Studies of the phospholipase A2 activity of rat ileal mucosa

Effects of endotoxin and dexamethasone on group I and II phospholipase A2 in rat ileum and stomach

Phospholipase A2 (EC 3.1.1.4) is a key enzyme in inflammation and is thought to play an important part in inflammatory diseases of the gastrointestinal tract. To investigate the nature and regulation of phospholipase A2 activity in the gastrointestinal mucosa, the distribution of messenger ribonucleic acid (mRNA) for group II phospholipase A2 in various parts of the rat gastrointestinal tract was studied, as well as the influence of endotoxin or dexamethasone, or both, on the group I and II phospholipase A2 mRNA expression and activity in the rat glandular stomach and distal ileum. The results show that (a) group II phospholipase A2 is present along the whole gastrointestinal tract, but in particularly large amounts in the distal ileum, (b) endotoxin increases group II, but not group I, phospholipase A2 mRNA expression in the glandular stomach and distal ileum, and (c) dexamethasone reduces the endotoxin induced increases in group II phospholipase mRNA expression and activity in the gastrointestinal mucosa. These findings suggest that phospholipase A2 of type II is a mediator of endotoxin effects in the gastrointestinal mucosa and that its expression at the mRNA level can be inhibited by corticosteroids.

Initiation of anastomotic recurrence of Crohn's disease after ileocolic resection. Onset proximal to the junction and preceded by increased phospholipase A2 activity

Colono-ileoscopy was performed on 11 patients after ileocolic resection for Crohn's disease, to observe development of recurrent anastomotic inflammation and its relationship to mucosal phospholipase A2 (EC 3.1.1.4) activity. Ileal inflammation appeared soon after surgery in eight cases but in none of nine controls with noninflammatory bowel disease. The ileal inflammation was more severe 1-3 cm above than greater than 5 cm above the ileocolic junction (p less than 0.05), whereas the postanastomotic colonic mucosa remained unchanged. Ileal phospholipase A2 activity in the mucosa was equally raised at the two ileal sites (p less than 0.01 and less than 0.02), irrespective of the presence or absence of inflammation. In colonic postanastomotic mucosa the phospholipase A2 activity was the same as in the controls. Further followup showed preanastomotic ileal inflammation at both investigated levels in all patients with Crohn's disease but still with greater severity close to the mucosal junction (p less than 0.05). The study indicates that recurrent inflammation in Crohn's disease is initiated in the terminal ileum close to the ileocolic junction. Progression of severity is accompanied by greater proximal involvement. The increase in mucosal phospholipase A2 activity, which precedes endoscopically detectable inflammation, implies a role for this enzyme in Crohn's disease.

Phospholipase C from Clostridium perfringens stimulates phospholipase A2-mediated arachidonic acid release in cultured intestinal epithelial cells (INT 407)

The mechanisms by which phospholipase C from Clostridium perfringens stimulates release of arachidonic acid (AA) in cultured intestinal epithelial cells (INT-407) were investigated. INT-407 cells were first allowed to incorporate 14C-labeled AA into their phospholipids; the labeled cells were then exposed to phospholipase C, and the release of free 14C-AA was determined. Phospholipase C caused a rapid (3 min) intracellular rise of free 14C-AA, followed by a considerable, dose- and time-dependent release of 14C-AA into the extracellular medium. For comparison, the calcium ionophore A23187 also caused a rapid mobilization of free 14C-AA, but a much lower extracellular 14C-AA release than phospholipase C during longer (1 h) incubation. The 14C-AA release was accompanied by a degradation of 14C-myo-inositol-labeled phosphatidylinositols and was reduced by the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). Both phospholipase C- and A23187-stimulated 14C-AA release was associated with degradation of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol and was reduced by nordihydroguaiaretic acid and 4-bromophenacyl bromide, two known phospholipase A2 inhibitors. In addition, the 14C-AA release was reduced by the calmodulin inhibitors trifluoperazine, compound 48/80, and N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7). These findings indicate that phospholipase C from C. perfringens stimulates phospholipase A2-mediated AA release from human intestinal epithelial cells and suggest that this stimulation is brought about via processes involving phosphatidylinositol breakdown and activation of calmodulin and protein kinase C. It is possible that this phospholipase C-evoked AA release may contribute to the mucosal pathologic condition in diseases with altered intestinal microbial flora.

Glucocorticoid treatment in ileal Crohn's disease: relief of symptoms but not of endoscopically viewed inflammation

The effect of prednisolone (20-30 mg daily for six to nine weeks) was studied in eight patients with Crohn's disease and recurrent, preanastomotic ileal inflammation, in respect of symptoms, endoscopic findings and phospholipase A2 activity in the ileal mucosa. The Harvey-Bradshaw Crohn's disease activity index improved significantly, mainly because of reduced frequency of loose stools and diminution of abdominal pain. Endoscopy revealed no corresponding decrease in ileal inflammation, which in all cases persisted after treatment. The phospholipase A2 activity in the ileal mucosa was not altered by prednisolone. In two of three patients with concomitant colitis colonic inflammation improved. The study confirmed earlier reports of good symptomatic relief from glucocorticoid treatment in Crohn's disease of the small bowel, but endoscopy suggests that this improvement was not the result of resolution of small intestinal mucosal inflammation.

Increased lipid peroxidation in malignant tissues of patients with colorectal cancer

The authors investigated the level of lipid peroxidation and possible related phenomena in the activation of phospholipase A2 and the infiltration of granulocyte neutrophils in human colorectal cancer tissue samples. Malondialdehyde (an index of lipid peroxidation), phospholipase A2 activity, and myeloperoxidase activity (a marker of granulocyte neutrophils) were found to be increased significantly in cancerous compared with macroscopically normal tissues. They concluded that increased lipid peroxidation, phospholipase A2, and myeloperoxidase activity are associated with human colorectal cancer.

Role of phospholipase A2 and oxygenated free radicals in mucosal damage after small intestinal ischemia and reperfusion

The influence of quinacrine, a phospholipase A2 inhibitor, and enzymatic scavengers of active oxygen metabolites (superoxide dismutase and catalase) on ischemic small intestinal mucosal damage has been investigated. In the absence of an inhibitor, ischemia and reperfusion caused increased mucosal permeability to sodium fluorescein, increased N-acetyl-glucosaminidase release from the mucosa into the lumen, increased malondialdehyde content, and increased myeloperoxidase and phospholipase A2 (PLA2) activities in the mucosa. All these effects of ischemia were efficiently inhibited by the PLA2 inhibitor quinacrine. On the other hand, superoxide dismutase together with catalase, even if it totally prevented the increased formation of malondialdehyde, was only able to reduce 50 percent of the increases of the other parameters. These findings indicate that, in addition to free radicals, other factors are involved in the pathogenesis of small intestinal mucosal injury after ischemia and reperfusion. We suggest that one such factor is the activation of PLA2 and the generation of various PLA2-dependent compounds such as arachidonic acid metabolites, lysophosphatidyl choline, and platelet-activating factor.

Characterization of phospholipase A2 activity in aspirates of human pancreatic pseudocysts after isolation by reversed-phase high performance liquid chromatography

Phospholipase A (PLA) is able to attack membrane phospholipids and thereby plays a putative role in the pathogenesis of pancreatic pseudocysts. We looked for PLA2-like activity in aspirates from human pancreatic pseudocysts. In material originating from one cyst which occurred shortly after an acute pancreatitis attack, hydrolyzing enzymatic activity measured by a sensitive bioassay system for PLA2 activity was found without prior trypsin activation (67 x 10(3) U/min/100 microliters). A biochemical characterization of this hydrolyzing enzymatic activity was provided after resolution of the respective proteins contained in the cyst fluid by HPLC. High hydrolyzing activities were found in correspondence to one specific, early eluting peak. The purified enzyme had pH optima at 3.5 and 6. Addition of EDTA (5 mM) to the test system abolished the enzymatic activity which mirrored the requirement for calcium ions. The activity was optimal at calcium concentrations ranging from 1-2 mM. Higher calcium concentrations reduced the enzymatic activity. The enzyme showed high heat stability. SDS-gel analysis of the peak showed one single band with a molecular weight of about 20,000 Daltons. Our findings demonstrate the possibility of activated, PLA-like activity in human pancreatic pseudocyst fluid. We speculate that an inappropriate activation of this enzyme in peri- or intrapancreatic "fluid collections" could account for pseudocyst formation after an acute pancreatitis attack.

Quinacrine prevention of intestinal ischaemic mucosal damage is partly mediated through inhibition of intraluminal phospholipase A2

The influence of total ischaemia and revascularization on luminal phospholipid metabolism in the rat small intestine was investigated. Two hours of total ischaemia followed by five minutes of revascularization caused increases in phospholipase A2 activity, and lysophosphalidylcholine content in the gut lumen. The above treatment also resulted in mucosal damage expressed as an increase in N-acetyl-beta-glucosaminidase activity in the lumen. Pretreatment of animals with the phospholipase A2 inhibitor, quinacrine prevented the increases in luminal phospholipase A2 activity and mucosal damage following ischaemia and revascularization. Intraluminal injection of either phospholipase A2 purified from snake venom or Triton X-100 resulted in increased activity of N-acetyl-beta-glucosaminidase in the luminal content. Again, quinacrine pretreatment of animals prevented the increases in mucosal permeability and activity of N-acetyl-beta-glucosaminidase after intraluminal injection of purified phospholipase A2. On the other hand quinacrine pretreatment had no influence on the observed effects of Triton X-100 treatment. These findings suggest that an increase in luminal phospholipase A2 could be involved in mediating the mucosal injury caused by small intestinal ischaemia.

Dexamethasone-induced alterations in lipid composition and fluidity of rat proximal-small-intestinal brush-border membranes

A series of experiments were conducted to examine the possible effects of subcutaneous administration of the synthetic glucocorticoid dexamethasone (100 micrograms/day per 100 g body wt.) on the lipid fluidity and lipid composition of rat proximal-small-intestinal brush-border membranes. After 4 days of treatment, membranes and their liposomes prepared from treated animals possessed a greater fluidity than did their control (diluent, 0.9% NaCl) counterparts, as assessed by steady-state fluorescence-polarization techniques using several different fluorophores. Examination of the effects of temperature on the anisotropy values of 1,6-diphenylhexa-1,3,5-triene, using Arrhenius plots, moreover, revealed that the mean break-point temperatures of the treated preparations were approx. 3-4 degrees C lower than those of their control-preparation counterparts. Changes in the sphingomyelin/phosphatidylcholine (PC) molar ratio as well as in certain of the fatty acids of the PC fraction of treated membranes, secondary to alterations in membrane PC levels and in lysophosphatidylcholine acyltransferase activities respectively, were also noted after dexamethasone administration. These compositional alterations appeared to be responsible, at least in part, for the differences in fluidity noted between treated and control plasma membranes. These results therefore demonstrate that dexamethasone administration can modulate the lipid fluidity and lipid composition of rat proximal-small-intestinal brush-border membranes.

Increased phospholipase A2 and decreased lysophospholipase activity in the small intestinal mucosa after ischaemia and revascularisation

The influence of ischaemia and revascularisation on lipid peroxidation and phospholipid metabolism in the rat small intestinal mucosa was investigated. Two hours of total ischaemia followed by five minutes of revascularisation caused not only accumulation of malondialdehyde in the mucosa, but also increased activity of phospholipase A2, decreased activity of lysophospholipase, and increased ratio between lysophosphatidylcholine and phosphatidylcholine. Pretreatment with the phospholipase A2 inhibitor, quinacrine, prevented the increases in mucosal phospholipase A2 activity and lysophosphatidylcholine/phosphatidylcholine ratio after ischaemia and morphological examinations revealed that the mucosa was then also protected against ischaemic injury. These findings point to the possibility that activation of phospholipase A2 and accumulation of lysophosphoglycerides could be involved in mediating the mucosal injury caused by small intestinal ischaemia.

Development of phospholipase A2 and lysophosphatidylcholine metabolising enzyme activities in the neonatal rat intestine

We have studied the development of phospholipase A2 (PLA2) and lysophosphatidylcholine (lysoPC)-metabolising enzyme activities in the neonatal rat intestine and its relation to the intestinal permeability of macromolecules. The permeability was determined by feeding young rats a mixture of bovine serum albumin, bovine immunoglobulin G and fluorescein-isothiocyanate-conjugated dextran 70,000, and analysing the serum concentrations after six hours. The animals were then killed and the intestinal mucosa was homogenised and assessed for PLA2 and lysoPC-metabolising enzyme activities. The intestine was 'open' to the macromolecules in 14 day old animals, but 'closed' in 22 and 32 day old animals and in 14 day old rats treated with cortisone acetate on day 10, 11, and 12 postpartum. The activity of PLA2 (at pH 6 and 2 mM Ca2+) was higher in 32, 22, and cortisone treated 14 day old animals, than in untreated, 14 day old animals. Incubation of 14C-acyl-lysoPC with mucosa from 14 day old rats did not change the radioactivity pattern as shown by thin layer chromatography, whereas after incubation with mucosa from 22 or 32 day old animals all the radiolabel was found in free 14C-fatty acid and in 14C-phosphatidylcholine. These findings indicate that mucosal PLA2 activity increases during intestinal maturation and that the mucosa acquires the ability to acylate and deacylate lysoPC when it is 'closed' to macromolecules.

A rat gastric mucosal preparation for studying agents which affect the in vitro production of prostanoids

A convenient and reproducible method for measuring the production of prostanoids from either the rat gastric mucosa or whole corpus tissue is described. Use of selective radio-immunoassay was employed to measure production of 6-keto- PGF1 alpha, the stable hydrolysis product of PGI2, and PGE2. The mucosal tissue generated approximately 10 times more 6-keto-PGF1 alpha than PGE2. Gastric muscle tissue, however, produced 2-3 times more 6-keto-PGF1 alpha than the mucosa. Exogenous arachidonic acid was found to elevate the amount of both prostanoids although the rate of formation of 6-keto-PGF1 alpha appeared to be less than that of PGE2. A possible explanation for this was that PGE2 production, in part, was non-enzymic. Mepacrine, a phospholipase inhibitor, was found to inhibit both the control amounts of 6-keto-PGF1 alpha and the increased production of this prostanoid due to exogenous phospholipase activity. The generation of prostanoids was also inhibited by indomethacin in a concentration-related manner which suggested that the method is suitable for screening non-steroidal anti-inflammatory agents in vitro or ex vivo. Preliminary results with the above method have been reported elsewhere.