Increased IL-6 receptor expression and signaling in ageing cartilage can be explained by loss of TGF-ß-mediated IL-6 receptor suppression.

OBJECTIVE: Osteoarthritis (OA) development is strongly associated with ageing, possibly due to age-related changes in transforming growth factor-ß (TGF-ß) signaling in cartilage. Recently, we showed that TGF-ß suppresses interleukin (IL)-6 receptor (IL-6R) expression in chondrocytes. As IL-6 is involved in cartilage degeneration, we hypothesized that age-related loss of TGF-ß signaling results in increased IL-6R expression and signaling in ageing cartilage. DESIGN: Bovine articular cartilage was collected and immediately processed to study age-related changes in IL-6R expression using qPCR and IHC (age-range: 0.5-14 years). Moreover, cartilage from young and aged cows was stimulated with rhIL-6 and/or rhTGF-ß1 to measure IL-6-induced p-STAT3 using Western blot. Expression of STAT3-responsive genes was analyzed using qPCR. RESULTS: Expression of IL-6 receptor (bIL-6R) significantly increased in cartilage upon ageing (slope: 0.32, 95%CI: 0.20-0.45), while expression of glycoprotein 130 (bGP130) was unaffected. Cartilage stimulation with IL-6 showed increased induction of p-STAT3 upon ageing (slope: 0.14, 95%CI: 0.08-0.20). Furthermore, IL-6-mediated induction of STAT3-responsive genes like bSOCS3 and bMMP3 was increased in aged compared to young cartilage. Interestingly, the ability of TGF-ß to suppress bIL6R expression in young cartilage was lost upon ageing (slope: 0.21, 95%CI: 0.13-0.30). Concurrently, an age-related loss in TGF-ß-mediated suppression of IL-6-induced p-STAT3 and bSOCS3 expression was observed. CONCLUSIONS: Ageing results in enhanced IL-6R expression and subsequent IL-6-induced p-STAT3 signaling in articular cartilage. This is likely caused by age-related loss of protective TGF-ß signaling, resulting in loss of TGF-ß-mediated IL-6R suppression. Because of the detrimental role of IL-6 in cartilage, this mechanism may be involved in age-related OA development.

TGF-ß dampens IL-6 signaling in articular chondrocytes by decreasing IL-6 receptor expression.

OBJECTIVE: Transforming growth factor-ß (TGF-ß) is an important homeostatic regulator of cartilage. In contrast, interleukin-6 (IL-6) is a pro-inflammatory cytokine implicated in cartilage degeneration. Cross-talk between TGF-ß and IL-6 is reported in tissues other than articular cartilage. Here, we investigated regulation of IL-6 signaling by TGF-ß in articular chondrocytes. DESIGN: Human primary chondrocytes and the human G6 chondrocyte cell line were stimulated with TGF-ß1 or interleukin-1ß (IL-1ß). Expression of IL-6 and IL-6 receptor (IL-6R) was determined on mRNA and protein level. TGF-ß regulation of IL-6 signaling via phosho-STAT3 (p-STAT3) was determined using Western blot, in presence of inhibitors for IL-6R, and Janus kinase(JAK)- and activin receptor-like kinase ALK)5 kinase activity. Furthermore, induction of STAT3-responsive genes was used as a read-out for IL-6 induced gene expression. RESULTS: TGF-ß1 increased IL-6 mRNA and protein expression in both G6 and primary chondrocytes. Moreover, TGF-ß1 stimulation clearly induced p-STAT3), which was abolished by inhibition of either IL-6R, JAK- or ALK5 kinase activity. However, TGF-ß1 did not increase expression of the STAT3-responsive gene SOCS3 and pre-treatment with TGF-ß1 even inhibited induction of p-STAT3 and SOCS3 by rhIL-6. Interestingly, TGF-ß1 potently decreased IL-6R expression. In contrast, IL-1ß did increase IL-6 levels, but did not affect IL-6R expression. Finally, addition of recombinant IL-6R abolished the inhibitory effect of TGF-ß1 on IL-6-induced p-STAT3 and downstream SOCS3, BCL3, SAA1 and MMP1 expression. CONCLUSIONS: In this study we show that TGF-ß decreases IL-6R expression, thereby dampening IL-6 signaling in chondrocytes. This reveals a novel effect of TGF-ß, possibly important to restrict pro-inflammatory IL-6 effects to preserve cartilage homeostasis.