Glucocorticoid-induced proteins in rat thymus cells.

The possibility that hormone-induced changes in the synthesis of individual proteins may serve as initiating events for the rapidly evolving glucocorticoid-induced metabolic suppressions in rat thymus cells was examined. Synthetic rates of about 2500 individual proteins were screened using giant two-dimensional gel electrophoresis. Dexamethasone induces large, rapid increases in the rates of synthesis of at least three and more subtle increases in another three proteins. Increases in the rates of synthesis of four of these six proteins occur within 15 to 45 min. The time course suggests that changes in the synthesis of some or all of these four proteins may serve to generate the rapidly evolving hormone-induced inhibition of glucose transport. More slowly emerging changes in two other proteins parallel the latter metabolic effects of decreased mitochondrial ATP production and increased nuclear fragility. Cordycepin, an inhibitor of new mRNA processing, prevents the dexamethasone-induced increases in protein labeling. Also, when used alone, cordycepin selectively slows the synthesis of those early proteins that can be induced by dexamethasone, suggesting that the mRNAs coding for the early induced proteins have unusually short half-lives. Short-lived mRNAs for such putative regulatory proteins may allow the cell to respond rapidly to changing needs. Preliminary experiments using giant gel separations of subcellular fractions indicate that two of the induced proteins co-purify with crude plasma and nuclear membrane fractions. None of the induced proteins was detected in gel separations of mitochondria.