Tendon degeneration is not mediated by regulation of Toll‐like receptors 2 and 4 in human tenocytes

Abstract

We hypothesized that expression of Toll-like receptors (TLRs) 2 and 4 by tenocytes is involved in the catabolic processes of tendon degeneration. We investigated TLR2 and TLR4 expression by tenocytes in healthy and tendinotic Achilles tendons. We also investigated whether TLR2 and TLR4 could be upregulated in tendon explants using proinflammatory cytokines interleukin (IL)-1b and tumor necrosis factor alphpa (TNFa). Peroperatively harvested healthy (n ¼ 5) and tendinotic (n ¼ 13) Achilles tendon samples were examined by real-time RT-PCR and immunohistochemical staining for TLR2 and TLR4. In addition, the catabolic process in tendinopathy was analyzed by real-time RT-PCR for matrix metalloproteinases MMP1, MMP3, MMP9, and MMP13. Furthermore, healthy tendon explants were cultured in the presence of 20 ng/ml IL-1b (n ¼ 10) or 10 ng/mL TNFa (n ¼ 8) for 4, 24, 48, and 72 h before analysis of TLR and MMP expression levels. Although mRNA levels for both TLR2 and TLR4 were detected in healthy and tendinotic Achilles tendons, we could not confirm expression of these receptors by immunohistochemical staining in either healthy or tendinotic tendon samples. Both receptors did not show significant transcriptional regulation in tendinopathy, although MMP3 was downregulated and MMP9 was upregulated in tendinopathy. In tendon explant cultures TLR2 mRNA was upregulated by TNFa (p < 0.05) and IL-1b (not significant). TLR4 gene expression was not altered by addition of IL-1b or TNFa. Tendon tissue can be stimulated to increase TLR2 gene expression by addition of catabolic factors TNFa or IL-1b. However, the catabolic processes in Achilles tendinopathy cannot be attributed to regulation of TLR2 and TLR4 by tenocytes. Toll-like receptors (TLRs) are phylogenetically conserved transmembrane receptors belonging to the family of pattern-recognition receptors (PRR). In humans, 11 TLR homologs have been identified thus far, expressed by numerous cell types like monocytes and dendritic cells, 1 and for example by chondrocytes. 2 TLRs detect microbial components, but TLR2 and TLR4 also recognize endogenous ligands like heat-shock proteins, necrotic cells, hyaluronan, and fibronectin. 1 Ligand recognition elicits an immune response with upregulation and activation of proinflammatory cytokines and matrix-metalloproteinases (MMPs). 2 Hence, via endogenous ligand recognition the TLR-mediated catabolic signalling pathways can directly contribute to degradative tissue reactions even in the absence of microbial components. TLRs are implicated in inflammatory and autoimmune diseases, but also in disorders like atherosclerosis and cancer. 3,4 Furthermore they play a role in noninfected rheumatoid arthritis (RA) and in the primarily degenerative joint disease osteoarthritis (OA). [1][2][3]5,6 In degenerative tendinopathies the balance between matrix synthesis and degradation is disturbed, and increased MMP activity as well as an increased percentage of degraded collagen are found. 7-9 An increase in NO synthase expression was found in a sup...

Hyperuricemic PRP in Tendon Cells

“…In particular, our experiments show that tendon cells sense uric acid but whether it could occur via TLR2 as in chondrocytes is unexplored [25]. Actually, tenocytes express the main receptors involved in sterile inflammation, TLR2 and TLR4, [26], but it is not clear in what circumstances these receptors are functional. Uric acid can also enter the cell via specific transporters where it can modulate inflammatory and oxidative events.…”

Evidence of the TNF-α System in the Human Achilles Tendon: Expression of TNF-α and TNF Receptor at both Protein and mRNA Levels in the Tenocytes

“…Concerning the human Achilles tendon, which is a tendon that is very frequently affected by tendinopathy, especially in the midportion, in principle nothing is known. The only aspect that is known for this tendon is the finding described above concerning the effects of TNF-␣ stimulation on cultured tendon explants, these explants being from Achilles tendon [de Mos et al, 2009]. It is not known…”

“…Cultured avian tendon cells exposed to TNF-␣ , an increased temperature, and mechanical stress led to a reduction of procollagen mRNA expression [Pan and Halper, 2003]. Cultured human tenocytes treated with TNF-␣ reduced their type 1 collagen deposition and highly upregulated their expression of pro-and immunoregulatory cytokines [John et al, 2010], and culturing tendon explants in the presence of TNF-␣ led to upregulation of the expression of Toll-like receptor 2 mRNA [de Mos et al, 2009].…”

Gene Variants that Predispose to Achilles Tendon Injuries: An Update on Recent Advances

“…In addition to COL5A1, three variants within the MMP3 gene were also found to associate with Achilles tendinopathy in South Africans ). Interestingly previous (Alfredson et al, 2003, Ireland et al, 2001 and more recent data (de Mos et al, 2009, Jelinsky et al, 2011 have shown that MMP3 expression levels are significantly repressed in tissue obtained from Achilles tendinopathic material when compared to controls.…”