Differential Regulation of Pro- and Anti-Apoptotic Genes by Bcr-Abl in an In Vitro Experimental Model of Chronic Myelogenous Leukemia.

Leukemic cells from patients with Philadelphia chromosome-positive chronic myelogenous leukemia (CML) are very resistant to apoptosis induced by DNA-damaging agents and other chemotherapeutic drugs, due to the presence of Bcr-Abl, a chimeric cytoplasmic tyrosine-kinase that confers both malignancy and resistance to apoptosis. Efficient treatment of CML can be achieved with a normal bone marrow transplant, which induces a graft-versus-leukemia response, and more recently by the use of the specific inhibitor imatinib mesylate (glivec. Novartis). Glivec blocks Bcr-Abl kinase activity and, as a consequence, the malignant cell dies by apoptosis. However most glivec-treated patients, mainly in the acute and blast phases, develop resistant forms of the disease. Since resistance to apoptosis in Bcr-Abl⁺ cells is probably related to the inhibition of mitochondrial release of cytochrome c, an obligatory step in most apoptotic pathways, we sought to investigate expression of Bcl-2 family genes in Bcr-Abl⁺, glivec-treated cells. By semi-quantitative RT-PCR we analyzed the gene expression of several pro- and anti-apoptotic molecules in the transduced cell line HL-60.Bcr-Abl and the wild-type HL-60, after a 1, 4 and 8h treatment with 10μM glivec. Bcr-Abl′s kinase activity is promptly inhibited by glivec (within 5 to 15min) and HL-60.Bcr-Abl cells begin to show mitochondrial depolarization 24h after treatment with the drug, dying 48h later, whereas no effects are observed in HL-60. Soon after glivec addition some genes are transcriptionally regulated in HL-60.Bcr-Abl cells. The major differences were observed for bcl-x_L (2-fold reduction), c-flip (2-fold increase), bcl-w (30% increase) and mcl-1 (20% reduction). Some pro-apoptotic molecules such as noxa also displayed differential regulation in HL-60.Bcr-Abl cells. No differences were observed in HL-60 cells. In conclusion we describe a complex transcriptional regulation mechanism dependent on Bcr-Abl tyrosine-kinase activity, which has not been previously described by the use of microarrays, and could contribute to the understanding of the mechanisms involved in protection of apoptosis and drug resistance of Bcr-Abl⁺ cells.