Cytotoxicity and genotoxicity studies of two free-radical generators (AAPH and SIN-1) in human microvascular endothelial cells (HMEC-1) and human peripheral lymphocytes.

Vascular endothelial cells, smooth muscle cells, macrophages and other cell types in the arterial wall may develop oxidative/nitrosative damage by generation of reactive oxygen/nitrogen species, which could alter endothelial cell function. These changes could play a key role in acute inflammatory processes, atherosclerosis and neurodegenerative pathogenesis. A human microvascular endothelial cell line (HMEC-1) and human peripheral lymphocytes were employed to investigate the cytotoxic and genotoxic effects induced by reactive peroxyl radicals and peroxynitrite generated from 2,2'-azo-bis-(2-amidinopropane)-dihydrochloride (AAPH) and 3-morpholinosydnonimine (SIN-1), respectively. The peroxides generated by AAPH were cytotoxic but not genotoxic in HMEC-1 cells and in peripheral lymphocytes (in separate culture and in whole blood). SIN-1 showed progressive cytotoxicity to HMEC-1 at doses of 10-75µM. In the same range of concentrations a significant increase in apoptotic cells and micronuclei was observed. DNA flow-cytometric analysis indicated that 100 and 200µM SIN-1 significantly increased the proportion of cells in G(2) phase compared with the control. SIN-1 decomposition products, NO and superoxide anion or peroxynitrite, induced greater cytotoxicity in lymphocyte cultures (separately and in whole blood) supplemented with HEPES - the organic buffer that is widely used to maintain stable physiological pH in cell cultures -, due to H(2)O(2) production, than in cultures without HEPES. In contrast, increased genotoxicity was observed in both lymphocyte cultures in the absence of HEPES due to the reduced cytotoxicity. In the cell systems employed in this study the genotoxic effect appears closely dependent on the nature of radical species generated by SIN-1.

Erybraedin C, a natural compound from the plant Bituminaria bituminosa, inhibits both the cleavage and religation activities of human topoisomerase I.

The interaction of human topoisomerase I and erybraedin C, a pterocarpan purified from the plant Bituminaria bituminosa, that was shown to have an antitumour activity, was investigated through enzymatic activity assays and molecular docking procedures. Erybraedin C is able to inhibit both the cleavage and the religation steps of the enzyme reaction. In both cases, pre-incubation of the drug with the enzyme is required to produce a complete inhibition. Molecular docking simulations indicate that, when interacting with the enzyme alone, the preferential drug-binding site is localized in proximity to the active Tyr723 residue, with one of the two prenilic groups close to the active-site residues Arg488 and His632, essential for the catalytic reaction. When interacting with the cleavable complex, erybraedin C interacts with both the enzyme and DNA in a way similar to that found for topotecan. This is the first example of a natural compound able to act on both the cleavage and religation reaction of human topoisomerase I.

Protective effects of plicatin B on micronucleus induction in cultured human lymphocytes by different mutagens.

Among flavonoids, chalcones have been identified as interesting compounds having antioxidant, antimutagenic and antitumour properties. In this study we have evaluated the clastogenicity of plicatin B on human lymphocytes (whole blood and isolated lymphocytes) and its anticlastogenic activity, in the same cellular systems, using mitomycin C (MMC), radio-mimetic bleomycin (BL) and hydrogen peroxide (H2O2) as reference clastogens. Plicatin B per se resulted cytotoxic at high dose but non-clastogenic. The clastogenic effects induced by MMC and BL were significantly reduced (-33%) whereas with H2O2 the protective effect of plicatin B was observed (-62%) only at the greater H2O2 dose. The anticlastogen effects of plicatin B were effective when the chalcone was introduced in the culture, 1 h before the reference clastogens.

Erybraedin C and bitucarpin A, two structurally related pterocarpans purified from Bituminaria bituminosa, induced apoptosis in human colon adenocarcinoma cell lines MMR- and p53-proficient and -deficient in a dose-, time-, and structure-dependent fashion.

Pterocarpans, the second group of natural isoflavonoids, have received considerable interest on account of their medicinal properties. These drugs are employed as antitoxins, but display antifungal, antiviral and antibacterial properties as well. Erybraedin C and bitucarpin A are two new structurally related pterocarpans recently purified and characterized. Bitucarpin A differs from erybraedin C for the absence of a prenyl group in 5' position and the presence of a methoxylate hydroxyl group in 7, 4' positions. These compounds proved not to be clastogens in human lymphocytes per se but displayed anticlastogenic activity against mytomicin C and bleomycin C. Here we extended the study of their antiproliferative and apoptosis-inducing mechanism on human cell lines. Two human adenocarcinoma cell lines, LoVo and HT29, as examples of slow-growing solid tumors, proficient and deficient in mismatch repair system (MMR), p53 and Bcl-2, were used to evaluate the cytotoxicity of the drugs and their effects on the cell cycle, measured by flow cytometry. Erybraedin C similarly affects the survival of HT29 (MMR +/+, p53 -/- and Bcl-2 +/+) and LoVo (MMR -/-, p53 +/+ and Bcl-2 -/-) cells (LD(50): 1.94 and 1.73 microg/ml, respectively). By contrast, bitucarpin A exhibits a differential cytotoxicity in the cell lines (LD(50): 6.00 microg/ml, HT29, and 1.84 microg/ml, LoVo). The cell cycle distributions of the LoVo and HT29 cells treated with erybraedin C lacked a specific checkpoint arrest, whereas they underwent a characteristic sub-G(1) peak, time- and drug-concentration dependent. So that apoptotic process induced by erybraedin C in both adenocarcinoma cell lines is independent of cell cycle arrest and of phenotypic status of the cells as well. By contrast, bitucarpin A affects cell cycle progression on both cell lines, inducing a transient block in G(0)/G(1) along 24-96 h, and induces apoptosis with a cell density and treatment time dependency. Similar results were obtained with the positive control drug etoposide. The programmed cellular death on human adenocarcinoma cell lines may be efficiently activated, via a topoisomerase II poison pattern, by erybraedin C, the drug containing regio-specific hydroxyl and prenyl groups. The apoptotic effect induced by the methoxylated bitucarpin A proved to be conditioned by cell density and required higher dose (5-fold-LD(50)) and longer treatment time. The present study provides evidences that erybraedin C may act as a potent growth inhibitory compound, at low and high cell density, comparable to other clinically important antineoplastic natural drugs including etoposide, on human colon adenocarcinoma cells. Bitucarpin A proved less active because it was conditioned by cell density effect, but this finding may represent a clinical advantage against early micrometastatic diseases.