New role and molecular mechanism of Gadd45a in hepatic fibrosis

Contradictory Role of Gadd45β in Liver Diseases

There are three homologous proteins (α, β and γ) in the growth arrest and DNA damage 45 (Gadd45) family. These proteins act as cellular responders to physiological and environmental stimuli. Gadd45β plays a significant role in the pathogenesis of liver diseases. Liver injury and growth stimulation increase expression of Gadd45β, which promotes cell survival, growth and proliferation in normal liver cells. By contrast, Gadd45β plays a role in promoting apoptosis and inhibiting tumour function in hepatocellular carcinoma (HCC). Currently, it is believed that Gadd45β benefits the liver through two different pathways: binding to MAPK kinase 6 (MKK6) to increase PCD induced by p38 (inhibiting tumours) or binding to constitutive androstane receptor (CAR) to jointly activate transcription of liver synthesis metabolism (promoting liver regeneration). This article aims to systematically review the role of Gadd45β in liver diseases, including its regulatory mechanism on expression and involvement in liver cell damage, inflammation, fibrosis and HCC. In conclusion, we explore the potential of targeting Gadd45β as a therapeutic strategy for liver diseases.

Harmine inhibits pulmonary fibrosis through regulating DNA damage repair-related genes and activation of TP53-Gadd45α pathway

BACKGROUND

Harmine has many pharmacological activities and has been found to significantly inhibit the fibrosis of keloid fibroblasts. DNA damage repair (DDR) is essential to prevent fibrosis. This study aimed to investigate the effects of harmine on pulmonary fibrosis and its underlying mechanisms.

METHODS

Bleomycin and TGF-β1 were used to construct pulmonary fibrosis models in vivo and in vitro, then treated with harmine to explore harmine's effects in treating experimental pulmonary fibrosis and its related mechanisms. Then, RNA sequencing was applied to investigate further the crucial DDR-related genes and drug targets of harmine against pulmonary fibrosis. Finally, the expression levels of DDR-related genes were verified by real-time quantitative PCR (RT-qPCR) and western blot.

RESULTS

Our in vivo experiments showed that harmine treatment could improve weight loss and lung function and reduce tissue fibrosis in mice with pulmonary fibrosis. The results confirmed that harmine could inhibit the viability and migration of TGF-β1-induced MRC-5 cells, induce their apoptosis, and suppress the F-actin expression, suggesting that harmine could suppress the phenotypic transition from lung fibroblasts to lung myoblasts. In addition, RNA sequencing identified 1692 differential expressed genes (DEGs), and 10 DDR-related genes were screened as critical DDR-related genes. RT-qPCR and western blotting showed that harmine could down-regulate the expression of CHEK1, ERCC1, ERCC4, POLD1, RAD51, RPA1, TOP1, and TP53, while up-regulate FEN1, H2AX and GADD45α expression.

CONCLUSIONS

Harmine may inhibit pulmonary fibrosis by regulating DDR-related genes and activating the TP53-Gadd45α pathway.

GADD45A: With or without you

The growth arrest and DNA damage inducible (GADD)45 family includes three small and ubiquitously distributed proteins (GADD45A, GADD45B, and GADD45G) that regulate numerous cellular processes associated with stress signaling and injury response. Here, we provide a comprehensive review of the current literature investigating GADD45A, the first discovered member of the family. We first depict how its levels are regulated by a myriad of genotoxic and non-genotoxic stressors, and through the combined action of intricate transcriptional, posttranscriptional, and even, posttranslational mechanisms. GADD45A is a recognized tumor suppressor and, for this reason, we next summarize its role in cancer, as well as the different mechanisms by which it regulates cell cycle, DNA repair, and apoptosis. Beyond these most well-known actions, GADD45A may also influence catabolic and anabolic pathways in the liver, adipose tissue and skeletal muscle, among others. Not surprisingly, GADD45A may trigger AMP-activated protein kinase activity, a master regulator of metabolism, and is known to act as a transcriptional coregulator of numerous nuclear receptors. GADD45A has also been reported to display a cytoprotective role by regulating inflammation, fibrosis and oxidative stress in several organs and tissues, and is regarded an important contributor for the development of heart failure. Overall data point to that GADD45A may play an important role in metabolic, neurodegenerative and cardiovascular diseases, and also autoimmune-related disorders. Thus, the potential mechanisms by which dysregulation of GADD45A activity may contribute to the progression of these diseases are also reviewed below.

The ameliorative effect of cerium oxide nanoparticles on chlorpyrifos induced hepatotoxicity in a rat model: Biochemical, molecular and immunohistochemical study

BACKGROUND

Chlorpyrifos (CPF) is a widely used insecticide that causes toxicity to living organisms through the production of free radicals. Cerium oxide nanoparticles (CeO2NPs) are a new antioxidant agent that has proved therapeutic effects. We evaluated the effect of CeO2NPs on CPF hepatotoxicity.

METHODS

Forty rats were randomized into four groups. Group I: rats received 1 ml corn oil by gastric tube once daily and 0.5 ml PBS by intra-peritoneal injection twice a week for 4 weeks. Group II: received CeO2NPs 0.5 mg/kg in PBS by i.p. injection, twice weekly for four weeks. Group III: were treated with oral administration of CPF 13.5 mg/kg in corn oil daily for 4 weeks. Group IV: received CPF as in group III, then each animal received CeO2NPs twice weekly for four weeks as in group II. Twenty-four hours after the last dose, rats were anesthetized and sera were collected for liver enzymes assessment. Afterwards, rats were sacrificed, livers were excised, the right lobe of each liver was fixed for immunohistochemical studies, and the left lobe was homogenized for oxidative profile assessment and molecular analysis.

RESULTS

CPF group showed significant increase in liver transaminases, disturbance of the oxidative profile with up-regulation of BAX expression and down-regulation in the Bcl-2, Gadd45 and NFE2L2. CPF caused severe histopathological liver damage as well as significant increase in anti-Caspase 3 and TNF immunostaining. The CeO2NPs treated group revealed significant improvement of all previous parameters.

CONCLUSION

CeO2NPs could alleviate CPF hepatoxicity through decreasing expression of the inflammatory and apoptotic proteins and increasing the activity of antioxidant enzymes.

Stress and Liver Fibrogenesis: Understanding the Role and Regulation of Stress Response Pathways in Hepatic Stellate Cells

Stress response pathways are crucial for cells to adapt to physiological and pathologic conditions. Increased transcription and translation in response to stimuli place a strain on the cell, necessitating increased amino acid supply, protein production and folding, and disposal of misfolded proteins. Stress response pathways, such as the unfolded protein response (UPR) and the integrated stress response (ISR), allow cells to adapt to stress and restore homeostasis; however, their role and regulation in pathologic conditions, such as hepatic fibrogenesis, are unclear. Liver injury promotes fibrogenesis through activation of hepatic stellate cells (HSCs), which produce and secrete fibrogenic proteins to promote tissue repair. This process is exacerbated in chronic liver disease, leading to fibrosis and, if unchecked, cirrhosis. Fibrogenic HSCs exhibit activation of both the UPR and ISR, due in part to increased transcriptional and translational demands, and these stress responses play important roles in fibrogenesis. Targeting these pathways to limit fibrogenesis or promote HSC apoptosis is a potential antifibrotic strategy, but it is limited by our lack of mechanistic understanding of how the UPR and ISR regulate HSC activation and fibrogenesis. This article explores the role of the UPR and ISR in the progression of fibrogenesis, and highlights areas that require further investigation to better understand how the UPR and ISR can be targeted to limit hepatic fibrosis progression.

Polymorphisms within DIO2 and GADD45A genes increase the risk of liver disease progression in chronic hepatitis b carriers

The study enrolled 284 patients with chronic hepatitis B virus infection. Participants included people with mild fibrotic lesions (32.5%), moderate to severe fibrotic lesions (27.5%), cirrhotic lesions (22%), hepatocellular carcinoma (HCC) in 5%, and people with no fibrotic lesions in 13%. Eleven SNPs within DIO2, PPARG, ATF3, AKT, GADD45A, and TBX21 were genotyped by mass spectrometry. The rs225014 TT (DIO2) and rs10865710 CC (PPARG) genotypes were independently associated with susceptibility to advanced liver fibrosis. However, cirrhosis was more prevalent in individuals with the GADD45A rs532446 TT and ATF3 rs11119982 TT genotypes. In addition, the rs225014 CC variant of DIO2 was more frequently found in patients with a diagnosis of HCC. These findings suggest that the above SNPs may play a role in HBV-induced liver damage in a Caucasian population.

Resistin deletion protects against heart failure injury by targeting DNA damage response

AIMS

Increased resistin (Retn) levels are associated with development of cardiovascular diseases. However, the role of Retn in heart failure (HF) is still unclear. Here we probed the functional and molecular mechanism underlying the beneficial effect of Retn deletion in HF.

METHODS AND RESULTS

Wild-type (WT) and adipose tissue-specific Retn-knockout (RKO) mice were subjected to transverse aortic constriction (TAC)-induced HF. Cardiac function and haemodynamic changes were measured by echocardiography and left ventricular catheterization. Adipose tissue Retn deletion attenuated while Retn cardiac-selective overexpression, via a recombinant adeno-associated virus-9 vector, exacerbated TAC-induced hypertrophy, cardiac dysfunction, and myocardial fibrosis in WT and RKO mice. Mechanistically, we showed that Gadd45α was significantly increased in RKO HF mice while cardiac overexpression of Retn led to its downregulation. miR148b-3p directly targets Gadd45α and inhibits its expression. Retn overexpression upregulated miR148b-3p expression and triggered DNA damage response (DDR) in RKO-HF mice. Inhibition of miR148b-3p in vivo normalized Gadd45α expression, decreased DDR, and reversed cardiac dysfunction and fibrosis. In vitro Retn overexpression in adult mouse cardiomyocytes activated miR148b-3p and reduced Gadd45α expression. Gadd45α overexpression in H9C2-cardiomyoblasts protected against hydrogen peroxide- and Retn-induced DDR.

CONCLUSION

These findings reveal that diminution in circulating Retn reduced myocardial fibrosis and apoptosis, and improved heart function in a mouse model of HF, at least in part, through attenuation of miR148b-3p and DDR. The results of this study indicate that controlling Retn levels may provide a potential therapeutic approach for treating pressure overload-induced HF.

Deriving time-concordant event cascades from gene expression data: A case study for Drug-Induced Liver Injury (DILI)

Adverse event pathogenesis is often a complex process which compromises multiple events ranging from the molecular to the phenotypic level. In toxicology, Adverse Outcome Pathways (AOPs) aim to formalize this as temporal sequences of events, in which event relationships should be supported by causal evidence according to the tailored Bradford-Hill criteria. One of the criteria is whether events are consistently observed in a certain temporal order and, in this work, we study this time concordance using the concept of “first activation” as data-driven means to generate hypotheses on potentially causal mechanisms. As a case study, we analysed liver data from repeat-dose studies in rats from the TG-GATEs database which comprises measurements across eight timepoints, ranging from 3 hours to 4 weeks post-treatment. We identified time-concordant gene expression-derived events preceding adverse histopathology, which serves as surrogate readout for Drug-Induced Liver Injury (DILI). We find known mechanisms in DILI to be time-concordant, and show further that significance, frequency and log fold change (logFC) of differential expression are metrics which can additionally prioritize events although not necessary to be mechanistically relevant. Moreover, we used the temporal order of transcription factor (TF) expression and regulon activity to identify transcriptionally regulated TFs and subsequently combined this with prior knowledge on functional interactions to derive detailed gene-regulatory mechanisms, such as reduced Hnf4a activity leading to decreased expression and activity of Cebpa. At the same time, also potentially novel events are identified such as Sox13 which is highly significantly time-concordant and shows sustained activation over time. Overall, we demonstrate how time-resolved transcriptomics can derive and support mechanistic hypotheses by quantifying time concordance and how this can be combined with prior causal knowledge, with the aim of both understanding mechanisms of toxicity, as well as potential applications to the AOP framework. We make our results available in the form of a Shiny app (https://anikaliu.shinyapps.io/dili_cascades), which allows users to query events of interest in more detail.

Understanding mechanisms from systems-scale biological data is of great relevance in toxicology as well as drug discovery; however how to generate causal hypotheses instead of correlations is by no means clear. In this work, we study the conserved temporal order of events and present an automatable framework to quantify and characterize time concordance across a large set of time-series. We apply this concept to events derived from time-resolved gene expression and histopathology from the TG-GATEs in vivo liver data as a case study. We were able to recover known events involved in the pathogenesis of Drug-Induced Liver Injury (DILI), and identify potentially novel pathway and transcription factors (TFs) which precede adverse histopathology. As complementary sources of evidence for causality, we additionally show how time concordance and prior knowledge on plausible interactions between TFs can be combined to derive causal hypotheses on the TFs’ mode of regulation and interaction partners. Overall, the results derived in our case study can serve as valuable hypothesis-free starting points for the development of Adverse Outcome Pathways for DILI, and demonstrate that our approach provides a novel angle to prioritize mechanistically relevant events.

Integrative Analysis of Nanopore and Illumina Sequencing Reveals Alternative Splicing Complexity in Pig Longissimus Dorsi Muscle

Alternative splicing (AS) is a key step in the post-transcriptional regulation of gene expression that can affect intramuscular fat (IMF). In this study, longissimus dorsi muscles from 30 pigs in high- and low- IMF groups were used to perform Oxford Nanopore Technologies (ONT) full-length sequencing and Illumina strand-specific RNA-seq. A total of 43,688 full-length transcripts were identified, with 4,322 novel genes and 30,795 novel transcripts. Using AStalavista, a total of 14,728 AS events were detected in the longissimus dorsi muscle. About 17.79% of the genes produced splicing isoforms, in which exon skipping was the most frequent AS event. By analyzing the expression differences of mRNAs and splicing isoforms, we found that differentially expressed mRNAs with splicing isoforms could participate in skeletal muscle development and fatty acid metabolism, which might determine muscle-related traits. SERBP1, MYL1, TNNT3, and TNNT1 were identified with multiple splicing isoforms, with significant differences in expression. AS events occurring in IFI6 and GADD45G may cause significant differences in gene expression. Other AS events, such as ONT.15153.3, may regulate the function of ART1 by regulating the expression of different transcripts. Moreover, co-expression and protein-protein interaction (PPI) analysis indicated that several genes (MRPL27, AAR2, PYGM, PSMD4, SCNM1, and HNRNPDL) may be related to intramuscular fat. The splicing isoforms investigated in our research provide a reference for the study of alternative splicing regulation of intramuscular fat deposition.

Gadd45 in Senescence

Gadd45a, Gadd45b, and Gadd45g have been implicated in cell cycle arrest, DNA repair, apoptosis, innate immunity, genomic stability, and more recently in senescence. Evidence has accumulated that Gadd45a deficiency results in escape of mouse embryo fibroblasts from senescence, whereas Gadd45b deficiency promotes premature senescence and skin aging. Moreover, recently Gadd45b deficiency was found to promote senescence and attenuate liver fibrosis, whereas Gadd45a was observed to exert a protective effect against hepatic fibrosis. These findings indicate that the Gadd45 stress response proteins play important roles in modulating cellular responses to senescence. Thus, exploring how Gadd45 proteins modulate cellular senescence has the potential to provide new and innovative tools to treat cancer as well as liver disease.

Histone H3K27 methyltransferase EZH2 and demethylase JMJD3 regulate hepatic stellate cells activation and liver fibrosis

Rationale: As the central hallmark of liver fibrosis, transdifferentiation of hepatic stellate cells (HSCs), the predominant contributor to fibrogenic hepatic myofibroblast responsible for extracellular matrix (ECM) deposition, is characterized with transcriptional and epigenetic remodeling. We aimed to characterize the roles of H3K27 methyltransferase EZH2 and demethylase JMJD3 and identify their effective pathways and novel target genes in HSCs activation and liver fibrosis.

Methods: In primary HSCs, we analyzed effects of pharmacological inhibitions and genetic manipulations of EZH2 and JMJD3 on HSCs activation. In HSCs cell lines, we evaluated effects of EZH2 inhibition by DZNep on proliferation, cell cycling, senescence and apoptosis. In CCl₄ and BDL murine models of liver fibrosis, we assessed in vivo effects of DZNep administration and Ezh2 silencing. We profiled rat primary HSCs transcriptomes with RNA-seq, screened the pathways and genes associated with DZNep treatment, analyzed EZH2 and JMJD3 regulation towards target genes by ChIP-qPCR.

Results: EZH2 inhibition by DZNep resulted in retarded growth, lowered cell viability, cell cycle arrest in S and G2 phases, strengthened senescence, and enhanced apoptosis of HSCs, decreased hepatic collagen deposition and rescued the elevated serum ALT and AST activities of diseased mice, and downregulated cellular and hepatic expressions of H3K27me3, EZH2, α-SMA and COL1A. Ezh2 silencing by RNA interference in vitro and in vivo showed similar effects. JMJD3 inhibition by GSK-J4 and overexpression of wild-type but not mutant Jmjd3 enhanced or repressed HSCs activation respectively. EZH2 inhibition by DZNep transcriptionally inactivated TGF-β1 pathway, cell cycle pathways and vast ECM components in primary HSCs. EZH2 inhibition decreased H3K27me3 recruitment at target genes encoding TGF-β1 pseudoreceptor BAMBI, anti-inflammatory cytokine IL10 and cell cycle regulators CDKN1A, GADD45A and GADD45B, and increased their expressions, while Jmjd3 overexpression manifested alike effects.

Conclusions: EZH2 and JMJD3 antagonistically modulate HSCs activation. The therapeutic effects of DZNep as epigenetic drug in liver fibrosis are associated with the regulation of EZH2 towards direct target genes encoding TGF-β1 pseudoreceptor BAMBI, anti-inflammatory cytokine IL10 and cell cycle regulators CDKN1A, GADD45A and GADD45B, which are also regulated by JMJD3. Our present study provides new mechanistic insight into the epigenetic modulation of EZH2 and JMJD3 in HSCs biology and hepatic fibrogenesis.

PolyG mitigates silica-induced pulmonary fibrosis by inhibiting nucleolin and regulating DNA damage repair pathway

Polyguanylic acid potassium salt (PolyG) has an anti-fibrotic G-quadruplex (G4) structure. It could inhibit the expression of nucleolin, a protein involved in cell proliferation and apoptosis. However, its role in regulating nucleolin in silicosis is still unknown. After instillation of 50 μl of crystalline silica suspension (50 mg/ml) into the trachea of C57BL/6 mice, we show that nucleolin expression is upregulated in mouse pulmonary tissue following the treatment with silica and that PolyG, which were injected 2.5 mg/kg body weight into mice by abdomen, could alleviate pulmonary fibrosis through inhibiting the expression of nucleolin. Further, we demonstrated that the expression of the DNA double-strand break (DSB) marker, γ-H2AX, increased in response to silica treatment. PolyG could efficiently reduce the protein expression of γ-H2AX and decreased the level of fibrosis-related genes, such as Col1a1 and Col3a1, as well as the levels of fibrosis-associated proteins α-SMA and vimentin in the lungs of silica-treated mice. These findings show that PolyG could regulate nucleolin and DNA damage repair to control fibrotic response in experimental silicosis and provide a new target for preventive intervention.

Comparative analysis of the expression profiles of genes related to the Gadd45α signaling pathway in four kinds of liver diseases

Gadd45α (growth arrest and DNA damage inducible alpha) is a member of a group of genes whose transcript levels are increased following stressful conditions that lead to growth arrest and treatment with agents that lead to DNA damage. Gadd45α is upregulated in liver cirrhosis (LC), hepatic cancer (HC), acute liver failure (AHF) and non-alcoholic fatty liver disease(NAFLD). Here, we investigated the essential differences in the Gadd45α signaling pathway in these diseases at the transcriptional level. The results showed that 44, 46, 71 and 27 genes significant changes in these diseases, and the H-cluster showed that the expression of the Gadd45α signaling-related genes was significantly different in the four liver diseases. DAVID functional analysis showed that the Gadd45α signaling pathway-related genes were mainly involved in cell adhesion and migration, cell proliferation, apoptosis, stress and inflammatory responses, etc. Ingenuity pathway analysis (IPA) software was used to predict the functions of the Gadd45α signaling-related genes, and the results indicated that there were significant changes in cell differentiation, DNA damage repair, autophagy, apoptosis and necrosis. Gadd45α signaling pathway is involved in four kinds of liver disease and regulates a variety of activities via P38 MAPK, NF-κB, mTOR/STAT3, P21, PCNA, PI3K/Akt and other signaling pathways. Modulation of Gadd45α may be exploited to prevent the progression of liver disease, and to identify specific treatments for different stages of liver disease. In summary, the Gadd45α signaling pathway is involved in four kinds of liver disease and regulates a variety of physiological activities through various signaling pathways.

Unfolded protein response is an early, non-critical event during hepatic stellate cell activation

Hepatic stellate cells activate upon liver injury and help at restoring damaged tissue by producing extracellular matrix proteins. A drastic increase in matrix proteins results in liver fibrosis and we hypothesize that this sudden increase leads to accumulation of proteins in the endoplasmic reticulum and its compensatory mechanism, the unfolded protein response. We indeed observe a very early, but transient induction of unfolded protein response genes during activation of primary mouse hepatic stellate cells in vitro and in vivo, prior to induction of classical stellate cell activation genes. This unfolded protein response does not seem sufficient to drive stellate cell activation on its own, as chemical induction of endoplasmic reticulum stress with tunicamycin in 3D cultured, quiescent stellate cells is not able to induce stellate cell activation. Inhibition of Jnk is important for the transduction of the unfolded protein response. Stellate cells isolated from Jnk knockout mice do not activate as much as their wild-type counterparts and do not have an induced expression of unfolded protein response genes. A timely termination of the unfolded protein response is essential to prevent endoplasmic reticulum stress-related apoptosis. A pathway known to be involved in this termination is the non-sense-mediated decay pathway. Non-sense-mediated decay inhibitors influence the unfolded protein response at early time points during stellate cell activation. Our data suggest that UPR in HSCs is differentially regulated between acute and chronic stages of the activation process. In conclusion, our data demonstrates that the unfolded protein response is a JNK1-dependent early event during hepatic stellate cell activation, which is counteracted by non-sense-mediated decay and is not sufficient to drive the stellate cell activation process. Therapeutic strategies based on UPR or NMD modulation might interfere with fibrosis, but will remain challenging because of the feedback mechanisms between the stress pathways.

GADD45α alleviates acetaminophen-induced hepatotoxicity by promoting AMPK activation

As an analgesic and antipyretic drug, acetaminophen (APAP) is commonly used and known to be safe at therapeutic doses. In many countries, the overuse of APAP provokes acute liver injury and even liver failure. APAP-induced liver injury (AILI) is the most used experimental model of drug-induced liver injury (DILI). Here, we have demonstrated elevated levels of growth arrest and DNA damage-inducible 45α (GADD45α) in the livers of patients with DILI/AILI, in APAP-injured mouse livers and in APAP-treated hepatocytes. GADD45α exhibited a protective effect against APAP-induced liver injury and alleviated the accumulation of small lipid droplets in vitro and in vivo. We found that GADD45α promoted the activation of AMP-activated protein kinase α and induced fatty acid beta-oxidation, tricarboxylic acid cycle (TCA) and glycogenolysis-related gene expression after APAP exposure. Liquid chromatography-mass spectrometry (LC-MS) analysis showed that GADD45α increased the levels of TCA cycle metabolites. Co-immunoprecipitation analysis showed that Ppp2cb, a catalytic subunit of protein phosphatase 2A, could interact directly with GADD45α. Our results indicate that hepatocyte GADD45α might represent a therapeutic target to prevent and rescue liver injury caused by APAP.

GW0742 (PPAR-beta agonist) attenuates hepatic endoplasmic reticulum stress by improving hepatic energy metabolism in high-fat diet fed mice

Endoplasmic reticulum (ER) stress and hepatic steatosis are intertwined with insulin resistance. PPARs are at the crossroads of these pathways. This study aimed to investigate the effects of GW0742 (PPAR-beta agonist) on hepatic energy metabolism and ER stress in a murine diet-induced obesity model. HF diet caused overweight, hyperinsulinemia, hepatic inflammation (increased NF-kB, TNF-alpha, and IL-6 protein expression) and favored hepatic lipogenesis, leading to ER stress, with ultrastructural and molecular alterations, ending up in proapoptotic stimulus. GW0742 rescued the overweight and the glucose tolerance, tackled hepatic inflammation and favored hepatic beta-oxidation over lipogenesis. These results comply with ER ultrastructure improvement, reducing ER stress and apoptosis in treated animals. Our results indicate that the PPAR-beta/delta activation alleviated the ER stress by improving the insulin sensitivity and maximizing the hepatic energy metabolism with a shift towards beta-oxidation. PPAR-beta/delta activation could be an essential tool to avoid the NAFLD progression and other obesity constraints.

Growth arrest and DNA damage-inducible 45α protects against nonalcoholic steatohepatitis induced by methionine- and choline-deficient diet

Growth arrest and DNA damage-inducible 45 α (Gadd45α) is a stress-inducible protein that plays an important role in cell survival/death and DNA repair, but its contribution to the development of nonalcoholic steatohepatitis (NASH) has not been investigated. C57BL/6 Gadd45a-null and wild-type (WT) mice were treated with a methionine and choline-deficient diet (MCD) for eight weeks and phenotypic changes examined. Gadd45a-null mice had more severe hepatic inflammation and fibrosis, higher levels of mRNAs encoding pro-inflammatory, pro-fibrotic, and pro-apoptotic proteins, and greater oxidative and endoplasmic reticulum (ER) stress compared with WT mice. Indeed, Gadd45a mRNA was induced in response to ER stress in primary hepatocytes. Lipidomic analysis of NASH livers demonstrated decreased triacylglycerol (TG) in MCD-treated Gadd45a-null mice, which was associated with increased mRNAs encoding phospholipase D (Pld1/2), phosphatidic acid phosphatase type 2A, and choline/ethanolamine phosphotransferase 1 (Cept1), involved in the phosphatidylcholine-phosphatidic acid-diacylglycerol cycle, and decreased mRNAs encoding fatty acid (FA)-binding protein 1 (Fabp1) and FA transport protein 5. Treatment of cultured primary hepatocytes with tumor necrosis factor α, transforming growth factor β, and hydrogen peroxide led to the corresponding induction of Fabp1, Pld1/2, and Cept1 mRNAs. Collectively, Gadd45α plays protective roles against MCD-induced NASH likely due to attenuating cellular stress and ensuing inflammatory signaling. These results also suggest an interconnection between hepatocyte injury, inflammation and disrupted glycerophospholipid/FA metabolism that yields a possible mechanism for decreased TG accumulation with NASH progression (i.e., "burned-out" NASH).