Effector caspase activation, in the absence of a conspicuous apoptosis induction, in mononuclear cells treated with azidothymidine

In the present study we focused our attention on the effect of AZT, at pharmacological and suprapharmacological concentrations, on some apoptosis-related key events and, particularly, on caspase activation in fresh human peripheral blood mononuclear cells (PBMCs). The main results can be summarized as follows: (i) AZT induced a strong, dose-dependent antiproliferative effect in mitogen-stimulated PBMCs, but low levels of cytotoxicity. in comparison with 5FU; (ii) low levels of cytotoxicity were coupled with a poor increase of apoptosis after AZT treatment in PBMCs; (iii) despite low levels of apoptosis, remarkable signs of both initiator and effector caspase enhanced expression with respect to control were detected by immunoblot analysis in AZT-treated PBMCs; (iv) enhanced caspase expression was associated with an increased expression of both anti-apoptotic Bcl-2 and pro-apoptotic Fas and p53 proteins, as detected by flow cytometry analysis; (v) combination treatment in vitro with AZT and anti-Fas significantly increased apoptosis in PBMCs with respect to single treatments. Overall, these results suggest that AZT treatment activates a complex, and apparently contrasting apoptosis-related signaling activity in PBMCs and that additional events are necessary to disrupt the balance induced by AZT towards apoptosis, on these cells.