Distinct roles for MEKK2 and MEKK3 in cell signaling, gene regulation and development as defined by targeted gene disruption

Mitogen activated protein kinase kinase kinases (MAP3Ks) are required for the signal transduction of specific stimuli to activate MAPK cascades, ultimately resulting in a cellular response. MEKKs are a family of kinases at the level of the MAP3Ks comprised of four family members: MEKK1, MEKK2, MEKK3, and MEKK4. MEKK2 and MEKK3 are the most structurally similar of the MEKKs: 65% homologous in the amino-terminal region and 94% homologous within the kinase domain. To delineate the explicit signaling pathways mediated by MEKK2 and MEKK3 in a physiological setting, targeted gene disruption of the two kinases was employed.; The homozygous deletion of MEKK3 resulted in an embryonic lethality between day E3.5 and E6.5. Additionally, MEKK3 is necessary for proliferation of embryonic stem cells and trophoblast stem cells, but not for the initial cellular differentiation of the blastocyst into the embryonic stem cell and trophoblast lineages. Because of the early stage lethality and thus the inability to obtain MEKK3 knockout cells, RNAi was employed to address the role for MEKK3 in MEFs. In MEFs, MEKK3 is necessary for osmotic stress-induced signaling through p38 and ERK5, but not necessary for growth factor regulated induction of MAPK modules, unlike MEKK2.; MEKK2 knockout mice are viable and fertile. Demonstrated in knockout MEKK2 MEFs, MEKK2 expression is required for growth factor activation of the ERK5 and JNK MAPK signaling pathways. Moreover, MEKK2 is required for growth factor induced synthesis of c-Jun and Fra-1 as shown by RPA analysis. Additionally, cytokine expression of IL-1, IL-6 and TNFalpha is mediated by growth factor stimulation through MEKK2. Therefore, MEKK2 coordinately regulates two MAPK pathways in response to growth factors, ERK5 and JNK, resulting in the integrated regulation of AP-1 transcription complexes and ultimately cytokine expression.; As shown by targeted gene disruption and RNAi knockdown, MEKK3 is necessary for embryogenesis, whereas MEKK2-/- mice are viable and fertile. MEKK2 is necessary for growth factor-induced activation of ERK5 and JNK in MEFs, whereas, MEKK3 is necessary for stress-induced activation of ERK5 and p38 in MEFs. Therefore, MEKK2 and MEKK3 have distinct roles in terms of cell signaling, gene regulation and development.