The modulating effect of isoproterenol on DNA replication and protein synthesis

Within 96 h after initial isoproterenol administration, DNA replication and cell cycling were activated, as reflected in the bimodal distribution of nuclear fluorescence determined by flow-microfluorometric technques. A group of proteins, the cetyltrimethylammonium bromide extractable nuclear proteins (CTAB-proteins), isolated from electrostatically sorted nuclei of rat salivary glands, was shown by staining and autoradiography after two-dimensional electrophoresis to undergo differential synthesis during various phases of the in vivo cell cycle after isoproterenol administration. Stained chromatographs revealed quantitative differences in protein synthesis. Gel autoradiography was a more sensitive technique than staining for detecting nuclear protein synthesis during cell cycling. As observed in the autoradiographs of the CTAB-proteins, isoproterenol initiated two distinct periods of protein synthesis in the salivary gland cell cycle: one during the 2C population (G₀/G₁), and one during the 4C population (G₂/M). Protein synthesis after isoproterenol administration was much more dramatic in the 2C (isoproterenol) population, where 22 additional spots were observed, than in the 4C (isoproterenol) population, where five new spots were seen. There was less radioactive incorporation in the 4C (isoproterenol) population. Two spots ‘a’ and ‘b’ that demonstrate differential protein synthesis in stained gel chromatographs and gel autoradiographs were shown to have electrophoretic mobilities, molecular weights and amino acid compositions highly similar to those of HMG1 and HMG2, respectively. A positive correlation could also be drawn between quantitative levels of ‘a’ and ‘b’ and their levels of incorporation during cellular activity with HMG (high mobility group) proteins. For example protein ‘b’ (HMG2) was consistently more abundant in proliferating cell populations than in the quiescent ones. Autoradiographic patterns of the CTAB-proteins indicated that proteins ‘a’ and ‘b’ were synthesized during the $\text{G}{}_0\text{G}{}_1$ phase of the cell cycle, as were the majority of CTAB-proteins.