Cytoprotective antioxidant activity of serum albumin and autocrine catalase in chronic lymphocytic leukaemia.

Chronic lymphocytic leukaemia (CLL) cells are long lived in vivo but undergo spontaneous apoptosis when cultured in vitro. Intriguingly, CLL cells also appear to have a specific susceptibility to oxidative stress - a potent inducer of apoptosis. Here, we show that serum albumin can function as a cytoprotective antioxidant of potential relevance to circulating CLL cells, and that autocrine catalase - a hydrogen peroxide-inactivating enzyme that may be released extracellularly - can perform a similar role under the crowded conditions that prevail at sites of tissue involvement. Albumin lowered oxidative stress in cultured CLL cells and inhibited spontaneous and reactive oxidant-induced apoptosis. Maximal effects were observed at a concentration of 10 mg/ml - fourfold lower than that in plasma and twofold higher than that in standard culture medium containing 10% fetal calf serum. Oxidative stress and spontaneous apoptosis were also decreased by cell crowding and by conditioned medium (CM) from crowded CLL cells, indicating that these processes were subject to autocrine regulation. CLL cells were found to express catalase and release enzyme activity into the culture medium. Exogenous catalase decreased oxidative stress and spontaneous apoptosis, and the anti-apoptotic effect of CM from crowded CLL cells was abrogated by the specific catalase inhibitor, 3'-amino-1,2,4-triazole. Together, these data strongly implicate autocrine catalase as a cytoprotective antioxidant. Oxidative stress in CLL cells was greatly diminished by ruthenium red - an inhibitor of mitochondrial reactive oxidant production - and by the glutathione (GSH) precursor N-acetylcysteine, suggesting that the GSH peroxidase antioxidant system may be compromised by lack of available substrate. Our findings highlight the importance of endogenous reactive oxidants in regulating CLL-cell apoptosis, and help to explain why CLL cells survive for prolonged periods in vivo despite their vulnerability to oxidative stress and spontaneous apoptosis when cultured in vitro.