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Increased monitor peptide leads to Increased blood CCK level
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Trypsin inhibition leading to pancreatic acinar cell tumors||adjacent||High||Moderate||Shigeru Hisada (send email)||Under development: Not open for comment. Do not cite||Under Development|
Life Stage Applicability
|All life stages||High|
Key Event Relationship Description
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.
Evidence Collection Strategy
Evidence Supporting this KER
Regulation of pancreatic secretion
Pancreatic exocrine secretion is controlled mainly by the gastrointestinal hormone cholecystokinin (CCK), which is secreted by CCK-producing I cells located in the mucosa of the small intestine. Multiple mechanisms are involved in the stimulation of CCK release [Wang BJ and Cui ZJ, 2007; Caron J et al, 2017].
Regulation of CCK release mediated by monitor peptide (MP) in rats
In rats, CCK release from I cells in the duodenal mucosa of the small intestine is regulated actively by MP [Miyasaka K et al, 1989a; Fushiki T et al, 1989; Iwai K et al, 1988; Miyasaka K and Funakoshi A, 1998], which consists of 61 amino acids with a molecular weight of approximately 6000. It was first purified from rat pancreatic juice, and its amino acid sequence was subsequently determined [Iwai K et al, 1987].
In the empty intestine, secreted MP is bound to trypsin and thus free MP is maintained at a low level in the intestine; in this situation, CCK release is suppressed. However, after the gastric contents are transported to the small intestine, proteases are postulated to be used for protein hydrolysis, allowing the amount of free MP to increase [Iwai K et al, 1988; Liddle RA, 1995; Miyasaka K and Funakoshi A, 1998; Graf R, 2006]. The increased MP stimulates mucosal I cells to release CCK via their surface MP receptors, stimulating pancreatic exocrine secretion [Liddle RA et al, 1992; Guan D et al, 1990; Cuber JC et al, 1990]. MP binds to the surface of CCK-immunoreactive mucosal cells in the small intestine [Yamanishi R et al, 1993a; Yamanishi R et al, 1993b]. After proteolysis of the intestinal contents, the luminal level of free trypsin is increased, which causes the luminal MP level to return to a low level, followed by a decrease in CCK release [Liddle RA, 1995; Miyasaka K and Funakoshi A, 1998; Graf R, 2006].
Another role of MP as a pancreatic secretory trypsin inhibitor (TI)
Similar to other pancreatic soluble TIs, MP forms complexes with trypsin in the empty intestine to prevent auto-injury by trypsin [Lin YZ et al, 1990; Voet D and Voet JG, 1995]. Once TI is ingested, TI–trypsin complexes are formed, and the intestinal level of free MP is increased to stimulate CCK release [Yamanishi R et al, 1993b], increasing the blood CCK level even on an empty intestine. TIs other than MP show no effect on CCK release [Miyasaka K, 1989a; Tsuzuki S, 1991].
Effects of TIs on MP-mediated CCK release
In contrast, once TIs are ingested, the intestinal concentration of MP is increased due to inhibition of its binding with trypsin and degradation, and the increased MP directly stimulates I cells to release CCK into the blood. In turn, the increased CCK stimulates pancreatic acinar cells to secrete MP as well as pancreatic enzymes, and the secretion of MP further upregulates CCK release via a positive feedback mechanism, especially under trypsin inhibition [Wang BJ and Cui ZJ, 2007; Liddle RA, 1995; Miyasaka K and Funakoshi A, 1998; Liddle RA, 1995].
Some studies have reported that intraduodenal injection of MP stimulates CCK release in rats with external biliary and pancreatic fistulae [Miyasaka K et al, 1989a; Longnecker DS, 1987].
Raw soya flour containing TIs and protease inhibitors such as camostat directly inhibit trypsin activity, and rats treated with these agents showed an increased blood level of CCK [Liddle RA et al, 1984; Goke B et al, 1986; Calam J et al, 1987; Douglas BR et al, 1989; Cuber JC et al, 1990; Playford RJ et al, 1993; Obourn JD et al, 1997; Tashiro M et al, 2004; Komarnytsky S et al, 2011] . The mechanism underlying the increase in CCK release by TIs is thought to involve an increase in the intestinal MP level resulting from trypsin inhibition [Iwai K et al, 1988; Cuber JC et al, 1990; Miyasaka K et al, 1989a].
CCK is a peptide hormone secreted by I cells located in the mucosa of the small intestine, and it regulates pancreatic exocrine secretion. CCK is secreted as peptide prohormone consisting of 150 amino acids. Several CCK isoforms exist, composed of different numbers of amino acids due to post-transcriptional modifications, although the amino acid sequence of the C-terminal end is common among these isoforms [Rehfeld JF, 2017; Wang BJ and Cui ZJ, 2007].
In addition, MP receptors are thought to be expressed on I cells, based on the findings that MP binds to CCK-positive cells in the mucosa of the small intestine, and this binding is inhibited by TIs [Yamanishi R et al, 1993a; Yamanishi R et al, 1993b].
Uncertainties and Inconsistencies
Known modulating factors
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].
MP at a concentration range of 2–12 µg/mL induced a dose-dependent transient increase in portal CCK-like immunoreactivity in isolated vascularly perfused rat duodeojejunum MP at 36 µg/mL showed lower CCK release [Cuber JC et al, 1990].
MP stimulated CCK release from isolated mucosal cells from the rat duodenum, sorted CCK-positive rat intestinal mucosal cells, or isolated vascularly perfused rat duodenum/jejunum after or within several minutes from the incubation [Liddle RA et al, 1992; Bouras EP et al, 1992; Cuber JC et al, 1990].
Known Feedforward/Feedback loops influencing this KER
In rodents, monitor peptide, a pancreatic secretory trypsin inhibitor, is secreted by pancreatic acinar cells along with trypsin and other digestive enzymes stimulated by CCK [Iwai K et al, 1988; Tsuzuki S et al, 1991]. Because MP binds tightly to trypsin [Voet D and Voet JG, 1995], trypsin inhibition increases the intraluminal concentration of MP in a positive feedback manner [Liddle RA et al, 1984; Wang BJ and Cui ZJ, 2007].
Meanwhile, in mammalian species including rodents, TIs might stimulate CCK release into the bloodstream via an increased luminal concentration of trypsin-sensitive CCK-releasing peptides secreted by duodenal mucosal cells [Miyasaka K et al, 1989c; Lu L et al, 1989; Guan D et al, 1990; Owyang C, 1994; Liddle RA, 1995; Spannagel AW et al, 1996; Herzig KH et al, 1996; Miyasaka K and Funakoshi A, 1998; Marchbank T et al, 1998; Li Y et al, 2000; Owyang C, 1999; Wang Y et al, 2002] . Increased blood level of CCK does not stimulate further secretion of LCRF different from the positive feedback regulation of CCK release by MP.
Domain of Applicability
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].
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