Transactivation of Fra-1 and consequent activation of AP-1 occur extracellular signal-regulated kinase dependently

Mitogen-activated protein (MAP) kinase, extracellular-signal- regulated kinases (ERKs) play an important role in activating AP-1-dependent transcription. Studies using the JB6 mouse epidermal model and a transgenic mouse model have established a requirement for AP-1-dependent transcription in tumor promotion. Tumor promoters such as 12-O-tetradecanoylphorbol- 13-acetate (TPA) and epidermal growth factor induce activator protein 1 (AP-1) activity and neoplastic transformation in JB6 transformation-sensitive (P+) cells, but not in transformation- resistant (P-) variants. The resistance in one of the P- variants can be attributed to the low levels of the MAP kinases, ERKs 1 and 2, and consequent nonresponsiveness to AP-1 activation. The resistant variant is not deficient in c-fos transcription. The purpose of these studies was to define the targets of activated ERK that lead to AP-1 transactivation. The results establish that the transactivation domain of Fra-1 can be activated, that activation of Fra-1 is ERK dependent, and that a putative ERK phosphorylation site must be intact for activation to occur. Fra-1 was activated by TPA in ERK- sufficient P+ cells but not in ERK-deficient P- cells. A similar activation pattern was seen for c-Fos but not for Fra- 2. Gel shift analysis identified Fra-1 as distinguishing mitogen-activated (P+) from nonactivated (P-) AP-1 complexes. A second AP-1-nonresponsive P- variant that underexpresses Fra-1 gained AP-1 response upon introduction of a Fra-1 expression construct. These observations suggest that ERK-dependent activation of Fra-1 is required for AP-1 transactivation in JB6 cells.

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