A Novel Bcr-Abl Mediated Pro-Survival Pathway: Reduction of Releasable Calcium Levels in the Endoplasmic Reticulum Inhibits Calcium Dependent Apoptotic Signaling.

The Bcr-Abl oncoprotein plays a major role in the development and progression of chronic myeloid leukemia (CML). Several studies have suggested that the expression levels of Bcr-Abl are elevated at disease progression to blast crisis (BC) and that this plays significant role in the achievement of drug resistance. We have established cell lines expressing low (C2 cells) and high levels (C4 cells) of Bcr-Abl to study the molecular mechanisms involved in disease progression especially with regard to drug resistance. Microarray analysis revealed that Bcr-Abl expressing cells exhibit increased expression of various endoplasmic reticulum (ER) resident proteins indicating that the ER is a novel target of Bcr-Abl. Recently, this organelle has been described as an important apoptotic gateway. Indeed, by measuring calcium levels (steady state and following drug treatment) in the ER and cytoplasm we could elucidate that the calcium dependent apoptotic signaling is disturbed in Bcr-Abl transformed cells. Moreover, inhibition of Bcr-Abl activity by Imatinib reversed this phenotype indicating that Bcr-Abl is directly modulating calcium homeostasis by yet unknown mechanisms. Two Bcr-Abl expressing human cell lines, K562 and BV173, also showed reduced amounts of calcium in the ER compared to the Bcr-Abl negative cell lines HL60 and Jurkat. Etoposide was chosen for apoptosis induction as this drug was classified to target not only the mitochondria (classical mitochondrial pathway) but also the ER (calcium dependent apoptotic pathway). Cell death was induced in the parental and Bcr-Abl transformed cell lines and calcium changes in the cytosol and the mitochondria were measured using organelle specific calcium probes. In C2 and C4 cells an early mitochondrial calcium overload was absent possibly due to the decreased amount of free releasable calcium in the ER. Moreover, other ER/calcium activated pathways were greatly diminished, such as cytosolic calcium elevation and calpain activation during apoptosis. It has been shown that decreased levels of releasable calcium in the ER is associated with cell survival. It is possible therefore that Bcr-Abl may exert some of its pro-survival or anti-apoptotic effects at the ER, by directly influencing the levels of releasable calcium. This study demonstrates a novel downstream consequence of Bcr-Abl signaling. The ability to negate calcium dependent apoptotic signaling is likely to be a major pro-survival mechanism in Bcr-Abl transformed cells.