Regulation of the rate of glucose transport across plasma membranes is of fundamental importance to normal cell metabolism, glucose homeostasis, and hyperglycemia of diabetes. Expression of the ubiquitous GLUT1 glucose transporter is augmented by several stimuli including serum, thyroid hormone, rise in cytosolic calcium, hypoxia, and inhibition of oxidative phosphorylation. The investigators have shown that the increase in cell GLUT1 mRNA content in response to inhibition of oxidative phosphorylation by azide is mediated by both enhanced transcription and decreased degradation of GLUT1 mRNA. Recently the investigators have made the novel observation that GLUT1 expression in a number of cell lines is markedly augmented by cobalt(II) chloride [Co(II)], an agent which stimulates the expression of hypoxia-responsive genes; members of this gene family are upregulated by hypoxia but not by inhibitors of oxidative phosphorylation. Hence, GLUT1 appears to be the first example of a gene whose expression is regulated in a dual fashion by hypoxia, namely by a reduction in oxygen concentration per se, and by the attendant inhibition of oxidative phosphorylation. The investigators have also found that different regions of the GLUT1 promoter/5'-flanking region are necessary for the transcriptional response to azide and Co(II). In addition, a 480-bp DNA segment located approximately 3.5 kbp upstream of the GLUT1 transcription start-site functions as an """"""""enhancer"""""""", is essential for the response to Co(II), and contains an hypoxia-inducible-element (HIE). More recently the investigators have identified a 666-bp region located approximately 6 kbp upstream of the transcription start-site which appears to be responsible for the stimulation o GLUT1 gene transcription in response to azide. Based on the above findings, studies are proposed in this continuation grant application to test the following hypotheses: 1) Different elements in the GLUT1 promoter region mediate the transcriptional response to azide and Co(ll); 2) Sequences contained in GLUT1 mRNA and specific RNA binding factors mediate the observed stabilization of GLUT1 mRNA in cells treated with azide, calcium ionophores, and anisomycin. Results of these studies will enhance our understanding of the regulation of the GLUTl glucose transporter expression in a number of physiological and pathophysiological conditions, especially those of the adaptive response to hypoxia and to inhibition of oxidative phosphorylation.
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