Oxidized LDLs alter the activity of the ubiquitin-proteasome pathway: potential role in oxidized LDL-induced apoptosis

Oxidized low-density lipoproteins (oxLDL) play a role in the genesis of ath erosclerosis. OxLDL are able to induce apoptosis of vascular cells, which i s potentially involved in the formation of the necrotic center of atheroscl erotic lesions, plaque rupture, and subsequent thrombotic events, Because o xLDL may induce structural modifications of cell protein and altered protei ns may impair cell viability, the present work aimed to evaluate the extent of protein alterations, the degradation of modified proteins through the u biquitin-proteasome system (a major degradative pathway for altered and oxi datively modified proteins) and their role during apoptosis induced by oxLD L. This paper reports the following: 1) oxLDL induce derivatization of cell proteins by 4-hydroxynonenal (4-HNE) and ubiquitination. 2) Toxic concentr ations of oxLDL elicit a biphasic effect on proteasome activity. An early a nd transient activation of endogenous proteolysis is followed rapidly by a subsequent decay (resulting probably from the 26S proteasome inhibition) an d followed later by the inhibition of the 20S proteasome las assessed by in hibition of sLLVY-MCA hydrolysis). 3) Specific inhibitors of proteasome (la ctacystin and proteasome inhibitor I) potentiated considerably the toxicity of oxLDL (nontoxic doses of oxLDL became severely toxic). The defect of th e ubiquitination pathway tin temperature-sensitive mutants) also potentiate d the toxicity of oxLDL. This suggests that the ubiquitin-proteasome pathwa y plays a role in the cellular defenses against oxLDL-induced toxicity. 4) Dinitrophenylhydrazine (DNPH), an aldehyde reagent, prevented both the oxLD L- induced derivatization of cell proteins and subsequent cytotoxicity. Alt ogether, the reported data suggest that both derivatization of cell protein s (by 4-HNE and other oxidized lipids) and inhibition of the proteasome pat hway are involved in the mechanism of of oxLDL-induced apoptosis.