Paclitaxel increases p21 synthesis and accumulation of its AKT-phosphorylated form in the cytoplasm of cancer cells.

CKI p21 is a regulator of cellular responses to microtubule damage induced by drugs such as paclitaxel (PTX). It mediates the G1 4N arrest postactivation of the spindle assembly checkpoint and protects cancer cells against PTX-induced cytotoxicity. We demonstrated here that low doses of PTX that are unable to activate the spindle assembly checkpoint, upregulate p21 by a p53-dependent pathway and induce its translocation to the cytoplasm. This cytoplasmic accumulation of p21 resulted from an AKT-dependent p21 phosphorylation leading to an association of p21 with 14-3-3. Furthermore, the cytoplasmic p21 accumulation observed in PTX-treated cells was inhibited by LY 294002, a specific PI-3 kinase inhibitor or by the expression of a dominant-negative AKT mutant. However, the kinase activity of AKT was unchanged in PTX-treated cells, suggesting that low doses of PTX could regulate p21 phosphorylation via inhibition of its dephosphorylation. As a functional consequence, we found that cytoplasmic accumulation of the phosphorylated form of p21 prevents the inhibitory effect of p21, enabling these cells to escape to the p53-dependent Gl/S and G2/M checkpoints.

Farnesyltransferase inhibitor, R115777, reverses the resistance of human glioma cell lines to ionizing radiation.

We investigated for the first time the ability of farnesyltransferase inhibitors (FTI) to radiosensitize human glioma. For this, human glioma cell lines were treated with the specific FTI, R115777, 48 hr prior to a 2Gy irradiation. The treatment with R115777 decreased by 45% the SF2 value of the more radioresistant glioma cell lines (SF763 and U87) without any significant effect on the radioresistance of the radiosensitive ones (SF767 and U251-MG). This radiosensitizer effect was due to the induction of post-mitotic necrotic cell death. We then tested the hypothesis that wild-type Ras or RhoB, which has been proposed as potential FTI target, could control the glioma radioresistance. For this, we expressed inducible dominant negative forms of Ras (RasN17) and RhoB (RhoBN19) in radioresistant U87 glioma cell line and analyzed the survival after irradiation of the obtained clones. While blocking Ras pathways by expression of RasN17 did not affect the SF2 value of the U87 glioma cell line, the expression of RhoBN19 dramatically reduced the cell survival after irradiation of these cells. Taken together, these data demonstrated that RhoB, but not Ras, is implicated in glioma radioresistance. Furthermore, the R115777 differential radiosensitizer effect underlines the potential therapeutic interest of using this drug as a radiosensitizer of human glioma.