CD36 down regulation by the macrophage antioxidant 7,8-dihydroneopterin through modulation of PPAR-γ activity

Abstract

CD36 is the key scavenger receptor driving the formation of cholesterol-loaded foam cells, the principal cellular component of atherosclerotic plaques. CD36 is down regulated by 7,8-dihydroneopterin, a potent superoxide and hypochlorite scavenging antioxidant generated by interferon-γ stimulated macrophages. 7,8-dihydroneopterin downregulates CD36 mRNA and protein levels so inhibiting macrophage foam cell formation in vitro. We examined the mechanism of 7,8-dihydroneopterin downregulation of CD36 by measuring CD36 and PPAR-γ levels by Western blot analysis, in the monocyte-like U937 cells with a range of PPAR-γ stimulants and inhibitors. Lipoxygenase activity was measured by monitoring linoleic acid oxidation at 234 nm for diene formation. Between 100 and 200 μM, 7,8-dihydroneopterin decreased CD36 levels by 50% within 12 h with levels dropping below 25% by 24 h. CD36 levels returned to basal levels after 24 h. Inhibition of protein synthesis by cycloheximide shows 7,8-dihydroneopterin had no effect on CD36 degradation rates. PPAR-γ levels were not altered by the addition of 7,8-dihydroneopterin. MAP Kinase, P38 and NF-κB pathways inhibitors SP600125, PD98059, SB202190 and BAY 11-7082, respectively, did not restore the CD36 levels in the presence of 7,8-dihydroneopterin. The addition of the lipophilic PPAR-γ activators rosiglitazone and azelaoyl-PAF prevented the CD36 downregulation by 7,8-dihydroneopterin. 7,8-dihydroneopterin inhibited soybean lipoxygenase and reduced U937 cell basal levels of cellular lipid oxides as measured by HPLC-TBARS analysis. The data show 7,8-dihydroneopterin down regulates CD36 expression by decreasing the level of lipid oxide stimulation of PPAR-γ promotor activity, potentially through lipoxygenase inhibition.