Smad2 Overexpression Induces Alveolar Bone Loss Through TNF-a

Objectives: The aim of the current study was to investigate whether Smad2 overexpression in junctional epithelial (JE) cells induces alveolar bone loss.
Methods: The Cytokeratin 14 (K14) promoter was used to overexpress Smad2 in epithelial cells of transgenic mice (K14-Smad2). Tissue sections from Hemi maxilla samples were stained with Van Gieson’s and Ponceau S solutions and then photographed. Micro CT radiographs were used to assess bone loss, volume and density. The expression of TNFa, IL1-b, IFg, RANKL, and OPG was analyzed by RT-PCR. Western blots were used to detect the protein levels of TNFa, and IL1-b. Immunohistochemistry was done for Tartrate-resistant acid phosphatase (TRAP) as a marker for osteoclasts. Wild type (WT) mice were used as controls.
Results: Smad2 overexpression was associated with alveolar bone loss at the 12-month time point in K14-Smad2 mice. Micro CT results showed that K14-Smad2 mice had 52.5% (± 4.2) root exposure compared to 32.4%(± 3.2) in controls. There was a significant difference in bone volume in the K14-Smad2 mice compared to controls, 2.65mm^3 (± 0.3) and 4.3mm^3 (± 0.35) respectively. K14-Smad2 mice had lower bone density when compared to controls at 12 months of age, 845.9 mg/cc (± 10) and 696.8 mg/cc (± 70), respectively. The mRNA levels of TNFa and RANKL increased by 3.26 and 2.5 fold respectively in K14-Smad2 mice when compared to controls. The protein level of TNF-a increased significantly by 2.8 fold in K14-Smad2 mice when compared to WT mice. Finally, Smad2 overexpression increased the total numbers of osteoclasts in K14-Smad2 mice by 3.4 (± 02) fold when compared to WT mice.
Conclusions: Smad2 overexpression increased the osteoclastgenesis through the up regulation of TNF-a and RANKL to induce alveolar bone loss