Reassessment of GLUT7 and GLUT9 as Putative Fructose and Glucose Transporters

Although increased dietary fructose consumption is associated with metabolic impairments, the mechanisms and regulation of intestinal fructose absorption are poorly understood. GLUT5 is considered to be the main intestinal fructose transporter. Other GLUT family members, such as GLUT7 and GLUT9 are also expressed in the intestine and were shown to transport fructose and glucose. A conserved isoleucine-containing motif (NXI) was proposed to be essential for fructose transport capacity of GLUT7 and GLUT9 but also of GLUT2 and GLUT5. In assessing whether human GLUT2, GLUT5, GLUT7, and GLUT9 are indeed fructose transporters, we expressed these proteins in Xenopus laevis oocytes. Stably transfected NIH-3T3 fibroblasts were used as second expression system. In proving the role of the NXI motif, variants p.I322V of GLUT2 and p.I296V of GLUT5 were tested as well. Sugar transport was measured by radiotracer flux assays or by metabolomics analysis of cell extracts by GC-MS. Fructose and glucose uptakes by GLUT7 were not increased in both expression systems. In search for the physiological substrate of GLUT7, cells overexpressing the protein were exposed to various metabolite mixtures, but we failed to identify a substrate. Although urate transport by GLUT9 could be shown, neither fructose nor glucose transport was detectable. Fructose uptake was decreased by the GLUT2 p.I322V variant, but remained unaffected in the p.I296V GLUT5 variant. Thus, our work does not find evidence that GLUT7 or GLUT9 transport fructose or glucose or that the isoleucine residue determines fructose specificity. Rather, the physiological substrate of GLUT7 awaits to be discovered.