Researchers from the Washington University School of Medicine this week published a report suggesting that a microRNA — miR-33 — could play a key role in the buildup of cholesterol in macrophages that is associated with age-related macular degeneration and may serve as a therapeutic target for the condition.
Macrophage-mediated inflammation is known to be involved in several age-associated conditions including AMD, where the cells promote pathologic angiogenesis beneath the retina — a process known as choroidal neovascularization, or CNV — and atherosclerosis, wherein cholesterol-loaded foam cell macrophages can trigger the inflammatory rupture of arterial plaques.
Notably, AMD is diagnosed by the presence of lipid-rich deposits called drusen that develop outside of the retinal pigment epithelium underneath the retina and can served as a reservoir for monocytic inflammation.
In a paper describing their work, published this week in Cell Metabolism, the researchers wrote that “progression of AMD is often characterized by increased size and number of these lipid-rich drusen,” and that macrophage-mediated inflammation has been shown to accelerate the progression of AMD from its early “dry” form to its more advanced “wet" form by promoting neovascularization underneath drusen.
Various groups have conducted genome-wide association studies demonstrating single nucleotide polymorphisms in the innate immune system increase the risk of development and progression of AMD, and that there is an association between SNPs in genes involved in cholesterol metabolism and advanced AMD.
“These polymorphisms are especially interesting because multivariate analyses have shown that allelic associations with advanced AMD are independent of actual serum [high-density lipoprotein] or lipid levels,” the Washington University team wrote. “It is thus apparent that the biologic pathways that connect macrophage-mediated inflammation, cholesterol regulation, and age in eye disease and drusen are highly complex and cannot be explained by a simple association between increased risk and abnormal serum lipid component levels.”
In further exploring this issue, the scientists showed that ABCA1 and ABCG1, two members of the ATP binding cassette family of transporters that play critical roles in mobilizing cholesterol out of macrophages, are down-regulated in the white blood cells in both aging mice and humans.
In loss-of-function experiments in knockout mice, the team demonstrated that macrophages deficient in ABCA1 are unable to regulate CNV in vivo and vascular endothelial cell proliferation in vitro.
However, when ATP binding cassette transporter expression was up-regulated in senescent macrophages using LXR agonists delivered in eye drops to mice, the researchers were able to restore the cells’ ability to inhibit vascular endothelial cell proliferation and CNV.
“In essence, LXR agonists are able to reverse the senescent phenotype and restore their angioregulatory function to levels comparable to mice that are significantly younger,” they wrote in the paper.
Another key finding was that up-regulation of miR-33 in aged macrophages appeared responsible for the repression of ABCA1 causing defective cellular cholesterol metabolism.
The miRNA has previously been associated with the genes responsible for cellular cholesterol metabolism, and is being explored as a therapeutic target by a number of miRNA drug developers including Regulus Therapeutics and Santaris Pharma.
The Washington University investigators found that miR-33 expression was “significantly higher” in old macrophages as compared to young ones, and that both the miRNA and its host gene are co-expressed and coordinately regulated after LXR agonist treatment or cholesterol loading in both young and old macrophages.
“Based on these findings, we hypothesized that antagonism of miR-33 in macrophages might enhance their regulation of vascular proliferation,” they wrote. In mouse models, this was indeed the case, with “specific and robust” inhibition of CNV observed.
“Anti-miR-33 treatment polarizes macrophages to a classically activated phenotype that is reflected in the cytokine gene expression signature” — a finding with “significant therapeutic implications,” the team noted.
“Therapeutic intervention prior to the development of advanced disease with effective agents that up-regulate macrophage cholesterol efflux in the eye might prevent progression and can be used as prophylaxis against the development of CNV and its blinding complications,” they wrote.
Because the eye is an immune-privileged organ, treatment with an miRNA-targeting drug may be possible while avoiding the potential off-target complications associated with systemic treatment, they concluded.