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 PIGF
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.