The roles of somatostatin and ghrelin in pancreatic islet and beta cell function in type 2 diabetes

Abstract

Beta cell dysfunction and loss play central roles in the pathogenesis of type 2 diabetes. Strategies to overcome the detrimental effects of cellular stress factors is fundamental for the protection of beta cell mass and function. In this study two hormones, somatostatin (SST) and ghrelin, were assessed for their roles in regulating 1) beta cell and/or islet survival in response to cellular stressors typical of type 2 diabetes and 2) acute insulin release. In addition, the mechanisms underlying their actions were explored and it was tested whether any effects of ghrelin on beta cells was mediated via somatostatin receptor 3 (SSTR3) to identify a potential interaction between ghrelin and SST in islet function.

The rodent pancreatic beta cell lines MIN6 and INS-1 and primary pancreatic mouse islets were used as research models in this study. Cell viability was assessed by detecting metabolic activity of viable cells and apoptosis by measurement of caspase 3/7 activity. Static incubation experiments and radioimmunoassays were performed to detect hormone secretion. mRNA expression of genes of interest was detected by quantitative PCR (qPCR) and protein expression by Western blot.

Treatment with SST reduced apoptosis in MIN6 cells in response to lipotoxicity. SSTR3 was detected in beta cells and the receptor partly mediated an inhibitory effect of SST on insulin secretion, but not the observed protective effect of SST on MIN6 cell survival. Incubation with ghrelin did not modify the clonal beta cell response to apoptosis, despite the ghrelin receptor (GHSR1a) being expressed in these cells. In contrast, treatment with ghrelin reduced apoptosis in islets from female mice in a sex-dependent manner, but this effect did not appear to be mediated by the ghrelin receptor, GHSR1a. Antagonising GHSR1a by liver-expressed antimicrobial peptide 2 resulted in a sex-specific increase in insulin secretion in islets from male mice. Finally, SSTR3 did not appear to mediate any potential effects of ghrelin on islet insulin secretion.

In conclusion, the results presented in this thesis suggest that both SST and ghrelin play important roles in protecting the pancreatic beta cells and/or islets against cellular stresses leading to cell death. The underlying mechanisms are complex and may rely on direct interaction between SST and ghrelin. In addition, a novel, sex-dimorphic role for ghrelin in islet function was identified, that highlights a potential need to develop sex-specific treatments for type 2 diabetes.

Date of Award	9 Jul 2020
Original language	English
Awarding Institution	University of Roehampton
Sponsors	Roehampton VC Scholarship, Society for Endocrinology Practical Skills & Ede and Ravenscroft Research fund 2018
Supervisor	Sue Reeves (Director of Studies), Astrid Hauge Evans (Co-Supervisor) & Michael Patterson (Co-Supervisor)