The Megakaryocyte / Platelet - Specific Enhancer of the a 2 b 1 Integrin Gene : Two Tandem AP 1 Sites and the Mitogen - Activated Protein Kinase Signaling Cascade

The a2b1 integrin, a collagen receptor on platelets and megakaryocytes, is required for normal platelet function. Transcriptional regulation of the a2 integrin gene in cells undergoing megakaryocytic differentiation requires a core promoter between bp 230 and 292, a silencer between bp 292 and 2351, and megakaryocytic enhancers in the distal 58 flank. We have now identified a 229-bp region of the distal 58 flank of the a2 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 a2 integrin gene expression. Inhibition of the MAP kinase cascade with PD98059, a specific inhibitor of MAPK kinase 1, prevented the expression of the a2 integrin subunit in cells induced to become megakaryocytic. We provide a model of megakaryocytic differentiation in which expression of the a2 integrin gene requires signaling via the MAP kinase pathway to activate two tandem AP1 binding sites in the a2 integrin enhancer. r 1999 by The American Society of Hematology.