Immunoreactivity of apo B towards monoclonal antibodies that inhibit the LDL-receptor interaction: effects of LDL oxidation

The role of oxidative stress in the long-term glycation of LDL

Advanced glycation is a major pathway for the posttranslational modification of plasma and tissue proteins. The initiating reaction is the nonenzymatic addition of sugars such as glucose to the primary amino groups of proteins, i.e., mainly to lysine residues. These "early" Schiff base and Amadori products then undergo a series of inter- and intramolecular rearrangements to produce the "late" products termed advanced glycation end products (AGEs). Incubation of LDL with glucose or glucose-6-phosphate produces AGE moieties on both the lipid and apolipoprotein B components. In addition, we tried to generate AGE-LDL by reaction with AGE-peptides (< 10 kD) obtained by enzymatic digestion of long-term glycated fibronectin as a model for connective tissue AGE-peptides. AGE-formation can be assessed by monitoring of fluorescence (370/440 nm) which is easily differentiated from the much lower autofluorescence of oxidized low density lipoproteins (oxLDL). Alternatively, AGE formation was detected by an AGE-specific ELISA using antibodies elicited in rabbits against bovine AGE-RNAse. In the present study we investigated the influence of oxidative stress on the long-term glycation of LDL and the modulation of LDL-oxidation by AGE-modification. We observed (a) that the rate of AGE formation is reduced by BHT/EDTA both on LDL and serum albumin (glycation vs. glycoxidation), (b) long-term glycated LDL is more readily oxidized than unglycated LDL, (c) oxLDL is more prone to AGE-modification, (d) AGE-modification of LDL strongly alters its epitope spectrum and (e) that aminoguanidine at higher concentrations (1-10 mM) inhibits copper-catalyzed LDL oxidation in the way of a classical antioxidant.

Oxidation-specific epitopes in human coronary atherosclerosis are not limited to oxidized low-density lipoprotein

BACKGROUND

Previous small studies have demonstrated positive immunohistochemical staining in rabbit and human atherosclerotic plaques by antibodies that recognize oxidized low-density lipoprotein (OxLDL), but none have examined a large number of human coronary arteries or evaluated whether epitopes recognized by these antibodies might be present on plaque proteins other than OxLDL.

METHODS AND RESULTS

Immunohistochemistry was performed on atherosclerotic (n = 87) and nonatherosclerotic (n = 51) coronary arterial segments from 20 patients by use of monoclonal antibodies that recognize epitopes on macrophages, smooth muscle cells, apolipoprotein (apo) B, and OxLDL. Staining with the OxLDL antibody (Ox5) was much more prevalent in atherosclerotic than in control segments. Extracellular Ox5 staining colocalized with apo B, but cell-associated Ox5 staining occurred in the absence of cell-associated apo B staining, which suggests that cell-associated epitopes for Ox5 were on proteins other than LDL. Epitopes for Ox5 formed in vitro on two readily available non-apo B proteins, human serum albumin and apo A-I, when these proteins were incubated under conditions of oxidant stress with polyunsaturated but not monounsaturated fatty acids; furthermore, an antioxidant inhibited Ox5 epitope formation. Thus, epitopes for Ox5 can form on proteins other than apo B. Also, phorbol ester-treated macrophages cultured in apo B-free medium developed epitopes for Ox5.

CONCLUSIONS

These findings are consistent with the hypothesis that atherosclerosis is associated with oxidative modification of proteins in addition to LDL, particularly cell-associated proteins, and that the antiatherosclerotic effects of antioxidants seen in some studies may not be solely due to prevention of LDL oxidation.

Cigarette smoke extract inhibits oxidative modification of low density lipoprotein

Oxidative modification of low density lipoprotein (LDL) by cigarette smoke has been suggested in several recent studies. To characterize possible modification of LDL by cigarette smoke extract (CSE), we incubated LDL with CSE either in the presence or absence of the chemical pro-oxidants, cupric chloride or 2,2'-azo-bis(2-amidinopropane) hydrochloride (AAPH). Surprisingly, CSE inhibited oxidative modification of LDL induced by either copper or AAPH. Under such oxidant stress conditions, CSE inhibited formation of thiobarbituric acid-reactive substances and also inhibited the increased agarose gel electrophoretic mobility of LDL in a dose-response manner. In addition, CSE prevented degradation of phosphatidylcholine to lysophosphatidylcholine and also fragmentation of the apolipoprotein B-100 moiety of LDL. Finally, CSE inhibited loss of immunoreactivity of the treated LDL with a murine monoclonal antibody against human apolipoprotein B-100. On the other hand, at higher concentrations, CSE per se was still able to cause structural changes in LDL. After incubation with CSE for 24 h, LDL showed a slight increase in agarose gel electrophoretic mobility, a slight loss of immunoreactivity with monoclonal antibody, and a marked increase in protein carbonyl formation. Lipid peroxidation did not appear to be involved in the modification of LDL caused by CSE. It is suggested that reactive aldehydes present in cigarette smoke may cause direct chemical modification of LDL. Furthermore, the free radical-scavenging potential of the tar fraction of cigarette smoke may be responsible for the apparent antioxidant properties of CSE against LDL oxidation.