Suzanne Rogers

Molecular and cellular regulation of glucose transporter (GLUT) proteins in cancer

Malignant cells are known to have accelerated metabolism, high glucose requirements, and increased glucose uptake. Transport of glucose across the plasma membrane of mammalian cells is the first rate‐limiting step for glucose metabolism and is mediated by facilitative glucose transporter (GLUT) proteins. Increased glucose transport in malignant cells has been associated with increased and deregulated expression of glucose transporter proteins, with overexpression of GLUT1 and/or GLUT3 a characteristic feature. Oncogenic transformation of cultured mammalian cells causes a rapid increase of glucose transport and GLUT1 expression via interaction with GLUT1 promoter enhancer elements. In human studies, high levels of GLUT1 expression in tumors have been associated with poor survival. Studies indicate that glucose transport in breast cancer is not fully explained by GLUT1 or GLUT3 expression, suggesting involvement of another glucose transporter. Recently, a novel glucose transporter protein, GLUT12, has been found in breast and prostate cancers. In human breast and prostate tumors and cultured cells, GLUT12 is located intracellularly and at the cell surface. Trafficking of GLUT12 to the plasma membrane could therefore contribute to glucose uptake. Several factors have been implicated in the regulation of glucose transporter expression in breast cancer. Hypoxia can increase GLUT1 levels and glucose uptake. Estradiol and epidermal growth factor, both of which can play a role in breast cancer cell growth, increase glucose consumption. Estradiol and epidermal growth factor also increase GLUT12 protein levels in cultured breast cancer cells. Targeting GLUT12 could provide novel methods for detection and treatment of breast and prostate cancer.

Differential expression of GLUT12 in breast cancer and normal breast tissue

Increased and deregulated expression of glucose transporters is a characteristic of cancer cells. We previously identified a novel glucose transporter protein, GLUT12, in the MCF7 malignant breast epithelial cell line. Here we present the first demonstration of GLUT12 expression in human breast tumours. Using immunohistochemistry and reverse transcription polymerase chain reaction, GLUT12 was detected in eight of ten invasive tumours. Ductal cell carcinoma in situ cells also stained strongly for GLUT12. Immunohistochemical staining for GLUT12 in benign ducts was less intense, with few positively stained cells. GLUT12 may have a role in hexose supply to breast cancer cells.

Glucose transporter GLUT12-functional characterization in Xenopus laevis oocytes

We have recently identified and cloned the cDNA of a new member of the glucose transporter family that has been designated GLUT12. GLUT12 possesses the structural features critical to facilitative transport of glucose but the key to understanding the possible physiological roles of this novel protein requires analysis of functional glucose transport. In the current study, we have utilized the Xenopus laevis oocyte expression system to assay transport of the glucose analog 2-deoxy-D-glucose and characterize the glucose transport properties and hexose affinities of GLUT12. Our results demonstrate that GLUT12 facilitates transport of glucose with an apparent preferential substrate affinity for glucose over other hexoses assayed. The results are significant to understanding the potential role and importance of GLUT12 in insulin-sensitive tissues and also cells with high glucose utilization such as cancer cells.

Expression during rat fetal development of GLUT12 - a member of the class III hexose transporter family

Glucose is an essential molecule for most mammalian cells, and is particularly important during fetal development, when cells are rapidly dividing and differentiating. In rats, GLUT1 is present at high levels in most fetal tissues, with levels decreasing after birth. We used immunohistochemistry to localise GLUT12 protein, a recently identified member of the sugar transporter family, and GLUT1 during rat fetal development. GLUT12 staining was observed in heart muscle from gestational days 15 to 21. GLUT12 staining in skeletal muscle increased from gestational days 17 to 21, and GLUT12 was also detected in brown adipose tissue. The expression of GLUT12 in insulin-responsive tissues supports a potential role for GLUT12 in the provision of glucose to these tissues before the appearance of GLUT4. GLUT12 protein was also expressed in fetal chondrocytes from gestational day 15 onward, in kidney distal tubules and collecting ducts from day 19, and in lung bronchioles from day 19. The specific pattern of expression observed in the rat fetus suggests that GLUT12 may be important in hexose delivery to developing tissues.

Evolutionary ancestry and novel functions of the mammalian glucose transporter (GLUT) family

BackgroundIn general, sugar porters function by proton-coupled symport or facilitative transport modes. Symporters, coupled to electrochemical energy, transport nutrients against a substrate gradient. Facilitative carriers transport sugars along a concentration gradient, thus transport is dependent upon extracellular nutrient levels. Across bacteria, fungi, unicellular non-vertebrates and plants, proton-coupled hexose symport is a crucial process supplying energy under conditions of nutrient flux. In mammals it has been assumed that evolution of whole body regulatory mechanisms would eliminate this need. To determine whether any isoforms bearing this function might be conserved in mammals, we investigated the relationship between the transporters of animals and the proton-coupled hexose symporters found in other species.ResultsWe took a comparative genomic approach and have performed the first comprehensive and statistically supported phylogenetic analysis of all mammalian glucose transporter (GLUT) isoforms. Our data reveals the mammalian GLUT proteins segregate into five distinct classes. This evolutionary ancestry gives insight to structure, function and transport mechanisms within the groups. Combined with biological assays, we present novel evidence that, in response to changing nutrient availability and environmental pH, proton-coupled, active glucose symport function is maintained in mammalian cells.ConclusionsThe analyses show the ancestry, evolutionary conservation and biological importance of the GLUT classes. These findings significantly extend our understanding of the evolution of mammalian glucose transport systems. They also reveal that mammals may have conserved an adaptive response to nutrient demand that would have important physiological implications to cell survival and growth.

Retinoic acid stimulates glucose transporter expression in L6 muscle cells

Factors that regulate the tissue specific and developmental expression of the GLUT4 gene, whose transcribed protein is primarily responsible for mediating insulin stimulated glucose transport, are poorly defined. In this study we examined the effects of retinoic acid, a circulating factor that can promote cellular differentiation, on glucose uptake and glucose transporter expression in cultured L6 muscle cells. At the myoblast stage, treatment with 1 microM retinoic acid for 24 h increased both 1 h and 8 h insulin stimulated uptake of 2-deoxyglucose by more than twofold. A dose and time dependent effect of retinoic acid on 8 h insulin stimulated 2-deoxyglucose uptake was observed at both the myoblast and myocyte stage. Comparatively little effect from retinoic acid treatment was found on basal uptake at either stage. In myoblast cells, retinoic acid increased the content of GLUT4 mRNA in a dose and time dependent manner, an effect that was partially attenuated by insulin. In myocytes retinoic acid increased GLUT4 mRNA levels to 2.3 times basal. Nuclear run-on studies indicate that the increased GLUT4 mRNA represents enhanced transcriptional activity. The results suggest a role for retinoic acid in regulation of expression of the GLUT 4 gene in muscle cells.

Expression during rat fetal development of GLUT12 – a member of the class III hexose transporter family

Glucose is an essential molecule for most mammalian cells, and is particularly important during fetal development, when cells are rapidly dividing and differentiating. In rats, GLUT1 is present at high levels in most fetal tissues, with levels decreasing after birth. We used immunohistochemistry to localise GLUT12 protein, a recently identified member of the sugar transporter family, and GLUT1 during rat fetal development. GLUT12 staining was observed in heart muscle from gestational days 15 to 21. GLUT12 staining in skeletal muscle increased from gestational days 17 to 21, and GLUT12 was also detected in brown adipose tissue. The expression of GLUT12 in insulin-responsive tissues supports a potential role for GLUT12 in the provision of glucose to these tissues before the appearance of GLUT4. GLUT12 protein was also expressed in fetal chondrocytes from gestational day 15 onward, in kidney distal tubules and collecting ducts from day 19, and in lung bronchioles from day 19. The specific pattern of expression observed in the rat fetus suggests that GLUT12 may be important in hexose delivery to developing tissues.