Anisomycin abrogates repression of protooncogene c-fos transcription in E1A + cHa-ras-transformed cells through activation of MEK/ERK kinase cascade.

We have previously shown that transcription of immediate-early c-fos protooncogene is becoming strongly repressed in rat embryo fibroblasts transformed by oncogenes E1A and cHa-ras, so that serum only slightly stimulated c-fos transcription in these cells in contrast to high level of c-fos activation in non-transformed REF52 cells. Here we showed that stress-inducing agent anisomycin was able to override the c-fos repression and to induce c-fos transcription in E1A + ras transformants. In vitro kinase assay data demonstrated that anisomycin increased phosphorylation of transactivation domain of Elk-1 transcription factor--a key regulator of inducible c-fos transcription. Importantly, this activation was mediated through up-regulation of MEK/ERK but not stress-kinase cascades JNK or p38. The activating effect of anisomycin on c-fos transcription could be abrogated by a prior treatment with N-acetyl-L-cysteine. This indicates that anisomycin potentiates generation of reactive oxygen species (ROS), which, in turn, can modulate the activity of MAP kinase-specific phosphatases (MKPs). As anisomycin did not cause acetylation of nucleosome core histones, the present work focuses on the molecular mechanisms mediating the HDAC-independent induction of IEG c-fos by anisomycin in E1A + cHa-ras-transformed fibroblasts.