Denosumab and Male Infertility: a Prospective Intervention Study

The receptor activator of NF-kB ligand (RANKL) system is considered important for bone homeostasis . RANKL exist in three isoforms and the effects of all isoforms are mediated through binding to a specific receptor (receptor activator of NF-KB (RANK)). RANKL is predominantly found as a transmembrane protein and signalling is therefore dependent on cell-cell interaction to a neighbouring cell expressing RANK that subsequently activates NFKB and regulates cellular activation through regulation of cell cycle i.e proliferation, differentiation and apoptosis. RANKL-RANK interaction is modified by osteoprotegerin (OPG), which is an endogenous secreted protein that binds RANKL and inhibits its signalling.

RANK/RANKL triggers a network of TRAF-mediated kinase cascades that promote osteoclast differentiation. RANKL is expressed on osteoblast cells and its receptor, Rank, on pre- osteoclastic cells. RANKL expression is stimulated by a number of factors, such as IL-1 , IL-6, IL-11 , IL-17, TNF- α, vitamin D, Ca2+, parathyroid, glucocorticoids, prostaglandin E2, and immunosuppressive drugs, and is down-regulated by TGF-α. The RANK/RANKL interaction induces differentiation and formation of multinucleated mature osteoclasts, causing bone resorption. The third protein agonist, osteoprotegerin (OPG), is also produced by osteoblasts and is known to exert an inhibitory effect on the pre-osteoclastic differentiation process. By binding to RANKL also known as osteoprotegerin binding protein (OPGbp), OPG inhibits the RANK/RANKL interaction and subsequent osteoclastogenesis. OPG is thus a very efficient anti-resorptive agent. It also serves as a decoy receptor for the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) and increases cell survival by blocking the apoptotic effects of this ligand. The fact that the overexpression of OPG in mice results in severe osteopetrosis and that OPG-null mice are osteoporotic is testimony to the physiological importance of OPG. The lack of RANK or RANKL induces osteopetrosis in mice.

Thus, the RANK/RANKL system is vital for activation of the bone resorping cells (osteoclasts). In the skeleton the bone synthesizing cells (osteoblasts) express RANKL that signals to RANK on the immature osteoclasts. This induces proliferation and activation of the cells they start to proliferate and resorp bone. OPG is produced by somatic cells in the bone and this production is regulated by sex hormones, TGF-B and various other substances. Today a human made recombinant antibody against RANKL, Denosumab is used to treat osteoporosis as it inhibits RANKL signalling and causes less bone resorption in humans. RANKL signalling has only two other known additional functions in healthy humans where it is involved in lactation and the immune response.

The investigators have data showing that RANKL, RANK and OPG are expressed at both RNA and protein level in the human testis. The Sertoli cells express RANKL, while the germ cells express RANK and the peritubular cells express OPG. Normally, RANKL activates NFKB and activation of this pathway in the male gonad appears to regulate whether the testicular cells proliferate or undergo apoptosis in the testis. The investigators' in vitro, ex vivo and in vivo data from functional models support this suggestion and this pathway appears therefore to be a novel regulator of germ cell proliferation.

Decreased semen quality is a major factor of male infertility. Semen quality is an indirect measure of the ability of the sperm to accomplish fertilization. Evaluation of male fertility potential is evaluated by semen analysis. Semen analysis evaluates certain characteristics and the most common variables measured to evaluate sperm quality are: sperm count, motility and morphology.