Dissecting the activation platform of nuclear caspase-8 in cancer

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Mutations in p53 is a hallmark of tumor development and found in ~50% of all tumors, leading to loss-of-function or gain-of-function of this important tumor suppressor. As a consequence, cancer cells resist p53-dependent cell cycle checkpoints and intrinsic apoptosis. Intriguingly, the residual 50% of tumors can still progress with wildtype p53 (wtp53) indicating that these tumor cells must be able to adapt alternative mechanisms suppressing wtp53 functions. We have recently identified a non-canonical role of caspase-8 in the nucleus that allows tumor cells with wtp53 activity to establish a de facto p53 protein loss, shifting cell fate from cell cycle arrest and apoptosis towards mitotic cell division at the G2/M checkpoint. In the nucleus, caspase-8 cleaves and inactivates the deubiquitinase USP28, preventing it from de-ubiquitinating and stabilizing wtp53. Based on our recent finding inhibition of nuclear caspase-8 might provide an ideal alternative treatment option for these aggressive cancers. However, general inhibition of caspase-8 is not indicated because it would only affect a subpopulation within the tumor tissue that harbors nuclear caspase-8. In contrast, it may have adverse effects on tumor subpopulations still expressing cytosolic caspase-8, aiming to induce cell death. Using three different approaches (APEX2-based proximity biotinylation; moTAPtag-based pull down; genome-wide CRISPR-Cas9/sg RNA screen) we aim to dissect the platform of caspase-8 activation in the nucleus of cycling and chemo-resistant melanoma cells. This will improve our understanding about the impact of the caspase-8/USP28/p53 axis in therapy responsiveness of melanoma patients and may pave the way for the identification of new therapeutic targets.