Multiple forms of growth inhibitors secreted from cultured rat liver cells: purification and characterization

It was found that a non-tumorigenic epithelial cell line from the liver of a Buffalo-strain rat (BRL) secreted into the culture medium various inhibitors of the growth of BRL and RSV-BRL (tumorigenic BRL transformed by infection of Rous sarcoma virus). The secreted inhibitors were classified into two types: one inhibited the growth of BRL to a greater extent than that of RSV-BRL (non-tumorigenic BRL growth inhibitor, NGI), and the other, vice versa (tumorigenic BRL growth inhibitor, TGI). Two NGI (NGI-I and NGI-II) and two TGI (TGI-I and TGI-II) were highly purified from the serum-free conditioned medium. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis without 2-mercaptoethanol, NGI-I and II gave protein bands with molecular weights (Mr) of 56,000 and 21,000, respectively. TGI-I and II gave a band that migrated faster than bromophenol blue marker dye, but they did not pass through an ultrafiltration membrane with an Mr cutoff of 5,000. In the presence of a reducing reagent, only NGI-II showed a decrease of Mr, from 21,000 to 11,000. NGI and TGI showed 50% growth inhibition with BRL and RSV-BRL, respectively, at 5-15 ng/ml in the medium containing 10% fetal calf serum. NGI and TGI all were stable to 1 M acetic acid (pH 2.3) and 6 M urea, but labile to 5 mM dithiothreitol or trypsin. Of the eight cell lines tested, NGI-I was most effective on BRL, NGI-II on BRL and HSC-3 (human tongue squamous carcinoma), and both TGI-I and II on RSV-BRL.