Cell-free cytosolic extracts from the Yoshida (AH 130) rat
ascites hepatoma cell line, grown in vivo, showed high
ribosomal protein S6 kinase activity in vitro, as measured by transfer of 32P to exogenous 40S rat liver ribosomal subunits, in both exponential growing and stationary phase cells. A significant decrease of
protein synthesis (3H-
leucine incorporation into total cell
protein) was found to occur in cells reaching the stationary phase of growth, suggesting that S6 phosphorylation was not tightly coupled to the rate of the intraperitoneal cell growth and of
protein synthesis in these
tumor cells. When the cell-free cytosolic extracts were prepared from cells exposed to
amiloride, at concentrations that inhibit the Na+/H+ exchange, a decrease of
S6 kinase activity was observed only in exponential growing cells, suggesting the possibility of coupling of the Na+/H+ exchange with phosphorylation of intracellular
proteins in these
tumor cells. Actually, stationary phase cells showed unchanged
S6 kinase activity under the same conditions, possibly due to the extremely low Na+/H+ exchange activity, previously demonstrated (Cell Biol. Int. Rep., 1985, 9, 1017-1025). The present experiments support the hypothesis that the regulation of
protein synthesis is not tightly coupled to phosphorylation-dephosphorylation cycles, at least of
ribosomal protein S6, in cells characterized by a rather uncontrolled growth such as the Yoshida (AH 130) rat
ascites hepatoma. In this connection, an elevated degree of
protein phosphorylation, such as that of the
ribosomal protein S6, could be a general phenomenon of neoplastic transformation.