γ-Glutamyltransferase in adult rat hepatocyte during proliferation

γ-Glutamyltransferase (GGT) activity is known to increase during hepatocarcinogenesis induced by chemical carcinogens. To understand the possible significance of GGT, we have investigated GGT activities in hepatocytes cultured with or without metabolic inhibitors. Primary cultures of adult rat hepatocytes were established and the cell number, [³H]thymidine incorporation and GGT activity were measured at various points of hepatocyte proliferation. When insulin (0.1 μM) and EGF (0.1 μg/ml) were added to the culture, the initially low hepatocyte density (5-6 x 10⁴ cells/cm²) has increased 1.5-fold after 32 h of incubation. The change of GGT activity was correlated with that of [³H]thymidine incorporation: both the GGT activity and the [³H]thymidine incorporation have increased 2.6-fold and 2.5-fold, respectively, after 3 h of incubation, and then decreased rapidly followed by another increase after 24 h. The effect of inhibitors, such as serine-borate complex and acivicin for GGT during hepatocyte proliferation was also investigated. Serine-borate complex (reversible inhibitor) and acivicin (irreversible inhibitor) inhibited proliferation of hepatocyte in a dose-dependant manner: At the concentration of 1 mM serine-borate complex or 0.5 mM acivicin, hepatocyte proliferation was completely abolished, while GGT activity decreased slightly even at the 5 mM serine-borate complex. When hepatocytes were initially cultured in the presence of the growth factor with 5 mM serine-borate complex for 24 h, and then continued to culture further in the absence of the inhibitor, the cells restored proliferating capability completely, indicating the reversibility of the inhibitor. In conclusion, we have demonstrated that during hepatocyte proliferation increase in GGT activity is temporarily correlated with DNA synthesis, suggesting that GGT may play a role in hepatocyte growth.