Treatment-induced expression of anti-apoptotic proteins in WSU-CLL, a human chronic lymphocytic leukemia cell line

Bryostatin 1 (bryo 1) has been shown to potentiate the anti-tumor activity of 2-chloro-2-deoxyadenosine (2-CdA) in chronic lymphocytic leukemia (CLL) and in the WSU-CLL cell line. However, like resistant CLL, WSU-CLL cells lo se their sensitivity to bryo 1/2-CdA treatment. We report that 2-CdA-induce d IAP expression may be a possible mechanism whereby resistance to apoptosi s is acquired in these cells. In WSU-CLL cells, three members of the Inhibi tors of Apoptosis (IAP) family were identified. Bryo I treatment of WSU-CLL cells leads to initiation of the apoptotic cascade and induced a marginal increase in XIAP protein expression. In contrast, 2-CdA treatment, alone or in combination with bryo 1, induced a substantial increase in survivin and XIAP proteins and phosphorylation of BAD. Bryo I alone induced caspase-7 a nd -9 dependent [poly ADP-ribose] polymerase (PARP) cleavage, while sequent ial treatment with bryo 1(72 h) followed by 2-CdA (24 h) induced caspase-3, -7, and -9 dependent PARP cleavage and increased apoptosis. Although exposu re to bryo I initiated apoptotic events, apoptosis was first enhanced by 2- CdA, and then reversed in a time-dependent manner by 2-CdA-induced expressi on of survival proteins. Taken together, resistance to bryo 1/2-CdA treatme nt may be the result of 2-CdA-induced LAY inhibition of the intrinsic apopt otic pathway caspases.