PB0737 - Platelet Protein Phosphatase 1c alpha Reduces Tumor Anoikis and Promotes Metastasis.

Background: The survival of circulating tumors in blood relies on the crosstalk between tumors and blood platelets in part via platelet growth factors and microparticles. Platelet-tumor interaction led to protein phosphatase 1(PP1) mediated dephosphorylation of Ser 127 of Yes associated protein (YAP1), a transcriptional coactivator. Dephosphorylation of YAP1 activates genes that reduce detachment-induced apoptosis (anoikis) and facilitate metastasis. However, whether PP1 from platelets participate in signaling crosstalk remains unknown.

Aims: To investigate if alpha isoform of the catalytic subunit of PP1 (PP1c) from platelets facilitates tumor YAP1 dephosphorylation and impact anoikis and metastasis of lung and skin cancer.

Methods: Mice with conditional deletion of Ppp1ca in platelets was generated by crossing Pf4Cre with Ppp1ca f/f mice (PP1ca -/-). Anoikis of LLC and B16 F10 cells were induced by seeding in low attachment plates and the ability of WT and PP1ca -/- platelets to protect from anoikis was analyzed by flow cytometry and YAP phosphorylation. WT and PP1ca -/- mice were subjected to tail vein injection of LLC and B16F10 cells and then analyzed at three weeks for lung tumors.

Results: Consistent with quantitative changes in tumor educated platelets, we observed significantly (p=0.03) increased PP1ca protein in platelets from LLC bearing WT mice. In an experimental metastasis model, we observed significantly (P < 0.05) decreased tumor burden in PP1ca -/- mice, compared to WT mice. In coculture studies, WT but not PP1ca-/- platelets efficiently (P=0.01) dephosphorylated YAP1 Ser127 in LLC and B16F10 cells. Compared to the PP1ca -/-, WT platelets provided a significant protection of anoikis in LLC and B16 F10 cells. Coculture of WT platelets with tumors generated PP1ca bearing platelet microparticles.

Conclusion(s): A crosstalk between platelet PP1ca and tumors facilitate efficient dephosphorylation of tumor YAP1, thereby reducing anoikis and facilitating experimental metastasis.