GLUT1 glucose transporter gene transcription is repressed by Sp3. Evidencefor a regulatory role of Sp3 during myogenesis

GLUT1 glucose transporters are highly expressed in proliferating and transf ormed cells as well as in tissues during fetal life. However, the mechanism s that regulate GLUT1 gene expression remain largely unknown. Here, we demo nstrate that Sp3 proteins bind to the GLUT1 proximal promoter Gene and inhi bit transcriptional activity in muscle and non-muscle cells. Two different Sp3 translational products (110 and 74 kDa) derived from differential trans lational initiation were detected in nuclear extracts from myoblast cells, and both Sp3 protein species inhibited GLUT1 gene transcriptional activity. The inhibitory effect of Sp3 was dominant over the stimulatory effect of S pl on transcriptional activity of GLUT1 gene. Furthermore, abolition of Sp3 binding to the proximal promoter of GLUT1 gene completely blocked the resp onse to Sp3. We provide evidence that the expression of Sp3 protein is subject to regula tion:in muscle cells and that this is likely to control GLUT1. Thus, Sp3 pr otein was up-regulated in the absence of changes in Spl early after the ind uction of IGF-II-dependent myogenesis. Furthermore, forced overexpression o f MyoD caused an enhancement in the cellular Sp3/Sp1 ratio which was concom itant to a reduced GLUT1 expression. Later during myogenesis, Sp3 expressio n was substantial whereas Spl was markedly down-regulated. In summary, we provide direct evidence that the transcription factor Sp3 re presses gene expression in non-muscle and muscle cells and this is likely t o operate in fetal heart by binding to the GLUT1 gene promoter. This is the first description of a repressor of GLUT1 gene transcription. Furthermore, we propose that variations in the ratio of Sp3 versus Spl regulate GLUT1 p romoter activity and this is crucial in the down-regulation of GLUT1 associ ated to myogenesis. (C) 1999 Academic Press.