Patients with schizophrenia have a reduced life expectancy compared to the general population. Second generation antipsychotics are commonly prescribed to patients, but treatment contributes to the increased mortality through adverse events, such as increased rates of diabetes. Olanzapine is considered to be one of the highest risk diabetogenic antipsychotics; it is known to promote insulin resistance and hyperglycaemia through pharmacological interaction with peripheral metabolic organs. Glucose stimulated insulin secretion (GSIS) from the pancreatic beta cells is fundamental to overall glucose homeostasis, but the impact on the function of the beta cells has not been thoroughly explored. This thesis aimed to test the hypothesis that olanzapine pharmacologically alters the viability or function of the pancreatic beta cells leading to reduced insulin secretion.
The mouse MIN6 beta cell line was exposed to olanzapine for 24-72 hour and analysed for viability using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, fluorescent staining and cell counts. Following 72 hour exposure, insulin secretion in response to 2 hour glucose stimulation was determined by ELISA. Cellular lysates or mRNA was extracted and analysed by real-time qPCR for mRNA expression, or Western blot for protein analysis. Statistical analysis was carried out using two-way ANOVA and Bonferroni’s posttest.
There was no increase in cell death when exposed to 0.1–10 µM olanzapine, but these concentrations did cause a significant increase in proliferation in unstimulated conditions (p<0.01). Olanzapine treatment increased expression of cell cycle regulator cyclin D1 (p<0.05), and altered concentrations of components of the canonical Wnt/beta catenin signalling pathway (TCF7L2 and Axin2) indicating that olanzapine may disrupt activity in this pathway. Pre-treatment with 10 µM olanzapine significantly reduced GSIS (p<0.01) with no change in intracellular insulin concentration. There was significantly reduced cytoplasmic beta catenin (p<0.05) and increased axin2 (p<0.05), the central scaffold of the beta catenin destruction complex. Glycogen synthase kinase-3 (GSK3) is another component of the destruction complex, and addition of the GSK3 inhibitor 6-bromoindirubin-3'- oxime (BIO) reduced activity of the beta catenin destruction complex leading to significant increases in cytoplasmic accumulation and significantly improved insulin secretion (p<0.05).
Beta catenin has been linked to insulin vesicle trafficking and these data imply that olanzapine alters beta catenin dynamics in MIN6 cells leading to a decline in function, reduced insulin secretion and hypertrophy. In patients treated with olanzapine, reduced beta cell function may add to hyperglycaemia and prevent adaptation to peripheral insulin resistance, advancing the development of diabetes and contributing to increased mortality.