Structures and functions of growth inhibitors secreted from rat liver cell lines

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We have been studying growth-inhibitory substances present in various sera, tissues and cultured cells using a rat liver-derived epithelial cell line BRL and its tumorigenic transformant RSV-BRL. Previous studies revealed that the target cell lines themselves secreted growth inhibitors, and three kinds of growth inhibitors (M_r 56k, 21k and <10k) found in the conditioned medium of BRL were purified and characterized. In this study, we attempted to characterize growth inhibitors secreted from RSV-BRL.RSV-BRL secreted at least two kinds of growth inhibitors: one (Gl-1) was stable against acid and heat but not reducing reagents, whereas the other (Gl-2) was stable against acid and heat but labile against reducing reagents. Gl-1 was secreted as a latent complex with M_r 300-700k into the culture medium, and at least two active inhibitors with pl 9.2 and pl 7.9 were dissociated from the complex when the complex was treated with acid. Analysis of the N-terminal amino acid sequence of Gl-1 with pl 9.2 revealed that it was identical to TGF-beta1. Gl-2 was partially purified from the conditioned medium. The M_r of Gl-2 was estimated to be 10-40k. Both Gl-I and Gl-2 much more potently inhibited the growth of BRL than RSV-BRL. In addition to these growth inhibitors, we highly purified a novel growth inhibitor with M_r 60-70k from rabbit serum, which selectively inhibited the growth of tumorous or malignantly transformed cells.