Down-regulation of C/EBP alpha in Breast Cancer Cells by Hypoxia-Estrogen Combination is Mainly Due to Hypoxia

CCAAT/enhancer binding protein-alpha (C/EBP alpha) is involved in the control of cell differentiation and proliferation. It has been previously shown that hypoxia (H) down-regulates C/EBP alpha in breast cancer cells; here the effect of estrogen (E-2) during H on C/EBP alpha in T-47D cell line was examined. By quantitative RT-PCR the C/EBP alpha mRNA stability was analyzed at 21% O-2 and 1% O-2 under E-2. The H-E-2 combination but not E-2 alone reduced the half-life of the C/EBP alpha mRNA. C/EBP alpha promoter activity studies covering -576 bp do not show any effect of E-2; a significant decrease of C/EBP alpha transcriptional activity was found in the C/EBP alpha promoter between -576/416 bp as previously observed when cells were treated with hypoxia alone. By immunocytochemistry, H-E-2 combination alters the cellular distribution of C/EBP alpha in T-47D cells and locates this protein mainly at the cytosol. Therefore, the observed down-regulation of C/EBP alpha by the H-E-2 combination in T-47D cells is mainly due to the hypoxia effect.