HSP90: a rising star on the horizon of anticancer targets

Role of HSP90 in Cancer

HSP90 is a vital chaperone protein conserved across all organisms. As a chaperone protein, it correctly folds client proteins. Structurally, this protein is a dimer with monomer subunits that consist of three main conserved domains known as the N-terminal domain, middle domain, and the C-terminal domain. Multiple isoforms of HSP90 exist, and these isoforms share high homology. These isoforms are present both within the cell and outside the cell. Isoforms HSP90α and HSP90β are present in the cytoplasm; TRAP1 is present in the mitochondria; and GRP94 is present in the endoplasmic reticulum and is likely secreted due to post-translational modifications (PTM). HSP90 is also secreted into an extracellular environment via an exosome pathway that differs from the classic secretion pathway. Various co-chaperones are necessary for HSP90 to function. Elevated levels of HSP90 have been observed in patients with cancer. Despite this observation, the possible role of HSP90 in cancer was overlooked because the chaperone was also present in extreme amounts in normal cells and was vital to normal cell function, as observed when the drastic adverse effects resulting from gene knockout inhibited the production of this protein. Differences between normal HSP90 and HSP90 of the tumor phenotype have been better understood and have aided in making the chaperone protein a target for cancer drugs. One difference is in the conformation: HSP90 of the tumor phenotype is more susceptible to inhibitors. Since overexpression of HSP90 is a factor in tumorigenesis, HSP90 inhibitors have been studied to combat the adverse effects of HSP90 overexpression. Monotherapies using HSP90 inhibitors have shown some success; however, combination therapies have shown better results and are thus being studied for a more effective cancer treatment.

Molecular Chaperones in Osteosarcoma: Diagnosis and Therapeutic Issues

Osteosarcoma (OS) is the most common form of primary bone tumor affecting mainly children and young adults. Despite therapeutic progress, the 5-year survival rate is 70%, but it drops drastically to 30% for poor responders to therapies or for patients with metastases. Identifying new therapeutic targets is thus essential. Heat Shock Proteins (HSPs) are the main effectors of Heat Shock Response (HSR), the expression of which is induced by stressors. HSPs are a large family of proteins involved in the folding and maturation of other proteins in order to maintain proteostasis. HSP overexpression is observed in many cancers, including breast, prostate, colorectal, lung, and ovarian, as well as OS. In this article we reviewed the significant role played by HSPs in molecular mechanisms leading to OS development and progression. HSPs are directly involved in OS cell proliferation, apoptosis inhibition, migration, and drug resistance. We focused on HSP27, HSP60, HSP70 and HSP90 and summarized their potential clinical uses in OS as either biomarkers for diagnosis or therapeutic targets. Finally, based on different types of cancer, we consider the advantage of targeting heat shock factor 1 (HSF1), the major transcriptional regulator of HSPs in OS.

It's not magic - Hsp90 and its effects on genetic and epigenetic variation

Canalization, or phenotypic robustness in the face of environmental and genetic perturbation, is an emergent property of living systems. Although this phenomenon has long been recognized, its molecular underpinnings have remained enigmatic until recently. Here, we review the contributions of the molecular chaperone Hsp90, a protein that facilitates the folding of many key regulators of growth and development, to canalization of phenotype - and de-canalization in times of stress - drawing on studies in eukaryotes as diverse as baker's yeast, mouse ear cress, and blind Mexican cavefish. Hsp90 is a hub of hubs that interacts with many so-called 'client proteins,' which affect virtually every aspect of cell signaling and physiology. As Hsp90 facilitates client folding and stability, it can epistatically suppress or enable the expression of genetic variants in its clients and other proteins that acquire client status through mutation. Hsp90's vast interaction network explains the breadth of its phenotypic reach, including Hsp90-dependent de novo mutations and epigenetic effects on gene regulation. Intrinsic links between environmental stress and Hsp90 function thus endow living systems with phenotypic plasticity in fluctuating environments. As environmental perturbations alter Hsp90 function, they also alter Hsp90's interaction with its client proteins, thereby re-wiring networks that determine the genotype-to-phenotype map. Ensuing de-canalization of phenotype creates phenotypic diversity that is not simply stochastic, but often has an underlying genetic basis. Thus, extreme phenotypes can be selected, and assimilated so that they no longer require environmental stress to manifest. In addition to acting on standing genetic variation, Hsp90 perturbation has also been linked to increased frequency of de novo variation and several epigenetic phenomena, all with the potential to generate heritable phenotypic change. Here, we aim to clarify and discuss the multiple means by which Hsp90 can affect phenotype and possibly evolutionary change, and identify their underlying common feature: at its core, Hsp90 interacts epistatically through its chaperone function with many other genes and their gene products. Its influence on phenotypic diversification is thus not magic but rather a fundamental property of genetics.

Geldanamycin and Its Derivatives Inhibit the Growth of Myeloma Cells and Reduce the Expression of the MET Receptor

Introduction. Geldanamycin (GA) is an ansamycin antibiotic that exhibits potent anti-neoplastic properties. The aim of this study was to assess the impact of GA and its derivatives on the growth and invasiveness of myeloma cell lines and CD138+ cells derived from the bone marrow of patients with multiple myeloma. Materials and methods. We evaluated cell proliferation, survival, apoptosis, cell cycle of myeloma cells, and the expression of cell surface proteins after incubation with geldanamycin or its derivatives. Results. GA and its analogs have an effect on myeloma cells by inhibiting their growth in a time and dose-dependent manner. Myeloma cell lines demonstrated decreased proliferation after incubation with 10 nM of GA or 100 nM GA analogs. The first significant effects of GA on U266 cells was observed after 24 hours. After 24 hours, U266 cells incubated with 100 nM GA were in both early and late stages of apoptosis; 17AEP and 17DMAG caused apoptosis of similar intensity to GA. It has been observed that GA and its derivatives cause caspase-3 activation. Analysis of the activity of AKT and MAP 42/44 kinases was performed by incubating U266 cells for 24 and 48 hours in100 nM of GA and its derivatives. After 24 hours incubation, no significant changes in protein expression were observed, while after 48 hours, the strongest changes were seen in AKT protein expression after incubation with GA and 17AEP-GA. In studies of the cell cycle, it was found that 100 nM 17AEP-GA and 17-DMAP-GA cause cell cycle abnormalities. We observed a nearly two-fold increase in U266 cells in the G1 phase and a simultaneous decrease in the percentage of cells in the G2/M phase, indicating that cells were halted in the G1 phase. In the case of the INA6 cells, proliferation was halted in both the G1 and G2/M phases. Conclusions. GA and the analogues that we tested can inhibit myeloma cell growth by induction of apoptosis and blockage of cell cycle progression, and have an effect on the down-regulation of the MET receptor. The GA derivatives tested, despite their modifications still retain strong anticancer properties. Specifically, two analogues of GA, 17AEP-GA and 17DMAG due to their properties can be more effective and safer chemotherapeutic agents than 17AAG, which is currently used and described in literature.

Synthesis and biological evaluation of coumarin replacements of novobiocin as Hsp90 inhibitors

Since Hsp90 modulates all six hallmarks of cancer simultaneously, it has become an attractive target for the development of cancer chemotherapeutics. In an effort to develop more efficacious compounds for Hsp90 inhibition, novobiocin analogues were prepared by replacing the central coumarin core with naphthalene, quinolinone, and quinoline surrogates. These modifications allowed for modification of the 2-position, which was previously unexplored. Biological evaluation of these compounds suggests a hydrophobic pocket about the 2-position of novobiocin. Anti-proliferative activities of these analogues against multiple cancer cell lines identified 2-alkoxyquinoline derivatives to exhibit improved activity.

Hsp90 inhibitor 17-DMAG decreases expression of conserved herpesvirus protein kinases and reduces virus production in Epstein-Barr virus-infected cells

All eight human herpesviruses have a conserved herpesvirus protein kinase (CHPK) that is important for the lytic phase of the viral life cycle. In this study, we show that heat shock protein 90 (Hsp90) interacts directly with each of the eight CHPKs, and we demonstrate that an Hsp90 inhibitor drug, 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), decreases expression of all eight CHPKs in transfected HeLa cells. 17-DMAG also decreases expression the of the endogenous Epstein-Barr virus protein kinase (EBV PK, encoded by the BGLF4 gene) in lytically infected EBV-positive cells and inhibits phosphorylation of several different known EBV PK target proteins. Furthermore, 17-DMAG treatment abrogates expression of the human cytomegalovirus (HCMV) kinase UL97 in HCMV-infected human fibroblasts. Importantly, 17-DMAG treatment decreased the EBV titer approximately 100-fold in lytically infected AGS-Akata cells without causing significant cellular toxicity during the same time frame. Increased EBV PK expression in 17-DMAG-treated AGS-Akata cells did not restore EBV titers, suggesting that 17-DMAG simultaneously targets multiple viral and/or cellular proteins required for efficient viral replication. These results suggest that Hsp90 inhibitors, including 17-DMAG, may be a promising group of drugs that could have profound antiviral effects on herpesviruses.

Targeting malignant mitochondria with therapeutic peptides

The current status of peptides that target the mitochondria in the context of cancer is the focus of this review. Chemotherapy and radiotherapy used to kill tumor cells are principally mediated by the process of apoptosis that is governed by the mitochondria. The failure of anticancer therapy often resides at the level of the mitochondria. Therefore, the mitochondrion is a key pharmacological target in cancer due to many of the differences that arise between malignant and healthy cells at the level of this ubiquitous organelle. Additionally, targeting the characteristics of malignant mitochondira often rely on disruption of protein--protein interactions that are not generally amenable to small molecules. We discuss anticancer peptides that intersect with pathological changes in the mitochondrion.

Proteotoxic stress of cancer: implication of the heat-shock response in oncogenesis

Organisms frequently encounter a wide variety of proteotoxic stressors. The heat-shock response, an ancient cytoprotective mechanism, has evolved to augment organismal survival and longevity in the face of proteotoxic stress from without and within. These broadly recognized beneficial effects, ironically, contrast sharply with its emerging role as a culprit in the pathogenesis of cancers. Here, we present an overview of the normal biology of the heat-shock response and highlight its implications in oncogenic processes, including the proteotoxic stress phenotype of cancer; the function of this stress response in helping cancer survive and adapt to proteotoxic stress; and perturbation of proteome homeostasis in cancer as a potential therapeutic avenue.

High-content, high-throughput analysis of cell cycle perturbations induced by the HSP90 inhibitor XL888

BACKGROUND

Many proteins that are dysregulated or mutated in cancer cells rely on the molecular chaperone HSP90 for their proper folding and activity, which has led to considerable interest in HSP90 as a cancer drug target. The diverse array of HSP90 client proteins encompasses oncogenic drivers, cell cycle components, and a variety of regulatory factors, so inhibition of HSP90 perturbs multiple cellular processes, including mitogenic signaling and cell cycle control. Although many reports have investigated HSP90 inhibition in the context of the cell cycle, no large-scale studies have examined potential correlations between cell genotype and the cell cycle phenotypes of HSP90 inhibition.

METHODOLOGY/PRINCIPAL FINDINGS

To address this question, we developed a novel high-content, high-throughput cell cycle assay and profiled the effects of two distinct small molecule HSP90 inhibitors (XL888 and 17-AAG [17-allylamino-17-demethoxygeldanamycin]) in a large, genetically diverse panel of cancer cell lines. The cell cycle phenotypes of both inhibitors were strikingly similar and fell into three classes: accumulation in M-phase, G2-phase, or G1-phase. Accumulation in M-phase was the most prominent phenotype and notably, was also correlated with TP53 mutant status. We additionally observed unexpected complexity in the response of the cell cycle-associated client PLK1 to HSP90 inhibition, and we suggest that inhibitor-induced PLK1 depletion may contribute to the striking metaphase arrest phenotype seen in many of the M-arrested cell lines.

CONCLUSIONS/SIGNIFICANCE

Our analysis of the cell cycle phenotypes induced by HSP90 inhibition in 25 cancer cell lines revealed that the phenotypic response was highly dependent on cellular genotype as well as on the concentration of HSP90 inhibitor and the time of treatment. M-phase arrest correlated with the presence of TP53 mutations, while G2 or G1 arrest was more commonly seen in cells bearing wt TP53. We draw upon previous literature to suggest an integrated model that accounts for these varying observations.

Hsp90 inhibitors block outgrowth of EBV-infected malignant cells in vitro and in vivo through an EBNA1-dependent mechanism

EBV causes infectious mononucleosis and is associated with certain malignancies. EBV nuclear antigen 1 (EBNA1) mediates EBV genome replication, partition, and transcription, and is essential for persistence of the viral genome in host cells. Here we demonstrate that Hsp90 inhibitors decrease EBNA1 expression and translation, and that this effect requires the Gly-Ala repeat domain of EBNA1. Hsp90 inhibitors induce the death of established, EBV-transformed lymphoblastoid cell lines at doses nontoxic to normal cells, and this effect is substantially reversed when lymphoblastoid cell lines are stably infected with a retrovirus expressing a functional EBNA1 mutant lacking the Gly-Ala repeats. Hsp90 inhibitors prevent EBV transformation of primary B cells, and strongly inhibit the growth of EBV-induced lymphoproliferative disease in SCID mice. These results suggest that Hsp90 inhibitors may be particularly effective for treating EBV-induced diseases requiring the continued presence of the viral genome.

The neuroblastoma tumour-suppressor TrkAI and its oncogenic alternative TrkAIII splice variant exhibit geldanamycin-sensitive interactions with Hsp90 in human neuroblastoma cells

Hsp90 chaperones stabilize many tyrosine kinases including several oncogenes, which are inhibited or induced to degrade by the Hsp90 inhibitor geldanamycin (GA). As a consequence, GA has been developed for future chemotherapeutic use in several tumour types including neuroblastoma (NB). Alternative splicing of the neurotrophin receptor tyrosine kinase TrkA may have a pivotal function in regulating NB behaviour, with reports suggesting that tumour-suppressing signals from TrkA may be converted to oncogenic signals by stress-regulated alternative TrkAIII splicing. Within this context, it is important to know whether Hsp90 interacts with TrkA variants in NB cells and how GA influences this. Here, we report that both TrkAI and TrkAIII are Hsp90 clients in human NB cells. TrkAI exhibits GA-sensitive interaction with Hsp90 required for receptor endoplasmic reticulum export, maturation, cell surface stabilization and ligand-mediated activation, whereas TrkAIII exhibits GA-sensitive interactions with Hsp90 required for spontaneous activity and to a lesser extent stability. We show that GA inhibits proliferation and induces apoptosis of TrkAI expressing NB cells, whereas TrkAIII reduces the sensitivity of NB cells to GA-induced elimination. Our data suggest that GA-sensitive interactions with Hsp90 are critical for both TrkAI tumour suppressor and TrkAIII oncogenic function in NB and that TrkAIII expression exerts a negative impact on GA-induced NB cell eradication, which can be counteracted by a novel TrkAIII-specific peptide nucleic acid inhibitor.

New developments in Hsp90 inhibitors as anti-cancer therapeutics: mechanisms, clinical perspective and more potential

The molecular chaperone Hsp90 (heat shock protein 90) is a promising target in cancer therapy. Preclinical and clinical evaluations of a variety of Hsp90 inhibitors have shown anti-tumor effect as a single agent and in combination with chemotherapy. Current Hsp90 inhibitors are categorized into several classes based on distinct modes of inhibition, including (i) blockade of ATP binding, (ii) disruption of co-chaperone/Hsp90 interactions, (iii) antagonism of client/Hsp90 associations and (iv) interference with post-translational modifications of Hsp90. The different functions of Hsp90 isoforms and the isoform selectivity of drugs need further investigation. The correlation of cell surface Hsp90 with cancer metastasis and the emerging involvement of Hsp90 inhibition in cancer stem cells have become exciting areas that could be exploited. Therefore, the aim of this review is (1) to summarize the up-to-date knowledge of mechanistic studies and clinical prospect of currently available Hsp90 inhibitors, (2) to enhance our perspectives for designing and discovering novel Hsp90 inhibitors, and (3) to provide an insight into less-understood potential of Hsp90 inhibition in cancer therapy.

A proteomic approach for evaluating the cell response to a novel histone deacetylase inhibitor in colon cancer cells

Epigenetic inactivation of gene expression is a general phenomenon associated with malignant transformation. Recently, we have found that a novel series of histone deacetylases (HDAC) inhibitors exhibit a broad-spectrum inhibition profile characterized by a marked effect on acetylation of histone and non-histone proteins. RC307, a representative compound of this series, caused a growth-inhibitory effect in colon carcinoma cells HCT116 associated with G2 accumulation and induction of apoptosis. The present study was designed to investigate the effect of RC307 on protein expressions in the HCT116 cells following treatment with cytotoxic drug concentrations. HCT116 cells were cultured in the absence or presence of RC307 and total cell lysates, as well as nuclear proteins, were extracted. The protein samples were then subjected to two-dimensional polyacrylamide gel electrophoresis, and the 2D gel images were compared to discover the protein changes caused by RC307 treatment. A total of 48 and 46 different spots were found to be modulated by RC307 in total lysates and nuclear proteome of HCT116 cell line. The modulated proteins were identified by tandem mass spectrometry. We found that RC307 exposure modulates proteins that are involved in proliferation, cell cycle regulation, apoptosis, gene expression, as well as chromatin and cytoskeleton organization.

Akt shows variable sensitivity to an Hsp90 inhibitor depending on cell context

Hsp90 inhibitors are currently in clinical trials for cancer therapy based on their ability to promote proteasomal degradation of oncogenic protein kinases and nuclear receptors. Results from recent studies suggest that cancer cells are more sensitive to these inhibitors than cells from healthy tissues. We analyzed an immortalized cell line Ba/F3 for sensitivity to the Hsp90 inhibitor geldanamycin in the absence and presence of the oncogenic tyrosine fusion kinase NPM-ALK expressed from a retroviral vector. Our results showed that NPM-ALK expression makes Akt and Cdk4 more resistant to degradation in the presence of geldanamycin, and there was a slightly reduced amount of apoptosis. The mechanism underlying the effect of NPM-ALK on Akt stability was probed by comparison of the turnover of the kinase after translation inhibition and geldanamycin treatment. We observed that Akt was degraded more rapidly in the presence of GA than upon translation inhibition without NPM-ALK expression. This suggests that NPM-ALK protects the mature kinase. Furthermore, Akt failed to bind to the Cdc37 chaperone in cells expressing NPM-ALK, which also correlates with increased Akt stability.

Hsp90 regulates processing of NF-kappa B2 p100 involving protection of NF-kappa B-inducing kinase (NIK) from autophagy-mediated degradation

NF-kappaB-inducing kinase (NIK) is required for NF-kappaB activation based on the processing of NF-kappaB2 p100. Here we report a novel mechanism of NIK regulation involving the chaperone 90 kDa heat shock protein (Hsp90) and autophagy. Functional inhibition of Hsp90 by the anti-tumor agent geldanamycin (GA) efficiently disrupts its interaction with NIK, resulting in NIK degradation and subsequent blockage of p100 processing. Surprisingly, GA-induced NIK degradation is mediated by autophagy, but largely independent of the ubiquitin-proteasome system. Hsp90 seems to be specifically involved in the folding/stabilization of NIK protein, because GA inhibition does not affect NIK mRNA transcription and translation. Furthermore, Hsp90 is not required for NIK-mediated recruitment of the alpha subunit of IkappaB kinase to p100, a key step in induction of p100 processing. These findings define an alternative mechanism for Hsp90 client degradation and identify a novel function of autophagy in NF-kappaB regulation. These findings also suggest a new therapeutic strategy for diseases associated with p100 processing.

Autophagy and NF-kappaB: fight for fate

Autophagy/macroautophagy is known for its role in cellular homeostasis, development, cell survival, aging, immunity, cancer and neurodegeneration. However, until recently, the mechanisms by which autophagy contributes to these important processes were largely unknown. The demonstration of a direct cross-talk between autophagy and NF-kappaB opens up new frontiers for deciphering the role of autophagy in diverse biological processes. Here, we review our current understanding of autophagy, with a focus on its role in tumor suppression, NF-kappaB inactivation and selective protein degradation in mammals. We also list some most intriguing and outstanding questions that are likely to engage researchers in the near future.

Evolutionary constraints on chaperone-mediated folding provide an antiviral approach refractory to development of drug resistance

The genome diversity of RNA viruses allows for rapid adaptation to a wide variety of adverse conditions. Accordingly, viruses can escape inhibition by most antiviral compounds targeting either viral or host factors. Here we exploited the capacity of RNA viruses for rapid adaptation to explore the evolutionary constraints of chaperone-mediated protein folding. We hypothesized that inhibiting a host molecular chaperone required for folding of a viral protein would force the virus to evolve an alternate folding strategy. We identified the chaperone Hsp90 as an essential factor for folding and maturation of picornavirus capsid proteins. Pharmacological inhibition of Hsp90 impaired the replication of poliovirus, rhinovirus, and coxsackievirus in cell culture. Strikingly, anti-Hsp90 treatment did not yield drug-resistant viruses, suggesting that the complexity of capsid folding precludes the emergence of alternate folding pathways. These results reveal tight evolutionary constraints on chaperone-mediated protein folding, which may be exploited for viral inhibition in vivo. Indeed, Hsp90 inhibitors drastically reduced poliovirus replication in infected animals without the emergence of drug-resistant escape mutants. We propose that targeting folding of viral proteins may provide a general antiviral strategy that is refractory to development of drug resistance.

Apoptosis versus cell differentiation: role of heat shock proteins HSP90, HSP70 and HSP27

Heat shock proteins HSP27, HSP70 and HSP90 are molecular chaperones whose expression is increased after many different types of stress. They have a protective function helping the cell to cope with lethal conditions. The cytoprotective function of HSPs is largely explained by their anti-apoptotic function. HSPs have been shown to interact with different key apoptotic proteins. As a result, HSPs can block essentially all apoptotic pathways, most of them involving the activation of cystein proteases called caspases. Apoptosis and differentiation are physiological processes that share many common features, for instance, chromatin condensation and the activation of caspases are frequently observed. It is, therefore, not surprising that many recent reports imply HSPs in the differentiation process. This review will comment on the role of HSP90, HSP70 and HSP27 in apoptosis and cell differentiation. HSPs may determine de fate of the cells by orchestrating the decision of apoptosis versus differentiation.

A non-toxic Hsp90 inhibitor protects neurons from Abeta-induced toxicity

The molecular chaperones have been implicated in numerous neurodegenerative disorders in which the defining pathology is misfolded proteins and the accumulation of protein aggregates. In Alzheimer's disease, hyperphosphorylation of tau protein results in its dissociation from microtubules and the formation of pathogenic aggregates. An inverse relationship was demonstrated between Hsp90/Hsp70 levels and aggregated tau, suggesting that Hsp90 inhibitors that upregulate these chaperones could provide neuroprotection. We recently identified a small molecule novobiocin analogue, A4 that induces Hsp90 overexpression at low nanomolar concentrations and sought to test its neuroprotective properties. A4 protected neurons against Abeta-induced toxicity at low nanomolar concentrations that paralleled its ability to upregulate Hsp70 expression. A4 exhibited no cytotoxicity in neuronal cells at the highest concentration tested, 10 microM, thus providing a large therapeutic window for neuroprotection. In addition, A4 was transported across BMECs in vitro, suggesting the compound may permeate the blood-brain barrier in vivo. Taken together, these data establish A4, a C-terminal inhibitor of Hsp90, as a potent lead for the development of a novel class of compounds to treat Alzheimer's disease.

Small molecules, big players: the National Cancer Institute's Initiative for Chemical Genetics

In 2002, the National Cancer Institute created the Initiative for Chemical Genetics (ICG), to enable public research using small molecules to accelerate the discovery of cancer-relevant small-molecule probes. The ICG is a public-access research facility consisting of a tightly integrated team of synthetic and analytical chemists, assay developers, high-throughput screening and automation engineers, computational scientists, and software developers. The ICG seeks to facilitate the cross-fertilization of synthetic chemistry and cancer biology by creating a research environment in which new scientific collaborations are possible. To date, the ICG has interacted with 76 biology laboratories from 39 institutions and more than a dozen organic synthetic chemistry laboratories around the country and in Canada. All chemistry and screening data are deposited into the ChemBank web site (http://chembank.broad.harvard.edu/) and are available to the entire research community within a year of generation. ChemBank is both a data repository and a data analysis environment, facilitating the exploration of chemical and biological information across many different assays and small molecules. This report outlines how the ICG functions, how researchers can take advantage of its screening, chemistry and informatic capabilities, and provides a brief summary of some of the many important research findings.

Hsp90 inhibition results in autophagy-mediated proteasome-independent degradation of IκB kinase (IKK)

Autophagic and proteasomal proteolysis are two major pathways for degradation of cellular constituents. Current models suggest that autophagy is responsible for the nonselective bulk degradation of long-lived proteins and organelles while the proteasome specifically degrades short-lived proteins including misfolded proteins caused by the absence of Hsp90 function. Here, we show that the IkappaB kinase (IKK), an essential activator of NF-kappaB, is selectively degraded by autophagy when Hsp90 is inhibited by geldanamycin (GA), a specific Hsp90 inhibitor showing highly effective anti-tumor activity. We find that in this case inactivation of ubiquitination or proteasome fails to block IKK degradation. However, inhibition of autophagy by an autophagy inhibitor or knockout of Atg5, a key component of the autophagy pathway, significantly rescues IKK from GA-induced degradation. These findings provide the first evidence that an Hsp90 client may be degraded by a mechanism different from the proteasome pathway and establish a molecular link among Hsp90, NF-kappaB and autophagy.