Interactions of ATP, oestradiol, genistein and the anti-oestrogens, faslodex (ICI 182780) and tamoxifen, with the human erythrocyte glucose transporter, GLUT1

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Abstract
17β-Oestradiol (ED when subscript to K) and the phytoestrogen isoflavone genistein (GEN) inhibit glucose transport in human erythrocytes and erythrocyte ghosts. The selective oestrogen receptor modulators or anti-oestrogens, faslodex (ICI 182780) (FAS) and tamoxifen (TAM), competitively antagonize oestradiol inhibition of glucose exit from erythrocytes (Ki(ED/FAS) = 2.84±0.16μM and Ki(ED/TAM) = 100±2nM). Faslodex has no significant inhibitory effect on glucose exit, but tamoxifen alone inhibits glucose exit (Ki(TAM) = 300±100nM). In ghosts, ATP (1–4mM) competitively antagonizes oestradiol, genistein and cytochalasin B (CB)-dependent inhibitions of glucose exit, (Ki(ATP/ED) = 2.5±0.23mM, Ki(ATP/GEN) = 0.99±0.17mM and Ki(ATP/CB) = 0.76±0.08mM). Tamoxifen and faslodex reverse oestradiol-dependent inhibition of glucose exit with ATP>1mM (Ki(ED/TAM) = 130±5nM and Ki(ED/FAS) = 2.7±0.9μM). The cytoplasmic surface of the glucose transporter (GLUT)1 contains four sequences with close homologies to sequences in the ligand-binding domain of human oestrogen receptor β (hesr-2). One homology is adjacent to the Walker ATP-binding motif II (GLUT1, residues 225–229) in the large cytoplasmic segment linking transmembrane helices 6 and 7; another GLUT (residues 421–423) contains the Walker ATP-binding motif III. Mapping of these regions on to a three-dimensional template of GLUT indicates that a possible oestrogen-binding site lies between His337, Arg349 and Glu249 at the cytoplasmic entrance to the hydrophilic pore spanning GLUT, which have a similar topology to His475, Glu305 and Arg346 in hesr-2 that anchor the head and tail hydroxy groups of oestradiol and genistein, and thus are suitably placed to provide an ATP-sensitive oestrogen binding site that could modulate glucose export.

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