LRP1 Controls TNF Release via the TIMP-3/ADAM17 Axis in Endotoxin-Activated Macrophages

The metalloproteinase ADAM17 plays a pivotal role in initiating inflammation by releasing TNF from its precursor. Prolonged TNF release causes many chronic inflammatory diseases, indicating that tight regulation of ADAM17 activity is essential for resolution of inflammation. In this study, we report that the endogenous ADAM17 inhibitor TIMP-3 inhibits ADAM17 activity only when it is bound to the cell surface and that cell surface levels of TIMP-3 in endotoxin-activated human macrophages are dynamically controlled by the endocytic receptor LRP1. Pharmacological blockade of LRP1 inhibited endocytic clearance of TIMP-3, leading to an increase in cell surface levels of the inhibitor that blocked TNF release. Following LPS stimulation, TIMP-3 levels on the surface of macrophages increased 4-fold within 4 h and continued to accumulate at 6 h, before a return to baseline levels at 8 h. This dynamic regulation of cell surface TIMP-3 levels was independent of changes in TIMP-3 mRNA levels, but correlated with shedding of LRP1. These results shed light on the basic mechanisms that maintain a regulated inflammatory response and ensure its timely resolution.