Accurate assessment of the bioactivities of redox-active polyphenolics in cell culture

Phenolic compounds are widely known for their roles as antioxidants and anti-inflammatory agents, as well as for their epidemiological association with reduced risks for certain types of diseases. In the present study, we used rabbit peripheral blood mononuclear cells (PBMCs) to evaluate possible artifacts that result from the reactivity of polyphenolics. We evaluated several common methods for cytotoxicity tests using nine polyphenolics, representing several major classes of tannins and their subunits. For three of those phenolics, we investigated whether or not the bioactivities of the phenolics were altered by spontaneous oxidation. Our study showed that many of the nine tested tannins interfered with the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay, which is commonly used to measure cell viability. A better method for determining cell viability is the luciferin/luciferase ATP assay, and using that method, we found that several tannins are toxic to PBMCs. We measured TNF-alpha production to assess possible anti-inflammatory activity, and found that only apigenin inhibited TNF-alpha production in LPS-stimulated cells (EC 50 1.0 microg/mL). The other polyphenolic compounds we tested either had no effect on TNF-alpha or increased its production. However, our data indicated that spontaneous oxidation altered the activity of phenolics, eliminating their toxicity. This study shows that the chemical reactivity of phenolics can significantly affect attempts to evaluate bioactivity in cultured cells and that particular attention should be paid to both methods for determining toxicity and to spontaneous oxidation of tannins during cell testing.