Functional and genetic studies in type 2 diabetes and obesity

Abstract

Type 2 diabetes (T2D) and obesity are highly prevalent disorders reflecting a complex interplay of genetics, epigenetics, and environment. They constitute serious health problems and lead to significant morbidity and mortality. The overall aim of this thesis was to shed some light on the molecular mechanisms underlying pathogenesis of T2D and obesity by studies in rat and man.

In Paper I, adenylyl cyclase 3 (Ac3) mRNA expression levels in pancreatic islets and striatum/hypothalamus regions of brain of diabetic Goto-Kakizaki (GK) rats were higher compared with control Wistar rats, while its expression was intermediate in islets and brain regions of insulin-treated GK rats. This study proposes that increased Ac3 mRNA expression in these tissues is partially a primary and inherited defect and not solely secondary to hyperglycaemia, and that AC3 may participate in the regulation of glucose homeostasis via insulin secretion and CNS.

In Paper II, protein kinase Cα (PKCα) and PKCζ mRNA expressions in pancreatic islets were decreased in GK compared with Wistar rats, with intermediate expressions in the insulin-treated GK group. PKCα and phosphorylated PKCα (p-PKCα) protein expressions in islets were diminished in GK compared with insulin-treated GK and Wistar rats. Islet PKCζ protein expression was reduced in both GK and insulin-treated GK compared with Wistar rats, but p-PKCζ was reduced only in GK compared with insulin-treated GK and Wistar rats. PKCε was lower in islets of GK compared with insulin-treated GK and Wistar rats, at both the mRNA and protein levels. PKCδ and p-PKCδ protein expressions in islets were decreased in GK compared with insulin-treated GK and Wistar rats. In liver, PKCδ and PKCζ mRNA expressions were decreased in both GK and insulin-treated GK compared with Wistar rats. Hepatic PKCζ protein expression was diminished in GK rats with and without insulin treatment compared with Wistar rats. Although PKCδ showed no difference at the protein level, p-PKCδ/PKCδ was reduced in liver of GK compared with Wistar rats. PKCε mRNA expression in liver was down-regulated in insulin-treated GK compared with non-treated GK and Wistar rats. Therefore, this study suggests defects in PKCα and PKCε expressions in pancreatic islets of GK rats secondary to hyperglycaemia. In liver, PKCε mRNA expression in liver could be under control of insulin.

In Paper III, mRNA expressions, enzyme activities, and protein levels of succinyl-CoA:3-ketoacid-CoA transferase (SCOT) and ATP citrate lyase (ATPCL) were decreased in pancreatic islets of GK compared with Wistar rats. Two cell lines with the severest knockdown of SCOT enzyme activity and protein, SCOT 1676 and SCOT 1184, showed the severest reduction in secretagogue-stimulated insulin secretion. This study confirms that the mitochondrial pathways involving SCOT, instead of or synergizing with the pathway involving ATPCL, are important potentiators of glucose-stimulated insulin secretion (GSIS).

In Paper IV, we studied the association of adrenergic receptor alpha 2A (ADRA2A) genetic polymorphisms with obesity and/or T2D in a Swedish cohort. The single nucleotide polymorphism (SNP) rs553668 was associated with T2D in men, but this association disappeared after adjusting for age and body mass index (BMI). Associations were also detected when comparing obese subjects with normal glucose tolerance (NGT) and lean NGT subjects, and in obese but not lean T2D patients. In women, multiple logistic regression regarding SNP rs521674 demonstrated an association with T2D when including age as a covariant. However, correcting for BMI removed this association. When age was included in the model, the increased risk of rs521674 was seen in obese, but not in lean, T2D women. This study provides evidence that ADRA2A genetic polymorphisms are mainly associated with obesity and may also relate to T2D in a Swedish population.

In conclusion, this thesis has revealed molecular pathogenetic defects related to T2D and obesity, and may thus create a basis for more precise and improved therapeutic approaches.