Pretreatment with paclitaxel enhances Apo-2 ligand tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis of prostate cancer cellsby inducing death receptors 4 and 5 protein levels

We have demonstrated that Apo-2 ligand (Apo-2L)/tumor necrosis factor-relat ed apoptosis-inducing ligand (TRAIL) induces apoptosis of human prostate ca ncer PC-3, DU145, and LNCaP cells in a dose-dependent manner, with PC-3 cel ls displaying the greatest sensitivity to Apo-2L/TRAIL. Susceptibility of t he prostate cancer cell types to Apo-2L/TRAIL-induced apoptosis did not app ear to correlate with the levels of the Apo-2L/TRAIL receptors death recept or (DR) 4 (TRAIL receptor 1) or DR5 (TRAIL receptor 2), decoy receptor (DcR ) 1 and DcR2, Flame-1, or the inhibitors of apoptosis proteins family of pr oteins. Apo-2L/TRAIL-induced apoptosis of PC-3 cells was associated with th e processing of caspase-8, caspase-l0, and the proapoptotic Bid protein, re sulting in the cytosolic accumulation of cytochrome c as well as the proces sing of procaspase-9 and procaspase-3. Cotreatment with the caspase-8 inhib itor z-IETD-fmk or DR4: Fe significantly inhibited Apo-2L/TRAIL-induced apo ptosis. Treatment with paclitaxel or taxotere increased DR4 and/or DR5 prot ein levels (up to 8-fold) without affecting the protein levels of DcR1 and DcR2, Apo-2L/TRAIL, Fas, or Fas ligand. Up-regulation of DR4 and DR5 was no t preceded by the induction of their mRNA levels but was inhibited by cotre atment with cycloheximide. Importantly, sequential treatment of PC-3, DU145 , and LNCaP cells with paclitaxel followed by Apo-2L/TRAIL induced signific antly more apoptosis than Apo-2L/TRAIL treatment alone (P < 0.01). This was also associated with greater professing of procaspase-8 and Bid, as well a s greater cytosolic accumulation of cytochrome c and the processing of casp ase-3. These endings indicate that up-regulation of DR4 and DR5 protein lev els by treatment with paclitaxel enhances subsequent Apo-2L/TRAIL-induced a poptosis of human prostate cancer cells.