Properties and Physiological Roles of Cell Growth Inhibitors Present in Blood

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To understand the negative control mechanism of cell growth, growth inhibitory proteins (growth inhibitors) present in blood were surveyed using the non-malignant epithelial cell line from rat liver (BRL) and its malignant transformant (RSV-BRL) as the indicator cells.Rat serum contained a growth inhibitor that inhibited the growth of BRL but hardly that of RSV-BRL. This inhibitor existed as a protein complex with a <M_m> of about 220k and a pI of 5.2, but it was dissociated to an active component of a <M_r> of about 30k in the presence of denaturating reagents. The activity of the 220k inhibitor was increased more than 10-fold by treatment with a mixture of ethanol and acetone or with 1 M acetic acid. The growth inhibitor of 30k was purified about 6,000-fold.From human platelets, a growth inhibitor was extracted with acid-ethanol and purified to homogeniety. The purified growth inhibitor showed 50% inhibition on the growth of BRL at 20 pg/ml (0.7 pM), but hardly influenced the growth of RSV-BRL. Its inhibition was nearly specific for non-malignant epithelial cells. The purified inhibitor had a <M_r> of about 27k and was composed of two apparently identical subunits cross-linked with disulfide bonds. Isoelectric electrophoresis on polyacrylamide gel separated it into two active components with pI of 8.7 and 9.0, respectively.From rabbit serum, growth inhibitors with <M_r> of lower than 10k and pI of 5.0 and 6.5 were purified to homogeniety. Unlike the growth inhibitors from rat serum and human platelets, the growth inhibitors from rabbit serum more strongly inhibited the growth of RSV-BRL than that of BRL. Several human cancer cell lines were also sensive to the growth inhibitors. These results indicate that different species of growth inhibitors are involved in the control of cell growth in vivo.