Krüppel-like factor 10 protects against metabolic dysfunction-associated steatohepatitis by regulating HNF4α-mediated metabolic pathways

BACKGROUND

Krüppel-like factor 10 (KLF10), a zinc finger transcription factor, plays a pivotal role in modulating TGF-β-mediated cellular processes such as growth, apoptosis, and differentiation. Recent studies have implicated KLF10 in regulating lipid metabolism and glucose homeostasis. This study aimed to elucidate the precise role of hepatic KLF10 in developing metabolic dysfunction-associated steatohepatitis (MASH) in diet-induced obese mice.

METHODS

We investigated hepatic KLF10 expression under metabolic stress and the effects of overexpression or ablation of hepatic KLF10 on MASH development and lipidemia. We also determined whether hepatocyte nuclear factor 4α (HNF4α) mediated the metabolic effects of KLF10.

RESULTS

Hepatic KLF10 was downregulated in MASH patients and genetically or diet-induced obese mice. AAV8-mediated overexpression of KLF10 in hepatocytes prevented Western diet-induced hypercholesterolemia and steatohepatitis, whereas inactivation of hepatocyte KLF10 aggravated Western diet-induced steatohepatitis. Mechanistically, KLF10 reduced hepatic triglyceride and free fatty acid levels by inducing lipolysis and fatty acid oxidation and inhibiting lipogenesis, and reducing hepatic cholesterol levels by promoting bile acid synthesis. KLF10 highly induced HNF4α expression by directly binding to its promoter. The beneficial effect of KLF10 on MASH development was abolished in mice lacking hepatocyte HNF4α. In addition, the inactivation of KLF10 in hepatic stellate cells exacerbated Western diet-induced liver fibrosis by activating the TGF-β/SMAD2/3 pathway.

CONCLUSIONS

Our data collectively suggest that the transcription factor KLF10 plays a hepatoprotective role in MASH development by inducing HNF4α. Targeting hepatic KLF10 may offer a promising strategy for treating MASH.

Hepatic FOXA3 overexpression prevents Western diet-induced obesity and MASH through TGR5

Forkhead transcription factor 3 (FOXA3) has been shown to regulate metabolism and development. Hepatic FOXA3 is reduced in obesity and fatty liver disease. However, the role of hepatic FOXA3 in regulating obesity or steatohepatitis remains to be investigated. In this work, C57BL/6 mice were i.v. injected with AAV8-ALB-FOXA3 or the control virus. The mice were then fed a chow or Western diet for 16 weeks. The role of hepatic FOXA3 in energy metabolism and steatohepatitis was investigated. Plasma bile acid composition and the role of Takeda G protein-coupled receptor 5 (TGR5) in mediating the metabolic effects of FOXA3 were determined. Overexpression of hepatic FOXA3 reduced hepatic steatosis in chow-fed mice and attenuated Western diet-induced obesity and steatohepatitis. FOXA3 induced lipolysis and inhibited hepatic genes involved in bile acid uptake, resulting in elevated plasma bile acids. The beneficial effects of hepatic FOXA3 overexpression on Western diet-induced obesity and steatohepatitis were abolished in Tgr5-/- mice. Our data demonstrate that overexpression of hepatic FOXA3 prevents Western diet-induced obesity and steatohepatitis via activation of TGR5.

Adipocyte retinoic acid receptor α prevents obesity and steatohepatitis by regulating energy expenditure and lipogenesis

OBJECTIVE

The adipose tissue-liver axis is a major regulator of the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Retinoic acid signaling plays an important role in development and metabolism. However, little is known about the role of adipose retinoic acid signaling in the development of obesity-associated NAFLD. In this work, the aim was to investigate whether and how retinoic acid receptor alpha (RARα) regulated the development of obesity and NAFLD.

METHODS

RARα expression in adipose tissue of db/db or ob/ob mice was determined. Rarαfl/fl mice and adipocyte-specific Rarα-/- (RarαAdi-/- ) mice were fed a chow diet for 1 year or high-fat diet (HFD) for 20 weeks. Primary adipocytes and primary hepatocytes were co-cultured. Metabolic regulation and inflammatory response were characterized.

RESULTS

RARα expression was reduced in adipose tissue of db/db or ob/ob mice. RarαAdi-/- mice had increased obesity and steatohepatitis (NASH) when fed a chow diet or HFD. Loss of adipocyte RARα induced lipogenesis and inflammation in adipose tissue and the liver and reduced thermogenesis. In the co-culture studies, loss of RARα in adipocytes induced inflammatory and lipogenic programs in hepatocytes.

CONCLUSIONS

The data demonstrate that RARα in adipocytes prevents obesity and NASH via inhibiting lipogenesis and inflammation and inducing energy expenditure.

Hepatocyte Sirtuin 6 Protects against Atherosclerosis and Steatohepatitis by Regulating Lipid Homeostasis

Histone deacetylase Sirtuin 6 (SIRT6) regulates many biological processes. SIRT6 is known to regulate hepatic lipid metabolism and inhibit the development of nonalcoholic fatty liver disease (NAFLD). We aimed to investigate the role of hepatocyte SIRT6 in the development of atherosclerosis and further characterize the mechanism underlying SIRT6's effect on NAFLD. Ldlr-/- mice overexpressing or lacking hepatocyte SIRT6 were fed a Western diet for 16 weeks. The role of hepatic SIRT6 in the development of nonalcoholic steatohepatitis (NASH), atherosclerosis, and obesity was investigated. We also investigated whether p53 participates in the pathogenesis of NAFLD in mice overexpressing hepatic SIRT6. Our data show that loss of hepatocyte SIRT6 aggravated the development of NAFLD, atherosclerosis, and obesity in Ldlr-/- mice, whereas adeno-associated virus (AAV)-mediated overexpression of human SIRT6 in the liver had opposite effects. Mechanistically, hepatocyte SIRT6 likely inhibited the development of NAFLD by inhibiting lipogenesis, lipid droplet formation, and p53 signaling. Hepatocyte SIRT6 also likely inhibited the development of atherosclerosis by inhibiting intestinal lipid absorption and hepatic VLDL secretion. Hepatic SIRT6 also increased energy expenditure. In conclusion, our data indicate that hepatocyte SIRT6 protects against atherosclerosis, NAFLD, and obesity by regulating lipid metabolism in the liver and intestine.

Retinoic Acid Receptor Alpha (RARα) in Macrophages Protects from Diet-Induced Atherosclerosis in Mice

Retinoic acid signaling plays an important role in regulating lipid metabolism and inflammation. However, the role of retinoic acid receptor alpha (RARα) in atherosclerosis remains to be determined. In the current study, we investigated the role of macrophage RARα in the development of atherosclerosis. Macrophages isolated from myeloid-specific Rarα-/- (RarαMac-/-) mice showed increased lipid accumulation and inflammation and reduced cholesterol efflux compared to Rarαfl/fl (control) mice. All-trans retinoic acid (AtRA) induced ATP-binding cassette subfamily A member 1 (Abca1) and Abcg1 expression and cholesterol efflux in both RarαMac-/- mice and Rarαfl/fl mice. In Ldlr-/- mice, myeloid ablation of RARα significantly reduced macrophage Abca1 and Abcg1 expression and cholesterol efflux, induced inflammatory genes, and aggravated Western diet-induced atherosclerosis. Our data demonstrate that macrophage RARα protects against atherosclerosis, likely via inducing cholesterol efflux and inhibiting inflammation.

Hepatic retinoic acid receptor alpha mediates all-trans retinoic acid's effect on diet-induced hepatosteatosis

All-trans retinoic acid (AtRA) is an active metabolite of vitamin A that influences many biological processes in development, differentiation, and metabolism. AtRA functions through activation of retinoid acid receptors (RARs). AtRA is shown to ameliorate hepatic steatosis, but the underlying mechanism is not well understood. In this study, we investigated the role of hepatocyte RAR alpha (RARα) in mediating the effect of AtRA on hepatosteatosis in mice. Hepatocyte-specific Rarα-/- (L-Rarα-/- ) mice and their control mice were fed a chow diet, high-fat diet (HFD), or a high-fat/cholesterol/fructose (HFCF) diet. Some of the mice were also treated with AtRA. Loss of hepatocyte RARα-induced hepatosteatosis in chow-fed aged mice and HFD-fed mice. AtRA prevented and reversed HFCF diet-induced obesity and hepatosteatosis in the control mice but not in L-Rarα-/- mice. Furthermore, AtRA reduced hepatocyte fatty acid uptake and lipid droplet formation, dependent on hepatocyte RARα. Our data suggest that hepatocyte RARα plays an important role in preventing hepatosteatosis and mediates AtRA's effects on diet-induced hepatosteatosis.

Abstract MP01: Retinoic Acid Receptor Alpha (Rarα) In Macrophages Protects From Diet-induced Atherosclerosis In Mice

Atherosclerosis is a chronic vascular disease caused by inflammation and accumulation of lipids in the blood vessels. Macrophages in atherosclerotic lesions participate in lipid accumulation giving rise to formation of foam cells and production of mediating inflammatory cytokines. This makes them an attractive target for therapy. Previous studies have shown that retinoid signaling plays a broader role in modulating macrophage lipid metabolism and its inflammatory phenotype. Our preliminary results showed that when macrophages (RAW267.4 cells and mouse peritoneal macrophages) were treated with all-trans retinoic acid or retinoic acid receptor alpha (Rarα)-specific agonist, cholesterol efflux genes like Abca1 And Abcg1 and anti-inflammatory gene like Arg-1 were significantly increased. We hypothesized that retinoic acid receptor alpha (Rarα) may play a role in macrophages to protect against diet-induced atherosclerosis. To test this hypothesis, we conducted in-vivo studies using macrophage-specific Rarα knockout mice. Rarα floxed mice were crossed with lysozyme-cre transgenic mice to generate macrophage specific knockout mice (m-Rarα -/- ). Rarα fl/fl (control) and m-Rarα -/- were given high fat/high cholesterol (HFHC) diet for 16 weeks. Intracellular triglyceride and cholesterol levels were significantly increased in peritoneal macrophages of m-Rarα -/- compared to the control. Analysis of macrophage gene expression showed increase in the expression of pro-inflammatory genes (Il-1β, Tnfα and Tgm-2) with decrease in genes involved in cholesterol efflux (Abca1 and Abcg1) and anti-inflammation (Arg-1 and Mrc-1) in m-Rarα -/- macrophages. This implies that loss of Rarα in macrophages can aggravate atherosclerosis. To test this hypothesis, we generated mice deficient in macrophage Rarα and low-density lipoprotein receptor and their control mice. These mice were then fed an HFHC diet for 16 weeks. The Oil Red O staining of aortas showed that mice lacking macrophage Rar had a significant increase in atherosclerotic plaques compared to the control mice. Our data demonstrate that Rarα in macrophages is protective from atherosclerosis.

Hepatocyte Nuclear Factor 4α Prevents the Steatosis-to-NASH Progression by Regulating p53 and Bile Acid Signaling (in mice)

BACKGROUND AND AIMS

Hepatocyte nuclear factor 4α (HNF4α) is highly enriched in the liver, but its role in the progression of nonalcoholic liver steatosis (NAFL) to NASH has not been elucidated. In this study, we investigated the effect of gain or loss of HNF4α function on the development and progression of NAFLD in mice.

APPROACH AND RESULTS

Overexpression of human HNF4α protected against high-fat/cholesterol/fructose (HFCF) diet-induced steatohepatitis, whereas loss of Hnf4α had opposite effects. HNF4α prevented hepatic triglyceride accumulation by promoting hepatic triglyceride lipolysis, fatty acid oxidation, and VLDL secretion. Furthermore, HNF4α suppressed the progression of NAFL to NASH. Overexpression of human HNF4α inhibited HFCF diet-induced steatohepatitis in control mice but not in hepatocyte-specific p53^-/- mice. In HFCF diet-fed mice lacking hepatic Hnf4α, recapitulation of hepatic expression of HNF4α targets cholesterol 7α-hydroxylase and sterol 12α-hydroxylase and normalized hepatic triglyceride levels and attenuated steatohepatitis.

CONCLUSIONS

The current study indicates that HNF4α protects against diet-induced development and progression of NAFLD by coordinating the regulation of lipolytic, p53, and bile acid signaling pathways. Targeting hepatic HNF4α may be useful for treatment of NASH.

Metformin treatment prevents SREBP2-mediated cholesterol uptake and improves lipid homeostasis during oxidative stress-induced atherosclerosis

Lipids are responsible for the atheromatous plaque formation during atherosclerosis by their deposition in the subendothelial intima of the aorta, leading to infarction. Sterol regulatory element-binding protein 2 (SREBP2), regulating cholesterol homeostasis, is suggested to play a pivotal role during the early incidence of atherosclerosis through dysregulation of lipid homeostasis. Here we demonstrate that oxidative stress stimulates SREBP2-mediated cholesterol uptake via low density lipoprotein receptor (LDLR), rather than cholesterol synthesis, in mouse vascular aortic smooth muscle cells (MOVAS) and THP-1 monocytes. The enhancement of mature form of SREBP2 (SREBP2-M) during oxidative stress was associated with the inhibition of AMP-activated protein kinase (AMPK) activation. In contrast, inhibition of either SREBP2 by fatostatin or LDLR by siLDLR resulted in decreased cholesterol levels during oxidative stress. Thereby confirming the role of SREBP2 in cholesterol regulation via LDLR. Metformin-mediated activation of AMPK was able to significantly abrogate cholesterol uptake by inhibiting SREBP2-M. Interestingly, although metformin administration attenuated angiotensin (Ang)-II-impaired lipid homeostasis in both aorta and liver tissues of ApoE^-/- mice, the results indicate that SREBP2 through LDLR regulates lipid homeostasis in aorta but not in liver tissue. Taken together, AMPK activation inhibits oxidative stress-mediated SREBP2-dependent cholesterol uptake, and moreover, metformin-induced prevention of atheromatic events are in part due to its ability to regulate the SREBP2-LDLR axis.

Resveratrol attenuates monocyte-to-macrophage differentiation and associated inflammation via modulation of intracellular GSH homeostasis: Relevance in atherosclerosis

Monocyte-to-macrophage differentiation promotes an inflammatory environment within the arterial vessel wall that causes a mal-adaptive immune response, which contributes to the progression of atheromatous plaque formation. In the current study, we show that resveratrol, a well-known antioxidant, dose-dependently attenuated phorbol myristate acetate (PMA)-induced monocyte-to-macrophage differentiation, as measured by cell adhesion, increase in cell size, and scavenger receptor expression in THP-1 monocytes. Also, resveratrol significantly inhibited PMA-induced pro-inflammatory cytokine/chemokine and matrix metalloprotease (MMP-9) production. This inhibitory effect of resveratrol on monocyte differentiation results from its ability to restore intracellular glutathione (GSH) status, as resveratrol in the presence of buthionine sulfoximine (BSO) failed to affect monocyte differentiation. Furthermore, PMA-induced monocyte differentiation and inflammation was greatly inhibited when cells were co-treated with N-Acetyl-l-cysteine (NAC), a GSH precursor, while the presence of BSO aggravated these processes. These results also show that resveratrol mediated up-regulation of GSH is due to AMP-activated protein kinase (AMPK)-α activation, as compound C (AMPK inhibitor) treatment drastically depleted intracellular GSH and exacerbated PMA-induced monocyte differentiation and pro-inflammatory cytokine production. More importantly, chronic administration of resveratrol efficiently prevented monocyte infiltration and markedly diminished angiotensin (Ang)-II-induced atheromatous plaque formation in apolipoprotein-E knockout (ApoE(-/-)) mice. We conclude that, intracellular GSH status plays a critical role in regulating monocyte-to-macrophage differentiation and inflammation and resveratrol, by restoring GSH levels, inhibits these processes. Taken together, these results suggest that resveratrol can attenuate atherosclerosis, at least, in part, by inhibiting monocyte differentiation and pro-inflammatory cytokines production.

Mitochondria-targeted esculetin alleviates mitochondrial dysfunction by AMPK-mediated nitric oxide and SIRT3 regulation in endothelial cells: potential implications in atherosclerosis

Mitochondria-targeted compounds are emerging as a new class of drugs that can potentially alter the pathophysiology of those diseases where mitochondrial dysfunction plays a critical role. We have synthesized a novel mitochondria-targeted esculetin (Mito-Esc) with an aim to investigate its effect during oxidative stress-induced endothelial cell death and angiotensin (Ang)-II-induced atherosclerosis in ApoE^−/− mice. Mito-Esc but not natural esculetin treatment significantly inhibited H2O2- and Ang-II-induced cell death in human aortic endothelial cells by enhancing NO production via AMPK-mediated eNOS phosphorylation. While L-NAME (NOS inhibitor) significantly abrogated Mito-Esc-mediated protective effects, Compound c (inhibitor of AMPK) significantly decreased Mito-Esc-mediated increase in NO production. Notably, Mito-Esc promoted mitochondrial biogenesis by enhancing SIRT3 expression through AMPK activation; and restored H2O2-induced inhibition of mitochondrial respiration. siSIRT3 treatment not only completely reversed Mito-Esc-mediated mitochondrial biogenetic marker expressions but also caused endothelial cell death. Furthermore, Mito-Esc administration to ApoE^−/− mice greatly alleviated Ang-II-induced atheromatous plaque formation, monocyte infiltration and serum pro-inflammatory cytokines levels. We conclude that Mito-Esc is preferentially taken up by the mitochondria and preserves endothelial cell survival during oxidative stress by modulating NO generation via AMPK. Also, Mito-Esc-induced SIRT3 plays a pivotal role in mediating mitochondrial biogenesis and perhaps contributes to its anti-atherogenic effects.

Statin‐induced inhibition of breast cancer proliferation and invasion involves attenuation of iron transport: intermediacy of nitric oxide and antioxidant defence mechanisms

Accumulating evidence from in vitro, in vivo, clinical and epidemiological studies shows promising results for the use of statins against many cancers including breast carcinoma. However, the molecular mechanisms responsible for the anti‐proliferative and anti‐invasive properties of statins still remain elusive. In this study, we investigated the involvement of nitric oxide, iron homeostasis and antioxidant defence mechanisms in mediating the anti‐proliferative and anti‐invasive properties of hydrophobic statins in MDA‐MB‐231, MDA‐MB‐453 and BT‐549 metastatic triple negative breast cancer cells. Fluvastatin and simvastatin significantly increased cytotoxicity which was reversed with mevalonate. Interestingly, fluvastatin downregulated transferrin receptor (TfR1), with a concomitant depletion of intracellular iron levels in these cells. Statin‐induced effects were mimicked by geranylgeranyl transferase inhibitor (GGTI‐298) but not farnesyl transferase inhibitor (FTI‐277). Further, it was observed that TfR1 downregulation is mediated by increased nitric oxide levels via inducible nitric oxide synthase (iNOS) expression. NOS inhibitors (asymmetric dimethylarginine and 1400W) counteracted and sepiapterin, a precursor of tetrahydrobiopterin, exacerbated statin‐induced depletion of intracellular iron levels. Notably, fluvastatin increased manganese superoxide dismutase (by repressing the transcription factor DNA damage‐binding protein 2), catalase and glutathione which, in turn, diminished H2O2 levels. Fluvastatin‐induced downregulation of TfR1, matrix metalloproteinase‐2, ‐9 and inhibition of invasion were reversed in the presence of aminotriazole, a specific inhibitor of catalase. Finally, we conclude that fluvastatin, by altering iron homeostasis, nitric oxide generation and antioxidant defence mechanisms, induces triple negative breast cancer cell death.

Statin-induced inhibition of breast cancer proliferation and invasion involves attenuation of iron transport: intermediacy of nitric oxide and antioxidant defence mechanisms

Accumulating evidence from in vitro, in vivo, clinical and epidemiological studies shows promising results for the use of statins against many cancers including breast carcinoma. However, the molecular mechanisms responsible for the anti-proliferative and anti-invasive properties of statins still remain elusive. In this study, we investigated the involvement of nitric oxide, iron homeostasis and antioxidant defence mechanisms in mediating the anti-proliferative and anti-invasive properties of hydrophobic statins in MDA-MB-231, MDA-MB-453 and BT-549 metastatic triple negative breast cancer cells. Fluvastatin and simvastatin significantly increased cytotoxicity which was reversed with mevalonate. Interestingly, fluvastatin downregulated transferrin receptor (TfR1), with a concomitant depletion of intracellular iron levels in these cells. Statin-induced effects were mimicked by geranylgeranyl transferase inhibitor (GGTI-298) but not farnesyl transferase inhibitor (FTI-277). Further, it was observed that TfR1 downregulation is mediated by increased nitric oxide levels via inducible nitric oxide synthase (iNOS) expression. NOS inhibitors (asymmetric dimethylarginine and 1400W) counteracted and sepiapterin, a precursor of tetrahydrobiopterin, exacerbated statin-induced depletion of intracellular iron levels. Notably, fluvastatin increased manganese superoxide dismutase (by repressing the transcription factor DNA damage-binding protein 2), catalase and glutathione which, in turn, diminished H2 O2 levels. Fluvastatin-induced downregulation of TfR1, matrix metalloproteinase-2, -9 and inhibition of invasion were reversed in the presence of aminotriazole, a specific inhibitor of catalase. Finally, we conclude that fluvastatin, by altering iron homeostasis, nitric oxide generation and antioxidant defence mechanisms, induces triple negative breast cancer cell death.