The Role of miR-210 in the Biological System: A Current Overview

MicroRNAs (miRNAs) are endogenous RNAs measuring 19–23 nucleotides in length that control a wide range of cellular processes [1]. These molecules are severely deregulated in various diseases, including cancer [2, 3]. miRNAs make up approximately 1–2% of the eukaryotic cell transcriptome and play a crucial role in cell coordination, differentiation, proliferation, metabolism, and death [4‒6]. These molecules also stimulate the post-transcriptional regulation of gene expression via direct or indirect mechanisms [7]. miRNAs are found in plants, viruses, green algae, and deeply branching animals (starlet sea anemone and sponge). Other types of small non-coding RNAs are found in animals, plants, and fungi [1]. miRNAs regulate gene expression and control a broad range of physiological systems by targeting mRNAs and causing translational suppression or degradation of RNAs [8‒10], miRNAs are involved in regulating HIF pathways [11, 12].

Hypoxia is a unique environmental stress that induces substantial changes in the regulatory system of signaling proteins and transcriptional factors to organize cellular adaptation in various metabolic processes, DNA repair, proliferation, and apoptosis [13]. Hypoxia refers to the lack of oxygen supply to cells due to biological or pathological circumstances, including high altitude, abnormal vasculature, or anemia [8]. Hypoxic-ischemic encephalopathy (HIE) is the major cause of brain injury and long-term neurological sequelae in the prenatal period [14].

Hypoxia-inducible factor (HIF) controls the response of cells to hypoxia by regulating genes included in the metabolic process, cell differentiation, cell proliferation, angiogenesis, and apoptosis (Fig. 1) [8]. Induction of miR-210 expression is an important feature of the cellular low-oxygen response and the most consistent and vigorous target of HIF [8, 15]. This review summarizes the physiological functions of miR-210 under normal biological and pathological conditions, and the regulation of its expression by hypoxia.

Many studies have shown a direct relationship between hypoxia and miR-210 expression in normal and transformed cells; specifically, miR-210 is upregulated by hypoxia [16‒18]. miR-210 is highly upregulated in cells under hypoxia, thereby revealing its significance to cell endurance [16, 19]. Kelly et al. [20] identified a new regulator of HIF-1 called glycerol-3-phosphate dehydrogenase 1-like (GPD1L), which is regulated by HIF-1-inducible miR-210. Stimulation of miR-210 by HIF-1 induces a remarkable decrease in GPD1L protein expression, which leads to an increase in HIF-1 stabilization. Under normal biological circumstances, GPD1L increases the activity of prolyl-hydroxylase domain isoforms and hydrolyzes HIF-1 proline, eventually driving the degradation of HIF-1 via protein complexes called proteasomes. miR-210 overexpression increases the accumulation of HIF-1 under hypoxic conditions due to decreased GPD1L protein expression because the miRNA targets the GPD1L mRNA 3′ UTR. Low miR-210 levels and, consequently, upregulated GPD1L expression have been observed under low HIF-1 protein expression.

A decrease in oxygen levels increases HIF-1 protein and gene expression activity, thereby leading to miR-210 accumulation. As a result, GPD1L expression is downregulated and inactivates prolyl-hydroxylase domain isoforms, which leads to an increase in HIF-1 protein. The aforementioned process consists of an exacerbating feedback loop in which miR-210 stimulates and retains the amount of HIF-1 protein. Inhibition of miR-210 could affect this hypoxic loop [17, 20]. An earlier study showed that HIF-2α is involved in the regulatory process of miR-210 [21]. HIF-1α binds to hypoxia-responsive elements (HREs) at the miR-210 promoter closest to the transcription start site [22]. An active HRE is located approximately 40 base pairs upstream from the transcriptional starting site. This element is essential for inducing AK123483 (the miR-210 host gene) under hypoxia and may serve as a regulator of miR-210 gene expression under hypoxic conditions [8].

Comparison of the core promoters of miR-210 across different organisms suggests that the HRE region is deeply conserved and that hypoxia is essential to the regulation of miR-210 expression among species. miR-210 induction in rats under hypoxic conditions depends on HIF-1α [23]. One of the conserved HREs is essential for miR-210 induction in mice under hypoxia [15]. Previous studies showed that many additional binding sites of a number of transcription factors, such as PPARγ, E2F1, and Oct4, in very close proximity to the miR-210 HRE region are also strongly conserved across organisms [24, 25]. These factors could be involved in the regulatory process of miR-210 gene expression in tissues and cells; unfortunately, this involvement has not yet been extensively investigated [25].

Chan et al. [26] revealed that mitochondrial metabolism is regulated by miR-210 under hypoxic conditions. The cell metabolic process shifts from oxidative phosphorylation to glycolysis under hypoxia. Several hypoxia-induced proteins, including lactate dehydrogenase A, cytochrome c oxidase subunit 4-2, pyruvate dehydrogenase kinase, and mitochondrial protease LON, are engaged in this metabolic change [27]. HIF-1 upregulates the expression of numerous glycolytic enzymes and pyruvate dehydrogenase kinase and downregulates mitochondrial respiration [28]. miR-210 reduces the activity of TCA cycle enzymes and performs mitochondrial functions by downregulating iron-sulfur cluster assembly proteins, which leads to an increase in the generation of free radicals, enhancement of cell endurance under hypoxic conditions, induction of a shift to glycolysis in normoxia and hypoxia, and improvement of the iron intake needed for several cell functions [29].

miR-210 specifically targets iron-sulfur cluster assembly proteins 1/2 and reduces the activity of proteins regulating metabolic process in the mitochondria, such as aconitase and complex I, which leads to reduced oxidative phosphorylation [26]. Several studies have investigated the involvement of miR-210 in regulating mitochondrial function and discovered many targets for miR-210 [29‒32]. Mitochondria are the main sites for the production of reactive oxygen species (ROS). However, more research is required to understand the function of miR-210 in modulating ROS levels. miR-210 is a multi-faceted controller for many cellular features. Other roles of miR-210 under hypoxia or normoxia may be expected on account of research demonstrating its numerous regulatory functions since it was first established as a miRNA regulated by hypoxia.

Genome integrity is extremely important for cells because any defect in pivotal genes leads to various diseases. Multiple influences, including ROS, mutagens, ultraviolet rays, gamma rays, and chemical agents, lead to several forms of DNA damage, among which the breaking of the DNA double-strand (DSB) is the most severe [33]. Genetic instability is one of the characteristics of cancer [34]. Hypoxia can increase genetic instability by downregulating genes involved in DNA damage repair, such as RAD51, MSH2, and MLH1 [35]. The mechanism of suppression of these genes under hypoxic conditions includes histone deacetylation, which can alter the chromatin structure of the promoter of MLH1 [36]. miR-210 suppresses the translation, but not transcription, of RAD52, which is essential for the repair of DNA DSB. This protein is also important for homologous recombination and could provide a new regulatory mechanism for repairing damaged DNA by downregulating the cellular DNA repair process under hypoxic conditions [23]. In addition to downregulating DNA-repairing genes, including RAD52, miR-210 promotes DNA DSB repair after exposure to radiation, which increases genetic instability and cancer cell proliferation [32]. Non-small-cell lung cancer cell lines expressing miR-210 in normal oxygen levels are radiation resistant because of effective DNA repair; however, the basic mechanism behind this effect requires further exploration [18]. These findings confirm that miR-210 plays an important protective role in DNA repair.

The production of new blood vessels from current endothelial vascular cells provides oxygen and nutrients to different tissues and organs [37]. Hypoxic regions in all solid tumors induce angiogenesis and promote tumor growth [8]. The signaling pathway that controls angiogenesis includes many vascular growth factors [38]. One of these factors is vascular endothelial growth factor (VEGF), a hypoxia-regulated gene involved in the regulation of tumor angiogenesis [39]. miR-210-stimulated VEGF-driven cell migration and capillary-like structure formation occur through the inhibition of receptor tyrosine kinase ligand ephrin-A3 [19, 40]. miR-210 notably stimulated angiogenesis regulated by the signaling pathway of VEGF in a mouse renal ischemic/perfusion model in vivo [41].

Zeng et al. [42] revealed that miR-210-injected mice show increased CD31⁺ vessel numbers, but SMA⁺ vessel numbers did not show the same increase; these results indicate that miR-210 induces angiogenesis but not collateral growth or arteriogenesis. The Notch signaling pathway plays an essential regulatory role throughout normal vascular growth. In this pathway, ligand Dll4, which is necessary for the creation of functional vasculature, is normally expressed in the endothelia of new blood vessels [43, 44]. miR-210 can regulate angiogenesis and vasculature development in post-ischemic brain tissues by targeting Notch expression as one of the main cellular processes regulating post-ischemic angiogenesis [38].

In certain types of cells, hypoxia may induce arrest in cell cycle by targeting HIF-1α [45]. miR-210 is the main target of HIF-1α, which controls the progression of cell cycle by targeting E2F transcriptional factor 3 (E2F3) [21, 46]. E2F3 is one of the main proteins in the cell cycle and is regulated at the protein level through miR-210 induction [46]. E2F3a expression varies throughout the cell cycle; high E2F3a expression is found in the G1/S phase, while E2F3b expression is generally crucial during the cell cycle but remarkably increases in the G0 phase [47]. miR-210 regulates the G2/M transformation and is involved in mitotic development by modifying Fam83D, Bub1B, Pds5B, Cyclin F, and Plk1 expression levels. In the S phase, miR-210 is involved in centrosome replication by controlling E2F3 expression. miR-210 overexpression represses E2F3 expression and deregulates centrosome replication, which could lead to the amplification and aneuploidy of centrosomes [48].

Cyclin-dependent kinase7 (CDK7) is an activated kinase that targets and activates many other CDKs; it also regulates multiple cell-cycle checkpoints and is essential for S-phase entry [49]. miR-210 regulates CDK7 (the 3′ UTR of CDK7 contains a binding site for miR-210) and is essential for the progression of the normal neural progenitor cell cycle [50].

p53 is a crucial transcriptional factor that determines the fate of cells throughout cell cycle arrest or apoptotic activation in humans [51]. A previous study reported that miR-210 is upregulated in the p53-dependent protein and kinase B pathways on account of the regulatory effects of p53 on miR-210 [52]. However, additional work is needed to investigate the role of miR-210 in these pathways.

Inflammation is the predominant response to infection or injury. It allows the body to remove pathogens and injured tissue and initiate the repairing process. A key feature of inflamed cells/tissues is hypoxia or low oxygen levels, which is due to local vasculature damage and increased consumption of oxygen by pathogens and some immune cells [53]. Hypoxia also regulates and induces inflammatory reactions by inducing inflammatory cytokine production and directing immune cell infiltration [54]. Increasing evidence demonstrates that miR-210 is a key regulator of the inflammatory reaction under highly stressful conditions [53, 55]. Wang et al. [56] observed that miR-210 could play an important regulatory function in T-cell differentiation and stimulation and revealed that miR-210 is upregulated in the TH17 lineage (activated T cells) of helper T cells under hypoxic conditions. Other studies have confirmed miR-210 upregulation controlled by the T-cell receptor and the coreceptor CD28. A deficiency in miR-210 levels facilitates the differentiation of TH17 under hypoxic conditions. This differentiation is mediated by a balancing feedback circle where HIF-1α expression is inhibited by miR-210. One study reported that miR-210 enhances myeloid-derived suppressor cell-mediated T-cell repression by increasing the production of nitric oxide and arginase enzyme activity, thereby leading to an increase in tumor growth [57].

Interestingly, miR-210 upregulation in tumor cells reduces the latter’s resistance to the antigen-specific CD8+ T cells [58]. A recent study indicated that chemokine (e.g., CCL2 and CCL3) and pro-inflammatory cytokine (e.g., IL-6, NF-α, and IL-1β) expression levels are reduced by inhibiting miR-210 [59]. Germline miR-210 removal results in autoantibody development, whereas miR-210 overexpression in mice compromises class-switched antibody responses and enhances the immune role of the RNA in B lymphocytes [60]. Takeda et al. [61] reported that HIF-1α is expressed in murine M1 macrophages, while HIF-2α is expressed in M2 macrophages. Thus, miR-210 may be an essential regulator for another immune system cells. This miRNA has diverse effects on multiple cells in the immune system. Further studies could promote the use of miR-210 as a therapeutic target to improve the immune response to therapeutics.

The main cause of brain injury during the perinatal period is HIE. The developmental vulnerability of the brain to injury induced by hypoxia is probably related to HIF-1α, a critical regulator of physiopathological response to hypoxia stress that plays a vital role in brain and injury development [14]. Numerous studies have revealed that brain miR-210 is upregulated after a hypoxic insult [59, 62]. A study on neonatal rats by Ma et al. [62] confirmed the upregulation of miR-210 after 2.5 h of hypoxic-ischemic injury. By contrast, Zhao et al. [63] found that the miR-210 expression is downregulated after HIE. Another study reported that brain miR-210 is downregulated 72 h following hypoxic-ischemic injury; miR-210 actively represses neuronal apoptosis by inhibiting caspase activity and controlling the proper balance between bax and BCL-2 levels [14].

HIE levels in the brain of neonatal rats showed that miR-210 downregulates the glucocorticoid receptor gene by targeting its 3′ UTR, resulting in increased vulnerability to injury [62]. Deterioration of the blood-brain barrier (BBB) junction complex leads to cerebral edema, which is one of the major causes of neonatal HIE brain damage and brain tissue trauma [64, 65]. The BBB is an endothelial-specific structure [66]. miR-210 regulates the survival of endothelial cells [19, 26]. Ma et al. [67] showed that miR-210 negatively controls the integrity of the BBB in neonatal brain. Future studies should explore the possible effect of miR-210 on the BBB.

The experimental evidence reveals that miR-210 is an extensively investigated miRNA because of its involvement in many biological processes, such as DNA repair, angiogenesis, cell cycle, and immune system. miR-210 may be involved in macrophage regulation under the effect of HIF-1α because HIF-1α is expressed in M1 macrophages. Moreover, given that miR-210 is involved in tumor initiation and development, using it as a cancer biomarker (e.g., pancreatic or breast cancer) may help in tumor hypoxia. Hypoxia plays an important role in many diseases including cancer. It induces cell cycle arrest, inflammatory reactions, and promotes tumor growth in all solid tumors.

We are grateful to the National Natural Science Foundation of China, the Natural Science Foundation of Heilongjiang Province, the Supporting Plan Project for Youth Academic Backbone of General Colleges and Universities of Heilongjiang Province, the Jiamusi University Basic medical discipline team, and Natural Science major project of Jiamusi University for their supports.

The authors have no ethical conflicts to disclose.

The authors have no conflicts of interest to declare.

This work was supported by the National Natural Science Foundation of China (Nos. 30671803 and 81273174), the Natural Science Foundation of Heilongjiang Province (No. D201247), the Supporting Plan Project for Youth Academic Backbone of General Colleges and Universities of Heilongjiang Province (No. 1253G058), the Basic medical discipline team (JDXKTD-2019002), and the Natural Science major project of Jiamusi University (Sz2011-007).

X.H. proofread the manuscript. H.A-.W. and F.S. drafted the manuscript. C.-Y.L. and N.L. managed the references. All authors read and approved the final manuscript.

X.H. and H.A.-W. should both be regarded as first authors.