Osteocytic Connexin Hemichannels Modulate Oxidative Bone Microenvironment and Breast Cancer Growth.

Abstract

Simple SummaryEstrogen deficiency with increased oxidative stress is associated with various pathological conditions, including cancers. Oxidative stress (OS) is elevated in the bone microenvironment with estrogen deficiency, but the impact of OS on tumor growth in bone remains largely unknown. In our research, we found that elevated oxidative stress inhibits tumor cell growth and the lack of Cx43 hemichannel function in bone osteocytes could mediate this effect. Impairment of Cx43 hemichannels either under oxidative stress or in Cx43-hemichannel-impaired transgenic mice under estrogen deficiency led to the inhibition of breast cancer growth. Treatment with an antioxidant reverses the effect, showing an increase in tumor growth in mice. This study unveiled a new mechanism regarding the role of osteocytic Cx43 hemichannels in modulating an oxidative bone microenvironment and the response of breast cancer cells to local oxidative stress.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.