A Hepatocyte FOXN3-α Cell Glucagon Axis Regulates Fasting Glucose

Abstract

SUMMARY The common genetic variation at rs8004664 in the FOXN3 gene is independently and significantly associated with fasting blood glucose, but not insulin, in non-diabetic humans. Recently, we reported that primary hepatocytes from rs8004664 hyperglycemia risk allele carriers have increased FOXN3 transcript and protein levels and liver-limited overexpression of human FOXN3, a transcriptional repressor that had not been implicated in metabolic regulation previously, increases fasting blood glucose in zebrafish. Here, we find that injection of glucagon into mice and adult zebrafish decreases liver Foxn3 protein and transcript levels. Zebrafish foxn3 loss-of-function mutants have decreased fasting blood glucose, blood glucagon, liver gluconeogenic gene expression, and α cell mass. Conversely, liver-limited overexpression of foxn3 increases α cell mass. Supporting these genetic findings in model organisms, non-diabetic rs8004664 risk allele carriers have decreased suppression of glucagon during oral glucose tolerance testing. By reciprocally regulating each other, liver FOXN3 and glucagon control fasting glucose. In Brief Karanth et al. find that glucagon lowers liver expression of Foxn3. Deletion of the Foxn3 gene decreases fasting blood glucose and the number of glucagon-producing α cells in the primary islet of zebrafish. Human carriers of the hyperglycemia risk allele of FOXN3 gene fail to suppress glucagon during oral glucose challenge.

FOXN3 controls liver glucose metabolism by regulating gluconeogenic substrate selection

“…1C and D). These results mirror the effects seen in zebrafish mutants carrying a heterozygous foxn3 loss-of-function allele where a 50% reduction in foxn3 transcript was accompanied by a twofold increase in mycb transcript (Karanth et al, 2018). These effects are also similar to that observed when immortalized human cells are transduced with a dominant-negative FOXN3 cDNA that occurs in leukemia as a consequence of pathological intron polyadenylation (Lee et al, 2018).…”

“…We modeled this increase in liver FOXN3 by transgenic over-expression of human FOXN3 in zebrafish hepatocytes, and observed an increase in fasting blood glucose in animals fed their normal diets (Karanth et al, 2016). Conversely, deletion of the foxn3 ortholog in zebrafish decreases fasting glucose (Karanth et al, 2018). On a mechanistic level, FOXN3 regulates fasting blood glucose by directly repressing expression of MYC (Karanth et al, 2016), a transcription factor central to promoting glucose utilization in liver (Sloan and Ayer, 2010).…”

“…On a mechanistic level, FOXN3 regulates fasting blood glucose by directly repressing expression of MYC (Karanth et al, ), a transcription factor central to promoting glucose utilization in liver (Sloan and Ayer, ). Quite surprisingly, liver FOXN3 gene dose regulates pancreatic islet α cell biology, modulating both plasma glucagon and α cell abundance in zebrafish, with the liver‐transgenic model showing increased α cell mass and the deletion model showing decreased α cell mass (Karanth et al, ). Paralleling these zebrafish model studies, non‐diabetic human carriers of the hyperglycemia risk allele within the FOXN3 locus show blunted suppression of glucagon during an oral glucose tolerance test, while showing no differences in fasting glucagon; except for fasting glucose, these FOXN3 hyperglycemia risk allele carriers do not have different glucose or insulin parameters during the oral glucose challenge (Karanth et al, ).…”

“…Quite surprisingly, liver FOXN3 gene dose regulates pancreatic islet α cell biology, modulating both plasma glucagon and α cell abundance in zebrafish, with the liver‐transgenic model showing increased α cell mass and the deletion model showing decreased α cell mass (Karanth et al, ). Paralleling these zebrafish model studies, non‐diabetic human carriers of the hyperglycemia risk allele within the FOXN3 locus show blunted suppression of glucagon during an oral glucose tolerance test, while showing no differences in fasting glucagon; except for fasting glucose, these FOXN3 hyperglycemia risk allele carriers do not have different glucose or insulin parameters during the oral glucose challenge (Karanth et al, ). Further underscoring that liver FOXN3 does not appear to impact insulin action, carriers of the rs8004664 risk allele do not have altered glucose disposal rates when subjected to high‐dose euglycemic, hyperinsulinemic clamp (Erickson et al, ).…”

The clinical and prognostic significance of FOXN3 downregulation in acute myeloid leukaemia

“…The transcription factor forkhead box N3 (FOXN3), as a member of the forkhead box N superfamily, participates in several biological processes, including the cell cycle, cell differentiation, epithelial‐mesenchymal transition, gene transcription and glucose metabolism . Previous studies have shown that the downregulated expression of FOXN3 is observed in various malignancies, such as hepatocellular carcinoma (HCC), colon cancer, ERα‐positive breast cancer, Hodgkin lymphoma, head and neck cancer, lung cancer, adult glioblastoma multiforme, T cell acute lymphoblastic leukaemia (ALL) and osteosarcoma, and the FOXN3 expression level is associated with the prognosis of some cancers …”