2 MONOCLONAL-ANTIBODIES AGAINST PROLACTIN-RECEPTOR ARE INTERNALIZED IN EPITHELIAL MAMMARY CELLS WITHOUT MIMETIC PROLACTIN EFFECT ON CASEIN SECRETION

Prolactin exerts an early stimulatory effect on casein secretion which was qualified as a secretagogue effect. After binding to its receptor , the hormone transits intracellularly through the mammary epithelial cell. When this transit is slowed down the secretagogue effect does no t occur. Different monoclonal antibodies which bind to the rabbit prol actin receptor have been previously developed. One of them (A917) mimi cs prolactin effect on casein gene expression. Another (M110) blocks t his prolactin effect. In order to study the respective role of the hor mone and its receptor, we have examined the binding of the two monoclo nal antibodies (M110 and A917), labeled with biotin or colloidal gold, to the receptor of lactating rabbit mammary epithelial cells in incub ation. Subsequently, the intracellular movement of these antibodies an d the secretory response have been measured. Irrespective of the label ing (biotin or colloidal gold) or the preparation of tissues (fragment s or enzymatically dissociated cells), M110 and A917 bound to the basa l membrane of mammary epithelial cells. However, only M110 bound to ap ical membrane of dissociated cell when this membrane was in direct con tact with the incubation medium, showing that the two antibodies discr iminate the receptor located on the apical membrane. Following interna lization, each antibody was carried via a peculiar pathway. M110 remai ned associated with the cells during a l-h incubation, mainly in endos omes, multivesicular bodies and lysosomes Like vesicles. In contrast, A917 was very quickly detectable in endosomes, multivesicular bodies a nd vesicles of the Golgi region and was carried throughout the cell to the lumen of the acini. M110 and A917 were extremely rare in secretor y vesicles containing casein micelles. When mammary fragments pulse la beled for 3 min with H-3-leucine were chased for 60 min in the presenc e of prolactin, M110 or A917, only prolactin was able to increase case in secretion. These results show that two monoclonal antibodies agains t prolactin receptor are internalized after binding to the surface of the mammary cell. They are carried intracellularly by different routes . Internalization of these antibodies is not sufficient to mimic the s ecretagogue effect of prolactin.