Mitochondrial fusion limits breast cancer metastasis

Mitochondrial metabolism plays a central role in promoting cancer growth and metastatic progression. The transition between a hyperfused and fragmented mitochondrial network is termed mitochondrial dynamics and is important for many mitochondria-associated functions; however, little is known regarding how this process influences metastasis. Here, we show that breast cancer cells with low metastatic potential exhibit a more fused mitochondrial network compared to highly metastatic breast cancer cells. To examine whether a fused mitochondrial network could impair metastasis, we inhibited mitochondrial fission in metastatic breast cancer cells by individual genetic deletion of three key regulators of mitochondrial fission (Drp1, Fis1 and Mff) or pharmacological intervention using leflunomide, an anti-rheumatic drug. These cells displayed a fused mitochondrial network and limited survival under anoikis conditions, consistent with mitochondrial fusion limiting metastasis. Transcriptomics and metabolomics analyses revealed that mitochondrial fusion causes significant alterations in metabolic pathways and processes related to cell adhesion. Functional bioenergetics assays demonstrated that mitochondrial fusion limited the mitochondrial capacity of cancer cells. Mitochondrial fusion in breast cancer cells had no significant effect on primary tumor growth but almost completely ablated lung metastasis in vivo. Furthermore, the transcriptomics signature associated with enhanced mitochondrial fusion correlated with improved survival in patients with breast cancer. Overall, our findings highlight mitochondrial fusion as a therapeutic opportunity for breast cancer. We generated two independent CRISPR-Cas9 KO clones using the highly metastatic 4T1 cell line for Dynamin-related protein 1 (Drp1), mitochondrial fission 1 protein (Fis1) and mitochondrial fission factor (Mff). In addition, we treated 4T1 cells with leflunomide or vehicle control (DMSO) for 3 days. We then performed gene expression profiling analysis from RNA-Seq. We compared KO clones to parental 4T1 and leflunomide-treated cells to vehicle control.