Overexpression of Lrp5 enhanced the anti-breast cancer effects of osteocytes in bone

Abstract

Osteocytes are the most abundant cells in bone, which is a frequent site of breast cancer metastasis. Here, we focused on Wnt signaling and evaluated tumor-osteocyte interactions. In animal experiments, mammary tumor cells were inoculated into the mammary fat pad and tibia. The role of Lrp5-mediated Wnt signaling was examined by overexpressing and silencing Lrp5 in osteocytes and establishing a conditional knockout mouse model. The results revealed that administration of osteocytes or their conditioned medium (CM) inhibited tumor progression and osteolysis. Osteocytes overexpressing Lrp5 or β-catenin displayed strikingly elevated tumor-suppressive activity, accompanied by downregulation of tumor-promoting chemokines and upregulation of apoptosis-inducing and tumor-suppressing proteins such as p53. The antitumor effect was also observed with osteocyte-derived CM that was pretreated with a Wnt-activating compound. Notably, silencing Lrp5 in tumors inhibited tumor progression, while silencing Lrp5 in osteocytes in conditional knockout mice promoted tumor progression. Osteocytes exhibited elevated Lrp5 expression in response to tumor cells, implying that osteocytes protect bone through canonical Wnt signaling. Thus, our results suggest that the Lrp5/β-catenin axis activates tumor-promoting signaling in tumor cells but tumor-suppressive signaling in osteocytes. We envision that osteocytes with Wnt activation potentially offer a novel cell-based therapy for breast cancer and osteolytic bone metastasis.

Generation of the tumor-suppressive secretome from tumor cells

“…In Wnt signaling, we observed that the overexpression of Wnt1 and Fzd7 did not generate iTS cells. In the generation of iTS cells from osteocytes, we have previously shown that the overexpression of β-catenin and Lrp5 but not Lrp6 promoted the anti-tumor capabilities and the protection of tumor-invased bone 1 . In osteocytes and MSCs, the overexpression of multiple pro-tumorigenic genes and the administration of tumor-promoting compounds have been shown to generate iTSC that significantly inhibited the progression of adjacent tumors 1 - 3 .…”

“…In the generation of iTS cells from osteocytes, we have previously shown that the overexpression of β-catenin and Lrp5 but not Lrp6 promoted the anti-tumor capabilities and the protection of tumor-invased bone 1 . In osteocytes and MSCs, the overexpression of multiple pro-tumorigenic genes and the administration of tumor-promoting compounds have been shown to generate iTSC that significantly inhibited the progression of adjacent tumors 1 - 3 . Further analyses are needed to elucidate the requirement for creating effective iTS CM from varing tumor and non-tumor cells, considering types of target cancer cells, as well as pathways and genes to be activated and overexpressed.…”

“…The derailed activation of Wnt signaling is reported to promote the progression of tumors and many studies have been directed to inhibit Wnt activation for cancer treatments 6 , 7 . We have shown that the overexpression of Lrp5, a Wnt co-receptor, stimulated the proliferation and migration of breast cancer cells 1 . To our surprise, however, we have also shown that the conditioned medium, collected from Lrp5-overexpressing osteocytes and MSCs, can suppress tumor growth 2 .…”

“…To explore the unconventional territory, however, we evaluated the role of those activators in the tumor microenvironment. In tumor-osteocyte interactions, in which bone formation is promoted by Wnt signaling, we reported that the overexpression of β-catenin and Lrp5, a Wnt co-receptor, granted osteocytes anti-tumor capabilities 1 . This anti-tumor capability was also observed in MSCs that overexpressed varying protumorigenic genes such as β-catenin, Lrp5, snail and Akt.…”

“…We named those osteocytes and MSCs induced tumor-suppressing cells (iTSCs). Cell culture and preclinical studies have revealed that iTSCs generate anti-tumor secretomes and their complete and partial conditioned medium (CM) inhibits the progression of tumor cells in vitro , ex vivo and in vivo 1 - 3 . The question herein was whether the anti-tumor secretomes can be generated not only from non-tumor cells but also from tumor cells.…”