Inhibition of DNA synthesis by a farnesyltransferase inhibitor involves inhibition of the p70(s6k) pathway

Previously, the protein farnesyltransferase inhibitor (FTI), L-744,832, has been shown to inhibit the proliferation of a number of tumor cell lines in vitro in a manner that correlated with the inhibition of the mitogen-activ ated protein kinase cascade. Here we show that FTI inhibits p70(s6k) phosph orylation in mammary tumors in vivo in transgenic mice. Furthermore, in a m ouse keratinocyte cell line, FTI inhibits p70(s6k) phosphorylation and acti vity and inhibits PHAS-1 phosphorylation in vitro in both rapidly growing c ells and in growth factor-stimulated quiescent cells, Dominant-negative Ras expression inhibits p70(s6k) stimulation by epidermal growth factor, demon strating a requirement for Pas activity during p70(s6k) activation. FTI doe s not inhibit protein kinase B phosphorylation on Ser(473), indicating that FTI does not act by inhibiting phosphatidylinositol 3-kinase, FTI also inh ibits DNA synthesis in keratinocytes, and inhibition of DNA synthesis corre lates closely with p70(S6k) inhibition. Rapamycin, an inhibitor of p70(s6k) and PHAS-1 phosphorylation, causes a 30-45% reduction in DNA synthesis in keratinocytes, while FTI induces an 80-90% reduction in DNA synthesis. Thes e observations suggest that alteration of p70(s6k) and PHAS-1 function by F TI are responsible for a substantial portion of the growth-inhibitory prope rties of FTI. Together, these data demonstrate that p70S6k and PHAS-1 are n ovel downstream targets of FTI and suggest that the antitumor properties of FTI are probably due to the inhibition of multiple mitogenic pathways.