To date, a large number of long non-coding RNAs (lncRNAs) have been recently discovered through functional genomics studies. Importantly, lncRNAs have been shown, in many cases, to function as master regulators for gene expression and thus, they can play a critical role in various biological functions and disease processes including cancer. Although the lncRNA-mediated gene expression involves various mechanisms, such as regulation of transcription, translation, protein modification, and the formation of RNA-protein or protein-protein complexes, in this review, we discuss the latest developments primarily in important cell signaling pathways regulated by lncRNAs in cancer.

LncRNA-mediated regulation of cell signaling in cancer.

The discovery of regulated cell death processes has enabled advances in cancer treatment. In the past decade, ferroptosis, an iron-dependent form of regulated cell death driven by excessive lipid peroxidation, has been implicated in the development and therapeutic responses of various types of tumours. Experimental reagents (such as erastin and RSL3), approved drugs (for example, sorafenib, sulfasalazine, statins and artemisinin), ionizing radiation and cytokines (such as IFNγ and TGFβ1) can induce ferroptosis and suppress tumour growth. However, ferroptotic damage can trigger inflammation-associated immunosuppression in the tumour microenvironment, thus favouring tumour growth. The extent to which ferroptosis affects tumour biology is unclear, although several studies have found important correlations between mutations in cancer-relevant genes (for example, RAS and TP53), in genes encoding proteins involved in stress response pathways (such as NFE2L2 signalling, autophagy and hypoxia) and the epithelial-to-mesenchymal transition, and responses to treatments that activate ferroptosis. Herein, we present the key molecular mechanisms of ferroptosis, describe the crosstalk between ferroptosis and tumour-associated signalling pathways, and discuss the potential applications of ferroptosis in the context of systemic therapy, radiotherapy and immunotherapy. Ferroptosis is an iron-dependent form of regulated cell death driven by excessive lipid peroxidation. Pharmacological agents, ionizing radiation and cytokines can induce ferroptosis and thus suppress tumour growth, but ferroptosis can also trigger inflammation-associated immunosuppression. The authors describe the key molecular mechanisms of ferroptosis, including crosstalk with tumour-associated signalling pathways, and discuss potential therapeutic applications of ferroptosis.

Broadening horizons: the role of ferroptosis in cancer.

Phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling is one of the most important intracellular pathways, which can be considered as a master regulator for cancer. Enormous efforts have been dedicated to the development of drugs targeting PI3K signaling, many of which are currently employed in clinical trials evaluation, and it is becoming increasingly clear that PI3K inhibitors are effective in inhibiting tumor progression. PI3K inhibitors are subdivided into dual PI3K/mTOR inhibitors, pan-PI3K inhibitors and isoform-specific inhibitors. In this review, we performed a critical review to summarize the role of the PI3K pathway in tumor development, recent PI3K inhibitors development based on clinical trials, and the mechanisms of resistance to PI3K inhibition.

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Targeting PI3K in cancer: mechanisms and advances in clinical trials

The cystine/glutamate antiporter SLC7A11 (also commonly known as xCT) functions to import cystine for glutathione biosynthesis and antioxidant defense and is overexpressed in multiple human cancers. Recent studies revealed that SLC7A11 overexpression promotes tumor growth partly through suppressing ferroptosis, a form of regulated cell death induced by excessive lipid peroxidation. However, cancer cells with high expression of SLC7A11 (SLC7A11high) also have to endure the significant cost associated with SLC7A11-mediated metabolic reprogramming, leading to glucose- and glutamine-dependency in SLC7A11high cancer cells, which presents potential metabolic vulnerabilities for therapeutic targeting in SLC7A11high cancer. In this review, we summarize diverse regulatory mechanisms of SLC7A11 in cancer, discuss ferroptosis-dependent and -independent functions of SLC7A11 in promoting tumor development, explore the mechanistic basis of SLC7A11-induced nutrient dependency in cancer cells, and conceptualize therapeutic strategies to target SLC7A11 in cancer treatment. This review will provide the foundation for further understanding SLC7A11 in ferroptosis, nutrient dependency, and tumor biology and for developing novel effective cancer therapies.

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Cystine transporter SLC7A11/xCT in cancer: ferroptosis, nutrient dependency, and cancer therapy

Ferroptosis is an iron-dependent, non-apoptotic form of regulated cell death caused by lipid peroxidation, which is controlled by integrated oxidation and antioxidant systems. The iron-containing e...

Ferroptosis: machinery and regulation

Glutathione peroxidase 4 (GPX4) has been confirmed to inhibit ferroptosis in cancer cells, however, whether GPX4 serves as an oncogene is not clear. In this study, the expression of GPX4 and its influence to survival of patients with cancer were analyzed via public databases. Furthermore, the epigenetic regulation of GPX4 and the relation between GPX4 and chemoresistance of different anticancer drugs was also detected. Most importantly, cytological assays were performed to investigate the function of GPX4 in cancer cells. The results showed that GPX4 was higher expressed in cancer tissues than normal and was negatively associated with prognosis of patients. Furthermore, at upstream of GPX4 there was low DNA methylation sites and enhanced level of H3K4me3 and H3K27ac, indicating that high level of GPX4 in cancer may resulted from epigenetic regulation. Moreover, GPX4 was positively related to chemoresistance of anticancer drugs L-685458, lapatinib, palbociclib, and topotecan. In addition, GPX4 may potentially be involved in translation of protein, mitochondrial respiratory chain complex I assembly, electron transport oxidative phosphorylation, nonalcoholic fatty liver disease, and metabolic pathways. Finally, we detected that GPX4 inhibited ferroptosis in cancer cells, the inhibition of GPX4 via RSL3 could enhance the anticancer effect of cisplatin in vitro and in vivo. In conclusion, GPX4 acts as an oncogene and inhibits ferroptosis in cancer cells, the anticancer effect of cisplatin can be enhanced by GPX4 inhibition.

Inhibition of tumor propellant glutathione peroxidase 4 induces ferroptosis in cancer cells and enhances anticancer effect of cisplatin

The metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) regulates cell motility via the transcriptional or post-transcriptional control of motility-related genes. Whether MALAT1 plays a critical role in cancer progression in breast cancer remains unclear. In this study, we found that MALAT1 was downregulated in breast tumor cell lines and cancer tissue, and showed that knockdown of MALAT1 in breast cancer cell lines induced an epithelial-to-mesenchymal transition (EMT) program via phosphatidylinositide-3 kinase-AKT pathways. Furthermore, lower expression of MALAT1 in breast cancer patients was associated with shorter relapse-free survival. Thus, our results indicate for the first time that MALAT1 is a novel regulator of EMT in breast cancer and may be a potential therapeutic target for breast cancer metastasis.

Downregulation of long noncoding RNA MALAT1 induces epithelial-to-mesenchymal transition via the PI3K-AKT pathway in breast cancer

(+)-JQ1 is an inhibitor of the tumor-driver bromodomain protein BRD4 and produces satisfactory effects because it efficiently increases apoptosis. Ferroptosis is an oxidative cell death program differing from apoptosis. Ferroptosis is characterized by high levels of iron and reactive oxygen species and has been confirmed to suppress tumor growth. In this study, BRD4 expression in cancer and its influence on the prognosis of cancer patients were analyzed using data from public databases. In addition, the effect of the BRD4 inhibitor (+)-JQ1 on ferroptosis was investigated via a series of in vitro assays. A nude mouse model was used to evaluate the function of (+)-JQ1 in ferroptosis in vivo. The potential mechanisms by which (+)-JQ1 regulates ferroptosis were explored. The results showed that BRD4 expression levels were higher in cancer tissues than in normal tissues and were related to poor prognosis in cancer patients. Furthermore, ferroptosis was induced under (+)-JQ1 treatment and BRD4 knockdown, indicating that (+)-JQ1 induces ferroptosis via BRD4 inhibition. Moreover, the anticancer effect of (+)-JQ1 was enhanced by ferroptosis inducers. Further studies confirmed that (+)-JQ1 induced ferroptosis via ferritinophagy, which featured autophagy enhancement by (+)-JQ1 and increased iron levels. Subsequently, the reactive oxygen species levels were increased by iron via the Fenton reaction, leading to ferroptosis. In addition, expression of the ferroptosis-associated genes GPX4, SLC7A11, and SLC3A2 was downregulated under (+)-JQ1 treatment and BRD4 knockdown, indicating that (+)-JQ1 may regulate ferroptosis by controlling the expression of ferroptosis-associated genes regulated by BRD4. Finally, (+)-JQ1 regulated ferritinophagy and the expression of ferroptosis-associated genes via epigenetic inhibition of BRD4 by suppressing the expression of the histone methyltransferase G9a or enhancing the expression of the histone deacetylase SIRT1. In summary, the BRD4 inhibitor (+)-JQ1 induces ferroptosis via ferritinophagy or the regulation of ferroptosis-associated genes through epigenetic repression of BRD4.

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Ferritinophagy is required for the induction of ferroptosis by the bromodomain protein BRD4 inhibitor (+)-JQ1 in cancer cells

The biology function of antisense intronic long noncoding RNA (Ai-lncRNA) is still unknown. Meanwhile, cancer patients with paclitaxel resistance have limited therapeutic options in the clinic. However, the potential involvement of Ai-lncRNA in paclitaxel sensitivity remains unclear in human cancer. Whole transcriptome sequencing of 33 breast specimens was performed to identify Ai-lncRNA EGOT. Next, the role of EGOT in regulation of paclitaxel sensitivity was investigated. Moreover, the mechanism of EGOT enhancing autophagy sensitizes paclitaxel cytotoxicity via upregulation of ITPR1 expression by RNA-RNA and RNA-protein interactions was investigated in detail. Furthermore, upstream transcriptional regulation of EGOT expression was also investigated by co-immunoprecipitation and chromatin immunoprecipitation. Finally, clinical breast specimens in our cohort, TCGA and ICGC were applied to validate the role of EGOT in enhancing of paclitaxel sensitivity. EGOT enhances autophagosome accumulation via the up-regulation of ITPR1 expression, thereby sensitizing cells to paclitaxel toxicity. Mechanistically, on one hand, EGOT upregulates ITPR1 levels via formation of a pre-ITPR1/EGOT dsRNA that induces pre-ITPR1 accumulation to increase ITPR1 protein expression in cis. On the other hand, EGOT recruits hnRNPH1 to enhance the alternative splicing of pre-ITPR1 in trans via two binding motifs in EGOT segment 2 (324–645 nucleotides) in exon 1. Moreover, EGOT is transcriptionally regulated by stress conditions. Finally, EGOT expression enhances paclitaxel sensitivity via assessment of cancer specimens. These findings broaden comprehensive understanding of the biology function of Ai-lncRNAs. Proper regulation of EGOT may be a novel synergistic strategy for enhancing paclitaxel sensitivity in cancer therapy.

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Ai-lncRNA EGOT enhancing autophagy sensitizes paclitaxel cytotoxicity via upregulation of ITPR1 expression by RNA-RNA and RNA-protein interactions in human cancer

Eosinophil granule ontogeny transcript (EGOT) is a long noncoding RNA involved in the regulation of eosinophil granule protein transcript expression. However, little is known about the role of EGOT in malignant disease. This study aimed to assess the potential role of EGOT in the pathogenesis of breast cancer. Quantitative real-time polymerase chain reaction was performed to detect the expression levels of EGOT in 250 breast cancerous tissues and 50 adjacent noncancerous tissues. The correlation of EGOT expression with clinicopathological features and prognosis was also analyzed. EGOT expression was lower in breast cancer compared with the adjacent noncancerous tissues (P < 0.001), and low levels of EGOT expression were significantly correlated with larger tumor size (P = 0.022), more lymph node metastasis (P = 0.020), and higher Ki-67 expression (P = 0.017). Moreover, patients with low levels of EGOT expression showed significantly worse prognosis for overall survival (P = 0.040), and this result was further validated in a larger cohort from a public database. Multivariate analysis suggested that low levels of EGOT were a poor independent prognostic predictor for breast cancer patients (HR = 1.857, 95 % CI = 1.032–3.340, P = 0.039). In conclusion, EGOT may play an important role in breast cancer progression and prognosis and may serve as a new potential prognostic target in breast cancer patients.

Shiyao Sui

Papers

Inhibition of tumor propellant glutathione peroxidase 4 induces ferroptosis in cancer cells and enhances anticancer effect of cisplatin

Downregulation of long noncoding RNA MALAT1 induces epithelial-to-mesenchymal transition via the PI3K-AKT pathway in breast cancer

Ferritinophagy is required for the induction of ferroptosis by the bromodomain protein BRD4 inhibitor (+)-JQ1 in cancer cells

Ai-lncRNA EGOT enhancing autophagy sensitizes paclitaxel cytotoxicity via upregulation of ITPR1 expression by RNA-RNA and RNA-protein interactions in human cancer

Downregulation of the long noncoding RNA EGOT correlates with malignant status and poor prognosis in breast cancer