bcl-xL is an antiapoptotic protein that shares sequence homology with bcl-2 and seems to convey chemoresistance in many human tumor cell lines. bcl-xL protein is expressed at a 3-fold higher level in PC-3 cells than it is in LNCaP cells. Taxol causes apoptosis in both these cell lines, as measured by the formation of DNA ladders and by the observation of typical cellular morphological changes (chromatin condensation and nuclear fragmentation) after 4', 6-diamidino-2-phenylindole staining. Overexpression of bcl-2 in LNCaP cells did not prevent Taxol-induced apoptosis. Treatment of LNCaP cells with 10 nm Taxol led, after 24 h, to relatively specific and almost total down-regulation of bcl-xL protein in the absence of alteration of bax, bak, or bcl-2 levels. This change was paralleled by a similar decrease in the level of bcl-xL mRNA, as demonstrated by reverse transcription-PCR. The level of glyceraldehyde-3-phosphate dehydrogenase mRNA was not changed. In PC-3 cells, 48 h were required for both maximal bcl-xL protein down-regulation and cellular apoptosis. In contrast, treatment of LNCaP cells with estramustine induced apoptosis, but this was not associated with any change in the intracellular level of bcl-xL or bax protein. Instead, relatively specific 2-fold up-regulation of the proapoptotic protein bak was observed. In PC-3 cells, cellular apoptosis induced by estramustine was bak independent. These results augment our understanding of the importance of bcl-xL in prostate cancer and suggest that appropriate manipulation of cytotoxic chemotherapeutic agents may favorably alter the balance between pro- and antiapoptotic proteins in this tumor.