Type I insulin-like growth factor receptors in the BeWo choriocarcinoma cell (b30 clone) during cell differentiation

The expression of insulin-like growth factor (IGF) receptors in the differentiating human trophoblast was studied using the b30 clone of the BeWo choriocarcinoma cell line (BeWo^b30) as a model system. This clonally derived cell line differentiates over 48-72 h, in culture, to form syncytiotrophoblasts when intracellular cAMP levels are elevated by exposure to 100 μM forskolin (FSK). IGF receptors were studied at various times during the differentiation process by measuring the specific binding of [¹²⁵I]-IGF-I and [¹²⁵I]-IGF-II to attached cells. First, [¹²⁵I]-IGF-I bound to a single class of binding sites in the untreated cells (K(D) ≃ 1-2 x 10^-10 M) that exhibited binding specificity characteristic of the type I IGF receptor (IGF-I ≤ IGF-II >> Insulin). FSK treatment resulted in a two- to threefold increase in the number of these binding sites. Increased receptor expression was observed as early as 24 h after FSK treatment and remained elevated for at least 72 h. Next, [¹²⁵I]-IGF-II bound to two classes of binding sites in the untreated cells, a high-affinity (K(D) ≃ 2.5 x 10-¹⁰ M), low-capacity site and a low-affinity (K(D) ≃ 6 x 10^-9 M), high-capacity site. The B(max) and K(D) of the high-affinity site suggested that it represented the type I IGF receptor. Competition studies revealed that 15-20 per cent of total [¹²⁵I]-IGF-II binding only was sensitive to IGF-I competition in the untreated cells. After FSK treatment, however, unlabelled IGF-I inhibited 60-70 per cent of specific [¹²⁵I]-IGF-II binding. Scatchard analysis revealed a two- to fourfold increase in the number of both binding sites with no change in their respective binding affinities. Cross-linking analysis demonstrated that [¹²⁵I]-IGF-II bound to two structurally distinct binding sites in the untreated BeWo^b30 cell consistent with both the type I and II IGF receptors. After FSK treatment, however, there was an increase in the relative amount of [¹²⁵I]-IGF-II associated with the higher affinity type I IGF receptor. The BeWo^b30 cells expressed no insulin receptors at any stage of differentiation. These data demonstrate that the BeWo^b30 choriocarcinoma cell line expresses both type I and II IGF receptors. Induction of cell differentiation is associated with an increase in type I IGF receptors expressed at the cell surface. These receptors bind IGF-II with high-affinity, providing additional binding capacity for locally available IGF-II. These data are consistent with specific roles for the type I IGF receptor in regulating differentiated trophoblast cell function. Furthermore, the early rise in type I IGF receptor number suggests they may play a regulatory role in the differentiation process.