Abstract
Alternative processing of the precursor protein pro-GIP results in endogenously produced GIP(1–30)NH2, that by DPP-4 cleavage in vivo results in the metabolite GIP(3–30)NH2. We showed previously that GIP(3–30)NH2 is a high affinity antagonist of the human GIPR in vitro. Here we determine whether it is suitable for studies of GIP physiology in rats since effects of GIP agonists and antagonists are strictly species-dependent. Transiently transfected COS-7 cells were assessed for cAMP accumulation upon ligand stimulation or assayed in competition binding using human 125I-GIP(1–42) as radioligand. In isolated perfused rat pancreata, insulin, glucagon, and somatostatin-releasing properties were evaluated. Competition binding demonstrated that on the rat GIP receptor (GIPR), rat GIP(3–30)NH2 bound with high affinity (Ki of 17nM), in contrast to human GIP(3–30)NH2 (Ki of 250nM). In cAMP studies, rat GIP(3–30)NH2 inhibited GIP(1–42)-induced rat GIPR activation and schild-plot analysis showed competitive antagonism with a pA2 of 13nM and a slope of 0.9±0.09. Alone, rat GIP(3–30)NH2 displayed weak, low-potent partial agonistic properties (EC50>1μM) with an efficacy of 9.4% at 0.32μM compared to GIP(1–42). In perfused rat pancreata, rat GIP(3–30)NH2 efficiently antagonized rat GIP(1–42)-induced insulin, somatostatin, and glucagon secretion. In summary, rat GIP(3–30)NH2 is a high affinity competitive GIPR antagonist and effectively antagonizes GIP-mediated G protein-signaling as well as pancreatic hormone release, while human GIP(3–30)NH2, despite a difference of only one amino acid between the two (arginine in position 18 in rat GIP(3–30)NH2; histidine in human), is unsuitable in the rat system. This underlines the importance of species differences in the GIP system, and the limitations of testing human peptides in rodent systems.
Highlights
glucose-dependent insulinotropic polypeptide (GIP)(1–42) is known as a postprandial gut hormone secreted from enteroendocrine K cells of the small intestine [1] together with other gut hormones [2,3]
Our study demonstrates that rat GIP(3–30)NH2 is a high affinity competitive antagonist on the rat GIP receptor (GIPR) in vitro and in the surviving perfused rat pancreas, whereas human GIP(3–30)NH2 displays much lower affinity and a consequent lower antagonistic potency on the rat GIPR
This indicates that human GIP(3–30)NH2 is irrelevant in the rat GIP system, whereas rat GIP(3–30)NH2 can be used as a tool to study the GIP physiology when using the rat as a model system
Summary
GIP(1–42) is known as a postprandial gut hormone secreted from enteroendocrine K cells of the small intestine [1] together with other gut hormones [2,3]. Additional pancreatic effects may include stimulation of glucagon secretion from the a-cells [5,6] and somatostatin release from d-cells [7,8]. The GIP receptor (GIPR) is widely expressed in various tissues besides the pancreas including adipose, bone, and lung tissue [9,10]. GIPR knock out mice are resistant to diet-induced obesity and crossing this mouse with the leptin mutant (ob/ob) mouse, which is an established mouse model for hyperphagic obesity, reduced weight gain by 23% [11], whereas transgenic GIPR expression in adipose tissue in global GIPR knock out mice restores diet-induced body weight gain [12].
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