Mitochondrial permeabilization engages NF-κB-dependent anti-tumour activity under caspase deficiency

Abstract

Apoptosis represents a key anti-cancer therapeutic effector mechanism. During apoptosis, mitochondrial outer membrane permeabilisation (MOMP) typically kills cells even in the absence of caspase activity. Caspase activity can also have a variety of unwanted consequences that include DNA-damage. We therefore investigated whether MOMP-induced caspase-independent cell death (CICD) might be a better way to kill cancer cells. We find that cells undergoing CICD display potent pro-inflammatory effects relative to apoptosis. Underlying this, MOMP was found to stimulate NF-κB activity through the down-regulation of inhibitor of apoptosis (IAP) proteins. Strikingly, engagement of CICD displays potent anti-tumorigenic effects, often promoting complete tumour regression in a manner dependent on intact immunity. Our data demonstrate that by activating NF-κB, MOMP can exert additional signalling functions besides triggering cell death. Moreover, they support a rationale for engaging caspase-independent cell death in cell-killing anti-cancer therapies.

Mitochondrial inner membrane permeabilisation enables mt DNA release during apoptosis

“…We next investigated an association between mitochondrial mtDNA release and activation of a STING‐dependent interferon response. For this purpose, we used SVEC 4–10 murine endothelial cells, which we have previously shown have an intact cGAS‐STING signalling pathway (Giampazolias et al , ). To induce rapid mitochondrial apoptosis, SVEC cells were treated with ABT‐737 together with the MCL‐1 inhibitor S63845 (Kotschy et al , ), and cell death was monitored by SYTOX Green uptake and IncuCyte live‐cell imaging (Fig A).…”

“…Mitochondrial apoptosis is considered a non‐inflammatory form of cell death, allowing the host to quickly and efficiently clear away dead cell corpses without provoking an immune response (Arandjelovic & Ravichandran, ). Recently, others and ourselves have shown that caspase activity is essential for the non‐inflammatory nature of mitochondrial apoptosis; if caspase activity is blocked following MOMP, cell death still occurs, but a type I interferon (IFN) response and NF‐κB activation ensues (Rongvaux et al , ; White et al , ; Giampazolias et al , ). This leads to pro‐inflammatory cytokine production and an immune response towards the dying cell that can initiate anti‐tumour immunity (Giampazolias et al , ).…”

“…Recently, others and ourselves have shown that caspase activity is essential for the non‐inflammatory nature of mitochondrial apoptosis; if caspase activity is blocked following MOMP, cell death still occurs, but a type I interferon (IFN) response and NF‐κB activation ensues (Rongvaux et al , ; White et al , ; Giampazolias et al , ). This leads to pro‐inflammatory cytokine production and an immune response towards the dying cell that can initiate anti‐tumour immunity (Giampazolias et al , ). Therefore, as proposed by others, a main function of apoptotic caspase activity may be to silence inflammation during cell death (Martin et al , ).…”

“…The ability of MOMP to activate inflammation and a type I IFN response requires recognition of mtDNA by the cytosolic cGAS‐STING DNA sensing pathway (Rongvaux et al , ; White et al , ; Giampazolias et al , ). However, it is challenging to reconcile how matrix localised mtDNA can activate the cytosolic cGAS‐STING DNA sensing pathway since the mitochondrial inner membrane is thought to remain intact during apoptosis (von Ahsen et al , ; Waterhouse et al , ).…”

MIM through MOM : the awakening of Bax and Bak pores

“…It will be interesting to test whether mtDNA release also takes place in these organisms, and whether there is a role for MOMP in inflammation in invertebrates. Irrespective of mtDNA release, caspase‐deficient MOMP can also induce TNF production via SMAC‐mediated downregulation of IAP proteins and activation of the NF‐κB pathway (Giampazolias et al , ). Thus, Bax and Bak emerge as key modulators of cell death‐associated inflammation.…”