REGULATION OF THE FAS APOPTOTIC CELL-DEATH PATHWAY BY ABL

Relatively little is known about oncogene involvement in the regulatio n of Fas-mediated apoptosis. Inhibition of Pas-induced cell death by t he bcl-2 oncogene has been demonstrated to be only partial. In light o f a growing body of evidence for the Abl kinase as a negative regulato r of cell death, we sought to determine whether Abl expression could p rotect against Pas-mediated cell death. To address this question, we u tilized two separate strategies. In the first, we expressed human Fas in K562, a chronic myelogenous leukemia cell line, which constitutivel y expresses bcr-abl and examined the effects of Fas ligation in these cells. Fas-positive K562 transformants (K562.Fas) were found to be pro tected against Pas-mediated cell death. However, down-regulation of Bc r-Abl protein levels in K562,Fas cells using antisense oligonucleotide s targeted to bcr-abl mRNA rendered these cells highly susceptible to Fas-induced death. In the second approach we utilized a Fas-positive H L-60 cell line, which we transfected with a temperature-sensitive muta nt of v-Abl. HL-60.v-AbI(ts) transfectants were found to be protected from Fas-induced apoptosis at the permissive but not the restrictive t emperature for the Abl kinase. Taken together, these observations iden tify the Abl kinase as a negative regulator of Pas-mediated cell death . Since Abl was also found to block apoptosis mediated by ceramide, a recently proposed downstream effector of the apoptotic pathway initiat ed by Fas, we propose that Abl exerts its protective effects downstrea m of the early Fas-initiated signaling events.