HYPOXIA AND MITOCHONDRIAL INHIBITORS REGULATE EXPRESSION OF GLUCOSE TRANSPORTER-1 VIA DISTINCT CIS-ACTING SEQUENCES

Studies of gene regulation by oxygen have recently defined the existen ce of a widely operative system that responds to hypoxia but not mitoc hondrial inhibitors and involves the induction of a DNA-binding comple x termed hypoxia-inducible factor 1. This system has been implicated i n the regulation of erythropoietin, certain angiogenic growth factors, and particular glycolytic isoenzymes. The glucose transporter Glut-1 is induced by both hypoxia and mitochondrial inhibitors, implying the operation of a different mechanism of oxygen sensing. To explore that possibility, we analyzed the cis-acting sequences that convey these re sponses. An enhancer lying 5' to the mouse Glut-1 gene was found to co nvey responses both to hypoxia and to the mitochondrial inhibitors, az ide and rotenone. However, detailed analysis of this enhancer demonstr ated that distinct elements responded to hypoxia and the mitochondrial inhibitors. The response to hypoxia was mediated by sequences that co ntained a functionally critical, although atypical, hypoxia-inducible factor 1 binding site, whereas sequences lying approximately 100 nucle otides 5' to this site, which contained a critical serum response elem ent, conveyed responses to the mitochondrial inhibitors. Thus, rather than reflecting an entirely different mechanism of oxygen sensing, reg ulation of Glut-1 gene expression by hypoxia and mitochondrial inhibit ors arises from the function of two different sensing systems. One of these responds to hypoxia alone and resembles that involved in erythro poietin regulation, while the other responds to mitochondrial inhibito rs and involves activation of a serum response element.