Low density lipoprotein binding, internalization, and degradation in human adipose cells

Human adipose tissue relies on circulating lipoproteins for cholesterol supply. Prior research has shown that fat cells interact with various lipoproteins, but the exact mechanisms of LDL binding and processing remain unclear. While it was already known that adipocytes handle lipoproteins, the specifics of LDL metabolism had not been fully characterized. This gap motivated a detailed investigation into how human adipose cells manage LDL. No prior work had resolved the kinetic parameters of LDL uptake in these cells. Researchers sought to quantify binding rates and internalization mechanisms. The study aimed to clarify whether LDL metabolism in adipose tissue is energy dependent. This uncertainty drove the design of experiments using radiolabeled LDL. The goal was to determine the functional relevance of LDL interactions in fat cells.

The study aimed to examine how human adipose cells bind, internalize, and degrade low-density lipoprotein (LDL). Researchers focused on quantifying the kinetics of LDL uptake and identifying whether this process requires cellular energy. They also sought to determine if other lipoproteins, such as HDL or VLDL, could influence LDL metabolism. The motivation stemmed from the need to understand how fat cells manage cholesterol from circulating lipoproteins. The study aimed to assess the specificity of LDL binding and internalization mechanisms. Researchers wanted to compare the inhibitory effects of different lipoproteins on LDL processing. The goal was to evaluate the role of adipose tissue in overall LDL catabolism. This work could help clarify how fat cells contribute to systemic lipid metabolism.

The study used isolated human adipocytes and radiolabeled LDL to track binding and internalization. Researchers measured dose-dependent uptake of 125I-LDL and calculated kinetic parameters like Km and Vmax. Time-course experiments monitored LDL accumulation over four hours. The degradation of LDL was assessed by measuring trichloroacetic acid-soluble iodopeptides in the incubation medium. Energy dependence was tested using azide and F- to inhibit metabolic processes. The specificity of LDL binding was evaluated by adding excess unlabeled VLDL, HDL, or LDL. Researchers also used plasma membrane preparations to study LDL binding saturation. The inhibitory effects of different lipoproteins were compared on a protein-equivalent basis.

Human adipocytes bound and internalized 125I-LDL in a dose-dependent manner with an apparent Km of 6.9 ± 0.9 µg/mL and a Vmax of 15–80 µg/mg lipid per 2 hours. LDL uptake showed rapid initial binding followed by linear accumulation over four hours. Degradation products, including iodopeptides, accumulated progressively in the incubation medium. Azide and F- significantly inhibited LDL internalization and degradation, indicating energy dependence. Excess unlabeled VLDL, HDL, and LDL all reduced binding and internalization of 125I-LDL. On a protein basis, HDL was the most potent inhibitor of LDL binding. Plasma membrane binding of LDL was saturable and nearly abolished by higher concentrations of HDL. These findings suggest that adipocytes process LDL similarly to other mesenchymal cells.

The study found that human adipocytes bind, internalize, and degrade LDL in a manner resembling other mesenchymal cells. These findings suggest that adipose tissue contributes to systemic LDL catabolism. The energy dependence of LDL processing implies active cellular mechanisms are involved. The lack of lipoprotein class specificity indicates shared binding pathways for LDL, VLDL, and HDL. HDL was the most effective inhibitor of LDL binding on a protein-equivalent basis. These results support the idea that adipose tissue is a key site for LDL and HDL interactions. The observed kinetics and degradation patterns align with in vivo LDL metabolism. The authors suggest that these findings may help explain how fat cells manage cholesterol from circulating lipoproteins.