RAPID STIMULATION BY INSULIN OF RIBOSOME SYNTHESIS IN CULTURED CHICK-EMBRYO FIBROBLASTS

The effect of insulin on the synthesis of cytoplasmic ribosomes was studied in primary cultures of resting chick embryo fibroblasts. For obviation of problems with precursor nucleotide pools, ribosome formation was measured by the incorporation of [3H] leucine into ribosomal core proteins and the results have been expressed as the quotients of the specific activities of the core and total cell proteins. The following observations were made. (1) Insulin (5μg/mL) raised the incorporation of [3H] leucine into total chick cell protein by 1.3-1.5 times whereas ribosome production was stimulated by almost fourfold. (2) The ratios of the specific activities of the core and total cell proteins rose gradually as the concentration of insulin was raised from 0 to 1 μ/mL. (3) Insulin also elevated the incorporation of [3H]adenine into the 28S ribonucleic acid (RNA) of cytoplasmic ribosomes. A constant relationship was maintained between the increased labeling of the 28S RNA and the proteins of the 60S-derived cores at all the levels of the hormone tested, suggesting that insulin did not influence the specific activity of the adenosine 5'-triphosphate that served as a precursor for preribosomal RNA synthesis. (4) Insulin increased the uptake of [3H] adenine into 41S preribosomal RNA by two- to threefold. (5) The hormone did not, however, affect either the speed with which the 41S rRNA was processed or the time that labeled 60S sub-ribosomes first appeared in the cytoplasm. (6) The stimulation by insulin of the labeling of 41S preribosomal RNA and of 60S subribosomes was rapid and appeared to begin within 10 min after the resting cells were treated with the hormone. (7) Insulin increased the labeling of 41S preribosomal RNA in cells that had no exogenous source of glucose. © 1979, American Chemical Society. All rights reserved.