bcl-xL is a M(r) 26,000 bcl-2 homologue that is highly expressed in prostate cancer cells. In previous studies, the down-regulation of its expression by antisense oligonucleotides led to resistance. In this work, the 445-bp 5' terminus of the bcl-xL cDNA was cloned in the antisense orientation and stably transfected into DU145 and LNCaP prostate cancer cells. In the DU145 (and to a lesser extent the LNCaP) transfectants, phenotypic changes (versus mock-transfected cells) included an increase in doubling time (from 36 to 175 h) in the clone in which bcl-xL protein expression was 25% of control. The transfectants did not demonstrate characteristic apoptotic changes, as demonstrated by 4',6-diamidino-2-phenylindole staining, lack of either DNA laddering, caspase-3 activation, or poly(ADP)ribose and lamin cleavage, and the absence of a significant sub-G(0) population. Cell cycle analysis demonstrated an increase in a tetraploid population (from 28% to 66%), as well as the appearance of a hypertetraploid population. Levels of cIAP-1 protein were almost undetectable in the mock cells but increased at least 25-fold in the DU145 transfectants. The down-regulation of bcl-xL in both DU145 (and to a much lesser extent in LNCaP) cells led to their resistance to cytotoxic agents, including docetaxel, mitoxantrone, etoposide, vinblastine, and carboplatin. Reversion of bcl-xL expression in stable DU145 transfectants to nearly the levels found in the mock-transfected cells was accomplished by retroviral infection of the cells with a bcl-xL sense cDNA under control of a prolific promoter. This led to a dramatic increase in the growth rate and in BrdUrd incorporation, as well as a sharp decrease in the expression of cIAP-1 protein. Overall, these findings highlight the adaptability of prostate cancer cells to loss of bcl-xL and suggest that in addition to its prosurvival role, bcl-xL protein may also be involved in the regulation of the rate of cellular proliferation.