e Mitochondria are the main engine that generates ATP through oxidative phosphorylation within the respiratory chain. Mitochondrial respiration is regulated according to the metabolic needs of cells and can be modulated in response to metabolic changes. Little is known about the mechanisms that regulate this process. Here, we identify MCJ/DnaJC15 as a distinct cochaperone that localizes at the mitochondrial inner membrane, where it interacts preferentially with complex I of the electron transfer chain. We show that MCJ impairs the formation of supercomplexes and functions as a negative regulator of the respiratory chain. The loss of MCJ leads to increased complex I activity, mitochondrial membrane potential, and ATP production. Although MCJ is dispensable for mitochondrial function under normal physiological conditions, MCJ deficiency affects the pathophysiology resulting from metabolic alterations. Thus, enhanced mitochondrial respiration in the absence of MCJ prevents the pathological accumulation of lipids in the liver in response to both fasting and a high-cholesterol diet. Impaired expression or loss of MCJ expression may therefore result in a "rapid" metabolism that mitigates the consequences of metabolic disorders.

Development of metastasis in peripheral tissues is a major problem in the fight to cure breast cancer. Although it is becoming evident that chronic inflammation can contribute to tumor progression and metastasis, the effect of acute inflammation in primary tumor is less known. Using mouse models for breast cancer here we show that biopsy of mammary tumors increases the frequency of lung metastases. This effect is associated with the recruitment of inflammatory cells to the lung and elevated levels of certain cytokines such as IL-6 in the lung airways. Antiinflammatory treatment prior to and after the biopsy reduces the development of metastases triggered by the biopsy. In addition, while lack of IL-6 does not affect primary tumor development, it protects from increasing number of metastases upon biopsy. Thus, our studies show that in addition to chronic inflammation, acute immune response caused by invasive procedures in the primary tumor may cause an increased risk on peripheral metastases, but the risk could be decreased by anti-inflammatory treatments.

Chemotherapy remains the standard of care for most cancers worldwide, however development of chemoresistance due to the presence of the drug-effluxing ATP binding cassette (ABC) transporters remains a significant problem. The development of safe and effective means to overcome chemoresistance is critical for achieving durable remissions in many cancer patients. We have investigated the energetic demands of ABC transporters in the context of the metabolic adaptations of chemoresistant cancer cells. Here we show that ABC transporters use mitochondrial-derived ATP as a source of energy to efflux drugs out of cancer cells. We further demonstrate that the loss of methylation-controlled J protein (MCJ) (also named DnaJC15), an endogenous negative regulator of mitochondrial respiration, in chemoresistant cancer cells boosts their ability to produce ATP from mitochondria and fuel ABC transporters. We have developed MCJ mimetics that can attenuate mitochondrial respiration and safely overcome chemoresistance in vitro and in vivo. Administration of MCJ mimetics in combination with standard chemotherapeutic drugs could therefore become an alternative strategy for treatment of multiple cancers.