orldwide consumption of fructose in the form of high-fructose corn syrup has increased dramatically and excess fructose consumption is now known to be linked to adult metabolic disorders such as obesity and type 2 diabetes. Of equal concern, but less widely known, is the emergence of these fructose-linked metabolic disorders not only in teenagers but also in young children. There is very little information about fructose metabolism in the young, and the mechanisms regulating intestinal fructose absorption in neonates are unknown. In the small intestine, luminal fructose is absorbed by the apical fructose transporter GLUT5. Because development of the small intestine is similar in rats and humans, we used neonatal rats to demonstrate that GLUT5 expression and activity are very low between birth and weaning, but can be dramatically enhanced by precocious introduction of its substrate fructose into the intestinal lumen. GLUT5 upregulation by dietary fructose is possible only in rat pups older than 14 days, hence, absorption of fructose is strictly programmed to appear only at a certain age. In a series of studies, we determined that treatment with dexamethasone, a drug used to stimulate lung maturation in premature babies, can overcome natural programming and allow fructose to induce GLUT5 even in pups younger than 14 days. We also identified the receptor used by dexamethasone to modify GLUT5 programming, and tracked the movement of the bound receptor into the nucleus where reprogramming occurs. Information like this should increase our understanding of fructose metabolism in neonates.
For thousands of years, humans consumed about 16 - 24 g of fructose each day, mainly from fruits and honey. Recent modernization in food processing methods has altered consumption patterns and led to the gradual increase in the manufacture and consumption of fructose as sucrose and as high fructose corn syrup, containing 42 to 90 % fructose. Today, the per capita amount of fructose consumed in the U.S. each day is approximately 80 g/day, and about 330 to 380 kcal per day of the energy intake of average Americans is derived from fructose. This marked increase in fructose consumption has been tightly correlated by many clinical studies with the increased incidence of obesity and type II diabetes.
The final products of carbohydrate digestion are the monosaccharides glucose, fructose and galactose, but only processed (as opposed to natural) fructose seems to be associated with metabolic syndrome. These metabolic effects are possible only if ingested fructose is transported from the intestinal lumen to the blood at rates sufficient to lead to physiologically significant concentrations. Fructose transport is mediated on the apical side of intestinal cells by a member of the facilitative glucose transporter (GLUT) family, named GLUT5. The large GLUT family has seven members able to transport fructose, but GLUT5 is specific for fructose and has no ability to transport glucose or galactose. On the basolateral membrane of intestinal cells, GLUT2 transports cytosolic fructose and glucose into the blood.
Because young children and teenagers seem to be especially vulnerable to the metabolic effects of excessive fructose consumption, and because infants generally malabsorb fructose, we decided to study the regulation of intestinal fructose absorption using neonatal rats. GLUT5 is normally expressed at low baseline levels throughout the suckling (0 – 14 d of age) and weaning (14 – 28 d) stages in neonatal rats, stages during which pups receive sustenance mainly from milk, which does not contain any fructose. GLUT5 expression and activity, however, are dramatically enhanced by precocious introduction of fructose into the intestinal lumen before weaning is completed at ~ 28 days. We found that dietary fructose alone is sufficient to enhance expression of GLUT5 between 14 – 28 days, but not before ~14 days of age, indicating sharply defined developmental limits in regulation.
Fructose transport by the small intestine during the perinatal stage of mammalian development was a neglected subject area of research, because fructose has long been considered to be a minor contributor to metabolic diseases. However, the potential role of fructose in childhood obesity and diabetes has laid the groundwork for studies that will increase our understanding of fructose absorption and metabolism. Since intestinal transport is the critical first step involved in the metabolic effects of fructose, the main purpose of my project was to identify the signaling mechanisms underlying dietary and developmental regulation of the intestinal fructose transporter GLUT5 (Slc2A5).
First, using microarray techniques, we compared the expression levels of genes in the intestines of 10 (representing suckling stage when GLUT5 cannot be regulated by fructose alone) and 20 (representing the weaning stage when GLUT5 can be regulated by dietary fructose) day-old pups to identify fructose-responsive genes whose expression also changes with age. This first study identified a cluster of fructose-specific and age-responsive genes regulated by glucocorticoids, suggesting that glucocorticoids may modulate GLUT5 development. Fascinatingly, glucocorticoids are also known to have a vital role in modulation of many developmental events. Thus, in humans, glucocorticoids are administered to mothers to promote organ maturation (especially lung and gut) in fetuses in danger of preterm delivery. We then confirmed the role of glucocorticoids in the early regulation of GLUT5 by showing that when the gut is primed with dexamethasone (dex, a corticosterone analog) prior to fructose perfusion, dex would subsequently allow fructose to stimulate GLUT5 even in suckling pups younger than 14d. Hence, dexamethasone seems to mimic the effect of age and to induce a
precocious responsiveness of the intestine to luminal fructose. Glucocorticoid-modulated activities are classically explained by rapid (latency time < 10-30 min) non-genomic and deferred (latency time > 2-3 h) genomic mechanisms involving specific steroid nuclear receptors. We demonstrated later that the mechanism involved in the regulation of GLUT5 by dexamethasone requires a latency time > 12 hours and was turned off in the presence of transcription inhibitors. This indicated that dexamethasone utilizes genomic mechanisms in regulating GLUT5. Dex, like the endogenous cortisol or corticosterone, could bind the glucocorticoid (GR), mineralocorticoid (MR) and pregnane-X (PXR) receptors that we have shown to be expressed in the neonatal rat intestine. We then determined, by using a pharmacological approach, that the precocious fructose enhancement of GLUT5 by dex was RU486 (a specific GR antagonist) sensitive, indicating a specific role of the GR in mediating the effect of dex. Then we clearly observed by immunocytochemistry the translocation of the GR following dex administration and fructose perfusion.
These results portray the role of glucocorticoids in regulating the developmental maturation of some digestive and absorptive functions of the intestine and increase our understanding of mechanisms underlying the normal developmental pattern of intestinal fructose absorption. More generally, the capability of the intestine to reprogram indicates developmental plasticity, which provides organisms the ability to change function in response to environmental cues. Thus, one of the first functions of those adaptive mechanisms is to permit the small intestine of the neonate to digest and absorb nutrients obtained from the environment in order to enhance its survival in case of abnormal stressful conditions (e.g., if the dam is malnourished and provides little milk). Currently, preterm infants in the U.S. represent 10% of births, and as we mentioned previously, prenatal mothers at risk for premature delivery and/or preterm infants receive corticosteroids, which as we have clearly shown in this study also affect the maturation of the fructose transporter in the small intestine. We wonder what could be the long-term consequence of the interaction of glucocorticoids and fructose, the major sweetener currently used in human diets including baby foods and foods marketed specifically to children.
Veronique Douard received her BS and MS degrees from the Science University of Rennes in France. She then earned her PhD in biochemistry at the National Institute of Agronomic Research (INRA) in Nouzilly, France. After a short stint as a post-doctoral fellow in nutrition at INRA in St. Gilles, she joined the laboratory of Dr. Ferraris.