Effect of cobalt-mediated Toll-like receptor 4 activation on inflammatory responses in endothelial cells

Cobalt-containing metal-on-metal hip replacements are associated with adverse reactions to metal debris (ARMD), including inflammatory pseudotumours, osteolysis, and aseptic implant loosening. The exact cellular and molecular mechanisms leading to these responses are unknown. Cobaltions (Co(2+)) activate human Toll-like receptor 4 (TLR4), an innate immune receptor responsible for inflammatory responses to Gram negative bacterial lipopolysaccharide (LPS). We investigated the effect of Co(2+)-mediated TLR4 activation on human microvascular endothelial cells (HMEC-1), focusing on the secretion of key inflammatory cytokines and expression of adhesion molecules. We also studied the role of TLR4 in Co(2+)-mediated adhesion molecule expression in MonoMac 6 macrophages. We show that Co(2+) increases secretion of inflammatory cytokines, including IL-6 and IL-8, in HMEC-1. The effects are TLR4-dependent as they can be prevented with a small molecule TLR4 antagonist. Increased TLR4-dependent expression of intercellular adhesion molecule 1 (ICAM1) was also observed in endothelial cells and macrophages. Furthermore, we demonstrate for the first time that Co(2+) activation of TLR4 upregulates secretion of a soluble adhesion molecule, sICAM-1, in both endothelial cells and macrophages. Although sICAM-1 can be generated through activity of matrix metalloproteinase-9 (MMP-9), we did not find any changes in MMP9 expression following Co(2+) stimulation. In summary we show that Co(2+) can induce endothelial inflammation via activation of TLR4. We also identify a role for TLR4 in Co(2+)-mediated changes in adhesion molecule expression. Finally, sICAM-1 is a novel target for further investigation in ARMD studies.