Stimulation of type-X collagen gene transcription by retinoids occurs in part through the BMP signaling pathway.
BACKGROUND
Chondrocyte maturation and hypertrophy during endochondral bone formation are stimulated by both retinoids and bone morphogenetic proteins (BMPs). The type-X collagen gene, which is expressed only in hypertrophic chondrocytes, provides an excellent marker for chondrocyte maturation. We previously identified a 651-base-pair region of the type-X collagen promoter that is essential for its activation by BMP. We examined the relationship between the retinoid and BMP signaling pathways in transcriptional stimulation of the type-X collagen gene to determine whether they act independently or interact to regulate endochondral bone formation.


METHODS
Prehypertrophic chondrocytes from embryonic chick sterna cultured in the presence or absence of retinoic acid or BMP-2 were transiently transfected with plasmids containing various mutations of the type-X collagen promoter directing expression of a luciferase reporter gene. In addition, real-time polymerase chain reaction was used to examine the effects of retinoic acid on expression of genes encoding BMP-2, 4, and 6.


RESULTS
The previously identified BMP-responsive region of the type-X collagen promoter also mediated stimulation by physiological concentrations of retinoic acid in prehypertrophic chondrocytes. Systematic deletion mutagenesis of the BMP/retinoid-responsive region of the type-X collagen promoter identified distinct regions that are responsible for promoter stimulation by retinoids and BMP. Retinoic acid rapidly and dramatically stimulated accumulation of BMP-2 and BMP-6 messenger RNAs.


CONCLUSIONS
These results suggest that, while retinoic acid appears to stimulate type-X collagen gene transcription in part by stimulating the BMP signaling pathway, it also acts in part through mechanisms that are independent of BMP.
Adams
Sherrill L
SL
University of Pennsylvania School of Dental Medicine, Philadelphia 19104-6030, USA. sherri@biochem.dental.upenn.edu
Pallante
Kim M
KM
Niu
Zeling
Z
Cohen
Arthur J
AJ
Lu
Jane
J
LeBoy
Phoebe S
PS
D000818:Animals; D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; D055418:Bone Morphogenetic Protein 6; D019485:Bone Morphogenetic Proteins / Q000235:genetics / Q000502:physiology*; D002454:Cell Differentiation / Q000187:drug effects / Q000235:genetics*; D002478:Cells, Cultured; D002642:Chick Embryo; D019902:Chondrocytes / Q000166:cytology*; D024121:Collagen Type X / Q000235:genetics*; D005786:Gene Expression Regulation / Q000187:drug effects; D010012:Osteogenesis / Q000187:drug effects / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics; D012176:Retinoids / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects*; D014158:Transcription, Genetic / Q000187:drug effects*; D016212:Transforming Growth Factor beta; D014212:Tretinoin / Q000494:pharmacology
D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; D055418:Bone Morphogenetic Protein 6; D019485:Bone Morphogenetic Proteins; D024121:Collagen Type X; D012333:RNA, Messenger; D012176:Retinoids; D016212:Transforming Growth Factor beta; D014212:Tretinoin