Constitutively active Akt is an important regulator of TRAIL sensitivity in prostate cancer

TRAIL/Apo-2L is a member of the tumor necrosis factor superfamily and has r ecently been shown to induce apoptosis in cancer cells, but not in normal c ells. In nude mice injected with human tumors, TRAIL reduces the size of th ese tumors without side effects. Akt promotes cell survival and block apopt osis. Some prostate cancer cells express high levels of Akt due to lack of active lipid phosphatase PTEN, a negative regulator of PI-3 kinase pathway, which may be responsible for drug resistance. The objective of this paper is to investigate the intracellular molecules that regulate TRAIL resistanc e. We have examined caspase-8 activity, BID cleavage, Akt activity, mitocho ndrial membrane potential (AT.) and apoptosis in prostate cancer (LNCap, PC -3, PC-3M and DU145) cells treated with or without TRAIL. PC-3, PC-3M and D U145 cells are sensitive to TRAIL, whereas LNCap cells are resistant. LNCap cells express the highest level of constitutively active Akt, which is dir ectly correlated with TRAIL resistance. TRAIL activates caspase-8 in all th e cell lines. Downregulation of constitutively active Akt by PI-3 kinase in hibitors (wortmannin and LY-294002), dominant negative Akt or PTEN, renders LNCap cells sensitive to TRAIL. Inhibition of TRAIL sensitivity occurs at the level of BID cleavage. Inhibition of protein synthesis by cycloheximide also causes LNCap cells sensitive to TRAIL. Overexpression of Bcl-2 or Bcl -X-L inhibits TRAIL-induced Delta Psi (m) and apoptosis. Overexpression of constitutively active Akt in PC-3M cells (express very low levels of consti tutively active Akt) restores TRAIL resistance. These data suggest that ele vated Akt activity protects LNCap cells from TRAIL-induced apoptosis, and t he PI-3 kinase/Akt pathway may inhibit apoptotic signals by inhibiting proc essing of BID. Thus, constitutively active Akt is an important regulator of TRAIL sensitivity in prostate cancer.