GLP-1R is initially found expressing in pancreatic islets cells and activated GLP-1R induces insulin synthesis and release; thus, GLP-1R agonists or dipeptidyl peptidase 4 (DPP-4) inhibitors have been used in the treatment of type 2 diabetes mellitus (T2DM) [93]. Here, DPP4 is linked to type 2 diabetes mellitus.