Agonists and inverse agonists for the herpesvirus 8-encoded constitutivelyactive seven-transmembrane oncogene product, ORF-74

A number of CXC chemokines competed with similar, nanomolar affinity agains t I-125-interleukin-8 (IL-8) binding to ORF-74, a constitutively active sev en-transmembrane receptor encoded by human herpesvirus 8. How ever, in comp etition against I-125-labeled growth-related oncogene (GRO)-alpha, the ORF- 74 receptor was highly selective for GRO peptides, with IL-8 being 10,000-f old less potent. The constitutive stimulating activity of ORF-74 on phospha tidylinositol turnover was not influenced by, for example, IL-8 binding. In contrast, GRO peptides acted as potent agonists in stimulating ORF-74 sign aling, whereas IP-10 and stromal cell-derived factor-1 alpha surprisingly a cted as inverse agonists. These peptides had similar pharmacological proper ties with regard to enhancing or inhibiting, respectively, the stimulatory effect of ORF-74 on NIH-3T3 cell proliferation. Construction of a high affi nity zinc switch through introduction of two His residues at the extracellu lar end of transmembrane segment V enabled Zn2+ to act as a prototype non-p eptide inverse agonist, which eliminated the constitutive signaling. It is concluded that ORF-74, which is believed to be causally involved in the for mation of highly vascularized tumors, has been optimized for agonist and in verse agonist modulation by the endogenous angiogenic GRO peptides and angi ostatic IP-10 and stromal cell-derived factor-leg respectively. ORF-74 coul d serve as a target for the development of non-peptide inverse agonist drug s as demonstrated by the effect of Zn2+ on the metal ion site-engineered re ceptor.