Placenta growth factor gene expression is induced by hypoxia in fibroblasts: a central role for metal transcription factor-1.

Placenta growth factor (PlGF) is a mitogen for endothelial cells that can potentiate the growth and permeabilizing effects on endothelium of vascular endothelial growth factor. Here we report that hypoxia induces the expression of both PlGF mRNA and protein in immortalized/transformed mouse embryonic fibroblasts (mEFs) and in NIH 3T3 cells. Importantly, the magnitude of the induction of PlGF expression by hypoxia is enhanced by the presence of oncogenic Ras. To investigate the transcriptional component of hypoxia-inducible PlGF expression, we cloned and sequenced a 1350-bp fragment of the 5′-flanking region of the mouse gene. Analysis of the promoter region indicated the presence of putative consensus sequences for known hypoxia-responsive regulatory sites, including metal response elements and Sp1-like sites. In the present study, we show that the induction of PlGF expression by hypoxia is dependent on the presence of the metal response element-binding transcription factor 1 (MTF-1). Thus, in mEFs with targeted deletions of both MTF-1 alleles, hypoxia-induced increases of PlGF mRNA and protein levels were greatly attenuated compared with those in wild-type mEFs. Moreover, transient transfection of a PlGF promoter reporter gene into NIH 3T3 cells resulted in hypoxia-responsive transcriptional activation of the reporter. Finally, ectopic expression of MTF-1 resulted in increased basal transcriptional activity of a PlGF promoter reporter. Together, these findings demonstrate that the PlGF gene is responsive to hypoxia and that this response is mediated by MTF-1. It remains to be determined whether this activation is the result of direct and/or indirect transcriptional activation by MTF-1. The stimulatory effect of oncogenic Ras on the induction of PlGF expression in hypoxic cells suggests that PlGF could be an important proangiogenic factor in the tumor microenvironment.