Crosstalk between Breast Cancer Cells and Bone Marrow Endothelium Require the Engagement of PI3K/AKT Signaling.

The presence of breast cancer cells in the patient’s bone marrow (BM) at diagnosis is associated with resistance to treatment, disease relapse and poor prognosis. Identification of the factors implicated in the homing, survival and latency of breast cancer cells in the BM should contribute to the design of more efficient therapeutic strategies for breast cancer. There is evidence that breast cancer can recruit endothelial progenitors from the BM. Also, other epithelial tumors seem to preferentially adhere to BM endothelial cells. Therefore, we hypothesized that BM endothelium may play a significant role in the biology of breast cancer cells residing in the BM. Co-cultures in Matrigel showed that breast cancer cells interact with BM endothelium to form heterotypic multicellular networks. Moreover, breast cancer cells migrate towards BM endothelium assembled as capillary-like structures, but not to structures of BM mesenchymal stem cells or BM stroma. This migration was abrogated by pertussis toxin-mediated blockade of chemokine receptor signaling, suggesting the involvement of endothelium-secreted chemokine(s). We then evaluated the impact of breast cancer cells in the survival and proliferation of BM endothelium. All breast cancer lines tested (n=4) promoted the proliferation of BM-derived endothelial cells. This effect is mediated through the engagement of the PI3K/Akt pathway (phosphorylation of Akt at Ser437 and Thr308, and activation of its downstream substrates GSK3β, PRAS-40 and FKHRL1) since its specific blockade abrogated the stimulatory effects of breast cancer on BM endothelium. We next determined whether, reciprocally, BM endothelium impacts on breast cancer cell survival. These experiments were performed in serum-free media to enhance dependency of breast cancer cells from microenvironmental stimuli. In all cases tested, BM endothelium promoted survival/proliferation of breast cancer cells. This stimulation was accompanied by the engagement of the PI3K/Akt pathway in breast cancer cells and, in three of the four lines, the phosphorylation of Erk1/2. These effects were also observed for breast cancer cells that showed constitutive activation of Akt (MCF-7 and ZR-75-1 cells). Specific blockade of PI3K/Akt abrogated the BM endothelium-promoted survival of breast cancer cells, thus demonstrating the critical role of this pathway. These studies show that crosstalk between BM endothelial cells and breast cancer cells may impact on the survival of both cell types. These findings provide new light on the mechanisms that may facilitate the development of a tumor-permissive BM microenvironment in breast cancer, and the creation of breast cancer-supporting BM niches. Importantly, this study implicates BM endothelium as a therapeutic target in breast cancer and suggest that blockade of PI3K/Akt may impact the outcome of patients with metastatic breast cancer.