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 C1r and
C1s, 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 kappaB kinase-nuclear factor-kappaB (
NF-kappaB) pathway. LRP-1-deficient
macrophages, isolated from
mice, demonstrated increased expression of iNOS, C1r, and
monocyte chemoattractant protein-1 (
MCP-1);
MCP-1 expression was inhibited by
NF-kappaB antagonism. The mechanism by which LRP-1 inhibits
NF-kappaB 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-kappaB 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 kappaB kinase-NF-kappaB pathway.
