Role of Hepatocyte Growth Regulators in Liver Regeneration

Abstract

We have studied whether growth factors, cytokines, hormones, neurotransmitters, and local hormones (autacoids) promote the proliferation of hepatic parenchymal cells (i.e., hepatocytes) using in vitro primary cultured hepatocytes. The indicators used for this purpose include changes in DNA synthesis activity, nuclear number, cell number, cell cycle, and gene expression. In addition, the intracellular signaling pathways from the plasma membrane receptors to the nucleus have been examined in detail for representative growth-promoting factors that have been found to promote DNA synthesis and cell proliferation of hepatocytes. In examining intracellular signaling pathways, the effects of specific inhibitors of presumed signaling factors involved have been pharmacologically confirmed, and the phosphorylation activities of the signaling factors (e.g., RTK, ERK, mTOR, and p70 S6K) have been evaluated. As a result, it has been found that there are many factors that promote the proliferation of hepatocytes (e.g., HGF, EGF, TGF-α, IL-1β, TNF-α, insulin, growth hormone (GH), prostaglandin (PG)), and serotonin (5-HT)), while there are very few factors (e.g., TGF-β1 and glucocorticoids) that inhibit the effects of growth-promoting factors. We have also found that 5-HT and GH promote the proliferation of hepatocytes via different autocrine factors (e.g., TGF-α and IGF-I, respectively). Using primary cultured hepatocytes, it will be possible to further study the molecular and cellular aspects of liver regeneration.

Keywords: cytokine; growth factor; hepatocyte proliferation; signaling pathway.

Figure 1

Temporal sequence of events associated with 70% partial hepatectomy (PHx) in vivo. Changes in serum transaminase activity, DNA synthesis activity, and LW/BW ratio.

Figure 2

Cell cycle progression stimulated by mitogens in primary cultures. (A) Percent of total hepatocyte nuclei in the G₀/G₁ phase of the cell cycle. (B) Percent of total nuclei in the S phase of the cell cycle.

Figure 3

Time course of hepatocyte proliferation stimulated by mitogens in primary cultures. (A) Phosphorylation of signaling factors by mitogens, (B) DNA synthesis and hepatocyte proliferation induced by mitogens.

Figure 4

Co-mitogenic effects of α- and β-adrenergic receptor agonists on hepatocyte proliferation stimulated by growth factors.

Figure 5

Hepatocyte proliferative effects of autacoids and growth hormone via an autocrine mechanism.