A farnesyltransferase inhibitor induces tumor regression in transgenic mice harboring multiple oncogenic mutations by mediating alterations in both cell cycle control and apoptosis

The farnesyltransferase inhibitor L-744,832 selectively blocks the transformed phenotype of cultured cells expressing a mutated H-ras gene and induces dramatic regression of mammary and salivary carcinomas in mouse mammary tumor virus (MMTV)-v-Ha-ras transgenic mice. To better understand how the farnesyltransferase inhibitors might be used in the treatment of human tumors, we have further explored the mechanisms by which L-744,832 induces tumor regression in a variety of transgenic mouse tumor models, We assessed whether L-744,832 induces apoptosis or alterations in cell cycle distribution and found that the tumor regression in MMTV-v-Ha-ras mice could be attributed entirely to elevation of apoptosis levels, In contrast, treatment with doxorubicin, which induces apoptosis in many tumor types, had a minimal effect on apoptosis in these tumors and resulted in a less dramatic tumor response, To determine whether functional p53 is required for L-744,832-induced apoptosis and the resultant tumor regression, MMTV-v-Ha-ras mice were interbred with p53(-/-) mice, Tumors in ras/p53(-/-) mice treated with L-744,832 regressed as efficiently as MMTV-V-Ha-ras tumors, although this response was found to be mediated by both the induction of apoptosis and an increase in G(1) with a corresponding decrease in the S-phase fraction, MMTV-v-Ha-ras mice were also interbred with MMTV-c-myc mice to determine whether ras/myc tumors, which possess high levels of spontaneous apoptosis, have the potential to regress through a further increase in apoptosis levels, The ras/myc tumors were found to respond nearly as efficiently to L-744,832 treatment as the MMTV-v-Ha-ras tumors, although no induction of apoptosis was observed. Rather, the tumor regression in the ras/myc mice was found to be mediated by a large reduction in the S-phase fraction, In contrast, treatment of transgenic mice harboring an activated MMTV-c-neu gene did not result in tumor regression. These results demonstrate that a farnesyltransferase inhibitor can induce regression of v-Ha-ras-bearing tumors by multiple mechanisms, including the activation of a suppressed apoptotic pathway, which is largely p53 independent, or by cell cycle alterations, depending upon the presence of various other oncogenic genetic alterations.