Induced and repressed genes after irradiation sensitizing by pentoxyphylline

Abstract

Aim in cancer therapy is to increase the therapeutic ratio eliminating the disease while minimizing toxicity to normal tissues. Radiation therapy is a main component in targeting cancer. Radiosensitizing agents like pentoxyphylline (PTX) have been evaluated to improve radiotherapy. Commonly, cells respond to radiation by the activation of specific early and late response genes as well as by inhibition of genes, which are expressed under normal conditions. A display of the genetic distinctions at the level of transcription is given here to characterize the molecular events underlying the radiosensitizing mechanisms. The method of suppression subtractive hybridization allows the visualization of both induced and repressed genes in irradiated cells compared with cells sensitized immediately after irradiation. The genes were isolated by cDNA-cloning, differential analysis and sequence similarity search. Genes involved in protein synthesis, metabolism, proteolysis and transcriptional regulation were detected. It is important that genes like KIAA280, which were only known as unidentified EST sequences before without function, but inaccessible by array technology were recovered as functional genes. Database searches for PTX-induced genes detected a human mRNA completely unknown. In case of suppressed genes, we detected several mRNAs; one thereof shows homology to a hypothetical protein possibly involved in signal transduction. A further mRNA encodes the protein BM036 supposed to associate with the E2F transcription factor. A hypothetical protein H41 was detected, which may repress the Her-2/neu receptor influencing breast cancer, gliomas and prostate tumors. Radiation combined with PTX may lead to a better prognosis by down regulation of the Her-2/neu, which will be proven by clinical studies in the near future. The connection between cancerogenesis and cyclins has been demonstrated with the aberrant expression of members of the family of cyclins and the mechanisms of the cell cycle G1 arrest and apoptosis were well documented. [3][4][5][6][7] The mechanisms of G2 arrest in contrast have been described to a lesser extent, although this event could be of relevance to the cellular survival after photonirradiation. 8,9 Beyond doubt, the G2-phase arrest is generally activated in response to DNA damage independent of the p53 status. 8 It acts as an universal feedback for maintaining the genomic integrity and for survival after irradiation by repairing DNA breaks prior to mitosis. 10 Much of the underlying knowledge on the G2/M phase was obtained by studying genes involved in the regulation of DNA damage response because DNA-damaging processes, like g-radiation, respond by arresting at cell cycle checkpoints. 11 This ''G2 delay'' after photon-irradiation involves molecular mechanisms like reduced cyclin B levels and inhibition of the Cdk1 activity. 8 Formation and activation of the maturation promoting factor (MPF) are pivotal for the G2/M checkpoint regulation. The transition from G2 into the mitosis occurs by the activatio...