J. Biol. Chem., Vol. 262, Issue 30, 14571-14575, 10, 1987
Evidence that forskolin binds to the glucose transporter of human erythrocytes
VR Lavis, DP Lee and S Shenolikar
Department of Internal Medicine, University of Texas Medical School at Houston 77225.
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 KD = 0.11 microM, and maximum
binding approximately 550 pmol/mg of protein. Forskolin inhibited the
glucose-displaceable binding of cytochalasin B in an apparently competitive
manner, with K1 = 3 microM. Glucose- displaceable binding of
[12-3H]forskolin was also saturable, with KD = 2.6 microM and maximum
binding approximately equal to 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.
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