Protein kinase B (PKB), a serine/threonine protein kinase, prevents apoptosis and promotes cellular transformation. PKB activity is stimulated by insulin. In this report, we examined the relative amounts of expression, location, and translocation upon insulin stimulation of PKBalpha in normal primary hepatocytes and carcinoma cells, HepG2 cells. Non-phosphorylated PKBalpha was present in both types of unstimulated cells. The phosphorylated form of the enzyme was present in the nucleus of unstimulated HepG2 cells but not in normal hepatocytes. In the cytoplasm, PKBalpha was found in greater abundance in the hepatocytes as compared in HepG2 cells. Insulin induced the translocation of phosphorylated PKBalpha from the nucleus to the nuclear membrane in HepG2 cells. In contrast, insulin caused translocation and phosphorylation of PKBalpha from the cytosol to the plasma membrane in normal hepatocytes. In addition, there is a higher expression of PKBalpha in the HepG2 cells as compared to normal primary hepatocytes. These findings provide an important distinction between hepatocellular HepG2 cells and normal liver cells and suggest that the presence of constitutively active nuclear PKB in the transformed cells might be an important contributor in cell transformation and immortality of hepatoma cells.
Copyright 2002 Wiley-Liss, Inc.