Regulation of prostaglandin production in cultured gastric mucosal cells

Urinary tract obstruction induces transient accumulation of COX-2-derived prostanoids in kidney tissue

Inhibitors of cyclooxygenase (COX)-2 prevent suppression of aquaporin-2 and reduce polyuria in the acute phase after release of bilateral ureteral obstruction (BUO). We hypothesized that BUO leads to COX-2-mediated local accumulation of prostanoids in inner medulla (IM) tissue. To test this, rats were subjected to BUO and treated with selective COX-1 or COX-2 inhibitors. Tissue was examined at 2, 6, 12, and 24 h after BUO. COX-2 protein abundance increased in IM 12 and 24 h after onset of BUO but did not change in cortex. COX-1 did not change at any time points in any region. A full profile of all five primary prostanoids was obtained by mass spectrometric determination of PGE(2), PGF(2alpha), 6-keto-PGF(1alpha), PGD(2), and thromboxane (Tx) B(2) concentrations in kidney cortex/outer medulla and IM fractions. IM concentration of PGE(2), 6-keto-PGF(1alpha), and PGF(2alpha) was increased at 6 h BUO, and PGE(2) and PGF(2alpha) increased further at 12 h BUO. TxB(2) increased after 12 h BUO. 6-keto-PGF(1alpha) remained significantly increased after 24 h BUO. The COX-2 inhibitor parecoxib lowered IM PGE(2,) TxB(2), 6-keto-PGF(1alpha), and PGF(2alpha) below vehicle-treated BUO and sham rats at 6, 12 and, 24 h BUO. The COX-1 inhibitor SC-560 lowered PGE(2), PGF(2alpha), and PGD(2) in IM compared with untreated 12 h BUO, but levels remained significantly above sham. In cortex tissue, PGE(2) and 6-keto-PGF(1alpha) concentrations were elevated at 6 h only. In conclusion, COX-2 activity contributes to the transient increase in prostacyclin metabolite 6-keto-PGF(1alpha) and TxB(2) concentration in the kidney IM, and COX-2 is the predominant isoform that is responsible for accumulation of PGE(2) and PGF(2alpha) with minor, but significant, contributions from COX-1. PGD(2) synthesis is mediated exclusively by COX-1. In BUO, therapeutic interventions aimed at the COX-prostanoid pathway should target primarily COX-2.

Protective role of intracellular superoxide dismutase against extracellular oxidants in cultured rat gastric cells

We examined the role of intracellular superoxide dismutase (SOD) as an antioxidant by studying the effect of diethyldithiocarbamate (DDC) on extracellular H2O2-induced damage in cultured rat gastric mucosal cells. 51Cr-labeled monolayers from rat stomachs were exposed to glucose oxidase-generated H2O2 or reagent H2O2, which both caused a dose-dependent increase in 51Cr release. DDC dose-dependently enhanced 51Cr release by hydrogen peroxide, corresponding with inhibition of endogenous SOD activity. This inhibition was not associated either with modulation of other antioxidant defenses, or with potentiation of injury by nonoxidant toxic agents. Enhanced hydrogen peroxide damage by DDC was significantly prevented by chelating cellular iron with deferoxamine or phenanthroline. Inhibition of cellular xanthine oxidase (possible source of superoxide production) by oxypurinol neither prevented lysis by hydrogen peroxide nor diminished DDC-induced sensitization to H2O2. We conclude that (a) extracellular H2O2 induces dose dependent damage to cultured gastric mucosal cells; (b) intracellular SOD plays an important role in preventing H2O2 damage; (c) generation of superoxide seems to occur intracellularly after exposure to H2O2, but independent of cellular xanthine oxidase; and (d) cellular iron mediates the damage by catalyzing the production of more reactive species from superoxide and H2O2, the process which causes ultimate cell injury.

Roles of Ca2+ and protein kinase C in regulation of prostaglandin E2 release by cultured rabbit gastric epithelial cells

Prostaglandin (PG) has been reported to be one of the important protective factors in the gastric mucosa. However the mechanism of the regulation of endogenous PG production has not been well studied. We investigated the possible roles of Ca2+, cAMP, and protein kinase C (PKC) in the regulation of PGE2 release from cultured rabbit gastric mucosal cells. PGE2 was measured by radioimmunoassay. A23187 (Ca2+ ionophore) at 2 x 10(-6) M significantly increased PGE2 release. Deprivation of Ca2+ from the medium blocked the A23187-induced increase of PGE2. TMB-8 (a putative inhibitor of Ca2+ release from intracellular stores) did not have any significant effects on the increase of PGE2-induced by A23187. Thus, A23187 increased PGE2 through the influx of extracellular Ca2+. W7 or compound 48/80 (calmodulin inhibitors) did not alter the response of PGE2 caused by A23187. Exogenous administration of cAMP, forskolin (an activator of adenylate cyclase), or 2-chloroadenosine (a possible activator of adenylate cyclase through adenosine A2 receptor) had neither significant effects on PGE2 release nor an effect on A23187-induced increase of PGE2 release. 12-O-tetradecanoylphorbol 13-acetate (TPA, an activator of PKC) significantly stimulated PGE2 release in a dose-dependent fashion, whereas another phorbol ester with no biological activity did not. A23187 at 0.8 x 10(-6) M, but not cAMP, potentiated the TPA-induced increase of PGE2. Mepacrine (a phospholipase A2 inhibitor) reduced the A23187- and TPA-induced increase of PGE2. These results suggest that Ca2+ and protein kinase C may play important roles in the regulation of PGE2 release by cultured rabbit gastric cells.

Antioxidant protection against oxidant-induced damage in cultured gastric mucosal cells

Gastric epithelium is exposed not only to oxidants generated within the lumen, but also to those produced by ischemia/reperfusion. This study examined the mechanism(s) of oxidant-induced injury to cultured rat gastric mucosal cells, and characterized the antioxidant profile of these cells. Hydrogen peroxide (H2O2), generated by glucose oxidase, damaged cells dose-dependently, as assessed by increased leakage of labeled 51Cr. Glucose oxidase-induced damage was prevented by exogenous catalase (but not by exogenous superoxide dismutase). Chelation of cellular iron with desferrioxamine or phenanthroline specifically protected cells against H2O2, whereas binding of extracellular iron with apotransferrin failed to. Disruption of the glutathione redox cycle at three independent sites rendered cells less resistant to H2O2, whereas inhibition of cellular catalase did not result in sensitization of cells to H2O2. In conclusion, (1) oxidant injury induced by extracellular H2O2 is mediated by intracellular iron; (2) extracellular superoxide is not involved in the damaging process; and (3) the glutathione redox cycle plays a principal role in detoxifying H2O2 as a cellular antioxidant in cultured gastric mucosal cells.

Inhibition of human gastric cyclic AMP production by Helicobacter pylori protein--possible involvement of mucosal prostaglandin E2

The effect of Helicobacter pylori protein on cAMP and prostaglandin (PGE2) production was studied in incubates of human gastric fundic mucosa. At 24 hours, specimens incubated in the control fluid had a median cAMP value of 81 pmol/mg protein, compared to 28 pmol/mg (P less than 0.05) when incubated in H. pylori protein, 155 pmol/kg (P less than 0.006) in histamine, and 23 pmol/kg (P less than 0.05) in histamine plus H. pylori protein. A similar trend was observed at 48 hours. Although H. pylori protein had no direct effect on mucosal PGE2, it intensified the inhibitory effect of indomethacin and prevented the stimulatory effect of histamine on both PGE2 and cAMP production. Given the role of cAMP in various physiological responses, these results suggest that H. pylori protein might alter those functional aspects of the human gastric mucosa which rely on cAMP as a second messenger. Assuming that PGE2 is involved in mediating such effect, its role would appear to be either partial or indirect.

Effects of 16,16-dimethyl prostaglandin E2 on glycoprotein and lipid synthesis of gastric epithelial cells grown in a primary culture

We investigated the biosynthesis of phospholipid, neutral lipids, glycoproteins, and DNA in primary cultures of rat oxyntic mucosal cells. In addition, responses of these biosynthetic pathways to the gastric protective agent 16,16-dimethyl prostaglandin E2 (dmPGE2) were studied. Cultured gastric cells under control conditions synthesized glycoprotein in a linear manner over time. The cells responded to dmPGE2 with an increase in glycoprotein synthesis without an effect on DNA synthesis. Investigations of lipid synthesis showed that phospholipid was produced in a linear fashion by these cells, however, no effect of exogenously administered dmPGE2 on its rate of formation was discernible. In contrast, the incorporation of labeled palmitate into neutral lipids revealed that triglyceride biosynthesis was significantly increased by the addition of dmPGE2 to the culture medium, which could be further enhanced by the administration of the phosphodiesterase inhibitor, isobutyl methyl xanthine. Cyclic nucleotide involvement was further suggested by our finding that triglyceride synthesis in cultured gastric mucous cells could be increased a comparable amount by the addition of both dbcAMP and dbcGMP to the medium. The possible relationship between these biochemical alterations and the gastric protective action of dmPGE2 is discussed.