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

Heat shock proteins in neurodegenerative diseases: pathogenic roles and therapeutic implications

Neurodegenerative diseases including amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, and polyglutamine (polyQ) diseases are thought to be caused by protein misfolding. Heat shock proteins (HSPs), which function mainly as molecular chaperones, play an important role in the folding and quality control of proteins. The histopathological hallmark of neurodegenerative diseases is accumulation and/or inclusions of the disease-causing proteins in residual neurons in targeted regions of the nervous system. The inclusions combine with many components of molecular chaperone pathways and ubiquitin-proteasome, raising the possibility that misfolding and altered degradation of mutant proteins may be involved in the pathogenesis of neurodegenerative diseases. Overexpression of HSPs has been reported to reduce the number and size of inclusions and accumulation of disease-causing proteins, and ameliorate the phenotypes in neuronal cell and mouse models. Hsp90 inhibitors also exert therapeutic effects through selective proteasome degradation of its client proteins. Elucidation of its pathophysiology using animal models has led to the development of disease-modifying drugs, i.e., Hsp90 inhibitor and HSP inducer, which inhibit the pathogenic process of neuronal degeneration. These findings may provide the basis for development of an HSP-related therapy for neurodegenerative diseases.

Epigenetic repression of PDZ-LIM domain-containing protein 2: implications for the biology and treatment of breast cancer

The NF-kappaB transcription factor plays a pivotal role in breast cancer progression and therapy resistance. However, the mechanisms by which the tightly regulated NF-kappaB becomes constitutively activated during breast cancer pathogenesis remain obscure. Here, we report that PDZ-LIM domain-containing protein 2 (PDLIM2), an essential terminator of NF-kappaB activation, is repressed in both estrogen receptor-positive and estrogen receptor-negative breast cancer cells, suggesting one important mechanism for the constitutive activation of NF-kappaB. Indeed, PDLIM2 reexpression inhibited constitutive NF-kappaB activation and expression of NF-kappaB-targeted genes in those breast cancer cells. Importantly, PDLIM2, but not its mutants defective in NF-kappaB termination, could suppress in vitro anchorage-independent growth and in vivo tumor formation of those malignant breast cells. In addition, we have shown that PDLIM2 repression involves promoter methylation. Accordingly, treatment of the breast cancer cells with the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine reverses the methylation of the PDLIM2 promoter, restored PDLIM2 expression, and suppressed tumorigenicities of human breast cancer cells both in vitro and in vivo. These studies thus provide important mechanistic insights into breast cancer pathogenesis. These studies also suggest a tumor suppression function of PDLIM2 and a therapeutic strategy for breast cancer.

Autophagy and innate recognition systems

Autophagy is an ancient, highly conserved pathway responsible for the lysosomal degradation of cytosolic constituents and organelles that is critical in maintaining cellular homeostasis. Recent studies have illustrated an important interplay between autophagy and the innate immune system. Signaling through innate pattern recognition receptors leads to the induction of autophagy. Autophagy is utilized by the innate immune cells to survey for virus infection through delivery of cytosolic viral replication complexes to the endosomal viral sensors. In another case, key molecules in the autophagy pathway were found to negatively regulate cytosolic sensors of RNA viruses. Moreover, it has recently become apparent that the autophagic machinery is utilized by phagocytic cells for efficient phagocytosis and clearance of extracellular pathogens. These studies shed light on the possibility that molecules classically thought to be dedicated to the process of autophagy may function in important physiological processes independent of autophagy, whereby the double-membrane structures form within the cytosol to enclose organelles and long-lived proteins. In this chapter, we will highlight key findings relevant to the role of the autophagic machinery in the innate immune system.

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.

Human T-cell leukemia virus type I-mediated repression of PDZ-LIM domain-containing protein 2 involves DNA methylation but independent of the viral oncoprotein tax

Human T-cell leukemia virus type I (HTLV-I) is the etiological agent of adult T-cell leukemia (ATL). Our recent studies have shown that one important mechanism of HTLV-I-Mediated tumorigenesis is through PDZ-LIM domain-containing protein 2 (PDLIM2) repression, although the involved mechanism remains unknown. Here, we further report that HTLV-I-Mediated PDLIM2 repression was a pathophysiological event and the PDLIM2 repression involved DNA methylation. Whereas DNA methyltransferases 1 and 3b but not 3a were upregulated in HTLV-I-transformed T cells, the hypomethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) restored PDLIM2 expression and induced death of these malignant cells. Notably, the PDLIM2 repression was independent of the viral regulatory protein Tax because neither short-term induction nor long-term stable expression of Tax could downregulate PDLIM2 expression. These studies provide important insights into PDLIM2 regulation, HTLV-I leukemogenicity, long latency, and cancer health disparities. Given the efficient antitumor activity with no obvious toxicity of 5-aza-dC, these studies also suggest potential therapeutic strategies for ATL.

Role of oxidative stress in geldanamycin-induced cytotoxicity and disruption of Hsp90 signaling complex

Heat shock protein 90 (Hsp90) is a chaperone protein regulating PC-12 cell survival by binding and stabilizing Akt, Raf-1, and Cdc37. Hsp90 inhibitor geldanamycin (GA) cytotoxicity has been attributed to the disruption of Hsp90 binding, and the contribution of oxidative stress generated by its quinone group has not been studied in this context. Reactive oxygen species (ROS) and cell survival were assessed in PC-12 cells exposed to GA or menadione (MEN), and Akt, Raf-1, and Cdc37 expression and binding to Hsp90 were determined. GA disrupted Hsp90 binding and increased ROS production starting at 1 h, and cell death occurred at 6 h, inhibited by N-acetylcysteine (NAC) without preventing dissociation of proteins. At 24 h, NAC prevented cytotoxicity and Hsp90 complex disruption. However, MnTBAP antioxidant treatment failed to inhibit GA cytotoxicity, suggesting that NAC acts by restoring glutathione. In contrast, 24 h MEN treatment induced cytotoxicity without disrupting Hsp90 binding. GA and MEN decreased Hsp90-binding protein expression, and proteasomal inhibition prevented MEN-, but not GA-induced degradation. In conclusion, whereas MEN cytotoxicity is mediated by ROS and proteasomal degradation, GA-induced cytotoxicity requires ROS but induces Hsp90 complex dissociation and proteasome-independent protein degradation. These differences between MEN- and GA-induced cytotoxicity may allow more specific targeting of cancer cells.

Autophagy: from basic science to clinical application

Autophagy is a cellular pathway involved in protein and organelle degradation, which is likely to represent an innate adaptation to starvation. In times of nutrient deficiency, the cell can self-digest and recycle some nonessential components through nonselective autophagy, thus sustaining minimal growth requirements until a food source becomes available. Over recent years, autophagy has been implicated in an increasing number of clinical scenarios, notably infectious diseases, cancer, neurodegenerative diseases, and autoimmunity. The recent identification of the importance of autophagy genes in the genetic susceptibility to Crohn's disease suggests that a selective autophagic response may play a crucial role in the pathogenesis of common complex immune-mediated diseases. In this review, we discuss the autophagic mechanisms, their molecular regulation, and summarize their clinical relevance. This progress has led to great interest in the therapeutic potential of manipulation of both selective and nonselective autophagy in established disease.

Hsp27 inhibits IKKbeta-induced NF-kappaB activity and skeletal muscle atrophy

Heat shock protein 25/27 (Hsp25/27) is a cytoprotective protein that is ubiquitously expressed in most cells, and is up-regulated in response to cellular stress. Previous work, in nonmuscle cells, has shown that Hsp27 inhibits TNF-alpha-induced NF-kappaB activation. During skeletal muscle disuse, Hsp25/27 levels are decreased and NF-kappaB activity increased, and this increase in NF-kappaB activity is required for disuse muscle atrophy. Therefore, the purpose of the current study was to determine whether electrotransfer of Hsp27 into the soleus muscle of rats, prior to skeletal muscle disuse, is sufficient to inhibit skeletal muscle disuse atrophy and NF-kappaB activation. The 35% disuse muscle-fiber atrophy observed in nontransfected fibers was attenuated by 50% in fibers transfected with Hsp27. Hsp27 also inhibited the disuse-induced increase in MuRF1 and atrogin-1 transcription by 82 and 40%, respectively. Furthermore, disuse- and IKKbeta-induced NF-kappaB transactivation were abolished by Hsp27. In contrast, Hsp27 had no effect on Foxo transactivation. In conclusion, Hsp27 is a negative regulator of NF-kappaB in skeletal muscle, in vivo, and is sufficient to inhibit MuRF1 and atrogin-1 and attenuate skeletal muscle disuse atrophy.

Autophagy as an emerging dimension to adaptive and innate immunity

Autophagy is an evolutionary conserved cellular process during which cytoplasmic material is engulfed in double membrane vacuoles that then fuse with lysosomes, ultimately degrading their cargo. Emerging evidence, however, now suggests that autophagy can form part of our innate and adaptive immune defense programs. Recent studies have identified pattern recognition molecules as mediators of this process and shown that intracellular pathogens can interact with and even manipulate autophagy. Recent translational evidence has also implicated autophagy in the pathogenesis of several immune-mediated diseases, including Crohn disease. In this review, we present autophagy in the context of its role as an immune system component and effector and speculate on imminent and future research directions in this field.

Cyclodepsipeptide toxin promotes the degradation of Hsp90 client proteins through chaperone-mediated autophagy

Promoting the degradation of Hsp90 client proteins by inhibiting Hsp90, an important protein chaperone, has been shown to be a promising new anticancer strategy. In this study, we show that an oxazoline analogue of apratoxin A (oz-apraA), a cyclodepsipeptide isolated from a marine cyanobacterium, promotes the degradation of Hsp90 clients through chaperone-mediated autophagy (CMA). We identify a KFERQ-like motif as a conserved pentapeptide sequence in the kinase domain of epidermal growth factor receptor (EGFR) necessary for recognition as a CMA substrate. Mutation of this motif prevents EGFR degradation by CMA and promotes the degradation of EGFR through the proteasomal pathway in oz-apraA–treated cells. Oz-apraA binds to Hsc70/Hsp70. We propose that apratoxin A inhibits Hsp90 function by stabilizing the interaction of Hsp90 client proteins with Hsc70/Hsp70 and thus prevents their interactions with Hsp90. Our study provides the first examples for the ability of CMA to mediate degradation of membrane receptors and cross talks of CMA and proteasomal degradation mechanisms.

NFkappaB signaling in carcinogenesis and as a potential molecular target for cancer therapy

It has become increasingly clear that deregulation of the NFkappaB signaling cascade is a common underlying feature of many human ailments including cancers. The past two decades of intensive research on NFkappaB has identified the basic mechanisms that govern the functioning of this pathway but uncovering the details of why this pathway works differently in different cellular contexts or how it interacts with other signaling pathways remains a challenge. A thorough understanding of these processes is needed to design better and more efficient therapeutic approaches to treat complex diseases like cancer. In this review, we summarize the literature documenting the involvement of NFkappaB in cancer, and then focus on the approaches that are being undertaken to develop NFkappaB inhibitors towards treatment of human cancers.

Emerging roles for the ubiquitin-proteasome system and autophagy in pancreatic beta-cells

Protein degradation in eukaryotic cells is mediated primarily by the ubiquitin-proteasome system and autophagy. Turnover of protein aggregates and other cytoplasmic components, including organelles, is another function attributed to autophagy. The ubiquitin-proteasome system and autophagy are essential for normal cell function but under certain pathological conditions can be overwhelmed, which can lead to adverse effects in cells. In this review we will focus primarily on the insulin-producing pancreatic beta-cell. Pancreatic beta-cells respond to glucose levels by both producing and secreting insulin. The inability of beta-cells to secrete sufficient insulin is a major contributory factor in the development of type 2 diabetes. The aim of this review is to examine some of the crucial roles of the ubiquitin-proteasome system and autophagy in normal pancreatic beta-cell function and how these pathways may become dysfunctional under pathological conditions associated with metabolic syndromes.

Autophagy and lymphocyte homeostasis

Lymphocyte homeostasis is tightly regulated in vivo by various factors including cytokines, antigens, and costimulatory signals. Central to this regulation is the intricate balance between survival and apoptosis determined by pro- and antiapoptotic factors, including Bcl-2/Bcl-xL of the Bcl-2 family in the intrinsic death pathway and Fas/FADD of the TNF death receptor superfamily in the extrinsic death pathway. Recent studies have identified a critical role for autophagy, a well-conserved catabolic process in eukaryotic cells, in T and B lymphocyte homeostasis. Autophagy is essential for mature T lymphocyte survival and proliferation. In addition, autophagy can promote T cell death in defined physiologic or pathologic conditions. Autophagy also contributes to the survival of subsets of B lymphocytes, including developing pre-B cells as well as B1 B cells in vivo. Thus, autophagy represents a novel pathway regulating both developing and mature lymphocytes. Future studies are required to investigate the role of autophagy in regulating T and B cell homeostasis during immune responses to pathogens, as well as to define the mechanisms by which autophagy regulates lymphocyte death and survival.

Eating the enemy within: autophagy in infectious diseases

Autophagy is emerging as a central component of antimicrobial host defense against diverse viral, bacterial, and parasitic infections. In addition to pathogen degradation, autophagy has other functions during infection such as innate and adaptive immune activation. As an important host defense pathway, microbes have also evolved mechanisms to evade, subvert, or exploit autophagy. Additionally, some fungal pathogens harness autophagy within their own cells to promote pathogenesis. This review will highlight our current understanding of autophagy in infection, focusing on the most recent advances in the field, and will discuss the potential implications of these studies in the design of anti-infective therapeutics.

Roles of nuclear factor-kappaB in postischemic liver

Hepatic ischemia/reperfusion (I/R) results in a chain of events that culminate in liver dysfunction and injury. I/R injury is characterized by early oxidant stress followed by an intense acute inflammatory response that involves the transcription factor nuclear factor (NF)-kappaB. In addition to being a primary regulator of pro-inflammatory gene expression, NF-kappaB may play other roles in the hepatic response to I/R, such as mediating the expression of anti-apoptotic genes, preventing the accumulation of damaging reactive oxygen species, facilitating liver regeneration, and mediating the protective effects of ischemic preconditioning. In the present study, we review the diverse functions of NF-kappaB during hepatic I/R injury.

Autophagy: healthy eating and self-digestion for gastroenterologists

Autophagy is an evolutionary conserved mechanism in eukaryotic cells that is known to process redundant or defective cellular proteins and organelles. The recent renewal of interest in autophagy research has led to a significant expansion in our understanding of the importance of autophagy in cellular health and disease. This invited review summarizes key elements of autophagy research, emphasizes those of particular interest to gastroenterologists, and offers insights into present and future research directions.

Viral evasion of autophagy

Autophagy is an evolutionarily ancient pathway for survival during different forms of cellular stress, including infection with viruses and other intracellular pathogens. Autophagy may protect against viral infection through degradation of viral components (xenophagy), by promoting the survival or death of infected cells, through delivery of Toll-like receptor (TLR) ligands to endosomes to activate innate immunity, or by feeding antigens to MHC class II compartments to activate adaptive immunity. Given this integral role of autophagy in innate and adaptive antiviral immunity, selective pressure likely promoted the emergence of escape mechanisms by pathogenic viruses. This review will briefly summarize the current understanding of autophagy as an antiviral pathway, and then discuss strategies that viruses may utilize to evade this host defense mechanism.

A proteomic approach towards the Hsp90-dependent ubiquitinylated proteome

Since many Hsp90 client proteins are key players in tumour pathways, the ubiquitylation and subsequent degradation of Hsp90-substrates as a consequence of pharmacologically inhibiting Hsp90 represents an innovative approach for cancer therapy. We therefore identified Hsp90-binding proteins which accumulated as ubiquityl-tagged aggregates in the detergent insoluble fraction of HeLa cells as a consequence of simultaneously inhibiting Hsp90 and the proteasome. 2-DE followed by nanoLC-MS/MS of trypsinised protein spots provided the Hsp90-dependent ubiquitylated proteome which was finally annotated and functionally classified. The overall picture thus obtained emphasised the well-established role of Hsp90 in stabilising proteins involved in gene transcription and signal transduction. It also provided a novel Hsp90-related link to metabolic pathways as the inhibition of Hsp90 caused the ubiquitylation of a significant amount of metabolic enzymes. These findings serve to support cumulating indications which attribute Hsp90 to diverse stabilising functions beyond signal transduction and gene transcription.

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.