The MODY2 gene glucokinase is negatively regulated by FoxO forkhead transcription factors

Glucokinase (GK) plays a key role in the regulation of glucose utilization in hepatocytes. Defects in the glucokinase gene have been linked to maturity-onset diabetes of the young (non-insulin-dependent, also called MODY–2) permanent neonatal diabetes (PNDM) and persistent hyperinsulinemic hypoglycemia of infancy (PHHI). Insulin is a major regulator of GK expression, however, the knowledge of the involved transcription factors and signalling pathways is quite limited. Insulin signaling involves mitogen activated protein kinase (MAPK) casacades and phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB) which in turn phosphorylates a number of target proteins including transcription factors and coactivators. Among the transcription factors regulated by insulin/PKB are forkhead transcription factors (FOXO), however, so far it was unknown whether they are regulating GK transcription. Therefore it was our aim to investigate the role of FoxO1, FoxO3 and FoxO4 on GK expression.

We found that FoxoO1 repressed GK promoter activity while FoxO3 and FoxO4 had no effect. By contrast, the expression of another insulin-regulated gene, PAI–1, was enhanced only when FoxO4 was present. Detailed promoter analyses revealed, that an element in the promoter which we have shown to bind HNF–4a was critically involved. Mutation of this element abolished FoxO1-mediated GK repression. Thus, we hypothesized that FoxO1 may mediate its effect via interaction with HNF–4. Indeed, we found that HNF–4a interacted with FoxO1 and a mammalian Gal–4 two-hybrid assay showed, that the DNA binding and activation domain of HNF–4 appears to be crucial for that interaction. Thus, our data provide a new mechanism of GK transcriptional regulation which might be of importance in the pathogenesis of diabetes.

hepatocytes - insulin - metabolism