Background Breast malignancy comprises clinically and molecularly distinct tumor subgroups that differ in cell histology and biology and show divergent Ki16198 clinical phenotypes that impede phase III trials such as those utilizing cathepsin K inhibitors. platelets PAR-1 and ?4 are highly expressed but PAR-3 shows low expression and unclear functions. Methods Platelet aggregation was monitored by measuring changes in turbidity. Platelets were immunoblotted with anti-phospho and total p38 Src-Tyr-416 FAK-Tyr-397 and TGFβ monoclonal antibody. Activation was measured in a circulation cytometer and calcium mobilization in a confocal microscope. Mammary epithelial cells were prepared from the primary breast cancer samples of 15 women with Luminal-B subtype to produce primary cells. Results We demonstrate that platelets are aggregated by cathepsin K in a dose-dependent manner but not by other cysteine cathepsins. PARs-3 and ?4 were confirmed as the cathepsin K target by immunodetection and specific antagonists using a fibroblast cell collection derived from PARs deficient mice. Moreover through co-culture experiments we show that platelets activated by cathepsin K mediated the up-regulation of SHH PTHrP OPN and TGFβ in epithelial-mesenchymal-like cells from patients with Ki16198 Luminal B breast malignancy. Conclusions Cathepsin K induces platelet dysfunction and affects signaling in breast malignancy cells. Electronic supplementary material The online version of this article (doi:10.1186/s12885-016-2203-7) contains supplementary material which is available to authorized users. Keywords: Cathepsin K Platelets Breast cancer Protease activated receptors Background Proteases from epithelial myoepithelial stromal and tumor cells become activated during neoplastic progression and can display causal functions in tumor growth migration invasion angiogenesis and metastasis [1-5]. However identification of the exact tissue of origin temporal release and activation is not fully established. Human cysteine cathepsins (Cat) are proteases that are highly up-regulated in a wide variety of cancers. Active forms of cathepsins are localized in endosomal or lysosomal vesicles cell membranes and/or secreted and localized in pericellular environments as soluble enzymes that are Ki16198 involved in cleaving the extracellular matrix proteins laminin and type IV collagen and cell-adhesion proteins such as E-cadherin and matricellular proteins [2 6 Proteolytically activated receptors (PARs) constitute a family of G-protein-coupled receptors that are activated during one of several protease-generating pathways in humans such as inflammatory fibrinolytic and hemostatic pathways and Fzd10 malignancy; PARs are also activated by proteases particularly thrombin via a specific proteolytic cleavage of their amino-terminal exodomain [9-12]. The PAR-mediated mitogenic pathway regulates tumor cell growth and can promote tumor cell invasion . Several examples of PARs up-regulation and their potential in activating proteinases in tumor tissues including breast prostate and colon cancer and malignant melanomas have been reported [11 14 In addition abnormalities in blood coagulation are common in malignant tumors . Tumor cells have platelet aggregating activity that occurs through different mechanisms including the activation of PARs. PAR-1 and ?4 show the highest expression in human platelets among the four currently identified PARs [16 17 PAR-3 shows the lowest expression and appears to be preferentially expressed in cells of hematopoietic origin suggesting a function distinct Ki16198 from that of PAR-1 which is the major receptor involved in thrombin-mediated platelet activation . Furthermore PAR-3 has been shown to be a major thrombin receptor in mouse platelets; however its role in humans remains uncharacterized [11 19 In this scenario the link between human cysteine cathepsins and platelet functions in malignant conditions is usually underexplored. The cysteine cathepsins used in our study K L V S and B are particularly attractive drug targets [8 22 Cat K is usually of relevant interest because it is a cysteine protease implicated in bone remodeling breast malignancy progression and other diseases [23-26]. We investigated platelet aggregation using washed platelets which enabled the identification of PARs involved in this process to determine the role of cathepsins in human platelet aggregation and the detailed triggering signal produced by cathepsins on platelets. In addition we examined whether Cat K alone which was activated in epithelial-mesenchymal cells from women with breast malignancy or its co-culture with Cat K activated human.