During
atherogenesis,
low density lipoprotein (
LDL) particles in the arterial intima become modified and fuse to form extracellular lipid droplets. Proteolytic modification of
apolipoprotein (
apo) B-100 may be one mechanism of droplet formation from
LDL. Here we studied whether the newly described
acid protease cathepsin F can generate
LDL-derived lipid droplets in vitro. Treatment of
LDL particles with human recombinant
cathepsin F led to extensive degradation of
apoB-100, which, as determined by rate
zonal flotation, electron microscopy, and NMR spectroscopy, triggered both aggregation and fusion of the
LDL particles. Two other
acid cysteine proteases,
cathepsins S and K, which have been shown to be present in the arterial intima, were also capable of degrading
apoB-100, albeit less efficiently.
Cathepsin F treatment resulted also in enhanced retention of
LDL to human arterial
proteoglycans in vitro. Cultured monocyte-derived macrophages were found to secrete active
cathepsin F. In addition, similarly with
cathepsins S and K,
cathepsin F was found to be localized mainly within the macrophage-rich areas of the human coronary
atherosclerotic plaques. These results suggest that proteolytic modification of
LDL by
cathepsin F may be one mechanism leading to the extracellular accumulation of
LDL-derived lipid droplets within the
proteoglycan-rich extracellular matrix of the arterial intima during
atherogenesis.