Characterisation of cancer cell vulnerabilities acquired during senescence for cancer cell clearance

Abstract

Senescence induction is becoming a relevant strategy in tumour treatment. Besides induction with therapeutic compounds (TIS – therapy induced senescence) senescence can be induced in tumor cells in vitro and in vivo via cytokine (IFN-γ & TNF) or TH1 cell treatment (CIS – cytokine induced senescence). Here, vulnerabilities acquired by senescence induction in β-cancer cells were investigated. While most CIS cancer cells remain viable, cease proliferation, and show SA-beta Gal activity, the cells also accumulate features associated with early apoptotic events. The cells lose their original membrane composition indicated by an increased surface localization of negatively charged phospholipids. This is accompanied by reduced mitochondrial membrane potential (Δψm) as well as changes in opening of the mitochondrial permeability transition pore (PTP), without changes in mitochondrial mass. This in turn compromises the retention of cytochrome c, leading to a depletion of overall cytochrome c protein. While caspase 3 activity in CIS cancer cells is enhanced in comparison to non-senescent cancer cells, it was suspected that increased levels of anti-apoptotic protein Bcl-2 in CIS counteracts death induction in favor of senescence. Indeed, treatment of CIS cancer cells with ABT263 (pan-Bcl-2 inhibitor) induced apoptosis preferentially in senescent cancer cells. Furthermore, the changes in cellular status after senescence induction lifted the resistance of cancer cells to TRAIL induced cell death. This thesis shows that CIS and TIS β-cell tumour cells are susceptible to TRAIL induced cell death in comparison to non-senescent β-cell tumour cells. Further analysis showed that apoptosis induction in senescent cancer cells was a prerequisite to render β-cell cancer cells susceptible to phagocytosis by macrophages. Therefore, type I anti-cancer immune response can act as a relay between inducing a stable growth arrest in cancer cells and senescent cancer cell clearance by macrophages exploiting the acquired sensitivity to TRAIL of senescent cancer cells. Overall, introduction of senescence followed by secondary apoptosis induction may prove to be a relevant therapeutic strategy. The findings may be a first step in understanding new mechanisms in cancer immune therapy with the goal to clear senescent tumour cells.