Subtypes of betaglycan and of type I and type II transforming growth factor‐β (TGF‐β) receptors with different affinities for TGF‐β1 and TGF‐β2 are exhibited by human placental trophoblast cells

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Abstract
Transforming growth factor-β 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 125I-TGF-β1 or 125I-TGF-β2 and bis(sulfosuccinimidyl) suberate (BS3) to characterize TGF-β 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-β receptor) and relatively low levels of the Type I and Type II TGF-β receptor complexes. The results from affinity labelling saturation and competition experiments with TGF-β1 and TGF-β2 suggest the existence of two distinct subtypes of betaglycan: one subtype has a lower capacity and higher affinity, binds both TGF-β1 and TGF-β2, yet has a preferential affinity for TGF-β2; the second subtype has a higher capacity and lower affinity and binds TGF-β1 exclusively. In contrast, mesenchymal cell-enriched placental primary cultures possessed only one subtype of the betaglycan component that binds the two TGF-β 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-β2 than for TGF-β1, 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-β receptors were detected on the trophoblast-enriched cultures. In competition experiments, when 125I-TGF-β1 was used as the radiotracer, the Type I and II TGF-β receptors show a much higher affinity for TGF-β1 than for TGF-β2, as observed with other cell types. However, when 125I-TGF-β2 was used, low abundance subtypes of both the Type I and II receptors that show similar affinities for TGF-β1 and TGF-β2 were also revealed.