Synergistic stimulatory effect of glucocorticoid, EGF and insulin on the synthesis of ribosomal RNA and phosphorylation of nucleolin in primary cultured rat hepatocytes.

Effects of dexamethasone, EGF and insulin on the synthesis of rRNA and phosphorylation of nucleolin in primary cultures of adult rat hepatocytes were studied. Hepatocytes were incubated for 8 h with EGF (20 ng/ml) plus insulin (0.1 microM) and/or for 20 h with dexamethasone (1 microM) before the end of incubation. The incorporation of 3H]uridine into acid-insoluble materials and the nuclear activity of RNA polymerase I were stimulated approx. 2-fold with EGF plus insulin and these were further enhanced 2-3-times by dexamethasone, although dexamethasone alone exerted no stimulation. When hepatocytes were incubated with 32P]orthophosphate, similar enhancement by these hormones was also observed in the phosphorylation of a nucleolar protein, nucleolin, which was detected by immunoprecipitation with anti-nucleolin antibodies. The amount of nucleolin was slightly increased by EGF plus insulin in the presence of dexamethasone, but scarcely changed by treatment with EGF plus insulin or dexamethasone alone. Cycloheximide inhibited RNA synthesis to a greater or lesser degree in the case of all hepatocytes which were cultured with or without these hormonal treatments. These results indicate that the in vivo effect of glucocorticoid on rRNA synthesis and nucleolin phosphorylation in liver is primarily a direct action on parenchymal cells and requires other growth factors such as EGF and insulin.