Differential effect of hypoxia on etoposide-induced DNA damage response and p53 regulation in different cell types.

Among the main causes of cancer cell resistance to chemotherapy are p53 mutation and hypoxic tumor microenvironment. However, the effect of hypoxia can be very different from one cell type to the other. We studied the effect of hypoxia on the etoposide-induced cell death in two cancer cell lines, HepG2 and A549 cells. Hypoxia decreased etoposide-induced apoptosis in HepG2 cells but not in A549 cells. Here, we evidenced two pathways, known to play important roles in cancer cell resistance, that are differently affected by hypoxia in these two cell types. First, in HepG2 cells, hypoxia decreased p53 protein level and activity by acting post-transcriptionally and independently of HIF-1. The results suggest an effect of hypoxia on p53 translation. On the other hand, in A549 cells, no effect of hypoxia was observed on p53 level. Secondly, hypoxia decreased DNA damage response in HepG2 cells while this was not the case in A549 cells. Indeed, a decrease in the phosphorylation level of CHK2 and H2AX with a decrease in ATM activity was observed. Importantly, these results evidenced that hypoxia can prevent cancer cell apoptosis by acting at different levels in the cell and that these effects are strongly cell-type dependent.

Cycling hypoxia induces a specific amplified inflammatory phenotype in endothelial cells and enhances tumor-promoting inflammation in vivo.

Abnormal architecture of the tumor blood network, as well as heterogeneous erythrocyte flow, leads to temporal fluctuations in tissue oxygen tension exposing tumor and stromal cells to cycling hypoxia. Inflammation is another feature of tumor microenvironment and is considered as a new enabling characteristic of tumor progression. As cycling hypoxia is known to participate in tumor aggressiveness, the purpose of this study was to evaluate its role in tumor-promoting inflammation. Firstly, we assessed the impact of cycling hypoxia in vitro on endothelial inflammatory response induced by tumor necrosis factor α. Results showed that endothelial cells exposed to cycling hypoxia displayed an amplified proinflammatory phenotype, characterized by an increased expression of inflammatory cytokines, namely, interleukin (IL)-6 and IL-8; by an increased expression of adhesion molecules, in particular intercellular adhesion molecule-1 (ICAM-1); and consequently by an increase in THP-1 monocyte adhesion. This exacerbation of endothelial inflammatory phenotype occurs through nuclear factor-κB overactivation. Secondly, the role of cycling hypoxia was studied on overall tumor inflammation in vivo in tumor-bearing mice. Results showed that cycling hypoxia led to an enhanced inflammation in tumors as prostaglandin-endoperoxide synthase 2 (PTGS2), IL-6, CXCL1 (C-X-C motif ligand 1), and macrophage inflammatory protein 2 (murine IL-8 functional homologs) mRNA expression was increased and as a higher leukocyte infiltration was evidenced. Furthermore, cycling hypoxia-specific inflammatory phenotype, characterized by a simultaneous (baculoviral inhibitor of apoptosis repeat-containing 5)(low)/PTGS2(high)/ICAM-1(high)/IL-6(high)/IL-8(high) expression, is associated with a poor prognosis in human colon cancer. This new phenotype could thus be used in clinic to more precisely define prognosis for colon cancer patients. In conclusion, our findings evidenced for the first time the involvement of cycling hypoxia in tumor-promoting inflammation amplification.