|ISSN No. 1606-7754 Vol.10 No.1 April 2002|
Structure-function studies on enterostatin inhibition of insulin release
M Tadayyon1*, S Liou2, CP Briscoe1, G Badman3, DS Eggleston4, JRS Arch, DA York2
Department of 1Vascular Biology, 3Synthetic Usotope Chemistry and 4Medicinal Chemistry, GlaxoSmithKline Pharmaceuticals, Harlow, CM19 5 AD, UK. 2Pennington Biomedical Research Center, Louisiana State University, Louisiana, 70808-4124, USA.
The pentapeptide enterostatin (Val-Pro-Asp-Pro-Arg or VPDPR1) has been shown to inhibit insulin secretion both in vivo and in isolated rat islets. We report on structure-function studies of enterostatin in rat islets and in RINm5F insulinoma cells. In isolated rat islets, the tyrosinated version of enterostatin (YVPDPR) had ~10 fold reduced potency at lower concentrations (10nM versus 1nM) in inhibiting insulin release compared to nascent enterostatin and did not exhibit activity at 1m M concentrations. Replacement of the L-valine with D-valine caused total loss of biological activity. The peptide YPDPR was more efficacious than native enterostatin at 0.1nM but lost activity at higher concentrations. However, in RINm5F cells, the inhibitory effect of YPDPR was preserved. 3-iodo-YPDPR exhibited a similar inhibitory profile to enterostatin across a wide concentration. These studies demonstrate the functional importance of the N-terminal sequences of enterostatin in regulating insulin secretion but also suggest tolerance to some modifications. Radio-iodinated-YPDPR may be useful in characterising putative enterostatin receptors.
Keywords : Enterostatin, Enterostatin Analogues, Insulin Secretion, Rat Islets, RINm5F cell-line
The peptide enterostatin is generated via proteolytic cleavage of pancreatic procolipase in the intestinal lumen.1,2 Whilst pancreatic colipase acts as an essential cofactor in the hydrolysis and subsequent absorption of triglycerides, enterostatin may act as a feedback signal to inhibit fat consumption.3-5 Food intake, in particular dietary fat, stimulates the release of procolipase and presumably also of enterostatin.2,6 Peripheral3 as well as central7,8 administration of enterostatin have been shown to inhibit food-intake, particularly of fatty foods, and chronic intracerebroventricular infusion of enterostatin in the rat reduces food- intake,7,8 fat deposition and body weight gain.7
In addition to the central and vagally-mediated actions of enterostatin as an inhibitor of fat-intake, this peptide exerts a direct inhibitory effect on the endocrine pancreas, resulting in the inhibition of insulin release from islets 9-11 and perfused pancreas, 12,13 without affecting glucagon or somatostatin release.13 The inhibitory effect of enterostatin on insulin release appears to be mediated at least in part via suppression of cAMP accumulation.11 Consistent with this hypothesis was the recently reported inhibition of insulin release by enterostatin evoked by glucagon-like peptide-1 and gastric inhibitory peptide, both of which elevate cAMP levels.12 In contrast, enterostatin has no effect on insulin secretion in response to agents that activate the phospholipase C pathway, e.g. cholecystokinin.12 Despite these advances in understanding the mechanisms of enterostatin inhibition of insulin release, the molecular nature of putative enterostatin receptors is unknown and enterostatin receptors remain uncharacterised, in part due to the lack of suitable high affinity ligands.
The structure of enterostatin is fairly well conserved across different species. Based on colipase gene sequence as well as amino acid analysis three enterostatin sequences have been described to date: VPDPR,4 APGPR6,14 and VPGPR.15 Structure-function studies on central actions of enterostatin and its analogues indicate that several substitutions and deletions are tolerated.5 For example, 1nmol YPDPR or PDP were as potent as 1nmol VPDPR in inhibiting high-fat food intake.5 However, PDP had no effect on insulin secretion, raising the possibility that different receptors may be involved in mediating enterostatin actions in the brain (to regulate feeding) versus periphery (to regulate insulin secretion). The C-terminally truncated enterostatin peptide, VPDP was found to produce a potent inhibitory effect on food-intake, as well as on insulin release in vitro9 but no effect on plasma insulin was seen when VPDP was infused intraduodenally.16 Thus, the structural requirements for enterostatin inhibition of insulin release are incompletely understood.
The aim of the present study was to elucidate in more detail the necessary structural features for enterostatin's inhibitory effect on insulin release. We report that although some modifications in the N-terminus of VPDPR abrogate biological function, other modifications result in retention of biological activity. Notably, the enterostatin derivative 3-iodo-YPDPR retains almost full activity and could be useful for the pharmacological characterisation of putative enterostatin receptors in islets and other tissues.