Protease activated receptor-1 (PAR-1), also known as thrombin receptor, is an important cell surface receptor that in response to injury regulates diverse cellular functions. PAR-1 is expressed on activated vascular cells and on tumor cells. A role for PAR-1 in tumor biology is supported by the finding that the receptor is upregulated in the progression of breast cancer, but very little information is available on whether and how PAR-1 activity might influence the behavior of malignant tumors. To support the hypothesis that PAR-1 on vascular cells contributes to the generation of tumor neovessels, growth factor and tumor cell-induced angiogenesis, as well as tumor growth in PAR-1 deficient mice, will be compared to control mice. It will be determined which vascular cell types express PAR-1 and how PAR-1 activation influences vascular cell behavior in the growth and development of new vessels. To implicate PAR-1 as a mitogenic receptor for tumor cells, it will be tested whether PAR-1 activation on human breast cancer cells results in enhanced proliferation in vitro and tumor growth in the m.f.p. of SCID mice. Transformed cell lines from PAR-1 deficient mice will be transfected to express PAR-1 and studied for the effect of thrombin on cell growth in vitro and in s.c. tumors to implicate PAR-1 in tumor models where PAR-1 expression is genetically controlled. To investigate the contribution of PAR-1 to tumor cell invasion and metastasis, it will be tested whether PAR-1 activation in human breast carcinoma cell lines results in the expression of proteins associated with invasive behavior and in enhanced cell migration and invasion in vitro. It will be determine in SCID mice whether PAR-1 expression and activation on human breast carcinoma cells contributes to experimental and spontaneous metastasis and in transgenic MMTV-PymT mice whether PAR-1 is involved in mammary tumor development and progression. Finally, it will be addressed whether tumor cell generated thrombin is required for PAR-1 activation in the context of tumor metastasis. Together, these studies will critically assess the contribution of PAR-1 to tumor growth and dissemination and identify the mechanisms involved.
