Activation of mTOR modulates SREBP-2 to induce foam cell formation through increased retinoblastoma protein phosphorylation.
OBJECTIVE
Our previous studies demonstrated that inflammation contributes to atherosclerosis through disruption of the low density lipoprotein receptor (LDLr) pathway. However, this effect is overridden by rapamycin, which is an inhibitor of mammalian target of rapamycin (mTOR). This study investigated the role of the mTOR pathway in atherosclerosis in vivo and in vitro.


RESULTS
To induce inflammation, we used subcutaneous injection of 10% casein in apolipoprotein E knockout (ApoE KO) mice and lipopolysaccharide stimulation in rat vascular smooth muscle cells (VSMCs). Results showed that inflammation increased lipid accumulation in aortas of ApoE KO mice and in VSMCs, which were correlated with increased expressions of LDLr, sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP), and SREBP-2 as well as with enhanced translocation of SCAP/SREBP-2 complex from the endoplasmic reticulum (ER) to the Golgi. Furthermore, inflammation increased both the percentage of cells in the S phase of cell cycle and protein expressions of the phosphorylated forms of retinoblastoma tumour suppressor protein (Rb), mTOR, eukaryotic initiation factor 4E-binding protein 1 (4EBP1), and P70 S6 kinase. After treatment with rapamycin or mTOR siRNA, the activity of the mTOR pathway was blocked. Interestingly, the expression levels of LDLr, SCAP, and SREBP-2 and the translocation of SCAP/SREBP-2 complex from the ER to the Golgi in treated VSMCs were decreased even in the presence of inflammatory stress.


CONCLUSIONS
Our findings demonstrate for the first time that inflammation disrupts LDLr feedback regulation through the activation of the mTOR pathway. Increased mTORC1 activity was found to up-regulate SREBP-2-mediated cholesterol uptake through Rb phosphorylation.
Institute of Nephrology, Zhong Da Hospital, Southeast University School of Medicine, NO.87, Ding Jia Qiao Road, Nanjing City, Jiangsu Province 210009, P.R. China.
Kun Ling
KL
Ma
Jing
J
Liu
Chang Xian
CX
Wang
Jie
J
Ni
Yang
Y
Zhang
Yu
Y
Wu
Lin Li
LL
Lv
Xiong Zhong
XZ
Ruan
Bi Cheng
BC
Liu
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001011:Aorta / Q000201:enzymology / Q000473:pathology; D001018:Aortic Diseases / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D001057:Apolipoproteins E / Q000172:deficiency / Q000235:genetics; D050197:Atherosclerosis / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D002352:Carrier Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism; D004195:Disease Models, Animal; D004721:Endoplasmic Reticulum / Q000378:metabolism; D039642:Eukaryotic Initiation Factors; D005487:Foam Cells / Q000187:drug effects / Q000201:enzymology* / Q000473:pathology; D006056:Golgi Apparatus / Q000378:metabolism; D007249:Inflammation / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008297:Male; D000076222:Mechanistic Target of Rapamycin Complex 1; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D046912:Multiprotein Complexes / Q000378:metabolism; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000201:enzymology* / Q000473:pathology; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D034622:RNA Interference; D051381:Rats; D011973:Receptors, LDL / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000378:metabolism*; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D016196:S Phase; D015398:Signal Transduction; D051782:Sterol Regulatory Element Binding Protein 2 / Q000378:metabolism*; D051779:Sterol Regulatory Element Binding Proteins / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
D048868:Adaptor Proteins, Signal Transducing; D001057:Apolipoproteins E; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C499527:Eif4ebp1 protein, mouse; C499526:Eif4ebp1 protein, rat; D039642:Eukaryotic Initiation Factors; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D046912:Multiprotein Complexes; D010750:Phosphoproteins; D047428:Protein Kinase Inhibitors; D011973:Receptors, LDL; D016160:Retinoblastoma Protein; C102196:SREBP cleavage-activating protein; C497391:Srebf2 protein, mouse; C470194:Srebf2 protein, rat; D051782:Sterol Regulatory Element Binding Protein 2; D051779:Sterol Regulatory Element Binding Proteins; D002784:Cholesterol; C546843:mTOR protein, mouse; C546845:mTOR protein, rat; D000076222:Mechanistic Target of Rapamycin Complex 1; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases
Foam cell formation; Inflammation; LDL receptor pathway; Mammalian target of rapamycin