Oxygen-regulated Expression of TGF-$beta;3, a Growth Factor Involved in Trophoblast Differentiation

The transforming growth factor-β3 (TGF-β3) is involved in oxygen-dependent differentiation processes during placental development and pregnancy disorders. However, the importance of oxgen partial pressure for the regulation of TGF-β3 expression is presently unclear. We and others presented preliminary evidence that the hypoxia-inducible factor-1 (HIF-1) confers TGF-β3 transcription but it was unknown whether this occurred directly or indirectly. To analyze how HIF-1 regulates TGF-β3 gene transcription, we cloned and sequenced the mouse TGF-β3 promoter region. Multiple putative HIF-1 binding sites (HBSs) were identified, many of which co-localized with two G+C rich CpG islands 5′ to the TGF-β3 transcription start site. A 6.8 kb fragment of the TGF-β3 promoter induced reporter gene expression under hypoxic conditions or when treated with an iron chelator known to stabilize and activate the HIF-1α subunit. Deletion of a 2.4 kb fragment upstream of the distal CpG island abolished inducibility of reporter gene expression. Two HBSs (HBS1 and HBS6) that bound the HIF-1 protein could be identified within this 2.4 kb fragment. These results suggest that TGF-β3 gene expression is directly regulated by HIF-1.