The expression of some IL-6 family members was found to be up-regulated in pathological tendon. However, contrary to our expectation, painful AT and PTT showed different expression patterns, indicating a substantial difference between those two tendinopathies. The up-regulation of inflammatory markers in ruptured AT might point towards a role of inflammation in rupture healing. However, inflammatory markers were also up-regulated in painful AT, suggesting a potential role of inflammation in Achilles tendinopathy also. In contrast, the increase in expression of IL-6 and COX2 found in the painful AT does not occur in the painful PTT. Such a finding indicates that the inflammatory response may not simply be a cell response to degeneration or inflammation, but could also be influenced by differences in gross mechanical loading of the AT and PTT. The mechanical properties of the AT have been shown to be altered by tendinopathy, leading to lower tendon stiffness and higher in vivo
strains during isometric plantar flexion compared with healthy AT [10
]. Previous studies have demonstrated that the concentration of IL-6 and COX2 rises with increased mechanical strain [3
]. Hence the higher strains perceived by cells in tendinopathic AT may be the cause of the up-regulation of IL-6 and COX2 seen in the painful AT in our study. In contrast, the PTT appears to lengthen permanently with tendinopathy, resulting in a loss of support of the foot arch (A. Robinson, consultant orthopaedic surgeon, Addenbrooke's Hospital, Cambridge, personal observation). This elongation likely reduces the mechanical strain on the cells during loading and may explain the lack of inflammatory response in this tendon. The down-regulation of IL-6R in painful and ruptured AT could be a response to high levels of IL-6.
The expression of the IL-6 family member's OSM and LIF was increased in ruptured compared with normal and painful AT. It is unknown whether these changes preceded the rupture or were a result of the rupture and following healing process. Both OSM and LIF have been shown to stimulate release and suppress synthesis of proteoglycans in articular cartilage [11
Similarly CNTF, which was up-regulated in painful PTT, has also been shown to inhibit proteoglycan synthesis, although without affecting proteoglycan release [12
]. One might assume that OSM, LIF and CNTF might function in a similar way in tendon, thereby decreasing proteoglycan content. However, this seems surprising, since GAG content and mRNA expression of the proteoglycans aggrecan and biglycan has been shown to increase in AT tendinopathy [13
] while the expression of decorin either increased [14
] or decreased [13
] in AT rupture.
Collagen metabolism has also been shown to be influenced by IL-6 and OSM. Collagen degradation is stimulated in tendon tissue treated with OSM in combination with IL-1a [15
] and in cartilage tissue treated with the combination of IL-1a and either IL-6 (in the presence of its soluble receptor) or OSM [16
]. However, IL-6 can also stimulate collagen synthesis, as IL-6 injections led to an increased concentration of a procollagen marker in the peritendinous space around the human AT [17
]. An increased COL1A1 expression is a common response to tendinopathy [9
], and one we confirm in this study, showing an increased expression in both painful AT and PTT. Such a response seems to indicate an attempt of the tendon to restore normal tissue composition and function.
An increase in expression of the angiogenic factor VEGF has also been described as a common sign of pathological tendon, as it is critical for neovascularization that usually does not occur in healthy adult tendon [19–21
]. However, VEGF expression is highly variable in tissue samples of painful tendinopathy [19
], suggesting that expression might depend on the stage of the disease. A study on patellar tendinopathy showed that the subset of patients demonstrating VEGF expression suffered from tendinopathy for a considerably shorter time period than the patients with no detectable VEGF [21
]. This indicates that VEGF might only be up-regulated early in the onset of the disease or the healing process, and might explain why we found VEGF to be up-regulated in ruptured AT but not in painful AT or PTT.
Limitations of the study and potential future directions
The extent to which the altered gene expression of IL-6 family members in painful and ruptured tendon is translated into protein remains to be established. Immunohistochemistry could determine where in the tissue those proteins are expressed, focusing particularly on vascular expression, as blood vessels have been shown to be a major source of IL-6 expression in ruptured tendon [7
] and neovascularization has been linked to tendinopathy. Also, the response of the tissue could be affected by gender or age, e.g. mRNA expression levels tend to decrease with age (see supplementary data
available at Rheumatology