Comparison of the X-Radiation, Drug and Ultraviolet-Radiation Responses of Clones Isolated from a Human Colorectal Tumor Cell Line

APM2 is a novel mediator of cisplatin resistance in a variety of cancer cell types regardless of p53 or MMR status

Cisplatin is one of the most widely used chemotherapeutics in the world today. Unfortunately, chemoresistance often develops hindering the effectiveness of the drug. Mismatch-repair (MMR) and p53 have previously been shown to be important determinants of cisplatin resistance and can contribute to cisplatin resistance clinically. Here, we have used cDNA microarray to identify several genes as up or downregulated in a previously described, cisplatin resistant, clone of the HCT116 cell line (HCT116-K). On follow-up, one gene, APM2, was found to promote cisplatin resistance when overexpressed in sensitive HCT116 clones. Furthermore, silencing APM2 in a panel of cell lines encompassing all combinations of p53 status and MMR proficiency (HCT116-K, HCT116, SW620, MCF7, PC-3 and OV2008) resulted in sensitization regardless of these 2 factors. In addition, silencing APM2 stably using shRNA also resulted in the sensitization of cells to cisplatin. More importantly, cisplatin inhibited the growth of APM2 silenced tumor xenografts (HCT116-K or OV2008 cells) significantly better than it inhibited the growth of xenografts carrying nontargeting control shRNAs. These findings represent a novel strategy that could be exploited to overcome cisplatin resistance in patients regardless of p53 status or ability to perform MMR.

Death through a tragedy: mitotic catastrophe

Mitotic catastrophe (MC) has long been considered as a mode of cell death that results from premature or inappropriate entry of cells into mitosis and can be caused by chemical or physical stresses. Whereas it initially was depicted as the main form of cell death induced by ionizing radiation, it is today known to be triggered also by treatment with agents influencing the stability of microtubule, various anticancer drugs and mitotic failure caused by defective cell cycle checkpoints. Although various descriptions explaining MC exist, there is still no general accepted definition of this phenomenon. Here, we present evidences indicating that death-associated MC is not a separate mode of cell death, rather a process ('prestage') preceding cell death, which can occur through necrosis or apoptosis. The final outcome of MC depends on the molecular profile of the cell.

Fractionated X-radiation treatment can elicit an inducible-like radioprotective response that is not dependent on the intrinsic cellular X-radiation resistance/sensitivity

Inducible responses are well documented to play a role in the radiation response of cells. However, it is not known whether clinically relevant fractionated X-radiation treatment could elicit an inducible-like radioprotective response and whether there is a direct correlation between the inducible radiation response phenomenon and the intrinsic radiation response of the cell. Therefore, the purpose of this study was to determine whether closely related human colorectal tumor (HCT116) clones treated with fractionated X rays could elicit an inducible-like radiation response to a subsequent acute (i.e. single) X-ray challenge, and whether the magnitude of the inducible-like response correlates with the intrinsic X-ray resistance of the responding clones. After fractionated X irradiation, only the radiosensitive clone showed enhanced clonogenic survival with a subsequent acute X-ray exposure. Cell cycle changes or the selection of subclones with increased intrinsic radiation resistance induced by the fractionated X rays were excluded as the basis of this enhanced tolerance, suggesting the presence of an inducible-like radioprotective response. Using the comet assay, we found similar amounts of intrinsic DNA damage among the clones after acute X irradiation. Our findings demonstrate that fractionated X-ray treatment can elicit an inducible-like radioprotective response and represent the first evidence that this response is independent of the intrinsic radiation resistance/sensitivity of the responding cells.

Effects ofN1,N13-Diethylnorspermine (DENSPM) and X-Radiation Treatment on Human Colorectal Tumor Clones with Varying X-Radiation and Drug Responses

This study was designed to examine the effects of treatment with N1, N13-diethylnorspermine (DENSPM), a spermine analog, and X radiation on survival and on the polyamine and spermidine/spermine N1-acetyltransferase (SSAT) levels in closely related human colorectal tumor (HCT116) clones exhibiting a wide range of X-radiation and drug responses. After treatment with DENSPM and X radiation, clonogenic cell survival was measured. SSAT protein levels were measured by Western blot analysis and SSAT enzymatic activities by the conversion of [1-14C]acetyl-CoA into [1-14C]acetylspermidine. Polyamine [i.e. putrescine (PUT), spermine (SPM) and spermidine (SPD)] levels were measured with high-performance liquid chromatography. DENSPM enhanced the efficacy of radiation treatment in HCT116, HCT116-Clone2 (a radiation-resistant clone) and HCT116-Clone10 (a clone with similar X-radiation response as the parental HCT116 cells) but not in HCT116-CloneK (an X-radiation-sensitive but relatively drug-resistant clone). Treatment with DENSPM without X radiation caused the most significant increase in SSAT activity (approximately 22-fold) and an almost complete depletion of SPD levels in HCT116-CloneK. Our results suggest that (a) the lack of sensitization of X-radiation treatment by DENSPM in HCT116-CloneK was likely due to the prior depletion of SPD levels by DENSPM alone, (b) natural polyamine contents and/or inducibility of SSAT may be important factors influencing cellular response to combined X-radiation and DENSPM treatments, and (c) more importantly, there may be a potentially novel role for combining polyamine analogs such as DENSPM with X rays.