A novel orphan GPCR has been identified that appears to mediate an important chemotactic response in immune cells.
The directional response by motile cells to a chemical gradient, otherwise known as chemotaxis, is an important feature in many pathways, including inflammatory response. But although most chemotaxis pathways involve signal transduction mediated through G-protein-coupled receptors (GPCRs), GPCRs in certain areas of the inflammatory response pathway have remained unidentified. Now, in Molecular Pharmacology, Carol Jones and colleagues at Novartis report the identification of a novel orphan GPCR that appears to mediate an important chemotactic response in immune cells.
Searches for additional members of the GPCR family in human genome databases led to the identification of R527. R527 has high similarity to the orphan receptors HM74, GPR81 and GPR31 at the amino-acid level, and was found to be highly expressed in immune cells, including eosinophils, neutrophils and lung macrophages. A cell line expressing R527 and the G protein -subunit (a promiscuous G protein which has been used to generate screening platforms for GPCR ligand fishing) was generated to screen a collection of 2,000 known and putative GPCR agonists. Leukotrienes, one class of chemotactic agents, failed to elicit a response, but two lipids activated R527 — the most effective being the eicosanoid 5-oxo-6E, 8Z, 11Z, 14Z-eicosatetraenoic acid (5-oxo-ETE). Increasing concentrations of 5-oxo-ETE added to the cell line inhibited the production of cyclic AMP, showing that R527 is coupled to the inhibitory subtype of G proteins (Gi), and this was confirmed by showing that pertussis toxin (which inhibits Gi proteins) reverses this effect.
The findings could tie up some loose ends in this area. 5-oxo-ETE is known to be a potent simulator of chemotaxis and exerts its effects on eosinophils, neutrophils and monocytes. And the chemotactic responses induced by 5-oxo-ETE were thought to be mediated through a GPCR, the pharmacological properties of which appear to be similar to R527. Although it is possible that there might be other unidentified receptors that bind 5-oxo-ETE, evidence that 5-oxo-ETE could be a physiological mediator of inflammation in asthma indicates that R527 could have a key role in this and other inflammatory diseases (for example, psoriasis).
Simon Frantz
References
Jones, C. E. et al. Expression and characterization of a 5-oxo-6E, 8Z, 11Z, 14Z-eicosatetraenoic acid receptor highly expressed on human eosinophils and neutrophils. Mol. Pharmacol.63, 471–477 (2003) | Article | PubMed |
-subunit (a promiscuous G protein which has been used to generate screening platforms for GPCR ligand fishing) was generated to screen a collection of 2,000 known and putative GPCR agonists. Leukotrienes, one class of chemotactic agents, failed to elicit a response, but two lipids activated R527 — the most effective being the eicosanoid 5-oxo-6E, 8Z, 11Z, 14Z-eicosatetraenoic acid (5-oxo-ETE). Increasing concentrations of 5-oxo-ETE added to the cell line inhibited the production of cyclic AMP, showing that R527 is coupled to the inhibitory subtype of G proteins (G