Increased expression and translocation of lysosomal cathepsins contribute to macrophage apoptosis in atherogenesis.

It has been recently reported that atherosclerotic lesions in both humans and mice express several lysosomal proteases, including cathepsins B, D, L, and S, which may affect plaque development and stability. The mechanisms responsible for the extralysosomal expression of lysosomal cathepsins and the related atherogenic implications remain unknown. We find that the lesion-dependent expression of cathepsins B and L is mainly in macrophage-infiltrated areas of human carotid atheroma. These enzymes appear in the cytoplasm and nuclei of apoptotic macrophages (normally confined to the lysosomal compartment) and in the extracellular areas. After exposure to oxidized low-density lipoprotein (oxLDL) or 7beta-hydroxycholesterol (7beta-OH), macrophages initially transform into foam cells and then undergo apoptotic cell death. The oxidized lipids induce lysosomal destabilization, with leakage to the cytosol of lysosomal enzymes (cathepsins B, D, and L), as detected by cytochemistry and immunocytochemistry. A remarkable increase in cathepsin D mRNA levels was observed after 7beta-OH exposure. Like macrophages within atheroma, intralysosomal cathepsins B and L are translocated to the cytoplasm and nuclei of 7beta-OH-exposed cells. Our results suggest that endocytosed oxLDL and oxysterols not only destabilize the acidic vacuolar compartment but also cause the upregulation and translocation of lysosomal cathepsins, which may act as cleaving enzymes during the apoptotic process. The increased macrophage apoptosis and nuclear and matrix degradation by lysosomal enzymes in atheroma may play important roles in plaque development and rupture.