Hypoxia-inducible factor 2α (HIF-2α) (encoded by Epas1) causes osteoarthritic (OA) cartilage destruction by regulating the expression of catabolic factor genes. We undertook this study to explore the role of interleukin-6 (IL-6) in HIF-2α–mediated OA cartilage destruction in mice.
The expression of HIF-2α, IL-6, and catabolic factors was determined at the messenger RNA and protein levels in primary culture mouse chondrocytes, human OA cartilage, and mouse experimental OA cartilage. Experimental OA in wild-type, HIF-2α–knockdown (Epas1+/−), and Il6–/– mice was caused by intraarticular injection of Epas1 adenovirus or destabilization of the medial meniscus. The role of IL-6 was determined by treating with recombinant IL-6 protein or by injecting HIF-2α adenovirus (AdEpas1) intraarticularly in mice with or without IL-6–neutralizing antibody.
We found that Il6 is a direct target gene of HIF-2α in articular chondrocytes. Both Epas1 and Il6 were up-regulated in human and mouse OA cartilage, whereas HIF-2α knockdown in mice led to inhibition of both Il6 expression and cartilage destruction. Treatment with IL-6 enhanced Mmp3 and Mmp13 expression; conversely, Il6 knockdown inhibited HIF-2α–induced up-regulation of Mmp3 and Mmp13. Injection of IL-6 protein into mouse knee joints triggered OA cartilage destruction, whereas IL-6 neutralization led to blocking of HIF-2α–induced cartilage destruction with concomitant modulation of Mmp3 and Mmp13 expression. Moreover, Il6 knockout resulted in inhibition of AdEpas1-induced and destabilization of the medial meniscus–induced cartilage destruction as well as inhibition of Mmp3 and Mmp13 expression.
Our findings indicate that IL-6 acts as a crucial mediator of HIF-2α–induced experimental OA cartilage destruction in mice via regulation of Mmp3 and Mmp13 levels.