Requirement of Hsp90 for centrosomal function reflects its regulation of Polo kinase stability

Association of Hsp90 with p53 and Fizzy related homolog (Fzr) synchronizing Anaphase Promoting Complex (APC/C): An unexplored ally towards oncogenic pathway

The intricate molecular interactions leading to the oncogenic pathway are the consequence of cell cycle modification controlled by a bunch of cell cycle regulatory proteins. The tumor suppressor and cell cycle regulatory proteins work in coordination to maintain a healthy cellular environment. The integrity of this cellular protein pool is perpetuated by heat shock proteins/chaperones, which assist in proper protein folding during normal and cellular stress conditions. Among these versatile groups of chaperone proteins, Hsp90 is one of the significant ATP-dependent chaperones that aid in stabilizing many tumor suppressors and cell cycle regulator protein targets. Recently, studies have revealed that in cancerous cell lines, Hsp90 stabilizes mutant p53, 'the guardian of the genome.' Hsp90 also has a significant impact on Fzr, an essential regulator of the cell cycle having an important role in the developmental process of various organisms, including Drosophila, yeast, Caenorhabditis elegans, and plants. During cell cycle progression, p53 and Fzr coordinately regulate the Anaphase Promoting Complex (APC/C) from metaphase to anaphase transition up to cell cycle exit. APC/C mediates proper centrosome function in the dividing cell. The centrosome acts as the microtubule organizing center for the correct segregation of the sister chromatids to ensure perfect cell division. This review examines the structure of Hsp90 and its co-chaperones, which work in synergy to stabilize proteins such as p53 and Fizzy-related homolog (Fzr) to synchronize the Anaphase Promoting Complex (APC/C). Dysfunction of this process activates the oncogenic pathway leading to the development of cancer. Additionally, an overview of current drugs targeting Hsp90 at various phases of clinical trials has been included.

Roles of RACK1 in centrosome regulation and carcinogenesis

Receptor for activated C kinase 1 (RACK1) regulates various cellular functions and signaling pathways by interacting with different proteins. Recently, we showed that RACK1 interacts with breast cancer gene 1 (BRCA1), which regulates centrosome duplication. RACK1 localizes to centrosomes and spindle poles and is involved in the proper centrosomal localization of BRCA1. The interaction between RACK1 and BRCA1 is critical for the regulation of centrosome number. In addition, RACK1 contributes to centriole duplication by regulating polo-like kinase 1 (PLK1) activity in S phase. RACK1 binds directly to PLK1 and Aurora A, promoting the phosphorylation of PLK1 and activating the Aurora A/PLK1 signaling axis. Overexpression of RACK1 causes centrosome amplification, especially in mammary gland epithelial cells, inducing overactivation of PLK1 followed by premature centriole disengagement and centriole re-duplication. Other proteins, including hypoxia-inducible factor α, von Hippel-Lindau protein, heat-shock protein 90, β-catenin, and glycogen synthase kinase-3β, interact with RACK1 and play roles in centrosome regulation. In this review, we focus on the roles and underlying molecular mechanisms of RACK1 in centrosome regulation mediated by its interaction with different proteins and the modulation of their functions.

Over-expression of Hsp83 in grossly depleted hsrω lncRNA background causes synthetic lethality and l(2)gl phenocopy in Drosophila

We examined interactions between the 83 kDa heat-shock protein (Hsp83) and hsrω long noncoding RNAs (lncRNAs) in hsrω⁶⁶ Hsp90GFP homozygotes, which almost completely lack hsrω lncRNAs but over-express Hsp83. All +/+; hsrω⁶⁶ Hsp90GFP progeny died before the third instar. Rare Sp/CyO; hsrω⁶⁶ Hsp90GFP reached the third instar stage but phenocopied l(2)gl mutants, becoming progressively bulbous and transparent with enlarged brain and died after prolonged larval life. Additionally, ventral ganglia too were elongated. However, hsrω⁶⁶ Hsp90GFP/TM6B heterozygotes, carrying +/+ or Sp/CyO second chromosomes, developed normally. Total RNA sequencing (+/+, +/+; hsrω66/hsrω⁶⁶, Sp/CyO; hsrω⁶⁶/ hsrω⁶⁶, +/+; Hsp90GFP/Hsp90GFP and Sp/CyO; hsrω66 Hsp90GFP/hsrω66 Hsp90GFP late third instar larvae) revealed similar effects on many genes in hsrω66 and Hsp90GFP homozygotes. Besides additive effect on many of them, numerous additional genes were affected in Sp/CyO; hsrω66 Hsp90GFP larvae, with l(2)gl and several genes regulating the central nervous system being highly down-regulated in surviving Sp/CyO; hsrω66 Hsp90GFP larvae, but not in hsrω⁶⁶ or Hsp90GFP single mutants. Hsp83 and several omega speckle-associated hnRNPs were bioinformatically found to potentially bind with these gene promoters and transcripts. Since Hsp83 and hnRNPs are also known to interact, elevated Hsp83 in an altered background of hnRNP distribution and dynamics, due to near absence of hsrω lncRNAs and omega speckles, can severely perturb regulatory circuits with unexpected consequences, including down-regulation of tumoursuppressor genes such as l(2)gl.

HSP70 is required for the proper assembly of pericentriolar material and function of mitotic centrosomes

Background

At the onset of mitosis, the centrosome expands and matures, acquiring enhanced activities for microtubule nucleation and assembly of a functional bipolar mitotic spindle. However, the mechanisms that regulate centrosome expansion and maturation are largely unknown. Previously, we demonstrated in an immortalized human cell line CGL2 and cancer cell line HeLa that the inducible form of heat shock protein 70 (HSP70) accumulates at the mitotic centrosome and is required for centrosome maturation and bipolar spindle assembly.

Results

In this study, we further show that HSP70 accumulated at the spindle pole in a PLK1-dependent manner. HSP70 colocalized with pericentrin (PCNT), CEP215 and γ-tubulin at the spindle pole and was required for the 3D assembly of these three proteins, which supports mitotic centrosome function. Loss of HSP70 disrupted mitotic centrosome structure, reduced pericentriolar material recruitment and induced fragmentation of spindle poles. In addition, HSP70 was necessary for the interaction between PCNT and CEP215 and also facilitated PLK1 accumulation and function at the spindle pole. Furthermore, we found that HSP70 chaperone activity is required for PCNT accumulation at the mitotic centrosome and assembly of mitotic spindles.

Conclusion

Our current results demonstrate that HSP70 is required for the accurate assembly of the pericentriolar material and proper functioning of mitotic centrosomes.

Spotlight on 17-AAG as an Hsp90 inhibitor for molecular targeted cancer treatment

Hsp90 is a ubiquitous chaperone with important roles in the organization and maturation of client proteins that are involved in the progression and survival of cancer cells. Multiple oncogenic pathways can be affected by inhibition of Hsp90 function through degradation of its client proteins. That makes Hsp90 a therapeutic target for cancer treatment. 17-allylamino-17-demethoxy-geldanamycin (17-AAG) is a potent Hsp90 inhibitor that binds to Hsp90 and inhibits its chaperoning function, which results in the degradation of Hsp90's client proteins. There have been several preclinical studies of 17-AAG as a single agent or in combination with other anticancer agents for a wide range of human cancers. Data from various phases of clinical trials show that 17-AAG can be given safely at biologically active dosages with mild toxicity. Even though 17-AAG has suitable pharmacological potency, its low water solubility and high hepatotoxicity could significantly restrict its clinical use. Nanomaterials-based drug delivery carriers may overcome these drawbacks. In this paper, we review preclinical and clinical research on 17-AAG as a single agent and in combination with other anticancer agents. In addition, we highlight the potential of using nanocarriers and nanocombination therapy to improve therapeutic effects of 17-AAG.

Targeted cancer therapy through 17-DMAG as an Hsp90 inhibitor: Overview and current state of the art

Heat shock protein 90 (Hsp90) is an evolutionary preserved molecular chaperone which mediates many cellular processes such as cell transformation, proliferation, and survival in normal and stress conditions. Hsp90 plays an important role in folding, maturation, stabilization and activation of Hsp90 client proteins which all contribute to the development, and proliferation of cancer as well as other inflammatory diseases. Functional inhibition of Hsp90 can have a massive effect on various oncogenic and inflammatory pathways, and will result in the degradation of their client proteins. This turns it into an interesting target in the treatment of different malignancies. 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) as a semi-synthetic derivative of geldanamycin, has several advantages over 17-Allylamino-17-demethoxygeldanamycin (17-AAG) such as higher water solubility, good bioavailability, reduced metabolism, and greater anti-tumour capability. 17-DMAG binds to the Hsp90, and inhibits its function which eventually results in the degradation of Hsp90 client proteins. Here, we reviewed the pre-clinical data and clinical trial data on 17-DMAG as a single agent, in combination with other agents and loaded on nanomaterials in various cancers and inflammatory diseases.

Rapid progression through the cell cycle ensures efficient migration of primordial germ cells - The role of Hsp90

Zebrafish primordial germ cells (PGCs) constitute a useful in vivo model to study cell migration and to elucidate the role of specific proteins in this process. Here we report on the role of the heat shock protein Hsp90aa1.2, a protein whose RNA level is elevated in the PGCs during their migration. Reducing Hsp90aa1.2 activity slows down the progression through the cell cycle and leads to defects in the control over the MTOC number in the migrating cells. These defects result in a slower migration rate and compromise the arrival of PGCs at their target, the region where the gonad develops. Our results emphasize the importance of ensuring rapid progression through the cell cycle during single-cell migration and highlight the role of heat shock proteins in the process.

Plk1 is essential for proper chromosome segregation during meiosis I/meiosis II transition in pig oocytes

BACKGROUND

Polo-like kinase 1 (Plk1), as a characteristic regulator in meiosis, organizes multiple biological events of cell division. Although Plk1 has been implicated in various functions in somatic cell mitotic processes, considerably less is known regarding its function during the transition from metaphase I (MI) to metaphase II (MII) stage in oocyte meiotic progression.

METHODS

In this study, the possible role of Plk1 during the MI-to-MII stage transition in pig oocytes was addressed. Initially, the spatiotemporal expression and subcellular localization pattern of Plk1 were revealed in pig oocytes from MI to MII stage using indirect immunofluorescence and confocal microscopy imaging techniques combined with western blot analyses. Moreover, a highly selective Plk1 inhibitor, GSK461364, was used to determine the potential role of Plk1 during this MI-to-MII transition progression.

RESULTS

Upon expression, Plk1 exhibited a specific dynamic intracellular localization, and co-localization of Plk1 with α-tubulin was revealed in the meiotic spindle of pig oocyte during the transition from MI to MII stage. GSK461364 treatment significantly blocked the first polar body (pbI) emission in a dose-dependent manner and resulted in a failure of meiotic maturation, with a larger percentage of the GSK461364-treated oocytes arresting in the anaphase-telophase I (ATI) stage. Further subcellular structure examination results showed that inhibition of Plk1 with GSK461364 had no visible effect on spindle assembly but caused a significantly higher proportion of the treated oocytes to have obvious defects in homologous chromosome segregation at ATI stage.

CONCLUSIONS

Thus, these results indicate that Plk1 plays an essential role during the meiosis I/meiosis II transition in porcine oocytes, and the regulation is associated with Plk1's effects on homologous chromosome segregation in the ATI stage.

G1/S phase progression is regulated by PLK1 degradation through the CDK1/βTrCP axis

Polo-like kinase 1 (PLK1) is a serine/threonine kinase involved in several stages of the cell cycle, including the entry and exit from mitosis, and cytokinesis. Furthermore, it has an essential role in the regulation of DNA replication. Together with cyclin A, PLK1 also promotes CDH1 phosphorylation to trigger its ubiquitination and degradation, allowing cell cycle progression. The PLK1 levels in different type of tumors are very high compared to normal tissues, which is consistent with its role in promoting proliferation. Therefore, several PLK1 inhibitors have been developed and tested for the treatment of cancer. Here, we further analyzed PLK1 degradation and found that cytoplasmic PLK1 is ubiquitinated and subsequently degraded by the SCFβTrCP/proteasome. This procedure is triggered when heat shock protein (HSP) 90 is inhibited with geldanamycin, which results in misfolding of PLK1. We also identified CDK1 as the major kinase involved in this degradation. Our work shows for the first time that HSP90 inhibition arrests cell cycle progression at the G1/S transition. This novel mechanism inhibits CDH1 degradation through CDK1-dependent PLK1 destruction by the SCFβTrCP/proteasome. In these conditions, CDH1 substrates do not accumulate and cell cycle arrests, providing a novel pathway for regulation of the cell cycle at the G1-to-S boundary.-Giráldez, S., Galindo-Moreno, M., Limón-Mortés, M. C., Rivas, A. C., Herrero-Ruiz, J., Mora-Santos, M., Sáez, C., Japón, M. Á., Tortolero, M., Romero, F. G1/S phase progression is regulated by PLK1 degradation through the CDK1/βTrCP axis.

The Centrioles, Centrosomes, Basal Bodies, and Cilia of Drosophila melanogaster

Centrioles play a key role in the development of the fly. They are needed for the correct formation of centrosomes, the organelles at the poles of the spindle that can persist as microtubule organizing centers (MTOCs) into interphase. The ability to nucleate cytoplasmic microtubules (MTs) is a property of the surrounding pericentriolar material (PCM). The centriole has a dual life, existing not only as the core of the centrosome but also as the basal body, the structure that templates the formation of cilia and flagellae. Thus the structure and functions of the centriole, the centrosome, and the basal body have an impact upon many aspects of development and physiology that can readily be modeled in Drosophila Centrosomes are essential to give organization to the rapidly increasing numbers of nuclei in the syncytial embryo and for the spatially precise execution of cell division in numerous tissues, particularly during male meiosis. Although mitotic cell cycles can take place in the absence of centrosomes, this is an error-prone process that opens up the fly to developmental defects and the potential of tumor formation. Here, we review the structure and functions of the centriole, the centrosome, and the basal body in different tissues and cultured cells of Drosophila melanogaster, highlighting their contributions to different aspects of development and cell division.

Targeting polo-like kinase 1 suppresses essential functions of alloreactive T cells

Acute graft-versus-host disease (aGvHD) is still a major cause of transplant-related mortality after allogeneic stem cell transplantation (ASCT). It requires immunosuppressive treatments that broadly abrogate T cell responses including beneficial ones directed against tumor cells or infective pathogens. Polo-like kinase 1 (PLK1) is overexpressed in many cancer types including leukemia, and clinical studies demonstrated that targeting PLK1 using selective PLK1 inhibitors resulted in inhibition of proliferation and induction of apoptosis predominantly in tumor cells, supporting the feasibility of PLK1 as target for anticancer therapy. Here, we show that activation of alloreactive T cells (Tallo) up-regulate expression of PLK1, suggesting that PLK1 is a potential new candidate for dual therapy of aGvHD and leukemia after ASCT. Inhibition of PLK1, using PLK1-specific inhibitor GSK461364A selectively depletes Tallo by preventing activation and by inducing apoptosis in already activated Tallo, while memory T cells are preserved. Activated Tallo cells which survive exposure to PLK1 undergo inhibition of proliferation by induction of G2/M cell cycle arrest, which is accompanied by accumulation of cell cycle regulator proteins p21(WAF/CIP1), p27(Kip1), p53 and cyclin B1, whereas abundance of CDK4 decreased. We also show that suppressive effects of PLK1 inhibition on Tallo were synergistically enhanced by concomitant inhibition of molecular chaperone Hsp90. Taken together, our data suggest that PLK1 inhibition represents a reasonable dual strategy to suppress residual tumor growth and efficiently deplete Tallo, and thus provide a rationale to selectively prevent and treat aGvHD.

Multifunctional Microtubule-Associated Proteins in Plants

Microtubules (MTs) are involved in key processes in plant cells, including cell division, growth and development. MT-interacting proteins modulate MT dynamics and organization, mediating functional and structural interaction of MTs with other cell structures. In addition to conventional microtubule-associated proteins (MAPs) in plants, there are many other MT-binding proteins whose primary function is not related to the regulation of MTs. This review focuses on enzymes, chaperones, or proteins primarily involved in other processes that also bind to MTs. The MT-binding activity of these multifunctional MAPs is often performed only under specific environmental or physiological conditions, or they bind to MTs only as components of a larger MT-binding protein complex. The involvement of multifunctional MAPs in these interactions may underlie physiological and morphogenetic events, e.g., under specific environmental or developmental conditions. Uncovering MT-binding activity of these proteins, although challenging, may contribute to understanding of the novel functions of the MT cytoskeleton in plant biological processes.

Regulation of Greatwall kinase by protein stabilization and nuclear localization

Greatwall (Gwl) functions as an essential mitotic kinase by antagonizing protein phosphatase 2A. In this study we identified Hsp90, Cdc37 and members of the importin α and β families as the major binding partners of Gwl. Both Hsp90/Cdc37 chaperone and importin complexes associated with the N-terminal kinase domain of Gwl, whereas an intact glycine-rich loop at the N-terminus of Gwl was essential for binding of Hsp90/Cdc37 but not importins. We found that Hsp90 inhibition led to destabilization of Gwl, a mechanism that may partially contribute to the emerging role of Hsp90 in cell cycle progression and the anti-proliferative potential of Hsp90 inhibition. Moreover, in agreement with its importin association, Gwl exhibited nuclear localization in interphase Xenopus S3 cells, and dynamic nucleocytoplasmic distribution during mitosis. We identified KR456/457 as the locus of importin binding and the functional NLS of Gwl. Mutation of this site resulted in exclusion of Gwl from the nucleus. Finally, we showed that the Gwl nuclear localization is indispensable for the biochemical function of Gwl in promoting mitotic entry.

Understanding the Polo Kinase machine

The Polo Kinase is a central regulator of cell division required for several events of mitosis and cytokinesis. In addition to a kinase domain (KD), Polo-like kinases (Plks) comprise a Polo-Box domain (PBD), which mediates protein interactions with targets and regulators of Plks. In all organisms that contain Plks, one Plk family member fulfills several essential functions in the regulation of cell division, and here we refer to this conserved protein as Polo Kinase (Plk1 in humans). The PBD and the KD are capable of both cooperation and mutual inhibition in their functions. Crystal structures of the PBD, the KD and, recently, a PBD-KD complex have helped understanding the inner workings of the Polo Kinase. In parallel, an impressive array of molecular mechanisms has been found to mediate the regulation of the protein. Moreover, the targeting of Polo Kinase in the development of anti-cancer drugs has yielded several molecules with which to chemically modulate Polo Kinase to study its biological functions. Here we review our current understanding of the protein function and regulation of Polo Kinase as a fascinating molecular device in control of cell division.

HSP90 inhibition enhances antimitotic drug-induced mitotic arrest and cell death in preclinical models of non-small cell lung cancer

HSP90 inhibitors are currently undergoing clinical evaluation in combination with antimitotic drugs in non-small cell lung cancer (NSCLC), but little is known about the cellular effects of this novel drug combination. Therefore, we investigated the molecular mechanism of action of IPI-504 (retaspimycin HCl), a potent and selective inhibitor of HSP90, in combination with the microtubule targeting agent (MTA) docetaxel, in preclinical models of NSCLC. We identified a subset of NSCLC cell lines in which these drugs act in synergy to enhance cell death. Xenograft models of NSCLC demonstrated tumor growth inhibition, and in some cases, regression in response to combination treatment. Treatment with IPI-504 enhanced the antimitotic effects of docetaxel leading to the hypothesis that the mitotic checkpoint is required for the response to drug combination. Supporting this hypothesis, overriding the checkpoint with an Aurora kinase inhibitor diminished the cell death synergy of IPI-504 and docetaxel. To investigate the molecular basis of synergy, an unbiased stable isotope labeling by amino acids in cell culture (SILAC) proteomic approach was employed. Several mitotic regulators, including components of the ubiquitin ligase, anaphase promoting complex (APC/C), were specifically down-regulated in response to combination treatment. Loss of APC/C by RNAi sensitized cells to docetaxel and enhanced its antimitotic effects. Treatment with a PLK1 inhibitor (BI2536) also sensitized cells to IPI-504, indicating that combination effects may be broadly applicable to other classes of mitotic inhibitors. Our data provide a preclinical rationale for testing the combination of IPI-504 and docetaxel in NSCLC.

Hsp90α forms a stable complex at the cilium neck for the interaction of signalling molecules in IGF-1 receptor signalling

The primary cilium is composed of an axoneme that protrudes from the cell surface, a basal body beneath the membrane and a transition neck in between. It is a sensory organelle on the plasma membrane, involved in mediating extracellular signals. In the transition neck region of the cilium, the microtubules change from triplet to doublet microtubules. This region also contains the transition fibres that crosslink the axoneme with the membrane and the necklace proteins that regulate molecules being transported into and out of the cilium. In this protein-enriched, complex area it is important to maintain the correct assembly of all of these proteins. Here, through immunofluorescent staining and protein isolation, we identify the molecular chaperone Hsp90α clustered at the periciliary base. At the transition neck region, phosphorylated Hsp90α forms a stable ring around the axoneme. Heat shock treatment causes Hsp90α to dissipate and induces resorption of cilia. We further identify that Hsp90α at the transition neck region represents a signalling platform on which IRS-1 interacts with intracellular downstream signalling molecules involved in IGF-1 receptor signalling.

Centrosomes at M phase act as a scaffold for the accumulation of intracellular ubiquitinated proteins

Centrosome size varies considerably during the cell cycle; it is greatest during metaphase, partly because of pericentriolar matrix recruitment and an increase in microtubule-organizing activity. However, the mechanism of centrosome maturation during M phase is poorly defined. In the present study, we identified and quantified centrosomal proteins during S and M phases using stable isotope labeling by amino acids in cell culture (SILAC) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS). We identified 991 proteins, of which 310 and 325 proteins were upregulated during S and M phases, respectively. Ubiquitinated proteins containing K48- and K63-linked polyubiquitin chains accumulated in the centrosomes during M phase, although 26S proteasome activity in the centrosomes did not markedly differ between S and M phases. Conversely, cytoplasmic dynein, which transports ubiquitinated proteins to the centrosomes, increased 2-fold in the centrosomes during M phase relative to S phase. Furthermore, PYR-41, a ubiquitin E1 inhibitor, reduced centrosome size during metaphase, causing increased aneuploidy. RNA interference suppression of Ecm29, which inhibits proteasome activity, decreased the accumulation of ubiquitinated proteins in the centrosomes. These results show that accumulation of ubiquitinated proteins promotes centrosome maturation during M phase and further suggest a novel function of centrosomes as a scaffold temporarily gathering intracellular ubiquitinated proteins.

The therapeutic target Hsp90 and cancer hallmarks

Hsp90 is a major molecular chaperone that is expressed abundantly and plays a pivotal role in assisting correct folding and functionality of its client proteins in cells. The Hsp90 client proteins include a wide variety of signal transducing molecules such as protein kinases and steroid hormone receptors. Cancer is a complex disease, but most types of human cancer share common hallmarks, including self-sufficiency in growth signals, insensitivity to growth-inhibitory mechanism, evasion of programmed cell death, limitless replicative potential, sustained angiogenesis, and tissue invasion and metastasis. A surprisingly large number of Hsp90-client proteins play crucial roles in establishing cancer cell hallmarks. We start the review by describing the structure and function of Hsp90 since conformational changes during the ATPase cycle of Hsp90 are closely related to its function. Many co-chaperones, including Hop, p23, Cdc37, Aha1, and PP5, work together with Hsp90 by modulating the chaperone machinery. Post-translational modifications of Hsp90 and its cochaperones are vital for their function. Many tumor-related Hsp90-client proteins, including signaling kinases, steroid hormone receptors, p53, and telomerase, are described. Hsp90 and its co-chaperones are required for the function of these tumor-promoting client proteins; therefore, inhibition of Hsp90 by specific inhibitors such as geldanamycin and its derivatives attenuates the tumor progression. Hsp90 inhibitors can be potential and effective cancer chemotherapeutic drugs with a unique profile and have been examined in clinical trials. We describe possible mechanisms why Hsp90 inhibitors show selectivity to cancer cells even though Hsp90 is essential also for normal cells. Finally, we discuss the "Hsp90-addiction" of cancer cells, and suggest a role for Hsp90 in tumor evolution.

The therapeutic target Hsp90 and cancer hallmarks

Hsp90 is a major molecular chaperone that is expressed abundantly and plays a pivotal role in assisting correct folding and functionality of its client proteins in cells. The Hsp90 client proteins include a wide variety of signal transducing molecules such as protein kinases and steroid hormone receptors. Cancer is a complex disease, but most types of human cancer share common hallmarks, including self-sufficiency in growth signals, insensitivity to growth-inhibitory mechanism, evasion of programmed cell death, limitless replicative potential, sustained angiogenesis, and tissue invasion and metastasis. A surprisingly large number of Hsp90-client proteins play crucial roles in establishing cancer cell hallmarks. We start the review by describing the structure and function of Hsp90 since conformational changes during the ATPase cycle of Hsp90 are closely related to its function. Many co-chaperones, including Hop, p23, Cdc37, Aha1, and PP5, work together with Hsp90 by modulating the chaperone machinery. Post-translational modifications of Hsp90 and its cochaperones are vital for their function. Many tumor-related Hsp90-client proteins, including signaling kinases, steroid hormone receptors, p53, and telomerase, are described. Hsp90 and its co-chaperones are required for the function of these tumor-promoting client proteins; therefore, inhibition of Hsp90 by specific inhibitors such as geldanamycin and its derivatives attenuates the tumor progression. Hsp90 inhibitors can be potential and effective cancer chemotherapeutic drugs with a unique profile and have been examined in clinical trials. We describe possible mechanisms why Hsp90 inhibitors show selectivity to cancer cells even though Hsp90 is essential also for normal cells. Finally, we discuss the "Hsp90-addiction" of cancer cells, and suggest a role for Hsp90 in tumor evolution.

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.

MLL5 maintains genomic integrity by regulating the stability of the chromosomal passenger complex through a functional interaction with Borealin

Mixed lineage leukemia 5 (MLL5) is a versatile nuclear protein associated with many cellular events. We have shown previously that phosphorylation of MLL5 by Cdk1 is required for mitotic entry. In this paper, the function of MLL5 in mitotic regulation is further explored. SiRNA-mediated downregulation of MLL5 caused improper chromosome alignment at metaphase and resulted in failure of DNA segregation and cytokinesis. Mechanistic studies revealed that the chromosomal passenger complex (CPC), which plays a key role in chromosomal bi-orientation, was delocalized from the inner centromere region because of proteasome-mediated degradation in MLL5-depleted cells. Biochemical analyses further demonstrated that the central domain of MLL5 interacted with the C-terminus of Borealin, and the interaction is essential to maintain the stability of Borealin. Moreover, the mitotic defects in MLL5-depleted cells were rescued by overexpression of FLAG-MLL5, but not by a FLAG-MLL5 mutant that did not contain the central domain. Collectively, our results suggest that MLL5 functionally interacts with Borealin, facilitates the expression of CPC, and hence contributes to mitotic fidelity and genomic integrity.

Cdc28 provides a molecular link between Hsp90, morphogenesis, and cell cycle progression in Candida albicans

The molecular chaperone Hsp90 regulates morphogenesis of the leading human fungal pathogen Candida albicans. Hsp90 inhibition induces filaments with a delay in mitotic exit mediated by the checkpoint protein Bub2. Hsp90 depletion destabilizes the cyclin-dependent kinase Cdc28, providing a link between Hsp90, cell cycle regulation, and morphogenesis.

The trimorphic fungus Candida albicans is the leading cause of systemic candidiasis, a disease with poor prognosis affecting immunocompromised individuals. The capacity of C. albicans to transition between morphological states is a key determinant of its ability to cause life-threatening infection. Recently the molecular chaperone heat shock protein 90 (Hsp90) was implicated as a major regulator of temperature-dependent C. albicans morphogenesis; compromising Hsp90 function induces filamentation and relieves repression of Ras1–protein kinase A (PKA) signaling, although the mechanism involved remains unknown. Here we demonstrate that filaments generated by compromise of Hsp90 function are neither pseudohyphae nor hyphae but closely resemble filaments formed in response to cell cycle arrest. Closer examination revealed that these filaments exhibit a delay in mitotic exit mediated by the checkpoint protein Bub2. Furthermore, Hsp90 inhibition also led to a distinct morphology with defects in cytokinesis. We found that the cyclin-dependent kinase Cdc28 was destabilized in response to depletion of Hsp90 and that Cdc28 physically interacts with Hsp90, implicating this major cell cycle regulator as a novel Hsp90 client protein in C. albicans. Taken together, our results suggest that Hsp90 is instrumental in the regulation of cell division during yeast-form growth in C. albicans and exerts its major effects during late cell cycle events.

Hsp90 in non-mammalian metazoan model systems

The molecular chaperone Hsp90 has been discovered in the heat-shock response of the fruit fly more than 30years ago. Today, it is becoming clear that Hsp90 is in the middle of a regulatory system, participating in the modulation of many essential client proteins and signaling pathways. Exerting these activities, Hsp90 works together with about a dozen of cochaperones. Due to their organismal simplicity and the possibility to influence their genetics on a large scale, many studies have addressed the function of Hsp90 in several multicellular model systems. Defined pathways involving Hsp90 client proteins have been identified in the metazoan model systems of Caenorhabditis elegans, Drosophila melanogaster and the zebrafish Danio rerio. Here, we summarize the functions of Hsp90 during muscle maintenance, development of phenotypic traits and the involvement of Hsp90 in stress responses, all of which were largely uncovered using the model organisms covered in this review. These findings highlight the many specific and general actions of the Hsp90 chaperone machinery. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Mammalian polo-like kinase 1-dependent regulation of the PBIP1-CENP-Q complex at kinetochores

Mammalian polo-like kinase 1 (Plk1) plays a pivotal role during M-phase progression. Plk1 localizes to specific subcellular structures through the targeting activity of the C-terminal polo-box domain (PBD). Disruption of the PBD function results in improper bipolar spindle formation, chromosome missegregation, and cytokinesis defect that ultimately lead to the generation of aneuploidy. It has been shown that Plk1 recruits itself to centromeres by phosphorylating and binding to a centromere scaffold, PBIP1 (also called MLF1IP and CENP-U[50]) through its PBD. However, how PBIP1 itself is targeted to centromeres and what roles it plays in the regulation of Plk1-dependent mitotic events remain unknown. Here, we demonstrated that PBIP1 directly interacts with CENP-Q, and this interaction was mutually required not only for their stability but also for their centromere localization. Plk1 did not appear to interact with CENP-Q directly. However, Plk1 formed a ternary complex with PBIP1 and CENP-Q through a self-generated p-T78 motif on PBIP1. This complex formation was central for Plk1-dependent phosphorylation of PBIP1-bound CENP-Q and delocalization of the PBIP1-CENP-Q complex from mitotic centromeres. This study reveals a unique mechanism of how PBIP1 mediates Plk1-dependent phosphorylation event onto a third protein, and provides new insights into the mechanism of how Plk1 and its recruitment scaffold, PBIP1-CENP-Q complex, are localized to and delocalized from centromeres.

Establishment of a paclitaxel resistant human breast cancer cell strain (MCF-7/Taxol) and intracellular paclitaxel binding protein analysis

Multidrug resistance of tumours is one of the most important factors that leads to chemotherapy failure. A multidrug-resistant breast cancer cell line, MCF-7/Taxol, was established from the drug-sensitive parent cell line MCF-7. The biological properties of MCF-7/Taxol, including its drug resistance profile and profile of paclitaxel binding proteins, were analysed and compared with the parent cell line. A number of paclitaxel binding proteins were present in MCF-7 cells but absent from MCF-7/Taxol cells, namely heat shock protein 90, actinin and dermcidin precursor. The identification of differential paclitaxel binding proteins between the multidrug-resistant MCF-7/Taxol cell line and the parent drug-sensitive cell line MCF-7 provides insight into possible mechanisms involved in resistance to these chemotherapy drugs.

Natural product Celastrol destabilizes tubulin heterodimer and facilitates mitotic cell death triggered by microtubule-targeting anti-cancer drugs

BACKGROUND

Microtubule drugs are effective anti-cancer agents, primarily due to their ability to induce mitotic arrest and subsequent cell death. However, some cancer cells are intrinsically resistant or acquire a resistance. Lack of apoptosis following mitotic arrest is thought to contribute to drug resistance that limits the efficacy of the microtubule-targeting anti-cancer drugs. Genetic or pharmacological agents that selectively facilitate the apoptosis of mitotic arrested cells present opportunities to strengthen the therapeutic efficacy.

METHODOLOGY AND PRINCIPAL FINDINGS

We report a natural product Celastrol targets tubulin and facilitates mitotic cell death caused by microtubule drugs. First, in a small molecule screening effort, we identify Celastrol as an inhibitor of neutrophil chemotaxis. Subsequent time-lapse imaging analyses reveal that inhibition of microtubule-mediated cellular processes, including cell migration and mitotic chromosome alignment, is the earliest events affected by Celastrol. Disorganization, not depolymerization, of mitotic spindles appears responsible for mitotic defects. Celastrol directly affects the biochemical properties of tubulin heterodimer in vitro and reduces its protein level in vivo. At the cellular level, Celastrol induces a synergistic apoptosis when combined with conventional microtubule-targeting drugs and manifests an efficacy toward Taxol-resistant cancer cells. Finally, by time-lapse imaging and tracking of microtubule drug-treated cells, we show that Celastrol preferentially induces apoptosis of mitotic arrested cells in a caspase-dependent manner. This selective effect is not due to inhibition of general cell survival pathways or mitotic kinases that have been shown to enhance microtubule drug-induced cell death.

CONCLUSIONS AND SIGNIFICANCE

We provide evidence for new cellular pathways that, when perturbed, selectively induce the apoptosis of mitotic arrested cancer cells, identifying a potential new strategy to enhance the therapeutic efficacy of conventional microtubule-targeting anti-cancer drugs.

Plk1-dependent and -independent roles of an ODF2 splice variant, hCenexin1, at the centrosome of somatic cells

Outer dense fiber 2 (ODF2) was initially identified as a major component of the sperm tail cytoskeleton, and was later suggested to be localized to somatic centrosomes and required for the formation of primary cilia. Here we show that a splice variant of hODF2 called hCenexin1, but not hODF2 itself, efficiently localizes to somatic centrosomes via a variant-specific C-terminal extension and recruits Plk1 through a Cdc2-dependent phospho-S796 motif within the extension. This interaction and Plk1 activity were important for proper recruitment of pericentrin and gamma-tubulin, and, ultimately, for formation of normal bipolar spindles. Earlier in the cell cycle, hCenexin1, but again not hODF2, also contributed to centrosomal recruitment of ninein and primary cilia formation independent of Plk1 interaction. These findings provide a striking example of how a splice-generated C-terminal extension of a sperm tail-associating protein mediates unanticipated centrosomal events at distinct stages of the somatic cell cycle.

Heat shock protein inhibitors, 17-DMAG and KNK437, enhance arsenic trioxide-induced mitotic apoptosis

Arsenic trioxide (ATO) has recently emerged as a promising therapeutic agent in leukemia because of its ability to induce apoptosis. However, there is no sufficient evidence to support its therapeutic use for other types of cancers. In this study, we investigated if, and how, 17-dimethylaminoethylamino-17-demethoxy-geldanamycin (17-DMAG), an antagonist of heat shock protein 90 (HSP90), and KNK437, a HSP synthesis inhibitor, potentiated the cytotoxic effect of ATO. Our results showed that cotreatment with ATO and either 17-DMAG or KNK437 significantly increased ATO-induced cell death and apoptosis. siRNA-mediated attenuation of the expression of the inducible isoform of HSP70 (HSP70i) or HSP90alpha/beta also enhanced ATO-induced apoptosis. In addition, cotreatment with ATO and 17-DMAG or KNK437 significantly increased ATO-induced mitotic arrest and ATO-induced BUBR1 phosphorylation and PDS1 accumulation. Cotreatment also significantly increased the percentage of mitotic cells with abnormal mitotic spindles and promoted metaphase arrest as compared to ATO treatment alone. These results indicated that 17-DMAG or KNK437 may enhance ATO cytotoxicity by potentiating mitotic arrest and mitotic apoptosis possibly through increased activation of the spindle checkpoint.

Sgt1, a co-chaperone of Hsp90 stabilizes Polo and is required for centrosome organization

Sgt1 was described previously in yeast and humans to be a Hsp90 co-chaperone and required for kinetochore assembly. We have identified a mutant allele of Sgt1 in Drosophila and characterized its function. Mutations in sgt1 do not affect overall kinetochore assembly or spindle assembly checkpoint. sgt1 mutant cells enter less frequently into mitosis and arrest in a prometaphase-like state. Mutations in sgt1 severely compromise the organization and function of the mitotic apparatus. In these cells, centrioles replicate but centrosomes fail to mature, and pericentriolar material components do not localize normally resulting in highly abnormal spindles. Interestingly, a similar phenotype was described previously in Hsp90 mutant cells and correlated with a decrease in Polo protein levels. In sgt1 mutant neuroblasts, we also observe a decrease in overall levels of Polo. Overexpression of the kinase results in a substantial rescue of the centrosome defects; most cells form normal bipolar spindles and progress through mitosis normally. Taken together, these findings suggest that Sgt1 is involved in the stabilization of Polo allowing normal centrosome maturation, entry and progression though mitosis.

Heat shock proteins in animal neoplasms and human tumours--a comparison

Heat shock proteins (HSPs) are implicated in all phases of cancer from proliferation, impaired apoptosis and sustained angiogenesis to invasion and metastasis. The presence of abnormal HSP levels in several human tumours suggests that these proteins could be used as diagnostic and/or prognostic markers, whilst the direct correlation between HSP expression and drug resistance in neoplastic tissues means they could also be used to predict cancer response to specific treatment. HSPs have also been successfully targeted in clinical trials modifying their expression or chaperone activity. Preliminary studies in veterinary medicine have also demonstrated the presence of altered HSP expression in neoplasms, and the study of carcinogenesis and the role of HSPs in animal models will surely be an additional source of information for clinical cancer research.

Arsenite-induced mitotic death involves stress response and is independent of tubulin polymerization

Arsenite, a known mitotic disruptor, causes cell cycle arrest and cell death at anaphase. The mechanism causing mitotic arrest is highly disputed. We compared arsenite to the spindle poisons nocodazole and paclitaxel. Immunofluorescence analysis of alpha-tubulin in interphase cells demonstrated that, while nocodazole and paclitaxel disrupt microtubule polymerization through destabilization and hyperpolymerization, respectively, microtubules in arsenite-treated cells remain comparable to untreated cells even at supra-therapeutic concentrations. Immunofluorescence analysis of alpha-tubulin in mitotic cells showed spindle formation in arsenite- and paclitaxel-treated cells but not in nocodazole-treated cells. Spindle formation in arsenite-treated cells appeared irregular and multi-polar. gamma-tubulin staining showed that cells treated with nocodazole and therapeutic concentrations of paclitaxel contained two centrosomes. In contrast, most arsenite-treated mitotic cells contained more than two centrosomes, similar to centrosome abnormalities induced by heat shock. Of the three drugs tested, only arsenite treatment increased expression of the inducible isoform of heat shock protein 70 (HSP70i). HSP70 and HSP90 proteins are intimately involved in centrosome regulation and mitotic spindle formation. HSP90 inhibitor 17-DMAG sensitized cells to arsenite treatment and increased arsenite-induced centrosome abnormalities. Combined treatment of 17-DMAG and arsenite resulted in a supra-additive effect on viability, mitotic arrest, and centrosome abnormalities. Thus, arsenite-induced abnormal centrosome amplification and subsequent mitotic arrest is independent of effects on tubulin polymerization and may be due to specific stresses that are protected against by HSP90 and HSP70.

The Opitz syndrome gene product MID1 assembles a microtubule-associated ribonucleoprotein complex

Opitz BBB/G syndrome (OS) is a heterogenous malformation syndrome mainly characterised by hypertelorism and hypospadias. In addition, patients may present with several other defects of the ventral midline such as cleft lip and palate and congenital heart defects. The syndrome-causing gene encodes the X-linked E3 ubiquitin ligase MID1 that mediates ubiquitin-specific modification and degradation of the catalytic subunit of the translation regulator protein phosphatase 2A (PP2A). Here, we show that the MID1 protein also associates with elongation factor 1alpha (EF-1alpha) and several other proteins involved in mRNA transport and translation, including RACK1, Annexin A2, Nucleophosmin and proteins of the small ribosomal subunits. Mutant MID1 proteins as found in OS patients lose the ability to interact with EF-1alpha. The composition of the MID1 protein complex was determined by several independent methods: (1) yeast two-hybrid screening and (2) immunofluorescence, (3) a biochemical approach involving affinity purification of the complex, (4) co-fractionation in a microtubule assembly assay and (5) immunoprecipitation. Moreover, we show that the cytoskeleton-bound MID1/translation factor complex specifically associates with G- and U-rich RNAs and incorporates MID1 mRNA, thus forming a microtubule-associated ribonucleoprotein (RNP) complex. Our data suggest a novel function of the OS gene product in directing translational control to the cytoskeleton. The dysfunction of this mechanism would lead to malfunction of microtubule-associated protein translation and to the development of OS.

Requirement for the budding yeast polo kinase Cdc5 in proper microtubule growth and dynamics

In many organisms, polo kinases appear to play multiple roles during M-phase progression. To provide new insights into the function of the budding yeast polo kinase Cdc5, we generated novel temperature-sensitive cdc5 mutants by mutagenizing the C-terminal noncatalytic polo box domain, a region that is critical for proper subcellular localization. One of these mutants, cdc5-11, exhibited a temperature-sensitive growth defect with an abnormal spindle morphology. Strikingly, provision of a moderate level of benomyl, a microtubule-depolymerizing drug, permitted cdc5-11 cells to grow significantly better than the isogenic CDC5 wild type in a FEAR (cdc Fourteen Early Anaphase Release)-independent manner. In addition, cdc5-11 required MAD2 for both cell growth and the benomyl-remedial phenotype. These results suggest that cdc5-11 is defective in proper spindle function. Consistent with this view, cdc5-11 exhibited abnormal spindle morphology, shorter spindle length, and delayed microtubule regrowth at the nonpermissive temperature. Overexpression of CDC5 moderately rescued the spc98-2 growth defect. Interestingly, both Cdc28 and Cdc5 were required for the proper modification of the spindle pole body components Nud1, Slk19, and Stu2 in vivo. They also phosphorylated these three proteins in vitro. Taken together, these observations suggest that concerted action of Cdc28 and Cdc5 on Nud1, Slk19, and Stu2 is important for proper spindle functions.

Polo-like kinase 1 may regulate G2/M transition of mouse fertilized eggs by means of inhibiting the phosphorylation of Tyr 15 of Cdc2

Polo-like kinase 1(Plk1) has been reported to be a multifunctional kinase that plays pivotal regulatory roles in microtubule assembly during mammalian early embryonic mitosis. In the present study, we examined the expression of Plk1 at protein and mRNA level in mouse fertilized eggs by Western blot and RT-PCR. We also examined the kinase activity of Plk1. At various developmental phases of mouse one-cell stage embryos, both the protein and the mRNA of Plk1 were uniformly distributed; but the kinase activity of Plk1 increased at G2/M phase and decreased at the end of M phase. At the meantime, the phosphorylation of Tyr 15 of Cdc2 was inhibited at M phase. To investigate its function in mammalian fertilized eggs further, we used specific short hairpin RNAs (shRNA) and scytonemin, the putative inhibitor of Plk1 to suppress the activity of Plk1 in mouse fertilized eggs. Upon blockage of the activation of with Plk1 shRNA and scytonemin in mouse one-cell stage embryos, the cleavage rate decreased and the phosphorylation level of Tyr 15 of Cdc2 increased. These results imply that the Plk1 may regulate cell cycle progression of mouse fertilized eggs by means of inhibiting the phosphorylation of Tyr 15 of Cdc2.

Targeting mitosis for anti-cancer therapy

Basic research that has focused on achieving a mechanistic understanding of mitosis has provided unprecedented molecular and biochemical insights into this highly complex phase of the cell cycle. The discovery process has uncovered an ever-expanding list of novel proteins that orchestrate and coordinate spindle formation and chromosome dynamics during mitosis. That many of these proteins appear to function solely in mitosis makes them ideal targets for the development of mitosis-specific cancer drugs. The clinical successes seen with anti-microtubule drugs such as taxanes and the vinca alkaloids have also encouraged the development of drugs that specifically target mitosis. Drugs that selectively inhibit mitotic kinesins involved in spindle and kinetochore functions, as well as kinases that regulate these activities, are currently in various stages of clinical trials. Our increased understanding of mitosis has also revealed that this process is targeted by inhibitors of farnesyl transferase, histone deacetylase, and Hsp90. Although these drugs were originally designed to block cell proliferation by inhibiting signaling pathways and altering gene expression, it is clear now that these drugs can also directly interfere with the mitotic process. The increased attention to mitosis as a chemotherapeutic target has also raised an important issue regarding the cellular determinants that specify drug sensitivity. One likely contribution is the mitotic checkpoint, a failsafe mechanism that delays mitotic exit so that cells whose chromosomes are not properly attached to the spindle have extra time to correct their errors. As the biochemical activity of the mitotic checkpoint is finite, cells cannot indefinitely sustain the delay, as in cases where cells are treated with anti-mitotic drugs. When the mitotic checkpoint activity is eventually lost, cells will exit mitosis and become aneuploid. While many of the aneuploid cells may die because of massive chromosome imbalance, survivors that continue to proliferate will no doubt be selected. This is clearly an undesirable outcome, thus efforts to obtain fundamental insights into why some cells that arrest in mitosis die without exiting mitosis will be exceedingly important in enhancing our understanding of the drug sensitivity of cancer cells.

Hsp90 is required to localise cyclin B and Msps/ch-TOG to the mitotic spindle in Drosophila and humans

During mitosis, cyclin B is extremely dynamic and although it is concentrated at the centrosomes and spindle microtubules (MTs) in organisms ranging from yeast to humans, the mechanisms that determine its localisation are poorly understood. To understand how cyclin B is targeted to different locations in the cell we have isolated proteins that interact with cyclin B in Drosophila embryo extracts. Here we show that cyclin B interacts with the molecular chaperone Hsp90 and with the MT-associated protein (MAP) Mini spindles (Msps; the Drosophila orthologue of XMAP215/ch-TOG). Both Hsp90 and Msps are concentrated at centrosomes and spindles, and we show that Hsp90, but not Msps, is required for the efficient localisation of cyclin B to these structures. We find that, unlike what happens with other cell cycle proteins, Hsp90 is not required to stabilise cyclin B or Msps during mitosis. Thus, we propose that Hsp90 plays a novel role in regulating the localisation of cyclin B and Msps during mitosis.

Identification of novel centrosomal proteins in Dictyostelium discoideum by comparative proteomic approaches

The centrosome functions as the main microtubule-organization center of the cell and is of importance for all microtubule-dependent processes such as organelle transport and directionality of cell migration. One of the major model organisms in centrosome research is the slime mold Dictyostelium discoideum. Since only 10 centrosomal proteins are known so far in Dictyostelium discoideum, the elucidation of new centrosomal components may give a more comprehensive understanding of centrosomal function. To distinguish between centrosomal and contaminating proteins we established different separation and relative quantification strategies including techniques such as iTRAQ and DIGE. In this work, we present the identification of several known components as well as more than 70 new candidates--currently subject of further investigations--for the protein inventory of the Dictyostelium centrosome. Among these protein identifications, 44% represent hypothetical proteins of still unknown function associated with the centrosome.

Analysis of expression of heat shock protein-90 (HSP90) and the effects of HSP90 inhibitor (17-AAG) in multiple myeloma

Heat shock protein 90 (HSP90) is required for structural folding and maintenance of conformational integrity of various proteins, including several associated with cellular signaling. Recent studies utilizing 17-allylamino-17-demethoxygeldanamycin (17-AAG), an inhibitor of HSP90, demonstrated an antitumor effect in solid tumors. To test whether HSP90 could be targeted in multiple myeloma (MM) patients, we first investigated expression of HSP90 by immunofluorescence and flow cytometric analysis in a myeloma cell line (U266) and primary myeloma cells. Following demonstration of HSP90 expression in myeloma cells, archival samples of 32 MM patients were analysed by immunoperoxidase staining. Myeloma cells in all patients showed strong cytoplasmic expression of HSP90 in all samples and 55% also demonstrated concurrent nuclear immunopositivity. Treatment of U266 and primary MM cells with 17AAG resulted in significantly increased apoptosis compared to untreated control cells. Analysis of anti-apoptotic BCL2 family proteins and akt in MM cells incubated with 17-AAG revealed down-regulation of BCL-2, BCL-XL, MCL-1 and akt. Furthermore, although a low concentration of bortezomib resulted in no cell death, a combination of 17AAG and bortezomib treatment revealed a synergistic apoptotic effect on the U266 cell line. These data suggest that targeted inhibition of HSP90 may prove to be a valid and innovative strategy for the development of future therapeutic options for MM patients.

Polo-like Kinases Inhibited by Wortmannin

Polo-like kinases play crucial roles throughout mitosis. We previously reported that wortmannin potently inhibits Polo-like kinase 1 (Plk1). In this study, we show that wortmannin also strongly inhibits Polo-like kinase 3 (Plk3). To further characterize this inhibition, we identified the sites of labeling on Plk1 and Plk3 targeted by AX7503, a tetramethylrhodamine-wortmannin conjugate. AX7503 labeling on Plk1 and Plk3 was found to occur on a conserved ATP binding site residue. In addition, we show that wortmannin inhibits Plk3 activity in live cells at concentrations commonly used to inhibit the more well known targets of wortmannin, the phosphoinositide 3-kinases. Importantly, we found that inhibition of Plk3 by wortmannin lead to a decrease in phosphorylation of p53 on serine 20 induced by DNA damage, demonstrating the effect of wortmannin on a downstream Plk3 target. Taken together, our results suggest that wortmannin can affect multiple functions of Plk3 in cell cycle progression and at the DNA damage check point. The identification of the labeling sites of Plk1 and Plk3 by AX7503 may be useful in designing more effective compounds to target Polo-like kinases for cancer treatment and also may be useful for the structural study of Plk domains.

Hsp90 inhibitors: small molecules that transform the Hsp90 protein folding machinery into a catalyst for protein degradation

The 90 kDa heat shock proteins (Hsp90) are responsible for the conformational maturation of nascent polypeptides and the renaturation of denatured proteins. In transformed cells, numerous mutated and overexpressed proteins rely on the Hsp90 protein folding machinery for tumor progression. The Hsp90-mediated protein folding process is dependent upon ATP, and when inhibitors of ATP are present, the Hsp90 machinery is unable to fold client proteins into their biologically active form, which results in the degradation of protein substrates via the ubiquitin-proteasome pathway. Consequently, Hsp90 has evolved into a promising anti-cancer target because multiple oncogenic proteins can be simultaneously degraded as a consequence of Hsp90 inhibition. This review serves to explain the Hsp90 protein folding process, the impact of Hsp90 inhibition, the identification of natural product inhibitors, and the development of rationally designed inhibitors of the Hsp90 protein folding machinery.

Inhibitors of Polo-like kinase reveal roles in spindle-pole maintenance

Polo-like kinases (Plks) have several functions in mitotic progression and are upregulated in many tumor types. Small-molecule Plk inhibitors would be valuable as tools for studying Plk biology and for developing antitumor agents. Guided by homology modeling of the Plk1 kinase domain, we have discovered a chemical series that shows potent and selective Plk1 inhibition. The effects of one such optimized benzthiazole N-oxide, cyclapolin 1 (1), on purified centrosomes indicate that Plks are required to generate MPM2 epitopes, recruit gamma-tubulin and enable nucleation of microtubules. The compound can also promote loss of centrosome integrity and microtubule nucleating ability apparently through increased accessibility of protein phosphatases. We show that treatment of living S2 cells with cyclapolin 1 leads to collapsed spindles, in contrast to the metaphase-arrested bipolar spindles observed after RNAi. This different response to protein depletion and protein inhibition may have significance in the development of antitumor agents.

Requirement of hCenexin for proper mitotic functions of polo-like kinase 1 at the centrosomes

Outer dense fiber 2 (Odf2) was initially identified as a major component of sperm tail cytoskeleton and later was suggested to be a widespread component of centrosomal scaffold that preferentially associates with the appendages of the mother centrioles in somatic cells. Here we report the identification of two Odf2-related centrosomal components, hCenexin1 and hCenexin1 variant 1, that possess a unique C-terminal extension. Our results showed that hCenexin1 is the major isoform expressed in HeLa cells, whereas hOdf2 is not detectably expressed. Mammalian polo-like kinase 1 (Plk1) is critical for proper mitotic progression, and its association with the centrosome is important for microtubule nucleation and function. Interestingly, depletion of hCenexin1 by RNA interference (RNAi) delocalized Plk1 from the centrosomes and the C-terminal extension of hCenexin1 was crucial to recruit Plk1 to the centrosomes through a direct interaction with the polo-box domain of Plk1. Consistent with these findings, the hCenexin1 RNAi cells exhibited weakened gamma-tubulin localization and chromosome segregation defects. We propose that hCenexin1 is a critical centrosomal component whose C-terminal extension is required for proper recruitment of Plk1 and other components crucial for normal mitosis. Our results further suggest that the anti-Odf2 immunoreactive centrosomal antigen previously detected in non-germ line cells is likely hCenexin1.

Heat shock protein expression in canine malignant mammary tumours

Background

Abnormal levels of Heat Shock Proteins (HSPs) have been observed in many human neoplasms including breast cancer and it has been demonstrated that they have both prognostic and therapeutic implications. In this study, we evaluated immunohistochemical expression of HSPs in normal and neoplastic canine mammary glands and confronted these results with overall survival (OS), in order to understand the role of HSPs in carcinogenesis and to establish their potential prognostic and/or therapeutic value.

Methods

Immunohistochemical expression of Hsp27, Hsp72, Hsp73 and Hsp90 was evaluated in 3 normal canine mammary glands and 30 malignant mammary tumours (10 in situ carcinomas, 10 invasive carcinomas limited to local structures without identifiable invasion of blood or lymphatic vessels, 10 carcinomas with invasion of blood or lymphatic vessels and/or metastases to regional lymph nodes). A semi-quantitative method was used for the analysis of the results.

Results

Widespread constitutive expression of Hsp73 and Hsp90 was detected in normal tissue, Hsp72 appeared to be focally distributed and Hsp27 showed a negative to rare weak immunostaining. In mammary tumours, a significant increase in Hsp27 (P < 0.01), Hsp72 (P < 0.05) and Hsp90 (P < 0.01) expression was observed as well as a significant reduction in Hsp73 (P < 0.01) immunoreactivity compared to normal mammary gland tissue. Hsp27 demonstrated a strong positivity in infiltrating tumour cells and metaplastic squamous elements of invasive groups. High Hsp27 expression also appeared to be significantly correlated to a shorter OS (P = 0.00087). Intense immunolabelling of Hsp72 and Hsp73 was frequently detected in infiltrative or inflammatory tumour areas. Hsp90 expression was high in all tumours and, like Hsp73, it also showed an intense positivity in lymphatic emboli.

Conclusion

These results suggest that Hsp27, Hsp72 and Hsp90 are involved in canine mammary gland carcinogenesis. In addition, Hsp27 appears to be implicated in tumour invasiveness and its high immunodetection in invasive tumours is indicative of a poorer clinical outcome.

HSP90: a rising star on the horizon of anticancer targets

Over the past decade, heat-shock protein (HSP)90 has begun to draw increasing attention as a novel anticancer target with unique features. As a molecular chaperone, HSP90 promotes the maturation and maintains the stability of a large number of conformationally labile client proteins, most of which are involved in biologic processes that are often deranged within tumor cells, such as signal transduction, cell-cycle progression and apoptosis. As a result, and in contrast to other molecular targeted therapeutics, inhibitors of HSP90 achieve their promising anticancer activity through simultaneous disruption of many oncogenic substrates within cancer cells. This review provides a brief summary of HSP90 biology and its association with cancer. It describes the discovery and development of HSP90 inhibitors as anticancer agents and their current status in the clinic. Finally, it closes with a discussion of the unique challenges confronting the further development of these agents and their prospects for the future.

Thermoregulation in the life cycle of nematodes

An unanswered question in the biology of many parasites is the mechanism by which environmental (or external) and intrinsic signals are integrated to determine the switch from one developmental stage to the next. This is particularly pertinent for nematode parasites, many of which have a free-living stage in the environment prior to infection of the mammalian host, or for parasites such as filarial nematodes, which utilise an insect vector for transmission. The environmental changes experienced by a parasite upon infection of a mammalian host are extremely complex and poorly understood. However, the ability of a parasite to sense its new environment must be intrinsically linked to its developmental programme, as progression of the life cycle is dependent upon the infection event. In this review, the relationship between temperature and development in filarial nematodes and in the free-living species Caenorhabditis elegans is summarised, with a focus on the role of heat shock factor and heat shock protein 90 in the nematode life cycle.

Hsp90 inhibitors in the clinic

Specific inhibitors of Hsp90 have recently entered human clinical trials. At the time of writing, trials have been initiated only in metastatic cancer, although a rationale exists for using these agents in a variety of human diseases where protein (mis)folding is involved in the disease pathophysiology. Hsp90 inhibitors offer a unique anti-cancer opportunity because they provide simultaneous combinatorial blockade of multiple oncogenic pathways. The first compound in this class, 17-AAG, has completed phase I trials and phase II trials are in progress. The toxicity has been manageable and evidence of possible clinical activity has been seen in metastatic melanoma, prostate cancer and multiple myeloma. Other inhibitors with improved properties are approaching clinical trials. This chapter presents an update of the current clinical trials using Hsp90 inhibitors, focussing on the areas that will be increasingly relevant in the next 5 years.