MiR-155
MiR-155 is a microRNA that in humans is encoded by the MIR155 host gene or MIR155HG. MiR-155 plays a role in various physiological and pathological processes. Exogenous molecular control in vivo of miR-155 expression may inhibit malignant growth, viral infections, and enhance the progression of cardiovascular diseases.
Discovery
The MIR155HG was initially identified as a gene that was transcriptionally activated by promoter insertion at a common retroviral integration site in B-cell lymphomas and was formerly called BIC. The MIR155HG is transcribed by RNA polymerase II and the resulting ~1,500 nucleotide RNA is capped and polyadenylated. The 23 nucleotide single-stranded miR-155, which is harbored in exon 3, is subsequently processed from the parent RNA molecule.Biogenesis
The MIR155HG RNA transcript does not contain a long open reading frame, however, it does include an imperfectly base-paired stem loop that is conserved across species. This non-coding RNA is now defined as a primary-miRNA. Once miR-155 pri-miRNA is transcribed, this transcript is cleaved by the nuclear microprocessor complex, of which the core components are the RNase III type endonuclease Drosha and the DiGeorge critical region 8 protein, to produce a 65 nucleotide stem-loop precursor miRNA .Following export from the nucleus by exportin-5, pre-mir-155 molecules are cleaved near the terminal loop by Dicer resulting in RNA duplexes of ~22nucleotides. Following Dicer cleavage, an Argonaute protein binds to the short RNA duplexes, forming the core of a multi-subunit complex called the RNA-induced silencing complex. In a manner similar to siRNA duplexes, one of the two strands, the "passenger miRNA", is released and degraded while the other strand, designated the "guide strand" or "mature miRNA", is retained within the RISC.
Recent data suggest that both arms of the pre-miRNA hairpin can give rise to mature miRNAs. Due to the increasing number of examples where two functional mature miRNAs are processed from opposite arms of the same pre-miRNA, pre-mir-155 products are now denoted with the suffix -5p and -3p following their name.
Once miR-155-5p/-3p is assembled into the RISC, these molecules subsequently recognize their target messenger RNA by base pairing interactions between nucleotides 2 and 8 of miR-155-5p/-3p and complementary nucleotides predominantly in the 3′-untranslated region of mRNAs. Finally, with the miR-155-5p/-3p acting as an adaptor for the RISC, complex-bound mRNAs are subjected to translational repression and/or degradation following deadenylation.
Evolutionary conservation
Early phylogenetic analyses demonstrated that the sequence of pre-mir-155 and miR-155-5p was conserved between human, mouse, and chicken. Recent annotated sequencing data found that 22 different organisms including, mammals, amphibians, birds, reptiles, sea squirts, and sea lampreys, express a conserved miR-155-5p. Currently much less sequence data is available regarding miR-155-3p, therefore, it is not clear how conserved this miRNA is across species.Tissue distribution
analysis found that miR-155 pri-miRNA was abundantly expressed in the human spleen and thymus and detectable in the liver, lung, and kidney. Sequence analysis of small RNA clone libraries comparing miRNA expression to all other organ systems examined established that miR-155-5p was one of five miRNAs that was specific for hematopoietic cells including B-cells, T-cells, monocytes and granulocytes. Together these results suggest that miR-155-5p is expressed in a number of tissues and cell types and, therefore, may play a critical role in a wide variety of biological processes, including hematopoiesisAlthough very few studies have investigated the expression levels of miR-155-3p, Landgraf et al. established that expression levels of this miRNA was very low in hematopoietic cells. Additionally, PCR analyses found that while miR-155-3p was detectable in a number of human tissues the expression levels of this miRNA were 20–200 fold less when compared to miR-155-5p levels. Even though the function of miR-155-3p has been largely ignored, several studies now suggest that, in some cases, both miR-155-5p and -3p can be functionally matured from pre-mir-155.
Targets
analysis using TargetScan 6.2 revealed at least 4,174 putative human miR-155-5p mRNA targets exist, with a total of 918 conserved sites and 4,249 poorly conserved sites. Although the TargetScan 6.2 algorithm cannot be utilized to determine the miR-155-3p putative targets, one would speculate that this miRNA may also potentially regulate the expression of thousands of mRNA targets.A comprehensive list of miR-155-5p/mRNA targets that were experimentally authenticated by both the demonstration of endogenous transcript regulation by miR-155-5p and validation of the miR-155-5p seed sequence through a reporter assay was recently assembled. This list included 140 genes and included regulatory proteins for myelopoiesis and leukemogenesis, inflammation and known tumor suppressors. The validated miR-155-5p binding site harbored in the SPI1 mRNA and the validated miR-155-3p binding site harbored in the IRAK3 mRNA are shown in Figures 4 and 5 respectively.
Physiological roles
Hematopoiesis
is defined as the formation and development of blood cells, all of which are derived from hematopoietic stem-progenitor cells. HSPCs are primitive cells capable of self-renewal and initially differentiate into common myeloid progenitor or common lymphoid progenitor cells. CMPs represent the cellular population that has become myeloid lineage and it is the point that myelopoiesis begins. During myelopoiesis further cellular differentiation takes place including thrombopoiesis, erythropoiesis, granulopoiesis, and monocytopoiesis. CLPs subsequently differentiate into B-cells and T-cells in a process designated lymphopoiesis. Given that miR-155-5p is expressed in hematopoietic cells it was hypothesized that this miRNA plays a critical role in these cellular differentiation processes. In support of this premise, miR-155-5p was found to be expressed in CD34 human HSPCs, and it was speculated that this miRNA may hold these cells at an early stem-progenitor stage, inhibiting their differentiation into a more mature cell. This hypothesis was substantiated when pre-mir-155 transduced HSPCs generated 5-fold fewer myeloid and 3-fold fewer erythroid colonies. Additionally, Hu et al. demonstrated that the homeobox protein, HOXA9, regulated MIR155HG expression in myeloid cells and that this miRNA played a functional role in hematopoiesis. These investigators found that forced expression of miR-155-5p in bone marrow cells resulted in a ~50% decrease in SPI1, a transcription factor and a regulator of myelopoiesis, and a validated target of this miRNA. It was also established that in vitro differentiation of purified human erythroid progenitor cells resulted in a progressive decrease of miR-155-5p expression in mature red cells. Additionally, mice deficient in pre-mir-155 showed clear defects in lymphocyte development and generation of B- and T-cell responses in vivo. Finally, it was established that regulatory T-cell development required miR-155-5p and this miRNA was shown to play a role in Treg homeostasis and overall survival by directly targeting SOCS1, a negative regulator for IL-2 signaling. Taken together, these results strongly suggest that miR-155-5p is an essential molecule in the control of several aspects of hematopoiesis including myelopoiesis, erythropoiesis, and lymphopoiesis.Immune system
The innate immune system constitutes the first line of defense against invading pathogens and is regarded as the major initiator of inflammatory responses. Its cellular component involves primarily monocyte/macrophages, granulocytes, and dendritic cells, which are activated upon sensing of conserved pathogen structures by pattern recognition receptors such as Toll-like receptors ). MIR155HG expression is greatly enhanced by TLR agonist stimulation of macrophages and dendritic cells. Since microbial lipopolysaccharide activates a chain of events that lead to the stimulation of the NF-κB and AP-1 transcription factors, it was hypothesized that endotoxin activation of MIR155HG may be mediated by those transcription factors. Indeed, MIR155HG expression was found to be activated in LPS treated murine macrophage cells by an NF-κB-mediated mechanism. Furthermore, H. pylori infection of primary murine bone marrow-derived macrophages resulted in a NF-κB dependent up-regulation of MIR155HG. In the context of viral infection vesicular stomatitis virus challenge of murine peritoneal macrophages was reported to result in miR-155-5p over-expression via a retinoic acid-inducible gene I/JNK/NF-κB–dependent pathway. Support for a role of AP-1 in MIR155HG activation comes from studies using stimuli relevant to viral infection such as TLR3 ligand poly or interferon beta. Downstream of those stimuli AP-1 seems to play a major role in MIR155HG activation.Upon its initiation via activation of e.g. TLRs by pathogen stimuli miR-155-5p functions as a post-transcriptional regulator of innate immune signaling pathways. Importantly, miR-155-5p displays a similar responsiveness to pathogen stimuli as major pro-inflammatory marker mRNAs. Once activated, miR-155-5p suppresses negative regulators of inflammation. These include inositol polyphosphate-5-phosphatase and suppressor of cytokine signaling 1, suppression of which promotes cell survival, growth, migration, and anti-pathogen responses. Besides supporting the activation of defense pathways miR-155-5p may also limit the strength of the resulting NF-κB dependent inflammatory response, suggesting varying functions of miR-155 at different stages of inflammation.
Taken together, these observations imply that the activation of the MIR155HG may be context-dependent given that both AP-1- and NF-κB-mediated mechanisms regulate the expression of this gene. These studies also suggest that a broad range of viral and bacterial inflammatory mediators can stimulate the expression of miR-155-5p and indicate that there is an intimate relationship between inflammation, innate immunity and MIR155HG expression.