Exercise reduces intramuscular stress and counteracts muscle weakness in mice with breast cancer

Background Patients with breast cancer exhibit muscle weakness, which is associated with increased mortality risk and reduced quality of life. Muscle weakness is experienced even in the absence of loss of muscle mass in breast cancer patients, indicating intrinsic muscle dysfunction. Physical activity is correlated with reduced cancer mortality and disease recurrence. However, the molecular processes underlying breast cancer‐induced muscle weakness and the beneficial effect of exercise are largely unknown. Methods Eight‐week‐old breast cancer (MMTV‐PyMT, PyMT) and control (WT) mice had access to active or inactive in‐cage voluntary running wheels for 4 weeks. Mice were also subjected to a treadmill test. Muscle force was measured ex vivo. Tumour markers were determined with immunohistochemistry. Mitochondrial biogenesis and function were assessed with transcriptional analyses of PGC‐1α, the electron transport chain (ETC) and antioxidants superoxide dismutase (Sod) and catalase (Cat), combined with activity measurements of SOD, citrate synthase (CS) and β‐hydroxyacyl‐CoA‐dehydrogenase (βHAD). Serum and intramuscular stress levels were evaluated by enzymatic assays, immunoblotting, and transcriptional analyses of, for example, tumour necrosis factor‐α (TNF‐α) and p38 mitogen‐activated protein kinase (MAPK) signalling. Results PyMT mice endured shorter time and distance during the treadmill test (~30%, P