"Society for Free Radical Biology and Medicine Abstract: Mitochondrial Alterations in Apoptosis-Resistant Lymphoma"
ABSTRACT
The acquired ability of a cell to evade apoptosis is considered a prerequisite for progression to cancer. As such, malignancies often present with, or acquire, resistance to apoptosis-inducing chemotherapies. Determining the mechanism of resistance is the first step in devising more effective treatments. Our model system of glucocorticoid-induced apoptosis comprises WEHI7.2 murine thymic lymphoma-derived cells treated with 1 microM Dexamethasone, a synthetic glucocorticoid. WEHI7.2 cells transfected with catalase (CAT2, CAT38) or selected for resistance to hydrogen peroxide (200R), demonstrate resistance to glucocorticoid-induced apoptosis, suggesting that the ability to remove ROS and/or tolerate oxidative stress contributes to apoptosis resistance. Previous work has demonstrated that the mechanism of resistance occurs in the signaling phase of the intrinsic apoptotic pathway, upstream of cytochrome c release from the mitochondria. The present study demonstrates that mitochondria isolated from the glucocorticoid-resistant variants are also resistant to cytochrome c release induced in vitro by addition of recombinant pro-apoptotic tBid protein. Of interest, the amount of cytochrome c expressed in mitochondria varies, with the resistant CAT2 and CAT38 expressing more cyt c than sensitive WEHI7.2 cells. The interplay of the various pro- and anti-apoptotic Bcl-2 family members can control mitochondrial integrity and cytochrome c release; however, Western blots for Bcl-2 family protein expression levels do not explain the difference in apoptosis sensitivity in our model system. Differences in Uncoupler Protein 2 (UCP-2) expression between sensitive a nd resistant variants could provide a mechanism that links ROS signaling and apoptosis, and western blots for UCP-2 demonstrate a reduced expression of protein in the resistant 200R cells, as compared to sensitive WEHI7.2. 2-D gel analyses of mitochondrial proteins have shown differences between sensitive and resistant cells, and have afforded additional proteins of interest for further study. We expect that results from these experiments will suggest targets for modulation of the apoptotic response in lymphoma therapy.
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