Serum-mediated Stimulation of Protein Synthesis in Ehrlich Ascites Tumor Cells
Edvardas Kaminskas 1
From the
1 From the Departments of Medicine, Harvard Medical School and Beth Israel Hospital, Boston, Massachusetts 02215
Ehrlich ascites tumor cells, grown in suspension cultures, could be cultivated for a period of time in serum-free medium, in which they stopped growing but remained viable. Their rate of protein synthesis was about 
that of growing cells. Following serum addition protein synthetic rates in these cells rose to values characteristic of growing cells. These high rates were sustained and the cells went on to divide; thus stimulation of protein synthesis was the initial part of a growth response. Accompanying the increase in protein synthetic rate was a marked redistribution of ribosomes, with ribosomes in the inactive monomer pool entering the polyribosomes. The faster sedimenting polyribosomes increased to a greater extent than the slower sedimenting ones, suggesting that mRNAs became more completely loaded with ribosomes. During this process the pools of ribosomal sub-units remained constant. Rates of protein synthesis calculated as per polyribosomal ribosome increased; thus, both the rates of initiation of translation and the rates of peptide chain elongation were enhanced following serum addition. The role of increased RNA synthesis in serum stimulation of protein synthesis was investigated by testing the ability of cells previously treated with actinomycin, cordycepin, or 5-bromo-2'-deoxyuridine to respond to serum. The results are best interpreted as showing that in each case the mRNA pool was significantly diminished, but that cells stimulated with serum were able to translate it with about double the efficiency of serum-starved cells. The possible mechanisms by which serum growth factor or factors may regulate the efficiency of translation are discussed.
Submitted on March 24, 1972
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