title: Functional analyses reveal the essential role of SOX6 and RUNX2 in the communication of chondrocyte and osteoblast.
abstract: CONCLUSIONS
  This study provides novel evidence that sex determining region Y (SRY)-box (SOX6) and runt-related transcription factor 2 (RUNX2) play essential roles in the communication of chondrocyte and osteoblast. Our findings open a new avenue to the limited understanding of the coordination effect between chondrogenesis and osteogenesis.
  
  
  BACKGROUND
  Sox6 and Runx2 are two new susceptibility genes for osteoporosis identified by genome-wide association studies, but the functions of these genes in osteogenesis remain unclear. Both genes are essential transcription factors in chondrogenesis, which reminds us that SOX6 and RUNX2 might be involved in the coordination of chondrogenesis and osteogenesis. Therefore, this study aimed to investigate the functions of SOX6 and RUNX2 in the coupling regulation of chondrogenesis and osteogenesis.
  
  
  METHODS
  We established a chondrogenic differentiation model of ATDC5 cell and profiled the expression of SOX6 and RUNX2 during chondroblast differentiation. We co-cultured osteoblast cells with ATDC5 cells in different differentiation stages and examined the proliferation, cell cycle progression, apoptosis, and differentiation of osteoblast cells. SOX6 or RUNX2 was knocked down using specific siRNA and the effect was examined.
  
  
  RESULTS
  During chondrogenic differentiation, SOX6 and RUNX2 expressed sequentially in the proliferating and hypertrophic stages. Proliferative ATDC5 cells stimulated the multiplication of osteoblasts and promoted more osteoblasts to enter S-phase. Hypertrophic ATDC5 cells enhanced the differentiation of osteoblasts. Yet, the apoptosis of osteoblasts was neither affected by proliferating nor hypertrophic ATDC5 cells. Knockdown of SOX6 in proliferating ATDC5 cells significantly repressed the stimulation of osteoblast multiplication, whereas depletion of RUNX2 in hypertrophic ATDC5 cells retarded the expression of osteoblastic markers.
  
  
  CONCLUSIONS
  Our study first reveals that SOX6 and RUNX2 play important roles in the chondrogenesis-osteogenesis coordination. This finding enriches the limited understanding about this coordination and unravels the novel function of SOX6 and RUNX2 in the endochondral ossification.
authors.affiliation: Center for Translational Medicine, The First Affiliated Hospital of Xi'an Jiaotong University College of Medicine, The Key Laboratory of Biomedical Information Engineering of Ministry of Education, Xi'an Jiaotong University, Xi'an, People's Republic of China.
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: T-L
authors.initials: TL
authors.lastname: Yang
authors.forename: X
authors.initials: X
authors.lastname: Li
authors.forename: Y
authors.initials: Y
authors.lastname: Guo
mesh_terms: D002453:Cell Cycle; D019902:Chondrocytes / Q000378:metabolism*; D020219:Chondrogenesis / Q000502:physiology*; D005434:Flow Cytometry; D006801:Humans; D010006:Osteoblasts / Q000378:metabolism*; D010012:Osteogenesis / Q000502:physiology*; D016133:Polymerase Chain Reaction / Q000379:methods; D034741:RNA, Small Interfering; D055755:SOXD Transcription Factors / Q000378:metabolism*
chemical_list: D034741:RNA, Small Interfering; D055755:SOXD Transcription Factors