TGF-β1 as possible link between loss of bone mineral density and chronic inflammation.
BACKGROUND
The TGF family plays a key role in bone homeostasis. Systemic or topic application of proteins of this family apparently positively affects bone healing in vivo. However, patients with chronic inflammation, having increased TGF-β(1) serum-levels, often show reduced bone mineral content and disturbed bone healing. Therefore, we wanted to identify intracellular mechanisms induced by chronic presence of TGF-β(1) and their possible role in bone homeostasis in primary human osteoblasts.


RESULTS
Osteoblasts were isolated from femur heads of patients undergoing total hip replacement. Adenoviral reporter assays showed that in primary human osteoblasts TGF-β(1) mediates its signal via Smad2/3 and not Smad1/5/8. It induces proliferation as an intermediate response but decreases AP-activity and inorganic matrix production as a late response. In addition, expression levels of osteoblastic markers were strongly regulated (AP↓; Osteocalcin↓; Osteopontin↑; MGP↓; BMP 2↓; BSP2↓; OSF2↓; Osteoprotegerin↓; RANKL↑) towards an osteoclast recruiting phenotype. All effects were blocked by inhibition of Smad2/3 signaling with the Alk5-Inhibitor (SB431542). Interestingly, a rescue experiment showed that reduced AP-activities did not recover to base line levels, even 8 days after stopping the TGF-β(1) application.


CONCLUSIONS
In spite of the initial positive effects on cell proliferation, it is questionable if continuous Smad2/3 phosphorylation is beneficial for bone healing, because decreased AP-activity and BMP2 levels indicate a loss of function of the osteoblasts. Thus, inhibition of Smad2/3 phosphorylation might positively influence functional activity of osteoblasts in patients with chronically elevated TGF-β(1) levels and thus, could lead to an improved bone healing in vivo.
Department of Traumatology, MRI, Technische Universität München, München, Germany. ehnert@uchir.me.tum.de
Sabrina
S
Ehnert
Johannes
J
Baur
Andreas
A
Schmitt
Markus
M
Neumaier
Martin
M
Lucke
Steven
S
Dooley
Helen
H
Vester
Britt
B
Wildemann
Ulrich
U
Stöckle
Andreas K
AK
Nussler
D000368:Aged; D000469:Alkaline Phosphatase / Q000235:genetics / Q000378:metabolism; D001549:Benzamides / Q000494:pharmacology; D015519:Bone Density / Q000502:physiology; D055396:Bone Morphogenetic Protein 2 / Q000235:genetics / Q000378:metabolism; D055419:Bone Morphogenetic Protein 7 / Q000235:genetics / Q000378:metabolism; D001862:Bone Resorption / Q000503:physiopathology; D049109:Cell Proliferation / Q000187:drug effects*; D002478:Cells, Cultured; D002908:Chronic Disease; D004149:Dioxoles / Q000494:pharmacology; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D007249:Inflammation / Q000097:blood / Q000473:pathology; D008297:Male; D010006:Osteoblasts / Q000187:drug effects* / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D053245:RANK Ligand / Q000235:genetics / Q000378:metabolism; D000077293:Receptor, Transforming Growth Factor-beta Type I; D018125:Receptors, Transforming Growth Factor beta / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects; D051785:Smad Proteins / Q000235:genetics / Q000378:metabolism; D013997:Time Factors; D053773:Transforming Growth Factor beta1 / Q000097:blood / Q000494:pharmacology* / Q000502:physiology
C459179:4-(5-benzo(1,3)dioxol-5-yl-4-pyridin-2-yl-1H-imidazol-2-yl)benzamide; C525718:BMP2 protein, human; C525819:BMP7 protein, human; D001549:Benzamides; D055396:Bone Morphogenetic Protein 2; D055419:Bone Morphogenetic Protein 7; D004149:Dioxoles; D053245:RANK Ligand; D018125:Receptors, Transforming Growth Factor beta; D051785:Smad Proteins; D053773:Transforming Growth Factor beta1; D017346:Protein Serine-Threonine Kinases; D000077293:Receptor, Transforming Growth Factor-beta Type I; C000625841:TGFBR1 protein, human; D000469:Alkaline Phosphatase