Establishment of novel specific assay for short-form glucose-dependent insulinotropic polypeptide and evaluation of its secretion in nondiabetic subjects

The naturally occurring GIP(1-30)NH2 is a GIP receptor agonist in humans

OBJECTIVE

The gut hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of glucose and bone metabolism. In rodents, the naturally occurring GIP variant, GIP(1-30)NH2, has shown similar effects as full-length GIP (GIP(1-42)), but its effects in humans are unsettled. Here, we investigated the actions of GIP(1-30)NH2 compared to GIP(1-42) on glucose and bone metabolism in healthy men and in isolated human pancreatic islets.

METHODS

Nine healthy men completed three separate three-step glucose clamps (0-60 minutes at fasting plasma glucose (FPG) level, 60-120 minutes at 1.5× FPG, and 120-180 minutes at 2× FPG) with infusion of GIP(1-42) (4 pmol/kg/min), GIP(1-30)NH2 (4 pmol/kg/min), and saline (9 mg/mL) in randomised order. Blood was sampled for measurement of relevant hormones and bone turnover markers. Human islets were incubated with low (2 mmol/L) or high (20 mmol/L) d-glucose with or without GIP(1-42) or GIP(1-30)NH2 in three different concentrations for 30 minutes, and secreted insulin and glucagon were measured.

RESULTS

Plasma glucose (PG) levels at FPG, 1.5× FPG, and 2× FPG were obtained by infusion of 1.45 g/kg, 0.97 g/kg, and 0.6 g/kg of glucose during GIP(1-42), GIP(1-30)NH2, and saline, respectively (P = .18), and were similar on the three experimental days. Compared to placebo, GIP(1-30)NH2 resulted in similar glucagonotropic, insulinotropic, and carboxy-terminal type 1 collagen crosslinks-suppressing effects as GIP(1-42). In vitro experiments on human islets showed similar insulinotropic and glucagonotropic effects of the two GIP variants.

CONCLUSIONS

GIP(1-30)NH2 has similar effects on glucose and bone metabolism in healthy individuals and in human islets in vitro as GIP(1-42).

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

Establishment of novel specific assay for short-form glucose-dependent insulinotropic polypeptide and evaluation of its secretion in nondiabetic subjects

The short‐form glucose‐dependent insulinotropic polypeptide (GIP) (1–30) is released from islet alpha cells and promotes insulin secretion in a paracrine manner in vitro. However, it is not well elucidated how GIP (1–30) is involved in glucose metabolism in vivo, since a specific assay system for GIP (1–30) has not yet been established. We first developed a sandwich enzyme‐linked immunosorbent assay (ELISA) specific for GIP (1–30) by combining a novel antibody specific to the GIP (1–30) C terminus with the common antibody against GIP N terminus. Then, we explored cross‐reactivities with incretins and glucagon‐related peptides in this ELISA. GIP (1–30) amide, but not GIP (1–42), GLP‐1, or glucagon increased absorbance in a dose‐dependent manner. We next measured plasma GIP (1–30) concentrations in nondiabetic participants (ND) during a 75‐g oral glucose tolerance test or cookie meal test (carbohydrates 75 g, lipids 28.5 g, proteins 8.5 g). Both glucose and cookie load increased GIP (1–30) concentrations in ND, but the increases were much lower than those of GIP (1–42). Furthermore, the DPP‐4 inhibitor significantly increased GIP (1–30) concentrations similarly to GIP (1–42) in ND. In conclusion, we for the first time developed an ELISA specific for GIP (1–30) and revealed its secretion in ND.