The Megakaryocyte/Platelet-Specific Enhancer of the 2β1 Integrin Gene: Two Tandem AP1 Sites and the Mitogen-Activated Protein Kinase Signaling Cascade

Abstract

The 2β1 integrin, a collagen receptor on platelets and megakaryocytes, is required for normal platelet function. Transcriptional regulation of the 2 integrin gene in cells undergoing megakaryocytic differentiation requires a core promoter between bp −30 and −92, a silencer between bp −92 and −351, and megakaryocytic enhancers in the distal 5′ flank. We have now identified a 229-bp region of the distal 5′ flank of the 2 integrin gene required for high-level enhancer activity in cells with megakaryocytic features. Two tandem AP1 binding sites with dyad symmetry are required for enhancer activity and for DNA-protein complex formation with members of the c-fos/c-jun family. The requirement for AP1 activation suggested a role for the mitogen-activated protein kinase (MAPK) signaling pathway in regulating 2 integrin gene expression. Inhibition of the MAP kinase cascade with PD98059, a specific inhibitor of MAPK kinase 1, prevented the expression of the 2 integrin subunit in cells induced to become megakaryocytic. We provide a model of megakaryocytic differentiation in which expression of the 2 integrin gene requires signaling via the MAP kinase pathway to activate two tandem AP1 binding sites in the 2 integrin enhancer.