Background: Bone is a frequent site of
metastases from
breast cancer, but existing therapeutic options are not satisfactory. Although osteoblasts have active roles in
cancer progression by assisting the vicious bone-destructive cycle, we employed a counterintuitive approach of activating pro-tumorigenic Wnt signaling and examined the paradoxical possibility of developing osteoblast-derived
tumor-suppressive, bone-protective secretomes. Methods: Wnt signaling was activated by the overexpression of Lrp5 and β-
catenin in osteoblasts as well as a pharmacological agent (BML284), and the
therapeutic effects of their
conditioned medium (CM) were evaluated using in vitro cell cultures, ex vivo
breast cancer tissues, and a mouse model of
osteolysis. To explore the unconventional regulatory mechanism of the action of Wnt-activated osteoblasts, whole-genome proteomics analysis was conducted, followed by immunoprecipitation and gain- and loss-of-function assays. Results: While osteoblasts did not present any innate
tumor-suppressing ability, we observed that the overexpression of Lrp5 and β-
catenin in Wnt signaling made their CM
tumor-suppressive and bone-protective. The growth of
breast cancer cells and tissues was inhibited by Lrp5-overexpressing CM (Lrp5 CM), which suppressed mammary
tumors and
tumor-driven bone destruction in a mouse model. Lrp5 CM also inhibited the differentiation and maturation of bone-resorbing osteoclasts by downregulating NFATc1 and
cathepsin K. The overexpression of Lrp5 upregulated
osteopontin that enriched Hsp90ab1 (Hsp90 beta) and
moesin (MSN) in Lrp5 CM. Hsp90ab1 and MSN are atypical
tumor-suppressing
proteins since they are multi-tasking, moonlighting
proteins that promote
tumorigenesis in
tumor cells. Importantly, Hsp90ab1 immuno-precipitated latent TGFβ and inactivated TGFβ, whereas MSN interacted with CD44, a cancer stem-cell marker, as well as
fibronectin 1, an ECM
protein. Furthermore, Hsp90ab1 and MSN downregulated KDM3A that demethylated
histones, together with PDL1 that inhibited immune responses. Conclusion: In contrast to inducing
tumor-enhancing secretomes and chemoresistance in general by inhibiting varying oncogenic pathways in
chemotherapy, this study presented the unexpected outcome of generation
tumor-suppressive secretomes by activating the pro-tumorigenic Wnt pathway. The results shed light on the contrasting role of oncogenic signaling in
tumor cells and osteoblast-derived secretomes, suggesting a counterintuitive option for the treatment of
breast cancer-associated bone
metastasis.