The mitochondrial ATP transporter VDAC1 /Porin is required for TRAIL-induced apoptosis in non-small cell lung cancer


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Alex Chacko, Ian Paul, Fabio Liberante, Dean Fennell

Queen's University Belfast, Belfast, UK

Abstract

Proffered paper presentation

Resistance to apoptosis is both a hallmark of cancer and a critical problem limiting the efficacy of chemotherapy, particularly in non-small cell lung cancer (NSCLC). Activation of death receptors by TRAIL receptor agonists is a strategy currently under investigation in non-small cell lung cancer such as the upcoming EORTC 08071 clinical trial. Identification of resistance biomarkers will be important for optimal tailoring of TRAIL receptor agonist therapy.

A panel of H460 NSCLC cells were selected with stable shRNA knockdown of BAX, BAK, BAX and BAK, BID, BIM and VDAC1. Cells were then exposed to TRAIL ligand, which agonises the DR4 and DR5 surface receptors, leading to activation of Caspase-8. Previously, the pro-apoptotic BCL-2 family protein BID has been shown to be important for TRAIL-induced toxicity. Two shBID clones abrogated TRAIL-induced cell death and partially blocked Caspase-3 activation consistent with a requirement for mitochondria for full TRAIL induced death signalling. Unexpectedly, stable knockdown of mitochondrial VDAC1 to virtually undetectable levels protected cells against TRAIL induced apoptosis using viability assay. Indicators of apoptosis such as Caspase-3 activity and PARP were also markedly reduced by VDAC1 knockdown, and Caspase-8 activation was also attenuated. Previously, full Caspase-8 activation was reported to take place at the mitochondria, requiring mitochondrial cardiolipin, and is inhibitable by the anti-apoptotic mitochondrial factor BCL-XL, which is itself known to regulate the VDAC1 channel. We observe that in VDAC1 knockdown cells, translocation of Caspase-8 to the mitochondria is unaffected, but final activation to p18-Caspase-8 does not occur. Our findings suggest that activation of type II apoptosis by TRAIL ligand may be regulated at the mitochondrial surface by VDAC1. Loss of VDAC1 activity in primary tumours could therefore be clinically associated with resistance to TRAIL receptor agonists.