Effect of medium composition on protein degradation and DNA synthesis in rat embryo fibroblasts.

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Abstract
Fibroblasts in medium deficient in serum, amino acids, phosphate or glucose stop synthesizing DNA and increase the rate of degradation of their long-lived cellular proteins .apprx. 2-fold. There is no difference in rate of degradation of short-lived proteins under these conditions. Insulin, dexamethasone and fibroblast growth factor act synergistically to inhibit protein degradation and to stimulate thymidine incorporation to about the same extent as serum. When the medium content in serum or fibroblast growth factor is varied over a wide range, there is a close, inverse correlation between rate of protein degradation and extent of thymidine incorporation. When serum is added to cells that were deprived of serum, the inhibition of protein degradation is immediate, whereas the enhanced rate of protein degradation in serum-free medium is attained within 1 h after serum removal. A 30 min exposure to serum followed by incubation in serum-free medium was as effective as continuous exposure to serum in stimulating thymidine incorporation after 8-24 h.