Peptide hormones as regulators of glucose homeostasis

Abstract

Glucose homeostasis is very tightly controlled to allow constant availability of fuel to cells in fed, fasted and exercise states. The control of glucose requires a co-ordinated effort over several organs. For example, maintenance of glucose homeostasis relies on neuronal and hormonal control of the liver to switch between glucose uptake and glucose release. Major hormonal control of the liver comes from the secretion of glucagon and insulin from the pancreas. However it is now known that other hormones in the entero-insular axis, which includes the pancreas and the gastrointestinal (GI), also contribute to the regulation of glucose homeostasis. The importance of glycaemic control becomes apparent when glucose homeostasis is disrupted leading to metabolic diseases such as diabetes mellitus. Currently, the major aim for diabetic patients is to maintain adequate control of glycaemia to lower the risk of complications associated with glucose fluctuations including kidney disease, cardiovascular complications and blindness. New understanding of the roles of other hormones has led to a multi-hormonal view of glucose homeostasis and as such, increased the potential for therapeutic targets to be developed. Three peptides were investigated in this thesis for their potential as glucoregulatory hormones. Preptin is a 34 amino acid peptide isolated from the granules of βTC6-F7 cells, an islet β-cell line and corresponds to the E domain of proIGF-2. Although isolated from a β-cell line, preliminary studies indicated preptin-like immunoreactivity (PLIM) is confined to the α-cell in the pancreas and as yet unidentified cells in the mucosa of the stomach of rats. In addition, preptin has been shown to be co-secreted with insulin in βTC6-F7 cells and to increase glucose-stimulated insulin secretion (GSIS) in the rat pancreas. This thesis investigated the location of preptin in the stomach. However PLIM was not able to be confirmed due to lack of a suitable antibody. Furthermore, two alternative methods, MALDI-TOF and LC-MS/MS did not detect preptin in the rat pancreas. Therefore the presence of preptin in rat islets is inconclusive. Preptin effects on liver function were also investigated in an isolated perfused rat liver model. While preptin decreased glucose output, its effects were insubstantial and inconsistent compared to the well-characterised glucoregulatory hormone, insulin. However, the presence of other PLIM products in disease indicate that proIGF-2 may have a physiological role and further investigation is warranted to elucidate proIGF-2 products in the pathology of metabolic diseases. Two peptides were also investigated as glucoregulatory peptides. GRPP was purified from the porcine pancreas and studies using MALDI-TOF also suggested its presence in rat and mouse islets. Glicentin-related pancreatic polypeptide-like peptide (GRPP-LP) was discovered in rat islets using MALDI-TOF and LC-MS/MS techniques. This thesis studied the effect of rat GRPP and rat GRPP-LP in the isolated perfused rat liver, however no significant changes to glucose or lactate levels, or portal pressure was found. Effects of GRPP on insulin secretion have previously been explored and suggest that rat GRPP may have an effect on insulin secretion. This thesis confirmed the presence of GRPP and GRPP-LP in the α-cell of the rat pancreas. Furthermore exogenous rat GRPP inhibited GSIS in the rat pancreas. To our knowledge the effect of rat GRPP and GRPP-LP on GSIS is novel and the findings suggest that GRPP has the potential to be a glucoregulatory peptide. Therefore it will be important to explore GRPP biology and its characteristics in order to better understand its physiological role and to explore the possibility that it could contribute to the development of potential treatments for metabolic diseases such as diabetes.