Cell-serum factor interaction. Characteristics of stimulation of protein synthesis in Ehrlich ascites cells by factors in serum and in cell extracts.

Stimulation of protein synthesis induced by serum in serum-starved Ehrlich ascites tumor cells was directly proportional to the concentration of added serum, inversely proportional to the concentration of cells, in the culture, and dependent on the length of exposure of cells to serum. Stimulation was markedly decreased in cells incubated with serum at temperatures lower than 37 °C. During the exposure of cells to serum, active protein synthesis was not required in order for subsequent stimulation of protein synthesis to take place. These characteristics were consistent with the possibility that stimulation of protein synthesis followed uptake of serum factors by cells. Extracts of cells stimulated protein synthesis in a similar fashion to serum. Stimulations by extracts and by serum were additive. The factors in cell extracts were macromolecular, associated with articulate fractions, and inactivated by trypsin, but not by RNAase, DNAase, ether or chloroform. Extracts of serum-grown cells were more stimulatory than extracts of serum-starved cells. When serum-starved cells were incubated with serum, stimulatory activities of their extracts increased as a function of time of incubation with serum.