Increased Superoxide Levels, HO-1 and Fyn Protein Expression in Bcr/abl Overexpressing Cells: Implications for Adaptation and Survival in an Oxidatively Challenging Environment.

The bcr/abl oncogene is present in the majority of CML and in a subset of ALL patients. In addition to anti-apoptotic properties, proliferative advantages and growth factor independence, the presence of the Bcr/abl kinase raises intracellular levels of reactive oxygen species (ROS). Increased ROS in CML have been linked to DNA damage and DNA repair deficiencies with implications for promoting genomic instability and secondary mutations associated with disease progression. Here we show that superoxide is the specific ROS entity that is elevated when Bcr/abl is overexpressed in an IL-3 dependent pre-B cell line, BaF3. Strikingly, treatment with classical oxidants, such as hydrogen peroxide or tert-butyl peroxide, demonstrated an inherent resistance to oxidative stress induced apoptosis in Bcr/abl-containing cells, suggesting that antioxidant defenses activated by Bcr/abl as a response to the increased endogenous ROS production may promote survival in Bcr/abl positive populations.

In an effort to characterize the redox environment in Bcr/abl containing cells, we examined several antioxidant enzymes that are known downstream substrates of c-abl or bcr/abl, including peroxiredoxin, catalase and heme oxygenase-1 (HO-1). Of these, HO-1 protein expression was three fold higher in Bcr/abl transductants, consistent with a reported role for HO-1 as a Bcr/abl dependent survival factor. Interestingly, the Src family kinase, Fyn, which can be activated by oxidative stress, was increased four fold in Bcr/abl overexpressing cells. This is consistent with reported microarray data in which fyn mRNA levels are higher in Philadelphia positive leukemias than non-Philapdelphia positive leukemias. Other components of ROS responsive signaling pathways were also interrogated including the Mapk pathway and Akt pathway, with no difference noted between parental and Bcr/abl overexpressing cells.

Our data suggest a relationship between Bcr/abl-induced superoxide elevation, resistance to oxidative stress, elevation of HO-1 (which has antioxidant properties) and elevated Fyn (a putative downstream target for ROS production). These redox alterations and their consequences may equip Bcr/abl positive cells with a survival and/or proliferative advantage in an oxidatively challenging environment, thus promoting disease progression and blast crisis.