Daitoku Sakamuro

Clinical evidence suggests that hepatitis C virus (HCV) is etiologically involved in hepatic cancer and liver cirrhosis. To investigate whether the HCV nonstructural protein NS3 has oncogenic activity, NIH 3T3 cells were transfected with an expression vector containing cDNA for the 5'- or 3'-half sequence of the HCV genome segment encoding NS3. Only cells transfected with the 5'-half cDNA rapidly proliferated, lost contact inhibition, grew anchorage independently in soft agar, and formed tumors in nude mice. PCR analysis confirmed the presence of the 5'-half DNA in the transfectants. These results suggest that the 5' region of the HCV genome segment encoding NS3 is involved in cell transformation.

Farnesyl transferase inhibitors (FTIs) are a novel class of antitumor drugs that block the oncogenic activity of Ras. Because FTIs lack significant cell toxicity in vitro and in vivo, a significant question is how they cause tumor regression. We now report that FTIs are in fact potent activators of apoptosis in Ras-transformed cells if attachment to substratum is prevented. When cultured at high density or on polyHEMA, a nonadherent substrate, Ras-transformed cells exhibited massive DNA degradation and cell death within 24 h of treatment with the FTI L-739,749. Death was p53-independent and was inhibited by the apoptosis suppressor BCL-XL. Furthermore, apoptosis was significantly attenuated by ectopic expression of a farnesyl-independent form of RhoB, a Rho protein previously implicated as a critical target for inhibition by FTIs. The findings suggest a link between FTIs and Rho-dependent adhesion signaling. Furthermore, our work indicates that FTIs revert cells to a state in which cell-substratum attachment is necessary for viability and suggests that apoptosis forms the basis for drug-induced tumor regression.