Medicine (St Vincent's) - Theses
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ItemThe effect of inhibiting pro-apoptotic BH3-only proteins on beta cell loss and glucose homeostasis in type 2 diabetes( 2014)Type 2 diabetes is caused by a combination of insulin resistance, as well as dysfunction and loss of beta cell mass. Around 30-70% of beta cell mass is lost in type 2 diabetes and this is due to apoptosis, induced mainly by chronic hyperglycaemia. High concentrations of glucose activate the intrinsic apoptosis pathway in beta cells and this requires the pro-apoptotic Bcl-2 family proteins Bim and Puma. Glucose-induced production of reactive oxygen species may also activate the NLRP3-inflammasome pathway, resulting in caspase-1 cleavage, and production of IL-1β. This thesis aimed to identify the stress pathways that mediate glucose toxicity of beta cells. In addition, possible benefits of apoptosis inhibition on glucose homeostasis and beta cell function were examined using mouse models. Chapter 2 describes the role of endoplasmic reticulum (ER) stress and oxidative stress in mediating glucose toxicity of islets. Exposure of islets to high concentrations of glucose induced oxidative and ER stress and increased expression of the pro-apoptotic ER factor CHOP. This caused downstream activation of Bim and Puma-mediated apoptosis and was inhibited by chemical chaperones or antioxidant treatment. Further, increased expression of Bim and Puma mRNA was observed in human islets in type 2 diabetes. Inhibition of ER and oxidative stress could be an effective strategy to reduce beta cell apoptosis in type 2 diabetes. In Chapter 3, the controversial subject of activation of the NLRP3-inflammasome in islets by glucose toxicity-induced ER and oxidative stress was studied. Apoptosis induced by ER or oxidative stress inducing agents was similar in islets isolated from wild-type, NLRP3 or caspase-1 deficient mice. Loss of NLRP3 did not protect islets from glucose, ribose or gluco-lipotoxicity. The presence of a hyperactive mutant of NLRP3 in beta cells had no effect on ribose toxicity or ribose-induced IL-1β secretion. These results suggest that activation of the inflammsome does not mediate glucose toxicity in islets. In Chapter 4, the metabolic effects of apoptosis inhibition were studied in Bimdeficient mice. Bim deficiency resulted in reduced adiposity, improved insulin sensitivity and glucose tolerance in mice. Islet size and function was normal suggesting that phenotype was not beta cell-dependent. Study of mice in metabolic cages revealed that loss of Bim resulted in increased lipid oxidation and this may explain the metabolic changes. The effects of apoptosis inhibiton in beta cells were studied in Chapter 5 by analysing Bim-deficient Leprdb/db mice. Loss of Bim resulted in increased body weight but normalized fasting blood glucose and improved glucose tolerance in Leprdb/db mice. These changes were likely due to a two-fold increase in islet area and volume, and a similar increase in serum insulin concentration. Therefore, loss of Bim alters the balance between apoptosis and replication, thereby making it a possible therapeutic target to prevent beta cell loss in type 2 diabetes. Overall, work presented in this thesis shows that inhibition of apoptosis or its upstream activators ER and oxidative stress may prevent beta cell loss in type 2 diabetes. Inhibiting pro-apoptotic proteins could also have additional benefits such as increased lipid oxidation and improved insulin sensitivity.