Sox6, A Potential Target for MicroRNAs in Cardiometabolic Disease

Abstract

Purpose of Review

The study aims to review recent advances in knowledge on the interplay between miRNAs and the sex-determining Region Y (SRY)-related high-mobility-group box 6 (Sox6) in physiology and pathophysiology, highlighting an important role in autoimmune and cardiometabolic conditions.

Recent Findings

The transcription factor Sox6 is an important member of the SoxD family and plays an indispensable role in adult tissue homeostasis, regeneration, and physiology. Abnormal expression of the Sox6 gene has been implicated in several disease conditions including diabetes, cardiomyopathy, autoimmune diseases, and hypertension. Expression of Sox6 is regulated by miRNAs, which are RNAs of about 22 nucleotides, and have also been implicated in several pathophysiological conditions where Sox6 plays a role.

Summary

Regulation of Sox6 by miRNAs is important in diverse physiological tissues and organs. Dysregulation of the interplay between miRNAs and Sox6 is an important determinant of various disease conditions and may be actionable for therapeutic purposes.

Introduction

The Sex-determining Region Y (SRY)-related high-mobility-group box 6 (Sox6) is a transcription factor that was discovered in 1990s while searching for Sry-related genes [1, 2, 3, 4]. Based on similarities of amino acid sequences, the Sox family of transcription factors has been classified into 10 groups, from A to J, though some inconsistencies in the classification have been reported [5••]. In vertebrates, the SoxD subfamily consists of Sox6, Sox5, and Sox13 [6, 7]. Sox102F, another member of SoxD family, was found in Drosophila melanogaster and in other invertebrates.

The genes and protein structures of the SoxD subfamily members are highly similar. In contrast, they only match the other Sox family members in the high-mobility-group (HMG) box, a signature feature found in all Sox family members [8]. The members of each subfamily share about 80% of sequence identity in the DNA-binding HMG domain and a fair amount of sequences in other well-conserved regions [9, 10]. The human Sox5 and Sox6 are in paralogous chromosomal regions on 12p12.1 and 11p15.3–15.2, respectively. Both genes are more closely related to each other compared to Sox13 which is located on 1q32. The three genes possess 12–16 coding exons. Sox5 and Sox6 are spread across 300–400 kb, whereas Sox13 is spread across 12 kb of genomic DNA [9]. The SoxD proteins are among the largest Sox proteins, weighing between 48 and 89 kDa (Fig. 1). Unlike other Sox family members which generally harbor one functional domain, SoxD subfamily members possess two functional domains. The C-terminal half of the proteins possesses the HMG-DNA binding domain (Sox family specific), whereas the N-terminal half of the proteins harbors the coiled-coil domain [10]. The latter is SoxD specific and about 76% identical among all human and mouse subfamily members [9, 10]. The SoxD subfamily members also exhibit short stretches of identity outside the two highly identical domains [9].

Fig. 1. A representation of the human SOX family members, protein structure, and domains.

Sox family showing proteins from each family from A to H with distinct domains and three-dimensional structures for each protein. Each protein differs by the number of specific amino acid residues with sox6 containing the highest. All contain the homology (HMG) box domain. The D family contains the coiled-coil domain while family A and C contain the transactivation domains. B1 and B2 all contain a B homologous domain but with additional transactivation and transrepression domains, respectively. The E family contains the dimerization domain in addition to the HMG box and transactivation domain. The F family contains the transactivation and DxxEF/EQYL domains. The G and H family only contain the HMG box. At the right, the three-dimensional structures for each protein are represented. No three-dimensional structure was available to represent for Sox15. Figure created with BioRender.com

The proteins of the SoxD subfamily do not harbor any transactivation or transrepression domains; however, they participate in both processes by different mechanisms [11, 12]. For example, Sox5 and Sox6 interact with Sox9 to synergize several chondrocyte-specific extracellular matrix genes [11, 12], whereas they repress the Sox9 and Sox10 activation of myelin genes in oligodendrocytes [13] and the Sox10-induced activation of Mitf and Dct marker genes in melanocytes [14]. This interference or repression by SoxD proteins is exerted by competing for Sox binding sites with other proteins in the promoter of differentiation markers, ultimately leading to blockade of transactivation. Several studies revealed that Sox6 possesses the DNA binding domain and that in contrast to other transcription factors that bind to the major grove of DNA, Sox6 binds to the minor grove to widen it and exert its DNA-bending action [15]. Other studies showed that Sox6 binds to various proteins, cofactors, and miRNAs (miRs), and modulates the transcription and functions of a variety of genes and molecular pathways [12, 16, 17, 18, 19, 20, 21]. Differential expression of Sox6 at different times and places makes Sox6 a versatile protein [8, 22••], which probably accounts for the functional importance of Sox6 in various diseases such as diabetic nephropathy [23, 24], cardiomyopathy [25••, 26], adipogenesis [16], cancer [27], and inflammatory bowel disease [28].

A recent report by McCarthy et al. showed that Sox6 is a target for miRNAs in a model of striated muscle atrophy in the rat in which the Myh7 (β-MHC) gene is regulated by miRNA-499 and −208b. Diminished expression of these two miRNAs was associated with 2.2-fold increased expression of Sox6 [29••]. Another study showed that miR-1 and −499 regulate the differentiation and proliferation of cardiomyocyte progenitor cells into beating cardiomyocytes by modulating expression of histone deacetylase 4 and Sox6 [30]. Moreover, some studies showed that miRNAs, by modulating Sox6 expression, affect the meat quality and quantity of fish, chicken, and pork [31]. After these pioneering studies showing Sox6 as an important target for miRNAs, a plethora of studies supporting these findings have been published in the field of diabetes, cardiomyopathy, diabetic nephropathy, cancer, and other diseases. Importantly, a great deal of research has shown that Sox6 is a key target for miRNAs in diabetes, cardiomyopathy, and skeletal myopathy. In contrast, even though Sox6 has been implicated in experimental and human hypertension, there are no studies to date showing that Sox6 is targeted by miRNAs in regulating blood pressure in these conditions. We strongly believe that studying Sox6 as a target of miRNAs in the field of hypertension will be a very important addition to the understanding of blood pressure regulation. In this minireview, we summarize the information about Sox6 transcription factor as an important target for miRNAs in cardiovascular and associated diseases, to facilitate understanding of the importance of Sox6 as a target for miRNAs in these disease entities and their complications.

Micro-Ribonucleic Acids (mi-RNAs): Mechanism of Synthesis

The miRNAs are a class of endogenous non-coding single-stranded ribonucleic acid (RNA) molecules encoded by intronic parts of the genome [32]. They are about 22 nucleotides (nt) long and play an orchestrated role in the regulation of gene expression, cell signaling pathways, and various cell functions in animals and plants [33, 34, 35]. Thus, miRNAs may be as important as other transcription factors, and may hold great potential for future biomedical research. The miRNAs differ in the levels, spatial and temporal expression patterns among various tissues or organs, and during physiological versus pathological conditions, suggesting that they may regulate several genes during physiological and pathophysiological conditions. The current assumption is that miRNAs regulate about 35% of the human genome [36].

The first miRNA was discovered serendipitously in the nematode Caenorhabditis elegans by molecular geneticists in the early 1990s, when Lee and coworkers were studying the role of the lin-4 gene in the development of C. elegans. They noticed that transcription of this gene resulted in a non-coding rather than a protein-synthesizing mRNA [37••]. In metazoans, the canonical miRNAs are transcribed by RNA polymerase II (Pol II) in a longer form known as primary miRNAs (pri-miRNAs) [38, 39, 40], which are truncated into small functional 19–23 nt with the help of an obligate microprocessor, a heterotrimeric protein complex possessing one molecule of Drosha endonuclease and two molecules of partner protein DGCR8 (Pasha). Drosha harbors two RNA III domains, each truncates one strand and liberates a stem-loop of about 60 nt named precursor miRNAs (pre-miRNAs) [41]. Once transported to the cytoplasm through the actions of Exportin 5 and RAN-GTP, the pre-miRNA is further processed by dicer, an endonuclease with two RNA III domains, [42, 43] into a RNA duplex which becomes ready for loading into Argonaute (AGO) proteins to form the basic core of RNA-induced silencing complexes (RISCs) [44]. RISC-loaded miRNA recognizes a complementary sequence in the 3′ untranslated region (3′-UTR) of the protein-coding messenger RNA (mRNA) and either degrades it or represses protein translation [45, 46] (Fig. 2). Detailed descriptions of the structure, synthesis, mechanism of action, and short history of miRNAs, have been previously published [47, 48, 49•, 50].

Fig. 2. A general overview of miRNA synthesis and mechanism of action.

Synthesis of miRNA is initiated by the transcription of miRNA gene by RNA polymerase II. The resulting primary miRNA (pri-miRNA) is processed by Drosha and DiGeorge syndrome critical region in gene 8 (DGCR8) into pre-miRNA, which is transported to the cytoplasm by the RanGTP-dependent nuclear transport receptor exportin-5. In the cytoplasm, Dicer trims the pre-miRNA to yield miRNA:miRNA duplex. Argonaute 2 (Ago2) mediates further pre-miRNA processing and RISC (RNA-induced silencing complex) assembly. One strand of the duplex remains the mature miRNA (miRNA), while the other strand is degraded. Once ready, mature miRNAs can either repress target mRNA translation or facilitate target mRNA degradation. Figure created with BioRender.com

SOX6 and Cardiovascular Disease

Heart failure, coronary artery disease, and myocardial infarction are the most common cardiovascular diseases (CVDs) and contribute significantly to morbidity and mortality worldwide [51]. Significant advances have been made over the last few decades in the understanding of their molecular mechanism, pathophysiology, and targeted pharmacological treatments but there are still no reliable curative treatments.

The renin angiotensin aldosterone system (RAAS) and adrenergic pathways have proven to be very important and useful pharmacological targets in terms of treatments and biomarkers for cardiovascular diseases. However, more definitive and molecular target-based treatments and biomarkers are needed. In vivo and in vitro studies in the early and late 2000s predicted the possible importance of miRNAs as molecular targets for the treatments of cardiovascular diseases. This was followed by a deluge of research studies on these tiny molecules [30, 32, 52, 53, 54, 55]. In the cardiovascular field, much attention has been devoted to miRs that are either specific for or highly expressed in skeletal and cardiac muscle. These so-called myomiRs (e.g., miR-1, miR-133, miR-206, and miR-208) have been implicated in cardiac fibrosis, atherosclerosis, neonatal formation, and cardiac regeneration [56••].

Several pioneering studies showed that Sox6, among other transcription factors, is one of the main targets of cardiac-specific myomiRs and plays an important role in the regulation of optimal functioning of cardiac muscle and other cardiac functions [17, 57, 58]. Sox6 controls the balance between gene expression of fast- and slow-twitch myofiber isoforms by repressing expression of the latter [25••, 59, 60].

Several studies have confirmed a role for Sox6 in diverse forms of cardiomyocyte or whole heart function. A recent study on the mechanism by which fetal hypothyroidism results in cardiac dysfunction of rat offspring showed that the latter was due to excess β-MHC expression with concomitant decreased expression of α-MHC. This imbalance is produced by changes in the fetal gene expression of myomiRs (increased expression of miR-499 and miR-208b and decreased expression of miR-208a) encoded by the intronic sequences of α-MHC, β-MHC, and Myh7b genes, respectively [54, 60]. These changes in myomiR expression result in diminished expression of their target genes, which include Sox6, as well as the thyroid hormone receptor 1 (Thrap1), and purine-rich element-binding protein b (Purb) genes [61], confirming the role of Sox6 as a regulator of cardiac contractility. Similarly, Huang et al. [62] found increased levels of miR-208 gene expression in the peripheral blood from patients with progressive cardiac hypertrophy. Moreover, using an in vitro model of phenylephrine-induced rat cardiomyocyte hypertrophy, they showed overexpression of miR-802 with concomitant decrease in levels of Sox6 proteins, which could be reversed by treating the cells with antagomir-802, a miR-802 antagonist [62], establishing the miR-208/Sox6 axis as a mediator of cardiomyocyte hypertrophy. In a study using double-transgenic mice (AS-Ren) with cardiac hyperaldosteronism (AS mice) and systemic hypertension (Ren), Azibani et al. showed that inhibition of miR 208a was associated with increased Sox 6 mRNA compared to the AS control strain [63]. The resulting increase in Sox6 repressed β-myosin heavy chain expression. Therefore, aldosterone inhibits the fetal program and increases cardiac hypertrophy in hypertensive mice by inhibiting MiR-208a expression which in turn sustains Sox6 expression [63].

Soci et al. (2016) studied the effect of aerobic exercise-induced physiological cardiac hypertrophy on the network of myomiRs, their downstream target genes, and the fine-tuning of the expression of α-MHC:β-MHC. Aerobic exercise-induced cardiac hypertrophy is the normal physiological adaptation to exercise. It involves orchestrated fine-tuning of gene and protein expression of contractile and metabolic components with absence of markers of pathological cardiac hypertrophy such as collagen III, extracellular matrix proteins and re-expression of fetal genes, and with improvement in cardiac function. In a rat model of swim training, they reported decreased expression of miR-208a and −208b and increased expression of target genes Sox6, Purβ, Sp3, Med13, and HP1β confirming the participation of Sox6 in modulation of the physiological cardiac metabolic and contractile response to exercise [62].

Another study showed the involvement of Sox6 in lipopolysaccharide (LPS)-induced apoptosis. When triggering this process in cardiomyocytes by exposure to LPS, cells exhibited decreased expression of miR-499 and increased levels of Sox6 and programmed cell death 4 (PDCD4). Overexpression of miR-499 decreased the overexpression of Sox6 and PDCD4 and rescued the apoptosis [64].

Trbp (Tarbp2) is an RNA-binding protein indispensable for maintenance of normal heart function. miR-208a is a critical target of Tarbp2 and mice with heart-specific Trbp KO (Trbp^cKO) showed repressed expression of miR-208a and increased expression of Sox6 protein leading to cardiomyopathy and heart failure. Transgenic overexpression of miR-208a rescued heart function by inhibiting Sox6 expression in Trbp^cKO mice, confirming a role for Sox6 in the pathways leading to heart failure [25••].

Human umbilical cord mesenchymal stem cells (hucMSCs) secrete exosomes that are protective against anoxiareoxygenation (H/R) injury in cardiomyocytes. The protection is due to miRs that possess anti-inflammatory properties in acute myocardial injury. A study showed that miR-19a derived from hucMSC exosomes attenuated H/R injury by downregulating cardiomyocyte Sox6 expression and the AKT/JNK3/caspase-3 pathway [65]. Others showed that miR-499, which is exclusively expressed in cardiomyocytes, is repressed during H/R injury, with concomitant overexpression of Sox6. Mir-499 is protective against this injury and reverses it if overexpressed. This is associated with decreased expression of Sox6 [66] confirming its involvement in this process.

Sox6 and BCL11A, which are potential targets for 2 and 12 miRs, are overexpressed in two forms of thalassemia, hereditary persistence of fetal hemoglobin deletion type-2 (HPFH-2) and Sicilian-β. These are deletion disorders of the β-like human globin cluster resulting in lack of β-chain globin chains and compensatory increase in γ-globin. Because both the Sox6 and BCL11A genes regulate γ-globin, their upregulation in thalassemia suggest that they have an important compensatory role in this disease [67] (Fig. 3).

Fig. 3. A pictorial depiction of miRNA-mediated regulation of SOX6 and associated diseases.

Various miRNAs have been identified to modulate the expression of SOX6 in models of different pathologies. The miRNAs shown in the figure modulate gene expression of SOX6, which contribute to the processes described next to each miRNA. Please go to the references for detailed description of mechanism of action between Sox6 and miRNAs. Figure created with BioRender.com

SOX6 and Hypertension

Hypertension is a multifactorial disease and a cardinal risk factor for renal and cardiovascular diseases. Recent studies showed that Sox6 regulates the renin angiotensin system and therefore has the potential for contributing to blood pressure regulation [68]. Further studies implicated the role of Sox6 in experimental [69] and human hypertension [70]. The former study showed that Sox6 by controlling renal renin and prorenin expression has protective function against hypertension and kidney injury during renal artery stenosis in a Sox6 knockout mouse model [69]. There is little known about the effects of miRs on the interaction between hypertension and the Sox6 genes. In a genome-wide association study (GWAS) in China [71], and in another study by Johnson et al. [70], Sox6 is associated with novel loci for blood pressure and hypertension. However, the mechanism and clinical implications for these associations remain unknown. Dong et al. implicated single-nucleotide polymorphisms (SNPs) in or near Sox6 and other transcription factors with carotid plaque formation [72]. GWAS studies associated Sox6 SNPs with human hypertension across ethnicities. This provides impetus to continue studying the underlying mechanisms and molecular pathways by which Sox6 participates in the pathogenesis of experimental and human hypertension, including regulation of its expression by miRs.

Sox6 and Skeletal Muscle

A heterogenous population of slow- and fast-twitch myofibers is the main component of the skeletal muscle and is imperative for the fine-tuning of muscle function. These two types of myofibers exhibit distinct metabolic and contractile properties. The muscle fiber-type specificity is orchestrated in a coordinated manner by a number of genes, transcription factors, and miRs [73].

In pioneering studies in the early 2000s, Hagiwara et al. demonstrated that Sox6 is vital for cardiac and skeletal muscle development and discovered the distinct genes for the slow and fast fiber isoforms in mice [26, 74, 75]. These studies have been replicated in zebra fish and mice [76, 77]. A study by Lin et al. (2018) revealed that Sox6 is highly expressed during the skeletal muscle cell differentiation in chickens and that its expression is positively associated with copy number variations (CNVs), which are large DNA fragments ranging from 50 base pairs (bp) to 3 mega bases that may have been deleted, inserted, duplicated, or translocated [78].

Over the last two decades, several myomiRs have been discovered which play pivotal roles in skeletal myogenesis [79, 80, 81]. For example, the skeletal muscle of mice with muscle-specific KO of vestigial-like factor-2 (VgII2, a pivotal transcription factors for expression of muscle-specific genes) has increased number of fast-twitch type IIb fibers and downregulation of slow-type I myosin heavy chain gene (Myh7), causing exercise intolerance. These mice also have decreased levels of miR-208b (encoded by an intronic region of myh7), which leads to increased expression of its target genes, including Sox6, Sp3, and purβ [82], confirming roles for miRs and Sox6 in myogenesis.

Several studies in fish, from which slow and fast fiber myotubules can be cultured, have further characterized relationships between miRs and Sox6. For example, in myotubules from rainbow trout Sox6 is preferentially expressed in fast fibers but negatively regulated by miR-133 and miR-499 expression in slow fiber cultures [31]. In the Amazonian fish Pacu (Piaractus mesopotamicus) several miRs (miR-1, miRs-133a, miR-133b, and miR-206) increased expression during growth whereas the expression of target mRNAs (hdac4, srf, and pax7) decreased. This resulted in enhanced differentiation of myoblasts and muscle growth. In contrast, miR-499 expression decreased and Sox6 expression increased in fast-twitch muscle during growth whereas the opposite occurred in slow-twitch muscles, indicating a role for miR-499/Sox6 specification and maintenance of muscle fiber phenotypes [83]. A similar inverse relationship between the expression of miR-499 and its main targets, Sox6 and rod1, was shown in the red skeletal muscle of Nile tilapia [84].

Analogous observations were made in other species. For example, in Sprague–Dawley rats, myricetin, a natural flavonoid that improves endurance capacity and alleviates fatigue by increasing the proportion of slow-twitch myofibers, increased expression of miR-499 and down-regulated that of Sox6 expression both in vivo and in cultured L6 myotubes. Inhibition of miR-499 overturned the effect of myricetin on Sox6 downregulation [85]. Also, in this rat strain, animals were subjected to intermittent hypoxia-hypercapnia to mimic the skeletal muscle dysfunction of human chronic obstructive pulmonary disease (COPD). Decreased type I fibers and increased Sox6 expression were produced by the intervention. The effect on Sox6 was mediated by a peroxisome proliferator-activating receptor β/estrogen-related receptor γ/miR pathway. Electrical stimulation enhanced type-I fiber numbers through suppressing Sox6, confirming its involvement in hypoxia-induced muscle dysfunction [86]. In chicken sartorius muscle, miR-499–5p negatively regulated Sox6 mRNA and in conjunction with other miR-mRNA networks, affected muscle fiber expression and meat quality [87]. A similar inverse correlation between miR-499–5p and Sox6 mRNA was observed in porcine satellite cells, which are multipotent cells involved in postnatal muscle growth and skeletal muscle hypertrophy and regeneration [88]. This is important because it has been reported that the effects of a butyrate diet in improving muscle fiber-type composition (preponderance of slow muscle fibers), mitochondrial biogenesis, and meat quality of finishing pigs involves increased expression of miR-499–5p and miR-208b and decreased expression of their target genes Sox6 and Sp3 [89]. Also, single-nucleotide polymorphisms in the Sox6 gene correlate with markers of growth, carcass, and meat quality traits in pigs, suggesting that its protein product could be targeted to enhance the proportion of slow fiber-type isoforms which are characteristic of good quality pork meat [90] (Fig. 3).

Sox6 and Diabetes

The incidence of diabetes and associated microvascular and macrovascular complication is increasing worldwide. The most common and prevalent vascular complications caused by type 1 and type 2 diabetes mellitus are nephropathy, retinopathy, neuropathy, and cardiovascular diseases such as atherosclerosis, stroke, and hypertension [91, 92]. Type 1 diabetes (T1DM) is an autoimmune disease that is clinically silent until most of the β-cells are destroyed [93]. Type 2 diabetes (T2DM) is characterized by both insulin resistance and impaired insulin secretion. Several miRs have been implicated in both T1DM and T2DM metabolic disorders [36, 94, 95]. Elegant reviews have been published regarding the role of miRs in diabetes and associated complications such as endothelial dysfunction, inflammation, hypertension, and atherosclerosis [94, 96••, 97, 98, 99, 100, 101, 102].

Some groundbreaking studies in early 2000 showed the functional importance of Sox6 in beta cell activity and insulin secretion, thereby implicating it in the pathogenesis of diabetes. These studies revealed that the insulin gene promoter possesses a binding site for Sox6 and that Sox6 reduces insulin secretion by inhibiting pancreatic-duodenal homeobox factor-1 (PDX1), and cyclin D1 [20, 103, 104]. In 2011, the first report was published describing Sox6 as a promising and important target gene for several miRs that control pancreatic development, beta cell integrity and function, and insulin secretion [105•].

In 2017, a study showed that miRs-375 and −26a cooperate to differentiate nestin-expressing umbilical cord-derived mesenchymal stem cells (N-UCMSC) into insulin-producing cells (IPCs). N-UCMSCs are retrieved from discarded umbilical cord tissue and their manipulation poses no risk to the donors, making them a useful experimental platform. In this study, authors used N-UCMSCs and the small subset of pancreatic mesenchymal stem cells that express nestin and are the precursors of the differentiated pancreatic endocrine glands. Differentiation of N-UCMSC into IPCs was associated with inhibition of Sox6, ccnd1, mtpn, and bhlhe22 [106].

Another study revealed that miR-223 is also important for β-cell function and insulin secretion. miR-223 KO mice exhibit insulin resistance and impaired glucose tolerance because its deficiency dramatically inhibits β-cell proliferation and insulin secretion. Employing luciferase reporter gene assays, they showed that forkhead box O1 (FOXO1) and Sox6 are the target genes for miR-223 in β-cells. Conversely, overexpression of miR-223 in MIN6 (isolated islet) cells decreases the expression of Sox6 and FOXO1 and promotes β-cell proliferation and function [107•].

miR-96 also regulates the Sox6 and FOXO1 genes in vivo and in vitro in a transgenic mouse model of T2DM. The study showed that miR-96 KO mice exhibit β-cell dysfunction and insulin resistance, which were further aggravated by feeding high-fat diet. Using dual-luciferase reported gene assays authors found that Sox6 and FOXO1 are the target genes for miR-96 and that they are negatively regulated. Overexpression of miR-96 upregulated cell proliferation and inhibited apoptosis by targeting FOXO1 and Sox6 in MIN6 cells [24]. This was associated with increases in expression of Pdx1, Nkx6.1, Cyclin D1, and Cyclin E1 in these cells.

miR-21 and its target genes play an important role in the formation of IPCs from the pancreatic progenitor cells. Using microarray and deep sequencing approaches, this study showed that Sox6 inhibited the PDX1 and PDX1-mediated stimulation of the insulin gene promoter and inhibited IPCs. On the other hand, HES1 a potent inhibitor of islet hormone genes is repressed by RBPJ. Both Sox6 and RBPJ are the important target genes for miR-21 during IPCs formation. Therefore, miR-21 functions as a bidirectional switch in the formation of IPCs by regulating the expression of the Sox6 and RBPJ genes [108].

In contrast to the many observations above, in other models of diabetes miRs are involved whereas Sox6 expression is not affected. For example, β-cell specific deletion of Dicer (an enzyme necessary for miR processing and maturation) leads to abnormal insulin secretion, reduction in β-cell number, and diabetes mellitus. Martinez-Sanchez et al. showed that out of the 14 disallowed genes in β-cells, mRNAs of 6 genes (Fcgrt, Igfbp4, Maf, Oat, Pdgfra, and Slc16a1) were overexpressed in Dicer-null β-cells, and that three of them (Fcgrt, Oat, and Pdgfra) are direct targets of miRs. However, they did not find any effect of Dicer deletion on Sox6 expression [109].

DKD mice are a strain used experimentally to study diabetic kidney disease (DKD), the major microvascular complication of T1DM and T2DM. These mice showed lower expression of miR-342–3p and higher expression of Sox6 measured with Western blot and RT-qPCR. Mouse mesangial cells treated with high glucose exhibited a similar gene pattern for this miR and its target gene. Induced expression of miR-342–3p in mesangial cells led to decreased expression of Sox6, cell proliferation, inhibition of apoptosis, and attenuated kidney injury and fibrosis [23] (Fig. 3).

Sox6 as a Target for miRNAs in other Organs and Diseases

Sox6 has been key target for miRNAs in neural development and Alzheimer’s disease [110, 111, 112, 113], bone development and osteoporosis [114, 115, 116], male and female fertility [117, 118, 119], skin diseases [120], and various forms of cancers [121, 122, 123, 124, 125, 126, 127]. These topics have been extensively reviewed and beyond the scope of our review.

Perspective/Conclusion

In this review, we have described the functional importance of Sox6 as an important target for miRs, and the roles for both in cardiovascular pathophysiology, skeletal muscle development, and diabetes. Sox6 and other members of SoxD family (Sox5, Sox13) play vital functions in adult tissue homeostasis, regeneration, and physiology both in vertebrates and invertebrates. Abnormal expression or dysregulation in Sox6 gene function has been implicated in cardiovascular diseases, cancer, chondrodysplasia, osteoarthritis, and autoimmune diseases. miRNAs play an orchestrated function in gene expression, cell signaling pathways, and various cell functions in animals and plants. Dysregulated expression of miRNAs has been reported in pathophysiological conditions such as cardiovascular diseases, cancer, chondrodysplasia, and autoimmune diseases. The current assumption is that miRNAs regulate about 35% of the human genome (50); thus, miRNAs may be as important as transcription factors are, and may hold great potential for future biomedical research. Sox6 being a multifaceted transcription factor and a target molecule for miRNAs holds a great potential to reveal undiscovered avenues in cardiovascular and associated diseases.