Mechanism of stimulation of polyamine synthesis by growth hormone in rat liver

Abstract

The effect of growth hormone on the synthesis of polyamines, RNA and protein was studied in the normal rat liver. The hormone markedly stimulated the synthesis of spermidine from [ 14C]-methionine. The time-course of this stimulation was close to that observed in RNA and protein. In addition to stimulating spermidine synthesis, growth hormone also caused an early increase in the concentration of hepatic putrescine and markedly stimulated the incorporation of label from [ 14C]ornithine into putrescine. The formation of putrescine from [ 14C]ornithine was also demonstrated in vitro, using the 100 000 × g supernatant fraction of liver homogenate as the source of the enzyme (ornithine decarboxylase). A 4-fold increase in this enzyme activity was observed after growth-hormone treatment. The effects of growth hormone on the ornithine decarboxylase activity, as well as on the accumulation of putrescine and the synthesis of spermidine, could be partially reversed by actinomycin D or puromycin. No change was observed in the capacity of the liver to synthesize spermidine from [ 14C]putrescine after growth-hormone treatment. It is suggested that the stimulation of spermidine synthesis after growth-hormone treatment depends on a primary increase in the concentration of putrescine, a precursor of spermidine.