Type I collagen is composed of two chains, alpha1(I) and alpha2(I), encoded
by two distinct genes, the alpha1(I) and alpha2(I) collagen genes, that ar
e highly expressed in osteoblasts. In most physiological situations, alpha1
(I) and alpha2(I) collagen expression is coregulated, suggesting that ident
ical transcription factors control their expression. Here, we studied the r
ole of Cbfa1, an osteoblast-specific transcription factor, in the control o
f alpha1(I) and alpha2(I) collagen expression in osteoblasts. A consensus C
bfa1-binding site, termed OSE2, is present at the same location in the alph
a1(I) collagen promoter at approximately -1347 base pairs (bp) of the rat,
mouse, and human genes Cbfa1 can bind to this site, as demonstrated by elec
trophoretic mobility shift assay (EMSA) and supershift experiments using an
anti-Cbfa1 antibody, Mutagenesis of the alpha1(I) collagen OSE2 at - 1347
bp reduced the activity of a alpha1(I) collagen promoter fragment 2- to 3-f
old. Moreover, multimers of this OSE2 at -1347bp confer osteoblast-specific
activity to a minimum alpha1(I) collagen promoter fragment in DNA transfec
tion experiments as well as in transgenic mice, An additional Cbfa1-binding
element is present in the alpha1(I) collagen promoter of mouse, rat, and h
uman at approximately position -372, This site binds Cbfa1 only weakly and
does not act as a cis-acting activator of transcription when tested in DNA
transfection experiments. Similar to alpha1(I) collagen, the mouse alpha2(I
) collagen gene contains multiple OSE2 sites, of which one is conserved acr
oss multiple species. In EMSA, Cbfa1 binds to this site and multimers of th
is alpha2(I) OSE2 element confer osteoblast-specific activity to the minimu
m alpha1(I) collagen promoter in DNA transfection experiments. Thus, our re
sults suggest that Cbfa1 is one of the positive regulators of the osteoblas
t-specific expression of both type I collagen genes.