SIRT4 suppresses the inflammatory response and oxidative stress in osteoarthritis.

Sirtuins have been involved in the osteoarthritis (OA) process. However, the functions of SIRT4 in the degeneration of human chondrocytes and OA are not fully understood. This study aimed to explore the role of SIRT4 during OA and mechanisms implicated. We extracted total protein and mRNA of the cartilage from OA patients and isolated the chondrocytes from the cartilage in different degenerated degrees for cell culture. Collagen II and SIRT4 levels of the tissues were analyzed by quantitative reverse-transcription polymerase chain reaction (RT-PCR) and Western blot. Chondrocytes were transferred with SIRT4-siRNA, treated with recombinant human SIRT4 protein for 24 h, respectively. Aggrecan, collagen I, collagen II, MMP-13, IL-6, TNF-α, SOD1, SOD2, and CAT expression, and ROS levels were investigated by Western blot, RT-PCR, immunofluorescence, enzyme-linked immunosorbent assay (ELISA), or flow cytometry. Collagen II decreased significantly in severely degenerated cartilage compared to the mild one, paralleling with SIRT4 expression both in protein and mRNA levels. Chondrocytes in severe OA grade were observed with a decrease in aggrecan, collagen II, SOD1, SOD2, CAT expression, nonetheless, an increase in collagen I, reactive oxygen species (ROS), MMP-13, IL-6, and TNF-α levels. However, SRIT4 protein treatment significantly upregulated aggrecan, collagen II, an antioxidant enzyme, and suppressed ROS and inflammatory response. Further analysis revealed that silencing of SIRT4 expression induced healthy chondrocytes, a decrease in aggrecan, collagen II and antioxidant enzyme expression, and an increase in ROS and inflammatory response, importantly, which can be reversed by SIRT4 protein stimuli. Our results elucidated that SIRT4 was tangled with the development of OA, and SIRT4 overexpression contributes to suppresses the inflammatory response and oxidative stress.