Hepatic proliferation inhibitor.

Although a long held tenet of biology has been that endogenous inhibitors can modulate cell proliferation, little progress was made in purifying any such inhibitor. This was largely due to the rarity of non-malignant cell cultures in which regulation of cell division was still operative, and to problems in separating cytotoxic and cytostatic effects in the complex biological extracts which were being studied. During the last decade, hepatic proliferation inhibitors of varying degrees of purity have been isolated using regenerating rat liver or hepatoma cell cultures as test systems. In these early studies, a number of inhibitors with differing molecular weights, physicochemical properties and biological responses were purified from liver cytosol and/or serum. Some of them could inhibit DNA synthesis or mitosis and thus were considered to be G1 or G2 inhibitors. However, experiments which could give precise answers about mechanisms of action could not be done until an inhibitor purified to homogeneity was available. Using well-characterized rat liver diploid epithelial cell cultures, which maintain a number of liver properties and which do not possess any transformation markers or malignant properties, we recently purified an hepatic proliferation inhibitor to a homogenous protein. It has a molecular weight of 26 000 daltons and an isoelectric point of 4.65. It specifically inhibits cell division and DNA synthesis in a number of non-malignant rat liver epithelial cell types, and has no effect on transformed liver cells, or hepatoma cells, in culture. Its effect is not mediated through destruction or sequestration of essential nutrients or calcium ions. Nor have preliminary experiments shown the hepatic proliferation inhibitor to interfere with the binding of epidermal growth factor to its receptors. The majority of the cells treated with the inhibitor are blocked in the G1 phase. Further experiments to study its mechanism of action and the inter-relationship, if any, between the cell cycle block induced by serum or nutrient deprivation, and the inhibitor-induced cycle block are in progress.