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"The role of transport, metabolism and trafficking in the regulation of GLUT5 by its substrate fructose in mouse small intestine"

by
Chirag Patel
Pharmacology & Physiology Program
B. Pharmacy 2005, L.M. College of Pharmacy, India
M.S. 2008, Long Island University, Brooklyn

Thesis Advisor: Ronaldo P. Ferraris, Ph.D.
Professor
Department of Pharmacology & Physiology

Friday, November 14, 2014
12:00 P.M. , MSB Room H-609b


Abstract

Increases in our understanding of the regulation of the intestinal fructose transporter GLUT5 (Slc2a5) are essential in understanding fructose intolerance and the many metabolic diseases ascribed to excessive fructose intake. Previous work in rats has shown that GLUT5 regulation has developmental constraints. In adults, the mere presence of fructose in the intestinal lumen increases GLUT5 expression, whereas in neonatal suckling rats, corticosterone treatments are required for fructose-induced regulation of GLUT5. In this project, I used mutant mouse models to test the hypotheses that fructose transport via GLUT5, metabolism via ketohexokinase (KHK), and GLUT5 intracellular trafficking to the apical membrane via the GTPase Rab11a-dependent endosomes are each required for GLUT5 upregulation in the small intestine of adults and neonates. To induce GLUT5, 30% fructose and control (glucose or lysine) solutions @ ~0.25 ml/mouse were gavage-fed twice a day for 2.5 d to adult wildtype (WT) and mutant mice that otherwise had ad libitum access to commercial feeds and exhibited normal feeding as well as growth rates. In WT mice, fructose but not lysine or glucose dramatically increased the hnRNA, mRNA, and protein expression as well as activity of GLUT5 but not those of GLUTs2, 7, 8 and 12 as well as of SGLT1, suggesting that luminal fructose activated the transcription of Slc2a5 without affecting other transporters claimed by others to also absorb fructose and other sugars. Fructose also increased the intestinal expression of specific enzymes involved in fructose metabolism and of representative enzymes involved in gluconeogenesis. Deletion of GLUT5 abolished facilitated fructose uptake, suggesting that GLUT5 mediates most fructose absorption across the apical membrane. Deletion also prevented the fructose-induced upregulation of fructose-metabolizing and gluconeogenic enzymes, suggesting that transapical fructose transport is required and that luminal contact between fructose and intestinal sweet taste receptors is insufficient stimulation for activating fructose-responsive genes. Inhibiting intracellular fructose metabolism by deletion of KHK also prevented upregulation, indicating that fructose metabolism is required for fructose-induced upregulation. The regulatory signal is likely not the downstream fructose metabolite, glyceraldehyde, which, when gavage-fed, modestly increased fructose uptake but did not increase GLUT5 mRNA and protein expression. Finally, impeding GLUT5 trafficking to the apical membrane using Rab11a─IEC mice that had no Rab11a in enterocytes, prevented not only the fructose-induced upregulation of GLUT5 and other fructose-responsive genes, but also reduced the baseline transport activity of GLUT5 and other apical sugar transporters, suggesting that trafficking of GLUT5 to the apical membrane is necessary for fructose-induced upregulation of fructose responsive genes, and for maintenance of baseline transporter activity. I then investigated whether glucocorticoid-dependent, fructose-induced GLUT5 upregulation in neonates also have the same metabolic requirements as that in adults. Although the corticosterone analog dexamethasone dramatically increased the intestinal expression of known glucocorticoid responsive genes, fructose failed to induce GLUT5 expression and activity in suckling mice, unlike results obtained from suckling rats. Thus, the developmental crosstalk between endocrine and luminal signals regulating GLUT5 may be markedly different between rats and mice. In summary, dietary regulation of the primary intestinal fructose transporter GLUT5 and fructose-responsive genes in adult mice requires fructose uptake from the lumen, fructose metabolism by KHK, and Rab11a-mediated GLUT5 trafficking to the apical membrane.


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