We have used an expression cloning strategy to isolate the cDNAs encoding the types III and II cell-surface receptors for transforming growth factor beta (TGF-beta). One major goal is to clone the multiple type I THP-1 cells, IEC-18 ras7 cells, and in HL-60 cells. Using the derived amino acid sequences of the types II and III receptors, we have generated anti-peptide sera that are specific for the encoded proteins. These reagents have allowed us to determine that the types of cell-surface TGF- beta binding-proteins expressed on a variety of human and animal tumors and cell lines that are or are not responsive to growth inhibition by TGF-beta is far more than previously appreciated. Thus a second major aim is to molecularly clone and characterize many of the other cell- surface TGF-beta receptors we and others have identified, and to study in detail their abilities to affect cell proliferation and gene expression. These include TGF-beta binding proteins attached to the plasma membrane through a glycosylphosphatidylinositol anchor, the major 70-74 kDa cell surface TGF-beta receptor in GH3 pituitary cells that also binds activin and inhibin, the major 38-kDa cell surface protein on several cells that binds TGF-beta2 at high affinity, and the about 50kDa proteins expressed on the surface of many cells that bind TGF-beta1, beta2, and beta3 with equal high affinity. As there are many cell- surface molecules that bind TGF-beta specifically, we need to understand how these several receptors interact with each other and with each other and with TGF-beta homo- and hetero-dimers, to understand why expression of certain receptor polypeptides enhances or inhibits binding of TGF-beta to other receptors, to determine the subcellular metabolism of the receptor polypeptides, and to determine whether receptor homo- or hetero- oligomerization is induced by TGF-beta and whether this is part of the signaling response. Finally, we and others have shown that a number of human tumor cell lines are resistant to growth-inhibition by TGF-beta and, as shown by affinity-labeling with [125I] TGF-beta, lack the type II and often other cell-surface receptors. We have shown, surprisingly, that several express mRNA encoding the type II TGF-beta receptor and that the encoded receptors contain the same point mutation in the predicted kinase domain, yet the cells express no surface receptor. We will clone and characterize these mutant receptor cDNAs, determine whether the mutations are functional and/or generate a dominant-negative phenotype, and cDNAs that we have and will clone. All of these studies are fundamental to our understanding of how TGF-beta exerts its multiple functions on a diverse array of cells, and in understanding how this novel class of cell-surface receptors transduces signals to the cell nucleus.
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