Studies in beta cells and adipocytes in the context of obesity and T2D - focusing on PDE3B, OPN and SCFAs

Abstract: Type 2 diabetes (T2D) is a heterogeneous disease characterized by altered lipid parameters and elevated glucose levels, as a direct consequence of impaired insulin signaling in target tissues and reduced insulin exocytosis from pancreatic ?-cells. Obesity, which dramatically increases worldwide, is associated with insulin resistance and T2D. In this thesis, we elucidate the effects of hormones and nutrients on biological responses and regulatory mechanisms in pancreatic ?-cells and adipocytes. The information is scarce with regard to the regulation of cyclic nucleotide phosphodiesterase 3B (PDE3B) in pancreatic ?-cells. We show that PDE3B is activated in response to glucose, insulin and forskolin, which is coupled to a decrease, no apparent change or an increase in total phosphorylation of the enzyme in rat pancreatic ?-cells. Furthermore, PDE1A, PDE3, PDE4C, PDE5A, PDE7A, PDE7B, PDE8A, PDE10A and PDE11A are detected in human pancreatic islets. Islets from RIP-PDE3B mice, exhibiting ?-cell specific overexpression of the cAMP-degrading enzyme phosphodiesterase 3B (PDE3B) and dysregulated insulin secretion, show induced OPN protein expression. In addition, in silico and functional approaches demonstrate that PDE3B and OPN are connected and follow a similar protein expression pattern in response to e.g. cAMP-elevating agents and insulin. Little is known regarding the direct effects of short-chain fatty acids (SCFAs), produced through bacterial fermentation of dietary fibers, on glucose and lipid metabolism in adipocytes. We show that the SCFAs propionic acid and butyric acid inhibit cAMP-stimulated lipolysis, a mechanism that is not dependent on the cAMP-degrading enzyme PDE3B. Moreover, both SCFAs inhibit basal and insulin-stimulated de novo lipogenesis, which is associated with increased phosphorylation of acetyl CoA carboxylase, the rate-limiting enzyme in fatty acid synthesis. Propionic acid and butyric acid are also able to potentiate insulin-stimulated glucose uptake. In summary, we demonstrate that agents relevant for ?-cell function regulate PDE3B activity and phosphorylation levels. Based on several strategies, we demonstrate a connection between PDE3B and OPN, the latter having a protective role in pancreatic ?-cells. Further investigations are required to identify downstream targets of PDE3B that are involved in the regulation of insulin exocytosis and also to elucidate the relationship with OPN. Moreover, several PDEs are present in human pancreatic islets. The majority of these PDEs have been described as insulin secretagogues in animal models and it is thus promising to also confirm their presence in humans, as it can be advantageous for the 8 treatment of T2D. SCFAs inhibit fatty acid mobilization and potentiates insulininduced glucose uptake; observed effects that might be beneficial for preventing ectopic lipid accumulation, lipotoxicity and insulin resistance. Thus, it remains to be verified if anti-obesity properties can be conferred to SCFAs.