High soluble endoglin levels regulate cholesterol homeostasis and bile acids turnover in the liver of transgenic mice

AIMS

Increased plasma soluble endoglin concentrations (sEng) are frequently detected in metabolic disorders accompanied with hypercholesterolemia in serum, but effect of sEng on the cholesterol biochemistry is unknown. Cholesterol and bile acids (BA) are important products of liver metabolism with numerous functions within the organism. Turnover of these substances requires precise regulation due to potential toxicities during their cumulation. In this study, we hypothesized that high sEng levels affect cholesterol homeostasis and BA turnover in mice liver.

MAIN METHODS

Nine-month-old transgenic male mice overexpressing human sEng and wild-type mice underwent plasma, bile, stool, and organ samples analysis by analytical, qRT-PCT and Western blot methods.

KEY FINDINGS

sEng mice demonstrated decreased plasma total and LDL cholesterol concentrations due to upregulation of hepatic Sr-b1 and Ldlr receptors, increased liver cholesterol content, and increased Abcg8-mediated cholesterol efflux into bile. sEng also increased conversion of cholesterol into bile acids (BA) via upregulation of Cyp7a1 and increased Mdr1 expression. Plasma concentrations of BA were increased in sEng mice due to their enhanced reabsorption via ileum. Increased hepatic disposition of BA led to their increased biliary excretion coupled with choleretic activity.

SIGNIFICANCE

For the first time, we have shown that high sEng plasma levels affect cholesterol and BA homeostasis on the basis of complex liver and intestinal effects. The significance of these findings for pathophysiology of diseases associated with increased sEng concentrations remains to be elucidated in prospective studies.

Iron overload reduces synthesis and elimination of bile acids in rat liver

Excessive iron accumulation in the liver, which accompanies certain genetic or metabolic diseases, impairs bile acids (BA) synthesis, but the influence of iron on the complex process of BA homeostasis is unknown. Thus, we evaluated the effect of iron overload (IO) on BA turnover in rats. Compared with control rats, IO (8 intraperitoneal doses of 100 mg/kg every other day) significantly decreased bile flow as a consequence of decreased biliary BA secretion. This decrease was associated with reduced expression of Cyp7a1, the rate limiting enzyme in the conversion of cholesterol to BA, and decreased expression of Bsep, the transporter responsible for BA efflux into bile. However, IO did not change net BA content in faeces in response to increased intestinal conversion of BA into hyodeoxycholic acid. In addition, IO increased plasma cholesterol concentrations, which corresponded with reduced Cyp7a1 expression and increased expression of Hmgcr, the rate-limiting enzyme in de novo cholesterol synthesis. In summary, this study describes the mechanisms impairing synthesis, biliary secretion and intestinal processing of BA during IO. Altered elimination pathways for BA and cholesterol may interfere with the pathophysiology of liver damage accompanying liver diseases with excessive iron deposition.

Regulation and role of endoglin in cholesterol-induced endothelial and vascular dysfunction in vivo and in vitro

Our objective was to investigate the effect of cholesterol [hypercholesterolemia and 7-ketocholesterol (7K)] on endoglin (Eng) expression and regulation with respect to endothelial or vascular dysfunction in vivo and in vitro. In vivo experiments were performed in 2-mo-old atherosclerosis-prone apolipoprotein E-deficient/LDL receptor-deficient (ApoE^-/-/LDLR^-/-) female mice and their wild-type C57BL/6J littermates. In in vitro experiments, human aortic endothelial cells (HAECs) were treated with 7K. ApoE^-/-/LDLR^-/- mice developed hypercholesterolemia accompanied by increased circulating levels of P-selectin and Eng and a disruption of NO metabolism. Functional analysis of the aorta demonstrated impaired vascular reactivity, and Western blot analysis revealed down-regulation of membrane Eng/Smad2/3/eNOS signaling in ApoE^-/-/LDLR^-/- mice. 7K increased Eng expression via Krüppel-like factor 6 (KLF6), liver X nuclear receptor, and NF-κB in HAECs. 7K-induced Eng expression was prevented by the treatment with 2-hydroxypropyl-β-cyclodextrin; 8-{[5-chloro-2-(4-methylpiperazin-1-yl) pyridine-4-carbonyl] amino}-1-(4-fluorophenyl)-4, 5-dihydrobenzo[g]indazole-3-carboxamide; or by KLF6 silencing. 7K induced increased adhesion and transmigration of monocytic human leukemia promonocytic cell line cells and was prevented by Eng silencing. We concluded that hypercholesterolemia altered Eng expression and signaling, followed by endothelial or vascular dysfunction before formation of atherosclerotic lesions in ApoE^-/-/LDLR^-/- mice. By contrast, 7K increased Eng expression and induced inflammation in HAECs, which was followed by an increased adhesion and transmigration of monocytes via endothelium, which was prevented by Eng inhibition. Thus, we propose a relevant role for Eng in endothelial or vascular dysfunction or inflammation when exposed to cholesterol.-Vicen, M., Vitverova, B., Havelek, R., Blazickova, K., Machacek, M., Rathouska, J., Najmanová, I., Dolezelova, E., Prasnicka, A., Sternak, M., Bernabeu, C., Nachtigal, P. Regulation and role of endoglin in cholesterol-induced endothelial and vascular dysfunction in vivo and in vitro.

Hepatic stellate cell-derived platelet-derived growth factor receptor-alpha-enriched extracellular vesicles promote liver fibrosis in mice through SHP2

Liver fibrosis is characterized by the activation and migration of hepatic stellate cells (HSCs), followed by matrix deposition. Recently, several studies have shown the importance of extracellular vesicles (EVs) derived from liver cells, such as hepatocytes and endothelial cells, in liver pathobiology. While most of the studies describe how liver cells modulate HSC behavior, an important gap exists in the understanding of HSC-derived signals and more specifically HSC-derived EVs in liver fibrosis. Here, we investigated the molecules released through HSC-derived EVs, the mechanism of their release, and the role of these EVs in fibrosis. Mass spectrometric analysis showed that platelet-derived growth factor (PDGF) receptor-alpha (PDGFRα) was enriched in EVs derived from PDGF-BB-treated HSCs. Moreover, patients with liver fibrosis had increased PDGFRα levels in serum EVs compared to healthy individuals. Mechanistically, in vitro tyrosine720-to-phenylalanine mutation on the PDGFRα sequence abolished enrichment of PDGFRα in EVs and redirected the receptor toward degradation. Congruently, the inhibition of Src homology 2 domain tyrosine phosphatase 2, the regulatory binding partner of phosphorylated tyrosine720, also inhibited PDGFRα enrichment in EVs. EVs derived from PDGFRα-overexpressing cells promoted in vitro HSC migration and in vivo liver fibrosis. Finally, administration of Src homology 2 domain tyrosine phosphatase 2inhibitor, SHP099, to carbon tetrachloride-administered mice inhibited PDGFRα enrichment in serum EVs and reduced liver fibrosis.

CONCLUSION

PDGFRα is enriched in EVs derived from PDGF-BB-treated HSCs in an Src homology 2 domain tyrosine phosphatase 2-dependent manner and these PDGFRα-enriched EVs participate in development of liver fibrosis. (Hepatology 2018;68:333-348).

Resveratrol modifies biliary secretion of cholephilic compounds in sham-operated and cholestatic rats

AIM

To investigate the effect of resveratrol on biliary secretion of cholephilic compounds in healthy and bile duct-obstructed rats.

METHODS

Resveratrol (RSV) or saline were administered to rats by daily oral gavage for 28 d after sham operation or reversible bile duct obstruction (BDO). Bile was collected 24 h after the last gavage during an intravenous bolus dose of the Mdr1/Mrp2 substrate azithromycin. Bile acids, glutathione and azithromycin were measured in bile to quantify their level of biliary secretion. Liver expression of enzymes and transporters relevant for bile production and biliary secretion of major bile constituents and drugs were analyzed at the mRNA and protein levels using qRT-PCR and Western blot analysis, respectively. The TR-FRET PXR Competitive Binding Assay kit was used to determine the agonism of RSV at the pregnane X receptor.

RESULTS

RSV increased bile flow in sham-operated rats due to increased biliary secretion of bile acids (BA) and glutathione. This effect was accompanied by the induction of the hepatic rate-limiting transporters for bile acids and glutathione, Bsep and Mrp2, respectively. RSV also induced Cyp7a1, an enzyme that is crucial for bile acid synthesis; Mrp4, a transporter important for BA secretion from hepatocytes to blood; and Mdr1, the major apical transporter for xenobiotics. The findings were supported by increased biliary secretion of azithromycin. The TR-FRET PXR competitive binding assay confirmed RSV as a weak agonist of the human nuclear receptor PXR, which is a transcriptional regulator of Mdr1/Mrp2. RSV demonstrated significant hepatoprotective properties against BDO-induced cirrhosis. RSV also reduced bile flow in BDO rats without any corresponding change in the levels of the transporters and enzymes involved in RSV-mediated hepatoprotection.

CONCLUSION

Resveratrol administration for 28 d has a distinct effect on bile flow and biliary secretion of cholephilic compounds in healthy and bile duct-obstructed rats.

Iron depletion induces hepatic secretion of biliary lipids and glutathione in rats

Iron depletion (ID) has been shown to induce the liver expression of Cyp7a1, the rate-limiting enzyme initiating conversion of cholesterol to bile acids (BA), although the effect on bile acids metabolism and bile production is unknown. Therefore, we investigated changes in bile secretion and BA synthesis during diet-induced iron depletion (ID) in rats. ID increased bile flow along with augmented biliary excretion of bile acids, glutathione, cholesterol and phospholipids. Accordingly, we found transcriptional upregulation of the Cyp7a1, Cyp8b1, and Cyp27a1 BA synthetic enzymes, as well as induction of the Abcg5/8 cholesterol transporters in ID rat livers. In contrast, intravenous infusion of ³H-taurocholate failed to elicit any difference in biliary secretion of this compound in the ID rats. This corresponded with unchanged expression of canalicular rate-limiting transporters for BA as well as glutathione. We also observed that ID substantially changed the spectrum of BA in bile and decreased plasma concentrations of BA and cholesterol. Experiments with differentiated human hepatic HepaRG cells confirmed human CYP7A1 orthologue upregulation resulting from reduced iron concentrations. Results employing a luciferase reporter gene assay suggest that the transcriptional activation of the CYP7A1 promoter under ID conditions works independent of farnesoid X (FXR), pregnane X (PXR) and liver X (LXRα) receptors activation. It can be concluded that this study characterizes the molecular mechanisms of modified bile production as well as cholesterol as along with BA homeostasis during ID. We propose complex upregulation of BA synthesis, and biliary cholesterol secretion as the key factors affected by ID.