Glucose transporter-8 (GLUT8) mediates glucose intolerance and dyslipidemia in high-fructose diet-fed male mice - PubMed (original) (raw)

Glucose transporter-8 (GLUT8) mediates glucose intolerance and dyslipidemia in high-fructose diet-fed male mice

Brian J DeBosch et al. Mol Endocrinol. 2013 Nov.

Abstract

Members of the glucose transporter (GLUT) family of membrane-spanning hexose transporters are subjects of intensive investigation for their potential as modifiable targets to treat or prevent obesity, metabolic syndrome, and type 2 diabetes mellitus. Mounting evidence suggests that the ubiquitously expressed class III dual-specificity glucose and fructose transporter, GLUT8, has important metabolic homeostatic functions. We therefore tested the hypothesis that GLUT8 mediates the deleterious metabolic effects of chronic high-fructose diet exposure. Here we demonstrate resistance to high-fructose diet-induced glucose intolerance and dyslipidemia concomitant with enhanced oxygen consumption and thermogenesis in GLUT8-deficient male mice. Independent of diet, significantly lower systolic blood pressure both at baseline and after high-fructose diet feeding was also observed by tail-cuff plethysmography in GLUT8-deficient mice vs wild-type controls. Resistance to fructose-induced metabolic dysregulation occurred in the context of enhanced hepatic peroxisome proliferator antigen receptor-γ (PPARγ) protein abundance, whereas in vivo hepatic adenoviral GLUT8 overexpression suppressed hepatic PPARγ expression. Taken together, these findings suggest that GLUT8 blockade prevents fructose-induced metabolic dysregulation, potentially by enhancing hepatic fatty acid metabolism through PPARγ and its downstream targets. We thus establish GLUT8 as a promising target in the prevention of diet-induced obesity, metabolic syndrome, and type 2 diabetes mellitus in males.

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Figures

Figure 1.

Figure 1.

Normal jejunal fructose uptake and fructose transporter abundance in GLUT8KO male intestine. A, Mean jejunal fructose uptake measurements in WT and GLUT8KO mice fed chow or HFrD for 5 days. Data represent pooled means from 8–15 independent measurements per group. #, P < .01 vs chow-fed WT. n.s., not significantly different vs WT HFrD. B, Immunoblot analysis of crude jejunal lysates from chow-fed mice. Densitometric quantifications shown at right are normalized for GAPDH band density. C, Serum fructose concentrations after 2 g/kg oral fructose gavage in 4-hour fasted WT and GLUT8KO mice. Serum fructose concentrations were not significantly different at any time point.

Figure 2.

Figure 2.

Improved glucose homeostasis in HFrD-fed GLUT8KO mice. A and B, Mean fasting (16 h) blood glucose and plasma insulin in chow- and HFrD-fed WT and GLUT8KO mice. #, P < .05 vs chow-fed controls; *, P < .05 vs HFrD-fed WT. Oral glucose tolerance (C) and calculated area under the curve (AUC) (D) in HFrD-fed WT and GLUT8KO mice. Oral glucose solution (2 mg/g) was gavaged into fasted (4 h) WT and GLUT8KO mice. Blood glucose was measured at each interval shown. #, P < .05 vs chow-fed control; **, P < .01 and ***, P < .001 for HFrD-fed WT vs GLUT8KO. Insulin tolerance testing (E) and calculated AUC (F) in HFrD-treated WT and GLUT8KO males. Recombinant human insulin (0.6 U/kg) was injected ip in fasted (4 h) mice, and blood glucose was measured at each interval. *, P < .05 vs WT by 2-tail t test. Significant 2-way ANOVA diet-genotype interactions were detected in panels A–D.

Figure 3.

Figure 3.

Resistance to HFrD-induced dyslipidemia in GLUT8KO males. A, Fasting (16 h) plasma triglycerides in chow- and HFrD-fed GLUT8KO mice. B, Fasting (16 h) plasma free fatty acids in chow- and HFrD-fed GLUT8KO mice. #, P < .05 vs chow-fed controls; **, P < .01 vs HFrD-fed WT (n = 3, 6, 5, and 8 mice for WT and GLUT8KO chow and WT and GLUT8KO HFrD, respectively). Significant 2-way ANOVA diet-genotype interactions were detected in panels A and B.

Figure 4.

Figure 4.

Lower blood pressures and AngII in GLUT8KO mice. Nonfasting (A) and 4-hour fasting (B) systolic and diastolic BPs (SBP, upper panels, and DBP, lower panels) in chow- and HFrD-fed WT and GLUT8KO mice (n = 5–14 mice per group with a minimum of three to five readings taken per mouse). *, P < .05 vs WT control. C, Fasting (16 h) plasma AngII in HFrD-fed WT and GLUT8KO mice (n = 5–9 mice per group). *, P < .05 vs WT by 2-tailed t test.

Figure 5.

Figure 5.

Enhanced HFrD-induced thermogenesis and altered hepatic metabolic programming in GLUT8KO males. A, Oxygen consumption in chow- and HFrD-fed WT and GLUT8KO mice normalized to total body weight. ***, P < .001 vs WT control on an equivalent die from n = 6–8 mice per diet per genotype. B, Heat generated in chow- and HFrD-fed WT and GLUT8KO mice. ***, P < .001 vs WT mice on equivalent diet. C, Palmitate oxidation rates in cultured primary WT and GLUT8KO hepatocytes preincubated overnight in 5 mM fructose. ***, P < .001 vs WT (2 tailed t test). D, Immunoblot of PPARγ (upper panel) or GAPDH (lower panel) in 5-day chow- and HFrD-fed WT and GLUT8KO mice. E, Mean PPARγ-GAPDH band density in 5-day chow- and HFrD-fed WT and GLUT8KO mice. #, P < .05 vs chow-fed controls; ***, P < .001 vs same-diet WT controls. F, Immunoblot analysis of PPARγ and GLUT8 abundance in HFrD-fed mice orogastrically gavaged with adenovirus encoding green fluorescent protein (far left lane) or low- or high-titer adenovirus encoding SLC2A8 (middle and far right lanes). Shown are the mean PPARγ-GAPDH densities above each lane. G, Enhanced UCP2 abundance in HFrD-fed WT and GLUT8KO mouse crude hepatic lysates. Left panel, Quantification of UCP2 to GAPDH band density ratios, represented in right panel. **, P < .01 vs WT. Significant 2-way ANOVA diet-genotype interactions were detected in A, B, and E.

Figure 6.

Figure 6.

UCP isoform analysis in extrahepatic tissues. A, Unchanged skeletal muscle UCP2 and UCP3 in GLUT8KO males. Left: Gastrocnemius lysates from 4-hour fasted WT and GLUT8KO males fed 24-week chow or HFrD were immunoblotted for UCP2 (upper panel) or UCP3 (lower panels). GAPDH was probed as a loading control. B, Quantifications of immunoblot bands shown in (A). C, Visceral adipose tissue UCP2 and UCP3 in GLUT8KO males. Left: Visceral adipose tissue from 4-hour fasted WT and GLUT8KO males fed 24-week chow or HFrD were immunoblotted for UCP2 (upper panel) or UCP3 (middle panels). GAPDH was probed as a loading control to normalize densitometric quantifications shown in (D). *, P < .05 HFrD-fed GLUT8KO vs same-diet WT. Significant 2-way ANOVA diet-genotype interactions were detected in D for UCP2 and UCP3.

Figure 7.

Figure 7.

Lower ChREBP and PGC1β in GLUT8KO males fed HFrD. Relative ChREBP (A) and relative PGC1β (B) transcript abundance in 10-day chow- and HFrD-fed WT and GLUT8KO mice. #, P < .05 vs chow-fed control; *, P < .05 vs control for n ≥ 7 animals per treatment group. Significant interactions were observed for A and B. C, Working model regarding the role of GLUT8 in fructose-induced metabolic syndrome. Left graphic, In the presence of GLUT8, WT hepatocytes have maximal hexose entry into the cell, resulting in increased ChREBP and PGC1β mRNA and decreased PPARγ. In other models, ChREBP (31) and PGC1β promote downstream hepatic triglyceride (TG) synthesis (–34). Right graphic, GLUT8KO hepatocytes have diminished substrate entry into the cell. The net effect of this is to decrease ChREBP and PGC1β mRNA while increasing PPARγ and UCP2, promoting a metabolic program switch (28) toward greater fatty acid oxidation (FAO).

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