title: Constitutively active MAP kinase kinase (MEK1) stimulates SAP kinase and c-Jun transcriptional activity in U937 human leukemic cells.
abstract: Treatment of U937 human leukemic cells with the phorbol ester PMA, activates both mitogen-activated protein kinase (MAPK) and stress-activated protein kinase (SAPK), stimulates c-Jun phosphorylation and transcriptional activity, and induces a macrophage-like differentiation of U937 cells. The involvement of the MAPK pathway in mediating both the early phosphorylation and transcriptional activation events and the chronic differentiation of U937 cells was examined utilizing constitutively active MAPK kinase (MEK1) mutants. Transient expression of an activated MEK1 construct in U937 cells was found to stimulate MAPK and SAPK activity, as well as enhancing AP1-, SRE- and c-Jun-mediated transcriptional activity. Transient transfection of MAPK phosphatase-1 (MKP-1), a protein phosphatase which preferentially dephosphorylates and inactivates MAPK, inhibited the functional effects of both PMA and the constitutively active MEK1 mutants. To determine whether specific activation of the MEK/MAPK pathway was sufficient to induce hematopoietic differentiation, U937 cell lines were established that conditionally expressed the activated MEK1 mutant under the control of the human IIa metallothionein promoter. The induction of constitutively active MEK1 protein expression resulted in an increase in MEK1 activity, c-Jun and AP-1 transcriptional activity and an inhibition of U937 cell growth. However, this growth inhibition was not accompanied by U937 cell differentiation. These results suggest that a cross-talk mechanism exists between the MAPK and SAPK signal transduction pathways in U937 cells and that PMA-mediated SAPK activation may involve the MAPK pathway. Furthermore, selective activation of the MEK/MAPK pathway utilizing a constitutively active MEK1 mutant, while growth inhibitory, was not sufficient to induce the macrophage-like differentiation of U937 cells.
authors.affiliation: Division of Hematology/Oncology, University of Alabama, Birmingham 35294, USA.
authors.forename: C C
authors.initials: CC
authors.lastname: Franklin
authors.forename: A S
authors.initials: AS
authors.lastname: Kraft
mesh_terms: D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002454:Cell Differentiation / Q000235:genetics; D002455:Cell Division / Q000235:genetics; D002460:Cell Line; D054638:Dual Specificity Phosphatase 1; D004789:Enzyme Activation; D004790:Enzyme Induction; D000882:Haplorhini; D006801:Humans; D017874:Immediate-Early Proteins / Q000378:metabolism; D048031:JNK Mitogen-Activated Protein Kinases; D007938:Leukemia / Q000201:enzymology*; D020929:Mitogen-Activated Protein Kinase Kinases; D020928:Mitogen-Activated Protein Kinases; D009154:Mutation; D010749:Phosphoprotein Phosphatases; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D054645:Protein Phosphatase 1; D017027:Protein Tyrosine Phosphatases / Q000378:metabolism; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
chemical_list: D018797:Cell Cycle Proteins; D017874:Immediate-Early Proteins; D047428:Protein Kinase Inhibitors; D016755:Proto-Oncogene Proteins c-jun; D011494:Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D020929:Mitogen-Activated Protein Kinase Kinases; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; C517019:DUSP1 protein, human; D054638:Dual Specificity Phosphatase 1; D017027:Protein Tyrosine Phosphatases; D013755:Tetradecanoylphorbol Acetate