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A recycling pathway for resecretion of internalized apolipoprotein E in liver cells
L L Swift1, M H Farkas, A S Major
1Departments of Pathology, Medicine, and Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, USA. larry.swift@mcmail.vanderbilt.edu
Abstract:
We have investigated the recycling of apoE in livers of apoE(-)/- mice transplanted with wild type bone marrow (apoE(+/+) --> apoE(-)/-), a model in which circulating apoE is derived exclusively from macrophages. Nascent Golgi lipoproteins were recovered from livers of apoE(+/+) --> apoE(-)/- mice 8 weeks after transplantation. ApoE was identified with nascent d < 1.006 and with d 1.006-1.210 g/ml lipoproteins at a level approximately 6% that of nascent lipoproteins from C57BL/6 mice. Hepatocytes from apoE(+/+) --> apoE(-)/- mice were isolated and cultured in media free of exogenous apoE. ApoE was found in the media primarily on the d < 1.006 g/ml fraction, indicating a resecretion of internalized apoprotein. Secretion of apoE from C57BL/6 hepatocytes was consistent with constitutive production, whereas the majority of apoE secreted from apoE(+/+) --> apoE(-)/- hepatocytes was recovered in the last 24 h of culture. This suggests that release may be triggered by accumulation of an acceptor, such as very low density lipoproteins, in the media. In agreement with the in vivo data, total recovery of apoE from apoE(+/+) --> apoE(-)/- hepatocytes was approximately 6% that of the apoE recovered from C57BL/6 hepatocytes. Since plasma apoE levels in the transplanted mice are approximately 10% of control levels, the findings indicate that up to 60% of the internalized apoE may be reutilized under physiologic conditions. These studies provide definitive evidence for the sparing of apoE and its routing through the secretory pathway and demonstrate that internalized apoE can be resecreted in a quantitatively significant fashion.
Insights
Apolipoprotein E (apoE) recycling in mouse livers shows significant reuse. Internalized apoE is resecreted, indicating efficient sparing and reuse within the secretory pathway.
Area of Science:
- Lipid metabolism and lipoprotein research
- Cellular biology and protein trafficking
- Biochemistry of apolipoproteins
Background:
- Apolipoprotein E (apoE) plays a crucial role in lipoprotein metabolism and clearance.
- Understanding apoE recycling is vital for comprehending lipid homeostasis and potential therapeutic strategies.
- Previous studies suggested apoE might be recycled, but direct evidence and quantitative analysis were limited.
Purpose of the Study:
- To investigate the in vivo and in vitro recycling of apolipoprotein E (apoE) in a mouse model.
- To determine the quantitative significance of apoE reutilization through the secretory pathway.
- To elucidate the mechanism of apoE resecretion from hepatocytes.
Main Methods:
- Utilized apoE-deficient mice transplanted with wild-type bone marrow (apoE(+/+) --> apoE(-)/-) to study macrophage-derived apoE.
- Isolated nascent lipoproteins from mouse livers and characterized apoE content.
- Cultured primary hepatocytes from transplanted mice in apoE-free media to assess secretion.
- Quantified apoE levels in different lipoprotein fractions and media.
Main Results:
- ApoE was detected in nascent lipoproteins from transplanted mouse livers at approximately 6% of control levels.
- Isolated hepatocytes from transplanted mice resecreted internalized apoE, primarily in the d < 1.006 g/ml fraction.
- Secretion patterns suggested a triggered release of apoE, possibly by acceptor molecules like VLDL.
- Total apoE recovery from transplanted hepatocytes was ~6% of controls, with up to 60% of internalized apoE potentially reutilized.
Conclusions:
- Internalized apoE can be resecreted in a quantitatively significant manner, demonstrating efficient sparing.
- The secretory pathway plays a key role in the recycling and reuse of apoE.
- These findings provide definitive evidence for apoE reutilization under physiological conditions.