Bile acids inhibit duodenal secretin expression via orphan nuclear receptor small heterodimer partner (SHP)

Knockout of secretin ameliorates biliary and liver phenotypes during alcohol-induced hepatotoxicity

BACKGROUND

Alcohol-related liver disease (ALD) is characterized by ductular reaction (DR), liver inflammation, steatosis, fibrosis, and cirrhosis. The secretin (Sct)/secretin receptor (SR) axis (expressed only by cholangiocytes) regulates liver phenotypes in cholestasis. We evaluated the role of Sct signaling on ALD phenotypes.

METHODS

We used male wild-type and Sct^-/- mice fed a control diet (CD) or ethanol (EtOH) for 8 wk. Changes in liver phenotypes were measured in mice, female/male healthy controls, and patients with alcoholic cirrhosis. Since Cyp4a10 and Cyp4a11/22 regulate EtOH liver metabolism, we measured their expression in mouse/human liver. We evaluated: (i) the immunoreactivity of the lipogenesis enzyme elongation of very-long-chain fatty acids 1 (Elovl, mainly expressed by hepatocytes) in mouse/human liver sections by immunostaining; (ii) the expression of miR-125b (that is downregulated in cholestasis by Sct) in mouse liver by qPCR; and (iii) total bile acid (BA) levels in mouse liver by enzymatic assay, and the mRNA expression of genes regulating BA synthesis (cholesterol 7a-hydroxylase, Cyp27a1, 12a-hydroxylase, Cyp8b1, and oxysterol 7a-hydroxylase, Cyp7b11) and transport (bile salt export pump, Bsep, Na⁺-taurocholate cotransporting polypeptide, NTCP, and the organic solute transporter alpha (OSTa) in mouse liver by qPCR.

RESULTS

In EtOH-fed WT mice there was increased biliary and liver damage compared to control mice, but decreased miR-125b expression, phenotypes that were blunted in EtOH-fed Sct^-/- mice. The expression of Cyp4a10 increased in cholangiocytes and hepatocytes from EtOH-fed WT compared to control mice but decreased in EtOH-fed Sct^-/- mice. There was increased immunoreactivity of Cyp4a11/22 in patients with alcoholic cirrhosis compared to controls. The expression of miR-125b decreased in EtOH-fed WT mice but returned at normal values in EtOH-fed Sct^-/- mice. Elovl1 immunoreactivity increased in patients with alcoholic cirrhosis compared to controls. There was no difference in BA levels between WT mice fed CD or EtOH; BA levels decreased in EtOH-fed Sct^-/- compared to EtOH-fed WT mice. There was increased expression of Cyp27a1, Cyp8b1, Cyp7b1, Bsep, NTCP and Osta in total liver from EtOH-fed WT compared to control mice, which decreased in EtOH-fed Sct^-/- compared to EtOH-fed WT mice.

CONCLUSIONS

Targeting Sct/SR signaling may be important for modulating ALD phenotypes.

Metabolic adaptation of adherent-invasive Escherichia coli to exposure to bile salts

The adherent-invasive Escherichia coli (AIEC), which colonize the ileal mucosa of Crohn’s disease patients, adhere to intestinal epithelial cells, invade them and exacerbate intestinal inflammation. The high nutrient competition between the commensal microbiota and AIEC pathobiont requires the latter to occupy their own metabolic niches to survive and proliferate within the gut. In this study, a global RNA sequencing of AIEC strain LF82 has been used to observe the impact of bile salts on the expression of metabolic genes. The results showed a global up-regulation of genes involved in degradation and a down-regulation of those implicated in biosynthesis. The main up-regulated degradation pathways were ethanolamine, 1,2-propanediol and citrate utilization, as well as the methyl-citrate pathway. Our study reveals that ethanolamine utilization bestows a competitive advantage of AIEC strains that are metabolically capable of its degradation in the presence of bile salts. We observed that bile salts activated secondary metabolism pathways that communicate to provide an energy benefit to AIEC. Bile salts may be used by AIEC as an environmental signal to promote their colonization.

The therapeutic landscape of non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is characterized by lobular inflammation and hepatocellular ballooning, and may be associated with liver fibrosis leading to cirrhosis and its complications. A pharmacological approach is necessary to treat NASH because of failure to change dietary habits and lifestyle in most patients. Insulin resistance with an increased release of free fatty acids, oxidative stress and activation of inflammatory cytokines seem to be key features for disease progression. Thiazolidinediones, such as pioglitazone and antioxidant agents, such as vitamin E, were the first pharmacological options to be evaluated for NASH. In recent years, several new molecules that target different pathways related to NASH pathogenesis, such as liver metabolic homeostasis, inflammation, oxidative stress and fibrosis, have been developed. Obeticholic acid (INT-747) and elafibranor (GFT-505) have provided promising results in phase IIb, randomized, placebo-controlled clinical trials and they are being evaluated in ongoing phase III studies. Most of the potential treatments for NASH are under investigation in phase II studies, with some at phase I. This diversity in possible treatments calls for a better understanding of NASH in order to enrich trial populations with patients more susceptible to progress and to respond. This manuscript aims to review the pharmacological NASH treatment landscape.

The secretin/secretin receptor axis modulates liver fibrosis through changes in transforming growth factor-β1 biliary secretion in mice

The secretin/secretin receptor (SR) axis is up-regulated by proliferating cholangiocytes during cholestasis. Secretin stimulates biliary proliferation by down-regulation of let-7a and subsequent up-regulation of the growth-promoting factor, nerve growth factor (NGF). It is not known whether the secretin/SR axis plays a role in subepithelial fibrosis observed during cholestasis. Our aim was to determine the role of the secretin/SR axis in activation of biliary fibrosis in animal models and human primary sclerosing cholangitis (PSC). Studies were performed in wild-type (WT) mice with bile duct ligation (BDL), BDL SR(-/-) mice, or Mdr2(-/-) mouse models of cholestatic liver injury. In selected studies, the SR antagonist (Sec 5-27) was used to block the secretin/SR axis. Biliary proliferation and fibrosis were evaluated as well as secretion of secretin (by cholangiocytes and S cells), expression of markers of fibrosis, transforming growth factor-β1 (TGF-β1), transforming growth factor-β1 receptor (TGF-β1R), let-7a, and downstream expression of NGF. Correlative studies were performed in human control and PSC liver tissue biopsies, serum, and bile. SR antagonist reduced biliary proliferation and hepatic fibrosis in BDL WT and Mdr2(-/-) mice. There was decreased expression of let-7a in BDL and Mdr2(-/-) cholangiocytes that was associated with increased NGF expression. Inhibition of let-7a accelerated liver fibrosis was attributed to cholestasis. There was increased expression of TGF-β1 and TGF-β1R. Significantly higher expression of secretin, SR, and TGF-β1 was observed in PSC patient liver samples compared to healthy controls. In addition, there was higher expression of fibrosis genes and remarkably decreased expression of let-7a and increased expression of NGF compared to the control.

CONCLUSION

The secretin/SR axis plays a key role in regulating the biliary contribution to cholestasis-induced hepatic fibrosis. (Hepatology 2016;64:865-879).

The estrogen-related receptor alpha upregulates secretin expressions in response to hypertonicity and angiotensin II stimulation

Osmoregulation via maintenance of water and salt homeostasis is a vital process. In the brain, a functional secretin (SCT) and secretin receptor (SCTR) axis has recently been shown to mediate central actions of angiotensin II (ANGII), including initiation of water intake and stimulation of vasopressin (VP) expression and release. In this report, we provide evidence that estrogen-related receptor α (ERRα, NR3B1), a transcription factor mainly involved in metabolism, acts as an upstream activator of the SCT gene. In vitro studies using mouse hypothalamic cell line N-42 show that ERRα upregulates SCT promoter and gene expression. More importantly, knockdown of endogenous ERRα abolishes SCT promoter activation in response to hypertonic and ANGII stimulations. In mouse brain, ERRα coexpresses with SCT in various osmoregulatory brain regions, including the lamina terminalis and the paraventricular nucleus of the hypothalamus, and its expression is induced by hyperosmotic and ANGII treatments. Based on our data, we propose that both the upregulation of ERRα and/or the increased binding of ERRα to the mouse SCT promoter are two possible mechanisms for the elevated SCT expression upon hyperosmolality and central ANGII stimulation.

The orphan nuclear receptor SHP acts as a negative regulator in inflammatory signaling triggered by Toll-like receptors

The orphan nuclear receptor SHP (small heterodimer partner) is a transcriptional corepressor that regulates hepatic metabolic pathways. Here we identified a role for SHP as an intrinsic negative regulator of Toll-like receptor (TLR)-triggered inflammatory responses. SHP-deficient mice were more susceptible to endotoxin-induced sepsis. SHP had dual regulatory functions in a canonical transcription factor NF-κB signaling pathway, acting as both a repressor of transactivation of the NF-κB subunit p65 and an inhibitor of polyubiquitination of the adaptor TRAF6. SHP-mediated inhibition of signaling via the TLR was mimicked by macrophage-stimulating protein (MSP), a strong inducer of SHP expression, via an AMP-activated protein kinase-dependent signaling pathway. Our data identify a previously unrecognized role for SHP in the regulation of TLR signaling.

Protective effect of bile acids on the onset of fructose-induced hepatic steatosis in mice

Fructose intake is being discussed as a key dietary factor in the development of nonalcoholic fatty liver disease (NAFLD). Bile acids have been shown to modulate energy metabolism. We tested the effects of bile acids on fructose-induced hepatic steatosis. In C57BL/6J mice treated with a combination of chenodeoxycholic acid and cholic acid (100 mg/kg body weight each) while drinking water or a 30% fructose solution for eight weeks and appropriate controls, markers of hepatic steatosis, portal endotoxin levels, and markers of hepatic lipogenesis were determined. In mice concomitantly treated with bile acids, the onset of fructose-induced hepatic steatosis was markedly attenuated compared to mice only fed fructose. The protective effects of the bile acid treatment were associated with a downregulation of tumor necrosis factor (TNF)α, sterol regulatory element-binding protein (SREBP)1, FAS mRNA expression, and lipid peroxidation in the liver, whereas hepatic farnesoid X receptor (FXR) or short heterodimer partner (SHP) protein concentration did not differ between groups fed fructose. Rather, bile acid treatment normalized occludin protein concentration in the duodenum, portal endotoxin levels, and markers of Kupffer cell activation to the level of water controls. Taken together, these data suggest that bile acids prevent fructose-induced hepatic steatosis in mice through mechanisms involving protection against the fructose-induced translocation of intestinal bacterial endotoxin.

Bile salts and cholestasis

Bile salts have a crucial role in hepatobiliary and intestinal homeostasis and digestion. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Bile salts are efficiently reabsorbed from the intestinal lumen to undergo enterohepatic circulation. In addition to their function as a surfactant involved in the absorption of dietary lipids and fat-soluble vitamins bile salts are potent signaling molecules in both the liver and intestine. Under physiological conditions the bile salt pool is tightly regulated, but the adaptive capacity may fall short under cholestatic conditions. Elevated serum and tissue levels of potentially toxic hydrophobic bile salts during cholestasis may cause mitochondrial damage, apoptosis or necrosis in susceptible cell types. Therapeutic nontoxic bile salts may restore impaired hepatobiliary secretion in cholestatic disorders. The hydrophilic bile salt ursodeoxycholate is today regarded as the effective standard treatment of primary biliary cirrhosis and intrahepatic cholestasis of pregnancy, and is implicated for use in various other cholestatic conditions. Novel therapeutic bile salts that are currently under evaluation may also prove valuable in the treatment of these diseases.