Saquinavir-NO-targeted S6 protein mediates sensitivity of androgen-dependent prostate cancer cells to TRAIL

We previously reported that the NO-modified form of HIV protease inhibitor Saquinavir ( Saq) is a potent antitumoral agent efficient against numerous tumor cell lines in vitro and in vivo. In acute toxicity studies, doses of Saq-NO equivalent to DL100 of the parental drug were completely nontoxic. Beside direct effect on malignant cell growth, Saq-NO sensitizes certain type of cells to tumor necrosis factor-related apoptosis-inducing ligand ( TRAIL)-mediated cell death. In this study, we evaluated the effects of Saq-NO on androgen-dependent prostate cancer LNCaP. Saq-NO inhibited both the growth of LNCaP cells in vitro and in xenograft models. Suppression of tumor growth was accompanied with cell cycle arrest in G(0)/G(1) phase and established a persistent inhibition of proliferation. Furthermore, Saq-NO reverted sensitivity of LNCaP cells to TRAIL but not to TNF. Treatment of cells with Saq-NO induced transient upregulation of Akt and ERK1/2. This, however, did not represent the primary mode of action of Saq-NO, as elimination with specific inhibitors did not compromise the chemotherapic efficacy of the drug. However, permanent abrogation of phosphorylation of the S6 protein, which is the downstream target of both signaling pathways, was observed. Diminished S6 phosphorylation was associated with re-established sensitivity to TRAIL and reduction of X-linked inhibitor of apoptosis protein (XIAP). In summary, NO modification of Saq led to a new chemical entity with stronger and more pleiotropic antitumor activity than the parental drug.