Oxidized low density lipoprotein is resistant to cathepsins and accumulates within macrophages

Marilee Lougheed, Hanfang Zhang, Urs P. Steinbrecher

Research output: Contribution to journalArticle

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Abstract

The rate of uptake of oxidized low density lipoprotein (LDL) by mouse peritoneal macrophages is similar to that of acetyl LDL; but only ∼50% of the internalized oxidized LDL is ultimately degraded, in contrast to the near-complete degradation seen with acetyl LDL. The objectives of this study were to determine if this was due to increased surface binding of oxidized LDL, different uptake pathways for oxidized LDL and acetyl LDL, lysosomal dysfunction caused by oxidized LDL, or resistance of oxidized LDL to hydrolysis by lysosomal proteinases. LDL binding studies at 4 °C showed that the increased cell association with oxidized LDL could not be explained by differences in cell-surface binding. Immunofluorescence microscopy confirmed intracellular accumulation of apoB-immunoreactive material in macrophages incubated with oxidized LDL, but not with acetyl LDL. The scavenger receptor ligand polyinosinic acid inhibited both the cell association and degradation of oxidized LDL in macrophages by >75%, suggesting a common uptake pathway for degraded LDL and nondegraded LDL. Studies in THP-1 cells also did not reveal more than one specific uptake pathway for oxidized LDL. LDL derivatized by incubation with oxidized arachidonic acid (under conditions that prevented oxidation of the LDL itself) showed inefficient degradation, similar to oxidized LDL. When macrophages were incubated with oxidized LDL together with acetyl 125I-LDL, the acetyl LDL was degraded normally, excluding lysosomal dysfunction as the explanation for the accumulation of oxidized LDL. Generation of trichloroacetic acid-soluble products from oxidized 125I-LDL by exposure to cathepsins B and D was less than that observed with native 125I-LDL. LDL modified by exposure to reactive products derived from oxidized arachidonic acid was also degraded more slowly than native 125I-LDL by cathepsins. In contrast, acetyl 125I-LDL was degraded more rapidly by cathepsins than native 125I-LDL, and aggregated LDL and malondialdehyde-modified LDL were degraded at the same rate as native 125I-LDL. It is concluded that the intracellular accumulation of oxidized LDL in macrophages can be explained at least in part by the resistance of oxidatively modified apolipoprotein B to cathepsins. This resistance to cathepsins does not appear to be due to aggregation of oxidized LDL, but may be a consequence of modification of apolipoprotein B by lipid peroxidation products.

Original languageEnglish (US)
Pages (from-to)14519-14525
Number of pages7
JournalJournal of Biological Chemistry
Volume266
Issue number22
StatePublished - Dec 1 1991

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Cathepsins
Macrophages
LDL Lipoproteins
Apolipoproteins B
oxidized low density lipoprotein
Arachidonic Acid
Degradation
Poly I
Association reactions
Scavenger Receptors
Lipoproteins
Cathepsin B
Cathepsin D
Trichloroacetic Acid
Peritoneal Macrophages
Malondialdehyde

ASJC Scopus subject areas

  • Biochemistry

Cite this

Oxidized low density lipoprotein is resistant to cathepsins and accumulates within macrophages. / Lougheed, Marilee; Zhang, Hanfang; Steinbrecher, Urs P.

In: Journal of Biological Chemistry, Vol. 266, No. 22, 01.12.1991, p. 14519-14525.

Research output: Contribution to journalArticle

Lougheed, M, Zhang, H & Steinbrecher, UP 1991, 'Oxidized low density lipoprotein is resistant to cathepsins and accumulates within macrophages', Journal of Biological Chemistry, vol. 266, no. 22, pp. 14519-14525.
Lougheed, Marilee ; Zhang, Hanfang ; Steinbrecher, Urs P. / Oxidized low density lipoprotein is resistant to cathepsins and accumulates within macrophages. In: Journal of Biological Chemistry. 1991 ; Vol. 266, No. 22. pp. 14519-14525.
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abstract = "The rate of uptake of oxidized low density lipoprotein (LDL) by mouse peritoneal macrophages is similar to that of acetyl LDL; but only ∼50{\%} of the internalized oxidized LDL is ultimately degraded, in contrast to the near-complete degradation seen with acetyl LDL. The objectives of this study were to determine if this was due to increased surface binding of oxidized LDL, different uptake pathways for oxidized LDL and acetyl LDL, lysosomal dysfunction caused by oxidized LDL, or resistance of oxidized LDL to hydrolysis by lysosomal proteinases. LDL binding studies at 4 °C showed that the increased cell association with oxidized LDL could not be explained by differences in cell-surface binding. Immunofluorescence microscopy confirmed intracellular accumulation of apoB-immunoreactive material in macrophages incubated with oxidized LDL, but not with acetyl LDL. The scavenger receptor ligand polyinosinic acid inhibited both the cell association and degradation of oxidized LDL in macrophages by >75{\%}, suggesting a common uptake pathway for degraded LDL and nondegraded LDL. Studies in THP-1 cells also did not reveal more than one specific uptake pathway for oxidized LDL. LDL derivatized by incubation with oxidized arachidonic acid (under conditions that prevented oxidation of the LDL itself) showed inefficient degradation, similar to oxidized LDL. When macrophages were incubated with oxidized LDL together with acetyl 125I-LDL, the acetyl LDL was degraded normally, excluding lysosomal dysfunction as the explanation for the accumulation of oxidized LDL. Generation of trichloroacetic acid-soluble products from oxidized 125I-LDL by exposure to cathepsins B and D was less than that observed with native 125I-LDL. LDL modified by exposure to reactive products derived from oxidized arachidonic acid was also degraded more slowly than native 125I-LDL by cathepsins. In contrast, acetyl 125I-LDL was degraded more rapidly by cathepsins than native 125I-LDL, and aggregated LDL and malondialdehyde-modified LDL were degraded at the same rate as native 125I-LDL. It is concluded that the intracellular accumulation of oxidized LDL in macrophages can be explained at least in part by the resistance of oxidatively modified apolipoprotein B to cathepsins. This resistance to cathepsins does not appear to be due to aggregation of oxidized LDL, but may be a consequence of modification of apolipoprotein B by lipid peroxidation products.",
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