Differential impact of adenosine nucleotides released by osteocytes on breast cancer growth and bone metastasis.

Extracellular ATP has been shown to either inhibit or promote cancer growth and migration; however, the mechanism underlying this discrepancy remained elusive. Here we demonstrate the divergent roles of ATP and adenosine released by bone osteocytes on breast cancers. We showed that conditioned media (CM) collected from osteocytes treated with alendronate (AD), a bisphosphonate drug, inhibited the migration of human breast cancer MDA-MB-231 cells. Removal of the extracellular ATP by apyrase in CM abolished this effect, suggesting the involvement of ATP. ATP exerted its inhibitory effect through the activation of purinergic P2X receptor signaling in breast cancer cells evidenced by the attenuation of the inhibition by an antagonist, oxidized ATP, as well as knocking down P2X7 with small interfering RNA (siRNA), and the inhibition of migration by an agonist, BzATP. Intriguingly, ATP had a biphasic effect on breast cancer cells-lower dosage inhibited but higher dosage promoted its migration. The stimulatory effect on migration was blocked by an adenosine receptor antagonist, MRS1754, ARL67156, an ecto-ATPase inhibitor, and A2A receptor siRNA, suggesting that in contrast to ATP, adenosine, a metabolic product of ATP, promoted migration of breast cancer cells. Consistently, non-hydrolyzable ATP, ATPγS, only inhibited but did not promote cancer cell migration. ATP also had a similar inhibitory effect on the Py8119 mouse mammary carcinoma cells; however, adenosine had no effect owing to the absence of the A2A receptor. Consistently, ATPγS inhibited, whereas adenosine promoted anchorage-independent growth of MDA-MB-231 cells. Our in vivo xenograft study showed a significant delay of tumor growth with the treatment of ATPγS. Moreover, the extent of bone metastasis in a mouse intratibial model was significantly reduced with the treatment of ATPγS. Together, our results suggest the distinct roles of ATP and adenosine released by osteocytes and the activation of corresponding receptors P2X7 and A2A signaling on breast cancer cell growth, migration and bone metastasis.