Intracellular phosphorylation of chloro‐adenosine analogs is a prerequisite for activation of caspase‐3 and induction of apoptosis in human astrocytoma cells

We previously demonstrated that both Cladribine (2‐chloro‐deoxyadenosine) and the related compound 2‐chloroadenosine induce apoptosis of human astrocytoma cells (Ceruti et al. [2000] J Neurosci Res 60:388–400). In the present study we have characterized the role of the most important “effector” caspase (caspase‐3) in the same experimental model and the relationship between the intracellular phosphorylation of the two adenosine analogs and activation of this enzyme. Data show that: 1) both adenosine analogs can activate caspase‐3 in a time‐dependent fashion; 2) significant activation of the enzyme (i.e., 6–7‐fold over control level) can be detected before any appearance of nuclear signs of apoptosis; 3) the degree of caspase‐3 activation is proportional to the percentage of apoptosis at the end of the incubation period, suggesting a causal relationship between enzyme activation and induction of cell death; and 4) both activation of caspase‐3 and induction of cell death can be completely prevented by preexposure of cultures to inhibitors of the intracellular phosphorylation of 2‐chloroadenosine and 2‐chloro‐2′‐deoxyadenosine (namely, 5‐iodotubercidin and 2′‐deoxycytidine, respectively). Adenosine analogs act, therefore, as prodrugs and the toxic species are represented by the corresponding chloro‐nucleotides. The pathways upstream of caspase‐3 activation in human astrocytoma cells still remain to be elucidated. Taken together, these data strongly suggest a possible use of adenosine analogs in the pharmacological treatment of central nervous system tumors and of all tumors characterized by a high activity of nucleoside kinases leading to selective intracellular accumulation of toxic chloro‐nucleotides. Drug Dev. Res. 58:396–404, 2003. © 2003 Wiley‐Liss, Inc.