Mammary gland tissue targeted overexpression of human protease-activated receptor 1 reveals a novel link to β-catenin stabilization

Protease-activated receptor 1 (PAR1) is emerging with distinct assignments in tumor biology. We show that tissue targeted overexpression of hPar1 in mice mammary glands results in precocious hyperplasia, characterized by a dense network of ductal side branching and accelerated proliferation. These glands exhibit increased levels of wnt-4 and wnt-7b and a striking beta-catenin stabilization. Nuclear localization of beta-catenin is observed in hPar1 transgenic mouse tissue sections but not in the wild-type, age-matched counterparts. PARI induces beta-catenin nuclear localization also in established epithelial tumor cell lines of intact beta-catenin system (transformed on the background of mismatch repair system; RKO cells). We propose hereby that PARI-mediated beta-catenin stabilization is taking place primarily via the increase of Wnt expression. Enforced expression of a specific Wnt antagonist family member, secreted frizzled receptor protein 5 (SFRP5), efficiently inhibited PAR1-induced beta-catenin stabilization. Likewise, application of either SFRP2 or SFRP5 on epithelial tumor cells completely abrogated PAR1-induced beta-catenin nuclear accumulation. This takes place most likely via inhibition of Wnt signaling at the level of cell surface (forming a neutralizing complex of Receptors-SFRP-Wnt). Furthermore, depletion of hPar1 by small interfering RNA (siRNA) vectors markedly inhibited PARI-induced Wnt-4. The striking stabilization of beta-catenin, inhibited by SFRPs on one hand and Wnt-4 silencing by hPar-1 siRNA on the other hand, points to a novel role of hPar1 in Wnt-mediated beta-catenin stabilization. This link between PAR1 and beta-catenin may bear substantial implications both in developmental and tumor progression processes.