Hsp90-binding Immunophilins Link p53 to Dynein During p53 Transport to the Nucleus

Hsp90 binds microtubules and is involved in the reorganization of the microtubular network in angiosperms

Microtubules (MTs) are essential for many processes in plant cells. MT-associated proteins (MAPs) influence MT polymerization dynamics and enable them to perform their functions. The molecular chaperone Hsp90 has been shown to associate with MTs in animal and plant cells. However, the role of Hsp90-MT binding in plants has not yet been investigated. Here, we show that Hsp90 associates with cortical MTs in tobacco cells and decorates MTs in the phragmoplast. Further, we show that tobacco Hsp90_MT binds directly to polymerized MTs in vitro. The inhibition of Hsp90 by geldanamycin (GDA) severely impairs MT re-assembly after cold-induced de-polymerization. Our results indicate that the plant Hsp90 interaction with MTs plays a key role in cellular events, where MT re-organization is needed.

Role of molecular chaperones and TPR-domain proteins in the cytoplasmic transport of steroid receptors and their passage through the nuclear pore

In the absence of hormone, corticosteroid receptors such as GR (glucocorticoid receptor) and (mineralocorticoid receptor) are primarily located in the cytoplasm. Upon steroid-binding, they rapidly accumulate in the nucleus. Regardless of their primary location, these receptors and many other nuclear factors undergo a constant and dynamic nucleocytoplasmic shuttling. All members of the steroid receptor family are known to form large oligomeric structures with the heat-shock proteins of 90-kDa (hsp90) and 70-kDa (hsp70), the small acidic protein p23, and a tetratricopeptide repeat (TPR) -domain protein such as FK506-binding proteins (FKBPs), cyclophilins (CyPs) or the serine/threonine protein phosphatase 5 (PP5). It has always been stated that the dissociation of the chaperone heterocomplex (a process normally referred to as receptor "transformation") is the first step that permits the nuclear import of steroid receptors. However the experimental evidence is consistent with a model where the chaperone machinery is required for the retrotransport of the receptor through the cytoplasm and also facilitates the passage through the nuclear pore. Recent evidence indicates that the hsp90-based chaperone system also interacts with structures of the nuclear pore such as importin β and the integral nuclear pore glycoprotein Nup62 facilitating the passage of the untransformed receptor through the nuclear pore.

Cytoplasmic travels of the ecdysteroid receptor in target cells: pathways for both genomic and non-genomic actions

Signal transduction of the insect steroid hormones, ecdysteroids, is mediated by the ecdysteroid receptor, EcR. In various cells of the insect Rhodnius prolixus, EcR is present in both the nucleus and the cytoplasm, where it undergoes daily cycling in abundance and cellular location at particular developmental times of the last larval instar that are specific to different cell types. EcR favors a cytoplasmic location in the day and a nuclear location in the night. This study is the first to examine the potential mechanisms of intracellular transport of EcR and reveals close similarities with some of its mammalian counterparts. In double and triple labels using several antibodies, immunohistochemistry, and confocal laser scanning microscopy, we observed co-localization of EcR with the microtubules (MTs). Treatments with either the MT-stabilizing agent taxol or with colchicine, which depolymerizes MTs, resulted in considerable reduction in nuclear EcR with a concomitant increase in cytoplasmic EcR suggesting that MT disruption inhibits receptor accumulation in the nucleus. EcR also co-localizes with the chaperone Hsp90, the immunophilin FKBP52, and the light chain 1 of the motor protein dynein. All these factors also co-localize with MTs. We propose that in Rhodnius, EcR exerts its genomic effects by forming a complex with Hsp90 and FKBP52, which uses dynein on MTs as a mechanism for daily nucleocytoplasmic shuttling. The complex is transported intact to the nucleus and dissociates within it. We propose that EcR utilizes the cytoskeletal tracks for movement in a manner closely similar to that used by the glucocorticoid receptor. We also observed co-localization of EcR with mitochondria which suggests that EcR, like its mammalian counterparts, may be involved in the coordination of non-genomic responses of ecdysteroids in mitochondria.

Microtubules regulate hypoxia-inducible factor-1α protein trafficking and activity: implications for taxane therapy

Disruption of the microtubule cytoskeleton impairs tumor angiogenesis by inhibiting the hypoxia-inducible factor (HIF-1α) pathway. However, the signaling cascade linking microtubule disruption to HIF-1α inactivation has not been elucidated. Here, we show that microtubule-targeting drug (MTD) treatment impaired HIF-1α protein nuclear translocation, which significantly down-regulated HIF transcriptional activity. We provide strong evidence that HIF-1α protein associates with polymerized microtubules and traffics to the nucleus, with the aid of the dynein motor protein. Together, these data suggest that microtubules are critically involved in the nuclear trafficking and transcriptional activity of HIF-1α. We also show that the connection between the microtubule cytoskeleton and HIF-1α regulation is lost in renal cell carcinoma (RCC), where HIF-1α is overexpressed because of mutations in the von Hippel Lindau (VHL) tumor suppressor protein. Specifically, we show that MTD treatment of RCC cells did not impair HIF-1α nuclear accumulation or transcriptional activity, and had no effect on the polysome association profile of HIF-1α. Interestingly, we found that HIF-1α protein did not bind microtubules in RCC. Moreover, restoration of VHL function failed to restore the ability of MTDs to inhibit HIF-1α, suggesting that VHL does not contribute to this phenotype. Together, these results suggest that HIF-1α regulation is microtubule-independent, and likely contributes to the chemoresistant nature of RCCs. Further understanding of the microtubule-dependent HIF-1α regulation, and lack thereof in RCC, is essential given the importance of HIF-1α in tumor biology, and the widespread use of MTDs in clinical oncology.

FKBP51 and FKBP52 in signaling and disease

FKBP51 and FKBP52 are diverse regulators of steroid hormone receptor signaling, including receptor maturation, hormone binding and nuclear translocation. Although structurally similar, they are functionally divergent, which is largely attributed to differences in the FK1 domain and the proline-rich loop. FKBP51 and FKBP52 have emerged as likely contributors to a variety of hormone-dependent diseases, including stress-related diseases, immune function, reproductive functions and a variety of cancers. In addition, recent studies have implicated FKBP51 and FKBP52 in Alzheimer's disease and other protein aggregation disorders. This review summarizes our current understanding of FKBP51 and FKBP52 interactions within the receptor-chaperone complex, their contributions to health and disease, and their potential as therapeutic targets for the treatment of these diseases.

Characterization of inhibitors of glucocorticoid receptor nuclear translocation: a model of cytoplasmic dynein-mediated cargo transport

Agonist-induced glucocorticoid receptor [GR] transport from the cytoplasm to the nucleus was used as a model to identify dynein-mediated cargo transport inhibitors. Cell-based screening of the library of pharmacologically active compound (LOPAC)-1280 collection identified several small molecules that stalled the agonist-induced transport of GR-green fluorescent protein (GFP) in a concentration-dependent manner. Fluorescent images of microtubule organization, nuclear DNA staining, expression of GR-GFP, and its subcellular distribution were inspected and quantified by image analysis to evaluate the impact of compounds on cell morphology, toxicity, and GR transport. Given the complexity of the multi-protein complex involved in dynein-mediated cargo transport and the variety of potential mechanisms for interruption of that process, we therefore developed and validated a panel of biochemical assays to investigate some of the more likely intracellular target(s) of the GR transport inhibitors. Although the apomorphine enantiomers exhibited the most potency toward the ATPase activities of cytoplasmic dynein, myosin, and the heat-shock proteins (HSPs), their apparent lack of specificity made them unattractive for further study in our quest. Other molecules appeared to be nonspecific inhibitors that targeted reactive cysteines of proteins. Ideally, specific retrograde transport inhibitors would either target dynein itself or one of the other important proteins associated with the transport process. Although the hits from the cell-based screen of the LOPAC-1280 collection did not exhibit this desired profile, this screening platform provided a promising phenotypic system for the discovery of dynein/HSP modulators.

Management of cytoskeleton architecture by molecular chaperones and immunophilins

Cytoskeletal structure is continually remodeled to accommodate normal cell growth and to respond to pathophysiological cues. As a consequence, several cytoskeleton-interacting proteins become involved in a variety of cellular processes such as cell growth and division, cell movement, vesicle transportation, cellular organelle location and function, localization and distribution of membrane receptors, and cell-cell communication. Molecular chaperones and immunophilins are counted among the most important proteins that interact closely with the cytoskeleton network, in particular with microtubules and microtubule-associated factors. In several situations, heat-shock proteins and immunophilins work together as a functionally active heterocomplex, although both types of proteins also show independent actions. In circumstances where homeostasis is affected by environmental stresses or due to genetic alterations, chaperone proteins help to stabilize the system. Molecular chaperones facilitate the assembly, disassembly and/or folding/refolding of cytoskeletal proteins, so they prevent aberrant protein aggregation. Nonetheless, the roles of heat-shock proteins and immunophilins are not only limited to solve abnormal situations, but they also have an active participation during the normal differentiation process of the cell and are key factors for many structural and functional rearrangements during this course of action. Cytoskeleton modifications leading to altered localization of nuclear factors may result in loss- or gain-of-function of such factors, which affects the cell cycle and cell development. Therefore, cytoskeletal components are attractive therapeutic targets, particularly microtubules, to prevent pathological situations such as rapidly dividing tumor cells or to favor the process of cell differentiation in other cases. In this review we will address some classical and novel aspects of key regulatory functions of heat-shock proteins and immunophilins as housekeeping factors of the cytoskeletal network.

DYNLL/LC8: a light chain subunit of the dynein motor complex and beyond

The LC8 family members of dynein light chains (DYNLL1 and DYNLL2 in vertebrates) are highly conserved ubiquitous eukaryotic homodimer proteins that interact, besides dynein and myosin 5a motor proteins, with a large (and still incomplete) number of proteins involved in diverse biological functions. Despite an earlier suggestion that LC8 light chains function as cargo adapters of the above molecular motors, they are now recognized as regulatory hub proteins that interact with short linear motifs located in intrinsically disordered protein segments. The most prominent LC8 function is to promote dimerization of their binding partners that are often scaffold proteins of various complexes, including the intermediate chains of the dynein motor complex. Structural and functional aspects of this intriguing hub protein will be highlighted in this minireview.

The 90-kDa heat-shock protein (Hsp90)-binding immunophilin FKBP51 is a mitochondrial protein that translocates to the nucleus to protect cells against oxidative stress

Confocal microscopy images revealed that the tetratricopeptide repeat motif (TPR) domain immunophilin FKBP51 shows colocalization with the specific mitochondrial marker MitoTracker. Signal specificity was tested with different antibodies and by FKBP51 knockdown. This unexpected subcellular localization of FKBP51 was confirmed by colocalization studies with other mitochondrial proteins, biochemical fractionation, and electron microscopy imaging. Interestingly, FKBP51 forms complexes in mitochondria with the glucocorticoid receptor and the Hsp90/Hsp70-based chaperone heterocomplex. Although Hsp90 inhibitors favor FKBP51 translocation from mitochondria to the nucleus in a reversible manner, TPR domain-deficient mutants of FKBP51 are constitutively nuclear and fully excluded from mitochondria, suggesting that a functional TPR domain is required for its mitochondrial localization. FKBP51 overexpression protects cells against oxidative stress, whereas FKBP51 knockdown makes them more sensitive to injury. In summary, this is the first demonstration that FKBP51 is a major mitochondrial factor that undergoes nuclear-mitochondrial shuttling, an observation that may be related to antiapoptotic mechanisms triggered during the stress response.

Structural characterization of the PPIase domain of FKBP51, a cochaperone of human Hsp90

Steroid hormone receptors are key components of mammalian stress and sex hormone systems. Many of them rely on the Hsp90 chaperone system for full function and are further fine-tuned by Hsp90-associated peptidyl-prolyl isomerases such as FK506-binding proteins 51 and 52. FK506-binding protein 51 (FKBP51) has been shown to reduce glucocorticoid receptor signalling and has been genetically associated with human stress resilience and with numerous psychiatric disorders. The peptidyl-prolyl isomerase domain of FKBP51 contains a high-affinity binding site for the natural products FK506 and rapamycin and has further been shown to convey most of the inhibitory activity on the glucocorticoid receptor. FKBP51 has therefore become a prime new target for the treatment of stress-related affective disorders that could be amenable to structure-based drug design. Here, a series of high-resolution structures of the peptidyl-prolyl isomerase domain of FKBP51 as well as a cocrystal structure with the prototypic ligand FK506 are described. These structures provide a detailed picture of the drug-binding domain of FKBP51 and the molecular binding mode of its ligand as a starting point for the rational design of improved inhibitors.

Structural features and chaperone activity of the NudC protein family

The NudC family consists of four conserved proteins with representatives in all eukaryotes. The archetypal nudC gene from Aspergillus nidulans is a member of the nud gene family that is involved in the maintenance of nuclear migration. This family also includes nudF, whose human orthologue, Lis1, codes for a protein essential for brain cortex development. Three paralogues of NudC are known in vertebrates: NudC, NudC-like (NudCL), and NudC-like 2 (NudCL2). The fourth distantly related member of the family, CML66, contains a NudC-like domain. The three principal NudC proteins have no catalytic activity but appear to play as yet poorly defined roles in proliferating and dividing cells. We present crystallographic and NMR studies of the human NudC protein and discuss the results in the context of structures recently deposited by structural genomics centers (i.e., NudCL and mouse NudCL2). All proteins share the same core CS domain characteristic of proteins acting either as cochaperones of Hsp90 or as independent small heat shock proteins. However, while NudC and NudCL dimerize via an N-terminally located coiled coil, the smaller NudCL2 lacks this motif and instead dimerizes as a result of unique domain swapping. We show that NudC and NudCL, but not NudCL2, inhibit the aggregation of several target proteins, consistent with an Hsp90-independent heat shock protein function. Importantly, and in contrast to several previous reports, none of the three proteins is able to form binary complexes with Lis1. The availability of structural information will be of help in further studies on the cellular functions of the NudC family.

Identification of clinically relevant protein targets in prostate cancer with 2D-DIGE coupled mass spectrometry and systems biology network platform

Prostate cancer (PCa) is the most common type of cancer found in men and among the leading causes of cancer death in the western world. In the present study, we compared the individual protein expression patterns from histologically characterized PCa and the surrounding benign tissue obtained by manual micro dissection using highly sensitive two-dimensional differential gel electrophoresis (2D-DIGE) coupled with mass spectrometry. Proteomic data revealed 118 protein spots to be differentially expressed in cancer (n = 24) compared to benign (n = 21) prostate tissue. These spots were analysed by MALDI-TOF-MS/MS and 79 different proteins were identified. Using principal component analysis we could clearly separate tumor and normal tissue and two distinct tumor groups based on the protein expression pattern. By using a systems biology approach, we could map many of these proteins both into major pathways involved in PCa progression as well as into a group of potential diagnostic and/or prognostic markers. Due to complexity of the highly interconnected shortest pathway network, the functional sub networks revealed some of the potential candidate biomarker proteins for further validation. By using a systems biology approach, our study revealed novel proteins and molecular networks with altered expression in PCa. Further functional validation of individual proteins is ongoing and might provide new insights in PCa progression potentially leading to the design of novel diagnostic and therapeutic strategies.

p53 Superfamily proteins in marine bivalve cancer and stress biology

The human p53 tumour suppressor protein is inactivated in many cancers and is also a major player in apoptotic responses to cellular stress. The p53 protein and the two other members of this protein family (p63, p73) are encoded by distinct genes and their functions have been extensively documented for humans and some other vertebrates. The structure and relative expression levels for members of the p53 superfamily have also been reported for most major invertebrate taxa. The functions of homologous proteins have been investigated for only a few invertebrates (specifically, p53 in flies, nematodes and recently a sea anemone). These studies of classical model organisms all suggest that the gene family originally evolved to mediate apoptosis of damaged germ cells or to protect germ cells from genotoxic stress. Here, we have correlated data from a number of molluscan and other invertebrate sequencing projects to provide a framework for understanding p53 signalling pathways in marine bivalve cancer and stress biology. These data suggest that (a) the two identified p53 and p63/73-like proteins in soft shell clam (Mya arenaria), blue mussel (Mytilus edulis) and Northern European squid (Loligo forbesi) have identical core sequences and may be splice variants of a single gene, while some molluscs and most other invertebrates have two or more distinct genes expressing different p53 family members; (b) transcriptional activation domains (TADs) in bivalve p53 and p63/73-like protein sequences are 67-69% conserved with human p53, while those in ecdysozoan, cnidarian, placozoan and choanozoan eukaryotes are ≤33% conserved; (c) the Mdm2 binding site in the transcriptional activation domain is 100% conserved in all sequenced bivalve p53 proteins (e.g. Mya, Mytilus, Crassostrea and Spisula) but is not present in other non-deuterostome invertebrates; (d) an Mdm2 homologue has been cloned for Mytilus trossulus; (e) homologues for both human p53 upstream regulatory and transcriptional target genes exist in molluscan genomes (missing are ARF, CIP1 and BH3 only proteins) and (f) p53 is demonstrably involved in bivalve haemocyte and germinoma cancers. We usually do not know enough about the molecular biology of marine invertebrates to address molecular mechanisms that characterize particular diseases. Understanding the molecular basis of naturally occurring diseases in marine bivalves is a virtually unexplored aspect of toxicoproteomics and genomics and related drug discovery. Additionally, increases in coastal development and concomitant increases in aquatic pollutants have driven interest in developing models appropriate for evaluating potential hazardous compounds or conditions found in the aquatic environment. Data reviewed in this study are coupled with recent developments in our understanding the molecular biology of the marine bivalve p53 superfamily. Taken together, they suggest that both structurally and functionally, bivalve p53 family proteins are the most highly conserved members of this gene superfamily so far identified outside of higher vertebrates and invertebrate chordates. Marine bivalves provide some of the most relevant and best understood models currently available for experimental studies by biomedical and marine environmental researchers.

P53 is transported into the nucleus via an Hsf1-dependent nuclear localization mechanism

Loss of p53 function can occur through disruption of its ability to localize to the nucleus. Previously we showed through characterization a set of mutant cell lines that lacked the ability to import p53 into the nucleus that nuclear translocation of p53 appeared to be mechanistically different from that of the SV40 T-antigen (SV40TAg). Here we extend that work by examining nuclear importation of p53 and SV40TAg using both in vivo and in vitro assays for nuclear localization. We show that disruption of microtubule polymerization using colchicine suppresses nuclear localization of p53 but not of SV40TAg. We also show, for the first time, that the heat shock transcription factor (Hsf1), is required for establishment of the microtubule network in cells and for nuclear localization of p53. In contrast, SV40TAg does not interact with polymerized microtubules suggesting that it is transported into the nucleus through an alternative mechanism. Interestingly, lacking of Hsf1 expression and suppressing Hsf1 by siRNA also made cells more resistant to the cytotoxic effects of paclitaxel. Hence, loss of Hsf1 activity not only suppressed p53 function, but also led to reduced sensitivity to killing by drugs that target microtubules.

Regulation of activation-induced deaminase stability and antibody gene diversification by Hsp90

Activation-induced deaminase (AID) is the mutator enzyme that initiates somatic hypermutation and isotype switching of the antibody genes in B lymphocytes. Undesired byproducts of AID function are oncogenic mutations. AID expression levels seem to correlate with the extent of its physiological and pathological functions. In this study, we identify AID as a novel Hsp90 (heat shock protein 90 kD) client. We find that cytoplasmic AID is in a dynamic equilibrium regulated by Hsp90. Hsp90 stabilizes cytoplasmic AID, as specific Hsp90 inhibition leads to cytoplasmic polyubiquitination and proteasomal degradation of AID. Consequently, Hsp90 inhibition results in a proportional reduction in antibody gene diversification and off-target mutation. This evolutionarily conserved regulatory mechanism determines the functional steady-state levels of AID in normal B cells and B cell lymphoma lines. Thus, Hsp90 assists AID-mediated antibody diversification by stabilizing AID. Hsp90 inhibition provides the first pharmacological means to down-regulate AID expression and activity, which could be relevant for therapy of some lymphomas and leukemias.

Subcellular rearrangement of hsp90-binding immunophilins accompanies neuronal differentiation and neurite outgrowth

FKBP51 and FKBP52 (FK506-binding protein 51 and 52) are tetratricopeptide repeat-domain immunophilins belonging to the tetratricopeptide-protein•hsp90•hsp70•p23 heterocomplex bound to steroid receptors. Immunophilins are related to receptor folding, subcellular localization, and hormone-dependent transcription. Also, they bind the immunosuppressant macrolide FK506, which shows neuroregenerative and neuroprotective actions by a still unknown mechanism. In this study, we demonstrate that in both, undifferentiated neuroblastoma cells and embryonic hippocampal neurons, the FKBP52•hsp90•p23 heterocomplex concentrates in a perinuclear structure. Upon cell stimulation with FK506, this structure disassembles and this perinuclear area becomes transcriptionally active. The acquisition of a neuronal phenotype is accompanied by increased expression of βIII-tubulin, Map-2, Tau-1, but also hsp90, hsp70, p23, and FKBP52. During the early differentiation steps, the perinuclear heterocomplex redistributes along the cytoplasm and nascent neurites, p23 binds to intermediate filaments and microtubules acquired higher filamentary organization. While FKBP52 moves towards neurites and concentrates in arborization bodies and terminal axons, FKBP51, whose expression remains constant, replaces FKBP52 in the perinuclear structure. Importantly, neurite outgrowth is favored by FKBP52 over-expression or FKBP51 knock-down, and is impaired by FKBP52 knock-down or FKBP51 over-expression, indicating that the balance between these FK506-binding proteins plays a key role during the early mechanism of neuronal differentiation.

Effects of HSP70 antisense oligonucleotide on the proliferation and apoptosis of human hepatocellular carcinoma cells

The study investigated the effects of heat shock protein 70 (HSP70) antisense oligonucleotide (ASODN) on the proliferation and apoptosis of a human hepatocellular carcinoma cell line (SMMC-7721 cells) in vitro. HSP70 oligonucleotide was transfected into SMMC-7721 cells by the mediation of Sofast transfection reagent. Inhibition rate of SMMC-7721 cells was determined by using MTT method. Apoptosis rate and cell cycle distribution were measured by flow cytometry. Immunocytochemistry staining was used to observe the expression of HSP70, Bcl-2 and Bax. The results showed that HSP70 ASODN at various concentrations could significantly inhibit the growth of SMMC-7721 cells, and the inhibition effect peaked 48 h after transfection with 400-nmol/L HSP70 ASODN. Cytometric analysis showed the apoptotic rate was increased in a dose- and time-dependent manner in the HSP70 ASODN-treated cells. The percentage of cells in the G2/M and S phases was significantly decreased and that in the G0/G1 phase increased as the HSP70 ASODN concentration was elevated and the exposure time prolonged. Immunocytochemistry showed that treatment of SMMC-7721 cells with HSP70 ASODN resulted in decreased expressions of HSP70 and Bcl-2 proteins, and an increased expression of Bax protein. It was concluded that the HSP70 ASODN can inhibit the growth of the SMMC-7721 cells and increase cell apoptosis by down-regulating the expression of HSP70. HSP70 ASODN holds promise for the treatment of hepatocellular carcinoma.

Proposal for a role of the Hsp90/Hsp70-based chaperone machinery in making triage decisions when proteins undergo oxidative and toxic damage

The Hsp90/Hsp70-based chaperone machinery plays a well-established role in signaling protein function, trafficking and turnover. A number of recent observations also support the notion that Hsp90 and Hsp70 play key roles in the triage of damaged and aberrant proteins for degradation via the ubiquitin-proteasome pathway. In the mid-1990s, it was discovered that Hsp70 is required for ubiquitin-dependent degradation of short-lived and abnormal proteins, and it became clear that inhibition of Hsp90 uniformly leads to the proteasomal degradation of Hsp90 client proteins. Subsequently, CHIP and parkin were shown to be Hsp70-binding ubiquitin E3 ligases that direct ubiquitin-charged E2 enzymes to the Hsp70-bound client protein. Stabilization by Hsp90 reflects the interaction of the chaperone with the ligand binding cleft of the client protein. These hydrophobic clefts must be open to allow passage of ligands to binding sites in the protein interior, and they are inherent sites of conformational instability. Hsp90 stabilizes the open state of the cleft and prevents Hsp70-dependent ubiquitination. In the model we propose here, progressive oxidative events result in cleft opening as the initial step in protein unfolding, and as long as Hsp90 can interact to stabilize the cleft, it will buffer the effect of oxidative damage. When cleft opening is such that Hsp90 can no longer interact, Hsp70-dependent ubiquitination occurs. We summarize evidence that Hsp90 interacts very dynamically with a variety of proteins that are not classic Hsp90 clients, and we show that this dynamic cycling of Hsp90 with nitric oxide synthase protects against CHIP-mediated ubiquitination. Scientific interest to date has focused on stringent regulation of the classic client proteins, which have metastable clefts and are inherently short lived. But, the recognition that Hsp90 cycles dynamically with longer lived proteins with more stable clefts may permit extension of the triage model to the quality control of damaged proteins in general.

Extracellular Hsp90 serves as a co-factor for MAPK activation and latent viral gene expression during de novo infection by KSHV

The Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi's sarcoma (KS), an important cause of morbidity and mortality in immunocompromised patients. KSHV interaction with the cell membrane triggers activation of specific intracellular signal transduction pathways to facilitate virus entry, nuclear trafficking, and ultimately viral oncogene expression. Extracellular heat shock protein 90 localizes to the cell surface (csHsp90) and facilitates signal transduction in cancer cell lines, but whether csHsp90 assists in the coordination of KSHV gene expression through these or other mechanisms is unknown. Using a recently characterized non-permeable inhibitor specifically targeting csHsp90 and Hsp90-specific antibodies, we show that csHsp90 inhibition suppresses KSHV gene expression during de novo infection, and that this effect is mediated largely through the inhibition of mitogen-activated protein kinase (MAPK) activation by KSHV. Moreover, we show that targeting csHsp90 reduces constitutive MAPK expression and the release of infectious viral particles by patient-derived, KSHV-infected primary effusion lymphoma cells. These data suggest that csHsp90 serves as an important co-factor for KSHV-initiated MAPK activation and provide proof-of-concept for the potential benefit of targeting csHsp90 for the treatment or prevention of KSHV-associated illnesses.

Alterations in the steroid hormone receptor co-chaperone FKBPL are associated with male infertility: a case-control study

BACKGROUND

Male infertility is a common cause of reproductive failure in humans. In mice, targeted deletions of the genes coding for FKBP6 or FKBP52, members of the FK506 binding protein family, can result in male infertility. In the case of FKBP52, this reflects an important role in potentiating Androgen Receptor (AR) signalling in the prostate and accessory glands, but not the testis. In infertile men, no mutations of FKBP52 or FKBP6 have been found so far, but the gene for FKBP-like (FKBPL) maps to chromosome 6p21.3, an area linked to azoospermia in a group of Japanese patients.

METHODS

To determine whether mutations in FKBPL could contribute to the azoospermic phenotype, we examined expression in mouse and human tissues by RNA array blot, RT-PCR and immunohistochemistry and sequenced the complete gene from two azoospermic patient cohorts and matching control groups. FKBPL-AR interaction was assayed using reporter constructs in vitro.

RESULTS

FKBPL is strongly expressed in mouse testis, with expression upregulated at puberty. The protein is expressed in human testis in a pattern similar to FKBP52 and also enhanced AR transcriptional activity in reporter assays. We examined sixty patients from the Japanese patient group and found one inactivating mutation and one coding change, as well as a number of non-coding changes, all absent in fifty-six controls. A second, Irish patient cohort of thirty showed another two coding changes not present in thirty proven fertile controls.

CONCLUSIONS

Our results describe the first alterations in the gene for FKBPL in azoospermic patients and indicate a potential role in AR-mediated signalling in the testis.

Genome-wide analysis of genes encoding FK506-binding proteins in rice

The FK506-binding proteins (FKBPs) are a class of peptidyl-prolyl cis/trans isomerase enzymes, some of which can also operate as molecular chaperones. FKBPs comprise a large ubiquitous family, found in virtually every part of the cell and involved in diverse processes from protein folding to stress response. Higher plant genomes typically encode about 20 FKBPs, half of these found in the chloroplast thylakoid lumen. Several FKBPs in plants are regulators of hormone signalling pathways, with important roles in seed germination, plant growth and stress response. Some FKBP isoforms exists as homologous duplicates operating in finely tuned mechanisms to cope with abiotic stress. In order to understand the roles of the plant FKBPs, especially in view of the warming environment, we have identified and analysed the gene families encoding these proteins in rice using computational approaches. The work has led to identification of all FKBPs from the rice genome, including novel high molecular weight forms. The rice FKBP family appears to have evolved by duplications of FKBP genes, which may be a strategy for increased stress tolerance.

The hsp90-FKBP52 complex links the mineralocorticoid receptor to motor proteins and persists bound to the receptor in early nuclear events

In this study, we demonstrate that the subcellular localization of the mineralocorticoid receptor (MR) is regulated by tetratricopeptide domain (TPR) proteins. The high-molecular-weight immunophilin (IMM) FKBP52 links the MR-hsp90 complex to dynein/dynactin motors favoring the cytoplasmic transport of MR to the nucleus. Replacement of this hsp90-binding IMM by FKBP51 or the TPR peptide favored the cytoplasmic localization of MR. The complete movement machinery, including dynein and tubulin, could be recovered from paclitaxel/GTP-stabilized cytosol and was fully reassembled on stripped MR immune pellets. The whole MR-hsp90-based heterocomplex was transiently recovered in the soluble fraction of the nucleus after 10 min of incubation with aldosterone. Moreover, cross-linked MR-hsp90 heterocomplexes accumulated in the nucleus in a hormone-dependent manner, demonstrating that the heterocomplex can pass undissociated through the nuclear pore. On the other hand, a peptide that comprises the DNA-binding domain of MR impaired the nuclear export of MR, suggesting the involvement of this domain in the process. This study represents the first report describing the entire molecular system that commands MR nucleocytoplasmic trafficking and proposes that the MR-hsp90-TPR protein heterocomplex is dissociated in the nucleus rather than in the cytoplasm.

Molecular chaperones, essential partners of steroid hormone receptors for activity and mobility

Steroid hormone receptors (SHRs) are notorious intracellular travellers, transiting among different cellular compartments as they mature, are subjected to regulation and exert their biological functions. Understanding the processes governing the intracellular traffic of SHRs is important, since their unbalanced or erroneous localization could lead to the development of diseases. In this review, we not only explore the functions of the heat-shock protein 90 (Hsp90) molecular chaperone machine for the intracellular transport of SHRs, but also for the regulation of their nuclear mobility, for their recycling and for the regulation of their transcriptional output.

Basal endothelial nitric oxide synthase (eNOS) phosphorylation on Ser(1177) occurs in a stable microtubule- and tubulin acetylation-dependent manner

To better understand the relationship between the subcellular compartmentalization of endothelial nitric oxide synthase (eNOS) and its function in endothelial cells, we addressed the roles of the microtubule network and of its dynamics in organizing Golgi-bound eNOS. We found that part of Golgi-bound eNOS localizes to the trans-Golgi network and/or to trans-Golgi network-derived vesicles and membrane tubules that are organized preferentially by stable microtubules. Also, while most of cellular eNOS was recovered in a detergent-resistant microtubule-enriched subcellular fraction, its recovery was impaired after total microtubule disassembly, but not after selective disassembly of dynamic microtubules or after microtubule stabilization. Basal eNOS phosphorylation on Ser(1177) further required the association of the trans-Golgi network to stable microtubules and was enhanced by microtubule stabilization. We finally show that the involvement of stable microtubules in basal eNOS phosphorylation involved alpha-tubulin acetylation. Microtubule-dependent organization of subcellular eNOS and control over its phosphorylation would thus be essential for endothelial cells to maintain their basal eNOS function.

Nuclear import of the glucocorticoid receptor-hsp90 complex through the nuclear pore complex is mediated by its interaction with Nup62 and importin beta

Glucocorticoid receptor (GR) is cytoplasmic in the absence of ligand and localizes to the nucleus after steroid binding. Previous evidence demonstrated that the hsp90-based heterocomplex bound to GR is required for the efficient retrotransport of the receptor to the nuclear compartment. We examined the putative association of GR and its associated chaperone heterocomplex with structures of the nuclear pore. We found that importin beta and the integral nuclear pore glycoprotein Nup62 interact with hsp90, hsp70, p23, and the TPR domain proteins FKBP52 and PP5. Nup62 and GR were able to interact in a more efficient manner when chaperoned by the hsp90-based heterocomplex. Interestingly, the binding of hsp70 and p23 to Nup62 does not require the presence of hsp90, whereas the association of FKBP52 and PP5 is hsp90 dependent, as indicated by the results of experiments where the hsp90 function was disrupted with radicicol. The ability of both FKBP52 and PP5 to interact with Nup62 was abrogated in cells overexpressing the TPR peptide. Importantly, GR cross-linked to the hsp90 heterocomplex was able to translocate to the nucleus in digitonin-permeabilized cells treated with steroid, suggesting that GR could pass through the pore in its untransformed state.

Cooperative regulation of the interferon regulatory factor-1 tumor suppressor protein by core components of the molecular chaperone machinery

Our understanding of the post-translational processes involved in regulating the interferon regulatory factor-1 (IRF-1) tumor suppressor protein is limited. The introduction of mutations within the C-terminal Mf1 domain (amino acids 301-325) impacts on IRF-1-mediated gene repression and growth suppression as well as the rate of IRF-1 degradation. However, nothing is known about the proteins that interact with this region to modulate IRF-1 function. A biochemical screen for Mf1-interacting proteins has identified an LXXLL motif that is required for binding of Hsp70 family members and cooperation with Hsp90 to regulate IRF-1 turnover and activity. These conclusions are supported by the finding that Hsp90 inhibitors suppress IRF-1-dependent transcription shortly after treatment, although at later time points inhibition of Hsp90 leads to an Hsp70-dependent depletion of nuclear IRF-1. Conversely, the half-life of IRF-1 is increased by Hsp90 in an ATPase-dependent manner leading to the accumulation of nuclear but not cytoplasmic IRF-1. This study begins to elucidate the role of the Mf1 domain of IRF-1 in orchestrating the recruitment of regulatory factors that can impact on both its turnover and transcriptional activity.

Tubulin acetylation favors Hsp90 recruitment to microtubules and stimulates the signaling function of the Hsp90 clients Akt/PKB and p53

Involved in a wide range of cellular processes such as signal transduction, microtubules are highly dynamic polymers that accumulate various post-translational modifications including polyglutamylation, polyglycylation, carboxyterminal cleavage and acetylation, the functions of which just begin to be uncovered. The molecular chaperone Hsp90, which is essential for the folding and activity of numerous client proteins involved in cell proliferation and apoptosis, associates with the microtubule network but the effects of tubulin post-translational modifications on its microtubule binding has not yet been investigated. Herein, we show that both the constitutive (beta) and the inducible (alpha) Hsp90 isoforms bind to microtubules in a way that depends on the level of tubulin acetylation. Tubulin acetylation also stimulates the binding and the signaling function of at least two of its client proteins, the kinase Akt/PKB and the transcription factor p53. This study highlights the role of tubulin acetylation in modulating microtubule-based transport of Hsp90-chaperoned proteins and thus in regulating signaling dynamics in the cytoplasm.

Isolation and characterization of two cytoplasmic hsp90s from Mytilus galloprovincialis (Mollusca: Bivalvia) that contain a complex promoter with a p53 binding site

The commercially important marine bivalve Mytilus galloprovincialis (Mediterranean mussel) is considered a valuable bioindicator, due to its exposure to various pollutants and extreme environmental conditions. Environmental responsive genes, such as the hsp90s, protect the structure and function of cells and accomplish a significant task in cellular homeostasis. To study the hsp90s in M. galloprovincialis a genomic library was screened and two hsp90s were isolated. Sequence analysis revealed that the two genes exhibit great similarities in both the 5' non-coding and the coding region but differ in the 3' non-coding region, as well as in three introns, due to the presence of repeated sequences. Few synonymous substitutions in the coding region of the genes result to an identical predicted polypeptide, which belongs to the cytoplasmic HSP90 subfamily. The 5' non-coding region contains a non-translated exon and multiple binding sites for various transcription factors. The presence of a p53 binding site in the promoter of the isolated genes raises questions about the possible implication of hsp90s in the molluscan leukemia.

A novel FK506-like binding protein interacts with the glucocorticoid receptor and regulates steroid receptor signaling

FKBP-like (FKBPL) protein is a novel immunophilin-like protein that plays a role in the cellular stress response. Its three tetratricopeptide repeat motifs are homologous to the heat shock protein 90 interaction sites of other immunophilins that have roles in steroid hormone receptor signaling. In this study, using biomolecular complementation and coimmunoprecipitation techniques, we show that FKBPL also colocalizes and interacts with the components of the heat shock protein 90-glucocorticoid receptor (GR) complex and demonstrate that the PPIase domain of FKBPL is important for the interaction between this complex and the dynein motor protein, dynamitin. Treatment of DU145 cells with the GR ligand, dexamethasone, induced a rapid and coordinated translocation of both GR and FKBPL to the nucleus; this response was perturbed when FKBPL was knocked down with a targeted small interfering RNA. Furthermore, overexpression of FKBPL increased GR protein levels and transactivation of a luciferase reporter gene in response to dexamethasone in DU145 cells. However, these responses were cell line dependent. In summary, these data suggest that FKBPL can be classed as a new member of the FKBP protein family with a role in steroid receptor complexes and signaling.

Transcriptome sequencing of malignant pleural mesothelioma tumors

Cancers arise by the gradual accumulation of mutations in multiple genes. We now use shotgun pyrosequencing to characterize RNA mutations and expression levels unique to malignant pleural mesotheliomas (MPMs) and not present in control tissues. On average, 266 Mb of cDNA were sequenced from each of four MPMs, from a control pulmonary adenocarcinoma (ADCA), and from normal lung tissue. Previously observed differences in MPM RNA expression levels were confirmed. Point mutations were identified by using criteria that require the presence of the mutation in at least four reads and in both cDNA strands and the absence of the mutation from sequence databases, normal adjacent tissues, and other controls. In the four MPMs, 15 nonsynonymous mutations were discovered: 7 were point mutations, 3 were deletions, 4 were exclusively expressed as a consequence of imputed epigenetic silencing, and 1 was putatively expressed as a consequence of RNA editing. Notably, each MPM had a different mutation profile, and no mutated gene was previously implicated in MPM. Of the seven point mutations, three were observed in at least one tumor from 49 other MPM patients. The mutations were in genes that could be causally related to cancer and included XRCC6, PDZK1IP1, ACTR1A, and AVEN.

Protein phosphatase 5 is required for Hsp90 function during proteotoxic stresses in Trypanosoma brucei

Trypanosoma brucei, a parasitic protozoan that causes African trypanosomiasis in human and domestic animals, adapt in various environments during their digenetic life cycle. In this study, we found that Hsp90 is crucial for the survival of this parasite. Inhibition of Hsp90 activity by geldanamycin (GA) reduced cell growth and increased the level of Hsp90. Both the bloodstream and procyclic forms of T. brucei showed a several-fold greater sensitivity than the mammalian cells to GA and also to 17-AAG, a less toxic derivative of GA, suggesting that Hsp90 could be a potential chemotherapeuric target for African trypanosomiasis. T. brucei Hsp90 interacts with the protein phosphatase 5 (PP5) in vivo. Under normal growth conditions, T. brucei PP5 (TbPP5) and Hsp90 are primarily localized in the cytosol. However, with increase in growth temperature and GA treatment, these proteins translocate to the nucleus. Overproduction of TbPP5 by genetic manipulation reduced the growth inhibitory effect of GA, while knockdown of TbPP5 reduced cell growth more in the presence of GA, as compared to parental control. Depletion of TbPP5, however, did not prevent the induction of Hsp90 protein level during GA treatment. Together, these results suggest that TbPP5 positively regulates the function of Hsp90 to maintain cellular homeostasis during proteotoxic stresses in T. brucei.

Identification of candidate prostate cancer biomarkers in prostate needle biopsy specimens using proteomic analysis

Although serum prostate specific antigen (PSA) is a well-established diagnostic tool for prostate cancer (PCa) detection, the definitive diagnosis of PCa is based on the information contained in prostate needle biopsy (PNBX) specimens. To define the proteomic features of PNBX specimens to identify candidate biomarkers for PCa, PNBX specimens from patients with PCa or benign prostatic hyperplasia (BPH) were subjected to comparative proteomic analysis. 2-DE revealed that 52 protein spots exhibited statistically significantly changes among PCa and BPH groups. Interesting spots were identified by MALDI-TOF-MS/MS. The 2 most notable groups of proteins identified included latent androgen receptor coregulators [FLNA(7-15) and FKBP4] and enzymes involved in mitochondrial fatty acid beta-oxidation (DCI and ECHS1). An imbalance in the expression of peroxiredoxin subtypes was noted in PCa specimens. Furthermore, different post-translationally modified isoforms of HSP27 and HSP70.1 were identified. Importantly, changes in FLNA(7-15), FKBP4, and PRDX4 expression were confirmed by immunoblot analyses. Our results suggest that a proteomics-based approach is useful for developing a more complete picture of the protein profile of PNBX specimen. The proteins identified by this approach may be useful molecular targets for PCa diagnostics and therapeutics.

Breast cancer molecular signatures as determined by SAGE: correlation with lymph node status

Global gene expression measured by DNA microarray platforms have been extensively used to classify breast carcinomas correlating with clinical characteristics, including outcome. We generated a breast cancer Serial Analysis of Gene Expression (SAGE) high-resolution database of approximately 2.7 million tags to perform unsupervised statistical analyses to obtain the molecular classification of breast-invasive ductal carcinomas in correlation with clinicopathologic features. Unsupervised statistical analysis by means of a random forest approach identified two main clusters of breast carcinomas, which differed in their lymph node status (P=0.01); this suggested that lymph node status leads to globally distinct expression profiles. A total of 245 (55 up-modulated and 190 down-modulated) transcripts were differentially expressed between lymph node (+) and lymph node (-) primary breast tumors (fold change, >or=2; P<0.05). Various lymph node (+) up-modulated transcripts were validated in independent sets of human breast tumors by means of real-time reverse transcription-PCR (RT-PCR). We validated significant overexpression of transcripts for HOXC10 (P=0.001), TPD52L1 (P=0.007), ZFP36L1 (P=0.011), PLINP1 (P=0.013), DCTN3 (P=0.025), DEK (P=0.031), and CSNK1D (P=0.04) in lymph node (+) breast carcinomas. Moreover, the DCTN3 (P=0.022) and RHBDD2 (P=0.002) transcripts were confirmed to be overexpressed in tumors that recurred within 6 years of follow-up by real-time RT-PCR. In addition, meta-analysis was used to compare SAGE data associated with lymph node (+) status with publicly available breast cancer DNA microarray data sets. We have generated evidence indicating that the pattern of gene expression in primary breast cancers at the time of surgical removal could discriminate those tumors with lymph node metastatic involvement using SAGE to identify specific transcripts that behave as predictors of recurrence as well.

The glucocorticoid responses are shaped by molecular chaperones

The glucocorticoid receptor is a known regulator of a variety of physiological processes. Its mode of action is well defined: upon hormone binding, it undergoes a conformational change, translocates to the nucleus and modulates the transcription of target genes. Molecular chaperones have a widely recognized role in the folding of newly made proteins, but their participation in further maturation of folded proteins to their active states and beyond tends to be underestimated. This review presents the current knowledge on how the Hsp70 and Hsp90 chaperone machines help to shape the responses to glucocorticoids. We discuss the contributions of these molecular chaperones to folding, activation, intracellular transport, transcriptional regulation, and decay of the glucocorticoid receptor.

Chaperones as Parts of Organelle Networks

The efficiency, divergence, and specificity of virtually all intracellular metabolic and signalling pathways largely depend on their compartmentalized organization. A corollary of the requirement of compartmentalization is the dynamic structural partition of the intracellular space by endomembrane systems. A branch of these membranes communicate with the extracellular space through the endo- and exocytotic processes. Others, like the mitochondrial and endoplasmic reticulum networks accomplish a further role, being fundamental for the maintenance of cellular energy balance and for determination of cell fate under stress conditions. Recent structural and functional studies revealed that the interaction of these networks and the connectivity state of mitochondria controls metabolic flow, protein transport, intracellular Ca2+ signalling, and cell death. Moreover, reflecting the fact that the above processes are accomplished in a microdomain between collaborating organelle membranes, the existence of macromolecular complexes at their contact sites have also been revealed. Being not only assistants of nascent protein folding, chaperones are proposed to participate in assembling and maintaining the function of the above complexes. In this chapter we discuss recently found examples of such an assembly of protein interactions driven by chaperone proteins, and their role in regulating physiological and pathological processes.

A microtubule-facilitated nuclear import pathway for cancer regulatory proteins

Nuclear protein import is dependent on specific targeting signals within cargo proteins recognized by importins (IMPs) that mediate translocation through the nuclear pore. Recent evidence, however, implicates a role for the microtubule (MT) network in facilitating nuclear import of the cancer regulatory proteins parathyroid hormone-related protein (PTHrP) and p53 tumor suppressor. Here we assess the extent to which MT and actin integrity may be generally required for nuclear protein import for the first time. We examine 10 nuclear-localizing proteins with diverse IMP-dependent nuclear import pathways, our results indicating that the cytoskeleton does not have a general mechanistic role in nuclear localization sequence-dependent nuclear protein import. Of the proteins examined, only the p110(Rb) tumor suppressor protein Rb, together with p53 and PTHrP, was found to require MT integrity for optimal nuclear import. Fluorescence recovery after photobleaching experiments indicated that the MT-dependent nuclear transport pathway increases both the rate and extent of Rb nuclear import but does not affect Rb nuclear export. Dynamitin overexpression experiments implicate the MT motor dynein in the import process. The results indicate that, additional to IMP/diffusion-dependent processes, certain cancer regulatory proteins utilize an MT-enhanced pathway for accelerated nuclear import that is presumably required for their nuclear functions.

Retrograde activation of STAT3 by leukemia inhibitory factor in sympathetic neurons

Leukemia inhibitory factor (LIF) is a member of the interleukin-6 family of cytokines and signals through the glycoprotein 130 and LIF receptor beta subunits. Binding of cytokines to these subunits activates multiple signaling cascades, including the Janus kinase (Jak)/signal transducers and activators of transcription (STAT) pathway. We used compartmentalized cultures of sympathetic neurons and immunocytochemical analyses of STAT3 to examine the mechanisms involved in retrograde signaling of LIF from distal neurites (DN) to cell bodies. Addition of LIF to the DN of these neurons triggers the activation and nuclear translocation of STAT3. Inhibition of Jak activity in the cell bodies prevented LIF-induced retrograde activation of STAT3, while block of Jak activity in the DN had no effect on the appearance of activated STAT3 in the nucleus. These results show that the transport of activated Jak is not the main mechanism mediating retrograde signaling. Although there is an increase in phosphorylated STAT3 in the neurites after distal stimulation, the transport of activated STAT3 is not necessary for retrograde signaling. Our results are consistent with a signaling endosome model for retrograde signaling, in which the LIF/glycoprotein 130/LIF receptor/Jak complex is internalized and transported to activate STAT3 in the cell body.

The immunophilin FKBP52 specifically binds to tubulin and prevents microtubule formation

The FK506 binding protein FKBP52 belongs to the large family of immunophilins and is known as a steroid receptor-associated protein. Previous data suggest that FKBP52 is associated with the motor protein dynein and with the cytoskeleton during mitosis. Here we demonstrate a specific and direct interaction between FKBP52 and tubulin. The region of FKBP52 located between aa 267 and 400, which includes the tetratricopeptide repeat domain, is required for tubulin binding. We provide evidence that FKBP52 prevents tubulin polymerization and that an 84 residue sequence located in the C-terminal part of the molecule (aa 375-458) is necessary and sufficient for its microtubule depolymerization activity. In colocalization experiments in PC12 cells, FKBP52 is associated with tubulin in motile cellular compartments. Furthermore, we suggest that, by using siRNA, a decrease of FKBP52 expression in PC12 cells may lead to differentiated cell phenotype characterized by neurite extensions. Collectively, our data define an unexpected property of FKBP52 as a novel regulator of microtubule dynamics. The possible role of microtubule formation and tubulin binding of other immunophilins such as FKBP12 and FKBP51 is discussed.

MDM2 binding induces a conformational change in p53 that is opposed by heat-shock protein 90 and precedes p53 proteasomal degradation

p53 protein conformation is an important determinant of its localization and activity. Changes in p53 conformation can be monitored by reactivity with wild-type conformation-specific (pAb-1620) or mutant conformation-specific (pAb-240) p53 antibodies. Wild-type p53 accumulated in a mutant (pAb-240 reactive) form when its proteasome-dependent degradation was blocked during recovery from stress treatment and in cells co-expressing p53 and MDM2. This suggests that conformational change precedes wild-type p53 degradation by the proteasome. MDM2 binding to the p53 N terminus could induce a conformational change in wild-type p53. Interestingly, this conformational change was opposed by heat-shock protein 90 and did not require the MDM2 RING-finger domain and p53 ubiquitination. Finally, ubiquitinated p53 accumulated in a pAb-240 reactive form when p53 degradation was blocked by proteasome inhibition, and a p53-ubiquitin fusion protein displayed a mutant-only conformation in MDM2-null cells. These results support a model in which MDM2 binding induces a conformational change that is opposed by heat-shock protein 90 and precedes p53 ubiquitination. The covalent attachment of ubiquitin may "lock" p53 in a mutant conformation in the absence of MDM2-binding and prior to its degradation by the proteasome.

Hsp90 regulates the Fanconi anemia DNA damage response pathway

Heat shock protein 90 (Hsp90) regulates diverse signaling pathways. Emerging evidence suggests that Hsp90 inhibitors, such as 17-allylamino-17-demethoxygeldanamycin (17-AAG), enhance DNA damage-induced cell death, suggesting that Hsp90 may regulate cellular responses to genotoxic stress. However, the underlying mechanisms are poorly understood. Here, we show that the Fanconi anemia (FA) pathway is involved in the Hsp90-mediated regulation of genotoxic stress response. In the FA pathway, assembly of 8 FA proteins including FANCA into a nuclear multiprotein complex, and the complex-dependent activation of FANCD2 are critical events for cellular tolerance against DNA cross-linkers. Hsp90 associates with FANCA, in vivo and in vitro, in a 17-AAG-sensitive manner. Disruption of the FANCA/Hsp90 association by cellular treatment with 17-AAG induces rapid proteasomal degradation and cytoplasmic relocalization of FANCA, leading to impaired activation of FANCD2. Furthermore, 17-AAG promotes DNA cross-linker-induced cytotoxicity, but this effect is much less pronounced in FA pathway-defective cells. Notably, 17-AAG enhances DNA cross-linker-induced chromosome aberrations. In conclusion, our results identify FANCA as a novel client of Hsp90, suggesting that Hsp90 promotes activation of the FA pathway through regulation of intracellular turnover and trafficking of FANCA, which is critical for cellular tolerance against genotoxic stress.

Cell-type-specific disruption and recovery of the cytoskeleton in Arabidopsis thaliana epidermal root cells upon heat shock stress

The cytoskeleton in plant cells plays an important role in controlling cell shape and mediating intracellular signalling. However, almost nothing is known about the reactions of cytoskeletal elements to heat stress, which represents one of the major environmental challenges for plants. Here we show that living epidermal root cells of Arabidopsis thaliana could cope with short-term heat shock stress showing disruption and subsequent recovery of microtubules and actin microfilaments in a time-dependent manner. Time-lapse imaging revealed a very dynamic behavior of both cytoskeletal elements including transient depolymerization and disassembly upon heat shock (40-41 degrees C) followed by full recovery at room temperature (20 degrees C) within 1-3 h. Reaction of microtubules, but not actin filaments, to heat shock was dependent on cell type and developmental stage. On the other hand, recovery of actin filaments, but not microtubules, from heat shock stress was dependent on the same parameters. The relevance of this adaptive cytoskeletal behavior to intracellular signalling is discussed.

Phenotypic changes associated with DYNACTIN-2 (DCTN2) over expression characterise SJSA-1 osteosarcoma cells

DYNACTIN-2 (DCTN2) localises to chromosome 12q13-q15, a region prone to stable amplification in several cancers. Transient DCTN2 overexpression has a significant impact on cellular phenotype primarily due to disruption of the DYNEIN-dynactin motor. Changes reported include alterations of microtubule-directed movement of molecular (e.g. TP53) and organelle (e.g. Golgi) cargoes towards the nucleus, centrosome biology, cellular movement and mitosis with a potential predisposition to mitotic block and polyploidy. These changes would be expected to be of relevance to carcinogenesis. To investigate this, we report the first study of DCTN2 genomic amplification and sustained DCTN2 overexpression in cancer cells. QFMPCR was employed to characterise the extent of chromosome 12q13-q15 amplicons in SJSA-1, SJRH30, U373MG and CCF-STTG1 cancer cells. DCTN2 amplification was present in SJSA-1, U373MG and SJRH30 cells, yet was incomplete at the 5'-end in SJRH30 cells. Only SJSA-1 cells were characterised by DCTN2 overexpression on Western blot analyses. Microscopy studies distinguished SJSA-1 cells by greater DCTN2 immunofluorescence and diminished centrosome and 58K protein Golgi-marker focus compared to SJRH30 cells. Indirect evidence derived from the published work of others indicated that TP53 transport into the nucleus was unimpaired. Furthermore, we observed that SJSA-1 cells were easy to propagate. In conclusion, persistent DCTN2 overexpression can be tolerated in SJSA-1 cancer cells despite phenotypic abnormalities predicted from transient overexpression studies. This preliminary study does not support a major role for DCTN2 overexpression in carcinogenesis, although further studies would be necessary to confirm this.