Platelet C3G protects from liver fibrosis, while enhancing tumor growth through regulation of the immune response

Abstract

Primary liver cancer usually occurs in the context of chronic liver disease (CLD), being associated with fibrosis. Platelets have emerged as important regulators of CLD and liver cancer, although their precise function and mechanism of action need to be clarified. C3G (RapGEF1) regulates platelet activation, adhesion and secretion. Here, we have evaluated the role of platelet C3G in chemically-induced fibrosis and liver cancer associated with fibrosis using genetically modified mouse models. We found that while overexpression of full-length C3G in platelets decreased liver fibrosis induced by chronic treatment with CCl4, overexpressed C3G lacking the catalytic domain did not, although in both cases platelet recruitment to the liver was similar. In addition, C3G deletion in platelets (PF4-C3GKO mouse model) increased CCl4-induced liver damage and hepatic fibrosis, reducing liver platelets and macrophages. Moreover, early liver immune response to CCl4 was altered in PF4-C3GKO mice, being remarkable the lower activation of macrophages and the increased monocyte-derived macrophages compared to wt mice. On the other hand, in response to DEN+CCl4, PF4-C3Gwt mice exhibited more, and larger liver tumors than PF4-C3GKO mice, accompanied by the presence of more platelets, despite having less fibrosis in previous steps. Liver immune cell populations were also differentially regulated in PF4-C3GKO mice, highlighting the higher number of macrophages, likely with a pro-inflammatory phenotype, present in the liver in response to chronic DEN+CCl4 treatment. Proteins upregulated or downregulated in platelet-rich plasma from PF4-C3GKO compared to wt mice might regulate the immune response and tumor development. In this regard, enrichment analyses using proteomic data showed changes in several proteins involved in platelet activation and immune response pathways. Additionally, the higher secretion of CD40L by PF4-C3GKO platelets could contribute to their anti-tumor effect. Therefore, platelet C3G presents anti-fibrotic and pro-tumoral effects in the liver, likely mediated by changes in the immune response.