Regulation of the Fructose Transporter Gene Slc2a5 Expression by Glucose in Cultured Microglial Cells

Abstract

Microglia play a role in the regulation of metabolism and pathogenesis of obesity. Microglial activity is altered in response to changes in diet and the body’s metabolic state. Solute carrier family 2 member 5 (Slc2a5) that encodes glucose transporter 5 (GLUT5) is a fructose transporter primarily expressed in microglia within the central nervous system. However, little is known about the nutritional regulation of Slc2a5 expression in microglia and its role in the regulation of metabolism. The present study aimed to address the hypothesis that nutrients affect microglial activity by altering the expression of glucose transporter genes. Murine microglial cell line SIM-A9 cells and primary microglia from mouse brain were exposed to different concentrations of glucose and levels of microglial activation markers and glucose transporter genes were measured. High concentration of glucose increased levels of the immediate-early gene product c-Fos, a marker of cell activation, Slc2a5 mRNA, and pro-inflammatory cytokine genes in microglial cells in a time-dependent manner, while fructose failed to cause these changes. Glucose-induced changes in pro-inflammatory gene expression were partially attenuated in SIM-A9 cells treated with the GLUT5 inhibitor. These findings suggest that an increase in local glucose availability leads to the activation of microglia by controlling their carbohydrate sensing mechanism through both GLUT5-dependent and –independent mechanisms.

Glucose Stimulates Glial Cell Line-Derived Neurotrophic Factor Gene Expression in Microglia through a GLUT5-Independent Mechanism

“…It was reported that microglia are responsive to changes in local glucose levels. For example, microglia alter their morphology and expression levels of inflammatory and activation markers following a change in glucose concentration in a culture medium in vitro [20][21][22][23][24]29]. We found that high concentration of glucose exerts a stimulatory effect on Gdnf mRNA in SIM-A9 cells in the present study.…”

“…Previous studies reported that fructose induces pro-inflammatory response in microglial BV-2 cells, suggesting that a direct action of fructose increases microglial activity [53,54]. Moreover, glucose increases levels of GLUT5-encoding Slc2a5 mRNA in SIM-A9 cells and glucose-induced pro-inflammatory gene expression is attenuated in the presence of GLUT5 inhibitor [29]. These findings raise the possibility that glucose stimulates Gdnf and Manf gene expression through its conversion to fructose.…”

“…To determine the effect of glucose on neurotrophic factor gene expression in SIM-A9 cells, levels of Gdnf and Manf mRNA were compared between cells exposed to 17.5 mM glucose, which is required for the maintenance of SIM-A9 cells [34], and 25 mM glucose. We recently showed that a transition of glucose concentration from 17.5 mM to 25 mM induces expression of gene and protein markers of microglia activation in SIM-A9 cells [29]. Exposure to 25 mM glucose significantly increased Gdnf mRNA levels at 40 min (63.0% increase), 2 h (642.5% increase), 4 h (314.7% increase), 6 h (337.3% increase), and 24 h (236.5% increase) compared to the control 17.5 mM glucose (Figure 2C).…”

“…The immortalized mouse microglial cell line SIM-A9 (ATCC ® , CRL-3265™) cells were maintained in a DMEM/F-12 supplemented with 10% FBS (A3160702, Gibco), 5% heat inactivated horse serum (H1138, Sigma-Aldrich, St. Louis, MO, USA), 50 U/mL penicillin, 50 µg/mL streptomycin and 100 µg/mL neomycin (P4083, Sigma-Aldrich) as described previously [29]. To determine the time-dependent effect of glucose, cells were seeded in 12-well plates at 10 6 cells/well and incubated in a maintenance medium containing 17.5 mM glucose for 24 h followed by incubation in a culture medium containing either 17.5 glucose (control) or 25 mM glucose for various periods of time (40 min, 2, 4, 6 or 24 h).…”

“…Fructose feeding leads to an increased expression of GLUT5 in the brain and an increased activity of microglia [27,28]. We recently found that levels of GLUT5-encoding Slc2a5 mRNA are up-regulated in microglia when glucose concentration is raised, and glucose-induced pro-inflammatory gene expression is attenuated by GLUT5 inhibition [29]. These findings suggest that microglia serve as an important component of the hypothalamic glucose-sensing mechanism by altering their activity in response to fluctuations of glucose levels partly via GLUT5.…”