SUBTYPES OF BETAGLYCAN AND OF TYPE-I AND TYPE-II TRANSFORMING GROWTH-FACTOR-BETA (TGF-BETA) RECEPTORS WITH DIFFERENT AFFINITIES FOR TGF-BETA-1 AND TGF-BETA-2 ARE EXHIBITED BY HUMAN PLACENTAL TROPHOBLAST CELLS

Transforming growth factor-beta is likely to be an important factor controlling placental activities, including growth, differentiation, invasiveness, hormone production, and immunosuppression. We have used a chemical cross-linking technique with either I-125-TGF-beta-1 or I-125-TGF-beta-2 and bis(sulfosuccinimidyl) suberate (BS3) to characterize TGF-beta binding components on human placental cells in primary culture. Trophoblast-enriched primary cultures exhibited a predominant affinity-labelled complex characteristic of membrane-anchored betaglycan (formerly termed the Type III TGF-beta receptor) and relatively low levels of the Type I and Type II TGF-beta receptor complexes. The results from affinity labelling saturation and competition experiments with TGF-beta-1 and TGF-beta-2 suggest the existence of two distinct subtypes of betaglycan: one subtype has a lower capacity and higher affinity, binds both TGF-beta-1 and TGF-beta-2, yet has a preferential affinity for TGF-beta-2; the second subtype has a higher capacity and lower affinity and binds TGF-beta-1 exclusively. In contrast, mesenchymal cell-enriched placental primary cultures possessed only one subtype of the betaglycan component that binds the two TGF-beta isoforms with similar affinities and capacities as observed on most cell lines. These experiments demonstrate that the betaglycan component which exhibits a higher affinity for TGF-beta-2 than for TGF-beta-1, that we had observed previously on term placental membranes, is actually present on trophoblast cells. In addition to the two distinctive betaglycan subtypes, subtypes of the Type I and II TGF-beta receptors were detected on the trophoblast-enriched cultures. In competition experiments, when I-125-TGF-beta-1 was used as the radiotracer, the Type I and II TGF-beta receptors show a much higher affinity for TGF-beta-1 than for TGF-beta-2, as observed with other cell types. However, when I-125-TGF-beta-2 was used, low abundance subtypes of both the Type I and II receptors that show similar affinities for TGF-beta-1 and TGF-beta-2 were also revealed.