PARP1-mediated PPARα poly(ADP-ribosyl)ation suppresses fatty acid oxidation in non-alcoholic fatty liver disease.

BACKGROUND & AIMS: PARP1 is a key mediator of cellular stress responses and critical in multiple physiological and pathophysiological processes of cells. However, whether it is involved in the pathogenesis of non-alcoholic fatty liver disease (NAFLD) remains elusive. METHODS: We analysed PARP1 activity in the liver of mice on a high fat diet (HFD), and samples from NAFLD patients. Gain- or loss-of-function approaches were used to investigate the roles and mechanisms of hepatic PARP1 in the pathogenesis of NAFLD. RESULTS: PARP1 is activated in fatty liver of HFD-fed mice. Pharmacological or genetic manipulations of PARP1 are sufficient to alter the HFD-induced hepatic steatosis and inflammation. Mechanistically we identified peroxisome proliferator-activated receptor α (PPARα) as a substrate of PARP1-mediated poly(ADP-ribosyl)ation. This poly(ADP-ribosyl)ation of PPARα inhibits its recruitment to target gene promoters and its interaction with SIRT1, a key regulator of PPARα signaling, resulting in suppression of fatty acid oxidation upregulation induced by fatty acids. Moreover, we show that PARP1 is a transcriptional repressor of PPARα gene in human hepatocytes, and its activation suppresses the ligand (fenofibrate)-induced PPARα transactivation and target gene expression. Importantly we demonstrate that liver biopsies of NAFLD patients display robust increases in PARP activity and PPARα poly(ADP-ribosyl)ation levels. CONCLUSIONS: Our data indicate that PARP1 is activated in fatty liver, which prevents maximal activation of fatty acid oxidation by suppressing PPARα signaling. Pharmacological inhibition of PARP1 may alleviate PPARα suppression and therefore have therapeutic potential for NAFLD. LAY SUMMARY: PARP1 is activated in the non-alcoholic fatty liver of mice and patients. Inhibition of PARP1 activation alleviates lipid accumulation and inflammation in fatty liver of mice.

Inhibition of the lncRNA Mirt1 Attenuates Acute Myocardial Infarction by Suppressing NF-κB Activation.

BACKGROUND/AIMS: The expression of a novel lncRNA, myocardial infarction associated transcript 1(Mirt1), has been shown to be upregulated in acute myocardial infarction (AMI). However, the role of Mirt1 in AMI is not clear. METHODS: In this study, we analyzed the level of Mirt1 in cardiomyocytes and cardiac fibroblasts in AMI mice. Moreover, adenovirus mediated knockdown of Mirt1 was employed to clarify its roles in AMI mice or cultured cardiac fibroblasts. The cardiac functions and infarct size of AMI mice were examined, and tissues and cultured cells were collected and processed for histology and biochemical examination. RESULTS: We demonstrated that Mirt1 was mainly expressed in cardiac fibroblasts, and that knockdown of Mirt1 improved cardiac functions, decreased cardiomyocytes apoptosis and attenuated inflammatory cell infiltration in vivo. Furthermore, knockdown of Mirt1 in cardiac fibroblasts not only attenuated the apoptosis of cardiomyocytes, but also suppressed the migration of macrophages under hypoxia in vitro. NF-κB signaling pathway, activated under hypoxia, was also inhibited by Mirt1 knockdown in fibroblasts. CONCLUSIONS: Knockdown of Mirt1 attenuates AMI injury presumably by decreasing cardiomyocytes apoptosis and reducing inflammatory cell infiltration. These effects could be attributed, at least partly, to inhibition of the NF-κB pathway, resulting in decreased expression of inflammatory factors.

Inhibition of Poly(ADP-Ribose) Polymerase-1 Protects Chronic Alcoholic Liver Injury.

Activation of Kupffer cells (KCs) by gut-derived endotoxin plays a pivotal role in the pathogenesis of alcoholic liver diseases (ALD). Limiting the activation of resident KCs attenuates chronic ethanol-induced liver steatosis and injury. Poly (ADP-ribose) polymerase (PARP)-1 is suggested to play a role in a number of chronic inflammatory diseases. In this study, we found a significant increase of hepatic PARP activity in mice with short-term and long-term ethanol-induced ALD. Male mice on a long-term ethanol diet exhibited severe hepatic steatosis and apoptosis and enhanced KC activation and neutrophil infiltration. However, pharmacologic inhibition of PARP activity or genetic depletion of PARP1 significantly attenuated these detrimental effects in vivo. We found that inhibition of PARP1 effectively reduced hepatic expression of genes involved in lipogenesis and elevated hepatic expression of genes involved in lipolysis. Moreover, limited KC activation and neutrophil infiltration were observed in PARP1 knockout mice or PARP inhibitor-treated mice. Furthermore, in vitro experiments found that LPS-induced macrophage activation was limited by PARP inhibitor, and exposure of ethanol-treated hepatocytes to this conditioned medium further decreased the number of apoptotic and steatotic cells. Taken together, these findings suggest that PARP1 inhibition protects against long-term ethanol-induced liver injury, as indicated by limited hepatocytes steatosis, apoptosis, inflammation levels, and neutrophil infiltration, mainly by limiting KC activation during the initiation of ALD.

IL-38 attenuates myocardial ischemia-reperfusion injury by inhibiting macrophage inflammation.

BACKGROUND: Reperfusion therapy is the most effective approach to resolve coronary occlusion, but myocardial injury caused by excessive inflammation during myocardial ischemia-reperfusion will also pose a new threat to health. Our prior study revealed the expression pattern of interleukin-38 (IL-38) in the peripheral blood serum of patients with ischemic cardiomyopathy and the role of IL-38 in acute myocardial infarction in mice. However, its role and potential mechanisms in myocardial ischemia/reperfusion injury (MIRI) remain to be determined. METHODS AND RESULTS: The left anterior descending artery of C57BL/6 mice was transiently ligated to induce the MIRI model. We found that MIRI induced the expression of endogenous IL-38, which was mainly produced by locally infiltrating macrophages. Overexpression of IL-38 in C57BL/6 mice attenuated inflammatory injury and decreased myocardial apoptosis after myocardial ischemia-reperfusion. Furthermore, IL-38 inhibited lipopolysaccharide-induced macrophage inflammation in vitro. Cardiomyocytes cocultured with the supernatant of IL-38- and troponin I-treated macrophages showed a lower rate of apoptosis than controls. CONCLUSIONS: IL-38 attenuates MIRI by inhibiting macrophage inflammation. This inhibitory effect may be partially achieved by inhibiting the activation of NOD-like receptor pyrin domain-related protein 3 inflammasome, resulting in decreased expression of inflammatory factors and reduced cardiomyocyte apoptosis.

CD9 exacerbates pathological cardiac hypertrophy through regulating GP130/STAT3 signaling pathway.

CD9 is a member of the tetraspanin protein family, which has been widely studied in inflammation and cancer, but not in pathological cardiac hypertrophy. In this study, we found that the expression of CD9 was increased in transaortic constriction (TAC) myocardial tissue. Knockdown of CD9 alleviated damage to cardiac function in the TAC model and reduced heart weight, cardiomyocyte size, and degree of fibrosis, and vice versa. Mechanistically, co-immunoprecipitation results showed that CD9 and GP130 can bind to each other in cardiomyocytes, and knockdown of CD9 can reduce the protein level of GP130 and the phosphorylation of STAT3 in vivo and in vitro, and vice versa. GP130 knockdown reversed the aggravating effects of CD9 on pathological cardiac hypertrophy. Therefore, we conclude that CD9 exacerbates pathological cardiac hypertrophy by regulating the GP130/STAT3 signaling pathway and may serve as a therapeutic target for pathological cardiac hypertrophy.

Role of CCR2 in the Development of Streptozotocin-Treated Diabetic Cardiomyopathy.

CCR2 has been proven to play an important role in diabetes. However, the role of CCR2 in diabetic cardiomyopathy has not been examined. In this study, we investigated the effects of cardiac CCR2 on diabetic cardiomyopathy. We created a model of streptozotocin (STZ)-induced diabetic cardiomyopathy. Expression of CCR2 was upregulated in the hearts of STZ-induced diabetic mice. CCR2 knockout significantly improved STZ-induced cardiac dysfunction and fibrosis. Moreover, deletion of CCR2 inhibited STZ-induced apoptosis and the production of STZ-induced reactive oxygen species in the heart. CCR2 knockout resulted in M2 polarization in hearts of STZ-treated mice. Treatment with a CCR2 inhibitor reversed hyperglycemia-induced cardiac dysfunction in db/db mice. These results suggest that CCR2-induced inflammation and oxidative stress in the heart are involved in the development of diabetic cardiomyopathy and that CCR2 could be a novel target for therapy.

The LPS-inducible lncRNA Mirt2 is a negative regulator of inflammation.

Toll-like receptors (TLRs) are a family of pattern recognition receptors (PRR) with a crucial function in innate immune responses. Activation of TLR4 signaling at the plasma membrane by lipopolysaccharide (LPS) stimulates proinflammatory signaling pathways dependent on the E3 ubiquitin ligase TRAF6. Here we show the LPS-induced long non-coding RNA (lncRNA) Mirt2 functions as a checkpoint to prevent aberrant activation of inflammation, and is a potential regulator of macrophage polarization. Mirt2 associates with, and attenuates Lys63 (K63)-linked ubiquitination of, TRAF6, thus inhibiting activation of NF-κB and MAPK pathways and limiting production of proinflammatory cytokines. Adenovirus mediated gene transfer of Mirt2 protects mice from endotoxemia induced fatality and multi-organ dysfunction. These findings identify lncRNA Mirt2 as a negative feedback regulator of excessive inflammation.

Nkx2-5 Is Expressed in Atherosclerotic Plaques and Attenuates Development of Atherosclerosis in Apolipoprotein E-Deficient Mice.

BACKGROUND: NK2 homeobox 5 (Nkx2-5) is a cardiac homeobox transcription factor that is expressed in a broad range of cardiac sublineages. Embryos lacking Nkx2-5 are nonviable attributed to growth retardation and gross abnormalities of the heart. However, the role of Nkx2-5 in atherosclerosis remains elusive. This study aims to elucidate the specific functions of Nkx2-5 during atherogenesis and in established atherosclerotic plaques. METHODS AND RESULTS: Two types of atherosclerotic lesions were created in ApoE-/- mice through 2 different dietary manipulations. Mice fed a standard chow diet were sacrificed at 20 weeks old, a time point at which mice developed early-stage atherosclerotic lesions. The other half of mice were fed a western diet from 6 to 22 weeks old and then sacrificed. These mice demonstrated advanced atherosclerosis. No Nkx2-5 was detected in normal arteries; however, it was abundantly present in the intima of atherosclerotic lesions and localized in macrophages and smooth muscle cells. Adenovirus gene transfer of Nkx2-5 markedly ameliorated and stabilized the atherosclerotic plaques, and knockdown of Nkx2-5 significantly exacerbated the disease. Molecular studies indicated that expression of specific members of matrix metalloproteinases and tissue inhibitor of metalloproteinases, which play a crucial role in the progression of atherosclerosis, were directly regulated by Nkx2-5. Furthermore, we demonstrated that the compromised endothelial function, which was considered as a hallmark of early atherosclerosis, could be improved by Nkx2-5 gene transfer. CONCLUSIONS: Nkx2-5 exerts antiatherogenic effects, which may partly be attributed to regulation on matrix metalloproteinases and tissue inhibitor of metalloproteinases, thus stabilizing atherosclerotic plaque; besides, it improves endothelial function by inhibiting leukocyte adhesion to the endothelium.

Renalase is a novel target gene of hypoxia-inducible factor-1 in protection against cardiac ischaemia-reperfusion injury.

AIMS: Renalase, an enzyme that can metabolize catecholamine, was recently reported to attenuate the ischaemia/reperfusion (I/R)-induced cardiac injury. This work was undertaken to investigate the functions and regulation mechanisms of renalase in protection against cardiac I/R injury. METHODS AND RESULTS: An elevated level of renalase was found in C57BL/6 mice challenged with I/R injury. Then, we generated a mouse model with cardiac administration of cholesterol-conjugated renalase siRNA followed by I/R operation. The mice treated with renalase siRNA exhibited increased infarction size and decreased cardiac function compared with the scramble siRNA group. Subsequently, we identified four potential hypoxia-inducible factor-1 alpha (HIF-1α)-binding motifs in the promoter of renalase through bioinformatics approaches. Dual-luciferase reporter assay, electrophoretic mobility shift assay, chromatin immunoprecipitation assay, and western blot were conducted and demonstrated that renalase was a novel target gene of HIF-1α. Furthermore, administration of renalase reduced the infarct area and rescued the deterioration of cardiac function in myocardial HIF-1α knockdown mice subjected to I/R injury. In addition, the levels of norepinephrine in serum as well as nicotinamide adenine dinucleotide (NAD(+)) and ATP in myocardium were determined, which implied that cardiac protection of renalase against I/R may be related, at least in part, to its metabolism of catecholamine and regulation of energy. CONCLUSION: These findings have revealed renalase as a novel target gene of HIF-1α in protection against myocardial I/R injury, which provided a basis for therapeutic strategies for enhancing cardiomyocyte survival in patients associated with ischaemic heart diseases.

Identification of poly(ADP-ribose) polymerase-1 as a cell cycle regulator through modulating Sp1 mediated transcription in human hepatoma cells.

The transcription factor Sp1 is implicated in the activation of G0/G1 phase genes. Modulation of Sp1 transcription activities may affect G1-S checkpoint, resulting in changes in cell proliferation. In this study, our results demonstrated that activated poly(ADP-ribose) polymerase 1 (PARP-1) promoted cell proliferation by inhibiting Sp1 signaling pathway. Cell proliferation and cell cycle assays demonstrated that PARP inhibitors or PARP-1 siRNA treatment significantly inhibited proliferation of hepatoma cells and induced G0/G1 cell cycle arrest in hepatoma cells, while overexpression of PARP-1 or PARP-1 activator treatment promoted cell cycle progression. Simultaneously, inhibition of PARP-1 enhanced the expression of Sp1-mediated checkpoint proteins, such as p21 and p27. In this study, we also showed that Sp1 was poly(ADP-ribosyl)ated by PARP-1 in hepatoma cells. Poly(ADP-ribosyl)ation suppressed Sp1 mediated transcription through preventing Sp1 binding to the Sp1 response element present in the promoters of target genes. Taken together, these data indicated that PARP-1 inhibition attenuated the poly(ADP-ribosyl)ation of Sp1 and significantly increased the expression of Sp1 target genes, resulting in G0/G1 cell cycle arrest and the decreased proliferative ability of the hepatoma cells.

Poly(ADP-ribose) polymerase 1 promotes oxidative-stress-induced liver cell death via suppressing farnesoid X receptor α.

Farnesoid X receptor α (FXR) is highly expressed in the liver and regulates the expression of various genes involved in liver repair. In this study, we demonstrated that activated poly(ADP-ribose) polymerase 1 (PARP1) promoted hepatic cell death by inhibiting the expression of FXR-dependent hepatoprotective genes. PARP1 could bind to and poly(ADP-ribosyl)ate FXR. Poly(ADP-ribosyl)ation dissociated FXR from the FXR response element (FXRE), present in the promoters of target genes, and suppressed FXR-mediated gene transcription. Moreover, treatment with a FXR agonist attenuated poly(ADP-ribosyl)ation of FXR and promoted FXR-dependent gene expression. We further established the CCl4-induced acute liver injury model in wild-type and FXR-knockout mice and identified an essential role of FXR poly(ADP-ribosyl)ation in CCl4-induced liver injury. Thus, our results identified poly(ADP-ribosyl)ation of FXR by PARP1 as a key step in oxidative-stress-induced hepatic cell death. The molecular association between PARP1 and FXR provides new insight into the mechanism, suggesting that inhibition of PARP1 could prevent liver injury.