**4.2. microRNAs structural forms**

For the reason that miRNAs are produced from the differential gene expression, heterogeneous structures are obtained with different length and nucleotide sequences. Sequences of two biochemically stable forms with asymmetrical structural motifs, the immature form (premiRNA) and the mature form (miRNA) have been reported.

The pre-miRNAs have a length of more 60 nucleotides, obtained from pri-miRNAs by the division in the union on the opposite side of the double chain loop. The recurrent structure is a double-helix that ends in a loop that joins both chains, the chain that goes from 5' to 3' and the chain of the direction from 3' to 5', which are 100% non-complementary and the length varies according to the mature stage due to the cuts made by the enzymes [81]. This double chain contains a monophosphate group in the end 5', and a free OH group in end 3'.

The mature microRNAs are approximately of 18–25 nucleotides in length, and differ from one another in their nucleotide sequence and length. Finally, two chains are generated in sense (5') and antisense (3') directions, emerging from the 5' arm of the hairpin microRNA or 3' arm of the microRNA hairpin, these sequences are called -5p or -3p, respectively. Generally, only one of them is dominant while the other is considered as a minor product, due to the intracellular concentration. However, both sequences are functional, as this depends on the tissue and species in which they are located [76, 82].

#### **4.3. The functional diversity of miRNAs**

The study of miRNAs is relevant because it has been involved in diverse functions and their expression levels have been associated with different diseases, such as insulin resistance, diabetes, atherosclerosis or cancer. While the new miRNAs sequence is identified, their participation in biological processes at the molecular level is to be defined. The mechanisms described include adipocyte differentiation, metabolic integration and appetite regulation [80, 83].

Among the metabolic diseases associated with the expression of miRNAs, their role in diabetes mellitus type 2 has been described, where it was shown that the miR-375 is directly involved in the regulation of insulin secretion. These data suggest their participation in the metabolic pathways and the possible association with inflammatory markers and their expression in individuals with phenotypic characteristics associated with obesity inflammation [84].

On the other hand, the miRNAs can also be found in the soluble form and it is important to remark that they are stable in serum for long periods of time due to two mechanisms, formation of a complex of ribonucleoprotein with argonaut proteins and the addition of exosomes and micro-particles. Besides, it has been discovered that HDLc can transport miRNAs in the plasma and take them to the target cells where they will be captured [85].

Related to their location in the serum of the miRNAs mature form, Slack has proposed two hypotheses: first, the miRNAs of tissues can be present in the circulation as a result of cell death and lysis, and second, the tissue cells actively secrete miRNAs in their microenvironment, where they get into the blood vessels so that they get their way up the circulation, so that it has been also suggested their potential usage as markers for different diseases [86].

Due to the strong correlation between the expression patterns of miRNAs and the state of the diseases, that has been showed not only in animal models but also in human patients, the miRNAs have been considered as promising candidates for the next generation of biomarkers.

#### **4.4. miRNAs and immunometabolism**

**4.2. microRNAs structural forms**

168 Adiposity - Omics and Molecular Understanding

species in which they are located [76, 82].

**4.3. The functional diversity of miRNAs**

[80, 83].

miRNA) and the mature form (miRNA) have been reported.

For the reason that miRNAs are produced from the differential gene expression, heterogeneous structures are obtained with different length and nucleotide sequences. Sequences of two biochemically stable forms with asymmetrical structural motifs, the immature form (pre-

The pre-miRNAs have a length of more 60 nucleotides, obtained from pri-miRNAs by the division in the union on the opposite side of the double chain loop. The recurrent structure is a double-helix that ends in a loop that joins both chains, the chain that goes from 5' to 3' and the chain of the direction from 3' to 5', which are 100% non-complementary and the length varies according to the mature stage due to the cuts made by the enzymes [81]. This double

The mature microRNAs are approximately of 18–25 nucleotides in length, and differ from one another in their nucleotide sequence and length. Finally, two chains are generated in sense (5') and antisense (3') directions, emerging from the 5' arm of the hairpin microRNA or 3' arm of the microRNA hairpin, these sequences are called -5p or -3p, respectively. Generally, only one of them is dominant while the other is considered as a minor product, due to the intracellular concentration. However, both sequences are functional, as this depends on the tissue and

The study of miRNAs is relevant because it has been involved in diverse functions and their expression levels have been associated with different diseases, such as insulin resistance, diabetes, atherosclerosis or cancer. While the new miRNAs sequence is identified, their participation in biological processes at the molecular level is to be defined. The mechanisms described include adipocyte differentiation, metabolic integration and appetite regulation

Among the metabolic diseases associated with the expression of miRNAs, their role in diabetes mellitus type 2 has been described, where it was shown that the miR-375 is directly involved in the regulation of insulin secretion. These data suggest their participation in the metabolic pathways and the possible association with inflammatory markers and their expression in individuals with phenotypic characteristics associated with obesity inflammation [84].

On the other hand, the miRNAs can also be found in the soluble form and it is important to remark that they are stable in serum for long periods of time due to two mechanisms, formation of a complex of ribonucleoprotein with argonaut proteins and the addition of exosomes and micro-particles. Besides, it has been discovered that HDLc can transport miRNAs in the plasma

Related to their location in the serum of the miRNAs mature form, Slack has proposed two hypotheses: first, the miRNAs of tissues can be present in the circulation as a result of cell death and lysis, and second, the tissue cells actively secrete miRNAs in their microenvironment,

and take them to the target cells where they will be captured [85].

chain contains a monophosphate group in the end 5', and a free OH group in end 3'.

It is important to recognize that metabolic diseases have a strong immune component derived from inflammation and oxidative stress in which microRNAs regulate different ways (**Figure 3**).

**Figure 3.** miRNA and immunometabolism. In immunometabolism, the microRNAs have an important role in regulation of different levels of metabolism. Further, the oxidative stress is the link between inflammation and obesity, since the accumulation of adipose tissue stimulates the expression of inflammation markers allowing the establishment of pathologies like DM2 and insulin resistance. Otherwise, there are microRNAs that improve glucose tolerance and lipid metabolism. M1: macrophages phenotype 1; HDLc: high-density lipoprotein cholesterol and miR: microRNA.

In diabetes mellitus type 2 patients, associations of miR-146 and miR-126 with markers of endoplasmic reticulum stress and inflammation as well as decreased miR-20b, miR-21, miR-24, miR-15a, miR-126, miR-191, miR-197, miR-223, miR-320 and miR-486 have been reported. Evenly, in obesity, the miR-152 increases, whereas miR-17 and miR-138 decreases [75, 87, 88].

With regard to metabolic functions in the skeletal muscle and the deregulation of the miRNA species, it is known that it can lead to deep alterations of glucose and the lipid metabolism in the adipose tissue, which is an important metabolic regulator. Regarding this, it has been shown that the expression of miR-14 decreases the levels of triacylglycerol and diacylclycerol, so that miR-14 could be an important lipid regulator in this level, besides other involved miRNAs such as miR-143 [89], miR-181a [90], miR-103 and miR-107 [91] have all been shown to affect hepatic insulin sensitivity, and more recently, miR-802 has been shown to be increased with obesity and that its reduction improves glucose tolerance and insulin action [92].

According to the association between obesity and the fatty acids different microRNAs have been associated, among them miR33a and miR33b, which are found in the intronic regions of the genes *SREBF2* and *SREBF1* that code for the transcription factors SREBP2 and SREBP1 and control the expression of the genes involved on the synthesis of cholesterol and fatty acids [93].

*miR33a/b* act as suppressing genes that oppose the functions of SREBP, for instance, the cholesterol efflux and the oxidation of fatty acids; so that under low-cholesterol conditions the transcription of SREBP2 and the regulation of the genes involved in the synthesis of cholesterol and absorption is activated, therefore, co-transcription of miR-33a acts in exporting cell cholesterol inhibiting the transcription of ABCA1 [93].

On the other hand, it is known that high levels of free fatty acids (FFA) and different adipokines, such as leptin and resistin, have a strong regulator role in other microRNAs; while expression of miR-143 can modulate the differentiation of the preadipocytes by increasing the storing of lipids, likewise, inhibition of miR-143 blocks differentiation of adipocytes through the kinase 5 regulator of extracellular signals (ERK5) [94].
