Separate glucocorticoid, heavy metal, and heat shock domains in thymic lymphocytes

Some recent studies suggest that eukaryotic cells may respond to glucocorticoids, heavy metals, or heat shock by selective increases in the synthesis of a common set of stress-related proteins, implying the existence of a common initiating mechanism. This circumstance could alter the interpretation of the mechanisms involved in other protein inductions that seemed to reflect agent-specific inductions of individual genes. To rule out the possibility of such a common induction mechanism, nearly 3,000 cellular proteins from normal rat thymic lymphocytes were separated by two-dimensional giant gel electrophoresis and examined for changes in rates of synthesis after the cells were exposed to dexamethasone, cadmium, or elevated temperatures. The three agents each induce characteristic, nonoverlapping sets of gene products. Dexamethasone increases six proteins, four within the first 15 to 45 min, and another two after 1 h; typically, these reach a 5- to 10-fold level of induction by 4 h. Cadmium induces two proteins: metallothionein (7-fold by 4 h) and a 27,000-dalton protein (appearing after 1 h, 4-fold induction by 4 h). Heat shock, besides inducing the 70,000-dalton heat shock protein often reported by others, also substantially increases at least 24 other proteins (typically 50- to 200-fold). In no case was a protein found to be inducible by more than one agent. These observations demonstrate that each agent initiates its characteristic response through a separate gene-induction mechanism. In addition to demonstrating that the heavy metal and heat shock responses are more extensive than previously realized, the results imply an agent specificity for the recently observed rapid glucocorticoid inductions that is consistent with their proposed function as initiators of the earliest metabolic hormone effects.