Quantitation of resistance to cytosine arabinoside by myeloid leukemic cells expressing bcl‐2

The presence of bcl-2 in myeloid leukemias has been associated with a decrease in therapy-induced apoptosis, reduced patient survival and in vitro autonomous growth of leukemic cells. The present study focuses on the quantitation of resistance to increasing doses of 1-β-d-arabinofuranosylcytosine (Ara-C) by using hematological tumors expressing different levels of bcl-2. Scanning densitometry of Western blots demonstrated that the myeloid U-937 cells express low levels of bcl-2 (RD=0.008), whereas the follicular lymphoma RL-7 expressed very high levels (RD=3.084). Colony formation was also examined following incubation with Ara-C and RL-7 cells demonstrated a higher clonogenic survival (LD 50 =0.5 μm) when compared with U-937 cells (LD 50 =0.005 μM). Similarly, the level of bcl-2 expression in each cell line was also related to apoptosis with U-937 cells demonstrating increased DNA fragmentation when compared with RL-7 cells. To further evaluate the effect of upregulated bcl-2 on Ara-C treatment, U-937 cells were transfected with a retroviral vector carrying the murine bcl-2 or vector alone. Upregulation of bcl-2 by myeloid leukemic cells increased the resistance by 3 logs to Ara-C when comparing LD 50 values from clonogenic assays, and decreased apoptosis by at least 3 logs when measuring dUTP positive cells by flow cytometry.