Endotoxin-resistant mice are protected from PAF-induced bowel injury and death. Role of TNF, complement activation, and endogenous PAF production

The benefits, limitations and opportunities of preclinical models for neonatal drug development

Increased research to improve preclinical models to inform the development of therapeutics for neonatal diseases is an area of great need. This article reviews five common neonatal diseases – bronchopulmonary dysplasia, retinopathy of prematurity, necrotizing enterocolitis, perinatal hypoxic–ischemic encephalopathy and neonatal sepsis – and the available in vivo, in vitro and in silico preclinical models for studying these diseases. Better understanding of the strengths and weaknesses of specialized neonatal disease models will help to improve their utility, may add to the understanding of the mode of action and efficacy of a therapeutic, and/or may improve the understanding of the disease pathology to aid in identification of new therapeutic targets. Although the diseases covered in this article are diverse and require specific approaches, several high-level, overarching key lessons can be learned by evaluating the strengths, weaknesses and gaps in the available models. This Review is intended to help guide current and future researchers toward successful development of therapeutics in these areas of high unmet medical need.

Summary: This article reviews and analyzes the available preclinical models for five common neonatal diseases to direct therapeutic development in these areas of high unmet medical need.

Precision-based modeling approaches for necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is the leading cause of death from gastrointestinal disease in premature infants and remains stubbornly difficult to treat in many cases. Much of our understanding of NEC pathogenesis has been gained through the study of highly translational animal models. However, most models of NEC are limited by their overall complexity and by the fact that they do not incorporate human tissue. To address these limitations, investigators have recently developed precision-based ex vivo models of NEC, also termed ‘NEC-in-a-dish’ models, which provide the opportunity to increase our understanding of this disease and for drug discovery. These approaches involve exposing intestinal cells from either humans or animals with or without NEC to a combination of environmental and microbial factors associated with NEC pathogenesis. This Review highlights the current progress in the field of NEC model development, introduces NEC-in-a-dish models as a means to understand NEC pathogenesis and examines the fundamental questions that remain unanswered in NEC research. By answering these questions, and through a renewed focus on precision model development, the research community may finally achieve enduring success in improving the outcome of patients with this devastating disease.

Summary: Much of our understanding of necrotizing enterocolitis (NEC) pathogenesis has been achieved through animal models. Here, we discuss the development of advanced precision-based models to improve outcomes for patients with NEC.

Pathogenesis of necrotizing enterocolitis: modeling the innate immune response

Necrotizing enterocolitis (NEC) is a major cause of morbidity and mortality in premature infants. The pathophysiology is likely secondary to innate immune responses to intestinal microbiota by the premature infant's intestinal tract, leading to inflammation and injury. This review provides an updated summary of the components of the innate immune system involved in NEC pathogenesis. In addition, we evaluate the animal models that have been used to study NEC with regard to the involvement of innate immune factors and histopathological changes as compared to those seen in infants with NEC. Finally, we discuss new approaches to studying NEC, including mathematical models of intestinal injury and the use of humanized mice.

Paneth cells and necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a common and devastating disease of premature infants. Immaturity of the innate immune system of the gut is central to the pathogenesis of NEC. Recent studies suggest a key role for Paneth cells in this disease. Addressing basic questions on the development and function of immature Paneth cells may shed light on the puzzling pathophysiology of NEC. Current animal models of NEC are limited in their capacity to answer these questions.

Polyunsaturated fatty acid supplementation alters proinflammatory gene expression and reduces the incidence of necrotizing enterocolitis in a neonatal rat model

Although supplementation of preterm formula with polyunsaturated fatty acids (PUFA) has been shown to reduce the incidence of necrotizing enterocolitis (NEC) in animal models and clinical trials, the mechanisms remain elusive. We hypothesized that the protective effect of PUFA on NEC may be due to the ability of PUFA to suppress Toll-like receptor (TLR) 4 and platelet-activating factor receptor (PAFR) gene expression (molecules that are important in the pathogenesis of NEC) in epithelial cells. To investigate the efficacy of different PUFA preparations on NEC in a neonatal rat model, we compared the incidence of NEC among the four PUFA supplemented groups--A: arachidonic acid and docosahexaenoic acid (AA+DHA), B: egg phospholipids (EP), C: DHA, and D: control without PUFA. PUFA supplementation reduced the incidence of NEC and inhibited intestinal PAFR and TLR4 gene expression compared with the controls. To validate the in vivo observations, IEC-6 cells were exposed to PAF after pretreatment with AA or DHA. Both AA and DHA supplementation blocked PAF-induced TLR4 and PAFR mRNA expression in these enterocytes. These results suggest that PUFA modulates gene expression of key factors involved in experimental NEC pathogenesis. These effects might in part explain the protective effect of PUFA on neonatal NEC.

Lipopolysaccharide induces CXCL2/macrophage inflammatory protein-2 gene expression in enterocytes via NF-kappaB activation: independence from endogenous TNF-alpha and platelet-activating factor

CXCL2 (macrophage inflammatory protein-2 (MIP-2)), a critical chemokine for neutrophils, has been shown to be produced in the rat intestine in response to platelet-activating factor (PAF) and to mediate intestinal inflammation and injury. The intestinal epithelium, constantly exposed to bacterial products, is the first line of defence against micro-organisms. It has been reported that enterocytes produce proinflammatory mediators, including tumour necrosis factor (TNF) and PAF, and we showed that lipopolysaccharide (LPS) and TNF activate nuclear factor (NF)-kappaB in enterocytes. However, it remains elusive whether enterocytes release CXCL2 in response to LPS and TNF via a NF-kappaB-dependent pathway and whether this involves the endogenous production of TNF and PAF. In this study, we found that TNF and LPS markedly induced CXCL2 gene expression in IEC-6 cells, TNF within 30 min, peaking at 45 min, while LPS more slowly, peaking after 2 hr. TNF- and LPS- induced CXCL2 gene expression and protein release were completely blocked by pyrrolidine dithiocarbamate (PDTC) and helenalin, two potent NF-kappaB inhibitors. NEMO-binding domain peptide, a specific inhibitor of inhibitor protein kappaB kinase (IKK) activation, a major upstream kinase mediating NF-kappaB activation, significantly blocked CXCL2 gene expression and protein release induced by LPS. WEB2170 (PAF antagonist) and anti-TNF antibodies had no effect on LPS-induced CXCL2 expression. In conclusion, CXCL2 gene is expressed in enterocytes in response to both TNF and LPS. LPS-induced CXCL2 expression is dependent on NF-kappaB activation via the IKK pathway. The effect of LPS is independent of endogenous TNF and PAF.

Effect of platelet activating factor on function of intestinal immunological barrier in young rats

AIM: To investigate the effect of platelet activating factor (PAF) on the content of secretory IgA (SIgA) in intestinal mucosa.

METHODS: The rats were intraperitoneally injected with different concentrations of PAF (50 and 65 µg/kg) at a dose of 1 µL/g. Double antibody-PEG radioimmunoassay was used to determine the amount of SIgA in the intestinal mucosa. The histological changes were detected by hematoxylin and Eosin staining under light microscope.

RESULTS: In the PAF65 group, the histological examination showed edema of the villus, capillary congestion of the lamina propria, extension of the subepithelial lymphatic channel, polymorphonuclear infiltration in enteric cavity, and shedding of the epithelial layer at 0.5, 1.5, and 3 h. Edema of the villus were still shown at 6 and 24 h. In the PAF50 group, edema of the villus, capillary congestion of the lamina propria were showed at 0.5 and 1.5 h. Edema of the villus were still shown at 3, 6 and 24 h. The content of SIgA was obviously decreased in the experimental group than that in the control group (0.31±0.03 mg/L, 0.40±0.10 mg/L, P < 0.01; 0.43±0.13 mg/L, 0.46±0.11mg/L, P < 0.05, in PAF50 group; 0.28±0.07 mg/L, 0.36±0.08 mg/L, P < 0.01, 0.40±0.11 mg/L, 0.42±0.06 mg/L, P < 0.05, in PAF65 group vs 0.66±0.10 mg/L in the control) at 0.5, 1.5, 3 and 6 h, respectively. The SIgA content decreased most at 0.5 h, but then gradually increased.

CONCLUSION: PAF can lead to damages of the intestinal immunologic barrier by decreasing the SIgA content.

Azithromycin and erythromycin ameliorate the extent of colonic damage induced by acetic acid in rats

Ulcerative colitis is a common inflammatory bowel disease (IBD) of unknown etiology. Recent studies have revealed the role of some microorganisms in the initiation and perpetuation of IBD. The role of antibiotics in the possible modulation of colon inflammation is still uncertain. In this study, we evaluated the effects of two macrolides, namely azithromycin and erythromycin, at different doses on the extent and severity of ulcerative colitis caused by intracolonic administration of 3% acetic acid in rats. The lesions and the inflammatory response were assessed by histology and measurement of myeloperoxidase (MPO) activity, nitric oxide synthetase (NOS) and tumor necrosis factor alpha (TNFalpha) in colonic tissues. Inflammation following acetic acid instillation was characterized by oedema, diffuse inflammatory cell infiltration and necrosis. Increase in MPO, NOS and TNFalpha was detected in the colonic tissues. Administration of either azithromycin or erythromycin at different dosage (10, 20 and 40 mg/kg orally, daily for 5 consecutive days) significantly (P < 0.05) reduced the colonic damage, MPO and NOS activities as well as TNFalpha level. This reduction was highly significant with azithromycin when given at a dose of 40 mg/kg. It is concluded that azithromycin and erythromycin may have a beneficial therapeutic role in ulcerative colitis.

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.

Effect of PAF receptor antagonists on adrenocortical secretion induced by ACTH in normal and athymic nude mice

The effect of SM12502 and CV6209, platelet-activating factor (PAF) receptor antagonists on corticosterone (B) secretion induced by ACTH was examined in the perfused adrenals of CD1 ICR (normal) and CD1 ICR nu/nu (athymic) mice. Bilateral adrenals were perfused in situ with an artificial medium equilibrated by 95% O2 + 5% CO2. Continuous infusion of 10 microM SM12502 or CV6209 inhibited the B response to 100 pg/ml ACTH markedly in normal mice but insignificantly in athymic mice. Infusion of PAF did not significantly affect B secretion in either normal or athymic mice. Administration of 0.1 microM of N-methylcarbamyl PAF, a nonmetabolizable PAF agonist, significantly increased B secretion in normal mice, but not in athymic mice. Infusion of SM12502 significantly depressed the B response to 10 microM forskolin or 1 mM dibutyryl cyclicAMP (cAMP) in normal mice, but not in athymic mice. The results indicate that endogenous PAF and its receptor may play a role in the ACTH-initiated signaling pathway at the phase after responsiveness to cAMP and its receptor may have little function in athymic mice.

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 microg/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.

The role of platelet-activating factor in conscious, normotensive endotoxemia

The effects of endotoxin have been postulated to be mediated in large part by release of endogenous platelet-activating factor (PAF) due to the similarity of their hemodynamic and gastric effects in anesthetized animals, and to the ability of PAF inhibitors to ameliorate endotoxin's effects. We chose to examine the relationship with doses that would not produce circulatory shock, in unrestrained conscious animals, in order to mimic clinical situations. Adult male rats were prepared with vascular access, hemodynamic and temperature monitors, and gastric strain gauge transducers. After an overnight fast, rats received a 4-hr infusion of saline (0.5 ml/hr), endotoxin (12.5 mg/kg/hr), PAF (36 micrograms/kg/hr, or 600 ng/kg/min), or endotoxin plus the PAF inhibitor CV 3988 (1 mg/kg/hr, after an initial pretreatment of 1 mg/kg). Rats were killed, stomachs were harvested, and contents were analyzed at the end of the infusions. Blood pressure was not affected by any treatment, but all treated groups developed diarrhea and vasodilatation. Endotoxin and PAF infusions decreased heart rate and body temperature to a similar extent, although the PAF effect on temperature was delayed. The PAF inhibitor did not prevent the body temperature effect, but did reverse it. Gastric secretions were affected by PAF to a lesser extent than by endotoxin, and the PAF inhibitor did not decrease endotoxin's gastric secretory effects. PAF has similar systemic and gastric effects to endotoxin in conscious, unrestrained, normotensive animals. The systemic effects of endotoxin at 12.5 mg/kg/hr were prevented or reversed by the PAF inhibitor CV-3988 at 1 mg/kg/hr, but not the gastric secretory effects.

Inhibition of lipopolysaccharide-induced bowel erythrocyte extravasation in rats, and of mesenteric hypoperfusion in dogs, by phosphodiesterase inhibitors

Sepsis is intricately associated with mesenteric ischemia. The remote complications of mesenteric ischemia are essentially those of sepsis, whether as a cause or as a consequence. Experimental endotoxic shock induces bowel hypoperfusion, erythrocyte extravasation and intestinal necrosis. The effects of pentoxifylline, rolipram and denbufylline, three phosphodiesterase inhibitors, were studied on endotoxin-induced bowel erythrocyte extravasation and intestinal and renal hypoperfusion, in conscious rats and anaesthetized dogs, respectively. Two hours after lipopolysaccharide i.v. injection in rats, erythrocyte extravasation was evident throughout the intestinal musculature and mucosa, apparently without affecting lungs, heart, kidneys, liver or pancreas. Pretreatment with the non-selective phosphodiesterase inhibitor, pentoxifylline, or selective phosphodiesterase IV inhibitors such as denbufylline or rolipram reduced intestinal haemoconcentration. In the anaesthetized dog, pentoxifylline and denbufylline both inhibited the E. coli lipopolysaccharide-induced mesenteric blood flow fall, without affecting renal blood flow or cardiac index. In conclusion, phosphodiesterase inhibitors protected from intestinal damage and bowel hypoperfusion after lipopolysaccharide challenge. This action may thus play a role in the protective effects against endotoxin-induced lethal toxicity previously described for phosphodiesterase inhibitors.

Endogenous bacterial toxins are required for the injurious action of platelet-activating factor in rats

BACKGROUND & AIMS

Platelet-activating factor (PAF), an endogenous mediator for experimental sepsis, has been shown to induce shock and intestinal necrosis in vivo. However, it is unclear whether PAF exerts its injurious effects on the intestinal tissue directly or via synergism with other endogenous products. The aim of this study was to examine the role of endogenous bacterial products, such as endotoxin, in PAF-induced intestinal injury.

METHODS

PAF (3 micrograms/kg) was injected intravenously into normally colonized rats, germfree rats, and normal rats pretreated with a combination of antibiotics, and the systemic response and intestinal injury were assessed.

RESULTS

PAF did not cause prolonged shock, leukopenia, hemoconcentration, and bowel necrosis in germfree rats. When germfree rats were primed with a low dose (0.5 mg/kg) of endotoxin, the protection was lost. Combined treatment of the normally colonized rats with neomycin, polymyxin B, and metronidazole for 7 days largely protected the animal from PAF-induced shock and intestinal necrosis.

CONCLUSIONS

PAF does not directly induce prolonged hypotension, hemoconcentration, persistent leukopenia, and gross intestinal necrosis but causes these changes via a synergism with endogenous bacterial toxins, presumably from the gut flora.