Purification and Properties of Epithelial Growth Inhibitor (EGI) from Human Platelets: Its Separation from Type β Transforming Growth Factor (TGF-β)1

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Abstract
We previously reported that sera from various kinds of animals contain a protein(s) capable of inhibiting the growth of the non-malignant epithelial cell line derived from Buffalo rat liver (BRL). In the present study, a similar epithelial cell-specific growth inhibitor (EGI) was purified to homogeneity from an acid-ethanol extract of human platelets. During purification, EGI was separated from the major component of type β transforming growth factor (TGF-β), which can stimulate the colony formation of the non-malignant fibroblastic cell line derived from rat kidney (NRK) in soft agar in the presence of epidermal growth factor (EGF). The purified EGI had an M r of 27,000, and was composed of two subunits identical in M r . It significantly inhibited the growth in monolayer cultures of three non-malignant epithelial cell lines, BRL, MDCK (from Madin-Darby canine kidney) and BSC-1 (from African green monkey kidney), at doses lower than 40 pg/ml in medium containing 10% fetal calf serum. Its inhibitory activity was stable against heating at 90°C for 3min, but not against treatment with 50 mM dithiothreitol. In addition, TGF-β was also partially purified from the same extract. The purified TGF-β did not show any inhibitory activity toward the growth of BRL, MDCK, BSC-1, or NRK.