Studies on in Vitro Synthesis and Secretion of Human Chorionic Gonadotropin and Its Subunits

Abstract

Human chorionic tissues were cultivated in vitro in the presence of ³H-proline in order to study the synthesis of hCG and its subunits by the placenta. After terminating the culture, tissue extracts and media were individually gel-filtrated on Sephadex G-100. The eluted fractions were radioimmunoassayed for hCG, hCGα and hCGβ and were measured for ³H-radioactivity. Label incorporation was determined by immunoprecipitation. Elution profiles of tissue extracts showed the existence of large immunologic forms of hCG, hCGα and hCGβ emerging near the void volume. The amounts of these large immunologic species in chorionic tissue gradually decreased during the course of cultivation. ³H-proline was almost exclusively incorporated into the large immunologic forms of hCG, hCGα and hCGβ within the chorionic tissue during the 5-hour exposure. A great quantity of hCGα was found in the media after the 3-day culture, while the amount of hCGβ found in the media was minute. After a 15-minute pulse, the ³H-radioactivity peak within the chorionic tissue appeared in tha void volume, coincidental with hCG immunoreactivity. During the chase period, there was a shift of the ³H-radioactivity peak associated with hCG immunoreactivity from the void volume to the more retarded area. In the media until after the 60-minute chase, no labeled hCG and its subunits appeared. Within the media after the 3-hour chase, the hCG peak associated with ³H-radioactivity was more retarded on Sephadex G-100 than that within the tissue extract after the 15-minute pulse. These results suggest that the large immunologic forms of hCG, hCGα and hCGβ are synthesized as the earliest detectable biosynthetic forms and that they may then be converted to small molecule species.
In the culture of molar trophoblastic tissues after a 15-minute pulse, considerable amounts of hCG and its subunits accompanied by high ³H-radioactivity had already been secreted into the media. These observations suggest that protein synthesis by molar trophoblastic tissue is markedly enhanced as compared with that by normal chorionic tissue and that immunoreactive materials synthesized in molar trophoblastic tissue may be secreted more readily than those synthesized in normal chorionic tissue.