Marloes Dekker Nitert
From the pancreatic beta cell to the endothelium: Pathophysiological aspects of Type 2 diabetes
The incidence of Diabetes Mellitus increases globally in epidemic proportions. Type 2 Diabetes is the most prevalent form of Diabetes, comprising 90% of the patients. In Type 2 Diabetes, two processes contribute to the development of the disease: insufficient insulin secretion from the pancreatic ?-cell and insulin resistance of the target organs. This leads to loss of control of blood glucose levels, which characterize Diabetes. Even while blood glucose levels can be controlled by a variety of life-style and pharmacological interventions, complications often arise. These complications include cardiovascular disease, retinopathy, neuropathy, and nephropathy. In this thesis, different aspects of pathophysiological mechanisms in Type 2 Diabetes were studied. The aims were (i) to identify the voltage-gated calcium channel that is coupled to glucose-stimulated insulin secretion in the rat clonal ?-cell line INS-1 832/13; (ii) to investigate the mechanism of ?-cell adaptation in the C57BL/6J mouse model of insulin resistance; (iii) to determine whether spontaneous glucose tolerance was a feature in the RIP2-Cre mouse model which is often used for ?-cell specific knockout of genes; and (iv) to study the presence of insulin receptors and IGF-I receptors in human endothelial cells of different origin. It was established that CaV1.2 was the main voltage-gated calcium channel coupled to glucose-stimulated insulin secretion in INS-1 832/13 cells, confirming previous results obtained from mouse ?-cells. C57BL/6J mice on a high-fat diet become insulin resistant but do not develop Diabetes. The hypersecretion of insulin from the ?-cells of these animals is due to a shift in metabolic fuels from glucose to fatty acids and amino acids. The ?-cells of these mice have a high fat content that might interfere with the function of glucose transporters. Furthermore, an increase in mitochondrial mass was observed in the ?-cells of insulin-resistant C57BL/6J mice. All these alterations are part of the ?-cell adaptation, which enables the mice to secrete sufficient insulin in order to prevent the development of overt Diabetes. C57BL/6J mice were also used to backcross RIP2-Cre mice onto. Absence of the recently reported five-exon deletion in the nnt gene in the C57BL/6J mice used, contributed to normal glucose tolerance in both mice strains studied. The expression of Cre recombinase did not affect glucose tolerance and this mouse strain on this background can be used in ?-cell specific knockout studies. Human endothelial cells from coronary artery and umbilical vein expressed more IGF-I receptors than insulin receptors. Indications for the presence of insulin/IGF-I hybrid receptors were found in both endothelial cell types. These results reflect the importance of IGF-I in the development of vascular complications of Diabetes Mellitus.