Integration of Ras subeffector signaling in TGF-β mediated late stage hepatocarcinogenesis

Immortalized p19(ARF) null hepatocytes (MIM) feature a high degree of functional differentiation and are susceptible to transforming growth factor (TGF)-beta driven growth arrest and apoptosis. In contrast, polarized MIM hepatocytes expressing hyperactive Ha-Ras continue proliferation in cooperation with TGF-beta, and adopt an invasive phenotype by executing an epithelial to mesenchymal transition (EMT). In this study, we analyzed the involvement of Ras subeffectors in TGF-beta mediated hepatocellular EMT by employing MIM hepatocytes, which express Ras mutants allowing selective activation of either mitogen-activated protein kinase (MAPK) signaling (V12-S35) or phosphoinositide 3-OH (PI3)3 kinase (PI3K) signaling (V12-C40). We found that MAPK signaling in MIM-S35 hepatocytes was necessary and sufficient to promote resistance to TGF-beta mediated inhibition of proliferation in vitro and in vivo. MIM-S35 hepatocytes showed also PI3K activation during EMT, however, MAPK signaling on its own protected hepatocytes from apoptosis. Yet, MIM-C40 hepatocytes failed to form tumors and required additional MAPK stimulation to overcome TGF-beta mediated growth arrest. In vivo, the collaboration of MAPK signaling and TGF-beta activity drastically accelerated the cell-cycle progression of the hepatocytes, leading to vast tumor formation. From these data we conclude that MAPK is crucial for the cooperation with TGF-beta to regulate the proliferation as well as the survival of hepatocytes during EMT, and causes the fatal increase in hepatocellular tumor progression.