Porphyromonas gingivalis–Derived Virulence Lipids Accelerate Osteoclastogenesis Independently of High Mobility Group Box Protein‐1 Canonical Signaling

TL;DR

Virulence lipids from the periodontal bacterium Porphyromonas gingivalis promote bone loss by accelerating osteoclast formation, independent of HMGB1 signaling.

Contribution

The study reveals that P. gingivalis virulence lipids accelerate osteoclastogenesis via pathways independent of HMGB1, challenging prior assumptions.

Findings

PGDHC and LPS Pg accelerate osteoclastogenesis even in the absence of HMGB1.

PGDHC reduces interferon-inducible GBP gene expression, while LPS Pg increases MMP gene expression.

HMGB1 is crucial for RANKL-induced osteoclastogenesis in vitro.

Abstract

Periodontal bacterial pathogen‐associated molecular patterns (PAMPs) and damage‐associated molecular patterns (DAMPs) accelerate inflammatory osteoclastogenesis, resulting in alveolar bone loss. The core PAMP and DAMP prototype molecules are periodontal bacterium Porphyromonas gingivalis–derived virulence lipids, for example, phosphoglycerol dihydroceramide (PGDHC) and lipopolysaccharide (LPS Pg), and the host non‐histone alarmin high mobility group box protein‐1 (HMGB1), respectively. Although it was reported that extracellularly released HMGB1 is critical for the promotion of sepsis inflammation in response to non‐periodontal bacterial LPS, our understanding of the crosstalk between HMGB1 and P. gingivalis–derived virulence lipids remains limited. Therefore, we used Hmgb1fl/fl LysM‐Cre+ mice with ablated HMGB1 mRNA and littermate Hmgb1fl/fl LysM‐Cre− controls. We observed limited Hmgb1fl/fl LysM‐Cre+ osteoclastogenesis compared to Hmgb1fl/fl in response to RANKL in vitro. Furthermore, recombinant HMGB1 protein restored osteoclast formation in Hmgb1fl/fl LysM‐Cre+ cells, indicating the pivotal role of extracellular HMGB1 in osteoclastogenesis in vitro. Using bulk RNA‐sequencing, we identified the diminished osteoclastogenesis in Hmgb1fl/fl LysM‐Cre+ cells are linked to accelerated expression of canonical osteoclast‐suppressing interferon genes. We surprisingly detected that PGDHC and LPS Pg accelerate osteoclastogenesis in Hmgb1fl/fl LysM‐Cre+ cells in vitro. Using bulk RNA‐sequencing and real‐time PCR assays, we confirmed that PGDHC diminishes the expression patterns of different interferon‐inducible guanylate‐binding proteins (GBP 3, 4, 5, 9). At the same time, LPS Pg accelerates the expression of osteoclast‐promoting matrix metalloproteases (MMP 8 and 12) mRNAs. The results suggest that the RANKL‐primed osteoclastogenesis accelerated by P. gingivalis–derived virulence lipids is mediated by different MMP or GBP signaling pathways independently from canonical HMGB1 signaling.

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