Cells exposed to a variety of stresses such as heat or ethanol respond by increasing their rate of synthesis of a set of proteins termed heat shock proteins (HSP). These proteins then appear to offer protection against the stressor and many other insults. The HSP also play important roles in unstressed cells. They are involved in the regulation of the cell cycle and during specific stages in the development of organisms. Exposure to stress during development (e.g., in pupal stages of insects or during gestation in mammals) leads to birth defects that are specific to the timing of the stress. It has been hypothesized that the ill-timed induction of HSP is responsible for this phenomenon. Epidemiological studies in humans have related teratogenic events to maternal exposures to hyperthermia or ethanol during pregnancy. The rate of alcohol-induced birth defects is greatly enhanced by smoking, suggesting a role for nicotine. Nicotine by itself, however, is said not to induce HSP. We hypothesized that nicotine may act as a coinducer (or facilitator) of stress responses. This possibility we tested on three levels: protection against heat (thermotolerance), induction of specific HSP, and binding of the heat shock transcription factor to the heat shock element. Each of these tests showed clearly that nicotine does indeed play such a role. This places nicotine in a novel position; to date, no other coinducers of stress responses have been reported. Our results may offer an explanation for the epidemiological data cited earlier.Stress responses in mammalian cells are characterized by two readily measured events. First, the protein synthesis pattern of the stressed cell is modified. The rates of synthesis of many normal cell proteins are reduced, while the synthesis of a specific set of proteins (and the relevant mRNAs) is either enhanced or induced de novo. These latter proteins are termed stress proteins and are of two types: proteins common to many stresses and proteins specific to the individual inducer (23). The second event is the development, in the surviving cells, of a transient resistance to the stressor; this is frequently accompanied by cross-resistance to other inducers (10, 13, 21). Many investigators have suggested that the stress proteins are responsible for this induced resistance, although direct proof is lacking (20,36).Among the inducers of stress responses are heat shock; exposure to aliphatic alcohols (including ethanol), metals such as cadmium, or sodium arsenite; and release of cells from hypoxia (23). A common feature of these treatments is protein damage. Indeed, several studies have shown that damaged proteins or the intracellular presence of proteins deemed "foreign" by the cell can lead to the induction of a response that is similar to that seen after heat shock (1, 14, 19). The heat shock response has been studied far more extensively than any other stress response. The reasons for this include the current interest in the use of hyperthermia in cancer therapy, the ease of induction of the resp...