Regulation of tumor necrosis factor receptor-1 and the IKK-NF-κB pathway by LDL receptor-related protein explains the antiinflammatory activity of this receptor

Low-density lipoprotein receptor-related protein (LRP-1) functions in endocytosis and in cell signaling directly (by binding signaling adaptor proteins) or indirectly (by regulating levels of other cell-surface receptors). Because recent studies in rodents suggest that LRP-1 inhibits inflammation, we conducted activity-based protein profiling experiments to discover novel proteases, involved in inflammation, that are regulated by LRP-1. We found that activated complement proteases accumulate at increased levels when LRP-1 is absent. Although LRP-1 functions as an endocytic receptor for Clr and Cls, complement protease mRNA expression was increased in LRP-1-deficient cells, as was expression of inducible nitric oxide synthase (iNOS) and interleukin-6. Regulation of expression of inflammatory mediators was explained by the ability of LRP-1 to suppress basal cell signaling through the I kappa B kinase-nuclear factor-kappa B (NF-kappa B) pathway. LRP-1-deficient macrophages, isolated from mice, demonstrated increased expression of iNOS, Clr, and monocyte chemoattractant protein-1 (MCP-1); MCP-1 expression was inhibited by NF-kappa B antagonism. The mechanism by which LRP-1 inhibits NF-kappa B activity involves down-regulating cell-surface tumor necrosis factor receptor-1 (TNFR1) and thus. inhibition of autocrine TNFR1-initiated cell signaling. TNF-alpha-neutralizing antibody inhibited NF-kappa B activity selectively in LRP-1-deficient cells. We propose that LRP-1 suppresses expression of inflammatory mediators indirectly, by regulating TNFR1-dependent cell signaling through the I kappa B kinase-NF-kappa B pathway.