Relationship between RNA synthesis and the Ca 2+-filled state of the nuclear envelope store

Abstract

RNA synthesis and ATP-dependent 45Ca 2+ uptake were measured simultaneously in isolated nuclear fraction of rat liver nuclei. Maximal level of RNA synthesis was obtained under ATP-dependent 45Ca 2+-uptake conditions (1 μM free [Ca 2+] and 1 mM ATP in the bathing solution). This experimental condition was defined as “stimulated nuclei” condition. ATP-dependent 45Ca 2+ uptake was inhibited using different strategies including: (a) eliminating Ca 2+ (1 mM EGTA); (b) lowering the ATP concentration; (c) modifying nuclear envelope membranes Ca 2+ permeability (Ca 2+ ionophores); or (d) inhibiting the nuclear Ca 2+ pump (thapsigargin and 3′,3″,5′,5″-tetraiodophenolsulfonephthalein). Under all the above conditions, RNA synthesis was lower than in “stimulated nuclei” condition. In the presence of ionomycin, RNA synthesis was significantly higher at 500 nM free [Ca 2+], as compared with RNA synthesis in a Ca 2+-free medium or at 1 μM free [Ca 2+]. However, even in such condition (500 nM free [Ca 2+]), RNA synthesis was lower than RNA synthesis obtained in “stimulated nuclei” condition. We suggest two components for the effect of Ca 2+ on RNA synthesis: (A) a direct effect of nucleoplasmic [Ca 2+]; and (B) an effect dependent on the accumulation of Ca 2+ in the nuclear envelope store mediated by the SERCA nuclear Ca 2+ pump.