Pro-survival function of Akt/protein kinase B in prostate cancer cells - Relationship with trail resistance

Tumor necrosis factor superfamily member TRAIL/ Apo-2L has recently been sh own to induce apoptosis in transformed and cancer cells. Some prostate canc er cells express constitutively active Akt/protein kinase B due to a comple te loss of lipid phosphatase PTEN gene, a negative regulator of phosphatidy linositol 3-kinase pathway. Constitutively active Akt promotes cellular sur vival and resistance to chemotherapy and radiation. We have recently notice d that some human prostate cancer cells are resistant to TRAIL. We therefor e examined the intracellular mechanisms of cellular resistance to TRAIL. Th e cell lines expressing the highest level of constitutively active Akt were more resistant to undergo apoptosis by TRAIL than those expressing the low est level. Down-regulation of constitutively active Akt by phosphatidylinos itol 3-kinase inhibitors, wortmannin and LY294002, reversed cellular resist ance to TRAIL. Treatment of resistant cells with cycloheximide (a protein s ynthesis inhibitor) rendered cells sensitive to TRAIL. Transfecting dominan t negative Akt decreased Akt activity and increased TRAIL-induced apoptosis in cells with high Akt activity. Conversely, transfecting constitutively a ctive Akt into cells with low Akt activity increased Akt activity and atten uated TRAIL-induced apoptosis. Inhibition of TRAIL sensitivity occurs at th e level of BID cleavage, as caspase-8 activity was not affected. Enforced e xpression of anti-apoptotic protein Bcl-2 or Bcl-X-L inhibited TRAIL-induce d mitochondrial dysfunction and apoptosis. We therefore identify Akt as a c onstitutively active kinase that promotes survival of prostate cancer cells and demonstrate that modulation of Akt activity, by pharmacological or gen etic approaches, alters the cellular responsiveness to TRAIL. Thus, TRAIL i n combination with agents that down-regulate Akt activity can be used to tr eat prostate cancer.