The relaxin receptor has so far avoided molecular cloning and characterizat
ion. We have therefore characterized the signalling events activated by rel
axin (RLX), using two different cell culture-based bioassay systems: primar
y human endometrial stromal cells from the cycle (ESC) and the human monocy
te cell line THP-1. Upon RLX stimulation, both cell types showed a rapid in
crease in cAMP accumulation, which could be inhibited by an inhibitor of G-
protein activation, GDP-beta -S. However, evolutionarily one would expect t
he RLX receptor, like those for the structurally related hormones insulin a
nd insulin-like growth factor-I, to involve tyrosine kinase activity. The s
pecific tyrphostins AG 1478, AG 527 and AG 879 inhibited the RLX-stimulated
cAMP response in human ESC and THP-1 cells in a dose-dependent manner, tho
ugh the potent broad range tyrphostin AG 213 had no effect. Also, treatment
of THP-1 cells with the potent phosphotyrosine phosphatase inhibitors bpV(
phen) and mpV(pic) increased RLX-stimulated cAMP accumulation in a dose-dep
endent manner. The effect of the general tyrosine kinase inhibitor genistei
n (which can also inhibit some phosphodiesterases) on RLX-mediated cAMP acc
umulation strongly depended on the activity status of phosphodiesterase. In
the absence of a phosphodiesterase inhibitor, genistein enhanced RLX-stimu
lated cAMP accumulation in both bioassays. When phosphodiesterase was inhib
ited by isobutylmethylxanthine, this effect was not observed. The results i
mply that activation of the RLX receptor uses tyrosine kinase signalling to
control phosphodiesterase activity, and hence to up-regulate intracellular
cAMP.