Reassessment of GLUT7 and GLUT9 as Putative Fructose and Glucose Transporters

Abstract

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.

Identification of essential amino acids for glucose transporter 5 (GLUT5)-mediated fructose transport

“…For all cloning procedures we used the vector pMXs with human GLUT5-GFP or GLUT7-GFP (GLUT7: NM_207420.2, GLUT5: NM_003039.2) insert as described elsewhere [9]. In the first round, we divided GLUT5 into 26 fragments covering the complete protein sequence and constructed GLUT5-GLUT7-GFP chimeras by overlap extension PCR.…”

“…Although their protein sequence is approximately 60 % identical, the transport characteristics are different. GLUT5 represents a high capacity fructose transporter [2], whereas GLUT7 does not transport fructose and its physiological substrate remains unknown [9]. We thus considered that the generation of chimeras of these two transporters might be a suitable model to identify amino acids and protein domains involved in fructose transport.…”

<p>Hyperuricemia and Hypertension: Links and Risks</p>

“…There is controversy as to whether GLUT9 may function as both a fructose and urate transporter, thus being responsible for urate reabsorption in the context of increased fructose intake. 57,58 Evidence suggests that hyperuricemia risk may be partially programmed in utero. Preeclampsia is associated with increased maternal and fetal SUA levels, believed to occur secondary to endothelial dysfunction and reduced glomerular filtration rate (GFR).…”

Starch digestion in the upper gastrointestinal tract of humans

“…Glucose Transport proteins (GLUTs) are a family of integral membrane proteins that participate in the facilitated diffusion of pentose and hexose sugars. Most mammalian cells express at least one isoform of these proteins, and often also have lower levels of one or more of the remaining 13 isoforms . GLUT2 is classically described as a facilitative transporter located on the basolateral membrane of absorptive small intestinal epithelial cells.…”