Genetic Inactivation of ZCCHC6 Suppresses Interleukin-6 Expression and Reduces the Severity of Experimental Osteoarthritis in Mice

Objective. Cytokine expression is tightly regulated posttranscriptionally, but high levels of interleukin-6 (IL-6) in patients with osteoarthritis (OA) indicate that regulatory mechanisms are disrupted in this disorder. The enzyme ZCCHC6 (zinc-finger CCHC domain-containing protein 6; TUT-7) has been implicated in posttranscriptional regulation of inflammatory cytokine expression, but its role in OA pathogenesis is unknown. The present study was under-taken to investigate whether ZCCHC6 directs the expression of IL-6 and influences OA pathogenesis in vivo. Methods. Human and mouse chondrocytes were stimulated with recombinant IL-1 beta. Expression of ZCCHC6 in human chondrocytes was knocked down using small interfering RNAs. IL-6 transcript stability was determined by actinomycin D chase, and 3'-uridylation of microRNAs was determined by deep sequencing. Zcchc6(-/-) mice were produced by gene targeting. OA was surgically induced in the knee joints of mice, and disease severity was scored using a semiquantitative grading system. Results. ZCCHC6 was markedly up-regulated in damaged cartilage from human OA patients and from wild-type mice with surgically induced OA. Overexpression of ZCCHC6 induced the expression of IL-6, and its knockdown reduced IL-6 transcript stability and IL-1 beta-induced IL-6 expression in chondrocytes. Reintroduction of Zcchc6 in Zcchc6(-/-) mouse chondrocytes rescued the IL-1 beta-induced IL-6 expression. Knockdown of ZCCHC6 reduced the population of micro-RNA 26b (miR-26b) with 3'-uridylation by 60%. Zcchc6(-/-) mice with surgically induced OA produced low levels of IL-6 and exhibited reduced cartilage damage and synovitis in the joints. Conclusion. These findings indicate that ZCCHC6 enhances IL-6 expression in chondrocytes through transcript stabilization and by uridylating miR-26b, which abrogates repression of IL-6. Inhibition of IL-6 expression and significantly reduced OA severity in Zcchc6(-/-) mice identify ZCCHC6 as a novel therapeutic target to inhibit disease pathogenesis.