Porphyromonas gingivalis lipids enhance RANKL-mediated osteoclast formation

Periodontitis is associated with microbial lipid accumulation in gingival tissues at sites where bone destruction occurs. The primary lipid analytes identified in diseased periodontal tissues are known to be produced by Porphyromonas gingivalis (Pg), a pathogen strongly associated with periodontitis. These compounds accumulate in tissues despite clearance by gingival fibroblasts and macrophages in specific contexts. This investigation quantified lipid-dependent modulation of RANKL-mediated osteoclast formation in bone marrow macrophages and RAW 264.7 cells. The effects of Pg lipids were tested in both repeat-exposure studies, i.e. lipid priming prior to RANKL administration, and concomitant treatment with RANKL. For these studies, we used RANKL concentrations that mirror concentrations measured at disease sites. Osteoclast formation was evaluated by quantifying TRAP-positive cells. Our findings show that RANKL-induced osteoclast formation from bone marrow macrophages and RAW cells is enhanced, particularly in cells primed with lipids prior to RANKL administration. These results show a 2- to 10-fold increased in numbers of osteoclasts formed in cultures pre-exposed to Pg lipids with significance levels often at p < 0.0001. Enhanced osteoclast formation occurs despite loss of TNF-α secretory responses after repeated lipid exposure. These results indicate that RANKL at levels observed in periodontitis gingival tissues will promote osteoclast formation when bacterial lipids are also present. This suggests that lipid deposition could be used to identify at-risk sites for bone destruction in periodontitis. Collectively, our data support the notion that osteoclast formation is enhanced by the site-specific deposition of Pg lipids which contribute to localized bone loss around teeth in human periodontitis.