Post-transcriptional regulation in metabolic diseases

Abstract

Post-transcriptional gene regulation by microRNAs (miRNAs) and RNA-binding proteins (RBPs) is central to many biological functions.  Aberrant gene expression patterns underlie many metabolic diseases that represent major public health concerns and formidable therapeutic challenges.  Several studies have established a number of post-transcriptional regulators implicated in metabolic diseases such as diabetes and obesity.  In addition, emerging knowledge of metabolically active and insulin-sensitive organs, such as the pancreas, liver, muscle and adipose compartment, is rapidly expanding the panel of potential therapeutic targets for the treatment of metabolic diseases.  Here, we review our current understanding of miRNAs and RBPs that affect glucose and lipid homeostasis, and their roles in normal physiology and metabolic disorders, especially type 2 diabetes and obesity.

Keywords: :

• posttranscriptional gene regulation
• microRNAs
• RNA-binding proteins
• glucose homeostasis
• lipid homeostasis
• diabetes
• metabolic disease

Introduction

Type 2 diabetes mellitus (T2DM) is a metabolic disease characterized by insulin resistance in insulin-sensitive tissues including white adipose tissue, skeletal muscle and liver as well as impaired insulin secretion from pancreatic β cells, resulting in hyperglycemia.^1,2 Insulin resistance is defined as a state in which insulin becomes less effective at lowering blood glucose and occurs when increasing amounts of insulin in circulation are needed to maintain glucose homeostasis.³ It commonly leads to impaired glucose and lipid metabolism, which are major risk factors for development of metabolic disease.⁴ It is important to characterize the molecular mechanisms of insulin resistance and subsequent pancreatic β-cell failure in order to develop approaches to prevent metabolic diseases. Despite considerable gains in our understanding of causes and risk factors for metabolic disease, the molecular mechanisms underlying insulin resistance and the pathogenesis of other metabolic diseases are unclear. Recent studies have extended this notion to posttranscriptional gene regulation.

Acute and prompt changes in mRNA levels are critical for altering the level of proteins in response to various metabolic stimuli and maintain cellular homeostasis. At the post-transcriptional level, gene regulation is governed by two major types of trans-binding factors: microRNAs (miRNAs) and RNA binding proteins (RBPs). miRNAs are a large group of small (19–23-nt-long) non-coding RNAs that critically influence gene expression by forming incomplete base pairing with target mRNAs.^5,6 miRNAs are involved in diverse biological progresses, such as differentiation and development, and their dysregulation has been linked to many diseases.⁷ Generally, miRNAs repress protein synthesis by enhancing mRNA decay or inhibiting translation through their interaction with cis-elements of target mRNAs, although, some miRNAs were reported to activate mRNA translation.^8-10 RBPs (especially, turnover- and translation-regulatory RBPs, TTR-RBPs) regulate mRNA stability and translation rate by interacting with target mRNAs via different RNA interaction motifs.¹¹ RBPs are varied in both structure and complexity, and perform diverse functions affecting all aspects of RNA metabolism (reviewed in ref. 12). miRNAs and RBPs generally interact with the 3′-untranslated region (UTR) of target mRNAs and modulate mRNA stability and translation. However, increasing evidences indicate that those trans-binding factors also interact with the coding region and with the 5′-UTR.^13,14

Recently, significant progress has been made in understanding the cellular roles of post-transcriptional regulators in metabolically active and insulin-sensitive tissues. Their influence on metabolism and cellular homeostasis in vivo needs to be further characterized systematically. Here, we review the most recent findings of miRNAs and RBPs involved in the maintenance of glucose and lipid homeostasis, and their implication in normal physiology and in metabolic disorders, especially T2DM and obesity.

Glucose homeostasis

In mammals, glucose homeostasis is determined by the minute-to-minute control of the amount of circulating insulin levels and the number of insulin-secreting β- cells in islets. An inadequate expansion of β-cell mass or failure of the existing β-cell mass to compensate for increased insulin demand, associated with eventual loss of β- cells due to apoptosis, are hallmarks of T2DM. These prominent features of T2DM result from a defect in insulin action mediated by the insulin receptor (IR).^15-20 Insulin binding to IR activates a intracellular signaling pathway including the IR, insulin receptor substrate (IRS), phosphoinositide 3-kinase (PI3K), phosphoinositide-dependent kinase 1 (PDK1), and the protein kinase AKT, and thereby regulates not only glucose and lipid metabolism but also cellular proliferation and apoptosis.^15-23 Indeed, studies of animal models with targeted mutations in genes that encode mediators of insulin signaling have revealed that insulin signaling plays a central role in metabolic actions in insulin-sensitive tissues and contributes to the pathogenesis of T2DM.^21-23 Dysregulation at any step of this fine tuning is responsible for insulin resistance and impaired glucose homeostasis, resulting from diminished glucose uptake and utilization in insulin-sensitive tissues.

miR-375 in glucose homeostasis

The link between miRNA and diabetes was brought to light with the discovery of miR-375, which is one of the most abundant miRNAs present in pancreatic islets and is a major mediator of β-cell function and hence glucose homeostasis.^24,25 The expression of miR-375 is under the control of PDX-1, a major transcriptional regulator of β-cell function and mass.²⁶ The first study of miR-375 in insulin-secreting MIN6 and primary β-cells revealed that miR-375 negatively regulates glucose-stimulated insulin secretion with no effect on either ATP production or intracellular Ca²⁺ levels.²⁴ It influences a late step of insulin exocytosis through a reduction in the expression of myotrophin, a protein involved in insulin granule fusion. Subsequently, miR-375 was shown to be involved in glucose-mediated actions on insulin gene expression through the regulation of PDK1, a key protein in PI3K signaling in pancreatic β- cells.²⁵ Moreover, miR-375 expression is regulated by glucose²⁷ and its levels in pancreatic islets are aberrant in obese ob/ob mice,²⁸ a model of T2DM. These findings suggest that miR-375 is involved in the regulation of glucose responsiveness in β-cells and therefore it has implication in the pathogenesis of T2DM. Indeed, mice lacking miR-375 exhibited fasting and non-fasting hyperglycemia and glucose intolerance; this phenotype was a result of upregulated gluconeogenesis and increased hepatic glucose output due to the higher total pancreatic α-cell compartment and resultant hyperglucagonemia.²⁸ In contrast, pancreatic β-cell mass is decreased in these mice as a result of impaired proliferation. In agreement with these observations, genetic deletion of miR-375 in obese ob/ob mice led to a severe diabetic state, because of a profoundly diminished proliferative capacity of β cells. Thus, miR-375 is at present the best-studied miRNA implicated in β-cell function as well as glucose homeostasis.

Lin28/ let-7a in glucose homeostasis

The tumor-suppressor miRNA let-7 negatively regulates the expression of oncogenes and cell cycle regulators,^29-32 while, the RNA-binding proteins Lin28a and Lin28b are activated in many cancers to selectively and post-transcriptionally inhibit let-7 biogenesis.^32-35 A recent study by Zhu and colleagues uncovered a connection between lin28/let-7 signaling and glucose metabolism.³⁶ Both Lin28a and Lin28b transgenic mice have improved insulin sensitivity and glucose homeostasis, and are resistant to obesity, whereas muscle-specific Lin28a knockout mice display insulin resistance and glucose intolerance. These effects are mediated by an increase in insulin-PI3K-mTOR signaling due in part to the let-7-mediated repression of multiple targets in the pathway, including insulin-like growth factor 1 receptor (IGF1R), IR, IRS2 and AKT2. Moreover, overexpression of let-7 caused impaired insulin sensitivity and glucose homeostasis, suggesting that Lin28a/b exerts effects on glucose metabolism at least in part by suppressing let-7 levels. The connection between lin28/let-7 signaling and glucose homeostasis is further supported by the observation that many genes associated with T2DM and control of fasting glucose in human are known or predicted let-7 targets.³⁶ Thus, although the function and regulation of lin28 and let-7 remain to be studied in more detail in animal models, the lin28/let-7 signaling axis may represent a valuable therapeutic target in obesity and diabetes.

microRNAs in insulin sensitivity

It is clear that various miRNAs contribute to the development of insulin resistance due to alteration of gene expression in insulin-sensitive tissues (reviewed in ref. 37). Recently, several microRNAs have been found to directly regulate insulin sensitivity.^38,39 Trajkovski et al. found that miR-103 and miR-107 negatively regulate insulin sensitivity: both were upregulated in both genetic and diet-induced obese mice.³⁸ The expression of miR-103/107 in either liver or white adipose tissue disrupted glucose homeostasis and, conversely, global miR-103/107 silencing resulted in improved glucose homeostasis and insulin sensitivity, implicating these miRNAs as novel therapeutic targets for the treatment of diabetes. The same investigation also found that these effects are mediated by targeting caveolin-1, resulting in a reduction in the abundance of IRs in caveolae-enriched plasma membrane microdomains and reducing downstream insulin signaling.

The upregulation of miR-143 was also confirmed in the liver of both genetic- and diet-induced obese mice.³⁹ This observation is consistent with other reports showing that miR-143 is dysregulated in tissues of ob/ob,⁴⁰ diet-induced obese^41,42 and diabetic mice⁴³ as well as in human patients.⁴⁴ Gain- and loss-of-function studies also confirmed miR-143 as a negative regulator of insulin sensitivity and glucose homeostasis, resulting from impaired insulin signaling via AKT.³⁹

microRNAs in insulin biosynthesis

A review of RBPs and microRNAs that target the mRNAs encoding insulin and its receptors was lately published⁴⁵ and additional RBPs and microRNAs have since been reported. miR-24, miR-26, miR-148 and miR-182 were found to be involved in increasing insulin promoter activity and insulin mRNA levels, while miRNA-dependent regulation of insulin expression was associated with upregulation of transcriptional repressors such as Bhlhe22 (basic helix-loop-helix family member 22) and Sox6 (Sry-related HMG box 6).⁴⁶

RBPs in glucose homeostasis

Several RBPs were reported to be involved in insulin biosynthesis and glucose homeostasis. Kulkarni and coworkers reported protein-disulfide isomerase (PDI) and poly(A)-binding protein as trans-acting factors that bind to the 5′ UTR of insulin mRNA and mediate glucose-stimulated translation of insulin mRNA in pancreatic islets.⁴⁷ ARE/poly(U)-binding factor 1 (AUF1), the best-characterized RBP that controls mRNA stability and translation by associating with AU- or U-rich sequence elements (ARE) frequently found within the 3′-UTR, was found to be involved in the cytotoxic effects of proinflammatory cytokines on pancreatic β cells.⁴⁸ Exposure of pancreatic β cells to cytokines caused translocalization of AUF1 to the cytoplasm. Overexpression of AUF1 induced β-cell death due to a decrease in the expression of the anti-apoptotic proteins, whereas knockdown of AUF1 restored the levels of the anti-apoptotic proteins, attenuated the activation of the nuclear factor-κB pathway, and protect the β cells from cytokine-induced apoptosis.

Studies by Welsh group have shown that glucose-stimulated binding of the polypyrimidine tract-binding protein (PTB), also known as heterogeneous nuclear ribonucleoprotein I (hnRNP I), to the 3′-UTR of the preproinsulin mRNA, stabilizes the messengers, resulting in the glucose-stimulated increase in preproinsulin mRNA.^49,50 Additionally, PTB plays a role in insulin secretion by regulating the biogenesis of insulin secretory granules.⁵¹ Glucose stimulation of pancreatic β cells induced the cytosolic localization of PTB, thereby participating in the stabilization of mRNAs encoding proteins on secretory granules. Conversely, knockdown of PTB expression led to the depletion of secretory granules as well as decreased levels of intracellular and secreted insulin.

Recently, we identified HuD/ELAVL4 (human antigen D/ embryonic lethal abnormal vision-like 4) as an RBP that binds to the insulin mRNA and controls its translation.⁵² Similar to two other Hu/ELAV family members (HuB and HuC), HuD was believed to be expressed specifically in neurons.^53,54 However, we found that HuD is also present in pancreatic β cells, where its levels are controlled by the IR pathway.⁵² In β cells, HuD binding to the Ins2 5′UTR repressed Ins2 mRNA translation and decreased insulin production. Accordingly, HuD-null mice expressed higher levels of insulin in β cells, while HuD-transgenic mice expressed lower insulin levels in β cells. Importantly, HuD-transgenic mice displayed impaired glucose homeostasis because of decreased insulin secretion, thus defining HuD as a pivotal regulator of insulin translation.

Lipid homeostasis

Cholesterol is a major component of cell membranes and is synthesized endogenously in a highly regulated enzymatic reaction. Cellular lipid levels are tightly regulated by critical factors; peroxisome proliferator-activated receptors (PPARs), sterol regulatory element-binding proteins (SREBPs) and liver X receptor (LXR) (reviewed in refs. 55-57). To maintain cholesterol homeostasis, excess cholesterol is exported from cells to apoA1 acceptors via the adenosine triphosphate-binding cassette (ABC) transporters, ABCA1 and ABCG1.⁵⁸ Dysregulation of lipid homeostasis is associated with the development of several diseases such as cancers and atherosclerosis.^59-61

miR-122 in cholesterol metabolism

Several groups have assessed the role of miR-122 in lipid metabolism by using antisense methods to inhibit the expression of miR-122.^62-64 miR-122 is highly conserved and the most abundantly expressed miRNA which regulate cholesterol and lipid homeostasis in the liver.^62-64 Krütfeldt and colleagues showed that inhibition of miR-122 using 2′-O-methoxyethyl (2′-O-MOE) phosphorothioate antisense oligonucleotide resulted in complete loss of miR-122 in the liver leading to the upregulation of a number of genes involved in cholesterol biosynthesis.⁶² Although they observed an increase of several genes that contain putative binding sites for miR-122, it needs to be determined which direct target mRNAs are responsible to miR-122-mediated cholesterol biosynthesis. Esau et al. also reported that miR-122 knockdown reduced circulating cholesterol level and fatty acid synthesis, while it elevated liver fatty acid β-oxidation, indicating that inhibition of miR-122 reduces hepatic cholesterol accumulation and improves liver steatosis in diet-induced obesity.⁶³ Similarly, locked-nucleic acid (LNA)-antagonism of miR-122 in mice increased the expression of predicted target genes involved in lipid biogenesis and affected cholesterol metabolism without liver toxicity,⁶⁴ suggesting that inhibition of miR-122 has a feasible therapeutic target .

miR-33 in cholesterol metabolism

SREBPs are helix-loop-helix-leucine zipper transcription factors that play critical roles in cholesterol biosynthesis, while miR-33a and miR-33b, encoded within the introns of the srebp-2 and srebp-1 genes, respectively, are highly conserved and ubiquitously expressed miRNAs.⁶⁵ Recently, it was reported that miR-33a/b are co-regulated with their host genes under metabolic stimuli and they were identified as critical regulators of cholesterol homeostasis by three different groups.^65-68 In mouse and human, miR-33 inhibited the expression of the ABCA1, thereby inhibiting cholesterol trafficking. Overexpression of miR-33 in macrophages and hepatocytes decreased cholesterol efflux to apoA1, while inhibition of miR-33 resulted in increased ABCA expression and cholesterol efflux. In mouse macrophages, miR-33 also regulated ABCG1, which mobilized cellular free cholesterol to high density lipoprotein (HDL) particles.^65,66 In human macrophages and hepatocytes, miR-33 regulated the expression of the protein NPC1 (Neimann Pick C1), which acts in concert with ABCA1 to cause efflux of cholesterol to apoA1, suggesting that miR-33 suppresses another component of the cholesterol export pathway in humans.⁶⁶ It was further demonstrated that miR-33 also affects β-oxidation of fatty acids by targeting CPT1a, CROT and HADHB, which are critical enzymes in the β-oxidation pathway.^67,69 Overexpression of miR-33 in hepatocytes reduced CPT1a and HADHB expression and thereby decreased cellular β-oxidation of fatty acids. Taken together, these results underscore the potential usefulness of anti-miR-33 as a therapeutic target for metabolic disease. Further in vivo studies will be necessary to test this possibility.

miR-378/378* in lipid metabolism

Recently, miR-378/378* was found to play a role in the regulation of lipid metabolism.⁷⁰ Both are intronic miRNAs cotranscribed within the first intron of the PPARγ coactivator-1 β (PGC-1β) and are highly induced during adipogenesis.⁷⁰ ST2 mesenchymal precursor cells infected with miR-378/378* had increased lipid droplet size and accumulation of triacylglycerol, accompanied by increased expression of lipogenic genes including Krüppel-like factor 5, fatty acid binding protein 4, fatty acid synthase, stearoyl-coenzymeA desaturase, and resistin. Conversely, knockdown of miR-378/378* in 3T3-L1 adipocytes led to decreased accumulation of triacylglycerol. They also found that miR-378/378* also appeared to increase the transcriptional activity of CCAAT-enhancer-binding protein (C/EBP) α and β on adipocyte gene promoter.

RBPs in lipid metabolism

HuR (human antigen R) is a ubiquitously expressed RBP belonging to Hu/ELAV family and modulates the stability and translational efficiency of mRNAs by interacting with ARE in the 3′-UTR. Gantt and colleagues showed that HuR plays a role during acquisition of the adipocyte phenotype.⁷¹ HuR was constitutively expressed and localized predominantly to the nucleus in the preadipocytes, whereas adipogenic stimulus increased the HuR content in the cytosol and rapid formation of HuR complexes containing C/EBPβ mRNA. In the 3T3-L1 cell, suppression of HuR expression resulted in a decrease of C/EBPβ expression and accumulation of lipid droplets and, in turn, an inhibition of adipogenesis, suggesting that HuR functions in establishing the adipocyte phenotype.

Similar to HuR, hematopoietic zinc finger (Hzf) was also found to be required for efficient adipogenesis through the translational regulation of C/EBPα mRNA.⁷² Hzf was highly expressed in mouse adipose tissue and was markedly induced during adipogenesis. Silencing of Hzf by shRNA in 3T3-L1 cells caused impaired expression of C/EBPα and decreased triglyceride levels and glucose uptake, resulting in attenuated adipogenesis. The effect of Hzf on adipogenesis was mediated by its association with the 3′-UTR of C/EBPα mRNA to enhance its efficient translation and to thereby reinforce adipogenesis. Consistent with studies using cell lines, C/EBPα protein level, but not mRNA level, was lower in Hzf-null mice than in wildtype, suggesting that C/EBPα is translationally regulated by Hzf in vivo. In addition, Hzf-null mice showed an impaired glucose metabolism and reduced insulin sensitivity.

Others

Other miRNAs and RBPs involved in glucose and lipid homeostasis, but not described here, are summarized in Table 1.^73-99

Table 1. miRNAs and RBPs involved in glucose and lipid homeostasis.

Regulators	Target		System				Functions	Ref.
Glucose homeostasis
Lin28/let-7		IGF1R, IRs				mouse	↑ Insulin sensitivity, ↑ Glucose metabolism	36
miR-9		Onecut2				INS-1	↓ Insulin secretion	73
miR-15a		UCP-2				MIN6	↑ Insulin biosynthesis	74
miR-21						MIN6	↓ Insulin secretion	75
		PDCD4				mouse	↑ β-cell death	76
miR-24		Bhlhe22 and Sox6				mouse	↑ Insulin biosynthesis	46
miR-26		Bhlhe22 and Sox6				mouse	↑ Insulin biosynthesis	46
miR-29						3T3-L1	↑ Insulin resistance	77
miR-30d						MIN6	↑ Insulin biosynthesis	78
miR-33		IRS2				Huh7, HepG2	↓ IR signaling	60
miR-34a		Vamp2, Bcl2				MIN6	↓ Insulin secretion, ↑ β-cell death	75,79
miR-96		Granuphilin				MIN6	↓ Insulin secretion	80
miR-103		Caveolin-1				mouse	↑ Insulin resistance	38
miR-107		Caveolin-1				mouse	↑ Insulin resistance	38
miR-124a		Rab27A				MIN6	↓ Insulin secretion	80
miR-130a						MIN6	↑ Insulin secretion	81
miR-143		ORP8				mouse	↑ Insulin resistance	39
miR-146						MIN6	↓ Insulin secretion, ↑ β-cell death	75
miR-148		Bhlhe22 and Sox6				mouse	↑ Insulin biosynthesis	46
miR-182		Bhlhe22 and Sox6				mouse	↑ Insulin biosynthesis	46
miR-200a						MIN6	↑ Insulin secretion	81
miR-278						Drosophila	↓ Insulin resistance	82
miR-375		Gphn, Cadm1, HuD				mouse	↑ β-cell mass and glucose metabolism	28
		Mtpn				MIN6	↓ Insulin secretion	24
miR-410						MIN6	↑ Insulin secretion	81
AUF1		Bcl2, MCL1				MIN6	↑ β-cell death	48
HuD		Insulin				mouse, βTC6	↓ Insulin biosynthesis	52
Hzf						mouse	↑ Glucose metabolism, ↑ Insulin sensitivity	72
PDI		Insulin				mouse, βTC6	↑ Insulin biosynthesis	47
PTB		Insulin				rat, βTC6	↑ Insulin biosynthesis	49, 50
		ICA512, PC1/3, PC2				rat, Ins-1	↑ Insulin secretion	51
Lipid homeostsasis
miR-21		TGFBR2				hASC	↑ Adipogenesis	83
miR-24						C3H10T1/2	↑ Adipogenesis	84
miR-27		PPARγ				3T3-L1, hASC	↓ Adipogenesis	85, 86
miR-31		C/EBPα				hASC	↓ Adipogenesis		87
miR-33		ABCA1, PC1,ABCG1				human, mouse	↓Cholesterol efflux andfatty acid oxidation	58–61
miR-103						3T3-L1	↑ Adipogenesis	88, 89
miR-107						3T3-L1	↑ Adipogenesis	88, 89
miR-122		Aldoa,Gys1, Ccng1				mouse	↑ Cholesterol synthesis	56,57
miR-130		PPARγ		Preadipocyte			↓ Adipogenesis	90
miR-138		EID1				hASC	↓ Adipogenesis	91
miR-143		Erk5				Pre-adipocyte	↑ Adipogenesis	92
miR-181d						Huh7	↓ Adipogenesis	93
miR-200						ST2	↑ Adipogenesis	94
miR-210		TCF7L2				3T3-L1	↑ Adipogenesis	95
miR-221						3T3-L1	↓ Adipogenesis	88
miR-222						3T3-L1	↓ Adipogenesis	88
miR-326						Preadipocyte	↓ Adipogenesis	87
miR-335					mouse, 3T3-L1		↑ TG and cholesterol	96
miR-370		miR-122, Cpt1α			mouse, HepG2		↓ fatty acid oxidation, cholesterol synthesis	97
miR-378/378*		C/EBPα and β				ST2, 3T3-L1	↑ Adipogenesis	70
miR-448		KLF5				3T3-L1	↓ Adipogenesis	98
Let-7		Hmga2				3T3-L1	↓ Adipogenesis	99
HuR		C/EBPβ				3T3-L1	↑ Adipogenesis	71
Hzf		C/EBPα				3T3-L1	↑ Adipogenesis	72

INS-1, MIN6 and βTC6: Murine insulinoma cell lines; HepG2 and Huh7: Human hepatocarcinoma cell lines; 3T3-L1: Murine preadipocyte cell line; hASC: Human adipose-derived stem cell; C3H10T1/2: Murine mesenchymal stem cell line; ST2: Murine bone marrow stromal cell line.

Conclusion

Since metabolic diseases are major public health concerns, the importance of developing methods for managing and treating metabolic diseases is extremely urgent, particularly when one considers the social and economic strain that results if optimal therapy is not available. As the present review shows, numerous post-transcriptional regulators of these metabolic processes have been identified. There is a broad agreement that regulators such as miRNAs and RBPs are linked to the pathogenesis and pathophysiology of metabolic diseases, especially T2DM. Altered patterns of miRNA expression have been seen in T2DM patients,^100,101 and a unique miRNA signature was detected in blood of T2DM patients¹⁰¹ and these molecules are stable in stored samples.^100,102 Although the determination of their potential to diagnose T2DM remains to be confirmed by more systematic and large independent studies of well-characterized cohorts of patients, miRNAs circulating in the blood may have the potential to serve as novel biomarkers for the detection and diagnosis of T2DM. Moreover, modifying their behavior by miRNAs or their mimics, as well as inhibitors of miRNA function is also widely recognized and could be the potential therapeutics for targeting metabolic diseases. miRNA activity can be enhanced by transfecting synthetic miRNA mimetics or by using plasmids to transcribe miRNAs or suppressed by antisence oligonucleotides, termed antagomirs,⁶² whose stability and binding affinity can be increased by chemical modification such as 2′-O-methyl (2′-O-Me), 2′-O-MOE, and LNA.¹⁰³ Indeed, intravenous administration of antagomirs against miR-16, miR-122, miR-192, and miR-194 resulted in suppression of corresponding miRNA levels in liver, lung, kidney, heart, intestine, fat, skin, bone marrow, muscle, ovaries and adrenals.⁶² Of relevance to metabolism, several studies have further evaluated the potency of different chemically modified antagomir designs in targeting of miR-122,^62-64 miR-103/107,³⁸ and miR-33.^65-68 Clinical trials are currently underway that target miR-122 with antisense LNA technology as a therapeutic approach for hypercholesterolemia.¹⁰⁴ Besides antagomirs, miRNA sponges¹⁰⁵ and small-molecule inhibitors¹⁰⁶ might be an alternative approach to suppress miRNA activity. Despite recent advances in the design of therapeutic strategies for miRNAs, it is important to recognize that miRNAs likely function by simultaneously targeting groups of mRNAs (instead of a single mRNA), and therein lies their effectiveness, as they can jointly affect expression of many proteins within a given pathway. Also, novel and precisely delivery systems of the miRNAs and its inhibitors into the tissue of interest need to be explored and developed. Therefore, a more comprehensive knowledge of the molecular post-transcriptional influence of RBPs and miRNAs in metabolic diseases and better understanding of the upstream factors regulating their expression would be useful so as to design valid therapeutic strategies to treat metabolic diseases. In the future, approaches to target these molecules selectively in a cell- or tissue-specific manner will need to be developed and evaluated carefully for safety and therapeutic effectiveness (Fig. 1).

Figure 1. Schematic overview of the miRNAs and RBPs involved in glucose or lipid homeostasis in insulin-sensitive tissues.

Acknowledgments

We appreciate M.Gorospe and J.M.Egan for critical reading of the manuscript. W. Kim is supported by the Intramural Research Program of the National Institute on Aging (NIA)/NIH. E.K.Lee is supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (20110013116) and the Catholic Medical Center Research Foundation.