Incretin peptides are hormones and peptide mimetics that cause an increase in the amount of insulin released when glucose levels are normal or particularly when they are elevated. These incretin peptides have other actions beyond the initial incretin action defined by insulin secretion. For instance, they may also have actions to reduce glucagon production and delay gastric emptying. In addition, they may have actions to improve insulin sensitivity, and they may increase islet cell neogenesis—the formation of new islets.
The concept of the incretin effect developed from the observation that insulin responses to oral glucose exceeded those measured after intravenous administration of equivalent amounts of glucose. It was concluded that gut-derived factors, or incretins, influenced postprandial insulin release. Nutrient entry into the stomach and proximal gastrointestinal tract causes release of incretin hormones, which then stimulate insulin secretion. This insulinotropism, or ability to stimulate insulin secretion, can be quantified by comparing insulin or C-peptide responses to oral vs. intravenous glucose loads. In this way, it has been shown that the incretin effect is responsible for about 50% to 70% of the insulin response to oral glucose in healthy individuals.
Although many postprandial hormones have incretin-like activity, predominant incretin peptides include glucose-dependent insulinotropic polypeptide, also known as gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and exendin peptides (which are non-endogenous incretin mimetics). GIP and GLP-1 both belong to the glucagon peptide superfamily and thus share amino acid sequence homology. GIP and GLP-1 are secreted by specialized cells in the gastrointestinal tract and have receptors located on islet cells as well as other tissues. As incretins, both are secreted from the intestine in response to ingestion of nutrients, which results in enhanced insulin secretion. The insulinotropic effect of GIP and GLP-1 is dependent on elevations in ambient glucose. Both are rapidly inactivated by the ubiquitous enzyme dipeptidyl peptidase IV (DPP-IV).
Native human GIP is a single 42-amino acid peptide synthesized in and secreted by specialized enteroendocrine K-cells. These cells are concentrated primarily in the duodenum and proximal jejunum, although they also can be found throughout the intestine. The main stimulant for GIP secretion is ingestion of carbohydrate- and lipid-rich meals. Following ingestion, circulating plasma GIP levels increase 10- to 20-fold. The half-life of intact GIP is estimated to be approximately 7.3 minutes in healthy subjects and 5.2 minutes in diabetic subjects.
The physiologic effects of GIP have been elucidated using GIP receptor antagonists, GIP peptide antagonists, and GIP receptor knockout mice, in addition to GIP infusion protocols. Blocking GIP binding to its receptor results in attenuated glucose-dependent insulin secretion following oral glucose load in rats and mice. Similarly, administration of GIP antagonists or GIP antisera markedly reduces the postprandial insulin release in rats. GIP receptor knockout mice demonstrate normal fasting glucose levels but mild glucose intolerance following oral glucose loads. Interestingly, they also exhibit resistance to diet-induced obesity following months of high-fat feeding. Additionally, in the leptin-deficient ob/ob mouse, the GIP receptor knockout genotype appears to decrease the extent of obesity that develops.
GIP infusion has consistently demonstrated stimulation of insulin secretion in isolated rat islets, isolated perfused rat pancreas, dogs, and humans. During stepwise euglycemic, mild hyperglycemic (54 mg/dL above basal), and moderate hyperglycemic (143 mg/dL above basal) clamps, it has been demonstrated that GIP infusion results in insulin secretion only in the presence of elevated glucose concentrations. Furthermore, it has been demonstrated that GIP is not glucagonotropic in normal humans during either euglycemic or hyperglycemic conditions. Thus, the effect of endogenously released GIP appears to be an important mechanism of postprandial insulin secretion and does not appear to play a role in the fasting state.
GIP has many non-incretin effects as well. Unlike other insulin secretagogues, GIP stimulates beta-cell proliferation and cell survival in INS-1 islet cell-line studies. Furthermore, animal studies have suggested a role for GIP in lipid metabolism by stimulating lipoprotein lipase activity, inducing fatty acid incorporation into adipose tissue and stimulating fatty acid synthesis. However, in humans, there is no clear evidence for an effect of GIP on lipid metabolism. GIP also appears to stimulate glucagon secretion from the isolated perfused rat pancreas, although human studies have not demonstrated any significant influence on glucagon secretion. Furthermore, unlike GLP-1, GIP appears to act by accelerating emptying of the stomach rather than by inhibiting gastrointestinal motility.
The GIP-receptor, a member of the G-protein-coupled receptor family has a high specificity for GIP and does not bind other peptides of the glucagon family. For this reason, GLP-1/GIP chimeric peptides show nearly no affinity for the GIP-receptor. From such studies it has been concluded that the GIP(1-30) sequence of the GIP(1-42) is sufficient for recognizing the receptor. GIP(6-30)-amide contains the high affinity binding region of GIP(1-42) but exhibits antagonist activity.
Despite potent glucoregulatory actions through glucose-dependant stimulation of insulin secretion, the insulinotropic effect of GIP is significantly reduced in diabetic subjects compared to normal individuals (16-18). Consequently, clinical use of GIP has not been significantly advanced. Further, there remains a need to develop additional diabetic treatment modalities as well as treatments for metabolic diseases, conditions, and disorders. Accordingly, it is an object of the present invention to provide GIP analog and GIP-containing hybrid polypeptides and methods for their use to treat or prevent metabolic diseases, conditions, and disorders.
The issued patents, applications, and references that are cited herein are hereby incorporated by reference to the same extent as if each was specifically and individually indicated to be incorporated by reference and as though fully set forth herein.