Platelet factor 4 selectively inhibits binding of TGF‐β1 to the type I TGF‐β1 receptor

Abstract

A low molecular weight inhibitor of TGF-β₁ binding was detected in partially purified human platelet extracts by using Hep 3B hepatoma cells in the binding assays. The inhibitory protein was purified to homogeneity and was identified as platelet factor 4 on the basis of its amino acid sequence. TGF-β₁ binding to Hep 3B cells was almost completely inhibited by 100 nM concentrations of platelet factor 4, but TGF-β₁ binding to NRK 49F fibroblasts was inhibited only slightly. Affinity cross-linking experiments revealed that these differences in the inhibition of TGF-β₁ binding by platelet factor 4 were due to differences in the complements of TGF-β₁ binding proteins present on these two cell types. In Hep 3B cells the majority of bound TGF-β₁ was cross-linked to a complex which had an apparent molecular weight of 70 kDa. TGF-β₁ binding to this protein was the most sensitive to inhibition by platelet factor 4. Based on its size and TGF-β₁ binding properties, we believe this protein is the type I TGF-β₁ receptor. Hep 3B cells also had a high-affinity TGF-β₁ binding protein which appeared as an 80 kDa complex, and which we believe to be the type II TGF-β₁ receptor. TGF-β₁ binding to this protein was not inhibited by platelet factor 4. TGF-β₁ was also cross-linked to complexes of higher molecular weights in Hep 3B cells, but it was not clear whether any of them represented the type III TGF-β₁ receptor. In NRK 49F cells, the majority of bound TGF-β₁ was cross-linked to a high molecular weight complex which probably represented the type III TGF-β₁ receptor. NRK 49F cells also had type I TGF-β₁ receptors and platelet factor 4 inhibited binding to these receptors in the NRK cells. Since the type I receptor contributed only a small percentage of total TGF-β₁ binding, however, the overall effects of platelet factor 4 on TGF-β₁ binding to NRK 49F cells were negligible. We were unable to demonstrate specific or saturable binding of platelet factor 4 to Hep 3B cells using either direct binding or affinity cross-linking assays. Thus, it is not clear whether platelet factor 4 inhibits TGF-β₁ binding by competition for binding to the type I receptor. Modest concentrations of TGF-β₁ reduced the adherence of Hep 3B cells to tissue culture dishes. Platelet factor 4 did not mimic this effect of TGF-β₁, nor did it inhibit the effect, even at concentrations which were sufficient to completely inhibit binding to the type I TGF-β₁ binding protein/receptor. This suggests that the type I binding protein does not mediate the effect of TGF-β₁ on Hep 3B cell adhesion.