Sw. Volk et al., A BMP RESPONSIVE TRANSCRIPTIONAL REGION IN THE CHICKEN TYPE-X COLLAGEN GENE, Journal of bone and mineral research, 13(10), 1998, pp. 1521-1529
Bone morphogenetic proteins (BMPs) were originally identified by their
ability to induce ectopic bone formation and have been shown to promo
te both chondrogenesis and chondrocyte hypertrophy, BMPs have recently
been found to activate a membrane serine/threonine kinase signaling m
echanism in a variety of cell types, but the downstream effecters of B
MP signaling in chondrocyte differentiation remain unidentified. We ha
ve previously reported that BMP-2 markedly stimulates type X collagen
expression in prehypertrophic chick sternal chondrocytes, and that typ
e X collagen mRNA levels in chondrocytes cultured under serum-free (SF
) conditions are elevated 3- to 5-fold within 24 h, To better define t
he molecular mechanisms of induction of chondrocyte hypertrophy by BMP
s, we examined the effect of BMPs on type X collagen production by 15-
day chick embryo sternal chondrocytes cultured under SF conditions in
the presence or absence of 30 ng/ml BMP-2, BMP-4, or BMP-7, Two popula
tions of chondrocytes were used: one representing resting cartilage is
olated from the caudal third of the sterna and the second representing
prehypertrophic cartilage from the cephalic third of the sterna, BMP-
2, BMP-4 and BMP-7 all effectively promoted chondrocyte maturation of
cephalic sternal chondrocytes as measured by high levels of alkaline p
hosphatase, diminished levels of type II collagen, and induction of th
e hypertrophic chondrocyte-specific marker, type X collagen. To test w
hether BMP control of type X collagen expression occurs at the transcr
iptional level, we utilized plasmid constructs containing the chicken
collagen X promoter and 5' flanking regions fused to a reporter gene.
Constructs were transiently transfected into sternal chondrocytes cult
ured under SF conditions in the presence or absence of 30 ng/ml BMP-2,
BMP-4, or BMP-7, A 533 bp region located 2.4-2.9 kb upstream from the
type X collagen transcriptional start site was both necessary and suf
ficient for strong BMP responsiveness in cells destined for hypertroph
y, but not in chondrocytes derived from the lower sterna.