The cysteine protease legumain decreases glucose metabolism and enhances fatty acid uptake in human myotubes.

Skeletal muscle has an important role in whole body energy metabolism and various proteases are involved in skeletal muscle functions. We have previously identified the cysteine protease legumain in cultured human skeletal muscle cells. However, the potential role of legumain in regulation of energy metabolism remains unexplored. This study aimed to investigate cellular uptake, processing, and activation of prolegumain in human myotubes. Additionally, we sought to determine the effects of prolegumain on energy substrate metabolism in these cells. During differentiation of human myoblast to myotubes, legumain mRNA expression and activity were upregulated. Interestingly, legumain activity in myotubes was inversely correlated with the body mass index (BMI) of the obese cell donors. Myotubes exposed to conditioned medium enriched in prolegumain during the last two days of differentiation demonstrated the capacity to internalize and process prolegumain into its active form. Pre-treatment with prolegumain induced a metabolic shift towards increased fatty acid uptake in myotubes, as evidenced by elevated oleic acid uptake whereas glucose uptake and oxidation were reduced. The metabolic changes were not reversed by a legumain inhibitor, indicating a different mechanism for this effect. The metabolic alterations were accompanied by increased mRNA expression of the fatty acid transporter CD36, whereas the glucose transporter GLUT1 mRNA level remained unchanged. These findings suggest that legumain may play a regulatory role in skeletal muscle energy metabolism, highlighting its potential as a novel therapeutic target of metabolic disorders.