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The G protein-coupled protease activated receptors (PARs) are key signaling components of protease pathways in vascular biology. The coagulation protease thrombin specifically cleaves and activates PAR1, whereas the complex of tissue factor (TF) with coagulation factor VIIa activates PAR2. In breast cancer, PAR1 expression is correlated with the invasive phenotype and PAR1 or PAR2 signaling promote breast cancer migration and invasion, indicating redundancy. However, in a xenograft model of highly aggressive breast cancer cells (MDA-MB-231mfp), we have recently shown that blockade of TF-VIIa signaling as well as inhibition of PAR2 activation by antibodies is sufficient to attenuate subcutaneous and orthotopic tumor growth. As an independent evaluation of the contributions of PAR1 and PAR2 to breast cancer progression, PAR1-/- and PAR2-/- mice were crossed with MMTV-PyMT mice (expressing the polyoma middle T antigen specifically in the mammary gland) to generate cohorts of transgene-positive C57BL/6 wild-type, PAR1-/- and PAR2-/- mice. In the PyMT model mice develop breast tumors spontaneously and tumor progression recapitulates several aspects of human disease. Cohorts were weekly monitored for the appearance of palpable tumors, tumor growth and at sacrifice lung metastatic burden was quantified by quantitative RT PCR of the breast cancer specific transgene. Appearance of palpable tumors, tumor expansion and metastasis was indistinguishable between wild-type and PAR1-/- mice. Although PAR1 has oncogenic potential, these data indicate that thrombin and PAR1 signaling in the tumor microenvironment are dispensable for breast cancer progression. In contrast, the appearance of first palpable tumors and multifocal disease was delayed in PAR2-/- mice. Metastasis was reduced in PAR2-/- mice consistent with delayed tumor onset. Whole mount analysis did not reveal differences between wild-type, PAR1-/-, and PAR2-/- mice in mammary gland development and all strains developed adenomas of similar morphological appearance. Analysis of early tumors showed persistence of adenomas with impaired vascularization in PAR2-/- mice. These data corroborate our findings in the human breast cancer xenograft model that tumor cell TF-PAR2 signaling contributes to the angiogenic switch and promotes angiogenesis. This study establishes that protease signaling contributes to breast cancer development and that PAR2, rather than the thrombin receptor PAR1 plays crucial roles in the communication of tumor cells with the tumor microenvironment.

99th AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA