The molecular basis for breast cancer metastasis to the brain is largely unknown 1,2 . Brain relapse typically occurs years after the removal of a breast tumour [2][3][4] , suggesting that disseminated cancer cells must acquire specialized functions to overtake this organ. Here we show that breast cancer metastasis to the brain involves mediators of extravasation through non-fenestrated capillaries, complemented by specific enhancers of blood-brain barrier crossing and brain colonization. We isolated cells that preferentially infiltrate the brain from patients with advanced disease. Gene expression analysis of these cells and of clinical samples, coupled with functional analysis, identified the cyclooxygenase COX2 (also known as PTGS2), the epidermal growth factor receptor (EGFR) ligand HBEGF, and the α2,6-sialyltransferase ST6GALNAC5 as mediators of cancer cell passage through the blood-brain barrier. EGFR ligands and COX2 were previously linked to breast cancer infiltration of the lungs, but not the bones or liver 5,6 , suggesting a sharing of these mediators in cerebral and pulmonary metastases. In contrast, ST6GALNAC5 specifically mediates brain metastasis. Normally restricted to the brain 7 , the expression of ST6GALNAC5 in breast cancer cells enhances their adhesion to brain endothelial cells and their passage through the blood-brain barrier. This co-option of a brain sialyltransferase highlights the role of cell-surface glycosylation in organspecific metastatic interactions.Brain metastasis affects an estimated 10% of cancer patients with disseminated disease 2,8,9 . Even small lesions can cause neurological disability, and the median survival time of patientsCorrespondence and requests for materials should be addressed to J.M. (E-mail: j-massague@ski.mskcc.org). † Present addresses: Institut de Malalties Hemato-Oncològiques, Hospital Clínic, 08036 Barcelona, Spain (C.N.); Oncology Programme, Institute for Research in Biomedicine, 08028 Barcelona, Spain (R.R.G.). Author InformationThe clinical microarray data on the brain metastatic cell lines have been deposited in NCBI's Gene Expression Omnibus (GEO, http://www.ncbi.nlm.nih.gov/geo) under the GEO series accession number GSE12237.Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. 11 and also by tight junctions and astrocyte foot processes in the blood-brain barrier (BBB) 2,8 , whereas the capillaries in the bone marrow and the liver are fenestrated 11,12 . The composition of the parenchyma also varies extensively between these organs. The protracted progression of disseminated cancer cells in different environments may give rise to metastatic speciation, as suggested by the coexistence of malignant cells with different organ tropisms in fluids from patients with advanced disease 5,13 . Analysis of such malignant cell populations has revealed genes that selectively mediate breast cance...

Cells released from primary tumors seed metastases to specific organs by a nonrandom process, implying the involvement of biologically selective mechanisms. Based on clinical, functional, and molecular evidence, we show that the cytokine TGFbeta in the breast tumor microenvironment primes cancer cells for metastasis to the lungs. Central to this process is the induction of angiopoietin-like 4 (ANGPTL4) by TGFbeta via the Smad signaling pathway. TGFbeta induction of Angptl4 in cancer cells that are about to enter the circulation enhances their subsequent retention in the lungs, but not in the bone. Tumor cell-derived Angptl4 disrupts vascular endothelial cell-cell junctions, increases the permeability of lung capillaries, and facilitates the trans-endothelial passage of tumor cells. These results suggest a mechanism for metastasis whereby a cytokine in the primary tumor microenvironment induces the expression of another cytokine in departing tumor cells, empowering these cells to disrupt lung capillary walls and seed pulmonary metastases.

The identification of the TGFb cytokine signaling pathway, including membrane receptor serine/threonine kinases and Smad transcription factors as their substrates, has allowed the delineation of a process for conversion of these signals into programs of gene activation and repression that underlie critical cell fate and developmental decisions. The deconstruction of one of these responses -the cell cycle arrest response -into its elemental molecular parts has shed light into the mechanisms used by tumors to evade surveillance and cause metastasis.