CuIIIons and the Stilbene-Chroman Hybrid with a Catechol Moiety Synergistically Induced DNA Damage, and Cell Cycle Arrest and Apoptosis of HepG2 Cells: An Interesting Acid/Base-Promoted Prooxidant Reaction

Designing salicylaldehyde isonicotinoyl hydrazones as Cu(II) ionophores with tunable chelation and release of copper for hitting redox Achilles heel of cancer cells

Higher levels of copper, reduced glutathione (GSH) and reactive oxygen species (ROS) observed in cancer cells than in normal cells, favor the idea of developing copper ionophores as prooxidative anticancer agents (PAAs) to hit the altered redox homeostasis (redox Achilles heel) of cancer cells. In this work, we used salicylaldehyde isonicotinoyl hydrazone (SIH-1) as a basic scaffold to design Cu(II) ionophores with tunable chelation and release of Cu(II) by introducing electron-withdrawing nitro and electron-donating methoxyl groups in the para position to phenolic hydroxyl, or by blocking the phenolic hydroxyl site using methyl. These molecules were used to probe how chelation and release of copper influence their ionophoric role and ability to target redox Achilles heel of cancer cells. Among these molecules, SIH-1 was identified as the most potent Cu(II) ionophore to kill preferentially HepG2 cells over HUVEC cells, and also superior to clioquinol, a copper ionophore evaluated in clinical trials, in terms of its relatively higher cytotoxicity and better selectivity. Higher oxidative potential, despite of lower stability constant, of the Cu(II) complex formed by SIH-1 than by the other molecules, is responsible for its stronger ability in releasing copper by GSH, inducing redox imbalance and triggering mitochondria-mediated apoptosis of HepG2 cells. This work gives useful information on how to design copper ionophores as PAAs for selective killing of cancer cells.

Targeting redox vulnerability of cancer cells by prooxidative intervention of a glutathione-activated Cu(II) pro-ionophore: Hitting three birds with one stone

Altered redox homeostasis including higher levels of copper, reduced glutathione (GSH) and reactive oxygen species (ROS) in cancer cells than in normal cells illustrates their redox vulnerability, and has opened a window for developing prooxidative anticancer agents (PAAs) to hit this status. However, how to design PAAs with high selectivity in killing cancer cells over normal cells remains a challenge. Herein we designed a 3-hydroxyflavone-inspired copper pro-ionophore (PHF) as a potent PAA based on the GSH-mediated conversion of 2,4-dinitrobenzenesulfonates to enols. Mechanistic investigation reveals that it is capable of exploiting increased levels of GSH in cancer cells to in situ release an active ionophore, 3-hydroxyflavone, inducing redox imbalance (copper accumulation, GSH depletion and ROS generation) and achieving highly selective killing of cancer cells upon specific transport of small amounts of Cu(II). To the best of our knowledge, it is the first example of Cu(II) pro-ionophore type of PAA which hits (changes) the three birds (abnormal copper, GSH and ROS levels in cancer cells) with one stone (PHF) in terms of its ability to induce preferentially redox imbalance of cancer cells by copper accumulation, GSH depletion and ROS generation.

Keto-enol-based modification on piperlongumine to generate a potent Cu(II) ionophore that triggers redox imbalance and death of HepG2 cells

Altered redox status including higher levels of copper in cancer cells than in normal cells inspired many researchers to develop copper ionophores targeting this status. We have recently found that flavon-3-ol (3-HF) works as a potent Cu(II) ionophore by virtue of its keto-enol moiety. To further emphasize the significance of this moiety for developing Cu(II) ionophores, we herein designed a β-diketo analog of piperlongumine, PL-I, characterized by the presence of high proportion of the keto-enol form in dimethylsulfoxide and chloroform, and identified its keto-enol structure by NMR and theoretical calculations. Benefiting from deprotonation of its enolic hydroxyl group, this molecule is capable of facilitating the transport of Cu(II) through cellular membranes to disrupt redox homeostasis of human hepatoma HepG2 cells and trigger their death.

Structural basis, chemical driving forces and biological implications of flavones as Cu(II) ionophores

A main biochemical property of cancer cells, compared with normal cells, is altered redox status including increased levels of copper to maintain their malignant phenotypes. Thus, increasing copper accumulation, by using ionophores, to disrupt abnormal redox homeostasis of cancer cells may be an important anticancer strategy. Naturally occurring molecules with extraordinarily diverse chemical scaffolds are an important source of inspiration for developing copper ionophores. Dietary flavonoids are well-characterized copper chelators and show cancer chemopreventive potential, but their ionophoric role for redox-active copper and the related biological implications have remained unknown. This study reports, for the first time, the structural basis, chemical driving forces and biological implications of flavones (a widely distributed subgroup of flavonoids) as Cu(II) ionophores, and also provides new insights into cancer chemopreventive mechanism of flavones bearing 3(or 5)-hydroxy-4-keto group. 3-Hydroxyflavone surfaced as a potent Cu(II) ionophore to induce the mitochondria-dependent apoptosis of cancer cells in a redox intervention fashion via sequential proton-loss Cu(II) chelation, GSH-driving releasing of copper and protonation-dependent efflux of the neutral ligand.

Tetrahydropyrrolization of Resveratrol and Other Stilbenes Improves Inhibitory Effects on DNA Oxidation

The inhibitory effect of resveratrol on DNA oxidation caused by 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH) was found to be enhanced if the C=C bond in resveratrol was converted into tetrahydropyrrole by reaction with azomethine ylide (CH2 =N(+) (CH3 )CH2 (-) ). This encouraged us to explore whether the inhibitory activities of other stilbenes could also be increased by the same method. We found that the inhibitory effects of the tetrahydropyrrole derivatives on AAPH-induced oxidation of DNA were higher than those of the corresponding stilbenes, because the tetrahydropyrrole motif can provide hydrogen atoms to be abstracted by radicals. Therefore, the tetrahydropyrrolization offered an advantage for enhancing the antioxidant effects of stilbenes. Notably, (CH3 )3 SiCH2 N(CH3 )CH2 OCH3 (in the presence of CF3 COOH) and (CH3 )3 NO (in the presence of LiN(iPr)2 ) can be used to generate azomethine ylide for the tetrahydropyrrolization of stilbenes containing electron-withdrawing and -donating groups, respectively.

Toward an understanding of the role of a catechol moiety in cancer chemoprevention: The case of copper- and o-quinone-dependent Nrf2 activation by a catechol-type resveratrol analog

SCOPE

Catechol moieties are commonly present in dietary natural products that exert cancer chemopreventive activity. While the oxidative conversion of catechols into their corresponding o-quinones is generally considered to contribute to their cancer chemopreventive effects, the mechanism of the intracellular conversion has not been fully elucidated.

METHODS AND RESULTS

Among resveratrol and its hydroxylated analogs examined, only 3,4-dihydroxy-trans-stilbene exerted cytoprotective effects against t-butylhydroperoxide-induced death of HepG2 cells. This resveratrol analog activated the nuclear factor erythroid-2-related factor 2 (Nrf2) pathway through stimulating phosphorylation of Akt and inducing keap1 modification, thereby resulting in its nuclear translocation and subsequent transcriptional induction of phase II detoxifying enzymes. Its cytoprotective effect through Nrf2 activation was largely abrogated by pretreatment of cells with DTT, a sulfhydryl-containing nucleophile, and neocuproine, a specific chelating agent for copper ions.

CONCLUSION

We identified 3,4-dihydroxy-trans-stilbene as a novel Nrf2 activator that is converted intracellularly into its corresponding o-quinone electrophile by copper ions. The copper-mediated oxidation was required for the Nrf2 activation, subsequent transcriptional induction of phase II detoxifying enzymes and ultimately for cytoprotection. The findings demonstrate a previously underrecognized role for intracellular copper ions in the cancer chemopreventive effects of catechol-containing dietary natural products.

Hexamethoxylated Monocarbonyl Analogues of Curcumin Cause G2/M Cell Cycle Arrest in NCI-H460 Cells via Michael Acceptor-Dependent Redox Intervention

Curcumin, derived from the dietary spice turmeric, holds promise for cancer prevention. This prompts much interest in investigating the action mechanisms of curcumin and its analogues. Two symmetrical hexamethoxy-diarylpentadienones (1 and 2) as cucumin analogues were reported to possess significantly enhanced cytotoxicity compared with the parent molecule. However, the detailed mechanisms remain unclear. In this study, compounds 1 and 2 were identified as the G2/M cell cycle arrest agents to mediate the cytotoxicity toward NCI-H460 cells via Michael acceptor-dependent redox intervention. Compared with curcumin, they could more easily induce a burst of reactive oxygen species (ROS) and collapse of the redox buffering system. One possible reason is that they could more effectively target intracellular TrxR to convert this antioxidant enzyme into a ROS promoter. Additionally, they caused up-regulation of p53 and p21 and down-regulation of redox-sensitive Cdc25C along with cyclin B1/Cdk1 in a Michael acceptor- and ROS-dependent fashion. Interestingly, in comparison with compound 2, compound 1 displayed a relatively weak ability to generate ROS but increased cell cycle arrest activity and cytotoxicity probably due to its Michael acceptor-dependent microtubule-destabilizing effect and greater GST-inhibitory activity, as well as its enhanced cellular uptake. This work provides useful information for understanding Michael acceptor-dependent and redox-mediated cytotoxic mechanisms of curcumin and its active analogues.

Insights into the importance for designing curcumin-inspired anticancer agents by a prooxidant strategy: The case of diarylpentanoids

Developing anticancer agents by a prooxidant strategy has attracted increasing attention in recent years, although it is not conventional in medicinal chemistry and is completely opposite to antioxidant therapy. In this work, a panel of diarylpentanoids as the curcumin mono-carbonyl analogs were designed and synthesized, and their cytotoxic and proapoptotic mechanisms against human lung cancer A549 cells were investigated at the frontiers of chemistry and biology. It was found that compared with curcumin, the compounds (A1, B1, and C1) bearing two ortho substituents on the aromatic rings, especially A1, exhibit significantly increased cytotoxic and proapoptotic activities through a Michael acceptor unit-dependent prooxidant-mediated mechanism. The prooxidative ability is governed not only by their electrophilicity but also by their geometry, cellular uptake and metabolic stability, and TrxR-inhibitory activity. Mechanistic investigation reveals that the compound A1 could effectively and irreversibly modify the TrxR by virtue of the above optimal biochemical parameters, and convert this antioxidant enzyme into a reactive oxygen species (ROS) promoter, resulting in a burst of the intracellular ROS including H2O2 and O2(-)•. The ROS generation is associated with falling apart in the redox buffering system, and subsequently induces increases in Ca(2+) influx and oxidative stress, collapse of mitochondrial membrane potential, and activation of caspase-9 and caspase-3, ultimately leading to cell apoptosis. This work highlights the feasibility in designing curcumin-inspired anticancer agents by a prooxidant strategy, and gives us useful information on how to design them.

Development and mechanism investigation of a new piperlongumine derivative as a potent anti-inflammatory agent

Inflammation, especially chronic inflammation, is directly involvement in the pathogenesis of many diseases including cancer. An effective approach for managing inflammation is to employ chemicals to block activation of nuclear factor-κB (NF-κB), a key regulator for inflammatory processes. Piperlongumine (piplartine, PL), an electrophilic molecule isolated from Piper longum L., possesses excellent anti-cancer and anti-inflammatory properties. In this study, a new PL analogue (PL-0N) was designed by replacing nitrogen atom of lactam in PL with carbon atom to increase its electrophilicity and thus anti-inflammatory activity. It was found that PL-0N is more potent than the parent compound in suppressing lipopolysaccharide (LPS)-induced secretion of nitric oxide and prostaglandin E2 as well as expression of inducible nitric oxide synthase and cyclooxygenase-2 in RAW264.7 macrophages. Mechanistic investigation implies that PL-0N exerts anti-inflammatory activity through inhibition of LPS-induced NF-κB transduction pathway, down-regulation of LPS-induced MAPKs activation and impairment of proteasomal activity, but also enhancement of LPS-induced autophagy; the inhibition of NF-κB by PL-0N is achieved at various stages by: (i) preventing phosphorylation of IKKα/β, (ii) stabilizing the suppressor protein IκBα, (iii) interfering with the nuclear translocation of NF-κB, and (iv) inhibiting the DNA-binding of NF-κB. These data indicate that nitrogen-atom-lacking pattern is a successful strategy to improve anti-inflammatory property of PL, and that the novel molecule, PL-0N may be served as a promising lead for developing natural product-directed anti-inflammatory agents.

Influence of glucuronidation and reduction modifications of resveratrol on its biological activities

Resveratrol (3,5,4'-trihydroxystilbene, RES), a star among dietary polyphenols, shows a wide range of biological activities, but it is rapidly and extensively metabolized into its glucuronide and sulfate conjugates as well as to the corresponding reduced products. This begs the question of whether the metabolites of RES contribute to its in vivo biological activity. To explore this possibility, we synthesized its glucuronidation (3-GR and 4'-GR) and reduction (DHR) metabolites, and evaluated the effect of these structure modifications on biological activities, including binding ability with human serum albumin (HSA), antioxidant activity in homogeneous solutions and heterogeneous media, anti-inflammatory activity, and cytotoxicity against various cancer cell lines. We found that 1) 4'-GR, DHR and RES show nearly equal binding to HSA, mainly through hydrogen bonding, whereas 3-GR adopts a quite different orientation mode upon binding, thereby resulting in reduced ability; 2) 3-GR shows comparable (even equal) ability to RES in FRAP- and AAPH-induced DNA strand breakage assays; DHR, 3-GR, and 4'-GR exhibit anti-hemolysis activity comparable to that of RES; additionally, 3-GR and DHR retain some degree activity of the parent molecule in DPPH.-scavenging and cupric ion-initiated oxidation of LDL assays, respectively; 3) compared to RES, 4'-GR displays equipotent ability in the inhibition of COX-2, and DHR presents comparable activity in inhibiting NO production and growth of SMMC-7721 cells. Relative to RES, its glucuronidation and reduction metabolites showed equal, comparable, or some degree of activity in the above assays, depending on the specific compound and test model, which probably supports their roles in contributing to the in vivo biological activities of the parent molecule.