Uptake of Oxidized LDL by Macrophages Results in Partial Lysosomal Enzyme Inactivation and Relocation

Abstract

Abstract-The cytotoxicity of oxidized LDL (oxLDL) to several types of artery wall cells might contribute to atherosclerosis by causing cell death, presumably by both apoptosis and necrosis. After its uptake into macrophage lysosomes by receptor-mediated endocytosis, oxLDL is poorly degraded, resulting in ceroid-containing foam cells. We studied the influence of oxLDL on lysosomal enzyme activity and, in particular, on lysosomal membrane stability and the modulation of these cellular characteristics by HDL and vitamin E (vit-E). Unexposed cells and cells exposed to acetylated LDL (AcLDL) were used as controls. The lysosomal marker enzymes cathepsin L and N-acetyl-␤-glucosaminidase (NA␤Gase) were biochemically assayed in J-774 cells after fractionation. Lysosomal integrity in living cells was assayed by the acridine orange (AO) relocation test. Cathepsin D was immunocytochemically demonstrated in J-774 cells and human monocyte-derived macrophages. We found that the total activities of NA␤Gase and cathepsin L were significantly decreased, whereas their relative cytosolic activities were enhanced, after oxLDL exposure. Labilization of the lysosomal membranes was further proven by decreased lysosomal AO uptake and relocation to the cytosol of cathepsin D, as estimated by light and electron microscopic immunocytochemistry. HDL and vit-E diminished the cytotoxicity of oxLDL by decreasing the lysosomal damage. The results indicate that endocytosed oxLDL not only partially inactivates lysosomal enzymes but also destabilizes the acidic vacuolar compartment, causing relocation of lysosomal enzymes to the cytosol. Exposure to AcLDL resulted in its uptake with enlargement of the lysosomal apparatus, but the stability of the lysosomal membranes was not changed. (Arterioscler Thromb Vasc Biol. 1998;18:177-184.)

Cathepsin D links TNF-induced acid sphingomyelinase to Bid-mediated caspase-9 and -3 activation

“…Numerous studies outline the possibility of translocation of lysosomal proteases and their participation in apoptotic signalling. Oxidative stress induced by the redox cycling quinone naphtazarin (5,8-dihydroxy-1,4-naphthoquinone) leads to the relocation of CTSD with ensuing apoptosis in rat cardiomyocytes 26 and to the relocation of cathepsins D, B and L. 18 Furthermore, there is evidence for a lysosomal pathway of apoptosis induced by the synthetic retinoid CD437, triggering lysosomal leakage and CTSD relocation. 44 Recently, TNF-mediated translocation of CTSD in vascular endothelial cells was reported.…”

“…The induction of cathepsin release results in apoptosis. 13,[25][26][27] Notably, the oxLDL-induced apoptosis has been shown to depend on A-SMase expression and ceramide production. 28 The functional link between TNF, A-SMase-generated ceramide and CTSD activation in endolysosomal compartments, and the connection of CTSD with further downstream apoptotic signalling remains an unsolved issue and is subject of the present study.…”

The role of lysosomes in iron metabolism and recycling

“…While it is unclear what factors cause LDL oxidation and why macrophages generally do not clear themselves from their lysosomal content of oxidized LDL but rather are converted to foam cells, many of which die by apoptosis and undergo calcification, it is obvious that further LDL oxidation takes place intralysosomally (Li et al, 1998a) and that macrophage lysosomal stability is affected (Li et al, 1998b).…”

Involvement of cathepsin D in chemotherapy-induced cytochrome c release, caspase activation, and cell death

“…Mice that are deficient in cathepsin D die on postnatal day 26 F 1 due to atrophy of the intestinal mucosa and consequent anorexia (24). Cathepsin D has been shown to be relocalized to the cytoplasm following treatment of cells with hydrogen peroxide (25), oxidized low-density lipoprotein (26), the quinone naphthazarin (27), and the protein kinase C inhibitor staurosporine (28,29). Reports demonstrating participation of cathepsin D in apoptosis execution have relied heavily on the pharmacologic inhibitor pepstatin A.…”