Altered processing of procholecystokinin in carboxypeptidase E‐deficient fat mice: differential synthesis in neurons and endocrine cells

Abstract

The fat mouse strain exhibits a late-onset obesity syndrome associated with a mutation in the gene encoding carboxypeptidase E (CPE). CPE plays a central role in the biosynthesis of many regulatory peptides. Therefore, we examined the processing of procholecystokinin (proCCK) in the brain (neurons) and small intestine (endocrine cells) of fat/fat mice. In the brain, bioactive CCK was markedly reduced (7.9 þ 1.0 pmol/g in fat/fat mice vs. 82.5 þ 11.2 pmol/g in controls), but the concentration of the CPE substrate, glycylarginine-extended CCK, was elevated 105-fold. In contrast, the concentration of bioactive CCK in intestinal endocrine cells was unaffected. Endocrine cell processing was, nevertheless, altered with a 33-fold increase in glycyl-arginine-extended CCK. Interestingly, although total proCCK products were normal in the brain they were elevated 3-fold in the intestine, indicating that biosynthesis is upregulated in endocrine cells but not neurons to compensate for the processing defect. These results demonstrate that the CPE mutation differentially affects CCK processing in these two cell types. Intestinal CCK synthesis more closely resembles progastrin processing, suggesting the presence of an endocrine-specific biosynthetic regulatory mechanism not present in neurons.z 1998 Federation of European Biochemical Societies.

Cholesterol gallstone formation in overweight mice establishes that obesity per se is not linked directly to cholelithiasis risk

“…2) establishes Cpe fat as a particularly novel "Lith" gene. Mice with the Cpe fat mutation lack functional carboxypeptidase E and therefore cannot hydrolyze many prohormones efficiently to their bioactive form, including procholecystokinin to cholecystokinin (CCK) (37)(38)(39). We have shown elsewhere that inactivation of CCK's hormonal axis, by targeted mutation of the murine cholecystokinin A receptor (Cckar) gene, results in heightened susceptibility to ChGS in mice with an otherwise gallstone-resistant genetic background (40).…”

“…We have shown elsewhere that inactivation of CCK's hormonal axis, by targeted mutation of the murine cholecystokinin A receptor (Cckar) gene, results in heightened susceptibility to ChGS in mice with an otherwise gallstone-resistant genetic background (40). Nonetheless, mice with the Cpe fat mutation are not totally deficient in hormonally-active CCK because some hydrolysis of pro-CCK occurs by other, less specific, intracellular carboxypeptidases (37)(38)(39). Despite such ancillary mechanisms, the "rescue" hydrolases fail to produce a normal post-prandial elevation of circulating levels of bioactive CCK in response to a meal (37).…”

“…Nonetheless, mice with the Cpe fat mutation are not totally deficient in hormonally-active CCK because some hydrolysis of pro-CCK occurs by other, less specific, intracellular carboxypeptidases (37)(38)(39). Despite such ancillary mechanisms, the "rescue" hydrolases fail to produce a normal post-prandial elevation of circulating levels of bioactive CCK in response to a meal (37). Not only does circulating CCK promote gallbladder contractility and relaxation of Oddi's sphincter, but physiological CCK levels are a powerful stimulant of small intestinal motility (41).…”

Deficiencies in Pro-thyrotropin-releasing Hormone Processing and Abnormalities in Thermoregulation in CpeMice

“…Additionally, concentrations of pro-insulin in pancreas (6) and pro-opiomelanocortin in pituitary are enhanced (29,31,32). Despite these findings, impaired endoproteolysis of pro-peptides is not necessarily a common feature of the CPE mutation, because the levels of pro-cholecystokinin in brain are unchanged (33,34), whereas those in intestine are reported to be either enhanced (34) or unaltered (33) from the WT controls.…”

“…It should be noted that besides TRH intermediates, an increase in additional C-terminal basic amino acid-extended peptides has been observed for insulin (6), cholecystokinin (33,34), gastrin (39,40), and neurotensin in the Cpe fat mutant (41). In murine hypothalamus, the Cpe fat mutation depresses the levels of fully processed TRH.…”