**4. Novel mechanisms involved on immunometabolic regulation: miRNAs**

The microRNAs (miRNAs) were discovered in 1993 by Lee, Feinbaum and Ambros, when it was shown that their expression involves negative regulation at the post‐transcriptional level and their biogenesis was a result of two unrelated molecular routes.

About miRNAs biogenesis, the non‐coding region into the genes is transcribed, hence a small non‐coding RNA is obtained, these molecules synchronized the downregulation of protein expression both at the transcriptional and translational levels. However, upregulation of translation has also been reported. In the negative regulation processes, the miRNAs bind to their complementary sites within the 3'‐untranslated regions (UTRs) of target mRNA through‐ out sequence recognition, resulting in mRNA translational repression or degradation of the mRNA transcripts [74, 75].

The miRNAs are a kind of non‐coding RNA of specific genes, whose products are single‐ stranded RNA molecules between 19 and 25 nucleotides, their sequences were identified by Northern blot analysis, microarrays or the real‐time PCR method. Nucleotide sequences of miRNAs are reported in miRBase registry (http://mirbase.org/), and the correct nomenclature is discussed by several authors [76, 77].

#### **4.1. microRNAs biogenesis**

The molecular biosynthesis process of microRNAs involving multiple pathways however is possible to characterize a general mechanism, in which sequential routes of a particular method are identified (**Figure 2**):

**1.** miRNAs are transcribed in the cell nucleus by the RNA polymerase II, based on three main gene sequences: intronic regions, polycistronic clusters or from intergenic areas; the molecules obtained are called pri‐miRNAs.

**2.** pri-miRNAs are improved by RNasa type III (Drosha) to become pre-miRNA, which is recognized by the XPO5 and RanGTP complex and transported to the cytoplasm through a nuclear pores [78].

At the cellular level, there are two main mechanisms responsible of IR development: (1) cellular stress in the endoplasmic reticulum, and in the mitochondria of adipocytes, hepatocytes and myocytes; and (2) release of pro‐inflammatory cytokines, principally, TNF‐α and IL‐6 by activation of the Toll‐like receptor 4 (TLR‐4) on the surface of infiltrated macrophages of white adipose tissue and liver [72, 73]. Obtained data point towards multiple triggering paths for this processes to be started, however, it is the obesity‐associated chronic low‐grade inflamma‐

Moreover, in obesity, the amount and the size of adipocytes increase (hyperplasia and hypertrophy); furthermore, macrophage infiltration in white adipose tissue is higher and these processes together deregulate the secretion of adipokines. One of them, adiponectin, is negatively correlated with WAT accumulation (as mentioned before), this diminishes insulin signalling, already affected by the pro‐inflammatory milieu. To add complexity to the IR phenomenon, they have been recently described novel mechanisms of immunometabolic

**4. Novel mechanisms involved on immunometabolic regulation: miRNAs**

The microRNAs (miRNAs) were discovered in 1993 by Lee, Feinbaum and Ambros, when it was shown that their expression involves negative regulation at the post‐transcriptional level

About miRNAs biogenesis, the non‐coding region into the genes is transcribed, hence a small non‐coding RNA is obtained, these molecules synchronized the downregulation of protein expression both at the transcriptional and translational levels. However, upregulation of translation has also been reported. In the negative regulation processes, the miRNAs bind to their complementary sites within the 3'‐untranslated regions (UTRs) of target mRNA through‐ out sequence recognition, resulting in mRNA translational repression or degradation of the

The miRNAs are a kind of non‐coding RNA of specific genes, whose products are single‐ stranded RNA molecules between 19 and 25 nucleotides, their sequences were identified by Northern blot analysis, microarrays or the real‐time PCR method. Nucleotide sequences of miRNAs are reported in miRBase registry (http://mirbase.org/), and the correct nomenclature

The molecular biosynthesis process of microRNAs involving multiple pathways however is possible to characterize a general mechanism, in which sequential routes of a particular method

**1.** miRNAs are transcribed in the cell nucleus by the RNA polymerase II, based on three main gene sequences: intronic regions, polycistronic clusters or from intergenic areas; the

and their biogenesis was a result of two unrelated molecular routes.

tion the most linked one [72, 73].

166 Adiposity - Omics and Molecular Understanding

regulation, the miRNAs.

mRNA transcripts [74, 75].

**4.1. microRNAs biogenesis**

are identified (**Figure 2**):

is discussed by several authors [76, 77].

molecules obtained are called pri‐miRNAs.


Subsequently when the mature miRNA is formed, their function will be to recognize by 3'UTR complementary sequences in the target mRNA. The level of coincidence in these sequences determines the degree of regulation of transcription, with one of two options, when the sequence of the complex is 100% complementary to the sequence in the region of the target (perfect complementarity), leading to denaturation and degradation of mRNA, while incomplete complementarity triggers silencing of mRNA through different molecular mechanisms, as repression of translation, degradation and/or sequestration of target [78, 79].

Since its deregulation has been related to different illnesses and it is estimated that more than 60% of the human genes expression are regulated by microRNAs [80].

**Figure 2.** miRNA biogenesis. MicroRNA is transcribed by the polymerase II from (a) genes, (b) polycistronic clusters and (c) intronic regions. These pri-miRNAs are processed by the RNase Drosha which shortens them. The pre-miRNA formed is transported to cytoplasm by the XPO5 and then Dicer cleaves the loop leaving a double chain fragment that is recognized for the miRISC complex and targets the mRNA. POL II: polymerase II; XPO5: exportin 5; AGO2: argonaute protein 2; UTR: untranslated region; CAP: caperuse and AAAA: poly A chain.
