Identification of the pregnancy hormone relaxin as glucocorticoid receptor agonist

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Abstract
The insulin-like peptide relaxin is a central hormone of pregnancy, but it also produces anti-fibrotic, myocardial, renal, central-nervous, and vascular effects. Recently, two G protein-coupled receptors, LGR7 and LGR8, have been identified as relaxin receptors. Prompted by reports on immunoregulatory effects of relaxin, we investigated possible interactions with the human glucocorticoid receptor (GR). Relaxin blunted the endotoxin-induced production of inflammatory cytokines (IL-1, IL-6, TNF-alpha) by human macrophages--an effect that was suppressed by the GR antagonist RU-486. In three different cell lines, relaxin induced GR activation, nuclear translocation, and DNA binding as assessed in GRE-luciferase assays. Co-immunoprecipitation experiments revealed physical interaction of endogenous and exogenous relaxin with cytoplasmic and nuclear GR. Relaxin competed with GR agonists for GR binding, both in vivo in whole-cell assays, and in vitro in fluorescence polarization assays. Relaxin was shown to up-regulate GR protein expression as well as the number of functionally active GR sites. In LGR7/8-free cells, the relaxin-mediated activation of GR was preserved. In conclusion, relaxin acts as GR agonist--a pathway pivotal to its effects on cytokine secretion by human macrophages. These findings may deepen our understanding of relaxin's abundant physiological actions, as well as our insights into general principles of hormone signaling.