Human choriogonadotropin entrapped into liposomes: characterization, biologic effects and interaction with purified...

Liposomes containing human choriogonadotropin (hCG) were prepared from phosphatidylserine by the ether injection method. hCG adsorbed to the outer surface of the liposomes (77% of total liposome-associated hCG) was removed by proteolytic digestion with subtilisin. hCG-containing liposomes digested and not digested with subtilisin stimulated testosterone biosynthesis by Leydig cells in a dose-dependent way; both preparations had identical biologic activities (32% of the activity of free, not liposome-associated hCG) when equal doses of liposome-associated hCG were applied. The onset of stimulation was delayed when compared to the action of free hCG. Liposomes without hCG did not stimulate testosterone biosynthesis. Association of liposomes with Leydig cells was determined by measurement of transfer of radioactive label from liposomes to Leydig cells. The association was not mediated by the hormone receptor. hCG entrapped in liposomes was incorporated by Leydig cells and translated to the cellular surface. This process was impaired by colchicine (10(-5) M). hCG translocated to the external surface of the cell membrane contained a modified alpha-subunit (Mr 16,200 instead of 20,600) which was not detected in unentrapped hCG bound to Leydig cells. We suggest that liposomally entrapped hCG is taken up by Leydig cells and re-exported to the cell membrane by a mechanism resembling retroendocytosis.