Non-regulated and stimulated mechanisms cooperate in the nuclear accumulation of MEK1

AKTs do not translocate to the nucleus upon stimulation but AKT3 can constitutively signal from the nuclear envelope

AKT is a central signaling protein kinase that plays a role in the regulation of cellular survival metabolism and cell growth, as well as in pathologies such as diabetes and cancer. Human AKT consists of three isoforms (AKT1-3) that may fulfill different functions. Here, we report that distinct subcellular localization of the isoforms directly influences their activity and function. AKT1 is localized primarily in the cytoplasm, AKT2 in the nucleus, and AKT3 in the nucleus or nuclear envelope. None of the isoforms actively translocates into the nucleus upon stimulation. Interestingly, AKT3 at the nuclear envelope is constitutively phosphorylated, enabling a constant phosphorylation of TSC2 at this location. Knockdown of AKT3 induces moderate attenuation of cell proliferation of breast cancer cells. We suggest that in addition to the stimulation-induced activation of the lysosomal/cytoplasmic AKT1-TSC2 pathway, a subpopulation of TSC2 is constitutively inactivated by AKT3 at the nuclear envelope of transformed cells.

LIMK1 nuclear translocation promotes hepatocellular carcinoma progression by increasing p-ERK nuclear shuttling and by activating c-Myc signalling upon EGF stimulation

LIM kinase 1 (LIMK1) is a serine/threonine and tyrosine kinase that is predominantly located in the cytoplasm. In our study, nuclear translocation of LIMK1 in clinical hepatocellular carcinoma (HCC) samples was demonstrated for the first time, especially in samples from those with intravascular tumour thrombus. LIMK1 was overexpressed in HCC tissues, and nuclear LIMK1 expression was associated with poor prognosis in HCC patients. Although the effects of cytoplasmic LIMK1 on cofilin phosphorylation and actin filament dynamics have been well studied, the function of nuclear LIMK1 is still unclear. Gain- and loss-of-function experiments were performed both in vitro and in vivo and demonstrated a correlation between nuclear LIMK1 and the enhanced aggressive phenotype of HCC. EGF could drive the nuclear translocation of LIMK1 by activating the interaction of p-ERK and LIMK1 and facilitating their roles in nuclear shuttling. Moreover, nuclear LIMK1 could directly bind to the promoter region of c-Myc and stimulate c-Myc transcription. Although the EGFR monoclonal antibody cetuximab has a poor therapeutic effect on advanced HCC patients, in vivo animal study showed that cetuximab achieved a significant inhibitory effect on the progression of nuclear LIMK1-overexpressing HCC cells. In addition, recent data have demonstrated the potential of cetuximab in combination therapy for HCC patients with LIMK1 nuclear translocation.

MEK nuclear localization promotes YAP stability via sequestering β-TrCP in KRAS mutant cancer cells

Tumours manage to survive the ablation of mutant KRAS, despite the development of KRAS-targeted drugs. Here we describe that inhibition of mutant KRAS promotes MEK nuclear localization as an alternative mechanism of KRAS-targeted drugs resistance. Tissue microarray analysis in colon tumours shows that aberrant MEK nuclear localization is closely related to YAP levels and tumour malignancy. MEK nuclear localization could sequester β-TrCP from cytoplasmic inactive YAP, then stabilizing YAP. Mutant KRAS restrains MEK within the cytoplasm via IQGAP1, inhibiting MEK nuclear translocation. Trametinib, an allosteric MEK inhibitor, could prevent MEK nuclear localization and subsequently promote YAP degradation. In vitro and in vivo results suggests that inhibition of MEK nuclear localization by trametinib synergizes with KRAS knockdown or deltarasin treatment in suppressing the viability of KRAS mutant colon cancer cells. Our study provides new insights into the mechanisms of resistance to KRAS ablation, and suggests novel strategies for the treatment of KRAS-mutant colon cancers.

HAS2-AS1 is a novel LH/hCG target gene regulating HAS2 expression and enhancing cumulus cells migration

Background

The cumulus expansion process is one of the LH mediated ovulatory processes. Hyaluronan synthase 2 (HAS2) regulates the synthesis of hyaluronic acid, the main component of the cumulus expansion process. Recently, the lncRNA HAS2 antisense RNA 1 (HAS2-AS1) was identified in our global transcriptome RNA-sequencing of novel ovulation associated genes. The role of HAS2-AS1 in HAS2 regulation w.as studied previously with contradictive results in different models but not in the ovary. Taken together the induction of HAS2-AS1 and the important role of HAS2 in the cumulus expansion process, we hypothesize that HAS2-AS1 regulate HAS2 expression and function in the ovary. Therefore we undertook to study the expression, regulation, and possible functional role of HAS2-AS1 in the human ovary.

Results

HAS2-AS1, located within the HAS2 gene that was highly regulated in our library. We found that HAS2-AS1 express mainly in cumulus cells (CCs). Furthermore, HAS2-AS1 showed low expression in immature CCs and a significant increase expression in mature CCs. Functional studies reveal that inhibition of HAS2-AS1 by siRNA caused decrease expression of HAS2. Furthermore, inhibition of HAS2-AS1 by siRNA results in decrease migration of granulosa cells.

Conclusions

Our results suggest that HAS2-AS1 is an LH/hCG target gene that plays a positive role in HAS2 expression and thus might play a role in regulating cumulus expansion and migration.

Clinical resistance associated with a novel MAP2K1 mutation in a patient with Langerhans cell histiocytosis

Patients with Langerhans cell histiocytosis (LCH) harbor BRAF V600E and activating mutations of MAP2K1/MEK1 in 50% and 25% of cases, respectively. We evaluated a patient with treatment-refractory LCH for mutations in the RAS-RAF-MEK-ERK pathway and identified a novel mutation in the MAP2K1 gene resulting in a p.L98_K104 > Q deletion and predicted to be auto-activating. During treatment with the MEK inhibitor trametinib, the patient's disease showed significant progression. In vitro characterization of the MAP2K1 p.L98_K104 > Q deletion confirmed its effect on cellular activation of the ERK pathway and drug resistance.

Intrinsically active MEK variants are differentially regulated by proteinases and phosphatases

MAPK/ERK kinase (MEK) 1/2 are central signaling proteins that serve as specificity determinants of the MAPK/ERK cascade. More than twenty activating mutations have been reported for MEK1/2, and many of them are known to cause diseases such as cancers, arteriovenous malformation and RASopathies. Changes in their intrinsic activity do not seem to correlate with the severity of the diseases. Here we studied four MEK1/2 mutations using biochemical and molecular dynamic methods. Although the studied mutants elevated the activating phosphorylation of MEK they had no effect on the stimulated ERK1/2 phosphorylation. Studying the regulatory mechanism that may explain this lack of effect, we found that one type of mutation affects MEK stability and two types of mutations demonstrate a reduced sensitivity to PP2A. Together, our results indicate that some MEK mutations exert their function not only by their elevated intrinsic activity, but also by modulation of regulatory elements such as protein stability or dephosphorylation.

The nuclear translocation of the kinases p38 and JNK promotes inflammation-induced cancer

The stimulated nuclear translocation of signaling proteins, such as MAPKs, is a necessity for the initiation and regulation of their physiological functions. Previously, we determined that nuclear translocation of the MAPKs p38 and JNK involves binding to heterodimers comprising importin 3 and either importin 7 or importin 9. Here, we identified the importin-binding region in p38 and JNK and developed a myristoylated peptide targeting this site that we called PERY. The PERY peptide specifically blocked the interaction of p38 and JNK with the importins, restricted their nuclear translocation, and inhibited phosphorylation of their nuclear (but not cytoplasmic) substrates. Through these effects, the PERY peptide reduced the proliferation of several (but not all) cancer cell lines in culture and inhibited the growth of a human breast cancer xenograft in mice. In addition, the PERY peptide substantially inhibited inflammation in mice, as manifested in models of colitis and colitis-associated colon cancer. The PERY peptide more effectively prevented colon cancer development than did a commercial p38 inhibitor. In vivo analysis further suggested that this effect was mediated by PERY peptide-induced prevention of the nuclear translocation of p38 in macrophages. Together, these results support the use of the nuclear translocation of p38 and JNK as a novel drug target to treat various cancers and inflammation-induced diseases.

Molecular effectors in the chronic exposure to arsenic as early and sensitive biomarkers in developing Rhinella arenarum toads

Arsenic, a natural element of ecological relevance, is one of the most toxic elements present in various regions of the world. It can be found in natural water sources throughout Argentina in concentrations between 0.01 and 15mgL^-1. The Argentinean autochthonous toad Rhinella arenarum was selected to study the molecular mechanisms involved in the effects and response to the chronic As exposure along its embryonic and larval development. We evaluated the effects on MAPK signal transduction pathway and transcription factors c-FOS and c-JUN, and the regulation of the expression at protein levels of different antioxidant enzymes. Our results indicated that As is modulating the MAPK pathway, increasing MEK and ERK levels both in the nuclear and post-nuclear fraction along the embryonic development and mainly at the beginning of the larval stage. Through this pathway, As can upregulate transcription factors like c-FOS and c-JUN, impacting the antioxidant response of the exposed embryos and larvae through antioxidant enzymes and recycling of GSH. Arsenic triggered specifically the synthesis of antioxidant enzymes in exposed R. arenarum embryo and larvae. In particular, the expression levels of SOD, CAT and GST enzymes analyzed by Western blot showed a similar behavior to their enzymatic activities in our previous work. This fact suggests that not only the synthesis of these antioxidant enzymes but also their rapid degradation after inactivation would be regulated in response to ROS levels. Antioxidant enzymes may show dual responses of induction and inactivation followed by degradation depending on the levels of oxidative stress and impact on ROS targets when the exposure is sustained in time and intensity. We also performed a probability of exceedence analysis including our previous results to visualize a progression of the response in time and also established the best early-responding biomarkers at the lowest As concentrations. As a conclusion, the molecular biomarkers such as the MAPKs MEK and ERK and transcription factors c-FOS and c-JUN are early induced in the response of developing toad embryos exposed to very low As concentrations in water. The advantage of counting with molecular biomarkers early responding to low concentrations of As in a chronic exposure is that they may anticipate the irreversible damage at later developmental stages due to the constant oxidative challenge.

Novel Reporter for Faithful Monitoring of ERK2 Dynamics in Living Cells and Model Organisms

Uncoupling of ERK1/2 phosphorylation from subcellular localization is essential towards the understanding of molecular mechanisms that control ERK1/2-mediated cell-fate decision. ERK1/2 non-catalytic functions and discoveries of new specific anchors responsible of the subcellular compartmentalization of ERK1/2 signaling pathway have been proposed as regulation mechanisms for which dynamic monitoring of ERK1/2 localization is necessary. However, studying the spatiotemporal features of ERK2, for instance, in different cellular processes in living cells and tissues requires a tool that can faithfully report on its subcellular distribution. We developed a novel molecular tool, ERK2-LOC, based on the T2A-mediated coexpression of strictly equimolar levels of eGFP-ERK2 and MEK1, to faithfully visualize ERK2 localization patterns. MEK1 and eGFP-ERK2 were expressed reliably and functionally both in vitro and in single living cells. We then assessed the subcellular distribution and mobility of ERK2-LOC using fluorescence microscopy in non-stimulated conditions and after activation/inhibition of the MAPK/ERK1/2 signaling pathway. Finally, we used our coexpression system in Xenopus laevis embryos during the early stages of development. This is the first report on MEK1/ERK2 T2A-mediated coexpression in living embryos, and we show that there is a strong correlation between the spatiotemporal subcellular distribution of ERK2-LOC and the phosphorylation patterns of ERK1/2. Our approach can be used to study the spatiotemporal localization of ERK2 and its dynamics in a variety of processes in living cells and embryonic tissues.

Nuclear MEK1 sequesters PPARγ and bisects MEK1/ERK signaling: a non-canonical pathway of retinoic acid inhibition of adipocyte differentiation

Uncontrolled adipogenesis and adipocyte proliferation have been connected to human comorbidities. Retinoic acid (RA) is known to inhibit adipocyte differentiation, however the underlying mechanisms have not been adequately understood. This study reports that RA acting as a ligand to RA receptors (RARs and RXRs) is not a sine qua non to the inhibition of adipogenesis. Our intriguing observation of a negative correlation between increased retinoylation and adipogenesis led us to explore retinoylated proteins in adipocytes. Exportin (CRM1) was found to be retinoylated, which in turn can affect the spatio-temporal regulation of the important signaling molecule mitogen-activated protein kinase kinase 1 (MEK1), likely by disrupting its export from the nucleus. Nuclear enrichment of MEK1 physically sequesters peroxisome proliferator-activated receptor gamma (PPARγ), the master regulator of adipogenesis, from its target genes and thus inhibits adipogenesis while also disrupting the MEK1-extracellular-signal regulated kinase (ERK) signaling cascade. This study is first to report the inhibition of adipocyte differentiation by retinoylation.

Small GTPase RBJ mediates nuclear entrapment of MEK1/MEK2 in tumor progression

Ras-related small GTPases play important roles in cancer. However, the roles of RBJ, a representative of the sixth subfamily of Ras-related small GTPases, in tumorigenesis and tumor progression remain unknown. Here, we report that RBJ is dysregulated in human gastrointestinal cancers and can promote carcinogenesis and tumor progression via nuclear entrapment of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK)1/MEK2 and activation of ERK1/ERK2. Nucleus-localized RBJ interacts with MEK/ERK and prolongs the duration of MEK/ERK activation. Rbj deficiency abrogates nuclear accumulation of MEK1/MEK2, attenuates ERK1/ERK2 activation, and impairs AOM/DSS-induced colonic carcinogenesis. Moreover, Rbj knockdown inhibits growth of established tumors. Our data suggest that RBJ may be an oncogenic Ras-related small GTPase mediating nuclear accumulation of active MEK1/MEK2 in tumor progression.

Stimulated nuclear import by β-like importins

Classic nuclear shuttling is mediated by an importin-α∙β heterodimer that binds to cargoes containing a nuclear localization signal, and shuttles most nuclear proteins immediately after their translation. Aside from this canonical mechanism, kariopheryn-βs or β-like importins operate by binding to non-canonical nuclear localization signals to mediate translocation without the assistance of importin-α. The mechanism by which these components operate is much less understood and is currently under investigation. Recently, several β-like importins have been implicated in the stimulated nuclear translocation of signaling proteins. Here, we propose that this group of importins might be responsible for the swift nuclear shuttling of many proteins following various stimuli.

High-Content Screening of Natural Products Reveals Novel Nuclear Export Inhibitors

Natural products are considered an extremely valuable source for the discovery of new drugs against diverse pathologies. As yet, we have only explored a fraction of the diversity of bioactive compounds, and opportunities for discovering new natural products leading to new drugs are huge. In the present study, U2nesRELOC, a previously established cell-based imaging assay, was employed to screen a collection of extracts of microbial origin for nuclear export inhibition activity. The fluorescent signal of untreated U2nesRELOC cells localizes predominantly to the cytoplasm. Upon treatment with the nuclear export inhibitor leptomycin B, the fluorescent-tagged reporter proteins appear as speckles in the nucleus. A proprietary collection of extracts from fungi, actinomycetes, and unicellular bacteria that covers an uncommonly broad chemical space was used to interrogate this nuclear export assay system. A two-step image-based analysis allowed us to identify 12 extracts with biological activities that are not associated with previously known active metabolites. The fractionation and structural elucidation of active compounds revealed several chemical structures with nuclear export inhibition activity. Here we show that substrates of the nuclear export receptor CRM1, such as Rev, FOXO3a and NF-κB, accumulate in the nucleus in the presence of the fungal metabolite MDN-0105 with an IC50 value of 3.4 µM. Many important processes in tumor formation and progression, as well as in many viral infections, critically depend on the nucleocytoplasmic trafficking of proteins and RNA molecules. Therefore, the disruption of nuclear export is emerging as a novel therapeutic approach with enormous clinical potential. Our work highlights the potential of applying high-throughput phenotypic imaging on natural product extracts to identify novel nuclear export inhibitors.

Sumoylation of SAE2 C terminus regulates SAE nuclear localization

SUMOylation occurs predominantly in the nucleus, but non-nuclear proteins can also be SUMOylated. It is unclear how intracellular trafficking of the SUMOylation enzymes is regulated to catalyze SUMOylation in different cellular compartments. Here we report that the SAE2 subunit of human SUMO activation enzyme (SAE) underwent rapid nucleocytoplasmic shuttling and its nuclear accumulation depended on SUMO modification at the C terminus. The SUMOylation sites included three Lys residues on the bipartite nuclear localization sequence (NLS) and two Lys residues outside of but adjacent to the NLS, and their SUMOylation was catalyzed by Ubc9. Because SAE2 forms a tight heterodimer with SAE1 and it controls the trafficking of the heterodimer, this study has identified the mechanism used to localize SAE to the nucleus. Similar mechanisms are likely to exist for other proteins that depend on SUMOylation for nuclear localization.

Sef downregulation by Ras causes MEK1/2 to become aberrantly nuclear localized leading to polyploidy and neoplastic transformation

Subcellular trafficking of key oncogenic signal pathway components is likely to be crucial for neoplastic transformation, but little is known about how such trafficking processes are spatially controlled. In this study, we show how Ras activation causes aberrant nuclear localization of phosphorylated mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK; MEK) MEK1/2 to drive neoplastic transformation. Phosphorylated MEK1/2 was aberrantly located within the nucleus of primary colorectal tumors and human colon cancer cells, and oncogenic activation of Ras was sufficient to induce nuclear accumulation of phosphorylated MEK1/2 and ERK1/2 in intestinal epithelial cells. Enforced nuclear localization of MEK1 in epithelial cells or fibroblasts was sufficient for hyperactivation of ERK1/2, thereby driving cell proliferation, chromosomal polyploidy, and tumorigenesis. Notably, Ras-induced nuclear accumulation of activated MEK1/2 was reliant on downregulation of the spatial regulator Sef, the reexpression of which was sufficient to restore normal MEK1/2 localization and a reversal of Ras-induced proliferation and tumorigenesis. Taken together, our findings indicate that Ras-induced downregulation of Sef is an early oncogenic event that contributes to genetic instability and tumor progression by sustaining nuclear ERK1/2 signaling.

Insulin-like growth factor 1 attenuates antiestrogen- and antiprogestin-induced apoptosis in ER+ breast cancer cells by MEK1 regulation of the BH3-only pro-apoptotic protein Bim

Introduction

In this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF). Our goal was to identify new molecular targets to improve the efficacy of hormonal therapy in breast cancer patients that have a poor response to hormonal therapy, in part, due to high circulating levels of unbound insulinIGF-1.

Methods

IGF-1-mediated effects on cytostasis and apoptotic cell death were determined with cell counts conducted in the presence and absence of trypan blue; enzyme-linked immunosorbent assays to determine the intracellular levels of cleaved cytokeratin 18, a marker of epithelial cancer cell apoptosis; and immunoblot analysis to determine the levels of cleaved poly-ADP ribose polymerase (PARP) and lamin A that result from caspase-dependent apoptosis. Cytotoxicity was further characterized by determination of the levels of reactive oxygen species (ROS) and the percent of mitochondrial membrane depolarization in cell populations treated with the different hormones in the presence and absence of IGF-1. Small molecule inhibitors of the dual-specificity protein kinase MEK1, MEK1 siRNA, Bim siRNA, and vectors overexpressing MEK1 wild type and mutant, dominant negative cDNA were used to identify key IGF-1 downstream prosurvival effectors.

Results

IGF-1, at physiologically relevant levels, blocked the cytotoxic action(s) of the antiestrogens 4-hydroxytamoxifen (4-OHT) and tamoxifen (TAM) when used as single agents or in combination with the antiprogestin MIF. The antiapoptotic action of IGF-1 was mediated primarily through the action of MEK1. MEK1 expression reduced the levels of ROS and mitochondrial membrane depolarization induced by the hormonal treatments via a mechanism that involved the phosphorylation and proteasomal turnover of the proapoptotic BH3-only Bcl-2 family member Bim. Importantly, small-molecule inhibitors of MEK1 circumvented the prosurvival action of IGF-1 by restoring Bim to levels that more effectively mediated apoptosis in ER⁺ breast cancer cells.

Conclusion

his study provides strong support for the use of MEK1 inhibitors in combination with hormonal therapy to effectively affect cytostasis and activate a Bim-dependent apoptotic pathway in ER⁺ breast cancer cells. We discuss that MEK1 blockade may be a particularly effective treatment for women with high circulating levels of IGF-1, which have been correlated to a poor prognosis.

New algorithm to determine true colocalization in combination with image restoration and time-lapse confocal microscopy to MAP kinases in mitochondria

The subcellular localization and physiological functions of biomolecules are closely related and thus it is crucial to precisely determine the distribution of different molecules inside the intracellular structures. This is frequently accomplished by fluorescence microscopy with well-characterized markers and posterior evaluation of the signal colocalization. Rigorous study of colocalization requires statistical analysis of the data, albeit yet no single technique has been established as a standard method. Indeed, the few methods currently available are only accurate in images with particular characteristics. Here, we introduce a new algorithm to automatically obtain the true colocalization between images that is suitable for a wide variety of biological situations. To proceed, the algorithm contemplates the individual contribution of each pixel's fluorescence intensity in a pair of images to the overall Pearsońs correlation and Manders' overlap coefficients. The accuracy and reliability of the algorithm was validated on both simulated and real images that reflected the characteristics of a range of biological samples. We used this algorithm in combination with image restoration by deconvolution and time-lapse confocal microscopy to address the localization of MEK1 in the mitochondria of different cell lines. Appraising the previously described behavior of Akt1 corroborated the reliability of the combined use of these techniques. Together, the present work provides a novel statistical approach to accurately and reliably determine the colocalization in a variety of biological images.

The MAPK cascades: signaling components, nuclear roles and mechanisms of nuclear translocation

The MAPK cascades are central signaling pathways that regulate a wide variety of stimulated cellular processes, including proliferation, differentiation, apoptosis and stress response. Therefore, dysregulation, or improper functioning of these cascades, is involved in the induction and progression of diseases such as cancer, diabetes, autoimmune diseases, and developmental abnormalities. Many of these physiological, and pathological functions are mediated by MAPK-dependent transcription of various regulatory genes. In order to induce transcription and the consequent functions, the signals transmitted via the cascades need to enter the nucleus, where they may modulate the activity of transcription factors and chromatin remodeling enzymes. In this review, we briefly cover the composition of the MAPK cascades, as well as their physiological and pathological functions. We describe, in more detail, many of the important nuclear activities of the MAPK cascades, and we elaborate on the mechanisms of ERK1/2 translocation into the nucleus, including the identification of their nuclear translocation sequence (NTS) binding to the shuttling protein importin7. Overall, the nuclear translocation of signaling components may emerge as an important regulatory layer in the induction of cellular processes, and therefore, may serve as targets for therapeutic intervention in signaling-related diseases such as cancer and diabetes. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Insulin-like growth factor-I activates extracellularly regulated kinase to regulate the p450 side-chain cleavage insulin-like response element in granulosa cells

IGF regulates steroidogenesis in granulosa cells through expression of the cytochrome P450 side-chain cleavage enzyme (P450scc) (CYP11A1), the rate-limiting enzyme in this biosynthetic process. We showed previously that the polypyrimidine tract-binding protein-associated splicing factor (PSF) acts as a repressor, whereas Sp1 is an activator, of P450 gene expression. The aim of the present study was to investigate IGF-stimulated ERK signaling regulating P450scc gene expression in the immortalized porcine granulosa cell line JC-410. We used a reporter gene under control of the IGF response element from the P450scc promoter. Inhibition of ERK phosphorylation with U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] blocked IGF-I induction of IGF response element reporter gene activity. Western blotting revealed that IGF-I treatment resulted in phosphorylation of ERK that was specifically inhibited by U0126. ERK activation led to phosphorylation of T739 (an ERK site) on Sp1 that was diminished by U0126 or overexpression of PSF. Coimmunoprecipitation and Western blotting of nuclear extracts showed that phosphorylated ERK (pERK) bound PSF under basal conditions. IGF-I caused dissociation of pERK from PSF. Finally, chromatin immunoprecipitation analysis showed that PSF and Sp1 constitutively occupy the P450scc promoter independent of IGF-I treatment. These events provide a potential molecular mechanism for release of PSF repression of P450scc expression by dissociation of pERK and subsequent pERK-mediated phosphorylation of Sp1 to drive transcriptional induction of the P450scc gene in the absence of altered binding of PSF or Sp1 to the promoter. Understanding IGF-I regulation of these critical ovarian signaling pathways is the first step to delineating ovarian hyperstimulation syndromes such as polycystic ovarian syndrome.

The subcellular localization of MEK and ERK--a novel nuclear translocation signal (NTS) paves a way to the nucleus

The ERK cascade is a central signaling pathway that regulates a large number of intracellular processes including proliferation, differentiation, development and also survival or apoptosis. The induction of so many distinct and even opposing cellular processes raises the question as to how the signaling specificity of the cascade is regulated. In the past few years, subcellular localization of components of the ERK cascade was shown to play an important role in specificity determination. Here we describe the dynamic subcellular localization of Raf kinases, MEKs, and particularly ERKs, which translocate into the nucleus during many cellular processes to induce transcription. We also describe in details the recent identification of a novel nuclear translocation mechanism for ERKs, which is based on a nuclear translocation sequence (NTS) within their kinase insert domain (KID). Phosphorylation of this domain, mainly upon stimulation, allows ERKs to interact with the nuclear importing protein - importin7, which mediates the penetration of the interacting ERKs into the nucleus via nuclear pores. Interestingly, the NTS is not specific to ERKs, and seems to be a general signal for regulating nuclear accumulation of various proteins, including MEKs, upon their stimulation. Better understanding of this mechanism may clarify the role of the massive nuclear translocation of many regulatory proteins shortly after their stimulation.

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.

Specific phosphorylation and activation of ERK1c by MEK1b: a unique route in the ERK cascade

Extracellular signal-regulated kinases (ERKs) are key signaling molecules that regulate a large number of cellular processes, including mitosis. We showed previously that ERK1c, an alternatively spliced form of ERK1, facilitates mitotic Golgi fragmentation without the involvement of ERK1 and ERK2. Here we demonstrate that activation of ERK1c is mainly mediated by mitogen-activated protein kinase (MAPK)/ERK kinase 1b (MEK1b), which is an alternatively spliced form of MEK1 that was previously considered an inactive kinase. MEK1b phosphorylation and activity are preferentially stimulated by nocodazole, to induce its specific activity toward ERK1c. MEK1/2, on the other hand, preferentially target ERK1/2 in response to growth factors, such as EGF. As previously demonstrated for ERK1c, also MEK1b expression and activity are elevated during mitosis, and thereby enhance Golgi fragmentation and mitotic rate. MEK1 activity is also increased during mitosis, but this isoform facilitates mitotic progression without affecting the Golgi architecture. These results illustrate that the ERK cascade is divided into two routes: the classic MEK1/2-ERK1/2 and the splice-variant MEK1b-ERK1c, each of which regulates distinct cellular processes and thus extends the cascade specificity.

How ERK1/2 activation controls cell proliferation and cell death: Is subcellular localization the answer?

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) are members of the mitogen-activated protein kinase super family that can mediate cell proliferation and apoptosis. The Ras-Raf-MEK-ERK signaling cascade controlling cell proliferation has been well studied but the mechanisms involved in ERK1/2-mediated cell death are largely unknown. This review focuses on recent papers that define ERK1/2 translocation to the nucleus and the proteins involved in the cytosolic retention of activated ERK1/2. Cytosolic retention of ERK1/2 denies access to the transcription factor substrates that are responsible for the mitogenic response. In addition, cytosolic ERK1/2, besides inhibiting survival and proliferative signals in the nucleus, potentiates the catalytic activity of some proapoptotic proteins such as DAP kinase in the cytoplasm. Studies that further define the function of cytosolic ERK1/2 and its cytosolic substrates that enhance cell death will be essential to harness this pathway for developing effective treatments for cancer and chronic inflammatory diseases.

Nuclear import of Pin1 is mediated by a novel sequence in the PPIase domain

Pin1 actively regulates diverse biological/pathological processes, but little is known about the regulatory mechanisms of its cellular localization. In this study, we report that the endogenous Pin1 is distributed in both nucleus and cytoplasm. We found that point mutations of several basic amino acids in the PPIase domain of Pin1 significantly compromise its nuclear localization. Such inhibition is independent of Pin1 enzymatic activity, and is mainly due to the defects in the nuclear import. A novel sequence harboring these residues was identified as a putative nuclear localization signal (NLS) of Pin1. Importin alpha5 of the nuclear import machinery was found to interact with Pin1.

Identification and characterization of a general nuclear translocation signal in signaling proteins

Upon stimulation, many proteins translocate into the nucleus in order to regulate a variety of cellular processes. The mechanism underlying the translocation is not clear since many of these proteins lack a canonical nuclear localization signal (NLS). We searched for an alternative mechanism in extracellular signal-regulated kinase (ERK)-2 and identified a 3 amino acid domain (SPS) that is phosphorylated upon stimulation to induce nuclear translocation of ERK2. A 19 amino acid stretch containing this phosphorylated domain inserts nondiffusible proteins to the nucleus autonomously. The phosphorylated SPS acts by binding to importin7 and the release from nuclear pore proteins. This allows its functioning both in passive and active ERK transports. A similar domain appears in many cytonuclear shuttling proteins, and we found that phosphorylation of similar sequences in SMAD3 or MEK1 also induces their nuclear accumulation. Therefore, our findings show that this phosphorylated domain acts as a general nuclear translocation signal (NTS).

Role of Non-phosphorylated Activation Loop Residues in Determining ERK2 Dephosphorylation, Activity, and Subcellular Localization

Extracellular signal-regulated kinases (ERKs) activity is regulated by MAPK/ERK kinases (MEKs), which phosphorylate the regulatory Tyr and Thr residues in ERKs activation loop, and by various phosphatases that remove the incorporated phosphates. Although the role of the phosphorylated residues in the activation loop of ERKs is well studied, much less is known about the role of other residues within this loop. Here we substituted several residues within amino acids 173-177 of ERK2 and studied their role in ERK2 phosphorylation, substrate recognition, and subcellular localization. We found that substitution of residues 173-175 and particularly Pro(174) to alanines reduces the EGF-induced ERK2 phosphorylation, without modifying its in vitro phosphorylation by MEK1. Examining the ability of these mutants to be dephosphorylated revealed that 173-5A mutants are hypersensitive to phosphatases, indicating that these residues are important for setting the phosphorylation/dephosphorylation balance of ERKs. In addition, 173-5A mutants reduced ERK2 activity toward Elk-1, without affecting the activity of ERK2 toward MBP, while substitution of residues 176-8 decreased ERK2 activity toward both substrates. Substitution of Asp(177) to alanine increased nuclear localization of the construct in MEK1-overexpressing cells, suggesting that this residue together with His(176) is involved in the dissociation of ERK2 from MEKs. Combining CRS/CD motif and the activation loop mutations revealed that these two regions cooperate in determining the net phosphorylation of ERK2, but the role of the CRS/CD motif predominates that of the activation loop residues. Thus, we show here that residues 173-177 of ERK2 join other regulatory regions of ERKs in governing ERK activity.

An HTS Approach to Screen for Antagonists of the Nuclear Export Machinery Using High Content Cell-Based Assays

Intracellular localization is essential for the regulated activity of many signaling molecules associated with disease-relevant pathways. High content screening is a powerful technology to monitor the impact of small molecules or interfering RNAs on translocation of proteins within intact cells. Several assays have been developed to measure the nucleocytoplasmic shuttling of proteins like nuclear factor kappaB, FoxO, or nuclear factor of activated T-cells involved in distinct signaling networks. However, since all these proteins bear a leucine-rich nuclear export signal (NES), modulators of the NES-dependent export machinery can lead to misinterpretation of the assay readout. Here we report the generation of U2nesRELOC, a cell-based system for the identification of nuclear export inhibitors and specific silencers of the nuclear export machinery, and its adaptation to high throughput screening. The assay is based on mammalian cells stably expressing green fluorescent protein (GFP)-labeled Rev protein, which contains a strong heterologous NES. The fluorescent signal of untreated U2nesRELOC cells localizes exclusively to the cytoplasm. Upon treatment with the nuclear export inhibitor leptomycin B the GFP-labeled reporter protein accumulates rapidly in the cell nucleus. The assay has been adapted to 96-multiwell format and fully automated. Pilot experiments with a panel of 50 test compounds using three different concentrations per compound resulted in very consistent data sets with excellent reproducibility and an average Z' value of 0.76. In summary, U2nesRELOC is a cell-based nuclear export assay suitable for high throughput screening, providing counterscreens for pathway deconvolution.

Interaction with MEK causes nuclear export and downregulation of peroxisome proliferator-activated receptor gamma

The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) cascade plays a central role in intracellular signaling by many extracellular stimuli. One target of the ERK cascade is peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor that promotes differentiation and apoptosis. It was previously demonstrated that PPARgamma activity is attenuated upon mitogenic stimulation due to phosphorylation of its Ser84 by ERKs. Here we show that stimulation by tetradecanoyl phorbol acetate (TPA) attenuates PPARgamma's activity in a MEK-dependent manner, even when Ser84 is mutated to Ala. To elucidate the mechanism of attenuation, we found that PPARgamma directly interacts with MEKs, which are the activators of ERKs, but not with ERKs themselves, both in vivo and in vitro. This interaction is facilitated by MEKs' phosphorylation and is mediated by the basic D domain of MEK1 and the AF2 domain of PPARgamma. Immunofluorescence microscopy and subcellular fractionation revealed that MEK1 exports PPARgamma from the nucleus, and this finding was supported by small interfering RNA knockdown of MEK1 and use of a cell-permeable interaction-blocking peptide, which prevented TPA-induced export of PPARgamma from the nucleus. Thus, we show here a novel mode of downregulation of PPARgamma by its MEK-dependent redistribution from the nucleus to the cytosol. This unanticipated role for the stimulation-induced nuclear shuttling of MEKs shows that MEKs can regulate additional signaling components besides the ERK cascade.

The MEK/ERK cascade: from signaling specificity to diverse functions

The ERK signaling cascade is a central MAPK pathway that plays a role in the regulation of various cellular processes such as proliferation, differentiation, development, learning, survival and, under some conditions, also apoptosis. The ability of this cascade to regulate so many distinct, and even opposing, cellular processes, raises the question of signaling specificity determination by this cascade. Here we describe mechanisms that cooperate to direct MEK-ERK signals to their appropriate downstream destinations. These include duration and strength of the signals, interaction with specific scaffolds, changes in subcellular localization, crosstalk with other signaling pathways, and presence of multiple components with distinct functions in each tier of the cascade. Since many of the mechanisms do not function properly in cancer cells, understanding them may shed light not only on the regulation of normal cell proliferation, but also on mechanisms of oncogenic transformation.

MAPK signalling: ERK5 versus ERK1/2

Extracellular-signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase (MAPK) family and, similar to ERK1/2, has the Thr-Glu-Tyr (TEY) activation motif. Both ERK5 and ERK1/2 are activated by growth factors and have an important role in the regulation of cell proliferation and cell differentiation. Moreover, both the ERK5 and the ERK1/2 pathways are sensitive to PD98059 and U0126, which are two well-known inhibitors of the ERK pathway. Despite these similarities, recent studies have revealed distinctive features of the ERK5 pathway: ERK5 has a key role in cardiovascular development and neural differentiation; ERK5 nuclear translocation is controlled by its own nuclear localizing and nuclear export activities; and the carboxy-terminal half of ERK5, which follows its kinase catalytic domain, has a unique function.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

The nuclear localization of ERK2 occurs by mechanisms both independent of and dependent on energy

The mitogen-activated protein (MAP) kinases ERK1 and ERK2 often accumulate in the nuclei of stimulated cells to mediate changes in transcription. The mechanisms underlying stimulus-dependent redistribution of these kinases remain unclear. We have used a permeabilized cell reconstitution assay in HeLa cells and human foreskin fibroblasts to explore the processes by which ERK2 enters and exits the nucleus. We previously reported that entry of unphosphorylated ERK2 into the nucleus occurs by facilitated diffusion not requiring cytosolic transport factors. We find that export, like import, can occur by an energy- and carrier-independent mechanism. An energy-dependent mechanism of ERK2 export can also be distinguished, mediated at least in part through the exportin CRM1. We have also examined import and export of thiophosphorylated, active ERK2. Import of active ERK2 is significantly enhanced by the addition of exogenous transport factors and an energy regeneration system. These studies support a model in which multiple constitutive and regulated processes control the subcellular distribution of ERK2.

Adhesion-dependent redistribution of MAP kinase and MEK promotes muscarinic receptor-mediated signaling to the nucleus

The mitogen-activated protein kinases (MAPKs) are activated by extracellular signals, and translocate to the nucleus where they modulate transcription. Integrin-mediated cell adhesion to extracellular matrix (ECM) proteins is required for efficient transmission of MAPK-based signals initiated by growth factors. However, the modulation of G protein-coupled receptor (GPCR) signaling by adhesion is less well understood. In the present study, we assessed the impact of cell adhesion on MAPK activation by muscarinic M3 receptors. The muscarinic agonist carbachol more efficiently promoted stress fiber formation and tyrosine phosphorylation of focal adhesion-associated proteins in M3 receptor-expressing cells adherent to fibronectin or collagen type I, as compared to polylysine. Overall MAPK activation was robust in cells adherent to all three substrata. However, total levels of MAPK and mitogen-activated protein kinase kinase (MEK) in the nucleus were significantly greater in cells adherent to ECM proteins for 2.5 h, and levels of activated MAPK and MEK in the nuclei of these cells were higher following carbachol stimulation, relative to levels in cells adherent to polylysine. MEK inhibitors did not prevent adhesion-dependent translocation of MAPK and MEK to the nucleus, and increased nuclear phospho-MEK levels in carbachol-stimulated cells. The results suggest that adhesion of cells to ECM triggers the redistribution of MAPK and MEK to the nucleus, possibly as a result of the cytoskeletal rearrangements that accompany cell spreading. This may represent a mechanism for priming the nucleus with MEK and MAPK, leading to more rapid and pronounced increases in intranuclear phospho-MAPK upon GPCR stimulation.

Live Cell Imaging of ERK and MEK: simple binding equilibrium explains the regulated nucleocytoplasmic distribution of ERK

In response to epidermal growth factor (EGF), the mitogen-activated protein kinase ERK2 translocates into the nucleus. To probe the mechanisms regulating the subcellular localization of ERK2, we used live cell imaging to examine the interaction between MEK1 and ERK2. Fluorescence resonance energy transfer (FRET) studies show that MEK1 and ERK2 directly interact and demonstrate that this interaction in the cytoplasm is largely responsible for cytoplasmic retention of ERK2. Stimulation with EGF caused loss of FRET as ERK separated from MEK and moved into the nucleus. FRET was recovered as ERK returned to the cytosol, indicating ERK reassociation with MEK in the cytoplasm. The EGF-induced transit of ERK through the nucleus was complete within 20 min, and there was no significant movement of MEK into the nucleus. Fluorescence recovery after photobleaching experiments was used to assess the rate of movement of MEK and ERK. The steady-state rate of ERK entry into the nucleus in resting cells was energy-independent and greater than the rate of ERK entry upon EGF stimulation. This suggests that the rate constant for ERK transport across the nuclear membrane is not limiting nuclear entry. Thus, we suggest that the movement of ERK into and out of the nucleus in response to agonist occurs primarily by diffusion and is controlled by interactions with binding partners in the cytosol and nucleus. No evidence of ERK dimerization was detected by FRET methods; the kinetics for nucleocytoplasmic transport were unaffected by mutations in the ERK putative dimerization domain.

Adapter protein SH2-B beta undergoes nucleocytoplasmic shuttling: implications for nerve growth factor induction of neuronal differentiation

The adapter protein SH2-B has been shown to bind to activated nerve growth factor (NGF) receptor TrkA and has been implicated in NGF-induced neuronal differentiation and the survival of sympathetic neurons. However, the mechanism by which SH2-B enhances and maintains neurite outgrowth is unclear. We examined the ability of truncation mutants to regulate neuronal differentiation and observed that certain truncation mutants localized in the nucleus rather than in the cytoplasm or at the plasma membrane as reported for wild-type SH2-B beta. Addition of the nuclear export inhibitor leptomycin B caused both overexpressed wild-type and endogenous SH2-B beta to accumulate in the nucleus of both PC12 cells and COS-7 cells as did deletion of a putative nuclear export sequence (amino acids 224 to 233) or mutation of two critical lysines in that sequence. Deleting or mutating the nuclear export signal caused SH2-B beta to lose its ability to enhance NGF-induced differentiation of PC12 cells. Neither the NGF-induced phosphorylation of ERKs 1 and 2 nor their subcellular distribution was altered in PC12 cells stably expressing the nuclear export-defective SH2-B beta(L231A, L233A). These data provide strong evidence that SH2-B beta shuttles constitutively between the nucleus and cytoplasm. However, SH2-B beta needs continuous access to the cytoplasm and/or plasma membrane to participate in NGF-induced neurite outgrowth. These data also suggest that the stimulatory effect of SH2-B beta on NGF-induced neurite outgrowth of PC12 cells is either downstream of ERKs or via some other pathway yet to be identified.

Roles of Src and epidermal growth factor receptor transactivation in transient and sustained ERK1/2 responses to gonadotropin-releasing hormone receptor activation

The duration as well as the magnitude of mitogen-activated protein kinase activation has been proposed to regulate gene expression and other specific intracellular responses in individual cell types. Activation of ERK1/2 by the hypothalamic neuropeptide gonadotropin-releasing hormone (GnRH) is relatively sustained in alpha T3-1 pituitary gonadotropes and HEK293 cells but is transient in immortalized GT1-7 neurons. Each of these cell types expresses the epidermal growth factor receptor (EGFR) and responds to EGF stimulation with significant but transient ERK1/2 phosphorylation. However, GnRH-induced ERK1/2 phosphorylation caused by EGFR transactivation was confined to GT1-7 cells and was attenuated by EGFR kinase inhibition. Neither EGF nor GnRH receptor activation caused translocation of phospho-ERK1/2 into the nucleus in GT1-7 cells. In contrast, agonist stimulation of GnRH receptors expressed in HEK293 cells caused sustained phosphorylation and nuclear translocation of ERK1/2 by a protein kinase C-dependent but EGFR-independent pathway. GnRH-induced activation of ERK1/2 was attenuated by the selective Src kinase inhibitor PP2 and the negative regulatory C-terminal Src kinase in GT1-7 cells but not in HEK293 cells. In GT1-7 cells, GnRH stimulated phosphorylation and nuclear translocation of the ERK1/2-dependent protein, p90RSK-1 (RSK-1). These results indicate that the duration of ERK1/2 activation depends on the signaling pathways utilized by GnRH in specific target cells. Whereas activation of the Gq/protein kinase C pathway in HEK293 cells causes sustained phosphorylation and translocation of ERK1/2 to the nucleus, transactivation of the EGFR by GnRH in GT1-7 cells elicits transient ERK1/2 signals without nuclear accumulation. These findings suggest that transactivation of the tightly regulated EGFR can account for the transient ERK1/2 responses that are elicited by stimulation of certain G protein-coupled receptors.

Runx2 (Cbfa1, AML-3) interacts with histone deacetylase 6 and represses the p21(CIP1/WAF1) promoter

Runx2 (Cbfa1, AML-3) is multifunctional transcription factor that is essential for osteoblast development. Runx2 binds specific DNA sequences and interacts with transcriptional coactivators and corepressors to either activate or repress transcription of tissue-specific genes. In this study, the p21(CIP/WAF1) promoter was identified as a repressible target of Runx2. A carboxy-terminal repression domain distinct from the well-characterized TLE/Groucho-binding domain contributed to Runx2-mediated p21 repression. This carboxy-terminal domain was sufficient to repress a heterologous GAL reporter. The repressive activity of this domain was sensitive to the histone deacetylase inhibitor trichostatin A but not to trapoxin B. HDAC6, which is insensitive to trapoxin B, specifically interacted with the carboxy terminus of Runx2. The HDAC6 interaction domain of Runx2 was mapped to a region overlapping the nuclear matrix-targeting signal. The Runx2 carboxy terminus was necessary for recruitment of HDAC6 from the cytoplasm to chromatin. HDAC6 also colocalized and coimmunoprecipitated with the nuclear matrix-associated protein Runx2 in osteoblasts. Finally, we show that HDAC6 is expressed in differentiating osteoblasts and that the Runx2 carboxy terminus is necessary for maximal repression of the p21 promoter in preosteoblasts. These data identify Runx2 as the first transcription factor to interact with HDAC6 and suggest that HDAC6 may bind to Runx2 in differentiating osteoblasts to regulate tissue-specific gene expression.