In OVCAR3 cells, IFNAR1/IFNAR2-RNAi (80-90% knock down) or IRF9-RNAi (80% knockdown) completely inhibited the antiproliferative activity of IFN-alpha. On the other hand, the IRF9-RNAi demonstrated no inhibition of the antiproliferative activity of IFN-gamma. These results suggest that IFN-alpha signals through IFNAR1/IFNAR2 and utilizes IRF9 to elicit the anti-cancer activity in OVCAR3 cells. Furthermore, TRAIL gene expression (related to apoptosis) was inhibited by IFNAR1-RNAi or IRF9-RNAi, but not by Stat1-RNAi following IFN-alpha treatment, suggesting that induction of the TRAIL gene plays a role in eliciting the antiproliferative activity of IFN-alpha. It was shown that the recruitment of mitochondria in interferon (IFN)-alpha treated OVCAR3 cells leads to apoptosis. IFN alpha treatment results in the cleavage of BH3 interacting domain death agonist (Bid) to truncated Bid (tBid) which then activates mitochondrial Bak to trigger the loss of mitochondrial membrane integrity, and consequently causing release of apoptosis-inducing factor (AIF). AIF translocates from the mitochondria to the nucleus and induces nuclear fragmentation and cell death. Bid inhibition preserved mitochondrial membrane potential, prevented nuclear translocation of AIF, and abrogated IFN alpha2a-induced cell death. In addition, cell death induced by tBid was inhibited by AIF-RNAi, indicating that caspase-independent AIF signaling is the main pathway through which Bid mediates cell death. In conclusion, IFN alpha2a-induced apoptosis is mediated via the mitochondria-associated pathway involving the cleavage of Bid followed by AIF release that involves Bak activation and translocation from mitochondria to the nucleus in OVCAR3 cells. We have reported that low concentrations of interferon (IFN-alpha and -gamma) activated monocytes exert near eradicative cytocidal activity against low concentrations of several human tumor cells in vitro. In addition, confirmed strong cytocidal activity against seven human tumor cell lines. Then, to model larger in vivo tumors, we increased the target cell concentration one hundred-fold and found that increasing the tumor cell concentration increased the concentration of IFNs required for eradication by over 100 fold and the required concentration of monocytes by 10 fold. Thus in vitro modeling of high concentrations of target cells required high concentrations of monocytes and IFNs to approach tumor cell eradication. Recent in vivo studies on human ovarian tumors in immunosuppressed mice showed potent tumor eradication and enhanced survival after treatment with human monocytes and interferon alpha.
