DNA-protective effects of two components of essential plant oils carvacrol and thymol on mammalian cells cultured in vitro.

Many components of essential volatile oils show antioxidant activity and may serve e.g. as a natural replacement of synthetic antioxidant food additives. However, it is important to evaluate such compounds also for their pro-oxidant and toxic properties as their plant origin doesn't secure their safety for living beings, including humans. The aim of this study was therefore to investigate cytotoxic, genotoxic and DNA-protective effects of the long-term (24 h) incubation of mammalian cells with two components of essential plant oils (carvacrol and thymol) in in vitro conditions. Cytotoxicity testing was in all cell lines (human hepatoma cells HepG2, human colonic cells Caco-2 and hamster lung cells V79) performed on the basis of trypan blue exclusion. Plating efficiency was evaluated only in V79 cells which manifest a high colony forming ability. The amount of DNA lesions induced in cells treated with hydrogen peroxide, carvacrol, thymol or combinations of carvacrol or thymol with hydrogen peroxide was measured by standard alkaline single cell gel electrophoresis in human cells HepG2 and Caco-2. Trypan blue exclusion test showed that carvacrol was mildly more cytotoxic than thymol and that Caco-2 cells were mildly more resistant to both carvacrol and thymol than HepG2 and V79 cells. At concentrations = IC20-40, the compounds studied did not induce DNA strand breaks either in human cells HepG2 or in cells Caco-2. Incubation of HepG2 and Caco- 2 cells in the presence of the whole scale of concentrations of carvacrol or thymol led in both cases to a significant protection of the cells studied toward DNA strand breaks induced by a potent oxidant hydrogen peroxide.