Luciferase reporter assay

HEK293T cells were co-transfected with an NF-κB luciferase plasmid (Beyotime Biotechnology, Shanghai, China) or a DPP4 luciferase plasmid (Sangon Biotech, Shanghai, China) together with other plasmids according to the manufacturers’ protocols and experimental design. After 24 h, luciferase activity was assayed using the Dual-Luciferase Reporter Assay System (Promega, Madison, WI, USA) and a luminometer (Glomax 20/20, Promega). Luciferase activity was normalized to Renilla luciferase activity driven by a constitutively expressed promoter in the phRL vector. Basal promoter activity was measured as the fold change relative to that detected using the basic pGL3 vector alone.