Facilitative glucose transporter 9, a unique hexose and urate transporter

Abstract

GLUT9 is a novel, facilitative glucose transporter isoform that exists as two alternative splice variants encoding two proteins that differ in their NH₂-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.