Inhibitor of Apoptosis proteins (IAPs) are a group of E3 ligases involved in tumor necrosis factor (TNF) signalling. Upon TNF stimulation IAPs are needed for proper activation of NF-κB signalling, thereby protecting cells from cytotoxic activity of TNF.
IAPs have emerged as attractive targets for anti-cancer therapeutics since it has been shown that overexpression of the IAP antagonist Smac sensitises tumour cells to TNF driven apoptosis as do the synthetic counterparts, Smac mimetics. We demonstrated that cell death induced by inhibition of IAPs through Smac mimetics is not strictly TNF dependent. The combined treatment of Interferon γ (IFNγ), a potent pro-inflammatory cytokine, and Smac mimetics shows potent synergistic killing of a wide range of tumour cell lines as well as primary and transformed cells.
Further investigation showed that IFNγ and Smac mimetic induced killing requires NF-κB as well as Jak/STAT signalling and displayed classic apoptotic hallmarkers. Whereas killing of several cell lines could be blocked by the inhibition of caspases, other cell lines like HT29, KATOIII, mouse dermal fibroblasts (MDFs) and keratinocytes remained sensitive.
In order to elucidate the involvement of the necroptotic pathway in this apoptosis independent IFNγ and Smac mimetic combined killing we performed knock down experiments and analysed primary and transformed knock out MDFs and keratinocytes lacking members of the necroptotic pathway like RIPK3-/- and MLKL-/-.
The results suggest that IFNγ and Smac mimetic combined treatment can activate both apoptotic and non-apoptotic death pathways, involving members of the necroptotic pathway. This may provide new insights into yet unknown cell death mechanisms and extend the clinical use of Smac mimetics.