APLN and diabetes mellitus: The aforementioned data indicate that the exogenous injection of Apelin-13 can improve glucose and lipid metabolism disorders, endothelial dysfunction, inflammatory reactions, and pancreatic islet functions in the context of diabetes by regulating multiple signaling pathways, β-cell quality, ER stress, and oxidative stress, and this protein could therefore become a biomarker for the prevention and treatment of metabolic diseases such as DM (Figure 2).