An Examination of the Role of Sphingosine-1-Phosphate in High Density Lipoprotein Mediated Protection of Macrophages Against Apoptosis

Abstract

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 (S1P1) 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, tunicamycin, thapsigargin, staurosporine, or UV irradiation. Apoptosis was measured by terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) or with cleaved caspase-3 (CC3) staining. Treatment of cells with HDL or S1P protected them against apoptosis induced by a variety of stimuli. In contrast, pre-treatment of HDL with S1P lyase, which irreversibly cleaves S1P, eliminated the ability of HDL to protect macrophages. Inhibition of SR-B1’s lipid transport activity reduced HDL dependant protection against apoptosis. Furthermore, HDL dependent protection against apoptosis induced by tunicamycin was prevented when the S1P receptor S1P1 was knocked out. However, this protection was not prevented when apoptosis was induced by staurosporine. These results suggest that the HDL mediated protection of macrophages against apoptosis is multi-faceted and one approach may involve SR-B1 mediated delivery of S1P from HDL to the S1P1. Understanding the mechanisms by which HDL elicits atheroprotective signalling in macrophages will provide insight into new targets for therapeutic intervention in atherosclerotic disease.