Runx2 expression and action in chondrocytes are regulated by retinoid signaling and parathyroid hormone-related peptide (PTHrP).
OBJECTIVE
Runx2 (also known as Cbfa1) is a transcription factor required for chondrocyte maturation and osteoblast differentiation. While there is information on the regulation of its expression during osteogenesis, much less is known about it during cartilage maturation. Here we asked whether Runx2 expression and function are affected by retinoic acid (RA) and parathyroid hormone-related peptide (PTHrP), which represent an important stimulator and inhibitor of chondrocyte maturation, respectively.


METHODS
We first cloned and characterized Runx2 expressed by chick chondrocytes (cRunx2). We then constructed expression vectors of cRunx2 and a dominant-negative form (DN-cRunx2) and determined their effects on chondrocyte maturation in culture before and during retinoid and PTHrP treatment.


RESULTS
cRunx2 showed similar transactivation activity to that of its mammalian counterparts although it has a very short QA domain and lacks a small portion of the PST domain. cRunx2 over-expression stimulated chondrocyte maturation, as indicated by increases in alkaline phosphatase activity (APase), mineralization, and type X collagen and MMP-13 expression, and by maintenance of Indian hedgehog (Ihh) expression. RA treatment stimulated cRunx2 gene expression and boosted its pro-maturation effects. PTHrP treatment blocked Runx2 expression and its pro-maturation effects. Over-expression of DN-cRunx2 inhibited maturation and even prevented RA from exerting its pro-maturation role.


CONCLUSIONS
As previously indicated by mammalian studies, cRunx2 has chondrocyte pro-maturation activity. Its expression and roles are favorably modulated by retinoid signaling but are completely inhibited by PTHrP. A model integrating cRunx2 with PTHrP, Ihh and retinoid signaling and operating during skeletogenesis is proposed.
Iwamoto
M
M
Department of Oral Anatomy & Developmental Biology, Osaka University Faculty of Dentistry, 1-8 Yamadaoka, Suita, Osaka 565-0871, Japan. masahiro.iwamoto@mail.tju.edu
Kitagaki
J
J
Tamamura
Y
Y
Gentili
C
C
Koyama
E
E
Enomoto
H
H
Komori
T
T
Pacifici
M
M
Enomoto-Iwamoto
M
M
D000818:Animals; D015152:Blotting, Northern; D002356:Cartilage / Q000166:cytology / Q000378:metabolism; D002478:Cells, Cultured; D002642:Chick Embryo; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism*; D050659:Core Binding Factor Alpha 1 Subunit; D017403:In Situ Hybridization; D007641:Keratolytic Agents / Q000494:pharmacology*; D009363:Neoplasm Proteins; D044162:Parathyroid Hormone-Related Protein; D036361:Peptide Hormones / Q000494:pharmacology*; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D020539:Sequence Analysis, Protein; D014157:Transcription Factors / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D014212:Tretinoin / Q000494:pharmacology*
D050659:Core Binding Factor Alpha 1 Subunit; D007641:Keratolytic Agents; D009363:Neoplasm Proteins; D044162:Parathyroid Hormone-Related Protein; D036361:Peptide Hormones; D014157:Transcription Factors; D014212:Tretinoin