Objectives: To investigate the expression of Sam68 in articular cartilage of knee osteoarthritis (OA) and the relationship between Sam68 and NF-κB activation and apoptosis signaling in OA articular chondrocytes.
Methods: Sam68 expression in normal and osteoarthritic cartilage was assessed by immunohistochemistry and RT-PCR on both meniscal/ligamentous injury (MLI)-induced OA rat model and the clinical human OA cartilage tissues. Sam68 expression in chondrocytes under tumor necrosis factor-alpha (TNF-α) stimuli was also assessed by immunoblot. Inhibiting Sam68 in chondrocytes under TNF-α stimuli was conducted using small interfering RNA (siRNA) and its influence on the expression of apoptotic marker and catabolic genes was examined by immunoblot. The mechanism of how Sam68 stimulates NF-κB activity was determined by co-immunoprecipitation and immunoblot analysis of nuclear and cytoplasmic fractions of TNF-α-treated chondrocytes for p65 and Sam68.
Results: Sam68 expression was increased in OA cartilage tissues and chondrocytes under TNF-α stimuli. Inhibition of Sam68 by siRNA significantly decreased the expression of apoptotic markers (cleaved caspase-3 and cleaved PARP) in chondrocytes following TNF-α-stimulation. Sam68 knockdown suppressed Iκ-B degradation and p65 nuclear transportation in TNF-α-treated chondrocytes, indicating a suppressed NF-κB activation. Upon TNF-α exposure, the nuclear transportation of Sam68 and its interaction with p65 was detected in chondrocytes. Furthermore, Sam68 knockdown also alleviated the TNF-α-induced catabolic marker (MMP13, ADAMTS5, iNOS and IL-6) expression.
Conclusions: The highly expressed Sam68 promotes NF-κB signaling activation, catabolic gene expression and cellular apoptosis in TNF-α-treated chondrocytes, which may provide better insights into the pathophysiology of OA and a potential target for its treatment.
Keywords: Apoptosis; NF-κB; Osteoarthritis; Sam68; TNF-α.