PACLITAXEL (TAXOL) INHIBITS PROTEIN ISOPRENYLATION AND INDUCES APOPTOSIS IN PC-3 HUMAN PROSTATE-CANCER CELLS

Paclitaxel was examined for its effects on cell survival, internucleos omal DNA fragmentation, and protein isoprenylation in the human prosta te cancer cell line PC-3. Treatment of cells with paclitaxel at 5-60 n M for 24 hr resulted in a dose-dependent inhibition of cell viability (IC50, 31.2 nM), which was partially prevented by supplementing the ce ll culture medium with two nonsterol polyisoprenyl compounds, farnesyl -pyrophosphate (-PP) and geranylgeranyl-PP (3 mu M each). Furthermore, agarose gel electrophoresis of DNA extracted from cells treated with paclitaxel (15-60 nM) for 24 hr showed DNA laddering with production o f fragments of 180-base pair multiples, indicating the occurrence of a poptotic cell death. Internucleosomal DNA fragmentation by paclitaxel was also detected by a photometric enzyme immunoassay using antihiston e antibodies; if culture medium was supplemented with farnesyl-PP and geranylgeranyl-PP (3 mu M each), a reduction in mono- and oligoucleoso me production was observed. The post-translational incorporation of me tabolites of (RS)-[5-H-3]mevalonolactone (100 mu Ci/ml) into prenylate d proteins of PC-3 cells was inhibited by paclitaxel at 30 and 60 nM. In addition, the immunoprecipitation of p21ras and p21rap-1 proteins f rom PC-3 cells exposed to paclitaxel (30 and 60 nM) and labeled with ( RS)-[5-H-3]mevalonolactone showed a substantial inhibition of the inco rporation of farnesyl and geranylgeranyl prenoid groups, respectively, into the aforementioned proteins. These results indicate that the inh ibition of protein isoprenylation is a novel component of the complex biochemical effects of the drug and plays an important role in the mec hanism of paclitaxel cytotoxicity in PC-3 cells.