Radiation-induced reductions in transporter mRNA levels parallel reductions in intestinal sugar transport

Abstract

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.

Fig. 1.

Design of the 2 experiments where mice were acutely irradiated with low linear energy transfer (LET) gamma rays and fed specific diets: vitamin A-enriched diet (A) or high-sugar (d-fructose or d-glucose) diets (B). Arrows, postirradiation days when mice were killed; d, day.

Fig. 2.

Light micrograph of a cross section (hematoxylin and eosin stained) of proximal jejunum of mice fed with control diet and killed at day 8 postirradiation: 0 Gy (A), 8.5 Gy (B), and 10 Gy (C). Bars are 75 μm. Villi were intact in unirradiated and irradiated mice, independent of diet. Mice irradiated with 7 Gy and mice consuming vitamin A-supplemented diet not shown.

Fig. 3.

Retinoid concentrations in mouse proximal jejunum (A) and serum (B) after 5 days of the vitamin A-enriched diet or the control diet. Results are means ± SE (n = 6 for A; n = 3 for B). Clearly, a much greater amount of vitamin A was absorbed and retained in mice fed vitamin A-supplemented diets.

Fig. 4.

Proximal intestinal d-fructose uptakes per centimeter at days 2 (A), 5 (B), 8 (C), and 14 (D) postirradiation, and uptake per milligram at days 2 (E), 5 (F), 8 (G), and 14 (H) postirradiation at different absorbed doses. Results are means ± SE of 6 independent experiments. See text for P values of two-way and one-way ANOVAs. Similar per centimeter and per milligram results suggest that reductions were likely due to decreased transporters per cell and not to decreases in number of cells.

Fig. 5.

Proximal intestinal d-glucose uptakes per centimeter at days 2 (A), 5 (B), 8 (C), and 14 (D) postirradiation and uptake per milligram at days 2 (E), 5 (F), 8 (G), and 14 (H) postirradiation at different absorbed doses. Results are means ± SE of 6 independent experiments. See text for P values of two-way and one-way ANOVAs. There were similar trends in radiation-induced reductions of d-glucose uptake per centimeter and per milligram.

Fig. 6.

Effect of gamma rays on relative (to 0 Gy) heterogeneous nuclear (hnRNA) and mRNA abundance of GLUT5 (A) and SGLT1 (B) measured by real-time PCR in mice fed with the control diet and killed 8 days postirradiation. Results are means ± SE (n = 6). Bars with different letters are significantly different (P < 0.05). See text for P values. Levels of transporter hnRNA increase, while levels of transporter mRNA decrease with increasing radiation dose.

Fig. 7.

Effect of irradiation on rates of intestinal sugar uptake and on relative mRNA abundance of the sugars transporters of mice fed with high-sugar diets (experimental design in Fig. 1B). d-Fructose uptake (A) and d-glucose uptake (B) per centimeter at 8 days postirradiation at the proximal part of the small intestine of mice acutely gamma-irradiated at 0 Gy and 8.5 Gy. Mice were fed with 60% d-fructose or 60% d-glucose diet for 24 h (start specific diet at 7 days postirradiation), then killed. Relative (to d-glucose diet at 0 Gy) mRNA abundance of GLUT5 (C) and SGLT1 (D) is shown. Results are means ± SE (n = 6). Bars with different letters are significantly different (P < 0.05). See text for P values. d-Fructose uptake rate and GLUT5 mRNA abundance increase in both irradiated and unirradiated mice fed a d-fructose diet. d-Fructose uptake rate and GLUT5 mRNA abundance decreased markedly in irradiated mice fed d-fructose.