Autoradiographic localization of the sites of uptake, cellular transport, and catabolism of low density lipoproteins in the liver of normal and estrogen-treated rats

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Abstract
The hepatic uptake and catabolism of low density lipoproteins [LDL] are stimulated severalfold in rats treated with large amounts of 17.alpha.-ethinylestradiol. To determine the sites within the liver at which these processes occur, 125I-labeled human LDL were injected i.v. into intact control and estradiol-treated rats or added to perfusates of their isolated livers. The livers were fixed by perfusion and processed for light microscopy and EM autoradiography. Distribution of autoradiographic silver grains was estimated qualitatively in light micrographs and quantitatively in EM. Many more silver grains were seen in livers from estradiol-treated than from control rats, but the processing of labeled LDL was indistinguishable. At 3 min after i.v. injection or perfusion of livers, the grains were concentrated over the microvillous surface of parenchymal cells bordering the space of Disse. Many of these grains were within 2 half-distances from endocytic pits. Only 5-15% of the grains were seen over endothelial and Kupffer cells. Silver grains were also observed over vesicles beneath the plasma membrane whose size and shape suggested that they were derived from fusion of endocytic vesicles. By 15 min, grains were predominantly located in structures like multivesicular bodies in the region of the GERL (Golgi complex-endoplasmic reticulum-lysosomes) near the bile canaliculi. These bodies were packed with small vesicle-like structures and a few larger vesicles, the latter possessing a unit membrane. Between 15 and 30 min, when proteolysis of LDL begins, the initially clear matrix of the multivesicular body-like structures became dark and the structures frequently had a dense tail-like appendage. At the same time, silver grains began to appear over secondary lysosomes. Apparently the hepatic uptake of LDL that is stimulated in rats given large amounts of estradiol follows a pathway that closely resembles that of the well-defined LDL receptor in cultured cells. In the liver these lipoproteins appear to be transported in endocytic vesicles; the vesicles fuse to form multivesicular body-like structures that acquire lysosomal enzymes and are converted to secondary lysosomes as the lipoproteins are degraded.