Relationship: 2029



Increased monitor peptide leads to Increased blood CCK level

Upstream event


Increased monitor peptide

Downstream event


Increased blood CCK level

Key Event Relationship Overview


AOPs Referencing Relationship


AOP Name Adjacency Weight of Evidence Quantitative Understanding
Trypsin inhibition leading to pancreatic acinar cell tumors adjacent High Moderate

Taxonomic Applicability


Term Scientific Term Evidence Link
Homo sapiens Homo sapiens Low NCBI
Macaca fascicularis Macaca fascicularis Low NCBI
Rattus norvegicus Rattus norvegicus High NCBI
Mus musculus Mus musculus High NCBI

Sex Applicability


Sex Evidence
Mixed High

Life Stage Applicability


Term Evidence
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 Supporting this KER



Biological Plausibility


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].

Empirical Evidence


MP at concentrations ranging from 3 x 10-12 to 3 x 10-8 M stimulated CCK release from isolated mucosal cells from the rat duodenum in a dose-dependent manner with highest level at 15 minutes after stimulation [Bouras EP et al, 1992].

MP at a concentration range of 2–12 µg/mL induced within a few minutes a dose-dependent transient increase in portal CCK-like immunoreactivity in isolated vascularly perfused rat duodenum/ jejunum [Cuber JC et al, 1990].

In rats with biliary and pancreatic fistula, duodenal infusion of MP at 0.9 µg/rat increased pancreatic secretion and the plasma CCK level [Miyasaka K et al, 1989a].

Sorted CCK-positive rat intestinal mucosal cells stimulated with 30nM MP increased the secretion of CCK in a time-dependent manner starting at 5 min after the start of MP incubation [Liddle RA et al, 1992].

Uncertainties and Inconsistencies



Quantitative Understanding of the Linkage



Response-response Relationship


MP at concentrations ranging from 3 x 10-12 to 3 x 10-8 M stimulated mucosal cells isolated from the rat duodenum to release CCK in a dose-dependent manner [Bouras EP et al, 1992].

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 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].

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].



 1.    Bouras EP, Misukonis MA, Liddle RA: Role of calcium in monitor peptide-stimulated cholecystokinin release from perfused intestinal cells. Am J Physiol 262:G791-6,1992

 2.    Calam J, Bojarski JC, Springer CJ: Raw soya-bean flour increases cholecystokinin release in man. Br J Nutr 58:175-179,1987

 3.    Caron J, Domenger D, Dhulster P, Ravallec R, Cudennec B: Protein digestion-derived peptides and the peripheral regulation of food intake. Front Endocrinol (Lausanne) 8:85,2017

 4.    Cuber JC, Bernard G, Fushiki T, Bernard C, Yamanishi R, Sugimoto E, Chayvialle JA: Luminal CCK-releasing factors in the isolated vascularly perfused rat duodenojejunum. Am J Physiol 259:G191-197,1990

 5.    Douglas BR, Woutersen RA, Jansen JB, de Jong AJ, Rovati LC, Lamers CB: Modulation by CR-1409 (lorglumide), a cholecystokinin receptor antagonist, of trypsin inhibitor-enhanced growth of azaserine-induced putative preneoplastic lesions in rat pancreas. Cancer Res 49:2438-2441,1989

 6.    Eddeland A, Ohlsson K: Purification of canine pancreatic secretory trypsin inhibitor and interaction in vitro with complexes of trypsin-alpha-macroglobulin. Scand J Clin Lab Invest 36:815-820,1976

 7.    Fukuda M, Fujiyama Y, Sasaki M, Andoh A, Bamba T, Fushiki T: Monitor peptide (rat pancreatic secretory trypsin inhibitor) directly stimulates the proliferation of the nontransformed intestinal epithelial cell line, IEC-6. Digestion 59:326-330,1998

 8.    Fushiki T, Kajiura H, Fukuoka S, Kido K, Semba T, Iwai K: Evidence for an intraluminal mediator in rat pancreatic enzyme secretion: reconstitution of the pancreatic response with dietary protein, trypsin and the monitor peptide. J Nutr 119:622-627,1989 9.    Goke B, Printz H, Koop I, Rausch U, Richter G, Arnold R, Adler G: Endogenous CCK release and pancreatic growth in rats after feeding a proteinase inhibitor (camostate). Pancreas 1:509-515,1986

10.    Graf R, Bimmler D: Biochemistry and biology of SPINK-PSTI and monitor peptide. Endocrinol Metab Clin North Am 35:333-43, ix,2006

11.    Greene LJ, DiCarlo JJ, Sussman AJ, Bartelt DC: Two trypsin inhibitors from porcine pancreatic juice. J Biol Chem 243:1804-1815,1968

12.    Guan D, Ohta H, Tawil T, Liddle RA, Green GM: CCK-releasing activity of rat intestinal secretion: effect of atropine and comparison with monitor peptide. Pancreas 5:677-684,1990

13.    Herzig KH, Schon I, Tatemoto K, Ohe Y, Li Y, Folsch UR, Owyang C: Diazepam binding inhibitor is a potent cholecystokinin-releasing peptide in the intestine. Proc Natl Acad Sci U S A 93:7927-7932,1996

14.    Iwai K, Fukuoka S, Fushiki T, Tsujikawa M, Hirose M, Tsunasawa S, Sakiyama F: Purification and sequencing of a trypsin-sensitive cholecystokinin-releasing peptide from rat pancreatic juice. Its homology with pancreatic secretory trypsin inhibitor. J Biol Chem 262:8956-8959,1987

15     Iwai K, Fushiki T, Fukuoka S: Pancreatic enzyme secretion mediated by novel peptide: monitor peptide hypothesis. Pancreas 3:720-728,1988

16.    Kikuchi N, Nagata K, Yoshida N, Ogawa M: The multiplicity of human pancreatic secretory trypsin inhibitor. J Biochem 98:687-694,1985

17.    Komarnytsky S, Cook A, Raskin I: Potato protease inhibitors inhibit food intake and increase circulating cholecystokinin levels by a trypsin-dependent mechanism. Int J Obes (Lond) 35:236-243,2011

18.    Li Y, Hao Y, Owyang C: Diazepam-binding inhibitor mediates feedback regulation of pancreatic secretion and postprandial release of cholecystokinin. J Clin Invest 105:351-359,2000

19.    Liddle RA, Goldfine ID, Williams JA: Bioassay of plasma cholecystokinin in rats: effects of food, trypsin inhibitor, and alcohol. Gastroenterology 87:542-549,1984

20.    Liddle RA, Misukonis MA, Pacy L, Balber AE: Cholecystokinin cells purified by fluorescence-activated cell sorting respond to monitor peptide with an increase in intracellular calcium. Proc Natl Acad Sci U S A 89:5147-5151,1992

21.    Liddle RA: Regulation of cholecystokinin secretion by intraluminal releasing factors. Am J Physiol 269:G319-27,1995

22.    Lin YZ, Isaac DD, Tam JP: Synthesis and properties of cholecystokinin-releasing peptide (monitor peptide), a 61-residue trypsin inhibitor. Int J Pept Protein Res 36:433-439,1990

23.    Longnecker DS: Interface between adaptive and neoplastic growth in the pancreas. Gut 28 Suppl:253-258,1987

24.    Lu L, Louie D, Owyang C: A cholecystokinin releasing peptide mediates feedback regulation of pancreatic secretion. Am J Physiol 256:G430-435,1989

25.    Marchbank T, Freeman TC, Playford RJ: Human pancreatic secretory trypsin inhibitor. Distribution, actions and possible role in mucosal integrity and repair. Digestion 59:167-174,1998

26.    Miyasaka K, Nakamura R, Funakoshi A, Kitani K: Stimulatory effect of monitor peptide and human pancreatic secretory trypsin inhibitor on pancreatic secretion and cholecystokinin release in conscious rats. Pancreas 4:139-144,1989a

27.    Miyasaka K, Funakoshi A, Nakamura R, Kitani K, Uda K, Murata A, Ogawa M: Differences in stimulatory effects between rat pancreatic secretory trypsin inhibitor-61 and -56 on rat pancreas. Jpn J Physiol 39:891-899,1989b

28.    Miyasaka K, Guan DF, Liddle RA, Green GM: Feedback regulation by trypsin: evidence for intraluminal CCK-releasing peptide. Am J Physiol 257:G175-81,1989c

29.    Miyasaka K, Funakoshi A: Luminal feedback regulation, monitor peptide, CCK-releasing peptide, and CCK receptors. Pancreas 16:277-283,1998

30.    Obourn JD, Frame SR, Chiu T, Solomon TE, Cook JC: Evidence that A8947 enhances pancreas growth via a trypsin inhibitor mechanism. Toxicol Appl Pharmacol 146:116-126,1997

31.    Owyang C: Negative feedback control of exocrine pancreatic secretion: role of cholecystokinin and cholinergic pathway. J Nutr 124:1321S-1326S,1994

32.    Owyang C: Discovery of a cholecystokinin-releasing peptide: biochemical characterization and physiological implications. Chin J Physiol 42:113-120,1999

33.    Playford RJ, King AW, Deprez PH, De-Belleroche J, Freeman TC, Calam J: Effects of diet and the cholecystokinin antagonist; devazepide (L364,718) on CCK mRNA, and tissue and plasma CCK concentrations. Eur J Clin Invest 23:641-647,1993

34.    Pubols MH, Bartelt DC, Greene LJ: Trypsin inhibitor from human pancreas and pancreatic juice. J Biol Chem 249:2235-2242,1974

35.    Rehfeld JF: Cholecystokinin-from local gut hormone to ubiquitous messenger. Front Endocrinol (Lausanne) 8:47,2017

36.    Singer MV, Niebergall-Roth E: Secretion from acinar cells of the exocrine pancreas: role of enteropancreatic reflexes and cholecystokinin. Cell Biol Int 33:1-9,2009

37.    Spannagel AW, Green GM, Guan D, Liddle RA, Faull K, Reeve JR Jr: Purification and characterization of a luminal cholecystokinin-releasing factor from rat intestinal secretion. Proc Natl Acad Sci U S A 93:4415-4420,1996

38.    Tashiro M, Samuelson LC, Liddle RA, Williams JA: Calcineurin mediates pancreatic growth in protease inhibitor-treated mice. Am J Physiol Gastrointest Liver Physiol 286:G784-790,2004

39.    Tsuzuki S, Fushiki T, Kondo A, Murayama H, Sugimoto E: Effect of a high-protein diet on the gene expression of a trypsin-sensitive, cholecystokinin-releasing peptide (monitor peptide) in the pancreas. Eur J Biochem 199:245-252,1991

40     Tsuzuki S, Miura Y, Fushiki T, Oomori T, Satoh T, Natori Y, Sugimoto E: Molecular cloning and characterization of genes encoding rat pancreatic cholecystokinin (CCK)-releasing peptide (monitor peptide) and pancreatic secretory trypsin inhibitor (PSTI). Biochim Biophys Acta 1132:199-202,1992

41.    Voet D, Voet JG: Biochemistry (2nd ed.). John Wiley & Sons (pp) 396-400,1995

42.    Wang BJ, Cui ZJ: How does cholecystokinin stimulate exocrine pancreatic secretion? From birds, rodents, to humans. Am J Physiol Regul Integr Comp Physiol 292:R666-78,2007

43.    Wang Y, Prpic V, Green GM, Reeve JR Jr, Liddle RA: Luminal CCK-releasing factor stimulates CCK release from human intestinal endocrine and STC-1 cells. Am J Physiol Gastrointest Liver Physiol 282:G16-22,2002

44.    Yamanishi R, Kotera J, Fushiki T, Soneda T, Iwanaga T, Sugimoto E: Characteristic and localization of the monitor peptide receptor. Biosci Biotechnol Biochem 57:1153-1156,1993a

45.    Yamanishi R, Kotera J, Fushiki T, Soneda T, Saitoh T, Oomori T, Satoh T, Sugimoto E: A specific binding of the cholecystokinin-releasing peptide (monitor peptide) to isolated rat small-intestinal cells. Biochem J 291 ( Pt 1):57-63,1993b