CCK-58: a novel reagent for studying the regulation of cholecystokinin bioactivity

Physiology of energy homeostasis: Models, actors, challenges and the glucoadipostatic loop

The aim of this review is to discuss the physiology of energy homeostasis (EH), which is a debated concept. Thus, we will see that the set-point theory is highly challenged and that other models integrating an anticipative component, such as energy allostasis, seem more relevant to experimental reports and life preservation. Moreover, the current obesity epidemic suggests that EH is poorly efficient in the modern human dietary environment. Non-homeostatic phenomena linked to hedonism and reward seem to profoundly impair EH. In this review, the apparent failed homeostatic responses to energy challenges such as exercise, cafeteria diet, overfeeding and diet-induced weight loss, as well as their putative determinants, are analyzed to highlight the mechanisms of EH. Then, the hormonal, neuronal, and metabolic factors of energy intake or energy expenditure are briefly presented. Last, this review focuses on the contributions of two of the most pivotal and often overlooked determinants of EH: the availability of endogenous energy and the pattern of energy intake. A glucoadipostatic loop model is finally proposed to link energy stored in adipose tissue to EH through changes in eating behavior via leptin and sympathetic nervous system activity.

Cholecystokinin-58 and cholecystokinin-8 produce similar but not identical activations of myenteric plexus and dorsal vagal complex

The enteric nervous system (ENS: myenteric and submucosal plexuses) of the gastrointestinal tract may have a role in the reduction of food intake by cholecystokinin (CCK). Exogenous cholecystokinin-8 (CCK-8) activates the myenteric plexus and the feeding control areas of the dorsal vagal complex (DVC) of the brainstem. An increasing number of reports, however, have shown that CCK-58 is the sole or the major circulating form of CCK in rat, human and dog, and that it is qualitatively different from CCK-8 in evoking various gastrointestinal physiological responses (e.g., contraction of the gallbladder and exocrine pancreatic secretion). In the current report, we compared the abilities of exogenous CCK-58 to activate the myenteric plexus and the dorsal vagal complex with those of exogenous CCK-8 by quantifying Fos-like immunoreactivity (Fos-LI; a marker for neuronal activation). We report that CCK-58 (1, 3, and 5 nmol/kg) increased Fos-LI in the myenteric plexus (p<0.001) and in the DVC (p<0.001) compared to the saline vehicle. The highest dose of CCK-58 increased Fos-LI more than an equimolar dose of CCK-8 in the myenteric plexus and the area postrema. Thus, CCK-8 and CCK-58 produce the same qualitative pattern of activation of central and peripheral neurons, but do not provoke identical quantitative patterns at higher doses. The different patterns produced by the two peptides at higher doses, in areas open to the circulation (myenteric plexus and area postrema) may reflect endocrine actions not observed at lower doses.

Sequence variation outside the "active" region of dog and rabbit cholecystokinin-58 results in bioactivity differences

OBJECTIVES

We propose that regions outside the bioactive 7-amino acid carboxyl terminus of cholecystokinin (CCK)-58 influence its biological activity. Here we evaluate if sequence variation of the N-terminal regions of rabbit and canine CCK-58 changes their biological activities.

METHODS

Cholecystokinin-like immunoreactivity was purified from rabbit intestinal extracts by reverse phase and ion-exchange high-performance liquid chromatography steps. The peptide was characterized by microsequence and mass spectral characterizations of the intact and tryptic peptides. Canine and rabbit CCK-58 were evaluated for their CCK1 and CCK2 receptor binding, receptor activation, and immunologic properties.

RESULTS

The sequence of rabbit CCK-58 differs from that of canine CCK-58 in 9 of the amino terminal 40 residues. Canine CCK-58 was approximately 3-fold more potent than rabbit CCK-58 for CCK1 receptor binding and CCK2 receptor binding, but about the same potency for stimulation of amylase release from purified acinar cells. The canine peptide was 9-fold more immunoreactive than rabbit CCK-58.

CONCLUSIONS

Canine and rabbit CCK-58 have different biological and immunologic properties that can only result from differences in their N-terminal sequences which influence the properties of their identical carboxyl termini. These results are the first direct demonstration that amino acids outside the C-terminus of CCK-58 influence CCK biological activity.

Identification of nonsulfated cholecystokinin-58 in canine intestinal extracts and its biological properties

Nonsulfated CCK(58) [CCK(58)(ns)] has not been considered to be of biological importance because CCK(58)(ns) binds poorly to the CCK(A) receptor and has only been identified once in intestinal extracts. In this work, a radioimmunoassay specific for the COOH-terminal region of gastrin and CCK (antibody 5135) was used to monitor the purification of CCK molecular forms from canine intestinal extracts. A minor immunoreactive peak was associated with a major absorbance peak during an ion-exchange, HPLC step. Characterization of this minor immunoreactive peak demonstrated that it was CCK(58)(ns). CCK(58)(ns) is 14% as immunoreactive as sulfated CCK(8) [CCK(8)(s)]. Amino acid analysis demonstrated that CCK(58)(ns) was present at 50% the amount of CCK(58)(s). In addition, we found that CCK(58)(ns) does not potently displace an (125)I-labeled CCK(10) analog from the CCK(A) receptor in mouse pancreatic membranes and does not stimulate amylase release from isolated pancreatic acini, or stimulate pancreatic secretion in an anesthetized rat model. By contrast, CCK(58)(ns) does bind to CCK(B) receptors and stimulates gastric acid secretion via this receptor. The presence of CCK(58)(ns) and its ability to selectively stimulate the CCK(B) receptor without stimulation of the CCK(A) receptor suggest that CCK(58)(ns) may have unique physiological properties, especially tissues where the nonsulfated peptide can act as a paracrine or neurocrine agent.

Differential bile-pancreatic secretory effects of CCK-58 and CCK-8

In this work, we 1) synthesized rat CCK-58, 2) determined the amounts and forms of rat CCK in whole blood after stimulation of its release by casein, 3) determined the potency of CCK-8 and CCK-58 peptides to displace labeled CCK-8 from CCK(A) and CCK(B) receptors transfected into Chinese hamster ovary (CHO) cells, and 4) examined the biological actions of CCK-8 and rat CCK-58 in an anesthetized rat model. CCK-58 was the only detected endocrine form of CCK in rat blood. Synthetic rat CCK-58 was less potent than CCK-8 for displacing the label from CCK(A) and CCK(B) receptors in transfected CHO cells. However, rat CCK-58 was more potent than CCK-8 for stimulation of pancreatic protein secretion in the anesthetized rat. In addition, CCK-58 but not CCK-8 stimulated fluid secretion in this anesthetized rat model. These data suggest that regions outside the COOH terminus of rat CCK-58 influence the expression of CCK biological activity. The presence of only CCK-58 in the circulation and the fact that its biological activity differs from CCK-8 suggests that CCK-58 deserves scrutiny in other physiological models of CCK activity.

Role of cholecystokinin in the intestinal phase of pancreatic circulation in dogs

The regulatory mechanisms of postprandial pancreatic hyperemia are not well characterized. The aim of this study is to clarify the role of cholecystokinin (CCK) in the intestinal phase of pancreatic circulation. Pancreatic, gastric, and intestinal blood flows were measured by ultrasound transit-time blood flowmeters in five conscious dogs. Pancreatic and gastric secretion and blood pressure were also monitored. Synthetic CCK octapeptide (CCK-8) or gastrin heptadecapeptide (gastrin-17) was infused intravenously, and milk was infused into the duodenum with or without loxiglumide, a specific CCK-A receptor antagonist. CCK-8 induced dose-related increases of pancreatic, but not gastric or intestinal, blood flow and protein secretion without affecting systemic blood pressure. Gastrin-17 did not affect pancreatic blood flow. An intraduodenal infusion of milk increased pancreatic and intestinal blood flows and pancreatic protein secretion. Loxiglumide completely inhibited pancreatic blood flow and protein responses to CCK-8 and milk but not the intestinal blood flow response. CCK is a potent and specific pancreatic vasodilator, with its effect mediated by CCK-A receptors. CCK plays an important role in the regulation of the intestinal phase of the pancreatic circulation in dogs.

Identical primary sequence but different conformations of the bioactive regions of canine CCK-8 and CCK-58

The C-terminal bioactive regions of CCK-8 and CCK-58 are identical (DY*MGWMDF-NH(2), Y* denotes a sulfated tyrosine residue), but these peptides have different patterns of bioactivity. Specifically, CCK-58 binds more avidly to the CCK(A) receptor while CCK-8 is more potent for stimulation of amylase secretion from pancreatic acini. We postulate that these seemingly contradictory observations reflect a stable conformational change in CCK-58 that enhances binding, but diminishes activation of second messenger. We used CD and NMR spectra to evaluate postulated structural differences between CCK-8 and the carboxy-terminus of synthetic CCK-58. The CD spectra indicate the presence of turns in CCK-8 but a mixture of helical and random coil structures for CCK-58. Comparisons of partial NMR chemical shift assignments of CCK-58 and full assignments for CCK-8 also indicate differences in the backbone conformations for these residues. The data support the hypothesis that these peptides have different, stable, carboxy-terminal structures that may influence bioactivity.

Isolation and characterization of sulphated and nonsulphated forms of cholecystokinin-58 and their action on gallbladder contraction

Cholecystokinin (CCK) exists in multiple molecular forms with different polypeptide lengths and the absence or presence of sulphation. We have isolated sulphated and nonsulphated forms of CCK-58 from porcine intestine and have determined their bioactivities in a guinea-pig gallbladder contraction assay. Both forms co-eluted in cation-exchange chromatography and in several rounds of reverse-phase (RP)-HPLC, but separated upon RP-HPLC using a water/acetonitrile system with heptafluorobutyric acid as counter ion. Nonsulphated CCK-58 was the form detected by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry because of desulphation in that process. The biological activity of CCK-58 and CCK-33 is equipotent, although the kinetics of the response differ. Sulphated CCK-58 was found to be 35 times more potent than nonsulphated CCK-58. In contrast, sulphated CCK-8 is 150 times more potent than nonsulphated CCK-8, and for sulphated and nonsulphated CCK-33, the activities differ by a factor of 100. This type of correlation indicates that the N-terminal end of CCK-58 partially compensates for the decrease in activity arising from the lack of sulphated tyrosine. Given its fairly high bioactivity, nonsulphated CCK-58 may have a physiological significance.

Prolonged intestinal afferent nerve discharge in response to cholecystokinin-58 compared to cholecystokinin-8 in rats

Intestinal afferents are sensitive to cholecystokinin (CCK) octapeptide. However, CCK-58 may be a more biologically relevant molecule. Therefore, recordings from jejunal mesenteric afferent bundles were secured for extracellular multi-unit recording and the responses to CCK-8 and CCK-58 compared. CCK-8 and CCK-58 (i.v.) evoked a dose-dependent, devazepide sensitive, increase in afferent nerve discharge. Peak discharge frequency was higher for CCK-8 at all doses (P < 0.05). However, response duration was more prolonged for CCK-58 (P < 0.05) at 200 pmol.kg(-1). This resulted in an overall increase in area under the curve of CCK-58 compared to CCK-8 (P < 0.05). CCK-58 stimulates afferent discharge in a different pattern than CCK-8 and, therefore, may have differential biological effects.