OA is a clinically common chronic degenerative disease, which mainly manifests as joint pain, swelling and/or restricted movement, and is the main cause of decreased mobility and dysfunction in the elderly[26, 27]. Damage and loss of articular cartilage is the key to the pathological process in OA[28, 29]. Inflammatory cytokines such as IL-1β can cause a series of inflammatory cascade reaction in chondrocytes, and ultimately leads to the destruction of chondrocytes and degradation of EMC[30, 31]. Therefore, IL-1β-stimulated chondrocytes was used to establish a cell model of OA in this study. It has been reported that LSD1 expression is increased in OA cartilage compared with normal cartilage[11], and our data confirmed this. Besides, our data also found that LSD1 expression was up-regulated in IL-1β-induced OA model in vitro.
Autophagy plays an important role in the homeostasis maintenance of chondrocytes for clearing dysfunctional organelles and macromolecules[32, 33]. With the degeneration of cartilage, autophagy is inhibited and induces the homeostasis imbalance of chondrocytes, eventually leading to cell death[34, 35]. Consistent with previous studies[36, 37], we found that autophagy was down-regulated in IL-1β-induced chondrocytes. mTOR, a serine/threonine protein kinase, is a key repressor of autophagy, which is modulated by multiple upstream signaling pathways to regulate autophagy level[24, 38]. It has been found that parathyroid hormone (PTH)-(1–34) and Astragaloside IV ameliorate chondrocyte apoptosis via increasing autophagy by repressing mTOR[7, 39]; Xue et al. and Kong et al. have reported that inhibition of mTOR attenuates inflammation of chondrocytes in rats with OA[40, 41]. What is more, numerous experimental data indicate that activating mTOR-mediated autophagy can inhibit the ECM degradation in IL-1β-induced chondrocytes[39, 40, 42]. These evidences suggest that mTOR-mediated autophagy plays an important role in the progression of OA.
LSD1 is the first confirmed histone demethylase that can specifically remove the methyl modification of H3K9me1/2 and restore it to the unmethylated state, thereby affecting the transcriptional activation of target genes[8, 12]. Previous studies have shown that LSD1 regulates the level of autophagy to programme oocyte death and reduce cancer cell viability via modulating p62 expression [43, 44]. Furthermore, Wei et al. found that LSD1 mediates autophagy to regulate ovarian cancer progression through the mTOR signaling pathway[45]. Shi et al. reported that LSD1 dowm-regulates autophagy of myoblast cells via the activity the mTOR signaling pathway[46]. A recent study has revealed that knockdown of LSD1 attenuates ox-LDL-induced inflammation of RAW264.7 cells by promoting mTOR-mediated autophagy[47]. In this study, we found that knockdown of LSD1 suppressed IL-1β-induced chondrocyte apoptosis, inflammation and ECM degradation by activating mTOR-mediated autophagy.
TRIM32 has been reported that be overexpressed in fibroblast-like synoviocytes (FLS) of OA patients[22], and the present study indicated that TRIM32 expression was up-regulated in OA cartilage tissues and IL-1β-induced chondrocytes. It is noted that LSD1 modulates TRIM37 expression in luminal breast cancer cells[12], our study showed that knockdown of LSD1 down-regulated TRIM32 expression in IL-1β-induced chondrocytes. Recently, papers have found that TRIM32 contributes to autophagy in muscle cells through the regulation of p62 activity[48]. Moreover, a in vivo experimental data shows that TRIM32 deficiency mice increases autophagy level by reducing mTOR activity[23]. In this study, we found that LSD1 knockdown activated mTOR-mediated autophagy by modulating TRIM32, and that may be the molecular mechanism of LSD1 knockdown on chondrocyte apoptosis, inflammation and ECM degradation.
In conclusion, our study found that LSD1 and TRIM32 were overexpressed in OA cartilage tissues and IL-1β-induced chondrocytes, and their expression showed a strong positive correlation. Knockdown of LSD1 enhanced chondrocyte viability, and repressed apoptosis, inflammation and ECM degradation via activating mTOR-mediated autophagy by regulating TRIM32. Therefore, our study may provide potential therapeutic targets for OA therapy.