It has been reported that oxidized LDL (oxLDL) are involved in the pathogenesis of atherosclerosis, and that macrophages as well as other cells of the arterial wall can oxidize LDL in vitro, depending on the balance between intracellular prooxidant generation and antioxidant defense efficiency. Because of their possible beneficial role in the prevention of atherosclerosis and other oxidative stress-associated diseases, phenolic compounds naturally occurring in vegetables, fruits, and beverages are receiving increased attention. In the present work, we investigated the mechanisms underlying the protective effect exerted by extra virgin olive oil biophenols, namely, protocatechuic acid and oleuropein, on LDL oxidation mediated by murine J774 A.1 macrophage-like cells. The biophenols were added to the cells with LDL and left in the medium during the entire experimental period, or for a period of 2 h and then removed from the medium before the addition of LDL. The effect of biophenols alone was also tested. In both experimental procedures, these antioxidants had the following effects: 1). completely prevented the J774 A.1-mediated oxidation of LDL; 2). counteracted the time-dependent variations in intracellular redox balance, inhibiting the production of O(2)(.-) and H(2)O(2) and the decrease in glutathione (GSH) content; 3). restored glutathione reductase (GR) and peroxidase (GPx) activities; and 4). restored the mRNA expression of gamma-glutamylcisteine synthetase (gammaGCS), GR, and GPx to control values. More importantly, we observed significant overtranscription and increased activities of two antioxidative enzymes, GPx and GR, compared with controls when the biophenols were present in the medium for 2 h and then removed before LDL exposure, or when the cells were exposed to the antioxidants alone for up to 24 h. Our findings suggest that the activation of mRNA transcription of GSH-related enzymes represents an important mechanism in phenolic antioxidative action.