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.

Exploring the potential of BH3 mimetic therapy in squamous cell carcinoma of the head and neck.

Squamous cell carcinoma of the head and neck (SCCHN) is the sixth most common cancer worldwide, with overall survival of less than 50%. Current therapeutic strategies involving a combination of surgery, radiation, and/or chemotherapy are associated with debilitating side effects, highlighting the need for more specific and efficacious therapies. Inhibitors of BCL-2 family proteins (BH3 mimetics) are under investigation or in clinical practice for several hematological malignancies and show promise in solid tumors. In order to explore the therapeutic potential of BH3 mimetics in the treatment of SCCHN, we assessed the expression levels of BCL-2, BCL-XL, and MCL-1 via Western blots and immunohistochemistry, in cell lines, primary cells derived from SCCHN patients and in tissue microarrays containing tumor tissue from a cohort of 191 SCCHN patients. All preclinical models exhibited moderate to high levels of BCL-XL and MCL-1, with little or no BCL-2. Although expression levels of BCL-XL and MCL-1 did not correlate with patient outcome, a combination of BH3 mimetics to target these proteins resulted in decreased clonogenic potential and enhanced apoptosis in all preclinical models, including tumor tissue resected from patients, as well as a reduction of tumor volume in a zebrafish xenograft model of SCCHN. Our results show that SCCHN is dependent on both BCL-XL and MCL-1 for apoptosis evasion and combination therapy targeting both proteins may offer significant therapeutic benefits in this disease.