Potency of ligands correlates with affinity measured against agonist and inverse agonists but not against neutral ligand in constitutively active chemokine receptor.

ORF-74, a 7TM receptor oncogene encoded by human herpes virus 8, shows 50% constitutive activity in stimulating phosphatidylinositol turnover and binds a large variety of CXC chemokines. These endogenous ligands cover a full spectrum of pharmacological properties with growth-related oncogene (GRO)-alpha and -gamma functioning as full agonists; GRObeta as a partial agonist; interleukin (IL)-8, neutrophil-activating peptide (NAP)-2, and epithelial cell-derived activating peptide (ENA)-78 as neutral ligands; granulocyte colony-stimulating factor (GCP)-2 as a partial inverse agonist; and interferon-gamma inducible protein (IP)-10 and stromal cell-derived factor (SDF)-1alpha as full inverse agonists. The affinity for the agonists was independent of whether it was determined in competition binding against the agonist (125)I-GROalpha, against the inverse agonist (125)I-IP-10, or against the neutral ligand (125)I-IL-8. Similarly, the affinities of the inverse agonists were within 1 order of magnitude independent of the choice of radioligand. In contrast, the neutral ligands IL-8, NAP-2, and ENA-78, which all displaced (125)I-IL-8 with single-digit nanomolar affinity showed up to 1000-fold lower affinity against both the radioactive agonist and against the radioactive inverse agonist. A close correlation was observed between the EC(50) values for the ligands and their IC(50) values measured against either radioactive agonist or radioactive inverse agonist, but a poor correlation was found to the IC(50) value measured against the neutral ligand. It is concluded that in ORF-74, ligands compete for binding more according to pharmacological property than to structural homology and that both agonists and inverse agonists, in contrast to neutral ligands, apparently bind with high affinity either to a common conformation of the receptor or to readily interconvertible states, not available for the neutral ligands.