5'-UTR and 3'-UTR Regulation of MICB Expression in Human Cancer Cells by Novel microRNAs

Natural killer cells in cancer immunotherapy

Natural killer (NK) cells, as innate lymphocytes, possess cytotoxic capabilities and engage target cells through a repertoire of activating and inhibitory receptors. Particularly, natural killer group 2, member D (NKG2D) receptor on NK cells recognizes stress-induced ligands-the MHC class I chain-related molecules A and B (MICA/B) presented on tumor cells and is key to trigger the cytolytic response of NK cells. However, tumors have developed sophisticated strategies to evade NK cell surveillance, which lead to failure of tumor immunotherapy. In this paper, we summarized these immune escaping strategies, including the downregulation of ligands for activating receptors, upregulation of ligands for inhibitory receptors, secretion of immunosuppressive compounds, and the development of apoptosis resistance. Then, we focus on recent advancements in NK cell immune therapies, which include engaging activating NK cell receptors, upregulating NKG2D ligand MICA/B expression, blocking inhibitory NK cell receptors, adoptive NK cell therapy, chimeric antigen receptor (CAR)-engineered NK cells (CAR-NK), and NKG2D CAR-T cells, especially several vaccines targeting MICA/B. This review will inspire the research in NK cell biology in tumor and provide significant hope for improving cancer treatment outcomes by harnessing the potent cytotoxic activity of NK cells.

Regulatory and therapeutic implications of competing endogenous RNA network in breast cancer progression and metastasis: A review

Breast cancer (BC) is a global health concern, and development of diagnostic tools and targeted treatments for BC remains challenging. Therapeutic approaches for BC often involve a combination of surgery, radiation therapy, chemotherapy, targeted therapy, and hormone therapy. In recent years, there has been a growing interest in the role of noncoding RNAs (ncRNAs), including long ncRNAs (lncRNAs) and microRNAs (miRNAs), in BC and their therapeutic implications. Various biological processes such as cell proliferation, migration, and apoptosis rely on the activities of these ncRNAs, and their dysregulation has been implicated in BC progression. The regulatory function of the competitive endogenous RNA (ceRNA) network, which comprises lncRNAs, miRNAs, and mRNAs, has been the subject of extensive pathophysiological research. Most lncRNAs serve as molecular sponges for miRNAs and sequester their activities, thereby regulating the expression of target mRNAs and contributing to the promotion or inhibition of BC progression. This review summarizes recent findings on the role of ceRNA networks in BC progression, metastasis, and therapeutic resistance, and highlights the association of ceRNA networks with transcription factors and signaling pathways. Understanding the ceRNA network can lead to the discovery of biomarkers and targeted treatment methods to prevent the spread and metastasis of BC.

Copy number variations and their effect on the plasma proteome

Structural variations, including copy number variations (CNVs), affect around 20 million bases in the human genome and are common causes of rare conditions. CNVs are rarely investigated in complex disease research because most CNVs are not targeted on the genotyping arrays or the reference panels for genetic imputation. In this study, we characterize CNVs in a Swedish cohort (N = 1,021) using short-read whole-genome sequencing (WGS) and use long-read WGS for validation in a subcohort (N = 15), and explore their effect on 438 plasma proteins. We detected 184,182 polymorphic CNVs and identified 15 CNVs to be associated with 16 proteins (P < 8.22×10-10). Of these, 5 CNVs could be perfectly validated using long-read sequencing, including a CNV which was associated with measurements of the osteoclast-associated immunoglobulin-like receptor (OSCAR) and located upstream of OSCAR, a gene important for bone health. Two other CNVs were identified to be clusters of many short repetitive elements and another represented a complex rearrangement including an inversion. Our findings provide insights into the structure of common CNVs and their effects on the plasma proteome, and highlights the importance of investigating common CNVs, also in relation to complex diseases.

MBS: a genome browser annotation track for high-confident microRNA binding sites in whole human transcriptome

MicroRNAs (miRNAs) are small non-coding ribonucleic acids (RNAs) that play a role in many regulatory pathways in eukaryotes. They usually exert their functions by binding mature messenger RNAs. The prediction of the binding targets of the endogenous miRNAs is crucial to unravel the processes they are involved in. In this work, we performed an extensive miRNA binding sites (MBS) prediction over all the annotated transcript sequences and made them available through an UCSC track. MBS annotation track allows to study and visualize the human miRNA binding sites transcriptome-wide in a genome browser, together with any other available information the user is interested in. In the creation of the database that underlies the MBS track, three consolidated algorithms of miRNA binding prediction have been used: PITA, miRanda and TargetScan, and information about the binding sites predicted by all of them has been collected. MBS track displays high-confident miRNA binding sites for the whole length of each human transcript, both coding and non-coding ones. Each annotation can redirect to a web page with the details of the miRNA binding and the involved transcripts. MBS can be easily applied to retrieve specific information such as the effects of alternative splicing on miRNA binding or when a specific miRNA binds an exon-exon junction in the mature RNA. Overall, MBS will be of great help for studying and visualizing, in a user-friendly mode, the predicted miRNA binding sites on all the transcripts arising from a gene or a region of interest. Database URL https://datasharingada.fondazionerimed.com:8080/MBS.

Variations in BCO2 Coding Sequence Causing a Difference in Carotenoid Concentration in the Skin of Chinese Indigenous Chicken

Carotenoid consumption decreases the risk of cancer, osteoporosis, or neurodegenerative diseases through interrupting the formation of free radicals. The deposition of carotenoids in chicken skin makes the skin color turn from white into yellow. The enzyme β-carotene oxygenase 2 (BCO2) plays a key role during the degradation process of carotenoids in skin. How the BCO2 affects the skin color of the chicken and whether it is the key factor that results in the phenotypic difference between yellow- and white-skin chickens are still unclear. In this research, the measurement of the concentration of carotenoids in chicken skin by HPLC showed that the carotenoid concentration in chickens with a yellow skin was significantly higher than that in white-skin chickens. Moreover, there were significant differences in BCO2 gene expression in the back skin between yellow- and white-skin chickens. Scanning the SNPs in BCO2 gene revealed a G/A mutation in exon 6 of the BCO2 gene in white and yellow skin chicken. Generally, one SNP c.890A>G was found to be associated with the chicken skin color and may be used as a genetic marker in breeding for yellow skin in Chinese indigenous chickens.

Extracellular vesicle expansion of PMIS-miR-210 expression inhibits colorectal tumour growth via apoptosis and an XIST/NME1 regulatory mechanism

BACKGROUND

Colorectal cancer (CRC) has a high mortality rate, and therapeutic approaches to treat these cancers are varied and depend on the metabolic state of the tumour. Profiles of CRC tumours have identified several biomarkers, including microRNAs. microRNA-210 (miR-210) levels are directly correlated with CRC survival. miR-210 expression is higher in metastatic colon cancer cells versus non-metastatic and normal colon epithelium. Therefore, efficient methods to inhibit miR-210 expression in CRC may provide new advances in treatments.

METHODS

Expression of miRs was determined in several metastatic and non-metastatic cell lines. miR-210 expression was inhibited using PMIS-miR-210 in transduced cells, which were transplanted into xenograft mice. In separate experiments, CRC tumours were allowed to grow in xenograft mice and treated with therapeutic injections of PMIS-miR-210. Molecular and biochemical experiments identified several new pathways targeted by miR-210 inhibition.

RESULTS

miR-210 inhibition can significantly reduce tumour growth of implanted colon cancer cells in xenograft mouse models. The direct administration of PMIS-miR-210 to existing tumours can inhibit tumour growth in both NSG and Foxn1nu/j mouse models and is more efficacious than capecitabine treatments. Tumour cells further transfer the PMIS-miR-210 inhibitor to neighbouring cells by extracellular vesicles to inhibit miR-210 throughout the tumour. miR-210 inhibition activates the cleaved caspase 3 apoptotic pathway to reduce tumour formation. We demonstrate that the long non-coding transcript XIST is regulated by miR-210 correlating with decreased XIST expression in CRC tumours. XIST acts as a competing endogenous RNA for miR-210, which reduces XIST levels and miR-210 inhibition increases XIST transcripts in the nucleus and cytoplasm. The increased expression of NME1 is associated with H3K4me3 and H3K27ac modifications in the NME1 proximal promoter by XIST.

CONCLUSION

Direct application of the PMIS-miR-210 inhibitor to growing tumours may be an effective colorectal cancer therapeutic.

MicroRNA‑142‑3p suppresses cell proliferation, invasion and epithelial‑to‑mesenchymal transition via RAC1‑ERK1/2 signaling in colorectal cancer

Aberrant expression of microRNAs (miRNAs/miRs) is associated with the initiation and progression of colorectal cancer (CRC), but how they regulate colorectal tumorigenesis is still unknown. The present study was designed to investigate the expression profile of miRNAs in human CRC tissues, and to reveal the molecular mechanism of miRNA-142-3p in suppressing colon cancer cell proliferation. The expression of miRNA was examined using an Exiqon miRNA array. Bioinformatics was used to predict the target genes of differentially expressed miRNAs and to analyze their biological function in CRC. The effect of miR-142-3p in colon cancer cells was evaluated in vitro using cell proliferation, colony formation and Transwell assays. Dual-luciferase reporter gene assays were performed to investigate the association between miR-142-3p and Rac family small GTPase 1 (RAC1). The effect of miR-142-3p regulation on colon cancer proliferation was assessed through western blotting and quantitative polymerase chain reaction analysis. Compared with their expression in adjacent non-cancer mucosal tissues, 76 miRNAs were upregulated and 102 miRNAs were downregulated in CRC. One of the most significantly and differentially regulated miRNAs was miR-142-3p, which was downregulated in 81.0% (51/63) of primary CRC tissues. After transfection of miR-142-3p mimics into colon cancer cells, proliferation and colony formation were decreased, and migration and invasion were markedly suppressed. RAC1 was a possible target of miR-142-3p, which was confirmed by dual-luciferase reporter assay. Transfection of miR-142-3p mimics decreased the levels of RAC1 and suppressed epithelial-to-mesenchymal transition in colon cancer cells. The phosphorylation of extraceullar signal-regulated kinase (ERK) was decreased significantly by the inhibition of RAC1 or transfection of miR-142-3p mimics in colon cancer cells. In conclusion, aberrant miRNAs are implicated in CRC. Decreased expression of miR-142-3p may be associated with CRC tumorigenesis via Rac1-ERK signaling.

Functional analysis of PCSK9 3'UTR variants and mRNA-miRNA interactions in patients with familial hypercholesterolemia

Aim: Functional analysis of PCSK9 3'UTR variants and mRNA-miRNA interactions were explored in patients with familial hypercholesterolemia (FH). Materials & methods: PCSK9 3'UTR variants were identified by exon-targeted gene sequencing. Functional effects of 3'UTR variants and mRNA-miRNA interactions were analyzed using in silico and in vitro studies in HEK293FT and HepG2 cells. Results: Twelve PCSK9 3'UTR variants were detected in 88 FH patients. c.*75C >T and c.*345C >T disrupted interactions with miR-6875, miR-4721 and miR-564. Transient transfection of the c.*345C >T decreased luciferase activity in HEK293FT cells. miR-4721 and miR-564 mimics reduced PCSK9 expression in HepG2 cells. Conclusion: PCSK9 c.*345C >T has a possible role as loss-of-function variant. miR-4721 and miR-564 downregulate PCSK9 and may be useful to improve lipid profile in FH patients.

The effect of miRNA and autophagy on colorectal cancer

Colorectal cancer (CRC) has become a concern because of its high recurrence rate and metastasis rate, low early diagnosis rate and poor therapeutic effect. At present, various studies have shown that autophagy is closely connected with the occurrence and progression of CRC. Autophagy is a highly cytosolic catabolic process involved in lysosomes in biological evolution. Cells degrade proteins and damaged organelles by autophagy to achieve material circulation and maintain cell homeostasis. Moreover, microRNAs are key regulators of autophagy, and their mediated regulation of transcriptional and post-transcriptional levels plays an important role in autophagy in CRC cells. This review focuses on the recent research advances of how autophagy and related microRNAs are involved in affecting occurrence and progression of CRC and provides a new perspective for the study of CRC treatment strategies.

Regulatory noncoding RNAs and the major histocompatibility complex

The Major Histocompatibility Complex (MHC) is a 4 Mbp genomic region located on the short arm of chromosome 6. The MHC region contains many key immune-related genes such as Human Leukocyte Antigens (HLAs). There has been a growing realization that, apart from MHC encoded proteins, RNAs derived from noncoding regions of the MHC-specifically microRNAs (miRNAs) and long noncoding RNAs (lncRNAs)-play a significant role in cellular regulation. Furthermore, regulatory noncoding RNAs (ncRNAs) derived from other parts of the genome fine-tune the expression of many immune-related MHC proteins. Although the field of ncRNAs of the MHC is a research area that is still in its infancy, ncRNA regulation of MHC genes has already been shown to be vital for immune function, healthy pregnancy and cellular homeostasis. Dysregulation of this intricate network of ncRNAs can lead to serious perturbations in homeostasis and subsequent disease.

Maternal blood EBF1-based microRNA transcripts as biomarkers for detecting risk of spontaneous preterm birth: a nested case-control study

Objective: Both genetic variants and maternal blood mRNA levels of EBF1 gene have been linked to sPTB. Animal and human studies suggest that specific EBF1-based miRNAs are involved in various physiological and pathophysiological processes. However, to date, we did not find any reports of EBF1-based miRNAs or miRNA transcripts in relation to sPTB. We therefore aimed to examine whether maternal blood early B cell factor 1 (EBF1) gene-based microRNA (miRNA) transcripts can be used for detecting risk of spontaneous preterm birth (sPTB).Methods: We conducted a nested case-control study within a Canadian cohort consisting of 1878 singleton pregnancies enrolled from May 2008 to December 2010 in Calgary, Alberta, Canada. We used a public gene expression dataset (GSE59491) derived from maternal blood in trimesters 2-3 that included women with sPTB (n = 51) and term births (n = 106) matched for maternal age, race/ethnicity, pre-pregnancy body mass index, smoking during pregnancy, and parity within the Canadian cohort. Two bioinformatics tools, miRWalk and STarMirDB, with different algorithms were applied to retrieve miRNA transcripts that putatively target the EBF1 gene (i.e. EBF1-based). Limma moderated t-tests were used to examine differentially expressed (DE) miRNA transcripts (sPTB vs term) within trimesters. Logistic regression models with miRNA transcript tertiles were applied to assess threshold associations between candidate miRNA transcripts' levels and sPTB. Receiver operating characteristic (ROC) analyses were used to identify the maximum Youden Index and its corresponding optimal sensitivity/specificity cut-point of EBF1-based miRNA transcripts for classifying sPTB, and to compare the classification performance of a linear combination (score) of miRNA transcripts with that of individual miRNA transcripts. A five-fold cross-validation was applied to examine the possible overfitting problem of the final ROC model.Results: Four maternal blood EBF1-based miRNA transcripts (MIR4266, MIR1251, MIR601, MIR3612) in the 3rd trimester were significantly associated with sPTB. The odds ratios (95%CIs) for highest versus lowest tertile of the four miRNA transcripts were 3.01-5.25(1.21-13.14, p ≤ .018). The combined 4-miRNA transcripts' score significantly improved the classification of sPTB compared to individual miRNA transcripts (AUC increased from 0.65-0.69 to 0.82, p ≤ .0034) and showed a sensitivity for sPTB of 0.81 and a specificity of 0.72. The final ROC model of the EBF1-based 4 miRNA transcripts' score in cases and controls had no significant overfitting issue.Conclusions: Maternal blood EBF1-based miRNA transcripts may, along with other biomarkers, be useful in screening for sPTB risk in 3rd trimester. Our results also provide clues for further study of potential molecular mechanisms underlying the relationship between EBF1 gene and sPTB, e.g. connecting genetic variants, mRNA expression, and miRNA regulation.

Association of 5'UTR polymorphism of secretory phospholipase A2 group IIA (PLA2G2A) gene with prostate cancer metastasis

Phospholipase A2 (PLA2) enzymes are small lipolytic hydrolases that can regulate immune responses through generation of Arachidonic Acid (AA), a precursor molecule of lipid mediators like prostaglandins, leukotrienes and thromboxanes. One of the family members of PLA2, secretory Phospholipase A2 Group IIA (PLA2G2A), was associated with different types of malignancies including prostate cancer. Elevated serum levels of PLA2G2A was found in prostate cancer (PCa) patients and associated with increased tumor grade in literature. 5'UTR regions have regulatory role in protein expression by controlling the accessibility of factors necessary for the translation initiation. Single nucleotide polymorphisms at 5'UTR regions have the potential to affect mRNA translation efficiency resulting in altered protein levels depending on structure and nucleotide content. Given that the 5'UTR polymorphism in PLA2G2A gene (rs11573156) is associated with increased serum levels of PLA2G2A, the association of this 5'UTR polymorphism with PCa susceptibility and metastasis was investigated in this study. Total of 261 PCa patients and 128 control individuals were genotyped with polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Individuals with heterozygous CG genotype was found to have significantly reduced risk of PCa metastasis with an Odds Ratio (OR) of 0.405 (p = 0.028, 95%CI = 0.181-0.906), compared to the carriers of homozygous CC genotype (p > 0.05) suggesting an anti-metastatic effect for the G allele. No association was found between PCa susceptibility and Gleason score (p > 0.05) in Turkish population.

Potential role of microRNA-424 in regulating ERRγ to suppress trophoblast proliferation and invasion in fetal growth restriction

BACKGROUND

Abnormal expression of estrogen-related receptor γ (ERRγ) protein is associated with fetal growth restriction (FGR). The upstream regulators of ERRγ are still unknown.

OBJECTIVE

To evaluate the placental expression level of microRNA-424 (miR-424) and to demonstrate the relationship between miR-424 and FGR.

METHODS

The expression levels of miR-424 were detected in FGR and control placentas. HTR-8/SVneo cells were transfected with mimics or inhibitors to increase or decrease the miR-424 expression level, respectively. The transwell and CCK-8 assays were used to determine trophoblast-derived cell line invasion and proliferation. The expression levels of miR-424, ERRγ, and 17 beta-hydroxysteroid dehydrogenase type 1 (HSD17B1) were detected by qRT-PCR and Western blotting. The relationship between miR-424, ERRγ, and HSD17B1 was determined by luciferase reporter assay.

RESULTS

Compared to the normal pregnancy group, FGR placental tissues showed a significantly higher expression level of miR-424. The up-regulation of miR-424 decreased trophoblast-derived cell line invasion and proliferation. Down-regulation of miR-424 enhanced invasive and proliferative abilities of the cell lines. Over-expression of miR-424 reduced ERRγ protein levels and decreased both mRNA and protein levels of HSD17B1. Thus down-regulation of miR-424 induced protein expression of ERRγ and enhanced the mRNA and protein expressions of HSD17B1. MiR-424 probably mediated the expression of ERRγ via binding to sites other than mRNA 3'UTR.

CONCLUSION

MiR-424 may be associated with the pathogenesis of FGR by modulating trophoblast-derived cell line proliferation and invasion. MiR-424 may play a role in mediating the protein expressions of ERRγ and HSD17B1.

Expression profile and prognostic value of SFN in human ovarian cancer

Ovarian cancer is a highly lethal cancer in females. Therefore, it is necessary to explore effective biomarkers for the diagnosis and prognosis of the disease. Stratifin (SFN) is a cell cycle checkpoint protein that has been reported to be involved in oncogenesis. Our studies detected the expression of SFN in ovarian cancer by Oncomine, Human Protein Atlas database and ULCAN database. Meanwhile, we found its coexpression gene by cBioPortal online tool and validated their expression in different ovarian cancer cells by western blot and reverse transcription quantitative PCR. Then, we also investigated their prognostic values via the Kaplan–Meier plotter database in different subtypes of ovarian cancer patients. The results demonstrated that SFN was found to be increased in ten various ovarian cancer datasets, compared with healthy tissues. Additionally, up-regulation of SFN expression is associated with age and cancer grades. The higher expression of SFN in all patients with ovarian cancers is significantly correlated with worse postprogression survival. In addition, high SFN expression is associated with significantly worse overall survival in patients who received chemotherapy contains gemcitabine, taxol, taxol+platin, paclitaxel and avastin. In human ovarian carcinoma SKOV3 and A2780 cells, the expression of SFN and its coexpression gene MICB were also increased at protein and mRNA levels compared with the normal ovarian epithelial cells. Based on above results, overexpression of SFN was correlated with the prognosis in ovarian cancer. The present study might be useful for better understanding the clinical significance of SFN mRNA.

Downregulation of miR-222-3p Reverses Doxorubicin-Resistance in LoVo Cells Through Upregulating Forkhead Box Protein P2 (FOXP2) Protein

BACKGROUND Doxorubicin (DOX) is a potent chemotherapeutic agent used to treat colon cancer. Despite impressive initial clinical responses, drug resistance has dramatically compromised the effectiveness of DOX. However, the underlying mechanisms of chemotherapeutic resistance in colon cancer remain poorly understood. MATERIAL AND METHODS In this study, we compared the expression of miR-222-3p in DOX-resistant colon cancer cells (LoVo/ADR) with the corresponding DOX-sensitive parental cells (LoVo/S) using quantitative real-time PCR. In addition, miR-222-3p inhibitors were infected into LoVo/ADR cell lines and the effects of this treatment were assessed. The Cell Counting Kit 8 assay was employed to verify the sensitivity of colon cancer cell lines to DOX. EdU (5-ethynyl-2'-deoxyuridine) assay, flow cytometry, and in vivo subcutaneous tumorigenesis were used to assess cell proliferation and apoptosis. Transwell and wound healing assays were used to investigate cell migration after adding DOX. Additionally, the expression of forkhead box protein P2 (FOXP2), P-glycoprotein (P-gp) and caspase pathway-associated markers was assessed by western blotting. RESULTS Our results showed that miR-222-3p was upregulated in LoVo/ADR compared with the expression in LoVo/S cells. Additionally, downregulation of miR-222-3p in LoVo/ADR cells increased their sensitivity to DOX, reduced P-gp expression, and activated the caspase pathway. However, the downregulation of FOXP2 could efficiently reverse the effect of miR-222-3p inhibitors on LoVo/ADR cells. CONCLUSIONS Taken together, our results showed that miR-222-3p induced DOX resistance via suppressing FOXP2, upregulating P-gp, and inhibiting the caspase pathway.

Aberrant expression of miR-153 is associated with the poor prognosis of cervical cancer

Previous studies have demonstrated that microRNAs (miRNAs) are frequently dysregulated in tumors and are associated with the initiation and progression of various types of cancer. miR-153 has been previously shown to have an anti-tumor effect in the majority of cancer types. However, to date, the expression status and function of miR-153 in cervical cancer (CC) remains unclear. In the present study, the expression of miR-153 in CC tissues and cell lines was examined, revealing that the expression of miR-153 was markedly downregulated in the CC tissues and cell lines investigated, when compared with matched noncancerous tissues and normal cervical epithelial cell line. Furthermore, ectopic expression of miR-153 by miR-153 mimic inhibited cell proliferation; however, transfection with the miR-153 inhibitor promoted the cell proliferation in CC cell lines. Finally, the results showed that the downregulation of miR-153 was associated with poor 5-year over survival in CC patients and it could be regarded as an independent biomarker to predict the prognosis of CC patients. Collectively, these results indicated that miR-153 may function as a tumor suppressor in CC, and it may be a potential novel therapeutic target for CC.

Genetic variants in mRNA untranslated regions

Genome Wide Association Studies (GWAS) have mapped thousands of genetic variants associated with complex disease risk and regulating quantitative traits, thus exploiting an unprecedented high-resolution genetic characterization of the human genome. A small fraction (3.7%) of the identified associations is located in untranslated regions (UTRs), and the molecular mechanism has been elucidated for few of them. Genetic variations at UTRs may modify regulatory elements affecting the interaction of the UTRs with proteins and microRNAs. The overall functional consequences include modulation of messenger RNA (mRNA) transcription, secondary structure, stability, localization, translation, and access to regulators like microRNAs (miRNAs) and RNA-binding proteins (RBPs). Alterations of these regulatory mechanisms are known to modify molecular pathways and cellular processes, potentially leading to disease processes. Here, we analyze some examples of genetic risk variants mapping in the UTR regulatory elements. We describe a recently identified genetic variant localized in the 3'UTR of the TNFSF13B gene, associated with autoimmunity risk and responsible of an increased stability and translation of TNFSF13B mRNA. We discuss how the correct use and interpretation of public GWAS repositories could lead to a better understanding of etiopathogenetic mechanisms and the generation of robust biological hypothesis as starting point for further functional studies. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry RNA Evolution and Genomics > Computational Analyses of RNA RNA in Disease and Development > RNA in Disease.

NKG2D and Its Ligands: "One for All, All for One"

The activating receptor NKG2D is peculiar in its capability to bind to numerous and highly diversified MHC class I-like self-molecules. These ligands are poorly expressed on normal cells but can be induced on damaged, transformed or infected cells, with the final NKG2D ligand expression resulting from multiple levels of regulation. Although redundant molecular mechanisms can converge in the regulation of all NKG2D ligands, different stimuli can induce specific cellular responses, leading to the expression of one or few ligands. A large body of evidence demonstrates that NK cell activation can be triggered by different NKG2D ligands, often expressed on the same cell, suggesting a functional redundancy of these molecules. However, since a number of evasion mechanisms can reduce membrane expression of these molecules both on virus-infected and tumor cells, the co-expression of different ligands and/or the presence of allelic forms of the same ligand guarantee NKG2D activation in various stressful conditions and cell contexts. Noteworthy, NKG2D ligands can differ in their ability to down-modulate NKG2D membrane expression in human NK cells supporting the idea that NKG2D transduces different signals upon binding various ligands. Moreover, whether proteolytically shed and exosome-associated soluble NKG2D ligands share with their membrane-bound counterparts the same ability to induce NKG2D-mediated signaling is still a matter of debate. Here, we will review recent studies on the NKG2D/NKG2D ligand biology to summarize and discuss the redundancy and/or diversity in ligand expression, regulation, and receptor specificity.