AGE and Porphyromonas gingivalis-LPS Increase Sclerostin Expression in Osteocytes

Objectives: Diabetes patients are known to have a tendency to suffer from periodontitis, and it is called DM-associated periodontitis (DM-Perio) which shows a severe inflammation and bone destruction in the periodontal tissue. Osteocytes are the abundant cells in bone, and sclerostin (SOST) is mainly expressed by osteocytes and inhibits osteoblast function and bone formation. It is reported that the serum SOST levels in DM patients are higher than in healthy subjects. Advanced glycation end-products (AGE) accumulate in various tissues under diabetic conditions, but the role of AGE in DM-Perio has not been fully clarified. In the present study, to reveal the mechanisms of the severity of DM-Perio, we investigated the effects of AGE and lipopolysaccharide of P.gingivalis (P-LPS) on SOST expression in osteocytes.
Methods: Mouse osteocyte cell line, MLO-Y4-A2 cells, were cultured in a-MEM containing 10% fatal bovine serum in the presence of AGE (100 mg/ml) and/or P-LPS (250 ng/ml). After the evaluation of cell viability, quantitative real-time PCR were performed to determine the expressions of SOST and receptor for AGE (RAGE). Protein level of SOST was measured using ELISA. Additionally, we examined the effects of AGE and P-LPS on signaling pathways of SOST expression using MAPK inhibitors (SB203580, U0126, SP600125) and NF-kB inhibitor (BAY11-7082).
Results: The addition of AGE significantly increased SOST and RAGE expressions in osteocytes. The combination of AGE and P-LPS enhanced the expressions of both SOST and RAGE. AGE-induced SOST expressions were significantly inhibited by ERK, JNK and NF-kB inhibitors. In contrast, P-LPS-induced SOST expressions were significantly inhibited by p38, JNK and NF-kB inhibitors.
Conclusions: AGE and P-LPS up-regulated SOST expression via the pathways of MAPK and NF-kB in osteocytes. Our results suggest that the accumulations of both AGE and P-LPS in the periodontal tissues may lead to aggravation of DM-Perio through SOST expression.