Shimizu et al. showed that intrasplenically injected tumor cells migrated into the space of Disse at 2 days after injection, where they proliferated in close association with HSCs, suggesting that tumor cells may interact with and activate HSCs directly in vivo.17 Their hypothesis was later supported by data showing that conditioned medium of tumor cells was able to induce HSC activation in vitro.14 Conditioned medium of tumor cells promoted HSC proliferation in a dose-dependent manner and induced the expression of α-SMA and formation of α-SMA–positive stress find more fibers in HSCs, which are characteristic of transdifferentiated myofibroblasts.14 In our laboratory,

we found that treatment of quiescent HSCs with TGF-β1, a cytokine released by cancer cells that is abundant in the hepatic tumor microenvironment, induced myofibroblast transdifferentiation of HSCs in vitro.20 Taken together, these data suggest bidirectional interactions between tumor cells and HSCs in vivo. The activation of HSCs in the tumor microenvironment is a complex process that requires participation of paracrine stimuli of tumor cells and intracellular factors within HSCs. TGF-β and PDGF are the two most potent factors regulating HSC activation in vivo. The action of TGF-β on HSC activation is mediated by the canonical TGF-β/Smad-dependent signaling pathway.20 PDGF is one of most powerful mitogens and survival factors for

HSCs, which acts by MK-8669 cost activating key signaling pathways such as Ras/Erk (extracellular signal-regulated kinase) and phosphoinositide 3-kinase in HSCs.40, 41 In addition to TGF-β and PDGF, intracellular factors

promoting HSC responsiveness to external stimuli include receptor-mediated signaling cascades, ECM-mediated integrin activation signaling, the Rho family of small guanosine triphosphatases, and transcription factors. Their roles in HSC activation remain active research topics and are reviewed in detail medchemexpress elsewhere.40, 42, 43 Given the complex nature of the hepatic microenvironment, it is likely that other components of liver may interact with HSCs and tumor cells, thus contributing to HSC activation and metastatic growth. For example, Kupffer cells may regulate HSC activation and tumor growth by releasing TGF-β1,44 and endothelial cells may suppress HSC activation by producing nitric oxide,45, 46 a multifunctional signaling molecule that possesses antifibrotic activity. Current in vivo models that are employed to study the role of HSCs in liver metastases include subcutaneous coimplantation of tumor cells and HSCs/myofibroblasts in mice, portal vein implantation of tumor cells into the liver of mice, and portal vein coimplantation of tumor cells and HSCs/myofibroblasts into the liver of mice. Subcutaneous or portal vein coimplantation of HSCs/myofibroblasts and tumor cells in mice often resulted in larger tumors.