Increasing complexity of farnesyltransferase inhibitors activity: role in chromosome instability

Oncogenic Ras proteins have been seen as an important target for novel anticancer drugs. Due to the functional role of Ras farnesylation, fanesyltransferase (FTase) inhibition was thought to be a strategy for interfering with Ras-dependent transformation. When farnesylation is blocked, the function of Ras protein is severely impaired because of the inability of the nonfarnesylated protein to anchor to the membrane. Although it has been clearly demonstrated that FTase inhibitors (FTIs) inhibit Ras farnesylation, it is uncertain whether the antiproliferative effects of these compounds result exclusively from the effects on Ras. Moreover, no consensus has been reached as to the relevant targets(s) of FTIs that can explain their mosaic pharmacology. In searching for downstream targets for FTIs effects, CENP-E and CENP-F/mitosin were identified. Different studies showed that the inhibition of farnesylation interferes with CENP-E-microtubule association. In the presence of FTIs, chromosome alignment to the metaphase plate is delayed, suggesting that farnesylated proteins are involved in a step critical to bipolar spindle formation and chromosome alignment. An important question is whether these biological effects might contribute to the chemotherapeutic effects of the FTIs. However, FTIs, triggering the spindle checkpoint, might elevate the rate of cellular missegregation to levels that are incompatible with cell viability, as well as have a reduced (but still significant?) effect on checkpoint-proficient normal cells. As an example, RPR-115135 induced micronuclei (MN) increase in cancer cells displaying high chromosome instability (CIN) levels, whereas in normal cells it is devoid of activity. Cancer cells showing high CIN level might represent an ideal target for the activity of some FTIs.