Lipopolysaccharide augments the uptake of oxidized LDL by up-regulating lectin-like oxidized LDL receptor-1 in macrophages
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There is a growing body of evidence supporting an intimate association of immune activation with the pathogenesis of cardiovascular diseases, including atherosclerosis. Uptake of oxidized low-density lipoprotein (oxLDL) through scavenging receptors promotes the formation of mature lipid-laden macrophages, which subsequently leads to exacerbation of regional inflammation and atherosclerotic plaque formation. In this study, we first examined changes in the mRNA level of the lectin-like oxLDL receptor-1 (LOX-1) in the mouse macrophage cell line RAW264.7 and the human PMA-induced macrophage cell line THP-1 after LPS stimulation. LPS significantly up-regulated LOX-1 mRNA in RAW264.7 cells; LOX-1 cell-surface protein expression was also increased. Flow cytometry and fluorescence microscopy analyses showed that cellular uptake of fluorescence (Dil)-labeled oxLDL was significantly augmented with LPS stimulation. The augmented uptake of Dil-oxLDL was almost completely abrogated by treatment with an anti-LOX-1 antibody. Of note, knockdown of Erk1/2 resulted in a significant reduction of LPS-induced LOX-1 up-regulation. Treatment with U0126, a specific inhibitor of MEK, significantly suppressed LPS-induced expression of LOX-1 at both the mRNA and protein levels. Furthermore, LOX-1 promoter activity was significantly augmented by LPS stimulation; this augmentation was prevented by U0126 treatment. Similar results were also observed in human PMA-induced THP-1 macrophages. Taken together, our results indicate that LPS up-regulates LOX-1, at least in part through activation of the Erk1/2 signaling pathway, followed by augmented cellular oxLDL uptake, thus highlighting a critical role of TLR4-mediated aberrant LOX-1 signaling in the pathogenesis of atherosclerosis.
KeywordsLPS LOX-1 Atherosclerosis Erk1/2 Molecular biology
ATP-binding cassette, sub-family A, member 1
Activator protein 1
Caffeic acid phenethylester
Extracellular signal-regulated protein kinases 1 and 2
Fluorescence-activated cell sorting
C-Jun N-terminal kinases
Lectin-like oxidized low-density lipoprotein receptor-1
Mitogen-activated protein kinase
Nuclear factor of kappa light polypeptide gene enhancer in B cells
Oxidized low-density lipoprotein
Scavenger receptor A
This work was partly supported by a grant from the Strategic Research Foundation Grant-aided Project for Private Universities from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) [S1101027 to S. K., H. K., and Y. H.]; and the AIKEIKAI Foundation [to A. O.]. We would like to thank Dr. Takashi Yokochi and Dr. Hiroshi Miwa at Aichi Medical University (Aichi, Japan) for kindly providing the RAW264.7 mouse macrophage cell line and THP-1 human acute monocytic leukemia cell line, respectively.
Conflict of interest disclosure
We declare that we have no conflict of interest.
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