Multiple effects of paclitaxel are modulated by a high c- myc amplification level

Abstract

Paclitaxel affects microtubule stability by binding to β-tubulin, thus leading to cell accumulation in the G 2/M phase, polyploidization, and apoptosis. Because both cell proliferation and apoptosis could be somehow regulated by the protooncogene c- myc, in this work we have investigated whether the c- myc amplification level could modulate the multiple effects of paclitaxel. To this aim, paclitaxel was administered to SW613-12A1 and -B3 human colon carcinoma cell lines (which are characterized by a high and low c- myc endogenous amplification level, respectively), and to the B3mycC5 cell line, with an enforced exogenous expression of c- myc copies. In this experimental system, we previously demonstrated that a high endogenous/exogenous level of amplification of c- myc enhances serum deprivation- and DNA damage-induced apoptosis. Accordingly, the present results indicate that a high c- myc amplification level potentiates paclitaxel cytotoxicity, confers a multinucleated phenotype, and promotes apoptosis to a great extent, thus suggesting that c- myc expression level is relevant in modulating the cellular responses to paclitaxel. We have recently shown in HeLa cells that the phosphorylated form of c-Myc accumulates in the nucleus, as distinct nucleolar and extranucleolar spots; here, we demonstrated that, after the treatment with paclitaxel, phosphorylated c-Myc undergoes redistribution, becoming diffused in the nucleoplasm.