Foxa2 regulates multiple pathways of insulin secretion

Abstract

The regulation of insulin secretion by pancreatic β cells is perturbed in several diseases, including adult-onset (type 2) diabetes and persistent hyperinsulinemic hypoglycemia of infancy (PHHI). The first mouse model for PHHI has a conditional deletion of the gene encoding the winged-helix transcription factor Foxa2 (Forkhead box a2, formerly Hepatocyte nuclear factor 3β) in pancreatic β cells. Using isolated islets, we found that Foxa2 deficiency resulted in excessive insulin release in response to amino acids and complete loss of glucose-stimulated insulin secretion. Most PHHI cases are associated with mutations in SUR1 (Sulfonylurea receptor 1) or KIR6.2 (Inward rectifier K + channel member 6.2), which encode the subunits of the ATP-sensitive K + channel, and RNA in situ hybridization of mutant mouse islets revealed that expression of both genes is Foxa2 dependent. We utilized expression profiling to identify additional targets of Foxa2. Strikingly, one of these genes, Hadhsc, encodes short-chain L-3-hydroxyacyl-coenzyme A dehydrogenase, deficiency of which has been shown to cause PHHI in humans. Hadhsc is a direct target of Foxa2, as demonstrated by cotransfection as well as in vivo chromatin immunoprecipitation experiments using isolated islets. Thus, we have established Foxa2 as an essential activator of genes that function in multiple pathways governing insulin secretion.

The MODY1 gene HNF-4α regulates selected genes involved in insulin secretion

“…In addi- tion, we did not find significant differences in mRNA levels of other MODY genes, including HNF-1β, Pdx-1, and NeuroD1, or those encoding other pancreatic transcription factors that regulate insulin secretion ( Figure 5, A and B). Levels of mRNA in the gene encoding Foxa2, an essential regulator of Kir6.2 expression, were not altered significantly (18,25). However, expression of the nuclear receptor HNF-4γ was significantly reduced by approximately 34%, indicating that HNF-4α is required for normal expression of its related family member.…”

The MODY1 gene HNF-4α regulates selected genes involved in insulin secretion

“…Primer sequences are available upon request. Islet purity was assessed as previous described (25). For Western blots, islet extracts were prepared as described previously (53), were separated by SDS-PAGE, were transferred to immobilin P membranes (Millipore), and were probed with rabbit polyclonal anti-Kir6.2 (Chemicon International Inc.), rabbit anti-HNF-1α (Santa Cruz Biotechnology Inc.), and monoclonal anti-α-tubulin (Sigma-Aldrich).…”

The Foxa family of transcription factors in development and metabolism

“…An evolutionarily conserved function of all Foxa genes appears to be the protection of the organism from hypoglycemia [28,29,33,40,41]. Mice lacking Foxa1 have severe hypoglycemia due at least in part to diminished proglucagon (Gcg) expression.…”

“…Islets isolated from these mice have two defects: they do not secrete insulin in response to elevated glucose concentrations, and they inappropriately secrete insulin in response to amino acids. The former is explained by diminished expression of the Foxa2 target genes Kir6.2 (Kcnj11) and Sur1 (Abcc8), which together comprise the K ATP glucose-sensing channel [41]. A novel Foxa2 target gene, Hadhsc, which encodes a short-chain fatty acid dehydrogenase has been identified; it is likely that loss of this gene product also contributes to the Foxa2 loxP/loxP ; Ins.Cre phenotype because humans with mutations in the orthologous gene SCHAD also have hyperinsulinemic hypoglycemia [42][43][44][45].…”