Ferroptosis is a recently recognized form of regulated cell death. It is characterized morphologically by the presence of smaller than normal mitochondria with condensed mitochondrial membrane densities, reduction or vanishing of mitochondria crista, and outer mitochondrial membrane rupture. It can be induced by experimental compounds (e.g., erastin, Ras-selective lethal small molecule 3, and buthionine sulfoximine) or clinical drugs (e.g., sulfasalazine, sorafenib, and artesunate) in cancer cells and certain normal cells (e.g., kidney tubule cells, neurons, fibroblasts, and T cells). Activation of mitochondrial voltage-dependent anion channels and mitogen-activated protein kinases, upregulation of endoplasmic reticulum stress, and inhibition of cystine/glutamate antiporter is involved in the induction of ferroptosis. This process is characterized by the accumulation of lipid peroxidation products and lethal reactive oxygen species (ROS) derived from iron metabolism and can be pharmacologically inhibited by iron chelators (e.g., deferoxamine and desferrioxamine mesylate) and lipid peroxidation inhibitors (e.g., ferrostatin, liproxstatin, and zileuton). Glutathione peroxidase 4, heat shock protein beta-1, and nuclear factor erythroid 2-related factor 2 function as negative regulators of ferroptosis by limiting ROS production and reducing cellular iron uptake, respectively. In contrast, NADPH oxidase and p53 (especially acetylation-defective mutant p53) act as positive regulators of ferroptosis by promotion of ROS production and inhibition of expression of SLC7A11 (a specific light-chain subunit of the cystine/glutamate antiporter), respectively. Misregulated ferroptosis has been implicated in multiple physiological and pathological processes, including cancer cell death, neurotoxicity, neurodegenerative diseases, acute renal failure, drug-induced hepatotoxicity, hepatic and heart ischemia/reperfusion injury, and T-cell immunity. In this review, we summarize the regulation mechanisms and signaling pathways of ferroptosis and discuss the role of ferroptosis in disease.

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Ferroptosis: process and function.

Ferroptosis is a newly discovered form of regulated cell death that is the nexus between metabolism, redox biology, and human health. Emerging evidence shows the potential of triggering ferroptosis for cancer therapy, particularly for eradicating aggressive malignancies that are resistant to traditional therapies. Recently, there has been a great deal of effort to design and develop anticancer drugs based on ferroptosis induction. Recent advances of ferroptosis-inducing agents at the intersection of chemistry, materials science, and cancer biology are presented. The basis of ferroptosis is summarized first to highlight the feasibility and characteristics of triggering ferroptosis for cancer therapy. A literature review of ferroptosis inducers (including small molecules and nanomaterials) is then presented to delineate their design, action mechanisms, and anticancer applications. Finally, some considerations for research on ferroptosis inducers are spotlighted, followed by a discussion on the challenges and future development directions of this burgeoning field.

Recent Progress in Ferroptosis Inducers for Cancer Therapy

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Roles, Functions, and Mechanisms of Long Non-coding RNAs in Cancer.

Ferroptosis is a novel type of cell death with distinct properties and recognizing functions involved in physical conditions or various diseases including cancers. The fast-growing studies of ferroptosis in cancer have boosted a perspective for its usage in cancer therapeutics. Here, we review the current findings of ferroptosis regulation and especially focus on the function of ncRNAs in mediating the process of cell ferroptotic death and on how ferroptosis was in relation to other regulated cell deaths. Aberrant ferroptosis in diverse cancer types and tissues were summarized, and we elaborated recent data about the novel actors of some “conventional” drugs or natural compounds as ferroptosis inducers in cancer. Finally, we deliberate future orientation for ferroptosis in cancer cells and current unsettled issues, which may forward the speed of clinical use of ferroptosis induction in cancer treatment.

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Ferroptosis, a new form of cell death: opportunities and challenges in cancer

The identification of RNAs that are not translated into proteins was an important breakthrough, defining the diversity of molecules involved in eukaryotic regulation of gene expression. These non-coding RNAs can be divided into two main classes according to their length: short non-coding RNAs, such as microRNAs (miRNAs), and long non-coding RNAs (lncRNAs). The lncRNAs in association with other molecules can coordinate several physiological processes and their dysfunction may impact in several pathologies, including cancer and infectious diseases. They can control the flux of genetic information, such as chromosome structure modulation, transcription, splicing, messenger RNA (mRNA) stability, mRNA availability, and post-translational modifications. Long non-coding RNAs present interaction domains for DNA, mRNAs, miRNAs, and proteins, depending on both sequence and secondary structure. The advent of new generation sequencing has provided evidences of putative lncRNAs existence; however, the analysis of transcriptomes for their functional characterization remains a challenge. Here, we review some important aspects of lncRNA biology, focusing on their role as regulatory elements in gene expression modulation during physiological and disease processes, with implications in host and pathogens physiology, and their role in immune response modulation.

https://www.mdpi.com/2311-553X/5/1/17/pdf

Long Non-Coding RNAs in the Regulation of Gene Expression: Physiology and Disease

Knockdown of long non-coding RNA XIST exerts tumor-suppressive functions in human glioblastoma stem cells by up-regulating miR-152

Glioblastoma multiforme (GBM) is one of the most common encephalic malignant tumors. Due to a high recurrence rate and a lack of effective treatments, the average survival rate remains low. Temozolomide (TMZ), a class of alkylating agent, is widely used as a first-line therapeutic drug during the adjuvant treatment for GBM patients. However, most patients exhibit a palpable resistance to TMZ treatment. Additionally, the underlying mechanism remains to be clarified. In this study, glutathione (GSH) and reactive oxygen species (ROS) levels were found to be closely associated with the sensitivity of GBM cells to TMZ. We also found that TMZ markedly induced xCT, the subunit of glutamate/cystine transporter system xc- expression, which together with the GSH synthesis was increased while the TMZ-inducible ROS level was decreased in GBM cells. In addition, the cystathionine γ-lyase (CTH) acivity, a key enzyme in the transsulfuration pathway was enhanced by TMZ, which insured a cysteine supply and GSH synthesis in a compensatory manner when xCT was blocked. Thus, the individual inhibition of xCT by siRNA and a pharmacological inhibitor (sulfasalazine) only partially inhibited GSH synthesis and moderately enhanced the GBM cell sensitivity to TMZ. However, the TMZ‑induced cytotoxicity was markedly increased along with a marked decrease in GSH levels as result of co-treatment with erastin, which inhibited cysteine uptake from xCT transporter and suppressed CTH activity, leading to impaired transformation from methionine to cysteine. In conclusion, to GBM therapy with a drug combination of TMZ and erastin may be beneficial.

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Erastin sensitizes glioblastoma cells to temozolomide by restraining xCT and cystathionine-γ-lyase function

Glioma is the most prevalent primary brain tumors in adults. In addition to the high incidence and mortality rate, the 5-year survival rate of glioma is also extremely low. MicroRNAs (miRNAs), as a class of small non-coding RNAs, may play an important role in carcinogenesis. It was also proposed that miRNAs might also be associated with glioma diagnosis and prognosis. In this study, we aimed at investigating the predictive and prognostic values of miR-125b, miR-221, and miR-222 in glioma and, hopefully, to provide some evidence for novel therapy of glioma. Tissue specimens were obtained from tumor tissue and adjacent non-tumor tissue. RNA was extracted and qRT-PCR was performed with U6 being the internal control. Receiver-operating characteristic (ROC) curves were constructed, and the area under the ROC curves (AUC) was calculated to evaluate the significance of candidate miRNAs in distinguishing glioma tumor tissues and adjacent normal tissues. Survival curves of Kaplan-Meier method were constructed for both high expression group and low expression group, and the difference between curves was evaluated by log-rank test. All the statistical analyses were performed using Stata version 12.0 software, and graphs were generated by GraphPad Prism 5.0. The significance of miR-125b, miR-221, and miR-222 expression level in distinguishing glioma tumor from adjacent non-tumor tissues was further validated. Combination of miR-125b, miR-221, and miR-22 was significantly superior compared to the clinical standard of using these miRNAs alone. A clear demarcation was shown by survival analysis between patients with high miR-125b, miR-221, and miR-222 expression and patients with poor prognosis. Similarly, panel of these miRNAs could play a better prognostic role in glioma. In this study, we confirmed the significance of miR-125b, miR-221, and miR-222 in distinguishing glioma tumor from adjacent non-tumor tissues. Higher expressions of miR-125b and miR-222 have also been proved to be associated with glioma. Furthermore, glioma patients with higher miR-125b, miR-221, and miR-222 expression were manifested to have poorer prognostic status, which might be attributed to their attenuated sensitivity to chemotherapy and radiotherapy.

Predictive and Prognostic Roles of Abnormal Expression of Tissue miR-125b, miR-221, and miR-222 in Glioma

The blood-tumor barrier (BTB) hinders delivery of chemotherapeutic drugs to tumors in the brain; previous studies have shown that the BTB can be selectively opened by endothelial monocyte activating polypeptide-II (EMAP-II), but the specific mechanism involved remains elusive. In this study, we found that microRNA-429 (miR-429) expression in glioma vascular endothelial cells (GECs) was far lower than in human brain microvascular endothelial cells (ECs). miR-429 had lower expression in GECs and glioma tissues compared to ECs or normal tissues of the brain. Furthermore, miR-429 had lower expression in high grade glioma (HGG) than in low grade glioma (LGG). In in vitro BTB models, we also found that EMAP-II significantly increased BTB permeability, decreased expression of ZO-1, occludin and claudin-5 in GECs, in a time- and dose-dependent manner. EMAP-II greatly increased miR-429 expression in GECs of the BTB models in vitro. Overexpression of miR-429 in GECs significantly decreased the transepithelial electric resistance (TEER) values in BTB models, and led to enhanced horseradish peroxidase (HRP) flux. Overexpression of miR-429 in GECs significantly decreased the expression of tight junction (TJ)-associated proteins (ZO-1, occludin and claudin-5), and decreased the distribution continuity. Silencing of miR-429 in GECs increased the expression of TJ-associated proteins and the distribution continuity. The dual-luciferase reporter assay revealed that ZO-1 and occludin were target genes of miR-429, and we demonstrated that miR-429 overexpression markedly down-regulated protein expression of p70S6K, as well as its phosphorylation levels. The dual-luciferase reporter assay also showed that p70S6K was a target gene of miR-429; miR-429 overexpression down-regulated expression and phosphorylation levels of p70S6K, and also decreased phosphorylation levels of S6 and increased BTB permeability. Conversely, silencing of miR-429 increased the expression and phosphorylation levels of p70S6K, and increased phosphorylation levels of S6, while decreasing BTB permeability. In conclusion, the results indicated that EMAP-II caused an increase in miR-429 expression that directly targeted TJ-associated proteins, which were negatively regulated; on the other hand, miR-429 down-regulated the expression of TJ-associated proteins by targeting p70S6K, also negatively regulated. As a result, the BTB permeability increased.

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MiR-429 Regulated by Endothelial Monocyte Activating Polypeptide-II (EMAP-II) Influences Blood-Tumor Barrier Permeability by Inhibiting the Expressions of ZO-1, Occludin and Claudin-5.

MicroRNA-34a (miR-34a) functions to regulate protein expression at the posttranscriptional level by binding the 3′ UTR of target genes and regulates functions of vascular endothelial cells. However, the role of miR-34a in regulating blood–tumor barrier (BTB) permeability remains unknown. In this study, we show that miR-34a overexpression leads to significantly increased permeability of BTB, whereas miR-34a silencing reduces the permeability of the BTB. In addition, miR-34a overexpression significantly down-regulates the expression and distribution of tight junction–related proteins in glioma endothelial cells (GECs), paralleled by protein kinase Ce (PKCe) reduction. Moreover, luciferase reporter gene analysis shows that PKCe is the target gene of miR-34a. We also show that cotransfection of miR-34a and PKCe inversely coregulates BTB permeability and protein expression levels of tight junction–related proteins. Pretreatment of ψeRACK, a PKCe-specific activator, decreases BTB permeability in miR-34a–overexpressed GECs and up-regulates expression levels of tight junction proteins. In contrast, pretreatment of eV1-2, a specific PKCe inhibitor, gives opposite results. Collectively, our findings indicate that miR-34a regulates BTB function by targeting PKCe; after phosphorylation, PKCe is activated and contributes to regulation of the expression of tight junction–related proteins, ultimately altering BTB permeability.

Liangyu Chen

Papers

Knockdown of long non-coding RNA XIST exerts tumor-suppressive functions in human glioblastoma stem cells by up-regulating miR-152

Erastin sensitizes glioblastoma cells to temozolomide by restraining xCT and cystathionine-γ-lyase function

Predictive and Prognostic Roles of Abnormal Expression of Tissue miR-125b, miR-221, and miR-222 in Glioma

MiR-429 Regulated by Endothelial Monocyte Activating Polypeptide-II (EMAP-II) Influences Blood-Tumor Barrier Permeability by Inhibiting the Expressions of ZO-1, Occludin and Claudin-5.

MiR-34a regulates blood–tumor barrier function by targeting protein kinase Cε