Hormonal Control of Serine Dehydratase mRNA in Primary Cultures of Adult Rat Hepatocytes 1

Abstract

The mechanism of hormonal induction of serine dehydratase [EC 4.2.1.13, SDH] was studied in primary cultures of adult rat hepatocytes by measuring the rates of syntheses of the enzyme protein and its translatable mRNA. The rate of synthesis of enzyme protein, measured as incorporation of [ ³ H]leucine into the enzyme protein in hepatocytes, was increased 4–5 times by dexamethasone (Dex) plus glucagon. Neither hormone alone increased the rate. The increased rate induced by the two hormones was suppressed by insulin and epinephrine. The decay curves of [3H]-leucine-labeled SDH showed that these hormones did not affect the rate of enzyme degradation. The level of translatable mRNA was determined by measuring cell-free synthesis of SDH in a reticulocyte lysate system. Dex plus glucagon increased the level of mRNA of SDH in hepatocytes. Insulin and epinephrine suppressed this increase without changing the rate of mRNA degradation. The level of mRNA changed in parallel with that of the rate of synthesis of the enzyme protein. These results suggest that these hormones regulate transcription of SDH, rather than its translation. After pretreatment of hepatocytes with Dex, further addition of glucagon caused more rapid induction of mRNA of SDH than addition of both hormones together. The effect of glucagon after pretreatment with Dex was inhibited by actinomycin D and α-amanitin, suggesting that glucagon does not affect post-tran-scription, but transcription per se . The requirement for both Dex and glucagon for induction of this enzyme is discussed in comparison with the requirements for either hormone alone for inductions of other gluconeogenic enzymes.