SIGNAL-TRANSDUCTION IN FIBROBLASTS STABLY TRANSFORMED BY [VAL12]RAS -THE ACTIVITIES OF EXTRACELLULAR-SIGNAL-REGULATED KINASE AND JUN N-TERMINAL KINASE ARE ONLY MODERATELY INCREASED, AND THE ACTIVITY OF THE DELTA-INHIBITOR OF C-JUN IS NOT ALLEVIATED

Ras-transformed cells often show high levels of expression of activati ng protein-1 and Ets and of genes regulated by these transcription fac tors. In analogy with the effects of transient stimulation of Ras, it is assumed that the increase in transcription-factor transactivation i n stably transformed cells is due to Ras-induced constitutive activati on of mitogen-activated protein kinases. However, this has not been ex tensively studied. Using specific substrate peptides, we have examined here the activities of two types of mitogen activated protein kinase, extracellular-signal-regulated kinase (ERK) and Jun N-terminal kinase (JNK), in [Val12]Ras transformed rat embryo fibroblast cell lines. Th ese activities were elevated 2-3-fold in Ras-transformed cells compare d with non-transformed cells with a similar growth rate. Increased ERK activity was not necessarily accompanied by a similar increase in JNK activity. In transformed cells, ERK and JNK activities could be stimu lated fourfold and ninefold by phorbol ester and ultraviolet-light tre atment, respectively, indicating that only a fraction of these enzymes were constitutively activated in these cells. It has been suggested t hat inactive JNK downregulates c-Jun transcriptional activity by bindi ng to the c-Jun delta-domain. No decrease in delta-inhibitor activity could be demonstrated in Ras-transformed cells compared with control c ells, consistent with the presence of mainly inactive JNK in transform ed cells. Treatment of transformed cells wih benzodiazepine 5B, an inh ibitor of Ras farnesylation, decreased ERK and JNK activities, and con comitantly caused morphological reversion, reduced growth rate, and no rmalization of transformation-related gene expression. We conclude tha t in stably Ras-transformed cells the moderately increased ERK/JNK act ivities are not coregulated, and that ERK rather than JNK activity cor related with transformation-related gene expression.