Advanced breast cancer predominantly metastasizes to the skeleton, at which point patients suffer bone loss, pain, heightened fracture risk and their prognosis significantly declines. The skeleton is sensitive to its highly dynamic mechanical environment whereby bone mass is increased when applied loads are increased whereas bone loss occurs when applied loads are reduced. Increased mechanical loading inhibited bone metastatic tumor formation and progression in vivo, but the underlying mechanisms are currently under investigation. Here, we focus on the osteocyte, a specialized bone cell well-known as the primary mechanosensor and director of remodeling in the skeleton. Osteocytic dendrites are important for mechanosensing, and their number and length increase with applied loading. Dendrite connections are also known to suppress breast cancer growth and bone metastasis. How breast cancer cells affect loading-induced changes in dendrites, or downstream effects on mechanosensing and remodeling, is unknown.

To examine how breast cancer cells modulate osteocyte function, we exposed osteocytes (MLO-Y4 cells) to medium conditioned by breast cancer cells (MDA-MB231 cells) and to fluid shear stress using a rocker platform (2 hrs per day for 3 days, shear stress 1.1 Pa). In the absence of loading, treatment with tumor conditioned media alone did not alter dendrite number per MLO or overall cell number. When loading was applied to MLOs treated with conditioned media, dendrite formation increased in MLOs despite the presence of tumor-derived factors. Conditioned media and loading together reduced MLO cell number, suggesting that the combination of these two factors may be stimulating apoptosis and dendrite formation increases in the remaining viable cells. Next, to model the more physiological situation in which both cell populations undergo loading, we exposed MLOs to loading as well as media conditioned by breast cancers that were also similarly loaded. When control (nonloaded) MLOs were treated with conditioned media from loaded breast cancer cells, their dendrite formation increased in a manner similar to that observed due to loading alone. When MLOs simultaneously underwent loading and treatment with loaded conditioned media, dendrite formation was greatest. Loaded conditioned media also decreased MLO cell number independent of MLO loading status. The results suggested that loading stimulates breast cancer cells to induce both osteocyte dendrite formation and apoptosis, which is a possible mechanism for the inhibitory role of applied loading on bone metastasis.