Polyamine influences on the prolactin stimulation of phosphoprotein synthesis in hydroxyurea synchronized MCF-7 human...

The actions of prolactin on the rate of synthesis of an isoelectrically precipitable (pH 4.6) phosphoprotein fraction of the MCF-7 human mammary epithelial cell line were determined in cells synchronized at the G1:S interphase of the cell cycle employing hydroxyurea in a serum-free defined medium. Cells not allowed to enter the S-phase of DNA replication, by maintaining hydroxyurea in the incubation medium, exhibited an increased rate of [3H] leucine incorporation into the isoelectrically precipitable phosphoprotein fraction when exposed to prolactin and 1-5 mM spermidine. Cells released from the hydroxyurea induced synchrony exhibited an increased rate of [3H] leucine incorporation in response to prolactin when ornithine, putrescine, or spermidine were present. The polyamine spermine was ineffective in allowing prolactin's action on phosphoprotein synthesis. In synchronized cells released from the hydroxyurea block, prolactin was shown to effect an increased rate of phosphoprotein synthesis at the posttranscriptional G1 stage of the cell cycle. All prolactin responses were attained with physiological concentrations of the hormone. During and subsequent to the synchrony period with hydroxyurea, the presence or absence of insulin was found to be useful for the "staging" of the cell cycle to maintain cell synchrony and obtain prolactin effects on phosphoprotein synthesis.