The Function And Mechanism Of Interleukin-34 In The Regulation Of Macrophage-drived Foam Cells

Background and Objective: The macrophage-drived foam cells and smooth muscle cell-drived foam cells are the crucial steps in the pathogenesis of atherosclerosis. The formation of foam cells can be regulated by various kinds of cytokines .Interleukin-34(IL-34),a new cytokine discovered in 2008, has closely associated with various chronicinflammatory and autoimmune diseases, including Rheumatoid Arthritis,Ulcerative Colitis and so on. IL-34 has been recognized as an inflammatory cytokine.Recently, studies suggest that expression of IL-34 incoronary artery diseases (CAD)are significantly elevated, and there is a positive correlation between the elevated level and severity of coronary artery lesions. However, there havebeen no studies currently on whether IL-34 involves in macrophage lipid metabolism and foam cell formation. In our present study we explored the contribution of IL-34 to foam cell formation and the underlying mechanism for the first time.Methods and Results:1. Bonemarrow-derived macrophages (BMDMs) were isolated from Apolipoprotein E deficient mice (apoE-/-), and induced to foam cells by co-incubated with oxLDL. BMDMswere divided intocontrol group,oxLDL group(40 ?g/ml), oxLDL+20ng/ml IL-34 group and oxLDL+50ng/ml IL-34group. Formation of foam cells was detected with Oil Red O staining and cholesterols in macrophages were measured by colorimetry. Results show that the severity of foam cells was significantly greater and the levels of TC, and cholesterol ester were significantly higher in oxLDL + 50ng/ml IL-34 group than in oxLDL group, suggesting that IL-34 50ng/ml could promote the formation of foam cells.2. The formation of foam cells is the disturbance of lipid uptake and cholesterol efflux. We explored the two progresses respectively. The results show that IL-34 treatment markedly increased the uptake of oxLDL but not cholesterol efflux.3. We examined the effects of IL-34 on the expression of CD36, SR-A and Lox-1,major scavenger receptors that mediate the uptake of modified LDL.Treatment with IL-34 (50ng/ml) markedly increased the protein and mRNA expression of CD36, but no such increase of SR-A and Lox-1 were seen. Antibody-dependent blocking assay was also performed, and the re-incubation with anti-CD36 antibody decreased IL-34-induced oxLDL uptake. Our results indicate that IL-34 (50ng/ml) enhanced oxLDL uptake in BMDMs through the up-regulation of CD36 expression. Besides, IL-34 also has a dramatic up-regulation effect on the expression of pro-inflammatory molecules induced by oxLDL such as IL-6?TNF-??IL-1 ? in BMDMs, consequently , leading to the deterioration of foam cells synergistically.4. Considering the importance of MAPK and NF-?B in regulating CD36 scavenger expression and many inflammatory genes, we analyzed the expression of molecules of MAPK and NF-?B, and we observed a higher phosphorylation of p38 after treatment with IL-34.After pretreatment with p38 inhibitor, the increased lipid-uptake induced by IL-34 was blocked.Conclusion:IL-34 facilitatesfoam cell formation by increasing CD36-mediated lipid uptake via p38 MAPK signal pathway in bone marrow-drived macrophages,subsequently promoted foam cells formation.