Zoledronate enhances lipopolysaccharide-induced inflammation via toll-like receptor 4 upregulation

Bisphosphonates (BPs) are commonly used to treat osteoporosis and bone metastases, but their use is associated with bisphosphonate-related osteonecrosis of the jaw (BRONJ), a condition characterized by chronic inflammation and impaired healing. Since bacterial infection is a major risk factor for BRONJ, this study investigated whether zoledronate (Zol), a nitrogen-containing BP, modulates Toll-like receptor 4 (TLR4) expression and enhances lipopolysaccharide (LPS)-induced inflammation.

Zol or other BPs were injected into the ear pinnae of mice. Five days later, tissues were harvested and analyzed by flow cytometry and ELISA to assess TLR4 expression and cytokine production. TLR4 expression in immune cells and keratinocytes was evaluated, and inflammatory cytokine production (IL-1β, TNF-α) was measured after LPS stimulation.

TLR4 expression was significantly increased in keratinocytes and neutrophils after Zol administration, whereas it was decreased in macrophages due to an increased proportion of inflammatory macrophage subsets with intrinsically low TLR4 expression. Zol-treated neutrophils exhibited elevated IL-1β and TNF-α production following LPS stimulation. Co-administration of Zol and LPS markedly enhanced local inflammation compared to either agent alone.

Zoledronate upregulates TLR4 expression in neutrophils and keratinocytes and augments LPS-induced inflammation, suggesting a mechanism by which BPs exacerbate oral inflammatory responses. These findings provide new insight into the pathogenesis of BRONJ and suggest that targeting TLR4 signaling may be a potential strategy to mitigate BP-associated oral complications.