Capturing Changes in the Brain Microenvironment during Initial Tumor Cell Invasion.

Abstract

Abstract Brain metastases occur mostly in patients with melanoma or primary tumors in the lung, breast or prostate. As therapies against primary tumors and extracranial metastases are improving and patients live longer, the incidence of brain metastases is increasing and brain lesions are seen in 20 to 40 % of patients with advanced metastatic disease. Unfortunately, no therapies are available to treat brain metastasis efficiently.Existing in vivo models of brain metastasis focus mainly on established macrometastases, while very little is known about the initial events of tumor cell brain colonization. In the present study, we used non-invasive bioluminescence imaging and detailed immunohistochemistry to capture the early events of tumor cell interaction with the cerebral microvasculature and invasion of the brain tissue. We used MDA-MB-435 metastatic cancer cells as a model because of their aggressive metastatic phenotype, and injected the cells into the internal carotid artery of immune deficient mice. Our results show that single cancer cells extravasate into the brain parenchyma 3 to 7 days after they arrest within brain capillaries and disrupt the vessel wall, bursting the microvasculature. Cancer cell arrest and extravasation occur exclusively in small vessels that are surrounded by pericytes and lack smooth-muscle cells. Early brain colonization consistently induces local activation of astrocytes, involving heterogeneous astrocyte populations. Reactive astrocytes express high levels of MMP-9 that may assist tumor cell invasion and provide first pro-angiogenic clues. Activated astrocytes consistently persist in the vicinity of growing metastases, and potentially contribute to metastatic proliferation. Occasionally, early host responses also include microglia with stellate or a reactive amoeboid morphology. Importantly, in addition to yielding actively proliferating macrometastases, tumor cell extravasation also leads to widespread single tumor cells dispersed throughout the brain tissue. The majority of these cells remains viable but stays quiescent and does not proliferate. We speculate that the diversity in early local responses of the brain microenvironment to initial cancer cell invasion contributes to the fate of extravasated tumor cells. An early involvement of diverse host cell types might provide distinct environmental clues and co-determine whether a single cell progresses to macrometastasis or remains dormant. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4162.