Ethanolamine is a co-mitogenic factor for proliferation of primary hepatocytes

Mature adult parenchymal hepatocytes can enter the S phase in the presence of growth factors such as HGF and EGF, but rarely proliferate in culture. We hypothesized that the cell cycle of hepatocytes in culture is restricted before G(2)/M phase and we attempted to identify the factor that induces cell cycle progression. We found that the conditioned medium from long-term cultured hepatocytes contained co-mitogenic activity with other growth factors, which was attributed to ethanolamine (Etn). Etn induced not only DNA synthesis but also cell replication of cultured hepatocytes with various other growth factors. Etn and HGF synergistically induced cyclin D-1, A and B expression, however, only cyclin B but not cyclin A formed a complex with Cdc2. In addition, Etn combined with HGF enhanced PKC beta II expression and translocated PKC beta II to the plasma membrane, and induced filopodia formation, which was inhibited by an antisense oligonucleotide against PKC beta II. In addition, blocking the cytoskeleton rearrangement with inhibitors (colchicine, cytochalasin D, or chlerythrine (a specific PKC inhibitor)) inhibited cyclin expression and cell proliferation. Although Etn enhanced the downstream product, cellular phosphatidylethanolamine (PE), PE itself did not show any Etn-like activities on hepatocytes. Taken together, our results indicate that Etn functions as a co-replication factor to promote the cell cycle of mature hepatocytes to G(2)/M phase in the presence of growth factors. The activity is thought to be mediated by PKC beta II-dependent cyclin S expression. (C) 2001 Wiley-Liss, Inc.