13 Furthermore, TGF-β derived from HSCs acted on tumor cells and governed tumorigenesis in a paracrine fashion, leading to tumor-progressive and autocrine TGF-β signaling in tumor cells.18 Recently, stromal cell-derived factor 1 (SDF-1) was found to be released by find more activated HSCs within the liver metastases, and CXCR4 (chemokine [C-X-C motif] receptor 4), the ligand of SDF-1, was found to be expressed in colorectal cancer cells.22In

vitro, this SDF-1/CXCR4 paracrine signaling promoted tumor cell invasion and protected tumor cells from apoptosis.22 In unpublished data, we have also demonstrated that myofibroblast-derived PDGF-BB is a potent survival factor for cholangiocarcinoma cells. Taken together, these data support the concept that activated PS-341 cost HSCs promote tumor cell growth by supplying them with growth factors and cytokines. A high degree of ECM remodeling favors tumor invasion and progression in the liver.23 Both MMP and TIMP2 play a key role in degrading basement membranes, thereby allowing cancer cells to cross tissue boundaries and develop into metastases. By performing

in situ hybridization and zymography, Musso et al. found that both MMP2 and TIMP2 messenger RNA were expressed in activated HSCs at the invasive front of liver metastases, and a higher level of MMP2 messenger RNA and enzymatic activity was detected in liver metastases than in nontumoral liver samples.24, 25 In addition, activated MCE HSCs at the invasive front of human liver metastases were found to express a secreted form of ADAM9 (a disintegrin and metallopeptidase 9).16 This molecule was shown to be able to cleave laminin and bind to tumor cells, thus promoting invasion of tumor cells.16 These data indicate that HSCs may facilitate tumor invasion by producing proteolytic enzymes involved in the degradation of ECM. Activated HSCs are a major cell type for ECM production during the pathogenesis of liver fibrosis,4, 5 and this process may

also contribute to the prometastatic growth effects of HSCs. In the liver tumor microenvironment, TGF-β1 released by tumor cells induces HSCs to produce increased amounts of ECM constituents such as fibronectin and collagen I. These ECM components constitute a microenvironment in which tumor cells adhere and grow. In addition to providing a physical support to tumor cells, these ECM components also regulate the adhesion, migration, and survival of tumor cells by binding to and activating integrins on the surface of tumor cells.26, 27 For example, ECM-mediated activation of phosphoinositide 3-kinase and its downstream targets in tumor cells protects tumor cells from genotoxin-induced cell cycle arrest and subsequent apoptosis, contributing to tumor chemoresistance.28 In addition, the poorly vascularized architecture associated with desmoplasia contributes to tumor chemoresistance by imposing a barrier to drug delivery.