The ubiquitin ligase Nedd4 mediates oxidized low-density lipoprotein-induced downregulation of insulin-like growth factor-1 receptor.
Oxidized low-density lipoprotein (LDL) is proatherogenic and induces smooth muscle cell apoptosis, which contributes to atherosclerotic plaque destabilization. We showed previously that oxidized LDL downregulates insulin-like growth factor-1 receptor in human smooth muscle cells and that this is critical for induction of apoptosis. To identify mechanisms, we exposed smooth muscle cells to 60 mug/ml oxidized LDL or native LDL and assessed insulin-like growth factor-1 receptor mRNA levels, protein synthesis rate, and receptor protein stability. Oxidized LDL decreased insulin-like growth factor-1 receptor mRNA levels by 30% at 8 h compared with native LDL, and this decrease was maintained for up to 20 h. However, insulin-like growth factor-1 receptor protein synthesis rate was not altered by oxidized LDL. Pulse-chase labeling experiments revealed that oxidized LDL reduced insulin-like growth factor-1 receptor protein half-life to 12.2+/-1.7 h from 24.4+/-4.7 h with native LDL. This destabilization of insulin-like growth factor-1 receptor protein was accompanied by enhanced receptor ubiquitination. Overexpression of dominant-negative Nedd4 prevented oxidized LDL-induced downregulation of insulin-like growth factor-1 receptor, suggesting that Nedd4 was the ubiquitin ligase that mediated receptor downregulation. However, the proteasome inhibitors lactacystin, MG-132, and proteasome inhibitor-1 failed to block oxidized LDL-induced downregulation of insulin-like growth factor-1 receptor. Thus oxidized LDL downregulates insulin-like growth factor-1 receptor by destabilizing the protein via Nedd4-enhanced ubiquitination, leading to degradation via a proteasome-independent pathway. This finding provides novel insights into oxidized LDL-triggered oxidant signaling and mechanisms of smooth muscle cell depletion that contribute to plaque destabilization and coronary events.
Higashi
Yusuke
Y
Section of Cardiology, Tulane University School of Medicine, New Orleans, Louisiana 70112, USA.
Sukhanov
Sergiy
S
Parthasarathy
Sampath
S
Delafontaine
Patrice
P
D050197:Atherosclerosis / Q000201:enzymology; D002478:Cells, Cultured; D015536:Down-Regulation; D056827:Endosomal Sorting Complexes Required for Transport; D006207:Half-Life; D006801:Humans; D008077:Lipoproteins, LDL / Q000378:metabolism*; D009131:Muscle, Smooth, Vascular / Q000201:enzymology*; D032389:Myocytes, Smooth Muscle / Q000201:enzymology; D000075702:Nedd4 Ubiquitin Protein Ligases; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*; D013997:Time Factors; D014162:Transfection; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism*; D054875:Ubiquitination
D056827:Endosomal Sorting Complexes Required for Transport; D008077:Lipoproteins, LDL; D012333:RNA, Messenger; C099252:oxidized low density lipoprotein; D000075702:Nedd4 Ubiquitin Protein Ligases; C000618483:Nedd4 protein, human; D044767:Ubiquitin-Protein Ligases; D017526:Receptor, IGF Type 1; D046988:Proteasome Endopeptidase Complex