Effects of lysosomal low-density lipoprotein oxidation by ferritin on macrophage function

Abstract

We have previously demonstrated that low-density lipoprotein (LDL) can be oxidized by iron in the lysosomes of macrophages. Some of the iron content of lysosomes might be delivered through autophagy of ferritin (the main iron-storage protein in the body). We have now investigated the effects of ferritin-mediated LDL oxidation on macrophage function. The addition of ferritin to human THP-1 cells and human monocyte-derived macrophages increased lysosomal lipid peroxidation, as shown by LPO-Foam, a fluorescent probe targeted to lysosomes. Incubating THP-1 cells with ferritin and native LDL or LDL aggregated by sphingomyelinase, to allow their endocytosis and delivery to lysosomes, led to the formation of lysosomal ceroid (an advanced lipid oxidation product), indicative of lysosomal LDL oxidation. Incubating THP-1 cells with ferritin and LDL caused metabolic activation of the cells, as shown by increased extracellular acidification and oxygen consumption measured by a Seahorse analyzer. LDL oxidized by ferritin in lysosomes might be released from macrophages when the cells die and lyse and affect neighboring cells in atherosclerotic lesions. Adding LDL oxidized by ferritin at lysosomal pH (pH 4.5) to macrophages increased their intracellular reactive oxygen species formation, shown using dihydroethidium, and increased apoptosis. Ferritin might therefore contribute to LDL oxidation in the lysosomes of macrophages and have atherogenic effects.

Graphical abstract

Keywords:

• LDL oxidation
• ferritin
• lysosome
• macrophage
• atherosclerosis

Acknowledgements

The authors are grateful to Tertiary Education Trust Fund (TETFund) of Nigeria for a PhD studentship for O.O.O. We thank our colleagues, who volunteered to donate blood and Dr. Kim G. Jackson and Ms. Rada G. Mihaylova for skillfully taking blood for LDL isolation. The authors are grateful to Professor Yi Xiao, Dalian University of Technology, PR China for kindly giving us Foam-LPO.

Author contributions

O.O.O. carried out the experiments and O.O.O. and D.S.L. wrote the paper.

Disclosure statement

No potential conflict of interest was reported by the authors.