RELAXIN AND RELATED PEPTIDES: FIFTH INTERNATIONAL CONFERENCE Role of Relaxin in Human Osteoclastogenesis Arianna Facciolli, Alberto Ferlin, Lisa Gianesello, Anastasia Pepe, and Carlo Foresta Section of Clinical Pathology & Centre for Male Gamete Cryopreservation, Department of Histology, Microbiology and Medical Biotechnologies, University of Padova, Padova, Italy Recently, we have demonstrated that INSL3 is important for bone metabolism, and in this study we analyzed the possible role of the cognate hormone relaxin on human osteoclasts. In fact, previous studies showed an effect of relaxin on peripheral blood mononuclear cells (PBMCs), the precursors of osteoclasts. Analysis of the expression of the relaxin receptor RXFP1 and RLN-2 mRNA in primary cell cultures of human osteo- clasts obtained from PBMCs showed by reverse transcriptase PCR and immunofluores- cence only the presence of RXFP1 transcripts. Analysis of the in vitro effects of relaxin on osteoclastogenesis showed that relaxin is able alone to induce the differentiation of PBMCs into mature osteoclasts. This study shows, for the first time, that relaxin has an effect on bone metabolism, facilitating the differentiation of osteoclasts. A possible role for relaxin in osteolytic bone metastasis is also proposed. Key words: bone metastasis; osteoclasts; osteoclastogenesis; relaxin; RXFP1 Introduction The peptide hormone relaxin is a multifunc- tional factor in a broad range of target tissues, including several nonreproductive organs, in addition to its historical role as a hormone of pregnancy. This effector mediates its action in binding to and activating the leucine-rich repeat-containing G-protein-coupled receptor RXFP1. Previous studies 1 demonstrated that peripheral blood mononuclear cells (PBMCs) express mRNA for the relaxin receptor and re- spond to relaxin at physiologic levels, suggest- ing that PBMCs could be studied in relation to the effects of relaxin on inflammation and bone destruction caused by osteoclasts. Osteoclasts are giant, multinucleated cells that derive from the differentiation of hemato- poietic mononuclear precursors. In specific mi- Address for correspondence: Prof. Carlo Foresta, University of Padova, Department of Histology, Microbiology and Medical Biotechnologies, Section of Clinical Pathology & Centre for Male Gamete Cryopreser- vation, Via Gabelli 63, 35121 Padova, Italy. Voice: +39 049 8218517; fax: +39 049 8218520. carlo.foresta@unipd.it croenvironments, osteoclastic precursors differ- entiate into mature osteoclasts, thanks to a tight interaction with osteoblastic and stromal cells: cell-to-cell interactions are necessary, as is the production of factors by osteoblasts. Os- teoblasts express the key factors modulating the differentiation of PBMCs into osteoclasts (os- teoclastogenesis): the receptor activator of nu- clear factor-κB ligand (RANKL), a member of the tumor necrosis factor ligand family, and os- teoprotegerin (OPG). By binding to its recep- tor, RANK, on osteoclast progenitors, RANKL stimulates their differentiation and activity. On the contrary, OPG acts as a nonsignaling decoy receptor, and by binding to RANKL it prevents the activation of RANK, resulting in decreased osteoclast recruitment. Osteoclastogenesis also requires the presence of macrophage colony- stimulating factor (M-CSF), which is released by osteoblasts as well and acts on osteo- clasts through its receptor, c-fms. The RANK– RANKL–OPG and M-CSF–c-fms regulatory axes couple osteoblast and osteoclast activity, thereby controlling the balance between bone formation and resorption. Relaxin and Related Peptides: Fifth International Conference: Ann. N.Y. Acad. Sci. 1160: 221–225 (2009). doi: 10.1111/j.1749-6632.2008.03788.x C  2009 New York Academy of Sciences. 221