Evidence that forskolin binds to the glucose transporter of human erythrocytes

Binding of [4-³H]cytochalasin B and [12-³H]forskolin to human erythrocyte membranes was measured by a centrifugation method. Glucose-displaceable binding of cytochalasin B was saturable, with K(D) = 0.11 μM, and maximum binding ≃ 550 pmol/mg of protein. Forskolin inhibited the glucose-displaceable binding of cytochalasin B in an apparently competitive manner, with K(I) = 3 μM. Glucose-displaceable binding of [12-³H]forskolin was also saturable, with K(D) = 2.6 μM and maximum binding ≃ 400 pmol/mg of protein. The following compounds inhibited binding of [12-³H]forskolin and [4-³H]cytochalasin B equivalently, with relative potencies parallel to their reported affinities for the glucose transport system: cytochalasins A and D, dihydrocytochalasin B, L-rhamnose, L-glucose, D-galactose, D-mannose, D-glucose, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, phloretin, and phlorizin. A water-soluble derivative of forskolin, 7-hemisuccinyl-7-desacetylforskolin, displaced equivalent amounts of [4-³H]cytochalasin B or [12-³H]forskolin. Rabbit erythrocyte membranes, which are deficient in glucose transporter, did not bind either [4-³H]cytochalasin B or [12-³H]forskolin in a glucose-displaceable manner. These results indicate that forskolin, in concentrations routinely employed for stimulation of adenylate cyclase, binds to the glucose transporter. Endogenous ligands with similar specificities could be important modulators of cellular metabolism.