Background: Bone
metastasis is a frequent symptom of
breast cancer and current targeted
therapy has limited efficacy. Osteoclasts play critical roles to drive
osteolysis and metastatic outgrowth of
tumor cells in bone. Previously we identified CST6 as a secretory
protein significantly downregulated in bone-metastatic
breast cancer cells. Functional analysis showed that CST6 suppresses breast-to-bone
metastasis in animal models. However, the functional mechanism and therapeutic potential of CST6 in bone
metastasis is unknown. Methods: Using in vitro osteoclastogenesis and in vivo
metastasis assays, we studied the effect and mechanism of extracellular CST6
protein in suppressing osteoclastic niches and bone
metastasis of
breast cancer. A number of
peptides containing the functional domain of CST6 were screened to inhibit bone
metastasis. The efficacy, stability and toxicity of CST6
recombinant protein and
peptides were evaluated in preclinical
metastasis models. Results: We show here that CST6 inhibits osteolytic bone
metastasis by inhibiting osteoclastogenesis.
Cancer cell-derived CST6 enters osteoclasts by endocytosis and suppresses the
cysteine protease CTSB, leading to up-regulation of the CTSB hydrolytic substrate SPHK1. SPHK1 suppresses osteoclast maturation by inhibiting the RANKL-induced p38 activation. Importantly, recombinant CST6
protein effectively suppresses bone
metastasis in vitro and in vivo. We further identified several
peptides mimicking the function of CST6 to suppress
cancer cell-induced osteoclastogenesis and bone
metastasis. Pre-clinical analyses of CTS6
recombinant protein and
peptides demonstrated their potentials in treatment of
breast cancer bone
metastasis. Conclusion: These findings reveal the CST6-CTSB-SPHK1 signaling axis in osteoclast differentiation and provide a promising approach to treat
bone diseases with CST6-based
peptides.