Lipopolysaccharide derived from Aggregatibacter actinomycetemcomitans inhibits differentiation of osteoblasts

Background and objectiveAlveolar bone defects in aggressive periodontitis are generally caused by Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) through osteoclast differentiation facilitated by the interaction between the host's immune system, stimulated by the bacterial outer components of A. actinomycetemcomitans, and osteoblasts. Recently, some reports on the direct effects of A. actinomycetemcomitans on osteoblast differentiation have been published. However, the mechanisms in detail have still remained unknown. We herein demonstrated that A. actinomycetemcomitans might inhibit differentiation of osteoblasts and identified a causative substance.Materials and methodsThe following culture supernatants of A. actinomycetemcomitans were added to mouse osteoblasts, MC3T3-E1: supernatants fractionated by the molecular weight, supernatant supplemented with proteinase K and then heated, and supernatant supplemented with polymyxin B. Subsequently, RNA was extracted to determine the expressions of bone differentiation markers, alkaline phosphatase (ALP), osteocalcin (OCN), and bone sialoprotein (BSP). Additionally, lipopolysaccharide (LPS) was added to MC3T3-E1 culture to measure the markers.ResultsA. actinomycetemcomitans culture supernatant, added to the MC3T3-E1, inhibited the expressions of bone differentiation markers at all concentrations. Only the fraction with a molecular weight of >100 kDa inhibited the bone differentiation. Neither proteinase K nor heating had an effect. However, polymyxin B completely abrogated the differentiation inhibitory activity. A. actinomycetemcomitans LPS concentration dependently inhibited the expressions of the bone differentiation markers.ConclusionsA. actinomycetemcomitans LPS suppressed osteoblast differentiation. This suggests that suppressed bone differentiation is involved in the alveolar bone destruction.