THE ETS-RELATED TRANSCRIPTION FACTOR ERM IS A NUCLEAR TARGET OF SIGNALING CASCADES INVOLVING MAPK AND PKA

Recent studies support a model for signal transduction from activated receptor tyrosine kinases to Ras which, in turn, activates the pathway of the mitogen-activated protein kinase (MARK), Although some members of the Ets transcription factor family have been shown to be activate d by this signaling pathway, no data are available on the activation o f the PEA3 group of Ets proteins, This group is composed of three memb ers PEA3, ER81 and ERM - which are very similar in the DNA-binding dom ain, the ETS domain, in the 32 residue amino-terminal acidic domain an d in the 61 residue carboxy-terminal domain, First of all we demonstra ted that ERM-transfected cells contain a positive labeling in the nucl eus, and rye concluded that a nuclear localization signal might be sit uated in the ETS domain, We then showed that of four putative reporter plasmids, ERM activated the artificial 3 x TORU plasmid which contain s an Ets binding site contiguous to an AP1 one, This transactivation e nhancement requires the presence of the ERM amino-terminal domain, In contrast, although the lack of the carboxy-terminal domain induced a d ecrease in transactivation, this latter domain is not crucial, By usin g the E74-reporter plasmid system which is not basically activated by ERM, we showed that the activation of the Ras/Raf-1/MAPK pathway signi ficantly enhanced ERM-mediated transactivation, The deletion of the am ino-terminal transactivation domain abolished the capacity of stimulat ed MAPK to activate ERM, We also demonstrated that ERM can also be act ivated through the protein kinase A (PKA), another signaling pathway, Nevertheless, the MAPK and PKA activation of ERM are not synergistic, Finally, we showed that this Ets transcription factor is in vitro phos phorylated by both activated ERK-2 and activated PKA, ERM has thus bee n identified as a transcription factor which is a target for two diffe rent signaling pathways and might therefore be involved in the mitogen ic response of cells.