Hypoxia induces protection against etoposide-induced apoptosis: molecular profiling of changes in gene expression and transcription factor activity.
BACKGROUND
it is now well established that hypoxia renders tumor cells resistant to radio- but also chemotherapy. However, few elements are currently available as for the mechanisms underlying this protection.


RESULTS
in this study, physiological hypoxia was shown to inhibit apoptosis induced in HepG2 cells by etoposide. Indeed, hypoxia reduced DNA fragmentation, caspase activation and PARP cleavage. The DNA binding activity of 10 transcription factors was followed while the actual transcriptional activity was measured using specific reporter plasmids. Of note is the inhibition of the etoposide-induced activation of p53 under hypoxia. In parallel, data from low density DNA microarrays indicate that the expression of several pro- and anti-apoptotic genes was modified, among which are Bax and Bak whose expression profile paralleled p53 activity. Cluster analysis of data unravels several possible pathways involved in the hypoxia-induced protection against etoposide-induced apoptosis: one of them could be the inhibition of p53 activity under hypoxia since caspase 3 activity parallels Bax and Bak expression profile. Moreover, specific downregulation of HIF-1alpha by RNA interference significantly enhanced apoptosis under hypoxia possibly by preventing the hypoxia mediated decrease in Bak expression without altering Bax expression.


CONCLUSIONS
these results are a clear demonstration that hypoxia has a direct protective effect on apoptotic cell death. Moreover, molecular profiling points to putative pathways responsible for tumor growth in challenging environmental conditions and cancer cell resistance to chemotherapeutic agents.
Sermeus
Audrey
A
URBC, FUNDP-University of Namur, 61 rue de Bruxelles, 5000 Namur, Belgium. audrey.sermeus@fundp.ac.be
Cosse
Jean-Philippe
JP
Crespin
Marianne
M
Mainfroid
Veronique
V
de Longueville
Francoise
F
Ninane
Noelle
N
Raes
Martine
M
Remacle
Jose
J
Michiels
Carine
C
D017209:Apoptosis / Q000187:drug effects*; D015687:Cell Hypoxia / Q000187:drug effects; D045744:Cell Line, Tumor; D016000:Cluster Analysis; D019610:Cytoprotection / Q000187:drug effects*; D004273:DNA, Neoplasm / Q000378:metabolism; D005047:Etoposide / Q000494:pharmacology*; D020869:Gene Expression Profiling; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects*; D020868:Gene Silencing; D017930:Genes, Reporter; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000235:genetics; D020411:Oligonucleotide Array Sequence Analysis; D034741:RNA, Small Interfering / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014157:Transcription Factors / Q000378:metabolism*; D014162:Transfection
D004273:DNA, Neoplasm; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D034741:RNA, Small Interfering; D014157:Transcription Factors; D005047:Etoposide