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Nucleoside transporters, bcl‐2 and apoptosis in CLL cells exposed to nucleoside analogues in vitro

The purine nucleoside analogues fludarabine (Fl) and chlorodeoxyadenosine (2‐CdA) are considered to be cell cycle specific agents which require DNA synthesis for cytotoxicity. However, their efficacy in the treatment of CLL, an indolent lymphoid malignancy suggests additional mechanisms of action. Like cytosine arabinoside (AraC), Fl and 2‐CdA gain access to the cell via a specific nucleoside transporter (NST) protein. To investigate the mode of action of these drugs in CLL, we used a fluorescent ligand for the NST (5‘‐(SAENTA‐ x 8)‐fluorescein) and 3‐colour flow cytometry to determine NST expression on CD5+/CD19+ B‐cells from the peripheral blood (PB) of patients with CLL. NST levels on these cells was found to be not significantly different from normal control lymphocytes (mean = 485±425) vs. (mean = 553±178). Exposure to varying concentrations (0, 3 μM and 30 μM) of Fl and 2‐CdA, however, resulted in an upregulation of NST (mean = 1552±775 with 30 μM FL; mean = 3392±2197 with 30 μM 2‐CdA) after 48 h. “Large” lymphoid cells (not present in normal PB) were found to express significantly more NST (mean = 2540±2861) and have a higher proliferative capacity than “small” cells (mean = 357±517 NST/cell). Incubation of CLL cells with Fl (n = 6) and 2‐CdA (n = 8) in vitro over 48 h also resulted in an increase in the proportion of cells in S‐phase (0 μM = 0.2±0.1; 30 μM FL = 2.4±2.0; 30 μM 2‐CdA = 3.3±1.3) and a significant increase in morphologically identifiable apoptosis. Apoptosis was confirmed by flow cytometric DNA analysis (0 μM = 13±8%; 30 μM FL = 40±20%; 30 μM 2‐CdA = 48±11%). In situ hybridization using a biotinylated cDNA bcl‐2 probe demonstrated that bcl‐2 mRNA expression was markedly decreased in treated cells after 24 h. These studies have demonstrated that: (1) NST expression on CLL lymphocytes is low; (2) in vitro exposure to the analogues increases both the level of NST expression and the % cells in S‐phase; (3) exposure to the analogues downregulates bcl‐2 expression and increases apoptosis.