Human, but not bovine, oxidized cerebral spinal fluid lipoproteins disrupt neuronal microtubules

beta-Amyloid promotes accumulation of lipid peroxides by inhibiting CD36-mediated clearance of oxidized lipoproteins

Background

Recent studies suggest that hypercholesterolemia, an established risk factor for atherosclerosis, is also a risk factor for Alzheimer's disease. The myeloid scavenger receptor CD36 binds oxidized lipoproteins that accumulate with hypercholesterolemia and mediates their clearance from the circulation and peripheral tissues. Recently, we demonstrated that CD36 also binds fibrillar β-amyloid and initiates a signaling cascade that regulates microglial recruitment and activation. As increased lipoprotein oxidation and accumulation of lipid peroxidation products have been reported in Alzheimer's disease, we investigated whether β-amyloid altered oxidized lipoprotein clearance via CD36.

Methods

The availability of mice genetically deficient in class A (SRAI & II) and class B (CD36) scavenger receptors has facilitated studies to discriminate their individual actions. Using primary microglia and macrophages, we assessed the impact of Aβ on: (a) cholesterol ester accumulation by GC-MS and neutral lipid staining, (b) binding, uptake and degradation of ¹²⁵I-labeled oxidized lipoproteins via CD36, SR-A and CD36/SR-A-independent pathways, (c) expression of SR-A and CD36. In addition, using mice with targeted deletions in essential kinases in the CD36-signaling cascade, we investigated whether Aβ-CD36 signaling altered metabolism of oxidized lipoproteins.

Results

In primary microglia and macrophages, Aβ inhibited binding, uptake and degradation of oxidized low density lipoprotein (oxLDL) in a dose-dependent manner. While untreated cells accumulated abundant cholesterol ester in the presence of oxLDL, cells treated with Aβ were devoid of cholesterol ester. Pretreatment of cells with Aβ did not affect subsequent degradation of oxidized lipoproteins, indicating that lysosomal accumulation of Aβ did not disrupt this degradation pathway. Using mice with targeted deletions of the scavenger receptors, we demonstrated that Aβ inhibited oxidized lipoprotein binding and its subsequent degradation via CD36, but not SRA, and this was independent of Aβ-CD36-signaling. Furthermore, Aβ treatment decreased CD36, but not SRA, mRNA and protein, thereby reducing cell surface expression of this oxLDL receptor.

Conclusions

Together, these data demonstrate that in the presence of β-amyloid, CD36-mediated clearance of oxidized lipoproteins is abrogated, which would promote the extracellular accumulation of these pro-inflammatory lipids and perpetuate lipid peroxidation.

Oxidized low-density lipoprotein is present in astrocytes surrounding cerebral infarcts and stimulates astrocyte interleukin-6 secretion

Ischemic injury to brain is associated with both disruption of the blood-brain barrier and increased oxidative stress. Given the neurotoxicity associated with exposure to oxidized low-density lipoprotein (oxLDL) in vitro, we tested the hypothesis that oxLDL may be present in parenchymal cells of cerebrum after infarction and that oxLDL may influence the pathophysiology of cerebral infarction. Our results showed that the subacute phase of cerebral infarction in patients was characterized by the appearance of oxLDL epitopes in astrocytes, but not neurons or microglia, in the perinecrotic zone. We further demonstrated that minimally oxLDL was most effectively internalized by primary cultures of rat astrocytes, and that exposure to minimal oxLDL stimulated astrocyte interleukin-6 secretion but did not alter nitric oxide production. These results demonstrate for the first time that oxLDL is present in brain parenchyma of patients with ischemic infarction and suggest a potential mechanism by which oxLDL may activate innate immunity and thereby indirectly influence neuronal survival.