Nelfinavir inhibits regulated intramembrane proteolysis of sterol regulatory element binding protein-1 and activating transcription factor 6 in castration-resistant prostate cancer.

Nelfinavir induces apoptosis in liposarcoma by inhibiting site-2 protease (S2P) activity, which leads to suppression of regulated intramembrane proteolysis. We postulate similar effects in castration-resistant prostate cancer because it exhibits a lipogenic phenotype. Nelfinavir inhibited androgen receptor activation in androgen-sensitive prostate cancer and the nuclear translocation of the fusion proteins sterol regulatory element binding protein-1 (SREBP-1)-enhanced green fluorescence protein (EGFP) and activating transcription factor 6 (ATF6)-EGFP in castration-resistant prostate cancer cells, viewed under confocal microscopy. Nelfinavir and site-1 protease (S1P) and S2P small interfering RNAs (siRNAs) reduced the proliferation of castration-resistant prostate cancer and induced apoptosis, which was opposed by autophagy. Inhibition of autophagy with hydroxychloroquine was additive to the apoptotic effect of nelfinavir. Western blotting of S1P and S2P siRNA knockdown and/or nelfinavir-treated cells confirmed the accumulation of precursor SREBP-1 and ATF6. 3,4-Dichloroisocoumarin, an S1P inhibitor, did not affect SREBP-1 processing. In contrast, 1,10-phenanthroline, an S2P inhibitor, reproduced the nelfinavir-treated molecular and biological phenotype. Nelfinavir-mediated inhibition of regulated intramembrane proteolysis led to the accumulation of unprocessed SREBP-1 and ATF6. This resulted in sequential endoplasmic reticulum stress, inhibition of the unfolded protein response, reduced fatty acid synthase expression and apoptosis, which was countered by autophagy. Inhibition of autophagy was at least additive to this pro-apoptotic effect. These findings provide new insights into nelfinavir-induced endoplasmic reticulum stress and cancer cell death, and lead us to propose investigating its clinical activity in castration-resistant prostate cancer. This report validates S2P as a therapeutic target in castration-resistant prostate cancer.