TNF-α enhances apoptosis by promoting chop expression in nucleus pulposus cells: role of the MAPK and NF-κB pathways.

CHOP has been shown to be involved in AF cells apoptosis and disc degeneration in a rat model. The aim of this study was to investigate the regulatory effects of TNF-α on C/EBP homologous protein (CHOP) and the role of CHOP in nucleus pulposus (NP) cell apoptosis. The effects of TNF-α on chop were measured by qPCR, Western blot, and immunofluorescence. TNF receptor involvement was analyzed by small interfering RNA (siRNA), Western blotting, immunofluorescence, and qPCR. The effects of NF-κB and MAPK on TNF-α-mediated chop promoter activity were studied using siRNAs, Western blotting, immunofluorescence, and qPCR. The regulatory effects of TNF-α-induced CHOP on Bcl-2 and Bax were studied using siRNAs, Western blotting, immunofluorescence, and qPCR. Flow cytometric and TUNEL analyses were performed to investigate the effects of chop on NP cell apoptosis. Increased CHOP expression was observed in NP cells after TNF-α treatment. Treatment of cells with TNF receptor, NF-κB, and ERK/JNK-MAPK inhibitors or siRNAs abolished the effects of cytokines on CHOP expression. Pharmacological siRNA knockdown of chop promoted Bax, decreased Bcl-2, and attenuated TNF-α-mediated cell apoptosis. During intervertebral disc degeneration (IVDD), TNF-α binds to TNF receptors and controls the JNK/ERK-MAPK, and NF-κB signaling pathways in NP cells, increasing CHOP expression. This change up-regulates the pro-apoptotic protein Bax and down-regulates the anti-apoptosis protein Bcl-2, inducing cell apoptosis. This study suggests a potential therapeutic target for controlling the inflammatory-induced apoptosis associated with IVDD.