Correlation between FOXO1a (FKHR) and FOXO3a (FKHRL1) Binding and the Inhibition of Basal Glucose-6-Phosphatase Catalytic Subunit Gene Transcription by Insulin

Abstract

Insulin inhibits transcription of the genes encoding the glucose-6-phosphatase catalytic subunit (G6Pase), phosphoenolpyruvate carboxykinase, and IGF binding protein-1 through insulin response sequences (IRSs) that share the same core sequence, T(G/A)TTTT(G/T). The transcription factors FOXO1a and FOXO3a have been shown to bind these elements, but there are conflicting reports as to whether this binding correlates with the action of insulin on gene transcription. Some researchers concluded, from overexpression experiments using FOXO1a, that binding correlated with the insulin response, whereas others concluded, mainly from gel retardation competition experiments using FOXO3a, that it did not. We show here that, although these factors can differentially activate gene transcription in a context-dependent manner, these conflicting data are not explained by a difference in FOXO1a and FOXO3a binding specificity. Instead, we find that gel retardation competition and binding experiments give different results; the latter reveal a correlation between FOXO1a/3a binding and the inhibition of basal G6Pase gene transcription by insulin. In addition, these data show that the binding of FOXO1a/3a to two adjacent IRSs in the G6Pase promoter is cooperative and that promoter context alters the specific IRS base requirements for FOXO1a-stimulated fusion gene expression. Surprisingly, an analysis of insulin action mediated through the G6Pase and IGF binding protein-1 IRSs in the context of a heterologous thymidine kinase promoter reveals that signaling through the latter does not support the accepted model for insulin-stimulated FOXO nuclear exclusion.

Sequence variation between the mouse and human glucose-6-phosphatase catalytic subunit gene promoters results in differential activation by peroxisome proliferator activated receptor gamma coactivator-1α

“…Presumably this reflects the low affinity of HNF-4α binding to the half-site, such that HNF-4α dissociates from this site during electrophoretic separation of the bound and free probe. Similar situations have been reported in studies on the glucocorticoid receptor [36] and FOXO1 [37], where binding to low-affinity sites could only be implied through competition experiments and not directly demonstrated using labelled probes.…”

The hepatic FOXQ1 transcription factor regulates glucose metabolism in mice

“…FOXO1 has been shown to activate expression of Pck1 and G6pc through directly binding to IREs mapped in the promoters of these target genes [14,15]. However, FOXQ1 may act as a transcriptional repressor [18,22].…”

“…Phosphorylated FOXO1 induced by increased insulin levels is excluded from the nucleus, thereby decreasing its transcriptional activity [5,12,13]. In contrast, the decreased blood insulin levels during fasting promote FOXO1 nuclear localisation, where it collaborates with peroxisome proliferator-activated receptor, γ, co-activator 1 α (PGC-1α) to increase the expression of the key gluconeogenic genes, Pck1 and G6pc, via direct binding to insulin response elements (IREs) in their promoters [2,11,14,15]. Hepatic FOXO1 deficiency in mice impairs fasting-induced gluconeogenesis, subsequently leading to lowered blood glucose level [8].…”

Effects of chronic ingestion of catechin-rich green tea on hepatic gene expression of gluconeogenic enzymes in rats

“…Forkhead/Winged helix transcription factors such as FOXO1a have been implicated in the regulation of a number of insulin-responsive genes including the PEPCK and G6Pase genes (16). For example, knockdown of FOXO1a by an antisense oligonucleotide induced partial repression of PEPCK and G6Pase (18) and liver-specific expression of FOXO1a transgene elevated hepatic mRNA levels of both enzymes (24).…”