The 42 amino acid peptide GIP (also known as glucose-dependent insulinotropic polypeptide) is secreted by enteroendocrine K cells in the duodenum and proximal jejunum, where it promotes lipid absorption from the intestinal lumen and inhibits gastric acid secretion and gastrointestinal motility. It is released in response to nutrient ingestion and, at supraphysiologic concentrations, is also a potent glucagon-like peptide.
In humans, the plasma levels of GIP rise immediately upon fat or glucose ingestion and are rapidly cleaved by DPP-4. This enzymatic inactivation is due to the presence of an alanine at position 2, which makes it susceptible to sulfhydrylation by DPP-4. GIP is a potent insulinotropic peptide with a short half-life (less than 2 min in rats), and its inactivation by DPP-4 results in diminished insulin secretion in response to fasting.
GIP also stimulates gene expression of proinflammatory cytokines and chemokines in cultured adipocytes, contributing to peripheral low-grade inflammation and insulin resistance, hallmarks of obesity. In mice, GIP-deficient animals are resistant to diet-induced obesity and extreme fat deposition with insulin resistance.
GIP is also a potent glucagon-like receptor agonist, stimulating insulin secretion in response to meals. However, the glucose-lowering effects of GIP are diminished in diabetic patients, presumably because of impaired insulinotropic activity in diabetes. Treatment of LD-STZ mice with the long-acting GIP analogue [d-Ala2]GIP(1-30)-PEG restored pancreatic beta cell area and normalised blood glucose excursions during an OGTT, although chronic treatment did not affect body weight gain or non-fasted glycaemia.