The beta-PDGF receptor induces neuronal differentiation of PC12 cells

Optogenetic control of YAP reveals a dynamic communication code for stem cell fate and proliferation

YAP is a transcriptional regulator that controls pluripotency, cell fate, and proliferation. How cells ensure the selective activation of YAP effector genes is unknown. This knowledge is essential to rationally control cellular decision-making. Here we leverage optogenetics, live-imaging of transcription, and cell fate analysis to understand and control gene activation and cell behavior. We reveal that cells decode the steady-state concentrations and timing of YAP activation to control proliferation, cell fate, and expression of the pluripotency regulators Oct4 and Nanog. While oscillatory YAP inputs induce Oct4 expression and proliferation optimally at frequencies that mimic native dynamics, cellular differentiation requires persistently low YAP levels. We identify the molecular logic of the Oct4 dynamic decoder, which acts through an adaptive change sensor. Our work reveals how YAP levels and dynamics enable multiplexing of information transmission for the regulation of developmental decision-making and establishes a platform for the rational control of these behaviors.

Tyrphostin A9 protects axons in experimental autoimmune encephalomyelitis through activation of ERKs

AIMS

Small molecule compound tyrphostin A9 (A9), an inhibitor of platelet-derived growth factor (PDGF) receptor, was previously reported by our group to stimulate extracellular signal-regulated kinase 1 (ERK1) and 2 (ERK2) in neuronal cells in a PDGF receptor-irrelevant manner. The study aimed to investigate whether A9 could protect axons in experimental autoimmune encephalomyelitis through activation of ERKs.

MAIN METHODS

A9 treatment on the protection on neurite outgrowth in SH-SY5Y neuroblastoma cells and primary substantia nigra neuron cultures from the neurotoxin MPP+ were analyzed. Then, clinical symptoms as well as ERK1/2 activation, axonal protection induction, and the abundance increases of the regeneration biomarker GAP-43 in the CNS in the relapsing-remitting experimental autoimmune encephalomyelitis (EAE) model were verified.

KEY FINDINGS

A9 treatment could stimulate neurite outgrowth in SH-SY5Y neuroblastoma cells and protect primary substantia nigra neuron cultures from the neurotoxin MPP+. In the relapsing-remitting EAE model, oral administration of A9 successfully ameliorated clinical symptoms, activated ERK1/2, induced axonal protection, and increased the abundance of the regeneration biomarker GAP-43 in the CNS. Interestingly, gene deficiency of ERK1 or ERK2 disrupted the beneficial effects of A9 in MOG-35-55-induced EAE.

SIGNIFICANCE

These results demonstrated that small molecule compounds that stimulate persistent ERK activation in vitro and in vivo may be useful in protective or restorative treatment for neurodegenerative diseases.

Feedback activation of neurofibromin terminates growth factor-induced Ras activation

Background

Growth factors induce a characteristically short-lived Ras activation in cells emerging from quiescence. Extensive work has shown that transient as opposed to sustained Ras activation is critical for the induction of mitogenic programs. Mitogen-induced accumulation of active Ras-GTP results from increased nucleotide exchange driven by the nucleotide exchange factor Sos. In contrast, the mechanism accounting for signal termination and prompt restoration of basal Ras-GTP levels is unclear, but has been inferred to involve feedback inhibition of Sos. Remarkably, how GTP-hydrolase activating proteins (GAPs) participate in controlling the rise and fall of Ras-GTP levels is unknown.

Results

Monitoring nucleotide exchange of Ras in permeabilized cells we find, unexpectedly, that the decline of growth factor-induced Ras-GTP levels proceeds in the presence of unabated high nucleotide exchange, pointing to GAP activation as a major mechanism of signal termination. Experiments with non-hydrolysable GTP analogues and mathematical modeling confirmed and rationalized the presence of high GAP activity as Ras-GTP levels decline in a background of high nucleotide exchange. Using pharmacological and genetic approaches we document a raised activity of the neurofibromatosis type I tumor suppressor Ras-GAP neurofibromin and an involvement of Rsk1 and Rsk2 in the down-regulation of Ras-GTP levels.

Conclusions

Our findings show that, in addition to feedback inhibition of Sos, feedback stimulation of the RasGAP neurofibromin enforces termination of the Ras signal in the context of growth-factor signaling. These findings ascribe a precise role to neurofibromin in growth factor-dependent control of Ras activity and illustrate how, by engaging Ras-GAP activity, mitogen-challenged cells play safe to ensure a timely termination of the Ras signal irrespectively of the reigning rate of nucleotide exchange.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0128-z) contains supplementary material, which is available to authorized users.

Light-mediated kinetic control reveals the temporal effect of the Raf/MEK/ERK pathway in PC12 cell neurite outgrowth

It has been proposed that differential activation kinetics allows cells to use a common set of signaling pathways to specify distinct cellular outcomes. For example, nerve growth factor (NGF) and epidermal growth factor (EGF) induce different activation kinetics of the Raf/MEK/ERK signaling pathway and result in differentiation and proliferation, respectively. However, a direct and quantitative linkage between the temporal profile of Raf/MEK/ERK activation and the cellular outputs has not been established due to a lack of means to precisely perturb its signaling kinetics. Here, we construct a light-gated protein-protein interaction system to regulate the activation pattern of the Raf/MEK/ERK signaling pathway. Light-induced activation of the Raf/MEK/ERK cascade leads to significant neurite outgrowth in rat PC12 pheochromocytoma cell lines in the absence of growth factors. Compared with NGF stimulation, light stimulation induces longer but fewer neurites. Intermittent on/off illumination reveals that cells achieve maximum neurite outgrowth if the off-time duration per cycle is shorter than 45 min. Overall, light-mediated kinetic control enables precise dissection of the temporal dimension within the intracellular signal transduction network.

Low-density lipoprotein receptor related protein-1 (LRP1)-dependent cell signaling promotes neurotrophic activity in embryonic sensory neurons

Developing sensory neurons require neurotrophic support for survival, neurite outgrowth and myelination. The low-density lipoprotein receptor-related protein-1 (LRP1) transactivates Trk receptors and thereby functions as a putative neurotrophin. Herein, we show that LRP1 is abundantly expressed in developing dorsal root ganglia (DRG) and that LRP1-dependent cell signaling supports survival, neurite extension and receptivity to Schwann cells even in the absence of neurotrophins. Cultured embryonic DRG neurons (E15) were treated with previously characterized LRP1 ligands, LRP1-receptor binding domain of α2-macroglobulin (RBD), hemopexin domain of MMP-9 (PEX) or controls (GST) for two weeks. These structurally diverse LRP1 ligands significantly activated and sustained extracellular signal-regulated kinases (ERK1/2) 5-fold (p<0.05), increased expression of growth-associated protein-43(GAP43) 15-fold (P<0.01), and increased neurite outgrowth 20-fold (P<0.01). Primary sensory neurons treated with LRP1 ligands survived > 2 weeks in vitro, to an extent equaling NGF, a finding associated with canonical signaling mechanisms and blockade of caspase-3 cleavage. LRP1 ligand-induced survival and sprouting were blocked by co-incubation with the LRP1 antagonist, receptor associated protein (RAP), whereas RAP had no effect on NGF-induced activity. Site directed mutagenesis of the LRP1 ligand, RBD, in which Lys¹³⁷⁰ and Lys¹³⁷⁴ are converted to alanine to preclude LRP1 binding, were ineffective in promoting cell signaling, survival or inducing neurite extension in primary sensory neurons, confirming LRP1 specificity. Furthermore, LRP1-induced neurite sprouting was mediated by Src-family kinase (SFK) activation, suggesting transactivation of Trk receptors. Co-cultures of primary embryonic neurons and Schwann cells showed that LRP1 agonists promoted axonal receptivity to myelination to Schwann cells. Collectively, these findings identify LRP1 as a novel and perhaps essential trophic molecule for sensory neuronal survival and development.

AAK1 identified as an inhibitor of neuregulin-1/ErbB4-dependent neurotrophic factor signaling using integrative chemical genomics and proteomics

Target identification remains challenging for the field of chemical biology. We describe an integrative chemical genomic and proteomic approach combining the use of differentially active analogs of small molecule probes with stable isotope labeling by amino acids in cell culture-mediated affinity enrichment, followed by subsequent testing of candidate targets using RNA interference-mediated gene silencing. We applied this approach to characterizing the natural product K252a and its ability to potentiate neuregulin-1 (Nrg1)/ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4)-dependent neurotrophic factor signaling and neuritogenesis. We show that AAK1 (adaptor-associated kinase 1) is a relevant target of K252a, and that the loss of AAK1 alters ErbB4 trafficking and expression levels, providing evidence for a previously unrecognized role for AAK1 in Nrg1-mediated neurotrophic factor signaling. Similar strategies should lead to the discovery of novel targets for therapeutic development.

Involvement of GPR12 in the induction of neurite outgrowth in PC12 cells

GPR12, an orphan G protein-coupled receptor, constitutively activates the Gs signaling pathway and further increases intracellular cyclic AMP. GPR12 overexpression has been reported to promote neurite extension in neurons or transform neuro2a neuroblastoma cells into neuron-like cells. However, the possible effects and mechanisms of GPR12 in the differentiation of PC12 cells are still unknown. The present study shows that GPR12 overexpression induced PC12 cells differentiation into neuron-like cells with enlarged cell sizes and neuritogenesis possibly via activation of Erk1/2 signaling and significantly increased the expression of several neurite outgrowth-related genes, including Bcl-xL, Bcl-2 and synaptophysin. These findings indicate that GPR12 may play a role in neurite outgrowth during PC12 cell differentiation.

Detection of phosphorylated mitogen-activated protein kinase in the developing spinal cord of the mouse embryo

Global understanding of the proteome is a major research topic. The comprehensive visualization of the distribution of proteins in vivo or the construction of in situ protein atlases may be a valuable strategy for proteomic researchers. Information about the distribution of various proteins under physiological and pathological conditions should be extremely valuable for the basic and clinical sciences. The mitogen-activated protein kinase (MAPK) cascade plays an essential role in intracellular signaling in organisms. This cascade also regulates biological processes involving development, differentiation, and proliferation. Phosphorylation and dephosphorylation are integral reactions in regulating the activity of MAPKs. Changes in the phosphorylation state of MAPKs are rapid and reversible; therefore, the localizations of physiologically phosphorylated MAPKs in vivo are difficult to accurately detect. Furthermore, phosphorylated MAPKs are likely to change phosphorylated states through commonly used experimental manipulations. In the present study, as a step toward the construction of in situ phosphoprotein atlases, we attempted to detect physiologically phosphorylated MAPKs in vivo in developing spinal cords of mice. We previously reported an improved immunohistochemical method for detecting unstable phosphorylated MAPKs. The distribution patterns of phosphorylated MAPKs in the spinal cords of embryonic mice from embryonic day 13 (E13) to E17 were observed with an improved immunohistochemical method. Phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2) and phosphorylated c-Jun N-terminal kinase 1/2 (p-JNK1/2) were strongly observed in the marginal layer and the dorsal horn from E13 to E17. Our results suggest that p-ERK1/2 and p-JNK1/2 play critical roles in the developing spinal cord. Constructing phosphoprotein atlases will be possible in the future if this work is systematically developed on a larger scale than we presented here.

Mechanism of hydrogen peroxide-induced keratinocyte migration in a scratch-wound model

Recent studies have shown that low concentrations of H(2)O(2) are produced endogenously by nonphagocytes after wounding. We observed that H(2)O(2) at such concentrations can stimulate proliferation as well as migration of keratinocytes in a scratch-wound assay. Both wounding and H(2)O(2) can induce phosphorylation of ERK1/2 via EGFR, but the activation of ERK1/2 by H(2)O(2) is more sustained and can last more than 8h. Sustained ERK1/2 activation is required for the increased proliferation and migration induced by H(2)O(2). The p38 MAPK was also found to be phosphorylated upon treatment with H(2)O(2) but it was not required for H(2)O(2)-induced migration or proliferation. Furthermore, it was observed that there is a cross talk between the ERK1/2 and the p38 pathways whereby inhibition of either pathway can lead to activation of the other. As a result, the motogenic effects of H(2)O(2) were further enhanced when p38 was inhibited. Our data are consistent with the view that H(2)O(2) may play an important signaling role in wound healing.

Feedback regulation of EGFR signalling: decision making by early and delayed loops

Human-made information relay systems invariably incorporate central regulatory components, which are mirrored in biological systems by dense feedback and feedforward loops. This type of system control is exemplified by positive and negative feedback loops (for example, receptor endocytosis and dephosphorylation) that enable growth factors and receptor Tyr kinases of the epidermal growth factor receptor (EGFR)/ERBB family to regulate cellular function. Recent studies show that the collection of feedback regulatory loops can perform computational tasks - such as decoding ligand specificity, transforming graded input signals into a digital output and regulating response kinetics. Aberrant signal processing and feedback regulation can lead to defects associated with pathologies such as cancer.

Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

Reactive oxygen species, Ki-Ras, and mitochondrial superoxide dismutase cooperate in nerve growth factor-induced differentiation of PC12 cells

Nerve growth factor (NGF) induces terminal differentiation in PC12, a pheochromocytoma-derived cell line. NGF binds a specific receptor on the membrane and triggers the ERK1/2 cascade, which stimulates the transcription of neural genes. We report that NGF significantly affects mitochondrial metabolism by reducing mitochondrial-produced reactive oxygen species and stabilizing the electrochemical gradient. This is accomplished by stimulation of mitochondrial manganese superoxide dismutase (MnSOD) both transcriptionally and post-transcriptionally via Ki-Ras and ERK1/2. Activation of MnSOD is essential for completion of neuronal differentiation because 1) expression of MnSOD induces the transcription of a neuronal specific promoter and neurite outgrowth, 2) silencing of endogenous MnSOD by small interfering RNA significantly reduces transcription induced by NGF, and 3) a Ki-Ras mutant in the polylysine stretch at the COOH terminus, unable to stimulate MnSOD, fails to induce complete differentiation. Overexpression of MnSOD restores differentiation in cells expressing this mutant. ERK1/2 is also downstream of MnSOD, as a SOD mimetic drug stimulates ERK1/2 with the same kinetics of NGF and silencing of MnSOD reduces NGF-induced late ERK1/2. Long term activation of ERK1/2 by NGF requires SOD activation, low levels of hydrogen peroxide, and the integrity of the microtubular cytoskeleton. Confocal immunofluorescence shows that NGF stimulates the formation of a complex containing membrane-bound Ki-Ras, microtubules, and mitochondria. We propose that active NGF receptor induces association of mitochondria with plasma membrane. Local activation of ERK1/2 by Ki-Ras stimulates mitochondrial SOD, which reduces reactive oxygen species and produces H(2)O(2). Low and spatially restricted levels of H(2)O(2) induce and maintain long term ERK1/2 activity and ultimately differentiation of PC12 cells.

Regulator of G-protein signaling 14 (RGS14) is a selective H-Ras effector

Background

Regulator of G-protein signaling (RGS) proteins have been well-described as accelerators of Gα-mediated GTP hydrolysis (“GTPase-accelerating proteins” or GAPs). However, RGS proteins with complex domain architectures are now known to regulate much more than Gα GTPase activity. RGS14 contains tandem Ras-binding domains that have been reported to bind to Rap- but not Ras GTPases in vitro, leading to the suggestion that RGS14 is a Rap-specific effector. However, more recent data from mammals and Drosophila imply that, in vivo, RGS14 may instead be an effector of Ras.

Methodology/Principal Findings

Full-length and truncated forms of purified RGS14 protein were found to bind indiscriminately in vitro to both Rap- and Ras-family GTPases, consistent with prior literature reports. In stark contrast, however, we found that in a cellular context RGS14 selectively binds to activated H-Ras and not to Rap isoforms. Co-transfection / co-immunoprecipitation experiments demonstrated the ability of full-length RGS14 to assemble a multiprotein complex with components of the ERK MAPK pathway in a manner dependent on activated H-Ras. Small interfering RNA-mediated knockdown of RGS14 inhibited both nerve growth factor- and basic fibrobast growth factor-mediated neuronal differentiation of PC12 cells, a process which is known to be dependent on Ras-ERK signaling.

Conclusions/Significance

In cells, RGS14 facilitates the formation of a selective Ras·GTP-Raf-MEK-ERK multiprotein complex to promote sustained ERK activation and regulate H-Ras-dependent neuritogenesis. This cellular function for RGS14 is similar but distinct from that recently described for its closely-related paralogue, RGS12, which shares the tandem Ras-binding domain architecture with RGS14.

Transcriptional regulation of proline-rich membrane anchor (PRiMA) of globular form acetylcholinesterase in neuron: an inductive effect of neuron differentiation

The transcriptional regulation of proline-rich membrane anchor (PRiMA), an anchoring protein of tetrameric globular form of acetylcholinesterase (G(4) AChE), was revealed in cultured cortical neurons during differentiation. The level of AChE(T) protein, total enzymatic activity and the amount of G(4) AChE were dramatically increased during the neuron differentiation. RT-PCR analyses revealed that the transcript encoding PRiMA was significantly up-regulated in the differentiated neurons. To investigate the transcriptional mechanism on PRiMA regulation, a reporter construct of human PRiMA promoter-tagged luciferase was employed in this study. Upon the neuronal differentiation in cortical neurons, a mitogen-activated protein (MAP) kinase-dependent pathway was stimulated: this signaling cascade was shown to regulate the transcriptional activity of PRiMA. In addition, both PRiMA and AChE(T) transcripts were induced by the over expression of an active mutant of Raf in the cultured neurons. The treatment of a MAP kinase inhibitor (U0126) significantly blocked the expression of PRiMA transcript and promoter-driven luciferase activity as induced by the differentiation of cortical neurons. These results suggested that a MAP kinase signaling pathway served as one of the transcriptional regulators in controlling PRiMA gene expression during the neuronal differentiation process.

Identification of receptor-tyrosine-kinase-signaling target genes reveals receptor-specific activities and pathway branchpoints during Drosophila development

Receptor tyrosine kinases (RTKs) are an important family of signaling molecules with the unusual property that they are able to transduce their signals using the same downstream pathways. This has led to an unresolved debate as to whether individual receptors are interchangeable, or if each receptor can mediate specific downstream responses. To address this question, we have conducted a screen to identify target genes whose expression is differentially modulated by RTKs and their downstream pathway components. Using whole-mount in situ hybridization in Drosophila embryos exposed to constitutively active RTK pathway signaling, along with quantitative RT-PCR, we found that a significant fraction of target genes respond differentially in a spatial and/or quantitative manner. This includes differential responses to EGF receptor vs. fibroblast growth factor receptor signaling as well as to more downstream components such as Ras1 and pointed. We show that not only genes but also individual alternative transcripts can respond differently to signaling, and we present evidence that the differential responses can be mediated at the transcriptional level. Our results demonstrate that different RTKs can elicit distinct transcriptional responses, and the target genes obtained from our screen provide a valuable resource for further exploration of the mechanisms underlying this signaling specificity.

FGF15 promotes neurogenesis and opposes FGF8 function during neocortical development

BACKGROUND

Growth, differentiation and regional specification of telencephalic domains, such as the cerebral cortex, are regulated by the interplay of secreted proteins produced by patterning centers and signal transduction systems deployed in the surrounding neuroepithelium. Among other signaling molecules, members of the fibroblast growth factor (FGF) family have a prominent role in regulating growth, differentiation and regional specification. In the mouse telencephalon the rostral patterning center expresses members of the Fgf family (Fgf8, Fgf15, Fgf17, Fgf18). FGF8 and FGF17 signaling have major roles in specification and morphogenesis of the rostroventral telencephalon, whereas the functions of FGF15 and FGF18 in the rostral patterning center have not been established.

RESULTS

Using Fgf15-/- mutant mice, we provide evidence that FGF15 suppresses proliferation, and that it promotes differentiation, expression of CoupTF1 and caudoventral fate; thus, reducing Fgf15 and Fgf8 dosage have opposite effects. Furthermore, we show that FGF15 and FGF8 differentially phosphorylate ERK (p42/44), AKT and S6 in cultures of embryonic cortex. Finally, we show that FGF15 inhibits proliferation in cortical cultures.

CONCLUSION

FGF15 and FGF8 have distinct signaling properties, and opposite effects on neocortical patterning and differentiation; FGF15 promotes CoupTF1 expression, represses proliferation and promotes neural differentiation.

Human chorionic gonadotropin induces neuronal differentiation of PC12 cells through activation of stably expressed lutropin/choriogonadotropin receptor

Human chorionic gonadotropin (hCG) and LH play an important role in reproductive physiology. Both hCG and LH bind to the same LH/choriogonadotropin receptor (LH/CG-R). Recent reports documented the temporal and spatial expression of LH/CG-R in the developing and mature mammalian brain. Administration of hCG promoted nerve regeneration in vivo and neurite outgrowth and survival of primary neurons in vitro. The function of hCG/LH and LH/CG-R in the nervous system remains unclear. In this study, we report that hCG/LH induced distinct morphological and biochemical changes, characteristic of neuronal differentiation, in PC12 cells stably expressing LH/CG-R and that the differentiation effect is ligand dose and time dependent. Western blot analysis revealed that both the ERKs and p38 MAPK are activated after hCG treatment. Inhibitor studies showed both the ERK and p38 MAPK signal transduction pathways are required for this differentiation process, which is cAMP dependent and protein kinase A independent. These findings imply a potential role for hCG/LH and LH/CG-R in the development, maintenance, and regeneration of the mammalian nervous system, and in the neuropathogenesis of genetic diseases caused by a mutated LH/CG-R.

Hyperactive Ras in developmental disorders and cancer

Ras genes are the most common targets for somatic gain-of-function mutations in human cancer. Recently, germline mutations that affect components of the Ras-Raf-mitogen-activated and extracellular-signal regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) pathway were shown to cause several developmental disorders, including Noonan, Costello and cardio-facio-cutaneous syndromes. Many of these mutant alleles encode proteins with aberrant biochemical and functional properties. Here we will discuss the implications of germline mutations in the Ras-Raf-MEK-ERK pathway for understanding normal developmental processes and cancer pathogenesis.

Selective role for RGS12 as a Ras/Raf/MEK scaffold in nerve growth factor-mediated differentiation

Regulator of G-protein signaling (RGS) proteins accelerate GTP hydrolysis by heterotrimeric G-protein alpha subunits and thus inhibit signaling by many G protein-coupled receptors. Several RGS proteins have a multidomain architecture that adds further complexity to their roles in cell signaling in addition to their GTPase-accelerating activity. RGS12 contains a tandem repeat of Ras-binding domains but, to date, the role of this protein in Ras-mediated signal transduction has not been reported. Here, we show that RGS12 associates with the nerve growth factor (NGF) receptor tyrosine kinase TrkA, activated H-Ras, B-Raf, and MEK2 and facilitates their coordinated signaling to prolonged ERK activation. RGS12 is required for NGF-mediated neurite outgrowth of PC12 cells, but not outgrowth stimulated by basic fibroblast growth factor. siRNA-mediated knockdown of RGS12 expression also inhibits NGF-induced axonal growth in dissociated cultures of primary dorsal root ganglia neurons. These data suggest that RGS12 may play a critical, and receptor-selective, role in coordinating Ras-dependent signals that are required for promoting and/or maintaining neuronal differentiation.

Cdk5 Modulation of mitogen-activated protein kinase signaling regulates neuronal survival

Cdk5, a cyclin-dependent kinase, is critical for neuronal development, neuronal migration, cortical lamination, and survival. Its survival role is based, in part, on "cross-talk" interactions with apoptotic and survival signaling pathways. Previously, we showed that Cdk5 phosphorylation of mitogen-activated protein kinase kinase (MEK)1 inhibits transient activation induced by nerve growth factor (NGF) in PC12 cells. To further explore the nature of this inhibition, we studied the kinetics of NGF activation of extracellular signal-regulated kinase (Erk)1/2 in cortical neurons with or without roscovitine, an inhibitor of Cdk5. NGF alone induced an Erk1/2-transient activation that peaked in 15 min and declined rapidly to baseline. Roscovitine, alone or with NGF, reached peak Erk1/2 activation in 30 min that was sustained for 48 h. Moreover, the sustained Erk1/2 activation induced apoptosis in cortical neurons. Significantly, pharmacological application of the MEK1 inhibitor PD98095 to roscovitine-treated cortical neurons prevented apoptosis. These results were also confirmed by knocking down Cdk5 activity in cortical neurons with Cdk5 small interference RNA. Apoptosis was correlated with a significant shift of phosphorylated tau and neurofilaments from axons to neuronal cell bodies. These results suggest that survival of cortical neurons is also dependent on tight Cdk5 modulation of the mitogen-activated protein kinase signaling pathway.

Coupling receptor tyrosine kinases to Rho GTPases--GEFs what's the link

Rho GTPases are molecular switches involved in the regulation of many cellular processes. This review summarizes work examining how stimulation of receptor tyrosine kinases (RTKs) leads to the activation of Rho guanine nucleotide exchange factors (GEFs) and their Rho GTPase substrates. The collective findings strongly suggest that RTK signaling to Rho proteins is a general signal transduction mechanism, like RTK mediated activation of phosphatidyl inositol 3-kinase, phospholipase Cgamma, and the mitogen activated protein kinase (MAPK) pathway. More than half of the 58 known human RTKs activate at least one Rho family member. Likewise, 16 Rho GEFs directly interact with and/or are phosphorylated by a RTK. The specificity of receptor tyrosine kinase/Rho GEF signaling seems to be somewhat promiscuous. There several cases where multiple RTKs activate the same Rho GEF and where a single RTK can activate multiple Rho GEFs. Expression analysis indicates that the average human tissue contains transcripts for 33 RTKs, 34 Rho GEFs, and 14 Rho GTPases with each tissue containing a unique complement of these proteins. Given the promiscuity of RTKs for Rho GEFs, Rho GEFs for Rho GTPases, and the large number of these proteins expressed in cells, a complex combinatorial network of proteins in these families may contribute to coding specific signals and cell responses from RTKs.

Mitogen activated protein kinase-dependent activation of c-Jun and c-Fos is required for neuronal differentiation but not for growth and stress response in PC12 cells

MAPK-dependent activation of AP-1 protein c-Jun is involved in PC12 cell differentiation and apoptosis. However, the role of other AP-1 proteins and their connection to MAPKs during growth, differentiation and apoptosis has remained elusive. Here we studied the activation of AP-1 proteins in response to ERK, JNK, and p38 signaling upon NGF, EGF and anisomycin exposures. All treatments caused different kinetics and strength of MAPK and AP-1 activities. NGF induced persistent ERK and AP-1 activities, whereas upon EGF and anisomycin exposures, their activities were only weakly and transiently induced. The sustained AP-1 activity was associated with concomitant c-Fos and c-Jun expression and phoshorylation, which were JNK and ERK dependent. While inhibition of the ERK, JNK, and p38 activities partially prevented AP-1 activity and suppressed differentiation, none of them was required for anisomycin-induced apoptosis. The importance of c-Fos and c-Jun as mediators of differentiation was demonstrated by the findings that the corresponding siRNAs suppressed NGF-induced neurite outgrowth. However, the capacity of c-Fos to promote differentiation required cooperation with Jun proteins. In contrast, Fra-2 expression was not required for the differentiation response. Together, the results show that sustained c-Jun and c-Fos activities mediate MAPK signaling and are essential for differentiation of PC12 cells.

Microarray analysis of early adipogenesis in C3H10T1/2 cells: cooperative inhibitory effects of growth factors and 2,3,7,8-tetrachlorodibenzo-p-dioxin

C3H10T1/2 mouse embryo fibroblasts differentiate into adipocytes when stimulated by a standard hormonal mixture (IDMB). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), via the aryl hydrocarbon receptor (AhR), inhibits induction of the key adipogenic gene peroxisome proliferator-activated receptor gamma (PPARgamma) and subsequent adipogenesis. This TCDD-mediated inhibition requires activation of the extracellular signal-regulated kinase (ERK) pathway, which can be accomplished by serum, epidermal growth factor (EGF), or fibroblast growth factor (FGF). In the absence of serum or growth factors, IDMB induced adipogenesis without mitosis. Microarray analysis identified 200 genes that exhibited expression changes of at least twofold after 24 h of IDMB treatment. This time precedes most PPARgamma stimulation but follows the period of TCDD/ERK cooperation and periods of increased cell contraction and DNA synthesis. Functionally related gene clusters include genes associated with cell structure, triglyceride and cholesterol metabolism, oxidative regulation, and secreted proteins. In the absence of growth factors TCDD inhibited 30% of these IDMB responses without inhibiting the process of differentiation. A combination of EGF and TCDD that blocks differentiation cooperatively blocked a further 44 IDMB-responsive genes, most of which have functional links to differentiation, including PPARgamma. Cell cycle regulators that are stimulated by EGF were substantially inhibited by IDMB but these responses were unaffected by TCDD. By contrast, TCDD and EGF cooperatively reversed IDMB-induced changes in cell adhesion complexes immediately prior to increases in PPARgamma1 expression. Changes in adhesion-linked signaling may play a key role in TCDD affects on differentiation.

PACAP promotes sensory neuron differentiation: blockade by neurotrophic factors

Developing neurons encounter a panoply of extracellular signals as they differentiate. A major goal is to identify these extrinsic cues and define the mechanisms by which neurons simultaneously integrate stimulation by multiple factors yet initiate one specific biological response. Factors that are known to exert potent activities in the developing nervous system include the NGF family of neurotrophic factors, ciliary neurotrophic factor (CNTF), and pituitary adenylate cyclase-activating peptide (PACAP). Here we demonstrate that PACAP promotes the differentiation of nascent dorsal root ganglion (DRG) neurons in that it increases both the number of neural-marker-positive cells and axonogenesis without affecting the proliferation of neural progenitor cells. This response is mediated through the PAC1 receptor and requires MAP kinase activation. Moreover, we find that, in the absence of exogenously added PACAP, blockade of the PAC1 receptor inhibits neuronal differentiation. These data coupled with our finding that both PACAP and the PAC1 receptor are expressed during the peak period of neuronal differentiation in the DRG suggest that PACAP functions in vivo to promote the differentiation of nascent sensory neurons. Interestingly, we also demonstrate that the neurotrophic factors NT-3 and CNTF completely block the PACAP-induced neuronal differentiation. This points to the intricate integration of cellular signals by nascent neurons and, to our knowledge, is the first evidence for neurotrophic factor abrogation of a pathway regulated by G-protein-coupled receptors (GPCRs).

Inhibition of extracellular signal-regulated protein kinase-2 phosphorylation by dihydrotestosterone reduces follicle-stimulating hormone-mediated cyclin D2 messenger ribonucleic acid expression in rat granulosa cells

Granulosa cell mitogenesis is critical for the development of normal ovarian follicles. FSH and other mitogenic stimuli play a crucial role in this process. We have shown that exposing granulosa cells to 5alpha-dihydrotestosterone (DHT) reduces forskolin-stimulated cyclin D2 mRNA expression, which leads to cell cycle arrest resulting in reduced cell proliferation. The present study investigated the signaling molecules upstream of cyclin D2 in FSH-mediated, cAMP-dependent signaling pathway that may be negatively affected by DHT, leading to inhibition of cell cycle progression. Because ERK is an important molecule in mitogenic signaling, the possible effect of DHT on its phosphorylation was examined. Granulosa cells from 3-d estradiol-primed immature rats were treated with DHT (90 ng/ml) for 24 h and subsequently stimulated with forskolin. DHT treatment reduced forskolin stimulation of ERK phosphorylation. Although DHT exposure did not affect cellular cAMP production in response to forskolin, treating the cells with DHT for 24 h significantly reduced protein kinase A activity. DHT also caused a reduction in ERK-2 phosphorylation in response to FSH similar to that seen with forskolin. Furthermore, blocking ERK phosphorylation as well as DHT treatment resulted in a reduction in FSH-stimulated cyclin D2 mRNA expression. From these results, we conclude that DHT treatment reduces the FSH-mediated ERK phosphorylation in granulosa cells, leading to reduced cyclin D2 mRNA expression that culminates in cell cycle arrest.

The neurotrophins and their receptors

The neurotrophins, which include nerve growth factor (NGF) and its relatives, were discovered and characterized for their distinctive ability to promote survival and differentiation of postmitotic neurons. Perhaps surprisingly, the neurotrophins have recently been found to utilize a family of receptor tyrosine kinases (the Trks) similar to those used by normally mitogenic growth factors. In fact, ectopic expression of the Trks in non-neuronal cells allows them to mediate conventional mitogenic responses to the neurotrophins. Despite similarities with other receptor tyrosine kinases, the Trks are rather unique in that they are almost exclusively expressed in the nervous system, and they also display a number of novel structural features. In addition to the Trks, the neurotrophins all bind to another cell surface receptor (known as p75 or the low-affinity NGF receptor), whose role remains quite controversial.

Domain-dependent modulation of PDGFRbeta by ganglioside GM1

The regulation of receptor tyrosine kinases (RTKs) is important in several cellular events, including proliferation, differentiation, and apoptosis. Gangliosides are sialic acid-containing glycosphingolipids that can regulate RTK activity. The addition of ganglioside GM1 to the medium of Swiss 3T3 fibroblasts inhibits both platelet-derived growth factor (PDGF)-mediated tyrosine phosphorylation of PDGF receptor beta (PDGFRbeta) and receptor-mediated endocytosis. However, GM1 did not affect PDGF-mediated receptor phosphorylation, neuritogenesis, or endocytosis in PC12 cells stably transfected with the gene for PDGFRbeta. The ability of GM1 to modulate PDGFRbeta in 3T3 cells but not in transfected PC12 cells indicates a cell context-dependent response. We hypothesized that this inhibition of PDGFRbeta by GM1 must map to one or more domains of the receptor. Thus, a chimeric receptor was created that possessed the extracellular and transmembrane domains of the nerve growth factor (NGF) receptor TrkA and the cytoplasmic domain of PDGFRbeta (TTbeta). In 3T3 cells transfected with the TTbeta construct, GM1 did not inhibit NGF-induced tyrosine phosphorylation of the chimeric receptor or of Erk1/2 in this cell line. GM1 still inhibited PDGF-mediated tyrosine phosphorylation of endogenous PDGFRbeta and of Erk1/2 in Swiss TTbeta cells. Thus, the cytoplasmic domain of PDGFRbeta is not required for GM1-dependent inhibition of PDGFRbeta in 3T3 cells. This suggests that the inhibition of PDGFRbeta by GM1 in Swiss 3T3 fibroblasts maps to either the extracellular and/or transmembrane domain of PDGFRbeta.

Adenosine triphosphate activates mitogen-activated protein kinase in pre-neoplastic and neoplastic ovarian surface epithelial cells

To investigate the role of ATP in ovarian tumorigenesis, the present study examined the expression of the P2U purinoceptor (P2U-R) and effect of ATP on growth stimulation in pre-neoplastic and neoplastic ovarian surface epithelial (OSE) cells. The immortalized OSE (IOSE) cell lines, including IOSE-29 (pre-neoplastic), IOSE-29EC (neoplastic), and OVCAR-3 (ovarian adenocarcinoma cell line) were used. Our results indicated that P2U-R mRNA was expressed and that ATP exerted a growth-stimulatory effect in IOSE-29, IOSE-29EC, and OVCAR-3. To investigate the mechanism of the growth-stimulatory effect, the activation of mitogen-activated protein kinases (MAPKs) by ATP was examined. Treatment with ATP resulted in MAPK activation in IOSE-29 and IOSE-29EC cells, whereas the stimulatory effect of ATP in cellular proliferation and MAPK activation was completely abolished in the presence of PD98059 (an MAPK/ERK kinase inhibitor) and staurosporin (a protein kinase C inhibitor), suggesting that the growth stimulatory effect of ATP is mediated via protein kinase C-dependent MAPK activation in pre-neoplastic and neoplastic OSE cells. In a time-dependent study, ATP significantly increased MAPK activity at 5-20 min in IOSE-29 cells. Activated MAPK declined to control levels after 20 min in these cells. Treatment with ATP significantly induced MAPK activation after 5 min and was sustained for 60 min in IOSE-29EC cells. In addition, treatment with ATP resulted in substantial phosphorylation of Elk-1, the Ets family transcriptional factor, confirming that ATP action is mediated by activation of MAPK. In conclusion, we have demonstrated that P2U-R was expressed and that ATP induced growth stimulation in IOSE and OVCAR-3 cells. Furthermore, treatment with ATP resulted in the activation of an MAPK cascade and phosphorylation of Elk-1 in IOSE-29 and IOSE-29EC cells. These results suggest that the MAPK cascade may be involved in growth stimulation in response to ATP in pre-neoplastic and neoplastic OSE cells.

U0126 and PD98059, specific inhibitors of MEK, accelerate differentiation of RAW264.7 cells into osteoclast-like cells

Osteoclasts are multinucleated cells that differentiate from hematopoietic cells and possess characteristics responsible for bone resorption. To study the involvement of mitogen-activated protein kinases (MAPKs) in osteoclastogenesis of the murine monocytic cell line RAW264.7, which can differentiate into osteoclast-like cells in the presence of the receptor activator of nuclear factor kappa B ligand (RANKL), we treated the cells with specific inhibitors of p38 MAPK, PD169316 and SB203580, and specific inhibitors of MAPK extracellular signaling-regulated kinase (ERK) kinase (MEK), U0126 and PD98059. Each inhibitor blocked differentiation into osteoclast-like cells when the cells were plated at the standard cell density (2000-4000 cells per well (96-well)). However, the effect of MEK inhibitors on osteoclastogenesis varied according to the initial cell density during culture, because cell growth was clearly inhibited by them. When the cells were plated at more than 8000 cells per well, marked enhancement and acceleration of the differentiation were observed. In addition, immunoblot analysis revealed that phosphorylation of ERK was increased by treatment with the p38 inhibitors, whereas the MEK inhibitors increased phosphorylation of p38, which implies a seesaw-like balance between ERK and p38 phosphorylation. We suggest that osteoclastogenesis is regulated under a balance between ERK and p38 pathways and that the MEK/ERK pathway negatively regulates osteoclastogenesis while the p38 pathway does so positively. This is the first report that an inhibitor of signal transduction enhanced osteoclastogenesis.

Lysophospholipid receptors in the nervous system

The lysophospholipid mediators, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P), are responsible for cell signaling in diverse pathways including survival, proliferation, motility, and differentiation. Most of this signaling occurs through an eight-member family of G-protein coupled receptors once known as the endothelial differentiation gene (EDG) family. More recently, the EDG receptors have been divided into two subfamilies: the lysophosphatidic acid subfamily, which includes LPA1, (EDG-2/VZG-1), LPA2 (EDG-4), and LPA3 (EDG-7), and the sphingosine-1-phosphate receptor subfamily, which includes S1P1 (EDG-1), S1P2 (EDG-5/H218/AGR16), S1P3 (EDG-3), S1P4 (EDG-6), and S1P5 (EDG-8/NRG-1). The ubiquitous expression of these receptors across species, coupled with their diverse cellular functions, has made lysophospholipid receptors an important focus of signal transduction research. Neuroscientists have recently begun to explore the role of lysophospholipid receptors in a number of cell types; this research has implicated these receptors in the survival, migration, and differentiation of cells in the mammalian nervous system.

Molecular interpretation of ERK signal duration by immediate early gene products

The duration of intracellular signalling is associated with distinct biological responses, but how cells interpret differences in signal duration are unknown. We show that the immediate early gene product c-Fos functions as a sensor for ERK1 (extracellular-signal-regulated kinase 1) and ERK2 signal duration. When ERK activation is transient, its activity declines before the c-Fos protein accumulates, and under these conditions c-Fos is unstable. However, when ERK signalling is sustained, c-Fos is phosphorylated by still-active ERK and RSK (90K-ribosomal S6 kinase). Carboxy-terminal phosphorylation stabilizes c-Fos and primes additional phosphorylation by exposing a docking site for ERK, termed the FXFP (DEF) domain. Mutating the DEF domain disrupts the c-Fos sensor and c-Fos-mediated signalling. Other immediate early gene products that control cell cycle progression, neuronal differentiation and circadium rhythms also contain putative DEF domains, indicating that multiple sensors exist for sustained ERK signalling. Together, our data identify a general mechanism by which cells can interpret differences in ERK activation kinetics.

Neutral endopeptidase/CD10 expression during phorbol ester-induced differentiation of choriocarcinoma cells through the protein kinase C- and extracellular signal-regulated kinase-dependent signalling pathway

Neutral endopeptidase 24.11 (NEP) is identical to CD10, which is a differentiation antigen for early B-lymphoid progenitors in the B-cell differentiation pathway. This ectoenzyme is known to have a key role in the control of growth, differentiation, and signal transduction of many cellular systems by regulating bioactive peptides and cytokines. Recently, we demonstrated that NEP/CD10 is upregulated during forskolin-induced choriocarcinoma cell differentiation, suggesting that NEP/CD10 is a trophoblast differentiation marker. The purpose of this study was to clarify the enhancement of NEP/CD10 expression and its signal transduction pathway during phorbol ester (PMA)-induced differentiation of BeWo choriocarcinoma cells. PMA-induced differentiation of BeWo cells was confirmed by morphological change and human chorionic gonadotropin (hCG) secretion, which was completely blocked by a protein kinase C (PKC) inhibitor, Bisindolylmaleimide I (Bis I). On immunoblot analysis, PMA enhanced NEP/CD10 expression in a dose- and time-dependent manner, which was completely abolished by Bis I and a mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor, PD98059. PMA also induced phosphorylation of p44/p42 extracellular signal-regulated kinases (ERK) 1 and 2. These observations indicated that activation of PKC by PMA induced differentiation of BeWo cells, and that PMA activated MAPK/ERK, which resulted in the enhancement of NEP/CD10 expression. Furthermore, immunocytochemical analysis showed that NEP/CD10 expression was detected on the membranes of PMA-treated differentiated BeWo cells. In summary, we demonstrated that NEP/CD10 was enhanced during PMA-induced differentiation of BeWo choriocarcinoma cells through a PKC-dependent MEK/ERK signalling pathway. Our findings also suggest that NEP/CD10 may play a functional role in the process of trophoblast differentiation.

Follicle-stimulating hormone activates mitogen-activated protein kinase in preneoplastic and neoplastic ovarian surface epithelial cells

To investigate the role of FSH in ovarian cancer development, the present study examined the expression of FSH receptor (FSH-R) and the effect of FSH on proliferation of normal, preneoplastic, and neoplastic ovarian surface epithelium (OSE) cells. Recently, immortalized OSE (IOSE) cell lines, including IOSE-29 (preneoplastic) and IOSE-29EC (neoplastic), were used. Our results indicated that FSH-R mRNA was expressed and that FSH exerted a growth stimulatory effect in normal, preneoplastic, and neoplastic OSE cells. To investigate the mechanism of the growth stimulatory effect, the activation of MAPKs by FSH was examined in preneoplastic and neoplastic OSE cells. Treatment with FSH resulted in MAPK activation of IOSE-29 and IOSE-29EC cells, whereas the stimulatory effect of FSH on cellular proliferation and MAPK activation was completely abolished in the presence of PD98059, a MAPK kinase inhibitor, suggesting that the growth stimulatory effect of FSH is mediated through MAPK activation in these OSE cells. In a time-dependent study, FSH significantly increased MAPK activity at 5-10 min in IOSE-29 cells. The activated MAPK declined to the control level after 20 min in these cells. Similarly, treatment with FSH significantly induced MAPK activation after 5 min and sustained it for 60 min in IOSE-29EC cells. In addition, treatment with FSH resulted in substantial phosphorylation of Elk-1, confirming that FSH action is mediated via activation of MAPK. In conclusion, we have demonstrated that FSH-R was expressed, and FSH induced growth stimulation in normal, preneoplastic, and neoplastic OSE cells. Furthermore, treatment with FSH stimulated activation of the MAPK cascade and phosphorylated Elk-1 in neoplastic OSE cells. These results suggest that the MAPK cascade may be involved in cellular functions such as growth stimulation in response to FSH in preneoplastic and neoplastic OSE cells.

Role of extracellular signal-regulated kinase pathway in RRR-alpha-tocopheryl succinate-induced differentiation of human MDA-MB-435 breast cancer cells

RRR-alpha-tocopheryl succinate (vitamin E succinate, VES) induces differentiation of human breast cancer cells. Previous studies ruled out transforming growth factor-beta and c-jun N-terminal kinase involvement in VES-induced differentiation but implicated extracellular signal-regulated kinases (ERKs). Here we show that dominant-negative mutants of either mitogen-activated protein kinase kinase (MEK) 1 or ERK1 blocked VES-induced differentiation of MDA-MB-435 cells, as measured by induction of cytokeratin 18 and p21 (Waf1/Cip1) proteins. Blockage of c-jun protein expression using c-jun antisense oligonucleotides or expression of an inducible dominant-negative c-jun mutant protein inhibited VES-induced differentiation. Elevated expression of wild-type c-jun alone was sufficient to induce cellular differentiation. A role for p21 (Waf1/Cip1) is implicated, in that p21 antisense oligomers blocked VES-induced differentiation. In summary, MEK1, ERK1, the transcription factor c-jun, and the cyclin-dependent kinase inhibitor p21 (Waf1/Cip1) play a part in VES-induced differentiation of human MDA-MB-435 breast cancer cells.

P2X7-like receptor activation in astrocytes increases chemokine monocyte chemoattractant protein-1 expression via mitogen-activated protein kinase

Leukocyte infiltration in the CNS after trauma or inflammation is triggered in part by upregulation of the chemokine, monocyte chemoattractant protein-1 (MCP-1), in astrocytes. However the signals that induce the upregulation of MCP-1 in astrocytes are unknown. We have investigated the roles for ATP P2X7 receptor activation because ATP is an intercellular signaling transmitter that is released in both trauma and inflammation and P2X7 receptors are involved in immune system signaling. Astrocytes in primary cell culture and acutely isolated from the hippocampus were immunopositive for P2X7 receptors. In astrocyte cultures, application of the selective P2X7 agonist, benzoyl-benzoyl ATP (Bz-ATP), activated MAP kinases extracellular signal receptor-activated kinase 1 (ERK1), ERK2, and p38. Purinergic antagonists depressed this activation with a profile suggesting P2X7 receptors. Bz-ATP also increased MCP-1 expression in cultured astrocytes, and again P2X7 antagonists prevented this increase. Blocking either the ERK1/ERK2 or the p38 pathway (with PD98059 or SB203580, respectively) significantly inhibited Bz-ATP-induced MCP-1 expression. Coapplication of both antagonists caused a greater depression. We also tested the roles for ATP receptor activation in inducing MCP-1 upregulation in corticectomy, an in vivo model of trauma. This model of cortical trauma was previously shown to increase MCP-1 expression in vivo principally in astrocytes. Suramin, a wide-spectrum purinergic receptor antagonist, significantly depressed the rapid (3 hr) trauma-induced increase in MCP-1 mRNA. These data indicate that purinergic transmitter receptors in astrocytes are important in regulating chemokine synthesis. The regulation of MCP-1 in astrocytes by ATP may be important in mediating communication with hematopoietic inflammatory cells.

Functional components of basic fibroblast growth factor signaling that inhibit lung elastin gene expression

Previously, we have demonstrated that basic fibroblast growth factor (bFGF) decreases elastin gene transcription in confluent rat lung fibroblasts via the binding of a Fra-1-c-Jun heterodimer to an activator protein-1-cAMP response element in the distal region of the elastin promoter. In the present study, we show that bFGF activates the mitogen-activated protein kinase extracellular signal-regulated kinase 1/2, resulting in the translocation of phosphorylated extracellular signal-regulated kinase 1/2 into the nucleus followed by increased binding of Elk-1 to the serum response element of the c-Fos promoter, transient induction of c-Fos mRNA, and sustained induction of Fra-1 mRNA. The addition of PD-98059, an inhibitor of mitogen-activated protein kinase kinase, abrogates the bFGF-dependent repression of elastin mRNA expression. Comparative analyses of confluent and subconfluent fibroblast cultures reveal significant differences in elastin mRNA levels and activator protein-1 protein factors involved in the regulation of elastin transcription. These findings suggest that bFGF modulates specific cellular events that are dependent on the state of the cell and provide a rationale for the differential responses that can be expected in development and injury or repair situations.

P2X7-like receptor activation in astrocytes increases chemokine monocyte chemoattractant protein-1 expression via mitogen-activated protein kinase

Leukocyte infiltration in the CNS after trauma or inflammation is triggered in part by upregulation of the chemokine, monocyte chemoattractant protein-1 (MCP-1), in astrocytes. However the signals that induce the upregulation of MCP-1 in astrocytes are unknown. We have investigated the roles for ATP P2X7 receptor activation because ATP is an intercellular signaling transmitter that is released in both trauma and inflammation and P2X7 receptors are involved in immune system signaling. Astrocytes in primary cell culture and acutely isolated from the hippocampus were immunopositive for P2X7 receptors. In astrocyte cultures, application of the selective P2X7 agonist, benzoyl-benzoyl ATP (Bz-ATP), activated MAP kinases extracellular signal receptor-activated kinase 1 (ERK1), ERK2, and p38. Purinergic antagonists depressed this activation with a profile suggesting P2X7 receptors. Bz-ATP also increased MCP-1 expression in cultured astrocytes, and again P2X7 antagonists prevented this increase. Blocking either the ERK1/ERK2 or the p38 pathway (with PD98059 or SB203580, respectively) significantly inhibited Bz-ATP-induced MCP-1 expression. Coapplication of both antagonists caused a greater depression. We also tested the roles for ATP receptor activation in inducing MCP-1 upregulation in corticectomy, an in vivo model of trauma. This model of cortical trauma was previously shown to increase MCP-1 expression in vivo principally in astrocytes. Suramin, a wide-spectrum purinergic receptor antagonist, significantly depressed the rapid (3 hr) trauma-induced increase in MCP-1 mRNA. These data indicate that purinergic transmitter receptors in astrocytes are important in regulating chemokine synthesis. The regulation of MCP-1 in astrocytes by ATP may be important in mediating communication with hematopoietic inflammatory cells.

β-Migrating Very Low Density Lipoprotein (βVLDL) Activates Smooth Muscle Cell Mitogen-activated Protein (MAP) Kinase via G Protein-coupled Receptor-mediated Transactivation of the Epidermal Growth Factor (EGF) Receptor

This study examined the premise that the atherogenic lipoprotein, beta-migrating very low density lipoprotein (betaVLDL), might activate the mitogen-activated protein (MAP) kinases ERK1/ERK2, thereby contributing to the induction of smooth muscle cell proliferation in atherosclerosis. The data show that betaVLDL activates rabbit smooth muscle cell ERK1/ERK2. Interestingly, ERK1/ERK2 activation is mediated by G protein-coupled receptors that transactivate the epidermal growth factor (EGF) receptor. betaVLDL-induced MAP kinase activation depends on Ras and Src activity as well as protein kinase C. The inhibition of lysosomal degradation of betaVLDL has no effect on ERK1/ERK2 activation. The contribution of betaVLDL-induced activation of ERK1/ERK2 to smooth muscle cell proliferation was also explored. betaVLDL induces expression of egr-1 and c-fos mRNA. Despite its ability to stimulate early gene expression, betaVLDL alone is unable to inspire quiescent cells into S phase. When added in conjunction with EGF, however, stimulation of [(3)H]thymidine incorporation into DNA and an increase in histone gene expression are observed. Moreover, betaVLDL plus EGF synergistically induce cyclin D1 expression and down-regulate p27(KIP1) expression. The addition of either betaVLDL or EGF stimulates a robust activation of ERK1/ERK2, but the addition of both agents simultaneously sustains the activation for a longer time period. Inhibition of MAP kinase kinase, pertussis toxin-sensitive G proteins, the EGF receptor, or protein kinase C blocks betaVLDL plus EGF-induced proliferation, demonstrating that activation of the betaVLDL-induced signaling pathway results in smooth muscle cell proliferation.

The Drosophila cell shape regulator c-Jun N-terminal kinase also functions as a stress-activated protein kinase

Mammalian c-Jun N-terminal kinases (JNKs) are members of a group of stress-activated intracellular signalling molecules within the MAP kinase family. Molecular genetic analysis of a highly evolutionarily conserved Drosophila JNK homologue, DJNK, has demonstrated that this molecule plays an essential developmental role in cell shape regulation. However, it remains to be determined whether DJNK also responds to the broad range of cellular stresses and other stimuli that affect its mammalian counterpart. Here we demonstrate that c-Jun, a substrate for mammalian JNKs, is a specific substrate for DJNK and that an antiserum that cross-reacts with activated mammalian JNK at the conserved threonyl-prolyl-tyrosyl (TPY) motif within the activation loop also specifically recognises the activated form of DJNK. Using these two assays, we show that DJNK activity is stimulated in cultured cells by several treatments that activate mammalian JNKs, including addition of arsenite, vanadate and ceramide derivatives. It is therefore concluded that in addition to its essential developmental functions, DJNK plays an important role in stress responses that mirrors its mammalian counterpart.

Stimulation of mitogen-activated protein kinase by gonadotropin-releasing hormone in human granulosa-luteal cells

The present study investigated the activation of mitogen-activated protein kinases (MAPKs) by a GnRH agonist (GnRHa) in human granulosa-luteal cells (hGLCs). The phosphorylation state of p44 and p42 MAPK was examined using antibodies that distinguish phospho-p44/42 MAPK (Thr(202)/Tyr(204)) from total p44/42 MAPK (activated plus inactivated). Activation of MAPK by GnRHa was observed within 5 min and was sustained for 60 min after treatment. GnRHa stimulated MAPK activation in a dose-dependent manner, with maximum stimulation (6.7-fold over basal levels) at 10(-7) M. Pretreatment with a protein kinase C (PKC) inhibitor, GF109203X, completely blocked GnRHa-induced MAPK activation. In addition, pretreatment with a PKC activator, phorbol-12-myristate 13-acetate, potentiated GnRH-induced MAPK activation. These results indicate that GnRHa stimulates MAPK activation through a PKC-dependent pathway in hGLCs, possibly coupled to G(q)alpha protein. MAPK activation was also observed in response to 8-bromo-cAMP or cholera toxin, but not pertussis toxin. Forskolin (50 microM) substantially stimulated a rapid cAMP accumulation, whereas GnRHa (10(-7) M) or pertussis toxin (100 mg/ml) did not affect basal intracellular cAMP levels. Cotreatment of GnRHa (10(-7) M) did not attenuate forskolin- or hCG-stimulated cAMP accumulation. These results suggest that the GnRH receptor is probably not coupled to G(s)alpha or G(i)alpha in hGLCs. Finally, GnRHa (10(-7) M) stimulated a significant increase in Elk-1 phosphorylation and c-fos messenger RNA expression, as revealed by an in vitro kinase assay and Northern blot analysis, respectively. These results clearly demonstrate that GnRH activates the MAPK cascade through a PKC-dependent pathway in the human ovary.

Activation and protein kinase C-dependent nuclear accumulation of ERK in differentiating human neuroblastoma cells

The human neuroblastoma cell line SH-SY5Y is a well characterized model for sympathetic neuronal differentiation in vitro. Several differentiation protocols exist, one of which, the addition of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) in the presence of serum, has been thoroughly studied. Wild-type SH-SY5Y cells are unresponsive to nerve growth factor (NGF), but cells transfected with the high-affinity NGF receptor TrkA (SH-SY5Y/TrkA) differentiate in response to NGF. In the present study, we have addressed the existence of a differentiation-specific mode of activation and subcellular distribution of the extracellular signal-regulated kinases ERK1 and ERK2 in SH-SY5Y/wt and SH-SY5Y/TrkA. Both TPA and NGF induced a sustained activation and nuclear accumulation of ERK that was accompanied by transactivation of a serum response element (SRE)-driven reporter and of the c-fos gene. However, activation and nuclear accumulation of ERK were not sufficient to induce neuronal differentiation in SH-SY5Y, as demonstrated by the response to TPA in serum-free cultures. Nuclear accumulation but not activation of ERK was demonstrated to require active protein kinase C (PKC). The effect of specific PKC inhibitors on subcellular distribution of ERK and ERK-dependent transcription suggests a functional role for PKC in the regulation of nuclear ERK activity in SH-SY5Y neuroblastoma cells.

MAPK-dependent expression of p21(WAF) and p27(kip1) in PMA-induced differentiation of HL60 cells

Treatment of HL60 cells with phorbol 12-myristate 13-acetate (PMA) results in growth arrest and differentiation towards the macrophage lineage. PMA-induced changes are easily monitored by morphological changes while cells in suspension start adhering onto substrate. PMA induces rapid activation of the extracellular signal-regulated kinases (ERKs). Activation of the ERK pathway is essential to PMA-induced differentiation of HL60 cells. PMA also induces the expression of the cyclin-dependent kinase inhibitors p21(WAF) and p27(kip1), which is modulated by the use of an inhibitor of the ERK cascade. This implies that a link exists between ERK activation and p21(WAF) and p27(kip1) induction in the process of terminal differentiation.

Neural cell adhesion molecule-stimulated neurite outgrowth depends on activation of protein kinase C and the Ras-mitogen-activated protein kinase pathway

The signal transduction pathways associated with neural cell adhesion molecule (NCAM)-induced neuritogenesis are only partially characterized. We here demonstrate that NCAM-induced neurite outgrowth depends on activation of p59(fyn), focal adhesion kinase (FAK), phospholipase Cgamma (PLCgamma), protein kinase C (PKC), and the Ras-mitogen-activated protein (MAP) kinase pathway. This was done using a coculture system consisting of PC12-E2 cells grown on fibroblasts, with or without NCAM expression, allowing NCAM-NCAM interactions resulting in neurite outgrowth. PC12-E2 cells were transiently transfected with expression plasmids encoding constitutively active forms of Ras, Raf, MAP kinase kinases MEK1 and 2, dominant negative forms of Ras and Raf, and the FAK-related nonkinase. Alternatively, PC12-E2 cells were submitted to treatment with antibodies to the fibroblast growth factor (FGF) receptor, inhibitors of the nonreceptor tyrosine kinase p59(fyn), PLC, PKC and MEK and an activator of PKC, phorbol-12-myristate-13-acetate (PMA). MEK2 transfection rescued cells treated with all inhibitors. The same was found for PMA treatment, except when cells concomitantly were treated with the MEK inhibitor. Arachidonic acid rescued cells treated with antibodies to the FGF receptor or the PLC inhibitor, but not cells in which the activity of PKC, p59(fyn), FAK, Ras, or MEK was inhibited. Interaction of NCAM with a synthetic NCAM peptide ligand, known to induce neurite outgrowth, was shown to stimulate phosphorylation of the MAP kinases extracellular signal-regulated kinases ERK1 and ERK2. The MAP kinase activation was sustained, because ERK1 and ERK2 were phosphorylated in PC12-E2 cells and primary hippocampal neurons even after 24 hr of cultivation on NCAM-expressing fibroblasts. Based on these results, we propose a model of NCAM signaling involving two pathways: NCAM-Ras-MAP kinase and NCAM-FGF receptor-PLCgamma-PKC, and we propose that PKC serves as the link between the two pathways activating Raf and thereby creating the sustained activity of the MAP kinases necessary for neuronal differentiation.

Neural cell adhesion molecule-stimulated neurite outgrowth depends on activation of protein kinase C and the Ras-mitogen-activated protein kinase pathway

The signal transduction pathways associated with neural cell adhesion molecule (NCAM)-induced neuritogenesis are only partially characterized. We here demonstrate that NCAM-induced neurite outgrowth depends on activation of p59(fyn), focal adhesion kinase (FAK), phospholipase Cgamma (PLCgamma), protein kinase C (PKC), and the Ras-mitogen-activated protein (MAP) kinase pathway. This was done using a coculture system consisting of PC12-E2 cells grown on fibroblasts, with or without NCAM expression, allowing NCAM-NCAM interactions resulting in neurite outgrowth. PC12-E2 cells were transiently transfected with expression plasmids encoding constitutively active forms of Ras, Raf, MAP kinase kinases MEK1 and 2, dominant negative forms of Ras and Raf, and the FAK-related nonkinase. Alternatively, PC12-E2 cells were submitted to treatment with antibodies to the fibroblast growth factor (FGF) receptor, inhibitors of the nonreceptor tyrosine kinase p59(fyn), PLC, PKC and MEK and an activator of PKC, phorbol-12-myristate-13-acetate (PMA). MEK2 transfection rescued cells treated with all inhibitors. The same was found for PMA treatment, except when cells concomitantly were treated with the MEK inhibitor. Arachidonic acid rescued cells treated with antibodies to the FGF receptor or the PLC inhibitor, but not cells in which the activity of PKC, p59(fyn), FAK, Ras, or MEK was inhibited. Interaction of NCAM with a synthetic NCAM peptide ligand, known to induce neurite outgrowth, was shown to stimulate phosphorylation of the MAP kinases extracellular signal-regulated kinases ERK1 and ERK2. The MAP kinase activation was sustained, because ERK1 and ERK2 were phosphorylated in PC12-E2 cells and primary hippocampal neurons even after 24 hr of cultivation on NCAM-expressing fibroblasts. Based on these results, we propose a model of NCAM signaling involving two pathways: NCAM-Ras-MAP kinase and NCAM-FGF receptor-PLCgamma-PKC, and we propose that PKC serves as the link between the two pathways activating Raf and thereby creating the sustained activity of the MAP kinases necessary for neuronal differentiation.

Role of Ras and Mapks in TGFbeta signaling

Normal signaling by TGFbeta, in the absence of serum or exogenous factors, involves a rapid activation of Ras, Erks, and Sapks in proliferating cultures of TGFbeta-sensitive untransformed epithelial cells and human carcinoma cells. Expression of either RasN17 or dominant-negative (DN) MKK4, or addition of the MEK1 inhibitor PD98059, can block the ability of TGFbeta to induce AP-1 complex formation at the TGFbeta(1) promoter and to autoinduce its own production. The primary components present in this TGFbeta-stimulated AP-1 complex are JunD and Fra-2, although c-Jun, and possibly Fos B, may also be present. While there are two potential Smad binding elements (SBE's) in the TGFbeta(1) promoter, supershift assays suggest that at least one of these does not bind Smad4, and the other is unable to bind factors activated by TGFbeta. In contrast, TGFbeta autoinduction is Smad3-dependent, as DN Smad3 inhibits the ability of TGFbeta to stimulate TGFbeta(1) promoter activity. Our results indicate that TGFbeta can activate both the MKK4/Sapk and MEK/Erk pathways, through Ras and TGFbeta R(I) and R(II), to induce TGFbeta(1) production; Smad4 does not appear to be involved, and Smad3 appears to function independently of this Smad4. We also demonstrate that activation of the Ras/Mapk pathway by TGFbeta positively modulates Smad1-signaling-pathway activation by TGFbeta. In addition, Smad1 could enhance TGFbeta activation of the SBE reporter SBE-luc and this effect could be blocked by co-expression of a DN TGFbeta R(I) receptor or by the MEK1 inhibitor PD98059. This cross-talk between the MEK/Erk and Smad1 pathways was mediated through the four Erk consensus phosphorylation sites in the linker region of Smad1. Mutation of these sites resulted in a loss of the ligand-dependence of both Smad1-Smad4 interactions and nuclear accumulation of Smad1, as well as a loss of the ability of Smad1 to enhance TGFbeta-mediated SBE activation. Our results provide evidence that Erk-mediated phosphorylation of Smad1 in response to TGFbeta is critical for regulating Smad1 subcellular localization; this may be a key determinant in maintaining TGFbeta-dependent transcriptional activation.

Tyrosine phosphorylation of the Bcl-2-associated protein BNIP-2 by fibroblast growth factor receptor-1 prevents its binding to Cdc42GAP and Cdc42

Fibroblast growth factor (FGF) receptor tyrosine kinases are involved in the regulation of cell growth, development, and differentiation in a variety of tissues. To isolate potential signaling molecules in the FGF signaling pathway, we have initiated a yeast two-hybrid screening using the cytosolic domain of FGF receptor-1 (Flg). Here we report the identification of BNIP-2, a previously cloned Bcl-2- and adenovirus E1B-associated protein, as a putative substrate of the receptor. When cotransfected in 293T cells, BNIP-2 was tyrosine-phosphorylated via Flg, but their interaction was transient and could only be seen by "capture" experiments with catalytically inert kinase mutants. When responsive cells were challenged with basic FGF, endogenous tyrosine-phosphorylated BNIP-2 could be precipitated with a BNIP-2 antibody. In addition, the recombinant BNIP-2 expressed in bacteria could be phosphorylated by active Flg in vitro. BNIP-2 shares a region of homology with the noncatalytic domain of Cdc42GAP, a GTPase-activating protein for the small GTP-binding molecule, Cdc42. We show here that BNIP-2 and Cdc42GAP could directly bind to each other and they also compete for the binding to the same target, Cdc42. Unexpectedly, BNIP-2, either produced as a bacterial recombinant protein or expressed in 293T cells, could stimulate the intrinsic GTPase activity of Cdc42. In all cases, tyrosine phosphorylation of BNIP-2 severely impaired its association with Cdc42GAP and its induced GTPase-activating protein-like activity toward Cdc42. These findings should allow us to further characterize the integration of signaling between receptor tyrosine kinases, GTP-binding molecules, and apoptotic pathways.

Mitogen and substrate differentially affect the lineage restriction of adult rat subventricular zone neural precursor cell populations

The effects of specific mitogens and substrates on the proliferative capacity and the differentiated phenotypic plasticity of neural precursor cell populations isolated from the adult rat subventricular zone (SVZ) were examined. SVZ cells were grown on uncoated tissue culture plastic, extracellular matrix, or poly-D-ornithine with either laminin or fibronectin. SVZ neural precursor cells could not be generated with platelet-derived growth factor (PDGF), granulocyte macrophage colony stimulating factor, stem cell factor, heparin-binding epidermal growth factor (HB-EGF), granulocyte colony stimulating factor, or ciliary neurotrophic factor (CNTF), but could be with EGF, fibroblast growth factor 2 (FGF2), and FGF2 plus heparin. Varying combinations of substrate and mitogen resulted in very different expansion rates and/or lineage potential. Neurons, oligodendrocytes, and astrocytes differentiated from all cultures, but EGF-generated neural precursor cells were more restricted to an astrocytic lineage and FGF2-generated neural precursor cells had a greater capacity for neuronal differentiation. In both EGF- and FGF2-generated cell populations, CNTF increased the number of differentiated astrocytes, triiodothyronine oligodendrocytes, PDGF neurons, and brain-derived neurotrophic factor neurons only from EGF cells. Electrophysiological analysis of differentiated cells showed three distinct phenotypes, glial, neuronal, and presumed precursor cells, although the neuronal properties were immature. Collectively, these data indicate that CNS neural precursor cell populations isolated with different mitogens and substrates are intrinsically different and their characteristics cannot be directly compared.