The pancreatic duct epithelium in vitro: bile acid injury and the effect of epidermal growth factor

Hepatic Lipidosis in Ruminants

Hepatic lipidosis (ie, fatty liver) occurs primarily during the first weeks of lactation in dairy cows because of excessive lipolysis overwhelming the concomitant capacity for beta-oxidation and hepatic export of triglycerides. Besides economic losses due to reduced lactational and reproductive performance, close associations with concomitantly occurring infectious and metabolic health disorders, in particular ketosis, exist. Hepatic lipidosis is not only a consequence from the postpartal negative energy balance but also acts as a disease component for further health disorders.

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.

Invited review: pathology, etiology, prevention, and treatment of fatty liver in dairy cows

Fatty liver (i.e., hepatic lipidosis) is a major metabolic disorder of many dairy cows in early lactation and is associated with decreased health status and reproductive performance. In severe cases, milk production and feed intake are decreased. Therefore, a practical preventative or an efficacious treatment of fatty liver could save millions of dollars yearly in treatment, replacement, and production losses for dairy farmers. Fatty liver develops when the hepatic uptake of lipids exceeds the oxidation and secretion of lipids by the liver, which usually is preceded by high concentrations of plasma NEFA mobilized from adipose tissue. Excess lipids are stored as triacylglycerol in the liver and are associated with decreased metabolic functions of the liver. Liver can be categorized into normal liver or mild, moderate, or severe fatty liver; the latter can be subdivided further into nonencephalopathic severe fatty liver and hepatic encephalopathy. Insufficient or unbalanced dietary intake, obesity, and elevated estrogen concentrations are involved in the etiology of fatty liver, which is associated with greater incidence of dystocia, diseases, infections, and inflammations. Because even mild fatty liver is associated with decreased health status and reproductive performance of dairy cows, prevention of fatty liver by supplying cows with sufficient nutrients and a clean and health-promoting environment in the peripartal period would reduce production losses of cows more than would any treatment of fatty liver. This, however, might not be enough for cows that are obese or do not eat well, had calving difficulties or twins, have metabolic or infectious diseases, or are in severe negative energy balance because of high milk production immediately after calving. Potential and commonly used preventatives, as well as treatments, are discussed in the review. Currently, detection of fatty liver is possible only by minor surgery. Ultrasonic techniques offer a potential tool to noninvasively detect fatty liver. Future gene-array and proteomic studies may provide means to detect early molecular events in the etiology of fatty liver plus their connection with immune function and reproductive performance so that more effective treatments and preventatives of fatty liver can be developed. Such advances hopefully will make fatty liver a problem of the past.

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.

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.

Effects of epidermal growth factor (EGF) on fetal islet B cells in vitro

ABSTRACT. It has been reported that when the rat fetus is treated with streptozotocin (STZ) in vivo, islet B cells are destroyed but later recover. To investigate the process of the recovery of B cells after in vitro treatment of the fetal pancreas with STZ and the role of epidermal growth factor (EGF) in the recovery of B cells, we measured the level of insulin released from the cultured fetal pancreas and examined it histologically. As a result, we immunohistologically confirmed the regeneration of B cells in the pancreas that had been cultured for 48 hr after destruction of islet B cells by STZ treatment. An immunohistologic study using proliferating cell nuclear antigen (PCNA) showed that without the addition of EGF, the cell division index was significantly higher in the STZ-treated group (STZ group) than in the untreated group (intact group), whereas with the addition of EGF, the cell division index increased in both groups, but EGF did not have a significant cell division-promoting effect on the pancreas in the STZ group. The addition of EGF caused a significant decrease in the concentration of insulin in culture medium in both groups. These results indicate that EGF has a cell growth-promoting effect on intact fetal pancreas in vitro but has the effect of inhibiting the release of insulin, and thus suggest that EGF does not trigger the regeneration of islet B cells.

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

BACKGROUND

Acute pancreatitis is associated with passage of gallstones, although the mechanism(s) linking the two processes remains undefined. Bile reflux into the pancreatic duct could play a role but the experimental conditions often employed to induce pancreatitis rarely develop clinically. Here we examined whether low concentrations of bile affect ductal electrophysiology as an indirect measure of ductal epithelial integrity and function in vitro.

METHODS

The main duct was dissected out of freshly harvested bovine pancreata, cut into 1- x 2-cm sections, placed in tissue culture for 48-72 h, then placed in Ussing chambers. Changes in tissue resistance (Rt) and short-circuit current (Isc) were monitored. The responses to forskolin and bile (taurodeoxycholic acid, TDCA) were examined separately and together.

RESULTS

Forskolin (10 microM) produced a decrease in the Isc without a significant change in Rt, suggesting a secretory response, followed by a return to baseline. TDCA caused a similarly reversible decrease in the Isc at low doses, but a persistent drop at higher concentrations. A concurrent drop in Rt was noted at all TDCA concentrations, the duration of which correlated with dosage and degree of histological damage. Prior exposure to low (0.5 mM) doses of TDCA significantly blunted the response to subsequent forskolin challenge.

CONCLUSIONS

Acute exposure to TDCA in vitro causes epithelial damage at levels lower than those normally used to induce experimental pancreatitis. At the lower concentrations, Rt returns to baseline rapidly, suggesting recovery (restitution) from epithelial damage but with a persistent loss of the response to forskolin. Reflux of minute amounts of bile into the pancreatic duct could play a significant role in the pathogenesis of gallstone pancreatitis by uncoupling the normal stimulus-secretion apparatus of the ductal system and breaking down the epithelial barrier.