Hormone-dependent Formation of Polysomes in Mammary Cells in Vitro

Growth-promoting actions of peptide hormones

In common with other growth-promoting hormones, peptide hormones evoke multiple biochemical responses in their target tissues. These can be divided into two groups: (a) rapid effects involving permeability properties of the target cell to amino acids, sugars and ions or changes in key intracellular metabolites like cyclic nucleotides; (b) slow responses based on the stimulation of RNA and protein synthesis. The impossibility of explaining all the late events as the results of early changes raises the possibility that more than one species of hormone receptor exists. It is proposed that the final expression of growth and maturation results from the cooperative interaction of rapid and slow responses of the target cell to the hormone.

The activation of phosphatidylinositol-specific phospholipase C by insulin in mammary epithelial cells of lactating mouse

We have previously demonstrated in vitro that, in the endoplasmic reticulum and Golgi apparatus of mammary epithelial cells of lactating and pregnant mice, inositol 1,4,5-trisphosphate releases Ca2+ that has been stored in these organelles. In this study, we examined whether insulin and prolactin, essential for the growth of mammary gland and for lactation, influenced the activity of phosphatidylinositol-specific phospholipase C in mammary cells. In the plasma membrane fraction of mammary epithelial cells of the DDY mouse strain 5 days after the start of lactation after the first pregnancy, and with phosphatidylinositol as substrate, it was shown that the activity of phospholipase C was enhanced by about four times in the presence of insulin compared with the control. Such enhancement was not found in the membrane fraction treated with prolactin.

A method for isolation of undegraded free and membrane-bound ribosomes from rat lactating mammary gland

Mammary gland polysomes are difficult to isolate from the lactating rat using methods developed for other species and tissues, most likely due to high calcium-stimulated ribonuclease activity in that tissue. A new method, utilizing ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) to bind calcium, yields highly aggregated polysomes from lactating rat mammary gland. Fresh mammary tissue is pulverized under liquid nitrogen. Free and membrane-bound polysomes are isolated by differential centrifugation in solutions containing 100 mM KCl, 100 mM MgCl2, 75 mM EGTA, 500 micrograms/ml heparin and 50 mM Tris buffer, pH 8.2 at 5 degrees C. Bound polysomes are released from the endoplasmic reticulum using Triton X-100 and deoxycholate. Polysome profiles are obtained on linear sucrose gradients and scanned at 254 nm. The method gives quantitative recovery of homogenate total RNA. To demonstrate that the method can be used to study nutritional effects on mammary gland polysome aggregation, lactating rats were fasted 22-66 h and then refed a stock diet for 71-95 h. Refeeding increased the percentage of polysomes (trimers or larger) in the bound fraction from 84 +/- 1 to 93 +/- 1% (P less than 0.001) and in the free fraction from 42 +/- 2 to 55 +/- 3% (P less than 0.001).

RNA synthesis in isolated bovine thyroid nuclei and nucleoli. alpha-Amanitin effect, a hint to the existence of a specific regulatory system

DNA dependent RNA polymerase activities in isolated bovine thyroid nuclei and nucleoli have been studied. They retain their RNA synthetic activity for an extended period of time. This RNA synthetic activity is sensitive to actinomycin D and requires the presence of all four ribonucleoside triphosphates. The optimal conditions have been determined. Polyacrylamide gel electrophoresis reveals that the RNA synthesized has a size distribution ranging from 34S to 4S. The production of 18S-8S RNA is very sensitive to low concentrations of alpha-amanitin. However, in isolated bovine thyroid nuclei (not in nucleoli) this drug displays an effect on all RNA classes produced. The alpha-amanitin induced drastic decrease of [3H]-UMP incorporation in RNA of all sizes synthesized by isolated bovine thyroid nuclei is discussed.

RNA synthesis in isolated nuclei of Xenopus laevis emproyos

Nuclei were isolated from tailbud embryos of Xenopus laevis and their RNA synthetic activity was studied. The activity was sensitive to actinomycin D, and required the presence of all four ribonucleotide triphosphates and a divalent cation (Mg2+ or Mn2+). The effect of the divalent cation varied depending on the presence of 0.4 M KCl: in its absence more RNA was synthesized with Mg2+ than with Mn2+, but in its presence the situation was reversed. About 50% of the RNA synthesized was sensitive to the presence of alpha-amanitin. Polyacrylamide gel electrophoresis of the labeled RNA revealed that the RNA synthesized had a very heterogeneous size distribution ranging from 40 to 4 S, with a broad peak at around 28--18 S. The RNA which was distributed at around 28--18 S and 4 S was resistant to the presence of a low dose of alpha-amanitin, suggesting its ribosomal and soluble RNA nature. The RNA which was sensitive to this drug and hence may be assumed to be messenger RNA was distributed mainly between 18 and 4 S. From these results it appears that the isolated embryonic nuclei retain their activity to synthesize all of the major classes of RNA.