Cytoplasmic retention of mutant tsp53 is dependent on an intermediate filament protein (vimentin) scaffold

Gene alterations in the nuclear transport receptor superfamily: A study of head and neck cancer

In cancer cells, the nuclear transport system is often disrupted, leading to abnormal localization of nuclear proteins and altered gene expression. This disruption can arise from various mechanisms such as mutations in genes that regulate nuclear transport, altered expression of transport proteins, and changes in nuclear envelope structure. Oncogenic protein build-up in the nucleus due to the disturbance in nuclear transport can also boost tumor growth and cell proliferation. In this study, we performed bioinformatic analyses of 23 key nuclear transport receptors using genomic and transcriptomic data from pancancer and head and neck squamous cell carcinoma (HNSCC) datasets from The Cancer Genome Atlas (TCGA) and Cancer Cell Line Encyclopedia and found that the total alteration frequency of 23 nuclear transport receptors in 2691 samples of the PCAWG Consortium was 42.1% and a high levels of genetic alterations was significantly associated with poor overall survival. Amplification was the most common type of genetic alterations, and results in the overexpression of nuclear transport receptors in HNSCC compared to normal tissues. Furthermore, our study revealed that seven out of eight cell cycle genes (CDK1, CDK2, CDK4, CDK6, CCNA1, CCNB1, and CCNE2) were significantly and positively correlated with nuclear transport receptor genes in TCGA pancancer and CCLE datasets. Additionally, functional enrichment analysis showed that nuclear transport receptor genes were mainly enriched in the adhesion junction, cell cycle, ERBB, MAPK, MTOR and WNT signaling pathways.

Roles of the Caspase-11 Non-Canonical Inflammasome in Rheumatic Diseases

Inflammasomes are intracellular multiprotein complexes that activate inflammatory signaling pathways. Inflammasomes comprise two major classes: canonical inflammasomes, which were discovered first and are activated in response to a variety of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs), and non-canonical inflammasomes, which were discovered recently and are only activated in response to intracellular lipopolysaccharide (LPS). Although a larger number of studies have successfully demonstrated that canonical inflammasomes, particularly the NLRP3 inflammasome, play roles in various rheumatic diseases, including rheumatoid arthritis (RA), infectious arthritis (IR), gouty arthritis (GA), osteoarthritis (OA), systemic lupus erythematosus (SLE), psoriatic arthritis (PA), ankylosing spondylitis (AS), and Sjögren's syndrome (SjS), the regulatory roles of non-canonical inflammasomes, such as mouse caspase-11 and human caspase-4 non-canonical inflammasomes, in these diseases are still largely unknown. Interestingly, an increasing number of studies have reported possible roles for non-canonical inflammasomes in the pathogenesis of various mouse models of rheumatic disease. This review comprehensively summarizes and discusses recent emerging studies demonstrating the regulatory roles of non-canonical inflammasomes, particularly focusing on the caspase-11 non-canonical inflammasome, in the pathogenesis and progression of various types of rheumatic diseases and provides new insights into strategies for developing potential therapeutics to prevent and treat rheumatic diseases as well as associated diseases by targeting non-canonical inflammasomes.

GFAP alternative splicing regulates glioma cell-ECM interaction in a DUSP4-dependent manner

Gliomas are the most common primary brain tumors. Their highly invasive character and the heterogeneity of active oncogenic pathways within single tumors complicate the development of curative therapies and cause poor patient prognosis. Glioma cells express the intermediate filament protein glial fibrillary acidic protein (GFAP), and the level of its alternative splice variant GFAP-δ, relative to its canonical splice variant GFAP-α, is higher in grade IV compared with lower-grade and lower malignant glioma. In this study we show that a high GFAP-δ/α ratio induces the expression of the dual-specificity phosphatase 4 (DUSP4) in focal adhesions. By focusing on pathways up- and downstream of DUSP4 that are involved in the cell-extracellular matrix interaction, we show that a high GFAP-δ/α ratio equips glioma cells to better invade the brain. This study supports the hypothesis that glioma cells with a high GFAP-δ/α ratio are highly invasive and more malignant cells, thus making GFAP alternative splicing a potential therapeutic target.-Van Bodegraven, E. J., van Asperen, J. V., Sluijs, J. A., van Deursen, C. B. J., van Strien, M. E., Stassen, O. M. J. A., Robe, P. A. J., Hol, E. M. GFAP alternative splicing regulates glioma cell-ECM interaction in a DUSP4-dependent manner.

Protein mislocalization: mechanisms, functions and clinical applications in cancer

The changes from normal cells to cancer cells are primarily regulated by genome instability, which foster hallmark functions of cancer through multiple mechanisms including protein mislocalization. Mislocalization of these proteins, including oncoproteins, tumor suppressors, and other cancer-related proteins, can interfere with normal cellular function and cooperatively drive tumor development and metastasis. This review describes the cancer-related effects of protein subcellular mislocalization, the related mislocalization mechanisms, and the potential application of this knowledge to cancer diagnosis, prognosis, and therapy.

Predisposition to apoptosis in keratin 8-null liver is related to inactivation of NF-κB and SAPKs but not decreased c-Flip

Keratin 8 and 18 (K8/K18) are major intermediate filament proteins of liver hepatocytes. They provide mechanical and nonmechanical stability, thereby protecting cells from stress. Hence, K8-null mice are highly sensitive to Fas-mediated liver cell apoptosis. However, the role of c-Flip protein in K8-null related susceptibility to liver injury is controversial. Here we analyzed c-Flip protein expression in various K8 or K18 null/mutant transgenic livers and show that they are similar in all analyzed transgenic livers and that previously reported c-Flip protein changes are due to antibody cross-reaction with mouse K18. Furthermore, analysis of various apoptosis- or cell survival-related proteins demonstrated that inhibition of phosphorylation of NF-κB and various stress activated protein kinases (SAPKs), such as p38 MAPK, p44/42 MAPK and JNK1/2, is related to the higher sensitivity of K8-null hepatocytes whose nuclear NF-κB is rapidly depleted through Fas-mediated apoptosis. Notably, we found that NF-κB and the studied protein kinases are associated with the K8/K18 complex and are released upon phosphorylation. Therefore, interaction of keratins with cell survival-related protein kinases and transcription factors is another important factor for hepatocyte survival.

A novel p53 phosphorylation site within the MDM2 ubiquitination signal: II. a model in which phosphorylation at SER269 induces a mutant conformation to p53

The p53 DNA-binding domain harbors a conformationally flexible multiprotein binding site that regulates p53 ubiquitination. A novel phosphorylation site exists within this region at Ser(269), whose phosphomimetic mutation inactivates p53. The phosphomimetic p53 (S269D) exhibits characteristics of mutant p53: stable binding to Hsp70 in vivo, elevated ubiquitination in vivo, inactivity in DNA binding and transcription, increased thermoinstability using thermal shift assays, and λ(max) of intrinsic tryptophan fluorescence at 403 nm rather than 346 nm, characteristic of wild type p53. These data indicate that p53 conformational stability is regulated by a phosphoacceptor site within an exposed flexible surface loop and that this can be destabilized by phosphorylation. To test whether other motifs within p53 have similarly evolved, we analyzed the effect of Ser(215) mutation on p53 function because Ser(215) is another inactivating phosphorylation site in the conformationally flexible PAb240 epitope. The p53(S215D) protein is inactive like p53(S269D), whereas p53(S215A) is as active as p53(S269A). However, the double mutant p53(S215A/S269A) was transcriptionally inactive and more thermally unstable than either individual Ser-Ala loop mutant. Molecular dynamics simulations suggest that (i) solvation of phospho-Ser(215) and phospho-Ser(269) by positive charged residues or solvent water leads to local unfolding, which is accompanied by local destabilization of the N-terminal loop and global destabilization of p53, and (ii) the double alanine 215/269 mutation disrupts hydrogen bonding normally stabilized by both Ser(215) and Ser(269). These data indicate that p53 has evolved two serine phosphoacceptor residues within conformationally flexible epitopes that normally stabilize the p53 DNA-binding domain but whose phosphorylation induces a mutant conformation to wild type p53.

The ATDC (TRIM29) protein binds p53 and antagonizes p53-mediated functions

The ataxia telangiectasia group D-complementing (ATDC) gene product, also known as TRIM29, is a member of the tripartite motif (TRIM) protein family. ATDC has been proposed to form homo- or heterodimers and to bind nucleic acids. In cell cultures, ATDC expression leads to rapid growth and resistance to ionizing radiation (IR), whereas silencing of ATDC expression decreases growth rates and increases sensitivity to IR. Although ATDC is overexpressed in many human cancers, the biological significance of ATDC overexpression remains obscure. We report here that ATDC increases cell proliferation via inhibition of p53 nuclear activities. ATDC represses the expression of p53-regulated genes, including p21 and NOXA. Mechanistically, ATDC binds p53, and this interaction is potentially fine-tuned by posttranslational acetylation of lysine 116 on ATDC. The association of p53 and ATDC results in p53 sequestration outside of the nucleus. Together, these results provide novel mechanistic insights into the function of ATDC and offer an explanation for how ATDC promotes cancer cell proliferation.

The subcellular localization of IGFBP5 affects its cell growth and migration functions in breast cancer

Background

Insulin-like growth factor binding protein 5 (IGFBP5) has been shown to be associated with breast cancer metastasis in clinical marker studies. However, a major difficulty in understanding how IGFBP5 functions in this capacity is the paradoxical observation that ectopic overexpression of IGFBP5 in breast cancer cell lines results in suppressed cellular proliferation. In cancer tissues, IGFBP5 resides mainly in the cytoplasm; however, in transfected cells, IGFBP5 is mainly located in the nucleus. We hypothesized that subcellular localization of IGFBP5 affects its functions in host cells.

Methods

To test this hypothesis, we generated wild-type and mutant IGFBP5 expression constructs. The mutation occurs within the nuclear localization sequence (NLS) of the protein and is generated by site-directed mutagenesis using the wild-type IGFBP5 expression construct as a template. Next, we transfected each expression construct into MDA-MB-435 breast cancer cells to establish stable clones overexpressing either wild-type or mutant IGFBP5.

Results

Functional analysis revealed that cells overexpressing wild-type IGFBP5 had significantly lower cell growth rate and motility than the vector-transfected cells, whereas cells overexpressing mutant IGFBP5 demonstrated a significantly higher ability to proliferate and migrate. To illustrate the subcellular localization of the proteins, we generated wild-type and mutant IGFBP5-pDsRed fluorescence fusion constructs. Fluorescence microscopy imaging revealed that mutation of the NLS in IGFBP5 switched the accumulation of IGFBP5 from the nucleus to the cytoplasm of the protein.

Conclusion

Together, these findings imply that the mutant form of IGFBP5 increases proliferation and motility of breast cancer cells and that mutation of the NLS in IGFBP5 results in localization of IGFBP5 in the cytoplasm, suggesting that subcellular localization of IGFBP5 affects its cell growth and migration functions in the breast cancer cells.

NPM-ALK inhibits the p53 tumor suppressor pathway in an MDM2 and JNK-dependent manner

Anaplastic large cell lymphoma (ALCL) is characterized by the presence of the t(2;5)(p23;q35) generating the nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) fusion protein, a hyperactive kinase with transforming properties. Among these properties is the ability to regulate activity of the p53 tumor suppressor protein. In many human cancers, p53 is inactivated by mutation or other means, in some cases as a result of up-regulation of the negative regulator MDM2. However, the majority of ALK-expressing ALCL carry wild-type p53 and do not over express MDM2. We demonstrate a novel p53-dependent pathogenetic mechanism in ALK-expressing lymphoma. We confirm previously published reports of NPM-ALK-induced activation of the phosphoinositide (PI) 3-kinase and Jun N-terminal kinase (JNK) stress-activated protein (SAP) kinase proteins, but in this study demonstrate a role for these in the regulation of p53 activity in an intricate signaling system. Specifically, constitutive ALK signaling leads to the functional inactivation and/or degradation of p53 in JNK and MDM2 dependent manners. We also show nuclear exclusion of p53 in a PI 3-kinase-dependent manner. Furthermore, we demonstrate that reactivation of p53 in ALK-expressing cells as a result of pharmacologic inhibition of JNK, PI 3-kinase, and/or MDM2 activities results in the induction of apoptosis suggesting a novel therapeutic modality.

Nuclear to cytoplasmic shift of p33(ING1b) protein from normal oral mucosa to oral squamous cell carcinoma in relation to clinicopathological variables

PURPOSE

p33(ING1b), as a candidate tumour suppressor gene, has been found to be expressed a proportion of oral squamous cell carcinomas (OSCCs), however, its clinicopathological significance is not studied yet. Our aim was to investigate association of p33(ING1b) expression with clinicopathological variables and particularly interesting new cysteine-histidine rich protein (PINCH) in OSCCs.

METHODS

p33(ING1b) expression was immunohistochemically examined in 20 normal oral mucosa specimens and 49 OSCCs.

RESULTS

Normal squamous cells showed only p33(ING1b )nuclear expression (no cytoplasmic expression), with a rate of 90% positive cases. While 24% of OSCCs appeared cytoplasmic expression (11 of them with weak nuclear staining) and the rest tumours (76%) were negative for p33(ING1b). Furthermore, the cases having lymph node metastasis showed a higher frequency of positive cytoplasmic expression than those without metastasis (P = 0.03). The p33(ING1b) cytoplasmic expression was positively related to PINCH expression (P = 0.04), the cases positive for both proteins had a high rate of the metastasis (P = 0.03).

CONCLUSIONS

The transfer of p33(ING1b) protein from the nucleus to the cytoplasm may result in loss of normal cellular function of the protein, which might play a role in the tumourigenesis and metastasis of OSCCs.

Vimentin binding to phosphorylated Erk sterically hinders enzymatic dephosphorylation of the kinase

Cleavage fragments of de novo synthesized vimentin were recently reported to interact with phosphorylated Erk1 and Erk2 MAP kinases (pErk) in injured sciatic nerve, thus linking pErk to a signaling complex retrogradely transported on importins and dynein. Here we clarify the structural basis for this interaction, which explains how pErk is protected from dephosphorylation while bound to vimentin. Pull-down and ELISA experiments revealed robust calcium-dependent binding of pErk to the second coiled-coil domain of vimentin, with observed affinities of binding increasing from 180 nM at 0.1 microM calcium to 15 nM at 10 microM calcium. In contrast there was little or no binding of non-phosphorylated Erk to vimentin under these conditions. Geometric and electrostatic complementarity docking generated a number of solutions wherein vimentin binding to pErk occludes the lip containing the phosphorylated residues in the kinase. Binding competition experiments with Erk peptides confirmed a solution in which vimentin covers the phosphorylation lip in pErk, interacting with residues above and below the lip. The same peptides inhibited pErk binding to the dynein complex in sciatic nerve axoplasm, and interfered with protection from phosphatases by vimentin. Thus, a soluble intermediate filament fragment interacts with a signaling kinase and protects it from dephosphorylation by calcium-dependent steric hindrance.

Multiple pathways regulate intracellular shuttling of MoKA, a co-activator of transcription factor KLF7

MoKA is a novel F-box containing protein that interacts with and stimulates the activity of transcription factor KLF7, a regulator of neuronal differentiation. MoKA accumulates throughout the cell and predominantly in the cytosol, consistent with the presence of several putative nuclear localization and export signals (NLSs and NESs). The present study was designed to refine the identity and location of the sequences responsible for MoKA intracellular shuttling and transcriptional activity. Forced expression of fusion proteins in mammalian cells demonstrated that only one of three putative NLSs potentially recognized by karyopherin receptors is involved in nuclear localization of MoKA. By contrast, three distinct sequences were found to participate in mediating cytoplasmic accumulation. One of them is structurally and functionally related to the leucine-rich export signal that interacts with the exportin 1 (CRM1) receptor. The other two export signals instead display either a novel leucine-rich sequence or an undefined peptide motif, and both appear to act through CRM1-independent pathways. Finally, transcriptional analyses using the chimeric GAL4 system mapped the major activation domain of MoKA to a highly acidic sequence that resides between the NLS and NES clusters.

Control of cell migration: a tumour suppressor function for p53?

Much remains to be learned about how cancer cells acquire the property of migration, a prerequisite for invasiveness and metastasis. Loss of p53 functions is assumed to be a crucial step in the development of many types of cancers, leading to dysregulation of cell cycle checkpoint controls and apoptosis. However, emerging evidence shows that the contribution of the tumour suppressor p53 to the control of tumorigenesis is not restricted to its well-known anti-proliferative activities, but is extended to other stages of cancer development, i.e. the modulation of cell migration. This interesting alternative function has been proposed in light of the effect of p53 on specific features of migrating cells, including cell spreading, establishment of cell polarization and the production of protrusions. The effects of p53 on cell motility are largely mediated through the regulation of Rho signalling, thereby controlling actin cytoskeletal organization. These recent studies connect the regulation of proliferation to the control of cell migration and define a new concept of p53 function as a tumour suppressor gene, suggesting that p53 might be involved in tumour invasion and metastasis. This review focuses on emerging data concerning the properties of p53 that contribute to its atypical role in the regulation of cell migration.

Stable formation of mutated p53 multimers in a Chinese hamster cell line causes defective p53 nuclear localization and abrogates its residual function

We have previously described a methotrexate-resistant cell line (MTX M) characterized by amplified dihydrofolate reductase (DHFR) genes, cytoplasmic p53 localization, and p53 stable tetramers. To investigate the p53 functionality in MTX M, the effect of chemical/physical agents was studied. In MTX M cells, DNA damage did not induce p53 or mdm-2 protein, while in the parental V79 cells, a residual p53 activity was found. cDNA sequencing showed that V79 and MTX M cells share the same mutations, indicating that the complete loss of p53 function in MTX M cells was due to cytoplasmic sequestration of a mutated p53 with residual activity. In Chinese hamster, both p53 and DHFR genes map on short arm of chromosome 2 suggesting that p53 itself might be amplified. However, fluorescence in situ hybridization with a hamster p53 probe showed only a single signal. Thus, the presence of p53 stable tetramers in MTX M cells, although correlated with DNA amplification, could not be the consequence of either p53 or DHFR gene amplification. Expression of a C-terminal human p53 peptide does not induce p53 nuclear accumulation, indicating that the cytoplasmic localization is due to a mechanism different from that already described in cancer cell lines. Treatments with Sodium Butyrate induced beta-tubulin polymerization, but did not apparently organize a normal microtubule network, which is shown to be important for the p53 localization. Our data indicated that in MTX M cells, p53 is sequestered in the cytoplasm by a novel mechanism that abrogates p53 residual function.

Cleavage of p53-vimentin complex enhances tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis of rheumatoid arthritis synovial fibroblasts

Rheumatoid arthritis synovial fibroblasts (RASFs) contribute to arthritic cartilage degradation. Although RASFs are normally resistant to apoptosis, Apo2L/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-based gene therapy has been successfully used in a mouse model of arthritis. We investigated this further by treating human RASFs with nontoxic doses of the proteasome inhibitor lactacystin. Treatment induced cytosolic accumulation of p53 and enhanced the susceptibility of RASFs to apoptosis mediated by TRAIL-R2 (DR5) but not Fas. A specific role for p53 in TRAIL-R2-mediated apoptosis was indicated by the ability of p53 siRNA to significantly reduce RASF apoptosis and by the reduced apoptosis of RASFs bearing p53 mutations on treatment with anti-DR5 antibody or anti-DR5 antibody plus lactacystin. p53 immunoprecipitation followed by mass spectrometry identified a vimentin-p53 complex, an interaction that was confirmed by reciprocal vimentin-p53 immunoprecipitation and by co-immunofluorescence. Interestingly, human caspase-4 cleaved human vimentin, and blockade of caspase-4 with a chemical inhibitor or with specific siRNA significantly inhibited TRAIL-R2-mediated apoptosis of RASFs. Furthermore, blockade of caspase-4 was paralleled by persistence of a cytosolic pattern of p53 and absence of p53 translocation to the nucleus. Taken together, our findings suggest a unique role for caspase-4 in cleaving vimentin and releasing cytosolic p53 for nuclear translocation, events that may regulate the sensitivity of RASFs to receptor-mediated apoptosis.

Cooperation between p53 and p130(Rb2) in induction of cellular senescence

To determine pathways cooperating with p53 in cellular senescence when the retinoblastoma protein (pRb)/p16INK4a pathway is defunct, we stably transfected the p16INK4a-negative C6 rat glioma cell line with a temperature-sensitive mutant p53. Activation of p53(Val-135) induces a switch in pocket protein expression from pRb and p107 to p130(Rb2) and stalls the cells in late G1, early S-phase at high levels of cyclin E. Maintenance of the arrest depends on the functions of p130(Rb2) repressing cyclin A. Inactivation of p53 in senescent cultures restores the pocket proteins to initial levels and initiates progression into S-phase, but the cells fail to resume proliferation, likely due to DNA damage becoming apparent in the arrest and activating apoptosis subsequent to the release from p53-dependent growth suppression. The data indicate that p53 can cooperate selectively with p130(Rb2) to induce cellular senescence, a pathway that may be relevant when the pRb/p16INK4a pathway is defunct.

Senescence-associated alterations of cytoskeleton: extraordinary production of vimentin that anchors cytoplasmic p53 in senescent human fibroblasts

The cytoskeleton of senescent cells was systematically studied using senescent and young fibroblasts. In the cell senescence, skin fibroblasts extraordinarily produced vimentin in contrast to actin and tubulin, which were down-regulated. Among the focal adhesion proteins, paxillin and c-Src decreased also. Senescent cells developed a long and dense vimentin network, long and thin actin fibers, and numerous small focal contact sites, which contrasted with young cells with short and thick actin stress fibers and prominently large focal adhesions. Noticeably, senescent fibroblasts markedly produced p53 molecules and anchored them to vimentin-cytoskeleton in the cytoplasm. The vimentin-anchored p53 was detected with antibody PAb240 that specifically recognizes a conformation variant of p53. A GFP-tagged wild type p53 cDNA was expressed by transfection and shown also to be retained in the cytoplasm in senescent cells, suggesting that p53 is structurally modified to be recognized by PAb240 and anchored to vimentin filaments. We discuss the correlation of the marked alteration of cytoskeleton and senescent cells' diminished proliferation and migration, as well as the significance of cytoskeletal anchorage of tumor suppressor p53.

Physiological and pathological consequences of the interactions of the p53 tumor suppressor with the glucocorticoid, androgen, and estrogen receptors

The p53 tumor suppressor plays a key role in protection from the effects of different physiological stresses (DNA damage, hypoxia, transcriptional defects, etc.), and loss of its activity has dire consequences, such as cancer. Its activity is finely tuned through interactions with other important regulatory circuits in the cell. Recently, striking evidence has emerged for crosstalk with another class of important regulators, the steroid hormone receptors, and in particular the glucocorticoid (GR), androgen (AR), and estrogen (ER) receptors. These receptors are important in maintaining homeostasis in response to internal and external stresses (GR) and in the development, growth, and maintenance of the male and female reproductive systems (AR and ER, respectively). We review how p53 interacts closely with these receptors, to the extent that they share the same E3 ubiquitin ligase, the MDM2 oncoprotein. We discuss the different physiological contexts in which such interactions occur, and also how these interactions have been undermined in various pathological situations. We will describe future areas for research, with special emphasis on GR, and how certain common features, such as cytoplasmic anchoring of p53 by the receptors, may become targets for the development of therapeutic interventions. Given the importance of GR in inflammation, erythropoiesis, and autoimmune diseases, and the importance of AR and ER in prostate and breast cancer (respectively), the studies on p53 interactions with the steroid receptors will be an important domain in the near future.

The importance of p53 location: nuclear or cytoplasmic zip code?

The regulation of p53 functions is tightly controlled through several mechanisms including p53 transcription and translation, protein stability, post-translational modifications, and subcellular localization. Despite intensive study of p53, the regulation of p53 subcellular localization although important for its function is still poorly understood. The regulation of p53 localization depends on factors that influence its nuclear import and export, subnuclear localization and cytoplasmic tethering and sequestration. In this review, we will focus on various proteins and modifications that regulate the location and therefore the activity of p53. For example, MDM2 is the most important regulator of p53 nuclear export and degradation. Cytoplasmic p53 associates with the microtubule cytoskeleton and the dynein family of motor proteins; while Parc and mot2 are involved in its cytoplasmic sequestration. Finally, a portion of p53 is localized to the mitochondria as part of the non-transcriptional apoptotic response. In this review we strive to present the most recent data on how the activity of p53 is regulated by its location.

Genomic alterations in spontaneous and carcinogen-induced murine melanoma cell lines

We have conducted an analysis of genetic alterations in spontaneous murine melanoma cell line B16F0 and its two metastatic clones, B16F1 and B16F10 and the carcinogen-induced murine melanoma cell lines CM519, CM3205, and K1735. We found that unlike human melanomas, the murine melanoma cell lines did not have activating mutations in the Braf oncogene at exon 11 or 15. However, there were distinct patterns of alterations in the ras, Ink4a/Arf, and p53 genes in the two melanoma groups. In the spontaneous B16 melanoma cell lines, expression of p16Ink4a and p19Arf tumor suppressor proteins was lost as a consequence of a large deletion spanning Ink4a/Arf exons 1alpha, 1beta, and 2. In contrast, the carcinogen-induced melanoma cell lines expressed p16Ink4a but had inactivating mutations in either p19Arf (K1735) or p53 (CM519 and CM3205). Inactivation of p19Arf or p53 in carcinogen-induced melanomas was accompanied by constitutive activation of mitogen-activated protein kinases (MAPKs) and/or mutation-associated activation of N-ras. These results indicate that genetic alterations in p16Ink4a/p19Arf, p53 and ras-MAPK pathways can cooperate in the development of murine melanoma.

Parc: a cytoplasmic anchor for p53

Nuclear localization of p53 is essential for its tumor suppressor function. Here, we have identified Parc, a Parkin-like ubiquitin ligase, as a cytoplasmic anchor protein in p53-associated protein complexes. Parc directly interacts and forms a approximately 1 MDa complex with p53 in the cytoplasm of unstressed cells. In the absence of stress, inactivation of Parc induces nuclear localization of endogenous p53 and activates p53-dependent apoptosis. Overexpression of Parc promotes cytoplasmic sequestration of ectopic p53. Furthermore, abnormal cytoplasmic localization of p53 was observed in a number of neuroblastoma cell lines; RNAi-mediated reduction of endogenous Parc significantly sensitizes these neuroblastoma cells in the DNA damage response. These results reveal that Parc is a critical regulator in controlling p53 subcellular localization and subsequent function.

Parc-ing p53 in the cytoplasm

Nikolaev et al. (this issue of Cell) report the identification of a parkin-like protein, Parc, and its role in anchoring the tumor suppressor protein p53 in the cytoplasm reveals yet another level of control of p53 function. Regulation of the subcellular localization of p53 by Parc may serve as a novel target in treatment of certain types of tumors.

Clustering of CD40 ligand is required to form a functional contact with CD40

Receptor clustering is a key event in the initiation of signaling by many types of receptor molecules. Here, we provide evidence for the novel concept that clustering of a ligand is a prerequisite for clustering of the cognate receptor. We show that clustering of the CD40 receptor depends on reciprocal clustering of the CD40 ligand (gp39, CD154). Clustering of the CD40 ligand is mediated by an association of the ligand with p53, a translocation of acid sphingomyelinase (ASM) to the cell membrane, an activation of the ASM, and a formation of ceramide. Ceramide appears to modify preexisting sphingolipid-rich membrane microdomains to fuse and form ceramide-enriched signaling platforms that serve to cluster CD40 ligand. Genetic deficiency of p53 or ASM or disruption of ceramide-enriched membrane domains prevents clustering of CD40 ligand. The functional significance of CD40 ligand clustering is indicated by the finding that clustering of CD40 on B lymphocytes upon co-incubation with CD40 ligand-expressing T cells depends on clustering of the CD40 ligand and is abrogated by inhibition of CD40 ligand clustering.

Homeostatic control of uridine and the role of uridine phosphorylase: a biological and clinical update

Uridine, a pyrimidine nucleoside essential for the synthesis of RNA and bio-membranes, is a crucial element in the regulation of normal physiological processes as well as pathological states. The biological effects of uridine have been associated with the regulation of the cardio-circulatory system, at the reproduction level, with both peripheral and central nervous system modulation and with the functionality of the respiratory system. Furthermore, uridine plays a role at the clinical level in modulating the cytotoxic effects of fluoropyrimidines in both normal and neoplastic tissues. The concentration of uridine in plasma and tissues is tightly regulated by cellular transport mechanisms and by the activity of uridine phosphorylase (UPase), responsible for the reversible phosphorolysis of uridine to uracil. We have recently completed several studies designed to define the mechanisms regulating UPase expression and better characterize the multiple biological effects of uridine. Immunohistochemical analysis and co-purification studies have revealed the association of UPase with the cytoskeleton and the cellular membrane. The characterization of the promoter region of UPase has indicated a direct regulation of its expression by the tumor suppressor gene p53. The evaluation of human surgical specimens has shown elevated UPase activity in tumor tissue compared to paired normal tissue.

p53 binds the nuclear matrix in normal cells: binding involves the proline-rich domain of p53 and increases following genotoxic stress

The tumour suppressor p53 is a multifunctional protein important for the maintenance of genomic integrity. It is able to form molecular complexes with different DNA targets and also with cellular proteins involved in DNA transcription and DNA repair. In mammalian cells the biochemical processing of DNA occurs on a nuclear sub-structure termed the nuclear matrix. Previously Deppert and co-workers have identified p53 in association with the nuclear matrix in viral- and non-viral transformed cell lines. In the present study we demonstrate, for the first time, that p53 is bound to the nuclear matrix in primary cultures of normal mammalian cells and that this binding increases following DNA damage. Analysis of cell lines expressing structural mutants of p53 revealed that association with the nuclear matrix is independent of the tertiary and quaternary structure of p53. However, the proline-rich domain towards the N-terminus of p53 (residues 67 to 98) appeared important for binding to the nuclear matrix. This was demonstrated by TET-ON regulated expression of p53-derived constructs in p53(-/-) murine embryonic fibroblasts (MEF p53(-/-)). The proline-rich domain of p53 has potential for SH3 protein-protein interaction, and has a role in p53-mediated apoptosis and possibly base excision repair of DNA damage. We discuss our observations in relation to the ability of p53 to facilitate DNA repair and also review evidence indicating that matrix-bound p53 in SV40-transformed cells may facilitate the transforming potential of SV40 large T antigen.

Caspase-resistant vimentin suppresses apoptosis after photodynamic treatment with a silicon phthalocyanine in Jurkat cells

Oxidative stress, such as photodynamic therapy, is an apoptosis inducer. Apoptosis, as well as photosensitization, have been associated with disruption of the cytoskeletal network. The purpose of the present study was to assess the role of vimentin, a major cytoskeletal protein, in apoptosis after photodynamic treatment (PDT) with the silicon phthalocyanine Pc 4 in human Jurkat T cells. Here we show for the first time that photosensitization with Pc 4 initiates vimentin cleavage and that this event precedes poly(ADP-ribose) polymerase (PARP) degradation. Similar findings were obtained in the presence of C2-ceramide, an inducer of oxidative stress and apoptosis. In the presence of benzyloxycarbonyl-Val-Ala-Asp(O-methyl)-fluoromethylketone, a pan-caspase inhibitor, Pc 4-PDT-induced vimentin and PARP cleavage were abolished. In Jurkat cells transfected with a caspase-resistant vimentin apoptosis was partly suppressed and delayed post-Pc 4-PDT. We suggest that the full-length vimentin confers resistance to nuclear apoptosis after PDT with Pc 4.

A role for Hsc70 in regulating nucleocytoplasmic transport of a temperature-sensitive p53 (p53Val-135)

Mouse temperature-sensitive p53(Val-135) accumulates in the nucleus and acts as a "wild-type" at 32 degrees C while it is sequestered in the cytoplasm at 37 degrees C. The cytoplasmic p53(Val-135) relocalized into the nucleus upon inhibition of the nuclear export at 37 degrees C, whereas a mutation in a major bipartite nuclear localization signal (NLS) caused constitutive cytoplasmic localization, indicating that it shuttled between the cytoplasm and the nucleus by its own nuclear export signal and NLS rather than tethered to cytoplasmic structures. Although the full-length p53(Val-135) did not bind the import receptor at 37 degrees C, a C-terminally truncated p53(Val-135) lacking residues 326-390 did bind it. Molecular chaperones such as Hsc70 were associated with p53(Val-135) at 37 degrees C but not at 32 degrees C. When the nuclear export was blocked by leptomycin B, only a fraction lacking Hsc70 was specifically accumulated in the nucleus. Immunodepletion of Hsc70 from the reticulocyte lysate caused p53(Val-135) to bind the import receptor. This binding was blocked by supplying the cell extract containing Hsc70 but not by the addition of recombinant Hsc70 alone. We suggest that the association with the Hsc70-containing complex prevents the NLS from the access of the import receptor through the C-terminal region of p53(Val-135) at 37 degrees C, whereas its dissociation at 32 degrees C allows rapid nuclear import.

p53 Mutations are present in colorectal cancer with cytoplasmic p53 accumulation

Previous studies have shown that nuclear p53 over-expression is an indicator of p53 mutations whereas cytoplasmic p53 accumulation is related to wild-type p53 in several kinds of tumors. Cytoplasmic p53 accumulation has been demonstrated to be an independent prognostic factor in colorectal adenocarcinomas. The purpose was to examine whether mutations occur in cases with p53 accumulated in the cytoplasm and whether there are any differences in the frequency and characteristics of p53 mutations in different staining patterns. In the present study, we identified p53 mutations using PCR single-strand conformation polymorphism (SSCP) and DNA sequencing in 75 primary colorectal adenocarcinomas with different staining patterns (negative, nucleus, cytoplasm, nucleus and cytoplasm). The results show that the frequency and nature of mutations in tumors with cytoplasmic p53 accumulation were similar to those with nuclear p53 expression. However, the tumors with accumulation in both the nucleus and cytoplasm demonstrated a higher mutation rate. We suppose that the role of cytoplasmic p53 accumulation in predicting prognosis in patients with colorectal cancer may be dependent on both mutational and non-mutational mechanisms.

p53 is associated with cellular microtubules and is transported to the nucleus by dynein

Here we show that p53 protein is physically associated with tubulin in vivo and in vitro, and that it localizes to cellular microtubules. Treatment with vincristine or paclitaxel before DNA-damage or before leptomycin B treatment reduces nuclear accumulation of p53 and expression of mdm2 and p21. Overexpression of dynamitin or microinjection of anti-dynein antibody before DNA damage abrogates nuclear accumulation of p53. Our results indicate that transport of p53 along microtubules is dynein-dependent. The first 25 amino acids of p53 contain the residues that are essential for binding to microtubules. We propose that functional microtubules and the dynein motor protein participate in transport of p53 and facilitate its accumulation in the nucleus after DNA damage.

Isolation and characterization of sixteen novel p53 response genes

The EB-1 cell line is a stable transfectant of EB, a p53 null colon carcinoma cell line, with an inducible promoter controlling expression of a wild type p53 cDNA. The induced p53 is transcriptionally active and gives rise to apoptosis in these cells. Using this cellular model for presence or absence of the transcription factor p53 and transactivated genes, the Suppression Subtractive Hybridization (SSH) technique permitted the isolation of 17 mRNA candidates (GIPs-Genes induced by p53), whose expression appears to be p53-dependent. Identity has been established for nine of the 17 isolated candidates. These are HGFL/MSP, Zap-70, APOBEC2, Ponsin/SH3P12/CAP/FLAF2, CDCrel2b/H5/Pnutl2, IgG, lats 2, cytokeratin 15 and PIG-3 (quinone oxidoreductase). The latter gene is the only GIP previously demonstrated to be p53 regulated. Of the eight remaining GIPs, six correspond to Unigene clusters. One candidate, GIP #1, is significantly homologous (72% identity) to a chicken zinc finger protein, CTCF, which binds to insulator elements and thus attenuates enhancer cross-talk between physically adjacent promoters. The p53-dependent expression of GIPs was confirmed by dependence of expression upon induction of wt p53 expression in the EB-1 cellular model and by up-regulation following activation of an endogenous wt p53 by treatment with adriamycin. Oncogene (2000) 19, 3978 - 3987.

p53 regulation by post-translational modification and nuclear retention in response to diverse stresses

p53 activation by diverse stresses involves post-translational modifications that alter its structure and result in its nuclear accumulation. We will discuss several unresolved topics regarding p53 regulation which are currently under investigation. DNA damage is perhaps the best-studied stress which activates p53, and recent data implicate phosphorylation at N-terminal serine residues as critical in this process. We discuss recent data regarding the potential kinases which modify p53 and the possible role of the resulting phosphorylation events. By contrast, much less is understood about agents which disrupt the mitotic spindle. The cell cycle phase, induction signal, and biochemical mechanism of the reversible arrest induced by microtubule disruption are currently under investigation. Finally, a key event in response to any genotoxic stress is the accumulation of p53 in the nucleus. The factors which determine the steady state level of p53 are starting to be elucidated, but the mechanisms responsible for nuclear accumulation and nuclear export remain controversial. We discuss new studies revealing a mechanism for nuclear retention of p53, and the potential contributions of MDM2 to this process.

Interaction partners of Dlk/ZIP kinase: co-expression of Dlk/ZIP kinase and Par-4 results in cytoplasmic retention and apoptosis

Dlk/ZIP kinase is a newly discovered serine/threonine kinase which, due to its homology to DAP kinase, was named DAP like kinase, Dlk. This kinase is tightly associated with nuclear structures, it undergoes extensive autophosphorylation and phosphorylates myosin light chain and core histones H3, H2A and H4 in vitro. Moreover, it possesses a leucine zipper which mediates interaction with transcription factor ATF-4, therefore it was called ZIP kinase. We employed the yeast two-hybrid system to identify interaction partners of Dlk that might serve as regulators or targets. Besides ATF-4 and others we found Par-4, a modulator of transcription factor WT1 and mediator of apoptosis. Complex formation between Dlk and Par-4 was confirmed by GST pull-down experiments and kinase reactions in vitro and coexpression experiments in vivo. The interaction domain within Dlk was mapped to an arginine-rich region between residues 338 - 417, rather than to the leucine zipper. Strikingly, coexpression of Dlk and Par-4 lead to relocation of Dlk from the nucleus to the cytoplasm, particularly to actin filaments. These interactions provoked a dramatic reorganization of the cytoskeleton and morphological symptoms of apoptosis, thus suggesting a functional relationship between Dlk and Par-4 in the control of apoptosis.

C-terminal truncation of Dlk/ZIP kinase leads to abrogation of nuclear transport and high apoptotic activity

Dlk (also termed ZIP kinase) is a novel serine/threonine kinase with a unique C-terminal domain that is rich in arginine and contains three putative NLS motifs and a functional lecuine zipper. Dlk is indeed localized in the nucleus where it shows a speckled distribution. To elucidate the biological functions of Dlk, we wanted to identify the signals relevant for nuclear transport and further the nuclear structures which Dlk binds to. Expression of various deletion and point mutations of Dlk as GFP fusion proteins revealed that the leucine zipper is required for association with speckles and the most C-terminal NLS is necessary and sufficient for nuclear transport. Interestingly, a C-terminal deletion mutant defective for nuclear transport exhibited a pronounced colocalization with actin filaments and, even more strikingly, was a very potent inducer of apoptosis. This apoptotic activity was abrogated, however, when this mutant was retargeted to the nucleus via a heterologous NLS from large T, indicating that Dlk only exerts an apoptotic activity in the cytoplasm. To identify the speckle like structures to which Dlk binds we performed immunofluorescence analyses with antibodies directed against representative marker proteins of replication, transcription, or splicing centers. None of these marker proteins revealed a colocalization with Dlk. Instead, we found a partial colocalization with PML bodies which seem to play a key role in regulation of apoptosis. Taken together, these data strongly suggest a functional role for Dlk in control of cell survival which is dependent on its subcellular localization.

The novel human HUEL (C4orf1) gene maps to chromosome 4p12-p13 and encodes a nuclear protein containing the nuclear receptor interaction motif

A 3250-bp novel human cDNA sequence was isolated from the MRC-5 human embryonic lung cell line by the rapid amplification of cDNA ends technique. This gene was designated HUEL and given the symbol C4orf1 by the HUGO Nomenclature Committee. Within HUEL was identified a continuous ORF of 1704 bp encoding a predicted hydrophilic protein of 568 amino acids with a calculated molecular mass of 63,410 Da. The putative protein contains the LXXLL signature motif considered necessary and sufficient for binding of certain coactivators to liganded nuclear receptors, as well as nuclear localization signals, a nuclear export-like signal, a zinc finger-like motif, an acidic region, and two leucine zipper-like domains. Northern blot analysis of human fetal tissues revealed 3. 4-kb transcripts, while RT-PCR demonstrated HUEL expression in a wide range of human adult tissues and cancer cell lines. In the SiHa, HT-1080, and G-401 cancer lines was detected an alternative transcript in which a 166-bp segment was excluded by exon skipping, which is predicted to culminate in a protein with a modified and truncated C-terminus. HUEL was localized to chromosome region 4p12-p13 by fluorescence in situ hybridization. In Western blots, affinity-purified antibodies raised against a HUEL-specific synthetic peptide could recognize a distinct protein band of approximately 70 kDa. Immunoblotting of subcellular fractions and indirect immunofluorescence of human embryonic lung cells demonstrated the distribution of HUEL predominantly in the cytoplasm, with an apparently cytoskeletal association. However, in smaller or dividing PLC/PRF/5 and TONG liver carcinoma cells, there was a translocation of HUEL from the cytoplasm to the nucleus. Taken together, these data suggest that HUEL plays a role in transcriptional regulation.

Nuclear transport and transcriptional regulation

Studies over the past 10 years have provided major insights into the molecular mechanisms responsible for active transport of macromolecules in and out of the nucleus. Nucleocytoplasmic transport pathways correspond to active and signal-mediated processes that involve substrates, adaptors and receptors. Regulation of both nuclear import and nuclear export is mainly exerted at the level of transport complex formation and has emerged as one of the most efficient mechanisms to adapt gene expression to the cell environment by restricting the access of transcriptional regulators to their target genes.

The p53 pathway

Abnormalities of the p53 tumour suppressor gene are among the most frequent molecular events in human and animal neoplasia. Moreover, p53 is one of the most studied proteins in the whole of contemporary biology, with more than 12,500 papers so far written! In this review the choice has been deliberately made not to be fully comprehensive in the coverage of the huge p53 literature. Rather attention is focused on a small number of recent developments which are reviewed in the context of modern models of p53 function. Progress in the analysis of signalling to p53 including phosphorylation cascades, and interactions with proteins such as mdm2 and ARF are highlighted. The plethora of protein-protein interactions is discussed, as are the strategies for defining downstream targets of p53. Finally, the emerging biology of p53 homologues is considered. The need for bridging the gap between reductionist, biochemical and biophysical studies and biological and genetic analysis is emphasized. Only this will provide the needed framework for utilizing the information in clinical care.

Wild-type p53 protein shows calcium-dependent binding to F-actin

Nuclear localization of p53 is required for p53 to detect and respond to DNA strand abnormalities and breaks following DNA damage. This leads to activation of the tumour suppressive functions of p53 resulting in either cell cycle arrest and DNA repair; or apoptosis. Critical functional changes in DNA which require strand breaks, including gene rearrangement, may transiently mimic DNA damage: here it is important not to trigger a p53 response. The fine control of p53 in these different circumstances is unknown but may include transient sequestering of p53 in the cytoplasm. Reversible nuclear-cytoplasmic shuttling is an intrinsic property of p53 (Middeler et al., 1997) associated with cell cycle-related changes in p53's subcellular distribution. Takahashi and Suzuki (1994) described p53 inactivation by shuttling to the cytoplasm and Katsumoto et al. (1995) found wild-type p53 to be closely associated with cytoplasmic actin filaments during DNA synthesis. Here we show that, in the presence of free calcium ions, p53 binds directly to F-actin with a dissocation constant of about 10 microM. Thus, part of the regulatory machinery in normal cell cycling may involve p53-actin interactions regulated by calcium fluxes and the dynamic turnover of F-actin.

A leucine-rich nuclear export signal in the p53 tetramerization domain: regulation of subcellular localization and p53 activity by NES masking

Appropriate subcellular localization is crucial for regulating p53 function. We show that p53 export is mediated by a highly conserved leucine-rich nuclear export signal (NES) located in its tetramerization domain. Mutation of NES residues prevented p53 export and hampered tetramer formation. Although the p53-binding protein MDM2 has an NES and has been proposed to mediate p53 export, we show that the intrinsic p53 NES is both necessary and sufficient for export. This report also demonstrates that the cytoplasmic localization of p53 in neuroblastoma cells is due to its hyperactive nuclear export: p53 in these cells can be trapped in the nucleus by the export-inhibiting drug leptomycin B or by binding a p53-tetramerization domain peptide that masks the NES. We propose a model in which regulated p53 tetramerization occludes its NES, thereby ensuring nuclear retention of the DNA-binding form. We suggest that attenuation of p53 function involves the conversion of tetramers into monomers or dimers, in which the NES is exposed to the proteins which mediate their export to the cytoplasm.

The p53 pathway

Abnormalities of the p53 tumour suppressor gene are among the most frequent molecular events in human and animal neoplasia. Moreover, p53 is one of the most studied proteins in the whole of contemporary biology, with more than 12,500 papers so far written! In this review the choice has been deliberately made not to be fully comprehensive in the coverage of the huge p53 literature. Rather attention is focused on a small number of recent developments which are reviewed in the context of modern models of p53 function. Progress in the analysis of signalling to p53 including phosphorylation cascades, and interactions with proteins such as mdm2 and ARF are highlighted. The plethora of protein-protein interactions is discussed, as are the strategies for defining downstream targets of p53. Finally, the emerging biology of p53 homologues is considered. The need for bridging the gap between reductionist, biochemical and biophysical studies and biological and genetic analysis is emphasized. Only this will provide the needed framework for utilizing the information in clinical care.