Cellular events involved in hormonal control of receptor-mediated endocytosis: regulation occurs at multiple sites in the low density lipoprotein pathway, including steps beyond the receptor

Abstract

We used biochemical and quantitative structural approaches to analyze hormonal regulation of receptor-mediated endocytosis of human low density lipoprotein (LDL) by differentiating rat granulosa cells in culture. We studied uptake and metabolism of 125I-labeled LDL and distribution of gold-labeled LDL in nontreated and FSH- or FSH/androstenedione-treated granulosa cells. FSH and androstenedione worked together to enlarge the segment of the granulosa cell population capable of accumulating LDL and to increase the number of LDL receptors at the cell surface. More importantly, FSH affected postreceptor aspects of the LDL pathway, including 1) accelerating the apparent rate of LDL internalization, and 2) decreasing the time needed for LDL to reach lysosomes. In both cases we eliminated the possibility that the observed effects merely reflected differences in receptor numbers between hormone-treated and nontreated cells. FSH also increased the number of both gold-labeled lysosomes per cellular profile and gold particles per lysosome. In all, FSH expanded the capacity of the LDL pathway to accommodate more intracellular ligand by augmenting the numbers of organelles participating in the endocytic process. However, it did not affect cell size, sizes of individual organelles comprising the LDL pathway, or numbers of gold particles per organelle (except lysosomes). Our findings indicate that hormonal regulation of the LDL pathway extends beyond simply causing expression of cell surface LDL receptors to encompass postreceptor events, including enhancing the apparent rates at which ligand is internalized and transported to lysosomes.

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