Role of the alpha2-integrin in osteoblast-specific gene expression and activation of the Osf2 transcription factor.
Extracellular matrix molecules such as type I collagen are required for the adhesion, migration, proliferation, and differentiation of a number of cell types including osteoblasts. Matrix components often affect cell function by interacting with members of the integrin family of cell surface receptors. Previous work showed that collagen matrix synthesis, induced by addition of ascorbic acid to cells, precedes and is essential for the expression of osteoblast markers and induction of the osteocalcin promoter in murine MC3T3-E1 cells. This later response requires OSE2, the promoter element recognized by Osf2 (also called Cbfa1/AML3/PEBP2alphaA), a recently identified osteoblast-specific transcription factor. Osteoblasts express several integrins including alpha2beta1 which is a major receptor for type I collagen. This paper examines the role of the alpha2-integrin subunit in osteocalcin promoter activation and osteoblast differentiation. Disruption of alpha2-integrin-ECM interactions with a blocking antibody or DGEA peptide containing the cell-binding domain of type I collagen blocked activation of the mouse osteocalcin gene 2 promoter by ascorbic acid as well as induction of endogenous osteocalcin mRNA and mineralization. Furthermore, anti-alpha2-integrin blocking antibody or peptide reduced ascorbic acid-dependent binding of Osf2 to OSE2 without affecting levels of transcription factor mRNA. Time course studies revealed that ascorbic acid-dependent binding of Osf2 to OSE2 preceded increases in osteocalcin and bone sialoprotein expression and this increase in Osf2 binding was not accompanied by comparable changes in levels of transcription factor mRNA or protein. Taken together, these studies demonstrate that an alpha2-integrin-collagen interaction is required for activation of Osf2 and induction of osteoblast-specific gene expression. Furthermore, matrix signals may regulate Osf2 through a post-translational pathway or via an accessory factor.
Xiao
G
G
Department of Periodontics, Prevention, and Geriatrics, School of Dentistry and Department of Biological Chemistry, School of Medicine, University of Michigan, Ann Arbor, Michigan 48109-1078, USA.
Wang
D
D
Benson
M D
MD
Karsenty
G
G
Franceschi
R T
RT
D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D015703:Antigens, CD / Q000378:metabolism / Q000502:physiology*; D002454:Cell Differentiation / Q000502:physiology; D003094:Collagen / Q000378:metabolism; D050659:Core Binding Factor Alpha 1 Subunit; D005109:Extracellular Matrix / Q000378:metabolism; D005786:Gene Expression Regulation / Q000502:physiology*; D039421:Integrin alpha2; D051379:Mice; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D012333:RNA, Messenger / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic / Q000502:physiology*
D015703:Antigens, CD; D050659:Core Binding Factor Alpha 1 Subunit; D039421:Integrin alpha2; D009363:Neoplasm Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; D003094:Collagen