ABSTRACT
During the last few years, an increasing number of vitamin-mediated effects has been discovered at the level of gene expression in addition to their well-known roles as substrates and cofactors; The best recognized examples are the lipophilic vitamins A and D. Although little is known about water-soluble vitamins as genetic modulators, there are increasing examples of their effect on gene expression. Biotin is a hydro soluble vitamin that acts as a prosthetic group of carboxylases. Besides its role as a carboxylase cofactor, biotin affects several systemic functions such as development, immunity and metabolism. In recent years, significant progress has been made in the identification of genes that are affected by biotin at the transcriptional and post-transcriptional levels as well as in the elucidation of mechanisms that mediate the effects of biotin on the gene expression. These studies bring new insights into biotin mediated gene expression and will lead to a better under-standing of biotin roles in the metabolism and in systemic functions.
Amazon Brand - Solimo Biotin 5000mcg, 300 Tablets, Value Size - Ten Month Supply
SUMMARY
In recent decades, various investigations have shown that vitamins affect gene expression. The best studied cases are those of vitamins A and D. There is less information for water-soluble vitamins on their effect on the expression of genes, however, it is known that these also modify them. Biotin is a water-soluble vitamin that acts as a prosthetic group of carboxylase. In addition to its role as a cofactor of enzymes, it participates in embryonic development, cell proliferation, immunological functions and metabolism. There has been a notable breakthrough in the identification of genes whose expression is regulated by biotin. Likewise, the molecular mechanisms through which biotin carries out these actions have been investigated. These studies provide new clues to understand the role of biotin in gene expression, metabolism, and other biological functions of this vitamin.
INTRODUCTION
Biotin is a water-soluble vitamin B complex whose most known function in eukaryotic organisms is to participate as a prosthetic group of the enzymes acetyl-CoA Carboxylase (ACC) (EC 6.4.1.2), both of the cytosolic isoform (ACC1) and of the mitochondrial (ACC2); and of the mitochondrial enzymes pyruvate carboxylase (PC) (EC 6.4.1.1); propionyl-CoA carboxylase (PCC) (EC 6.4.1.3) and methylcrotonyl-CoA carboxylase (MCC) (EC6.4.1.4). 1 These enzymes participate in various metabolic processes such as gluconeogenesis, lipogenesis and amino acid catabolism.
In addition to the participation of biotin in metabolic processes as a prosthetic group, biotin modifies biological functions such as cell proliferation, embryonic development, immunological functions and metabolism through an effect on gene expression. 2,3 In this article we review the current knowledge of the molecular actions of this vitamin on the expression of genes, which serves as a basis in the understanding of the participation of biotin in metabolism and in various biological functions.
METABOLISM OF BIOTIN IN MAMMALS
Mammals can not synthesize biotin, so it is necessary to consume it in the daily diet. Biotin is found in foods, in most of them bound to the ε-amino group of a lysine forming the dimer known as biocytin, biotinylated peptides, or in free form. 4 For its absorption it is required to break this semi-peptide bond by the action of pancreatic biotinidase. 5 Free biotin is absorbed by the enterocytes of the distal portion of the duodenum and proximal of the jejunum, and subsequently passes into the bloodstream. Entry into the cells is carried out through a multiple sodium-dependent vitamin transporter (SMVT) that mainly recognizes the valeric acid portion of biotin. 6,7 SMVT is a transmembrane protein that functions as an electroneutro symporter, introducing biotin and pantothenic acid along with sodium, in favor of a concentration gradient.
Carboxylases are synthesized as apocarboxylases, without enzymatic activity, in the cytoplasm. When biotin is covalently bound by the action of the holocarboxylase synthetase, the active protein or holoenzyme is formed. 8 This reaction is carried out in two stages: in the first, biotin is activated by reacting with an ATP molecule, forming the biotinyl-5'-adenylate intermediate. In the second stage, the biotinyl group is transferred to the apoenzyme forming a semi-peptide bond with a lysine residue, located within a highly conserved Met-Lys-Met sequence in all the apocarboxylases. 9 Biotin, as a prosthetic group of carboxylase, participates in the mechanism of transfer of an activated carboxyl group to the corresponding substrate. 10
Subsequently, the proteolysis of holocarboxylase releases lysine residues covalently bound to biotin (biocitin). This link is broken by the action of biotinidase, and in this way biotin can be recycled and integrated as a prosthetic group to new synthesized carboxylases, or it can be catabolized forming other derivative products and excreted. The synthesis of holocarboxylases and their catabolism is called the biotin cycle.
EFFECT OF BIOTIN ON GENE EXPRESSION
Observations made in the 1960s suggested that biotin intervened in various biological functions independently of its action as a prosthetic group of carboxylase. 11- 13 It has now been established that, in addition to its classical function as a prosthetic group, biotin modifies gene expression, both at the level of transcription and translation. This effect is analogous to that of other vitamins that, apart from their functions as substrates and cofactors, regulate gene expression. The best studied examples are those of vitamins A and D, which act as ligands of nuclear receptors of the hormonal receptor superfamily and thus affect various functions such as morphogenesis, immunity, differentiation and metabolism. 14
EFFECTS OF BIOTIN ON TRANSCRIPTION
Biotin participates in the regulation of the transcription of various genes. This has been demonstrated both for enzymes that require the vitamin as a prosthetic group and substrate, such as holocarboxylase synthetase (HCS), 1516 acetyl coenzyme A carboxylase -1 (ACC-1), propionyl coenzyme A carboxylase -A ( PCCA), 16 as for proteins that do not require it as a cofactor; among the latter, hepatic glucokinase, 17 hepatic phosphoenol-pyruvate carboxykinase, 18 pancreatic glucokinase, 19,20 insulin, 20.21 transcriptional factor PDX-1, 21 interleukin 2, and interleukin 2 receptor have been identified. , 22,23 the transcriptional factors NF-kB, 2 N-myc, c-myb, N-ras and raf. 24 The action of biotin on gene expression seems to be very broad: in a study of microarrays in mononuclear cells of human peripheral blood, it was found that biotin positively affects the expression of 139 genes, while decreasing that of another 131. 25 The molecular mechanisms through which biotin produces its action on the expression of some of these proteins have been studied, and are described in later sections of this review.
Effects of biotin on translation
Biotin affects the expression of genes at the post-transcriptional level. Investigations conducted by Stacker's group, 26-30 found that the vitamin modifies the expression of the asialoglycoprotein receptor through a pathway that requires cGMP and protein kinase G (PKG), 26,27 which leads to an increase in phosphorylation 28, 29 and activation of the a-COP subunit, 30 a 140kDa coatomeric protein associated with a translation complex in the trans region of the Golgi membrane. This subunit is bound to cis elements located in a fragment of 187 nucleotides of the 5 'untranslated region of the messenger RNA of the receptor, having a positive effect on the translation of this protein ( Figure 1 ). By a posttranscriptional mechanism that requires the activation of PKG, biotin also regulates the expression of the insulin receptor. 31
Molecular mechanisms of biotin
In recent years they have begun to delineate the molecular mechanisms through which biotin modifies the expression of genes. Different pathways have been identified, not necessarily excluding, that could participate in the genetic action of the vitamin:
- 1. Activation of soluble guanylate cyclase.
- 2. Biotinylation of histones.
Activation of soluble guanylate cyclase
Pioneering studies by Vesely, 32 in 1982, found that the addition of biotin to cell extracts increased the activity of soluble guanylate cyclase. Subsequently, Spence and Koudel ka, 33 found that the increase produced by biotin in hepatic glucokinase activity was preceded by an increase in intracellular concentrations of cGMP, which suggested that biotin exerted its gene effect through this second delivery courier. Since then, several studies have identified that a common denominator in the effect of biotin on gene expression involves the increase in the activity of soluble guanylate cyclase (GCs), the increase in the concentrations of cyclic guanosyl monophosphate (cGMP) intracellular, and the participation of protein kinase G (PKG). 16,27,31
Solórzano et al. have proposed that the biotinyl-AMP compound is the link in the cascade of phosphorylations involved in the regulation of gene expression by biotin. This compound is formed by the holocarboxylase synthetase in the first stage of its catalytic action (see Metabolism section of biotin in mammals). These researchers found that the regulation of the expression of acetyl-CoA carboxylase 1, propionyl CoA carboxylase and holocarboxylase synthetase itself requires the enzymatic activity of holocarboxylase synthetase. Based on their results, they propose that biotinyl-AMP, by a mechanism not yet known, activates soluble guanylate cyclase, and that, in this way, the cGMP content increases, which in turn activates PKG, thus favoring a series of phosphorylations that modify the expression of genes ( Figure 2 ).
Biotinylation of histones
Another molecular mechanism that could be involved in the effect of the vitamin on gene expression is the biotinylation of histones. Several observations in the 1960s and 70s suggested nuclear actions of biotin: the presence of biotin in the nucleus, 34 alterations in biotin-deficient animals in phosphorylation, methylation and acetylation of histones, and in the association of the latter with DNA 35 indicated a possible effect of the vitamin on chromatin. Subsequently, in vitro studies showed that histones are susceptible to being biotinylated, 36 which gave an explanation for the presence of biotin in the nucleus and the relationship between biotin and histones. In recent years it has been found that, indeed, histones in cells are biotinylated 37-39 and it has been proposed that this covalent modification, similar to covalent modifications such as methylation and / or acetylation of histones, could be part of the mechanisms through which biotin modifies gene expression. The presence in the nucleus of two key enzymes in the metabolism of biotin, biotinidase and holocarboxylase synthetase, 40,41 as well as their demonstrated ability to biotinylate histones, 42 also support this hypothesis. Recently, Narang, et al. 43 found that holocarboxylase synthetase is associated with chromatin and the nuclear lamina, and that during mitosis it is distributed in ring-like structures. In addition, fibroblasts from patients with enzyme deficiency have fewer biotinylated histones than fibroblasts from non-deficient individuals. Among the functions related to the biotinylation of histones are the increase in the abundance of these during cell proliferation of polymorphonuclear lymphocytes, 42 changes in histone biotinylation during the cell cycle 41 and the increase in the biotinylation of histones produced by damage to DNA caused by ultraviolet light. 39 These functions suggest that histone biotinylation could be linked to DNA repair and / or replication.
In summary, up to now the pieces that are part of the mechanisms involved in the genetic actions of biotin are: the formation of biotinyl-cAMP by the holocarboxylase synthetase, the modifications in the cGMP content and the biotinylation of histones. Currently, it has been established that hormones modify the expression of genes, 44 and it has been precisely the study of hormonal action that has contributed to the understanding of the effect of nutrients in genetic regulation. It is known that the effects of steroid hormones occur with different latencies and variable duration, and that hormones can exert more than one effect through several mechanisms that involve the participation of different molecules in different cellular compartments. 44 As observed in the genetic action of biotin: the metabolism of the effector, the presence of binding proteins in the nucleus, as well as the modification of chromatin are components of the regulation of steroid hormones on gene expression. Four. Five
BIOTIN ACTION ON VARIOUS BIOLOGICAL FUNCTIONS
Cell proliferation, 42,46 immunological function, 47,48 embryonic development, 49-52 and the metabolism of carbohydrates and lipids are affected by biotin. These effects seem to be mediated by the action of the vitamin on the expression of genes. It is interesting to note that it was the biotin studies on carbohydrate metabolism that laid the groundwork for the discovery of the effect of biotin on gene expression and that many of the genes regulated by biotin participate in regulation. of the metabolism of carbohydrates.
Effects of biotin on the metabolism of carbohydrates
The effects of biotin on metabolism have been demonstrated both in vivo and in vitro under different physiological conditions:
- • Biotin deficiency.
- • In normal physiological conditions.
- • In diabetic state.
Effect of biotin deficiency
The first evidences that suggested that biotin intervened in the metabolism of carbohydrates and allowed the discovery of the effect of biotin on gene expression were reported by Dakshinamurti, et al. 53 This group found that the biotin-deficient rats had significantly higher tolerance curves than the control animals, and that the hepatic glycogen content and glucose phosphorylation were lower in the biotin-deficient animals. 54 Subsequent studies showed that abnormalities in carbohydrate metabolism in deficient rats of the vitamin were due to a decrease in the activity of hepatic glucokinase, 55 a key enzyme in postprandial glucose uptake by the liver. The development of new molecular biology technologies allowed Chauhan and Dakshinamurti, 17 to demonstrate that the effect of biotin on hepatic glucokinase occurs through an increase in gene transcription. The biotin deficiency also served as a tool to reveal that biotin participates in the translation of the insulin receptor: In the HuH7 cell line derived from human hepatocytes cultured in the absence of biotin, it was found that the vitamin regulates the expression of the insulin receptor; 31 the mechanism of action indicates that the activation of PKG is required, through a cGMP elevation.
Biotin deficiency affects pancreatic islet metabolism. Studies carried out in our laboratory with biotin-deficient rats showed that vitamin deficiency produces a decrease in both the activity and the abundance of messenger RNA from pancreatic glucokinase, a key enzyme in the process that allows the beta cell to secrete insulin in response to glucose. 20 Our studies also found that pancreatic islets isolated from biotin-deficient rats have a decreased secretion of insulin in response to glucose. This detriment in the secretion of the hormone in response to glucose was also observed in the in vivo perfusion of pancreatic islets isolated from biotin-deficient rats. 56 It has also been reported that biotin deficiency in chickens affects serum glucagon concentrations. 57
Effects of biotin on the metabolism of carbohydrates in different physiological states
The administration of biotin is able to modify the metabolism of carbohydrates in non-deficient conditions of the vitamin. Studies conducted in rats showed that the administration of pharmacological doses of vitamin (1 mg / kg) increases the activity of hepatic glucokinase. This effect was observed both in postprandial conditions, metabolic situation in which glucokinase is normally increased, and in metabolic conditions in which the activity of the liver enzyme is normally reduced, such as fasting or a high-fat diet. 54, 55,58 The administration of biotin at a dose of 1 mg / kg produces a premature increase in the synthesis of hepatic glucokinase in lactating rats, a period in which this enzyme is not present. 55 In pregnant rats the administration of high doses of biotin decreases the amount of glycogen in the uterus and placenta, as well as the activity of glucose-6-phosphate dehydrogenase in the ovary, uterus and liver. 49
In in vitro cultures of cells isolated from non-vitamin deficient animals it has also been found that biotin has the ability to regulate gene expression. Spence and Kodelka 33 found that in hepatocytes isolated from normal rats, biotin increases the activity of glucokinase and this increase is preceded by an increase in intracellular concentrations of cGMP. Studies in our laboratory found that in primary cultures of islets from normal rats, treatment with biotin increases the activity and expression of pancreatic glucokinase. 20 This effect is also observed in the pancreatic cell line RIN1046-38. 19 Our studies and those of other researchers found that the expression of the insulin gene and the secretion of this hormone in response to glucose increase with biotin treatment. 59 It was recently reported that biotin increases the expression of the transcriptional factor PDX-1, which is determinant in pancreatic development 21 and in the expression of genes that participate in specific functions of the islet of Langerhans.
The effect of biotin on the metabolism of carbohydrates has been observed in yeast. In cultures of Saccharomyces cerevisiae it was found that in a medium with a high content of biotin the activities of pyruvate carboxylase and isocitrate lyase are increased while the glycogen content decreases, 60 suggesting that the effect of biotin on the metabolism appeared from early stages of evolution.
Effects of biotin in diabetic models
Several studies have found that the administration of pharmacological doses of biotin decreases hyperglycemia: patients with type 1 diabetes treated for a week with biotin (without receiving exogenous insulin), their fasting glucose concentrations decreased. 61 In a study in Japanese diabetic patients type 2, 62 it was found that oral administration of 9 mg of biotin daily for one month decreased blood fasting glucose, pyruvate and lactate concentrations; When the administration of the vitamin was discontinued, a return to the hyperglycemic concentrations observed before the start of the treatment occurred. Our group has found that in type 2 diabetic patients, treatment with biotin 15 mg / day for 28 days decreases the area of the glucose tolerance curves. 63
In animal models with type 2 diabetes it has also been reported that biotin decreases hyperglycemia. In mice of non-obese KK strain and in OLETF rats presenting spontaneous obesity, a decrease in hyperglycemia and in the glucose tolerance curve was observed in response to treatment with pharmacological doses of the vitamin. 27, 28 Studies in experimental models with diabetic rats generated by the treatment with alloxan or with streptozotocin found that biotin significantly increases the activity of laglucokinase-hepatic. 64,65 Treatment with the vitamin also increased the activities of the glycolytic enzymes phosphofructokinase and pyruvate kinase. In other studies in rats whose diabetes was induced by streptozotocin, biotin decreased by more than 50% the transcription of phosphoenolpyruvate carboxykinase, an enzyme limiting gluconeogenesis. 66
In summary, the nutritional status of biotin affects the metabolism of carbohydrates; biotin deficiency produces a hyperglycaemic effect, while pharmacological doses of biotin reverse hyperglycemia. This effect agrees with the action of biotin on the expression of genes that favor the uptake and catabolism of glucose, examples of which are hepatic and pancreatic glucokinase, insulin and insulin receptor; while it diminishes the expression of the enzyme phosphoenolpyruvate carboxykinase, a hyperglycemic action enzyme that regulates gluconeogenesis.
Biotin in lipid metabolism
There is less knowledge of the effect of biotin on lipid metabolism. Given that biotin intervenes directly as a cofactor of ACC (1 and 2), a crucial enzyme in the synthesis and oxidation of fatty acids, there is a direct relationship between biotin deficiency and lipid metabolism 67,68 through its function as a prosthetic group. However, under non-deficient conditions of the vitamin, effects of biotin that could be mediated through its action in gene regulation have been described.
Under normal biotin conditions, it has been observed that treatment with pharmacological doses of the vitamin can modify the concentrations of triglycerides and cholesterol. In patients with atherosclerosis and hypercholesterolemia, administration of 5 mg of biotin for four weeks produced a significant decrease in total cholesterol concentrations; A decrease, although not significant, in LDL concentrations was also observed. 69 Studies in healthy volunteers show that administration of 0.9 mg / day of biotin for 71 days reduced plasma lipid concentrations. 70 In our laboratory it was found that treatment with biotin 5 mg three times a day decreases plasma triglyceride concentrations in patients with hypertriglyceridemia. 63 Studies with animal models have also shown that biotin modifies hyperlipidemia. In the strain of BHE rats with genetic predisposition to develop high blood concentrations of glucose and lipids, treatment with biotin decreased plasma lipid concentrations. 71,72
Little is known about the genetic regulation of enzymes involved in lipid metabolism by biotin. However, recently, Levert, et al. 73 found that in the 3T3-L1 adipocyte cell line a biotin chloroacetylated analogue (CABI), in addition to inhibiting the activity of acetyl-CoA-carboxylase, reduces the expression of the differentiation factors STAT 1 and STAT 5a and PPARy , transcriptional factors that play a very important role in lipid metabolism. These early evidences suggest that the mechanism of action through which biotin affects the metabolism of lipids could be carried out on the transcription of these genes.
CONCLUSION
The role of biotin in genetic regulation has been confirmed in several studies, showing that this water-soluble vitamin has other biological functions in addition to its traditional role as a prosthetic group of carboxylase enzymes. The advance of the techniques in the areas of molecular and cellular biology has allowed to know the molecular bases of the effects of biotin on metabolism, reproduction and immunological function; However, there are still many unanswered questions today: by what mechanism does biotinyl-AMP activate soluble guanylate cyclase? What factors exist between the cGMP signal cascade and the regulation of gene expression? Does histone biotinylation participate in DNA repair and replication? Does histone biotinylation intervene in the expression of genes? Are there other molecular intermediaries involved in the mechanism of action of biotin? How many more genes and what other biological functions are affected by the presence or absence of biotin? These questions and others that will surely come from the investigation of these unknowns, make the study of biotin as an effector of gene expression an exciting area to discover.
