3.2. Class II GLUTs
Class II consists of GLUTs 5, 7, 9 and 11. GLUT5 is a fructose transporter protein with expression across many species [49]. According to the NCBI Gene and Protein databanks, human GLUT5 is thought to be a cytochalasin β-sensitive carrier with expression in human testis, spermatozoa, small intestine [49], adipose tissue and skeletal muscle [50]. More recent RNA-seq analyses found human GLUT5 expression in duodenum, bone marrow and kidney [51]. GLUT5 was found to have an association with malignant clear renal cell carcinoma [52]. According to the NCBI Gene database [32,37], orthologs of glut5/slc2a5 are found so far in 123 organisms across chicken, dog, cow, chimpanzee, Rhesus monkey, mouse, rat and X. tropicalis. Chicken GLUT5 has been shown to have mRNA expression in the small intestine [53] and may be regulated by glucocorticoids [54].
GLUT7 has been identified as a high affinity transporter for glucose and fructose. GLUT7 does not transport galactose, 2-deoxyglucose or xylose [55]. Human GLUT7 has expression in the small intestine and colon, with lower expression levels in the testis and prostate [55]. Based on our searches of the NCBI Gene and Protein Database and UniProt Database [37,46,56], there are no data for GLUT7 in chickens or other avian species, suggesting that the avian lineage has lost slc2a7 during evolution. Orthologs of slc2a7 are conserved in 55 organisms across mouse, rat, chimpanzee and Rhesus monkey, according to the NCBI Gene Database.
GLUT9 is a known transporter of fructose and urate and can transport glucose at a low rate. Mammalian GLUT9 plays a regulatory role in the development and survival of cartilage chondrocytes and may have a role in urate reabsorption by proximal tubules [57,58]. One study linked gout to GLUT9 deficiency in a population of Japanese males [59]. It is assumed that chicken GLUT9 mediates uric acid uptake, although substrate specificity for this GLUT transporter has not yet been identified [33]. Liver mRNA expression of GLUT9 was shown to be greater in obese chickens, possibly due to having a larger glucose uptake capacity with greater demand and glucose load in high bodyweight chickens [33]. Based on the NCBI Gene Database [32,37], two transcript variants with distinct isoforms have been identified for glut9/slc2a9. Orthologs of glut9/slc2a9 are present in 153 organisms including chicken, dog, cow, mouse, rat, chimpanzee, X. tropicalis and X. laevis.
According to the NCBI Gene Database, GLUT11 is also known as GLUT10. GLUT11 is a transporter of glucose and fructose, but does not transport galactose in humans. GLUT11 has roughly 42% amino acid sequence similarity to GLUT5 and 35% similarity to GLUT1 [60]. Alternative splicing results in multiple transcript variants, including GLUT11-A, GLUT11-B and GLUT11-C [61]. Mammalian GLUT11-A has expression in skeletal muscle, heart and kidney. Mammalian GLUT11-B is expressed in adipose tissue, kidney and placenta. Mammalian GLUT11-C has expression in skeletal muscle, heart, adipose tissue and pancreas [62]. Based on NCBI RefSeq, there is also evidence of a fourth GLUT11 isoform, known as GLUT11-D [46]. Human glut11/slc2a11 orthologs are present in 111 organisms and conserved across chicken, dog, cow, chimpanzee, Rhesus monkey, zebrafish and X. tropicalis, based on the NCBI Gene Database [37]. Rats and mice lack the glut11/slc2a11 gene [62].