Regulation of acetylated low density lipoprotein uptake in macrophages by pertussis toxin-sensitive G proteins

Class A scavenger receptors (SR-A) mediate the uptake of modified low density lipoprotein (LDL) by macrophages. Although not typically associated with the activation of intracellular signaling cascades, results with peritoneal macrophages indicate that the SR-A ligand acetylated LDL (AcLDL) promotes activation of cytosolic kinases and phospholipases. These signaling responses were blocked by the treatment of cells with pertussis toxin (PTX) indicating that SR-A activates G(i/o)-linked signaling pathways. The functional significance of SR-A-mediated G(i/o) activation is not clear. In this study, we investigated the potential role of G(i/o) activation in regulating SR-A-mediated lipoprotein uptake. Treatment of mouse peritoneal macrophages with PTX decreased association of fluorescently labeled AcLDL with cells. This inhibition was dependent on the catalytic activity of the toxin confirming that tile decrease in AcLDL uptake involved inhibiting G(i/o) activation. In contrast to the inhibitory effect on AcLDL uptake, PTX treatment did not alter β-VLDL-induced cholesterol esterification or deposition of cholesterol. The ability of polyinosine to completely inhibit AcEDL uptake, and the lack of PTX effect on β-VLDL uptake, demonstrated that the inhibitory effect is specific for SR-A and not the result of non-specific effects on lipoprotein metabolism. Despite having an effect on an SR-A- mediated lipoprotein uptake, there was no change in the relative abundance of SR-A protein after PTX treatment. These results demonstrate that activation of a PTX-sensitive G protein is involved in a feedback process that positively regulates SR-A function.