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Title: Effects of prominsulin C-peptide and other islet peptides on beta-cell function and insulin secretion
Author: Ng, Ming Tak
ISNI:       0000 0004 2721 5036
Awarding Body: University of Ulster
Current Institution: Ulster University
Date of Award: 2011
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Hyperglycaemia is a significant pathophysiological feature of diabetes mellitus. It has been considered to be a cause as well as a consequence of impaired pancreatic p-cell function and insulin action. The detrimental effects of a hyperglycaemic environment encourage the non-enzymatic glycation of regulatory and functional proteins. This thesis investigated the effects of pro insulin C-peptide and other important islet peptides such as insulin, somatostatin, islet amyloid polypeptide (IAPP) as well as the gut hormone gastric inhibitory polypeptide (GIP) in both native and glycated forms on insulin release and beta cell function in vitro using clonal pancreatic p-ce1ls and isolated mouse islets and in vivo, using Swiss TO lean mice. Pancreatic beta-cells not only secrete insulin into the bloodstream, but also release equimolar amounts of C-peptide. During the last decade, studies have provided definite evidence of C-peptide's active role in physiology and pathophysiology. However very little information currently exists on the direct effects of C-peptide on pancreatic beta cells. This study provides evidence that C-peptide is a biologically active endogenous peptide hormone that exerts tonic inhibitory effects on pancreatic beta cell function and these effects may be partially mediated through intracellular signalling pathways. Suppression of insulin secretion by C-peptide was observed at basal and stimulatory glucose concentrations and was shown to be concentration dependent. The ability of alanine and IBMX to potentiate glucose-induced insulin secretion was severely impaired in the pancreatic cell line and isolated islets in the presence of C-peptide. This inhibition of glucose stimulated insulin secretion may be associated with the mechanism by which cAMP and KATP channel potentiates insulin release. In vivo studies with the administration of C-peptide resulted in a decrease in plasma insulin levels and increase in plasma glucose concentrations. As proinsulin C-peptide clearly exhibited an inhibitory effect on insulin secretion, it was of interest to study the autocrine effect of the other important pancreatic p-cell peptide, insulin. Concentration dependent inhibitory effects of insulin on alanine- and IBMX-stimulated insulin secretion were observed in isolated islets. Validation of this result was confirmed in vivo using Swiss TO mice, where plasma C-peptide levels were decreased following insulin administration. In mechanistic studies, insulin not only inhibited cAMP production at stimulatory glucose concentration, but also decreased GLP-1 and forskolin-stimulated cAMP production in clonal pancreatic cells. Moreover, insulin exerted a negative effect on insulin biosynthesis by decreasing the alanine-stimulated insulin mRNA expression at basal and stimulatory glucose concentration. As expected, inhibitory effects on insulin secretion were also observed for the pancreatic peptides, somatostatin-14 and IAPP whilst the gut incretin hormone GIP exerted insulinotropic activity. Following incubation with glucose in vitro, glycated forms of C-peptide, insulin, somatostain-14 and GIP were all readily detectable by reverse-phase HPLC and MALDI- TOF mass spectrometry. The extent of glycation was shown to be time and glucose concentration dependent. G Iycated human C-peptide exerted an inhibitory effect on insulin secretion, but to a lesser extent than non-glycated C-peptide. Glycated human C- peptide in the presence of alanine, GIP and tolbutamide resulted in 22%, 12% and 27% inhibition, respectively compared to 50%, 52% and 42% inhibition with native human C- peptide. A similar trend was obtained when comparing glycated human insulin and non- glycated human insulin. Suppression of alanine- and IBMX-stimulated C-peptide release was observed with glycated insulin. Glycated insulin reduced cAMP production at stimulatory glucose concentration, GLP-l and forskolin-stimulated cAMP production. Both glycated C-peptide and glycated insulin exhibited a less potent inhibitory effect on insulin secretion. Contrastingly, glycation of somatostatin-14 increased its ability to inhibit insulin secretion. Likewise, glycated GIP was more insulinotropic than its native peptide. These studies indicate that C-peptide may be one of several players in the multifactorial regulation of the pancreatic beta cell. Insulin secretion is an important physiological process which is regulated by multiple regulatory mechanisms. C-peptide may have a compensatory regulatory effect through the fine control of insulin release. Collectively, these studies indicate the need for continuing investigation of the circulating levels of glycated peptides and their effects on the ability to modulate insulin secretion. Further investigations are thus necessary to find out the possible role of glycated peptides in the pathophysiology of type 2 diabetes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available