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Polymyxin B enhances low density lipoprotein catabolism in hepatic and extrahepatic cells
1Department of Internal Medicine, Lund University, Malmö General Hospital, Sweden.
Abstract:
We investigated the effects of polymyxin B (PMB), an antibiotic that binds to endotoxins, on the uptake and degradation of low density lipoproteins (LDLs) in HepG2 cells, a highly differentiated human hepatoma cell line. The results showed that PMB very effectively enhanced the binding, internalization, and degradation of LDL in HepG2 cells. The PMB-mediated enhancement of LDL uptake was not dependent on the LDL receptor-mediated pathway, as blockage of the LDL receptor by use of a monoclonal anti-LDL receptor antibody had no effect on the PMB-mediated cellular processing of LDL and PMB-mediated enhancement of LDL uptake did not cause an increase in cholesterol esterification. In addition, chloroquine and colchicine, which inhibit lysosomal degradation and cellular endocytosis, respectively, diminished PMB-enhanced degradation of LDL, indicating that PMB mediates uptake through a pathway similar to the LDL receptor-mediated pathway. The PMB-mediated uptake of LDL was sensitive to treatment with phospholipase C and pronase and was dependent on the presence of Ca2+. PMB caused similar changes in human skin fibroblasts, bovine smooth muscle cells, and bovine endothelial cells, which suggests that PMB-enhanced LDL uptake is a general cellular phenomenon. Our results thus indicate that PMB increases cellular catabolism of LDL through an endocytotic pathway not involving the LDL receptors.
Insights
Polymyxin B (PMB) significantly boosts the cellular uptake and breakdown of low-density lipoproteins (LDL) in liver cells. This antibiotic enhances LDL catabolism via an endocytotic pathway independent of LDL receptors.
Area of Science:
- Biochemistry
- Cell Biology
- Pharmacology
Background:
- Low-density lipoproteins (LDL) are crucial for cellular cholesterol transport.
- HepG2 cells, a human hepatoma cell line, are utilized to study cellular lipid metabolism.
- Polymyxin B (PMB) is an antibiotic known for its endotoxin-binding properties.
Purpose of the Study:
- To investigate the effects of Polymyxin B (PMB) on the uptake and degradation of low-density lipoproteins (LDL) in HepG2 cells.
- To elucidate the cellular pathway involved in PMB-mediated LDL processing.
- To determine if PMB-enhanced LDL uptake is a general cellular phenomenon.
Main Methods:
- Utilized HepG2 cells to assess LDL binding, internalization, and degradation.
- Employed LDL receptor blockage with monoclonal antibodies to investigate pathway dependence.
- Used inhibitors of lysosomal degradation (chloroquine) and endocytosis (colchicine).
- Tested sensitivity to phospholipase C, pronase, and calcium (Ca2+).
- Examined effects in human skin fibroblasts and bovine vascular cells.
Main Results:
- Polymyxin B (PMB) significantly enhanced LDL binding, internalization, and degradation in HepG2 cells.
- PMB-mediated LDL uptake was independent of the classical LDL receptor pathway.
- Inhibition of lysosomal degradation and endocytosis reduced PMB-enhanced LDL degradation.
- PMB-mediated LDL uptake was sensitive to phospholipase C, pronase, and Ca2+.
- Similar effects were observed in human skin fibroblasts and bovine vascular cells.
Conclusions:
- Polymyxin B (PMB) promotes cellular catabolism of LDL through an endocytotic pathway.
- This pathway does not involve the canonical LDL receptors.
- PMB-enhanced LDL uptake represents a general cellular mechanism observed across different cell types.