The insulin‐like growth factor binding proteins in uncultured human cartilage: Increases in insulin‐like growth factor binding protein 3 during osteoarthritis

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Abstract
Objective To assess changes in the insulin‐like growth factor binding proteins (IGFBPs) in uncultured cartilage during stages of osteoarthritis (OA), and to determine if OA cartilage is capable of autocrine secretion of IGFBPs. Methods Articular cartilage was dissected from fibrillated and nonfibrillated sites of 11 human femoral heads, and extracted in buffer containing 8M urea. IGFBPs were identified by immunoprecipitation and subsequent analysis by 125I–IGF‐2 Western ligand blotting (WLB), radioimmunoassay, or 2‐site immunoradiometric assay (IRMA). IGFBPs were assessed in cartilage extracts by WLB. IGFBP‐3 content was determined by IRMA and synthesis by metabolic labeling with 35S‐cysteine in organ cultures. Results Sample grouping into 3 distinct OA strata was supported by gross pathology of the femoral heads, histologic grading of cartilage slices, and biochemical analysis of the glycosaminoglycan and protein content of the extracts. Group I was normal/mild OA, group II was intermediate OA, and group III was severe OA. IGFBP‐2 was present in all samples, IGFBP‐4 in sporadic samples, and BP‐3 in group II–III samples. By IRMA, group I had a mean ± SD of 6.26 ± 2.6 ng IGFBP‐3/mg soluble protein (IGFBP‐3) (n = 6), group II had a mean ± SD 14 ± 7.5 IGFBP‐3 (n = 10), and group III had a mean ± SD 17.03 ± 8.94 IGFBP‐3 (n = 6). Analysis of variance showed group differences (F[3,19] = 3.84, P = 0.04), and post hoc tests revealed that IGFBP‐3 levels were higher for group III versus group I (P = 0.04). OA cartilage synthesized IGFBP‐3. Conclusion Increases in net cartilage content of IGFBP‐3 occurred in intact OA cartilage, reaching statistically significant elevation in severe disease. There was autocrine IGFBP‐3 production in OA cartilage.