Facilitative glucose transporter gene expression in human lymphocytes, monocytes, and macrophages: a role for GLUT isoforms 1, 3, and 5 in the immune response and foam cell formation
Introduction
Glucose transport proteins are the key functional units of the cellular glucose transport effector system. There exists a family of glucose transporter genes (GLUT 1–5) with variable tissue-specific expression, subcellular localization, and kinetics for glucose uptake [1], [2], [3], [4], [5]. However, glucose transporter gene expression in resting and activated leukocyte cells, particularly in lymphocytes and monocytes, has not been elucidated. This question is important for several reasons. First, activated leukocytes will require increased fuel delivery to participate in immune and inflammatory responses, and the biochemical determinants for these processes have not been fully defined. Secondly, macrophage transformation to foam cells is integral to atherosclerosis. The accumulation of lipid in foam cells represents an excess of stored substrate, and the role of glucose metabolism and GLUT isoforms involved in foam cell formation is unknown. Thirdly, leukocytes could be used to study glucose transport defects in relevant diseases, such as in patients with type 2 diabetes and atherosclerosis [6], [7], [8]; however, this will first require characterization of the glucose transport system in leukocytes. Finally, oncogenic transformation of leukocytes will lead to increased needs for fuel oxidation, and knowledge of the glucose transport system is required to understand how glucose transporters participate in this process.
Lymphocytes and monocytes play critical roles in mediating immune and inflammatory responses [9], [10], [11]. Monocytes are normally found in lymph nodes, spleen, bone marrow, and loose connective tissue. Lymphocytes comprise approximately 22–28% of total leukocytes in human blood, while monocytes normally account for 3–7% of circulating leukocytes. These cells can be isolated from human tissues in order to study glucose transporter (GLUT) gene expression. In addition, the THP-1 human monocyte cell line is a well-characterized model system appropriate for the study of differentiation of monocytes into macrophages and the transformation of macrophages to foam cells [12], [13], [14]. THP-1 monocyte cells can be induced in vitro to differentiate into macrophage cells following treatment with phorbol myristate acetate (PMA). The resulting macrophage cells can then be induced to transform into lipid-laden foam cells following treatment with oxidized low-density lipoprotein (oxLDL) [15], [16].
Here, we have determined which glucose transporter genes are expressed in resting and activated human lymphocytes, monocytes, and macrophages, including lipid-laden foam cells. Our data demonstrated that these cells exhibit differential regulation of glucose transporter gene isoforms depending on the cell metabolic activity. Different patterns of GLUT isoform expression characterize activation, differentiation, and transformation in leukocytes indicating that GLUT expression is regulated to meet specific metabolic needs during immune and inflammatory responses.
Section snippets
Reagents
Human monocytic leukemia THP-1 cells were purchased from American Type Culture Collection (ATCC) (Manassas, VA). Tissue culture media were purchased from Life Technologies (Gaithersburg, MD). Phytohemagglutinin (PHA), lipopolysaccharides (LPS), and phorbol myristate acetate (PMA) were purchased from Sigma (St. Louis, MO). RNA isolation solution was purchased from Biotecx Laboratory (Houston, TX). Purified goat polyclonal antibodies to human GLUT 1–5 were purchased from FabGennix, (Shreveport,
GLUT isoform expression in human lymphocytes and monocytes
We first studied GLUT isoform expression in lymphocytes and monocytes under baseline conditions and after nonspecific immune activation. Human lymphocytes and monocytes were isolated and then incubated in vitro with phytohemagglutinin (PHA) for lymphocytes or with lipopolysaccharides (LPS) for monocytes. GLUT protein levels were assessed in basal and activated cells. As shown in Fig. 1, only GLUT 1 and GLUT 3 proteins were detected in human lymphocytes and monocytes. Proteins for other members
Discussion
The present study is the first comprehensive examination of GLUT isoform expression and regulation in human leukocytes. The data demonstrated that GLUT 1 and GLUT 3 are expressed in isolated, resting, human lymphocytes and monocytes, with GLUT 1 protein levels higher in lymphocytes than monocytes and GLUT 3 levels higher in monocytes. Both lymphocytes and monocytes expressed low levels of GLUT 5 mRNA, however, GLUT 5 protein was not detected by immunoblot, indicating that GLUT 5 protein is
Acknowledgements
We are grateful to Dr. Maria F. Lopes-Virella for kindly providing oxLDL. This work was supported by grants from the National Institutes of Health (DK-38764 and PO1-HL55782 to WTG), by the Center for Biomedical Research Excellence at MUSC (NIH # P20 RR16434 to YF), and by the Merit Review program of the Department of Veterans Affairs (to WTG). We also gratefully acknowledge the support of the MUSC General Clinical Research Center (M01-RR-1070).
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