OR18-2 Hepatic FoxO1 Requires DNA-Binding Activity to Promote Glucose Production

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: OR18-Diabetes-Associated Genes & Pathways
Basic/Translational
Sunday, June 16, 2013: 11:15 AM-12:45 PM
Presentation Start Time: 11:30 AM
Room 304 (Moscone Center)
Joshua R. Cook* and Domenico Accili
Columbia University, New York, NY
A hallmark of type 2 diabetes mellitus is fasting hyperglycemia, due in part to inappropriate hepatic glucose production (HGP).  A key mediator of the effect of insulin resistance on HGP is FoxO1, a fasting-inducible transcriptional effector whose activity is normally suppressed by insulin.  Although mice lacking all hepatocellular FoxO1 (L-FoxO1) demonstrate diminished HGP, the mechanism responsible for FoxO1’s effects on HGP has not been proven.  Indeed, although FoxO1 is characterized by a forkhead DNA-binding domain, prior evidence suggests that FoxO1 is capable of co-regulatory actions independent of direct transcriptional transactivation.  In order to elucidate the mechanism of FoxO1’s action on HGP, we have generated a knock-in mouse heterozygous for an allele encoding a DNA binding-deficient FoxO1 (DBD-FoxO1) replacing one wild type (WT) allele.  When crossed onto a FoxO1-floxed background, we can produce a mouse bearing a conditional knockout of the one remaining WT allele of FoxO1, thereby isolating the DBD-FoxO1 allele in a cell type-specific manner; this allows for the dissection of DNA binding-dependent versus independent functions of FoxO1.   Global heterozygosity for WT-FoxO1 or DBD-FoxO1 does not alter glucose, insulin, or pyruvate tolerance relative to WT; however, the L-DBD model does exhibit an enhancement in glucose tolerance comparable to that of the L-FoxO1 mouse.  This effect is likely due to altered hepatic glucose production secondary to functional FoxO1 deficiency, as primary hepatocytes from L-DBD and L-FoxO1 mice produce significantly less glucose in culture than do WT hepatocytes; this effect occurs both in the basal and hormone-stimulated states.   This diminished capacity for glucose production in L-DBD and L-FoxO1 primary hepatocytes is coupled with decreased basal and stimulated expression levels of key fasting-inducible genes, such as glucose-6-phosphatase and IGF-binding protein-1.  Moreover, acute reconstitution of DNA binding-competent FoxO1 in L-FoxO1 primary hepatocytes rescues fasting-inducible gene expression while reconstitution with DBD-FoxO1 does not. Therefore, despite evidence demonstrating DNA binding-independent functions of FoxO1, FoxO1’s role in hepatic glucose production appears to require direct transcriptional transactivation.

Nothing to Disclose: JRC, DA

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm