AUTHOR=Yanagimachi Tsuyoshi , Fujita Yukihiro , Takeda Yasutaka , Honjo Jun , Yokoyama Hiroki , Haneda Masakazu 

TITLE=Receptor-Mediated Bioassay Reflects Dynamic Change of Glucose-Dependent Insulinotropic Polypeptide by Dipeptidyl Peptidase 4 Inhibitor Treatment in Subjects With Type 2 Diabetes

JOURNAL=Frontiers in Endocrinology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2020.00214

DOI=10.3389/fendo.2020.00214

ISSN=1664-2392

ABSTRACT=<p><bold>Objective:</bold> We recently observed a greater increase in plasma levels of bioactive glucose-dependent insulinotropic polypeptide (GIP) than glucagon-like peptide 1 (GLP-1) using the receptor-mediated bioassays in the subjects with normal glycemic tolerance (NGT) treated with dipeptidyl peptidase 4 (DPP-4) inhibitors, which may be unappreciated using conventional enzyme-linked immunosorbent assays (ELISAs) during oral glucose tolerance test. Thus, we determined incretin levels in addition to glucagon level using the bioassays in type 2 diabetes mellitus (T2DM) subjects with or without treatment of DPP-4 inhibitor, to evaluate whether these assays can accurately measure bioactivity of these peptides.</p><p><bold>Methods:</bold> We performed single meal tolerance test (MTT) by using a cookie meal (carbohydrate 75.0 g, protein 8.0 g, fat 28.5 g) in the subjects with NGT (<italic>n</italic> = 9), the subjects with T2DM treated without DPP-4 inhibitor (<italic>n</italic> = 7) and the subjects with T2DM treated with DPP-4 inhibitor (<italic>n</italic> = 10). All subjects fasted for 10–12 h before the MTT, and blood samples were collected at 0, 30, 60, and 120 min. We used the cell lines stably cotransfected with human-form GIP, GLP-1 or glucagon receptor, and a cyclic adenosine monophosphate–inducible luciferase expression construct for the bioassays. We measured active GIP, active GLP-1, and glucagon by the bioassays. To evaluate the efficacy of bioassay, we measured identical samples via ELISA kits.</p><p><bold>Results:</bold> During the single MTT study, postprandial active GIP <sub><italic>bioassay</italic></sub> levels of T2DM with DPP-4 inhibitor treatment were drastically higher than those of NGT and T2DM without DPP-4 inhibitor, although the DPP-4 inhibitor-treated group showed moderate increase of active GIP<sub>ELISA</sub> and active GLP-1<sub><italic>bioassay</italic></sub>, while active GLP-1<sub><italic>bioassay</italic></sub> levels of T2DM subjects without DPP-4 inhibitor were comparable to those of NGT subjects. During the serial MTT, administration of DPP-4 inhibitor significantly increased active GIP<sub><italic>bioassay</italic></sub> levels, but not active GLP-1<sub><italic>bioassay</italic></sub>.</p><p><bold>Conclusions:</bold> In comparison to conventional ELISA, receptor-mediated bioassay reflects dynamic change of GIP polypeptide by DPP-4 inhibitor treatment in subjects with type 2 diabetes.</p>