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

17beta-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 (K-i(ED/FAS) = 2.84 +/- 0.16 muM and K-i(ED/TAM) = 100 +/- 2 nM). Faslodex has no significant inhibitory effect on glucose exit, but tamoxifen alone inhibits glucose exit (K-i(TAM) = 300 +/- 100 nM). In ghosts, ATP (1-4 mM) competitively antagonizes oestradiol, genistein and cytochalasin B (CB)-dependent inhibitions of glucose exit, (K-i(ATP/ED) = 2.5 +/- 0.23 mM, K-i(ATP/GEN) = 0.99 +/- 0.17 mM and K-i(ATP/CB) = 0.76 +/- 0.08 mM). Tamoxifen and faslodex reverse oestradiol-dependent inhibition of glucose exit with ATP > 1 mM (K-i(ED/TAM) = 130 +/- 5 nM and K-i(ED/FAS) = 2.7 +/- 0.9 muM). The cytoplasmic surface of the glucose transporter (GLUT)l contains four sequences with close homologies to sequences in the ligand-binding domain of human oestrogen receptor beta (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 His(337), Arg(349) and Glu(249) at the cytoplasmic entrance to the hydrophilic pore spanning GLUT, which have a similar topology to His(475), Glu(305) and Arg(346) 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.