Prolactin internalisation by the epithelial mammary cell: effects of lysosomotropic agents and transglutaminase inhibitors

Prolactin endocytosis was studied by electron microscopy with 125I-prolactin 125I-hGH (human growth hormone) and prolactin-ferritin. Endocytosis and intracellular transit of the labelled hormone proceeded identically in epithelial cells isolated from the mammary glands of pseudopregnant rabbits and in surviving fragments from mammary glands of lactating rabbits. After binding of the hormone to its receptor, the labelled material was rapidly detectable in vesicles showing an homogeneous aspect; 15 min later part of the labelled material was still localized within the same kind of vesicles, but in addition it appeared to have migrated into microvesicles of the Golgi region and into vesicles of heterogeneous aspect tentatively identified with lysosomes. Endocytosis of bovine serum albumin, labelled with ferritin followed the same intracellular pathway. Native ferritin accumulated in vesicles of various sizes, but seemed excluded from the microvesicles of the Golgi zone. In the presence of lysosomotropic agents labelled prolactin accumulated in cytoplasmic vesicles. In the presence of dansylcadaverine, endocytosis of the labelled material proceeded unimpaired. Conversely, in the presence of bacitracin, the internalisation of labelled prolactin seemed to be reduced. These observations show that the endocytosis of the hormone/receptor complex is linked to membrane movements, which eventually lead to its location within both the Golgi apparatus and the lysosomes.