The olive leaf extract oleuropein exerts protective effects against oxidant-induced cell death, concurrently displaying pro-oxidant activity in human hepatocarcinoma cells.

OBJECTIVES: Oleuropein (OP), the predominant natural constituent of leaves of the olive tree, exerts anti-inflammatory and antioxidant effects. The purpose of this study was to assess the protective effects of OP under the conditions of paraquat (PQ)-induced oxidative stress in vitro, using the human hepatocarcinoma cell line, HepG2.
METHODS: Cell viability and death were determined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and 4',6-diamidino-2-phenylindole-propidium iodide staining, respectively. Superoxide anion and lipid peroxidation levels were evaluated using nitroblue tetrazolium and thiobarbituric acid-reactive substances assays, respectively. Apoptosis was assessed by measuring poly(ADP-ribose) polymerase (PARP) and caspase-3 (Casp-3) cleavage via immunoblotting and immunofluorescence analyses.
RESULTS: PQ induced a decrease in cellular viability by promoting necrosis through a mechanism involving superoxide generation and nuclear translocation of cleaved Casp-3. Co-treatment with OP afforded significant protection against the suppressive effects of PQ, as evident from increased cell viability, reduction of Casp-3 immunofluorescence, and normalization of β-tubulin expression levels. Unexpectedly, these OP-mediated protective effects were associated with increased superoxide and malondialdehyde generation and PARP cleavage. DISCUSSION: OP protects HepG2 cells against PQ-induced necrosis by suppressing Casp-3 cleavage while concomitantly acting as a pro-oxidant agent. This paradoxical mechanism of action of OP requires further investigation.