Xiao Wu Qu

Neonatal Necrotizing Enterocolitis: Clinical Considerations and Pathogenetic Concepts

Necrotizing enterocolitis (NEC), a disease affecting predominantly premature infants, is a leading cause of morbidity and mortality in neonatal intensive care units. Although several predisposing factors have been identified, such as prematurity, enteral feeding, and infection, its pathogenesis remains elusive. In the past 20 years, we have established several animal models of NEC in rats and found several endogenous mediators, especially platelet-activating factor (PAF), which may play a pivotal role in NEC. Injection of PAF induces intestinal necrosis, and PAF antagonists prevent the bowel injury induced by bacterial endotoxin, hypoxia, or challenge with tumor necrosis factor-a (TNF) plus endotoxin in adult rats. The same is true for lesions induced by hypoxia and enteral feeding in neonatal animals. Human patients with NEC show high levels of PAF and decreased plasma PAF-acetylhydrolase, the enzyme degrading PAF. The initial event in our experimental models of NEC is probably polymorphonuclear leukocyte (PMN) activation and adhesion to venules in the intestine, which initiates a local inflammatory reaction involving proinflammatory mediators including TNF, complement, prostaglandins, and leukotriene C4. Subsequent norepinephrine release and mesenteric vasoconstriction result in splanchnic ischemia and reperfusion. Bacterial products (e.g., endotoxin) enter the intestinal tissue during local mucosal barrier breakdown, and endotoxin synergizes with PAF to amplify the inflammation. Reactive oxygen species produced by the activated leukocytes and by intestinal epithelial xanthine oxidase may be the final pathway for tissue injury. Protective mechanisms include nitric oxide produced by the constitutive (mainly neuronal) nitric oxide synthase, and indigenous probiotics such as Bifidobacteria infantis. The former maintains intestinal perfusion and the integrity of the mucosal barrier, and the latter keep virulent bacteria in check. The development of tissue injury depends on the balance between injurious and protective mechanisms.

Neuronal nitric oxide synthase (NOS) regulates the expression of inducible NOS in rat small intestine via modulation of nuclear factor kappa B

We previously reported that neuronal nitric oxide synthase (nNOS) is the predominant NOS in the intestine. Inducible NOS (iNOS), an enzyme involved in the inflammatory response, is regulated by cytokines via the transcriptional factor NF‐κB. We examined a new mechanism of intestinal iNOS regulation with respect to the role of nNOS and its effect on NF‐κB. Young Sprague‐Dawley rats were treated for 4 days with 1) saline, 2) 7‐nitroindazole (7‐NI, specific nNOS inhibitor), 3) 7‐NI + pyrrolidine dithiocarbam‐ate (PDTC, NF‐κB inhibitor), or 4) PDTC. Intestinal iNOS mRNA, NF‐κB activity, and the tissue content of the regulatory IκBα were examined. We found that 7‐NI‐treated animals had higher intestinal NF‐κB (p50‐p65) activity, lower IκBα content, and increased intestinal iNOS mRNA, iNOS protein, and iNOS activity compared with controls. All of these changes were abolished when PDTC was given together with 7‐NI. PDTC alone had no effect. 7‐NI induces a delayed increase in intestinal myeloperoxidase activity (after elevation in NF‐κB and iNOS), which could be abrogated by PDTC. We conclude that in normal rat small intestine, nNOS suppresses the gene expression of iNOS through NF‐κB down‐regulation and that nNOS suppression leads to IκBα degradation, NF‐κB activation, and iNOS expression.—Qu, X.‐w., Wang, H., De Plaen, I. G., Rozenfeld, R. A., Hsueh, W. Neuronal nitric oxide synthase (NOS) regulates the expression of inducible NOS in rat small intestine via modulation of nuclear factor kappa B. FASEB J. 15, 439‐446 (2001)

Intestinal NF-κB is activated, mainly as p50 homodimers, by platelet-activating factor

Abstract NF-κB, a transcription factor, upregulates gene transcription of many inflammatory mediators. Here, we examined the activity of NF-κB in the rat small intestine, and how it may be affected by platelet-activating factor (PAF), an important mediator for intestinal injury and inflammation. Ileal nuclear extracts from sham-operated and PAF (1.5 μg/kg)-injected rats were prepared for the assessment of NF-κB DNA-binding activity, and the identification of NF-κB subunits. The experiment was also performed on neutrophil-depleted rats to examine whether the PAF effect is neutrophil-dependent. Cellular NF-κB was localized by immunohistochemistry. We found that: (a) NF-κB is constitutively active in rat small intestine; (b) PAF at a dose below that causing shock and bowel necrosis enhances DNA-binding activity of NF-κB within 30 min after injection; activated NF-κB contains predominantly p50 subunits; (c) immunohistochemistry showed that PAF induced translocation of p50 into the nucleus of cells of the lamina propria, as well as of the epithelium; and (d) the effect of PAF is abrogated by neutrophil depletion, suggesting a role of neutrophils in NF-κB activation. Our study suggests that NF-κB is weakly active constitutively in the intestine, and inflammatory stimuli such as PAF activate NF-κB and enhance its DNA-binding activity in the intestine, which contains predominantly p50 subunits.

Type I nitric oxide synthase (NOS) is the predominant NOS in rat small intestine. Regulation by platelet-activating factor

Constitutive nitric oxide synthase (cNOS) may play an important protective role in the intestine, since our previous study has shown that the degree of bowel injury induced by platelet-activating factor (PAF), a potent inflammatory mediator, is inversely related to the cNOS content of the intestine. This study aims to examine the composition of the cNOS system in rat small intestine, and its regulation by PAF. We found that an approximately 120 kDa NOS I (neuronal NOS) is the predominant NOS in rat intestine, as evidenced by the following: (a) immunoblotting with specific antibodies detected a NOS I of approximately 120 kDa, but little NOS III; (b) the Ca(2+)-dependent, constitutive NOS (cNOS) activity of the rat intestine was removed by immunoprecipitation with the anti-NOS I, but not anti-NOS II or anti-NOS III antibodies; (c) RT-PCR revealed constitutive expression of NOS I in the intestinal tissue, but only a minute amount of NOS III. Immunofluorescent staining with anti-NOS I located NOS in the Auerbach plexus and nerve fibers in the muscle layer. We also found that this 120 kDa NOS I is rapidly (within 1 h) down-regulated in response to PAF administration. The protein level, enzyme activity as well as mRNA of nNOS were all decreased in the intestine.

Endotoxin, but not platelet-activating factor, activates nuclear factor-κB and increases IκBα and IκBβ turnover in enterocytes

Bacterial endotoxin (lipopolysaccharide; LPS) and platelet‐activating factor (PAF) are important triggers of bowel inflammation and injury. We have previously shown that LPS activates the transcription factor nuclear factor (NF)‐κB in the intestine, which up‐regulates many pro‐inflammatory genes. This effect partly depends on neutrophils and endogenous PAF. However, whether LPS and PAF directly activate NF‐κB in enterocytes remains controversial. In this study, we first investigated the effect of LPS and PAF on NF‐κB activation in IEC‐6 (a non‐transformed rat small intestinal crypt cell line) cells, by electrophoresis mobility shift assay and supershift, and found that LPS, but not PAF, activates NF‐κB mostly as p50–p65 heterodimers. The effect was slower than tumour necrosis factor (TNF). Both LPS and TNF induce the expression of the NF‐κB‐dependent gene inducible nitric oxide synthase (iNOS), which occurs subsequent to NF‐κB activation. We then examined the effect of LPS and TNF on the inhibitory molecules IκBα and IκBβ. We found that TNF causes rapid degradation of IκBα and IκBβ. In contrast, LPS did not change the levels of IκBα and IκBβ up to 4 hr (by Western blot). However, in the presence of cycloheximide, there was a slow reduction of IκBα and IκBβ, which disappeared almost completely at 4 hr. These observations suggest that LPS causes slow degradation and synthesis of IκBα and IκBβ and therefore activates NF‐κΒ via at least two mechanisms: initially, through an IκB‐independent mechanism, and later, via an increased turnover of the inhibitor IκB. NF‐κΒ activation precedes the gene expression of iNOS (assayed by reverse transcription–polymerase chain reaction), suggesting that LPS up‐regulates iNOS via this transcription factor.

Roles of nitric oxide synthases in platelet-activating factor-induced intestinal necrosis in rats.

OBJECTIVE To examine the role of constitutive and inducible nitric oxide synthases (cNOS and iNOS) in platelet-activating factor (PAF)-induced shock and intestinal injury. DESIGN Prospective, randomized, controlled experimental study. SETTING Hospital research laboratory. SUBJECTS Young adult male Sprague-Dawley rats were anesthetized and studied. INTERVENTIONS Rats were injected with PAF, either alone or after the following pretreatments: a) selective iNOS inhibitors aminoguanidine or S-methylisothiourea; b) 3-morpholinosydnonimine, a NO donor; c) S-methylisothiourea + 3-morpholinosydnonimine; and d) antineutrophil antibody (to deplete neutrophils). MEASUREMENTS AND MAIN RESULTS Blood pressure, hematocrit, white blood cell counts, intestinal injury, and intestinal cNOS and iNOS activities were assessed. We found that: a) cNOS is the predominant NOS in the intestine and its activity is inversely correlated to the level of tissue injury; b) there is a time-dependent increase in cNOS activity in sham-operated animals, which was abolished by PAF; c) Western blotting and immunohistochemistry showed iNOS present in the normal intestine, localizing mainly in crypt cells; d) iNOS inhibitors attenuated PAF-induced injury in animals with high cNOS activity, but had no protective effect in animals with low cNOS activity; e) 3-morpholinosydnonimine, alone or together with S-methylisothiourea, alleviated PAF-induced injury; and f) neutrophil depletion blocked the suppressive effect of PAF on cNOS and prevented injury. CONCLUSIONS We conclude that cNOS and iNOS play different roles in PAF-induced intestinal injury. Caution should be exerted concerning potential therapeutic uses of iNOS inhibitors.

Platelet-activating factor increases mucosal permeability in rat intestine via tyrosine phosphorylation of E-cadherin.

Platelet‐activating factor (PAF), an inflammatory mediator, plays an important role in mediating intestinal injury. However, it remains unclear whether PAF has a function in the intestine. The production of PAF by normal intestine and by unstimulated intestinal epithelial cell lines suggests that PAF may have a regulatory function in the normal bowel. In this study we investigated the role of PAF in modulating intestinal mucosal permeability in rats. Lumen‐to‐blood transit of FD‐4 (dextran 4400), (an index of intestinal permeability), was assessed in sham‐operated rats and rats injected with PAF (1.25 μg kg−1, i.v., a dose insufficient to induce intestinal injury). PAF‐induced villus cytoskeletal changes were examined by staining the intestine for F‐actin. The effect of PAF on tyrosine phosphorylation of the junctional protein E‐cadherin was examined by immunoprecipitation. Some rats were pretreated with AG1288 (a tyrosine kinase inhibitor) before PAF injection, and mucosal permeability change was assessed. To investigate the role of endogenous PAF upon mucosal permeability, we studied the effect of PAF antagonists on (intraluminal) glucose‐induced increase in mucosal permeability. We found that low dose PAF: (a) alters the cytoskeletal structure of intestinal epithelium, (b) causes the influx of FD4 from intestinal lumen to systemic circulation, (c) induces tyrosine phosphorylation of E‐cadherin and cadherin‐associated proteins. Glucose‐induced mucosal permeability increase is abolished by using two structurally different PAF antagonists. These results suggest that endogenous PAF modulates macromolecular movement across the intestinal mucosal barrier, probably via tyrosine phosphorylation of E‐cadherin and cytoskeletal alteration of enterocytes.

Interaction of platelet-activating factor, spleen and atrial natriuretic peptide in plasma volume regulation during endotoxaemia in rats

1 We studied endotoxin (lipopolysaccharide, LPS)‐induced platelet‐activating factor (PAF) production in various visceral organs, and the effect of PAF antagonists or splenectomy on LPS‐induced changes. 2 PAF production in response to LPS was highest in the spleen, followed by ileum, heart, lung and kidneys. None was found in the liver. The splenic response was rapid, reaching 10 times the basal level at 30 min. The increased PAF content in each organ was unrelated to the enzyme activity of either macrophages or neutrophils. 3 LPS‐induced hypotension and haemoconcentration were largely prevented by PAF antagonists and splenectomy. 4 Plasma volume fell, and plasma atrial natriuretic peptide (ANP) rose, after LPS administration. Splenectomy or pretreatment with PAF antagonists almost completely prevented these LPS‐induced changes at 30 min, but only partially reversed them at 90 min. 5 These results suggest that during endotoxaemia: (a) the spleen is the site of the highest endogenous PAF production; (b) the initial release of ANP is dependent on the production of endogenous PAF, and a PAF‐ANP interaction mediates the early plasma volume reduction; (c) plasma volume reduction as well as ANP release depend on the spleen; (d) PAF mediated the hypotensive response and its action in the spleen; and (e) sequestered neutrophils are probably not the main source of PAF in the spleen.

Tetrahydrobiopterin prevents platelet-activating factor-induced intestinal hypoperfusion and necrosis: Role of neuronal nitric oxide synthase.

Objective:We reported previously that neuronal nitric oxide synthase (nNOS) is the predominant NOS in rat small intestine and is down-regulated by platelet-activating factor (PAF). The severity of the bowel injury induced by PAF is inversely related to its suppressing effect on nNOS. Here, we investigated whether intestinal perfusion is regulated by nNOS and whether tetrahydrobiopterin, a co-factor and stabilizer of nNOS, reverses PAF-induced intestinal hypoperfusion and injury. Setting:Animal laboratory. Design:We first examined nNOS regulation of splanchnic blood flow by measuring the perfusion of the heart, lung, ileum, and kidney in rats after a nNOS inhibitor. We then examined the protective effect of tetrahydrobiopterin on PAF-induced bowel injury, mesenteric hypoperfusion, and systemic inflammation. Subjects:Adult male Sprague-Dawley rats. Intervention:In part 1 of the experiment, rats were given 7-nitroindazole (a specific nNOS inhibitor, 50 mg·kg−1·day−1). In part 2 of the experiment, rats were treated with tetrahydrobiopterin (20 mg/kg) 5 mins before and 30 mins after PAF challenge (2.2 &mgr;g/kg, intravenously) Measurements:Perfusion of the heart, lung, ileum, and kidney was measured at 1 and 4 days after 7-nitroindazole, using fluorescent microspheres. Intestinal injury and inflammation (myeloperoxidase content), blood perfusion, calcium dependent-NOS activity, and systemic inflammation (hypotension and hematocrit increase) were assessed 1 hr after PAF with and without tetrahydrobiopterin treatment. Results:In part 1 of the experiment, 7-nitroindazole induced a long-lasting reduction of blood perfusion and inducible NOS expression selectively in the ileum but not in nonsplanchnic organs such as heart, lungs, and kidneys. In part 2, tetrahydrobiopterin protected against PAF-induced intestinal necrosis, hypoperfusion, neutrophil influx, and NOS suppression. It also reversed hypotension and hemoconcentration. Sepiapterin (2 mg/kg, stable tetrahydrobiopterin precursor) also attenuated PAF-induced intestinal injury. Conclusions:We conclude that nNOS selectively regulates intestinal perfusion. Tetrahydrobiopterin prevents PAF-induced intestinal injury, probably by stabilizing nNOS and maintaining intestinal perfusion.

Platelet-Activating Factor (PAF) Up-Regulates Plasma and Tissue PAF-Acetylhydrolase Activity in the Rat: Effect of Cycloheximide

Platelet-activating factor (PAF) is a proinflammatory phospholipid mediator implicated in necrotizing enterocolitis. Regulation of PAF-acetylhydrolase (AH), the enzyme degrading PAF, is poorly understood. In this study we found that administration of a dose of PAF (1.5 μg/kg, i.v.), which does not cause gross intestinal injury, increased plasma and intestinal PAF-AH in the rat. Cycloheximide (CHX, 5 mg/kg, i.v.) reduced the activity of plasma (but not intestinal tissue) AH in control, as well as in PAF-injected rats, and aggravated systemic inflammation and tissue injury in the latter. The intestinal necrosis induced by PAF and CHX was ameliorated by posttreatment with WEB2170 (a PAF antagonist), indicating a role of endogenous PAF in mediating injury. Both WEB2170 and anti-TNF antibody reduced PAF-induced AH activity in intestinal tissue, but not in the plasma. Allopurinol largely prevented the injury induced by PAF and CHX, but had no effect on the up-regulation of AH. We conclude: 1) de novo protein synthesis is required to maintain physiologic AH level in the plasma; 2) PAF up-regulates plasma and intestinal AH activity; 3) CHX enhances the injurious effect of PAF; 4) endogenous PAF and TNF also play a role in the up-regulation of intestinal AH; the former probably mediating the intestinal injury by PAF; and 5) reactive oxygen species may mediate the injurious effect of PAF plus CHX, but do not contribute to the regulation of AH by PAF.