Cancer Chemopreventive Activity And Mechanism Of Mercapto Compounds And Curcumin Analogs As Copper Reactors And Ionophores

Maintaining cellular redox homeostasis is crucial for normal cell growth and survival, and reactive oxygen species (ROS) are key mediators involved in the redox control. A moderate increase in ROS levels can promote cell proliferation and differentiation, whereas high levels of ROS reaching a toxic threshold could trigger cell death. Compared to normal cells, cancer cells exhibit increased levels of ROS and copper, which are responsible for the acquisition and maintenance of the hallmarks of cancer, and are thus likely to be more vulnerable to damage by further ROS and copper insults. This motivated us to investigate that if the prooxidant system constructed by Cu(?) ions and reducing molecules as the copper reactors, could selectively kill cancer cells. Moreover, curcumin, a natural compound present in the rhizomes of plant Curcuma longa Linn.,and its analogs were tested as copper ionophores to induce cell death. We believe that these works should contribute to understand redox-directed cancer chemoprevention and chemotherapy. The thesis includes the following details.(1) Ten mercapto compounds as copper reactors, that is, L-cysteine, glutathione, homocysteine, dithioerythritol, N-acetyl-L-cysteine, cysteamine, D-penicillamine, tiopronin, captopril and 2-mercaptoethanol, were chosen to construct the prooxidant systems with Cu(?) ions, respectively, and their effectiveness as DNA-cleaving agents was investigated. Cysteamine, tiopronin, N-acetyl-L-cysteine and L-cysteine were the most active ones among the compounds tested. The involvement of superoxide anion radical, hydrogen peroxide, hydroxyl radical and Cu(?) ions in the DNA damage was affirmed by the inhibition of the DNA breakage by using specific scavengers of ROS and a Cu(?) chelator. More importantly, it was found that L-cysteine/Cu(?) significantly inhibited growth and induced apoptosis in human hepatoma HepG2 cells by promoting intracellular ROS formation and the subsequent collapse of mitochondrial, but not in their normal counterparts, L02 cells. The super selectivity can be rationalized by the fact that in comparison with L02 cells, HepG2 cells have relatively high levels of intrinsic ROS,thus, a further increase of ROS stress by using L-cysteine/Cu(II) at faster rate cause easily elevation of ROS above the threshold level, resulting in their death. In other words, the promotion of intracellular ROS production can be more efficiently conducted by L-cysteine/Cu(II) in HepG2 cells than in L02 cells.(2) Curcumin and its analogs as copper reactors and ionophores were selected to probe their ability in inducing DNA damage and death of cancer cells.It was found that curcumin analogs bearing no hydroxyl group on the aromatic rings were not the effective DNA-cleaving agents in the presence of Cu(II) ions, but they could function as copper ionophores by their ?-diketo moiety to promote increase of intracellular copper and ROS levels, leading to death of HepG2.(3) In view of the importance of Michael acceptor moiety in redox-directed cancer therapeutics, we synthesized more than twenty chalcones with different substituents at different positions on the aromatic rings, evaluated preliminarily their cytotoxicity. Additionally, we also synthesized sixteen compounds with [6]-gingerol as the lead compound.(4) We previously initiated the study on the reaction of resveratrol with hypohalous acids under physiological conditions to mimic MPO-mediated fate of this molecule, and found that eight chlorinated and brominated resveratrols were formed in the reaction, and halogenation of resveratrol increased its anti-hemolysis activity and modified its antimicrobial profile (see also Master's thesis written by Xia Wei in our group).In the thesis, galvinoxyl radical (GO·)-scavenging activity of the halogenated resveratrol derivatives in methanol and ethyl acetate were further evaluated using UV/Vis spectroscopy. Interestingly, all the halogenated resveratrol derivatives were less active than resveratrol in ethyl acetate, a solvent that weakly supports ionization, whereas they were more active than this parent molecule in methanol, a solvent that supports ionization. This suggests that halogenation of resveratrol augments its bond dissociation enthalpy (BDE) of O-H bond, but decreases its acid dissociation constant.