Specific binding to cultured cells of 125I-labeled type beta transforming growth factor from human platelets

Purified type beta transforming growth factor from human platelets (TGF beta) radioiodinated with 125I-labeled Bolton and Hunter reagent was found to bind to a variety of cultured cells of both epithelial and mesenchymal origin, including normal human fibroblasts and keratinocytes. TGF beta binding sites have also been found on three mouse embryo-derived fibroblast-like cell lines with lower levels of TGF beta binding on the chemically transformed derivatives of these cell lines. A variety of human tumor cell lines was shown to have an inverse correlation between their level of TGF beta binding and their ability to form colonies in soft agar. The mouse embryo-derived AKR-2B (clone 84A) cells reached maximal binding of 125I-labeled TGF beta after 2 hr at 22 degrees C. Scatchard analysis of the equilibrium binding of TGF beta to AKR-2B (clone 84A) cells gives a Kd of 33 pM with approximately equal to 10,500 binding sites per cell. This Kd for TGF beta binding to AKR-2B (clone 84A) cells agreed well with the ED50 of 40 pM for stimulation of colony formation of these cells by TGF beta. The TGF beta binding sites on the AKR-2B cells were shown to be specific for TGF beta with no significant competition with epidermal growth factor, fibroblast growth factor, or insulin and only a small level of competition with high concentrations of platelet-derived growth factor. Partially purified preparations with TGF beta-like activity from mouse embryos and medium conditioned by mouse embryo-derived cells competed effectively for binding to the TGF beta receptor.