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.