Relaxin signalling in primary cultures of human myometrial cells

Abstract

In myometrium of pigs and rats, though not humans, relaxin appears to mediate an inhibition of spontaneous and oxytocin-induced contractility, presumably acting through a G-protein coupled receptor (RXFP1) to generate cAMP. In humans, circulating relaxin is highest in the first trimester, including the time of implantation, when transitory uterine quiescence could help a blastocyst to implant. We investigated whether relaxin can activate adenylate cyclase in primary human myometrial cells from non-pregnant tissue, and we show that relaxin is able to stimulate the generation of cAMP in a manner, which is dependent upon a tyrosine phosphorylation activity, as in the endometrium. We identified transcripts for the relaxin receptor RXFP1 as full-length variants, though a minor splice variant missing exon 2 was also present in low amounts. These cells also express transcripts encoding RXFP2, the receptor for the closely related hormone, INSL3. Although able to respond to relaxin at high concentrations, this receptor does not appear to function by contributing to the cAMP production in human myometrial cells, nor does INSL3 act as a functional agonist or antagonist of relaxin action. In conclusion, the inability of relaxin to inhibit contractility in human myometrial cells would appear to be due to events downstream of simple cAMP generation.

Regulation of the reproductive cycle and early pregnancy by relaxin family peptides

“…Additionally, in the rat it has been shown that relaxin acting on the myometrium may be involved in spacing of embryos in multiparous species (Rogers et al, 1983). In the human, relaxin does not appear to have any effect to inhibit spontaneous or oxytocin-induced contractility (MacLennan et al, 1986), although we have recently shown that human myometrial cells obtained at routine hysterectomy do indeed respond via RXFP1 to relaxin, and just like hESC cultures generate cAMP in a tyrosine kinase-dependent manner (Heng et al, 2008).…”

“…It may also activate the alternative receptor RXFP2 (which is specific for INSL3), but only in some species such as the human, and then only at highly supraphysiological concentrations. In transfected cells, and in some naturally receptor-expressing primary cells, such as human endometrial stromal cells (Bartsch et al, , 2004Ivell et al, 2007) or human myometrial cells (Heng et al, 2008), relaxin interacts with RXFP1 to activate G s -mediated adenylyl cyclase causing an elevation of intracellular cAMP. It may also in some circumstances activate PI3-kinase in a G i/o -dependent manner involving PKC-zeta (Nguyen and Dessauer, 2005).…”

“…What is the role of this first trimester relaxin? In addition to effects on the implanting embryo itself (see above), the uterus appears to be a major relaxin target, with RXFP1 relaxin-receptors expressed on uterine epithelial cells, on endometrial stromal cells, as well as in the myometrium Parry and Vodstrcil, 2007;Heng et al, 2008). Within the endometrium, the stromal cells appear to be the principal endocrine drivers dictating both the epithelial response as well as uterine remodeling essential for placentation (Ramathal et al, 2010).…”

“…Whilst superficially relaxin simply causes an elevation of cAMP because of the G s -mediated stimulation of adenylyl cyclase, as can be seen also in RXFP1-transfected over-expressing HEK293T cells Hsu et al, 2002;Halls et al, 2006;Ivell et al, 2007;Heng et al, 2008), relaxin's impact in the naturally receptor-expressing cells is more complex. Firstly, it additionally causes a modest induction of type 4 phosphodiesterases (Bartsch et al, 2004), in part to act as a feedback mechanism to regulate intracellular cAMP.…”

“…Human myometrial cells do not appear to respond to INSL3, although they do possess fulllength RXFP2 receptor mRNA (Heng et al, 2008). However, it is becoming very clear that the male fetus is a major producer of INSL3 already by mid-gestation.…”