Pancreatic beta-cell insulin signaling in genetic and dietary models of obesity and insulin resistance

Abstract: Type 2 Diabetes Mellitus (T2DM) is a heterogeneous metabolic disease characterized by elevated blood glucose levels that has reached pandemic proportions. Genome-wide association studies have linked T2DM to the function of the insulin-producing pancreatic βcell residing in the micro-organ islet of Langerhans. An individual´s risk to develop T2DM depends on genetic predisposition and environmental factors, e.g. life style. Central for disease development is the interplay between insulin resistance in insulin target tissues like muscle, liver and fat and deficient β-cell insulin secretion. Since the β-cell is an insulin target itself, βcell insulin resistance can contribute to β-cell dysfunction and the development of T2DM. This was shown in several genetic (knockout) mouse models, however the dynamics of β-cell insulin resistance and its relevance in a diet-induced context has so far not been explored. Furthermore the consequences of diet-induced β-cell insulin resistance for β-cell function remain to be understood. The difficulty to study β-cell insulin resistance in vivo has partly been due to the lack of a technique to monitor β-cell insulin resistance non-invasively and longitudinally in the living organism. In my thesis I employed the anterior chamber of the eye of mice as a transplantation site for biosensor-expressing reporter islets and the cornea as a natural body window to monitor β-cell insulin resistance non-invasively and longitudinally by microscopic imaging. The β-cell insulin resistance biosensor is based on GFP-labeled FoxO1, that changes its intracellular localization from cytoplasmic (insulin responsive) to nuclear (insulin resistant). With this technique we investigated β-cell insulin resistance dynamics in ob/ob and NZO mice and demonstrated that β-cell insulin resistance dynamics vary in animal models of insulin resistance and obesity. Furthermore, we showed that β-cell insulin resistance developed in the presence of whole-body insulin resistance, impaired glucose tolerance and increased body weight, but independently from liver insulin resistance. To study the relevance of β-cell insulin resistance in diet-induced T2DM development, we treated diabetes-prone male C57BL/6J mice with different combinations of solid high fat diet and drinking water containing either sucrose or fructose. Employing our new monitoring technique we showed that only mice that were fed a High-FatHigh-Sucrose-Diet developed β-cell insulin resistance. This demonstrated the importance of βcell insulin resistance in a model of diet-induced obesity and insulin resistance and highlighted the importance of diet composition for the development of T2DM. The β-cell insulin resistance was accompanied by a decreased functional β-cell mass and impaired insulin secretion downstream of glucose-stimulated Ca2+ influx, due to a reduction of syntaxin-1A. We were also able to show that β-cell insulin resistance in one insulin signaling cascade can re-route the insulin signal, thus allowing the co-existence of reduced and increased insulin response in the same cell. In conclusion, my in vivo studies of diet-induced β-cell insulin resistance and its consequences on β-cell function and survival contribute to better understanding of the development of T2DM.