80% of hepatocellular carcinomas (HCC) arise in fibrotic livers. Notably, chronic injury, inflammation and fibrosis are sufficient to trigger HCC i mice, suggesting that HCC truly represents "a wound that does not heal" and that the hepatic microenvironment exerts a profound tumor-promoting influence. In contrast to other organs, myofibroblast (MF) accumulation in the liver occurs not only as a desmoplastic response to established tumors with accumulation of cancer-associated fibroblasts (CAF), but there is also an abundance of MF in the precancerous liver. Thus, HCC may be affected by different MF subsets, namely tissue fibrosis-associated MF and CAF, at different stages. Despite the strong association between fibrosis and HCC, it is currently not known whether fibrosis promotes HCC development, or whether fibrosis and hepatocarcinogenesis represent two parallel but functionally independent responses. This is largely due to the lack of models, in which hepatic MF activation can be selectively modulated without confounding effects on the inflammatory and epithelial cell compartments that accompany all current models. Here, we seek to test the hypothesis that hepatic MF and CAF are important contributors to hepatocarcinogenesis. For this purpose, we will employ a novel Cre-transgenic mouse that not only marks 99% of hepatic stellate cells (HSC), the precursors of MF in the liver, but also enables us to selectively increas or decrease the number of activated MF in the liver. In Aim 1 of this proposal, we seek to test the hypothesis that HSC are the key source of fibrosis-associated MF and CAF in fibrosis-associated hepatocarcinogenesis using fate tracing via LratCre. We will furthermore establish the functional contribution of HSC-derived MF, employing LratCre to genetically deplete or activate HSC in combination with Cre-inducible diphtheria receptor and activated PDGFRß, respectively. In Aim 2, we seek to determine mechanisms by which MF promote carcinogenesis in the liver. For this purpose, we will determine time points at which HSC-derived MF promote HCC and analyze how MF activation or ablation change tumor proliferation, apoptosis and the tumor kinome. In addition, we seek to compare gene expression between highly-purified MF and CAF, and to investigate the involvement of candidate mediators from MF and CAF in hepatocarcinogenesis. In Aim 3, we will employ a novel and highly efficient anti-fibrotic, HSC-targeted IFNγ, to determine whether inhibiting MF activation and liver fibrosis reduces HCC development. We also seek to determine whether HSC-targeted IFNγ, either as monotherapy or in combination with sorafenib, inhibits HCC progression. The proposed studies will not only reveal origin and functions of MF in hepatocarcinogenesis, but may provide a basis for targeting MF for HCC prevention or therapy.