AbstractHere we show the increase of invasion of three breast cancer cell lines (8701‐BC, MDA‐MB‐231 and SKBR3) upon long‐term co‐incubation with culture medium of normal microvascular endothelial cells (MVEC) and normal breast epithelial cells (HB2). The enhancement of invasion relied on the interaction of microvascular endothelial cell and normal breast epithelial cell CXCL12 (SDF1) chemokine, whose expression by breast cancer cells was very low, with the cognate CXCR4 receptor of malignant cells, which resulted in over‐expression of the urokinase‐type plasminogen activator receptor (uPAR) on their surfaces. uPAR over‐expression, showed by RT–PCR and Western blotting, was paralleled by increased urokinase‐type plasminogen activator (uPA) partitioning on the cell surface with respect to the fluid phase, as demonstrated by zymography. Long‐term interaction of SDF1 with CXCR4 stimulated sustained activation of JNK phosphorylation. Blocking antibodies to CXCR4 were able to block the endothelial/epithelial cell‐dependent enhancement of invasion, as well as to inhibit SDF1‐CXCR4‐dependent JNK phosphorylation and uPAR over‐expression of malignant cells. We suggest that acquisition of the angiogenic phenotype by breast cancer cells triggers an amplification loop, in which endothelial cells and normal breast epithelial cells of the tumour cooperate to provide facilitated routes to cell invasion and metastasis and to enhance the aggressive phenotype of cancer cells. Copyright © 2008 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.