Kinase pathways involved in insulin gene regulation
Understanding how the insulin gene is regulated is essential to developing new treatments for diabetes mellitus. The aim of this study was to characterise, in detail, the effect of stress, glucose and insulin on three protein kinases, p38, JNK and PKB in pancreatic β cells. p38 was found to be active in both INS-1 and MIN-6 cells in response to the stress inducing agents anisomycin, UV and sodium arsenite. Physiologically high levels of glucose failed to result in p38 activation. Experiments were undertaken to characterise the effect of p38 overexpression on the activity of the rat insulin promoter. p38 overexpression resulted in a 10 fold increase in rat insulin promoter activity under conditions of high glucose however cells treated with the stress inducing agents UV and sodium arsenite showed a decrease in rat insulin promoter activity relative to controls. JNK was also detected in INS-1 and MIN-6 cells. JNK activity was increased by the cellular stresses of UV, sodium arsenite and anisomycin, but not by physiologically high levels of glucose. Overexpression of the transcription factor c-Jun inhibited the rat insulin gene promoter's response to glucose but overexpression of JNK had no effect. Furthermore JNK and c-Jun overexpression did not alter the size of the transcription factor PDX-1. PKB activity was found to be high in untreated INS-1 cells and could only be activated using significant quantities of insulin over a long time period. PKB overexpression in INS-1 cells led to a 10-20 fold increase in the activity of the rat insulin promoter under conditions of both low and high glucose. Further experiments were undertaken to evaluate the use of an adenoviral system to overexpress PKB in β cells in a controlled manner. Finally, a pilot study was undertaken to examine the effect of overexpressing Ngn3, a transcription factor essential to the development of the pancreas in rat liver cells.