Mononuclear phagocytes secrete a protein that directly resorbs devitalized bone particles

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Bone resorption is a complex process.
• Mononuclear phagocytes are implicated in bone remodeling.
• Understanding the molecular mechanisms of bone resorption is crucial for treating bone diseases.

Purpose of the Study:

• To investigate the role of human peripheral blood monocytes and U937 cells in bone resorption using an in vitro model.
• To characterize the factors released by these cells that mediate bone resorption.
• To explore the potential mechanisms underlying bone loss in inflammatory processes.

Main Methods:

• Utilized a radiolabeled bone particle assay as an in vitro model of bone resorption.
• Assessed 45Ca release from devitalized bone particles induced by monocytes and U937 cells.
• Investigated the effects of gamma interferon (INF-gamma), 1,25(OH)2D3, and lymphokines on cell activity and bone resorption.
• Analyzed the properties of the bone-resorbing activity in conditioned medium, including heat, pH, and enzymatic treatments.

Main Results:

• Monocytes and U937 cells significantly increased 45Ca release, indicating bone resorption.
• Neutrophils did not induce significant 45Ca release.
• INF-gamma inhibited 45Ca release from monocytes while stimulating hydrogen peroxide secretion.
• U937 cells, stimulated with 1,25(OH)2D3 and lymphokines, showed increased oxygen reduction products and enhanced 45Ca release.
• The bone-resorbing activity in conditioned medium was heat-stable, acid-stable, and partially sensitive to proteolysis, suggesting a proteinaceous nature.

Conclusions:

• Mononuclear phagocytes secrete a substance that directly mediates bone resorption.
• This substance is likely a protein, contributing to bone loss in inflammatory conditions.
• Further isolation and identification of this factor could elucidate mechanisms of inflammatory bone loss.