Depletion of METTL3 alters cellular and extracellular levels of miRNAs containing m6A consensus sequences

Beyond the Bubble: A Debate on microRNA Sorting Into Extracellular Vesicles

Over the past decade, a scientific field has been developed demonstrating microRNAs (miRNAs) to be actively sorted into extracellular vesicles via specific nucleotide motifs that interact with discrete RNA-binding proteins. These miRNAs are proposed to be transported into recipient cells in which they can regulate specific cellular pathways. This mechanism could have enormous potential in explaining how cells signal and regulate other cells nearby or at a distance. Tens of studies have built this theme of a regulated transport of miRNAs. However, some concerns exist about this field. Taken together, there are concerns of a lack of a consistent motif, RNA-binding protein, or preferential miRNA involved in this process. In this study, we provide an expert and extensive analysis of the field that makes the cases for and against an active sorting mechanism. We provide potential explanations on why there is a lack of agreement. Most importantly, we provide ideas on how to move this field forward with more rigor and reproducibility. It is hoped that by engaging in a scientific debate of the pros and cons of this field, more rigorous experiments can be performed to conclusively demonstrate this biological activity.

Epitranscriptomic RNA m6A Modification in Cancer Therapy Resistance: Challenges and Unrealized Opportunities

Significant advances in the development of new cancer therapies have given rise to multiple novel therapeutic options in chemotherapy, radiotherapy, immunotherapy, and targeted therapies. Although the development of resistance is often reported along with temporary disease remission, there is often tumor recurrence of an even more aggressive nature. Resistance to currently available anticancer drugs results in poor overall and disease-free survival rates for cancer patients. There are multiple mechanisms through which tumor cells develop resistance to therapeutic agents. To date, efforts to overcome resistance have only achieved limited success. Epitranscriptomics, especially related to m6A RNA modification dysregulation in cancer, is an emerging mechanism for cancer therapy resistance. Here, recent studies regarding the contributions of m6A modification and its regulatory proteins to the development of resistance to different cancer therapies are comprehensively reviewed. The promise and potential limitations of targeting these entities to overcome resistance to various anticancer therapies are also discussed.

Emerging roles of bases modifications and DNA repair proteins in onco-miRNA processing: novel insights in cancer biology

Onco-microRNAs (onco-miRNAs) are essential players in the post-transcriptional regulation of gene expression and exert a crucial role in tumorigenesis. Novel information about the epitranscriptomic modifications, involved in onco-miRNAs biogenesis, and in the modulation of their interplay with regulatory factors responsible for their processing and sorting are emerging. In this review, we highlight the contribution of bases modifications, sequence motifs, and secondary structures on miRNAs processing and sorting. We focus on several modes of action of RNA binding proteins (RBPs) on these processes. Moreover, we describe the new emerging scenario that shows an unexpected though essential role of selected DNA repair proteins in actively participating in these events, highlighting the original intervention represented by the non-canonical functions of Apurinic/apyrimidinic endodeoxyribonuclease 1 (APE1), a central player in Base Excision Repair (BER) pathway of DNA lesions. Taking advantage of this new knowledge will help in prospecting new cancer diagnostic and therapeutic strategies.

5-Fluorouracil treatment represses pseudouridine-containing miRNA export into extracellular vesicles

5-Fluorouracil (5-FU) has been used for chemotherapy for colorectal and other cancers for over 50 years. The prevailing view of its mechanism of action is inhibition of thymidine synthase leading to defects in DNA replication and repair. However, 5-FU is also incorporated into RNA causing defects in RNA metabolism, inhibition of pseudouridine modification, and altered ribosome function. We examined the impact of 5-FU on post-transcriptional small RNA modifications (PTxMs) and the expression and export of RNA into small extracellular vesicles (sEVs). EVs are secreted by all cells and contain a variety of proteins and RNAs that can function in cell-cell communication. We found that treatment of colorectal cancer (CRC) cells with 5-FU represses sEV export of miRNA and snRNA-derived RNAs, but promotes export of snoRNA-derived RNAs. Strikingly, 5-FU treatment significantly decreased the levels of pseudouridine on both cellular and sEV small RNA profiles. In contrast, 5-FU exposure led to increased levels of cellular small RNAs containing a variety of methyl-modified bases. These unexpected findings show that 5-FU exposure leads to altered RNA expression, base modification, and aberrant trafficking and localization of small RNAs.

Transfer of miR-100 and miR-125b increases 3D growth and invasiveness in recipient cancer cells

Aim

Extracellular communication via the transfer of vesicles and nanoparticles is now recognized to play an important role in tumor microenvironment interactions. Cancer cells upregulate and secrete abundant levels of miR-100 and miR-125b that can alter gene expression in donor and recipient cells. In this study, we sought to identify targets of miR-100 and miR-125b and conclusively demonstrate that microRNAs (miRNAs) can be functionally transferred from donor to recipient cells.

Methods

To identify targets of miR-100 and miR-125b, we used bioinformatic approaches comparing multiple colorectal cancer (CRC) cell lines, including knockout lines lacking one or both of these miRNAs. We also used spheroid and 3D growth conditions in collagen to test colony growth and invasiveness. We also used Transwell co-culture systems to demonstrate functional miRNA transfer.

Results

From an initial list of 96 potential mRNA targets, we identified and tested 15 targets, with 8 showing significant downregulation in the presence of miR-100 and miR-125b. Among these, cingulin (CGN) and protein tyrosine phosphatase receptor type-R (PTPRR) are downregulated in multiple cancers, consistent with regulation by increased levels of miR-100 and miR-125b. We also show that increased cellular levels of miR-100 and miR-125b enhance 3D growth and invasiveness in CRC and glioblastoma cell lines. Lastly, we demonstrate that extracellular transfer of miR-100 and miR-125b can silence both reporter and endogenous mRNA targets in recipient cells and also increase the invasiveness of recipient spheroid colonies when grown under 3D conditions in type I collagen.

Conclusion

miR-100 and miR-125b target multiple mRNAs that can regulate 3D cell-autonomous growth and invasiveness. By extracellular transfer, miR-100 and miR-125b can also increase colony growth and invasiveness in recipient cells through non-cell-autonomous mechanisms.

miR-100 and miR-125b Contribute to Enhanced 3D Growth and Invasiveness and can be Functionally Transferred to Silence Target Genes in Recipient Cells

Extracellular communication via the transfer of vesicles and nanoparticles is now recognized to play an important role in tumor microenvironment interactions. Cancer cells upregulate and secrete abundant levels of miR-100 and miR-125b that can alter gene expression by both cell- and non-cell-autonomous mechanisms. We previously showed that these miRNAs activate Wnt signaling in colorectal cancer (CRC) through noncanonical pairing with 5 negative regulators of Wnt signaling. To identify additional targets of miR-100 and miR-125b , we used bioinformatic approaches comparing multiple CRC cell lines, including knockout lines lacking one or both of these miRNAs. From an initial list of 96 potential mRNA targets, we tested 15 targets with 8 showing significant downregulation in the presence of miR-100 and miR-125b . Among these, Cingulin (CGN) and Protein tyrosine phosphatase receptor type-R (PTPRR) are downregulated in multiple cancers, consistent with regulation by increased levels of miR-100 and miR-125b. We also show that increased cellular levels of miR-100 and miR-125b enhance 3D growth and invasiveness in CRC and glioblastoma cell lines. Lastly, we demonstrate that extracellular transfer of miR-100 and miR-125b can silence both reporter and endogenous mRNA targets in recipient cells and also increase the invasiveness of recipient spheroid colonies when grown under 3D conditions in type I collagen.

5-Fluorouracil Treatment Represses Pseudouridine-Containing Small RNA Export into Extracellular Vesicles

5-fluorouracil (5-FU) has been used for chemotherapy for colorectal and other cancers for over 50 years. The prevailing view of its mechanism of action is inhibition of thymidine synthase leading to defects in DNA replication and repair. However, 5-FU is also incorporated into RNA causing toxicity due to defects in RNA metabolism, inhibition of pseudouridine modification, and altered ribosome function. Here, we examine the impact of 5-FU on the expression and export of small RNAs (sRNAs) into small extracellular vesicles (sEVs). Moreover, we assess the role of 5-FU in regulation of post-transcriptional sRNA modifications (PTxM) using mass spectrometry approaches. EVs are secreted by all cells and contain a variety of proteins and RNAs that can function in cell-cell communication. PTxMs on cellular and extracellular sRNAs provide yet another layer of gene regulation. We found that treatment of the colorectal cancer (CRC) cell line DLD-1 with 5-FU led to surprising differential export of miRNA snRNA, and snoRNA transcripts. Strikingly, 5-FU treatment significantly decreased the levels of pseudouridine on both cellular and secreted EV sRNAs. In contrast, 5-FU exposure led to increased levels of cellular sRNAs containing a variety of methyl-modified bases. Our results suggest that 5-FU exposure leads to altered expression, base modifications, and mislocalization of EV base-modified sRNAs.

Extracellular RNA in oncogenesis, metastasis and drug resistance

Extracellular vesicles and nanoparticles (EVPs) are now recognized as a novel form of cell-cell communication. All cells release a wide array of heterogeneous EVPs with distinct protein, lipid, and RNA content, dependent on the pathophysiological state of the donor cell. The overall cargo content in EVPs is not equivalent to cellular levels, implying a regulated pathway for selection and export. In cancer, release and uptake of EVPs within the tumour microenvironment can influence growth, proliferation, invasiveness, and immune evasion. Secreted EVPs can also have distant, systemic effects that can promote metastasis. Here, we review current knowledge of EVP biogenesis and cargo selection with a focus on the role that extracellular RNA plays in oncogenesis and metastasis. Almost all subtypes of RNA have been identified in EVPs, with miRNAs being the best characterized. We review the roles of specific miRNAs that have been detected in EVPs and that play a role in oncogenesis and metastasis.

The effects of intracellular and exosomal ncRNAs on cancer progression

Altered gene expression as well as mislocalization of a gene's encoded product (proteins or noncoding RNAs (ncRNAs)) can lead to disease and cancer formation. Multiple studies have indicated that exosomes and their contents act as cell-to-cell communicators and play a key role in cancer progression. Moreover, exosomes contain several functional molecules, including ncRNAs. NcRNAs function as master regulators to coordinate cell growth, cell motility and drug resistance. However, intracellular ncRNAs, which can be transferred to recipient cells via exosomes (exosomal ncRNAs), mediate common/distinct downstream molecules, signaling pathways and functions that are less emphasized concepts in cancer development research. In this study, by using exosomes as a model, we comprehensively discuss the current knowledge regarding (1) the functional role of ncRNAs, both their intracellular and exosomal forms, in cancer progression, (2) the possible mechanism of ncRNA incorporation into exosomes and (3) the therapeutic applications and limitations of exosomes based on current knowledge.

HNRNPA2B1-Mediated MicroRNA-92a Upregulation and Section Acts as a Promising Noninvasive Diagnostic Biomarker in Colorectal Cancer

MicroRNA-92a (miR-92a) may serve as a novel promising biomarker in multiple cancers, including colorectal cancer (CRC); however, the diagnostic accuracy and the underlying molecular mechanism of miR-92a in CRC is poorly understood. We first carried out meta-analysis and found that serum/plasma miR-92a yield better diagnostic efficacy when compared to stool samples and CRC tissues, and this finding was validated by our independent study through stool sample. Multiple bioinformatics assay indicated that miR-92a expression was positively correlated with heterogeneous nuclear ribonucleoproteins A2/B1 (HNRNPA2B1) expression and closely related with the clinical characteristics of CRC. Experimental evidence showed that knockdown of HNRNPA2B1 could significantly decrease miR-92a expression and secretion in RKO cells. HNRNPA2B1 mediated miR-92a via m6A RNA modification. These findings indicate that HNRNPA2B1-m6A RNA modification-derived MicroRNA-92a upregulation and section from the local CRC acts a candidate noninvasive serum biomarker in colorectal cancer. Our study provides a novel insight into miR-92a mechanisms in relation to both expression and secretion for CRC diagnosis.

Emerging roles of m6A RNA modification in cancer therapeutic resistance

Marvelous advancements have been made in cancer therapies to improve clinical outcomes over the years. However, therapeutic resistance has always been a major difficulty in cancer therapy, with extremely complicated mechanisms remain elusive. N6-methyladenosine (m6A) RNA modification, a hotspot in epigenetics, has gained growing attention as a potential determinant of therapeutic resistance. As the most prevalent RNA modification, m6A is involved in every links of RNA metabolism, including RNA splicing, nuclear export, translation and stability. Three kinds of regulators, "writer" (methyltransferase), "eraser" (demethylase) and "reader" (m6A binding proteins), together orchestrate the dynamic and reversible process of m6A modification. Herein, we primarily reviewed the regulatory mechanisms of m6A in therapeutic resistance, including chemotherapy, targeted therapy, radiotherapy and immunotherapy. Then we discussed the clinical potential of m6A modification to overcome resistance and optimize cancer therapy. Additionally, we proposed existing problems in current research and prospects for future research.

Recent Advances in the Study of Extracellular Vesicles in Colorectal Cancer

There has been significant progress in the study of extracellular vesicles (EVs) since the 2017 American Gastroenterological Association-sponsored Freston Conference "Extracellular Vesicles: Biology, Translation and Clinical Application in GI Disorders." The burgeoning interest in this field stems from the increasing recognition that EVs represent an understudied form of cell-to-cell communication and contain cargo replete with biomarkers and therapeutic targets. This short review will highlight recent advances in the field, with an emphasis on colorectal cancer. After a brief introduction to secreted particles, we will describe how our laboratory became interested in EVs, which led to refined methods of isolation and identification of 2 secreted nanoparticles. We will then summarize the cargo found in small EVs released from colorectal cancer cells and other cells in the tumor microenvironment, as well as those found in the circulation of patients with colorectal cancer. Finally, we will consider the continuing challenges and future opportunities in this rapidly evolving field.

The Epitranscriptome in miRNAs: Crosstalk, Detection, and Function in Cancer

The epitranscriptome encompasses all post-transcriptional modifications that occur on RNAs. These modifications can alter the function and regulation of their RNA targets, which, if dysregulated, result in various diseases and cancers. As with other RNAs, miRNAs are highly modified by epitranscriptomic modifications such as m6A methylation, 2'-O-methylation, m5C methylation, m7G methylation, polyuridine, and A-to-I editing. miRNAs are a class of small non-coding RNAs that regulates gene expression at the post-transcriptional level. miRNAs have gathered high clinical interest due to their role in disease, development, and cancer progression. Epitranscriptomic modifications alter the targeting, regulation, and biogenesis of miRNAs, increasing the complexity of miRNA regulation. In addition, emerging studies have revealed crosstalk between these modifications. In this review, we will summarize the epitranscriptomic modifications-focusing on those relevant to miRNAs-examine the recent crosstalk between these modifications, and give a perspective on how this crosstalk expands the complexity of miRNA biology.