Depletion of tumor-reactive HSCs reveals their significance during different stages of liver metastasis

Background: Activated HSCs play a major role in tissue repair, extracellular matrix regulation, immune response, and inflammation. However, their contributions to the hepatic tumor microenvironment are underexplored and need to be clarified. Methods: In vitro, we analyzed the responses of freshly isolated LSECs and HSCs to tumor cell supernatants and secretome-driven activation of both primary cell types. For in vivo HSC depletion, transgenic mice expressing the herpes simplex virus-thymidine kinase (HSV-Tk) gene driven by the mouse glial fibrillary acidic protein promoter were used. MC38 colon carcinoma or B16 melanoma was intrasplenically injected to generate liver metastasis to further analyze metastatic growth, collagen accumulation, angiogenesis, and immunosuppression. Results: Metastatic tumor cells arrest and adhere in the liver 48 hours after intrasplenic injection. The 65% of arrested tumor cells were surrounded by smooth muscle actin-expressing cells. In vitro, tumor-activated LSEC-derived secretomes stimulated smooth muscle actin expression, migration, VEGF, and LSEC promigratory factor release by HSCs. Tumor cell secretomes stimulated HSC proliferation and the secretion of proangiogenic and protumoral mediators. HSC depletion reduced the foci number and metastatic area in colorectal cancer and melanoma models. Moreover, livers from transgenic mice showed reduced key tumor microenvironment parameters, including intratumoral collagen accumulation, neoangiogenesis, and recruitment of myeloid-derived suppressor cells. Conclusions: Depletion of tumor-reactive proliferating HSCs implicates these cells as the required spark for the initiation and progression of liver metastasis, making them a good candidate for new therapies targeting the tumor microenvironment to treat liver metastasis of different primary origins.