Burkhard Krüger

The Role of Intracellular Calcium Signaling in Premature Protease Activation and the Onset of Pancreatitis

The exocrine pancreas synthesizes and secretes large amounts of digestive proteases as inactive precursor zymogens. Under physiological conditions a variety of cellular defense mechanisms protect the pancreatic acinar cell against a premature and intracellular activation of these zymogens. When these defenses fail, pancreatic autodigestion is initiated and acute pancreatitis can develop. A number of experimental observations suggest that extra- as well as intracellular calcium concentrations play an important part in the initiation of pancreatic protease activation, but the intracellular signaling events that regulate this process are unknown. Using a model system in which we used pancreatic acini (freshly prepared functional units of living acinar cells), we were able to simulate the conditions found during experimental pancreatitis in rodents. By means of a cell permeant fluorescent trypsin substrate we could demonstrate in these acini that premature protease activation is initiated at the apical acinar cell pole and occurs only in the presence of secretagogue concentrations that exceed those required for a maximum secretory response. By combining this technique with fluorescence ratio imaging for the Ca(2+)-sensitive dye fura-2, we could further show that this protease activation is highly dependent on the spatial as well as the temporal distribution of the corresponding Ca(2+) release from stores within the same subcellular compartment and that it is not propagated to neighboring acinar cells.

Autoantibodies against the exocrine pancreas in autoimmune pancreatitis: gene and protein expression profiling and immunoassays identify pancreatic enzymes as a major target of the inflammatory process

OBJECTIVES:Autoimmune pancreatitis (AIP) is thought to be an immune-mediated inflammatory process, directed against the epithelial components of the pancreas. The objective was to identify novel markers of disease and to unravel the pathogenesis of AIP.METHODS:To explore key targets of the inflammatory process, we analyzed the expression of proteins at the RNA and protein level using genomics and proteomics, immunohistochemistry, western blot, and immunoassay. An animal model of AIP with LP-BM5 murine leukemia virus-infected mice was studied in parallel. RNA microarrays of pancreatic tissue from 12 patients with AIP were compared with those of 8 patients with non-AIP chronic pancreatitis.RESULTS:Expression profiling showed 272 upregulated genes, including those encoding for immunoglobulins, chemokines and their receptors, and 86 downregulated genes, including those for pancreatic proteases such as three trypsinogen isoforms. Protein profiling showed that the expression of trypsinogens and other pancreatic enzymes was greatly reduced. Immunohistochemistry showed a near-loss of trypsin-positive acinar cells, which was also confirmed by western blotting. The serum of AIP patients contained high titers of autoantibodies against the trypsinogens PRSS1 and PRSS2 but not against PRSS3. In addition, there were autoantibodies against the trypsin inhibitor PSTI (the product of the SPINK1 gene). In the pancreas of AIP animals, we found similar protein patterns and a reduction in trypsinogen.CONCLUSIONS:These data indicate that the immune-mediated process characterizing AIP involves pancreatic acinar cells and their secretory enzymes such as trypsin isoforms. Demonstration of trypsinogen autoantibodies may be helpful for the diagnosis of AIP.

Tumour necrosis factor α secretion induces protease activation and acinar cell necrosis in acute experimental pancreatitis in mice

Background Acute pancreatitis has long been considered a disorder of pancreatic self-digestion, in which intracellular activation of digestive proteases induces tissue injury. Chemokines, released from damaged pancreatic cells then attract inflammatory cells, whose systemic action ultimately determines the disease severity. In the present work the opposite mechanism is investigated; that is, whether and how inflammatory cells can activate intracellular proteases. Design Using mice either deficient for the CD18-α subunit of the membrane attack complex-1 (MAC-1) complex or tumour necrosis factor (TNF)α, as well as after depletion of leucocyte subpopulations, pancreatitis was induced by 7-hourly caerulein injections (50 μg/kg, intraperitoneally). Pancreatic acini were coincubated in vitro from wild-type and cathepsin-B-deficient animals with phorbol-12-myristate-13-acetate (PMA)-activated neutrophils and macrophages, caerulein or TNFα, and activities of trypsin, cathepsin-B and caspase-3 were measured, as well as necrosis using fluorogenic substrates. TNFα was inhibited with monospecific antibodies. Results Deletion of CD18 prevented transmigration of leucocytes into the pancreas during pancreatitis, greatly reduced disease severity and abolished digestive protease activation. Depletion of neutrophils and macrophages equally reduced premature trypsinogen activation and disease severity. In vitro activated neutrophils and macrophages directly induced premature protease activation and cell death in pancreatic acini and stimulation of acini with TNFα induced caspase-3 activation and necrosis via a cathepsin-B and calcium-dependent mechanism. Neutralising antibodies against TNFα and genetic deletion of TNFα prevented leucocyte-induced trypsin activity and necrosis in isolated acini. Conclusions The soluble inflammatory cell mediator TNFα directly induces premature protease activation and necrosis in pancreatic acinar cells. This activation depends on calcium and cathepsin-B activity. The findings from the present work further suggest that targeting TNFα, for which pharmaceutical agents are readily available, could be an effective treatment strategy that directly addresses the cellular causes of pancreatitis.

The calcium binding protein S100A9 is essential for pancreatic leukocyte infiltration and induces disruption of cell–cell contacts†

Leukocyte infiltration is an early and critical event in the development of acute pancreatitis. However, the mechanism of leukocyte transmigration into the pancreas and the function of leukocytes in initiating acute pancreatitis are still poorly understood. Here, we studied the role of S100A9 (MRP14), a calcium binding protein specifically released by polymorph nuclear leukocytes (PMN), in the course of acute experimental pancreatitis. Acute pancreatitis was induced by repeated supramaximal caerulein injections in S100A9 deficient or S100A9 wild‐type mice. We then determined S100A9 expression, trypsinogen activation peptide (TAP) levels, serum amylase and lipase activities, and tissue myeloperoxidase (MPO) activity. Cell–cell contact dissociation was analyzed in vitro with biovolume measurements of isolated acini after incubation with purified S100A8/A9 heterodimers, and in vivo as measurement of Evans Blue extravasation after intravenous application of S100A8/A9. Pancreatitis induced increased levels of S100A9 in the pancreas. However, infiltration of leukocytes and MPO activity in the lungs and pancreas during acute pancreatitis was decreased in S100A9‐deficient mice and associated with significantly lower serum amylase and lipase activities as well as reduced intrapancreatic TAP‐levels. Incubation of isolated pancreatic acini with purified S100A8/A9‐heterodimers resulted in a rapid dissociation of acinar cell–cell contacts which was highly calcium‐dependent. Consistent with these findings, in vivo application of S100A8/A9 in mice was in itself sufficient to induce pancreatic cell–cell contract dissociation as indicated by Evans Blue extravasation. These data show that the degree of intrapancreatic trypsinogen activation is influenced by the extent of leukocyte infiltration into the pancreas which, in turn, depends on the presence of S100A9 that is secreted from PMN. S100A9 directly affects leukocyte tissue invasion and mediates cell contact dissociation via its calcium binding properties. J. Cell. Physiol. 216: 558–567, 2008.

Tauroursodeoxycholic acid reduces endoplasmic reticulum stress, trypsin activation, and acinar cell apoptosis while increasing secretion in rat pancreatic acini

Endoplasmic reticulum (ER) stress leads to accumulation of un- or misfolded proteins inside the ER and initiates the unfolded protein response (UPR). Several UPR components are physiologically involved in pancreatic development and are pathophysiologically activated during acute pancreatitis. However, the exact role of ER stress in exocrine pancreatic acini is mainly unclear. The present study examined the effects of tauroursodeoxycholic acid (TUDCA), a known ER chaperone, on acinar function and UPR components. Isolated rat pancreatic acini were stimulated by increasing concentrations of cholecystokinin (CCK-8) with or without preincubation of TUDCA. UPR components were analyzed, including chaperone binding protein (BiP), protein kinase-like ER kinase (PERK), X-box binding protein (XBP)-1, c-Jun NH(2)-terminal kinase (JNK), CCAAT/enhancer binding protein homologues protein (CHOP), caspase 3 activation, and apoptosis. In addition, TUDCA effects were measured on amylase secretion, calcium signaling, trypsin, and cathepsin B activation. TUDCA preincubation led to a significant increase in amylase secretion after CCK-8 stimulation, a 50% reduction of intracellular trypsin activation, and reduced cathepsin B activity, although the effects for cathepsin B were not statistical significant. Furthermore, TUDCA prevented the CCK-8-induced BiP upregulation, diminished PERK and JNK phosphorylation, and prohibited the expression of CHOP, caspase 3 activation and apoptosis. XBP-1 splicing was not altered. ER stress response mechanisms are activated in pancreatic inflammation. Chemical chaperones enhance enzyme secretion of pancreatic acini, reduce ER stress responses, and attenuate ER stress-associated apoptosis. These data hint new perspectives for an employment of chemical chaperones in the therapy of acute pancreatitis.

Protein tyrosine phosphatase κ and SHP-1 are involved in the regulation of cell-cell contacts at adherens junctions in the exocrine pancreas

Background: We have previously shown that cell contacts between pancreatic acinar cells dissociate early in pancreatitis and that this is a prerequisite for the development of pancreatic oedema. Here we studied the underlying mechanism. Methods: Employing experimental caerulein induced pancreatitis in vivo and isolated pancreatic acini ex vivo, in conjunction with protein chemistry, morphology, and electron microscopy, we determined whether cell contact regulation in the pancreas requires or involves: (1) changes in cadherin-catenin protein expression, (2) tyrosine phosphorylation of adhesion proteins, or (3) alterations in the actin cytoskeleton. Results: During initial cell-cell contact dissociation at adherens junctions, expression of adhesion proteins remained stable. At time points of dissociated adherens junctions, the cadherin-catenin complex was found to be tyrosine phosphorylated and internalised. The receptor type protein tyrosine phosphatase (PTP)κ was constitutively associated with the cadherin-catenin complex at intact cell contacts whereas following the dissociation of adherens junctions, the internalised components of the cadherin-catenin complex were tyrosine phosphorylated and associated with the cytosolic PTP SHP-1. In isolated acini, inhibition of endogenous protein tyrosine phosphatases alone was sufficient to induce dissociation of adherens junctions analogous to that found with supramaximal caerulein stimulation. Dissociation of actin microfilaments had no effect on adherens junction integrity. Conclusions: These data identify tyrosine phosphorylation as the key regulator for cell contacts at adherens junctions and suggest a definitive role for the protein tyrosine phosphatases PTPκ and SHP-1 in the regulation, maintenance, and restitution of cell adhesions in a complex epithelial organ such as the pancreas.

Pathophysiology of alcohol-induced pancreatitis

Excessive ethanol consumption is a common risk factor for acute and chronic pancreatitis. Ethanol could lead to the onset of pancreatitis in a number of ways; the most recently discovered is its effect on intrapancreatic digestive enzyme activation, by either sensitizing acinar cells to pathologic stimuli or stimulating the release of a secretagogue (cholecystokinin) from duodenal I cells. Recent advances in cell biologic and molecular techniques have permitted us to address the intracellular events involved in digestive enzyme activation in a manner that was previously considered impossible. Investigations that used these novel techniques found that (a) trypsin is, in contrast to its role in the small intestine, not necessarily involved in the premature intracellular activation of other digestive proteases such as proelastase; (b) trypsinogen does not autoactivate intracellularly but is instead largely activated by the lysosomal hydrolase cathepsin B; and (c) the role of trypsin in the intrapancreatic protease cascade is most likely one that involves the degradation, rather than the activation, of active digestive proteases including trypsin itself. These studies, as well as investigations that have addressed the role of mutant trypsin in the disease onset of hereditary pancreatitis, suggest that trypsin may not be critical for triggering pancreatitis but might have a protective role against the action of some of the other digestive proteases. While the specific role of different digestive enzymes in initiating pancreatitis is still a matter of debate and the topic of ongoing investigations, experimental evidence suggests that ethanol can directly interfere with the processes involved in digestive zymogen activation.

4-Phenylbutyric acid reduces endoplasmic reticulum stress, trypsin activation, and acinar cell apoptosis while increasing secretion in rat pancreatic acini.

Objectives Endoplasmic reticulum (ER) stress leads to misfolded proteins inside the ER and initiates unfolded protein response (UPR). Unfolded protein response components are involved in pancreatic function and activated during pancreatitis. However, the exact role of ER stress in the exocrine pancreas is unclear. The present study examined the effects of 4-phenylbutyric acid (4-PBA), an ER chaperone, on acini and UPR components. Methods Rat acini were stimulated with cholecystokinin (10 pmol/L to 10 nmol/L) with or without preincubation of 4-PBA. The UPR components were analyzed, including chaperone-binding protein, protein kinaselike ER kinase, X-box–binding protein 1, c-Jun NH2-terminal kinase, CCAAT/enhancer-binding protein homologous protein, caspase 3, and apoptosis. Effects of 4-PBA were measured on secretion, calcium, and trypsin activation. Results 4-Phenylbutyric acid led to an increase of secretion, whereas trypsin activation with supraphysiological cholecystokinin was significantly reduced. 4-Phenylbutyric acid prevented chaperone-binding protein up-regulation, diminished protein kinaselike ER kinase, and c-Jun NH2-terminal kinase phosphorylation, prohibited X-box–binding protein 1 splicing and CCAAT/enhancer-binding protein homologous protein expression, caspase 3 activation, and apoptosis caused by supraphysiological cholecystokinin. Conclusion By incubation with 4-PBA, beneficial in urea cycle deficiency, it was possible to enhance enzyme secretion to suppress trypsin activation, UPR activation, and proapoptotic pathways. The data hint new perspectives for the use of chemical chaperones in pancreatic diseases.

The Role of Cysteine Proteases in Intracellular Pancreatic Serine Protease Activation

Autodigestion by proteolytic enzymes is thought to represent the critical mechanism by which acute pancreatitis is initiated. Where and why pancreatic proteases, which are physiologically stored and secreted as inactive precursor zymogens, are activated within the pancreas has remained controversial. Here we present data which indicate that: the lysosomal protease cathepsin B can activate trypsinogen in vitro in a manner that is similar to trypsinogen activation by enterokinase; that cathepsin B colocalizes with trypsinogen in the secretory compartment of the rat pancreas and of the human pancreas; that trypsinogen activation begins in a secretory compartment that is distinct from mature zymogen granules; and that the inhibition of cathepsin B can either increase or decrease premature trypsinogen activation depending on the concentration of the inhibitor, its specificity and its site of action in the pancreatic acinar cell. These observations elucidate some of the complex relations between cysteine and serine proteases in the pancreas with respect to their mechanisms of activation, their subcellular sites of action, and their possible role in the onset of pancreatitis.

N-acetylcysteine impairs survival of luteal cells through mitochondrial dysfunction.

N‐acetylcysteine (NAC) is known as an antioxidant and used for mucus viscosity reduction. However, this drug prevents or induces cell death depending on the cell type. The response of steroidogenic luteal cells to NAC is unknown. Our data shows that NAC can behave as an antioxidant or prooxidant in dependency on the concentration and mitochondrial energization. NAC elevated the flowcytometric‐measured portion of hypodiploid (dying) cells. This rise was completely abolished by aurintricarboxylic acid, an inhibitor of topoisomerase II. NAC increased the secretion of nitric oxide and cellular nitrotyrosine. An image analysis indicated that cells pretreated with NAC and loaded with DHR showed a fluorescent structure probably elicited by the oxidative product of DHR, rhodamine 123 that sequesters mitochondrially. Pretreating luteal cells with NAC or adding NAC directly to mitochondrial fractions followed by assessing the mitochondrial transmembrane potential difference (Δψ) by the JC‐1 technique demonstrated a marked decrease in Δψ. A protonophore restored Δψ and rotenone (an inhibitor of respiratory chain complex I) inhibited mitochondrial recovering. Thus, in steroidogenic luteal cells from healthy mature corpus luteum, NAC impairs cellular survival by interfering with mitochondrial metabolism. The protonophore‐induced recovering of NAC‐provoked decrease in Δψ indicates that an ATP synthase‐favored route of H+ re‐entry to the matrix is essentially switched off by NAC while other respiratory chain complexes remain intact. These data may be important for therapeutic timing of treatments with NAC.