In this study we report for the first time that irisin can directly target hOAC and promote cell proliferation and GAG and type II collagen synthesis, while reducing type X collagen expression through inactivation of p38 MAPK and Akt signaling pathways. This is the first study showing that irisin can directly act on chondrocytes and attenuate OA-related cartilage degeneration in vitro, suggesting the existence of a cross-talk mechanism between muscle and cartilage. Irisin is secreted by skeletal muscle in response to physical exercise and may theoretically promote chondrocyte anabolism so that cartilage can better adapt to increased load and friction during prolonged exercise.
While irisin first reported effect was to promote adipocyte transdifferentiation and energy metabolism23, irisin-induced proliferation, differentiation and anabolic effects were also observed with other cell types, including osteoblasts24, bone marrow stromal cells25, and human umbilical vein endothelial cells26. Recent research efforts have described the wide biological activity of such myokine, whose effects are pleiotropically exerted on several organs, namely the brain27, the pancreas28, the liver12, the bone25 and the skeletal muscle29. Our data expands the knowledge base for irisin, reporting its role in promoting chondrocyte anabolism. We tested the anabolic effects of irisin by treating primary hOAC in a three-dimensional culture system with r-irisin for 7 days. As irisin effect on articular chondrocytes has not been reported before, we performed a dose-response experiment to assess the most effective concentration on GAG synthesis by using increasing doses within a range including intraarticular18 and serum irisin concentration15,21 as reported by previous studies. Our results showed that irisin increased the expression of type II collagen while reducing the expression of type X collagen, a marker of chondrocyte hypertrophy in osteoarthritic cartilage. In addition, we demonstrated that irisin was able to increase hOAC proliferation at all considered timepoints by disabling the PI3K-Akt pathways which plays a role in cartilage anabolic as well as catabolic processes in response to the activation of inflammatory processes of various origins. The reduced number of chondrocytes within osteoarthritic cartilage reduces the capacity of the tissue to counteract exogenous stresses and to maintain the original ECM composition. In this regard, increasing chondrocyte proliferation would enhance cartilage metabolism and capacity to react to stressful stimuli.
Our findings have demonstrated that irisin promotes chondrocyte anabolism by inhibiting the phosphorylation of p38 and Akt. Thus, we suggest that p38 and Akt signaling pathways may play a critical role in the chondrogenic effect of irisin.
p38 and ERK signaling pathways are crucial to cell proliferation and differentiation30 and may be the main pathways mediating irisin effects. Indeed, irisin can stimulate browning of white adipocytes through p38 and ERK MAPK31, promotes human umbilical vein endothelial cell proliferation through the ERK signaling pathway32 and osteoblast proliferation and differentiation via activating the phosphorylation of p38 and ERK15. Moreover, these pathways have been directly implied in OA pathogenesis. In osteoarthritic cartilage, excessive amounts of basic fibroblast growth factor are released upon mechanical loading and activate several transduction pathways involving different MAPK, including ERK and p38. This ultimately leads to upregulation of metalloproteinases, namely ADAMTS-5 and MMP-13, resulting in type II collagen degradation and aggrecan fragmentation33. Furthermore, p38 seems to be involved in promoting chondrocyte hypertrophy and apoptosis, inhibiting cartilage synthesis and downregulating chondrocyte autophagy34.
Extensive studies have revealed the function of Akt pathway in chondrocytes during endochondral ossification. Deletion of Akt1 results in delayed calcification35, while Akt activation in embryonic chondrocytes promotes chondrocyte proliferation and inhibits hypertrophic differentiation36. However, the in vivo function of Akt signaling in the maintenance of articular cartilage homeostasis and in OA development is largely undefined, with different in vitro studies reporting contradictory results. PI3K/Akt signaling has been shown to play a chondroprotective role by regulating chondrocyte survival, proliferation and extracellular matrix synthesis37,38. In contrast, some studies have reported a detrimental effect of PI3K/Akt pathway on OA, which might be achieved through transduction of procatabolic stimuli or inhibition of articular chondrocyte autophagy39,40. Our findings demonstrated that increased cell proliferation and ECM anabolism in chondrocytes treated with irisin were associated with a reduction of p-Akt and thus with a downregulation of the PI3K/Akt pathway. This is consistent with previous data regarding PI3K/Akt pathway involvement in irisin signaling on distinct cell types41, although other studies reported increased levels of p-Akt42. Contrariwise, no significant change in ERK activity was recorded in this study. This suggest that ERK and PI3K/Akt role in irisin signaling is probably cell-specific and conditioned by local stimuli.
Physical training yields recognized benefits in preserving joints health and is one of the main conservative approaches for preventing and treating OA43. Exercise, by strengthening periarticular muscles along with general aerobic conditioning can improve joint stability, reduce pain and ameliorate quality of life8,10. Moreover, the administration of physiological dynamic loads, as during physical exercise, enhances the production of ECM components, including collagens, proteoglycans and oligomers by articular chondrocytes44. Conversely, disuse and limited movement due to severe illness, cachexia and muscular diseases can favor joint degeneration and rigidity45.
However, the effect of physical training on both joint health and irisin serum concentration strictly depends on the type of exercise46. Several past studies have reported that resistance, anaerobic and high intensity exercise can increase irisin levels in the bloodstream47, while aerobic exercise and reduced load training do not significantly influence irisin concentration48. Duration of exercise training and environmental factors both influence the levels of circulating irisin. A large meta-analysis reported a decrease of circulating irisin in healthy individuals undergoing either endurance or resistance chronic exercise (> 8 weeks)49, whilst another study showed a reduction in irisin levels after two weeks of climbing at high altitude-hypoxia50. To date, no evidence concerning the ideal type of exercise or training protocol for osteoarthritic joints is available. A large meta-analysis comparing high-intensity versus low-intensity exercise for knee and hip OA was inconclusive51, although it is widely accepted that improving muscle strength, aerobic capacity and lowering body weight benefits joint maintenance and cardiovascular health5.
The major limitation of this study is that results have been obtained using an in vitro experimental design, even though human primary cells have been used. Currently, no reports are available correlating irisin synovial fluid concentration with the type of physical activity performed in either healthy subjects or patients with OA. Therefore, these data need to be further confirmed in an experimental animal model of OA exposed to physical exercise. In addition, as the effect of irisin on articular chondrocytes under physiological conditions has not been described yet, our understanding of its biological role on hOAC might not encompass all the effects that the myokine would have on the healthy tissue. A further consideration limiting this study is related to the posttranslational glycosylation of irisin after secretion that enhances its biological function31. Indeed, most of commercial r-irisin derived from Escherichia Coli, including the one used in this study, is non-glycosylated. Therefore, the biological activity may not exactly reflect the myokine action on chondrocytes in vivo, which may be even stronger.