Role of the alpha(2)-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 cel l types including osteoblasts, Matrix components often affect cell function by interacting with members of the integrin family of cell surface recepto rs, Previous work showed that collagen matrix synthesis, induced by additio n of ascorbic acid to cells, precedes and is essential for the expression o f osteoblast markers and induction of the osteocalcin promoter in murine MC 3T3-E1 cells. This later response requires OSE2, the promoter element recog nized by Osf2 (also called Cbfa1/AML3/PEBP2 alpha A), a recently identified osteoblast-specific transcription factor. Osteoblasts express several inte grins including alpha 2 beta 1 which is a major receptor for type I collage n. This paper examines the role of the alpha(2)-integrin subunit in osteoca lcin promoter activation and osteoblast differentiation. Disruption of alph a(2)-integrin-ECM interactions with a blocking antibody or DGEA peptide con taining the cell-binding domain of type I collagen blocked activation of th e mouse osteocalcin gene 2 promoter by ascorbic acid as well as induction o f endogenous osteocalcin mRNA and mineralization. Furthermore, anti-alpha(2 )-integrin blocking antibody or peptide reduced ascorbic acid-dependent bin ding of Osf2 to OSE2 without affecting levels of transcription factor mRNA Time course studies revealed that ascorbic acid-dependent binding of Osf2 t o OSE2 preceded increases in osteocalcin and bone sialoprotein expression a nd this increase in Osf2 binding was not accompanied by comparable changes in levels of transcription factor mRNA or protein. Taken together, these st udies demonstrate that an alpha(2)-integrin-collagen interaction is require d for activation of Osf2 and induction of osteoblast-specific gene expressi on. Furthermore, matrix signals may regulate Osf2 through a post-translatio nal pathway or via an accessory factor.