The microenvironment of a primary tumour has been recognized as an important player in cancer progression, but whether circulating tumour cells respond to additional metastatic signals during their transit through the vasculature is unclear. Given the high concentrations of growth factors and cytokines that are contained in platelets, Labelle, Begum and Hynes proposed that, in addition to their known roles in promoting cell adhesion and protecting tumour cells from death, platelets might provide pro-metastatic signals to circulating tumour cells.

To investigate this possibility, the authors first co-cultured platelets with mouse colon (MC38GFP) or breast (Ep5) carcinoma cells, and showed that these tumour cells (after the removal of the platelets) were able to induce more metastatic foci in the lungs following injection into the tail vein of mice, than tumour cells that had not been co-cultured with platelets. In vitro, the tumour cells seemed to undergo an epithelial to mesenchymal transition (EMT) when co-cultured with platelets and could invade through a layer of Matrigel. Examination of the changes in gene expression induced in Ep5 cells by platelets revealed the activation of EMT-related genes, especially those that depended on transforming growth factor-β (TGFβ) signalling. Indeed, increased levels of active TGFβ1 were found in the medium after co-culture of Ep5 or MC38GFP tumour cells with platelets, and the activation of the TGFβ effector SMAD2, and SMAD-dependent transcription, were both increased in the tumour cells.

platelet-mediated TGFβ1 and NF-κB signalling in circulating tumour cells can promote metastasis

Is TGFβ1 required for platelet-induced invasion and metastasis? Treatment with inhibitors of TGFβ signalling blocked invasion in vitro. In vivo, fewer metastatic foci were seen in the lungs following tail vein injection of MC38GFP cells into mice lacking TGFβ1 specifically in their platelets (Pf4-cre+;Tgfb1fl/fl mice) compared with littermate controls. Interestingly, the pretreatment of tumour cells with wild-type platelets was not sufficient to induce metastasis in Pf4-cre+;Tgfb1fl/fl mice, indicating a requirement for host-derived TGFβ1.

Is the TGFβ1 that is released from platelets sufficient to induce metastasis? Further experiments showed that direct platelet–tumour cell contact is required to induce the full range of gene expression changes in the tumour cells, indicating that, in addition to TGFβ1, platelet-bound factors are also required. To determine which molecular pathways are induced following platelet–tumour cell interactions, the authors conducted several luciferase reporter assays for known cancer-associated pathways. They discovered that the interaction with platelets resulted in the activation of the nuclear factor-κB (NF-κB) pathway in the tumour cells, which was independent of TGFβ1. In addition, inhibition of NF-κB in Ep5 cells prevented the formation of metastatic foci following pretreatment with platelets and tail vein injection, indicating that this pathway is also required for metastasis.

Overall, these data indicate that platelet-mediated TGFβ1 and NF-κB signalling in circulating tumour cells can promote metastasis, which may prove useful in the design of anti-metastatic therapies.