Thyroid Hormone-Regulated Cardiac microRNAs are Predicted to Suppress Pathological Hypertrophic Signaling

Remodelling of T-Tubules and Associated Calcium Handling Dysfunction in Heart Failure: Mechanisms and Therapeutic Insights

In cardiomyocytes, transverse tubules (T-tubules) are sarcolemmal invaginations that facilitate excitation-contraction coupling and diastolic function. The clinical significance of T-tubules has become evident in that their remodelling is recognised as a hallmark feature of heart failure (HF) and a key contributor to disrupted Ca2+ homeostasis, compromised cardiac function, and arrhythmogenesis. Further investigations have revealed that T-tubule remodelling is particularly pronounced in HF with reduced ejection fraction (HFrEF), but not in HF with preserved ejection fraction, implying that T-tubule remodelling may play a crucial pathophysiologic role in HFrEF. While research on the functional importance of T-tubules is ongoing, T-tubule remodelling has been found to be reversible. That finding has triggered a surge in studies aimed at identifying specific therapeutic approaches for HFrEF. This review discusses the functional importance of T-tubules and their microdomains, the pathophysiology of T-tubule remodelling, and the potential mechanisms of current HFrEF therapeutic approaches in reversing T-tubule alterations. We also highlight discrepancies regarding the roles of T-tubule proteins in the recovery process across studies to offer valuable insights for future research.

Experimental Modeling Of Hypothyroidism: Principles, Methods, Several Advanced Research Directions In Cardiology

Hypothyroidism is one of the most common pathological conditions in modern clinical practice. Due to the fact that the targets of thyroid hormones are virtually all organs and tissues, the morphological and clinical manifestations arising with a deficiency of thyroid hormones are quite diverse. Experimental models of hypothyroidism in laboratory animals are widely used for preclinical study of the fundamental pathophysiological mechanisms underlying hypothyroidism, as well as for assessing the effectiveness of treatment-and-prophylactic effects. Currently, several groups of effective models of hypothyroidism have been developed: dietary, surgical, medicamentous, genetic, radioactive and immunological. Each of the specified models is based on different principles, has advantages and disadvantages, and can be used depending on the goals and objectives of the experiment. In this review, we will consistently consider hypothyroidism modeling methods and indicate some promising areas of their use in cardiology.

Triiodothyronine maintains cardiac transverse-tubule structure and function

Subclinical hypothyroidism and low T3 syndrome are commonly associated with an increased risk of cardiovascular disease (CVD) and mortality. We examined effects of T3 on T-tubule (TT) structures, Ca²⁺ mobilization and contractility, and clustering of dyadic proteins. Thyroid hormone (TH) deficiency was induced in adult female rats by propyl-thiouracil (PTU; 0.025%) treatment for 8 weeks. Rats were then randomized to continued PTU or triiodo-L-thyronine (T3; 10 μg/kg/d) treatment for 2 weeks (PTU + T3). After in vivo echocardiographic and hemodynamic recordings, cardiomyocytes (CM) were isolated to record Ca²⁺ transients and contractility. TT organization was assessed by confocal microscopy, and STORM images were captured to measure ryanodine receptor (RyR2) cluster number and size, and L-type Ca²⁺ channel (LTCC, Ca_v1.2) co-localization. Expressed genes including two integral TT proteins, junctophilin-2 (Jph-2) and bridging integrator-1 (BIN1), were analyzed in left ventricular (LV) tissues and cultured CM using qPCR and RNA sequencing. The T3 dosage used normalized serum T3, and reversed adverse effects of TH deficiency on in vivo measures of cardiac function. Recordings of isolated CM indicated that T3 increased rates of Ca²⁺ release and re-uptake, resulting in increased velocities of sarcomere shortening and re-lengthening. TT periodicity was significantly decreased, with reduced transverse tubules but increased longitudinal tubules in TH-deficient CMs and LV tissue, and these structures were normalized by T3 treatment. Analysis of STORM data of PTU myocytes showed decreased RyR2 cluster numbers and RyR localizations within each cluster without significant changes in Ca_v1.2 localizations within RyR clusters. T3 treatment normalized RyR2 cluster size and number. qPCR and RNAseq analyses of LV and cultured CM showed that Jph2 expression was T3-responsive, and its increase with treatment may explain improved TT organization and RyR-LTCC coupling.

MicroRNAs and thyroid hormone action

MicroRNAs (miRNAs) are small noncoding RNAs that post-transcriptionally repress gene expression by binding generally to the 3'-untranslated regions of their target mRNAs. miRNAs regulate a large fraction of the genome, playing a key role in most physiological and pathological processes. The thyroid hormones (T4 and T3) are major regulators of development, metabolism and cell growth. The thyroid hormones (THs) are synthetized in the thyroid gland and enter the cells through transporter proteins. In the cells, T4 and T3 are metabolized by deiodinase enzymes and bind to nuclear receptors (TRs), which have a higher affinity by T3. TRs act as hormone dependent transcription factors by binding to thyroid hormone response elements (TREs) in the target genes and recruiting transcriptional coregulators. There is increasing evidence that a variety of miRNAs target deiodinases and the receptor, thus regulating TH signaling is different tissues. In turn, the THs have been shown to modulate the expression of specific miRNAs and their mRNA targets in different cell types and organs. In many cases, the existence of TREs in the regulatory regions of these miRNAs has been identified, and the hormone bound receptors transcriptionally regulate expression of these molecules. Changes in the levels of miRNAs have been demonstrated to mediate some of the important actions of the THs in processes such as muscle and heart function, lipid liver metabolism or skin physiology. In addition, miRNA regulation is involved in the effects of TRs on cell proliferation and cancer.

Thyroid Hormones Interaction With Immune Response, Inflammation and Non-thyroidal Illness Syndrome

The interdependence between thyroid hormones (THs), namely, thyroxine and triiodothyronine, and immune system is nowadays well-recognized, although not yet fully explored. Synthesis, conversion to a bioactive form, and release of THs in the circulation are events tightly supervised by the hypothalamic-pituitary-thyroid (HPT) axis. Newly synthesized THs induce leukocyte proliferation, migration, release of cytokines, and antibody production, triggering an immune response against either sterile or microbial insults. However, chronic patho-physiological alterations of the immune system, such as infection and inflammation, affect HPT axis and, as a direct consequence, THs mechanism of action. Herein, we revise the bidirectional crosstalk between THs and immune cells, required for the proper immune system feedback response among diverse circumstances. Available circulating THs do traffic in two distinct ways depending on the metabolic condition. Mechanistically, internalized THs form a stable complex with their specific receptors, which, upon direct or indirect binding to DNA, triggers a genomic response by activating transcriptional factors, such as those belonging to the Wnt/β-catenin pathway. Alternatively, THs engage integrin αvβ3 receptor on cell membrane and trigger a non-genomic response, which can also signal to the nucleus. In addition, we highlight THs-dependent inflammasome complex modulation and describe new crucial pathways involved in microRNA regulation by THs, in physiological and patho-physiological conditions, which modify the HPT axis and THs performances. Finally, we focus on the non-thyroidal illness syndrome in which the HPT axis is altered and, in turn, affects circulating levels of active THs as reported in viral infections, particularly in immunocompromised patients infected with human immunodeficiency virus.

Cultured Neonatal Rat Cardiomyocytes Continue Beating Through Upregulation of CTGF Gene Expression

BACKGROUND

Some cultured neonatal rat cardiomyocytes continue spontaneous beating even in serum-free medium. The present study explored the cause and genes responsible for this phenomenon.

METHODS

Ingenuity Pathway Analysis (IPA) software was used to analyze fold changes in gene expression in beating neonatal rat cardiomyocytes, as compared with non-beating cardiomyocytes, which were obtained from DNA microarray data of total RNA extracts of cardiomyocytes. To confirm the involvement of the 8 genes selected by IPA prediction, cellular protein abundances were determined by Western blot. The gene expression of connective tissue growth factor (CTGF) was substantially higher in beating cardiomyocytes than in non-beating cardiomyocytes; thus, CTGF protein content released from cardiomyocytes into the culture medium was examined.

RESULTS

IPA showed that the "Apelin Cardiac Fibroblast Signaling Pathway" was significantly inhibited and that microtubule dynamics and cytoskeleton organization were significantly activated. Each fluctuation in the cellular abundances of the 8 proteins in beating cardiomyocytes, as compared with non-beating cardiomyocytes, was primarily in the same direction as that of gene expression. However, the cellular CTGF protein abundance as well as CTGF content released into the medium did not substantially differ between beating and non-beating cardiomyocytes.

CONCLUSIONS

The present results suggest that the large increase in CTGF gene expression in beating cardiomyocytes is not a cause but a result of beating, which may provide a putative pathway for controlling beating. Beating is sustained by developed cardiomyofibrils and directly upregulates CTGF gene expression, which is not followed by CTGF protein synthesis.

T3 Critically Affects the Mhrt/Brg1 Axis to Regulate the Cardiac MHC Switch: Role of an Epigenetic Cross-Talk

The LncRNA my-heart (Mhrt) and the chromatin remodeler Brg1 inhibit each other to respectively prevent or favor the maladaptive α-myosin-heavy-chain (Myh6) to β-myosin-heavy-chain (Myh7) switch, so their balance crucially guides the outcome of cardiac remodeling under stress conditions. Even though triiodothyronine (T3) has long been recognized as a critical regulator of the cardiac Myh isoform composition, its role as a modulator of the Mhrt/Brg1 axis is still unexplored. Here the effect of T3 on the Mhrt/Brg1 regulatory circuit has been analyzed in relation with chromatin remodeling and previously identified T3-dependent miRNAs. The expression levels of Mhrt, Brg1 and Myh6/Myh7 have been assessed in rat models of hyperthyroidism or acute myocardial ischemia/reperfusion (IR) treated with T3 replacement therapy. To gain mechanistic insights, in silico analyses and site-directed mutagenesis have been adopted in combination with gene reporter assays and loss or gain of function strategies in cultured cardiomyocytes. Our results indicate a pivotal role of Mhrt over-expression in the T3-dependent regulation of Myh switch. Mechanistically, T3 activates the Mhrt promoter at two putative thyroid hormone responsive elements (TRE) located in a crucial region that is necessary for both Mhrt activation and Brg1-dependent Mhrt repression. This newly identified T3 mode of action requires DNA chromatinization and is critically involved in mitigating the repressive function of the Brg1 protein on Mhrt promoter. In addition, T3 is also able to prevent the Brg1 over-expression observed in the post-IR setting through a pathway that might entail the T3-mediated up-regulation of miR-208a. Taken together, our data evidence a novel T3-responsive network of cross-talking epigenetic factors that dictates the cardiac Myh composition and could be of great translational relevance.

Nonthyroidal Illness Syndrome and Hypothyroidism in Ischemic Heart Disease Population: A Systematic Review and Meta-Analysis

CONTEXT

The association of non-thyroidal illness syndrome (NTIS) and hypothyroidism with the prognosis in ischemic heart disease (IHD) population is inconclusive.

OBJECTIVE

We aimed to evaluate the influence of NTIS and hypothyroidism on all-cause mortality and major adverse cardiac events (MACE) in IHD population.

DATA SOURCES

We searched PubMed, EMBASE, Scopus, Web of Science, and Cochrane Library from inception through February 17, 2020.

STUDY SELECTION

Original articles enrolling IHD patients, comparing all-cause mortality and MACE of NTIS and hypothyroidism with those of euthyroidism, and providing sufficient information for meta-analysis were considered eligible.

DATA EXTRACTION

Relevant information and numerical data were extracted for methodological assessment and meta-analysis.

DATA SYNTHESIS

Twenty-three studies were included. The IHD population with NTIS was associated with higher risk of all-cause mortality (hazard ratio [HR] = 2.61; 95% confidence interval [CI] = 1.89-3.59) and MACE (HR = 2.22; 95% CI = 1.71-2.89) than that without. In addition, the IHD population with hypothyroidism was also associated with higher risk of all-cause mortality (HR = 1.47; 95% CI = 1.10-1.97) and MACE (HR = 1.53; 95% CI = 1.19-1.97) than that without. In the subgroup analysis, the acute coronary syndrome (ACS) subpopulation with NTIS was associated with higher risk of all-cause mortality (HR = 3.30; 95% CI = 2.43-4.48) and MACE (HR = 2.19; 95% CI = 1.45-3.30). The ACS subpopulation with hypothyroidism was also associated with higher risk of all-cause mortality (HR = 1.67; 95% CI = 1.17-2.39).

CONCLUSIONS

The IHD population with concomitant NTIS or hypothyroidism was associated with higher risk of all-cause mortality and MACE. Future research is required to provide evidence of the causal relationship and to elucidate whether normalizing thyroid function parameters can improve prognosis.

Adverse transverse-tubule remodeling in a rat model of heart failure is attenuated with low-dose triiodothyronine treatment

Abstract

Pre-clinical animal studies have shown that triiodothyronine (T3) replacement therapy improves cardiac contractile function after myocardial infarction (MI). We hypothesized that T3 treatment could prevent adverse post-infarction cardiomyocyte remodeling by maintaining transverse-tubule (TT) structures, thus improving calcium dynamics and contractility.

Methods

Myocardial infarction (MI) or sham surgeries were performed on female Sprague-Dawley rats (aged 12 wks), followed by treatment with T3 (5μg/kg/d) or vehicle in drinking water for 16 wks (n = 10–11/group). After in vivo echocardiographic and hemodynamic analyses, left ventricular myocytes were isolated by collagenase digestion and simultaneous calcium and contractile transients in single cardiomyocytes were recorded using IonOptix imaging. Live cardiomyocytes were stained with AlexaFluor-488 conjugated wheat germ agglutinin (WGA-488) or di-8-ANEPPS, and multiple z-stack images per cell were captured by confocal microscopy for analysis of TT organization. RTqPCR and immunoblot approaches determined expression of TT proteins.

Results

Echocardiography and in vivo hemodynamic measurements showed significant improvements in systolic and diastolic function in T3- vs vehicle-treated MI rats. Isolated cardiomyocyte analysis showed significant dysfunction in measurements of myocyte relengthening in MI hearts, and improvements with T3 treatment: max relengthening velocity (Vmax, um/s), 2.984 ± 1.410 vs 1.593 ± 0.325, p < 0.05 and time to Vmax (sec), 0.233 ± 0.037 vs 0.314 ± 0.019, p < 0.001; MI + T3 vs MI + Veh, respectively. Time to peak contraction was shortened by T3 treatment (0.161 ± 0.021 vs 0.197 ± 0.011 s., p < 0.01; MI + T3 vs MI + Veh, respectively). Analysis of TT periodicity of WGA- or ANEPPS-stained cardiomyocytes indicated significant TT disorganization in MI myocytes and improvement with T3 treatment (transverse-oriented tubules (TE%): 9.07 ± 0.39 sham, 6.94 ± 0.67 MI + Veh and 8.99 ± 0.38 MI + T3; sham vs MI + Veh, p < 0.001; MI + Veh vs MI + T3, p < 0.01). Quantitative RT-PCR showed that reduced expression of BIN1 (Bridging integrator-1), Jph2 (junctophilin-2), RyR2 (ryanodine receptor) and Ca_v1.2 (L-type calcium channel) in the failing myocardium were increased by T3 and immunoblot analysis further supporting a potential T3 effect on the TT-associated proteins, BIN1 and Jph2.

In conclusion, low dose T3 treatment initiated immediately after myocardial infarction attenuated adverse TT remodeling, improved calcium dynamics and contractility, thus supporting the potential therapeutic utility of T3 treatment in heart failure.

Novel Insight Into the Epigenetic and Post-transcriptional Control of Cardiac Gene Expression by Thyroid Hormone

Thyroid hormone (TH) signaling is critically involved in the regulation of cardiovascular physiology. Even mild reductions of myocardial TH levels, as occur in hypothyroidism or low T3 state conditions, are thought to play a role in the progression of cardiac disorders. Due to recent advances in molecular mechanisms underlying TH action, it is now accepted that TH-dependent modulation of gene expression is achieved at multiple transcriptional and post-transcriptional levels and involves the cooperation of many processes. Among them, the epigenetic remodeling of chromatin structure and the interplay with non-coding RNA have emerged as novel TH-dependent pathways that add further degrees of complexity and broaden the network of genes controlled by TH signaling. Increasing experimental and clinical findings indicate that aberrant function of these regulatory mechanisms promotes the evolution of cardiac disorders such as post-ischemic injury, pathological hypertrophy, and heart failure, which may be reversed by the correction of the underlying TH dyshomeostasis. To encourage the clinical implementation of a TH replacement strategy in cardiac disease, here we discuss the crucial effect of epigenetic modifications and control of non-coding RNA in TH-dependent regulation of biological processes relevant for cardiac disease evolution.

Integrative analysis of differentially expressed genes and miRNAs predicts complex T3-mediated protective circuits in a rat model of cardiac ischemia reperfusion

Thyroid hormone (T3) dyshomeostasis in the cardiac ischemia-reperfusion (IR) setting negatively impacts on mitochondria function and extracellular matrix remodeling. The modulation of cardiac miRNAs may represent the underlying molecular mechanisms, but a systems biology perspective investigating this critical issue in depth is still lacking. A rat model of myocardial IR, with or without an early short-term T3-replacement, was used to predict putative T3-dependent miRNA-gene interactions targeted to mitochondria quality control and wound healing repair. As evidenced by mRNA and miRNA expression profiling, the T3 supplementation reverted the expression of 87 genes and 11 miRNAs that were dysregulated in the untreated group. In silico crossing and functional analysis of the T3-associated differentially expressed transcripts, identified a signature of interconnected miRNA-gene regulatory circuits that confer resistance to noxious cascades of acute stress. In this network the T3-down-regulated Tp53, Jun and Sp1 transcription factors emerge as critical nodes linking intrinsic cell death and oxidative stress pathways to adverse remodeling cascades. The data presented here provide a novel insight into the molecular basis of T3 cardioprotection in the early post-IR phase and highlight the contribution of a previously unappreciated complex T3-regulatory network that may be helpful in translating T3 replacement into clinical practice.

Serum microRNA profiles in athyroid patients on and off levothyroxine therapy

Background

Levothyroxine replacement treatment in hypothyroidism is unable to restore physiological thyroxine and triiodothyronine concentrations in serum and tissues completely. Normal serum thyroid stimulating hormone (TSH) concentrations reflect only pituitary euthyroidism and, therefore, novel biomarkers representing tissue-specific thyroid state are needed. MicroRNAs (miRNAs), small non-coding regulatory RNAs, exhibit tissue-specific expression patterns and can be detectable in serum. Previous studies have demonstrated differential expression of (precursors of) miRNAs in tissues under the influence of thyroid hormone.

Objective

To study if serum miRNA profiles are changed in different thyroid states.

Design and methods

We studied 13 athyroid patients (6 males) during TSH suppressive therapy and after 4 weeks of thyroid hormone withdrawal. A magnetic bead capture system was used to isolate 384 defined miRNAs from serum. Subsequently, the TaqMan Array Card 3.0 platform was used for profiling after individual target amplification.

Results

Mean age of the subjects was 44.0 years (range 20–61 years). Median TSH levels were 88.9 mU/l during levothyroxine withdrawal and 0.006 mU/l during LT4 treatment with a median dosage of 2.1 μg/kg. After normalization to allow inter-sample analysis, a paired analysis did not demonstrate a significant difference in expression of any of the 384 miRNAs analyzed on and off LT4 treatment.

Conclusion

Although we previously showed an up-regulation of pri-miRNAs 133b and 206 in hypothyroid state in skeletal muscle, the present study does not supply evidence that thyroid state also affects serum miRNAs in humans.

Thyroid hormones and cardiac remodeling

Thyroid hormones have many cardioprotective actions expressed mainly through the action of T3 on thyroid receptors α1 and β1. They are procontractile anti-apoptotic, anti-inflammatory, and anti-fibrotic, promote angiogenesis and regeneration, and have beneficial effects on microRNA profiles. They have proven to be anti-remodeling in numerous animal studies, mostly in rodents; a specific action on the border zone has been described. Studies in humans with DIPTA have been in conclusion. Remodeling can be defined as an increase of ≥20 % of the end-diastolic or end-systolic volume, together with a return to the fetal phenotype. An overview of animal and clinical studies is given.

Role of thyroid hormone in hepatic gene regulation, chromatin remodeling, and autophagy

Thyroid hormone (TH) actions on development and metabolism have been studied ever since the discovery of thyroxine almost a century ago. Initial studies focused on the physiological and biochemical actions of TH. Later, the cloning of the thyroid hormone receptor (THR) isoforms and the development of techniques enabled the study of TH regulation of complex cellular processes (such as gene transcription). Recently we found that TH activates secondary transcription factors such as FOXO1, to amplify gene transcription; and also is a potent inducer of autophagy that was critical for fatty acid β-oxidation in the liver. This review summarizes the recent advancements in our understanding of TH regulation of gene expression of metabolic genes (via co-regulators/transcription factors and epigenetic control) and autophagy in the liver. Our deeper understanding of TH action recently has led to the development of tissue- and THR isoform-specific TH mimetics that may be useful for the treatment of metabolic disorders.

Cardiac Thyroid Hormone Metabolism and Heart Failure

The heart is a principal target of thyroid hormone, and a reduction of cardiac thyroid hormone signaling is thought to play a role in pathological ventricular remodeling and the development of heart failure. Studies in various rodent models of heart disease have identified increased activity of cardiac type III deiodinase as a possible cause of diminished levels and action of thyroid hormone. Recent data indicate novel mechanisms underlying the induction of this thyroid hormone-degrading enzyme in the heart as well as post-transcriptional regulation of its expression by microRNAs. In addition, the relevance of diminished thyroid hormone signaling for cardiac remodeling is suggested to include miRNA-mediated effects on pathological signaling pathways. These and other recent studies are reviewed and discussed in the context of other processes and factors that have been implicated in the reduction of cardiac thyroid hormone signaling in heart failure.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

MicroRNA 214 Is a Potential Regulator of Thyroid Hormone Levels in the Mouse Heart Following Myocardial Infarction, by Targeting the Thyroid-Hormone-Inactivating Enzyme Deiodinase Type III

Cardiac thyroid-hormone signaling is a critical determinant of cellular metabolism and function in health and disease. A local hypothyroid condition within the failing heart in rodents has been associated with the re-expression of the fetally expressed thyroid-hormone-inactivating enzyme deiodinase type III (Dio3). While this enzyme emerges as a common denominator in the development of heart failure, the mechanism underlying its regulation remains largely unclear. In the present study, we investigated the involvement of microRNAs (miRNAs) in the regulation of Dio3 mRNA expression in the remodeling left ventricle (LV) of the mouse heart following myocardial infarction (MI). In silico analysis indicated that of the miRNAs that are differentially expressed in the post-MI heart, miR-214 has the highest potential to target Dio3 mRNA. In accordance, a luciferase reporter assay, including the full-length 3'UTR of mouse Dio3 mRNA, showed a 30% suppression of luciferase activity by miR-214. In the post-MI mouse heart, miR-214 and Dio3 protein were shown to be co-expressed in cardiomyocytes, while time-course analysis revealed that Dio3 mRNA expression precedes miR-214 expression in the post-MI LV. This suggests that a Dio3-induced decrease of T3 levels is involved in the induction of miR-214, which was supported by the finding that cardiac miR-214 expression is down regulated by T3 in mice. In vitro analysis of human DIO3 mRNA furthermore showed that miR-214 is able to suppress both mRNA and protein expression. Dio3 mRNA is a target of miR-214 and the Dio3-dependent stimulation of miR-214 expression in post-MI cardiomyocytes supports the involvement of a negative feedback mechanism regulating Dio3 expression.

Role of thyroid hormones in ventricular remodeling

Cardiac remodeling includes alterations in molecular, cellular, and interstitial systems contributing to changes in size, shape, and function of the heart. This may be the result of injury, alterations in hemodynamic load, neurohormonal effects, electrical abnormalities, metabolic changes, etc. Thyroid hormones (THs) serve as master regulators for diverse remodeling processes of the cardiovascular system-from the prenatal period to death. THs promote a beneficial cardiomyocyte shape and improve contractility, relaxation, and survival via reversal of molecular remodeling. THs reduce fibrosis by decreasing interstitial collagen and reduce the incidence and duration of arrhythmias via remodeling ion channel expression and function. THs restore metabolic function and also improve blood flow both by direct effects on the vessel architecture and decreasing atherosclerosis. Optimal levels of THs both in the circulation and in cardiac tissues are critical for normal homeostasis. This review highlights TH-based remodeling and clinically translatable strategies for diverse cardiovascular disorders.

Early and Short-term Triiodothyronine Supplementation Prevents Adverse Postischemic Cardiac Remodeling: Role of Transforming Growth Factor-β1 and Antifibrotic miRNA Signaling

Activation of transforming growth factor (TGF)-β1 signaling in the ischemia/reperfusion (I/R) injured myocardium leads to dysregulation of miR-29-30-133, favoring the profibrotic process that leads to adverse cardiac remodeling (CR). We have previously shown that timely correction of the postischemic low-T3 syndrome (Low-T3S) exerts antifibrotic effects, but the underlying molecular players are still unknown. Here we hypothesize that a prompt, short-term infusion of T3 in a rat model of post I/R Low-T3S could hamper the early activation of the TGFβ1-dependent profibrotic cascade to confer long-lasting cardioprotection against adverse CR. Twenty-four hours after I/R, rats that developed the Low-T3S were randomly assigned to receive a 48-h infusion of 6 μg/kg/d T3 (I/R-L+T3) or saline (I/R-L) and sacrificed at 3 or 14 d post-I/R. Three days post-I/R, Low-T3S correction favored functional cardiac recovery. This effect was paralleled by a drop in TGFβ1 and increased miR-133a, miR-30c and miR-29c in the infarcted myocardium. Consistently, connective transforming growth factor (CTGF) and matrix metalloproteinase-2(MMP-2), validated targets of the above miRNAs, were significantly reduced. Fourteen days post-I/R, the I/R-L+T3 rats presented a significant reduction of scar size with a better preservation of cardiac performance and LV chamber geometry. At this time, TGFβ1 and miR-29c levels were in the normal range in both groups, whereas miR-30c-133a, MMP-2 and CTGF remained significantly altered in the I/R group. In conclusion, the antifibrotic effect exerted by T3 in the early phase of postischemic wound healing triggers a persistent cardioprotective response that hampers the progression of heart dysfunction and adverse CR.

MicroRNAs in the Myocyte Enhancer Factor 2 (MEF2)-regulated Gtl2-Dio3 Noncoding RNA Locus Promote Cardiomyocyte Proliferation by Targeting the Transcriptional Coactivator Cited2

Understanding cell cycle regulation in postmitotic cardiomyocytes may lead to new therapeutic approaches to regenerate damaged cardiac tissue. We have demonstrated previously that microRNAs encoded by the Gtl2-Dio3 noncoding RNA locus function downstream of the MEF2A transcription factor in skeletal muscle regeneration. We have also reported expression of these miRNAs in the heart. Here we investigated the role of two Gtl2-Dio3 miRNAs, miR-410 and miR-495, in cardiac muscle. Overexpression of miR-410 and miR-495 robustly stimulated cardiomyocyte DNA synthesis and proliferation. Interestingly, unlike our findings in skeletal muscle, these miRNAs did not modulate the activity of the WNT signaling pathway. Instead, these miRNAs targeted Cited2, a coactivator required for proper cardiac development. Consistent with miR-410 and miR-495 overexpression, siRNA knockdown of Cited2 in neonatal cardiomyocytes resulted in robust proliferation. This phenotype was associated with reduced expression of Cdkn1c/p57/Kip2, a cell cycle inhibitor, and increased expression of VEGFA, a growth factor with proliferation-promoting effects. Therefore, miR-410 and miR-495 are among a growing number of miRNAs that have the ability to potently stimulate neonatal cardiomyocyte proliferation.