Dipeptidyl peptidase-4 inhibitor treatment induces a greater increase in plasma levels of bioactive GIP than GLP-1 in non-diabetic subjects.
Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) possess multiple bioactive isoforms that are rendered non-insulinotropic by the enzyme dipeptidyl peptidase-4 (DPP-4). Recently, some ELISA kits have been developed to specifically measure "active" GIP and GLP-1, but it is unclear if these kits can accurately quantify all bioactive forms. Therefore, it remains uncertain to what extent treatment with a DPP-4 inhibitor boosts levels of biologically active GIP and GLP-1. Thus, we evaluated our novel receptor-mediated incretin bioassays in comparison to commercially available ELISA kits using plasma samples from healthy subjects before and after DPP-4 inhibitor administration.



We utilized cell lines stably co-transfected with human GIP or GLP-1 receptors and a cAMP-inducible luciferase expression construct for the bioassays and commercially available ELISA kits. Assays were tested with synthetic GIP and GLP-1 receptor agonists and plasma samples collected from subjects during a 75 g oral glucose tolerance test (OGTT) performed before or following 3-day administration of a DPP-4 inhibitor.



A GIP isoform GIP(1-30)NH2increased luciferase activity similarly to GIP(1-42) in the GIP bioassay but was not detectable by either a total or active GIP ELISA kit. During an OGTT, total GIP levels measured by ELISA rapidly increased from 0 min to 15 min, subsequently reaching a peak of 59.2 ± 8.3 pmol/l at 120 min. In contrast, active GIP levels measured by the bioassay peaked at 15 min (43.4 ± 6.4 pmol/l) and then progressively diminished at all subsequent time points. Strikingly, at 15 min, active GIP levels as determined by the bioassay reached levels approximately 20-fold higher after the DPP-4 inhibitor treatment, while total and active GIP levels determined by ELISA were increased just 1.5 and 2.1-fold, respectively. In the absence of DPP-4 inhibition, total GLP-1 levels measured by ELISA gradually increased up to 90 min, reaching 23.5 ± 2.4 pmol/l, and active GLP-1 levels determined by the bioassay did not show any apparent peak. Following administration of a DPP-4 inhibitor there was an observable peak of active GLP-1 levels as determined by the bioassay at 15 min after oral glucose load, reaching 11.0 ± 0.62 pmol/l, 1.4-fold greater than levels obtained without DPP-4 inhibitor treatment. In contrast, total GLP-1 levels determined by ELISA were decreased after DPP-4 inhibitor treatment.



Our results using bioassays indicate that there is a greater increase in plasma levels of bioactive GIP than GLP-1 in subjects treated with DPP-4 inhibitors, which may be unappreciated using conventional ELISAs.
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Tsuyoshi
T
Yanagimachi
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan. Electronic address: yfujita@asahikawa-med.ac.jp.
Yukihiro
Y
Fujita
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Yasutaka
Y
Takeda
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Jun
J
Honjo
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Hidemitsu
H
Sakagami
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Hiroya
H
Kitsunai
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Yumi
Y
Takiyama
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Atsuko
A
Abiko
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Yuichi
Y
Makino
Laboratory of Molecular & Cellular Medicine, Department of Cellular and Physiological Sciences, University of British Columbia, Vancouver, V6T 1Z3, Canada.
Timothy J
TJ
Kieffer
Division of Metabolism and Biosystemic Science, Department of Internal Medicine, Asahikawa Medical University, Asahikawa, Hokkaido, 078-8510, Japan.
Masakazu
M
Haneda
D000328:Adult; D001786:Blood Glucose / Q000378:metabolism; D018819:Dipeptidyl Peptidase 4 / Q000097:blood; D054873:Dipeptidyl-Peptidase IV Inhibitors / Q000008:administration & dosage*; D005749:Gastric Inhibitory Polypeptide / Q000097:blood; D005934:Glucagon / Q000097:blood; D052216:Glucagon-Like Peptide 1 / Q000097:blood*; D005951:Glucose Tolerance Test; D006801:Humans; D007328:Insulin / Q000097:blood; D008297:Male; D010446:Peptide Fragments / Q000097:blood; D020033:Protein Isoforms; D012680:Sensitivity and Specificity
D001786:Blood Glucose; D054873:Dipeptidyl-Peptidase IV Inhibitors; D007328:Insulin; D010446:Peptide Fragments; D020033:Protein Isoforms; D005749:Gastric Inhibitory Polypeptide; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; C042807:DPP4 protein, human; D018819:Dipeptidyl Peptidase 4
Dipeptidyl peptidase-4; Glucagon-like peptide-1; Glucose-dependent insulinotropic polypeptide; Receptor-mediated incretin bioassays