Brain-Derived Neurotrophic Factor Enhances Osteoclastogenesis in Multiple Myeloma via Upregulation of RANKL/OPG Ratio Both in Vitro and in a Murine Model of Myeloma Bone Disease

Abstract 568

Osteolytic bone disease is a prominent feature of multiple myeloma (MM), resulting from aberrant osteoclastic bone resorption uncoupled with osteoblastic bone formation. Myeloma-induced osteoclastogenesis is largely depending on the increase of receptor activator of NF-κB ligand (RANKL) and decrease of osteoprotegerin (OPG) within the bone marrow milieu. Recently, brain-derived neurotrophic factor (BDNF) was identified as an MM-derived factor correlated with increased RANKL level and contributed to myeloma bone destruction. On the other hand, tyrosine receptor kinase B (TrkB), the receptor of BDNF, was found to be abundantly expressed by osteoblasts (OBs). Since OBs are the main source of RANKL and OPG in bone, here we sought to evaluate the involvement of BDNF/TrkB in the crosstalk between myeloma cells and OBs, as well as the effects of BDNF on RANKL/OPG ratio and myeloma bone disease.

Co-cultures of OBs with pre-osteoclasts were performed in a non-contacted transwell system and treated with various concentration of BDNF. Osteoclast formation was detected with a tartrate-resistant acid phosphatase (TRAP) staining kit. Then, RANKL and OPG levels were measured when OBs cultures were exposed to BDNF or co-cultured with three human myeloma cell lines (RPMI8226, ARH-77 and U266). K252a (an inhibitor of TrkB) was present or absent in these systems to assess the effects of BDNF on RANKL/OPG expression in OBs. The involvement of downstream signaling molecules activated by BDNF in OBs was also investigated in this study, with the use of U0126 and a specific small interfering RNA (siRNA) for TrkB. For in vivo study, ARH-77 cells were stably transfected with an antisense short-hairpin RNA construct to BDNF (AS-ARH) or empty vector (EV-ARH). These cells were then intravenously injected to severe combined immunodeficiency (SCID) mice, to test their capacity to induce MM bone disease. Radiographs of mice tibiae and vertebrae were taken weekly by X ray. Changes in total body bone mineral density (BMD) of mice skeleton were recorded. At the end of the experiment, bone sections were stained with hematoxylin and eosin staining or TRAP staining. Secretion levels of RANKL and OPG in mice bone marrow were measured by ELISA.

We showed that BDNF increased RANKL and decreased OPG production in OBs in a time- and dose-dependent manner, thus contributing to osteoclast formation in vitro. In addition, these effects were completely abolished by K252a and TrkB-siRNA (P < 0.05). BDNF regulates RANKL/OPG expression in OBs through the TrkB/ERK signaling pathway. Our in vivo results indicated that mice injected with AS-ARH cells, which expressed low levels of endogenous BDNF, were preserved and exhibited no radiologically identifiable osteolytic lesions. In addition, mice in AS-ARH group also had a lower incidence of vertebral compression deformities and paralysis in comparison with mice in EV-ARH group (P < 0.05). Further more, bones harboring AS-ARH cells showed marked reduction of RANKL/OPG ratio and osteoclast density when compared to the controls harboring EV-ARH cells (P < 0.05).

Our results demonstrate that BDNF is an important contributor to osteoclastogenesis in MM. Antisense inhibition of BDNF in MM cells remarkably inhibited osteolytic bone destruction in SCID-ARH mice model. BDNF-induced bone destruction is partially mediated by MM-OB interactions via upregulation of RANKL/OPG ratio in the bone marrow milieu. These findings suggest targeting BDNF may become a new therapeutic strategy to improve patient outcome in MM.