Effects of differently oxidized LDL on the expression of pro-inflammatory molecules in human monocytes in vitro

Nuclear localization of active matrix metalloproteinase-2 in cigarette smoke-exposed apoptotic endothelial cells

Cigarette smoke (CS)-induced activation of proteases such as matrix metalloproteinases (MMPs) contributes to lung alveolar destruction due to cell death. The aim of this study was to determine whether MMPs are produced in pulmonary artery endothelial cells (PAECs) and whether CS activation of MMPs is associated with apoptosis. Cultured PAECs were exposed to CS and subjected to assessments of apoptosis and MMPs. Western blotting and in situ zymography were performed to localize gelatinolytic activity and to identify enzymes. CS-induced apoptosis, i.e., enhanced annexin V binding and cleaved poly-ADP-ribose-polymerase (PARP), correlated with increased degradation of gelatin, a substrate of MMPs. The levels of pro-MMP-2 and active MMP-2 were increased in cytosolic and nuclear fractions isolated from CS-exposed cells. MMP-2 protein colocalized with gelatinolytic activity in the nucleus of CS-exposed cells undergoing apoptosis. These observations support the notion that MMP-2 contributes to CS-induced gelatinase activity, which localizes in the nuclear region primarily and correlates with annexin V binding and PARP cleavage. This suggests a novel function of MMP-2 in the degradation of the nuclear matrix in CS-induced endothelial apoptosis.

Caspase-3 and Tissue Factor Expression in Lipid-Rich Plaque Macrophages

BACKGROUND

Macrophages associated with arterial wall lipid deposition contribute to inflammatory processes. Tissue factor (TF) has been implicated in the thrombogenicity of atherosclerotic plaques. Intimal cells undergoing apoptosis have been postulated as a source for TF. However, there is only limited knowledge of cell type, plaque component, and conditions associated with TF expression and apoptosis. We examined the hypothesis that macrophages exposed to conditions of lipid-rich plaque undergo apoptosis and express TF.

METHODS AND RESULTS

In human carotid (n=15) and coronary (n=6) atherosclerotic plaques, TF and caspase-3 mRNA and protein expression (evaluated by in situ hybridization and immunohistochemistry) were increased significantly in lipid-rich compared with fibrous plaque components (P<0.01) and correlated with high macrophage content (P<0.05). Double-labeling studies demonstrated colocalization of TF and active caspase-3. In hyperlipidemic mice, expression of TF and active caspase-3 was observed simultaneously and colocalized in neointimal macrophages after arterial injury. In neointima of normolipidemic animals, TF and active caspase-3 were absent after arterial injury. In monocytes cultured in the presence of oxidized LDL, strong induction and colocalization of TF and active caspase-3 were found compared with baseline (P<0.05). Both antigens were significantly decreased after cotreatment with a caspase inhibitor (P<0.05) and were absent in untreated control cells.

CONCLUSIONS

The expression of TF as the primary cell-associated activator of the coagulation pathway proves to be closely related to macrophages undergoing apoptosis in conditions of lipid-rich plaque, pointing to a key role of lipid content and inflammatory cell viability in determining plaque thrombogenicity.

Heme and lipid peroxides in hemoglobin-modified low-density lipoprotein mediate cell survival and adaptation to oxidative stress

Low-density lipoprotein (LDL) oxidation mediated by a variety of catalysts in atherosclerotic lesions plays a crucial role in the genesis and evolution of atherosclerotic plaques. In this study we focused on oxidative properties of hemoglobin (Hb)-modified LDL because Hb is present in atherosclerotic lesions. Under low oxygen tensions Hb was previously found to modify apolipoprotein B100 with covalent binding of Hb fragments and formation of electronegative LDL particles (LDL-). Here we show that HbLDL is highly susceptible to oxidation, but is not cytotoxic to vascular cells, as was found for LDL- isolated from human plasma. HbLDL and LDL- have similar levels of oxidized lipid products and low uptake rates; however, the virtual absence of HbLDL-induced toxicity depends on a marked adaptive oxidative stress response. This was evidenced by a time- and dose-dependent induction of heme oxygenase (HO-1). Cell survival was significantly decreased in the presence of HO-1 inhibitor, tin protoporphyrin (SnPPIX). HO-1 induction by HbLDL increased resistance of cells to toxic doses of hemin or t-BuOOH. The high sensitivity to oxidation and HO-1 induction was largely dependent on lipid hydroperoxides and heme associated with HbLDL. Reduction of pre-existing lipid peroxides using ebselen delayed HbLDL kinetics and inhibited HO-1 induction. Moreover, heme inactivation or its degradation inhibited HO-1 induction and provided an additive inhibitory effect to ebselen. We conclude that Hb-catalyzed reactions may modulate vascular cell survival and oxidative stress adaptation due to the presence of peroxides and heme, thus providing a possible mechanism for the evolution of atherosclerotic and hemorrhagic lesions.

Degree of oxidation of low density lipoprotein affects expression of CD36 and PPARgamma, but not cytokine production, by human monocyte-macrophages

Oxidized low-density lipoprotein (oxLDL) exhibits many atherogenic effects, including the promotion of monocyte recruitment to the arterial endothelium and the induction of scavenger receptor expression. However, while atherosclerosis involves chronic inflammation within the arterial intima, it is unclear whether oxLDL alone provides a direct inflammatory stimulus for monocyte-macrophages. Furthermore, oxLDL is not a single, well-defined entity, but has structural and physical properties which vary according to the degree of oxidation. We tested the hypothesis that the biological effects of oxLDL will vary according to its degree of oxidation and that some species of oxLDL will have atherogenic properties, while other species may be responsible for its inflammatory activity. The atherogenic and inflammatory properties of LDL oxidized to predetermined degrees (mild, moderate and extensive oxidation) were investigated in a single system using human monocyte-derived macrophages. Expression of CD36 mRNA was up-regulated by mildly- and moderately-oxLDL, but not highly-oxLDL. The expression of the transcription factor, proliferator-activated receptor-gamma (PPARgamma), which has been proposed to positively regulate the expression of CD36, was increased to the greatest degree by highly-oxLDL. However, the DNA binding activity of PPARgamma was increased only by mildly- and moderately-oxLDL. None of the oxLDL species appeared to be pro-inflammatory towards monocytes, either directly or indirectly through mediators derived from lymphocytes, regardless of the degree of oxidation.