The facilitative glucose transporter GLUT12: what do we know and what would we like to know?
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MINI REVIEW
The facilitative glucose transporter GLUT12: what do we know and what would we like to know? Jonai Pujol-Giménez & Jaione Barrenetxe & Pedro González-Muniesa & Maria Pilar Lostao
Received: 11 May 2012 / Accepted: 12 September 2012 / Published online: 3 October 2012 # University of Navarra 2012
Introduction Glucose, one of the most abundant molecules in nature, is used by most of the mammalian cells as their main energy source. Obtained from the diet, glucose is absorbed in the small intestine, incorporated into the circulating blood and stored as glycogen, mainly in the liver and muscle. Due to its hydrophilic nature, glucose cannot cross the plasma membrane by simple diffusion; instead, glucose enters the cell by specific membrane transporters. There are two families of glucose transporters, distinguished by their functional and structural properties: the Na+/glucose cotransporter family SGLT/SLC5A [47] and the facilitative glucose transporter family GLUT/SLC2A [11, 22]. GLUT protein family members transport monosaccharides across the plasma membrane without energetic requirement, using the favourable concentration gradient of the hexose generated in some physiological situations. GLUTs share a common structural feature of 12 transmembrane domains, with both amino and carboxy terminal domains located on the cytosolic side, and an Pedro González-Muniesa and Maria Pilar Lostao have contributed equally to this review. J. Pujol-Giménez : J. Barrenetxe : P. González-Muniesa : M. P. Lostao (*) Department of Nutrition Food Science and Physiology, University of Navarra, Irunlarrea 1, 31008 Pamplona, Spain e-mail: [email protected]
N-linked oligosaccharide site present either on the first or on the fifth extracellular loop [37]. At present, 14 different members of this family, divided in three classes according to its sequence homology, have been identified [11]. Class I is constituted by the well-characterized GLUT1, GLUT2, GLUT3, GLUT4 and GLUT14 (gene duplication of GLUT3); class II comprises the fructose transporter GLUT5, and GLUT7, GLUT9 and GLUT11; and class III includes GLUT6, GLUT8, GLUT10, GLUT12 and GLUT13 (HMIT) [12]. In relation to this classification, a recent phylogenetic analysis proposes that the proteins belonging to class III could be separated into three different groups and, therefore, suggests five structurally and/or functionally distinct GLUT classes (Fig. 1) [43]. Location, expression and regulation of the GLUT transporters are specific for each tissue and cellular type and are related to the cell metabolic needs. In many cases, the up- or downregulation of the GLUT proteins is directly linked to the development of diseases (Table 1). GLUT1, 2, 3, 4 and 5 were the first members of the GLUT family cloned and their physiological function has been well characterized. GLUT9 seems to be a urate transporter, newly described as electrogenic [44], while GLUT13 (HMIT) is a H+ -dependent myoinositol cotransporter [36]. However, the physiological significance of the rest of GLUT transporters of cl
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