Binding sites for epidermal growth factor in human fetal membranes

The present study demonstrates that human fetal membranes bind 125I-epidermal growth factor (125I-EGF), with chorion binding more than amnion. The chorion binding was also higher than that by decidua, but lower than in placenta. The lower binding by chorion compared to placenta was entirely attributable to a lower number of available EGF receptors and not to lower affinity. Chorion and placenta , but not amnion or decidua, from Cesarean section bound significantly more 125I-EGF when compared to tissues obtained after vaginal delivery. The binding of 125I-EGF to chorion exhibited dependency on time, temperature of incubation, pH of the incubation media, and amount of chorion protein. The 125I-EGF was not degraded during the binding reaction with chorion. The binding was specific in that unlabeled EGF inhibited 125I-EGF binding in a dose-dependent manner, whereas very high concentrations of other unlabeled hormones and growth factors had minimal effects on 125I-EGF binding. When some of these other hormones were tested for binding as tracers (125I-hCG, [3H]prostaglandin E1 and F2 alpha), neither chorion, amnion, decidua, nor placenta specifically bound these ligands. The 125I-EGF specific binding to chorion was saturable and a Scatchard plot of this data was curvilinear, which appears to be due to negative cooperativity. The apparent dissociation constant calculated from the initial slope of the plot was 0.20 nM, which is in excellent agreement with the concentration of unlabeled EGF required for half maximal inhibition of 125I-EGF binding, 0.26 nM. Autoradiography at the light microscope level revealed the presence of silver grains in amnion and chorion only when excess unlabeled EGF addition was withheld. The quantification of grains revealed the presence of a significantly (P less than 0.01) greater number of grains in chorion than in amnion, supporting the conclusion of the binding data. The physiological significance of EGF binding to fetal membranes and decidua is not known, but the considerable amount of EGF binding reported here suggests that they are target tissues for EGF.