INDUCTION OF APOPTOSIS BY CORDYCEPIN IN ADA-INHIBITED TDT-POSITIVE LEUKEMIA-CELLS

The nucleoside analogue cordycepin (3'-deoxyadenosine), when protected against ADA deamination, is specifically cytotoxic for TdT-positive l eukemia cells. Cordycepin-treated, ADA-inhibited, TdT-positive cells u ndergo the classic changes associated with drug-induced apoptosis: red uction in cell volume, chromatin clumping, membrane blebbing, and 180- bp multimer DNA laddering on agarose gels. In common with the apoptosi s seen in normal TdT-positive thymocytes, following exposure to variou s agents, apoptosis induced by cordycepin in TdT-positive leukemia cel ls was associated with increased protein kinase A (PK-A) activity. Unl ike thymocyte apoptosis however, no elevation in cAMP levels was seen preceding the rise in PK-A activity. Ex vivo we show that cordycepin m onophosphate can activate PK-A as efficiently as cAMP. On this basis w e speculate that cordycepin monophosphate in TdT-positive cells may be able to activate PK-A in place of cAMP, and that PK-A may phosphoryla te TdT, augmenting its activity as an endonuclease. In cell-free exper iments, the activity of recombinant TdT as an endonuclease digesting s upercoiled plasmid DNA into linear fragments was dramatically increase d following phosphorylation of TdT by PK-A. A role for TdT as an apopt otic endonuclease in TdT-positive leukemia cells following cordycepin exposure is now the subject of on-going work.