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Competitive inhibition of LDL binding and uptake by HDL in aortic endothelial cells
J J Alexander1, R Miguel, D Graham
1Case Western Reserve University School of Medicine, Cleveland Metropolitan General Hospital, Ohio 44109.
Insights
High-density lipoprotein (HDL) inhibits low-density lipoprotein (LDL) binding and uptake in bovine aortic endothelial cells. This effect is mediated by apolipoprotein A (Apo A), suggesting competitive receptor binding.
Area of Science:
- Cardiovascular Biology
- Lipid Metabolism
- Atherosclerosis Research
Background:
- High-density lipoprotein (HDL) may regulate exogenous cholesterol delivery to nonhepatic tissues.
- HDL's role in atherogenesis by altering arterial wall lipid metabolism is under investigation.
Purpose of the Study:
- To verify and characterize the inhibitory effect of HDL on low-density lipoprotein (LDL) binding and cellular uptake.
- To elucidate the mechanism behind HDL's influence on LDL interaction with endothelial cells.
Main Methods:
- Bovine aortic endothelial cells were cultured and incubated with radiolabeled LDL (125I-LDL) and varying concentrations of HDL or apolipoprotein A (Apo A).
- Cellular uptake and membrane-bound LDL were quantified by measuring intracellular and trypsin-released counts.
- Experiments were repeated using radiolabeled apolipoprotein B (125I-Apo B) to assess competitive binding.
Main Results:
- HDL significantly inhibited LDL binding and cellular uptake by endothelial cells (P < 0.005).
- Apolipoprotein A (Apo A) alone demonstrated a similar inhibitory effect (P < 0.005).
- Apo A inhibited the binding of apolipoprotein B (Apo B) to endothelial cells to the same extent as HDL (P < 0.0006).
Conclusions:
- HDL inhibits LDL binding and uptake in bovine aortic endothelial cells.
- The inhibitory effect is primarily mediated by apolipoprotein A (Apo A), suggesting competitive binding at the receptor level.
- This mechanism is likely responsible for HDL's role in regulating lipid metabolism in arterial wall cells and potentially influencing atherogenesis.
Abstract:
High-density lipoprotein (HDL) may inhibit the binding and cellular uptake of low-density lipoprotein (LDL) as one means of regulating the delivery of exogenous cholesterol to nonhepatic tissues. This may play an important role in atherogenesis, by altering lipid metabolism in cells of the arterial wall. To verify and better characterize this effect, endothelial cells were harvested from bovine aorta and maintained in tissue culture. Following initial preincubation in lipid-deficient culture media, these cells were incubated for 2 hr at 4 degrees C in media containing 125I-LDL (10 micrograms protein/ml) and varying concentrations of either HDL (0-400 micrograms protein/ml) or comparable amounts of Apoprotein A (Apo A), the major protein component of HDL. Intracellular and trypsin-released counts were assayed separately, as a measurement of cellular uptake and membrane bound LDL, respectively. Results of this study indicated an inhibition of LDL binding and uptake by HDL (P less than 0.005, ANOVA). A similar inhibition was found with Apo A alone (P less than 0.005). When identical studies were performed using 125I-Apoprotein B, the protein component of LDL, and Apo A, the latter was found to inhibit the binding of Apo B to the same extent (P less than 0.0006). These results indicate that HDL does inhibit LDL binding and uptake by bovine aortic endothelial cells and that, because this effect is seen equally with only the protein component of these lipoprotein particles, it is most likely due to competitive binding at the receptor level rather than to stearic hindrance or an alteration of the cell membrane.