PPARγ In Hepatocytes Research Articles

Acute murine cytomegalovirus infection boosts cell-type specific response and lipid metabolism changes in the liver of infant mice.

Human cytomegalovirus (HCMV) infection in infants can lead to severe diseases, including neonatal hepatitis. The single-cell dimensional changes in immune cells after the initial CMV infection remain elusive, as do the effects of CMV infection on hepatic lipid metabolism. We employed single-cell RNA-sequencing to investigate the changes in liver cell types and immune responses in infant mice following murine CMV (MCMV) infection. Additionally, we examined alterations in protein expression profiles related to lipid metabolism in hepatocytes and the role of the key transcription factor PPAR-γ in hepatocytes during CMV infection. Our study revealed that MCMV infects most liver cell types in infant mice, leading to an increase in the proportion of proliferating CD8 effector T cells and a subset of Nos2+ monocytes, potentially playing an essential role in early anti-viral responses. Furthermore, MCMV infection resulted in altered protein expression of lipid metabolism in hepatocytes. Knocking down the transcription factor PPAR-γ in hepatocytes effectively inhibited CMV infection. Our findings underscore the immune system's response to early-stage MCMV infection and the subsequent impact on hepatic lipid metabolism in infant mice. This research provides new insights into the mechanisms of CMV infection and could pave the way for novel therapeutic strategies.

Role of hepatic peroxisome proliferator-activated receptor γ in non-alcoholic fatty liver disease.

Peroxisome proliferator-activated receptor γ (PPARγ) belongs to a family of nuclear receptors that could serve as lipid sensors. PPARγ is the target of a group of insulin sensitizers called thiazolidinediones (TZDs) which regulate the expression of genes involved in glucose and lipid metabolism as well as adipokines that regulate metabolic function in other tissues. Non-alcoholic fatty liver disease (NAFLD) has a high prevalence worldwide and is even higher in patients with obesity and insulin resistance. TZD-mediated activation of PPARγ could serve as a good treatment for NAFLD because TZDs have shown anti-fibrogenic and anti-inflammatory effectsin vitro and increase insulin sensitivity in peripheral tissues which improves liver pathology. However, mechanistic studies in mouse models suggest that the activation of PPARγ in hepatocytes might reduce or limit the therapeutic potential of TZD against NAFLD. In this review, we briefly describe the short history of PPAR isoforms, the relevance of their expression in different tissues, as well as the pathogenesis and potential therapeutics for NAFLD. We also discuss some evidence derived from mouse models that could be useful for endocrinologists to assess tissue-specific roles of PPARs, complement reverse endocrinology approaches, and understand the direct role that PPARγ has in hepatocytes and non-parenchymal cells.

Hepatocyte PPARγ contributes to the progression of non-alcoholic steatohepatitis in male and female obese mice.

Non-alcoholic steatohepatitis (NASH) is associated with obesity and increased expression of hepatic peroxisome proliferator-activated receptor γ (PPARγ). However, the relevance of hepatocyte PPARγ in NASH associated with obesity is still poorly understood. In this study, hepatocyte PPARγ was knocked out (PpargΔHep) in male and female mice after the development of high-fat diet-induced obesity. The diet-induced obese mice were then maintained on their original diet or switched to a high fat, cholesterol, and fructose (HFCF) diet to induce NASH. Hepatic PPARγ expression was mostly derived from hepatocytes and increased by high fat diets. PpargΔHep reduced HFCF-induced NASH progression without altering steatosis, reduced the expression of key genes involved in hepatic fibrosis in HFCF-fed male and female mice, and decreased the area of collagen-stained fibrosis in the liver of HFCF-fed male mice. Moreover, transcriptomic and metabolomic data suggested that HFCF-diet regulated hepatic amino acid metabolism in a hepatocyte PPARγ-dependent manner. PpargΔHep increased betaine-homocysteine s-methyltransferase expression and reduced homocysteine levels in HFCF-fed male mice. In addition, in a cohort of 102 obese patients undergoing bariatric surgery with liver biopsies, 16 cases were scored with NASH and were associated with increased insulin resistance and hepatic PPARγexpression. Our study shows that hepatocyte PPARγ expression is associated with NASH in mice and humans. In male mice, hepatocyte PPARγ negatively regulates methionine metabolism and contributes to the progression of fibrosis.

Hawthorn or semen cassiae-alleviated high-fat diet-induced hepatic steatosis in rats via the reduction of endoplasmic reticulum stress.

Hepatic steatosis is a common pathological change of liver that manifests as abnormal lipid accumulation. Epidemiological findings support that diseases such as obesity, diabetes, and hyperlipidemia are mostly accompanied by the development of hepatic steatosis. By screening the disease targets of several traditional Chinese medicines (TCMs) with lipid-reducing effects (hawthorn, semen cassiae, etc.) through the Traditional Chinese Medicine Systems Pharmacology Database and Analysis Platform (TCMSP), we found that peroxisome-activated receptor gamma (PPAR-γ) is involved in regulating several lipid metabolism-related signaling pathways. Further experiments confirmed that PPAR-γ was correlated with aggravated endoplasmic reticulum (ER) stress in overnutrition-induced hepatic steatosis. The stimulation of hepatocytes by abnormal lipid metabolism signals causes an imbalance in ER homeostasis, which subsequently exacerbates hepatocyte lipid abnormalities. The inhibition of glucose regulatory protein 78 (GRP78, a master regulator of ER homeostasis) was effective in reducing hepatocyte PPAR-γ and lipid synthesis levels. In fact, the hawthorn/semen cassiae treatment effectively downregulated hepatocyte ER stress in high-fat-diet fed rats and reduced the PPAR-γ expression as well as related lipid synthesis. Herein, we confirmed that TCMs characterized by natural lipid-lowering effectively target hepatic PPAR-γ and GRP78, improve ER stress, and have a protective effect against obesity-related hepatic steatosis.

Rosiglitazone Requires Hepatocyte PPARγ Expression to Promote Steatosis in Male Mice With Diet-Induced Obesity.

Thiazolidinediones (TZD) are peroxisome proliferator-activated receptor γ (PPARγ) agonists that may reduce hepatic steatosis through their effects in adipose tissue and therefore have been assessed as potential therapies to treat nonalcoholic fatty liver disease (NAFLD) in humans. However, some studies suggest that expression and activation of hepatocyte PPARγ promotes steatosis and that would limit the benefits of TZD as a NAFLD therapy. To further explore this possibility, we examined the impact of short-term rosiglitazone maleate treatment after the development of moderate or severe diet-induced obesity, in both control and adult-onset hepatocyte-specific PPARγ knockout (PpargΔHep) mice. Independent of the level of obesity and hepatic PPARγ expression, the TZD treatment enhanced insulin sensitivity, associated with an increase in white adipose tissue (WAT) fat accumulation, consistent with clinical observations. However, TZD treatment increased hepatic triglyceride content only in control mice with severe obesity. Under these conditions, PpargΔHep reduced diet-induced steatosis and prevented the steatogenic effects of short-term TZD treatment. In these mice, subcutaneous WAT was enlarged and associated with increased levels of adiponectin, while hepatic levels of phosphorylated adenosine 5'-monophosphate-activated protein kinase were also increased. In addition, in mice with severe obesity, the expression of hepatic Cd36, Cidea, Cidec, Fabp4, Fasn, and Scd-1 was increased by TZD in a PPARγ-dependent manner. Taken together, these results demonstrate that hepatocyte PPARγ expression offsets the antisteatogenic actions of TZD in mice with severe obesity. Therefore, in obese and insulin resistant humans, TZD-mediated activation of hepatocyte PPARγ may limit the therapeutic potential of TZD to treat NAFLD.

Hepatocyte-Specific Loss of PPARγ Protects Mice From NASH and Increases the Therapeutic Effects of Rosiglitazone in the Liver.

Background & AimsNonalcoholic steatohepatitis (NASH) is commonly observed in patients with type 2 diabetes, and thiazolidinediones (TZD) are considered a potential therapy for NASH. Although TZD increase insulin sensitivity and partially reduce steatosis and alanine aminotransferase, the efficacy of TZD on resolving liver pathology is limited. In fact, TZD may activate peroxisome proliferator-activated receptor gamma (PPARγ) in hepatocytes and promote steatosis. Therefore, we assessed the role that hepatocyte-specific PPARγ plays in the development of NASH, and how it alters the therapeutic effects of TZD on the liver of mice with diet-induced NASH.MethodsHepatocyte-specific PPARγ expression was knocked out in adult mice before and after the development of NASH induced with a high fat, cholesterol, and fructose (HFCF) diet.ResultsHFCF diet increased PPARγ expression in hepatocytes, and rosiglitazone further activated PPARγ in hepatocytes of HFCF-fed mice in vivo and in vitro. Hepatocyte-specific loss of PPARγ reduced the progression of HFCF-induced NASH in male mice and increased the benefits derived from the effects of TZD on extrahepatic tissues and non-parenchymal cells. RNAseq and metabolomics indicated that HFCF diet promoted inflammation and fibrogenesis in a hepatocyte PPARγ–dependent manner and was associated with dysregulation of hepatic metabolism. Specifically, hepatocyte-specific loss of PPARγ plays a positive role in the regulation of methionine metabolism, and that could reduce the progression of NASH.ConclusionsBecause of the negative effect of hepatocyte PPARγ in NASH, inhibition of mechanisms promoted by endogenous PPARγ in hepatocytes may represent a novel strategy that increases the efficiency of therapies for NAFLD.

Loss of Hepatocyte-Specific PPARγ Expression Ameliorates Early Events of Steatohepatitis in Mice Fed the Methionine and Choline-Deficient Diet.

The prevalence of nonalcoholic fatty liver disease (NAFLD) is increasing worldwide. To date, there is not a specific and approved treatment for NAFLD yet, and therefore, it is important to understand the molecular mechanisms that lead to the progression of NAFLD. Methionine- and choline-deficient (MCD) diets are used to reproduce some features of NAFLD in mice. MCD diets increase the expression of hepatic peroxisome proliferator-activated receptor gamma (PPARγ, Pparg) and the fatty acid translocase (CD36, Cd36) which could increase hepatic fatty acid uptake and promote the progression of NAFLD in mice and humans. In this study, we assessed the contribution of hepatocyte-specific PPARγ and CD36 expression to the development of early events induced by the MCD diet. Specifically, mice with adult-onset, hepatocyte-specific PPARγ knockout with and without hepatocyte CD36 overexpression were fed a MCD diet for three weeks. Hepatocyte PPARγ and/or CD36 expression did not contribute to the development of steatosis induced by the MCD diet. However, the expression of inflammatory and fibrogenic genes seems to be dependent on the expression of hepatocyte PPARγ and CD36. The expression of PPARγ and CD36 in hepatocytes may be relevant in the regulation of some features of NAFLD and steatohepatitis.

Hepatocyte-specific Nrf2 deficiency mitigates high-fat diet-induced hepatic steatosis: Involvement of reduced PPARγ expression

Non-alcoholic fatty liver disease (NAFLD) is an emerging global disease with increasing prevalence. However, the mechanism of NAFLD development is not fully understood. To elucidate the cell-specific role of nuclear factor erythroid-derived 2-like 2 (NRF2) in the pathogenesis of NAFLD, we utilized hepatocyte- and macrophage-specific Nrf2-knockout [Nrf2(L)-KO and Nrf2(Mϕ)-KO] mice to examine the progress of NAFLD induced by high-fat diet (HFD). Compared to Nrf2-LoxP littermates, Nrf2(L)-KO mice showed less liver enlargement, milder inflammation and less hepatic steatosis after HFD feeding. In contrast, Nrf2(Mϕ)-KO mice displayed no significant difference in HFD-induced hepatic steatosis from Nrf2-LoxP control mice. Mechanistic investigations revealed that Nrf2 deficiency in hepatocytes dampens the expression of peroxisome proliferator-activated receptor γ (PPARγ) and its downstream lipogenic genes in the liver and/or primary hepatocytes induced by HFD and palmitate exposure, respectively. While PPARγ agonists augmented PPARγ expression and its transcriptional activity in primary hepatocytes in a NRF2-dependent manner, forced overexpression of PPARγ1 or γ2 distinctively reversed the decreased expression of their downstream genes fatty acid binding protein 4, lipoprotein lipase and/or fatty acid synthase caused by Nrf2 deficiency. We conclude that NRF2-dependent expression of PPARγ in hepatocytes is a critical initiating process in the development of NAFLD, suggesting that inhibition of NRF2 specifically in hepatocytes may be a valuable approach to prevent the disease.

SAT-152 Loss of Hepatocyte PPARγ Expression Ameliorates Steatosis in Mice Fed a High-Fat, Cholesterol, and Fructose Diet, but Not in Mice Fed a Methionine- and Choline-Deficient Diet

Hepatic PPARγ expression is positively associated with the progression of non-alcoholic fatty liver disease and non-alcoholic steatohepatitis (NASH) in mice and humans. Although PPARγ agonists, thiazolidinediones (TZD), could be used to treat patients with NASH, these drugs reduced steatosis only in some patients with NASH, while it exacerbated steatosis in obese mice. Several mouse models with congenital knockout or overexpression of hepatic PPARγ have been used to study the actions and relevance of hepatocyte PPARγ in the development of steatosis. However, the mechanisms that hepatocyte PPARγ regulates to promote steatosis are not fully understood. Our group has previously reported that adult-onset hepatocyte-specific PPARγ knockout (aHepPPARγKO) mice fed a high fat diet (60% Kcal from fat) developed adiposity similar to PPARγ-intact controls, but aHepPPARγKO mice showed a dramatic reduction in steatosis, associated with reduced hepatic Cd36-mediated fatty acid uptake. Here, we fed aHepPPARγKO mice two different diets that produce steatosis and NASH to determine if loss of hepatocyte PPARγ expression reduced hepatic lipid levels associated with a reduction in Cd36 expression. Specifically, aHepPPARγKO and control mice were generated by injecting 10 wk-old male PPARγ-floxed littermate mice with a single dose of adeno-associated virus serotype 8 (AAV8) expressing hepatocyte-specific Cre recombinase or a AAV8-null vector, respectively. Two weeks later, aHepPPARγKO and control mice were fed either a methionine- and choline-deficient diet (MCD) for 3 weeks or a high-fat, cholesterol and fructose diet (HFCFD) diet for 8 weeks. The expression of hepatic PPARγ and Cd36 was significantly increased in MCD- and HFCF-fed control mice as compared to their respective control diet-fed controls. Of note, aHepPPARγKO reduced the expression of hepatic PPARγ (by >85%) and Cd36 (by >35-65%). Overall, MCD-fed mice showed reduced body weight, fat mass, and plasma cholesterol levels, while HFCFD-fed mice showed increased body weight and gonadal fat mass, and reduced plasma NEFA levels as compared to their respective control diet-fed littermates. Independent of diet, aHepPPARγKO did not alter body weight, adiposity, or the levels of plasma lipids. However, aHepPPARγKO reduced steatosis in HFCFD-fed mice, but not in MCD-fed mice. Interestingly, the levels of hepatic oleic [18:1(n-9)] and linoleic [18:2(n-6)] acids were, in fact, increased in MCD-fed aHepPPARγKO mice, while the levels of hepatic palmitoleic [16:1(n-7)] and oleic [18:1(n-9)] acids were reduced in HFCFD-fed aHepPPARγKO mice. Taken together, these data suggest that hepatocyte PPARγ serves as a steatogenic factor only in HFCFD-fed mice and not in MCD-fed mice, and this may be due to diet-dependent effects: nature of the development of steatosis and/or changes in adipose tissue.

Su2089 - Hepatocyte PPARγ May Regulate Intestinal Absorption and/or Hepatic Clearance of Triacylglycerols, to Prevent Postprandial Dyslipidemia

Su2089 - Hepatocyte PPARγ May Regulate Intestinal Absorption and/or Hepatic Clearance of Triacylglycerols, to Prevent Postprandial Dyslipidemia

Hepatocyte-specific, PPARγ-regulated mechanisms to promote steatosis in adult mice.

Peroxisome proliferator-activated receptor γ (PPARγ) is the target for thiazolidinones (TZDs), drugs that improve insulin sensitivity and fatty liver in humans and rodent models, related to a reduction in hepatic de novo lipogenesis (DNL). The systemic effects of TZDs are in contrast to reports suggesting hepatocyte-specific activation of PPARγ promotes DNL, triacylglycerol (TAG) uptake and fatty acid (FA) esterification. As these hepatocyte-specific effects of PPARγ could counterbalance the positive therapeutic actions of systemic delivery of TZDs, the current study used a mouse model of adult-onset, liver (hepatocyte)-specific PPARγ knockdown (aLivPPARγkd). This model has advantages over existing congenital knockout models, by avoiding compensatory changes related to embryonic knockdown, thus better modeling the impact of altering PPARγ on adult physiology, where metabolic diseases most frequently develop. The impact of aLivPPARγkd on hepatic gene expression and endpoints in lipid metabolism was examined after 1 or 18 weeks (Chow-fed) or after 14 weeks of low- or high-fat (HF) diet. aLivPPARγkd reduced hepatic TAG content but did not impact endpoints in DNL or TAG uptake. However, aLivPPARγkd reduced the expression of the FA translocase (Cd36), in 18-week Chow- and HF-fed mice, associated with increased NEFA after HF feeding. Also, aLivPPARγkd dramatically reduced Mogat1 expression, that was reflected by an increase in hepatic monoacylglycerol (MAG) levels, indicative of reduced MOGAT activity. These results, coupled with previous reports, suggest that Cd36-mediated FA uptake and MAG pathway-mediated FA esterification are major targets of hepatocyte PPARγ, where loss of this control explains in part the protection against steatosis observed after aLivPPARγkd.

Rosiglitazone attenuates angiotensin II-induced C-reactive protein expression in hepatocytes via inhibiting AT1/ROS/MAPK signal pathway

Inflammation is a commonly pathological change in liver diseases, and promotes the development of acute and chronic liver diseases by excessive production of inflammatory cytokines. C-reactive protein (CRP) is primarily synthesized in the liver and participates in many chronic diseases. Our previous study confirmed that Ang II stimulates CRP generation in hepatocytes. This study investigated the effect of rosiglitazone (RSG) on Ang II-stimulated CRP expression and the molecular mechanism in hepatocytes. The results showed that the subcutaneous infusion of Ang II to rats for 7days through the osmotic minipumps increased CRP expression in the liver and serum CRP level. The simultaneous treatment of rats with RSG reduced CRP generation in the liver and CRP concentration in serum. Further study showed that RSG down-regulated Ang II-induced mRNA and protein expression of CRP and AT1 as well as phosphorylation of ERK1/2 and JNK, enhanced mRNA and protein expression of PPARγ in hepatocytes in vivo and in vitro. In addition, RSG increased superoxide dismutase activity in the liver of Ang II-infused rats in vivo, and decreased Ang II-stimulated reactive oxygen species (ROS) production in hepatocytes in vitro. In conclusion, the present study demonstrates that RSG is able to inhibit Ang II-induced CRP generation by interfering with AT1–ROS–ERK1/2/JNK signal pathway and up-regulating PPARγ expression in hepatocytes, which provides the new evidence and mechanisms for the beneficial effect of RSG to relieve hepatic inflammation and suggests the possibility that RSG is used for the inflammatory hepatic diseases.

PPARs and Metabolic Syndrome.

Peroxisome proliferator-activated receptors (PPARs) exert versatile biological effects, notably in energy metabolism. During the last two decades, numerous studies have demonstrated that PPARs act as pivotal regulators of metabolic syndrome, a series of disorders in energy utilization and storage that are implicated with type 2 diabetes, diabetic nephropathy, and cardiovascular diseases, to mention a few. PPARα and PPARγ are the molecular targets of a number of marketed drugs for the treatment of these diseases, and accumulating evidence suggested PPARβ/δ as a potential therapeutic drug target as well. Although energy metabolism and metabolic syndrome are the most intensively studied domain of PPARs, it has not been addressed specifically in any issue of PPAR Research ever since its launch. Here, we gathered 3 reviews and 5 research articles that encompass metabolic syndrome and its complications. M. Aprile et al. tackled the subject of PPARγ and human adipogenesis in their research article. Rather than focusing on canonical PPARγ transcripts, authors largely emphasized on the critical contribution of PPARγ dominant negative isoforms to adipogenesis and their implied potential role in pathological conditions. In addition, a novel of PPARγ dominant negative transcript, γ1ORF4, was first identified in this study. In regard to nonalcoholic fatty liver diseases, the hepatic expression of the metabolic syndrome, M. Sharif et al. conducted a thorough analysis of previously published data about the steatogenic role of PPARγ and summarized two probable PPARγ ligand-dependent toxicological modes of action: (i) activation of PPARγ in hepatocytes and (ii) inhibition in adipocytes. Two papers, one review and a research article, by Z. Jia and Y. Sun et al., appraised the role of PPARγ in diabetic nephropathy (DN). Their comprehensive review summarized the limitations of traditional PPARγ agonists, addressed the advantages of newly developed PPARγ agonists, and rendered new insights into the therapeutic potential of PPARγ agonists in the treatment of DN, while the research article suggested that a combination of PPARγ agonists with COX-2/PGE2 inhibitors may be an alternative way of dealing with DN. In another research article, J. Jin et al. analyzed the correlation between PPAR gene polymorphisms and pediatric primary nephrotic syndrome (PNS) by comparing children with PNS against healthy subjects. They found that PPARγ (Pro12Ala) and PGC-1α (Gly482Ser) polymorphisms are associated with abnormal insulin and triglyceride metabolism in pediatric PNS patients, suggesting that these polymorphisms may be relevant to the prognosis of this chronic disease. The knowledge of the role of PPARα in metabolic disorder-associated cardiovascular diseases was well recognized in this special issue. Z. Jia et al.'s research article asserted the involvement of HMGB1 (high mobility group box 1) in the protective effect of PPARα in cardiac hypertrophy and provided a novel approach to study the pathogenesis of cardiac hypertrophy. Although most studies showed that PPARα activation confers protection against atherogenesis, the intriguing possibility that PPARα might foster atherogenesis is also considered. In this current issue, M. Vechoropoulos et al. found that PPARα mediates the proatherogenic effect of chronic nitric oxide synthesis inhibition and this effect is independent of blood pressure and serum lipids alterations. These data further shaped the view that the role of PPARα in atherosclerosis needs to be reevaluated. Lastly, in the review article “PPARs Integrate the Mammalian Clock and Energy Metabolism,” we collected recent findings about the role of PPARs in biological clocks. This brand new function of PPARs bridges energy metabolism with circadian rhythm whose relationship has been known for long time, but not well understood. We summarized the circadian function of three PPAR subtypes one by one and concluded that the abnormality of PPARs and circadian rhythm could impinge on each other and thus leads to metabolic disorders. Further investigation of PPARs in this field will give us a new perspective on the therapeutic advances in the treatment of metabolic ailments. In conclusion, this special issue is packed with intriguing novel breakthroughs and insights into PPARs and metabolic syndrome. We hope that these advances will generate more interest from the scientific community in better understanding of the role of PPARs in metabolic syndrome and associated complications. Lihong Chen Zhanjun Jia Guangrui Yang

Role for PPARγ in obesity‐induced hepatic steatosis as determined by hepatocyte‐ and macrophage‐specific conditional knockouts

Peroxisome proliferator-activated receptor (PPAR) γ is a nuclear receptor central to glucose and lipid homeostasis. PPARγ role in nonalcoholic fatty liver disease is controversial because PPARγ overexpression is a general property of steatotic livers, but its activation by thiazolidinediones reduces hepatic steatosis. Here, we investigated hepatic PPARγ function by using Cre-loxP technology to generate hepatocyte (PPARγ(Δhep))- and macrophage (PPARγ(Δmac))-specific PPARγ-knockout mice. Targeted deletion of PPARγ in hepatocytes, and to a lesser extent in macrophages, protected mice against high-fat diet-induced hepatic steatosis. Down-regulated expression of genes involved in lipogenesis (SCD1, SREBP-1c, and ACC), lipid transport (CD36/FAT, L-FABP, and MTP), and β-oxidation (PPARα and ACO) was observed in PPARγ(Δhep) mice. Moreover, PPARγ(Δhep) mice showed improved glucose tolerance and reduced PEPCK expression without changes in Pcx, Fbp1, and G6Pc expression and CREB and JNK phosphorylation. In precision-cut liver slices (PCLSs) and hepatocytes, rosiglitazone either alone or in combination with oleic acid increased triglyceride accumulation, an effect that was blocked by the PPARγ antagonist biphenol A diglycidyl ether (BADGE). PCLSs and hepatocytes from PPARγ(Δhep) mice showed blunted responses to rosiglitazone and oleic acid, whereas the response to these compounds remained intact in PCLSs from PPARγ(Δmac) mice. Collectively, these findings establish PPARγ expression in hepatocytes as a prosteatotic factor in fatty liver disease.