Dissecting the role of mitochondrial fusion in maintaining breast cancer growth and survival

Abstract

Mitochondria are dynamic organelles which serve a wide range of cellular functions such as energy production through oxidative phosphorylation, serving as a calcium storage pool controlling ionic balance and overall support of cell survival. Inter-related with their function, mitochondria engage in interconnected pathways of fission and fusion which maintain overall mitochondrial quality control within the cell. As a hallmark of cancer biology, cancer cells undergo metabolic reprogramming to enable high energy production and increased metabolic biosynthetic flux to fuel cell proliferation. This has been understood to be achieved by a switch towards aerobic glycolysis as the main source of energy production and incorporation of bioavailable carbon. Here, we aimed to show that the mitochondrial fusion protein, optic atrophy 1 (Opa1), is involved in cancer cell metabolic reprogramming for the maintenance of plasticity and resilience of cancer cells. Database analyses suggested that Opa1 expression elevation across breast cancer patients correlate with decreased patient survival. Experimentally, we found that silencing of Opa1 led to metabolic reprogramming in which mechanistic target of rapamycin 1 (MTORC1) signaling was inhibited while the AMP-activated protein kinase (AMPK) pathway was activated. Genetic and pharmacological Opa1 inhibition resulted in growth impairments of both the 4T1 and MDA-MB-231 triple negative breast cancer (TNBC) cell lines. Finally, in vivo xenograft studies confirmed metabolic reprogramming in tumours derived from Opa1-silenced 4T1 cells including the re-activation of MTORC1 proliferative signaling to drive tumour growth. We carried out metabolomic studies of 4T1 cell media samples and cellular extracts following Opa1 silencing. Resulting metabolic profiling data suggested that cells following Opa1 silencing showed reprogrammed signaling with a block in glutaminolysis and increased reliance on aerobic glycolysis for efficient cell growth. Overall, Opa1 is required for breast cancer cell growth and survival via the maintenance of mitochondria-dependent metabolic capacity. Through gene targeting approaches, we have concluded that Opa1 function is a requirement for in vitro cell growth in both mouse and human TNBC cell models. These findings further support that mitochondrial fusion and overall maintenance are important processes required for the progression of breast cancer growth thereby proposing a potential therapeutic target for future studies.