Targeting the proteasome pathway

Background: The ubiquitin–proteasome pathway functions as a main pathway in intracellular protein degradation and plays a vital role in almost all cellular events. Various inhibitors of this pathway have been developed for research purposes. The recent approval of bortezomib (PS-341, Velcade^®), a proteasome inhibitor, for the treatment of multiple myeloma has opened the way to the discovery of drugs targeting the proteasome and other components of the ubiquitin–proteasome pathway. Objectives: We review the current understanding of the ubiquitin–proteasome pathway and inhibitors targeting this pathway, including proteasome inhibitors, as candidate drugs for chemical therapy. Methods: Preclinical and clinical data for inhibitors of the proteasome and the ubiquitin–proteasome pathway are discussed. Conclusions: The proteasome and other members in the ubiquitin–proteasome pathway have emerged as novel therapeutic targets.     

Targeting apoptosis in solid tumors: the role of bortezomib from preclinical to clinical evidence

The ubiquitin-proteasome pathway is the main proteolytic system present in the nucleus and cytoplasm of all eukaryotic cells. Apoptosis activation induced by ubiquitin-proteasome pathway inhibition makes the proteasome a new target of anticancer therapy. Bortezomib is the first proteasome inhibitor to be approved by the US FDA; in 2003 as a third line and in 2005 as a second line therapy for the treatment of multiple myeloma only. This review focuses on the use of bortezomib, not only in its therapeutic role but also, more specifically, in its biologic role and discusses the most recent applications of the drug in solid tumors, both at a preclinical and clinical level.     

Proteasomes as drug targets

The ubiquitin-proteasome pathway plays a role in the degradation of the bulk of proteins in the cytoplasmic and nuclear compartments. In this pathway proteins are targeted for degradation by covalent ligation with ubiquitin, a reaction that requires ATP. Following the binding of the first ubiquitin molecule with the epsilon-amino group of a lysine residue of the substrate protein, a polyubiquitin chain is usually formed, in which the C-terminus of each ubiquitin unit is linked to a specific Lys residue of the previous ubiquitin. Central to this pathway is the 26S proteasome, a high molecular mass multifunctional protease which requires ATP for its catalytic activity. Substrates of the 26S proteasome are not only old or damaged proteins, but also short lived proteins functioning as regulatory factors in a large array of cellular processes, such as cell cycle progression, cell growth and gene expression, inflammatory response and immune surveillance. A number of inhibitors of the catalytic activity of proteasomes have been developed and successfully employed in the study of their functional and structural properties, as well as of their involvement in different cellular processes. Some of these molecules due to their toxicity are used only as experimental research tools; others instead are now in clinical trials for treatment of a variety of hematologic malignancies and solid tumors and of reperfusion injury occurring after cerebral ischemia and myocardial infarction. Furthermore, proteasome inhibitors are described to interfere with HIV maturation, budding and aggressiveness, and cytostatic drugs, as well as antiretroviral agents used in HAART, have been shown to behave in vitro and in cultured cell lines as inhibitors of proteasome proteolytic activity at therapeutic dosages.     

泛素-蛋白酶体途径在病毒感染中的作用研究进展

泛素-蛋白酶体途径是真核生物中非溶酶体蛋白降解的主要系统,主要包括泛素,26S蛋白酶体和酶系统E1、E2、E3。泛素-蛋白酶体参与调节细胞周期进程、抗原递呈、转录和信号转导等多种细胞生理过程。研究发现,病毒可以利用泛素系统调控病毒的基因转录、抑制细胞凋亡、降解抗病毒蛋白、促使病毒出芽和释放等逃避宿主的免疫监视。深入理解泛素-蛋白酶体在病毒感染中的作用可以为抗病毒治疗提供新思路。     

Ubiquitin-proteasome pathway components as therapeutic targets for CNS maladies

In the central nervous system (CNS), abnormal deposition of insoluble protein aggregates or inclusion bodies within nerve cells is commonly observed in association with several neurodegenerative diseases. The ubiquitinated protein aggregates are believed to result from malfunction or overload of the ubiquitin-proteasome pathway or from structural changes in the protein substrates which prevent their recognition and degradation by the ubiquitin-proteasome pathway. Impaired proteolysis might also contribute to the synaptic dysfunction seen early in neurodegenerative diseases because the ubiquitin-proteasome pathway is known to play a role in normal functioning of synapses. Because specificity of the ubiquitin proteasome mediated proteolysis is determined by specific ubiquitin ligases (E3s), identification of specific E3s and their allosteric modulators are likely to provide effective therapeutic targets for the treatment of several CNS disorders. Another unexplored area for the discovery of drug targets is the proteasome. Although many inhibitors of the proteasome are available, no effective drugs exist that can stimulate the proteasome. Since abnormal protein aggregation is a common feature of different neurodegenerative diseases, enhancement of proteasome activity might be an efficient way to remove the aggregates that accumulate in the brain. In this review, we discuss how the components of the ubiquitin-proteasome pathway could be potential targets for therapy of CNS diseases and disorders.     

Macroautophagy modulates cellular response to proteasome inhibitors in cancer therapy

Macroautophagy and the ubiquitin–proteasome system are two complementary pathways for protein degradation. The former degrades long-lived proteins and damaged organelles while the later degrades short-lived proteins. Recent findings indicate that suppression of the ubiquitin–proteasome system by proteasome inhibitors induces macroautophagy through multiple pathways, including (1) accumulation of ubiquitinated proteins and activation of HDAC6; (2) activation of the IRE1-JNK pathway; (3) proteasomal stabilization of ATF4; (4) inhibition of mTOR complex 1 signaling; (5) reduced proteasomal degradation of LC3. Induction of macroautophagy attenuates the antitumor effect of proteasome inhibitors in various types of cancer. These findings suggest that inhibition of macroautophagy may represent a novel strategy to enhance cellular sensitivity to proteasome inhibition.     

Targeting the ubiquitin–proteasome system for cancer therapy

Introduction: The ubiquitin–proteasome system (UPS) degrades 80 – 90% of intracellular proteins. Cancer cells take advantage of the UPS for their increased growth and decreased apoptotic cell death. Thus, the components that make up the UPS represent a diverse group of potential anti-cancer targets. The success of the first-in-class proteasome inhibitor bortezomib not only proved that the proteasome is a feasible and valuable anti-cancer target, but also inspired researchers to extensively explore other potential targets of this pathway.

Areas covered: This review provides a broad overview of the UPS and its role in supporting cancer development and progression, especially in aspects of p53 inactivation, p27 turnover and NF-κB activation. Also, efforts toward the development of small molecule inhibitors (SMIs) targeting different steps in this pathway for cancer treatment are reviewed and discussed.

Expert opinion: Whereas some of the targets in the UPS, such as the 20S proteasome, Nedd8 activating enzyme and HDM2, have been well-established and validated, there remains a large pool of candidates waiting to be investigated. Development of SMIs targeting the UPS has been largely facilitated by state-of-the-art technologies such as high-throughput screening and computer-assisted drug design, both of which require a better understanding of the targets of interest.     

The ubiquitin-proteasome system and assays to determine responses to inhibitors

Importance of the field: Proteasome inhibition is an important therapeutic modality. Additionally, given the toxicities of direct proteasome inhibition, interest is increasing in modulating the ubiquitin ligases in the ubiquitin-proteasome system (UPS). Areas covered in this review: A detailed examination of the ubiquitin-proteasome pathway and an examination of methods of inhibiting this pathway from a variety of targets including the proteasome, the ubiquitin ligases and molecular biology techniques. Special attention is given to the assays used to measure modulation of the ubiquitin-proteasome pathway. What the reader will gain: A thorough examination of the UPS and its role in cells and disease and an overview of several assays for analyzing the effect of inhibitors on the UPS. Significant detail is given to assays of the ligase system and molecular approaches. These assays have their own advantages and disadvantages and will allow investigators to make informed choices on investigating the UPS. Take home message: Interrupting the UPS can have profound consequences for cellular health and disease progression. The ubiquitin-proteasome pathway contains multiple activities that cannot be definitively assayed by a single technique. Assaying the UPS requires investigators to use multiple corroborating techniques and avoid confounding issues within each technique.     

肝细胞癌中泛素蛋白表达及其与临床病理学的相关性

目的探讨肝细胞癌中泛素蛋白表达及其临床病理学意义。方法应用免疫组化方法在组织微阵列上检测泛素蛋白表达,并分析其与临床病理学指标之间相关性。结果肝细胞癌中泛素蛋白表达明显增强,并与肿瘤分级、分型、乙型肝炎病毒感染密切相关(P分别<0.05,0.01和0.05),与患者性别、年龄、肿瘤大小、癌旁状况、AFP水平等指标无相关性(P>0.05)。结论蛋白酶体功能增强是肝细胞癌发生、发展的重要机制,检测肿瘤中泛素蛋白表达可用于指导临床治疗。     

Evodiamine-induced human melanoma A375-S2 cell death was mediated by PI3K/Akt/caspase and Fas-L/NF-κB signaling pathways and augmented by ubiquitin–proteasome inhibition

Evodiamine, a major alkaloidal component of Evodiae fructus exhibits anti-tumor activities. We have previously reported that evodiamine has a marked inhibitory effect on IL-1 sensitive human melanoma A375-S2 cells proliferation, and this action might be through inactivation of PI3K signaling. However, the detailed molecular mechanisms of evodiamine-induced cell death remains poorly understood. In present study, we further confirmed that Akt is the main effector molecule involved in this pathway. Evodiamine also led to IκBα phosphorylation and degradation that reflect translocation of NF-κB. Pretreatment of A375-S2 cells with ubiquitin–proteasome inhibitor MG132 was shown to aggregate the evodiamine caused cell death at 24 h. In addition, MG132 reduced ERK phosphorylation, increased caspase-3 activation, Fas-L expression and Bcl-2 cleavage in evodiamine-treated A375-S2 cells. These results suggested the PI3K/Akt/caspase and Fas-L/NF-κB signaling pathways might account for the responses of A375-S2 cell death induced by evodiamine, and these signals could be augmented by ubiquitin–proteasome pathway.     

依托泊苷通过蛋白酶体通路诱导白血病细胞凋亡

目的研究拓扑异构酶Ⅱ抑制剂依托泊苷诱导白血病细胞HL-60凋亡的分子机制。方法细胞增殖和凋亡采用四唑氮盐（MTT）法和流式细胞仪测定,基因芯片技术检测依托泊苷作用于HL-60细胞2h后基因表达谱的变化。应用Gen—MAPP分析软件分析细胞内反应通路。结果依托泊苷半数抑制浓度（IC50）为（30．17±0．26）μmo]／L。依托泊苷促进HL-60细胞凋亡,凋亡百分率由对照组4．38％增加到药物处理组的53．96％,能显著抑制细胞内蛋白酶体降解通路（Z≥3．8）,抑制蛋白酶体通路中的PSMB5、PSMB7、PSMB8、PSMC3、PSMC5、RPN1和HIJA—A表达。结论依托泊苷抑制DNA拓扑异构酶Ⅱ可通过抑制细胞内蛋白酶体相关基因表达导致细胞凋亡。     

Cycle inhibiting factors (cifs): cyclomodulins that usurp the ubiquitin-dependent degradation pathway of host cells

Cycle inhibiting factors (Cifs) are type III secreted effectors produced by diverse pathogenic bacteria. Cifs are "cyclomodulins" that inhibit the eukaryotic host cell cycle and also hijack other key cellular processes such as those controlling the actin network and apoptosis. This review summarizes current knowledge on Cif since its first characterization in enteropathogenic Escherichia coli, the identification of several xenologues in distant pathogenic bacteria, to its structure elucidation and the recent deciphering of its mode of action. Cif impairs the host ubiquitin proteasome system through deamidation of ubiquitin or the ubiquitin-like protein NEDD8 that regulates Cullin-Ring-ubiquitin Ligase (CRL) complexes. The hijacking of the ubiquitin-dependent degradation pathway of host cells results in the modulation of various cellular functions such as epithelium renewal, apoptosis and immune response. Cif is therefore a powerful weapon in the continuous arm race that characterizes host-bacteria interactions.     

Proteasome inhibitors in the clinical setting: benefits and strategies to overcome multiple myeloma resistance to proteasome inhibitors

The majority of intracellular proteins undergo degradation through the ubiquitin-proteasome pathway. The proteasome pathway has a role in regulating cell proliferation, differentiation, survival and apoptosis. The naturally occurring proteasome inhibitor lactacystin was the first proteasome inhibitor noted to induce apoptosis in vitro. Compared with first-generation proteasome inhibitors, bortezomib (PS-341), a dipeptide boronic acid, has exhibited higher potency and specificity, and has been approved for the treatment of relapsed or refractory myeloma. However, there are some patients who do not respond to therapy or who respond briefly and then relapse. It is becoming increasingly clear that myeloma cells respond to the stress caused by proteasome inhibitors (bortezomib) via rapidly up-regulating pathways that suppress apoptosis, thus attenuating its antitumour activity. The delineation of these molecular pathways and mechanisms to circumvent them are needed to allow this important class of agents to remain vital in the armamentarium of the management of multiple myeloma and other malignancies.     

Potential effects of tetrodotoxin exposure to human glial cells postulated using microarray approach.

Sodium channels play an important role in many neurological disorders and also in prostate cancer. Tetrodotoxin (TTX), a blocker of voltage-gated sodium channels has been chiefly used as a molecular probe for the study and characterization of these channels. The regulation of gene expression in response for the exposure of TTX to glial cells which are reported to be involved in neurodegenerative process is poorly understood. Therefore, the present study aims to develop a repository of genes and map it on a few pivotal neurodegenerative pathways to speculate the effect of TTX. Using Affymetrix GeneChip (HG-U133A), we have selected a subset of 692 differentially expressed genes, several of which are-cullin 4A (CUL4A), ubiquitin carrier protein (E2-EPF), proteasome (prosome, macropain) subunit, beta type, 8 (large multifunctional protease 7) (PSMB8), protein tyrosine phosphatase type IVA (PTP4A1), intercellular adhesion molecule 1 (ICAM1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caspase 1 (CASP1). These genes, which facilitate some of the neurodegenerative pathways, such as ubiquitin, proteasome, inflammation and kinases, were identified to be up- or down-regulated for the TTX treatment. Thus, the selected genes were further examined on ubiquitin-proteasome mediated inflammatory responses pathway as ample evidence for the role of glial cell-mediated inflammation in the neurodegenerative process are available. In summary, our result provides a basic understanding of the differentially expressed genes along with one of the possible pathway which may have been modulated by the exposure of TTX.     

Targeting the ubiquitin-proteasome pathway in cancer therapy

The ubiquitin-proteasome pathway plays a central role in the degradation of proteins involved in several pathways including the cell cycle, cellular proliferation and apoptosis. Bortezomib is the first proteasome inhibitor to enter clinical use, and received approval by the Food and Drug Administration (FDA) for the treatment of patients with multiple myeloma, therefore validating inhibition of the proteasome as an anticancer target. The approval of Bortezomib was based on a large, international, multicenter phase III trial showing its efficacy and safety compared with conventional therapy. Preclinical data also demonstrates the synergistic effect of bortezomib with other chemotherapeutic agents and its ability to overcome drug resistance. Since then several other proteasome inhibitors have been developed. The anti-tumor activities of bortezomib have been attributed to its effect on pro-apoptotic pathways including the inhibition of NF-kappaB and induction of endoplasmic reticulum stress. However, the molecular mechanisms are not fully understood. In this review, we will summarize the molecular mechanism of apoptosis by bortezomib.     

Proteasome inhibitors as anti-cancer agents

The ubiquitin (Ub)-proteasome pathway is the major nonlysosomal pathway of proteolysis in human cells and accounts for the degradation of most short-lived, misfolded or damaged proteins. This pathway is important in the regulation of a number of key biological regulatory mechanisms. Proteins are usually targeted for proteasome-mediated degradation by polyubiquitinylation, the covalent addition of multiple units of the 76 amino acid protein Ub, which are ligated to 1-amino groups of lysine residues in the substrate. Polyubiquitinylated proteins are degraded by the 26S proteasome, a large, ATP-dependent multicatalytic protease complex, which also regenerates monomeric Ub. The targets of this pathway include key regulators of cell proliferation and cell death. An alternative form of the proteasome, termed the immunoproteasome, also has important functions in the generation of peptides for presentation by MHC class I molecules. In recent years there has been a great deal of interest in the possibility that proteasome inhibitors, through elevation of the levels of proteasome targets, might prove useful as a novel class of anti-cancer drugs. Here we review the progress made to date in this area and highlight the potential advantages and weaknesses of this approach.