Osteoblasts modulate Ca²⁺ signaling in bone-metastatic prostate and breast cancer cells

Metastatic prostate and breast cancers display a predilection for the skeleton. The high incidence of skeletal metastasis may be a reflection of favorable reciprocal interactions between the bone microenvironment and disseminated cancer cells. Here we show that bone-metastatic PC3-ML prostate cancer cells and MDA-231 breast cancer cells—when co-cultured with human osteoblasts—down-regulate the increase in cytosolic free calcium (Ca²⁺) induced by agonist stimulation. This osteoblast promoted alteration of Ca²⁺ signaling develops and reverts in a time-dependent manner. Most importantly, the Ca²⁺ responses of cancer cells lacking bone metastatic potential are not affected by osteoblasts. The limited increase in cytosolic Ca²⁺ observed in bone-metastatic cells does not result from depleted intracellular Ca²⁺ stores but rather a decreased entry of Ca²⁺ from the extracellular space. Interestingly, the inhibition of histone deacetylase in cancer cells replicates the changes in Ca²⁺ signaling induced by osteoblasts, suggesting the participation of epigenetic mechanisms. Finally, cancer cells harvested from skeletal metastases induced in mice showed Ca²⁺ responses identical to cells co-cultured with osteoblasts. However, Ca²⁺ signaling in cancer cells recovered from metastases to soft-tissues was not affected, emphasizing the role of the bone microenvironment in regulating the functional behavior of bone-metastatic cells. We propose that osteoblasts protect selected malignant phenotypes from cell death caused by an excessive increase in cytosolic Ca²⁺, thereby facilitating their progression into macroscopic skeletal metastases.