The Nkx6.1 homeodomain transcription factor suppresses glucagon expression and regulates glucose-stimulated insulin secretion in islet beta cells

We have previously described rat insulinoma INS-1-derived cell lines with robust or poor glucose-stimulated insulin secretion (GSIS). In the current study, we have further resolved these lines into three classes: class 1, glucose-unresponsive/glucagon-expressing; class 2, glucose-unresponsive/glucagon-negative; and class 3, glucose-responsive/glucagon-negative. The transcription factor Nkx2.2 was expressed with relative abundance of 3.3, 1.0, and 1.0 in class 1, class 2, and class 3 cells, respectively, whereas Nkx6.1 expression had the opposite trend: 1.0, 2.6, and 6.4 in class 1, class 2, and class 3 cells, respectively. In class 1 cells, overexpressed Nkx6.1 suppressed glucagon expression but did not affect the levels of several other prominent beta cell transcription factors. RNA interference (RNAi)-mediated suppression of Nkx6.1 in class 3 cells resulted in a doubling of glucagon mRNA, with no effect on Pdx1 levels, whereas suppression of Pdx1 in class 3 cells caused a 12-fold increase in glucagon transcript levels, demonstrating independent effects of Nkx6.1 and Pdx1 on glucagon expression in beta cell lines. RNAi-mediated suppression of Nkx6.1 expression in class 3 cells also caused a decrease in GSIS from 13.9- to 3.7-fold, whereas suppression of Pdx1 reduced absolute amounts of insulin secretion without affecting fold response. Finally, RNAi-mediated suppression of Nkx6.1 mRNA in primary rat islets was accompanied by a significant decrease in GSIS relative to control cells. In sum, our studies have revealed roles for Nkx6.1 in suppression of glucagon expression and control of GSIS in islet beta cells.