The development of therapy resistance continues to be a major problem in the treatment of patients with cancer. Treatment failure has been very recently identified as one of the four major issues in cancer research
. Identification of underlying mechanisms is thus of great value.
Some mechanisms underlying cancer resistance to chemotherapy have been unraveled
. One of the well characterized cellular factors of resistance is the overexpression of the P-glycoprotein encoded by the MDR1 gene
. This protein is an efflux pump that expulses the chemotherapeutic drug out of the tumor cells. Other efflux pumps have been identified, all belonging to the ABC (ATP-binding cassette) transporter family, which expression may also play a role in inducing chemoresistance
. Numerous molecules inhibiting efflux pump activity have been tested but without real therapeutic success or with unacceptable toxicity
. Other important causes of resistance are the molecular alterations of the drug targets. Other resistance mechanisms include enhanced DNA repair, loss of p53, inhibition of apoptosis, activation of cell survival pathways caused by mutations or epigenetic alterations occurring in the context of genetic instability (selection of resistant cells)
. There are however still numerous open questions regarding the mechanisms allowing cancer cells to escape drug-induced toxic effects.
Tumor hypoxia is often associated with resistance to chemotherapy
 and radiotherapy
, with tumor progression, aggressiveness and metastasis, and therefore with an increased probability of tumor recurrence
. Identification of the mechanisms responsible for this protection would therefore have significant clinical benefits.
Hypoxia, the reduction of the normal level of tissue oxygen tension, is a common feature of solid tumors caused by of the abnormal vascular network and the high proliferation rate of cancer cells
. Intratumoral hypoxia develops when cells are located further than 100–180 μm from a functional blood vessel. Indeed, oxygen is unable to diffuse beyond this distance from a capillary before it is completely metabolized.
It was shown that up to 50-60% of locally advanced solid tumors may exhibit hypoxic and/or anoxic tissue areas heterogeneously distributed within the tumor mass
. Hypoxia occurs in breast tumors, as in other solid tumors, mostly because of tumor outgrowing of the existing vasculature (reviewed in
). In breast cancer, hypoxia has been correlated with bad prognosis. Indeed, HIF-1α
 or HIF-2α
 expression, as surrogate markers of tumor hypoxia, correlates with distant recurrence and poor outcome. Furthermore, expression profiles of the hypoxic markers, GLUT1 and CAIX, also correlate with adverse prognostic factors in breast cancer
. Hepatocellular carcinomas were also reported to display hypoxia
. Hypoxic regions have also been identified in tumors of many other histotypes such as brain tumors
[17, 18], head and neck
 or cervical
 and lung
It is now thought that hypoxia, according to its severity, can either promote apoptosis and cell death or contrariwise induce cell growth and survival by provoking an adaptive response. To survive in the hypoxic environment which takes place in the centre of solid tumors, cells co-opt adaptive mechanisms leading to a variety of biological responses, i.e. switching to a glycolytic metabolism, proliferation, evasion of apoptosis, obtaining a limitless replicative potential, induction of angiogenesis, invasion of the immune system and tissue invasion and metastasis
If some mechanisms underlying the hypoxia-induced radio- and chemoresistance begin to be understood, the actual actors of the protection still need to be identified. The aim of this study was to characterize the mechanisms underlying the hypoxia protection against paclitaxel-induced apoptosis observed in MDA-MB-231 cells, but also the hypoxia protection against etoposide-induced apoptosis previously observed in HepG2 cells
We exposed MDA-MB-231 cells to two chemotherapeutics agents, paclitaxel and epirubicin, used as apoptosis inducers. These two drugs belong respectively to the class of taxoids and anthracyclines, which are considered to be the most active agents in the treatment of breast cancer
. Paclitaxel binds to microtubules and causes their stabilization, inducing cell cycle arrest at G2/M mitotic phase followed by apoptosis. It also induces modulation of the expression or posttranslational modification of pro- and anti-apoptotic proteins
. Epirubicin HCl, a derivative of doxorubicin, leads to the inhibition of DNA and RNA synthesis by intercalation between base pairs of the DNA/RNA. It also inhibits topoisomerase II by stabilizing DNA-topoisomerase complex, resulting in DNA damage and induction of apoptosis and cell death
. It was also proposed that epirubicin could induce formation of reactive oxygen species promoting apoptosis
. As a second experimental model, we used etoposide to induce cell death in HepG2 cells. This antineoplastic drug functions as a topoisomerase II inhibitor, hence inducing double strand breaks in DNA, leading to the activation of apoptosis in a p53 dependent manner
. The biochemical and molecular mechanisms of apoptosis activation by these drugs are complex and are still under investigation.
We recently highlighted two mechanisms by which hypoxia protects cells from apoptosis via the activation of hypoxia-inducible factor-1 (HIF-1) and AP-1 transcription factors and the consecutive changes in expression of anti- and pro-apoptotic proteins
. Here, we investigated other changes in gene expression that may explain how hypoxia exerts its protective effect against paclitaxel induced-apoptosis.