Methylation suppresses the proteasome-inhibitory function of green tea polyphenols

Under physiological conditions, biotransformation reactions, such as methylation, can modify green tea polyphenols (GTPs) and therefore limit their in vivo cancer-preventive activity. Although a recent study suggested that methylated polyphenols are less cancer-protective, the molecular basis is unknown. We previously reported that ester bond-containing GTPs, for example (-)-epigallocatechin-3-gallate [(-)-EGCG] or (-)-epicatechin-3-gallate [(-)-ECG], potently and specifically inhibit the proteasomal chymotrypsin-like activity. In this study, we hypothesize that methylated GTPs have decreased proteasome-inhibitory abilities. To test this hypothesis, methylated (-)-EGCG and (-)-ECG analogs that can be found in vivo were synthesized and studied for their structure-activity relationships (SARs) using a purified 20S proteasome. The addition of a single methyl group on (-)-EGCG or (-)-ECG led to decreased proteasome inhibition and, as the number of methyl groups increased, the inhibitory potencies further decreased. These SARs were supported by our findings from in silico docking analysis published recently. Previously, we synthesized a peracetate-protected (-)-EGCG molecule, Pro-EGCG (1), to enhance its cellular permeability and stability, and current HPLC analysis confirms conversion of Pro-EGCG (1) to (-)-EGCG in cultured human leukemic Jurkat T cells. Furthermore, in this study, peracetate-protected forms of methylated GTPs were added in intact Jurkat T cells to observe the intracellular effects of methylation. Peracetate-protected, monomethylated (-)-EGCG induced greater cellular proteasome inhibition and apoptosis than did peracetate-protected, trimethylated (-)-EGCG, consistent with the potencies of the parent methylated analogs against a purified 20S proteasome. Therefore, methylation on GTPs, under physiological conditions, could decrease their proteasome-inhibitory activity, contributing to decreased cancer-preventive effects of tea consumption.     

A para-amino substituent on the D-ring of green tea polyphenol epigallocatechin-3-gallate as a novel proteasome inhibitor and cancer cell apoptosis inducer

Analogs of (-)-EGCG containing a para-amino group on the D-ring in place of the hydroxyl groups have been synthesized and their proteasome inhibitory activities were studied. We found that, the O-acetylated (-)-EGCG analogs possessing a p-NH(2) or p-NHBoc (Boc; tert-butoxycarbonyl) D-ring (5 and 7) act as novel tumor cellular proteasome inhibitors and apoptosis inducers with potency similar to natural (-)-EGCG and similar to (-)-EGCG peracetate. These data suggest that the acetylated amino-GTP analogs have the potential to be developed into novel anticancer agents.     

Synthetic analogs of green tea polyphenols as proteasome inhibitors

BACKGROUND: Animal, epidemiological and clinical studies have demonstrated the anti-tumor activity of pharmacological proteasome inhibitors and the cancer-preventive effects of green tea consumption. Previously, one of our laboratories reported that natural ester bond-containing green tea polyphenols (GTPs), such as (-)-epigallocatechin-3-gallate [(-)-EGCG] and (-)-gallocatechin-3-gallate [(-)-GCG], are potent and specific proteasome inhibitors. Another of our groups, for the first time, was able to enantioselectively synthesize (-)-EGCG as well as other analogs of this natural GTP. Our interest in designing and developing novel synthetic GTPs as proteasome inhibitors and potential cancer-preventive agents prompted our current study. MATERIALS AND METHODS: GTP analogs, (+)-EGCG, (+)-GCG, and a fully benzyl-protected (+)-EGCG [Bn-(+)-EGCG], were prepared by enantioselective synthesis. Inhibition of the proteasome or calpain (as a control) activities under cell-free conditions were measured by fluorogenic substrate assay. Inhibition of intact tumor cell proteasome activity was measured by accumulation of some proteasome target proteins (p27, I kappa B-alpha and Bax) using Western blot analysis. Inhibition of tumor cell proliferation and induction of apoptosis by synthetic GTPs were determined by G(1) arrest and caspase activation, respectively. Finally, inhibition of the transforming activity of human prostate cancer cells by synthetic GTPs was measured by a colony formation assay. RESULTS: (+)-EGCG and (+)-GCG potently and specifically inhibit the chymotrypsin-like activity of purified 20S proteasome and the 26S proteasome in tumor cell lysates, while Bn-(+)-EGCG does not. Treatment of leukemic Jurkat T or prostate cancer LNCaP cells with either (+)-EGCG or (+)-GCG accumulated p27 and IkappaB-alpha proteins, associated with an increased G(1) population. (+)-EGCG treatment also accumulated the pro-apoptotic Bax protein and induced apoptosis in LNCaP cells expressing high basal levels of Bax, but not prostate cancer DU-145 cells with low Bax expression. Finally, synthetic GTPs significantly inhibited colony formation by LNCaP cancer cells. CONCLUSIONS: Enantiomeric analogs of natural GTPs, (+)-EGCG and (+)-GCG, are able to potently and specifically inhibit the proteasome both, in vitro and in vivo, while protection of the hydroxyl groups on (+)-EGCG renders the compound completely inactive.     

Pristimerin induces apoptosis by targeting the proteasome in prostate cancer cells

Pristimerin is a natural product derived from the Celastraceae and Hippocrateaceae families that were used as folk medicines for anti inflammation in ancient times. Although it has been shown that pristimerin induces apoptosis in breast cancer cells, the involved mechanism of action is unknown. The purpose of the current study is to investigate the primary target of pristimerin in human cancer cells, using prostate cancer cells as a working model. Nucleophilic susceptibility and in silico docking studies show that C6 of pristimerin is highly susceptible towards a nucleophilic attack by the hydroxyl group of N-terminal threonine of the proteasomal chymotrypsin subunit. Consistently, pristimerin potently inhibits the chymotrypsin-like activity of a purified rabbit 20S proteasome (IC50 2.2 micromol/L) and human prostate cancer 26S proteasome (IC50 3.0 micromol/L). The accumulation of ubiquitinated proteins and three proteasome target proteins, Bax, p27 and I kappa B-alpha, in androgen receptor (AR)-negative PC-3 prostate cancer cells supports the conclusion that proteasome inhibition by pristimerin is physiologically functional. This observed proteasome inhibition subsequently led to the induction of apoptotic cell death in a dose- and kinetic-dependent manner. Furthermore, in AR-positive, androgen-dependent LNCaP and AR-positive, androgen-independent C4-2B prostate cancer cells, proteasome inhibition by pristimerin results in suppression of AR protein prior to apoptosis. Our data demonstrate, for the first time, that the proteasome is a primary target of pristimerin in prostate cancer cells and inhibition of the proteasomal chymotrypsin-like activity by pristimerin is responsible for its cancer cell death-inducing property.     

A potential prodrug for a green tea polyphenol proteasome inhibitor: evaluation of the peracetate ester of (-)-epigallocatechin gallate [(-)-EGCG

Green tea has been shown to have many biological effects, including effects on metabolism, angiogenesis, oxidation, and cell proliferation. Unfortunately, the most abundant green tea polyphenol (-)-epigallocatechin gallate or (-)-EGCG is very unstable in neutral or alkaline medium. This instability leads to a low bioavailability. In an attempt to enhance the stability of (-)-EGCG, we introduced peracetate protection groups on the reactive hydroxyls of (-)-EGCG (noted in text as 1). HPLC analysis shows that the protected (-)-EGCG analog is six times more stable than natural (-)-EGCG under slightly alkaline conditions. A series of bioassays show that 1 has no inhibitory activity against a purified 20S proteasome in vitro, but exhibits increased proteasome-inhibitory activity in intact leukemic cells over natural (-)-EGCG, indicating an intercellular conversion. Inhibition of cellular proteasome activity by 1 is associated with induction of cell death. Therefore, our results indicate that the protected analog 1 may function as a prodrug of the green tea polyphenol proteasome inhibitor (-)-EGCG.     

(-)-Epigallocatechin gallate regulates CD3-mediated T cell receptor signaling in leukemia through the inhibition of ZAP-70 kinase

The zeta chain-associated 70-kDa protein (ZAP-70) of tyrosine kinase plays a critical role in T cell receptor-mediated signal transduction and the immune response. A high level of ZAP-70 expression is observed in leukemia, which suggests ZAP-70 as a logical target for immunomodulatory therapies. (-)-Epigallocatechin gallate (EGCG) is one of the major green tea catechins that is suggested to have a role as a preventive agent in cancer, obesity, diabetes, and cardiovascular disease. Here we identified ZAP-70 as an important and novel molecular target of EGCG in leukemia cells. ZAP-70 and EGCG displayed high binding affinity (Kd = 0.6207 micromol/liter), and additional results revealed that EGCG effectively suppressed ZAP-70, linker for the activation of T cells, phospholipase Cgamma1, extracellular signaling-regulated kinase, and MAPK kinase activities in CD3-activated T cell leukemia. Furthermore, the activation of activator protein-1 and interleukin-2 induced by CD3 was dose-dependently inhibited by EGCG treatment. Notably, EGCG dose-dependently induced caspase-mediated apoptosis in P116.cl39 ZAP-70-expressing leukemia cells, whereas P116 ZAP-70-deficient cells were resistant to EGCG treatment. Molecular docking studies, supported by site-directed mutagenesis experiments, showed that EGCG could form a series of intermolecular hydrogen bonds and hydrophobic interactions within the ATP binding domain, which may contribute to the stability of the ZAP-70-EGCG complex. Overall, these results strongly indicated that ZAP-70 activity was inhibited specifically by EGCG, which contributed to suppressing the CD3-mediated T cell-induced pathways in leukemia cells.     

Inhibitors of the eukaryotic 20S proteasome core particle: a structural approach

The ubiquitin-proteasome pathway is particularly important for the regulated degradation of various proteins which control a vast array of biological processes. Therefore, proteasome inhibitors are promising candidates for anti-tumoral or anti-inflammatory drugs. N-Acetyl-Leu-Leu-Norleucinal (Ac-LLN-al, also termed calpain inhibitor I) was one of the first proteasome inhibitors discovered and has been widely used to study the 20S proteasome core particle (CP) function in vivo, despite its lack of specificity. Vinyl sulfones, like Ac-PRLN-vs, show covalent binding of the beta-carbon atom of the vinyl sulfone group to the Thr1Ogamma only of subunit beta2. However, vinyl sulfones have similar limitations as peptide aldehydes as they have been reported also to bind and block intracellular cysteine proteases. A more specific proteasome inhibitor is the natural product lactacystin, which can be isolated from Streptomyces. It was found that this compound forms an ester bond only to the Thr1Ogamma of the chymotrypsin-like active subunit beta5 due to specific P1 interactions. In contrast to most other proteasome inhibitors, the natural alpha',beta'-epoxyketone peptide epoxomicin binds specifically to the small class of N-terminal nucleophilic (Ntn) hydrolases (CPs belong to this protease family) with the formation of a morpholino adduct. All previously described proteasome inhibitors bind covalently to the proteolytic active sites. However, as the proteasome is involved in a variety of biological important functions, it is of particular interest to block the CP only for limited time in order to reduce cytotoxic effects. Recently, the binding mode of the natural specific proteasome inhibitor TMC-95 obtained from Apiospora montagnei was investigated. The crystal structure revealed that the TMC-95 blocks the active sites of the CP noncovalently in the low nanomolar range. This review summarizes the current structural knowledge of inhibitory compounds bound to the CP, showing the proteasome as a potential target for drug development in medical research.     

Interaction of (-)-epigallocatechin-3-gallate with human serum albumin: fluorescence, fourier transform infrared, circular dichroism, and docking studies

(-)-Epigallocatechin-3-gallate (EGCG), the major constituent of green tea has been reported to prevent many diseases by virtue of its antioxidant properties. The binding of EGCG with human serum albumin (HSA) has been investigated for the first time by using fluorescence, circular dichroism (CD), Fourier transform infrared (FTIR) spectroscopy, and protein-ligand docking. We observed a quenching of fluorescence of HSA in the presence of EGCG. The binding parameters were determined by a Scatchard plot and the results were found to be consistent with those obtained from a modified Stern-Volmer equation. From the thermodynamic parameters calculated according to the van't Hoff equation, the enthalpy change deltaH degrees and entropy change deltaS degrees were found to be -22.59 and 16.23 J/mol K, respectively. These values suggest that apart from an initial hydrophobic association, the complex is held together by van der Waals interactions and hydrogen bonding. Data obtained by fluorescence spectroscopy, CD, and FTIR experiments along with the docking studies suggest that EGCG binds to residues located in subdomains IIa and IIIa of HSA. Specific interactions are observed with residues Trp 214, Arg 218, Gln 221, Asn 295 and Asp 451. We have also looked at changes in the accessible surface area of the interacting residues on binding EGCG for a better understanding of the interaction.     

(-)-Epigallocatechin-3-gallate, a polyphenolic compound from green tea, inhibits fibroblast adhesion and migration through multiple mechanisms

It is increasingly evident that the stromal cells are involved in key metastatic processes of melanoma and some malignant solid tumors. (-)-Epigallocatechin-3-gallate (EGCG), a polyphenolic compound from green tea, has been shown to have anti-tumor activity, inhibiting adhesion, migration, and proliferation of tumor cells. However, little attention has been paid on its effects on stromal cells. In the present study, we determined the effects of EGCG on stromal fibroblasts. We showed that fibroblast adhesion to collagen, fibronectin, and fibrinogen were inhibited by EGCG. One of the possible mechanisms is binding of EGCG to fibronectin and fibrinogen but not to collagen. We then focused how EGCG affected fibroblast adhesion to collagen. EGCG treatment attenuated the antibody binding to fibroblast's integrin alpha2beta1, indicating EGCG may affect the expression and affinity of integrin alpha2beta1. Moreover, intracellular H2O2 level was decreased by EGCG treatment, suggesting that the tonic maintenance of intracellular H2O2 may be required for cell adhesion to collagen. In parallel, collagen-induced FAK phosphorylation, actin cytoskeleton reorganization in fibroblasts, migration and matrix metalloproteinase(s) (MMPs) activity were also affected by EGCG. Tubular networks formed by melanoma cells grown on three-dimensional Matrigel were also disrupted when fibroblasts were treated with EGCG in a non-contact coculture system. Taken together, we provided here the first evidence that EGCG is an effective inhibitor on behaviors of the stromal fibroblasts, affecting their adhesion and migration. The inhibitory activity of EGCG may contribute to its anti-tumor activity. The findings and concepts disclosed here may provide important basis for a further experiment towards understanding tumor-stroma interaction.     

Inhibition of hemoglobin S polymerization in vitro by a novel 15-mer EF-helix beta73 histidine-containing peptide

Our mutational studies on Hb S showed that the Hb S beta73His variant (beta6Val and beta73His) promoted polymerization, while Hb S beta73Leu (beta6Val and beta73Leu) inhibited polymerization. On the basis of these results, we speculated that EF-helix peptides containing beta73His interact with beta4Thr in Hb S and compete with Hb S, resulting in inhibition of Hb S polymerization. We, therefore, studied inhibitory effects of 15-, 11-, 7-, and 3-mer EF-helix peptides containing beta73His on Hb S polymerization. The delay time prior to Hb S polymerization increased only in the presence of the 15-mer His peptide; the higher the amount, the longer the delay time. DIC image analysis also showed that the fiber elongation rate for Hb S polymers decreased with increasing concentration of the 15-mer His peptide. In contrast, the same 15-mer peptide containing beta73Leu instead of His and peptides shorter than 11 amino acids containing beta73His including His alone showed little effect on the kinetics of polymerization and elongation of polymers. Analysis by protein-chip arrays showed that only the 15-mer beta73His peptide interacted with Hb S. CD spectra of the 15-mer beta73His peptide did not show a specific helical structure; however, computer docking analysis suggested a lower energy for interaction of Hb S with the 15-mer beta73His peptide compared to peptides containing other amino acids at this position. These results suggest that the 15-mer beta73His peptide interacts with Hb S via the beta4Thr in the betaS-globin chain in Hb S. This interaction may influence hydrogen bond interaction between beta73Asp and beta4Thr in Hb S polymers and interfere in hydrophobic interactions of beta6Val, leading to inhibition of Hb S polymerization.     

The anticancer flavonoid chrysin induces the unfolded protein response in hepatoma cells

Chrysin is a natural and biologically active flavonoid with anticancer effects. However, little is known about the adaptive response of cancer cells to chrysin. Chrysin reportedly has proteasome inhibitor activity. Previous studies demonstrated that proteasome inhibitors might induce endoplasmic reticulum (ER) stress response. In this study, we aimed to determine the effects of chrysin on hepatoma cells and roles of the ER-resident protein GRP78 (glucose-regulated protein 78) in its action. Also, we investigated the effects of green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG), a natural GRP78 inhibitor, on the sensitivity of hepatoma cells to chrysin. Here, we report that chrysin inhibits hepatoma cells growth and induces apoptosis in a dose-dependent manner. Chrysin induces GRP78 overexpression, X-box binding protein-1 splicing and eukaryotic initiation factor 2α phosphorylation, hallmarks of the unfolded protein response. GRP78 knockdown potentiates chrysin-induced caspase-7 cleavage in hepatoma cells and enhances chrysin-induced apoptosis. EGCG overcomes chrysin-induced GRP78 expression. Combination of EGCG potentiates chrysin-induced caspase-7 and poly (ADP-ribose) polymerase (PARP) cleavage. Finally, EGCG sensitizes hepatoma cells to chrysin through caspase-mediated apoptosis. These data suggest that chrysin triggers the unfolded protein response. Abrogation of GRP78 induction may improve the anticancer effects of chrysin. Combination of EGCG and chrysin represents a new regimen for cancer chemoprevention and therapeutics.     

Synthesis and characterization of CAPE derivatives as xanthine oxidase inhibitors with radical scavenging properties

Inhibitors of the enzyme xanthine oxidase (XO) with radical scavenging properties hold promise as novel agents against reperfusion injuries after ischemic events. By suppressing the formation of damaging reactive oxygen species (ROS) by XO or scavenging ROS from other sources, these compounds may prevent a buildup of ROS in the aftermath of a heart attack or stroke. To combine these two properties in a single molecule, we synthesized and characterized the non-purine XO inhibitor caffeic acid phenethylester (CAPE) and 19 derivatives using a convenient microwave-assisted Knoevenagel condensation protocol. Varying systematically the number and positions of the hydroxyl groups at the two phenyl rings, we derived structure-activity relationships based on experimentally determined XO inhibition data. Molecular docking suggested that critical enzyme/inhibitor interactions involved π-π interactions between the phenolic inhibitor ring and Tyr914, hydrogen bonds between inhibitor hydroxyl groups and Glu802, and hydrophobic interactions between the CAPE phenyl ring and non-polar residues located at the entrance of the binding site. To effectively scavenge the stable radical DPPH, two hydroxyl groups in 1,2- or 1,4-position at the phenyl ring were required. Among all compounds tested, E-phenyl 3-(3,4-dihydroxyphenyl)acrylate, a CAPE analog without the ethyl tether, showed the most promising properties.     

Covalent modification of proteins by green tea polyphenol (-)-epigallocatechin-3-gallate through autoxidation

Green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) has various beneficial properties including chemopreventive, anticarcinogenic, and antioxidant actions. The interaction with proteins known as EGCG-binding targets may be related to the anticancer effects. However, the binding mechanisms for this activity remain poorly understood. Using mass spectrometry and chemical detection methods, we found that EGCG forms covalent adducts with cysteinyl thiol residues in proteins through autoxidation. To investigate the functional modulation caused by binding of EGCG, we examined the interaction between EGCG and a thiol enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Concentration-dependent covalent binding of EGCG to GAPDH was found to be coupled to the irreversible inhibition of GAPDH activity. Mutation experiments revealed that EGCG is primarily bound to the cysteinyl thiol group of the active center, indicating that the irreversible inhibition of GAPDH is due to the covalent attachment of EGCG to the active-center cysteine. Moreover, using EGCG-treated cancer cells, we identified GAPDH as a target of EGCG covalent binding through specific interactions between catechols and aminophenyl boronate agarose resin. Based on these findings, we propose that the covalent modification of proteins by EGCG may be a novel pathway related to the biological activity of EGCG.     

Investigate the Binding of Catechins to Trypsin Using Docking and Molecular Dynamics Simulation

To explore the inhibitory mechanism of catechins for digestive enzymes, we investigated the binding mode of catechins to a typical digestive enzyme-trypsin and analyzed the structure-activity relationship of catechins, using an integration of molecular docking, molecular dynamics simulation and binding free energy calculation. We found that catechins with different structures bound to a conservative pocket S1 of trypsin, which is comprised of residues 189–195, 214–220 and 225–228. In the trypsin-catechin complexes, Asp189 by forming strong hydrogen bonding, and Gln192, Trp215 and Gly216 through hydrophobic interactions, all significantly contribute to the binding of catechins. The number and the position of hydroxyl and aromatic groups, the structure of stereoisomers, and the orientation of catechins in the binding pocket S1 of trypsin all affect the binding affinity. The binding affinity is in the order of Epigallocatechin gallate (EGCG) > Epicatechin gallate (ECG) > Epicatechin (EC) > Epigallocatechin (EGC), and 2R-3R EGCG shows the strongest binding affinity out of other stereoisomers. Meanwhile, the synergic conformational changes of residues and catechins were also analyzed. These findings will be helpful in understanding the knowledge of interactions between catechins and trypsin and referable for the design of novel polyphenol based functional food and nutriceutical formulas.     

beta-Synuclein reduces proteasomal inhibition by alpha-synuclein but not gamma-synuclein

The accumulation of aggregated alpha-synuclein is thought to contribute to the pathogenesis of Parkinson's disease. Recent studies indicate that aggregated alpha-synuclein binds to S6', a component of the 19 S subunit in the 26 S proteasome and inhibits 26 S proteasomal degradation, both ubiquitin-independent and ubiquitin-dependent. The IC(50) of aggregated alpha-synuclein for inhibition of the 26 S ubiquitin-independent proteasomal activity is approximately 1 nm. alpha-Synuclein has two close homologues, termed beta-synuclein and gamma-synuclein. In the present study we compared the effects of the three synuclein homologues on proteasomal activity. The proteasome exists as a 26 S and a 20 S species, with the 26 S proteasome containing the 20 S core and 19 S cap. Monomeric alpha- and beta-synucleins inhibited the 20 S and 26 S proteasomal activities only weakly, but monomeric gamma-synuclein strongly inhibited ubiquitin-independent proteolysis. The IC(50) of monomeric gamma-synuclein for the 20 S proteolysis was 400 nm. In monomeric form, none of the three synuclein proteins inhibited 26 S ubiquitin-dependent proteasomal activity. Although beta-synuclein had no direct effect on proteasomal activity, co-incubating monomeric beta-synuclein with aggregated alpha-synuclein antagonized the inhibition of the 26 S ubiquitin-independent proteasome by aggregated alpha-synuclein when added before the aggregated alpha-synuclein. Co-incubating beta-synuclein with gamma-synuclein had no effect on the inhibition of the 20 S proteasome by monomeric gamma-synuclein. Immunoprecipitation and pull-down experiments suggested that antagonism by beta-synuclein resulted from binding to alpha-synuclein rather than binding to S6'. Pull-down experiments demonstrated that recombinant monomeric beta-synuclein does not interact with the proteasomal subunit S6', unlike alpha-synuclein, but beta-synuclein does bind alpha-synuclein and competes with S6' for binding to alpha-synuclein. Based on these data, we hypothesize that the alpha- and gamma-synucleins regulate proteasomal function and that beta-synuclein acts as a negative regulator of alpha-synuclein.     

Green tea polyphenol and curcumin inversely regulate human involucrin promoter activity via opposing effects on CCAAT/enhancer-binding protein function

Antioxidants are important candidate agents for the prevention of disease. However, the possibility that different antioxidants may produce opposing effects in tissues has not been adequately explored. We have reported previously that (-)-epigallocatechin-3-gallate (EGCG), a green tea polyphenol antioxidant, stimulates expression of the keratinocyte differentiation marker, involucrin (hINV), via a Ras, MEKK1, MEK3, p38delta signaling cascade (Balasubramanian, S., Efimova, T., and Eckert, R. L. (2002) J. Biol. Chem. 277, 1828-1836). We now show that EGCG activation of this pathway results in increased CCAAT/enhancer-binding protein (C/EBPalpha and C/EBPbeta) factor level and increased complex formation at the hINV promoter C/EBP DNA binding site. This binding is associated with increased promoter activity. Mutation of the hINV promoter C/EBP binding site eliminates the regulation as does expression of GADD153, a dominant-negative C/EBP factor. In contrast, a second antioxidant, curcumin, inhibits the EGCG-dependent promoter activation. This is associated with inhibition of the EGCG-dependent increase in C/EBP factor level and C/EBP factor binding to the hINV promoter. Curcumin also inhibits the EGCG-dependent increase in endogenous hINV levels. The curcumin-dependent suppression of C/EBP factor level is inhibited by treatment with the proteasome inhibitor MG132, suggesting that the proteasome function is required for curcumin action. We conclude that curcumin and EGCG produce opposing effects on involucrin gene expression via regulation of C/EBP factor function. The observation that two antioxidants can produce opposite effects is an important consideration in the context of therapeutic antioxidant use.     

Growth-associated protein of 43 kDa (GAP-43) is cleaved nonprocessively by the 20S proteasome.

Purified, nonubiquitinated growth-associated protein of 43 kDa (GAP-43) was attacked by purified reticulocyte 20S proteasome but not by the 26S proteasome. Cleavage yielded 12 N-terminally labelled GAP-43 fragments that could be resolved by SDS/PAGE. Inhibitor experiments suggested that proteasome beta1 activity yielded the resolved bands and that proteasomebeta5 activity generated nonresolvable fragments. Processive degradation, yielding only nonresolvable fragments, therefore did not occur. Most of the resolved fragments co-migrated with fragments formed in the reticulocyte lysate translation mixture used for GAP-43 synthesis, which suggested that the fragments were also produced in the translation mixture by the endogenous reticulocyte lysate proteasome. Consistent with this idea, the addition of proteasome inhibitors to translation mixtures blocked fragment production. Ubiquitinated GAP-43 appeared to be the source of the fragments in the presence of ATP, and nonubiquitinated GAP-43 the source in the absence of ATP. The results therefore suggest that the lack of processing seen with the 20S proteasome is not an artefact arising from the way in which the 20S proteasome was purified. In one purification protocol, the GAP-43 fragments formed in translation mixtures co-purified with full-length GAP-43. These fragments were digested to nonresolvable products upon addition of purified 20S proteasome. Addition of calmodulin or G-actin blocked the consumption of both full-length GAP-43 and the co-purified GAP-43 fragments. This showed that the resolved fragments can re-enter the proteasome and be cleaved to nonresolvable products, indicating that the lack of processivity is not a result of their resistance to further proteasome attack. The difficult step therefore appears to be the transfer of the large fragments within the proteasome from the beta1 to the beta5 activity for further attack.     

Relationship between the methylation status of dietary flavonoids and their growth-inhibitory and apoptosis-inducing activities in human cancer cells

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 cancer preventative agents 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 G(1) 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.