Macrophage expression of LRP1, a receptor for apoptotic cells and unopsonized erythrocytes, can be regulated by glucocorticoids.

Macrophage phagocytosis of apoptotic cells, or unopsonized viable CD47(-/-) red blood cells, can be mediated by the interaction between calreticulin (CRT) on the target cell and LDL receptor-related protein-1 (LRP1/CD91/α2-macroglobulin receptor) on the macrophage. Glucocorticoids (GC) are powerful in treatment of a range of inflammatory conditions, and were shown to enhance macrophage uptake of apoptotic cells. Here we investigated if the ability of GC to promote macrophage uptake of apoptotic cells could in part be mediated by an upregulation of macrophage LRP1 expression. Using both resident peritoneal and bone marrow-derived macrophages, we found that the GC dexamethasone could dose- and time-dependently increase macrophage LRP1 expression. The GC receptor-inhibitor RU486 could dose-dependently prevent LRP1 upregulation. Dexamethasone-treated macrophages did also show enhanced phagocytosis of apoptotic thymocytes as well as unopsonized viable CD47(-/-) red blood cells, which was sensitive to inhibition by the LRP1-agonist RAP. In conclusion, these data suggest that GC-stimulated macrophage uptake of apoptotic cells may involve an upregulation of macrophage LRP1 expression and enhanced LRP1-mediated phagocytosis.