br OPG RANK and RANKL contribute to the
OPG, RANK and RANKL contribute to the vicious cycle established between tumour Deferoxamine mesylate and bone microenvironment: evidences for sarcomas and carcinomas
RANK/RANKL axis is involved in the tumorigenic process RANK/RANKL axis is associated with the bone metastastic process, and as several arguments point out it is also involved in the tumorigenicity process itself. RANK/RANKL may participate in the initial oncogenic program as shown by high expression of RANK in melanoma-initiating cells compared to the other melanoma cells . Epithelial mesenchymal transition (EMT) is the first step allowing the extravasation and migration of carcinoma cells and RANKL appears clearly involved in this process. Indeed, Yamada et al. show that RANKL promotes EMT and induces angiogenesis independent of VEGF in a human head and neck squamous carinoma . RANKL has also a strong impact on normal epithelial cells as shown by its effect on mammary gland development evidenced by a lactation defect in RANKL knockout mice [79,80]. In fact, RANKL promotes the proliferation and survival of mammary epithelial cells [79–82] and RANK expression increases during the gestation more specifically at ductal branch points . More interestingly, whether RANKL or RANK overexpression in the mammary epithelial cells results in aberrant proliferation and hyperplasia of mammary glands, it directly correlates with preneoplasias and the development of spontaneous mammary tumours [83,84]. Several authors hypothesised that RANKL may act as a paracrine factor for mammary stem cells [85,86]. Consequently, blockade of RANKL significantly reduces the occurrence of mammary tumours . Overall, these data give clear evidences that the RANK/RANKL axis contributes to the initial steps of tumorigenesis at least for mammary glands, to the dissemination process of carcinoma cells and to the establishment of bone metastases.
Therapies targeting RANK/RANKL axis for patients suffering from bone tumours: pre-clinical and clinical arguments Given this context, targeting of RANKL signalling with its decoy receptor OPG or with a soluble form of its membranous receptor RANK (RANK-Fc) inhibits tumour associated osteolysis in several experimental bone tumour models, including rat and mouse primary bone tumours and bone metastases. Indeed, OPG and RANK-Fc administered by nonviral gene transfer or as recombinant molecules are effective in preventing the formation of osteolytic lesions associated with osteosarcoma development and in reducing the tumour incidence leading to a significant increase of animal survival [87,88]. Moreover, recent experiments demonstrated that RNA interference strategy targeting RANKL improved the tumour response to chemotherapy in a murine model of osteosarcoma . Similarly, administration of recombinant OPG-Fc or RANK-Fc has been investigated in numerous murine models of bone metastases [20,23,79] and confirms that blockade of the RANK/RANKL axis is extremely efficient in preclinical assessment to prevent tumour-induced osteolysis, to reduce tumour growth and to improve the survival rate. According these pre-clinical proofs of concept, recombinant OPG (OPG-Fc) has been evaluated in postmenopausal  and in patients suffering from myeloma and osteolytic bone metastases (Table 1). Results demonstrated that OPG was well tolerated and demonstrated the efficacy of a single injection of OPG, which strongly reduced bone turnover for a sustained period and suppressed bone resorption as indicated by the decrease of bone resorption markers (urinary NTX/creatinine); these effects were comparable to those obtained with pamidronate. However, due to the risk of immune modulation of OPG through its binding to TRAIL  and other ligands [32–36], a fully human monoclonal antibody (IgG2) specifically targeting soluble and membrane RANKL has been developed [92,93]. Clinical data in osteoporotic patients revealed that denosumab was well tolerated with no related serious adverse events occurred and that a single-dose (0.01–3.0mg/kg) resulted in a dose-dependent sustained decrease from baseline in bone turnover [92,93]. This antibody, named denosumab, only recognises the human protein and its nonhuman-primate homologue and its administration in chimeric mice expressing murine/human leads to a strong inhibition of bone resorption concomitantly to an increase of the bone mineral density .