Insulin Receptors of Human Term Placental Cells and Choriocarcinoma (JEG-3) Cells: Characteristics and Regulation*

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Abstract
Insulin has been implicated as a regulator of placental growth and function. In order to probe hormoneplacental interactions, an in vitro human placental cell culture model was employed in conjunction with the human choriocarcinoma cell line, JEG-3. Short term monolayer cultures of human term placentae were established from trypsinized chorionic villi and were presumed to be predominantly of syncytial origin on the basis of their electron microscopic morphology, their capacity to produce placental hormones, and their loss of replicative potential. Specific binding of [125I]iodoinsulin to both cell types was examined using whole cell radioreceptor assays, and was found to be a saturable process, time and temperature dependent, and proportional to cell protein and ligand concentration. At 4 C, a steady state was reached by 18 h and was maintained for 6 h. Specificity of the insulin receptor was evidenced by the inability of structurally unrelated hormones to inhibit [125I]iodoinsulin binding. Scatchard plots derived from equilibrium inhibition dose-response curves of [125I]iodoinsulin binding were curvilinear for both cell types, indicating retention of presumed site heterogeneity. The affinity constants obtained from Scatchard analysis were similar to those reported for [125I]iodoinsulin binding to placental membranes and other known targets of insulin action. Dissociation of [125I]iodoinsulin at 4 C was time dependent and accelerated in the presence of unlabeled insulin. In the normal placental cells, maximal specific binding was seen the first 2–4 days of culture and declined shortly thereafter. In contrast, [125I]iodoinsulin specific binding to JEG-3 cells increased progressively as the cells reached confluence. Preincubation of normal placental cell monolayers or JEG-3 monolayers with unlabeled insulin resulted in a dose-, time-, and temperature-dependent down-regulation which was reversible. However, both the rate and magnitude of the JEG-3 downregulation response was greater than the term cell response. These findings suggest that cultured human placental cells and JEG-3 cells provide a suitable system for determining the effects of insulin and possibly other growth factors on placental function, for evaluating the correlation between receptor regulation and placental metabolism, and for comparing receptor function and regulation in normal vs. tumor placentae.