Although vitamin C is critical to human physiology, it is not clear how it is taken up into cells. The kinetics of cell and tissue accumulation of ascorbic acid in vitro indicate that the process is mediated by specific transporters at the cell membrane. Some experimental observations have linked the transport of ascorbic acid with hexose transport systems in mammalian cells, although no clear information is available regarding the specific role(s) of these transporters, if any, in this process. Here we use the Xenopus laevis oocyte expression system to show that the mammalian facilitative hexose transporters are efficient transporters of the oxidized form of vitamin C (dehydroascorbic acid). Two transport pathways, one with low affinity and one with high affinity for dehydroascorbic acid, were found in oocytes expressing the mammalian transporters, and these oocytes accumulated vitamin C against a concentration gradient when supplied with dehydroascorbic acid. We obtained similar results in experiments using normal human neutrophils. These observations indicate that mammalian facilitative hexose transporters are a physiologically significant pathway for the uptake and accumulation of vitamin C by cells, and suggest a mechanism for the accumulation of ascorbic acid against a concentration gradient.

The acute chest syndrome (ACS), a pneumonia-like illness in sickle cell patients, is one of the most frequent causes of their morbidity and hospitalizations. Repeated ACS events may predict the development of chronic lung disease. ACS is reported as a frequent cause of death in these patients. We examine here the incidence and risk factors of ACS in 3,751 patients with sickle cell disease who were observed prospectively for at least 2 years (19,867 patient-years [pt-yrs]) as part of a multicenter national study group. The ACS, defined by a new pulmonary infiltrate on x-ray, occurred at least once in 1,085 patients (2,100 events). ACS incidence was higher in patients with homozygous sickle cell disease (SS; 12.8/100 pt-yrs) and in patients with sickle cell-beta(0) -thalassemic (9.4/100 pt-yrs), and lower in patients with hemoglobin (Hb) SC disease (5.2/100 pt-yrs) and patients with sickle cell-beta(+) thalassemia (3.9/100 pt-yrs). alpha-Thalassemia did not affect the rate of ACS incidence in SS patients. Within each Hb type the incidence was strongly but inversely related to age, being highest in children 2 to 4 years of age (25.3/100 pt-yrs in SS) and decreasing gradually to its lowest value in adults (8.8/100 pt-yrs in SS). In SS children (< 10 years of age), we documented an age-related within- person reduction in ACS attack rates. Adults with a higher ACS rate had a higher rate of mortality (from all causes) than those with low ACS rates. This increased rate of mortality might also have contributed to the decline in ACS rate with age. In multivariate analysis, other factors affecting incidence in SS patients were degree of anemia (lower ACS rates in patients with lower steady-state Hb levels) and fetal Hb (lower rates in patients with high fetal Hb). There was also a positive association between ACS rate and steady-state leukocyte count. The relationship of ACS rate to higher steady-state Hb levels in SS patients is unexplained but might be caused by increased blood viscosity.

It has been proposed that the enhanced metabolic activity of tumor cells is accompanied by an increased expression of facilitative hexose transporters (GLUTs). However, a previous immunohistochemical analysis of GLUT1 expression in 154 malignant human neoplasms failed to detect the GLUT1 isoform in 87 tumors. We used 146 normal human tissues and 215 tumor samples to reassess GLUT1 expression. A similar number of samples were used to compare the expression of GLUT2-6 and 9. The classical expression of GLUT1-5 in different normal human tissues was confirmed, however, we were unable to detect GLUT2 in human pancreatic islet cells. GLUT6 was principally detected in testis germinal cells and GLUT9 was localized in kidney, liver, heart, and adrenal. In tumor samples, GLUT1, 2, and 5 were the main transporters detected. GLUT1 was the most widely expressed transporter, however, 42% of the samples had very low-to-negative expression levels. GLUT2 was detected in 31% of the samples, being mainly expressed in breast, colon, and liver carcinoma. GLUT5 was detected in 27% of breast and colon adenocarcinoma, liver carcinoma, lymphomas, and testis seminoma samples. In situ RT-PCR and ultrastructural immunohistochemistry confirmed GLUT5 expression in breast cancer. GLUT6 and 9 are not clearly over-expressed in human cancer. The extensive expression of GLUT2 and 5 (glucose/fructose and fructose transporters, respectively) in malignant human tissues indicates that fructose may be a good energy substrate in tumor cells. Our functional data obtained in vitro in different tumor cells support this hypothesis. Additionally, these results suggest that fructose uptake could be used for positron emission tomography imaging and, may possibly represent a novel target for the development of therapeutic agents in different human cancers.

Vitamin C concentrations in the brain exceed those in blood by 10-fold. In both tissues, the vitamin is present primarily in the reduced form, ascorbic acid. We identified the chemical form of vitamin C that readily crosses the blood-brain barrier, and the mechanism of this process. Ascorbic acid was not able to cross the blood-brain barrier in our studies.

The primary metabolic characteristic of malignant cells is an increased uptake of glucose and its anaerobic metabolism. We studied the expression and function of the glucose transporters in human breast cancer cell lines and analyzed their expression in normal and neoplastic primary human breast tissue. Hexose uptake assays and immunoblotting experiments revealed that the breast carcinoma cell lines MCF-7 and MDA-468 express the glucose transporters GLUT1 and GLUT2, isoforms expressed in both normal and neoplastic breast tissue. We also found that the breast cancer cell lines transport fructose and express the fructose transporter GLUT5. Immunolocalization studies revealed that GLUT5 is highly expressed in vivo in human breast cancer but is absent in normal human breast tissue. These findings indicate that human breast cancer cells have a specialized capacity to transport fructose, a metabolic substrate believed to be used by few human tissues. Identification of a high-affinity fructose transporter on human breast cancer cells opens opportunities to develop novel strategies for early diagnosis and treatment of breast cancer.