Hepatocytes have the ability to go through specialized cell cycles, which, during normal developmental liver growth, result in the formation of binuclear and
polyploid cells. In the adult rat liver, the majority of the hepatocytes (about 70%) are
tetraploid, 15-20% are octoploid, and only 10-15% are diploid (about 50% in humans). One-third of the hepatocytes in either rats or humans are binuclear (with two diploid or two
tetraploid nuclei). Among cultured rat hepatocytes stimulated with
growth factors (
EGF and
insulin), one-half of the mitoses are of the binucleating type (suggesting a "quantal" mechanism), causing one-third of the postmitotic cells to become binuclear. In contrast, regenerative liver growth, induced by partial
hepatectomy, is predominantly nonbinucleating. During rat liver
carcinogenesis, the early populations of phenotypically altered cells (foci) are predominantly diploid, as are the later neoplastic nodules and
carcinomas, which can be shown to have a regeneration-like, largely nonbinucleating growth pattern. A negative correlation between growth capacity and ploidy can be demonstrated in cultured hepatocytes, regenerating livers, neoplastic nodules, and
hepatocellular carcinomas, suggesting that suppression of binucleation and polyploidization may carry a growth advantage, in addition to helping to maintain a large population of diploid, potential stem cells. Since a diploid genome is less protected against mutagenic change than a
polyploid genome, diploid
tumor cells may, furthermore, be more prone than
polyploid cells to undergo mutation-based progression toward increasing
malignancy. The ability of liver
tumor promoters like
2-acetylaminofluorene,
cyproterone acetate,
alpha-hexachlorocyclohexane and
methylclofenapate to induce nonbinucleating hepatocyte growth may, therefore, cooperate with the selective growth stimulation of
cancer cells and
cancer cell precursors to promote liver
carcinogenesis. Autophagy, a mechanism for the bulk degradation of cytoplasm, contributes to intracellular
protein turnover and serves to restrict cellular growth. Rat liver
carcinogenesis is accompanied by a progressive reduction of autophagic capacity, preneoplastic livers having 50% and
hepatocellular carcinoma cells only 20% as much autophagy as normal hepatocytes. The
ascites hepatoma cell line AH-130 has virtually no autophagy during logarithmic growth, but some autophagy is turned on when the cells become growth-arrested at high cell density. Ascitic fluid from AH-130 cells is able to completely inhibit autophagy in normal hepatocytes, suggesting that the
cancer cells may improve their growth ability through an autocrine, autophagy-suppressive mechanism. Hepatocytes from preneoplastic livers similarly maintain a low autophagic activity under restrictive culture conditions, thereby surviving much better than normal hepatocytes, which switch on their autophagy. In the presence of an autophagy inhibitor (3-methyladenine), normal and preneoplastic hepatocytes survive equally well, testifying to the importance of autophagy as a determinant of cell survival and growth.