Osteocytic connexin hemichannels suppress breast cancer growth and bone metastasis

Abstract

Although the skeleton is one of predominant sites for breast cancer metastasis, why breast cancer cells often become dormant after homing to bone is not well understood. Here, we reported an intrinsic self-defense mechanism of bone cells against breast cancer cells: a critical role of connexin (Cx) 43 hemichannels in osteocytes in the suppression of breast cancer bone metastasis. Cx43 hemichannels allow passage of small molecules between the intracellular and extracellular environments. The treatment of bisphosphonate drugs, either alendronate (ALN) or zoledronic acid (ZOL), opened Cx43 hemichannels in osteocytes. Conditioned media (CM) collected from MLO-Y4 osteocyte cells treated with bisphosphonates inhibited the anchorage-independent growth, migration and invasion of MDA-MB-231 human breast cancer cells and Py8119 mouse mammary carcinoma cells and this inhibitory effect was attenuated with Cx43(E2), a specific hemichannel blocking antibody. The opening of osteocytic Cx43 hemichannels by mechanical stimulation had similar inhibitory effects on breast cancer cells and this inhibition was attenuated by Cx43(E2) antibody as well. These inhibitory effects on cancer cells were mediated by ATP released from osteocyte Cx43 hemichannels. Furthermore, both Cx43 osteocyte-specific knockout mice and osteocyte-specific Δ130–136 transgenic mice with impaired Cx43 gap junctions and hemichannels showed significantly increased tumor growth and attenuated the inhibitory effect of ZOL. However, R76W transgenic mice with functional hemichannels but not gap junctions in osteocytes did not display a significant difference. Together, our studies establish the specific inhibitory role of osteocytic Cx43 hemichannels, and exploiting the activity of this channel could serve as a de novo therapeutic strategy.

Gap junctions and cancer: communicating for 50 years

“…ATP release and modulation of Ca 2+ concentrations regulate cell proliferation in a variety of cell types 154-156 , and inappropriate hemichannel opening may underlie some hyperproliferative disorders such as hidrotic ectodermal dysplasia 157 . Hemichannel functions have also been linked to vascular disruption and haemorrhage within tumours 158 , and recently Cx43 hemichannels of osteocytes are involved in suppression of breast cancer cell growth and bone metastasis 159 . However, the link between hemichannels and cancer is difficult to conclusively establish as most reagents that block gap junction channels also block connexin hemichannels making assignment of functional consequences specifically to hemichannel functions problematic 160 .…”

Influence of gap junction intercellular communication composed of connexin 43 on the antineoplastic effect of adriamycin in breast cancer cells

“…Numerous studies have revealed the significant role of GJs in tumors, since recovered expression of connexin that forms GJs or recovered function of GJs could reduce the cell malignant phenotype (11,34,35). The expression of connexin and its derived homotypic GJ suppressed the invasion of tumor cells (36,37).…”

The Emerging Role of Osteocytes in Cancer in Bone

“…Conditioned media from MLO‐Y4 osteocyte cells treated with bisphosphonates reduced the growth, migration, and invasion of MDA‐MB‐231 human breast cancer cells . These inhibitory effects appear to be mediated by Cx43, as mice with impaired Cx43 gap junctions showed significantly increased tumor burden and an attenuation of the inhibitory effect of bisphosphonates on tumor growth . This study suggests a role of osteocytes and Cx43 hemichannels in the response to bisphosphonates and identifies Cx43 as a novel therapeutic target.…”

“…The beneficial effects of bisphosphonates on the skeleton are also due to the prevention of osteoblast and osteocyte apoptosis, an effect that depends on the regulation of Connexin 43 (Cx43) signaling between neighboring cells . Conditioned media from MLO‐Y4 osteocyte cells treated with bisphosphonates reduced the growth, migration, and invasion of MDA‐MB‐231 human breast cancer cells . These inhibitory effects appear to be mediated by Cx43, as mice with impaired Cx43 gap junctions showed significantly increased tumor burden and an attenuation of the inhibitory effect of bisphosphonates on tumor growth .…”