Prevention of macrophage apoptosis in advanced atherosclerotic lesions can help stop atherosclerosis progression to vulnerable plaques. High density lipoprotein (HDL) can protect macrophages from apoptosis that has been induced by a variety of agents. We hypothesize that this is the consequence of the sphingolipid, sphingosine-1-phosphate (S1P), specifically carried by HDL, and transferred to S1P receptor 1 (S1PR1) on the cells via the HDL receptor, scavenger receptor class B type 1 (SR-B1).
Apoptosis was induced in murine peritoneal macrophages from wild type and different knockout mice with the ER stress inducing agent tunicamycin. Apoptosis was then observed and detected by terminal deoxynucleotidyl transferase mediated dUTP nick end labeling through fluorescent microscopy. All experiments were conducted with an n of 3 or 4.
Treatment of cells with HDL protected them against tunicamycin induced apoptosis. In contrast, pre-treatment of HDL with S1P lyase, which irreversibly cleaves S1P, eliminated the ability of HDL to protect macrophages. Furthermore, HDL-dependent protection of macrophages against apoptosis required both the HDL receptor SRB1 and the S1PR1. Inhibitor of SRB1’s lipid transport activity also prevented HDL dependant protection against apoptosis.
These results suggest that the HDL mediated protection of macrophages against apoptosis may involve SRB1 mediated delivery of S1P from HDL to the S1PR1. Understanding the mechanisms by which HDL elicits atheroprotective signalling in macrophages will provide insight into new targets for therapeutic intervention.