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.

Keywords:

• CD36
• neopterin
• 7,8-dihydroneopterin
• lipoxygenase
• PPAR-gamma

Acknowledgements

The authors thank the technical support staff in the School of Biological Sciences. This paper is dedecated to the memory of Dr Ghodsian’s mother for her endless love, support and encouragement.

Author contributions

Nooshin Ghodsian contributed to experimental design, data collection, results interpretation, figure preparation, manuscript preparation and editing. Anthony Yeandle contributed experimental design, data collection and results interpretation. Barry Hock advised on cell culture work, funding and manuscript editing. Steven Gieseg contributed to experimental direction, design, data interpretation, manuscript preparation and editing. He was principal investigator and supervisor of the research and was responsible for the securing the funding.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was partly funded through a project grant from the National Heart Foundation of New Zealand [Grant Number 1598] and Student Research Support from the School of Biological Sciences, University of Canterbury. Nooshin Ghodsian was supported by a University of Canterbury Post-graduate Scholarship.