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Abstract

Abnormalities in ion channels, such as voltage-gated sodium channels, that are directly involved in gastrointestinal motility and visceral pain may be a genetic factor in the pathogenesis of irritable bowel syndrome (IBS). One of these, the tetrodotoxin-resistant sodium channel NaV1.5 (encoded by SCN5A), is expressed in human smooth muscle cells and the interstitial cells of Cajal, the gastrointestinal pacemakers that provide an electrical stimulus for contraction. Inhibition of NaV1.5, which is also expressed in human myocardium, by the anti-anginal agent, ranolazine, is associated with constipation and patients with arrhythmia-predisposing mutations in SCN5A are more likely to have gastrointestinal symptoms compared with patients with other arrhythmia-associated ion channel defects. Recently, a SCN5A mutation was identified in a patient with IBS, but no cardiac conduction abnormalities. Based on these findings, in this issue of Gastroenterology, Beyder et al screened 584 patients with IBS to determine the prevalence of SCN5A polymorphisms and mutations in IBS and their functional implications. Missense mutations in SCN5A were found in 2.2% (13/584) of patients and, even though diarrhea-predominant IBS was more prevalent in their cohort, a greater percentage of patients with SCN5A mutations had constipation-predominant IBS. This subset of patients had no cardiac electrophysiologic abnormalities. Nine distinct SCN5A polymorphisms were detected in an additional 2.9% of patients. The association of SCN5A polymorphisms with IBS was confirmed in a genome-wide association study of an additional 1,745 patients from 4 different countries. Electrophysiologic studies performed in a cell line transfected with NaV1.5 mutants demonstrated reduced NaV1.5 function with 9 of 13 mutants, particularly in loss of activation, with the A997T mutant exhibiting the greatest reduction in function. Mexiletine, which is used in the treatment of patients with cardiac arrhythmias associated with NaV1.5 defects, was found to restore function in A997T-transfected cells. This in vitro experiment led to a trial of mexiletine treatment in a patient with constipation-predominant IBS and the NaV1.5-A997T mutation. After 5 days of treatment, frequency of bowel movements increased to normal range and persisted for 5 weeks (Figure 1). These novel findings suggest that a subset of patients with IBS may harbor mutations in SCN5A that result in an intestinal ion channelopathy, which may represent a potential target for new treatment options in these patients. See page 1659. The use of noncardioselective β-blockers (BBs), such as propranolol and carvedilol, is recommended as primary prophylaxis against variceal hemorrhage in patients with cirrhosis and esophageal varices because of their β1 and β2 adrenoreceptor inhibitory effects on cardiac output and splanchnic vasodilatation, respectively. In addition, BBs have been shown to reduce the risk of developing ascites, refractory ascites, spontaneous bacterial peritonitis (SBP), and the hepatorenal syndrome. This has led to the widespread use of BBs in patients with cirrhosis. However, patients with advanced cirrhosis and refractory ascites treated with BBs may have a worse outcome than untreated patients, suggesting that there is a therapeutic window in the natural history of cirrhosis beyond which the use of BBs may be contraindicated. Bacterial infections such as SBP also typically occur in patients with advanced cirrhosis and are associated with significant hemodynamic changes. To address the question of whether the development of SBP represents another condition in which BBs may have deleterious effects in patients with advanced cirrhosis, Mandorfer et al in this issue of Gastroenterology (accompanied by an editorial), have performed a retrospective study of 607 consecutive patients (245 on BBs and 362 not on BBs) with cirrhosis and ascites followed, after their first paracentesis, for 660 person-years. Treatment with BBs had no effect on the incidence of SBP; however, up to the development of SBP, which occurred in 182 patients, treatment with BBs increased transplant-free survival and was associated with a 25% reduction in mortality risk and a reduction in non-elective hospitalization (Figure 2). However, once a patient developed SBP, treatment with BBs was associated with a 58% increase in mortality risk and an increase in non-elective hospitalization. Similarly, the rate of the development of acute kidney injury and hepatorenal syndrome was higher in patients on BB treatment after their first diagnosis of SBP. Furthermore, at the time of the initial diagnosis of SBP, more patients on BBs had compromised hemodynamics (systolic arterial pressure, <100 mmHg; mean arterial pressure, <82 mmHg) compared with patients not taking BBs. These compelling, albeit retrospective, findings suggest that BBs should be discontinued in patients with cirrhosis and ascites complicated by SBP. See page 1680; editorial on page 1597. Noncoding RNA (ncRNA) describes RNA that does not code for protein, and is involved in a very wide spectrum of cellular functions. These include ribosomal RNA, transfer RNA, and ribonucleoproteins. Many ncRNAs also function in gene regulation. For example, microRNAs are small ncRNAs of 22 nucleotides that regulate gene expression at the transcriptional and post-transcriptional levels. A single microRNA species is able to regulate the expression of hundreds of different genes. Long intergenic ncRNAs (lincRNA) represent RNA species >200 nucleotides that are derived from noncoding DNA sequences situated in between protein-coding genes. lincRNA may regulate gene expression by a variety of means that include blocking protein production by binding mRNA or binding to chromatin-modifying complexes. For most lincRNAs, their functions are unknown. In this issue of Gastroenterology, Li et al report on the identification of a lincRNA that affects esophageal squamous carcinoma cells. The authors first searched for lincRNAs that showed differential expression between esophageal squamous cell tumors and the adjacent noncancerous normal tissue. One lincRNA (linc-POU3F3) located adjacent to the gene POU3F3, which encodes a transcription factor, was found to be expressed 2.8 times higher in the tumors compared to the normal adjacent tissue. Using subcellular fractionation approaches, linc-POU3F3 was determined to reside mainly in the nucleus. The authors go on to establish that linc-POU3F3 associates with EZH2, a member of the Polycomb Repressive Complex 2, which suggested an association with gene regulation. Overexpression of linc-POU3F3 resulted in decrease expression of POU3F3, whereas knockdown of linc-POU3F3 with shRNA resulted in increased POU3F3 expression. Additional experiments suggested that the observed effects were mediated by changes in methylation of the POU3F3 promoter. The functional effects of linc-POU3F3 expression were also studied with respect to esophageal squamous cell cancer cell lines. Associated with enhanced expression of linc-POU3F3 was an increase in the proportion of cells in the G2 phase of the cell cycle, which is associated with cell proliferation, and a decrease in apoptosis. Both findings are consistent with linc-POU3F3 supporting tumor growth. The authors also utilized classic assays for transformation in cancer cell lines and demonstrated that linc-POU3F3 expression promotes cell proliferation, colony formation, and xenograft growth in nude mice (Figure 3). The authors conclude that the linc-POU3F3 supports the growth of esophageal squamous cell cancers, which represents an important role for lincRNAs in human disease. See page 1714. Sterile inflammation is a source for damage in a variety of tissues and is often secondary to the innate immune response through the activation of Toll-like receptors on the cell surface and Nod-like receptors within the cytoplasm. This inflammatory response is mediated by the inflammasome, which is a complex of proteins that activates caspase-1, a protease that activates multiple proteins involved in the inflammatory response. Toll-like receptor activation in macrophages also has effects on metabolism, including enhanced glycolysis and decreased Krebs cycle activity, which results in increase lactate production. Lactate was recently found to serve as a ligand for an orphan G-protein receptor, GPR81, which through its interaction with Arrestin β-2 is able to negatively regulate Toll-like receptor–mediated inflammation. In this issue of Gastroenterology, Hoque et al report on studies that examine whether lactate supplementation can reduce organ injury from inflammasome-mediated inflammation. Using a variety of mononuclear cells, lactate exposure was found to suppress the inflammatory response to lipopolysaccharide exposure as assayed by decreased activation of caspase-1 or Pro-II1β. Consistent with previous work, RNA interference was used to demonstrate that lactate's effect on the inflammatory response was dependent on the presence of GPR81 and Arrestin β-2. The liver and pancreas are commonly compromised by sterile inflammation. The administration of lipopolysaccharide and galactosamine was used to induce hepatitis in 1 model, and experimental pancreatitis was induced with the administration of cerulein and lipopolysaccharide. In both models, the administration of lactate mitigated the resultant inflammatory response and tissue damage (Figure 4). The results provided support for lactate as a safe, inexpensive, and potent therapy for inflammatory diseases. Darwin Conwell and Markus Lerch discuss this study in greater detail in an accompanying editorial. See page 1763; editorial on page 1602. Nonselective β Blockers Increase Risk for Hepatorenal Syndrome and Death in Patients With Cirrhosis and Spontaneous Bacterial PeritonitisGastroenterologyVol. 146Issue 7PreviewNonselective β blockers (NSBBs) reduce portal pressure and the risk for variceal hemorrhage in patients with cirrhosis. However, development of spontaneous bacterial peritonitis (SBP) in these patients could preclude treatment with NSBBs because of their effects on the circulatory reserve. We investigated the effects of NSBBs in patients with cirrhosis and ascites with and without SBP. Full-Text PDF Increased Levels of the Long Intergenic Non–Protein Coding RNA POU3F3 Promote DNA Methylation in Esophageal Squamous Cell Carcinoma CellsGastroenterologyVol. 146Issue 7PreviewThousands of long intergenic non–protein coding RNAs (lincRNAs) have been identified in mammals via genome-wide sequencing studies. Many are functional, but are expressed aberrantly by cancer cells. We investigated whether levels of lincRNAs are altered during the development of esophageal squamous cell carcinoma (ESCC). Full-Text PDF Lactate Reduces Liver and Pancreatic Injury in Toll-Like Receptor– and Inflammasome-Mediated Inflammation via GPR81-Mediated Suppression of Innate ImmunityGastroenterologyVol. 146Issue 7PreviewThe NACHT, LRR, and pyrin domain–containing protein 3 (NLRP3) inflammasome induces inflammation in response to organ injury, but little is known about its regulation. Toll-like receptors (TLRs) provide the first signal required for activation of the inflammasome and stimulate aerobic glycolysis to generate lactate. We examined whether lactate and the lactate receptor, Gi-protein–coupled receptor 81 (GPR81), regulate TLR induction of signal 1 and limit inflammasome activation and organ injury. Full-Text PDF Loss-of-Function of the Voltage-Gated Sodium Channel NaV1.5 (Channelopathies) in Patients With Irritable Bowel SyndromeGastroenterologyVol. 146Issue 7PreviewSCN5A encodes the α-subunit of the voltage-gated sodium channel NaV1.5. Many patients with cardiac arrhythmias caused by mutations in SCN5A also have symptoms of irritable bowel syndrome (IBS). We investigated whether patients with IBS have SCN5A variants that affect the function of NaV1.5. Full-Text PDF When Should the β-Blocker Window in Cirrhosis Close?GastroenterologyVol. 146Issue 7PreviewThe β-blocker controversy continues.1 β Blockers are well-established in the primary and secondary prevention of variceal hemorrhage in patients with cirrhosis.2,3 Nonselective β blockers such as propranolol exert their influence via splanchnic vasoconstriction and the reduction of cardiac output, thereby reducing the hepatic venous pressure gradient. However, a series of recent studies from Sersté et al4,5 demonstrated reduced survival in patients with decompensated cirrhosis and refractory ascites who were treated with β blockers. Full-Text PDF The Anti-inflammasome Effect of Lactate and the Lactate GPR81-Receptor in Pancreatic and Liver InflammationGastroenterologyVol. 146Issue 7PreviewWhen organ injury occurs in the absence of an adequate oxygen supply, the glucose metabolism for energy generation is diverted to lactate, rather than pyruvate, and serum lactate levels rise. Any clinician regards this as an ominous sign for the patient's prognosis and suspect behind rising lactate levels organ ischemia or microcirculatory failure of some sort. He or she will also be aware that this is often paralleled by progressive acidification (a falling pH) and systemic inflammation. Recent cell biological studies have, indeed, suggested that lactate has a direct pro-inflammatory effect on the Toll-like receptor (TLR)/nuclear factor (NF)-κB/inflammasome signaling cascade that ultimately drives systemic inflammation via the activation of interleukin (IL)–1β. Full-Text PDF