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-beta-d-arab inofuranosylcytosine (Ara-C) by using hematological tumors expressing different levels of bcl-2. Scanning densitometry of Western blots demo nstrated that the myeloid U-937 cells express low levels of bcl-2 (RD= 0.008), whereas the follicular lymphoma RL-7 expressed very high level s (RD=3.084). Colony formation was also examined following incubation with Ara-C and RL-7 cells demonstrated a higher clonogenic survival (L D(50)=0.5 mu m) when compared with U-937 cells (LD(50)=0.005 mu M). Si milarly, the level of bcl-2 expression in each cell line was also rela ted to apoptosis with U-937 cells demonstrating increased DNA fragment ation when compared with RL-7 cells. To further evaluate the effect of upregulated bcl-2 on Ara-C treatment, U-937 cells were transfected wi th a retroviral vector carrying the murine bcl-2 or vector alone. Upre gulation of bcl-2 by myeloid leukemic cells increased the resistance b y 3 logs to Ara-C when comparing LD(50) values from clonogenic assays, and decreased apoptosis by at least 3 logs when measuring dUTP positi ve cells by flow cytometry.