TGFβ regulation of trophoblast function

Abstract

Placenta growth factor (PIGF) is a homodimeric glycoprotein, 46–50 kDa in size, belonging to the vascular endothelial growth factor (VEGF) sub-family. It exists as two isoforms, PIGF-1 and -2, the latter having a heparin-binding domain. Like VEGF, it is a potent angiogenic factor; however, PIGF homodimers interact with the VEGF receptor Flt-1 (fms-like tyrosine kinase), but not with the kinase domain-containing region (KDR). Since PIGF is made by the human placenta and extravillous trophoblast (EVT) cells of the human placenta express Flt-1 in situ, these cells may be responsive to PIGF. Therefore, this study examined whether first trimester EVT cells propagated in vitro expressed the mRNA or the protein of Flt-1 and PIGF, and whether exogenous PIGF-1 had any effect on EVT cell proliferation, migration or invasiveness. Immunocytochemical and RT-PCR analyses revealed that both normal and SV40 Tag-immortalized EVT cells expressed the protein and mRNA for Flt-1, but not for P1GF-1 or -2. Exogenous PIGF-1 stimulated proliferation (measured by 3H-thymidine uptake) of normal EVT cells in a concentration-dependent manner, but only in the presence of excess heparan sulphate proteoglycans (HSPGs). These results raise two possibilities: that exogenous PIGF-1 (in spite of having a low affinity for heparin) was sequestered away from its receptor because of binding to heparan sulphate proteoglycans on the EVT cell surface or the ECM, or that HSPGs could modify the interaction between Flt-1 and PIGF. PIGF-1, in the presence or absence of HSPGs, however, had no effect on EVT migration or invasiveness, when measured with a transwell invasion (in the presence of Matrigel™) or migration (in the absence of Matrigel™) assay. These findings place PIGF amongst a large group of growth factors that promote EVT cell proliferation without influencing their migratory or invasive behaviours, and suggest that PIGF-Flt-1 interactions may be regulated by HSPGs in situ.