Tumor hypoxia is a characteristic of the microenvironment of most solid tumors. It corresponds to the low oxygen tension observed in certain areas of the tumor and is the result of the rapid proliferation of tumor cells and the disordered structure of the tumoral vascular network. Tumor hypoxia is often correlated with poor prognosis for patients because it is a phenomenon that enhances the aggressiveness of cancer cells and their resistance to anticancer therapies such as radiotherapy and chemotherapy. Resistance to chemotherapy is due to a direct effect of hypoxia, because the drugs used in chemotherapy often require oxygen to perform their cytotoxicity, and to an indirect effect of hypoxia, as it induces a series of adaptations in cancer cells. Hypoxia modifies the metabolism, stimulates angiogenesis, erythropoiesis and regulates cell survival by acting on the regulation of apoptosis. Regulation of apoptosis by hypoxia is complex, depending on its severity. Indeed, hypoxia may stimulate or inhibit apoptosis. Understanding the regulation of apoptosis by hypoxia is important because apoptosis is involved in the selection of the tumor cells that are more aggressive and affects the effectiveness of cytotoxic drugs used in chemotherapy. This work aims to investigate the regulation of apoptosis by hypoxia. First, we compared the effect of hypoxia on apoptosis induced by etoposide, a cytotoxic drug used to treat different types of cancer, on three human cancer cell lines isolated from three different organs. We have shown that the effect of hypoxia on etoposide-induced apoptosis was dependent on cell lines as hypoxia stimulates etoposide-induced apoptosis in MCF7 cells while it inhibits the etoposide-induced apoptosis in HepG2 cells. These results show that the regulation of apoptosis by hypoxia is a complex mechanism that not only depends on the severity of hypoxia but also on other parameters such as cell lines. Next, we investigated how hypoxia is able to protect HepG2 cells from etoposide-induced apoptosis. For this, we evaluated the involvement of several transcription factors. The results showed that hypoxia protected HepG2 cells from etoposide-induced apoptosis by inhibiting the activity of p53 transcription factor and activating c-jun transcription factor. We also evaluated the involvement of autophagy that is a cellular process involved in recycling long-lived proteins and organelles and in the survival and cell death. In hypoxic conditions, autophagy can be induced by a mechanism involving the protein BNIP3. We showed that, in HepG2 cells, autophagy was induced by etoposide but not by hypoxia. However, etoposide-induced autophagy has different consequences depending on the oxygen tension as we observed that autophagy promoted etoposide-induced apoptosis under normoxia but not under hypoxia. Finally, we showed that BNIP3 did not influence autophagy but was involved in the protection against etoposide-induced apoptosis observed under hypoxia. In conclusion, these results showed that the regulation of apoptosis by hypoxia is a complex phenomenon that depends on several parameters including the cell line. Moreover, these results also indicate that hypoxia protects cancer cells against apoptosis by inducing several adaptative mechanisms. They are of importance as they enhance our understanding of the resistance process occurring in cancer patients.
Effet de l'hypoxie sur la résistance des cellules cancéreuses à l'étoposide: Etude de l'apoptose et de l'autophagie
Cosse, J. (Author). 25 May 2010
Student thesis: Doc types › Doctor of Sciences