Blood platelets stimulate cancer extravasation through 3 TGFβ-mediated down-regulation of PRH/HHEX

Cancer cells go through a process known as epithelial-mesenchymal transition (EMT) during which they acquire the ability to migrate and invade extracellular matrix. Some cells also acquire the ability to move across a layer of endothelial cells to enter and exit the bloodstream; intra- and extravasation, respectively. The transcription factor PRH/HHEX controls cell proliferation and cell migration/invasion in a range of cell types. Our previous work showed that PRH activity is down- regulated in prostate cancer cells due to increased inhibitory PRH phosphorylation and that this increases cell proliferation and invasion. PRH inhibits migration and invasion by prostate and breast epithelial cells in part by activating the transcription of Endoglin, a Transforming Growth Factor β (TGFβ) co-receptor. Here we show that depletion of PRH in immortalised prostate epithelial cells results in increased extravasation in vitro. We show that blood platelets stimulate extravasation of cells with depleted PRH and that inhibition of TGFβ signalling blocks the effects of platelets on these cells. Moreover, TGFβ induces changes characteristic of EMT including decreased E-Cadherin expression and increased Snail expression. We show that in prostate cells PRH regulates multiple genes involved in EMT and TGFβ signalling. However, both platelets and TGFβ increase PRH phosphorylation. Additionally, TGFβ increases binding of its effector pSMAD3 to the PRH/HHEX promoter and down-regulates PRH protein and mRNA levels. Thus TGFβ signalling own-regulates PRH activity by multiple mechanisms and induces an EMT that facilitates extravasation and sensitizes cells to TGFβ.