did not prevent the loss of DBH mRNA. CNTF decreased transcription of a DBH promoter-luciferase reporter construct in SK-N-BE(2)M17 cells, and this was also ERK-independent. Cytokine inhibition of DBH promoter activity did not require a silencer element but was prevented by overexpression of the transcriptional activator Phox2a. Inhibiting ERK activation increased basal DBH transcription in SK-N-BE(2)M17 cells, and DBH mRNA in sympathetic neurons. Transfection of Phox2a into PD98059-treated M17 cells resulted in a synergistic increase in DBH promoter activity compared with Phox2a or PD98059 alone. These data suggest that CDFs down-regulate DBH protein via an ERK-dependent pathway but inhibit DBH gene expression through an ERK-independent pathway. They further suggest that ERK activity inhibits basal DBH gene expression.

Quantification of transcriptional activities of reporter gene constructs in primary cultures of sympathetic neurons

Primary cultures of sympathetic neurons provide an attractive cellular model for investigating the mechanisms of neurotransmitter phenotypic plasticity. However, it has not been possible to transfect these neurons by conventional techniques, and this has been a major impediment to molecular investigations of neuronal gene expression in this system. Here, reporter plasmids were transferred into the nuclei of cultured sympathetic neurons by microinjection. We developed and improved this procedure and were able to measure the transcriptional activities of two coinjected promoters in small groups of neurons, and even from a single neuron. Promoter activities can thus be quantified and normalized relative to that of a constitutively expressed promoter, allowing correction for variability in the injection and assay procedures. High and low promoter activities can be reliably quantified. Importantly, this method can be used not only for reporter plasmids but also for DNA fragments containing only a promoter and reporter gene without any vector sequence that might interfere with promoter. Using this approach, we measured neuronal promoter activities and found that one promoter region of the gene encoding choline acetyltransferase was up-regulated by more than sevenfold by leukemia inhibitory factor. This method thus provides the means to investigate the function of neuronal genes and the mechanisms that regulate their transcription in cultured sympathetic neurons.

Independent roles of SOCS-3 and SHP-2 in the regulation of neuronal gene expression by leukemia inhibitory factor

The neurokine leukemia inhibitory factor (LIF) initiates signaling through heterodimerization of the low affinity LIF receptor (LIFR) and gp130. Tyrosine 759 of gp130 is required for the negative regulation of LIF-mediated signaling by both the protein tyrosine phosphatase SHP-2 and the suppressor of cytokine signaling-3 (SOCS-3). We find that SOCS-3 is expressed in the neuronal cell lines SN56 and IMR32 and negatively regulates LIF-stimulated neuronal gene expression. Studies using antisense oligonucleotides targeted to SHP-2 or SOCS-3 indicate that either protein can negatively regulate LIF-stimulated neuronal gene expression independently of the other. Mutagenesis of the cytoplasmic domain of gp130 demonstrates that the four signal transducer and activators of transcription (STAT) binding sites within gp130 are necessary for the induction of vasoactive intestinal peptide (VIP) and choline acetyltransferase (ChAT) reporter genes, with the sites surrounding tyrosines 905 and 915 (Y905 and Y915) being most important in gp130-mediated reporter gene expression. While there are four STAT binding sites within gp130, only those surrounding Y905 and Y915 can mediate STAT1 activation; these results indicate that STAT1 may be essential for normal gp130-stimulated VIP and ChAT expression. Additionally, the negative regulation of signaling mediated by Y759 of gp130 is dependent upon intact STAT sites within the receptor. This indicates that STAT signaling is necessary for LIF- and CNTF-stimulated VIP and ChAT expression and Y759 of gp130 mediates the activities of SHP-2 and SOCS-3, which act to negatively regulate STAT activity.

Regulation of acetylcholine synthesis and storage

Acetylcholine is one of the major modulators of brain functions and it is the main neurotransmitter at the peripheral nervous system. Modulation of acetylcholine release is crucial for nervous system function. Moreover, dysfunction of cholinergic transmission has been linked to a number of pathological conditions. In this manuscript, we review the cellular mechanisms involved with regulation of acetylcholine synthesis and storage. We focus on how phosphorylation of key cholinergic proteins can participate in the physiological regulation of cholinergic nerve-endings.

Mitogen-activated protein kinase kinase negatively modulates ciliary neurotrophic factor-activated choline acetyltransferase gene expression

The expression of the choline acetyltransferase (ChAT) enzyme that synthesizes the neurotransmitter acetylcholine (ACh) is upregulated by ciliary neurotrophic factor (CNTF). We studied the involvement of the mitogen-activated protein kinase (MAPK) pathway in regulating ChAT expression in a murine septal cell line. Surprisingly, we found that PD98059 and U0126, two structurally distinct inhibitors of MAPK kinase (MEK1), increased both basal and CNTF-induced ACh production. Transient transfections with ChAT promoter-luciferase reporter construct demonstrated synergy between PD98059 and CNTF at the transcriptional level. Moreover, in cotransfection studies, overexpression of constitutively activated MEK1 completely abrogated the CNTF-mediated induction of the reporter. Blocking MEK1 did not significantly alter CNTF-induced Tyr705 phosphorylation of the principal mediator of the CNTF pathway, the transcription factor Stat3. However, PD98059 inhibited Ser727 phosphorylation of Stat3, demonstrating that the latter is MEK1-dependent. Taken together, these results indicate that activation of the MEK1/MAPK pathway inhibits the CNTF-mediated stimulation of ChAT expression, possibly as a part of a feedback mechanism.

Identification of a novel gp130-responsive site in the vasoactive intestinal peptide cytokine response element

The neuropoietic cytokine ciliary neurotrophic factor (CNTF) potently induces transcription of the vasoactive intestinal peptide (VIP) gene through a 180-base pair (bp) cytokine response element (CyRE) in the VIP promoter. We have previously shown that CNTF induction of STAT and AP-1 protein binding within the CyRE is necessary to mediate CNTF induction of VIP gene transcription. We now show that a third, previously uncharacterized site at the 3'-end of the CyRE is also critical to CNTF induction of CyRE transcription. A 4-bp mutation in this 3'-region reduced CNTF-mediated induction of transcription approximately 80%. Whereas mutations in both the STAT and AP-1 sites substantially reduced CNTF induction of transcription, mutations in these sites together with the novel 3'-site completely abolished the ability of CNTF to induce CyRE-mediated transcription. Gel shift analysis indicated that a complex in neuroblastoma cells bound specifically to this 3'-site. This complex was not altered by CNTF treatment. Mutations in an 8-bp sequence (TTACTGGA) eliminated binding of this protein complex and markedly reduced transcriptional activation of the CyRE by CNTF. Thus, we have identified a protein complex binding to a novel DNA sequence that is necessary for full CNTF induction of VIP gene transcription.

Phosphorylation of human gp130 at Ser-782 adjacent to the Di-leucine internalization motif. Effects on expression and signaling

The receptor for leukemia inhibitory factor (LIF) consists of two polypeptides, the LIF receptor and gp130. Agonist stimulation has been shown previously to cause phosphorylation of gp130 on serine, threonine, and tyrosine residues. We found that gp130 fusion proteins were phosphorylated exclusively on Ser-782 by LIF- and growth factor-stimulated 3T3-L1 cell extracts. Ser-780 was required for phosphorylation of Ser-782 but was not itself phosphorylated. Ser-782 is located immediately N-terminal to the di-leucine motif of gp130, which regulates internalization of the receptor. Transient expression of chimeric granulocyte colony-stimulating factor receptor (G-CSFR)-gp130(S782A) receptors resulted in increased cell surface expression in COS-7 cells and increased ability to induce vasoactive intestinal peptide gene expression in IMR-32 neuroblastoma cells when compared with expression of chimeric receptors containing wild-type gp130 cytoplasmic domains. These results identify Ser-782 as the major phosphorylated serine residue in human gp130 and indicate that this site regulates cell surface expression of the receptor polypeptide.

Differential regulation of leukemia inhibitory factor-stimulated neuronal gene expression by protein phosphatases SHP-1 and SHP-2 through mitogen-activated protein kinase-dependent and -independent pathways

The neurally active cytokine leukemia inhibitory factor (LIF) signals through a bipartite receptor complex composed of LIF receptor alpha (LIFR) and gp130. gp130 and LIFR contain consensus binding motifs for the protein tyrosine phosphatase SHP-2 surrounding tyrosines 118 and 115 (Y118 and Y115) of their cytoplasmic domains, respectively. These sites are necessary for maximal activation of mitogen-activated protein kinase (MAPK). Coexpression of catalytically inactive, but not wild-type, SHP-2 reduced LIFR- and gp130-mediated activation of MAPK up to 75%. Conversely, coexpression of the wild-type, but not catalytically inactive, SHP-1, a related phosphatase, reduced activity up to 80%, demonstrating that SHP-2 and SHP-1 have opposing effects on the MAPK pathway. Mutation of Y115 of the cytoplasmic domain of LIFR eliminates receptor-mediated tyrosine phosphorylation of SHP-2. In contrast, SHP-1 association with gp130 and LIFR is constitutive and independent of Y118 and Y115, respectively. SHP-1 has a positive regulatory role on LIF-stimulated vasoactive intestinal peptide (VIP) reporter gene expression in neuronal cells, whereas the effect of SHP-2 is negative. Furthermore, LIF-stimulated MAPK activation negatively regulates this VIP reporter gene induction. SHP-2 also negatively regulates LIF-dependent expression of choline acetyltransferase, but this regulation could be dissociated from its effects on MAPK activation. These data indicate that SHP-1 and SHP-2 are important regulators of LIF-dependent neuronal gene expression via both MAPK-dependent and -independent pathways.

Cadmium-mediated activation of the metal response element in human neuroblastoma cells lacking functional metal response element-binding transcription factor-1

Metal response element-binding transcription factor-1 (MTF-1) binds specifically to metal response elements (MREs) and transactivates metallothionein (MT) gene expression in response to zinc and cadmium. This investigation contrasts the mechanism of mouse MT gene (mMT-I) promoter activation by cadmium and zinc in IMR-32 human neuroblastoma cells to determine whether MTF-1 binding to the MRE is necessary for activation by these metals. Cadmium activated a mMT-1 promoter (-150 base pairs) luciferase reporter 20-25-fold through a MRE-dependent mechanism. In contrast, zinc had little effect on the mMT-1 luciferase reporter. IMR-32 cells lacked MRE binding activity, and treatment with zinc in vitro or in vivo did not generate a MTF-1. MRE complex, suggesting that IMR-32 cells lack functional MTF-1. Overexpression of mMTF-1 regenerated a zinc-mediated induction of the MRE without affecting cadmium activation. Because no other transition metals tested activated the MRE, this effect appeared to be cadmium-specific. These data demonstrate that in IMR-32 human neuroblastoma cells, zinc and cadmium can use independent mechanisms for activation of the mMT-I promoter and cadmium-mediated MRE activation is independent of MTF-1 and zinc.

Participation of upstream stimulator factor (USF) in cadmium-induction of the mouse metallothionein-I gene

The roles of the bHLH-Zip protein, upstream stimulatory factor (USF), in mouse metallothionein-I (MT-I) gene expression were examined. The promoter contains a putative USF binding site which overlaps an antioxidant response element (ARE) located at -101 bp relative to the transcription start point. The USF/ARE composite element increases basal expression of the mouse MT-I gene, and partly mediates response to oxidative stress. However, other functions of this composite element and the in vivo roles for USF in MT-I promoter functions have not been examined. We report studies which indicate that USF participates via the USF/ARE element in cadmium responsiveness of the mouse MT-I promoter. During the course of these studies a second, higher affinity USF binding site at -223 bp was identified. Stable and transient transfection assays in mouse hepatoma cells, using the USF/ARE in the context of a minimal promoter and site-directed and truncation mutants of the MT-I promoter, revealed that the USF and the ARE sites contribute to cadmium (2-30 microM) but not zinc responsiveness, and to basal promoter activity. Overexpression of dominant-negative (dn)USF in co-transfection assays significantly attenuated cadmium induction of the USF/ARE in the context of a minimal promoter, and attenuated cadmium, but not zinc, induction of the intact MT-I promoter. A consensus E-box (CACATG) at -223 bp in the MT-I promoter was also found to bind USF in vitro , and to be constitutively footprinted in vivo . The interaction of USF with E-box1 was apparently 10-fold stronger than that with the USF/ARE. However, in contrast, E-box1 was not a strong basal promoter element nor was it metal ions responsive in mouse Hepa cells. In conclusion, these studies demonstrate a role for USF in cadmium-specific induction of the mouse MT-I gene, but bring into question an obligate role for USF in regulating basal activity of this gene. The data further suggest that USF interacts with ARE-binding proteins to influence MT-I gene expression.

Conclusions and comments. Xth International Symposium on Cholinergic Mechanisms

Ancient medicine men of Egypt and Arabia employed, under another name, the cholinergic agents, as did the hunters, warriors and shamans of Africa and South America. An explosion of cholinergic science occurred in the last and the current century, and the ISCMs witnessed and catalyzed this progress. The Xth ISCM emphasized the molecular characteristics of the receptors, cholinesterase and of the system engaged in liberation of Ach.

Cross talk between ERK and PKA is required for Ca2+ stimulation of CREB-dependent transcription and ERK nuclear translocation

Although Ca2+-stimulated cAMP response element binding protein- (CREB-) dependent transcription has been implicated in growth, differentiation, and neuroplasticity, mechanisms for Ca2+-activated transcription have not been defined. Here, we report that extracellular signal-related protein kinase (ERK) signaling is obligatory for Ca2+-stimulated transcription in PC12 cells and hippocampal neurons. The sequential activation of ERK and Rsk2 by Ca2+ leads to the phosphorylation and transactivation of CREB. Interestingly, the Ca2+-induced nuclear translocation of ERK and Rsk2 to the nucleus requires protein kinase A (PKA) activation. This may explain why PKA activity is required for Ca2+-stimulated CREB-dependent transcription. Furthermore, the full expression of the late phase of long-term potentiation (L-LTP) and L-LTP-associated CRE-mediated transcription requires ERK activation, suggesting that the activation of CREB by ERK plays a critical role in the formation of long lasting neuronal plasticity.

A crucial role for the mitogen-activated protein kinase pathway in nicotinic cholinergic signaling to secretory protein transcription in pheochromocytoma cells

The mitogen-activated protein kinase (MAPK) pathway plays a pivotal role in intracellular signaling, and this cascade may impinge on cAMP response elements (CREs) of target genes. Both the MAPK pathway and chromogranin A expression may be activated by cytosolic calcium influx, and calcium-dependent signals map onto the chromogranin A promoter proximal CRE. We therefore probed the role of the MAPK pathway in chromogranin A biosynthesis after secretory stimulation of PC12 pheochromocytoma cells by the nicotinic cholinergic pathway, the physiological secretory trigger. Chemical inhibition of either MAPK or MAPK kinase blocked the response of a transfected chromogranin A promoter to nicotine or protein kinase C activation [by phorbol-12-myristate-13-acetate (PMA)], although nicotine-evoked catecholamine secretion was unaffected. Activation of the MAP kinase cascade (Ras, Raf, MAPK, or CREB kinase) by cotransfection of pathway components stimulated the chromogranin A promoter. Cotransfection of MAPK pathway dominant negative mutants (for Raf, MAPK, or CREB kinase) blocked nicotinic or PMA activation of chromogranin A, although a dominant negative Ras mutant was without effect. MAPK pathway enzymatic activity was stimulated by both nicotine and PMA. Point mutations of the chromogranin A CRE suggested that this element was necessary in cis for stimulation by nicotine, PMA, or chemical activation of the MAPK pathway. Transfer of the CRE to a heterologous promoter conferred inducibility by not only nicotine or cAMP but also MAPK activation. Expression of the CREB antagonist KCREB blocked the response of the chromogranin A promoter to nicotine, cAMP, or MAPK pathway activation by either chemical stimulation or cotransfection of active cascade components. Chromogranin A mRNA responded to MAPK pathway manipulation in a fashion similar to the transfected chromogranin A promoter, in both direction and magnitude. We conclude that the MAPK pathway is a necessary intermediate in signaling from the nicotinic receptor to secretory protein transcription, although not to catecholamine secretion. In trans, this response seems to involve the following signal cascade: protein kinase C --> Raf --> MAPK kinase --> MAPK --> CREB kinase --> CREB. In cis, activation by the cascade maps onto the chromogranin A promoter proximal CRE, which is both necessary and sufficient to confer the response.

Coordinate regulation of STAT signaling and c-fos expression by the tyrosine phosphatase SHP-2

The src homology 2 (SH2) domain-containing protein-tyrosine phosphatase SHP-2 has been implicated as an important positive regulator of several mitogenic signaling pathways. SHP-2 has more recently been shown to be tyrosine phosphorylated and recruited to the gp130 component of the ciliary neurotrophic factor (CNTF) receptor complex upon stimulation with CNTF. CNTF does not, however, have a proliferative effect on responsive cells, but rather enhances the survival and differentiation of sympathetic, motor, and sensory neurons. In this study, expression of an interfering mutant of SHP-2 in the neuroblastoma cell line NBFL increased CNTF induction of a vasoactive intestinal peptide (VIP) reporter gene, and in cultures of sympathetic neurons, it resulted in an up-regulation of endogenous VIP and substance P (SP) gene expression. Members of the CNTF family of cytokines transmit their signal by activating signaling pathways involving both STAT and Fos-Jun transcription factors. In CNTF-stimulated NBFL cells that constitutively express the SHP-2 interfering mutant, there was increased and prolonged formation of STAT/DNA complexes, but decreased AP-1 binding activity, that mirrored a down-regulation of c-fos expression both at the mRNA and protein level. Taken together, these data indicate that SHP-2 has dual and opposing roles in a signaling cascade triggered by the same ligand, as illustrated by its ability to differentially regulate the levels of activity of both STAT and AP-1 transcription factors.

An antagonist for the leukemia inhibitory factor receptor inhibits leukemia inhibitory factor, cardiotrophin-1, ciliary neurotrophic factor, and oncostatin M

The leukemia inhibitory factor receptor (LIF-R) is activated not only by LIF, but also by cardiotrophin-1, ciliary neurotrophic factor with its receptor, and oncostatin M (OSM). Each of these cytokines induces the hetero-oligomerization of LIF-R with gp130, a signal-transducing subunit shared with interleukin-6 and interleukin-11. The introduction of mutations into human LIF that reduced the affinity for gp130 while retaining affinity for LIF-R has generated antagonists for LIF. In the current study, a LIF antagonist that was free of detectable agonistic activity was tested for antagonism against the family of LIF-R ligands. On cells that express LIF-R and gp130, all LIF-R ligands were antagonized. On cells that also express OSM receptor, OSM was not antagonized, demonstrating that the antagonist is specific for LIF-R. Ligand-triggered tyrosine phosphorylation of both LIF-R and gp130 was blocked by the antagonist. The antagonist is therefore likely to work by preventing receptor oligomerization.

Box 3-independent signaling mechanisms are involved in leukemia inhibitory factor receptor alpha- and gp130-mediated stimulation of mitogen-activated protein kinase. Evidence for participation of multiple signaling pathways which converge at Ras

Chimeric receptors containing the entire or various cytoplasmic domains of either gp130 or leukemia inhibitory factor receptor alpha (LIFR) were used to identify signaling molecules and regions of these polypeptides required for the stimulation of mitogen-activated protein kinase (MAPK). Coexpression of dominant-negative Jak2 inhibited chimeric receptor-stimulated MAPK activity by approximately 70%, while expression of dominant-negative Ras completely blocked MAPK activation by either receptor polypeptide. Deletion analysis identified a 24-amino acid region of gp130 that was necessary for maximal stimulation of MAPK, and contained box 3 (positions 120-129) and a consensus tyrosine binding motif (Tyr-118) for the protein-tyrosine phosphatase, SHP2. Expression of receptors lacking this region or of chimeric gp130(Y118F) point mutants inhibited MAPK activity by approximately 55%, suggesting that Tyr-118, but not box 3, was required during activation of MAPK by gp130. Similarly, expression of chimeric LIFR constructs lacking box 3 maximally stimulated MAPK activity, while those lacking Tyr-115, a putative SHP2 binding site, inhibited stimulation of MAPK by this polypeptide. Our results demonstrate that gp130 and LIFR stimulate MAPK activity through box 3-independent mechanisms involving: (i) effects at Tyr-118 and Tyr-115, respectively, for maximal stimulation of MAPK activity and (ii) a Jak/Tyk-dependent pathway that, together with Tyr-118- or Tyr-115-generated signals, converges at the level of Ras during activation of MAPK by cytokine.

Isolation and characterization of the chicken m2 acetylcholine receptor promoter region: induction of gene transcription by leukemia inhibitory factor and ciliary neurotrophic factor

We have isolated the promoter region and determined the start sites of transcription for the gene encoding the chicken m2 (cm2) muscarinic acetylcholine receptor. Transfection experiments, using cm2-luciferase reporter gene constructs, demonstrated that a 789-bp genomic fragment was sufficient to drive high level expression in chicken heart primary cultures, while an additional 1.2-kb region was required for maximal expression in mouse septal/ neuroblastoma (SN56) cells. Treatment of SN56 cells with the cytokines ciliary neurotrophic factor and leukemia inhibitory factor increases expression of endogenous muscarinic acetylcholine receptors and results in a 4- to 6-fold induction of cm2 promoter driven luciferase expression. We have mapped a region of the cm2 promoter that is necessary for induction by cytokines.

Coordinated up-regulation of choline acetyltransferase and vesicular acetylcholine transporter gene expression by the retinoic acid receptor alpha, cAMP, and leukemia inhibitory factor/ciliary neurotrophic factor signaling pathways in a murine septal cell line

The proteins responsible for acetylcholine (ACh) synthesis (choline acetyltransferase, ChAT) and storage (vesicular ACh transporter, VAChT) are encoded by two closely linked genes in vertebrates, with the VAChT coding sequence contained within the first intron of the ChAT gene. This unusual genomic organization suggests that the transcription of these two genes is coordinately regulated. Using Northern analysis we studied the modulation of ChAT and VAChT expression in a murine septal cell line (SN56) by three groups of agents: retinoids, trophic factors belonging to the leukemia inhibitory factor/ciliary neurotrophic factor (LIF/CNTF) family, and cAMP. All-trans-retinoic acid increased both ChAT and VAChT mRNA levels in SN56 cells up to 3.5-fold, and elevated intracellular ACh levels by 2.5-fold. This effect was mimicked by a retinoic acid receptor alpha (RAR alpha) agonist (Ro 40-6055) and prevented by a specific antagonist (Ro 41-5253), indicating that it was mediated by RAR alpha. ChAT- and VAChT-specific transcripts were also induced (up to 3-fold) by treatment with CNTF or LIF (20 ng/ml, 48 h), as well as by dibutyryl cAMP (1 mM). All these agents increased the ACh level in the cells (up to 2.5-fold). Dibutyryl cAMP had a greater effect on the level of VAChT mRNA (4-fold induction) than on the level of ChAT mRNA (2-fold induction), suggesting a quantitatively differential transcriptional regulation of the two genes by the cAMP pathway. The effects of the three groups of agents studied on ChAT and VAChT mRNA levels were additive, pointing to several independent mechanisms by which the cholinergic properties of septal neurons can be modulated.