TGFβ Induces Changes in Mitochondrial Homeostasis in Skeletal Muscle of Mice with Bone Metastases

Abstract

Cancer cachexia is a common neoplastic syndrome of advanced cancers characterized by a loss of skeletal muscle mass and decreased function. Patients with cancer cachexia have higher chances of morbidity and mortality. We have shown that mice inoculated with human MDA‐MB‐231 breast cancer cells, which metastasize to bone, have skeletal muscle weakness, cachexia, and altered skeletal muscle metabolism. Mice with bone metastases have increased serum levels of transforming growth factor beta (TGFβ). Serum TGFβ decreased after treatment with the bone anti‐resorptive agent, zoledronic acid (ZA), suggesting TGFβ is, in part, bone‐derived. Additionally, mice with bone metastases treated with ZA had increased skeletal muscle function. TGFβ has also been shown to decrease the master regulator of mitochondrial biogenesis, peroxisome‐activated receptor γ coactivator 1α (PGC1α), in C2C12 myotubes. Mitochondria provide muscle with energy (ATP) via oxidative phosphorylation, so we measured oxygen consumption and energy production using indirect calorimetry in mice with bone metastases. Oxygen consumption and energy production were both decreased in these mice suggesting a decrease in mitochondrial function. We then examined skeletal muscle mitochondria using transmission electron microscopy and western blotting. We found highly dysmorphic mitochondria and altered protein markers of mitochondrial biogenesis and dynamics. Additionally, LC3B protein was increased in skeletal muscle mitochondria demonstrating increased mitophagy in skeletal muscle of mice with bone metastases. From our previous knowledge and data of TGFβ and its role in mice with bone metastases, we hypothesize that TGFβ causes mitochondrial dysfunction in skeletal muscle of mice with bone metastases. We have shown that a 48‐hour treatment with TGFβ decreases protein expression in markers of mitochondrial biogenesis and dynamics in C2C12 myotubes. This data suggests that TGFβ could be causing the mitochondrial dysfunction and dysmorphia we see in our mouse model, but further experiments are needed to prove this hypothesis. Our data demonstrates the importance of mitochondrial function in skeletal muscle cells and skeletal muscle of mice with bone metastases. Understanding the role TGFβ plays in changes of skeletal muscle mitochondria in mice with bone metastases will help contribute to defining treatments for patients with cancer cachexia.