Antitumor quinols: Role of glutathione in modulating quinol-induced apoptosis and identification of putative cellular protein targets

Quinol derivatives as potential trypanocidal agents

Quinols have been developed as a class of potential anti-cancer compounds. They are thought to act as double Michael acceptors, forming two covalent bonds to their target protein(s). Quinols have also been shown to have activity against the parasite Trypanosoma brucei, the causative organism of human African trypanosomiasis, but they demonstrated little selectivity over mammalian MRC5 cells in a counter-screen. In this paper, we report screening of further examples of quinols against T. brucei. We were able to derive an SAR, but the compounds demonstrated little selectivity over MRC5 cells. In an approach to increase selectivity, we attached melamine and benzamidine motifs to the quinols, because these moieties are known to be selectively concentrated in the parasite by transporter proteins. In general these transporter motif-containing analogues showed increased selectivity; however they also showed reduced levels of potency against T. brucei.

Antitumor quinol PMX464 is a cytocidal anti-trypanosomal inhibitor targeting trypanothione metabolism

Better drugs are urgently needed for the treatment of African sleeping sickness. We tested a series of promising anticancer agents belonging to the 4-substituted 4-hydroxycyclohexa-2,5-dienones class (“quinols”) and identified several with potent trypanocidal activity (EC₅₀ < 100 nm). In mammalian cells, quinols are proposed to inhibit the thioredoxin/thioredoxin reductase system, which is absent from trypanosomes. Studies with the prototypical 4-benzothiazole-substituted quinol, PMX464, established that PMX464 is rapidly cytocidal, similar to the arsenical drug, melarsen oxide. Cell lysis by PMX464 was accelerated by addition of sublethal concentrations of glucose oxidase implicating oxidant defenses in the mechanism of action. Whole cells treated with PMX464 showed a loss of trypanothione (T(SH)₂), a unique dithiol in trypanosomes, and tryparedoxin peroxidase (TryP), a 2-Cys peroxiredoxin similar to mammalian thioredoxin peroxidase. Enzyme assays revealed that T(SH)₂, TryP, and a glutathione peroxidase-like tryparedoxin-dependent peroxidase were inhibited in time- and concentration-dependent manners. The inhibitory activities of various quinol analogues against these targets showed a good correlation with growth inhibition of Trypanosoma brucei. The monothiols glutathione and l-cysteine bound in a 2:1 ratio with PMX464 with K_d values of 6 and 27 μm, respectively, whereas T(SH)₂ bound more tightly in a 1:1 ratio with a K_d value of 430 nm. Overexpression of trypanothione synthetase in T. brucei decreased sensitivity to PMX464 indicating that the key metabolite T(SH)₂ is a target for quinols. Thus, the quinol pharmacophore represents a novel lead structure for the development of a new drug against African sleeping sickness.

Stabilization of hypoxia-inducible factor-1alpha protein in hypoxia occurs independently of mitochondrial reactive oxygen species production

The transcription factor hypoxia-inducible factor-1α (HIF-1α) is a master regulator of the cellular response to low oxygen. HIF-1α protein accumulates in hypoxia due to inhibition of prolyl hydroxylase enzymes, which under normoxic conditions use molecular oxygen to hydroxylate HIF-1α on two conserved proline residues (Pro(402) and Pro(564)), thus targeting the protein for 26 S proteasome-dependent degradation. A functional mitochondrial electron transport chain is known to be necessary for HIF-1α stabilization in hypoxia. It has been reported that reactive oxygen species (ROS), produced under hypoxia by complex III of the mitochondrial electron transport chain, play a critical role in the stabilization of the HIF-1α protein, possibly by directly inhibiting prolyl hydroxylase enzymes. In contrast, we found that ROS production by complex III is not required for hypoxia-induced HIF-1α stabilization. Thus, reestablishing mitochondrial oxygen consumption in the presence of a complex III inhibitor by using an artificial electron donor to complex IV or by overexpressing Ciona intestinalis alternative oxidase results in HIF-1α protein stabilization in hypoxia. Furthermore, five inhibitors that target different sites of the mitochondrial electron transport chain have similar effects on the HIF-1α protein half-life in hypoxia but vary in their effects on mitochondrial ROS production. Finally, ROS do not regulate prolyl hydroxylase activity directly. We conclude that HIF-1α protein stabilization in hypoxia occurs independently of mitochondrial ROS production. However, mitochondria can modulate the cellular hypoxic response through altered respiratory activity, likely by regulating the cellular oxygen availability.

Heteroaromatic 4-arylquinols are novel inducers of nuclear factor-erythroid 2-related factor 2 (Nrf2)

The induction of phase 2 and antioxidant enzymes via the transcription factor Nuclear factor-erythroid 2-related factor 2 (Nrf2) is an important chemopreventive strategy in cancer and neurodegenerative diseases. Nrf2 is mainly regulated at the level of its protein stability by the cytosolic protein Keap1, which functions as a substrate recruiting subunit of a Cullin3 E3 ubiquitin ligase to target Nrf2 for ubiquitination and subsequent degradation. Phase 2 inducing agents usually covalently modify cysteine residues in Keap1, leading to inhibition of Nrf2 ubiquitination. Quinols, which due to their Michael acceptor moiety react readily with cysteine residues in selective cellular proteins, are good candidates for potential Nrf2 inducing chemopreventive agents. Indeed, we found that similar to the known phase II inducer sulforaphane, the heteroaromatic 4-arylquinols PMX290 and PMX464 increase both Nrf2 protein concentrations and transcriptional activity. Interestingly, PMX290 had a much stronger effect on the Nrf2 protein concentration, but a weaker effect on Nrf2 transcriptional activity compared to PMX464. Given the marked effect of PMX290 on the Nrf2 protein concentration, we examined its effect on the interaction of Keap1 with its binding partners. While sulforaphane was found to decrease binding of Cullin3 to Keap1, PMX290 markedly increased the interaction between these two proteins in intact cells. PMX464, which increased Nrf2 protein only weakly, also had a much smaller effect on the binding between Keap1 and Cullin3. In conclusion, PMX290 is a novel phase 2 inducing agent which increases the interaction between Keap1 and Cullin3 and may inhibit Nrf2 ubiquitination via a novel mechanism.

Structure activity analysis of 2-methoxyestradiol analogues reveals targeting of microtubules as the major mechanism of antiproliferative and proapoptotic activity

2-Methoxyestradiol (2ME2) is an anticancer agent with antiproliferative, antiangiogenic, and proapoptotic effects. A major proposed mechanism of drug action is the disruption of the microtubule skeleton, leading to the induction of cell cycle arrest and apoptosis. In addition, other mechanisms of action have been proposed, including the generation of reactive oxygen species (ROS), inhibition of hypoxia-inducible factor (HIF), and interference with mitochondrial function. In this study, we used a selection of 2ME2 analogues to conduct structure activity analysis and correlated the antiproliferative and proapoptotic activity of the various analogues with their effects on different drug targets. A good correlation was observed between drug activity and effects on microtubule function. In contrast, our results indicate that effects on ROS, HIF, and mitochondria are unlikely to contribute significantly to the cellular activity of 2ME2. Thus, our data indicate that the structural requirements for inducing ROS and inhibition of complex I of the mitochondrial electron transport chain were different from those required for proapoptotic drug activity. Furthermore, antioxidant treatment or overexpression of catalase did not inhibit the cellular activity of 2ME2 in epithelial cancer cells. Inhibition of HIF required much higher concentrations of 2ME2 analogues compared with concentrations that inhibited cell proliferation and induced apoptosis. Our results thus provide a better insight into the mechanism of action of 2ME2 and reveal structural requirements that confer high cellular activity, which may aid future drug development.

Hydrolysis and photolysis of 4-Acetoxy-4-(benzothiazol-2-yl)-2,5-cyclohexadien-1-one, a model anti-tumor quinol ester

4-Acetoxy-4-(benzothiazol-2-yl)-2,5-cyclohexadien-1-one, 1, a quinol derivative that exhibits significant anti-tumor activity against human breast, colon, and renal cancer cell lines, undergoes hydrolysis in aqueous solution to generate an oxenium ion intermediate, 3, that is selectively trapped by N(3)(-) in an aqueous environment. The 4-(benzothiazol-2-yl) substituent slows the rate of ionization of 1 compared to analogues with 4-phenyl or 4-(p-tolyl) substituents, 4a or 4b. However, once generated, 3 is somewhat more selective than the 4-phenyl-substituted cation 5a. Calculations performed at the B3LYP/6-31G(d) level agree that the 4-(benzothiazol-2-yl) substituent does significantly stabilize 3. The structure of the major isolated azide adduct, 4-(6-azidobenzothiazol-2-yl)phenol, 9, confirms that the positive charge is highly delocalized in 3. The results of hydrolysis of 1 show that the 4-(benzothiazol-2-yl) substituent has a significant inductive electron-withdrawing effect as well as a significant resonance effect that is electron-donating. Photolysis of 1 in aqueous solution generates the quinol 2 as one of several photolysis products. The presence of the quinol suggests that photolysis also leads, in part, to generation of 3, but photoionization of 1 is significantly less efficient than is the case for the esters 4a and 4b. This study proves that 3 is generated by ionization of 1 in an aqueous environment. A significant number of other 2-benzothiazole derivatives that are not quinols, including ring-substituted derivatives of 2-(4-aminophenyl)benzothiazole 15, are under development as anti-tumor agents as well. The possible generation of the reactive intermediate 17 by hydrolysis of the putative metabolite 16 is under investigation.

The thioredoxin system: a key target in tumour and endothelial cells

Thioredoxin is a redox-sensitive molecule that has pleiotropic cellular effects, such as the control of proliferation, redox states and apoptosis, and is often upregulated in malignancy. The system controls the activation of a number of transcription factors through sulphydryl transfer and, through its activity on hypoxia inducible factor 1alpha, it is able to regulate vascular endothelial growth factor levels and hence angiogenesis. The thioredoxin protein has been shown to be upregulated in hypoxic regions of certain tumours, suggesting that inhibitors could potentially exhibit enhanced hypoxic toxicity and/or indirect anti-angiogenic effects. Evidence of this is becoming apparent in the literature. The current report reviews the thioredoxin system as an anticancer drug target and focuses upon two recent compounds, PMX464 and PX12, which reportedly inhibit this important pathway.

PMX464, a thiol-reactive quinol and putative thioredoxin inhibitor, inhibits NF-kappaB-dependent proinflammatory activation of alveolar epithelial cells

BACKGROUND AND PURPOSE

Subtle changes in the intracellular reduction-oxidation (redox) state can modulate nuclear factor-kappaB (NF-kappaB) activity. Thioredoxin-1 (Trx) is a small, ubiquitous, redox-active thiol (-SH) protein that, with thioredoxin reductase-1 (TrxR), modifies the redox status of NF-kappaB pathway components. PMX464 is a novel thiol-reactive quinol thought to inhibit the Trx/TrxR system. The aim of this work was to investigate whether PMX464 inhibited NF-kappaB-mediated proinflammatory activation of human type II alveolar epithelial cells (A549).

EXPERIMENTAL APPROACH

Intercellular adhesion molecule-1 (ICAM-1), granulocyte-macrophage colony-stimulating factor (GM-CSF) and CXCL8, NF-kappaB DNA binding, nuclear translocation of NF-kappaB p65 subunit, IkappaBalpha degradation, IkappaB phosphorylation and IkappaB kinase (IKK) activity were assessed in A549 cells stimulated with IL-1beta with or without PMX464 pretreatment. Effects of PMX464 on ICAM-1 expression in human lung microvascular endothelial cells (HLMVEC) were also investigated. For comparison, selected measurements (ICAM-1 and IkappaB-alpha phospho-IkappaB-alpha) were made on A549 cells after RNA interference-mediated silencing (siRNA) of Trx.

KEY RESULTS

PMX464 reduced ICAM-1, GM-CSF and CXCL8 expression in IL-1beta-stimulated A549 cells and ICAM-1 in HLMVEC. PMX464 inhibited IL-1beta-induced NF-kappaB DNA binding, nuclear translocation of NF-kappaB p65 subunit and factors involved in NF-kappaB activation; specifically, IkappaBalpha degradation, IkappaB phosphorylation and IkappaB kinase (IKK) activity in A549. By contrast, Trx siRNA did not alter ICAM-1 expression or IkappaBalpha degradation/phosphorylation in IL-1beta-stimulated A549 cells.

CONCLUSION AND IMPLICATIONS

PMX464 inhibits a proinflammatory response in A549 cells targeting the NFkappaB pathway above IKK. The lack of effect with Trx siRNA suggests that PMX464 acts on thiol proteins, in addition to Trx, to elicit anti-inflammatory responses in lung epithelial cells.

Targeting thioredoxin reductase is a basis for cancer therapy by arsenic trioxide

Arsenic trioxide (ATO) is an effective cancer therapeutic drug for acute promyelocytic leukemia and has potential anticancer activity against a wide range of solid tumors. ATO exerts its effect mainly through elevated oxidative stress, but the exact molecular mechanism remains elusive. The thioredoxin (Trx) system comprising NADPH, thioredoxin reductase (TrxR), and Trx and the glutathione (GSH) system composed of NADPH, glutathione reductase, and GSH supported by glutaredoxin are the two electron donor systems that control cellular proliferation, viability, and apoptosis. Recently, the selenocysteine-dependent TrxR enzyme has emerged as an important molecular target for anticancer drug development. Here, we have discovered that ATO irreversibly inhibits mammalian TrxR with an IC(50) of 0.25 microM. Both the N-terminal redox-active dithiol and the C-terminal selenothiol-active site of reduced TrxR may participate in the reaction with ATO. The inhibition of MCF-7 cell growth by ATO was correlated with irreversible inactivation of TrxR, which subsequently led to Trx oxidation. Furthermore, the inhibition of TrxR by ATO was attenuated by GSH, and GSH depletion by buthionine sulfoximine enhanced ATO-induced cell death. These results strongly suggest that the ATO anticancer activity is by means of a Trx system-mediated apoptosis. Blocking cancer cell DNA replication and repair and induction of oxidative stress by the inhibition of both Trx and GSH systems are suggested as cancer chemotherapeutic strategies.

A cellular and molecular investigation of the action of PMX464, a putative thioredoxin inhibitor, in normal and colorectal cancer cell lines

BACKGROUND AND PURPOSE

PMX464 is a novel benzothiazole substituted cyclohexadienone reportedly targeting the thioredoxin (Trx1)/thioredoxin reductase (TrxR1) system. We have previously shown that PMX464 has enhanced hypoxic anti-proliferative effects in colorectal tumour cells, with some non-tumour cells (quiescent endothelium and fibroblasts) being relatively resistant. The current study aimed to validate the Trx1 system as a molecular target of PMX464 in tumour cells and to investigate the differential sensitivities of normal cells at the molecular level.

EXPERIMENTAL APPROACH

Proliferation, clonogenic survival, protein expression and function, cell cycle and apoptosis assays were conducted using colorectal tumour (HT29), endothelial (HUVEC) and fibroblast (MRCV) cells treated with PMX464 under normoxic and hypoxic (1% O(2)) conditions.

KEY RESULTS

Protein and enzyme assays showed that PMX464, in HT29, inhibited Trx1 function without altering expression and that inhibition correlated with decreased proliferation and survival, and was more marked under hypoxia. In contrast, although hypoxic HUVEC were sensitive, in terms of proliferation and survival, inhibition of Trx1 function was not observed. Quiescent HUVEC and MRCVs (that have undetectable Trx1 protein) were relatively resistant. The effect on HT29 cells was essentially due to cell cycle inhibition, as apoptosis was modest. Anti-proliferative effects were lost after a lag period, suggesting a reversible phenomenon.

CONCLUSIONS AND IMPLICATIONS

The Trx1 system is an important target in tumour cells and can be inhibited by PMX464. Quiescent HUVEC and fibroblasts are relatively resistant conferring a therapeutic benefit when targeting Trx1.

A duplexed phenotypic screen for the simultaneous detection of inhibitors of the molecular chaperone heat shock protein 90 and modulators of cellular acetylation

Histone deacetylases (HDACs), histone acetyltransferases (HATs), and the molecular chaperone heat shock protein 90 (HSP90) are attractive anticancer drug targets. High-throughput screening plays a pivotal role in modern molecular mechanism-based drug discovery. Cell-based screens are particularly useful in that they identify compounds that are permeable and active against the selected target or pathway in a cellular context. We have previously developed time-resolved fluorescence cell immunosorbent assays (TRF-Cellisas) for compound screening and pharmacodynamic studies. These assays use a primary antibody to the single protein of interest and a matched secondary immunoglobulin labeled with an europium chelate (Eu). The availability of species-specific secondary antibodies labeled with different lanthanide chelates provides the potential for multiplexing this type of assay. The approach has been applied to the development of a 384-well duplexed cell-based screen to simultaneously detect compounds that induce the co-chaperone HSP70 as a molecular marker of potential inhibitors of HSP90 together with those that modulate cellular acetylation (i.e., potential inhibitors of histone deacetylase or histone acetyltransferase activity). The duplexed assay proved reliable in high-throughput format and approximately 64,000 compounds were screened. Following evaluation in secondary assays, 3 of 13 hits from the HSP70 arm were confirmed. Two of these directly inhibited the intrinsic ATPase activity of HSP90 whereas the third seems to have a different mechanism of action. In the acetylation arm, two compounds increased cellular acetylation, one of which inhibited histone deacetylase activity. A third compound decreased cellular histone acetylation, potentially through a novel mechanism of action.

Antitumour properties of fluorinated benzothiazole-substituted hydroxycyclohexa-2,5-dienones (‘quinols’)

The synthesis and in vitro antitumour evaluation of a new series of fluorinated benzothiazole-substituted 4-hydroxycyclohexa-2,5-dienones ('quinols') is described. The new compounds were found to be of comparable activity compared to the non-fluorinated precursor PMX 464, in terms of antiproliferative activity in sensitive human cancer cell lines (nanomolar GI(50) values) and inhibitory activity against the thioredoxin signalling system.