Chemokines in rheumatoid arthritis: novel, potential therapeutic targets of the CCR2 chemokine receptor signaling pathway

Several chemokine receptor antagonists are in phase I/II clinical trials. Macrophages play a pivotal role in rheumatoid arthritis, and the monocyte chemoattractant protein (MCP)-1/CCR2 axis determines the macrophage recruitment into inflamed joints. We have recently discovered molecules specifically associated with CCR2.

We incidentally discovered that an organic germanium, propagermanium (3-oxygermylpropionic acid polymer), which has been used as a therapeutic agent against chronic hepatitis associated with hepatitis B virus in Japan, very specifically inhibits CCR2-mediated monocyte chemotaxis. The effect of propagermanium requires glycosylphosphatidylinositol (GPI)-anchored proteins, as cleavage of GPI anchors by phosphatidylinositol-phospholipase C eliminated the inhibitory activity of propagermanium. In addition, anti-GPI-anchored protein CD55 antibody and anti-CD95 antibody selectively inhibited MCP-1-induced monocyte chemotaxis. Furthermore, GPI-anchored proteins were co-localized with CCR2 on monocytes under fluorescence microscopy. Moreover, a synthetic peptide corresponding to the N-terminal portion of CCR2 specifically blocked the action of propagermanium, suggesting that propagermanium may bridge GPI-anchored proteins and the N-terminal portion of CCR2, and may interfere with the action of MCP-1 (collaboration with Ishiwata et al., Sanwa Kagaku, Mie, Japan). The biological meaning of the close association of GPI-anchored proteins with CCR2 is unclear at present, but this finding corresponds well with the notion that some of chemokine receptors exist in the lipid raft of leukocyte cell membrane.

Critical roles of C-terminal portion of chemokine receptors in regulating leukocyte chemotaxis have been suggested. To seek CCR2-interacting proteins, we have adopted a yeast two-hybrid system and found a cytoplasmic protein, FROUNT, that specifically interacts with CCR2. Disrupting the interaction through the expression of FROUNT antisense mRNA or a truncated FROUNT mutant specifically abolished the chemotactic response to MCP-1 in human monocytes. FROUNT seems to be critically involved in receptosome formation based on confocal microscopic analysis. Suppression of endogenous FROUNT in a murine model of peritonitis markedly impaired CCR2-dependent recruitment of macrophages into the peritoneal cavity. Those results implicate that FROUNT regulates CCR2-mediated chemotactic signaling and the FROUNT could be a novel therapeutic target for macrophage-mediated chronic inflammatory diseases including rheumatoid arthritis.