Structure-function relationship of glucose transporters catalyzing facilitated diffusion

Molecular cloning and functional characterization of a glucose transporter (CsGLUT) in Clonorchis sinensis

A complementary DNA (cDNA) encoding a glucose transporter of Clonorchis sinensis (CsGLUT) was isolated from the adult C. sinensis cDNA library. The open reading frame of CsGLUT cDNA consists of 1653 base pairs that encode a 550-amino acid residue protein. Hydropathy analysis suggested that CsGLUT possess 12 putative membrane-spanning domains. The Northern blot analysis result using poly(A)(+)RNA showed a strong band at ~2.1 kb for CsGLUT. When expressed in Xenopus oocytes, CsGLUT mediated the transport of radiolabeled deoxy-D-glucose in a time-dependent but sodium-independent manner. Concentration-dependency results showed saturable kinetics and followed the Michaelis-Menten equation. Nonlinear regression analyses yielded a Km value of 588.5 ± 53.0 μM and a Vmax value of 1500.0 ± 67.5 pmol/oocyte/30 min for [1,2-(3)H]2-deoxy-D-glucose. No trans-uptakes of bile acid (taurocholic acid), amino acids (tryptophan and arginine), or p-aminohippuric acid were observed. CsGLUT-mediated transport of deoxyglucose was significantly and concentration-dependently inhibited by radio-unlabeled deoxyglucose and D-glucose. 3-O-Methylglucose at 10 and 100 μM inhibited deoxyglucose uptake by ~50 % without concentration dependence. No inhibitory effects by galactose, mannose, and fructose were observed. This work may contribute to the molecular biological study of carbohydrate metabolism and new drug development of C. sinensis.

Effect of β-casomorphin-7 from milk on the absorption of glucose in vitro and its mechanism

AIM: To detect the influence of major peptide β-casomorphin-7 (β-CM7) from milk on the absorption of glucose, and its mechanism in the diet and healthy food.

METHODS: In this study healthy adult SD rats were divided into control group (0 mol/L β-CM7), low-dose group (7.5 × 10^-7mol/L β-CM7), middle-dose group (7.5 × 10^-6 mol/L β-CM7) and high-dose group (7.5 × 10^-5 mol/L β-CM7). Reverted sacs of adult rats in vitro were performed which were dipped into culture sample including β-CM7 and glucose and we designed a few tests: (1) the concentration of glucose in reverted sacs was detected using oxidase method of glucose; (2) the activity of Na⁺-K⁺-ATPase in small intestinal mucosa was observed with colorimetric method; (3) the mRNA expressions of sodium-glucose transporter (SGLT-1) and facilitative glucose transporter (GLUT-2) in small intestinal mucosa were analyzed using the Real-time PCR.

RESULTS: Compared with control group, β-CM7 of 7.5 × 10^-7-7.5 × 10^-5 mol/L significantly induced glucose concentration in vitro(1.09, 1.24, 1.12 vs 1.74, P = 0.01, 0.04, 0.02); the activity of Na⁺-K⁺-ATPase was reduced (85.73, 112.06, 109.68 vs 114.93, P = 0.004, 0.73, 0.54); the mRNA expressions of SGLT-1 and GLUT-2 were also significantly reduced (0.77, 0.58, 0.46 vs 1.11, P = 0.20, 0.05, 0.02; 0.50, 0.66, 0.85 vs 1.14, P = 0.30, 0.14, 0.03), compared with control group.

CONCLUSION: The present data demonstrated that β-CM7 was able to reduce the absorption of glucose through inhibiting activity of Na⁺-K⁺-ATPase and the mRNA expression level of SGLT-1, GLUT-2 in small intestine of rats.

Modulation of glucose transport in fetal rat lung: a sexual dimorphism

Male fetuses exhibit delayed lung maturation and surfactant production in comparison with female fetuses. This delay may be related to sex hormone effects: estrogen enhances and androgens delay lung development. The uptake of glucose, an important precursor for surfactant synthesis, may be differently affected by estrogen and androgens. In these studies we determined the effects of these two hormones on glucose transport (glucose uptake, glucose transporter [Glut] 1 protein, and mRNA) and hexokinase activity in lung tissue of fetal rats. On Day 20 of gestation (term = 21.5 d) lung tissue was harvested from female and male fetal rats, minced into explants, and cultured for 24 h. Basal glucose uptake, measured in the absence of sex hormones, was 37% higher (P < 0.05) in female compared with male lungs. Explants were washed and cultured for an additional 3 h or 24 h in either estradiol or dihydrotestosterone (DHT) at 0, 1, 10, or 100 nM. Twenty-four-hour treatment with estradiol in both male and female explants increase 2-deoxyglucose uptake, Glut 1 protein, and mRNA levels (P < 0.05). However, explants from male fetuses were not as responsive to estradiol treatment as were those from females (P < 0.05). Treatment for 24 h with DHT decreased 2-deoxyglucose uptake, Glut 1 protein, and mRNA levels in females and males (P < 0.05). There was no difference in response between females and males. Short-term incubation (3 h) with sex hormones had no effect on glucose uptake. However, 3-h treatment with estradiol did increase Glut 1 mRNA levels (P < 0.05). Hexokinase activity was not affected by estradiol or DHT treatment. These findings indicate that estradiol and DHT differentially regulate glucose uptake in fetal rat lung tissue. This regulation of substrate supply (glucose) by estradiol and DHT may be another mechanism for the sexual dimorphism observed in lung development and surfactant synthesis.