Abstract
Prostaglandin (PG) E(2) (PGE(2)) is a potent prostanoid derived from arachidonic which can interact with EP(1), EP(2), EP(3) and EP(4) prostanoid receptor subtypes. Recombinant human EP(4) receptors expressed in human embryonic kidney (HEK-293) cells were evaluated for their binding characteristics using [(3)H]-PGE(2) and a broad panel of natural and synthetic prostanoids in order to define their pharmacological properties. [(3)H]-PGE(2) binding was optimal in 2-[N-Morpholino]ethanesulphonic acid (MES) buffer (pH 6.0) yielding 98+/-0.7% specific binding. The receptor displayed high affinity (K(d)=0.72+/-0.12 nM; n=3) for [(3)H]-PGE(2) and interacted with a saturable number of binding sites (B(max)=6.21+/-0.84 pmol mg(-1) protein). In competition studies, PGE(2) (K(i)=0.75+/-0.03 nM; n=12) and PGE(1) (K(i)=1.45+/-0.24 nM; n=3) displayed high affinities, as did two derivatives of PGE(1), namely 11-deoxy-PGE(1) (K(i)=1.36+/-0.34 nM) and 13,14-dihydro-PGE(1) (K(i)=3.07+/-0.29 nM). Interestingly, synthetic DP receptor-specific agonists such as BW245C (K(i)=64.7+/-1.0 nM; n=3) and ZK118182 (K(i)=425+/-42 nM; n=4), and the purported EP(3) receptor-specific ligand enprostil (K(i)=43.1+/-4.4 nM), also displayed high affinity for the EP(4) receptor. Two known EP(4) receptor antagonists were weak inhibitors of [(3)H]-PGE(2) binding akin to their known functional potencies, thus: AH23848 (K(i)=2690+/-232 nM); AH22921 (K(i)=31,800+/-4090 nM). These studies have provided a detailed pharmacological characterization of the recombinant human EP(4) receptor expressed in HEK-293 cells.