introduction
Ferroptosis is a programmed cell death mediated by excessive accumulation of iron-dependent lipid peroxidation[1]. Ferroptosis plays an important role in the occurrence and development of various diseases such as tumors. Recent studies have shown that immunotherapy or radiotherapy can induce ferroptosis in tumor cells, and ferroptosis plays an important role in promoting the efficacy of tumor immunotherapy and radiotherapy [1, 2], suggesting that targeted ferroptosis induction is a promising cancer treatment modality. However, the underlying regulatory mechanisms of ferroptosis in tumorigenesis and tumor treatment tolerance are still unclear, which limits the use of ferroptosis in cancer therapy.
2024年6月26日，中山大学肿瘤防治中心邓蓉/朱孝峰团队在Science Translational Medicine发表题为TRPML1 triggers ferroptosis defense and is a potential therapeutic target in AKT-hyperactivated cancer的研究成果。 该研究发现了溶酶体胞吐是新型的铁死亡抵抗系统；揭示了TRPML1-ARL8B介导的溶酶体胞吐通过抵抗铁死亡，促进AKT驱动的肿瘤发生和治疗耐受；开发的靶向TRPML1的特异性多肽通过抑制溶酶体胞吐促进铁死亡，在AKT驱动的肿瘤中表现出良好的抗瘤作用。
The Deng Rong/Zhu Xiaofeng team has long been committed to the study of the regulatory mechanism of ferroptosis and its role in tumorigenesis, development and treatment. They previously identified the "fuse" of ferroptosis:P KCβII is activated by sensing lipid peroxidation, which in turn directly phosphorylates downstream ACSL4, causing it to undergo dimerization activation and generate more lipid peroxides, thereby initiating the rapid amplification process of lipid peroxidation and ultimately inducing the occurrence of ferroptosis (Nat Cell Biol |). Xiaofeng Zhu/Rong Deng's team found that PKCβII-sensed lipid peroxidation promotes ferroptosis and its role in tumor treatment)[3]. The discovery of this core mechanism confirmed for the first time that ferroptosis was an active process, opening up a new direction for the study of ferroptosis [1, 2, 4]. However, tumors often exhibit ferroptosis resistance during the development process, and most tumors are not sensitive to ferroptosis induction therapy, so identifying the factors and pathways that promote tumor ferroptosis tolerance and developing targeted strategies are the key to applying ferroptosis to tumor therapy. On the basis of the above research, the researchers used the CRISPR-Cas9 genome-wide transcriptional activation library to screen the key factors that mediate tumor ferroptosis tolerance, and found that the sgRNAs of lysosomal exocytosis-related genes (TRPML1, TFEB, ARL8B, BORCS8, KIF1B, KIF5B, KIF1BP, etc.) were significantly enriched in ferroptosis-tolerant cells. Lysosomal exocytosis is the behavior of lysosomes secreting into the cell, which is essential for maintaining lysosomal homeostasis [5]. In order to explore the role of lysosomal exocytosis in ferroptosis resistance, researchers constructed a cell model of lysosomal exocytosis inhibition or enhancement by knocking out or supplementing lysosomal exocytosis genes in a variety of tumor cells or treating tumor cells with lysosomal exocytosis inhibitors.
Next, the researchers attempted to elucidate the molecular mechanism by which lysosomal exocytosis inhibits tumor ferroptosis. Using a library of kinase inhibitors, they identified AKT as a major upstream regulator of lysosomal exocytosis in tumor cells. It was found that AKT could directly phosphorylate the Ser343 locus of TRPML1 and inhibit the K552 ubiquitination and proteasomal degradation of TRPML1, thereby stabilizing the level of TRPML1 protein and enhancing the binding of TRPML1 to ARL8B to trigger lysosomal exocytosis. Lysosomal exocytosis ultimately mediates ferroptosis tolerance in tumor cells by decreasing intracellular Fe2+ and thereby reducing lipid peroxidation production while enhancing plasma membrane repair. So far, the researchers have confirmed for the first time that TRPML1-ARL8B-mediated lysosomal exocytosis leads to ferroptosis tolerance and its molecular mechanism, which is a new model of ferroptosis resistance in tumors.
The researchers then delved into the role of lysosomal exocytosis-mediated ferroptosis tolerance in tumorigenesis and tumor treatment tolerance. Through multi-level correlation analysis and functional experiments in tumor cells and tumor tissue specimens, it was found that ferroptosis resistance mediated by lysosomal exocytosis pathway activation was one of the characteristics of AKT highly activated tumors. Using xenograft tumor and hydrodynamic transgenic models, it was found that TRPML1-ARL8B-mediated ferroptosis resistance plays a key role in the malignant transformation of AKT-driven normal cells into tumor cells. Furthermore, the analysis of clinical tumor treatment data showed that ferroptosis resistance mediated by lysosomal exocytosis pathway activation was negatively correlated with the efficacy of tumor radiotherapy and immunotherapy. Using a mouse model of tumor therapy, TRPML1-ARL8B-mediated ferroptosis resistance was found to be an important reason for the resistance of AKT-driven tumors to radiotherapy and immunotherapy. These suggests that TRPML1 can be used as a new therapeutic target for AKT-highly activated tumors.
Finally, the researchers analyzed the structural basis of the formation of the TRPML1-ARL8B complex and found that the amino acids at positions 54-63 of ARL8B are the main regions that mediate its specific interaction with TRPML1, and then synthesized competitive peptides in this region. The polypeptide disrupts the interaction between TRPML1 and ARL8B by specifically binding TRPML1, thereby blocking lysosomal exocytosis to promote ferroptosis, ultimately inhibiting AKT-driven tumorigenesis, and enhancing the sensitivity of AKT-highly activated tumors to ferroptosis inducers, radiotherapy, immunotherapy, etc. The peptide has the advantages of high efficiency and low toxicity in in vivo and in vivo models, and has a good application prospect.
模式图（Credit: Science Translational Medicine）
Lysosomes play a pivotal role in the occurrence of ferroptosis. The abundance of ferric ions in lysosomes is a determinant of ferroptosis-associated lipid peroxidation [6, 7]. However, to date, the regulation of ferroptosis by lysosomal homeostasis regulation is unclear. This study is the first to reveal that TRPML1-ARL8B-mediated lysosomal exocytosis is an important pathway for inhibiting ferroptosis, which is different from the currently discovered ferroptosis inhibition systems (GPX4-GSH, FSP1-CoQH2, GCH1-BH4, and DHODH-CoQH2) [2]. The inhibitory effect of lysosomal exocytosis on ferroptosis is not through the removal of lipid peroxidation, but through lysosomal exocytosis, which excretes Fe2+, the catalyst that mediates the production of lipid peroxidation, from the cell, thereby inhibiting the production of lipid peroxidation from the source, which is a new mode of ferroptosis inhibition and an important cause of AKT-driven tumorigenesis and tumor treatment tolerance. In this study, a specific peptide targeting TRPML1 was developed and demonstrated its anti-tumor activity, and a prevention and treatment strategy for AKT-highly activated tumors was proposed from the perspective of lysosomal exocytosis regulating ferroptosis.
bibliography
1 Stockwell, B. R. Ferroptosis turns 10: Emerging mechanisms, physiological functions, and therapeutic applications. Cell 185, 2401-2421, doi:10.1016/j.cell.2022.06.003 (2022).2 Lei, G., Zhuang, L. & Gan, B. Targeting ferroptosis as a vulnerability in cancer. Nat Rev Cancer 22, 381-396, doi:10.1038/s41568-022-00459-0 (2022).3 Zhang, H. L. et al. PKCbetaII phosphorylates ACSL4 to amplify lipid peroxidation to induce ferroptosis. Nat Cell Biol 24, 88-98, doi:10.1038/s41556-021-00818-3 (2022).4 Rodencal, J. & Dixon, S. J. Positive feedback amplifies ferroptosis. Nat Cell Biol 24, 4-5, doi:10.1038/s41556-021-00824-5 (2022).5 Medina, D. L. et al. Transcriptional activation of lysosomal exocytosis promotes cellular clearance. Dev Cell 21, 421-430, doi:10.1016/j.devcel.2011.07.016 (2011).6 Gao, H. et al. Ferroptosis is a lysosomal cell death process. Biochem Biophys Res Commun 503, 1550-1556, doi:10.1016/j.bbrc.2018.07.078 (2018).7 Rizzollo, F., More, S., Vangheluwe, P. & Agostinis, P. The lysosome as a master regulator of iron metabolism. Trends Biochem Sci 46, 960-975, doi:10.1016/j.tibs.2021.07.003 (2021).http://doi.org/10.1126/scitranslmed.adk0330
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