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 di fferentiating human trophoblast was studied using the b30 done of the BeWo choriocarcinoma cell line (BeWo(b30)) as a model system. This clo nally derived cell line differentiates over 48-72 h, in culture, to fo rm syncytiotrophoblasts when intracellular cAMP levels are elevated by exposure to 100 mu M forskolin (FSK). IGF receptors were studied at v arious times during the differentiation process by measuring the speci fic binding of [I-125]-IGF-I and [I-125]-IGF-II to attached cells. Fir st, [I-125]-IGF-I bound to a single class of binding sites in the untr eated cells (K-D approximate to 1-2 x 10(-10) M) that exhibited bindin g: specificity characteristic of the type IIGF receptor (IGF-I greater than or equal to IGF-II much greater than Insulin). FSK treatment res ulted in a two- to threefold increase in the number of these binding s ites. Increased receptor expression was observed as early as 24 h afte r FSK treatment and remained elevated for at least 72 h. Next, [I-125] -IGF-II bound to two classes of binding sites in the untreated cells, a high-affinity (K-D approximate to 2.5 x 10-(10) M), low-capacity sit e and a low-affinity (K-D approximate to 6 x 10(-9) M), high-capacity site. The B-max and K-D of the high-affinity site suggested that it re presented the type I IGF receptor. Competition studies revealed that 1 5-20 per cent of total [I-125]-IGF-II binding only was sensitive to IG F-I competition in the untreated cells. After FSK treatment, however, unlabelled IGF-I inhibited 60-70 per cent of specific [I-125]-IGF-II b inding. Scatchard analysis revealed a two- to fourfold increase in the number of both binding sites with no change in their respective bindi ng affinities. Cross-linking analysis demonstrated that [I-125]-IGF-II bound to two structurally distinct binding sites in the untreated BeW o(b30) cell consistent with both the type I and II IGF receptors. Afte r FSK treatment, however, there was an increase in the relative amount of [I-125]-IGF-II associated with the higher affinity type I IGF rece ptor. The BeWo(b30) cells expressed no insulin receptors at any stage of differentiation. These data demonstrate that the BeWo(b30) chorioca rcinoma cell line expresses both type I and II IGF receptors. Inductio n 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 th e type I IGF receptor in regulating differentiated trophoblast cell fu nction. Furthermore, the early rise in type I IGF receptor number sugg ests they may play a regulatory role in the differentiation process it self. (C) 1996 W. B. Saunders Company Ltd