Regulation of Ras-MAPK pathway mitogenic activity by restricting nuclear entry of activated MAPK in endoderm differentiation of embryonic carcinoma and stem cells

Is it time for a retinoic acid-eluting stent or retinoic acid-coated balloon? Insights from experimental studies of systemic and local delivery of retinoids

Although the incidence of restenosis and stent thrombosis has substantially declined during the last decades, they still constitute the two major causes of stent failure. These complications are partially attributed to the currently used cytostatic drugs, which can cause local inflammation, delay or prevent re-endothelialization and essentially cause arterial cell toxicity. Retinoic acid (RA), a vitamin A (retinol) derivative, is a naturally occurring substance used for the treatment of cell proliferation disorders. The agent has pleiotropic effects on vascular smooth muscle cells and macrophages: it influences the proliferation, migration, and transition of smooth muscle cells to other cell types and modulates macrophage activation. These observations are supported by accumulated evidence from in vitro and in vivo experiments. In addition, systemic and topical administration of RA can decrease the development of atherosclerotic plaques and reduce or inhibit restenosis after vascular injury (caused by embolectomy, balloon catheters, or ligation of arteries) in various experimental models. Recently, an RA-drug eluting stent (DES) has been tested in an animal model. In this review, we explore the effects of RA in atherosclerosis and the potential of the local delivery of RA through an RA-DES or RA-coated balloon for targeted therapeutic percutaneous vascular interventions. Despite promising published results, further experimental study is warranted to examine the safety and efficacy of RA-eluting devices in vascular artery disease.

Immunohistochemical staining of phosphorylated-ERK in post-chemotherapeutic samples is a potential predictor of the prognosis of neuroblastoma

PURPOSE

The majority of relapsed neuroblastomas have mitogen-activated protein kinase (MAPK) pathway activating mutations. We previously showed the in vitro and in vivo anti-tumor effects of MAPK/ERK kinase (MEK) inhibitors in MAPK-activated neuroblastoma. We herein assessed the correlation between MAPK activation and the prognosis in neuroblastoma patients using phosphorylated extra-cellular signal-regulated kinase (pERK) immunohistochemistry to establish the protocol for the clinical administration of MEK inhibitors.

METHODS

Neuroblastoma samples from patients treated in our hospital were immunostained with pERK. The clinical outcomes were retrospectively collected from medical records. The correlation between pERK positivity and the prognosis was analyzed.

RESULTS

Regarding pre-chemotherapeutic specimens, there were no differences in the pERK status between tumors with a good and bad prognosis in both the nuclei and cytoplasm. Regarding post-chemotherapeutic specimens, one of eight tumors with a good prognosis and four of six tumors with a poor prognosis showed pERK-positive nuclear staining (p = 0.0909) and five of eight tumors with a good prognosis and four of six tumors with a poor prognosis showed pERK-positive cytoplasmic staining (p > 0.9999).

CONCLUSION

These findings suggested post-chemotherapeutic-not pre-chemotherapeutic-nuclear pERK-positive neuroblastoma tends to be associated with a poor prognosis and may be a potential therapeutic target for MEK inhibitor treatment.

The role of disabled-2 (Dab2) in diseases

Disabled-2 (Dab2/DOC-2) is a mitogen-responsive adaptor protein required for multiple cellular functions. It is involved in many signaling pathways and plays an integral role in vesicular uptake and trafficking, modulating immune function, protein-protein interactions, cellular homeostasis and differentiation, oncogenesis, and inflammatory processes in organ systems. It contains domains for binding to NPXY motif-containing and SH3 domain-containing adapter proteins, phosphoinositides, glycoprotein 100 (gp100, or megalin), integrins, clathrin, and myosin VI. However, the molecular mechanism(s) of Dab2's biological function still remain to be elucidated. In this review, we provide an extensive up-to-date understanding of the function of Dab2 and its regulation in cardiovascular diseases, immune disorders, tumorigenesis, and central nervous system disorders.

Heterogeneous Differentiation of Highly Proliferative Embryonal Carcinoma PCC4 Cells Induced by Curcumin: An In Vitro Study

Curcumin, the yellow pigment derived from turmeric rhizomes, exhibits antioxidant, anti-inflammatory, antimicrobial, and anticancer properties. We have previously reported in a study that curcumin could induce differentiation in embryonal carcinoma cell (EC). EC cells are the primary constituents of teratocarcinoma tumors, and hence differentiating them to a non-proliferative cell type may be useful in anticancer therapies. Here, we conducted a detailed study using various molecular approaches to characterize this differentiation at the cellular and molecular levels. The cells were treated with 20 µM curcumin, which was the optimal concentration to produce the highest amount of differentiated cells. Changes in protein and RNA expression, membrane dynamics, and migration of these cells after treatment with curcumin were then studied in a time-dependent manner. The differentiated cells were morphologically distinct from the precursor cells, and gene expression profiles were altered in curcumin-treated cells. Curcumin promoted cell motility and cell adhesion. Curcumin also induced changes in membrane fluidity and the lateral mobility of lipids in the plasma membrane. The findings of this study suggest that curcumin might have therapeutic potential in differentiation therapy for the treatment of teratocarcinomas or germ cell tumors (GCTs) such as testicular and ovarian GCTs.

Retinoic Acid Induces Differentiation of Mouse F9 Embryonic Carcinoma Cell by Modulating the miR-485 Targeting of Abhd2

Retinoic acid (RA) plays a key role in pluripotent cell differentiation. In F9 embryonic carcinoma cells, RA can induce differentiation towards somatic lineages via the Ras-extracellular signal-regulated kinase (Ras/Erk) pathway, but the mechanism through which it induces the Erk1/2 phosphorylation is unclear. Here, we show that miR-485 is a positive regulator that targets α/β-hydrolase domain-containing protein 2 (Abhd2), which can result in Erk1/2 phosphorylation and triggers differentiation. RA up-regulates miR-485 and concurrently down-regulates Abhd2. We verified that Abhd2 is targeted by miR-485 and they both can influence the phosphorylation of Erk1/2. In summary, RA can mediate cell differentiation by phosphorylating Erk1/2 via miR-485 and Abhd2.

Regulation of human airway smooth muscle cell migration and relevance to asthma

Airway remodelling is an important feature of asthma pathogenesis. A key structural change inherent in airway remodelling is increased airway smooth muscle mass. There is emerging evidence to suggest that the migration of airway smooth muscle cells may contribute to cellular hyperplasia, and thus increased airway smooth muscle mass. The precise source of these cells remains unknown. Increased airway smooth muscle mass may be collectively due to airway infiltration of myofibroblasts, neighbouring airway smooth muscle cells in the bundle, or circulating hemopoietic progenitor cells. However, the relative contribution of each cell type is not well understood. In addition, although many studies have identified pro and anti-migratory agents of airway smooth muscle cells, whether these agents can impact airway remodelling in the context of human asthma, remains to be elucidated. As such, further research is required to determine the exact mechanism behind airway smooth muscle cell migration within the airways, how much this contributes to airway smooth muscle mass in asthma, and whether attenuating this migration may provide a therapeutic avenue for asthma. In this review article, we will discuss the current evidence with respect to the regulation of airway smooth muscle cell migration in asthma.

Nuclear envelope structural proteins facilitate nuclear shape changes accompanying embryonic differentiation and fidelity of gene expression

BACKGROUND

Nuclear size and shape are specific to a cell type, function, and location, and can serve as indicators of disease and development. We previously found that lamin A/C and associated nuclear envelope structural proteins were upregulated when murine embryonic stem (ES) cells differentiated to primitive endoderm cells. Here we further investigated the morphological changes of nuclei that accompany this differentiation.

RESULTS

The nuclei of undifferentiated wild type cells were found shaped as flattened, irregular ovals, whereas nuclei of Gata4-positive endoderm cells were more spherical, less flattened, and with a slightly reduced volume. The morphological change was confirmed in the trophectoderm and primitive endoderm lineages of E4.5 blastocysts, compared to larger and more irregularly shaped of the nuclei of the inner cell mass. We established ES cells genetically null for the nuclear lamina proteins lamin A/C or the inner nuclear envelope protein emerin, or compound mutant for both lamin A/C and emerin. ES cells deficient in lamin A/C differentiated to endoderm but less efficiently, and the nuclei remained flattened and failed to condense. The size and shape of emerin-deficient nuclei also remained uncondensed after treatment with RA. The emerin/lamin A/C double knockout ES cells failed to differentiate to endoderm cells, though the nuclei condensed but retained a generally flattened ellipsoid shape. Additionally, ES cells deficient for lamin A/C and/or emerin had compromised ability to undergo endoderm differentiation, where the differentiating cells often exhibited coexpression of pluripotent and differentiation markers, such as Oct3/4 and Gata4, respectively, indicating an infidelity of gene regulation.

CONCLUSIONS

The results suggest that changes in nuclear size and shape, which are mediated by nuclear envelope structural proteins lamin A/C and/or emerin, also impact gene regulation and lineage differentiation in early embryos. Nevertheless, mice lacking both lamin A/C and emerin were born at the expected frequency, indicating their embryonic development is completed despite the observed protein deficiency.

Molecular signatures of mu opioid receptor and somatostatin receptor 2 in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC), a particularly aggressive malignancy, has been linked to atypical levels, certain mutations, and aberrant signaling of G-protein-coupled receptors (GPCRs). GPCRs have been challenging to target in cancer because they organize into complex networks in tumor cells. To dissect such networks with nanometer-scale precision, here we combine traditional biochemical approaches with superresolution microscopy methods. A novel interaction specific to PDAC is identified between mu opioid receptor (MOR) and somatostatin receptor 2 (SSTR2). Although MOR and SSTR2 did not colocalize in healthy pancreatic cells or matching healthy patient tissues, the pair did significantly colocalize in pancreatic cancer cells, multicellular tumor spheroids, and cancerous patient tissues. Moreover, this association in pancreatic cancer cells correlated with functional cross-talk and increased metastatic potential of cells. Coactivation of MOR and SSTR2 in PDAC cells led to increased expression of mesenchymal markers and decreased expression of an epithelial marker. Together these results suggest that the MOR-SSTR2 heteromer may constitute a novel therapeutic target for PDAC.

Multi-Compartmentalisation in the MAPK Signalling Pathway Contributes to the Emergence of Oscillatory Behaviour and to Ultrasensitivity

Signal transduction through the Mitogen Activated Protein Kinase (MAPK) pathways is evolutionarily highly conserved. Many cells use these pathways to interpret changes to their environment and respond accordingly. The pathways are central to triggering diverse cellular responses such as survival, apoptosis, differentiation and proliferation. Though the interactions between the different MAPK pathways are complex, nevertheless, they maintain a high level of fidelity and specificity to the original signal. There are numerous theories explaining how fidelity and specificity arise within this complex context; spatio-temporal regulation of the pathways and feedback loops are thought to be very important. This paper presents an agent based computational model addressing multi-compartmentalisation and how this influences the dynamics of MAPK cascade activation. The model suggests that multi-compartmentalisation coupled with periodic MAPK kinase (MAPKK) activation may be critical factors for the emergence of oscillation and ultrasensitivity in the system. Finally, the model also establishes a link between the spatial arrangements of the cascade components and temporal activation mechanisms, and how both contribute to fidelity and specificity of MAPK mediated signalling.

Defined spatiotemporal features of RAS-ERK signals dictate cell fate in MCF-7 mammary epithelial cells

Signals conveyed through the RAS-ERK pathway are essential for the determination of cell fate. It is well established that signal variability is achieved in the different microenvironments in which signals unfold. It is also known that signal duration is critical for decisions concerning cell commitment. However, it is unclear how RAS-ERK signals integrate time and space in order to elicit a given biological response. To investigate this, we used MCF-7 cells, in which EGF-induced transient ERK activation triggers proliferation, whereas sustained ERK activation in response to heregulin leads to adipocytic differentiation. We found that both proliferative and differentiating signals emanate exclusively from plasma membrane-disordered microdomains. Of interest, the EGF signal can be transformed into a differentiating stimulus by HRAS overexpression, which prolongs ERK activation, but only if HRAS localizes at disordered membrane. On the other hand, HRAS signals emanating from the Golgi complex induce apoptosis and can prevent heregulin-induced differentiation. Our results indicate that within the same cellular context, RAS can exert different, even antagonistic, effects, depending on its sublocalization. Thus cell destiny is defined by the ability of a stimulus to activate RAS at the appropriate sublocalization for an adequate period while avoiding switching on opposing RAS signals.

Regulation of cell proliferation by ERK and signal-dependent nuclear translocation of ERK is dependent on Tm5NM1-containing actin filaments

Tropomyosin Tm5NM1 regulates cell proliferation and organ size. It mediates this effect by regulating the interaction of pERK and Imp7, leading to the regulation of pERK nuclear translocation. This demonstrates a role for a specific population of actin filaments in regulating a critical step in the MAPK/ERK signaling pathway.

ERK-regulated cell proliferation requires multiple phosphorylation events catalyzed first by MEK and then by casein kinase 2 (CK2), followed by interaction with importin7 and subsequent nuclear translocation of pERK. We report that genetic manipulation of a core component of the actin filaments of cancer cells, the tropomyosin Tm5NM1, regulates the proliferation of normal cells both in vitro and in vivo. Mouse embryo fibroblasts (MEFs) lacking Tm5NM1, which have reduced proliferative capacity, are insensitive to inhibition of ERK by peptide and small-molecule inhibitors, indicating that ERK is unable to regulate proliferation of these knockout (KO) cells. Treatment of wild-type MEFs with a CK2 inhibitor to block phosphorylation of the nuclear translocation signal in pERK resulted in greatly decreased cell proliferation and a significant reduction in the nuclear translocation of pERK. In contrast, Tm5NM1 KO MEFs, which show reduced nuclear translocation of pERK, were unaffected by inhibition of CK2. This suggested that it is nuclear translocation of CK2-phosphorylated pERK that regulates cell proliferation and this capacity is absent in Tm5NM1 KO cells. Proximity ligation assays confirmed a growth factor–stimulated interaction of pERK with Tm5NM1 and that the interaction of pERK with importin7 is greatly reduced in the Tm5NM1 KO cells.

Differentiation into Endoderm Lineage: Pancreatic differentiation from Embryonic Stem Cells

The endoderm gives rise to digestive and respiratory tracts, thyroid, liver, and pancreas. Representative disease of endoderm lineages is type 1 diabetes resulting from destruction of the insulin-producing β cells. Generation of functional β cells from human embryonic stem (ES) cells in vitro can be practical, renewable cell source for replacement cell therapy for type 1 diabetes. It has been achieved by progressive instructive differentiation through each of the developmental stages. In this article, important studies of differentiation into pancreatic β cells from ES cells are reviewed through pancreatic developmental stages as definitive endoderm, primitive gut tube/foregut, and pancreatic cells. The investigation of differentiating ES cells from definitive endoderm to pancreas using signaling, arrays, and proteomics is also introduced.

Mitogen Activated Protein Kinase Family Proteins and c-jun Signaling in Injury-induced Schwann Cell Plasticity

Schwann cells (SCs) in the peripheral nerves myelinate axons during postnatal development to allow saltatory conduction of nerve impulses. Well-organized structures of myelin sheathes are maintained throughout life unless nerves are insulted. After peripheral nerve injury, unidentified signals from injured nerves drive SC dedifferentiation into an immature state. Dedifferentiated SCs participate in axonal regeneration by producing neurotrophic factors and removing degenerating nerve debris. In this review, we focus on the role of mitogen activated protein kinase family proteins (MAP kinases) in SC dedifferentiation. In addition, we will highlight neuregulin 1 and the transcription factor c-jun as upstream and downstream signals for MAP kinases in SC responses to nerve injury.

P53, MAPK, topoisomerase II alpha and Ki67 immunohistochemical expression and KRAS/BRAF mutation in ovarian serous carcinomas

Background

We investigated the immunohistochemical expression of p53, MAPK, topoisomerase II alpha (topoII alpha) and Ki67 in ovarian serous carcinomas (OSCs) along with mutational analysis for KRAS and BRAF.

Methods

Eighty one cases of OSCs were reviewed and examined immunohistochemically using antibodies against p53, MAPK, topoII alpha and Ki67. Staining was evaluated as a percentage of immunopositive cells with cut-off levels at 10% for p53 and topoII alpha, and 5% for MAPK. The Ki67 immunoexpression was assessed by means of Olympus Image Analysis System as a percentage of immunopositive cells in 1000 tumor cells. KRAS and BRAF mutational analysis was performed on 73 available microdissected samples.

Results

Of 81 cases of OSCs 13.6% were of low-grade and 86.4% were of high-grade morphology. In the high-grade group there was a significantly higher immunoexpression of p53 (P < 0.001) and topoII alpha (P = 0.001), with Ki67 median 56.5 vs. 19 in low-grade group (P < 0.001). The difference in immunoexpression of active MAPK between low- and high-grade group was also significant (P = 0.003). MAPK positive immunostaining was detected in 63.6% of low-grade vs. 17.1% of high-grade OSCs. The frequency of KRAS mutation was significantly higher in low-grade as compared to high-grade group (P = 0.006). None of the samples had BRAF mutation. In addition, we detected positive MAPK immunoexpression in 13/59 samples with wild-type KRAS, suggesting that activation of MAPK pathway is not ultimately related either to KRAS or BRAF mutation. Seven morphologically high-grade samples (11.7%) showed both KRAS mutation and p53 immunopositivity.

Conclusions

Although this study is limited by its humble number of low-grade samples, our data fit the proposed dualistic pathway of ovarian carcinogenesis. Mutational analysis for KRAS and BRAF discloses some possible interactions between different tumorigenic pathways of low- and high-grade carcinomas. Immunohistochemical staining for MAPK was not sufficiently sensitive, nor specific, to precisely predict the KRAS mutation. However, it appears to be quite reliable in ruling out a KRAS mutation if the staining is negative.

Virtual Slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/9283563368804632

Zusammenfassung

Hintergrund

Wir untersuchten die Immunohistochemische Expression der p53, MAPK, topoisomerase II alpha (topoII alpha) und Ki67 in Ovarialkarzinomen (OSCs) anbei mit Mutationsanalyse für KRAS und BRAF.

Methode

81 OSCs Fälle wurden analysiert und Immunohistochemisch untersucht mit Antikörper gegen p53, MAPK, topoII alpha und Ki67. Die Färbung war ausgewertet als der Prozent von immunopositiven Zellen mit den “cut-of” Niveau an 10% für p53 und topoII alpha und 5% für MAPK. Die Ki67 Expression war bewertet mittels Olympus Image Analysis System als der Prozent von immunopositiven Zellen in 1000 Tumorzellen. KRAS and BRAF Mutationsanalyse wurde in 73 verfügbaren microdissections Stichproben aufgeführt.

Ergebnisse

Von 81 OSCs Fälle 13.6% zeigte “low-grade” und 86.4% “high-grade” Morphologie. In der “high-grade” Gruppe war eine statistisch bedeutende höhere Expression von p53 (P < 0.001) und topoII alpha (P = 0.001) mit Ki67 median von 56.5 im Gegensatz zu 19 in der “low-grade” Gruppe (P < 0.001). Die Differenz in Immunoexpression von aktiver MAPK zwischen der “low-grade” und “high-grade” Gruppe war statistisch bedeutend (P = 0.003). MAPK positive Expression war in 63.6% der “low-grade” im Gegensatz von 17.1% der “high-grade” Karzinoms bemerkt. Die Häufigkeit der KRAS Mutation war bedeutend höher in “low-grade” im Verglich zu der “high-grade” Gruppe (P = 0.006). Keiner der Stichproben hate BRAF Mutation. Wir haben auch eine positive MAPK Expression in 13/59 der Stichproben mit “wild-type” KRAS bemerkt, was sugeriert das die Aktivation des MAPK Pfads ist nicht letztmalig mit KRAS oder BRAF verbunden. Sieben der “high-grade” Stichproben (11.7%) waren KRAS Mutation und p53 Expression positive.

Schlussworte

Obwohl diese Studie mit bescheiden Nummer von “low-grade” Stichproben limitiert ist, unsere Daten passen in das dualistische Modell von Ovarial Karzinogenesis. Mutationsanalyse für KRAS und BRAF enthüllen einige mögliche Interaktionen zwischen verschieden tumorigenen Wege von “low”- and “high-grade” Karcinomen.

Die Immunohistochemische Expression für MAPK war nicht empfindlich oder spezifisch genüg um den KRAS mutations Status des Tumor genau vorauszusagen.

Es scheint das die MAPK Expression ziemlich verlässlich ist in ausschließen der KRAS Mutation, wenn die Expression negative ist.

Oncogenic KRAS and BRAF activation of the MEK/ERK signaling pathway promotes expression of dual-specificity phosphatase 4 (DUSP4/MKP2) resulting in nuclear ERK1/2 inhibition

Gain-of-function mutations in KRAS and BRAF genes are found in up to 50% of colorectal cancers. These mutations result in the activation of the BRAF/MEK signaling pathway culminating in the stimulation of ERK1/2 mitogen-activated protein kinases. Upon activation, ERK1/2 translocate from the cytoplasm to the nucleus. This process has been shown to be required for the induction of many cellular responses, although the molecular mechanisms regulating ERK nuclear function, especially under oncogenic stimulation, remain to be explored. Herein, we examined the spatiotemporal regulation of ERK1/2 activity upon oncogenic activation of KRAS(G12V) and BRAF(V600E) in normal intestinal epithelial crypt cells (IECs). Results demonstrate that expression of these oncogenes markedly stimulated ERK1/2 activities and morphologically transformed IECs. Importantly however, ERK phosphorylation was not observed in the nucleus, but restricted to the cytoplasm of KRAS(G12V)- and BRAF(V600E)-transformed IECs. The absence of nuclear ERK phosphorylation was due to a vanadate-sensitive phosphatase activity. Nuclear ERK dephosphorylation was found to be tightly correlated with the rapid expression of DUSP4 phosphatase induced in an MEK-dependent manner. In addition, MEK-dependent phosphorylation of T361, T363, S390 and S395 residues highly stabilized DUSP4 protein. Finally, in human colorectal cancer cells, ERK1/2 activities were also confined to the cytoplasm and treatment with pervanadate reactivated ERK1/2 in the nucleus. Accordingly, DUSP4 mRNAs were found to be highly expressed, in an MEK-dependent manner, in all colorectal cancer cells analyzed. These findings indicate that DUSP4 functions as part of a negative feedback mechanism in the control of the duration and magnitude of nuclear ERK activation during intestinal tumorigenesis.

Receptor heteromerization expands the repertoire of cannabinoid signaling in rodent neurons

A fundamental question in G protein coupled receptor biology is how a single ligand acting at a specific receptor is able to induce a range of signaling that results in a variety of physiological responses. We focused on Type 1 cannabinoid receptor (CB₁R) as a model GPCR involved in a variety of processes spanning from analgesia and euphoria to neuronal development, survival and differentiation. We examined receptor dimerization as a possible mechanism underlying expanded signaling responses by a single ligand and focused on interactions between CB₁R and delta opioid receptor (DOR). Using co-immunoprecipitation assays as well as analysis of changes in receptor subcellular localization upon co-expression, we show that CB₁R and DOR form receptor heteromers. We find that heteromerization affects receptor signaling since the potency of the CB₁R ligand to stimulate G-protein activity is increased in the absence of DOR, suggesting that the decrease in CB₁R activity in the presence of DOR could, at least in part, be due to heteromerization. We also find that the decrease in activity is associated with enhanced PLC-dependent recruitment of arrestin3 to the CB₁R-DOR complex, suggesting that interaction with DOR enhances arrestin-mediated CB₁R desensitization. Additionally, presence of DOR facilitates signaling via a new CB₁R-mediated anti-apoptotic pathway leading to enhanced neuronal survival. Taken together, these results support a role for CB₁R-DOR heteromerization in diversification of endocannabinoid signaling and highlight the importance of heteromer-directed signal trafficking in enhancing the repertoire of GPCR signaling.

The ERK Cascade: Distinct Functions within Various Subcellular Organelles

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is a central signaling pathway that regulates a wide variety of stimulated cellular processes, including mainly proliferation, differentiation, and survival, but apoptosis and stress response as well. The ability of this linear cascade to induce so many distinct and even opposing effects after various stimulations raises the question as to how the signaling specificity of the cascade is regulated. Over the past years, several specificity-mediating mechanisms have been elucidated, including temporal regulation, scaffolding interactions, crosstalks with other signaling components, substrate competition, and multiple components in each tier of the cascade. In addition, spatial regulation of various components of the cascade is probably one of the main ways by which signals can be directed to some downstream targets and not to others. In this review, we describe first the components of the ERK1/2 cascade and their mode of regulation by kinases, phosphatases, and scaffold proteins. In the second part, we focus on the role of MEK1/2 and ERK1/2 compartmentalization in the nucleus, mitochondria, endosomes, plasma membrane, cytoskeleton, and Golgi apparatus. We explain that this spatial distribution may direct ERK1/2 signals to regulate the organelles' activities. However, it can also direct the activity of the cascade's components to the outer surface of the organelles in order to bring them to close proximity to specific cytoplasmic targets. We conclude that the dynamic localization of the ERK1/2 cascade components is an important regulatory mechanism in determining the signaling specificity of the cascade, and its understanding should shed a new light on the understanding of many stimulus-dependent processes.

ERK regulates strain-induced migration and proliferation from different subcellular locations

Repetitive deformation like that engendered by peristalsis or villous motility stimulates intestinal epithelial proliferation on collagenous substrates and motility across fibronectin, each requiring ERK. We hypothesized that ERK acts differently at different intracellular sites. We stably transfected Caco-2 cells with ERK decoy expression vectors that permit ERK activation but interfere with its downstream signaling. Targeting sequences constrained the decoy inside or outside the nucleus. We assayed proliferation by cell counting and migration by circular wound closure with or without 10% repetitive deformation at 10 cycles/min. Confocal microscopy confirmed localization of the fusion proteins. Inhibition of phosphorylation of cytoplasmic RSK or nuclear Elk confirmed functionality. Both the nuclear-localized and cytosolic-localized ERK decoys prevented deformation-induced proliferation on collagen. Deformation-induced migration on fibronectin was prevented by constraining the decoy in the nucleus but not in the cytosol. Like the nuclear-localized ERK decoy, a Sef-overexpressing adenovirus that sequesters ERK in the cytoplasm also blocked the motogenic and mitogenic effects of strain. Inhibiting RSK or reducing Elk ablated both the mitogenic and motogenic effects of strain. RSK isoform reduction revealed isoform specificity. These results suggest that ERK must translocate to the nucleus to stimulate cell motility while ERK must act in both the cytosol and the nucleus to stimulate proliferation in response to strain. Selectively targeting ERK within different subcellular compartments may modulate or replace physical force effects on the intestinal mucosa to maintain the intestinal mucosal barrier in settings when peristalsis or villous motility are altered and fibronectin is deposited into injured tissue.

Nuclear entry of activated MAPK is restricted in primary ovarian and mammary epithelial cells

Background

The MAPK/ERK1/2 serine kinases are primary mediators of the Ras mitogenic signaling pathway. Phosphorylation by MEK activates MAPK/ERK in the cytoplasm, and phospho-ERK is thought to enter the nucleus readily to modulate transcription.

Principal Findings

Here, however, we observe that in primary cultures of breast and ovarian epithelial cells, phosphorylation and activation of ERK1/2 are disassociated from nuclear translocalization and transcription of downstream targets, such as c-Fos, suggesting that nuclear translocation is limited in primary cells. Accordingly, in import assays in vitro, primary cells showed a lower import activity for ERK1/2 than cancer cells, in which activated MAPK readily translocated into the nucleus and activated c-Fos expression. Primary cells express lower levels of nuclear pore complex proteins and the nuclear transport factors, importin B1 and importin 7, which may explain the limiting ERK1/2 import found in primary cells. Additionally, reduction in expression of nucleoporin 153 by siRNA targeting reduced ERK1/2 nuclear activity in cancer cells.

Conclusion

ERK1/2 activation is dissociated from nuclear entry, which is a rate limiting step in primary cells and in vivo, and the restriction of nuclear entry is disrupted in transformed cells by the increased expression of nuclear pores and/or nuclear transport factors.

Revealing global regulatory perturbations across human cancers

The discovery of pathways and regulatory networks whose perturbation contributes to neoplastic transformation remains a fundamental challenge for cancer biology. We show that such pathway perturbations, and the cis-regulatory elements through which they operate, can be efficiently extracted from global gene expression profiles. Our approach utilizes information-theoretic analysis of expression levels, pathways, and genomic sequences. Analysis across a diverse set of human cancers reveals the majority of previously known cancer pathways. Through de novo motif discovery we associate these pathways with transcription-factor binding sites and miRNA targets, including those of E2F, NF-Y, p53, and let-7. Follow-up experiments confirmed that these predictions correspond to functional in vivo regulatory interactions. Strikingly, the majority of the perturbations, associated with putative cis-regulatory elements, fall outside of known cancer pathways. Our study provides a systems-level dissection of regulatory perturbations in cancer-an essential component of a rational strategy for therapeutic intervention and drug-target discovery.

Mu and kappa opioids modulate mouse embryonic stem cell-derived neural progenitor differentiation via MAP kinases

As embryonic stem cell-derived neural progenitors (NPs) have the potential to be used in cell replacement therapy, an understanding of the signaling mechanisms that regulate their terminal differentiation is imperative. In previous studies, we discovered the presence of functional mu opioid receptors (MOR) and kappa opioid receptors (KOR) in mouse embryonic stem cells and NPs. Here, MOR and KOR immunoreactivity was detected in NP-derived oligodendrocytes during three stages of their maturation in vitro. Moreover, we examined the modulation of retinoic acid-induced NP differentiation to astrocytes and neurons by mu, [D-ala(2), mephe(4), gly-ol(5)] enkephalin, or kappa, U69, 593, opioids. Both opioid agonists inhibited NP-derived neurogenesis and astrogenesis via their corresponding receptors as determined by immunocytochemistry. By administering selective inhibitors, we found that opioid inhibition of NP-derived astrogenesis was driven via extracellular-signal regulated kinase (ERK), while the p38 mitogen-activated protein kinase pathway was implicated in opioid attenuation of neurogenesis. In addition, mu and kappa opioids stimulated oligodendrogenesis from NP-derived NG2(+) oligodendrocyte progenitors via both ERK and p38 signaling pathways. Accordingly, both opioids induced ERK phosphorylation in NG2(+) cells. These results indicate that small molecules, such as MOR and KOR agonists may play a modulatory role in NP terminal differentiation.

The ERK signaling cascade--views from different subcellular compartments

The extracellular signal-regulated kinase cascade is a central signaling pathway that is stimulated by various extracellular stimuli. The signals of these stimuli are then transferred by the cascade's components to a large number of targets at distinct subcellular compartments, which in turn induce and regulate a large number of cellular processes. To achieve these functions, the cascade exhibits versatile and dynamic subcellular distribution that allows proper temporal and spatial modulation of the appropriate processes. In this review, we discuss the intracellular localizations of different components of the ERK cascade, and the impact of these localizations on their activation and specificity.

Identification of the B-Raf/Mek/Erk MAP kinase pathway as a target for all-trans retinoic acid during skin cancer promotion

Background

Retinoids have been studied extensively for their potential as therapeutic and chemopreventive agents for a variety of cancers, including nonmelanoma skin cancer (NMSC). Despite their use for many years, the mechanism of action of retinoids in the prevention of NMSC is still unclear. In this study we have attempted to understand the chemopreventive mechanism of all-trans retinoic acid (ATRA), a primary biologically active retinoid, in order to more efficiently utilize retinoids in the clinic.

Results

We have used the 2-stage dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis model to investigate the chemopreventive effects of ATRA. We have compared the gene expression profiles of control skin to skin subjected to the 2-stage protocol, with or without ATRA, using Affymetrix 430 2.0 DNA microarrays. Approximately 49% of the genes showing altered expression with TPA treatment are conversely affected when ATRA is co-administered. The activity of these genes, which we refer to as 'counter-regulated', may contribute to chemoprevention by ATRA. The counter-regulated genes have been clustered into functional categories and bioinformatic analysis has identified the B-Raf/Mek/Erk branch of the MAP kinase pathway as one containing several genes whose upregulation by TPA is blocked by ATRA. We also show that ATRA blocks signaling through this pathway, as revealed by immunohistochemistry and Western blotting. Finally, we found that blocking the B-Raf/Mek/Erk pathway with a pharmacological inhibitor, Sorafenib (BAY43-9006), induces squamous differentiation of existing skin SCCs formed in the 2-stage model.

Conclusion

These results indicate that ATRA targets the B-Raf/Mek/Erk signaling pathway in the 2-stage mouse skin carcinogenesis model and this activity coincides with its chemopreventive action. This demonstrates the potential for targeting the B-Raf/Mek/Erk pathway for chemoprevention and therapy of skin SCC in humans. In addition our DNA microarray results provide the first expression signature for the chemopreventive effect of ATRA in a mouse skin cancer model. This is a potential source for novel targets for ATRA and other chemopreventive and therapeutic agents that can eventually be tested in the clinic.

C-Fos elimination compensates for disabled-2 requirement in mouse extraembryonic endoderm development

Disabled-2 (Dab2) is expressed in primitive endoderm cells as they are differentiating from the inner cell mass and dab2 deficiency in mice results in lethality at E5.5-E6.5 due to the disorganization of the endoderm layers. Here we show that Dab2 suppresses c-Fos expression in endoderm cells. A morphological normal primitive endoderm layer was observed in putative E5.5 dab2 (-/-):c-fos (-/-) embryos, indicating that the primitive endoderm defect due to the loss of Dab2 is rescued by deletion of the c-fos gene. The lethality of the double knockout embryos was delayed until E9.5-E10.5 and the defective embryos failed to undergo organogenesis. We conclude that Dab2 plays a role in epithelial organization by suppression of c-Fos expression and suggest that unsuppressed c-Fos can lead to disruption of primitive endoderm epithelial organization, yet an additional dab2 function is required for later organogenesis.

Actin-targeting natural compounds as tools to study the role of actin cytoskeleton in signal transduction

Actin cytoskeleton controls a vast range of cellular processes such as motility, cytokinesis, differentiation, vesicle transport, phagocytosis, muscle contraction. A growing literature clearly demonstrated that actin cytoskeleton can play a regulating role in several signalling pathways. Cells tightly regulate actin dynamics through numerous specific proteins in order to rapidly and locally respond to various stimuli. An obvious approach to determine the involvement of actin cytoskeleton in signalling pathways is the use of actin-targeting natural compounds. These drugs modulate actin dynamics, accelerating either polymerization or depolymerization, through various mechanisms. This review focus on the use of these actin-targeting drugs as tools to demonstrate the role of actin cytoskeleton in several signal transduction pathways such as those initiated from antigen receptor in T and B cells or those involving mitogen-activated protein kinases (MAPKs) or transcription factors NF-kappaB and SRF (serum response factor). In this last case (SRF), the use of various actin-targeting drugs participated in the elucidation of the molecular mechanism by which actin regulates SRF-mediated transcription.

The effect of actin disrupting agents on contact guidance of human embryonic stem cells

Mammalian cells respond to their substrates by complex changes in gene expression profiles, morphology, proliferation and migration. We report that substrate nanotopography alters morpohology and proliferation of human embryonic stem cells (hESCs). Fibronectin-coated poly(di-methyl siloxane) substrates with line-grating (600nm ridges with 600nm spacing and 600+/-150nm feature height) induced hESC alignment and elongation, mediated the organization of cytoskeletal components including actin, vimentin, and alpha-tubulin, and reduced proliferation. Spatial polarization of gamma-tubulin complexes was also observed in response to nanotopography. Furthermore, the addition of actin disrupting agents attenuated the alignment and proliferative effects of nanotopography. These findings further demonstrate the importance of interplay between cytoskeleton and substrate interactions as a key modulator of morphological and proliferative cellular response in hESCs on nanotopography.

Developmental pluripotency-associated 4 (DPPA4) localized in active chromatin inhibits mouse embryonic stem cell differentiation into a primitive ectoderm lineage

Because embryonic stem (ES) cells can proliferate indefinitely in an undifferentiated state and differentiate into various cell types, ES cells are expected to be useful for cell replacement therapy and basic research on early embryogenesis. Although molecular mechanisms of ES cell self-renewal have been studied, many uncharacterized genes expressed in ES cells remain to be clarified. Developmental pluripotency associated 4 (Dppa4) is one such gene highly expressed in both ES cells and early embryos. Here, we investigated the role of Dppa4 in mouse ES cell self-renewal and differentiation. We generated Dppa4-overexpressing ES cells under the control of tetracycline. Dppa4 overexpression suppressed cell proliferation and formation of embryoid bodies and caused massive cell death in differentiating ES cells. Quantitative reverse transcription-PCR analysis showed that Dppa4 overexpression does not support ES cell self-renewal but partially inhibits ES cell differentiation. Suppression of Dppa4 expression by short hairpin RNA induced ES cell differentiation into a primitive ectoderm lineage. DPPA4 protein was localized in the ES cell nucleus associated with chromatin. Micrococcal nuclease digestion analysis and immunocytochemistry revealed that DPPA4 is associated with transcriptionally active chromatin. These findings indicate that DPPA4 is a nuclear factor associated with active chromatin and that it regulates differentiation of ES cells into a primitive ectoderm lineage.

Nuclear localization of the ERK MAP kinase mediated by Drosophila alphaPS2betaPS integrin and importin-7

The control of gene expression by the mitogen-activated protein (MAP) kinase extracellular signal-regulated kinase (ERK) requires its translocation into the nucleus. In Drosophila S2 cells nuclear accumulation of diphospho-ERK (dpERK) is greatly reduced by interfering double-stranded RNA against Drosophila importin-7 (DIM-7) or by the expression of integrin mutants, either during active cell spreading or after stimulation by insulin. In both cases, total ERK phosphorylation (on Westerns) is not significantly affected, and ERK accumulates in a perinuclear ring. Tyrosine phosphorylation of DIM-7 is reduced in cells expressing integrin mutants, indicating a mechanistic link between these components. DIM-7 and integrins localize to the same actin-containing peripheral regions in spreading cells, but DIM-7 is not concentrated in paxillin-positive focal contacts or stable focal adhesions. The Corkscrew (SHP-2) tyrosine phosphatase binds DIM-7, and Corkscrew is required for the cortical localization of DIM-7. These data suggest a model in which ERK phosphorylation must be spatially coupled to integrin-mediated DIM-7 activation to make a complex that can be imported efficiently. Moreover, dpERK nuclear import can be restored in DIM-7-deficient cells by Xenopus Importin-7, demonstrating that ERK import is an evolutionarily conserved function of this protein.

Catecholaminergic control of mitogen-activated protein kinase signaling in paraventricular neuroendocrine neurons in vivo and in vitro: a proposed role during glycemic challenges

Paraventricular hypothalamic (PVH) corticotropin-releasing hormone (CRH) neuroendocrine neurons mount neurosecretory and transcriptional responses to glycemic challenges [intravenous 2-deoxyglucose (2-DG) or insulin]. Although these responses require signals from intact afferents originating from hindbrain CA (catecholaminergic) neurons, the identity of these signals and the mechanisms by which they are transduced by PVH neurons during glycemic challenge remain unclear. Here, we tested whether the prototypical catecholamine, norepinephrine (NE), can reproduce PVH neuroendocrine responses to glycemic challenge. Because these responses include phosphorylation of p44/42 mitogen-activated protein (MAP) kinases [extracellular signal-regulated kinases 1/2 (ERK1/2)], we also determined whether NE activates ERK1/2 in PVH neurons and, if so, by what mechanism. We show that systemic insulin and 2-DG, and PVH-targeted NE microinjections, rapidly elevated PVH phospho-ERK1/2 levels. NE increased Crh and c-fos expression, together with circulating ACTH/corticosterone. However, because injections also increased c-Fos mRNA in other brain regions, we used hypothalamic slices maintained in vitro to clarify whether NE activates PVH neurons without contribution of inputs from distal regions. In slices, bath-applied NE triggered robust phospho-ERK1/2 immunoreactivity in PVH (including CRH) neurons, which attenuated markedly in the presence of the alpha1 adrenoceptor antagonist, prazosin, or the MAP kinase kinase (MEK) inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene). Therefore, at a systems level, local PVH delivery of NE is sufficient to account for hindbrain activation of CRH neuroendocrine neurons during glycemic challenge. At a cellular level, these data provide the first demonstration that MAP kinase signaling cascades (MEK-->ERK) are intracellular transducers of noradrenergic signals in CRH neurons, and implicate this transduction mechanism as an important component of central neuroendocrine responses during glycemic challenge.

Activation and nuclear translocation of ERK in response to ligand-dependent and -independent stimuli in liver and gill cells from rainbow trout

The mitogen-activated protein kinase ERK is an important signalling molecule involved in the control of cell proliferation, differentiation and cell death, targeting molecules at the cell membrane, in the cytosol, and in the nucleus. This study investigated the activation pattern and subcellular distribution of ERK in liver and gill cells of rainbow trout upon hypo-osmotic shock, addition of epidermal growth factor (EGF) and copper treatment. It further set out to characterize the hypothetical role of nuclear-export signal (NES)-dependent relocation of ERK after nuclear entry and the potential involvement of the ERK activator MEK. Although, in primary hepatocytes, ERK was activated in all conditions in a stimulus-specific manner, it did not accumulate in the nucleus, irrespective of the absence or presence of the inhibitor of NES-dependent export leptomycin B (LB). Similarly, in trout hepatoma cells, where pERK levels increased upon osmotic and mitotic stimulation, but not after toxic insult, no significant nuclear translocation was observed. In a gill cell line, levels of pERK increased after osmotic and mitotic stimulation and showed a decrease during incubation with a toxicant. Again, none of these conditions triggered nuclear accumulation of pERK in the gill cells in the absence of LB, but in contrast to the observation in liver cells, both osmotic and mitotic stimulation caused nuclear accumulation in the presence of the inhibitor. The ERK activator MEK, which possesses a NES-sequence, was apparently not involved in nuclear export, as it did not seem to enter the nucleus. Altogether, ERK is activated in trout cells in a stimulus- and cell type-specific manner, and our data suggest that it acutely acts primarily on cytoplasmic or membrane-situated targets in liver cells, whereas it presumably triggers rapid transcriptional activities in gill cells.

Activation of MAPK pathways links LMNA mutations to cardiomyopathy in Emery-Dreifuss muscular dystrophy

Mutations in LMNA, which encodes nuclear Lamins A and C cause diseases affecting various organs, including the heart. We have determined the effects of an Lmna H222P mutation on signaling pathways involved in the development of cardiomyopathy in a knockin mouse model of autosomal dominant Emery-Dreifuss muscular dystrophy. Analysis of genome-wide expression profiles in hearts using Affymetrix GeneChips showed statistically significant differences in expression of genes in the MAPK pathways at the incipience of the development of clinical disease. Using real-time PCR, we showed that activation of MAPK pathways preceded clinical signs or detectable molecular markers of cardiomyopathy. In heart tissue and isolated cardiomyocytes, there was activation of MAPK cascades and downstream targets, implicated previously in the pathogenesis of cardiomyopathy. Expression of H222P Lamin A in cultured cells activated MAPKs and downstream target genes. Activation of MAPK signaling by mutant A-type lamins could be a cornerstone in the development of heart disease in autosomal dominant Emery-Dreifuss muscular dystrophy.

Receptor heterodimerization leads to a switch in signaling: beta-arrestin2-mediated ERK activation by mu-delta opioid receptor heterodimers

Opiates are analgesics of choice in the treatment of chronic pain, but their long-term use leads to the development of physiological tolerance. Thus, understanding the mechanisms modulating the response of their receptor, the mu opioid receptor (muOR), is of great clinical relevance. Here we show that heterodimerization of muOR with delta opioid receptors (deltaOR) leads to a constitutive recruitment of beta-arrestin2 to the receptor complex resulting in changes in the spatio-temporal regulation of ERK1/2 signaling. The involvement of beta-arrestin2 is further supported by studies using beta-arrestin2 siRNA in cells endogenously expressing the heterodimers. The association of beta-arrestin2 with the heterodimer can be altered by treatment with a combination of muOR agonist (DAMGO) and deltaOR antagonist (Tipp(psi)), and this leads to a shift in the pattern of ERK1/2 phosphorylation to the pattern observed with muOR alone. These data indicate that, in the naive state, muOR-deltaOR heterodimers are in a conformation conducive to beta-arrestin-mediated signaling. Destabilization of this conformation by cotreatment with muOR and deltaOR ligands leads to a switch to a non-beta-arrestin-mediated signaling. Taken together, these results show for the first time that muOR-deltaOR heterodimers, by differentially recruiting beta-arrestin, modulate the spatio-temporal dynamics of opioid receptor signaling.

Mu- and kappa-opioids induce the differentiation of embryonic stem cells to neural progenitors

Growth factors, hormones, and neurotransmitters have been implicated in the regulation of stem cell fate. Since various neural precursors express functional neurotransmitter receptors, which include G protein-coupled receptors, it is anticipated that they are involved in cell fate decisions. We detected mu-opioid receptor (MOR-1) and kappa-opioid receptor (KOR-1) expression and immunoreactivity in embryonic stem (ES) cells and in retinoic acid-induced ES cell-derived, nestin-positive, neural progenitors. Moreover, these G protein-coupled receptors are functional, since [D-Ala(2),MePhe(4),Gly-ol(5)]enkephalin, a MOR-selective agonist, and U69,593, a KOR-selective agonist, induce a sustained activation of extracellular signal-regulated kinase (ERK) signaling throughout a 24-h treatment period in undifferentiated, self-renewing ES cells. Both opioids promote limited proliferation of undifferentiated ES cells via the ERK/MAP kinase signaling pathway. Importantly, biochemical and immunofluorescence data suggest that [D-Ala(2),MePhe(4),Gly-ol(5)]enkephalin and U69,593 divert ES cells from self-renewal and coax the cells to differentiate. In retinoic acid-differentiated ES cells, opioid-induced signaling features a biphasic ERK activation profile and an opioid-induced, ERK-independent inhibition of proliferation in these neural progenitors. Collectively, the data suggest that opioids may have opposite effects on ES cell self-renewal and ES cell differentiation and that ERK activation is only required by the latter. Finally, opioid modulation of ERK activity may play an important role in ES cell fate decisions by directing the cells to specific lineages.

DMRT-1 expression during NEC8 human embryonic carcinoma cell differentiation

To elucidate the relationship between dsx and mab-3 related transcription factor 1 (Dmrt-1) and differentiation, alteration in mRNA levels during differentiation of NEC8 human embryonic carcinoma cells was investigated. After stimulation with 50 nM phorbol 12-myristate 13-acetate (PMA), the cells differentiated into cells with mesenchymal characteristics and upregulated Dmrt-1 mRNA, possibly through the protein kinase C/mitogen-activated protein kinase/activated protein-1 signaling pathway. Conversely, knockdown of Dmrt-1 by small interfering RNA resulted in cell morphology that was different from that after PMA treatment. These results indicated that Dmrt-1 expression was apparently associated with the differentiation of NEC8, and this cell line may be a helpful in vitro tool to clarify the role of Dmrt-1 in the differentiation process.

smoothened and thickveins regulate Moleskin/Importin 7-mediated MAP kinase signaling in the developing Drosophila eye

The Drosophila Mitogen Activated Protein Kinase (MAPK) Rolled is a key regulator of developmental signaling, relaying information from the cytoplasm into the nucleus. Cytoplasmic MEK phosphorylates MAPK (pMAPK), which then dimerizes and translocates to the nucleus where it regulates transcription factors. In cell culture, MAPK nuclear translocation directly follows phosphorylation, but in developing tissues pMAPK can be held in the cytoplasm for extended periods (hours). Here, we show that Moleskin antigen (Drosophila Importin 7/Msk), a MAPK transport factor, is sequestered apically at a time when lateral inhibition is required for patterning in the developing eye. We suggest that this apical restriction of Msk limits MAPK nuclear translocation and blocks Ras pathway nuclear signaling. Ectopic expression of Msk overcomes this block and disrupts patterning. Additionally, the MAPK cytoplasmic hold is genetically dependent on the presence of Decapentaplegic (Dpp) and Hedgehog receptors.

Retinoic acid inhibits airway smooth muscle cell migration

Airway remodeling in chronic asthma is characterized by increased smooth muscle mass that is associated with the reduction of the bronchial lumen as well as airway hyperresponsiveness. The development of agents that inhibit smooth muscle growth is therefore of interest for therapy to prevent asthma-associated airway remodeling. All-trans retinoic acid (ATRA) suppresses growth of vascular smooth muscle cells (SMCs) from the systemic and pulmonary circulation. The present study investigated the effects of ATRA on human bronchial (airway) SMCs. Human bronchial SMCs were found to express mRNAs for retinoic acid receptor (RAR)-alpha, -beta, -gamma, and retinoid X receptor (RXR)-alpha, -beta, but not RXR-gamma. Although ATRA was not effective in inhibiting proliferation or in inducing apoptosis in airway SMCs, we found that ATRA (0.2-2 microM) inhibited the SMC migration in response to platelet-derived growth factor (PDGF), as determined in a modified Boyden chamber assay. Both RAR and RXR agonists also blocked PDGF-induced airway SMC migration. ATRA also inhibited PDGF-induced actin reorganization associated with migration. PDGF-induced actin reorganization and migration were blocked by inhibitors of phosphatidylinositol 3 kinase (PI3K) and Akt. However, migration was blocked by inhibitors of the MEK/ERK pathway, with no effect on cytoskeletal reorganization. ATRA suppressed PDGF-induced Akt activation without influencing ERK activation. RAR was found to form protein-protein interactions with the p85 PI3K subunit. These results suggest that retinoic acid inhibits airway SMC migration through the modulation of the PI3K/Akt pathway.

The duration, magnitude and compartmentalization of ERK MAP kinase activity: mechanisms for providing signaling specificity

ERK MAP kinase signaling plays a pivotal role in diverse cellular functions, including cell proliferation, differentiation, migration and survival. One of the central questions concerning this signaling is how activation of the same protein kinase, ERK, elicits distinct cellular outcomes. Recent progress has demonstrated that differences in the duration, magnitude and subcellular compartmentalization of ERK activity generate variations in signaling output that regulate cell fate decisions. Furthermore, several molecules have been identified as spatial, temporal or strength-controlling regulators of ERK activity. Signaling by various extracellular stimuli thus could be modulated by these regulators to give qualitative and quantitative differences in ERK activity, which are then interpreted by the cells as determinants for appropriate responses.

Ndrg4 enhances NGF-induced ERK activation uncoupled with Elk-1 activation

Ndrg4 is expressed predominantly in the early postnatal rat brain and may be related to neural cell differentiation. PC12 cell lines stably expressing increased levels of Ndrg4 protein display enhanced NGF-induced phosphorylation of MEK and ERK. In contrast, the Ndrg4-C2-overexpressed PC12 cell lines showed attenuated NGF-promoted phosphorylation of Elk-1, which is a nuclear target of ERK. A reporter assay also indicated that Ndrg4-C2 suppresses Elk-1-mediated transcriptional activation and SRE reporter expression. The suppressive effect of Ndrg4-C2 on NGF-induced activation of Elk-1 was abolished by colchicine but not by cytochalasin D, suggesting that microtubules are involved in the reduced activation of Elk-1 by Ndrg4. Ndrg4 may play a role in supporting the activation of ERK and its target proteins needed for neuronal differentiation and in reducing the activation of Elk-1 implicated in cell growth.

Differences between the growth regulatory pathways in primary rat adrenal cells and mouse tumor cell line

In this study, DNA synthesis, phosphorylation of ERK1/2 and CREB proteins, as well as induction of c-Fos protein, were examined in rat adrenocortical, glomerulosa and fasciculata/reticularis cells, as well as in the Y1 cell line. We found that FGF2 was mitogenic only in glomerulosa cells and although ACTH did not activate ERK1/2, it did activate CREB protein, indicating efficient transduction of signals initiated in the ACTH receptors of rat adrenocortical cells. The FGF2 activated ERK1/2 in rat adrenal cells by a mechanism that might be modulated by upstream PKA pathway phosphorylation of MEK and despite the nonmitogenic effect of ACTH on rat adrenal cells it effectively induces c-Fos protein. The results presented herein describe distinct differences between the ACTH and FGF2 signal transduction mechanisms seen in adrenocortical cells and those observed in the Y1 cell line, indicating that, in vitro, ACTH blockage of the mitogenic effect occurs in normal adrenal cells after induction of c-Fos protein.

MAP kinase subcellular localization controls both pattern and proliferation in the developing Drosophila wing

Mitogen-activated protein kinases (MAPKs) phosphorylate target proteins in both the cytoplasm and nucleus, and a strong correlation exists between the subcellular localization of MAPK and resulting cellular responses. It was thought that MAPK phosphorylation was always followed by rapid nuclear translocation. However, we and others have found that MAPK phosphorylation is not always sufficient for nuclear translocation in vivo. In the developing Drosophila wing, MAPK-mediated signaling is required both for patterning and for cell proliferation, although the mechanism of this differential control is not fully understood. Here, we show that phosphorylated MAPK (pMAPK) is held in the cytoplasm in differentiating larval and pupal wing vein cells, and we show that this cytoplasmic hold is required for vein cell fate. At the same time, we show that MAPK does move into the nucleus of other wing cells where it promotes cell proliferation. We propose a novel Ras pathway bifurcation in Drosophila and our results suggest a mechanism by which MAPK phosphorylation can signal two different cellular outcomes (differentiation versus proliferation) based on the subcellular localization of MAPK.

Regulation of CXCR4-Mediated Nuclear Translocation of Extracellular Signal-Related Kinases 1 and 2

Activation of the chemokine receptor CXCR4 by its agonist stromal cell-derived factor 1 (SDF-1) has been associated with cell migration and proliferation in many cell types, but the intracellular signaling cascades are incompletely defined. Here we show that CXCR4-dependent extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation was mediated through the Ras/Raf pathway, as demonstrated with a dominant-negative Ras mutant and pharmacological inhibitors. The Src inhibitor 4-amino-5-methylphenyl-7-(t-butyl)pyrazolo[3,4-d] pyrimidine (PP1) and the Rho-kinase (ROCK) inhibitor N-(4-pyridyl)-4-(1-aminoethyl)cyclohexanecarboxamide dihydrochloride (Y27632) also attenuated SDF-1-induced ERK1/2 phosphorylation. Involvement of Src could furthermore be demonstrated by Src phosphorylation and by the shortened ERK1/2 phosphorylation in SYF cells, which are Src/Yes/Fyn-deficient compared with Src-reconstituted Src(++) cells. Membrane translocation of RhoA could be detected similarly. A large portion of the SDF-1-mediated ERK phosphorylation was detected in the nucleus, as shown by Western blotting and confocal microscopy, and resulted in the phosphorylation of the transcription factor Elk. It is interesting that the nuclear accumulation of ERK1/2 and Elk phosphorylation was completely blocked by dominant-negative Rho, Y27632, PP1, and latrunculin B, indicating that the Rho/ROCK pathway, Src kinase, and the actin cytoskeleton were required in this process. In accordance, neither nuclear ERK phosphorylation nor Elk phosphorylation were observed in SYF cells stimulated with SDF-1 but were reconstituted in Src(++) cells. In summary, these results demonstrate that Src, Rho/ROCK, and an intact cytoskeleton contribute to overall ERK1/2 activation in SDF-1-stimulated cells and are indispensable for nuclear translocation of ERK1/2 and activation of transcription factors.

Three-dimensional nanofibrillar surfaces promote self-renewal in mouse embryonic stem cells

The regulation of mouse embryonic stem cell (mESC) fate is controlled by the interplay of signaling networks that either promote self-renewal or induce differentiation. Leukemia inhibitory factor (LIF) is a cytokine that is required for stem cell renewal in mouse but not in human embryonic stem cells. However, feeder layers of embryonic fibroblasts are capable of inducing stem cell renewal in both cell types, suggesting that the self-renewal signaling pathways may also be promoted by other triggers, such as alternative cytokines and/or chemical or physical properties of the extracellular matrix (ECM) secreted by feeder fibroblasts. We have recently used a synthetic polyamide matrix (Ultra-Web) whose three-dimensional (3D) nanofibrillar organization resembles the ECM/basement membrane. Growth of mESCs on this nanofibrillar surface greatly enhanced proliferation and self-renewal in comparison with growth on tissue culture surfaces without nanofibers, despite the presence of LIF in both systems. Enhanced proliferation and self-renewal of the stem cells on nanofibrillar surfaces were correlated with the activation of the small GTPase Rac, the activation of phosphoinositide 3-kinase (PI3K) pathway, and the enhanced expression of Nanog, a homeoprotein required for maintenance of pluripotency. Inhibitors of PI3K reduced the expression level of Nanog in mESCs cultured on 3D nanofibrillar surfaces. These results provide support for the view that the three-dimensionality of the culture surface may function as a cue for the activation of Rac and PI3K signaling pathways, resulting in stem cell proliferation and self-renewal.

Control of MAP kinase signaling to the nucleus

MAP kinase (MAPK) signaling is among central signaling pathways that regulate cell proliferation, cell differentiation and apoptosis. As MAPK should transmit extracellular signals to proper regions or compartments in cells, controlling subcellular localization of MAPK is important for regulating fidelity and specificity of MAPK signaling. The ERK1/2-type of MAPK is the best characterized member of the MAPK family. In response to extracellular stimulus, ERK1/2 translocates from the cytoplasm to the nucleus by passing through the nuclear pore by several independent mechanisms. Sef (similar expression to fgf genes), a transmembrane protein, has been shown to be a regulator of subcellular distribution of ERK1/2. Sef binds to activated MEK1/2, the specific activator of ERK1/2, and tethers the activated MEK1/2/activated ERK1/2 complex to the Golgi apparatus and the plasma membrane. Thus, Sef blocks ERK1/2 signaling to the nucleus and allows signaling to the cytoplasm. Here we review recent findings on spatial regulation of MAPK, especially on nucleocytoplasmic trafficking of ERK1/2.

Myosin light chain kinase and acto-myosin contractility modulate activation of the ERK cascade downstream of oncogenic Ras

The actin cytoskeleton is recognized as an important component of both adhesion- and growth factor-dependent signaling, but its role in oncogene-dependent signaling has received much less attention. In this study, we investigated the role played by the acto-myosin cytoskeleton and its main regulators, i.e., myosin light chain kinase and Rho kinase, in oncogenic Ki-Ras-induced signaling. We found that activation of the ERK cascade by Ras is dependent on acto-myosin contractility, under the regulation of myosin light chain kinase but not Rho kinase. Inhibition of myosin II or myosin light chain kinase caused a complete loss of ERK phosphorylation in a time- and dose-dependent manner, but proved dispensable for activation of the PI3K pathway. We also provide evidence that the target of myosin light chain kinase lays at the level of Raf activation. Since myosin light chain kinase is a target of ERK, these results suggest a previously uncharacterized signaling pathway involving Ras-mediated alterations of the actin cytoskeleton, which might play a critical role in ERK activation by the Ras oncogene and contribute to aberrant signaling and enhanced cell motility. In addition, restoration of stress fibers following ectopic expression of tropomyosin 2 resulted in reduced levels of ERK phosphorylation. Finally, these studies suggest that myosin light chain kinase but not Rho kinase plays an essential role in the generation of ERK signaling in transformed cells and indicate distinct cellular roles for Rho-kinase and myosin light chain kinase-dependent functions involving the regulation of acto-myosin contractility.

Bisphosphonates and estrogens inhibit osteocyte apoptosis via distinct molecular mechanisms downstream of extracellular signal-regulated kinase activation

Both estrogens and bisphosphonates attenuate osteocyte apoptosis by activating the extracellular signal-regulated kinases (ERKs). However, whereas estrogens activate ERKs via an extranuclear function of the estrogen receptor, bisphosphonates do so by opening connexin 43 hemichannels. Here, we demonstrated that the signaling events downstream of ERKs induced by these two stimuli are also distinct. Inhibition of osteocyte apoptosis by estrogens requires nuclear accumulation of ERKs and activation of downstream transcription factors. On the other hand, anti-apoptosis induced by bisphosphonates requires neither transcription nor ERK-dependent transcription factors. Instead, the effect of bisphosphonates is abolished when ERKs are restricted to the nucleus by blocking CRM1/exportin1-mediated nuclear protein export or by expressing nuclear-anchored ERKs, but it is unaffected in cells expressing cytoplasmic-anchored ERKs. Connexin 43/ERK-mediated anti-apoptosis induced by bisphosphonates requires the kinase activity of the cytoplasmic target of ERKs, p90(RSK), which in turn phosphorylates the pro-apoptotic protein BAD and C/EBPbeta. Phosphorylation of BAD renders it inactive, whereas phosphorylation of C/EBPbeta leads to binding of pro-caspases, thus inhibiting apoptosis independently of the transcriptional activity of this transcription factor. Consistent with the evidence that estrogens and bisphosphonates phosphorylate diverse targets of ERKs, probably resulting from activation of spatially distinct pools of these kinases, the two agents had additive effects on osteocyte survival.

Vinculin modulation of paxillin-FAK interactions regulates ERK to control survival and motility

Cells lacking vinculin are highly metastatic and motile. The reasons for this finding have remained unclear. Both enhanced survival and motility are critical to metastasis. Here, we show that vinculin null (vin-/-) cells and cells expressing a vinculin Y822F mutant have increased survival due to up-regulated activity of extracellular signal-regulated kinase (ERK). This increase is shown to result from vinculin's modulation of paxillin-FAK interactions. A vinculin fragment (amino acids 811-1066) containing the paxillin binding site restored apoptosis and suppressed ERK activity in vin-/- cells. Both vinY822F and vin-/- cells exhibit increased interaction between paxillin and focal adhesion kinase (FAK) and increased paxillin and FAK phosphorylation. Transfection with paxillin Y31FY118F dominant-negative mutant in these cells inhibits ERK activation and restores apoptosis. The enhanced motility of vin-/- and vinY822F cells is also shown to be due to a similar mechanism. Thus, vinculin regulates survival and motility via ERK by controlling the accessibility of paxillin for FAK interaction.