Conclusions
This study suggests for the first time a role for INSL3/RXFP2 signaling in bone metabolism and links RXFP2 gene mutations with human osteoporosis.
Methods
Extensive clinical, biochemical, and hormonal study, including bone densitometry by DXA, was performed on 25 young men (age, 27-41 yr) with the well-characterized T222P mutation in the RXFP2 gene. Expression analysis of INSL3 and RXFP2 on human bone biopsy and human and mouse osteoblast cell cultures was performed by RT-PCR, quantitative RT-PCR, and immunohistochemistry. Real-time cAMP imaging analysis and proliferation assay under the stimulus of INSL3 was performed on these cells. Lumbar spine and femoral bone of Rxfp2-deficient mice were studied by static and dynamic histomorphometry and muCT, respectively.
Results
Sixteen of 25 (64%) young men with RXFP2 mutations had significantly reduced BMD. No other apparent cause of osteoporosis was evident in these subjects, whose testosterone levels and gonadal function were normal. Expression analyses showed the presence of RXFP2 in human and mouse osteoblasts. Stimulation of these cells with INSL3 produced a dose- and time-dependent increase in cAMP and cell proliferation, confirming the functionality of the RXFP2/INSL3 receptor-ligand complex. Consistent with the human phenotype, bone histomorphometric and muCT analyses of Rxfp2(-/-) mice showed decreased bone mass, mineralizing surface, bone formation, and osteoclast surface compared with wildtype littermates. Conclusions: This study suggests for the first time a role for INSL3/RXFP2 signaling in bone metabolism and links RXFP2 gene mutations with human osteoporosis.