T1843 Fructose-Induced Translocation of the Glucocorticoid Receptor in Developing Rat Small Intestine

Abstract

Despite dramatic increases in per capita consumption of fructose (F) and its potential links to the obesity epidemic, particularly in young children, little is known about neonatal intestinal F absorptionmediated by the brushborder F transporter GLUT5. GLUT5 is normally expressed at low baseline levels throughout the suckling (0 ~ 13 d of age) and weaning (14 ~ 28 d) stages in neonatal rats. GLUT5 expression and activity, however, are dramatically enhanced by precocious perfusion of its substrate F for 4 h into the intestinal lumen, but only after ~14 d of age. Priming the gut with dexamethasone (D, a glucocorticoid analog) allowed F to stimulate GLUT5 even in suckling pups < 14 d old. Since an earlier study showed that the D effect is mediated by the glucocorticoid receptor (GR), we tested the hypothesis that GR must translocate to the nucleus to allow F to stimulate GLUT5 transcription. Four groups of 10 d old pups were injected with D or saline (S) before intestinal perfusion for 4h with F or glucose (G) solutions (10FD, 10FS, 10GD and 10GS). Using primary antibodies specific for GLUT5, GR or GR-P (phosphorylated GR), and propidium iodide (PI) as a nuclear marker we compared the (co)localization of GR and/or GLUT5 in the jejunum. In 10GS (control) or 10FS intestines double stained with anti GR and PI, we saw only cytoplasmic GR. GR remained in the cytoplasm of the 10GD intestines while in the 10FD pups, GR translocated into the nucleus. This GR translocation was blocked if the pups were treated with RU (specific antagonist of GR) before D injection and F perfusion. We then double stained the intestine with anti-rat GLUT5 and GR. The 10FD pups had the highest intensity of GLUT5 staining on the apical membrane, in correlation with a nuclear localization of GR. In the other groups, GR remained in the cytoplasm and the intensity of the GLUT5 staining was low or undetectable, suggesting that GR translocation may be necessary for D-allowed, F-mediated upregulation of GLUT5. We also probed the tissue with a primary antibody specific for GR phosphorylated at Ser-211 (GR-P), a known phosphorylation site that activates GR. However, GR-P was not detectable in the nucleus of any group of pups, indicating that GR phosphorylation is not necessary in the D-allowed, F-induced precocious increases in GLUT5 transcription. We conclude that the developmental regulation of GLUT5 expression in response to D and F requires translocation of GR to the nucleus but does not involve GR phosphorylation.