The primary function of the gastrointestinal tract is to supply nutrients to our bodies via the processes of ingestion, motility, secretion, digestion, and absorption; this occurs through complex coordination of digestive processes that are regulated by intrinsic endocrine and nervous systems. Although the nervous system exerts influence on many digestive processes, the GI tract is the largest endocrine organ in the human body and produces numerous mediators that play an integral role in regulating functions of the GI tract.

The GI hormones classify as endocrines, paracrine, or neurocrine based on the method by which the molecule gets delivered to its target cell(s). Endocrine hormones are secreted from enteroendocrine cells directly into the bloodstream, passing from the portal circulation to the systemic circulation, before being delivered to target cells with receptor-specificity for the hormone. The five GI hormones that qualify as endocrines are gastrin, cholecystokinin (CCK), secretin, glucose-dependent insulinotropic peptide (GIP), and motilin. Enteroendocrine cells also secrete paracrine hormones, but they diffuse through the extracellular space to act locally on target tissues and do not enter the systemic circulation. Two examples of paracrine hormones are somatostatin and histamine. Additionally, some hormones may operate via a combination of endocrine and paracrine mechanisms. These “candidate” hormones are glucagon-like peptide-1 (GLP-1), pancreatic polypeptide, and peptide YY. Lastly, neurocrine hormones get secreted by postganglionic non-cholinergic neurons of the enteric nervous system. Three neurocrine hormones with significant physiologic functions in the gut are vasoactive intestinal peptide (VIP), gastrin release peptide (GRP), and enkephalins.[1]

Gastrointestinal hormones undergo synthesis in specialized cells of the GI tract mucosa known as enteroendocrine cells. Enteroendocrine cells are specialized endoderm-derived epithelial cells that originate from stem cells located at the base of intestinal crypts. These cells are dispersed throughout the GI mucosa, sprinkled in between epithelial cells from the stomach all the way through to the colon. Also, these enteroendocrine cells possess hormone-containing granules concentrated at the basolateral membrane, adjacent to capillaries, that secrete their hormones via exocytosis in response to a wide range of stimuli related to food intake. These stimuli include small peptides, amino acids, fatty acids, oral glucose, distension of an organ, and vagal stimulation.[2]

G cells secrete gastrin in the antrum of the stomach and the duodenum in response to the presence of breakdown products of protein digestion (such as amino acids and small peptides), distention by food, and vagal nerve stimulation via GRP. More specifically, phenylalanine and tryptophan are the most potent stimulators of gastrin secretion among the protein digestion products. The vagal nerve stimulation of gastrin secretion is unique because gastrin and motilin are the only hormones released directly by neural stimulation.

CCK is secreted from I cells in the duodenum and jejunum in response to acids and monoglycerides (but not triglycerides), as well as the presence of protein digestion products.

Secretin is secreted from S cells in the duodenum in response to H+ and fatty acids in the lumen. Specifically, a pH less than 4.5 signals arrival of gastric contents, which initiates the release of secretin.

GIP is secreted by K cells in the duodenum and jejunum in response to glucose, amino acids, and fatty acids. GIP is the only GI hormone with a response to all three macronutrient types, and newer studies suggest that changes in intraluminal osmolarity may be what stimulates GIP secretion.[3]

GLP-1 is also produced in the small intestine and secreted from L cells. The presence of hexose and fat stimulate its release. Pancreatic polypeptide and peptide YY are secreted by protein and fat, respectively, although their functions are still relatively unknown.

The enteroendocrine cells responsible for synthesizing and secreting GI hormones derive from pluripotent intestinal stem cells in the intestinal crypts. As these stem cells move up the crypt-villus axis, they express specific transcription factors that give rise to absorptive enterocytes or cells of secretory lineages (Paneth cells, goblet cells, and enteroendocrine cells). The sequential expression of three basic loop helix loop (bHLH) transcription factors (Math1, Neurogenin3, and NeuroD1) is involved in specifying the enteroendocrine cell lineage. Math1 expression specifies cells that are fated for the secretory progenitor lineage and segregates them from the absorptive enterocyte lineage. Subsequent expression of Neurogenin3 represents a secretory progenitor cell that has initiated differentiation into the endocrine cell lineage. Lastly, NeuroD1 expression induces cell cycle arrest and commits a cell to an enteroendocrine fate.[4][5]

In addition to the bHLH transcription factors, numerous paired and homeodomain genes, including Isl-1, Pdx1, Nkx6.1, Nkx2.2, Pax4, and Pax6, are involved in differentiation of enteroendocrine cells into the distinct subtypes of hormone-secreting cells scattered throughout the GI tract. These subpopulations include G cells, I cells, S cells, K cells, Mo cells, L cells, and D cells, which are primarily responsible for secreting gastrin, CCK, secretin, GIP, motilin, GLP-1, and somatostatin, respectively. Interestingly, many subtypes of enteroendocrine cells are able to secrete multiple hormones, but the expression of hormonal genes is controlled by location in the GI tract.[6]

Gastrointestinal hormones are composed of polypeptides that can divide into two structurally homologous families that include the hormones responsible for a majority of regulation of GI function. The first hormone family consists of gastrin and CCK because both hormones share an identical 5 C-terminal amino acid sequence, also known as “pentagastrin.” This sequence includes the tetrapeptide that is minimally required for gastrin activity but is only about one-sixth as potent as the entire 17-amino acid gastrin peptide. Gastrin also exists in a 34-amino acid form called “big” gastrin which gets secreted during the inter-digestive period. During meal ingestion, the 17-amino acid form of gastrin, also called “little” gastrin, is secreted. Although each form of gastrin has its own distinct biosynthetic pathway, the mediation of the action of both gastrin peptides is via binding of cholecystokinin (CCK-2) receptors. The other member of the gastrin family, CCK, is a 33-amino acid peptide that includes the pentagastrin sequence and the C-terminal tetrapeptide sequence necessary for minimal gastrin activity; this enables CCK to demonstrate activity on gastrin (CCK-2) receptors, although it mediates a very weak stimulation of gastric acid secretion. Furthermore, the minimally active fragment for CCK activity is its C-terminal heptapeptide, which acts on CCK-1 receptors to mediate gallbladder contraction. Gastrin can also act on the CCK-1 receptor, but each hormone is more potent at its own receptor than those of its homolog.

The second hormone family consists of secretin, glucagon, GLP-1, and GIP. Secretin has 27 amino acids and is structurally similar to glucagon, which has 29 amino acids. However, in contrast with the gastrin-CCK family, all amino acids in the polypeptide are necessary for biological activity, and these two polypeptides only share 14 common amino acids. Glucagon derives from a 180-amino acid precursor peptide called proglucagon, which undergoes tissue-specific post-translation processing to produce different peptides in different cell types. Proglucagon is cleaved to form glucagon in the pancreas, while in the intestines, proglucagon undergoes processing to produce a 30-amino acid peptide called GLP-1. GIP is 42 amino acids long but only shares nine amino acids with secretin and 16 amino acids with glucagon.[7]

The digestive system is the primary site of action for most GI hormones and related polypeptides. The stomach is the primary site of gastrin production with some D-cells also populating the duodenum. Somatostatin and histamine are also produced in the stomach by enterochromaffin-like (ECL) cells, which is an enteroendocrine cell subtype. The small intestines, namely the duodenum and jejunum handle secretion of CCK, secretin, GIP, and motilin.