Distinct ex vivo biomarker profiles of calcified and non-calcified acetabular labrum tissues in primary hip OA

Abstract

Purpose: Hip OA has a prevalence of 7.4% in individuals older than 60 years and involves structural degeneration of several joint compartments, including femoral head cartilage, periarticular ligaments and the acetabular labrum. The latter is a ring-form fibrocartilage tissue that plays a fundamental role in hip joint stability and articular cartilage homeostasis. Degeneration of the labrum involves chondrocyte apoptosis, macrophage infiltration and calcification and is thought to promote progression of primary hip OA. Moreover, labrum is a vascularized and innervated tissue and therefore strongly implied in hip OA pain. While several histological studies have documented morpological tissue changes, biochemical changes have not been extensively reported. In this study, we characterized biomarker secretion of explanted human labrum tissues under basal and inflammatory conditions. Methods: Intact labrum specimens were obtained from patients undergoing total hip arthroplasty (n=11, mean age 65.5, 3 women) and dissected in equal-sized samples (200-400 mg wet weight). Labrum tissues were subjected to explant culture for one week in the presence or absence of an inflammatory stimulus (1 ug/mL LPS). To evaluate the role of TGF-beta signaling in labrum tissue metabolism, samples were treated with 10 uM SB-505124 (TGF-beta type I receptor inhibitor) in the presence or absence of LPS. Secretion of Aggrecan (ACAN), Cartilage Oligomeric Matrix Protein (COMP), MMP13, Pro-Collagen-Ia and IL-6 was assessed by ELISA. Tissue proteoglycans and calcification were evaluated by whole mount Alcian blue/Alizarin red staining followed by ethyl cinnamate-based optical clearing and fluorescence microscopy. Results: Whole mount staining for proteoglycans and calcium revealed different degrees of calcification of labrum tissues (Figure). Five samples contained numerous calcified nodules and one sample was completely ossified. Non-calcified specimens displayed strong proteoglycan tissue staining interspersed with fibrous tissue. Labrum tissues secreted low levels of IL-6 and ACAN (5-50 ng/gr tissue), moderate levels of MMP13 and COMP (100-300 ng/gr tissue) and high levels of Pro-Collagen-Ia (∼900 ng/gr tissue). Subgroup analysis revealed significantly higher Pro-Collagen-Ia (1170+230 vs. 596+305) and MMP13 (157+22 vs. 60+15) secretion by calcified labrum specimens. Inflammatory stimulation led to a ∼10-fold upregulation of IL-6 and ∼2-fold dowregulation of Pro-Collagen-Ia secretion, in both non-calcified and calcified specimens. Neither basal expression, nor effects of LPS stimulation were mediated through TGF-beta signaling. Conclusions: Biomarker profiling of degenerative acetabular labrum tissues revealed distinct secretion patterns in non-decalcified and calcified specimens, displaying elevated Pro-Collagen-Ia and MMP13 secretion by the latter. In contrast to articular cartilage, regulation of IL-6 and Pro-Collagen-Ia was not TGF-beta, but rather TLR4-dependent and calcification appeared IL-6-independent. Whether labrum fibrochondrytes or infiltrated macrophages are crucially involved in these effects remains to be elucidated. Due to its close proximity to articular cartilage, secreted mediators from degenerated labrum might contribute to or even accelerate the development of hip OA.