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Interaction of beta-very-low-density lipoproteins with rat liver cells
L Harkes1, A van Duijne, T J van Berkel
1Center for Bio-Pharmaceutical Sciences, Sylvius Laboratories, University of Leiden, The Netherlands.
Abstract:
Cholesteryl-ester-rich very-low-density lipoproteins (beta-VLDL) are considered to be atherogenic because in vitro they can provoke cholesterol accumulation in macrophages. The greatest population of macrophages resides inside the liver and in the present study the rat beta-VLDL uptake by the various rat liver cell types is determined in vivo and compared to the uptake of rat VLDL. beta-VLDL isolated from cholesterol-fed rats was iodinated and injected into the rat. After 10 min of circulation, 45% of the injected beta-VLDL was found in the liver. A low-temperature cell-isolation procedure shows that rat liver parenchymal cells form the major site for beta-VLDL uptake (96%) and, consequently, rat liver macrophages (nonparenchymal liver cells) do not perform a quantitatively significant role in the uptake of these lipoproteins. In vitro competition studies indicate that apolipoprotein (apo) E is the site recognised by liver parenchymal cells and even a 600-fold excess of apo-E-free human LDL was an ineffective competitor. Furthermore it can be demonstrated that induction of apo-B,E receptors on liver parenchymal cells by estrogen treatment does not result in a significant increased uptake of beta-VLDL. These data show that recognition of beta-VLDL is presumably exerted by the remnant receptor. Intracellular processing of both the apolipoproteins and phospholipids of beta-VLDL was followed by subcellular distribution studies. It appears that, within 45 min, 75% of the apolipoproteins are degraded and subsequently released from the liver. In contrast the phospholipids remain associated with the liver for a prolonged time and a specific transfer to the mitochondrial fraction is found. It can be concluded that liver parenchymal cells form in vivo the major site for beta-VLDL uptake and it appears that recognition of beta-VLDL is coupled to internalization and processing of both the apolipoproteins and phospholipids by a route which involves the lysosomes.
Insights
Cholesteryl-ester-rich very-low-density lipoproteins (beta-VLDL) are primarily taken up by liver parenchymal cells, not macrophages. This uptake involves the remnant receptor and lysosomal processing of lipoproteins.
Area of Science:
- Lipid metabolism and lipoprotein research.
- Cell biology and liver function.
- Atherosclerosis and cardiovascular disease.
Background:
- Cholesteryl-ester-rich very-low-density lipoproteins (beta-VLDL) are implicated in atherosclerosis due to in vitro cholesterol accumulation in macrophages.
- Macrophages are abundant in the liver, suggesting a potential key role in lipoprotein processing.
Purpose of the Study:
- To determine the in vivo uptake of rat beta-VLDL by different rat liver cell types.
- To compare beta-VLDL uptake with that of rat VLDL.
- To elucidate the cellular mechanisms and receptors involved in beta-VLDL processing by liver cells.
Main Methods:
- Isolation and iodination of beta-VLDL from cholesterol-fed rats.
- In vivo injection of labeled beta-VLDL into rats and subsequent liver cell isolation.
- In vitro competition studies using excess lipoproteins and receptor induction experiments.
- Subcellular distribution studies to track lipoprotein components.
Main Results:
- Liver parenchymal cells account for 96% of in vivo beta-VLDL uptake, with macrophages playing a minor role.
- Apolipoprotein E is identified as the key recognition site for beta-VLDL by liver parenchymal cells, likely via the remnant receptor.
- Estrogen-induced upregulation of apo-B,E receptors did not significantly increase beta-VLDL uptake.
- Apolipoproteins are rapidly degraded and released, while phospholipids are retained and transferred to mitochondria.
- Intracellular processing involves lysosomal pathways.
Conclusions:
- Liver parenchymal cells are the primary in vivo site for beta-VLDL uptake.
- Beta-VLDL recognition is mediated by the remnant receptor, involving apolipoprotein E.
- Lipoprotein processing involves lysosomal degradation of apolipoproteins and prolonged retention of phospholipids within liver cells.