Acute effects of bile acids on the pancreatic duct epithelium in vitro

Crosstalk of tight junction components with signaling pathways

Tight junctions (TJs) regulate the passage of ions and molecules through the paracellular pathway in epithelial and endothelial cells. TJs are highly dynamic structures whose degree of sealing varies according to external stimuli, physiological and pathological conditions. In this review we analyze how the crosstalk of protein kinase C, protein kinase A, myosin light chain kinase, mitogen-activated protein kinases, phosphoinositide 3-kinase and Rho signaling pathways is involved in TJ regulation triggered by diverse stimuli. We also report how the phosphorylation of the main TJ components, claudins, occludin and ZO proteins, impacts epithelial and endothelial cell function.

Paradoxical effects of hydrogen peroxide on human airway anion secretion

The present study concerns intriguing effects of hydrogen peroxide (H2O2) on cAMP-mediated anion secretion in polarized human airway epithelia. Although H2O2 applied to the apical and basolateral membrane increases short-circuit currents (ISC) with analogous properties, it has opposite effects on subsequent cAMP-activated ISC responses. Namely, forskolin (FK)-induced ISC responses were down-regulated by the apical presence of H2O2, whereas they were up-regulated by its basolateral presence. Despite this contrasting effect, oxidative stimuli from either aspect of the monolayer hindered FK-induced increments in cytosolic cAMP levels and apical membrane Cl- conductance. The site-dependent effects of H2O2 were reproduced in the responses to 8-bromo-cAMP. Estimation of the anionic composition of the ISC revealed that the FK up-regulated both bumetanide [an Na+-K+-2Cl- cotransporter (NKCC1) inhibitor]-sensitive and 4,4'-dinitrostilbene-2,2'-disulfonic acid [an HCO3--dependent anion transporter (NBC1/AE2) inhibitor]-sensitive ISC in the control, whereas the up-regulation evidently favored bumetanide-sensitive ISC in the basolateral presence of H2O2. The FK-induced NKCC1 augmentation after exposure to basolateral H2O2 was counteracted by cytochalasin D, an inhibitor of microfilament function, but not by charybdotoxin, a blocker of the intermediate conductance Ca2+-activated K+ channel, whose activation could be related to NKCC1-mediated Cl- secretion. These observations suggest that basolaterally but not apically applied H2O2 potentiates subsequent cAMP-mediated Cl- secretion by an increase in Cl- uptake via basolateral NKCC1, whose sensitivities to cAMP/protein kinase A are up-regulated, overcoming the H2O2-induced inhibition of cAMP-mediated apical anion conductance. The basolateral membrane-specific effects of H2O2 may be relevant to the basolateral cytoskeleton, which is believed to interact with NKCC1.

Protease-activated receptor-2 regulates bicarbonate secretion by pancreatic duct cells in vitro

BACKGROUND

Pancreatic duct cells (PDCs) are responsible for bicarbonate production by the pancreas. The trypsin-sensitive protease-activated receptor (PAR-2), also known as the trypsin receptor, is highly expressed in the pancreatic duct system and has been shown to regulate PDC ion transport. The possible role of this receptor on bicarbonate secretion, the central function of duct cells, is unknown. We hypothesize that PAR-2 may regulate pancreatic bicarbonate secretion during times of inappropriate pancreatic enzyme activation.

METHODS

To study this hypothesis in vitro, explants of the bovine main pancreatic duct were isolated and maintained in primary culture. They were then mounted in Ussing chambers, and bicarbonate secretion was determined with an autoburette titration. The response to luminal or serosal trypsin (10 micromol/L) and the synthetic trypsin receptor activating peptide (TRAP) (30 micromol/L) on spontaneous and secretin-stimulated bicarbonate secretion (10 nmol/L) was examined.

RESULTS

Serosal trypsin had no effect. Both luminal trypsin and TRAP significantly reduced the spontaneous bicarbonate secretion observed at luminal pH 7.4 (2.8 +/- 0.2 - 0.4 +/- 0.1 micromol/hr/cm(2) and 4.0 +/- 1.2 - 1.6 +/- 0.4 micromol/hr/cm(2), respectively) in a reversible manner. Baseline bicarbonate secretion at luminal pH 8.0 was reduced by trypsin and TRAP, but the increase in response to secretin stimulation observed with controls was unaffected.

CONCLUSIONS

PAR-2 activation may be the mechanism by which pancreatic juice secretion is inhibited during pancreatitis. We suggest that pharmacologic activation of PAR-2 receptors could suppress pancreatic exocrine secretion and thus serve as a potential agent in the treatment and prevention of pancreatic fistulas.

Effects of bile acids on dog pancreatic duct epithelial cell secretion and monolayer resistance

Pancreatic duct epithelial cells (PDEC) mediate the secretion of fluid and electrolytes and are exposed to refluxed bile. In nontransformed cultured dog PDEC, which express many ion transport pathways of PDEC, 1 mM taurodeoxycholic acid (TDCA) stimulated an (125)I(-) efflux inhibited by DIDS and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and a (86)Rb(+) efflux inhibited by charybdotoxin. Inhibition by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM suggests mediation via increased intracellular Ca(2+) concentration, whereas the absence of lactate dehydrogenase release excludes cellular toxicity. At 1 mM, TDCA stimulated a larger (125)I(-) efflux than glycodeoxycholate; two dihydroxy bile acids, taurochenodeoxycholate and TDCA, were similarly effective, whereas a trihydroxy bile acid, taurocholate, was ineffective. In Ussing chambers, 1 mM serosal or 2 mM luminal TDCA stimulated an I(sc) increase from confluent PDEC monolayers. TDCA also stimulated 1) a short-circuit current (I(sc)) increase from basolaterally permeabilized PDEC subject to a serosal-to-luminal Cl(-) gradient that was inhibited by BAPTA-AM, DIDS, and NPPB and 2) an I(sc) increase from apically permeabilized PDEC subject to a luminal-to-serosal K(+) gradient inhibited by BAPTA-AM and charybdotoxin. Along with the efflux studies, these findings suggest that TDCA interacts directly with PDEC to stimulate Ca(2+)-activated apical Cl(-) channels and basolateral K(+) channels. Monolayer transepithelial resistance was only minimally affected by 1 mM serosal and 2 mM luminal TDCA but decreased after exposure to higher TDCA concentrations (2 mM serosal and 4 mM luminal). A secretory role for bile acids should be considered in pancreatic diseases associated with bile reflux.

Influence of epidermal growth factor on bovine pancreatic duct cell bicarbonate

BACKGROUND AND AIMS

Epidermal growth factor (EGF) is secreted in pancreatic juice and its receptor is expressed on pancreatic duct cells (PDCs), suggesting a physiological role which has yet to be defined. Here we examined the effects of EGF on bicarbonate production and carbonic anhydrase (CA) activity in a PDC explant model.

METHODS

Bovine main PDCs were prepared and maintained in culture as explants. Levels of CA expression, phosphorylation, and enzymatic activity were measured in resting cells and compared to that of cells exposed to 10 nM secretin, 10 nM EGF, or both. Bicarbonate production was measured using the autoburette pH titration technique.

RESULTS

CA protein levels were unchanged with any treatment, but enzyme activity increased by 180% with secretin treatment and was reduced by 54% with EGF. The combination treatment led to a synergistic increase 240% above basal. EGF alone did not affect bicarbonate secretion, but the normal increase observed with secretin stimulation (1.3 +/- 0.4 to 2.9 +/- 0.6 micromol/h/cm(2)) was abolished by acute EGF pretreatment. On the other hand, EGF pretreatment for 24 h significantly increased basal and stimulated secretion (2.2 +/- 0.5 and 3.8 +/- 0.5, respectively) compared to controls.

CONCLUSIONS

EGF exerts a regulatory role on bicarbonate secretion by the pancreatic duct epithelium, independent of its effect on CA activity. Its inhibition of stimulated bicarbonate secretion could play a protective role in the setting of pancreatic inflammation, where increased levels of EGF are associated with reduced pancreatic juice production.

Protection by tetracyclines against ion transport disruption caused by nystatin in human airway epithelial cells

Polyene antifungal antibiotics like nystatin form monovalent cation pores on the plasma membrane that perturb the intracellular electrolyte milieu, resulting in cell damage. In the present study, we investigated the effects of tetracyclines (minocycline and tetracycline) on ion transporters disrupted by nystatin in cultured human airway Calu-3 cells. Apical application of nystatin (50 microM) on a monolayer of the cells stimulated Na(+)-K(+) pump activity as estimated by ouabain (1 mM)-sensitive short-circuit current (I(sc)). The nystatin-potentiated I(sc) was inhibited by minocycline (IC(50) = 25 microM) or tetracycline (IC(50) = 150 microM) applied only from the apical (nystatin-treated) side. Nystatin increased monolayer conductance that was reversed by the application of tetracyclines. In contrast, ouabain potentiated the nystatin-induced change in the conductance. Further, Na(+)-glucose transport affected by nystatin was also normalized by tetracyclines from the nystatin-treated side of the membrane. These data suggest that tetracyclines may lower the cell permeability potentiated by nystatin, protecting cells against damage.

The cytotoxic effects of bile acids in crude bile on human pancreatic cancer cell lines

Pancreatic cancer frequently causes extrahepatic cholestasis. To identify the direct effects of bile acids in jaundiced serum on pancreatic cancer, the proliferation of PANC-1 and MIA PaCa-2 cells as well as the ultrastructural alteration of PANC-1 cells cultured in crude bile modified media were studied. The growth of these cells in the RPMI-1640 media with or without 1%, 2%, and 4% of the refined crude bile was assessed after 48 and 96 h of incubation. The ultrastructure of PANC-1 cells was investigated by scanning and transmission electron microscopy after 24 and 48 h of incubation. The proliferation of both cell lines in the bile-treated media was greatly inhibited. The inhibitory rates of bile on PANC-1 ranged from 24.1% +/- 3.3% to 66.9% +/- 6.6% (P < 0.01) and those on MIA PaCa-2 ranged from 16.7% +/- 3.8% to 50.7% +/- 5.5%. (P < 0.01). When the bile-added media were replaced, the cells were able to restore their proliferating ability. The PANC-1 cells incubated in the bile-supplied media indicated that the mirovilli, mitochondria, and other organelles had thus been injured. These results suggest that bile acids appear to inhibit the proliferation of PANC-1 and MIA PaCa-2 cells, and the probable inhibitory mechanism is mainly considered to be due to the cytotoxicity of such bile acids.