Platelet-activating factor regulates chloride transport in colonic epithelial cell monolayers

Macrophage VLDL receptor promotes PAFAH secretion in mother's milk and suppresses systemic inflammation in nursing neonates

Mother’s milk is widely accepted as nutritious and protective to the newborn mammals by providing not only macronutrients but also immune-defensive factors. However, the mechanisms accounting for these benefits are not fully understood. Here we show that maternal very-low-density-lipoprotein receptor (VLDLR) deletion in mice causes the production of defective milk containing diminished level of platelet-activating factor acetylhydrolase (PAFAH). As a consequence, the nursing neonates suffer from alopecia, anemia and growth retardation owing to elevated levels of pro-inflammatory platelet-activating factors (PAFs). VLDLR deletion significantly impairs the expression of phospholipase A2 group 7 (Pla2g7) in macrophages, which decreases PAFAH secretion. Exogenous oral supplementation of neonates with PAFAH effectively rescues the toxicity. These findings not only reveal a novel role of VLDLR in suppressing inflammation by maintaining macrophage PAFAH secretion, but also identify the maternal VLDLR as a key genetic program that ensures milk quality and protects the newborns.

Toll-like receptor 2-mediated peptidoglycan uptake by immature intestinal epithelial cells from apical side and exosome-associated transcellular transcytosis

Peptidoglycan (PGN) is a potent immune adjuvant derived from bacterial cell walls. Previous investigations suggest that intestinal epithelium may absorb PGN from the lumen. Nonetheless, how PGN is taken up and crosses intestinal epithelium remains largely unclear. Here, we first characterized PGN transport in vitro using IEC-18 and HT29-CL19A cells, which represent less mature epithelial cells in intestinal crypts. With fluorescent microscopy, we visualized internalization of dual-labeled PGN by enterocytes. Engulfed PGN was found to form a complex with PGN recognition protein-3, which may facilitate delivering PGN in vivo. Utilizing electronic microscopy, we revealed that uptake of apical PGN across intestinal epithelial monolayers was involved in phagocytosis, multivesicular body formation, and exosome secretion. We also studied transport of PGN using the transwell system. Our data indicated that apically loaded PGN was exocytosed to the basolateral compartment with exosomes by HT29-CL19A cells. The PGN-contained basolateral exosome extracts induced macrophage activation. Through gavaging mice with labeled PGN, we found that luminal PGN was taken up by columnar epithelial cells in crypts of the small intestine. Furthermore, we showed that pre-confluent immature but not post-confluent mature C2BBe1 cells engulfed PGN via a toll-like receptor 2-dependent manner. Together, our findings suggest that (1) crypt-based immature intestinal epithelial cells play an important role in transport of luminal PGN over the intestinal epithelium; and (2) luminal PGN is transcytosed across intestinal epithelia via a toll-like receptor 2-mediated phagocytosis-multivesicular body-exosome pathway. The absorbed PGN and its derivatives may facilitate maintenance of intestinal immune homeostasis.

Hypoxia-inducible factor-dependent regulation of platelet-activating factor receptor as a route for gram-positive bacterial translocation across epithelia

Results from these studies reveal that some strains of Gram-positive bacteria exploit hypoxia-inducible factor-regulated platelet-activating factor receptor as a means for translocation through intestinal epithelial cells.

Mucosal surfaces, such as the lung and intestine, are lined by a monolayer of epithelia that provides tissue barrier and transport function. It is recently appreciated that a common feature of inflammatory processes within the mucosa is hypoxia (so-called inflammatory hypoxia). Given the strong association between bacterial translocation and mucosal inflammatory disease, we hypothesized that intestinal epithelial hypoxia influences bacterial translocation. Initial studies revealed that exposure of cultured intestinal epithelia to hypoxia (pO₂, 20 torr; 24–48 h) resulted in a increase of up to 40-fold in the translocation of some strains of Gram-positive bacteria, independently of epithelial barrier function. A screen of relevant pathway inhibitors identified a prominent role for the platelet-activating factor receptor (PAFr) in hypoxia-associated bacterial translocation, wherein pharmacologic antagonists of PAFr blocked bacterial translocation by as much as 80 ± 6%. Extensions of these studies revealed that hypoxia prominently induces PAFr through a hypoxia-inducible factor (HIF)-dependent mechanism. Indeed, HIF and PAFr loss of function studies (short hairpin RNA) revealed that apically expressed PAFr is central to the induction of translocation for the Gram-positive bacteria Enterococcus faecalis. Together, these findings reveal that some strains of Gram-positive bacteria exploit HIF-regulated PAFr as a means for translocation through intestinal epithelial cells.

Platelet-activating factor-induced chloride channel activation is associated with intracellular acidosis and apoptosis of intestinal epithelial cells

Platelet-activating factor (PAF) is a phospholipid inter- and intracellular mediator implicated in intestinal injury primarily via induction of an inflammatory cascade. We find that PAF also has direct pathological effects on intestinal epithelial cells (IEC). PAF induces Cl(-) channel activation, which is associated with intracellular acidosis and apoptosis. Using the rat small IEC line IEC-6, electrophysiological experiments demonstrated that PAF induces Cl(-) channel activation. This PAF-activated Cl(-) current was inhibited by Ca(2+) chelation and a calcium calmodulin kinase II inhibitor, suggesting PAF activation of a Ca(2+)-activated Cl(-) channel. To determine the pathological consequences of Cl(-) channel activation, microfluorimetry experiments were performed, which revealed PAF-induced intracellular acidosis, which is also inhibited by the Cl(-) channel inhibitor 4,4'diisothiocyanostilbene-2,2'disulfonic acid and Ca(2+) chelation. PAF-induced intracellular acidosis is associated with caspase 3 activation and DNA fragmentation. PAF-induced caspase activation was abolished in cells transfected with a pH compensatory Na/H exchanger construct to enhance H(+) extruding ability and prevent intracellular acidosis. As ClC-3 is a known intestinal Cl(-) channel dependent on both Ca(2+) and calcium calmodulin kinase II phosphorylation, we generated ClC-3 knockdown cells using short hairpin RNA. PAF induced Cl(-) current; acidosis and apoptosis were all significantly decreased in ClC-3 knockdown cells. Our data suggest a novel mechanism of PAF-induced injury by which PAF induces intracellular acidosis via activation of the Ca(2+)-dependent Cl(-) channel ClC-3, resulting in apoptosis of IEC.

Effects of alpha-linolenic acid on colonic secretion in rats with experimental colitis

BACKGROUND

Few studies have specifically examined the effects of n-3 polyunsaturated fatty acids (PUFAs) on intestinal water and ion secretion in ulcerative colitis (UC). The aim of this study was to examine the contribution of prostaglandins (PGs) and leukotrienes (LTs) to mucosal secretion in intestines with UC and to evaluate the effect of dietary n-3 PUFAs on diarrhea in UC.

METHODS

We measured the short-circuit current (Isc), using the Ussing chamber method, and fatty acid composition in the colonic mucosa of rats with dextran sulfate sodium (DSS)-induced experimental colitis. The DSS-treated rats were fed either a perilla oil-enriched diet (perilla group) or a soybean oil-enriched diet (soybean group); a control group did not undergo DSS administration.

RESULTS

The bradykinin-stimulated DeltaIsc in the soybean and perilla groups was significantly higher than that in the control group. The mucosal level of arachidonic acid in the perilla group was significantly lower than that in the soybean group. The mucosal levels of alpha-linolenic acid and EPA in the perilla group were significantly higher than those in the soybean group. The bradykinin-stimulated DeltaIsc was significantly suppressed after pretreatment with indomethacin in both the soybean and perilla groups, and was also significantly reduced in both groups after pretreatment with AA861. The suppression of bradykinin-stimulated DeltaIsc by the addition of AA861 was significantly higher in the perilla group than in the soybean group.

CONCLUSIONS

Our results suggest that supplementation with alpha-linolenic acid, in combination with a lipoxygenase inhibitor, could suppress the increase in Cl- secretion in patients with UC.

Effects of mouse and human lipocalin homologues 24p3/lcn2 and neutrophil gelatinase-associated lipocalin on gastrointestinal mucosal integrity and repair

BACKGROUND & AIMS

The lipocalin superfamily, including the mouse and human homologues 24p3/lcn2 and neutrophil gelatinase-associated lipocalin, show great functional diversity including roles in olfaction, transportation, and prostaglandin synthesis in mammals. Their potential role in maintaining gastrointestinal mucosal integrity and repair is, however, unclear.

METHODS

Changes in 24p3/lcn2 expression in the mouse gut in response to various noxious agents were examined using Northern blot, in situ hybridization, and immunohistochemistry. Effects of recombinant 24p3/lcn2 on proliferation ([3H]-thymidine uptake), and restitution (cell-wounding migration) were assessed using human colonic HT29 and HCT116 cells. In addition, the effects of recombinant 24p3/lcn2 on the amount of gastric damage were assessed in rats treated with indomethacin (20 mg/kg) and restraint.

RESULTS

Marked up-regulation of expression of 24p3/lcn2 was seen throughout the gut in response to indomethacin or dextran sodium sulfate treatment. Expression was increased particularly in the surface epithelial cells and infiltrating inflammatory cells. Proliferation and restitution assays in the presence of recombinant wild-type sequence neutrophil gelatinase-associated lipocalin, wild-type cys(98)-24p3/lcn2, and mutant ala98-24p3/lcn2 showed that all 3 peptides caused a 3- to 4-fold increase in promigratory activity (P < .01 vs control) but did not influence proliferation. The administration of wild-type cys98-, or mutant ala98-24p3/lcn2 (25 and 50 microg/kg/h, respectively), given via the subcutaneous route, both caused similar reductions in the rat gastric damage model (60% reduction at highest dose, P < .01 vs control), although oral administration was ineffective.

CONCLUSIONS

24p3/lcn2 facilitates mucosal regeneration by promoting cell migration.

The role of PAF, TLR, and the inflammatory response in neonatal necrotizing enterocolitis

The pathogenesis of neonatal necrotizing enterocolitis remains poorly understood. Recent evidence suggests that PAF (platelet activating factor) and human toll-like receptors (TLRs) contribute to the pro-inflammatory response that is characteristic of NEC pathology. Understanding the regulation of these molecular interactions may provide new approaches for prevention or treatment of this dreaded condition.

Clinical microbiology of bacterial and fungal sepsis in very-low-birth-weight infants

Twenty percent of very-low-birth-weight (<1500 g) preterm infants experience a serious systemic infection, and despite advances in neonatal intensive care and antimicrobials, mortality is as much as threefold higher for these infants who develop sepsis than their counterparts without sepsis during their hospitalization. Outcomes may be improved by preventative strategies, earlier and accurate diagnosis, and adjunct therapies to combat infection and protect the vulnerable preterm infant during an infection. Earlier diagnosis on the basis of factors such as abnormal heart rate characteristics may offer the ability to initiate treatment prior to the onset of clinical symptoms. Molecular and adjunctive diagnostics may also aid in diagnosing invasive infection when clinical symptoms indicate infection but no organisms are isolated in culture. Due to the high morbidity and mortality, preventative and adjunctive therapies are needed. Prophylaxis has been effective in preventing early-onset group B streptococcal sepsis and late-onset Candida sepsis. Future research in prophylaxis using active and passive immunization strategies offers prevention without the risk of resistance to antimicrobials. Identification of the differences in neonatal intensive care units with low and high infection rates and implementation of infection control measures remain paramount in each neonatal intensive care unit caring for preterm infants.

Platelet-activating factor-induced apoptosis is blocked by Bcl-2 in rat intestinal epithelial cells

Plateletactivating factor (PAF) is a key mediator in pathogenesis of inflammatory bowel diseases (IBDs) but mechanisms of PAF-induced mucosal injury are poorly understood. To determine whether apoptosis and the Bcl-2-family of apoptosis regulatory gene products play a role in PAF-induced mucosal injury, we stably and conditionally overexpressed bcl-2 in rat small intestinal epithelial cells-6 under the control of a lactose-inducible promoter. Western blot analysis and immuno-histochemistry were used to verify inducible Bcl-2 and to analyze Bcl-2 and a proapoptotic member of the Bcl-2 family, Bax, subcellular distribution. DNA fragmentation was quantified by ELISA, caspase activity was measured by using fluorogenic peptide substrates, and mitochondrial membrane potential was assayed by 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide (JC-1) and fluorescence digital imaging. Bcl-2 expression was highly inducible by lactose analog isopropyl-beta-(d)-thiogalactoside (IPTG) and was localized predominantly to mitochondria. In the absence of bcl-2 overexpression and after treatment with PAF, Bax translocated to mitochondria, and mitochondrial membrane potential collapsed within 1 h, followed by caspase-3 activation, which peaked at 6 h with an ensuing DNA fragmentation maximizing at 18 h. After IPTG-induction of bcl-2 expression, PAF failed to induce DNA fragmentation, caspase-3 activation, Bax translocation, or a collapse of mitochondrial membrane potential. These data are the first to show that PAF can activate apoptotic machinery in enterocytes via a mechanism involving Bax translocation and collapse of mitochondrial membrane potential and that both of these events are under control by bcl-2 expression levels. A better understanding of the role of PAF and Bcl-2 family of apoptosis regulators in epithelial cell death might aid design of better therapeutic or preventive strategies for IBDs.

Epithelial transport and gut barrier function in colitis

PURPOSE OF REVIEW

Colitis is an inflammatory bowel disease that is confined to the colon and is characterized by a watery diarrhea that can also be accompanied by blood in the stool. The inflammation associated with colitis is generally confined to the mucosal and submucosal layers, although Crohn's colitis may be transmural. The principal functions of the colonic mucosa are to act as a barrier to the luminal contents of the intestinal tract and to facilitate the bidirectional transport of water and electrolytes. It is well established that barrier and transport defects occur in colitis and may be involved in pathogenesis. Consequently, this review discusses recent evidence of potential mechanisms that may be involved in the perturbation of mucosal transport and barrier functions in colitis and therapeutic advances to counteract these defects.

RECENT FINDINGS

Mechanisms responsible for transport dysfunction and barrier defects in colitis are discussed, including decreased activity of transport proteins such as CFTR, bacterial interactions with the epithelium, including understanding of the regulation and function of NOD-2, and altered expression of components of the intestinal barrier, such as mucins and multidrug resistance proteins.

SUMMARY

Recent advances in our understanding of how changes in barrier and transport function occur in colitis may illuminate the pathophysiology of this condition. The work discussed may also identify novel targets that are functionally altered in colitis, which potentially can be modulated therapeutically either with existing medications or with newer agents that are in development.