Molecular events of acid-induced bone pain

Minodronic acid induces morphological changes in osteoclasts at bone resorption sites and reaches a level required for antagonism of purinergic P2X2/3 receptors

Minodronic acid is an aminobisphosphonate that is an antagonist of purinergic P2X2/3 receptors involved in pain. The aim of this study was to investigate the action and distribution of minodronic acid and the potential for P2X2/3 receptor antagonism based on the estimated concentration of minodronic acid. Microlocalization of radiolabeled minodronic acid was examined in the femur of neonatal rats. The bone-binding characteristics of minodronic acid and morphological changes in osteoclasts were analyzed in vitro. The minodronic acid concentration around bone resorption lacunae was predicted based on bone binding and the shape of lacunae. In microautoradiography, radioactive silver grains were abundant in bone-attached osteoclasts and were detected in calcified and ossification zones and in the cytoplasm of osteoclasts but not in the hypertrophic cartilage zone. In an osteoclast culture with 1 µM minodronic acid, 65% of minodronic acid was bound to bone, and C-terminal cross-linking telopeptide release was inhibited by 96%. Cultured osteoclasts without minodronic acid treatment formed ruffled borders and bone resorption lacunae and had rich cytoplasm, whereas those treated with 1 µM minodronic acid were not multinucleated, stained densely with toluidine blue, and were detached from the bone surface. In the 1 µM culture, the estimated minodronic acid concentration in resorption lacunae was 880 µM, which is higher than the IC₅₀ for minodronic acid antagonism of P2X2/3 receptors. Thus, inhibition of P2X2/3 receptors around osteoclasts may contribute to the analgesic effect of minodronic acid.

Integrin α6β4 and TRPV1 channel coordinately regulate directional keratinocyte migration

The directional migration of epithelial cells is crucial for wound healing. Among integrins, a family of cell adhesion receptors, integrin β4 has been assumed to be a promigratory factor, in addition to its role in stable adhesion. In turn, Ca(2+) signaling is also a key coordinator of migration. Keratinocytes reportedly express transient receptor potential vanilloid channels (TRPV1); however, the function of these channels as a regulator of intracellular Ca(2+) level in cell migration has remained uncharacterized. In the present study, we investigated the role of TRPV1 in directional migration related to integrin β4 using a scratch wound assay on a confluent monolayer sheet of murine keratinocytes (Pam212 cells). Double immunofluorescence staining revealed the de novo expression of integrin β4 and TRPV1 in migrating cells at the wound edge in response to scratch wounding, and both expression levels were almost matched. Epidermal growth factor (EGF) not only promoted keratinocyte migration, but also caused the further up-regulation of both integrin β4 and TRPV1. In addition, the knockdown of the integrin β4 or TRPV1 gene significantly impeded wound closure. The TRPV1 agonist capsaicin significantly promoted migration, while a selective TRPV1 antagonist inhibited it. The gene knockdown of TRPV1 inhibited the expression of the integrin β4 gene and that of β4 protein in migrating cells. These findings suggest that TRPV1 may stimulate directional migration directly by eliciting a Ca(2+) signal or indirectly via integrin β4 expression.

Role of RANKL/RANK in primary and secondary breast cancer

Bone is one of the most preferential metastatic target sites of breast cancer. Bone possesses unique biological microenvironments in which various growth factors are stored and continuously released through osteoclastic bone resorption, providing fertile soil for circulating breast cancer cells. Bone-disseminated breast cancer cells in turn produce osteotropic cytokines which modulate bone environments. Under the influences of breast cancer-produced cytokines, osteoblasts express elevated levels of Ligand for receptor activator of nuclear factor-κB (RANKL) and stimulate osteoclastogenesis via binding to the receptor receptor activator of nuclear factor-κB (RANK) and activating its downstream signaling pathways in hematopoietic osteoclast precursors, which causes further osteoclastic bone destruction. Establishment of crosstalk with bone microenvironments (so called vicious cycle) is an essential event for metastatic breast cancer cells to develop bone metastasis. RANKL and RANK play a central role in this crosstalk. Moreover, recent studies have demonstrated that RANKL and RANK are involved in tumorigenesis and distant metastasis independent of bone microenvironments. Pharmacological disruption of the RANKL/RANK interplay should be an effective therapeutic intervention for primary breast tumors and bone and non-bone metastasis. In this context, denosumab, which is neutralizing monoclonal antibody against RANKL, is a mechanism-based drug for the treatment of bone metastases and would be beneficial for breast cancer patients with bone metastases and potentially visceral organ metastases.