Signal-Regulated Protein Kinases/Protein Kinase B-p53-BH3-Interacting Domain Death Agonist Pathway Regulates Gingipain-Induced Apoptosis in Osteoblasts

Background: Gingipains are cysteine proteases produced by Porphyromonas gingivalis, the predominant pathogen in chronic periodontitis. The present study aims to examine the role of gingipains in promoting apoptosis in osteoblasts.

Methods: Human calvarial osteoblasts and osteoblast MC3T3-E1 cells were treated with 8.348 U/L gingipains. Flow cytometry analysis and terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick end labeling staining were used to detect cell apoptosis. Protein expression was examined by Western blotting, and gene expression was detected by real-time polymerase chain reaction. Small interfering (si)RNA transfection was used to knock down BH3-interacting domain death agonist (Bid) expression.

Results: Treatment with 8.348 U/L gingipains from 4 to 72 hours increased apoptosis, accompanied by elevated cleaved caspase-3 levels. Notably, gingipain-induced apoptosis was associated with increase of Bid and its truncated form, tBid, as well as p53. Transfection with Bid siRNA resulted in suppression of gingipain-induced apoptosis. The p53 inhibitor, Pifithrin-α, blocked the gingipain-induced Bid. The ability of gingipains to stimulate p53 and Bid expression was mimicked by PD-0325901 and MK-2206, the specific extracellular signal-regulated protein kinases (ERK) and protein kinase B (PKB) inhibitors, respectively. Furthermore, treatment with gingipains reduced phospho-ERK and phospho-PKB levels, an effect correlated to gingipain-induced increase in p53 and tBid expression.

Conclusion: The present findings suggest that Bid plays an essential role in gingipain-induced osteoblast apoptosis, which is dependent on inhibition of ERK and PKB phosphorylation, followed by the activation of p53.