Osteocytic Connexin Hemichannels Modulate Oxidative Bone Microenvironment and Breast Cancer Growth

Abstract

Osteocytes, the most abundant bone cell types embedded in the mineral matrix, express connexin 43 (Cx43) hemichannels that play important roles in bone remodeling and osteocyte survival. Estrogen deficiency decreases osteocytic Cx43 hemichannel activity and causes a loss in osteocytes’ resistance to oxidative stress (OS). In this study, we showed that OS reduced the growth of both human (MDA-MB-231) and murine (Py8119) breast cancer cells. However, co-culturing these cells with osteocytes reduced the inhibitory effect of OS on breast cancer cells, and this effect was ablated by the inhibition of Cx43 hemichannels. Py8119 cells were intratibially implanted in the bone marrow of ovariectomized (OVX) mice to determine the role of osteocytic Cx43 hemichannels in breast cancer bone metastasis in response to OS. Two transgenic mice overexpressing dominant-negative Cx43 mutants, R76W and Δ130-136, were adopted for this study; the former inhibits gap junctions while the latter inhibits gap junctions and hemichannels. Under normal conditions, Δ130-136 mice had significantly more tumor growth in bone than that in WT and R76W mice. OVX increased tumor growth in R76W but had no significant effect on WT mice. In contrast, OVX reduced tumor growth in Δ130-136 mice. To confirm the role of OS, WT and Δ130-136 mice were administered the antioxidant N-acetyl cysteine (NAC). NAC increased tumor burden and growth in Δ130-136 mice but not in WT mice. Together, the data suggest that osteocytes and Cx43 hemichannels play pivotal roles in modulating the oxidative microenvironment and breast cancer growth in the bone.

Osteocytes: New Kids on the Block for Cancer in Bone Therapy

“…Contrary to these studies, conditioned media from mechanically stimulated MLO-Y4 osteocyte-like cells increased breast cancer cell growth by producing CXCL1 and CXCL2 [38]. Lastly, osteocytes protected breast cancer cells from the proliferative inhibitory effects of oxidative stress, which are dependent on CX43 signaling [46].…”

“…As described for prostate cancer cells, breast cancer cells can also establish cell-to-cell contact with osteocytes [45]. However, in vitro studies have shown that cell-to-cell coculture with MLO-Y4 osteocyte-like cells did not increase the proliferation of breast cancer cells [46]. The role of osteocyte-derived factors in breast cancer tumor growth is ambiguous (recently reviewed by Pin et al [11]).…”

Role of the osteocyte in bone metastasis – The importance of networking

“…One of the important mechano-sensing mechanisms for osteocytes is connexin 43 (Cx43), a transmembrane protein that can form hemichannels sensitive to mechanical stimulation and that is highly enriched in osteocytes. Cx43 seems to play a protective role against breast cancer cell metastasis, and indeed has recently been show to do so via prevention of oxidative stress [35] . This study found, using an ovariectomised mouse model, that osteocytes can modulate the oxidative microenvironment via expression of Cx43, and thus can control the growth of tumours cells under oxidative stress in the local microenvironment [35] .…”

“…Cx43 seems to play a protective role against breast cancer cell metastasis, and indeed has recently been show to do so via prevention of oxidative stress [35] . This study found, using an ovariectomised mouse model, that osteocytes can modulate the oxidative microenvironment via expression of Cx43, and thus can control the growth of tumours cells under oxidative stress in the local microenvironment [35] .…”

Single-cell Transcriptome Analysis Identifies Senescent Osteocytes as Contributors to Bone Destruction in Breast Cancer Metastasis

“…Further, the knowledge acquired on the role of osteocytes and their derived factors in these cancers has provided the rationale for developing novel therapeutic interventions to treat cancer in bone (9,(14)(15)(16). In breast cancer metastases, mounting evidence suggests that osteocytes interact with cancer cells and can influence their proliferation, migration, and invasion abilities (17)(18)(19)(20). However, how metastatic cancer cells impact osteocytes in the tumor microenvironment is largely unknown.…”