Cytoprotection of Ulva rigida against hydrogen peroxide-induced apoptosis

Ulva rigida (Chlorophyta) has been suggested to contain cytoprotective agents against oxidative stress because of its antioxidant activity. However the antioxidant mechanisms of U. rigida have not yet been explored. In the present study, the protective effect of U. rigida against oxidative stress was evaluated, and the underlying molecular mechanisms were investigated in HeLa cervical carcinoma cells. Quantification of apoptosis by flow cytometry indicated that U. rigida extract inhibited H₂O₂-induced apoptosis in HeLa cells. In addition, using DiOC₆(3) localization as a measurment of mitochondrial integrity, we demonstrated that U. rigida extract prevented the decline of membrane mitochondrial potential (ΔΨ) induced by H₂O₂ stress. Western blot analysis revealed that the level of the pro-apoptotic protein p21 Bax decreased in H₂O₂ treated cells compared to the untreated cells, while it was consistely detected in cells co-treated with H₂O₂ and U. rigida extract. These can be associated to a cleavage, following H₂O₂ stress, of the full-length form of p21 Bax into a p18 Bax form accepted to be more potent in disrupting mitochondrial integrity. This study provided a possible explanation for the antioxidant activity of U. rigida, and implied an application of this alga for the therapy of diseases involving oxidative stress.