CS contains a lot of harmful substances, such as tar, nicotine, carbon monoxide, etc. When lung cells are stimulated by CS and other harmful substances, ERS will be triggered, and if ERS persists or is too strong, it will lead to cellular dysfunction and even apoptosis, which will promote the development of COPD[16].Many scholars have previously constructed COPD models using CS-exposed mice and cells and evaluated the expression profiles of ERS-related molecules.Jorgensen et al. reported the up-regulation of the expression of several ERS regulators, such as eIF2α, and ERS marker genes, such as XBP1, in CS-exposed human bronchial epithelioid-like cells by high-density microarray and protein blotting analysis, The expression levels of ERS-related proteins were gradually increased[17].Kenche et al. reported that the expression of eIF2α and XBP1 increased in mice exposed to CS[18].There are also studies reporting an increase in the expression of GRP78 in bronchoalveolar lavage fluid of smokers[19].In the present study, COPD rat and cell models were successfully constructed by smoking exposure system and medium containing cigarette extract, and ERS and ER-phagy were successfully induced by treating A549 cells with 10% CSE for 24 h. These indicated that CS could cause ERS and ER-phagy. dithiothreitol-induced ERS has been reported to increase the hepatocellular ER-phagy-related protein levels in hepatocytes[20],ERS can promote ER personality[21].Excess ER-phagy was found to cause increased apoptosis in HeLa cells and human embryonic kidney cells[22, 23],This suggests that ERS induced ER-phagy thereby promoting apoptosis.
TRIM13 is a non-classical ER-phagy receptor, and among more than 80 TRIM family members, TRIM13 is the only protein localized to the endoplasmic reticulum membrane[24],which is involved in a variety of endoplasmic reticulum and protein degradation processes, and is an important member of the endoplasmic reticulum to maintain homeostasis[25, 26].Many years ago, it was noted that members of the TRIM family, to which TRIM13 belongs, are involved in ER-phagy activities[24],The main mechanism of action is that TRIM protein interacts with p62 and utilizes the LIR structural domain of p62 protein to complete the recruitment of LC3 and promote the formation of isolation membrane and autophagosome[11].Tomar et al. observed in HEK293 cells that TRIM13 regulates the initiation of autophagy during ERS[27].Down-regulation of TRIM family members, including TRIM13, TRIM32, TRIM44 and TRIM59, induced cell cycle arrest and increased apoptosis in T-cell acute lymphoblastic leukemia. It was inferred that the up-regulation of TRIM13 expression should lead to a decrease in cell death[28].The results of this study demonstrate that overexpression of TRIM13 inhibits alveolar epithelial cell apoptosis by alleviating ERS and ER phagy, which may be a molecular mechanism by which TRIM13 protects against COPD.
Studies of TRIM13 and COPD have not been reported. However, previous studies have found that TRIM13 deficiency can lead to age-related autoinflammatory disease. 5-month-old TRIM13-deficient mice showed no obvious signs of inflammation; however, 10-month-old TRIM13-deficient mice showed signs of extensive inflammatory cell infiltration in the lungs[29],COPD is a typical age-related disease in which lung inflammation is involved in its pathogenesis, and TRIM13 may be directly or indirectly involved in the pathogenesis of COPD.This study observed for the first time that TRIM13 is downregulated and involved in the pathogenesis of COPD in both COPD patients and mouse lung tissues. Regulating TRIM13 expression can effectively regulate the levels of ERS and ER phagy in COPD patients, thereby reducing damage to alveolar epithelial cells. This may provide new directions and targets for the study of drugs for treating COPD.Upregulation of TRIM13 protein levels has been reported in macrophages after stimulation with TLR2 ligand[30],This encouraging result suggests that patients with COPD may benefit from similar treatments.
PI3K/AKT/mTOR as a widely reported pathway for the negative regulation of autophagy[31, 32].Enhanced autophagy present in COPD is associated with inhibition of the PI3K/AKT/MTOR pathway[33].Research has found that the protein expression of PI3K, p-AKT, and p-mTOR in the lung tissue cells of COPD mice is reduced, while LC3, ATG5, and cell number are reduced and cell apoptosis is increased[32].It can be seen that there is indeed inhibition of the PI3K/AKT/mTOR signaling pathway and enhanced autophagy in COPD, which also mediates cellular damage. It is not difficult to understand that the biggest risk factor for COPD is CS, which contains various toxic and harmful substances including particulate matter (PM). PM can cause cellular stress in human bronchial epithelial cells, macrophages, or endothelial cells and induce autophagy by inhibiting the PI3K/AKT/MTOR signaling pathway[34, 35].During cellular stress, the activity of mTOR complex is inhibited, leading to its release from the serine/threonine kinase (ULK1) complex and activation of autophagy. mTOR is a target molecule in the upstream pathway of cellular autophagy and the main inhibitory signal of autophagy.Research has found that under ERS conditions, the activity of the PI3K/AKT/mTOR signaling pathway is significantly reduced and autophagy is enhanced[36].The results of this study indicate that TRIM13 is an important regulatory factor in this pathway. Apoptosis experiments have shown that TRIM13 reduces damage to COPD alveolar epithelial cells by inhibiting ER phagy induced by ERS, and its mechanism may be the activation of the PI3K/AKT/mTOR signaling pathway.
The shortcoming of this study is that although this study showed that TRIM13 is closely related to ER-phagy and changes in the endoplasmic reticulum were observed before and after overexpression/disruption of TRIM13, it is not possible to determine whether these changes represent the occurrence of ER-phagy or not, which is the focus of the later study. In conclusion, the results of this study provide a theoretical basis for the use of TRIM13 as a target for ERS and ER-phagy therapy in COPD.