Bromodomain‐containing protein 4 inhibition alleviates matrix degradation by enhancing autophagy and suppressing NLRP3 inflammasome activity in NP cells

Abstract

An imbalance between matrix synthesis and degradation is the hallmark of intervertebral disc degeneration while inflammatory cytokines contribute to the imbalance. Bromodomain and extra‐terminal domain (BET) family is associated with the pathogenesis of inflammation, and inhibition of BRD4, a vital member of BET family, plays an anti‐inflammatory role in many diseases. However, it remains elusive whether BRD4 plays a similar role in nucleus pulposus (NP) cells and participates in the pathogenesis of intervertebral disc degeneration. The present study aims to observe whether BRD4 inhibition regulates matrix metabolism by controlling autophagy and NLRP3 inflammasome activity. Besides, the relationship was investigated among nuclear factor κB (NF‐κB) signaling, autophagy and NLRP3 inflammasome in NP cells. Here, real‐time polymerase chain reaction, western blot analysis and adenoviral GFP‐LC3 vector transduction in vitro were used, and it was revealed that BRD4 inhibition alleviated the matrix degradation and increased autophagy in the presence or absence of tumor necrosis factor α. Moreover, p65 knockdown or treatment with JQ1 and Bay11‐7082 demonstrated that BRD4 inhibition attenuated NLRP3 inflammasome activity through NF‐κB signaling, while autophagy inhibition by bafilomycin A1 promoted matrix degradation and NLRP3 inflammasome activity in NP cells. In addition, analysis of BRD4 messenger RNA expression in human NP tissues further verified the destructive function of BRD4. Simply, BRD4 inhibition alleviates matrix degradation by enhancing autophagy and suppressing NLRP3 inflammasome activity through NF‐κB signaling in NP cells.

Transcription Factors in Cartilage Homeostasis and Osteoarthritis

“…Whether BRD4 also plays a role in regulating ATG gene expression via these mechanisms in OA is not known but is likely because, in human OA knee cartilage, both mRNA and protein BRD4 levels were upregulated compared to normal cartilage, and BRD4 levels correlated positively with the severity of OA, indicating that BRD4 might also repress autophagy in OA chondrocytes [167]. Moreover, in a similar cell type as chondrocytes (nucleus pulposus cells), it was shown that inhibition of BRD4 by the protein inhibitor JQ1 resulted in enhanced autophagy [168]. Besides BRD4, SIRT1 is an important epigenetic remodeler that induces autophagy [169].…”

Pharmacological Modulation of BET Family in Sepsis

“…Normally, the expression of BRDT is restricted to the testis, while the expression of BRD2, BRD3, or BRD4 is commonly found in the nucleus of other cells (including immune cells). Under pathological conditions, the expression of BETs may be further changed (upregulation or downregulation) to meet the requirements for orchestrating a genetic regulatory response (Bachtel et al, 2019;Hong et al, 2020). In the case of sepsis, according to the type of pathogen infection, the expression of BRD2, BRD3 or BRD4 shows heterozygosity and diversity in immune cells.…”

“…Interestingly, BETs confer opposite effects on NRLP3 activation depending on cell types. For example, the inhibition of BRD4 alleviates the inflammatory response by blocking TNF-related NRLP3 activation in rat nucleus pulposus cells (Hong et al, 2020). However, the inhibition of BRD4 prevents proliferation and epithelial mesenchymal transition in kidney cancer cells by increasing RELA-mediated NLRP3 expression and subsequent pyroptosis (Tan et al, 2020).…”

The BET family in immunity and disease

“…BRD4 expression is markedly upregulated in different resident immune cells (e.g., macrophages, monocytes, T cells, and NK cells) and non-immune cells (e.g., pulmonary microvascular endothelial cells, bronchial epithelials, cardiomyocytes, and smooth muscle cells) under various stimuli (e.g., cigarette smoke extract, viruses, and listeriolysin-O). [28][29][30] In these cells, BRD4 mainly exerts proinflammatory roles through conferring transcription activation of a variety of immune and inflammatory genes, and it may serve as a detrimental stress protein that can be used to predict disease activity. Moreover, the upregulation of BRD4 is also found in the uterus and fetal membranes induced by labor and infection, 31 and severe early-onset preeclampsia placenta, 32 which may lead to adverse pregnancy outcomes.…”