Increased monitor peptide leads to Increased blood CCK level

Pancreatic exocrine secretion is regulated mainly by cholecystokinin (CCK) via multiple mechanisms. In the digestive system, CCK is released by I cells located in the duodenal mucosa of the small intestine. CCK release is at least in part under negative or positive feedback regulation mediated by trypsin-sensitive CCK-releasing peptides.

In rats, CCK release from I cells is regulated actively by monitor peptide (MP) secreted from pancreatic acinar cells in the presence of nutritional protein in the duodenum [Graf R, 2006].

In the empty intestine, secreted MP binds to trypsin and thus maintained at low intestinal levels; in this situation, CCK release is suppressed. Once the gastric contents are transported to the small intestine, secretion of pancreatic juice including trypsin and MP is stimulated, where trypsin is used for digestion, and the level of free MP is subsequently increased. The increased free MP level stimulates CCK release from I cells via MP receptors, and the resulting increase in CCK stimulates pancreatic exocrine secretion including MP. The resulting increased level of MP directly stimulates I cells to release CCK further; this positive feedback regulation might be continued as long as duodenal contents remain to consume trypsin for proteolysis.

Meanwhile, soon after nutritional protein is digested, free MP and excessive trypsin binds together to be subsequently degraded followed by decreases in blood level of CCK and pancreatic secretion. However, after ingestion of trypsin inhibitors, the intestinal concentration of MP is increased continuously with positive feedback manner due to inhibition of its degradation by trypsin.

On the other hand, in mammalian species including rodents, negative feedback regulation of trypsin secretion is mediated by trypsin-sensitive luminal CCK-releasing peptide (LCRF) secreted from the mucosa of the upper intestine into the intestinal lumen in response to dietary components such as amino acids and peptides. LCRF directly stimulates I cells to secrete CCK, with a resulting increase in trypsin secretion from pancreatic acinar cells, and trypsin then degrades LCRF, indicating negative feedback regulation of trypsin-mediated CCK release.

In addition to by MP in rats, CCK release from duodenal I cells is stimulated by gastric contents containing fatty acids and amino acids, either directly by specific receptors such as Ca-sensing receptors and the G protein-coupled receptor GPR93 or indirectly by luminal CCK-releasing factors (LCRF) in rats and humans[Caron J et al, 2017]. In humans, LCRF is released from intestinal mucosal cells in response to amino acids and fatty acids, and the LCRF mediate negative feedback regulation of CCK release via LCRF degradation by trypsin [Wang BJ and Cui ZJ, 2007].

TBD

Monitor peptide and related peptides with trypsin inhibitory activity

Pancreatic secretory trypsin inhibitors (PSTIs) are found in the pancreatic juice of multiple mammalian species, including rodents and humans [Greene LJ et al, 1968; Pubols MH et al, 1974; Eddeland A and Ohlsson K, 1976; Kikuchi N et al, 1985]. Secreted PSTIs bind tightly to trypsin to protect against trypsin-induced auto-injury in the pancreas and intestinal tracts [Voet D and Voet JG, 1995].

In rats, two types of PSTIs have been isolated: monitor peptide (MP, also known as PSTI-I) and PSTI-II [Tsuzuki S et al, 1991; Tsuzuki S et al, 1992]. Both are similar in amino acid sequence; however, the former directly stimulates CCK release from intestinal I cells via their surface MP receptors, whereas the latter does not [Miyasaka K et al, 1989b; Yamanishi R et al, 1993a]. Human PSTIs do not directly stimulate CCK release from intestinal mucosal cells [Miyasaka K et al, 1989a]. PSTIs from other mammalian species including dogs and pigs might neither directly stimulate CCK release although no related reports are found.

Species differences in the mechanism of CCK release

Pancreatic exocrine secretion is controlled mainly by CCK released into the bloodstream from intestinal mucosal I cells of the small intestine in response to the gastric contents transported to the intestine [Singer MV and Niebergall-Roth E, 2009; Rehfeld JF, 2017]. Peptides released from gastrointestinal digestion, along with fatty acids, are the main stimuli of CCK release involving several direct and indirect pathways [Caron J et al, 2017].

In humans and canines, amino acids and fatty acids in the gastric contents transported to the small intestine play a major role in stimulating CCK release, which regulates pancreatic exocrine secretion, but MP is not involved in exocrine regulation [Wang BJ and Cui ZJ, 2007].

In rats, in contrast to other mammalian species, MP secreted by pancreatic acinar cells plays a major role in protein-stimulated CCK release [Iwai K et al, 1988; Fushiki T et al, 1989]. Ingestion of TIs increases the intestinal level of MP, especially after all nutrient protein is digested in the intestines, causing a subsequent increase in the blood level of CCK. Increased levels of CCK stimulate pancreatic exocrine secretion of proteins including MP, which in turn further increases the release of CCK. This positive feedback response associated with MP secretion might lead to continuously elevated plasma levels of CCK [Liddle RA, 1995].

Species differences in CCKs

Several isoforms of CCK, including CCK-83, -58, -39, -33, -22, and -8, have been identified, and there are species differences in CCK isoforms (e.g., CCK-33, -22 and -58 are expressed in humans, CCK-58 in dogs, CCK-8, -33 and -58 in cats, CCK-22, -58, -3 and -8 in pigs, CCK-22 and -8 in rabbits, and CCK-58 in rats). All of these CCK isoforms have a highly conserved region of amino acids, and all are ligands of CCK1 receptors [Wang BJ and Cui ZJ, 2007].