Study of the effects of hypoxia on the apoptosis induced by different chemotherapeutics agents in different cancer cell types Despite the significant progresses made in the field of cancer screening and treatment, this pathology is still presently accountable for the death of more than 7 million people each year in the world. Chemotherapy is frequently used to induce cancer cell death. However, resistance to this kind of treatment is not rare. Several causes are involved in inducing this resistance. Tumours can indeed express proteins responsible for drug extrusion, display mutations in tumour suppressor genes such as TP53, or contain hypoxic areas. Hypoxic areas are frequently observed inside tumours due to the important distance between the cancer cells and the blood vessels, as well as due to the disorganized tumour vessel architecture. Tumour hypoxia is associated with a bad prognosis for patients in many cancer types. Hypoxia can indeed increase tumour aggressiveness as well as it can protect it from cell death induced by chemotherapeutic agents. However, severe hypoxia has an opposite effect since it can induce cell death. It has moreover been shown that, at the same oxygen percentage, some cell types are protected against apoptosis induced by a chemotherapeutic molecule, while it is not the case for other cell types. The molecular causes for these intercellular differences are not known. During this work, we studied the effect of hypoxia on the cell death induced by five chemotherapeutic molecules in seven cancer cell types originating from different organs and harbouring wild-type, mutated or null p53 protein. In accordance with the data reported in the literature, we observed that hypoxia inhibits the cell death induced by chemotherapeutic agents in the majority of the studied cell types, while it was not the case in all cell types. We next tried to identify the cellular mechanisms induced by hypoxia that are responsible for chemoresistance. We first studied proteins directly involved in apoptosis, and more particularly proteins belonging to the BCL-2 family proteins. We observed, for example, that the abundance of many pro-apoptotic proteins of the BCL-2 family was decreased by hypoxia in HepG2 cells that are strongly protected by hypoxia against etoposide-induced cell death, while it was not the case in A549 cells that are not protected by hypoxia against apoptosis. Moreover, a simultaneous silencing of BIM and NOXA induced a decrease in etoposide-induced cell death, underlying a possible mechanism for chemoresistance. We secondly studied the effect of hypoxia on p53 and on the activation of the signalling pathways activated downstream the etoposide-induced DNA damages. We observed that hypoxia decreased p53 abundance as well as the activation of a part of this pathway in HepG2 cells, while it was not the case in A549 cells. The causes of the hypoxia-induced decrease in p53 abundance in HepG2 cells have been studied and our results suggest a decrease in the translation of this protein during hypoxia. Finally, preliminary results have been obtained with regard to the involvement of miRNAs in the response of hypoxic cells to etoposide-induced cell death. miRNAs are small molecules, recently discovered, that regulate the expression of numerous genes, among which those involved in apoptosis. In conclusion, our results show how different the regulation of cell death by hypoxia could be according to the cell type. Moreover we showed that the response of a cell to hypoxia is very complex and that multiple cellular pathways are modulated by hypoxia in order to induce chemoresistance. These results underline therefore the importance of a better understanding of the intercellular variations of responses to hypoxia in order to improve anti-cancer treatments.
|Date of Award
|4 May 2012
|Carine Michiels (Supervisor), Patricia Renard (Jury), Catherine Lambert (Jury), Martine Raes (President), Patrick Dumont (Jury) & Jean-Christophe MARINE (Jury)