Resistance of Malignant Trophoblast Cells to both the Anti-proliferative and Anti-invasive Effects of Transforming Growth Factor-β

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Human placental trophoblast invasion of the uterus is a highly controlled event. We had shown that transforming growth factor-β (TGF-β) produced in the pregnant uterus controls invasiveness and reduces proliferation of first trimester placental trophoblasts in vitro. The anti-invasive effect of TGF-β was due, at least in part, to induction of tissue inhibitor of metalloproteinases (TIMP)-1. In the present study we compared the effects of TGF-β on proliferation ([3H]-TdR incorporation) and invasiveness (3-day Matrigel invasion assay) of JAR and JEG-3 choriocarcinoma cells vs normal first trimester human trophoblast cells. Transcripts of type IV collagenases (72- and 92-kDa enzymes, i.e., gelatinases A and B) and their inhibitors (TIMP-1 and TIMP-2) in these cells were measured by Northern analysis, and secretion of gelatinases and plasminogen activators (PAs) was evaluated by gel zymography. The results revealed that: (a) TGF-β inhibited invasiveness and proliferation of normal trophoblast but not JAR and JEG-3 choriocarcinoma cells; (b) gelatinase A mRNA, expressed by the normal trophoblast and JAR cells, was upregulated in the presence of TGF-β; (c) gelatinase B mRNA was not detected in the total RNA preparations of treated or untreated normal trophoblast or choriocarcinoma cells; (d) TGF-β significantly upregulated the levels of TIMP-1 mRNA in the normal trophoblasts, but this transcript was very low in treated as well as untreated choriocarcinoma cells; TGF-β also upregulated the 3.5-kb TIMP-2 message in the normal trophoblast; (e) gelatin zymography revealed a distinct band of approximately 68-kDa (gelatinase A) in the conditioned media of normal trophoblast and JAR cells; however, TGF-β did not change the level of secretion of this gelatinase; and (f) the normal trophoblast also exhibited significant PA secretion (casein zymography) which was reduced in the presence of TGF-β. PA secretion by the malignant trophoblast cells was low and unaffected by TGF-β. These findings suggest that choriocarcinoma cells may become refractory to the mechanisms which control normal trophoblast proliferation and invasiveness. Concurrent resistance to antiproliferative and anti-invasive molecules such as TGF-β may be highly relevant to tumor progression.