Reduced ER–mitochondria connectivity promotes neuroblastoma multidrug resistance

Abstract

Most cancer deaths result from progression of therapy resistant disease, yet our understanding of this phenotype is limited. Cancer therapies generate stress signals that act upon mitochondria to initiate apoptosis. Mitochondria isolated from neuroblastoma cells were exposed to tBid or Bim, death effectors activated by thera- peutic stress. Multidrug-resistant tumor cells obtained from chil- dren at relapse had markedly attenuated Bak and Bax oligomerization and cytochrome c release (surrogates for apopto- tic commitment) in comparison with patient-matched tumor cells obtained at diagnosis. Electron microscopy identified reduced ER– mitochondria-associated membranes (MAMs; ER–mitochondria contacts, ERMCs) in therapy-resistant cells, and genetically or bio- chemically reducing MAMs in therapy-sensitive tumors phenocopied resistance. MAMs serve as platforms to transfer Ca2+ and bioactive lipids to mitochondria. Reduced Ca2+ transfer was found in some but not all resistant cells, and inhibiting transfer did not attenuate apoptotic signaling. In contrast, reduced cer- amide synthesis and transfer was common to resistant cells and its inhibition induced stress resistance. We identify ER– mitochondria-associated membranes as physiologic regulators of apoptosis via ceramide transfer and uncover a previously unrecog- nized mechanism for cancer multidrug resistance.