Multiple mitogen-activated protein kinase signaling pathways connect the Cot oncoprotein to the c-jun promoter and to cellular transformation

The serine/threonine kinase Cot is a member of the mitogen-activated protei n kinase (MAPK) kinase kinase family implicated in cellular transformation. Enhanced expression of this protein has been shown to activate both the MA PK and the c-Jun N-terminal kinase (JNK) pathways and to stimulate the nucl ear factor of activated T cells and NF-kappa B-dependent transcription. How ever, the nature of the normal functions of the Cot protein and the molecul ar mechanisms responsible for its oncogenic potential are still largely unk nown. Here, we show that overexpression of the cot proto-oncogene is suffic ient to stimulate the expression of c-jun and that, in turn, the activity o f c-Jun is required for Cot-induced transformation. These observations prom pted us to explore the molecular events by which Cot regulates c-jun expres sion. We found that Cot potently stimulates the activity of the c-jun promo ter utilizing JNK-dependent and -independent pathways, the latter involving two novel members of the MAPK family, p38 gamma (ERK6) and ERK5. Molecular ly, this activity was found to be dependent on the ability of Cot to activa te, in vivo, members of each class of the MAPK kinase superfamily, includin g MEK, SEK, MKK6, and MEK5. Furthermore, the use of dominant interfering mo lecules revealed that Cot requires JNK, p38s, and ERK5 to stimulate the c-j un promoter fully and to induce neoplastic transformation. These findings i ndicate that Got represents the first example of a serine/threonine kinase acting simultaneously on all known MAPK cascades. Moreover, these observati ons strongly suggest that the transforming ability of Cot results from the coordinated activation of these pathways, which ultimately converge on the regulation of the expression and activity of the product of the c-jun proto -oncogene.