The sg2-targeted sequence in the APP-gene promoter conforms remarkably well to the consensus sequence of dimeric AP-1 transcription factors containing cFos, cJun, and/or other partners, suggesting that ApoE-stimulated MAP-kinase signaling activates APP transcription by stimulating AP-1 (Fig. 5A). To test this hypothesis, we examined cFos phosphorylation (Monje et al., 2003). Strikingly, ApoE2, ApoE3 and ApoE4 stimulated cFos phosphorylation 2- to 6-fold in human neurons co-cultured with MEFs, and acted with the same potency rank order of ApoE4>ApoE3>ApoE2 as DLK protein levels, MKK7- and ERK1/2-phosphorylation, APP transcription, and Aβ-synthesis (Fig. 5E). Moreover, using an APP promoter reporter construct with wild-type or mutant AP-1 transcription factor binding sites (Wade et al., 1992), we confirmed that ApoE2, ApoE3 and ApoE4 enhanced transcription from the APP promoter in an AP-1-dependent manner, again with the same ApoE4>ApoE3>ApoE2 potency rank order (Fig. 5F). Furthermore, ApoE3 not only stimulated transcription of APP, but also that of cFos (Fig. S6D), which is a target of AP-1 in a positive feedback loop (Karin, 1995). Again, stimulation of APP and cFos transcription by ApoE3 was specific, as it was not
