The cysteinyl
leukotrienes (cys-LTs),
leukotriene C4 (
LTC4), a conjugation product of
glutathione and eicosatetraenoic
acid, and its metabolites,
LTD4 and
LTE4, are
lipid mediators of smooth muscle constriction and
inflammation in
asthma.
LTD4 is the most potent
ligand for the type 1
cys-LT receptor (
CysLT1R), and
LTC4 and
LTD4 have similar lesser potency for CysLT2R, whereas
LTE4 has little potency for either receptor. Cysltr1/Cysltr2(-/-) mice, lacking the two defined receptors, exhibited a comparable dose-dependent vascular leak to
intradermal injection of
LTC4 or
LTD4 and an augmented response to
LTE4 as compared with WT mice. As
LTE4 retains a
cysteine residue and might provide recognition via a dicarboxylic
acid structure, we screened cDNAs within the P2Y
nucleotide receptor family containing CysLTRs and dicarboxylic
acid receptors with
trans-activator reporter gene assays. GPR99, previously described as an oxoglutarate receptor (Oxgr1), showed both a functional and a binding response to
LTE4 in these transfectants. We generated Gpr99(-/-) and Gpr99/Cysltr1/Cysltr2(-/-) mice for comparison with WT and Cysltr1/Cysltr2(-/-) mice. Strikingly, GPR99 deficiency in the Cysltr1/Cysltr2(-/-) mice virtually eliminated the vascular leak in response to the
cys-LT ligands, indicating GPR99 as a potential CysLT3R active in the Cysltr1/Cysltr2(-/-) mice. Importantly, the Gpr99(-/-) mice showed a dose-dependent loss of LTE4-mediated vascular permeability, but not to
LTC4 or
LTD4, revealing a preference of GPR99 for
LTE4 even when
CysLT1R is present. As
LTE4 is the predominant
cys-LT species in inflamed tissues, GPR99 may provide a new therapeutic target.