Mechanism Of Nuclear Translocation Research Articles

Analysis of the nuclear translocation mechanism of Dab1 required for layer formation of the cerebral cortex

Analysis of the nuclear translocation mechanism of Dab1 required for layer formation of the cerebral cortex

Genotoxic stress-induced nuclear localization of oncoprotein YB-1 in the absence of proteolytic processing

Y-box-binding protein 1 (YB-1) is an oncogenic transcription factor whose overexpression and nuclear localization is associated with tumor progression and drug resistance. Transcriptional activation of YB-1 in response to genotoxic stress is believed to occur in the cytoplasm through sequence-specific endoproteolytic cleavage by the 20S Proteasome, followed by nuclear translocation of cleaved YB-1. To study the proteolysis model, we developed a two-step affinity purification of endogenous YB-1 protein species and characterized the products using mass spectrometry. Whereas full-length YB-1 was readily identified, the smaller protein band thought to be activated YB-1 was identified as hnRNP A1. An antibody specific for YB-1 was generated, which revealed only one YB-1 species, even after genotoxic stress-induced nuclear YB-1 translocation. These findings warrant re-evaluation of the mechanism of YB-1 nuclear translocation and transcriptional activation. The relationship between nuclear YB-1 and tumor progression may also have to re-evaluated in some cases.

S1993 Tumor Suppressor At Motif Binding Factor 1 (ATBF1) Negatively Regulates Cell Growth Through the Nuclear Translocation with Runt Domain Transcription Factor 3 (RUNx3) in Cooperation with TGF-β Signal Transduction

Background and aims: AT motif binding factor 1 (ATBF1), a homeotic transcription factor, was identified as a tumor suppressor (Nat. Genet., 2005) and frequent loss of heterozygosity at ATBF1 locus in gastric cancer was recently reported (Clin Cancer Res., 2007). We previously showed that ATBF1 expression inversely correlated with the malignant character of gastric cancer (Oncogene, 2001) and ATBF1 enhanced the promoter activity of the p21 (Cancer Res., 2003). We also found that ATBF1 translocates between cytoplasm and nucleus (Int. J. Cancer, 2007), but its precise mechanism has not been fully elucidated. In this study, we investigated the mechanism of nuclear translocation of ATBF1 with runt domain transcription factor 3 (RUNX3) in cooperation with TGF-β signal transduction. Materials and Methods: 1. To analyze the expression of ATBF1 in gastric cancer cells, we performed immunohistochemistry of 123 resected gastric cancer tissues. 2. ATBF1 and RUNX3 were over expressed by transfecting myc-tagged ATBF1 expression vector and flag-tagged RUNX3 expression vector. Immunoprecipitation (IP) between ATBF1 and RUNX3 was performed. 3. To investigate the nuclear translocation of endogenous ATBF1 and RUNX3, we examined the subcellular localization of ATBF1 and RUNX3 in SNU-16 gastric cancer cells treated with from 0.2 to 2.0 ng/ml of recombinant TGF-β1 by confocal mirosope. 4. We confirmed ATBF1 and RUNX3 protein expression in cytoplasm and nucleus extracted fraction by western blot. 5. Cell cycle analyses by using FACScan were performed. 6. Dual-luciferase assays were performed by transfecting ATBF1 and RUNX3 with p21 reporter vector. Results: 1. The nuclear localization of ATBF1 was observed in 33 (26.8%) gastric cancers and the cytoplasm localization of ATBF1 was observed in 61 (49.6%) gastric cancers. ATBF1 expression was not observed in 29 (23.6%) gastric cancers. 2. IP revealed that the physical interaction of ATBF1-RUNX3. 3. In SNU-16 gastric cancer cells, ATBF1 and RUNX3 existed in the nucleus before recombinant TGF-β1 stimulation, but after 24 hours of TGF-β1 stimulation, endogenous ATBF1 and RUNX3 translocated to the nucleus by confocal microscopic observation. 4. Western blot also revealed ATBF1 and RUNX3 protein in the nucleus was increased after TGF-β1 stimulation. 5.ATBF1 induced the accumulation in the G0/G1 phase. 6. ATBF1 and RUNX3 synergistically up-regulated p21 promoter activity. Conclusion: ATBF1 associates with RUNX3 and translocates to the nucleus with RUNX3 by TGF-β signal transduction and works as tumor suppressor and transcription regulator in the nucleus.

Modes and Mishaps of Neuronal Migration in the Mammalian Brain

The ability of neurons to migrate to their appropriate positions in the developing brain is critical to brain architecture and function. Recent research has elucidated different modes of neuronal migration and the involvement of a host of signaling factors in orchestrating the migration, as well as vulnerabilities of this process to environmental and genetic factors. Here we discuss the role of cytoskeleton, motor proteins, and mechanisms of nuclear translocation in radial and tangential migration of neurons. We will also discuss how these and other events essential for normal migration of neurons can be disrupted by genetic and environmental factors that contribute to neurological disease in humans.

P4-285: Sumoylated alpha-synuclein translocates into nucleus by karyopherin alpha6

Autophagy is related to neurodegenerative disease such as AD(Alzheimer's disease), HD(Huntington's disease), ALS(Amyotrophic lateral sclerosis) as well as cancer, infectious disease, and autoimmune disease. Particularly, neuron cells in brains of neurodegenerative disease contained autophagic vacuoles as well as precipitates (aggregates) of various proteins characteristically. We want to know what kind of mechanism for survival break out in autophagic condition or for blocking of neuron cell death, what mechanism is triggered. In autophagic condition, we definitely detected that alpha-synuclein from cytoplasm is translocated to nucleus in neuron cells. In this study, we describe for the first time physical, direct interaction of alpha-synuclein with nuclear cargo protein KPNA that is the nuclear translocation mechanism of alpha-synuclein. We explored the functional consequences of this interaction on neuron cell survival by cell based transfection and gene knock-down experiments(Sh RNA). It was confirmed using GST-pull down assay, Immunoprecipitation, FRET methods. With confocal microscopy, we definitely detected a-synuclein can translocate to nucleus in autophagy. In the results of GST-pull down assay and Immunoprecipitation, a-synuclein can interact with KPNA6 and this interaction cannot detected to between recombinant proteins. We also observed this interaction by FRET (Fluorescence resonance energy transfer). For passing the wicket NPC (Nuclear Pore Complex) a-synuclein bind with KPNA 6. This interaction needs some modification because just recombinant a-synuclein cannot interact with KPNA 6. Nuclear translocation resulted to anti-proliferative effect and furthermore it was related to survival mechanism.

NEUROPROTECTION AGAINST ETHANOL MEDIATED OXIDATIVE STRESS AND APOPTOSIS VIA NRF2/ARE.

This study characterizes a neuroprotective pathway responsible for coordinated enhancement of astrocyte glutathione (GSH)‐homeostatic machinery. Injuries in fetal rat cortical neurons associated with Fetal Alcohol Syndrome are correlated to ethanol (E) exposure leading to increased oxidative stress, GSH depletion, and ultimately neuroapoptosis. Our lab has previously published that astrocytes protect neurons from E mediated apoptosis, via maintenance of GSH homeostasis. We hypothesize that neuroprotection is regulated by the activation of the anti‐oxidant response elements (ARE) contained in the 5’ regions of cytoprotective genes, via the master transcription factor Nrf2. To identify the role of E in astrocyte anti‐oxidant defense induction, we first employed an E time course to investigate Nrf2 protein expression. Primary cultured rat neonatal astrocytes were treated with 4mg/ml E at various time points. There was increased nuclear Nrf2 expression within 15 minutes after E treatment, and lasted up to 4 hours. Immunohistochemical studies corroborate cytoplasmic to nuclear translocation. Enhanced nuclear Nrf2 expression was further augmented by BSO pre‐treatment. In addition, the transcription factor Maf, which obligatorily dimerizes with Nrf2 to activate gene transcription, was increased by E or BSO. Known ARE inducers and oxidative stress mimicked the cytoplasmic to nuclear translocation seen in E treatment. To determine whether E increased GSH‐homeostatic machinery at the transcriptional level, cells were pre‐incubated with Actinomycin D, and subsequently treated with E. Nrf2 expression induced by E was abolished in the presence of AD suggesting that E increases Nrf2 by stimulating gene transcription. Transfection of Nrf2 siRNA completely ablated Nrf2 expression, and furthermore blocks the tert‐butyl hydroquinone induced Nrf2, GGT, and Mrp1 expression. Finally, we examined E's effect on Nrf2 ubiquitination. E decreased Nrf2 ubiquitination at 4 and 24 hours while not effecting overall ubiquitin levels in the cell suggesting another possible mechanism for Nrf2 protein level increase and subsequent nuclear translocation to activate ARE. Taken together, this suggests that Nrf2 is required for activation of ARE. The role of E in Nrf2‐ARE control of astrocyte neuroprotection needs to be further investigated. These points of regulation may allow for pharmacological augmentation of this crucial neuroprotective mechanism.

UV-B promotes rapid nuclear translocation of the Arabidopsis UV-B specific signaling component UVR8 and activates its function in the nucleus.

Arabidopsis thaliana UV RESISTANCE LOCUS8 (UVR8) is a UV-B-specific signaling component that binds to chromatin and regulates UV protection by orchestrating expression of a range of genes. Here, we studied how UV-B regulates UVR8. We show that UV-B stimulates the nuclear accumulation of both a green fluorescent protein (GFP)-UVR8 fusion and native UVR8. Nuclear accumulation leads to UV-B induction of the HY5 gene, encoding a key transcriptional effector of the UVR8 pathway. Nuclear accumulation of UVR8 is specific to UV-B, occurs at low fluence rates, and is observed within 5 min of UV-B exposure. Attachment of a nuclear export signal (NES) to GFP-UVR8 causes cytosolic localization in the absence of UV-B. However, UV-B promotes rapid nuclear accumulation of NES-GFP-UVR8, indicating a concerted mechanism for nuclear translocation. UVR8 lacking the N-terminal 23 amino acids is impaired in nuclear translocation. Attachment of a nuclear localization signal (NLS) to UVR8 causes constitutive nuclear localization. However, NLS-GFP-UVR8 only confers HY5 gene expression following UV-B illumination, indicating that nuclear localization, although necessary for UVR8 function, is insufficient to cause expression of target genes; UV-B is additionally required to stimulate UVR8 function in the nucleus. These findings provide new insights into the mechanisms through which UV-B regulates gene expression in plants.

Nuclear localization of coactivator RAC3 is mediated by a bipartite NLS and importin α3

The nuclear receptor coactivator RAC3 (also known as SRC-3/ACTR/AIB1/p/CIP/TRAM-1) belongs to the p160 coactivator family, which are involved in several physiological processes and diseases. Here we have investigated how RAC3 is translocated into the nucleus and show that it is mediated through a bipartite NLS and importin α3. This bipartite NLS is located within the conserved bHLH domain, and its mutation abolished nuclear localization. The NLS is also sufficient to cause nuclear import of EGFP, and the activity requires basic amino acids within the NLS. RAC3 binds strongly to importin α3, which also depends on the basic amino acids. Functionally, RAC3 cytoplasmic mutant loses its ability to enhance transcription, suggesting that nuclear localization is essential for coactivator function. Together, these results reveal a previous unknown mechanism for nuclear translocation of p160 coactivators and a critical function of the conserved bHLH within the coactivator.

Nuclear Translocation During the Cross-Talk Between Cellular Stress, Cell Cycle and Anticancer Agents

The function of many endogenous molecules in all eukaryotic cells depends on their subcellular localisation, being active when localized in one cellular compartment and inactive in another. Translocation or re-localization of mislocalized components in the optimal subcellular site may contribute to the development of novel cancer therapies and to the re-evaluation of conventional treatment. For instance, various agents are able to entrap cytoplasmic anti-apoptotic pathways to the nucleus, thus activating apoptosis. Moreover, amongst the factors identified so far, the optimal location of the tumor suppressor p53 for promoting cell arrest and apoptosis seems to be the nucleus, while the nuclear factor kappa B (NFkappaB) is desirable to stay in the cytoplasm. Thus, the mechanisms of nuclear translocation of endogenous signaling components, like p53, NFkappaB and various heat shock proteins (HSPs), may serve as targets for pharmacological intervention, without excluding the possible role of uptake and active transport into the nucleus of extracellular proteins.

A Pathway for Tumor Necrosis Factor-α-induced Bcl10 Nuclear Translocation: Bcl10 IS UP-REGULATED BY NF-κB AND PHOSPHORYLATED BY Akt1 AND THEN COMPLEXES WITH Bcl3 TO ENTER THE NUCLEUS

Bcl10 overexpression and nuclear translocation were originally identified in mucosa-associated lymphoid tissue lymphoma with t(1;14)(p32;q32) chromosome translocation. DNA amplification of Bcl10 was also found in other solid tumors. We have recently shown that nuclear translocation of Bcl10 is a specific molecular determinant of Helicobacter pylori-independent mucosa-associated lymphoid tissue lymphoma (Kuo, S.-H., Chen, L. T., Yeh, K.-H., Wu, M. S., Hsu, H. C., Yeh, P. Y., Mao, T. L., Chen, C. L., Doong, S. L., Lin, J. T., and Cheng, A.-L. (2004) J. Clin. Oncol. 22, 3491-3497). However, the molecular mechanism of Bcl10 nuclear translocation remains unknown. In this study, we observed that tumor necrosis factor-alpha (TNFalpha) up-regulates the expression of Bcl10 and induces a fraction of Bcl10 nuclear translocation in human breast carcinoma MCF7 cells. Chromatin immunoprecipitation assays and electrophoretic mobility shift assays indicated that an NF-kappaB-binding site resides in the Bcl10 5 '-untranslated region. This study also demonstrates that Akt1, activated by TNFalpha, phosphorylates Bcl10 at Ser218 and Ser231 and that phosphorylated Bcl10 subsequently complexes with Bcl3 to enter the nucleus. Either inhibition of Akt1 or depletion of Bcl3 blocks Bcl10 nuclear translocation. In summary, these findings characterize a molecular linkage that directs Bcl10 nuclear translocation in response to TNFalpha treatment.

A-Kinase-Anchoring Protein 95 Functions as a Potential Carrier for the Nuclear Translocation of Active Caspase 3 through an Enzyme-Substrate-Like Association

Caspase-mediated proteolysis is a critical and central element of the apoptotic process, and caspase 3, one of the effector caspases, is proposed to play essential roles in the nuclear morphological changes of apoptotic cells. Although many substrates for caspase 3 localize in the nucleus and caspase 3 translocates from the cytoplasm to the nuclei after activation in apoptotic cells, the molecular mechanisms of nuclear translocation of active caspase 3 have been unclear. Recently, we suggested that a substrate-like protein(s) served as a carrier to transport caspase 3 from the cytoplasm into the nucleus. In the present study, we identified A-kinase-anchoring protein 95 (AKAP95) as a caspase 3-binding protein. Small interfering RNA-mediated depletion of AKAP95 reduced apoptotic nuclear morphological changes, suggesting that AKAP95 is involved in the process of apoptotic nuclear morphological changes. The association of AKAP95 with active caspase 3 was analogous to an enzyme-substrate interaction. Furthermore, overexpression of AKAP95 with nuclear localization sequence mutations inhibited nuclear morphological changes in apoptotic cells. These results indicate that AKAP95 is a potential carrier protein for active caspase 3 from the cytoplasm into the nuclei in apoptotic cells.

Interactor-mediated Nuclear Translocation and Retention of Protein Phosphatase-1

Protein Ser/Thr phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that controls numerous cellular processes by the dephosphorylation of key regulatory proteins. PP1 is expressed in various cellular compartments but is most abundant in the nucleus. We have examined the determinants for the nuclear localization of enhanced green fluorescent protein-tagged PP1 in COS1 cells. Our studies show that PP1gamma(1) does not contain a functional nuclear localization signal and that its nuclear accumulation does not require Sds22, which has previously been implicated in the nuclear accumulation of PP1 in yeast (Peggie, M. W., MacKelvie, S. H., Bloecher, A., Knatko, E. V., Tatchell, K., and Stark, M. J. R. (2002) J. Cell Sci. 115, 195-206). However, the nuclear targeting of PP1 isoforms was alleviated by the mutation of their binding sites for proteins that interact via an RVXF motif. Moreover, one of the mutants with a cytoplasmic accumulation and decreased affinity for RVXF motifs (PP1gamma(1)-F257A) could be re-targeted to the nucleus by the overexpression of nuclear interactors (NIPP1 (nuclear inhibitor of PP1) and PNUTS (PP1 nuclear targeting subunit)) with a functional RVXF motif. Also, the addition of a synthetic RVXF-containing peptide to permeabilized cells resulted in the loss of nuclear enhanced green fluorescent protein-PP1gamma(1). Finally, NIPP1(-/-) mouse embryos showed a nuclear hyperphosphorylation on threonine, consistent with a role for NIPP1 in the nuclear targeting and/or retention of PP1. Our data suggest that both the nuclear translocation and the nuclear retention of PP1 depend on its binding to interactors with an RVXF motif.

Interaction of the PAS B Domain with HSP90 Accelerates Hypoxia-Inducible Factor-1α Stabilization

Hypoxia-inducible factor (HIF) α subunits are induced under hypoxic conditions, when limited oxygen supply prevents prolyl hydroxylation-dependent binding of the ubiquitin ligase pVHL and subsequent proteasomal degradation. A short normoxic half-life of HIF-α and a very rapid hypoxic protein stabilization are crucial to the cellular adaptation to changing oxygen supply. However, the molecular requirements for the unusually rapid mechanisms of protein synthesis, folding and nuclear translocation are not well understood. We and others previously found that the chaperone heat-shock protein 90 (HSP90) can interact with HIF-1α in vitro. Here we show that HSP90 also interacts with HIF-2α and HIF-3α, suggesting a general involvement of HSP90 in HIF-α stabilization. The PAS B domain, common to all three α subunits, was required for HSP90 interaction. ARNT competed with HSP90 for binding to the PAS B domain since an excess of either component inhibited the activity of the other. HSP90 as well as the heterocomplex members HSP70 and p23, but not HSP40, were detected in immunoprecipitations of endogenous cellular HIF-1α. While HSP90 and HSP70 bound to HIF-1α predominantly under normoxic conditions, ARNT bound to HIF-1α primarily under hypoxic conditions, suggesting that ARNT displaced HSP90 from HIF-1α following nuclear translocation. Hypoxic accumulation of HIF-1α was delayed in a novel cell model deficient for HSP90β as well as after treatment of wild-type cells with the HSP90 inhibitor geldanamycin, suggesting that HSP90 activity is involved in the rapid HIF-1α protein induction.

Molecular mechanism of activation and nuclear translocation of the mineralocorticoid receptor upon binding of pregnanesteroids

The mineralocorticoid receptor (MR) is primarily localized in the cytoplasm of the cell in the absence of ligand. The first step in the genomic-dependent mechanism of action of mineralocorticoids is the binding of steroid to the MR, which in turn triggers MR nuclear translocation. The regulation of hormone-binding to MR is complex and involves a multifactorial mechanism, making it difficult to determine the optimal structure of a steroid for activating the MR and promoting its nuclear translocation. Here we review the structure–activity relationship for several pregnanesteroids that possess various functional groups, and suggest that a flat conformation of the ligand rather than the presence of particular chemical groups is a critical parameter for the final biological effect in vivo. We also discuss how the MR undergoes differential conformational changes according to the nature of the bound ligand, which in turn affects the dynein-dependent retrograde rate of movement for the steroid/receptor complex.

Colocalization and interaction of the porcine arterivirus nucleocapsid protein with the small nucleolar RNA-associated protein fibrillarin.

Porcine reproductive and respiratory syndrome virus (PRRSV) replicates in the cytoplasm of infected cells, but its nucleocapsid (N) protein localizes specifically to the nucleus and nucleolus. The mechanism of nuclear translocation and whether N associates with particular nucleolar components are unknown. In the present study, we show by confocal microscopy that the PRRSV N protein colocalizes with the small nucleolar RNA (snoRNA)-associated protein fibrillarin. Direct and specific interaction of N with fibrillarin was demonstrated in vivo by the mammalian two-hybrid assay in cells cotransfected with the N and fibrillarin genes and in vitro by the glutathione S-transferase pull-down assay using the expressed fibrillarin protein. Using a series of deletion mutants, the interactive domain of N with fibrillarin was mapped to a region of amino acids 30 to 37. For fibrillarin, the first 80 amino acids, which contain the glycine-arginine-rich region (the GAR domain), was determined to be the domain interactive with N. The N protein was able to bind to the full-length genomic RNA of PRRSV, and the RNA binding domain was identified as the region overlapping with the nuclear localization signal situated at positions 41 to 47. These results suggest that the N protein nuclear transport may be controlled by the binding of RNA to N. The PRRSV N protein was also able to bind to both 28S and 18S ribosomal RNAs. The protein-protein interaction between N and fibrillarin was RNA dependent but independent of N protein phosphorylation. Taken together, our studies demonstrate a specific interaction of the PRRSV nucleocapsid protein with the host cell protein fibrillarin in the nucleolus, and they imply a potential linkage of viral strategies for the modulation of host cell functions, possibly through rRNA precursor processing and ribosome biogenesis.

Functional Domains of FOXJ2

FOXJ2 is a fork head transcriptional activator, the expression of which starts very early in embryonic development and it is distributed widely in the adult. Here, we describe the characterization of domains that are important for its function. FOXJ2 is localized constitutively at the nucleus of the cell. Two tyrosine residues and a stretch of basic amino acid residues at the N and C-terminal ends of the fork head domain, respectively, are important for its nuclear targeting. These residues are conserved strongly among all members of the fork head family, suggesting that they could be involved in the nuclear translocation mechanism of all fork head factors. In addition to the AB domain, we have found, at least, two other transactivation domains: Domain I, at the N terminus, and the H/P domain, rich in histidine and proline residues. Although the AB domain shows the strongest transactivation capacity, all three domains are required for full FOXJ2 transcriptional activity. Furthermore, a fourth region rich in proline and glutamine residues and with no intrinsic transactivation function, the P/Q domain, appears to play an important role in the FOXJ2-mediated transactivation mechanism. Although FOXJ2 can be phosphorylated in two serine residues, this post-translational modification did not appear to be essential for transactivation. Finally, we have found that the W2 wing of the fork head domain of FOXJ2 is dispensable for specific DNA binding, although it could have a weak stabilizing role for the DNA–FOXJ2 complex.

Examining the mechanism of Erk nuclear translocation using green fluorescent protein

In neuronal cells, the mitogen-activated protein kinase (MAP kinase) cascade is an important mediator of neurotrophin signaling from cell surface receptors to the nucleus, resulting in changes in gene expression. Nuclear localization of Erk is thought to be required for these effects. To examine the mechanism and regulation of Erk nuclear translocation, we have created a green fluorescent protein (GFP)-labeled Erk2 construct, which provides a sensitive means to follow the movement of Erk from the cytoplasm to the nucleus following receptor-mediated MAP kinase activation. Using this system in PC12 cells, we have examined a number of mechanisms that have been implicated in regulating the translocation of Erk. In PC12 cells, NGF and EGF induce a rapid translocation of GFP–Erk that requires Ras and Mek. We have found that prolonged phosphorylation of Erk is not required for the rapid and early influx of Erk into the nucleus following growth factor stimulation. Furthermore, following influx, GFP–Erk rapidly returned to the cytoplasm regardless of its phosphorylation state. The release of Erk from its cytoplasmic activator, Mek, followed by the dimerization of Erk, was sufficient to stimulate nuclear uptake, whereas Erk kinase activity was dispensable. PKA activity has been reported to be required for Erk translocation in PC12 cells. However, PKA activity was also not necessary for the early translocation of Erk into the nucleus by NGF or Ras, but it was able to induce a small influx of Erk that could be measured with GFP–Erk2.

1分子蛍光法によるインターロイキン6刺激後のStat3の追跡 : 2量体化と核膜上への移行について

1分子蛍光法によるインターロイキン6刺激後のStat3の追跡 : 2量体化と核膜上への移行について

Direct expression of fluorescent protein-tagged nuclear receptor CAR in mouse liver

This chapter describes a simple experimental procedure to directly express fluorescent protein-tagged constitutive active receptor (CAR) in mouse liver in vivo . The procedure includes construction of a fluorescent protein-tagged CAR expression plasmid, delivering the plasmid to the liver through tail vein injection, preparation of liver frozen sections, and detection of the expressed receptor under conventional and/or confocal fluorescent microscopes. Using the tail vein injection method, fluorescent protein-tagged CAR can be effectively expressed in mouse liver cells. The expressed receptor is properly retained in the cytoplasm of the control livers, and PB treatment translocates the receptor into the liver nucleus. The method is experimentally simple and is a practical tool to investigate intracellular localization of a given protein and its molecular/cellular mechanisms. Despite difficulties, primary hepatocytes have advantages in the experimental system. Development of primary hepatocytes suitable for the experiments is an urgent issue to examine further details of the regulatory mechanism of PB-elicited nuclear translocation of CAR.

Cut and Run to the Nucleus

The conventional view of receptor tyrosine kinase-mediated signal transduction holds that upon ligand binding, a signaling cascade initiated at the cell surface ultimately regulates gene expression. Although it has been proposed that such receptors may localize to the nucleus to affect transcription directly, the mechanism for nuclear translocation has not been clear. Ni et al. (see the Perspective by Heldin and Ericsson) show that cleavage of ErbB-4, an epidermal growth factor receptor family member, by presenilin-dependent secretase releases a transcriptionally active intracellular domain of ErbB-4 to the nucleus. In the absence of this cleavage, the ErbB-4 ligand could not modulate cell growth.C.-Y. Ni, M. P. Murphy, T. E. Golde, G. Carpenter, γ-Secretase cleavage and nuclear localization of ErbB-4 receptor tyrosine kinase. Science 294, 2179-2181 (2001).[Online Journal]C.-H. Helden, J. Ericsson, RIPping tyrosine kinase receptors apart. Science 294, 2111-2113 (2001).[Online Journal]