ABC | Volume 111, Nº5, November 2018

Review Article Silva et al miRNAs and cardiovascular disease Arq Bras Cardiol. 2018; 111(5):738-746 Figure1– SynthesisofmiRNAand itsactiononmessengerRNA(mRNA).Hairpinprimary-microRNAsaresynthesized in thenucleus,converted intopre-miRNAbyDroshaenzyme, and exported from the nucleus into the cytoplasm by the Exportin-5 protein. In the cytoplasm, pre-miRNA is recognized by the enzyme Dicer; RNA‑induced silencing complex (RISC) binds to a double-stranded RNA(dsRNA), generating mature miRNA. Mature miRNA interacts with the target mRNA, leading to either its degradation or its translation. they contain a hairpin structure, essential for its recognition by miRNA processing enzymes. Pri-miRNA is processed into pre-miRNA (~70pb) by the nuclear RNaseIII enzyme Drosha 9 which recognizes and cleaves the ends of the hairpin-shaped small RNA structures. 10 Following the nuclear processing, each pre-RNA is exported into the cytoplasm by the Exportin-5 protein. 11 Pre-miRNA is recognized by the enzyme Dicer, which cleaves the loop region into a double-stranded RNA (dsRNA:~22pb). This process recruits proteins of the Argonaute protein family to form the RNA-induced silencing complex (RISC). 12 RISC binds to one of the strands of the dsRNA and generates mature miRNA (canonical miR or miR-5p) which is involved in the regulation of a target mRNA. 13 The other strand (miR* or miR‑3p) is either degraded or involved in the generation of another RISC, acting in the regulation of another target mRNA. 14 The perfect matching between miRNA and the three prime untranslated region (3'-UTR) of the target mRNA leads to the cleavage of the mRNA and its transfer to mRNA processing bodies (p-bodies) and subsequent degradation. 15 On the other hand, a partial matching between miRNA and 3’-UTR inhibits translation, which is the main mechanism of action of the miRNAs in mammals. 16 Thus, by translation inhibition, miRNA has a direct effect on translation factors and on poly(A) tail functioning. 17 Although the primary location of miRNAs is cell cytoplasm, 8 some studies have confirmed the entry of these molecules into the circulatory system, possibly resulting from cell lysis. 18 Therefore, MiRNAs are also found in the circulatory system and several studies have shown its high stability in the extracellular environment. MiRNA degradation in the extracellular milieu could be prevented by its binding to proteins (e.g. lipoproteins) or its encapsulation into microvesicles and exosomes. Thereby, miRNAs can be reliably detected in plasma samples, and suggested as potential biomarkers of CVDs. 19 Since miRNAs are small sequences and do not require perfect matching, a unique miRNA can have tens of target mRNA, and a unique mRNA can be the target of multiple miRNAs, producing a broad regulatory power of genetic expression. 20 MiRNAs as circulating biomarkers Circulating levels of miRNAs have been shown to be altered in some CVDs. This fact has aroused interest in using it as a diagnostic and prognostic tool, since circulating miRNAs have high stability and are easily detected. MiRNAs could be used, for example, as biomarkers of heart failure (HF), atrial fibrillation, acute myocardial infarction (AMI) and atherosclerosis, by its detection in blood plasma (Figure 2). It has been well established that the presence of miR‑1 in the blood may be helpful in the detection of AMI. However, its long-term use as a biomarker has not been recommended, since it remains circulating in the blood only for a short period. The short half-life of MiR-1 is probably explained by its direct release from cardiac necrotic tissue into the circulation, not encapsulated in exosomes. 21 In case of heart injury, miRNAs would be released into the blood through exosomes or by cell rupture, associated or not with other molecules. These molecules could protect miRNAs from degradation, prolonging its time in circulation. Besides, the increase in blood flow, changes in pH and release of cytokines can also affect the half-life of circulating miRNAs. For example, it was shown that exogenously added mature miR-1 is rapidly degraded both in vitro and in vivo . 22 739