Abstract
Purpose of review: Glucose-dependent insulinotropic polypeptide (GIP) plays an important role in the enteroinsular axis, stimulating insulin secretion. In addition, it has extrapancreatic actions, which may have pathophysiological relevance. This review highlights recent findings with regard to GIP's actions both within the enteroinsular axis and beyond it, and discusses evidence for the therapeutic potential of GIP receptor agonists and antagonists for the treatment of type 2 diabetes mellitus and possibly obesity. Recent findings: GIP signalling pathways have been investigated using transgenic animal models, either lacking or overexpressing a defective GIP receptor. The dependence of early-phase insulin potentiation by GIP on KATP channel activity, and of late-phase insulin secretion on other signals, has been demonstrated. GIP receptor agonists and antagonists resistant to enzymatic degradation and with a greater potency than native GIP have recently been developed. Their activity in animal studies suggests a novel and effective treatment of type 2 diabetes. Extrapancreatic actions of GIP have received little recent attention, with notable exceptions being the investigation of aberrant GIP receptor expression in Cushing's disease, and a possible role for GIP in vascular endothelial function. Summary: The role of GIP in stimulating insulin secretion continues to be a primary focus for research, and the availability of various GIP-receptor knockout mice have helped to elucidate GIP's signalling pathways. A range of GIP receptor agonists and antagonists show promise in the treatment of type 2 diabetes, but as yet no clinical studies have been undertaken. Studies implicating GIP beyond the enteroinsular axis remain few and often negative, with the exception of effects on the vascular endothelium and the adrenal gland. © 2006 Lippincott Williams & Wilkins.