Catabolism of platelet-activating factor by human colonic mucosa. Calcium dependence of the catabolizing enzymes

Lysophospholipase D activity on oral mucosa cells in whole mixed human saliva involves in production of bioactive lysophosphatidic acid from lysophosphatidylcholine

We reported that lysophosphatidic acid (LPA) is present at 0.8 μM in mixed human saliva (MS). In this study, we examined the distribution, origin, and enzymatic generation pathways of LPA in MS. LPA was distributed in the medium and cell pellet fraction; a true level of soluble LPA in MS was about 150 nM. The soluble LPA was assumed to be generated by ecto-type lysophospholipase D on exfoliated cells in MS from LPC that originated mainly from the major salivary gland saliva. Our results with the albumin-back extraction procedures suggest that a significant pool of LPA is kept in the outer layer of the plasma membranes of detached oral mucosal cells. Such pool of LPA may contribute to wound healing in upper digestive organs including oral cavity. We obtained evidence that the choline-producing activity in MS was mainly due to Ca2+-activated lysophospholipase D activity of glycerophosphodiesterase 7.

Hypoxia increases cellular levels of phosphatidic acid and lysophospholipids in undifferentiated Caco-2 cells

Cancer cells are known to survive in a hypoxic microenvironment by altering their lipid metabolism as well as their energy metabolism. In this study, Caco-2 cells derived from human colon cancer, were found to have elevated intracellular levels of phosphatidic acid and its lysoform, lysophosphatidic acid (LPA), under hypoxic conditions. Our results suggested that the elevation of LPA in Caco-2 cells was mainly due to the combined increases in cellular levels of lysophosphatidylcholine and lysophosphatidylethanolamine by phospholipase A₂ and subsequent hydrolysis to LPA by lysophospholipase D. We detected the Ca²⁺ -stimulated choline-producing activities toward exogenous lysophosphatidylcholines in whole Caco-2 cell homogenates, indicating their involvement in the LPA production in intact Caco-2 cells.

Hypoxia induces a lipogenic cancer cell phenotype via HIF1α-dependent and -independent pathways

The biochemistry of cancer cells diverges significantly from normal cells as a result of a comprehensive reprogramming of metabolic pathways. A major factor influencing cancer metabolism is hypoxia, which is mediated by HIF1α and HIF2α. HIF1α represents one of the principal regulators of metabolism and energetic balance in cancer cells through its regulation of glycolysis, glycogen synthesis, Krebs cycle and the pentose phosphate shunt. However, less is known about the role of HIF1α in modulating lipid metabolism. Lipids serve cancer cells to provide molecules acting as oncogenic signals, energetic reserve, precursors for new membrane synthesis and to balance redox biological reactions. To study the role of HIF1α in these processes, we used HCT116 colorectal cancer cells expressing endogenous HIF1α and cells in which the hif1α gene was deleted to characterize HIF1α-dependent and independent effects on hypoxia regulated lipid metabolites. Untargeted metabolomics integrated with proteomics revealed that hypoxia induced many changes in lipids metabolites. Enzymatic steps in fatty acid synthesis and the Kennedy pathway were modified in a HIF1α-dependent fashion. Palmitate, stearate, PLD3 and PAFC16 were regulated in a HIF-independent manner. Our results demonstrate the impact of hypoxia on lipid metabolites, of which a distinct subset is regulated by HIF1α.

Intestinal alkaline sphingomyelinase hydrolyses and inactivates platelet-activating factor by a phospholipase C activity

Alkaline sphingomyelinase (alk-SMase) is a new member of the NPP (nucleotide pyrophosphatase/phosphodiesterase) family that hydrolyses SM (sphingomyelin) to generate ceramide in the intestinal tract. The enzyme may protect the intestinal mucosa from inflammation and tumorigenesis. PAF (platelet-activating factor) is a pro-inflammatory phospholipid involved in pathogenesis of inflammatory bowel diseases. We examined whether alk-SMase can hydrolyse and inactivate PAF. [3H]Octadecyl-labelled PAF was incubated with purified rat intestinal alk-SMase or recombinant human alk-SMase expressed in COS-7 cells. The hydrolytic products were assayed with TLC and MS. We found that alkSMase cleaved the phosphocholine head group from PAF and generated 1-O-alkyl-2-acetyl-sn-glycerol. Differing from the activity against SM, the activity against PAF was optimal at pH 7.5, inhibited by EDTA and stimulated by 0.1-0.25 mM Zn2+. The activity was abolished by site mutation of the predicted metal-binding sites that are conserved in all NPP members. Similar to the activity against SM, the activity against PAF was dependent on bile salt, particularly taurocholate and taurochenodeoxycholate. The V(max) for PAF hydrolysis was 374 mumol x h(-1) x (mg of protein)(-1). The hydrolysis of PAF and SM could be inhibited by the presence of SM and PAF respectively, the inhibition of PAF hydrolysis by SM being stronger. The PAF-induced MAPK (mitogen-activated protein kinase) activation and IL-8 (interleukin 8) release in HT-29 cells, and chemotaxis in leucocytes were abolished by alk-SMase treatment. In conclusion, alk-SMase hydrolyses and inactivates PAF by a phospholipase C activity. The finding reveals a novel function, by which alk-SMase may counteract the development of intestinal inflammation and colon cancer.

Diagnostic Value of the Serum Platelet-Activating Factor Acetylhydrolase Activity in Inflammatory Bowel Disease

Platelet-activating factor acetylhydrolase (PAF-AH) is an enzyme hydrolyzing platelet-activating factor (PAF), a potent inflammatory mediator, but the relationship between this enzyme and inflammatory bowel disease (IBD) is not fully elucidated. The aim of the present study was to examine the usefulness of the serum PAF-AH activity in order to differentiate ulcerative colitis (UC) from Crohn's disease (CD). The serum PAF-AH activity was measured in 57 patients with IBD (39 UC and 18 CD patients) and 13 control subjects by a spectrophotometric method. The serum PAF-AH activity was thus found to be significantly lower in patients with CD (median 265.5 U/l) than in those with UC (355 U/l) or control subjects (374 U/l). This marker at a cutoff level of 386 U/l demonstrated a sensitivity of 46%, a specificity of 100%, and a positive predictive value of 100% regarding its ability to distinguish UC from CD. Moreover, the marker responded inversely to the changes in the disease activity of IBD. These results suggest that measuring the serum PAF-AH activity is a useful diagnostic modality for making a differential diagnosis between UC and CD.

Substrate specificity of lysophospholipase D which produces bioactive lysophosphatidic acids in rat plasma

Previously we reported that lysophospholipase D in rat plasma hydrolyzes endogenous unsaturated lysophosphatidylcholines (LPCs) preferentially to saturated LPCs to lysophosphatidic acids with growth factor-like and hormone-like activities. In this study, we examined the possibility that association of LPCs with different proteins in rat plasma has an effect on the preference of lysophospholipase D for unsaturated LPCs. Large portions of various LPCs were found to be recovered in the lipoprotein-poor bottom fraction. Furthermore, the percentages of LPCs associated with albumin isolated from rat plasma were shown not to be consistent with their percentage conversions to lysophosphatidic acids by lysophospholipase D on incubation of rat plasma at 37 degrees C. These results indicate that distinct distributions of LPCs in the plasma protein fractions are not critical factors for the substrate specificity of lysophospholipase D. Experiments with Nagase analbuminemic rats suggested that albumin-LPC complexes are not necessarily required for the hydrolysis by lysophospholipase D; lipoprotein-associate LPCs appeared to be good substrates for the phospholipase. We found that both saturated and unsaturated LPCs are present mainly as 1-acyl isomers in rat plasma. This result indicates that the preference of lysophospholipase D for unsaturated LPCs is not attributable to a difference in position of the acyl group attached to the glycerol backbone of LPC. In addition, lysophospholipase D was also found to attack choline phospholipids with a long chain group and a short chain alkyl group, although their percentage hydrolyses were low. Taken altogether, these results suggest that lysophospholipase D shows higher affinities for free forms of unsaturated acyl type LPCs equilibrated with albumin-bound and lipoprotein-associated forms, than for free forms of saturated acyl type LPCs and analogs of platelet-activating factor.

Investigation of the effects of platelet-activating factor (PAF) on ion transport and prostaglandin synthesis in human colonic mucosa in vitro

1 We have investigated the effects of platelet-activating factor (PAF), an endogenous mediator of inflammation, on ion transport and prostaglandin synthesis in the human isolated colon. 2 Application of PAF to the serosal surface of human colonic mucosa induced a marked, concentration-dependent increase in ion transport. Mucosal application was without effect. 3 The secretory response to PAF was significantly inhibited by prior application of a specific PAF receptor antagonist WEB 2170, indicating that the response is dependent on PAF receptor activation. 4 The response to PAF was attenuated by prior application of indomethacin or piroxicam, implicating products of the cyclo-oxygenase pathway in the response. 5 The response to PAF was attenuated by the loop diuretic bumetanide, indicating an involvement of chloride ion secretion in the response. 6 Addition of PAF to the serosal surface induced a significant increase in serosal prostaglandin E2 (PGE2), but not 6-oxo-PGF1alpha release. There was no effect on mucosal application of PAF. 7 In summary, we have shown that PAF is a potent secretagogue in isolated preparations of human colon and that the response is dependent on a specific PAF receptor, cyclo-oxygenase products and bumetanide-sensitive chloride ion transport.

Platelet-activating factor acetylhydrolases in Caco-2 cells and epithelium of normal and ulcerative colitis patients

BACKGROUND & AIMS

Platelet-activating factor (PAF) is a potent inflammatory mediator implicated in the pathogenesis of inflammatory bowel disease and necrotizing enterocolitis. Metabolism by platelet-activating factor acetylhydrolase (PAF-AH) is the major pathway for platelet-activating factor degradation. The aim of this study was to investigate the possible role of intestinal epithelium as a source of PAF-AH.

METHODS

Intracellular and secreted PAF-AHs were characterized in human colonic epithelial cells isolated from histologically normal mucosa and inflamed mucosa from patients with ulcerative colitis and in the human intestinal epithelial cell line Caco-2 by measuring the metabolism of [3H]-PAF to [3H]lysoPAF.

RESULTS

Human colonic epithelial cells and Caco-2 cells synthesize and secrete PAF-AH as shown by in vitro hydrolysis of [3H]PAF to [3H]-lysoPAF in cell lysates and conditioned media. Both intracellular and secreted PAF-AHs are calcium-independent and substrate-specific for phospholipids similar to PAF. Epithelial cells from involved areas of resections for ulcerative colitis had increased levels of secreted PAF-AH and decreased levels of intracellular PAF-AH compared with epithelial cells from histologically normal areas.

CONCLUSIONS

Human colonic epithelial cells and Caco-2 cells produce intracellular and secreted PAF-AHs, which are distinct proteins. This is the first demonstration of PAF-AH production by epithelial cells.

Intraluminal excretion of PAF, lysoPAF, and acetylhydrolase in patients with ulcerative colitis

PAF-acether (PAF) is a phospholipid synthesized by numerous inflammatory cells. PAF can produce several pathological changes in various organs, especially in the colon. In animals PAF causes colonic ulceration and inflammation, which are similar to the anatomic lesions seen in human ulcerative colitis. The aim of this study was to measure in vivo colonic production of PAF in active ulcerative colitis using a modified colonic perfusion method. Ten patients with active ulcerative colitis and six control patients were investigated. A colonic segment was continuously perfused with a buffer and the liquid was recovered 20 cm distally, after a 45-min period of equilibration, at 20-min intervals. PAF, lysoPAF, and acetylhydrolase were measured in the colonic samples. PAF and lysoPAF outputs were significantly higher in patients with active ulcerative colitis compared to controls patients. There was a significant correlation between colonic PAF output and, respectively, macroscopic mucosal lesions and myeloperoxidase colonic output. We thus conclude: (1) the colonic perfusion method allows in vivo study of the metabolism of PAF during ulcerative colitis and could also be used to study the efficiency of PAF antagonists in UC; and (2) colonic production of PAF is increased during ulcerative colitis and correlated to local injury and inflammation. Whether or not PAF plays a role in the pathogenesis of ulcerative colitis remains open for further investigations.