Accumulation of TISl and TIS11 (Lim et al.: Oncogene 1:263-270, 1987) mRNAs in secondary cultures of rat neocortical astrocytes was much greater in response to tetradecanoyl phorbol acetate (TPA) than in response to either epidermal growth factor (EGF) or fibroblast growth factor (FGF). In contrast, EGF, FGF, and TPA were equally effective in inducing accumulation of TIS8 and TIS28/c-fos mRNAs. These data suggested that TPA and the polypeptide mitogens might induce TIS gene expression by distinct pathways. When maximally inducing concentrations of EGF and FGF were co-administered to astrocyte cultures, TIS mRNA accumulations were no greater than those observed for the individual growth factors, suggesting that EGF and FGF saturate a common, limiting step in their induction pathways. In contrast, when either EGF or FGF was presented to astrocytes in combination with maximally inducing levels of TPA, the resulting levels of accumulation of TIS mRNAs were at least as great as the sum of the levels induced by the individual mitogens. Stimulation of [3H]-thymidine incorporation demonstrated an identical pattern of interaction; EGF and FGF co-administration was no more effective than either polypeptide mitogen alone, but, when presented to astrocyte cultures along with maximally inducing concentrations of TPA, either EGF or FGF was able to increase incorporation of [3H]-thyrnidine. Superinduction of all the TIS genes occurred if cycloheximide (CHX) was present during TPA exposure. Once again, two distinct classes of responses of the various TIS genes occurred; superinduction of TIS1, TIS7, TISl1, and TIS2Sk-fos mRNA accumulation ranged from 10-to 20-fold, while CHX superinduction of TISS and TISlO was far more modest, ranging from 2-to 3-fold. Differential superinduction of TIS gene expression also occurred when astrocytes were challenged with TPA in the presence of the pe-ripherally active benzodiazepine (BZD) Ro5-4864, ranging from a high of fivefold (TIS28k-fos) to no superinduction effect (TISS). The temporal patterns and relative efficacies of BZD and CHX superinduction of the various TIS genes were quite distinct.
Induction of TIS gene accumulation occurs in response to a wide variety of ligands by several independent pathways and can be secondarily modulated by a number of additional factors. The differential expression of this set of transient prirnary-response gene products may play a major role in integrating information that impinges on cells and in directing their physiological responses.