Rotator cuff disease is common and can be caused by trauma as well as degenerative and auto-immune conditions.1 The severity of tendon disease, the natural history, and the prognosis associated with each of these factors may differ widely. However, inflammation plays a central role in the pathophysiology regardless of the underlying diagnosis.
Inflammation is portrayed in the literature as a double edge sword. It plays a crucial role in tissue healing and host protection against foreign antigens.2 However if the inflammation occurs in an unregulated fashion, secondary effects can occur that may result in irreversible tissue damage and other undesirable consequences.3 The intricate balance between these processes is coordinated by the complex interplay of various signaling pathways that are induced by inflammatory cytokines. Therefore, one of the objectives to effectively treat and alter the natural history of rotator cuff tendinopathy is to identify and target key cytokines in order to drive the inflammatory process towards tissue healing. Clinicians and scientists have made considerable efforts to characterise the profile of inflammatory cytokines produced in different disease processes as well as to map their signaling pathways. This approach has led to significant advancements in the treatment of inflammatory diseases, such as rheumatoid arthritis, through the development of biological modulators like Anakinra (Kineret) and Etanercept (Enbrel). These two pharmacological agents act by inhibiting the effects of IL-1 and TNFα respectively.4 Despite the positive impact these agents have on controlling the disease process, they can greatly interfere with tissue healing and are often withheld prior to major surgical procedures.5 These differences in clinical effects highlight the diverse roles inflammatory mediators, such as IL-1 and TNFα, can play in various clinical scenarios.
A key mechanism by which inflammatory cytokines influence tissue healing is through regulation of programmed cell death, or cellular apoptosis. Through a comparative microarray analysis between animal and human models, this research paper identified an important correlation between significant changes in gene expression of four important inflammatory cytokines (IL-6, IL-15, IL-18 and macrophage inhibitory factor (MIF)) and that of caspases 8 and 3. Caspases are enzymes belonging to the family of cysteine proteases which, when activated, lead to DNA fragmentation and cellular apoptosis. As noted by the authors, caspases 8 and 3 are downstream factors that are activated by the ligation of transmembrane ‘death receptors’ such as Fas and TNF receptors. Even though previous groups have identified similar correlations, the present article is unique in being able to identify similarities of the cytokine profile when comparing an in vivo rodent overuse model and ex vivo human tissue samples. This observation can help in setting a reliable platform for future use of this in vivo model to further study the pathophysiology of tendinopathies resulting from traumatic overuse.
This research again reflects on the complex interplay between inflammatory cytokines and cellular apoptosis. For example, it was noted that IL-15, a potent inhibitor of apoptosis, and IL-18, an inducer of apoptosis, were both upregulated in tendinopathy. Future work may involve developing a treatment model by using inhibitory RNA molecules (siRNA) to systematically down-regulate gene expression of the cytokines identified from the microarray analysis. The effect of this intervention on tendon pathology could then be assessed and thus characterise their importance as novel therapeutic targets. Another interesting observation noted by the authors is that the matched subscapularis samples obtained from shoulders with supraspinatus tears showed degeneration of grades 2 and 3 via histology despite having a normal appearing tendon on MRI and arthroscopy. It is indeed plausible that inflammatory cytokines released from diseased rotator cuff tissue could contribute to the degeneration of an adjacent rotator cuff tendon, due to the anatomical proximity. This finding indicates that MRI, at least with the sequences used, might not be sensitive enough to diagnose early rotator cuff tendinopathy in symptomatic patients. It is possible that joint aspiration and characterisation of its cytokine profile could provide an alternative to diagnosing early stages of tendinopathies and help guide our treatment in order to prevent clinical tendon rupture. An enzyme-linked immunosorbent array assay (ELISA) can be an effective and quick method to analyse the cytokine profile of these joint aspirates.
It is important to note that based on the chosen in vivo model and patient population in this study, the presented data can only be interpreted in the context of traumatic or degenerative tendon injury, and cannot be generalised to other rotator cuff disease processes such as rheumatoid arthritis. The authors acknowledge other limitations in their study, such as using a single time point in the disease process to evaluate the levels of gene expression in their in vivo model. Perhaps one way to circumvent this issue is by performing routine joint aspirates to analyse the cytokine profile of the joint fluid at different time points during the disease process. In addition, it is possible that in the selected patient population various confounders such as diabetes, peripheral vascular disease, smoking and certain medications may have affected the microassay results to some degree. Also, to understand the etiology of tendinopathy and the role of apoptosis, it is important to identify whether necrosis contributes to the diseased tendon. Recent studies have shown that triggering the Fas receptor can induce an alternative, caspase 8 independent cell death pathway which induces cellular necrosis.6-8 Since necrosis causes more tissue harm than apoptosis, it is important to decipher how and when during the disease process necrosis can contribute to tendon damage. Perhaps the disease process starts with tissue necrosis which recruits immune cells to the damaged area and subsequently triggers the release of toxic inflammatory interleukins that eventually induce apoptosis in adjacent cells. Further investigation in this area might help explain why certain cuff tears can be clinically more symptomatic than others despite being smaller in size.
Overall, the authors should be applauded for designing this study in a fashion which bridges clinical and bench research. This approach to translational research can allow for a constant iterative feedback loop between clinicians and basic scientists to help answer relevant clinical questions more effectively.
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8. Vanden Berghe T, Van Loo G, Saelens X, Van Gurp M. Differential signaling to apoptotic and necrotic cell death by Fas-associated death domain protein FADD. J Biol Chem 2004;279:7925-33.
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