MNRR1 is required for Triple Negative Breast Cancer growth and metastasis and can be targeted by repurposed drugs.

Despite progress in recent decades, breast cancers remain the most diagnosed malignancies, and second leading cause of cancer death, in women. Although improved screening and systemic and endocrine adjuvant approaches have contributed to major declines in breast cancer mortality, current standard of care drugs are extremely toxic, and many women continue to be overtreated. Although nearly two-thirds of breast cancers are hormone-responsive, aggressive subtypes, particularly Triple Negative Breast Cancer (TNBC), still lack safe oral medications. Recently the roles that mitochondria play in TNBC carcinogenesis, metastasis, and resistance to treatment have garnered a great deal of attention. Contrary to the popular dogma that cancer cells are powered by glycolysis, metastatic breast cancer cells have enhanced mitochondrial function. Our work identified that Mitochondrial Nuclear Retrograde Regulator 1 (MNRR1; also, CHCHD2, PARK22), a key coordinator of mitochondrial-nuclear crosstalk that is physically present in both compartments, is overexpressed in TNBC cells and is an important regulator of metastasis signaling. We have identified Heat Shock Factor 1 (HSF1) as the main transcription factor that activates the MNRR1 promoter in TNBC cell lines. In the mitochondria, MNRR1 protein facilitates ATP production and inhibits apoptosis, whereas in the nucleus it regulates the transcription of stress-responsive genes including several required for epithelial to mesenchymal transition (EMT), metabolic flexibility, and cell growth. Thus, each of the bi-organellar functions of MNRR1 constitutes processes regarded as hallmarks of cancer. For reasons that are not yet fully understood, MNRR1 levels display a significant and robust ancestry bias, showing increased expression in tumor samples from Non-Hispanic Black (NHB) women when compared to disease-matched tumors from Non-Hispanic White (NHW) patients. It is possible that increased levels of MNRR1 may underlie the aggressive metastatic phenotype observed in many NHB patients. In further support of this observation, loss of MNRR1 function, either genetically or by use of inhibitors, reduces TNBC growth and metastasis. MNRR1 therefore is an attractive therapeutic target that could be exploited for design of novel therapies or as adjuncts to existing ones.