Hu antigen R and tristetraprolin: counter-regulators of rat apical sodium-dependent bile acid transporter by way of effects on messenger RNA stability

Metabolic immaturity and breastmilk bile acid metabolites are central determinants of heightened newborn vulnerability to norovirus diarrhea

The pathogenic outcome of enteric virus infections is governed by a complex interplay between the virus, intestinal microbiota, and host immune factors, with metabolites serving as a key mediator. Noroviruses bind bile acid metabolites, which are produced by the host and then modified by commensal bacteria. Paradoxically, bile acids can have both proviral and antiviral roles during norovirus infections. Working in an infant mouse model of norovirus infection, we demonstrate that microbiota and their bile acid metabolites protect from norovirus diarrhea, whereas host bile acids promote disease. We also find that maternal bile acid metabolism determines the susceptibility of newborn mice to norovirus diarrhea during breastfeeding. Finally, targeting maternal and neonatal bile acid metabolism can protect newborn mice from norovirus disease. In summary, neonatal metabolic immaturity and breastmilk bile acids are central determinants of heightened newborn vulnerability to norovirus disease.

Metabolic immaturity of newborns and breast milk bile acid metabolites are the central determinants of heightened neonatal vulnerability to norovirus diarrhea

Noroviruses are the leading global cause of acute gastroenteritis, responsible for 685 million annual cases. While all age groups are susceptible to noroviruses, children are vulnerable to more severe infections than adults, underscored by 200 million pediatric cases and up to 200,000 deaths in children annually. Understanding the basis for the increased vulnerability of young hosts is critical to developing effective treatments. The pathogenic outcome of any enteric virus infection is governed by a complex interplay between the virus, intestinal microbiota, and host immune factors. A central mediator in these complex relationships are host- and microbiota-derived metabolites. Noroviruses bind a specific class of metabolites, bile acids, which are produced by the host and then modified by commensal bacterial enzymes. Paradoxically, bile acids can have both proviral and antiviral roles during norovirus infections. Considering these opposing effects, the microbiota-regulated balance of the bile acid pool may be a key determinant of the pathogenic outcome of a norovirus infection. The bile acid pool in newborns is unique due to immaturity of host metabolic pathways and developing gut microbiota, which could underlie the vulnerability of these hosts to severe norovirus infections. Supporting this concept, we demonstrate herein that microbiota and their bile acid metabolites protect from severe norovirus diarrhea whereas host-derived bile acids promote disease. Remarkably, we also report that maternal bile acid metabolism determines neonatal susceptibility to norovirus diarrhea during breastfeeding by delivering proviral bile acids to the newborn. Finally, directed targeting of maternal and neonatal bile acid metabolism can protect the neonatal host from norovirus disease. Altogether, these data support the conclusion that metabolic immaturity in newborns and ingestion of proviral maternal metabolites in breast milk are the central determinants of heightened neonatal vulnerability to norovirus disease.

Intestinal Absorption of Bile Acids in Health and Disease

The intestinal reclamation of bile acids is crucial for the maintenance of their enterohepatic circulation. The majority of bile acids are actively absorbed via specific transport proteins that are highly expressed in the distal ileum. The uptake of bile acids by intestinal epithelial cells modulates the activation of cytosolic and membrane receptors such as the farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (GPBAR1), which has a profound effect on hepatic synthesis of bile acids as well as glucose and lipid metabolism. Extensive research has focused on delineating the processes of bile acid absorption and determining the contribution of dysregulated ileal signaling in the development of intestinal and hepatic disorders. For example, a decrease in the levels of the bile acid-induced ileal hormone FGF15/19 is implicated in bile acid-induced diarrhea (BAD). Conversely, the increase in bile acid absorption with subsequent overload of bile acids could be involved in the pathophysiology of liver and metabolic disorders such as fatty liver diseases and type 2 diabetes mellitus. This review article will attempt to provide a comprehensive overview of the mechanisms involved in the intestinal handling of bile acids, the pathological implications of disrupted intestinal bile acid homeostasis, and the potential therapeutic targets for the treatment of bile acid-related disorders. Published 2020. Compr Physiol 10:21-56, 2020.

RNA binding protein, tristetraprolin in a murine model of recurrent pregnancy loss

Recurrent pregnancy loss is a major reproductive pathology affecting 1-5% of pregnant women worldwide. A distinct feature of this reproductive pathology is involvement of key inflammatory cytokines and transcription factors such as tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6) and nuclear factor kappa beta (NF-κB). Special classes of RNA-binding proteins regulate the transcripts of many of these important cytokines and regulatory factors via binding to the 3' untranslated regions (UTRs) and/or poly(A) tail and destabilizing/stabilizing the transcript. The tristetraprolin (TTP/ZFP36) family have been found to be potent destabilizers of the aforementioned inflammatory and cellular response cytokines. The aim of this study was to evaluate whether tristetraprolin is expressed in the placenta and involved in modulating inflammation in mouse model of lipopolysaccharide (LPS)-induced fetal loss. In this study, Swiss-albino mice were injected with LPS at gestational day 15.5 and placental tissues were harvested 6 hours post-LPS injection. Histopathology and immunohistochemistry analyses clearly revealed cellular stress and death in LPS treated placentas compared to controls. TTP protein was downregulated, while targets TNF-α and IL-6 were upregulated in LPS group compared to controls. We observed increased TTP nuclear immunolocalization corresponding with higher NF-κB nuclear localization in trophoblasts from LPS treated placentas. Our results suggest that RNA-binding proteins such as TTP are expressed and perhaps involved in the modulation of inflammation-induced pregnancy pathologies.

Organic Solute Transporter α-β Protects Ileal Enterocytes From Bile Acid-Induced Injury

BACKGROUND & AIMS

Ileal bile acid absorption is mediated by uptake via the apical sodium-dependent bile acid transporter (ASBT), and export via the basolateral heteromeric organic solute transporter α-β (OSTα-OSTβ). In this study, we investigated the cytotoxic effects of enterocyte bile acid stasis in Ostα-/- mice, including the temporal relationship between intestinal injury and initiation of the enterohepatic circulation of bile acids.

METHODS

Ileal tissue morphometry, histology, markers of cell proliferation, gene, and protein expression were analyzed in male and female wild-type and Ostα-/- mice at postnatal days 5, 10, 15, 20, and 30. Ostα-/-Asbt-/- mice were generated and analyzed. Bile acid activation of intestinal Nrf2-activated pathways was investigated in Drosophila.

RESULTS

As early as day 5, Ostα-/- mice showed significantly increased ileal weight per length, decreased villus height, and increased epithelial cell proliferation. This correlated with premature expression of the Asbt and induction of bile acid-activated farnesoid X receptor target genes in neonatal Ostα-/- mice. Expression of reduced nicotinamide adenine dinucleotide phosphate oxidase-1 and Nrf2-anti-oxidant responsive genes were increased significantly in neonatal Ostα-/- mice at these postnatal time points. Bile acids also activated Nrf2 in Drosophila enterocytes and enterocyte-specific knockdown of Nrf2 increased sensitivity of flies to bile acid-induced toxicity. Inactivation of the Asbt prevented the changes in ileal morphology and induction of anti-oxidant response genes in Ostα-/- mice.

CONCLUSIONS

Early in postnatal development, loss of Ostα leads to bile acid accumulation, oxidative stress, and a restitution response in ileum. In addition to its essential role in maintaining bile acid homeostasis, Ostα-Ostβ functions to protect the ileal epithelium against bile acid-induced injury. NCBI Gene Expression Omnibus: GSE99579.

Emerging role of HuR in inflammatory response in kidney diseases

Human antigen R (HuR) is a member of the embryonic lethal abnormal vision (ELAV) family which can bind to the A/U rich elements in 3' un-translated region of mRNA and regulate mRNA splicing, transportation, and stability. Unlike other members of the ELAV family, HuR is ubiquitously expressed. Early studies mainly focused on HuR function in malignant diseases. As researches proceed, more and more proofs demonstrate its relationship with inflammation. Since most kidney diseases involve pathological changes of inflammation, HuR is now suggested to play a pivotal role in glomerular nephropathy, tubular ischemia-reperfusion damage, renal fibrosis and even renal tumors. By regulating the mRNAs of target genes, HuR is causally linked to the onset and progression of kidney diseases. Reports on this topic are steadily increasing, however, the detailed function and mechanism of action of HuR are still not well understood. The aim of this review article is to summarize the present understanding of the role of HuR in inflammation in kidney diseases, and we anticipate that future research will ultimately elucidate the therapeutic value of this novel target.

A single element in the 3'UTR of the apical sodium-dependent bile acid transporter controls both stabilization and destabilization of mRNA

mRNA stability appears to play a key role in the ontogenic regulation of the apical sodium-dependent bile acid transporter (ASBT). The RNA-binding proteins Hu antigen R (HuR) and tristetraprolin (TTP) stabilize and destabilize ASBT mRNA, respectively. Potential HuR-binding sites were assessed by sequence analysis in the context of prior in vitro functional analyses of the rat ASBT 3'UTR. Wild-type and mutant-binding sites were investigated by gel-shift analysis using IEC-6 cell extracts. The functional consequences of binding site mutations were assessed using two different hybrid reporter constructs linking the 3'UTR element to either a luciferase or a β-globin coding mRNA sequence. A specific metastasis-associated gene 1 (MTA1) cis-element was identified in the ASBT 3'UTR that became associated with proteins in IEC-6 cell extracts and could be supershifted by anti-HuR or anti-TTP antibodies. Mutation of this cis-element abrogated the gel shift of IEC-6 proteins. Furthermore, hybrid constructs containing a mutant MTA1 element had reduced responses to modulation of HuR or TTP. For the first time, we have identified a single specific sequence element in the 3'UTR of the rat ASBT mRNA that mediates counter-regulatory changes in mRNA abundance in response to both HuR and TTP.

Intestinal epithelial HuR modulates distinct pathways of proliferation and apoptosis and attenuates small intestinal and colonic tumor development

HuR is a ubiquitous nucleocytoplasmic RNA-binding protein that exerts pleiotropic effects on cell growth and tumorigenesis. In this study, we explored the impact of conditional, tissue-specific genetic deletion of HuR on intestinal growth and tumorigenesis in mice. Mice lacking intestinal expression of HuR (Hur (IKO) mice) displayed reduced levels of cell proliferation in the small intestine and increased sensitivity to doxorubicin-induced acute intestinal injury, as evidenced by decreased villus height and a compensatory shift in proliferating cells. In the context of Apc(min/+) mice, a transgenic model of intestinal tumorigenesis, intestinal deletion of the HuR gene caused a three-fold decrease in tumor burden characterized by reduced proliferation, increased apoptosis, and decreased expression of transcripts encoding antiapoptotic HuR target RNAs. Similarly, Hur(IKO) mice subjected to an inflammatory colon carcinogenesis protocol [azoxymethane and dextran sodium sulfate (AOM-DSS) administration] exhibited a two-fold decrease in tumor burden. Hur(IKO) mice showed no change in ileal Asbt expression, fecal bile acid excretion, or enterohepatic pool size that might explain the phenotype. Moreover, none of the HuR targets identified in Apc(min/+)Hur(IKO) were altered in AOM-DSS-treated Hur(IKO) mice, the latter of which exhibited increased apoptosis of colonic epithelial cells, where elevation of a unique set of HuR-targeted proapoptotic factors was documented. Taken together, our results promote the concept of epithelial HuR as a contextual modifier of proapoptotic gene expression in intestinal cancers, acting independently of bile acid metabolism to promote cancer. In the small intestine, epithelial HuR promotes expression of prosurvival transcripts that support Wnt-dependent tumorigenesis, whereas in the large intestine epithelial HuR indirectly downregulates certain proapoptotic RNAs to attenuate colitis-associated cancer. Cancer Res; 74(18); 5322-35. ©2014 AACR.

c-Fos mediates repression of the apical sodium-dependent bile acid transporter by fibroblast growth factor-19 in mice

Fibroblast growth factor-19 (FGF-19), a bile acid-responsive enterokine, is secreted by the ileum and regulates a variety of metabolic processes. These studies examined the signal transduction pathways operant in FGF-19-mediated repression of the apical sodium-dependent bile acid transporter (ASBT). Responses to FGF-19 were assessed in Caco-2 and CT-26 cells and in mice where c-fos was conditionally silenced in the intestine by a cre-lox strategy. FGF-19 treatment of Caco-2 cells or wild-type mice led to a significant reduction in ASBT protein expression and enhanced phosphorylation of extracellular signaling kinase 1/2 (ERK1/2), c-Fos, and c-Jun. FGF-19 treatment of Caco-2 cells led to a reduction in activity of the human ASBT promoter and this repression could be blocked by treatment with a mitogen-activated protein kinase/ERK kinase (MEK1/2) inhibitor or by silencing jun kinase 1, jun kinase 2, c-fos, or c-jun. Site directed mutagenesis of a c-fos binding element in the ASBT promoter blocked FGF-19-mediated repression in luciferase reporter constructs. ASBT promoter activity was repressed by FGF-19 in CT-26 cells and this repression could be reduced by MEK1/2 inhibition or silencing c-fos. FGF-19-mediated repression of ASBT protein expression was abrogated in mice where c-fos was conditionally silenced in the intestine. In contrast, ASBT was repressed in the c-Fos expressing gallbladders of the same mice. The studies demonstrate that FGF-19 represses the expression of ASBT in the ileum and gallbladder via a signal transduction pathway involving MEK1/2, ERK1/2, JNK1, JNK2, and c-Fos.

HuR and other turnover- and translation-regulatory RNA-binding proteins: implications for the kidney

The posttranscriptional regulation of gene expression occurs through cis RNA regulatory elements by the action of trans factors, which are represented by noncoding RNAs (especially microRNAs) and turnover- and translation-regulatory (TTR) RNA-binding proteins (RBPs). These multifactorial proteins are a group of heterogeneous RBPs primarily implicated in controlling the decay and translation rates of target mRNAs. TTR-RBPs usually shuttle between cellular compartments (the nucleus and cytoplasm) in response to various stimuli and undergo posttranslational modifications such as phosphorylation or methylation to ensure their proper subcellular localization and function. TTR-RBPs are emerging as key regulators of a wide variety of genes influencing kidney physiology and pathology. This review summarizes the current knowledge of TTR-RBPs that influence renal metabolism. We will discuss the role of TTR-RBPs as regulators of kidney ischemia, fibrosis and matrix remodeling, angiogenesis, membrane transport, immunity, vascular tone, hypertension, and acid-base balance as well as anemia, bone mineral disease, and vascular calcification.

The solute carrier family 10 (SLC10): beyond bile acid transport

The solute carrier (SLC) family 10 (SLC10) comprises influx transporters of bile acids, steroidal hormones, various drugs, and several other substrates. Because the seminal transporters of this family, namely, sodium/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2), were primarily bile acid transporters, the term "sodium bile salt cotransporting family" was used for the SLC10 family. However, this notion became obsolete with the finding of other SLC10 members that do not transport bile acids. For example, the sodium-dependent organic anion transporter (SOAT; SLC10A6) transports primarily sulfated steroids. Moreover, NTCP was shown to also transport steroids and xenobiotics, including HMG-CoA inhibitors (statins). The SLC10 family contains four additional members, namely, P3 (SLC10A3; SLC10A3), P4 (SLC10A4; SLC10A4), P5 (SLC10A5; SLC10A5) and SLC10A7 (SLC10A7), several of which were unknown or considered hypothetical until approximately a decade ago. While their substrate specificity remains undetermined, great progress has been made towards their characterization in recent years. Explicitly, SLC10A4 may participate in vesicular storage or exocytosis of neurotransmitters or mastocyte mediators, whereas SLC10A5 and SLC10A7 may be involved in solute transport and SLC10A3 may have a role as a housekeeping protein. Finally, the newly found role of bile acids in glucose and energy homeostasis, via the TGR5 receptor, sheds new light on the clinical relevance of ASBT and NTCP. The present mini-review provides a brief summary of recent progress on members of the SLC10 family.

Tristetraprolin (TTP): interactions with mRNA and proteins, and current thoughts on mechanisms of action

Changes in mRNA stability and translation are critical control points in the regulation of gene expression, particularly genes encoding growth factors, inflammatory mediators, and proto-oncogenes. Adenosine and uridine (AU)-rich elements (ARE), often located in the 3' untranslated regions (3'UTR) of mRNAs, are known to target transcripts for rapid decay. They are also involved in the regulation of mRNA stability and translation in response to extracellular cues. This review focuses on one of the best characterized ARE binding proteins, tristetraprolin (TTP), the founding member of a small family of CCCH tandem zinc finger proteins. In this survey, we have reviewed the current status of TTP interactions with mRNA and proteins, and discussed current thinking about TTP's mechanism of action to promote mRNA decay. We also review the proposed regulation of TTP's functions by phosphorylation. Finally, we have discussed emerging evidence for TTP operating as a translational regulator. This article is part of a Special Issue entitled: RNA Decay mechanisms.

A role for RNA post-transcriptional regulation in satellite cell activation

Background

Satellite cells are resident skeletal muscle stem cells responsible for muscle maintenance and repair. In resting muscle, satellite cells are maintained in a quiescent state. Satellite cell activation induces the myogenic commitment factor, MyoD, and cell cycle entry to facilitate transition to a population of proliferating myoblasts that eventually exit the cycle and regenerate muscle tissue. The molecular mechanism involved in the transition of a quiescent satellite cell to a transit-amplifying myoblast is poorly understood.

Methods

Satellite cells isolated by FACS from uninjured skeletal muscle and 12 h post-muscle injury from wild type and Syndecan-4 null mice were probed using Affymetrix 430v2 gene chips and analyzed by Spotfire^tm and Ingenuity Pathway analysis to identify gene expression changes and networks associated with satellite cell activation, respectively. Additional analyses of target genes identify miRNAs exhibiting dynamic changes in expression during satellite cell activation. The function of the miRNAs was assessed using miRIDIAN hairpin inhibitors.

Results

An unbiased gene expression screen identified over 4,000 genes differentially expressed in satellite cells in vivo within 12 h following muscle damage and more than 50% of these decrease dramatically. RNA binding proteins and genes involved in post-transcriptional regulation were significantly over-represented whereas splicing factors were preferentially downregulated and mRNA stability genes preferentially upregulated. Furthermore, six computationally identified miRNAs demonstrated novel expression through muscle regeneration and in satellite cells. Three of the six miRNAs were found to regulate satellite cell fate.

Conclusions

The quiescent satellite cell is actively maintained in a state poised to activate in response to external signals. Satellite cell activation appears to be regulated by post-transcriptional gene regulation.

The biliary HCO(3)(-) umbrella: experimental evidence revisited

PURPOSE OF REVIEW

To critically review most recent experimental evidence for the protective action of biliary HCO(3)(-) secretion against bile acid-induced bile duct damage and development of fibrosing cholangiopathy in humans and experimental animals.

RECENT FINDINGS

Studies in human cholangiocytes in vitro indicate that a biliary HCO(3)(-) umbrella protects against bile acid-induced cholangiocyte damage and apoptosis in humans. The Cl(-)/HCO(3)(-) exchanger, AE2, and an intact biliary glycocalyx appear crucial for its stability. Related studies with experimental animal models in vivo have to be interpreted with caution as humans and mice differ not only with regard to bile salt pool, but also their expression patterns of transport proteins and signalling molecules.

SUMMARY

Adequate biliary HCO(3)(-) secretion may protect against bile salt-induced cholangiopathies. Future therapeutic strategies in biliary diseases will aim at stabilizing the biliary HCO(3)(-) umbrella.