Varun Nagpal

Molecular basis of cardiac endothelial-to-mesenchymal transition (EndMT): Differential expression of microRNAs during EndMT

Fibroblasts are responsible for producing the majority of collagen and other extracellular matrix (ECM) proteins in tissues. In the injured tissue, transforming growth factor-β (TGF-β)-activated fibroblasts or differentiated myofibroblasts synthesize excessive ECM proteins and play a pivotal role in the pathogenesis of fibrosis in heart, kidney and other organs. Recent studies suggest that fibroblast-like cells, derived from endothelial cells by endothelial-to-mesenchymal transition (EndMT), contribute to the pathogenesis of cardiac fibrosis. The molecular basis of EndMT, however, is poorly understood. Here, we investigated the molecular basis of EndMT in mouse cardiac endothelial cells (MCECs) in response to TGF-β2. MCECs exposed to TGF-β2 underwent EndMT as evidenced by morphologic changes, lack of acetylated-low density lipoprotein (Ac-LDL) uptake, and the presence of alpha-smooth muscle actin (α-SMA) staining. Treatment with SB431542, a small molecule inhibitor of TGF-β-receptor I (TβRI) kinase, but not PD98059, a MEK inhibitor, completely blocked TGF-β2-induced EndMT. The transcript and protein levels of α-SMA, Snail and β-catenin as well as acetyltransferase p300 (ATp300) were elevated in EndMT derived fibroblast-like cells. Importantly, microRNA (miRNA) array data revealed that the expression levels of specific miRNAs, known to be dysregulated in different cardiovascular diseases, were altered during EndMT. The protein level of cellular p53, a bonafide target of miR-125b, was downregulated in EndMT-derived fibroblast-like cells. Here, we report for the first time, the differential expression of miRNAs during cardiac EndMT. These results collectively suggest that TβRI serine-threonine kinase-induced TGF-β signaling and microRNAs, the epigenetic regulator of gene expression at the posttranscriptional level, are involved in EndMT and promote profibrotic signaling in EndMT-derived fibroblast-like cells. Pharmacologic agents that restrict the progression of cardiac EndMT, a phenomenon that is found in adults only in the pathological conditions, in targeting specific miRNA may be helpful in preventing and treating cardiac fibrosis.

microRNA-210 is upregulated in hypoxic cardiomyocytes through Akt- and p53-dependent pathways and exerts cytoprotective effects

microRNA-210 (miR-210) is upregulated in hypoxia, but its function in cardiomyocytes and its regulation in response to hypoxia are not well characterized. The purpose of this study was to identify upstream regulators of miR-210, as well as to characterize miR-210s function in cardiomyocytes. We first showed miR-210 is upregulated through both hypoxia-inducible factor (HIF)-dependent and -independent pathways, since aryl hydrocarbon nuclear translocator (ARNT) knockout mouse embryonic fibroblasts (MEF), lacking intact HIF signaling, still displayed increased miR-210 levels in hypoxia. To determine the mechanism for HIF-independent regulation of miR-210, we focused on p53 and protein kinase B (Akt). Overexpression of p53 in wild-type MEFs induced miR-210, whereas p53 overexpression in ARNT knockout MEFs did not, suggesting p53 regulates miR-210 in a HIF-dependent mechanism. Akt inhibition reduced miR-210 induction by hypoxia, whereas Akt overexpression increased miR-210 levels in both wild-type and ARNT knockout MEFs, indicating Akt regulation of miR-210 is HIF-independent. We then studied the effects of miR-210 in cardiomyocytes. Overexpression of miR-210 reduced cell death in response to oxidative stress and reduced reactive oxygen species (ROS) production both at baseline and after treatment with antimycin A. Furthermore, downregulation of miR-210 increased ROS after hypoxia-reoxygenation. To determine a mechanism for the cytoprotective effects of miR-210, we focused on the predicted target, apoptosis-inducing factor, mitochondrion-associated 3 (AIFM3), known to induce cell death. Although miR-210 reduced AIFM3 levels, overexpression of AIFM3 in the presence of miR-210 overexpression did not reduce cellular viability either at baseline or after hydrogen peroxide treatment, suggesting AIFM3 does not mediate miR-210s cytoprotective effects. Furthermore, HIF-3α, a negative regulator of HIF signaling, is targeted by miR-210, but miR-210 does not modulate HIF activity. In conclusion, we demonstrate a novel role for p53 and Akt in regulating miR-210 and demonstrate that, in cardiomyocytes, miR-210 exerts cytoprotective effects, potentially by reducing mitochondrial ROS production.

PAI-1–regulated extracellular proteolysis governs senescence and survival in Klotho mice

Significance Plasminogen activator inhibitor-1 (PAI-1) is an essential mediator of cellular senescence in vitro and is one of the biochemical fingerprints of senescence in vivo. Klotho-deficient (kl/kl) mice display a complex phenotype reminiscent of human aging and exhibit age-dependent increases in PAI-1 in tissues and in plasma. Thus, we hypothesized that PAI-1 contributes to the aging-like phenotype of kl/kl mice. We observed that either genetic deficiency or pharmacological inhibition of PAI-1 in kl/kl mice was associated with reduced evidence of senescence, preserved organ structure and function, and a fourfold increase in median lifespan. These findings indicate that PAI-1 is a critical mediator of senescence in vivo and defines a novel target for the prevention and treatment of age-related disorders in man. Cellular senescence restricts the proliferative capacity of cells and is accompanied by the production of several proteins, collectively termed the “senescence-messaging secretome” (SMS). As senescent cells accumulate in tissue, local effects of the SMS have been hypothesized to disrupt tissue regenerative capacity. Klotho functions as an aging-suppressor gene, and Klotho-deficient (kl/kl) mice exhibit an accelerated aging-like phenotype that includes a truncated lifespan, arteriosclerosis, and emphysema. Because plasminogen activator inhibitor-1 (PAI-1), a serine protease inhibitor (SERPIN), is elevated in kl/kl mice and is a critical determinant of replicative senescence in vitro, we hypothesized that a reduction in extracellular proteolytic activity contributes to the accelerated aging-like phenotype of kl/kl mice. Here we show that PAI-1 deficiency retards the development of senescence and protects organ structure and function while prolonging the lifespan of kl/kl mice. These findings indicate that a SERPIN-regulated cell-nonautonomous proteolytic cascade is a critical determinant of senescence in vivo.

Reduction in Hexokinase II Levels Results in Decreased Cardiac Function and Altered Remodeling After Ischemia/Reperfusion Injury

Rationale: Cardiomyocytes switch substrate utilization from fatty acid to glucose under ischemic conditions; however, it is unknown how perturbations in glycolytic enzymes affect cardiac response to ischemia/reperfusion (I/R). Hexokinase (HK)II is a HK isoform that is expressed in the heart and can bind to the mitochondrial outer membrane. Objective: We sought to define how HKII and its binding to mitochondria play a role in cardiac response and remodeling after I/R. Methods and Results: We first showed that HKII levels and its binding to mitochondria are reduced 2 days after I/R. We then subjected the hearts of wild-type and heterozygote HKII knockout (HKII+/−) mice to I/R by coronary ligation. At baseline, HKII+/− mice have normal cardiac function; however, they display lower systolic function after I/R compared to wild-type animals. The mechanism appears to be through an increase in cardiomyocyte death and fibrosis and a reduction in angiogenesis; the latter is through a decrease in hypoxia-inducible factor–dependent pathway signaling in cardiomyocytes. HKII mitochondrial binding is also critical for cardiomyocyte survival, because its displacement in tissue culture with a synthetic peptide increases cell death. Our results also suggest that HKII may be important for the remodeling of the viable cardiac tissue because its modulation in vitro alters cellular energy levels, O2 consumption, and contractility. Conclusions: These results suggest that reduction in HKII levels causes altered remodeling of the heart in I/R by increasing cell death and fibrosis and reducing angiogenesis and that mitochondrial binding is needed for protection of cardiomyocytes.

MiR-125b is Critical for Fibroblast-to-Myofibroblast Transition and Cardiac Fibrosis

Background— Cardiac fibrosis is the pathological consequence of stress-induced fibroblast proliferation and fibroblast-to-myofibroblast transition. MicroRNAs have been shown to play a central role in the pathogenesis of cardiac fibrosis. We identified a novel miRNA-driven mechanism that promotes cardiac fibrosis via regulation of multiple fibrogenic pathways. Methods and Results— Using a combination of in vitro and in vivo studies, we identified that miR-125b is a novel regulator of cardiac fibrosis, proliferation, and activation of cardiac fibroblasts. We demonstrate that miR-125b is induced in both fibrotic human heart and murine models of cardiac fibrosis. In addition, our results indicate that miR-125b is necessary and sufficient for the induction of fibroblast-to-myofibroblast transition by functionally targeting apelin, a critical repressor of fibrogenesis. Furthermore, we observed that miR-125b inhibits p53 to induce fibroblast proliferation. Most importantly, in vivo silencing of miR-125b by systemic delivery of locked nucleic acid rescued angiotensin II–induced perivascular and interstitial fibrosis. Finally, the RNA-sequencing analysis established that miR-125b altered the gene expression profiles of the key fibrosis-related genes and is a core component of fibrogenesis in the heart. Conclusions— In conclusion, miR-125b is critical for induction of cardiac fibrosis and acts as a potent repressor of multiple anti-fibrotic mechanisms. Inhibition of miR-125b may represent a novel therapeutic approach for the treatment of human cardiac fibrosis and other fibrotic diseases.

Response to Letter Regarding Article, "MiR-125b Is Critical for Fibroblast-to-Myofibroblast Transition and Cardiac Fibrosis".

We appreciate Li and colleagues for their interest in our recent publication on miR-125b and cardiac fibrogenesis.1 In their letter, the authors commented that “it is not known whether miR-125b is cell specific.” In fact, miR-125b is a highly conserved microRNA throughout diverse species from nematodes to humans and is expressed in different types of organs. Notably, we have previously reported upregulation of miR-125b in cardiac endothelial-to-mesenchymal transition, demonstrating that miR-125b is indeed not fibroblast specific.2 The authors were also concerned about potential side effects of inhibition of miR-125b in cardiomyocytes. We reported that miR-125b was upregulated during cardiac fibrosis, and the primary focus of our study was to normalize the levels of …

MicroRNA‐210 Decreases heme Levels by Targeting Ferrochelatase in Cardiomyocytes

Background MicroRNA‐210 (miR‐210) increases in hypoxia and regulates mitochondrial respiration through modulation of iron‐sulfur cluster assembly proteins (ISCU1/2), a protein that is involved in Fe/S cluster synthesis. However, it is not known how miR‐210 affects cellular iron levels or production of heme, another iron containing molecule that is also needed for cellular and mitochondrial function. Methods and Results To screen for micro‐ribonucleic acids (miRNAs) regulated by iron, we performed a miRNA gene array in neonatal rat cardiomyocytes treated with iron chelators. Levels of miR‐210 are significantly increased with iron chelation, however, this response was mediated entirely through the hypoxia‐inducible factor (HIF) pathway. Furthermore, miR‐210 reduced cellular heme levels and the activity of mitochondrial and cytosolic heme‐containing proteins by modulating ferrochelatase (FECH), the last enzyme in heme biosynthesis. Mutation of the 2 miR‐210 binding sites in the 3′ untranslated region (UTR) of FECH reversed the miR‐210 response, while mutation of either binding site in isolation did not exert any effects. Changes mediated by miR‐210 in heme and FECH were independent of ISCU, as overexpression of an ISCU construct lacking the 3′ UTR does not alter miR‐210 regulation of heme and FECH. Finally, FECH levels increased in hypoxia, and this effect was not reversed by miR‐210 knockdown, suggesting that the effects of miR‐210 on heme are restricted to normoxic conditions, and that the pathway is overriden in hypoxia. Conclusions Our results identify a role for miR‐210 in the regulation of heme production by targeting and inhibiting FECH under normoxic conditions.