Stimulation of Plasminogen Activator Inhibitor-1 Expression in Immortalized Human Trophoblast Cells Cultured Under Low Levels of Oxygen

Low levels of oxygen characterize certain biological settings such as the first trimester human placenta and rapidly growing tumors. Hypoxia in tumors and expression of high levels of tumor-associated plasminogen activator inhibitor-1 (PAI-1) indicate a poor prognosis for some cancer patients. Furthermore, PAI-1 may promote tumor invasion and metastasis by modulating cell adhesion and detachment from the extracellular matrix. In this study, we used immortalized human trophoblasts (HTR-8/SVneo cells) derived from first trimester placenta to study the effect of exposure to low levels of oxygen on PAI-1 expression. Cell viability following 24-h exposure to 1% oxygen was similar to that of cells cultured under 20% oxygen. Exposure to hypoxia resulted in time-dependent increases in PAI-1 mRNA and protein levels, as determined by Northern blot analysis and enzyme-linked immunosorbent assay. Culture with cobalt chloride or Tiron also resulted in increased PAI-1 mRNA levels, while carbon monoxide inhibited the hypoxia-mediated increase, thereby indicating that a heme protein is involved in the stimulation of PAI-1 expression by hypoxia. Incubation with transforming growth factor-beta1 (TGF-beta1) also resulted in increased levels of PAI-1 mRNA. However, addition of a neutralizing anti-TGF-beta antibody to hypoxic cultures did not abrogate the increase in PAI-1 mRNA levels, suggesting that hypoxia stimulates PAI-1 expression via a pathway that does not require TGF-beta production. These results indicate that, through their effect on PAI-1 expression, oxygen levels may play an important role in modulating cellular migration and invasion.