SGLT5 Reabsorbs Fructose in the Kidney but Its Deficiency Paradoxically Exacerbates Hepatic Steatosis Induced by Fructose

Although excessive fructose intake is epidemiologically linked with dyslipidemia, obesity, and diabetes, the mechanisms regulating plasma fructose are not well known. Cells transfected with sodium/glucose cotransporter 5 (SGLT5), which is expressed exclusively in the kidney, transport fructose in vitro; however, the physiological role of this transporter in fructose metabolism remains unclear. To determine whether SGLT5 functions as a fructose transporter in vivo, we established a line of mice lacking the gene encoding SGLT5. Sodium-dependent fructose uptake disappeared in renal brush border membrane vesicles from SGLT5-deficient mice, and the increased urinary fructose in SGLT5-deficient mice indicated that SGLT5 was the major fructose reabsorption transporter in the kidney. From this, we hypothesized that urinary fructose excretion induced by SGLT5 deficiency would ameliorate fructose-induced hepatic steatosis. To test this hypothesis we compared SGLT5-deficient mice with wild-type mice under conditions of long-term fructose consumption. Paradoxically, however, fructose-induced hepatic steatosis was exacerbated in the SGLT5-deficient mice, and the massive urinary fructose excretion was accompanied by reduced levels of plasma triglycerides and epididymal fat but fasting hyperinsulinemia compared with fructose-fed wild-type mice. There was no difference in food consumption, water intake, or plasma fructose between the two types of mice. No compensatory effect by other transporters reportedly involved in fructose uptake in the liver and kidney were indicated at the mRNA level. These surprising findings indicated a previously unrecognized link through SGLT5 between renal fructose reabsorption and hepatic lipid metabolism

Conditional deletion of Npt2b in phosphate transport

Background Hyperphosphatemia is common in chronic kidney disease and is associated with morbidity and mortality. The intestinal Na+-dependent phosphate transporter Npt2b is thought to be an important molecular target for the prevention of hyperphosphatemia. The role of Npt2b in the net absorption of inorganic phosphate (Pi), however, is controversial. Methods In the present study, we made tamoxifen-inducible Npt2b conditional knockout (CKO) mice to analyze systemic Pi metabolism, including intestinal Pi absorption. Results Although the Na+-dependent Pi transport in brush-border membrane vesicle uptake levels were significantly decreased in the distal intestine of Npt2b CKO mice compared with control mice, plasma Pi and fecal Pi excretion levels were not significantly different. Data obtained using the intestinal loop technique showed that Pi uptake in Npt2b CKO mice was not affected at a Pi concentration of 4 mM, which is considered the typical luminal Pi concentration after meals in mice. Claudin, which may be involved in paracellular pathways, as well as claudin-2, 12, and 15 protein levels were significantly decreased in the Npt2b CKO mice. Thus, Npt2b deficiency did not affect Pi absorption within the range of Pi concentrations that normally occurs after meals. Conclusion These findings indicate that abnormal Pi metabolism may also be involved in tight junction molecules such as Cldns that are affected by Npt2b deficiency

Generation of SGLT5-deficient mice and their fructose and mannose uptake by renal BBMV s.

<p>(A) Schematic representation of the strategy for targeting the <i>Slc5a10</i> gene. A targeting vector was constructed by inserting a neomycin resistant (<i>neo</i>) gene cassette to disrupt exons 3–6 of the <i>Slc5a10</i> genomic locus on a BAC genomic clone. Arrows indicate PCR primers for genotyping. (B) A representative result of genotyping the offspring obtained by intercrossing heterozygous-deficient mice. Wild type and null alleles are detected as signals of 900 bp and 350 bp, respectively. <i>Wt</i>: Wild type mice, <i>He</i>: Heterozygous null mutant, <i>Ho</i>: Homozygous null mutant. (C) Sodium-dependent uptake of fructose and (D) mannose in BBMVs of WT mice (+/+) and SGLT5-deficient mice (−/−). (E) Sodium-independent uptake of fructose and (F) mannose in BBMVs of WT mice (+/+) and SGLT5-deficient mice (−/−). Data are presented as means ± S.D. Data are derived from 3 independent experiments.</p

Oral glucose tolerance test with WT mice (+/+) and SGLT5-deficient mice (−/−) given plain water or fructose water (HF).

<p>Fasted 21-week-old male mice received an oral dose of glucose (2 g/kg). Plasma glucose levels were determined at the indicated time points. Data are presented as means ± S.E.M (<i>n</i> = 8–10). ### <i>P</i><0.001 versus respective water controls by analysis of covariance (ANCOVA).</p

Influence of the long-term consumption of fructose on tissue weight and lipid metabolism.

<p>(A) Plasma triglyceride levels of WT mice (+/+) and SGLT5-deficient mice (−/−). (B) Plasma total cholesterol levels. (C) Weight of epididymal fat. (D) Weight of the liver. (E) Hepatic triglyceride levels. (F) Histopathological analysis of the liver sections. Two sections per mouse were stained with Sudan III. Representative images are shown (scale bar, 50 µm). Data are presented as means ± S.E.M (<i>n</i> = 8–10). * <i>P</i><0.05, *** <i>P</i><0.001 versus WT mice given 30% fructose water. # <i>P</i><0.05, ## <i>P</i><0.01, ### <i>P</i><0.001 versus respective plain water controls.</p

Food and water intake in WT (+/+) mice and SGLT5-deficient mice (−/−). Daily intake of

<p>(<b>A</b>) <b>food and</b> (<b>B</b>) <b>water of mice at 17 weeks of age.</b> (C) Calculated daily energy intake. Data are presented as means ± S.E.M (n = 8–10). ### P<0.001 versus respective plain water control.</p

Gene expression analysis. Quantitative RT-PCR was performed with total RNA isolated from (A) the liver and (B) the kidney of WT mice (+/+) and SGLT5-deficient mice (−/−) given plain water or high fructose (HF) water.

<p><i>MAPK1</i> was used as an internal control. Data are presented as means ± S.E.M. <i>n</i> = 3 (A). <i>n</i> = 8–10 (B).</p

SGLT5 distribution and fructose uptake.

<p>(A) Tissue distribution of mouse SGLT5 and (B) SGLT5-mediated fructose uptake in COS-7 cells. Data are presented as means ± S.D. Data are derived from 3 independent experiments.</p

Effect of high fructose consumption in WT mice and SGLT5-deficient mice.

<p>(A) Plasma glucose levels of WT mice (+/+) and SGLT5-deficient mice (−/−) were measured every 2 weeks. <i>HF</i>: Mice given water containing high fructose. (B) Growth curves of WT mice and SGLT5-deficient mice. (C) Plasma samples were collected after 6 h fasting at 21 weeks of age, and immunoreactive insulin (<i>IRI</i>) was determined. (D) Plasma fructose concentrations measured in plasma samples collected under anesthesia after 3 h fasting. <i>Open circles</i> represent individual data. (E and F) WT mice and SGLT5-deficient mice given plain water or fructose water were maintained in metabolic cages and 24-h urine samples were collected. Urinary fructose excretion was calculated by multiplying urinary fructose concentration by the amount of urine. Data are presented as means ± S.E.M (<i>n</i> = 8–10). *** <i>P</i><0.001 versus WT mice given 30% fructose water. # <i>P</i><0.05, ## <i>P</i><0.01, ### <i>P</i><0.001 versus respective plain water controls. +++ <i>P</i><0.001 versus WT mice given plain water.</p