Growth hormone attenuates the transcriptional activity of Runx2 by facilitating its physical association with Stat3beta.
We document that GH controls osteoblast function by modulating the biological activity of the osteospecific transcription factor Runx2. Evidence is provided for a physical interaction between Runx2 and Stat3beta, which is enhanced by GH and downregulates the transcriptional properties of this key osteogenic regulator.


BACKGROUND
Growth hormone (GH) signals to bone either through insulin-like growth factor-1 or directly by influencing the function of osteoblasts, the bone-forming cells. This study aimed at exploring the molecular events that underlie the direct biological action of GH on osteoblastic cells, and specifically, the effects that it might exert on the function of the bone-specific transcriptional regulator Runx2.


METHODS
The GH-responsive human osteoblastic cell line Saos-2 was used as our experimental system. Western blot analyses were used to monitor the presence of several parameters known to be affected by GH in these cells (i.e., downregulation of GH receptor, induction of STATs, and extracellular signal-regulated kinase [ERK] mitogen-activated protein kinase [MAPK] pathways). Electrophoretic mobility shift assays were used to assess Runx2 and Stat3 binding activity on an osteoblast-specific element (OSE2) after GH treatment. A combination of yeast two-hybrid and co-immunoprecipitation assays were performed to test for the existence of a physical Runx2.Stat3beta association. Finally, co-transfection experiments were used to investigate the interplay of the two transcription factors on the activity of a p6OSE2-Luc promoter after GH stimulation.


RESULTS
We show that GH signaling through Stat3/ERK MAPK potentiates the DNA binding activity of Runx2 but, at the same time, restrains its transcriptional potential. Moreover, a novel physical interaction of Runx2 with transcription factor Stat3beta, which is enhanced by GH stimulation, was documented both in vitro and in vivo. Importantly, this interaction impairs the transcriptional activity of Runx2 without affecting its DNA binding capacity.


CONCLUSIONS
Our data provide the first evidence that GH modulates the transcriptional function of Runx2 in osteoblastic cells by promoting its inhibitory interaction with Stat3beta. Shedding light on such mechanisms will contribute to a better understanding of GH effects on skeletal homeostasis that may impact on decisions at the clinical level, especially in diseases affecting bone quantity and quality (e.g., osteoporosis).
Department of Biochemistry, School of Medicine, University of Patras, Patras, Greece.
Panos G
PG
Ziros
Tassos
T
Georgakopoulos
Ioannis
I
Habeos
Efthimia K
EK
Basdra
Athanasios G
AG
Papavassiliou
D015153:Blotting, Western; D001842:Bone and Bones / Q000378:metabolism; D002454:Cell Differentiation; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism; D050659:Core Binding Factor Alpha 1 Subunit; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D005982:Glutathione Transferase / Q000378:metabolism; D013006:Growth Hormone / Q000378:metabolism / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D047468:Immunoprecipitation; D007700:Kinetics; D008156:Luciferases / Q000378:metabolism; D020935:MAP Kinase Signaling System; D008894:Milk Proteins / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009363:Neoplasm Proteins / Q000378:metabolism*; D010006:Osteoblasts / Q000378:metabolism; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D050796:STAT3 Transcription Factor; D050799:STAT5 Transcription Factor; D015398:Signal Transduction; D013997:Time Factors; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic; D014162:Transfection; D020798:Two-Hybrid System Techniques; D014443:Tyrosine / Q000737:chemistry
D050659:Core Binding Factor Alpha 1 Subunit; D004268:DNA-Binding Proteins; D008894:Milk Proteins; D009363:Neoplasm Proteins; D050796:STAT3 Transcription Factor; C494086:STAT3 protein, human; D050799:STAT5 Transcription Factor; D015534:Trans-Activators; D014157:Transcription Factors; D014443:Tyrosine; D013006:Growth Hormone; D008156:Luciferases; D005982:Glutathione Transferase; D020928:Mitogen-Activated Protein Kinases