Expression and regulation of glucose transporter 8 in rat Leydig cells

Abstract

Basal and LH/human chorionic gonadotropin (hCG)-stimulated testosterone formation by Leydig cells is dependent on ambient glucose levels. Inhibition of glucose uptake is associated with decreased testosterone formation. Recently, glucose transporter 8 (GLUT8) has been shown to be highly expressed in the testis. In the present study, we have investigated the expression and regulation of the GLUT8 gene in rat Leydig cells. Primers were designed by using sequences that are not conserved in GLUT1 to GLUT5 and that contain the glycosylation region of GLUT8. This yielded an amplicon of 186 bp. The tIssue-specific expression experiments in adult rat (55- to 65-day-old) tIssues revealed that GLUT8 is expressed predominantly in the testis, in smaller amounts in heart and kidney, and in negligible amounts in liver and spleen. Furthermore, GLUT8 mRNA was found to be highly expressed in crude interstitial cells, Leydig cells and testicular and epididymal germ cells. In prepubertal rat (20-day-old) tIssues, GLUT8 expression was comparatively much lower than in the adult rat tIssues. By comparative RT-PCR, hCG caused dose- and time-dependent increases of GLUT8 mRNA levels. hCG and IGF-I had synergistic effects on GLUT8 mRNA and protein expression. GLUT1 and GLUT3 were also found to be expressed in Leydig cells. However, neither GLUT1 nor GLUT3 were affected by treatments with hCG, IGF-I or hCG and IGF-I combined. The addition of murine interleukin-1alpha (mIL-1alpha; 10 ng/ml), murine tumor necrosis factor-alpha (mTNF-alpha; 10 ng/ml), murine interferon-gamma (mIFN-gamma; 500 U/ml) separately or in combination decreased hCG-induced GLUT8 mRNA levels significantly. In conclusion, GLUT8 mRNA in Leydig cells was positively regulated by hCG and IGF-I and down-regulated by cytokines, mIL-1alpha, mTNF-alpha and mIFN-gamma. These results indicate that hCG, growth factors and cytokines affect Leydig cell steroidogenesis by modulating GLUT8 expression.