High-Content Microscopy Drug Screening Platform for Regulators of the Extracellular Digestion of Lipoprotein Aggregates by Macrophages

The recruitment of
macrophages to the intima of the arteries is
a critical event in atherosclerotic progression. These macrophages
accumulate excessive lipid droplets and become “foam cells”,
a hallmark of atherosclerosis. Most studies focus on lipid accumulation
through macrophage interaction with modified monomeric low-density
lipoprotein (LDL). However, in the intima, macrophages predominantly
encounter aggregated LDL (agLDL), an interaction that has been studied
far less. Macrophages digest agLDL and generate free cholesterol in
an extracellular, acidic, and hydrolytic compartment. They form a
tight seal around agLDL through actin polymerization and deliver lysosomal
contents into this space in a process termed digestive exophagy. There
is some evidence that inhibiting digestive exophagy to slow cholesterol
accumulation in macrophages protects them from becoming foam cells
and slows the progression of atherosclerotic lesions. Thus, understanding
the mechanisms of digestive exophagy is critical. Here, we describe
a high-content microscopy screen of a library of repurposed drugs
for compounds that inhibit lysosome exocytosis during digestive exophagy.
We identified many hit compounds and further characterized the effects
that five of these compounds have on various aspects of digestive
exophagy. In addition, three of the five compounds do not inhibit
oxidized LDL-induced foam cell formation, indicating that the two
pathways to foam cell formation can be targeted independently. We
demonstrate that this high-content screening platform has great potential
as a drug discovery tool with the ability to effectively and efficiently
screen for modulators of digestive exophagy.