Phospholipase activation and arachidonic acid release in cultured intestinal epithelial cells (INT 407)

Potential role of protease-activated receptor-2-stimulated activation of cytosolic phospholipase A(2) in intestinal myofibroblast proliferation: Implications for stricture formation in Crohn's disease

BACKGROUND AND AIMS

Myofibroblast hyperplasia contributes to muscularis mucosae thickening and stricture formation in Crohn's disease (CD). Protease-activated receptor-2 (PAR-2) and cytosolic phospholipase A(2) (cPLA(2)) are known regulators of cell growth, but their significance in intestinal myofibroblast proliferation remain to be elucidated. The principle aims of the present study were to investigate if PAR-2 is expressed in the expanded muscularis mucosa in ileal CD specimens, if inflammatory cytokines may stimulate PAR-2 expression in intestinal myofibroblasts, and if PAR-2 and cPLA(2) may regulate intestinal myofibroblast growth.

METHODS

Immunohistochemistry was used for detection of PAR-2 in ileal CD specimens. Studies on PAR-2 expression, PLA(2) activation and cell growth were performed in a human intestinal myofibroblast cell line, CCD-18Co. PAR-2 expression was investigated by RT-PCR and immunocytochemistry. PLA(2) activity was analyzed by quantification of released (14)C-arachidonic acid ((14)C-AA). Cell growth was examined by (3)H-thymidine incorporation and cell counting.

RESULTS

The thickened muscularis mucosae of the CD specimens showed strong PAR-2 expression. In cultured myofibroblasts, tumor necrosis factor-α (TNF-α) up-regulated PAR-2 mRNA and protein, and potentiated PAR-2-stimulated (14)C-AA release by two known PAR-2 activators, trypsin and SLIGRL-NH(2). The release of (14)C-AA was dependent on cPLA(2). Trypsin stimulated the proliferation of serum-starved cells, and inhibition of cPLA(2) reduced normal cell growth and abolished the growth-promoting effect of trypsin.

CONCLUSIONS

The results suggest that PAR-2-mediated cPLA(2) activation might be of importance in intestinal myofibroblast proliferation. The results also point to the possibility that PAR-2 up-regulation by inflammatory cytokines, like TNF-α, may modulate this effect.

Fatty acid-induced cytotoxicity: differences in susceptibility between MDCK cells and primary cultures of proximal tubular cells

We have compared the cytotoxicity of exogenously added fatty acid (oleic acid) and that of endogenous free fatty acids released from cell membranes by phospholipase A2 in primary cultures of mouse proximal tubular (MPT) cells and in Madine-Darby canine kidney (MDCK) cells. Exposure of MPT cell monolayers to oleic acid (125 mmol/L) for 2 hours resulted in severe irreversible injury to 70% +/- 4% of MPT cells. In striking contrast, only 8% +/- 3% of MDCK cells were killed by the same insult. This striking difference in the response to exogenous oleate by MPT and MDCK cells was associated with modest and comparable reductions in cell adenosine triphosphate (ATP) content in both cell types. Chemical anoxia induced by cyanide plus deoxyglucose (CN-DOG) in the absence of glucose was associated with greater injury in MPT cells (45% +/- 6% killed) than in MDCK cells (16% +/- 5% cells killed) despite severe and comparable depletion of cell ATP content in both MPT cells (96.0% +/- 0.6% reduction) and MDCK cells (96.0% +/- 0.5% reduction). The release of endogenous fatty acids by the exposure of cells to exogenous phospholipase A2 caused mild injury in both cell types that was more severe in MPT cells than in MDCK cells. The combined insult of phospholipase A2 and chemical anoxia for 2 hours caused substantially greater cell injury in both MPT and MDCK cells than either intervention alone, but the combined insult was still more damaging to MPT cells (73% +/- 4% killed) than to MDCK cells (30% +/- 4% killed). We conclude that the cell membrane in MDCK cells is intrinsically more resistant to fatty acid-induced injury than the lipid membrane in MPT cells.

Cytosolic phospholipase A2 and cyclooxygenase-2 mediate release and metabolism of arachidonic acid in tumor necrosis factor-alpha-primed cultured intestinal epithelial cells (INT 407)

BACKGROUND

We have recently reported that tumor necrosis factor-alpha (TNF-alpha), a pro-inflammatory cytokine that has been suggested to play a role in the pathogenesis of inflammatory bowel disease, potentiates phospholipase A2 (PLA2)-stimulated arachidonic acid (AA) release and prostaglandin E2 (PGE2) formation in cultured intestinal epithelial cells (INT 407). The aim of the present study was to investigate which particular isoforms of PLA2 and cyclooxygenase (COX) are involved in these processes.

METHODS

Cells were labeled with 14C-AA or 14C-oleic acid, and the amounts of released fatty acid and PGE2 were analyzed by thin-layer chromatography. mRNA was analyzed by reverse transcription and polymerase chain reaction.

RESULTS

The cells contained mainly mRNA for cytosolic PLA2 (cPLA2) and only trace amounts of mRNA for group I and II PLA2. TNF-alpha potentiated the release of 14C-AA but not of 14C-oleic acid. The TNF-alpha-potentiated PGE2 release was reduced after inhibition of cellular COX activity or mRNA synthesis. TNF-alpha increased the amounts of mRNA for COX-2 but not for COX-1.

CONCLUSIONS

The results point to the possibility that TNF-alpha may modulate the intestinal mucosal content of biologically active AA metabolites by priming cPLA2- and COX-2-mediated processes in the epithelial cells.

N-ethyl maleimide stimulates arachidonic acid release through activation of the signal-responsive phospholipase A2 in endothelial cells

Treatment of bovine endothelial cells with the alkylator N-ethyl maleimide results in arachidonic acid mobilization. N-ethyl maleimide-stimulated arachidonic acid release was dose and time dependent and maximum release was achieved after 10-15 min with 50 microM N-ethyl maleimide, N-ethyl maleimide-stimulated arachidonic acid release could be prevented by pretreating the cells with the phospholipase A2 inhibitor quinacrine. Based on the finding that N-ethyl maleimide was not able to release oleic acid from oleic acid-preloaded cells, it was clear that the effect of N-ethyl maleimide was limited to an arachidonic acid-specific phospholipase. The effect of N-ethyl maleimide does not appear to be dependent on calcium, as shown by the observation that N-ethyl maleimide was not able to increase intracellular calcium concentration in FURA2-loaded cells. Pretreatment of the cells with staurosporine totally inhibited N-ethyl maleimide-stimulated arachidonic acid liberation. The tyrosine kinase inhibitor genistein was also able to significantly inhibit arachidonic acid release. It is concluded from the results obtained in this study that N-ethyl maleimide stimulates arachidonic acid release by stimulating the activity of a specific, signal-responsive phospholipase A2. Furthermore this activation is not mediated by intracellular calcium fluxes but by a stimulation of intracellular kinase activity which eventually leads to the activation of this signal-responsive phospholipase A2.

Phospholipase C from Clostridium perfringens stimulates acetyltransferase-dependent formation of platelet-activating factor in cultured intestinal epithelial cells (INT 407)

The mechanisms by which phospholipase C from Clostridium perfringens stimulates the formation of platelet-activating factor (PAF-acether) in cultured intestinal epithelial cells (INT 407) were investigated. Although stimulation with phospholipase C caused a significant formation of PAF-acether, there was no significant increase in the cellular levels of lysoPAF-acether after stimulation. Moreover, when cells prelabeled with 3H-1-O-alkyl-2-acyl-sn-glycerophosphocholine were stimulated with phospholipase C, the 3H-lysoPAF-acether content was not increased in stimulated cells as compared with unstimulated cells. When cells were preincubated with the calmodulin inhibitor trifluoperazine (TFPA), the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), or the combined phospholipase A2-inhibitor and lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) before stimulation with phospholipase C, the PAF-acether formation was significantly decreased. The phospholipase A2 inhibitor 4-bromophenacyl bromide (BPB), on the other hand, had no significant effect on the PAF-acether formation. Preincubation with NDGA also decreased the levels of lysoPAF-acether, whereas BPB, H7, or TFPA had no such effect. These findings indicate that stimulation of acetyltransferase activity with increased acetylation of lysoPAF-acether may be one way by which phospholipase C from C. perfringens stimulates formation of PAF-acether in INT 407 cells.

Tumor necrosis factor-alpha potentiates phospholipase A2-stimulated release and metabolism of arachidonic acid in cultured intestinal epithelial cells (INT 407)

Tumor necrosis factor-alpha (TNF-alpha), a known pro-inflammatory cytokine, has been suggested to play a role in the pathogenesis of inflammatory bowel disease (IBD) by mediating damage to the intestinal epithelial cells. The present study demonstrates that TNF-alpha potentiates release and metabolism of 14C-labeled arachidonic acid (14C-AA) in cultured intestinal epithelial cells (INT 407). Although TNF-alpha on its own was but a weak stimulator of cellular 14C-AA turnover, it significantly potentiated the release of 14C-AA and 14C-labeled prostaglandin E2(14C-PGE2) after stimulation with three known phospholipase A2 activators: phospholipase. C from Clostridium perfringens, the calcium ionophore A23187, and the phorbol ester 4-beta-phorbol-12-myristate-13-acetate (PMA). The phospholipase A2 inhibitor quinacrine significantly reduced both AA and PGE2 release after combined stimulation with phospholipase C and TNF-alpha. In contrast to its effect on the AA turnover, TNF-alpha did not affect the phospholipase C-stimulated production of platelet-activating factor (PAF-acether). Taken together, these findings indicate that a) TNF-alpha potentiates phospholipase A2-stimulated AA release from cultured intestinal epithelial cells; b) TNF-alpha may stimulate phospholipase A2-dependent AA release without affecting the formation of PAF-acether and c) pretreatment with TNF-alpha potentiates the formation of PGE2 after stimulation with phospholipase A2 activators. In summary, the present investigation points to the possibility that TNF-alpha may stimulate intestinal epithelial cells to produce biologically active AA metabolites and that this stimulation may be modulated by components of the intestinal luminal content, like bacterial toxins.

Sequential incorporation of 3H- and 14C-arachidonic acid during 24 h in rabbit colonic explants cultured in vitro

Arachidonic acid (AA) might be released from a number of different intracellular pools. This aspect was studied by sequential incorporation of 3H- and 14C-AA during 24 h in rabbit colonic biopsies in culture. Mucosal explants were subjected to 3H-AA (0-6 h) and 14C-AA (6-24 h) followed by a 2-h washout. Lipid extraction of biopsies revealed a distinct pattern where 3H, in relative terms, dominated in phosphatidylinositol (PI) and phosphatidylethanolamine (PE) and 14C was more abundant in neutral lipids, whereas the remaining lipids had equal proportions of labellings. When stimulating double labelled biopsies with 1 microM phorbol 12-myristate 13-acetate (PMA), 10 micrograms/ml A23187 and a combination thereof, the 14C-labelling was in general released in larger amounts, i.e. a maximum of 7.5% with the combined drugs vs 5% for 3H. However, the relative release of 3H increased upon stimulation especially for PMA + A23187. The release of labelling was directly coupled to a stimulation dependent production of PGE2 in the order PMA less than A23187 less than PMA + A23187, maximum release = 8.2 +/- 2.4 ng/h and 3 biopsies. This study shows the existence of different AA-pools, where PI and PE (3H-labelling) appears to have a slow turnover and representing a terminus in a redistribution chain and where neutral lipids (14C-labelling) have a more rapid turnover. Upon stimulation more AA appears to be recruited from pools with slow turnover.

Phospholipase C from Clostridium perfringens stimulates formation and release of platelet-activating factor (PAF-acether) in cultured intestinal epithelial cells (INT 407)

This study demonstrates the ability of phospholipase C from Clostridium perfringens to stimulate the generation of platelet-activating factor (PAF-acether) in cultured intestinal epithelial cells (INT 407). Cells were exposed to phospholipase C for up to 60 min, and the content of PAF-acether within the cells and in the extracellular medium was determined. Phospholipase C caused a time-dependent formation of PAF-acether within the cells and also release of PAF-acether to the medium. In contrast, phospholipase C did not affect the cellular acetylhydrolase activity or the ability of the cells to metabolize extracellularly added 14C-PAF-acether. These findings suggest the possibility that intestinal epithelial cells, when stimulated with a naturally occurring intestinal bacterial toxin, generate and release PAF-acether. The possibility that this might contribute to the pathophysiology of inflammatory bowel disease is discussed.

Phospholipase C-induced anion secretion and its interaction with carbachol in the rat colonic mucosa

Phospholipase C (PLC) from clostridium perfringens induced a biphasic increase in short-circuit current (Isc) in the rat colon. The Isc rose rapidly to a transient peak, before it increased again to a plateau lasting for several hours. Ion replacement experiments and sensitivity to furosemide or a Cl- channel blocker indicated that PLC induced Cl- secretion. The first peak was suppressed by indomethacin, indicating mediation by prostaglandins. In contrast, the second phase was only partially sensitive to the cyclooxygenase blocker. The long-time action of PLC was dependent on intra- and extracellular Ca2+, although PLC did not induce an increase in the intracellular Ca2+ concentration of the enterocytes. The effect of PLC was blocked by the protein kinase inhibitor, staurosporine. Carbachol, when added during the second phase of the PLC response, induced a 'paradox' change in Isc: a rapid, transient increase in Isc was followed by a long-lasting decrease. This inhibition of the PLC response was more pronounced after elevation of the external Ca2+ concentration. A Ca2+ ionophore, ionomycin, and a Ca2+ channel activator, BAY K 8644, also inhibited the PLC response. The results suggest dual dependence of the action of PLC on the intracellular Ca2+ concentration.

Hydrogen peroxide stimulates phospholipase A2-mediated arachidonic acid release in cultured intestinal epithelial cells (INT 407)

The mechanisms by which hydrogen peroxide and, for comparison, 4-beta-phorbol-12-myristate-13-acetate (PMA) stimulate release of radiolabeled arachidonic acid (14C-AA) in cultured intestinal epithelial cells (INT 407) were investigated. Both hydrogen peroxide and PMA caused a rapid (3 min) and dose-related intracellular release of free 14C-AA, followed by a dose- and time-dependent release of 14C-AA into the extracellular medium, but hydrogen peroxide was about 50,000 times less effective than PMA in releasing 14C-AA. No 14C-AA was released on stimulation with 4-alpha-phorbol-12,13-di-decanoate (PDD), a phorbol ester that does not activate protein kinase C. The 14C-AA release was reduced by the phospholipase A2 inhibitors nordihydroguaiaretic acid and 4-bromophenacyl bromide and by the calmodulin/protein kinase C inhibitor trifluoperazine and the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). However, H-7 was less effective than the other inhibitors in reducing the hydrogen peroxide-stimulated 14C-AA release. The hydrogen peroxide-stimulated, but not the PMA-stimulated, rapid (3 min) 14C-AA release was associated with an increased influx of extracellular calcium. Stimulation of the cells with PMA resulted in phosphorylation of a cellular protein of about 32 kDa, whereas no phosphorylation of this protein was detected after stimulation with hydrogen peroxide. Taken together, these findings indicate that (i) both PMA and hydrogen peroxide may stimulate phospholipase A2-mediated AA release from human intestinal epithelial cells; (ii) this stimulation is brought about via protein kinase C and calmodulin-mediated events; (iii) PMA-stimulated 14C-AA release is associated with phosphorylation of a 32-kDa protein, possibly lipocortin, whereas the hydrogen peroxide-stimulated release is not; and (iv) calmodulin is more important for the hydrogen peroxide-stimulated 14C-AA release than is protein kinase C. The possibility that hydrogen peroxide-evoked AA release may contribute to the mucosal abnormality in Crohn's disease is discussed.

Phospholipase activation and arachidonic acid release in intestinal epithelial cells from patients with Crohn's disease

A method for studying the mobilization of free arachidonic acid (AA) in viable isolated human intestinal epithelial cells has been developed and applied to the study of patients with Crohn's disease. Cells were isolated from morphologically unaffected parts of the distal ileum and incubated with 14C-AA; most of the incorporated 14C-AA was then found in phospholipids (mainly phosphatidylcholine) and in a pool of neutral lipids (mainly triacylglycerols). Cells from patients with Crohn's disease incorporated more 14C-AA into their neutral lipids than did cells from control patients. When the labeled cells were stimulated with phospholipase C from Clostridium perfringens or with the calcium ionophore A23187, they released significant amounts of AA, mainly from phosphatidylcholine. There was no difference between cells from Crohn patients and controls in the 14C-AA amounts released, but unstimulated and phospholipase C-stimulated cells from prednisolone-treated Crohn patients released less AA than cells from control patients. The A23187-stimulated AA release was completely inhibited by the phospholipase A2 inhibitor 4-bromophenacyl bromide, whereas the phospholipase C-stimulated release was not. These findings suggest that AA release in human small-intestinal epithelial cells may be caused by calcium-mediated phospholipase A2 activation or by products of microbial phospholipase C activity and that prednisolone reduces the mobilization of free AA in intestinal epithelial cells. They also illustrate the potential use of isolated epithelial cells for revealing mechanisms underlying AA release in the intestinal mucosa in different disease states.

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.