Uptake of low density lipoproteins by rat tissues: special emphasis on the luteinized ovary

The aim of this study was to determine how luteal cells of the hormone-primed (luteinized) ovary process low density lipoproteins (LDL). Ovary uptake of perfused 125I-LDL was assessed by tissue levels of radioactivity; the distribution of LDL protein in cells was assessed on autoradiograms of the fixed tissue; and the level of stimulation of steroidogenesis, as well as degradation of LDL protein, was assessed on effluent perfusion samples. Human LDL ligand used in these studies was rigorously defined biochemically and physiologically. Homologous (rat) LDL was used as a special ligand control. Other tissue controls included the use of perfused or in vivo-infused luteinized ovaries from animals pretreated to reduce circulating lipoprotein levels, perfused ovaries from a second hormone-primed model, perfused liver from estrogen-treated rats, and isolated and cultured cells from the same ovarian tissues used in the perfusion experiments. The results show that perfused LDL promptly stimulates steroidogenesis. However, the labeled protein moiety of the LDL is not interiorized by the luteal cells, nor is there evidence of LDL protein degradation in the effluent samples. In contrast, internalization of the ligand occurs when luteal cells are incubated with the ligand in vitro. We have observed also that uptake of the 125I-LDL by the ovary can be displaced equally well by excess unlabeled LDL or HDL3. Overall, these experiments suggest that in the intact luteinized ovary, LDL binds to the same sites on the cell surface where HDL "binds," and that LDL cholesterol must be obtained by these steroid hormone-producing cells by a mechanism that does not require internalization of the intact lipoprotein particle.