"Mitochondria: A Crossroads for Oxidative Stress and Apoptosis Resistance in Lymphoma¡¨

ABSTRACT
Chronic inflammation increases the risk of lymphoma; adaptation to an environment of oxidative stress may contribute to the development of lymphoma and lead to resistance to treatment. Mitochondria could provide a point of convergence for oxidative stress and apoptosis, as they are a major source of endogenous ROS and are central to the intrinsic apoptotic pathway, in addition to their well-known role in energy generation. In our model of glucocorticoid-induced apoptosis, mitochondria isolated from WEHI7.2 lymphoma cells are sensitive to cytochrome c release induced by tBid, a pro-apoptotic member of the Bcl-2 family. These mitochondria are more sensitive to release following pre-treatment with dexamethasone (dex), a synthetic glucocorticoid. In contrast, mitochondria isolated from WEHI7.2 variants selected for resistance to H2O2, or transfected with catalase, demonstrate resistance to tBid-induced release, with little increase in sensitivity following dex treatment. Two steps are thought to govern cytochrome c release: oxidation of cardiolipin and permeabilization of the outer mitochondrial membrane by the Bcl-2 family of proteins. Western blots demonstrate that protein expression levels of Bcl-2 family members before and after drug treatment do not explain sensitivity to cytochrome c release and apoptosis. NAO fluorescence reveals that cardiolipin becomes oxidized in WEHI7.2 parental cells following dex-induced apoptosis, while oxidative stress-resistant variants demonstrate a delay in cardiolipin oxidation. This delay parallels the delay of cytochrome c release and apoptosis seen in vivo. Analysis of a diffuse large B-cell lymphoma microarray suggests a link between altered expression of mitochondrial proteins and patient outcome. We suggest that altering the cells' ability to handle oxidative stress has resulted in changes in mitochondrial function that affect cell survival. By understanding how oxidative stress and apoptosis converge at the mitochondria, we hope to identify targets for cancer therapy.