title: AP-1 regulates alpha2beta1 integrin expression by ERK-dependent signals during megakaryocytic differentiation of K562 cells.
abstract: Mitogen-activated protein kinases (MAPKs) have been implicated as regulators of cellular differentiation. The biological effect of MAPK signaling in the nucleus is achieved by signal-responsive transcription factors. Here, we have investigated the connection of MAPKs, transcription factor AP-1, and alpha2beta1 integrin expression in K562 cells undergoing differentiation along the megakaryocytic pathway. We report that three distinct MAPKs, ERK, JNK, and p38, are activated during the TPA-induced megakaryocytic differentiation. Activation of MAPK pathways is followed by acquisition of the AP-1 DNA-binding and transactivation capacities. AP-1 DNA-binding activity consists primarily of JunD, c-Fos, and Fra-1, and is accompanied with the increased expression and phosphorylation of these subunits. While inhibition of JNK mainly prevents expression and phosphorylation of JunD and c-Jun, inhibition of the ERK pathway suppresses both phosphorylation and expression of Jun proteins, and expression of c-Fos and Fra-1. Furthermore, only the activity of the ERK pathway is essential for the differentiation response, as determined by expression of alpha2beta1 (CD49b) integrin. The importance of AP-1 as a mediator ERK signaling during differentiation is demonstrated by the findings that expression of c-fos siRNA and dominant negative AP-1/c-Jun(bZIP) downregulate the TPA- and ERK-induced expression of alpha2beta1 integrin mRNAs and proteins. Conversely, coexpression of JunD or c-Jun and c-Fos can induce alpha2beta1 integrin expression independently of upstream signals. Taken together, the results show that AP-1 is a nuclear target of the ERK-pathway and mediates alpha2beta1 integrin expression during megakaryocytic differentiation of K562 cells.
authors.affiliation: Molecular Cancer Biology Research Program, Biomedicum Helsinki and Haartman Institute, P.O. Box 63, FIN-00014 Helsinki, Finland.
authors.forename: Minna
authors.initials: M
authors.lastname: Eriksson
authors.forename: Laura
authors.initials: L
authors.lastname: Arminen
authors.forename: Marja-Liisa
authors.initials: ML
authors.lastname: Karjalainen-Lindsberg
authors.forename: Sirpa
authors.initials: S
authors.lastname: Leppä
mesh_terms: D002454:Cell Differentiation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D006801:Humans; D038982:Integrin alpha2beta1 / Q000378:metabolism*; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D020014:K562 Cells; D020935:MAP Kinase Signaling System; D008533:Megakaryocytes / Q000166:cytology* / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000502:physiology; D012097:Repressor Proteins / Q000502:physiology; D018808:Transcription Factor AP-1 / Q000378:metabolism*
chemical_list: D038982:Integrin alpha2beta1; C491387:JDP2 protein, human; D016760:Proto-Oncogene Proteins c-fos; D012097:Repressor Proteins; D018808:Transcription Factor AP-1; D048049:Extracellular Signal-Regulated MAP Kinases; D048031:JNK Mitogen-Activated Protein Kinases