Chemoattractant receptors in drug discovery : FPR2 and CCR2-two potential targets

Abstract

Chemoattractants and chemoattractant receptors have many important functions in multicellular organisms, not least for their role in regulating migration of leukocytes. The receptors are also involved in many pathologies and they have, since they were cloned in the 1990ies, been regarded as attractive targets for development of drugs against cancer, viral infections, and inflammatory/autoimmune diseases.

Although chemotactic molecules are very heterogeneous in terms of chemical structure most of them mediate their actions through binding to a G-protein-coupled receptor (GPCR). The formyl peptide receptors (FPRs) are chemoattractant receptors expressed on phagocytes and have important implications in host defense and inflammation as well as in resolution of inflammation. One of the goals was to identify FPR2 specific ligands with similar anti-inflammatory and proresolution properties as the eicosanoid lipoxin A4 previously described. A small compound library was screened for both agonist and antagonists and selected compounds were analyzed with respect to anti-inflammatory properties such as inhibition of NADPH oxidase activity and degranulation in primary neutrophils. Receptor selectivity was investigated by compound stimulation and desensitization studies in presence of receptor-specific tools. While the agonist screen resulted in non selective hit compounds with proinflammatory properties, the antagonist screen led to identification of anti-inflammatory agents with different selectivity.

Another chemoattractant receptor binds CCL2, a member of the superfamily of chemokines. The function of CCL2 and its receptor (CCR2) is of importance for monocyte recruitment, and together this receptor ligand pair has many implications in inflammation. Accordingly, the importance of the CCL2/CCR2 axis in the recruitment of inflammatory cells and fibroblasts into fibrotic tissues was hypothesized as a mechanism for induction of fibrosis. There is, however, emerging evidence indicating that CCL2 may promote fibrosis also by other mechanisms such as activation of the fibroblast. The myofibroblast, synthesizing large amounts of extracellular matrix components (ECM) in response to profibrotic cytokines and growth factors, is the key cell in fibrosis. The effects of CCL2 on human fibroblasts with respect to ECM expression were studied and it was found that CCL2 might have a more multifaceted role in fibroblast activation than previously described.

The truncated form of CCL2, CCL2 (1,9-76), also known as 7ND, is a receptor antagonist and is described to have therapeutic effects in several animal disease models such as bleomycin-induced lung fibrosis. The interaction between the 7ND and the human as well as the murine CCR2 was studied in vitro and data showed that 7ND was an inhibitor of human CCR2 but a very low affinity binder of the murine receptor. 7ND was, however, shown to downregulate fibrotic markers in murine fibroblasts, but in accordance with its inability to bind CCR2 the downregulation was found to be independent of this receptor.