Flow cytometry and immunofluorescent staining were employed to examine the apoptosis signals, cytochrome-c and Caspase-3, in the primary leukemia cells with or without expression of VEGF and its receptors and normal healthy donor’s lymphocytes. We found that primary leukemic cells and normal lymphocytes didn’t secrete Cyt-c from mitochondria and no caspase-3 activation without the treatment of VP16. Instead, after incubation with VP16 (30mg/L) for 8 hours, normal lymphocytes and most of de novo leukemia cells secreted Cyt-c into cytosol from mitochondria and had the activation of caspase-3 in the cellular plasma. However, primary leukemia blasts with higher expression of VEGF and its receptors revealed lower secretion of Cyt-C and down-regulated activation of Caspase-3. Results showed that Cyt-c secretion and Caspase-3 activation were prone to occur in the VEGF negatively expressed blasts than positively expressed cells. This indicates that lower secretion of Cyt-c from mitochondria and inactivation of Caspase-3 are two of the pivotal mechanisms of anti-apoptosis for refractory leukemia clone with highly expressed VEGF and its receptors.

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