For years, the gut hormonal stimulation of insulin secretion (the incretin effect) has been attributed to two specific gut hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). Nobody questions whether GIP and GLP-1 are important incretins. However, as discussed below, there are reasons to question whether they are the only incretins when seen in a broader physiological context with clinical perspectives. This article will argue that cholecystokinin (CCK) and gastrin peptides should also be looked upon as incretins, and perhaps even more of the many gut hormones should also be taken into consideration. The issue about CCK and gastrin as incretins was described in detail half-a-decade ago 1. The present article is an update based on that review.
The first gut hormone to be discovered was secretin 2. The entire concept of endocrinology is founded on secretin. Starling, the co-discoverer of secretin, had already considered the possibility that secretin – in addition to its effect on pancreatic exocrine secretion – might also stimulate the ‘internal secretion’ of the pancreas. The term ‘incrétine’ was launched in 1932 by the Belgian physiologist La Barre 3 to match and distinguish the gut hormonal insulin-releasing activity from secretin.
The concept was, however, effectively revived in the 1960s, when insulin could be measured in plasma. Thus, McIntyre et al.4 and Elrick et al.5 showed that oral glucose induced a considerably larger insulin response compared with intravenous glucose. Subsequently, over several years, it was shown that GIP and truncated GLP-1 together could account for the observed difference in glucose-induced insulin secretion. In other words, GIP and GLP-1 sufficed to explain the intestinal insulin-releasing activity (incretin) after oral glucose ingestion. Since then, incretin effects of other gut hormones have been overlooked.
The problem is two-fold. First, a large oral intake of pure glucose is unphysiological. Neither humans nor other mammals obtain food and calories that way. Meals are mixtures of many components that release a multitude of gastrointestinal hormones. Second, the understanding of gut hormonal regulation has for more than a century been misled by the textbook concept of one hormone, one target: secretin stimulates pancreatic bicarbonate secretion, gastrin stimulates gastric acid secretion, CCK stimulates gallbladder contraction, GIP stimulates glucose-induced insulin secretion, motilin stimulates intestinal motility, etc. Today, the picture is different 6. We now know that the mammalian digestive tract expresses at least 20 different hormone genes, each prohormone being processed to several different bioactive forms so that more than a hundred hormonal bioactive peptides of intestinal origin participate in the regulation of digestion. Many hormone genes are expressed not only in enteroendocrine cells but also in gastrointestinal neurons and endocrine-like gut cells destined for local paracrine secretion 7. Moreover, the gut peptides often have multiple targets and thus more functions rather than a single or two cell types. Furthermore, gut peptides are not only acutely active as hormones and neurotransmitters but also have long-term growth effects. Finally, different gut peptides may interact in their activity, not only in terms of simple addition or inhibition but also in terms of having potentiating interactive effects; thus, a single hormone that in itself is without significant effect on a given target may be highly active in minute concentrations when acting together with other gut hormones. Such interaction is also suggested by the patterns of receptor expression on target cell membranes and by the subsequent intracellular cross-talking between different signal transduction pathways.
Gastrin and CCK peptides
The main production site of gastrin is antral G-cells wherein most bioactive gastrin is synthesized as gastrin-17 and gastrin-34, both of which occur in tyrosyl-sulphated and tyrosyl-nonsulphated form. Moreover, shorter and longer gastrins are synthesized, but only in small quantities. The synthesis of gastrin is cell-specific. Accordingly, foetal and neonatal G-cells in pancreatic islets synthesize only completely sulphated gastrin-17.
Similar to gastrin, CCK also occurs in different molecular forms. The main forms are synthesized in endocrine I-cells in the gut. They are CCK-58, CCK-33, CCK-22, and CCK-8, all of which are present in the circulation. Notably, the CCK gene is, however, also abundantly expressed in neurons, including pancreatic neurons that innervate islet cells and intrapancreatic ganglia 8. The major transmitter forms are sulphated CCK-8 and the short CCK-5. CCK-5 and perhaps CCK-4 may be of particular interest because of the high stimulatory potency for insulin release in the porcine and human pancreas 8–10.
Gastrin and CCK receptors
The targets for gastrin and CCK peptides are two 11,12 related G-protein-coupled receptors now called CCK-A (or CCK1) and CCK-B (or CCK2) receptors. The CCK1 receptor mediates gallbladder contraction, relaxation of the sphincter of Oddi, pancreatic growth and enzyme secretion, delay of gastric emptying, and inhibition of gastric acid secretion through somatostatin. The CCK1 receptor is also expressed in the pituitary, the myenteric plexus, and areas of the midbrain. The CCK receptor binds only CCK peptides that are amidated and sulphated with high affinity, whereas the affinity of nonsulphated CCK peptides and gastrins is negligible. Thus, nonsulphated CCKs, short CCKs (CCK-5) and the gastrins are not physiological ligands for the CCK1 receptor.
The CCK2 receptor is the predominant receptor for gastrin and CCK peptides in the central nervous system. It binds both sulphated and nonsulphated gastrin and CCK peptides, as well as short C-terminal fragments such as CCK-5 with high affinity. The CCK2 receptor is also abundantly expressed on enterochromaffin-like-cells in the stomach and on islet cells and ganglionic neurons in the pancreas of humans and pigs 13,14. Thus, islet cells are targets for both locally released gastrin (pancreatic G-cells) and CCK peptides (intrapancreatic CCK neurons), as well as from endocrine gastrin and CCK in circulation. Here, the concentrations of gastrin, however, are 10- to 20-fold above those of CCK 15. Notably, the CCK receptor expression in the pancreas is species specific. There are major discrepancies between humans and pigs (abundant islet-cell expression of the CCK2 receptor 13) and between rodents and dogs, where the CCK1 receptor is more abundant. Consequently, incretin results on CCK and gastrin obtained from rat, mice and dog studies do not necessarily apply to human physiology and pathophysiology.
Incretin studies of gastrin and CCK in humans and pigs
During the late 1960s and the 70s, a number of incretin studies on gastrin in humans were reported from several laboratories 8,9,16–20. The conclusions were that exogenous gastrin in dose–response studies does indeed release insulin, but that the endogenous gastrin release after oral glucose in normal individuals was too small to explain the insulin response during an oral glucose tolerance test. Therefore, using the oral-glucose-incretin definition, gastrin as such contributed only little to the incretin effect. However, review of these older studies suggests that this conclusion probably was false negative. Exogenous gastrin-17 in itself is quite a potent insulin-releaser together with intravenous glucose. An ordinary protein-rich meal releases, with expedient timing, both gastrin and insulin in substantial amounts, whereas the elevation in blood glucose concentration is small 18. Hence, in such daily meal-physiologic situation, gastrin is likely to stimulate the secretion of insulin significantly. Moreover, studies in endogenous hypergastrinaemia support an incretin effect of gastrin in humans 19.
The incretin effect of CCK has been less systematically examined in humans and pigs, because CCK studies entail several pitfalls in comparison with those of gastrin. Thus, sufficient amounts of pure CCK peptides have been difficult to obtain. Moreover, CCK peptides are less stable compared with the gastrins, and the studies have been hard to monitor due to shortage of reliable CCK assays for plasma measurements of CCK. Nevertheless, in studies of exogenous CCK, short peptides such as CCK-8, CCK-5, and CCK-4 have been shown to release insulin quite efficiently in humans and in the isolated perfused porcine pancreas 8,17,21.
Food and digestion are prerequisites for life. Accordingly, the gut is densely innervated and equipped with hormone-producing cells for neuroendocrine regulation of digestion and metabolic functions. The incretin function has, for years, been considered just one of the several extraintestinal activities of some gut hormones.
With the rapidly growing epidemics of obesity, diabetes mellitus and associated cardiovascular diseases, incretin has become a central issue, perhaps the clinically most important of all endocrine gut functions. The prospect of GLP-1 analogues as major drugs for the treatment of type 2 diabetes bears witness to this development and indicates that diabetes and obesity with derived cardiovascular complications should be considered gut endocrine diseases. Thus, the influence and control of insulin and glucagon secretion by gastrointestinal hormones and neurons have become a central issue. Perhaps the issue is too important to be left entirely to just two gut peptides, GLP-1 and GIP. Their history and the recognition of their functions are closely associated with unphysiological intake of large glucose loads. Considering that the daily food intake by normal individuals, diabetic patients and cardiovascular patients has different compositions, the time has come to look also at other gut hormones which may act in concert with one or both of the established incretins. In this context, it is interesting that in nonobese diabetic mice GLP-1, even in substantial doses, has no effect on pancreatic islet-cell/β-cell regeneration and hyperglycaemia. Moreover, gastrin on its own has no effect. However, when GLP-1 is combined with even very small doses of gastrin, the β-cells grow, insulin is secreted and the mice become normoglycaemic 22. The important factor in the observed mechanism is that interaction between gut hormones and gastrin, as well as CCK, seems worth while when insulin secretion is on the agenda. This point has also been emphasized in more recent combinatorial studies 23–25. Therefore, the early studies of the incretin effect of gastrin and CCK from the 1970s also deserve a second look.
Conflicts of interest
There are no conflicts of interest.
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