New Insights into biological roles of relaxin and relaxin-related peptides.

In the 1920s, a relaxin peptide was first identified in serum from pregnant guinea pigs and has been shown to play important roles in the regulation of uterine contraction and parturition [1]. The importance of relaxin signaling in the parturition process was demonstrated through a series of classic experiments involving passive immunization [2– 11]. Immunization with a monoclonal antirelaxin antibody to neutralize endogenous relaxin during the antepartum period (days 12–22 of gestation) reduces cervical growth and extensibility, and disrupts labor and birth in rats. Approximately 20–25% of fetuses and placenta were retained in utero on day 24 whereas neither fetuses nor placenta were retained in control rats [6,8]. Importantly, this treatment has no effect on the weights of other reproductive tissues (uterus, placenta, and ovary), the number of fetuses, or fetus viability [7]. Cervices obtained from antirelaxin antibody-treated rats were much smaller and less extensible than cervices obtained from control rats. Using the same approach, it was shown that relaxin also effects the growth of the vagina during pregnancy. Antirelaxin antibody treatment leads to lower vaginal wet weight, dry weight, length, diameter, inner surface area, and DNA content [8]. Biochemically, it has been shown that relaxin stimulates matrix metalloproteinase I activity in cultured guinea pig cervical cells [12], leading to degradation of the extracellular matrix. Likewise, relaxin significantly increases the secretion of active matrix metalloproteinase-2 and -9 protein into uterine fluid of gilt, leading to increased extracellular proteolysis during the remodeling of reproductive tissues [13]. In human, treatment of lower uterine segment fibroblasts with relaxin results in a higher expression of procollagenase protein and mRNA levels, stimulates prostromelysin-1 protein and mRNA levels, but inhibits expression of the tissue inhibitor of metalloproteinase1 protein [14]. In addition, treatment of human cervical stromal cells with relaxin leads to the secretion of several stromal metalloproteinases [15]. Thus, one of the major roles of relaxin in the reproductive tract is the activation of different proteases important for extracellular matrix degradation and tissue remodeling. Comparative genomic analysis showed that in humans there are a total of seven peptides relaxin1, relaxin2, INSL3/RLF, INSL4/EPIL, INSL5/RLF2, INSL6/RLF1, and relaxin3/INSL7 sharing close phylogenetic relatedness in the relaxin subfamily [16–22]. Among these peptides, relaxin1 and relaxin2 share >90% identity and represent co-orthologs of the relaxin peptide in other mammals. Relaxin2 is mainly expressed in the reproductive tissues such as ovarian corpus luteum from which the hormone circulates during the luteal phase of the menstrual cycle and during pregnancy in women [23]. In contrast, relaxin1 is produced by deciduae and trophoblasts; with relaxin2, they could play a paracrine function at the placental site [24]. Similar to relaxins, INSL3, INSL5, and INSL6 are expressed in gonads of a variety of mammals [17,18,20,21]. In contrast, relaxin3/INSL7 and the primate-specific INSL4 are mainly found in brain and placenta, respectively [19,22]. The physiological importance of relaxins and INSL3 has become more clear with advances in the studies of various genetic models; however, the roles of other family peptides remain to be investigated. Although the ovary is the main source of relaxin during pregnancy, relaxin is expressed in a variety of tissues, including brain, uterus, prostate, heart, and kidney [25], and has been shown to exert effects in a variety of nonreproductive tissues. Recent identification of relaxin