Etude de l'endommagement de l'ADN de cellules tumorales en culture par deux inhibiteurs de Topoisomérase II, la Doxorubicine et l'Etoposide. Recherche de sites de coupure dans les oncogènes de cellules sensibles et résistantes à la Doxorubicine

The mechanisms of resistance to anticancer drugs able to interfere with topoisomerase II generally involve a defect in drug availability at the cellular level (multidrug resistance) or alterations of drug target, topoisomerase II. However, mutations of genomic DNA, the third partner of this interaction, have never been directly identified. We have tried to know whether alterations at the level of genes involved in the control of cell proliferation could be found in cells selected for resistance to doxorubicin. In the first part of this thesis, we have shown that our hypothesis was compatible with the characteristics of the resistance found in a rat glioblastoma cell line, C6 0,5 : doxorubicin-induced DNA damage is not proportional to doxorubicin cytotoxicity in this cell line. In the second part, we have shown that the expression of two oncogenes, c-myc and c-jun, was decreased by doxorubicin in sensitive cells, but remained unchanged upon etoposide treatment. The expression of c-jun was maintained in the same conditions in resistant cells. Using the technique of PCR stop assay, we have also shown that a region of the c-jun oncogene was cleaved in sensitive cells in the presence of doxorubicin, but not of etoposide. This cleavage site was no longer found in resistant cells. These results confirm the existence of doxorubicin-specific cleavage site(s) within the oncogene sequences. They are in favour of the existence of a mutation, or of a specific protection, of this (these) site(s) in the doxorubicin-resistant line.