BackgroundSerum uric acid levels in humans are influenced by diet, cellular breakdown, and renal elimination, and correlate with blood pressure, metabolic syndrome, diabetes, gout, and cardiovascular disease. Recent genome-wide association scans have found common genetic variants of SLC2A9 to be associated with increased serum urate level and gout. The SLC2A9 gene encodes a facilitative glucose transporter, and it has two splice variants that are highly expressed in the proximal nephron, a key site for urate handling in the kidney. We investigated whether SLC2A9 is a functional urate transporter that contributes to the longstanding association between urate and blood pressure in man.Methods and FindingsWe expressed both SLC2A9 splice variants in Xenopus laevis oocytes and found both isoforms mediate rapid urate fluxes at concentration ranges similar to physiological serum levels (200–500 μM). Because SLC2A9 is a known facilitative glucose transporter, we also tested whether glucose or fructose influenced urate transport. We found that urate is transported by SLC2A9 at rates 45- to 60-fold faster than glucose, and demonstrated that SLC2A9-mediated urate transport is facilitated by glucose and, to a lesser extent, fructose. In addition, transport is inhibited by the uricosuric benzbromarone in a dose-dependent manner (K i = 27 μM). Furthermore, we found urate uptake was at least 2-fold greater in human embryonic kidney (HEK) cells overexpressing SLC2A9 splice variants than nontransfected kidney cells. To confirm that our findings were due to SLC2A9, and not another urate transporter, we showed that urate transport was diminished by SLC2A9-targeted siRNA in a second mammalian cell line. In a cohort of men we showed that genetic variants of SLC2A9 are associated with reduced urinary urate clearance, which fits with common variation at SLC2A9 leading to increased serum urate. We found no evidence of association with hypertension (odds ratio 0.98, 95% confidence interval [CI] 0.9 to 1.05, p > 0.33) by meta-analysis of an SLC2A9 variant in six case–control studies including 11,897 participants. In a separate meta-analysis of four population studies including 11,629 participants we found no association of SLC2A9 with systolic (effect size −0.12 mm Hg, 95% CI −0.68 to 0.43, p = 0.664) or diastolic blood pressure (effect size −0.03 mm Hg, 95% CI −0.39 to 0.31, p = 0.82).ConclusionsThis study provides evidence that SLC2A9 splice variants act as high-capacity urate transporters and is one of the first functional characterisations of findings from genome-wide association scans. We did not find an association of the SLC2A9 gene with blood pressure in this study. Our findings suggest potential pathogenic mechanisms that could offer a new drug target for gout.

is a novel, facilitative glucose transporter isoform that exists as two alternative splice variants encoding two proteins that differ in their NH 2-terminal sequence (GLUT9a and GLUT9b). Both forms of GLUT9 protein and mRNA are expressed in the epithelia of various tissues; however, the two splice variants are expressed differentially within polarized cells, with GLUT9a localized predominantly on the basolateral surfaces and GLUT9b expressed on apical surfaces. Protein expression of GLUT9 drops under conditions of starvation but increases with addition of glucose and under hyperglycemic conditions. The substrate specificity of GLUT9 is unique since, in addition to transporting hexose sugars, it also is a high-capacity uric acid transporter. Several recent large-scale human genetic studies show a correlation between SNPs mapped to GLUT9 and the serum uric acid levels in several different cohorts. The relationship between GLUT9 and uric acid is highly clinically significant. Elevated uric acid levels have been associated with metabolic syndrome, obesity, diabetes, hypertension, and chronic renal failure. Although some believe uric acid is elevated as a result of these diseases, there is now evidence that uric acid may play a role in the pathogenesis of these diseases. It is also known that GLUT9 is expressed in articular cartilage and is a uric acid transporter, and thus it is possible that GLUT9 plays a role in gout, a disease of uric acid deposition in the joints. In addition, some studies have suggested that intake of fructose plays an important role in causing elevated serum uric acid levels, especially in diabetes and obesity. It is possible that GLUT9, which seems to be both a fructose and a uric acid transporter, plays an important role in these conditions associated with hyperuricemia. splice variants; uric acid; fructose THE FACILITATIVE GLUCOSE TRANSPORTERS (SLC2A) belong to the facilitated transporter super gene family known as the glucose transporters (GLUTs). These integral membrane proteins are responsible for the facilitated diffusion of glucose and other hexoses into cells. Fourteen GLUTs have been identified. These transporters share several sequence and structural similarities, such as 12 transmembrane helices and conserved amino acid residues (18). The GLUT family is subdivided into three classes on the basis of comparison of amino acid sequences (35). GLUT1-4 are in class 1, GLUT5, -7, -9, and -11 are in class 2, and the last class consists of GLUT6, -8, -10, and -12 and the H ϩ /myoinositol transporter (35). GLUT14 is 95% identical to GLUT3 and thought to be a duplication of this class 1 transporter (37). The various GLUTs differ in their affinity for substrates, tissue distribution, and hormonal regulation. For example, GLUT4 is a glucose transporter found in muscle and adipose tissue that localizes to the plasma membrane in response to insulin, whereas GLUT11 is found in heart and skeletal muscle and appears to have a higher affinity for fructose than for glucose (16).All members of the GLUT...

Facilitative glucose transporters (GLUTs) including GLUT9, accelerate the facilitative diffusion of glucose across the plasma membrane. Studies in GLUT2-deficient mice suggested the existence of another GLUT in the mammalian beta-cell responsible for glucose sensing. The objective of this study was to determine the expression and function of GLUT9 in murine and human beta-cells. mRNA and protein expression levels were determined for both isoforms of GLUT9 in murine and human isolated islets as well as insulinoma cell lines (MIN6). Immunohistochemistry and subcellular localization were performed to localize the protein within the cell. Small interfering RNA knockdown of GLUT9 was used to determine the effect of this transporter, in the presence of GLUT2, on cell metabolism and insulin secretion in MIN6 and INS cells. In this report we demonstrate that GLUT9a and GLUT9b are expressed in pancreatic islets and that this expression localizes to insulin-containing beta-cells. Subcellular localization studies indicate that mGLUT9b is found associated with the plasma membrane as well as in the high-density microsome fraction and low-density microsome fraction, whereas mGLUT9a appears to be located only in the high-density microsome and low-density microsome under basal conditions. Functionally GLUT9 appears to participate in the regulation of glucose-stimulated insulin secretion in addition to GLUT2. small interfering RNA knockdown of GLUT9 results in reduced cellular ATP levels that correlate with reductions in glucose-stimulated insulin secretion in MIN6 and INS cells. These studies confirm the expression of GLUT9a and GLUT9b in murine and human beta-cells and suggest that GLUT9 may participate in glucose-sensing in beta-cells.

Polycystic ovary syndrome (PCOS) is a common reproductive endocrine disorder characterized by theca cell hyperplasia and excessive androgen production. An increasing body of evidence has pointed to a close association between PCOS and low-grade chronic systemic inflammation. However, the mechanistic basis for this linkage is unknown. Therefore, we evaluated the effects of the inflammatory agents lipopolysaccharide (LPS) and IL-1β on rat theca-interstitial cells (TICs). We found that incubation with either LPS or IL-1β elicited a dose-dependent increase in both TIC viability and androgen production. Using RNA sequencing analysis, we found that both of these inflammatory agents also triggered profound and widespread shifts in gene expression. Using a stringent statistical cutoff, LPS and IL-1β elicited differential expression of 5201 and 5953 genes, respectively. Among the genes upregulated by both LPS and IL-1β were key regulatory genes involved in the cholesterol and androgen biosynthesis pathways, including Cyp17a1, Cyp11a1, Hsd3b, and Hmgcr. This provides a molecular explanation for the mechanism of action of inflammatory agents leading to increased androgen production. Gene ontology and pathway analysis revealed that both LPS and IL-1β regulated genes highly enriched for many common functions, including the immune response and apoptosis. However, a large number of genes (n = 2222) were also uniquely regulated by LPS and IL-1β, indicating that these inflammatory mediators have substantial differences in their mechanism of action. Together, these findings highlight the potential molecular mechanisms through which chronic low-grade inflammation contributes to the pathogenesis of androgen excess in PCOS.

Twenty percent of patients with atrial fibrillation had ECs resistant to ostial ablation in one or more pulmonary veins. Isolating veins with ECs may require a different ablation approach. These connections are associated with an increased rate of early recurrence of conduction. (J Cardiovasc Electrophysiol, Vol. 22, pp. 149-159, February 2011).