Increased expression of a novel osteoclast-stimulating factor, ADAM8, in interface tissue around loosened hip prostheses.

OBJECTIVE: ADAM8 is a protein of a disintegrin and a metalloproteinase family that can induce osteoclast fusion and activity, perhaps via interactions involving integrin receptors and their cysteine-rich/disintegrin domains. Because loosening of hip replacement implants is characterized by foreign body giant cells and peri-implant osteoclasts, it was speculated that this molecule might be (over)expressed in the synovial membrane-like interface tissues.

METHODS: In situ hybridization; immunohistochemical staining with or without tartrate-resistant acid phosphatase (TRAP) staining; image analysis/morphometry; isolation, amplification, and cloning of ADAM8; nucleotide sequencing; quantitative reverse transcriptase-polymerase chain reaction (RT-PCR); and Western blot.

RESULTS: In situ hybridization disclosed ADAM8 mRNA in mono- and multinuclear cells in both interface and control synovial samples. Quantitative RT-PCR revealed high ADAM8 mRNA copy numbers in interface tissue (p < 0.01). Accordingly, extensive ADAM8 immunoreactivity was observed in the lining-like layers and sublining areas of interface tissue (p < 0.001). A 65 kDa ADAM8 band in Western blot of tissue extracts confirmed these findings. ADAM8/TRAP double staining showed close spatial relationships of ADAM8 positive precursor cells with other precursors and/or TRAP-positive multinuclear cells.

CONCLUSION: ADAM8 is (over)expressed in tissues around aseptically loosened total hip implants, which are characterized by chronic foreign body inflammation and peri-implant bone loss. This is compatible with a role for ADAM8 in the formation of foreign body giant cells and osteoclasts.