Metastases are responsible for 90% of deaths from cancer, and patients diagnosed with brain metastasis have only a dismal 20% probability of surviving for more than 1 year. In the case of breast cancer (BC), the incidence of brain metastasis has increased dramatically, possibly as an indirect result of improved therapies that effectively control extracranial disease. Due to the grave consequences of brain colonization by metastatic BC cells, we wished to elucidate the brain microenvironmental cues that promote secondary tumor formation.
We report that surgical specimens from a patient with breast-to-brain metastasis exhibit increased active gliosis (GFAP+, J131+, GLAST+/CD44+) around the peritumoral brain region, accompanied by infiltration of the tumor by neural progenitor cells (NPCs) (Emx2+, Sox2+, Pax6+, Numb+, Sox9+, Nestin+). Because signaling by the TGF-b superfamily is involved in gliogenesis and also exerts tumor-suppressive effects on BC cells, we investigated potential roles for these proteins in the brain microenvironment in the context of metastatic lesions. We found that metastatic BC cells exhibit increased bone morphogenic protein-2 (BMP-2) expression compared with primary breast tumors, and hypothesized that an interaction between BC cells that have metastasized to the brain and the resident peritumoral NPCs may be mediated by BMP-2. To examine the effect of BMP-2 overexpression, we created an in vitro model that utilized co-cultures of NPCs and primary or metastatic BC cells. We observed that during early co-culture (1-5 days) NPCs inhibited the proliferation of metastatic BC cells but later in co-culture (5-15 day) NPCs differentiated into astrocytes with a concomitant loss of their ability to inhibit metastatic BC growth. This result suggested an adaptive strategy adopted by metastatic BC cells for colonization of the brain may be the BMP-2-mediated differentiation of NPCs into reactive astrocytes. Inhibition of BMP-2 signaling by Noggin or the knockdown of BMP-2 expression in the metastatic breast tumor cells prevented NPC differentiation into astrocytes and maintained their ability to inhibit tumor cell proliferation. Conversely, BMP-2 overexpression in the primary tumor cells enhanced NPC differentiation into astrocytes and promoted tumor cell growth. Together, the results suggest that breast-brain metastatic tumor cells and NPCs co-evolve, creating a more permissive tumor microenvironment favoring tumor proliferation through increased expression of BMP-2.