Apogossypol-mediated reorganisation of the endoplasmic reticulum antagonises mitochondrial fission and apoptosis

The endoplasmic reticulum (ER) with its elaborate network of highly curved tubules and flat sheets interacts with several other organelles, including mitochondria, peroxisomes and endosomes, to play vital roles in their membrane dynamics and functions. Previously, we identified structurally diverse chemicals from different pharmacological classes, which induce a reversible reorganisation of ER membranes. Using apogossypol as a prototypic tool compound, we now show that ER membrane reorganisation occurs at the level of ER tubules but does not involve ER sheets. Reorganisation of ER membranes prevents DRP-1-mediated mitochondrial fission, thereby antagonising the functions of several mitochondrial fission-inducing agents. Previous reports have suggested that ER membranes mark the constriction sites of mitochondria by localising DRP-1, as well as BAX on mitochondrial membranes to facilitate both mitochondrial fission and outer membrane permeabilisation. Following ER membrane reorganisation and subsequent exposure to an apoptotic stimulus (BH3 mimetics), DRP-1 still colocalises with the reorganised ER membranes but BAX translocation and activation, cytochrome c release and phosphatidylserine externalisation are all inhibited, thereby diminishing the ability of BH3 mimetics to induce the intrinsic apoptotic pathway. Strikingly, both ER membrane reorganisation and its resulting inhibition of apoptosis could be reversed by inhibitors of dihydroorotate dehydrogenase (DHODH), namely teriflunomide and its active metabolite, leflunomide. However, neither genetic inhibition of DHODH using RNA interference nor metabolic supplementation with orotate or uridine to circumvent the consequences of a loss of DHODH activity rescued the effects of DHODH inhibitors, suggesting that the effects of these inhibitors in preventing ER membrane reorganisation is most likely independent of their ability to antagonise DHODH activity. Our results strengthen the hypothesis that ER is fundamental for key mitochondrial functions, such as fusion-fission dynamics and apoptosis.