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1.  Advances in the Understanding of Mechanisms and Therapeutic Use of Bortezomib 
Discovery medicine  2011;12(67):471-480.
The ubiquitin-proteasome pathway regulates many basic cellular processes and has been proven to be a promising target for cancer therapy. Bortezomib is the first U.S. Food and Drug Administration (FDA) approved proteasome inhibitor used in the treatment of newly diagnosed multiple myeloma, relapsed/refractory multiple myeloma, and mantle cell lymphoma. The anti-cancer mechanisms of bortezomib elucidated by preclinical studies include: upregulation of proapoptotic proteins (e.g., Noxa, IκB), inhibition of NFκB and its anti-apoptotic target genes, suppression of several anti-apoptotic proteins (e.g., Bcl-XL, Bcl-2, and STAT-3), down-regulation of expression of several proteins involved in DNA repair pathways, and induction of endoplasmic reticulum (ER) stress and pro-apoptotic Unfolded Protein Response (UPR). Bortezomib has potent chemo-/radio-sensitizing effects and can overcome traditional drug resistance in tumors when used in combination with potential chemotherapies. Although bortezomib has been successful in improving clinical outcomes when used in hematological malignancies, relapse may occur in those patients who responded initially. Furthermore, some cytotoxicities (such as peripheral neuropathy) were found to be associated with bortezomib treatment. These observations have encouraged researchers to search for the next generation proteasome inhibitors (including carfilzomib and marizomib) that could overcome bortezomib resistance and have improved properties, reduced toxicities, and broader anticancer activities, based on the lessons learned from the mechanisms and use of bortezomib. This review summarizes the current status of bortezomib as well as several other proteasome inhibitors that are currently under clinical and preclinical investigation.
PMCID: PMC4139918  PMID: 22204764
2.  Metal-based 2,3-indolinedione derivatives as proteasome inhibitors and inducers of apoptosis in human cancer cells 
Proliferation and apoptotic pathways are tightly regulated in cells by the ubiquitin-proteasome system (UPS). Alterations in the UPS may result in cellular transformation or other pathological conditions. The proteasome is indeed often found to be overactive in cancer cells. It has been reported that 2,3-indolinedione (L), which exists in marine organisms, as well as in mammals, is a proteasome inhibitor. Studies have shown that metal-based complexes inhibit proteasome activity and induce apoptosis in certain human cancer cells. In the current study, we synthesized six novel metal-based complexes with derivatives of 2,3-indolinedione: [Cd (C15H11O3N2) (CH3COO)] (C1), [Cd (C15H11O2N2) (CH3COO)] (C2), [Co (C15H9O4N2) (CH3COO)] (C3), [Co (C15H11O2N2) (CH3COO)] (C4), [Zn (C19H14O3N3) (CH3COO)] (C5) and [Zn (C17H13O3N2) (CH3COO)] (C6). We sought to characterize and assess the proteasome inhibitory and anti-proliferative effects of these metal-based complexes in human breast (MDA-MB-231) and prostate (LNCaP and PC-3) cancer cells, in order to determine whether specific structures contribute to the inhibition of tumor proteasome activity and the induction of apoptosis. The results revealed that the complexes, C1, C3 and C5, but not their counterparts, C2, C4 and C6, inhibited the chymotrypsin-like activity of the human cancer cellular 26S proteasome; in addition, these complexes promoted the accumulation of the proteasome target protein, Bax, inhibited cell growth and induced apoptosis in a concentration- and time-dependent manner due to their unique structures. Our data suggest that the study of metal-based complexes, including aromatic ring structures with electron-attracting groups, may be an interesting research direction for the development of anticancer drugs.
PMCID: PMC4121346  PMID: 25017797
ubiquitin-proteasome system; metal-based complexes; 2,3-indolinedione derivative; inhibitor; electron-attracting group
3.  The Interplay of AMP-activated Protein Kinase and Androgen Receptor in Prostate Cancer Cells† 
Journal of cellular physiology  2014;229(6):688-695.
AMP-activated protein kinase (AMPK) has recently emerged as a potential target for cancer therapy due to the observation that activation of AMPK inhibits tumor cell growth. It is well-known that androgen receptor (AR) signaling is a major driver for the development and progression of prostate cancer and that downregulation of AR is a critical step in the induction of apoptosis in prostate cancer cells. However, little is known about the potential interaction between AMPK and AR signaling pathways. In the current study, we showed that activation of AMPK by metformin caused decrease of AR protein level through suppression of AR mRNA expression and promotion of AR protein degradation, demonstrating that AMPK activation is upstream of AR downregulation. We also showed that inhibition of AR function by an anti-androgen or its siRNA enhanced AMPK activation and growth inhibition whereas overexpression of AR delayed AMPK activation and increased prostate cancer cellular resistance to metformin treatment, suggesting that AR suppresses AMPK signaling-mediated growth inhibition in a feedback mechanism. Our findings thus reveal a novel AMPK-AR regulatory loop in prostate cancer cells and should have a potential clinical significance.
PMCID: PMC3947449  PMID: 24129850
AMP-activated protein kinase; Androgen receptor; Prostate cancer; Metformin; Apoptosis
4.  Regulation of Metformin Response by Breast Cancer Associated Gene 2123 
Neoplasia (New York, N.Y.)  2013;15(12):1379-1390.
Adenosine monophosphate-activated protein kinase (AMPK), a master regulator of cellular energy homeostasis, has emerged as a promising molecular target in the prevention of breast cancer. Clinical trials using the United States Food and Drug Administration (FDA)-approved, AMPK-activating, antidiabetic drug metformin are promising in this regard, but the question of why metformin is protective for some women but not others still remains. Breast cancer associated gene 2 (BCA2/Rabring7/RNF115), a novel Really Interesting New Gene (RING) finger ubiquitin E3 ligase, is overexpressed in >50% of breast tumors. Herein, we report that BCA2 is an endogenous inhibitor of AMPK activation in breast cancer cells and that BCA2 inhibition increases the efficacy of metformin. BCA2 overexpression inhibited both basal and inducible Thr172 phosphorylation/activation of AMPKα1, while BCA2-specific small interfering RNA (siRNA) enhanced phosphorylated AMPKα1 (pAMPKα1). The AMPK-suppressive function of BCA2 requires its E3 ligase-specific RING domain, suggesting that BCA2 targets some protein controlling (de)phosphorylation of AMPKα1 for degradation. Activation of AMPK by metformin triggered a growth inhibitory signal but also increased BCA2 protein levels, which correlated with AKT activation and could be curbed by an AMPK inhibitor, suggesting a potential feedback mechanism from pAMPKα1 to pAkt to BCA2. Finally, BCA2 siRNA, or inhibition of its upstream stabilizing kinase AKT, increased the growth inhibitory effect of metformin in multiple breast cancer cell lines, supporting the conclusion that BCA2 weakens metformin's efficacy. Our data suggest that metformin in combination with a BCA2 inhibitor may be a more effective breast cancer treatment strategy than metformin alone.
PMCID: PMC3884529  PMID: 24403860
5.  Molecular study on copper-mediated tumor proteasome inhibition and cell death 
The metal ion copper is a cofactor essential for maintaining normal biological and physical functions in human beings. High copper levels have been found in variety of tumor tissues and are involved in tumor angiogenesis processes. The ubiquitin-proteasome system plays an important role in cell growth and apoptosis and has been shown as a novel target for cancer therapy. We previously reported that some organic copper complexes can inhibit the proteasomal chymotrypsin-like activity and induce apoptosis in human cancer cells and xenograft models. In the current study, we investigated the effect of oxidation status of copper, Cu(I) or Cu(II), on inhibition of proteasome activity, induction of apoptosis, and induction of reactive oxygen species (ROS) in human cancer cells. We report four major findings here: i) both Cu(I) and Cu(II) could inhibit the chymotrypsin-like activity of purified 20S proteasome, but Cu(I) was more potent than Cu(II), ii) purified 20S proteasome protein was able to reduce Cu(II) to Cu(I), suggesting that Cu(I) is the oxidation status of copper that directly reacts with the proteasome, iii) when complexed with the copper ligand neocuproine, Cu(I) showed higher ability to induce ROS production in cancer cells, compared with Cu(II), iv) addition of a ROS scavenger in the cancer cell culture-blocked copper-induced ROS generation, but did not overcome copper-mediated proteasome-inhibitory and cell death-inducing events, demonstrating the ROS-independent proteasome-inhibitory property of copper complexes.
PMCID: PMC3809063  PMID: 20514399
copper; oxidation status; proteasome inhibition; apoptosis; ROS
6.  Induction of tumor cell apoptosis by a novel class of N-thiolated β-lactam antibiotics with structural modifications at N1 and C3 of the lactam ring 
The investigation of novel anti-tumor agents that preferentially select for malignant cells with a tolerable toxicity level has been the focus of anti-cancer drug discovery. Our laboratories have previously reported that certain N-alkylthiolated β-lactams had DNA-damaging and apoptosis-inducing activity in various tumor lines but not in nontransformed cells. In the current study, we further delineated the effects of substitutions at C3 or N1 of the lactam ring for cell death-inducing capability with close attention paid to a discernible structure-activity relationship (SAR). We found that two β-lactam analogs (JG-5 and JG-19), both containing a branched-chain moiety at C3 of the lactam ring, exhibit potent apoptosis-inducing activity. Additionally, JG-5 exhibited superior in vitro biological activity over JG-19 owing to structural modifications made to substituents at the N1 and C3 positions of the lactam ring. Furthermore, the branched β-lactams were able to inhibit growth of mice bearing breast cancer xenografts, associated with induction of DNA damage and apoptosis in tumor tissues. Our results strongly warrant further investigation into these novel β-lactams as potential anti-cancer therapeutics.
PMCID: PMC3786433  PMID: 18506361
β-lactams; structure-activity relationship; apoptosis; DNA damage; tumor cell death
7.  Novel 8-hydroxylquinoline analogs induce copper-dependent proteasome inhibition and cell death in human breast cancer cells 
International journal of oncology  2009;35(6):1481-1491.
An elevated level of copper (Cu), which is necessary for the growth and metastasis of tumor cells, has been found in many types of cancer, including breast, prostate, lung and brain. Although its molecular basis is unclear, this tumor-specific Cu elevation has been proposed to be a novel target for developing selective anti-cancer therapies. We previously reported that 8-hydroxylquinoline (8-OHQ) is able to form a Cu complex that inhibits the proteasome and induces apoptosis in cultured cancer cells. Toward the goal of discovering novel 8-OHQ analogs as potential anti-copper and anti-cancer drugs, in the current study we synthesized several 8-OHQ analogs and their copper complexes and evaluated their biological activities in human breast cancer cells. We report that when substitutions are made on the hydroxyl group of 8-OHQ, their copper mixtures have profound effects on the proteasome-inhibitory and apoptosis-inducing abilities in breast cancer MDA-MB-231 cells. In addition, the proteasome-inhibitory and apoptosis-inducing activities of 8-OHQ analog-copper mixtures are determined by both the polarity and position of the substituents. Finally, a synthetic complex of 8-OHQ analog-copper was able to inhibit the proteasome activity, induce cell death and suppress the growth selectively in breast cancer MDA-MB-231 cells, but not in normal immortalized human breast MCF-10A cells. Our results support the concept that human cancer cells and tissues, which contain an elevated copper level and are highly dependent on proteasome activity for their survival, should be sensitive to treatment with anti-copper drugs such as the novel 8-OHQ analogs described here.
PMCID: PMC3779835  PMID: 19885572
8-hydroxylquinoline; copper; anti-copper drugs; structure activity relationship; proteasome inhibitors; apoptosis
8.  Induction of tumor cell apoptosis by taurine Schiff base copper complex is associated the with inhibition of proteasomal activity 
Schiff bases have been intensively investigated due to their antibacterial and antitumor properties. Copper is a cofactor essential for the tumor angiogenesis processes, whereas other transition metals are not. Consistently, high serum or tissue levels of copper were found in many types of human cancer including breast, prostate, colon, lung, and brain, supporting the idea that copper could be used as a novel selective target for cancer therapies. In the current study we hypothesize that a synthetic taurine Schiff base copper complex (Compound 1) could suppress tumor cell growth via the direct inhibition of proteasome activity. Compound 1 potently inhibits the activity of purified 20S and 26S proteasome in human breast cancer MDA-MB-231 and leukemia Jurkat T cells. Inhibition of tumor cellular proteasomal activity by Compound 1 results in the accumulation of ubiquitinated protein and the proteasome target proteins p27 and Bax, followed by the induction of apoptosis. Our results strongly suggest that taurine Schiff base copper complexes, as potent proteasome inhibitors, have great potential to be developed into novel anticancer drugs.
PMCID: PMC3777612  PMID: 18949390
taurine; Schiff base; copper; proteasome inhibitor; apoptosis; cancer
9.  Molecular Mechanisms of Green Tea Polyphenols 
Nutrition and cancer  2009;61(6):827-835.
Tea, next to water, is the most popular beverage in the world. It has been suggested that tea consumption has the cancer-preventive effects. Epidemiological studies have indicated decreased cancer occurrence in people who regularly drink green tea. Research has also discovered numerous mechanisms of action to explain the biological effects of tea. The most abundant and popular compound studied in tea research is (−)-epigallocatechin-3-gallate or (−)-EGCG, which is a powerful antioxidant and can inhibit a number of tumor cell proliferation and survival pathways. Tea polyphenols are known to inhibit metaloproteonases, various protein kinases, and proteins that regulate DNA replication and transformation. We also reported that ester bond-containing tea polyphenols, for example, (−)-EGCG, potently and specifically inhibited the tumor proteasomal activity. We further demonstrated that methylation on green tea polyphenols under physiological conditions decreased their proteasome-inhibitory activity, contributing to decreased cancer-preventive effects of tea consumption. Since (−)-EGCG is unstable under physiological conditions, we also developed the peracetate-protected or prodrug form of (−)-EGCG, Pro-EGCG (1), and showed that Pro-EGCG (1) increases the bioavailability, stability, and proteasome-inhibitory and anti-cancer activities of (−)-EGCG in human breast cancer cells and tumors, demonstrating its potential use for cancer prevention and treatment.
PMCID: PMC3769684  PMID: 20155623
10.  Inhibition of Tumor Proteasome Activity by Gold Dithiocarbamato Complexes via both Redox-Dependent and –Independent Processes 
Journal of cellular biochemistry  2010;109(1):162-172.
We have previously reported on a gold(III) complex, namely [AuBr2(DMDT)] (N,N-dimethyldithiocarbamate) showing potent in vitro and in vivo growth inhibitory activities toward human cancer cells and identifying the cellular proteasome as one of the major targets. However, the importance of the oxidation state of the gold center and the involved mechanism of action has yet to be established. Here we show that both gold(III)- and gold(I)-dithiocarbamato species, namely [AuBr2(ESDT)] (AUL12) and [Au(ESDT)]2 (AUL15), could inhibit the chymotrypsin-like activity of purified 20S proteasome and 26S proteasome in human breast cancer MDA-MB-231 cells, resulting in accumulation of ubiquitinated proteins and proteasome target proteins, and induction of cell death, but at significantly different levels. Gold(I) and gold(III) compounds-mediated proteasome inhibition and cell death induction were completely reversed by the addition of a reducing agent, dithiothreitol or N-acetyl-l-cysteine, suggesting the involvement of redox processes. Furthermore, treatment of MDA-MB-231 cells with gold(III) compound (AUL12), but not the gold(I) analogue (AUL15), resulted in the production of significant level of reactive oxygen species. Our study provides strong evidence that the cellular proteasome is an imporant target of both gold(I) and gold(III) dithiocarbamates, but distinct cellular mechanisms of action are responsible for their different overall effect.
PMCID: PMC3767936  PMID: 19911377
Apoptosis; gold compounds; proteasome inhibitor; redox; reactive oxygen species
11.  Disulfiram promotes the conversion of carcinogenic cadmium to a proteasome inhibitor with pro-apoptotic activity in human cancer cells 
Toxicology and applied pharmacology  2008;229(2):206-214.
The ubiquitinproteasome system is involved in various cellular processes, including transcription, apoptosis, and cell cycle. In vitro, in vivo, and clinical studies suggest the potential use of proteasome inhibitors as anticancer drugs. Cadmium (Cd) is a widespread environmental pollutant that has been classified as a human carcinogen. Recent study in our laboratory suggested that the clinically used anti-alcoholism drug disulfiram (DSF) could form a complex with tumor cellular copper, resulting in inhibition of the proteasomal chymotrypsin-like activity and induction of cancer cell apoptosis. In the current study, we report, for the first time, that DSF is able to convert the carcinogen Cd to a proteasome-inhibitor and cancer cell apoptosis inducer. Although the DSF–Cd complex inhibited the chymotrypsin-like activity of a purified 20S proteasome with an IC50 value of 32 μmol/L, this complex was much more potent in inhibiting the chymotrypsin-like activity of prostate cancer cellular 26S proteasome. Inhibition of cellular proteasome activity by the DSF–Cd complex resulted in the accumulation of ubiquitinated proteins and the natural proteasome substrate p27, which was followed by activation of calpain and induction of apoptosis. Importantly, human breast cancer MCF10DCIS cells were much more sensitive to the DSF–Cd treatment than immortalized but non-tumorigenic human breast MCF-10A cells, demonstrating that the DSF–Cd complex could selectively induce proteasome inhibition and apoptosis in human tumor cells. Our work suggests the potential use of DSF for treatment of cells with accumulated levels of carcinogen Cd.
PMCID: PMC3766637  PMID: 18304598
Cadmium; Disulfiram; Proteasome inhibitor; Calpain; Apoptosis
12.  Tea polyphenols, their biological effects and potential molecular targets 
Histology and histopathology  2008;23(4):487-496.
Tea is the most popular beverage in the world, second only to water. Tea contains an infusion of the leaves from the Camellia sinensis plant rich in polyphenolic compounds known as catechins, the most abundant of which is (−)-EGCG. Although tea has been consumed for centuries, it has only recently been studied extensively as a health-promoting beverage that may act to prevent a number of chronic diseases and cancers. The results of several investigations indicate that green tea consumption may be of modest benefit in reducing the plasma concentration of cholesterol and preventing atherosclerosis. Additionally, the cancer-preventive effects of green tea are widely supported by results from epidemiological, cell culture, animal and clinical studies. In vitro cell culture studies show that tea polyphenols potently induce apoptotic cell death and cell cycle arrest in tumor cells but not in their normal cell counterparts. Green tea polyphenols were shown to affect several biological pathways, including growth factor-mediated pathway, the mitogen-activated protein (MAP) kinase-dependent pathway, and ubiquitin/proteasome degradation pathways. Various animal studies have revealed that treatment with green tea inhibits tumor incidence and multiplicity in different organ sites such as skin, lung, liver, stomach, mammary gland and colon. Recently, phase I and II clinical trials have been conducted to explore the anticancer effects of green tea in humans. A major challenge of cancer prevention is to integrate new molecular findings into clinical practice. Therefore, identification of more molecular targets and biomarkers for tea polyphenols is essential for improving the design of green tea trials and will greatly assist in a better understanding of the mechanisms underlying its anti-cancer activity.
PMCID: PMC3763709  PMID: 18228206
Tea polyphenols; Molecular targets; Cancer prevention; Cancer treatment
13.  Rapid Communications (2nd Revision) Modulation of the tumor cell death pathway by androgen receptor in response to cytotoxic stimuli 
Journal of cellular physiology  2011;226(11):2731-2739.
Despite an initial response from androgen deprivation therapy, most prostate cancer patients relapse to a hormone-refractory state where tumors still remain dependent on androgen receptor (AR) function. We have previously shown that AR breakdown correlates with the induction of cancer cell apoptosis by proteasome inhibition. However, the involvement of AR in modulating the cell death pathway has remained elusive. To investigate this, we used an experimental model consisting of parental PC-3 prostate cancer cells that lack AR expression and PC-3 cells stably overexpressing wild type AR gene. Here, we report that both chemotherapeutic drugs (cisplatin) and proteasome inhibitors induced caspase-3-associated cell death in parental PC-3 cells whereas non-caspase-3 associated cell death in PC3-AR cells. The involvement of AR in modulating tumor cell death was further confirmed in PC-3 cells transiently expressing AR. Consistently, treatment with the clinically used proteasome inhibitor Bortezomib (Velcade/PS-341) of (AR+) LNCaP prostate cancer cells caused AR cleavage and cell death with low levels of caspase activation. However, co-treatment with Bortezomib and the AR antagonist Bicalutamide (Casodex) caused significant decrease in AR expression associated with an increase in caspase-3 activity in both LNCaP and PC3-AR cells. Thus our results provide compelling evidence for involvement of AR in deciding types of tumor cell death upon cytotoxic stimuli, and specifically, blockade of AR activities could change necrosis to apoptosis in tumor cells. Our findings may help guide clinicians based on AR status in the design of favorable treatment strategies for prostate cancer patients.
PMCID: PMC3134581  PMID: 21448923
Androgen receptor; proteasome inhibitor; cell death; caspase-3; necrosis; calpain
14.  Evaluation of curcumin acetates and amino acid conjugates as proteasome inhibitors 
Curcumin (diferuloylmethane) is the main active ingredient of turmeric, a traditional herbal medicine and food of south Asia. Curcumin has been found to have a wide range of biological activities, including antioxidant, anti-inflammatory, chemopreventive and chemotherapeutic activities. Curcumin is currently being tested in clinical trials for treatment of various types of cancers, including multiple myeloma, pancreatic cancer and colon cancer. Although no toxicity associated with curcumin (even at very high doses) has been observed, the effects of curcumin in other solid tumors have been modest, primarily due to poor water solubility and poor bioavailability in tissues remote from the gastrointestinal tract. Therefore, there is a need for the discovery of curcumin analogs with better water solubility or greater bioavailability for the treatment of solid tumors such as prostate cancer. In this study, curcumin acetates and amino acid conjugates of curcumin were studied in terms of their proteasome inhibitory and antiproliferative effects against several human cancer cell lines. It was found that the water soluble amino acid conjugates of curcumin showed a potent antiproliferative effect and are potent proteasome inhibitors. Docking studies of the curcumin amino acid conjugates for proteasome inhibition were carried out to explain their biological activities. It is suggested that they may serve as the water soluble analogs of curcumin.
PMCID: PMC3306612  PMID: 20818481
curcumin; proteasome inhibitor; cytotoxicity
15.  Celastrol and an EGCG pro-drug exhibit potent chemosensitizing activity in human leukemia cells 
Chemotherapy remains the staple of treatment for many types of leukemia. Despite the positive impact on extending overall survival in patients with hematological malignancies, new treatment strategies are needed to reduce the nonspecific toxicity and improve the efficacy of treatment. Celastrol, derived from the ‘Thunder God Vine’ and Pro-EGCG, a pre-drug version of green tea polyphenol EGCG have shown potent biological activity in vitro and in vivo. Whether these natural products augment the efficacy of conventional chemotherapy in the treatment of leukemia cells has yet to be demonstrated. Here we demonstrate that these natural products could sensitize the effect of chemotherapy in both K-562 and Jurkat T human leukemia cells. Accordingly, this potent biological activity was associated with increased levels of leukemia cell killing, caspase 3 activation, and poly(ADP-ribose) polymerase cleavage. Furthermore, the higher levels of apoptotic indices were associated with decreased levels of Bcr-Abl oncoprotein in K-562 cells. Taken together, our findings present a compelling rationale for the development of combination strategies using natural products in the treatment of hematological malignancies.
PMCID: PMC3304460  PMID: 20127053
natural products; pro-drug; chemosensitizing; chemotherapy; apoptosis; leukemia
Cancer-preventive effects of tea polyphenols, especially epigallocatechin-3-gallate (EGCG), have been demonstrated by epidemiological, preclinical, and clinical studies. Green tea polyphenols such as EGCG have the potential to affect multiple biological pathways, including gene expression, growth factor-mediated pathways, the mitogen-activated protein kinase-dependent pathway, and the ubiquitin/proteasome degradation pathway. Therefore, identification of the molecular targets of EGCG should greatly facilitate a better understanding of the mechanisms underlying its anticancer and cancer-preventive activities. Performing structure–activity relationship (SAR) studies could also greatly enhance the discovery of novel tea polyphenol analogs as potential anticancer and cancer-preventive agents. In this chapter, we review the relevant literature as it relates to the effects of natural and synthetic green tea polyphenols and EGCG analogs on human cancer cells and their potential molecular targets as well as their antitumor effects. We also discuss the implications of green tea polyphenols in cancer prevention.
PMCID: PMC3304302  PMID: 21404918
17.  Computational modeling of the potential interactions of the proteasome β5 subunit and catechol-O-methyltransferase-resistant EGCG analogs 
(−)-Epigallocatechin gallate [(−)-EGCG] has been implicated in cancer chemoprevention and has been shown as an inhibitor of tumor proteasomal chymotrypsin-like activity in vitro and in vivo. However, EGCG is subjected to rapid biotransforming modifications such as methylation by catechol-O-methyltransferase (COMT) that limits its action. We recently reported that structure 7, an EGCG analog which should be resistant to COMT-mediated methylation and inactivation in cells, was able to inhibit the activity of purified 20S proteasome and cellular 26S proteasome. However, the involved molecular mechanism is unknown. Herein, we applied computational solution to understand the possible interaction between EGCG analogs including structure 7 and the proteasome β5 subunit which is responsible for the chymotrypsin-like activity. We report that the ester carbonyls at C2 and C3 carbon atoms may be the active sites for nucleophilic attack in structure 7 and 5. Equally spaced carbon atoms in COMT-resistant structure 7 give more stable conformation and lower docked free energy than other EGCG analogs. The absence of a second gallate group in structure 16 and 21 significantly decreases the ability to inhibit the proteasome.
PMCID: PMC3304469  PMID: 20596600
catechol-O-methyltransferase; methylation; proteasome inhibitor; (−)-epigallocatechin gallate; computational docking; cancer
18.  Inhibition of catechol-O-methyltransferase activity in human breast cancer cells enhances the biological effect of the green tea polyphenol (-)-EGCG 
Oncology Reports  2010;24(2):563-569.
Tea is one of the most popular beverages in the world and has been studied extensively as a health-promoting beverage that may act to prevent a number of chronic diseases and cancers. (-)-Epigallocatechin gallate [(-)-EGCG], a major component in green tea, is unstable under physiological conditions and methylation of (-)-EGCG by catechol-O-methyltransferase (COMT) is a modification that reduces the biological activity of (-)-EGCG. In the current study, we hypothesized that suppression of COMT activity in human breast cancer cells could increase the proteasome-inhibitory potency of (-)-EGCG and therefore enhance its tumor cell growth-inhibitory activity. We first determined the COMT genotype and basal levels of COMT activity in various human breast cancer cell lines. Furthermore, when breast cancer MDA-MB-231 cells containing high COMT activity were tested, the diminished COMT activity apparently increased the effectiveness of (-)-EGCG via augmented proteasome inhibition and apoptosis induction. This study supplements the previous findings that methylated (-)-EGCG is less bioactive and supports the notion that COMT inhibition may increase the anti-cancer properties of tea polyphenols and the combination may serve as a novel approach or supplemental treatment for breast cancer chemotherapy.
PMCID: PMC3304301  PMID: 20596647
epigallocatechin gallate; catechol-O-methyltransferase; proteasome inhibition; apoptosis; breast cancer
19.  Inhibition of Tumor Cellular Proteasome Activity by Triptolide Extracted from the Chinese Medicinal Plant ‘Thunder God Vine’ 
Anticancer Research  2011;31(1):1-10.
The molecular mechanisms of triptolide responsible for its antitumor properties are not yet fully understood. The ubiquitin/proteasome system is an important pathway of protein degradation in cells. This study investigated whether triptolide may inhibit proteasomal activity and induce apoptosis in human cancer cells.
Materials and Methods
In vitro proteasome inhibition was measured by incubation of a purified 20S proteasome with triptolide. Human breast and prostate cancer cell lines were also treated with different doses of triptolide for different times, followed by measurement of proteasome inhibition (levels of the chymotrypsin-like activity, ubiquitinated proteins and three well-known proteasome target proteins, p27, IκB-α and Bax) and apoptosis induction (caspase-3 activity and PARP cleavage).
Triptolide did not inhibit the chymotrypsin-like activity of purified 20S proteasome. However, treatment of triptolide was able to cause decreased levels of cellular proteasomal chymotrypsin-like activity and accumulation of ubiquitinated proteins and three well-known proteasome target proteins in human breast and prostate cancer cells, associated with apoptosis induction.
It is possible that at least one of metabolites of triptolide has proteasome-inhibitory activity.
PMCID: PMC3303151  PMID: 21273574
Triptolide; medicinal compounds; proteasome inhibitors; apoptosis; cancer therapy
20.  Tumor Cellular Proteasome Inhibition and Growth Suppression by 8-Hydroxyquinoline and Clioquinol Requires Their Capabilities to Bind Copper and Transport Copper into Cells 
We have previously reported that when mixed with copper, 8-hydroxyquinoline (8-OHQ) and its analog clioquinol (CQ) inhibited the proteasomal activity and proliferation in cultured human cancer cells. CQ treatment of high copper-containing human tumor xenografts also caused cancer suppression, associated with proteasome inhibition in vivo. However, the nature of copper dependence of these events has not been elucidated experimentally. In the current study, by using chemical probe molecules that mimic structures of 8-OHQ and CQ, but have no copper binding capability, we dissected the complex cellular processes elicited by 8-OHQ-Cu or CQ-Cu mixture and revealed that copper-binding to 8-OHQ or CQ is required for transportation of copper complex into human breast cancer cells and the consequent proteasome-inhibitory, growth-suppressive and apoptosis-inducing activities. In contrast, the non-copper-binding analogs of 8-OHQ or CQ blocked the very first step – copper binding in this chain of events mediated by 8-OHQ-Cu or CQ-Cu.
PMCID: PMC2814311  PMID: 19809836
Copper-dependence; clioquinol; breast cancer; chemical probe; chemical biology
21.  Metals in Anticancer Therapy: Copper(II) Complexes as Inhibitors of the 20S Proteasome 
Selective 20S proteasomal inhibition and apoptosis induction were observed when several lines of cancer cells were treated with a series of copper complexes described as [Cu(LI)Cl] (1), [Cu(LI)OAc] (2), and [Cu(HLI)(LI)]OAc (3), where HLI is the ligand 2,4-diiodo-6-((pyridine-2-ylmethylamino)methyl)phenol. These complexes were synthesized, characterized by means of ESI spectrometry, infrared, UV-visible and EPR spectroscopies, and X-ray diffraction when possible. After full characterization species 1-3 were evaluated for their ability to function as proteasome inhibitors and apoptosis inducers in C4-2B and PC-3 human prostate cancer cells and MCF-10A normal cells. With distinct stoichiometries and protonation states, this series suggests the assignment of species [CuLI]+ as the minimal pharmacophore needed for proteasomal chymotryspin-like activity inhibition and permits some initial inference of mechanistic information.
Three well characterized discrete copper complexes with asymmetric phenol-substituted ligands are able to inhibit the proteolytic activity of the 20s proteasome. Evidence for a minimal pharmacophore suggests a potential basis for new cancer therapies with tunable and cost-effective metallodrugs.
PMCID: PMC2759842  PMID: 19559507
proteasome inhibition; anticancer therapy; copper(II); metallodrugs; bioinorganic
22.  Shikonin Exerts Antitumor Activity via Proteasome Inhibition and Cell Death Induction in vitro and in vivo 
Dysregulation of the ubiquitin-proteasome pathway plays an essential role in tumor growth and development. Shikonin, a natural naphthoquinone isolated from the traditional Chinese medicine Zi Cao (gromwell), has been reported to possess tumor cell-killing activity, and results from a clinical study using a shikonin-containing mixture demonstrated its safety and efficacy for the treatment of late-stage lung cancer. In the present study, we reported that shikonin is an inhibitor of tumor proteasome activity in vitro and in vivo. Our computational modeling predicts that the carbonyl carbons C1 and C4 of shikonin potentially interact with the catalytic site of β5 chymotryptic subunit of the proteasome. Indeed, shikonin potently inhibits the chymotrypsin-like activity of purified 20S proteasome (IC50 12.5 μmol/L) and tumor cellular 26S proteasome (IC50 between 2-16 μmol/L). Inhibition of the proteasome by shikonin in murine hepatoma H22, leukemia P388 and human prostate cancer PC-3 cultures resulted in accumulation of ubiquitinated proteins and several proteasome target proapoptotic proteins (IκB-α, Bax and p27), followed by induction of cell death. Shikonin treatment resulted in tumor growth inhibition in both H22 allografts and PC-3 xenografts, associated with suppression of the proteasomal activity and induction of cell death in vivo. Finally, shikonin treatment significantly prolonged the survival period of mice bearing P388 leukemia. Our results indicate that the tumor proteasome is one of the cellular targets of shikonin, and inhibition of the proteasome activity by shikonin contributes to its anti-tumor property.
PMCID: PMC2707765  PMID: 19165859
Shikonin; medicinal compounds; proteasome inhibitor; hepatoma; prostate cancer
23.  Calpain-mediated androgen receptor breakdown in apoptotic prostate cancer cells 
Journal of cellular physiology  2008;217(3):569-576.
Since androgen receptor (AR) plays an important role in prostate cancer development and progression, androgen-ablation has been the frontline therapy for treatment of advanced prostate cancer even though it is rarely curative. A curative strategy should involve functional and structural elimination of AR from prostate cancer cells. We have previously reported that apoptosis induced by medicinal proteasome-inhibitory compound celastrol is associated with a decrease in AR protein levels. However celastrol-stimulated events contributing to this AR decrease have not been elucidated. Here, we report that a variety of chemotherapeutic agents, including proteasome inhibitors, a topoisomerase inhibitor, DNA–damaging agents and docetaxel that cause cell death, decrease AR levels in LNCaP prostate cancer cells. This decrease in AR protein levels was not due to the suppression of AR mRNA expression in these cells. We observed that a proteolytic activity residing in cytosol of prostate cancer cells is responsible for AR breakdown and that this proteolytic activity was stimulated upon induction of apoptosis. Interestingly, proteasome inhibitor celastrol- and chemotherapeutic drug VP-16-stimulated AR breakdown was attenuated by calpain inhibitors calpastatin and N-Acetyl-L-leucyl-L-leucyl-L-methioninal. Furthermore, AR proteolytic activity pulled down by calmodulin-agarose beads from celastrol-treated PC-3 cells showed immunoreactivity to a calpain antibody. Taken together, these results demonstrate calpain involvement in proteasome inhibitor-induced AR breakdown, and suggest that AR degradation is intrinsic to the induction of apoptosis in prostate cancer cells.
PMCID: PMC2597227  PMID: 18726991
proteasome inhibitors; anticancer drugs; prostate cancer; apoptosis; cell death
24.  Relationship between the methylation status of dietary flavonoids and their growth-inhibitory and apoptosis-inducing activities in human cancer cells 
Journal of cellular biochemistry  2008;105(2):514-523.
Flavonoids are polyphenolic compounds widely distributed in the plant kingdom. Compelling research indicates that flavonoids have important roles in cancer chemoprevention and chemotherapy possibly due to biological activities that include action through anti-inflammation, free radical scavenging, modulation of survival/proliferation pathways, and inhibition of the ubiquitin-proteasome pathway. Plant polyphenols including the green tea polyphenol, (-)-epigallocatechin gallate or (-)-EGCG, and the flavonoids apigenin, luteolin, quercetin, and chrysin have been shown to inhibit proteasome activity and induce apoptosis in human leukemia cells. However, biotransformation reactions to the reactive hydroxyl groups on polyphenols could reduce their biological activities. Although methylated polyphenols have been suggested to be metabolically more stable than unmethylated polyphenols, the practical use of methylated polyphenols as a cancer preventative agent warrants further investigation. In the current study, methylated and unmethylated flavonoids were studied for their proteasome-inhibitory and apoptosis-inducing abilities in human leukemia HL60 cells. Methylated flavonoids displayed sustained bioavailability and inhibited cellular proliferation by arresting cells in the G1 phase. However, they did not act as proteasome inhibitors in either an in vitro system or an in silico model and only weakly induced apoptosis. In contrast, unmethylated flavonoids exhibited inhibition of the proteasomal activity in intact HL60 cells, accumulating proteasome target proteins and inducing caspase activation and poly (ADP-ribose) polymerase cleavage. We conclude that methylated flavonoids lack potent cytotoxicity against human leukemia cells and most likely have limited ability as chemopreventive agents.
PMCID: PMC2574743  PMID: 18636546
Methylated flavonoids; Proteasome inhibitors; Apoptosis; Cell cycle progression
25.  Curcumin inhibits the proteasome activity in human colon cancer cells in vitro and in vivo 
Cancer research  2008;68(18):7283-7292.
Curcumin (diferuloylmethane) is the major active ingredient of turmeric (curcuma longa) used in South Asian cuisine for centuries. Curcumin has been shown to inhibit the growth of transformed cells and to have a number of potential molecular targets. However, the essential molecular targets of curcumin under physiological conditions have not been completely defined. Herein, we report that the tumor cellular proteasome is most likely an important target of curcumin. Nucleophilic susceptibility and in silico docking studies show that both carbonyl carbons of the curcumin molecule are highly susceptible to a nucleophilic attack by the hydroxyl group of the N-terminal threonine of the proteasomal chymotrypsin-like subunit. Consistently, curcumin potently inhibits the chymotrypsin-like activity of a purified rabbit 20S proteasome (IC50=1.85 µM) and cellular 26S proteasome. Furthermore, inhibition of proteasome activity by curcumin in human colon cancer HCT-116 and SW480 cell lines leads to accumulation of ubiquitinated proteins and several proteasome target proteins, and subsequent induction of apoptosis. Furthermore, treatment of HCT-116 colon tumor–bearing ICR SCID mice with curcumin resulted in decreased tumor growth, associated with proteasome inhibition, proliferation suppression and apoptosis induction in tumor tissues. Our study demonstrates that proteasome inhibition could be one of the mechanisms for the chemopreventive and/or therapaeutic roles of curcumin in human colon cancer. Based on its ability to inhibit the proteasome and induce apoptosis in both HCT-116 and metastatic SW480 colon cancer cell lines, our study suggests that curcumin could potentially be used for treatment of both early stage and late stage/refractory colon cancer.
PMCID: PMC2556983  PMID: 18794115
Curcumin; polyphenols; proteasome inhibitors; colon cancer; apoptosis

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