Identification of novel collagen breakdown products by human osteoclasts in vitro and in vivo

Bone resorption involves dissolution of mineral and enzymatic degradation of bone matrix. The primary enzyme is cathepsin K but other proteases including matrix metalloproteinases are involved. Some cathepsin K cleavage products have been partially identified, including crossed-linked telopeptides of type I collagen. Here we aimed to characterise the entire complement of bone breakdown products resulting from osteoclast action under controlled conditions in vitro. We analysed extracellular media from human osteoclasts cultured on dentine substrates, using untargeted liquid chromatography mass spectrometry. We discovered 22 breakdown products resulting from osteoclastic action. These products were peptide fragment sequences that mapped to various collagen proteins present in bone and dentine. Nine peptide fragments mapped exclusively to collagen I alpha-1 chain (COL1A1), the most abundant protein in bone. We subsequently detected 21 of the fragment products, initially observed in vitro, in human serum and/or urine. Consistent positive correlations were observed between the COL1A1-specific peptide fragments and established bone biochemical markers in serum and urine. Ten urine fragments and two serum fragments markedly increased (p <0.05) following total hip arthroplasty, capturing the transient local peri-prosthetic osteolysis observed in these patients (serum, n=86 patients; urine, n=83 patients). Among these candidate osteolytic markers, four (two COL1A1-specific products) showed decreases from baseline (p <0.05) in patients on denosumab (n=10 patients). Additionally, two fragment peptides were higher (p <0.05, fold change >2) in urine from patients with bone metastasis (29/112) among a lung cancer cohort. The range of collagen peptide fragments we discovered as a direct result of osteoclast activity indicates a complexity of bone resorption pathways not previously known, extending beyond the known proteolytic cleavage events in bone collagen proteins. Monitoring biofluid concentrations of these novel bone markers has the potential to capture multiple pathways of bone resorption activity beyond the existing assays based on cathepsin K.