Bioactivity of recombinant prorelaxin from the marmoset monkey

The hormone relaxin (RLX) is generally present in the serum of humans and primates as a heterodimer, though some unprocessed prohormone may also be present. In order to test whether this proRLX is biologically relevant for human or primate physiology, recombinant marmoset monkey proRLX was synthesized in a baculovirus-infected cell system and tested in different bioassays. Marmoset proRLX is >70% identical to human H2 proRLX, especially in the so-called receptor-binding region of the B-peptide. The bioassay systems used were (a) cAMP production by human endometrial stromal cells and (b) cAMP production by the human monocyte cell line THP-1. In both bioassay systems recombinant proRLX showed comparable EC(50) values to pure porcine heterodimeric relaxin (porcine relaxin, 1.5-2.0 nM; marmoset prorelaxin 4.0-5.0 nM). Additionally, recombinant marmoset prorelaxin was shown to stimulate steroidogenesis in primary cultures of marmoset ovarian theca cells, though with a lower apparent activity than porcine relaxin. It thus appears that precursor processing of human or primate relaxin is not an essential prerequisite for the acquisition of bioactivity, as it is for the closely related hormone insulin, and that circulating prorelaxin is physiologically relevant.