Yoon Seok Nam

Regulation of Acetylation of Histone Deacetylase 2 by p300/CBP-Associated Factor/Histone Deacetylase 5 in the Development of Cardiac Hypertrophy

Rationale: Histone deacetylases (HDACs) are closely involved in cardiac reprogramming. Although the functional roles of class I and class IIa HDACs are well established, the significance of interclass crosstalk in the development of cardiac hypertrophy remains unclear. Objective: Recently, we suggested that casein kinase 2&agr;1–dependent phosphorylation of HDAC2 leads to enzymatic activation, which in turn induces cardiac hypertrophy. Here we report an alternative post-translational activation mechanism of HDAC2 that involves acetylation of HDAC2 mediated by p300/CBP-associated factor/HDAC5. Methods and Results: Hdac2 was acetylated in response to hypertrophic stresses in both cardiomyocytes and a mouse model. Acetylation was reduced by a histone acetyltransferase inhibitor but was increased by a nonspecific HDAC inhibitor. The enzymatic activity of Hdac2 was positively correlated with its acetylation status. p300/CBP-associated factor bound to Hdac2 and induced acetylation. The HDAC2 K75 residue was responsible for hypertrophic stress–induced acetylation. The acetylation-resistant Hdac2 K75R showed a significant decrease in phosphorylation on S394, which led to the loss of intrinsic activity. Hdac5, one of class IIa HDACs, directly deacetylated Hdac2. Acetylation of Hdac2 was increased in Hdac5-null mice. When an acetylation-mimicking mutant of Hdac2 was infected into cardiomyocytes, the antihypertrophic effect of either nuclear tethering of Hdac5 with leptomycin B or Hdac5 overexpression was reduced. Conclusions: Taken together, our results suggest a novel mechanism by which the balance of HDAC2 acetylation is regulated by p300/CBP-associated factor and HDAC5 in the development of cardiac hypertrophy.

Dietary protein restriction induces steatohepatitis and alters leptin/signal transducers and activators of transcription 3 signaling in lactating rats.

Dietary protein restriction during lactation affects lipid metabolism and food intake in rats. The goals of this study were to determine the effect of a low-protein diet on a liver damage in lactating rats, to determine whether dietary protein restriction of lactating dams affects the liver health of their offspring and to elucidate the molecular mechanisms underlying the development of hepatic damage. Lactating Sprague-Dawley rats were fed either a control 20% protein diet or an 8% low-protein diet for 11 or 23 days, respectively. After weaning, the offspring were continuously fed either the same control diet or the low-protein diet for an additional 22 days. Feeding a low-protein diet during lactation caused steatohepatitis with severe steatosis, lobular inflammation, ballooning degeneration and fibrosis. Offspring nourished by dams fed a low-protein diet showed simple hepatic steatosis. Combined effects of increased lipogenesis, decreased fatty acid oxidation and impaired very-low-density lipoprotein secretion were responsible for the development of hepatic steatosis. Hepatic up-regulation of genes linked to oxidative stress including nicotinamide adenine dinucleotide phosphate oxidase, inflammation and fibrogenesis supports the development of steatohepatitis in protein-restricted lactating rats. Furthermore, protein-restricted lactating rats showed activation of the leptin/signal transducers and activators of the transcription 3 signaling pathway. Taken together, oxidative stress induced by up-regulation of nicotinamide adenine dinucleotide phosphate oxidase with activation of leptin/signal transducers and activators of the transcription 3 signaling was responsible for development of steatohepatitis in protein-restricted lactating rats. Our findings suggest that protein malnutrition has a potential to induce steatohepatitis/hepatic steatosis in lactating mothers and infants during breast-feeding.

Estrogen-related receptor gamma induces cardiac hypertrophy by activating GATA4

Estrogen-related receptor gamma (ERRγ) is an orphan nuclear receptor that has biological roles mainly in metabolism and that controls metabolic switching in perinatal heart. In adult heart diseases, however, the functional roles of ERRγ have not yet been elucidated. In the present study, we aimed to characterize the role of ERRγ in cardiac hypertrophy. The functional roles of ERRγ in the development of cardiac hypertrophy were examined in primary cultured cardiomyocytes and in animal models. ERRγ expression was increased in hearts from human hypertrophic cardiomyopathy patients and in both cellular and animal models of cardiac hypertrophy. Transgenic overexpression in mouse heart as well as forced expression of ERRγ in cardiomyocytes induced hypertrophic phenotypes. Knock-down of ERRγ blocked agonist-induced hypertrophic phenotypes. ERRγ bound directly to the proximal ERR-responsive element in the GATA4 promoter in a sequence-specific manner and thereby induced transcription. ERRγ-induced hypertrophy was blocked by inhibition of GATA4. GSK-5182, an inverse agonist of ERRγ, completely blocked cardiac hypertrophy in cardiomyocytes. It also prevented aortic banding-induced cardiac hypertrophy and fibrosis in mouse heart. These findings demonstrate a novel ERRγ/GATA4 signal cascade in the development of cardiac hypertrophy and suggest GSK-5182 as a possible therapeutic.

MDM2 E3 ligase-mediated ubiquitination and degradation of HDAC1 in vascular calcification.

Vascular calcification (VC) is often associated with cardiovascular and metabolic diseases. However, the molecular mechanisms linking VC to these diseases have yet to be elucidated. Here we report that MDM2-induced ubiquitination of histone deacetylase 1 (HDAC1) mediates VC. Loss of HDAC1 activity via either chemical inhibitor or genetic ablation enhances VC. HDAC1 protein, but not mRNA, is reduced in cell and animal calcification models and in human calcified coronary artery. Under calcification-inducing conditions, proteasomal degradation of HDAC1 precedes VC and it is mediated by MDM2 E3 ubiquitin ligase that initiates HDAC1 K74 ubiquitination. Overexpression of MDM2 enhances VC, whereas loss of MDM2 blunts it. Decoy peptide spanning HDAC1 K74 and RG 7112, an MDM2 inhibitor, prevent VC in vivo and in vitro. These results uncover a previously unappreciated ubiquitination pathway and suggest MDM2-mediated HDAC1 ubiquitination as a new therapeutic target in VC.

Ret finger protein mediates Pax7-induced ubiquitination of MyoD in skeletal muscle atrophy.

Skeletal muscle atrophy results from the net loss of muscular proteins and organelles and is caused by pathologic conditions such as nerve injury, immobilization, cancer, and other metabolic diseases. Recently, ubiquitination-mediated degradation of skeletal-muscle-specific transcription factors was shown to be involved in muscle atrophy, although the mechanisms have yet to be defined. Here we report that ret finger protein (RFP), also known as TRIM27, works as an E3 ligase in Pax7-induced degradation of MyoD. Muscle injury induced by sciatic nerve transection up-regulated RFP and RFP physically interacted with both Pax7 and MyoD. RFP and Pax7 synergistically reduced the protein amounts of MyoD but not the mRNA. RFP-induced reduction of MyoD protein was blocked by proteasome inhibitors. The Pax7-induced reduction MyoD was attenuated by RFP siRNA and by MG132, a proteasome inhibitor. RFPΔR, an RFP construct that lacks the RING domain, failed to reduce MyoD amounts. RFP ubiquitinated MyoD, but RFPΔR failed to do so. Forced expression of RFP, but not RFPΔR, enhanced Pax7-induced ubiquitination of MyoD, whereas RFP siRNA blocked the ubiquitination. Sciatic nerve injury-induced muscle atrophy as well the reduction in MyoD was attenuated in RFP knockout mice. Taken together, our results show that RFP works as a novel E3 ligase in the Pax7-mediated degradation of MyoD in response to skeletal muscle atrophy.

Liver-specific deletion of the signal transducer and activator of transcription 5 gene aggravates fatty liver in response to a high-fat diet in mice

Growth hormone (GH) signal is mediated by signal transducer and activator of transcription 5 (STAT5), which controls hepatic lipid metabolism. Nonalcoholic fatty liver disease (NAFLD) is clinically associated with a deficiency in GH. This study was performed to understand the role of local STAT5 signaling on hepatic lipid and glucose metabolism utilizing liver-specific STAT5 gene deletion (STAT5 LKO) mice under both normal diet and high-fat diet (HFD) feeding conditions. STAT5 LKO induced hepatic steatosis under HFD feeding, while this change was not observed in mice on normal diet. STAT5 LKO caused hyperglycemia, hyperinsulinemia, hyperleptinemia and elevated free fatty acid and cholesterol concentrations under HFD feeding but induced only hyperglycemia on normal diet. At the molecular level, STAT5 LKO up-regulated the expression of genes involved in lipid uptake (CD36), very low-density lipoprotein receptor (VLDLR), lipogenic stearoyl-CoA desaturase and adipogenic peroxisome proliferator-activated receptor gamma, in both diet groups. In response to HFD feeding, further increases in CD36 and VLDLR expression were found in STAT5 LKO mice. In conclusion, our study suggests that low STAT5 signaling on normal diet predisposes STAT5 LKO mice to early development of fatty liver by hyperglycemia and activation of lipid uptake and adipogenesis. A deficiency in STAT5 signaling under HFD feeding deregulates hepatic and body glucose and lipid metabolism, leading to the development of hepatic steatosis. Our study indicates that low STAT5 signaling, due to low GH secretion, may increase a chance for NAFLD development in elderly people.

Small Heterodimer Partner Blocks Cardiac Hypertrophy by Interfering with GATA6 Signaling

Rationale: Small heterodimer partner (SHP; NR0B2) is an atypical orphan nuclear receptor that lacks a conventional DNA-binding domain. Through interactions with other transcription factors, SHP regulates diverse biological events, including glucose metabolism in liver. However, the role of SHP in adult heart diseases has not yet been demonstrated. Objective: We aimed to investigate the role of SHP in adult heart in association with cardiac hypertrophy. Methods and Results: The roles of SHP in cardiac hypertrophy were tested in primary cultured cardiomyocytes and in animal models. SHP-null mice showed a hypertrophic phenotype. Hypertrophic stresses repressed the expression of SHP, whereas forced expression of SHP blocked the development of hypertrophy in cardiomyocytes. SHP reduced the protein amount of Gata6 and, by direct physical interaction with Gata6, interfered with the binding of Gata6 to GATA-binding elements in the promoter regions of natriuretic peptide precursor type A. Metformin, an antidiabetic agent, induced SHP and suppressed cardiac hypertrophy. The metformin-induced antihypertrophic effect was attenuated either by SHP small interfering RNA in cardiomyocytes or in SHP-null mice. Conclusions: These results establish SHP as a novel antihypertrophic regulator that acts by interfering with GATA6 signaling. SHP may participate in the metformin-induced antihypertrophic response.