More than a century ago, ionizing radiation was observed to damage the radiosensitive small intestine. Although a large number of studies has since shown that radiation reduces rates of intestinal digestion and absorption of nutrients, no study has determined whether radiation affects mRNA expression and dietary regulation of nutrient transporters. Since radiation generates free radicals and disrupts DNA replication, we tested the hypotheses that at doses known to reduce sugar absorption, radiation decreases the mRNA abundance of sugar transporters SGLT1 and GLUT5, prevents substrate regulation of sugar transporter expression, and causes reductions in sugar absorption that can be prevented by consumption of the antioxidant vitamin A, previously shown by us to radioprotect the testes. Mice were acutely irradiated with (137)Cs gamma rays at doses of 0, 7, 8.5, or 10 Gy over the whole body. Mice were fed with vitamin A-supplemented diet (100x the control diet) for 5 days prior to irradiation after which the diet was continued until death. Intestinal sugar transport was studied at days 2, 5, 8, and 14 postirradiation. By day 8, d-glucose uptake decreased by approximately 10-20% and d-fructose uptake by 25-85%. With increasing radiation dose, the quantity of heterogeneous nuclear RNA increased for both transporters, whereas mRNA levels decreased, paralleling reductions in transport. Enterocytes of mice fed the vitamin A supplement had > or = 6-fold retinol concentrations than those of mice fed control diets, confirming considerable intestinal vitamin A uptake. However, vitamin A supplementation had no effect on clinical or transport parameters and afforded no protection against radiation-induced changes in intestinal sugar transport. Radiation markedly reduced GLUT5 activity and mRNA abundance, but high-d-fructose diets enhanced GLUT5 activity and mRNA expression in both unirradiated and irradiated mice. In conclusion, the effect of radiation may be posttranscriptional, and radiation-damaged intestines can still respond to dietary stimuli.

BACKGROUND While searching by microarray for sugar-responsive genes, we inadvertently discovered that sodium-phosphate cotransporter 2B (NaPi-2b) mRNA concentrations were much lower in fructose-perfused than in glucose-perfused intestines of neonatal rats. Changes in NaPi-2b mRNA abundance by sugars were accompanied by similar changes in NaPi-2b protein abundance and in rates of inorganic phosphate (Pi) uptake. OBJECTIVE We tested the hypothesis that luminal fructose regulates NaPi-2b. DESIGN We perfused into the intestine fructose, glucose, and nonmetabolizable or poorly transported glucose analogs as well as phlorizin. RESULTS NaPi-2b mRNA concentrations and Pi uptake rates in fructose-perfused intestines were approximately 30% of those in glucose and its analogs. NaPi-2b inhibition by fructose is specific because the mRNA abundance and activity of the fructose transporter GLUT5 (glucose transporter 5) increased with fructose perfusion, whereas those of other transporters were independent of the perfusate. Plasma Pi after 4 h of perfusion was independent of the perfusate, probably because normal kidneys can maintain normophosphatemia. Inhibiting glucose-6-phosphatase, another fructose-responsive gene, with tungstate or vanadate nonspecifically inhibited NaPi-2b mRNA expression and Pi uptake in both glucose- or fructose-perfused intestines. The AMP kinase (AMPK)-activator AICAR (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside) enhanced and the fatty acid synthase-AMPK inhibitor C75 (3-carboxy-4-octyl-2-methylenebutyrolactone trans-4-carboxy-5-octyl-3-methylenebutyrolactone) prevented fructose inhibition of NaPi-2b but had no effect on expression of other transporters. NaPi-2b expression decreased markedly with age and was inhibited by fructose in all age groups. CONCLUSIONS Energy levels in enterocytes may play a role in NaPi-2b inhibition by luminal fructose. Consumption of fructose that supplies approximately 10% of caloric intake by Americans clearly affects absorption of Pi and may promote Pi homeostasis in patients with impaired renal function.

Fructose is a potent stimulator of liver lipogenesis but little is known about the regulation of small intestinal (SI) genes involved in fatty acid synthesis, even though the SI contributes 10–40% of total body triglycerides. We pair‐fed two groups of 16 d old rats high fructose or glucose (control) solutions for 16 consecutive d (16F or 16G), and another two groups for 8 d followed by 8 d without sugars (8F or 8G) to evaluate the long‐term effects of fructose on the regulation of fatty acid metabolism. The expression of the fructose transporter GLUT5 was high in the liver, SI and kidney but only in the SI did fructose markedly upregulate GLUT5. In the liver but not in the SI of 16F fed rats, expression of lipogenic enzymes (FAS, HmGCoA, and Malic Enzyme 1) and the triglyceride transfer protein MTP were much greater than that in 16G, 8F and 8G. In the SI, withdrawal of sugar feeding in 16F and 16G markedly inhibited the expression of Malic Enzymes 1 and 3, MTP and HmGCoA in comparison with 8F and 8G. Expression of these genes in the kidney was not affected by sugar consumption regardless of feeding duration, indicating that lipogenic genes cannot be regulated in organs not involved in fatty acid synthesis. In contrast, genes involved in fatty acid synthesis are specifically stimulated by fructose in the liver but inhibited by both glucose and fructose in the small intestine. Support: NIH RDK075617A, NSF IOS‐0722365

Nutrient transporters are often specifically and solely regulated by their substrates. However, we recently showed that luminal G-perfusion, but not that of its non-metabolizable analogs or fructose (F), induces Pi uptake in weaning rat. We report here that this activation is mediated by a G-specific induction of both apical sodium-Pi co-transporter (NaPi2b) mRNA and brush-border protein levels. The role of G metabolism was evaluated by modulating the activity of G6Pase, a key enzyme of gluconeogenesis, and AMPK, the main cellular energy sensor. 20 d old rat intestines were perfused with G or F alone, or these sugars plus (1) a G6Pase inhibitor (Tungstate -T- or Vanadate -V-), or (2) an AMPK activator (AICAR) or inhibitor (C75). Perturbing glucose metabolism by inhibiting G6Pase with T or V also inhibits NaPi2b mRNA expression and Pi uptake. AICAR decreases NaPi2b mRNA levels. In contrast, C75 tends to increase NaPi2b gene expression and Pi uptake in both sugar-perfused groups. The effects are specific for NaPi2b since proline or G transporter mRNA levels and activities were not affected. The stimulating effect of G on Pi uptake may be specifically mediated by subsequent G metabolism in the enterocyte. Understanding the mechanisms underlying the potent stimulation of intestinal Pi uptake by G has implications in the management of hyperphosphatemia by dietary carbohydrate. (NSF-IBN 235011).

Intermediary signals, precociously enhancing GLUT5 transcription in response to perfusion of its substrate, fructose, in the small intestine of neonatal rats, are not known. Because glucose-6-phosphatase (G6Pase), glucose-6-phosphate translocase (G6PT), and fructose-1,6-bisphosphatase (FBPase) expression increases parallel to or precedes that of GLUT5, we investigated the link between these gluconeogenic genes and GLUT5 by using vanadate or tungstate, potent inhibitors of gluconeogenesis. Small intestinal perfusions of 20-d-old rats were performed with fructose alone, fructose + vanadate or tungstate, glucose alone, and glucose + vanadate or tungstate. As expected, fructose, but not glucose nor glucose + inhibitor perfusion, increased GLUT5 mRNA abundance and fructose transport. Fructose perfusion dramatically increased G6Pase mRNA abundance but had no effect on G6Pase activity. In sharp contrast, fructose perfusion did not increase FBPase gene expression but stimulated FBPase activity. Both vanadate and tungstate significantly inhibited G6Pase activity but did not prevent the fructose-induced increases in G6Pase and G6PT gene expression. Perfusion with fructose + vanadate prevented the fructose-induced increases in fructose transport and GLUT5 mRNA abundance, whereas perfusion with fructose + tungstate did not. Interestingly, vanadate, but not tungstate, inhibited the fructose-induced increase in FBPase activity. Thus, vanadate inhibition of fructose-induced increases in FBPase activity paralleled exactly vanadate inhibition of fructose-induced increases in GLUT5 mRNA abundance and activity. Fructose-induced changes in FBPase activity may regulate changes in GLUT5 expression and activity in the small intestine of neonatal rats. The marked increases in intestinal G6Pase and GLUT5 mRNA abundance may be a parallel response to different factors released during fructose perfusion.

Intermediary signals, precociously enhancing GLUT5 transcription in response to perfusion of its substrate, fructose, in the small intestine of neonatal rats, are not known. Because glucose-6-phosphatase (G6Pase), glucose-6-phosphate trans- locase (G6PT), and fructose-1,6-bisphosphatase (FBPase) expression increases parallel to or precedes that of GLUT5, we investigated the link between these gluconeogenic genes and GLUT5 by using vanadate or tungstate, potent inhibitors of gluconeogenesis. Small intestinal perfusions of 20-d-old rats were performed with fructose alone, fructose 1 vanadate or tungstate, glucose alone, and glucose 1 vanadate or tungstate. As expected, fructose, but not glucose nor glucose 1 inhibitor perfusion, increased GLUT5 mRNA abundance and fructose transport. Fructose perfusion dramatically increased G6Pase mRNA abundance but had no effect on G6Pase activity. In sharp contrast, fructose perfusion did not increase FBPase gene expression but stimulated FBPase activity. Both vanadate and tungstate significantly inhibited G6Pase activity but did not prevent the fructose-induced increases in G6Pase and G6PT gene expression. Perfusion with fructose 1 vanadate prevented the fructose-induced increases in fructose transport and GLUT5 mRNA abundance, whereas perfusion with fructose 1 tungstate did not. Interestingly, vanadate, but not tungstate, inhibited the fructose-induced increase in FBPase activity. Thus, vanadate inhibition of fructose-induced increases in FBPase activity paralleled exactly vanadate inhibition of fructose-induced increases in GLUT5 mRNA abundance and activity. Fructose-induced changes in FBPase activity may regulate changes in GLUT5 expression and activity in the small intestine of neonatal rats. The marked increases in intestinal G6Pase and GLUT5 mRNA abundance may be a parallel response to different factors released during fructose perfusion. J. Nutr. 136: 2308-2313, 2006.