sign in sign up
<<
Home , Digestive enzyme, Elastase, Enteropeptidase, Zymogen

Role of Pancreatic in Acute

M. V. SINGER, P. LAYER, and H. GOEBELL 1

Despite extensive clinical and experimental studies, the pathophysiology of acute pancreatitis is still poorly understood. Although in 80% of patients pancreatitis is associated with biliary tract disease and alcohol abuse, the precise mechanisms of induction and progression of pancreatic injury remain uncertain. There is clini• cal and experimental evidence that intrapancreatic activation of digestive en• zymes, and the subsequent "autodigestion", is the common underlying patholog• ical process for damage to the in acute pancreatitis [4]. However, one of the major mysteries of this disease remains unanswered, that is, how and where, within the pancreas, do digestive enzymes become activated during pancreatitis? We are still looking for the trigger mechanism of pancreatitis. In addition, we do not know whether the cardiac, pulmonary, and renal complications during acute pancreatitis are caused by the circulating pancreatic enzymes. Alternatively, these complications might be caused by toxic substances released from the inflamed pancreas or merely be a nonspecific response of different organs to severe intraab• dominal inflammation. In the present paper we shall give a short review of the pathogenetic role of pancreatic enzymes in acute pancreatitis. Three main areas will be discussed: 1. Why is it that intrapancreatic activation of pancreatic enzymes does not occur under normal conditions? What are the protective mechanisms? 2. What are the trigger mechanisms of intrapancreatic activation of digestive en• zymes? 3. What happens when digestive enzymes are activated? What are the damaging actions of the individual enzymes?

Protective Mechanisms

The pancreas protects itself against the potentially harmful effects of its own di• gestive enzymes in several ways. 1. All enzymes which can digest membranes (e.g., , , car• boxypeptidase, , and A) are synthesized and secreted as inactive precursors or and the activation of these zymogens nor• mally occurs only after they have been secreted into the . Enzymes which do not attack membranes (e.g., , ) are secreted in active forms.

1 Div. of Gastroenteroiogy, Dept. of Medicine, University of Essen, D-4300 Essen/FRG.

Diagnostic Procedures in Ed. by P. Malfertheiner and H. Ditschuneit © Springer-Verlag Berlin Heidelberg 1986 68 M. V. Singer et al.

2. The enzymes are stored in granules which are isolated from sur• rounding compartments of the acinar by phospholipid membranes. Should zymogens become inadvertently activated, their containment within membrane-enclosed intracellular spaces would, presumably, prevent cell in• JUry. 3. The acinar cell also synthesizes a which blocks any trypsin which is inadvertently present in the acinar cell. Not only pancreatic tissue, but also and serum contain proteolytic inhibitors (e.g., pancreatic trypsin inhibitor, alpha-I-antitrypsin, alpha-2-macroglobin [20]). Should become prematurely activated within the acinar cell, these trypsin inhibitors would be expected to bind and inactivate trypsin and, thus, prevent further zymogen activation within the cell. 4. The activating enzyme (enterokinase) is geographically separated from the pancreas. Upon reaching the duodenum, trypsinogen is activated by en• terokinase and trypsin activates the other zymogens. What are the initiators of the premature activation of zymogens to active en• zymes within the pancreas? There is good reason to believe that in addition to a special etiological mechanism some cofactors are needed to initiate actual de• struction of the pancreas and peripancreatic tissues (Tables 1,2). Some of the pro• posed mechanisms of pancreatic injury include: intrapancreatic reflux of or duodenal contents, direct disruption of the pancreatic parenchyma or duct, ob• struction of pancreatic ducts, altered pancreatic ductal permeability, ischemia, al-

Table 1. Etiological factors in pancreatitis. (Adapted from Schmidt and Creutzfeld [20])

I. Diseases of adjacent organs a) Biliary tract disease b) Duodenal disorders 2. Obstruction of pancreatic ducts 3. Alcoholism 4. Vascular disease 5. 6. Endocrine and metabolic disorders 7. Nervous factors 8. Allergy 9. Drugs, toxins 10. Hereditary pancreatitis 11. Trauma, operation

Table 2. Some initiators of acute pancreatitis. (Modified from Ranson [17])

Parenchymal or ductal disruption Obstruction of pancreatic duct or lymphatics Altered pancreatic ductal permeability Reflux of bile and/or duodenal contents Ischemia Altered acinar cell stability Role of Pancreatic Enzymes in Acute Pancreatitis 69 tered acinar cell stability, and activation of the . However, ob• jections have been made as to the significance of each of these factors. The mechanisms which most often have been incriminated in the initiation of pancreatic are reflux of duodenal contents and of bile into the pancreatic ducts. Reflux of duodenal contents (containing activated pancreatic enzymes, en• terokinase, bile, and lecithin) and bile into the pancreatic ducts leads to the gen• eration of bile monomers due to breakdown of micelles and of lysolecithin. These cytotoxic products initiate a positive feedback cycle of disruption of duct permeability, activation of enzymes, and acinar cell necrosis [13]. Recent studies in dogs [8, 16] have shown that transient pressure gradients occur across the pan• creatic duct sphincter of dogs which might favor reflux into the pancreatic duct without causing pancreatitis. Thus, reflux of bile and of duodenal content into the pancreatic duct may be a physiological event and unknown additional factors may be needed for initiating acute pancreatitis. How are the individual enzymes activated and what happens when they are activated? We cannot answer this question definitely since our knowledge is mainly derived from animal studies, but there are some good reasons to believe that the human pancreas acts in a similar way. A number of experimental models of pancreatitis have been developed (Table 3). From these studies we know that the critical ingredient must include disruption of acinar or ductal integrity, egress of pancreatic into tissue spaces, and activation of the enzymes. The similarity of experimental pancreatitis to the human disease is, obviously, always open to question, and the issue of how digestive enzymes become activated within the pancreas during clinical pancreati• tis remains both critical and unresolved. Mainly recent studies [2, 10, 12] in which noninvasive methods of inducing experimental pancreatitis have been used have provided some insight into the events which may occur during the early stages of pancreatitis. For example, studies in mice, in which a choline-deficient ethionine• supplemented diet was given to induce pancreatitis, have shown that in diet-in• duced pancreatitis, digestive enzyme secretion is blocked, zymogen granules accu• mulate, and zymogen granules fuse with by a process known as crino• phagy [7, 11]. As a result digestive enzymes and lysosomal are exposed to each other and the known ability oflysosomal enzymes to activate trypsinogen may explain the intrapancreatic activation of digestive enzymes observed in this

Table 3. Some models of experimental pancreatitis. (Adapted from Steer and Meldolesi [21))

Invasive models Retrograde ductal injection Intraparenchymal injection Closed duodenal loop Duct ligation and stimulation of secretion Noninvasive models Anticholinesterase insecticide Hyperstimulating doses of secretagogues Choline-deficient ethionine-supplemented diet 70 M. V. Singer et al. model of pancreatitis. Adler and Kern [1] concluded from their data obtained from six patients who died from acute pancreatitis that the heavy involvement of lysosomes in the autophagic removal of secretory product and cellular organelles is a mechanism which rids the cell of unused secretory and altered in• tracellular structures. In certain cases this process leads to cellular necrosis while in other cases it leads only to the progressive removal of distal secretory compart• ments from the exocrine cell.

Trypsin It is probably trypsin that plays a key role in the pathogenesis of acute pancreati• tis. This enzyme is able to activate the majority of proenzymes taking part in the process of autodigestion, such as trypsinogen (by autocatalysis), proelastase, pro• phospholipase A, and [20]. Traces of trypsin or chymotrypsin activity have been detected in human pancreatic juice or ascitic fluid in acute pancreatitis [3,6]. In experimental studies, significant amounts of trypsin, chymotrypsin, and elastase were found in pancreatic tissue in the early phase of the disease [18]. It is therefore thought that the activation of trypsinogen is the important triggering event in acute pancreatitis [9]. There is some indication that under certain circumstances only traces of active trypsin which may not be detected with enzymatic methods and which are rapidly inactivated by inhibitors of pancreatic tissue or plasma are sufficient to give ac• tivation of other pancreatic zymogens and to trigger the cascade of autodigestion known as acute pancreatitis. As mentioned above, this may occur when cell injury or necrosis results from a variety of causes. Trypsin may autocatalyze its own ac• tivation, although this is a very slow process. A lysosomal acid , cathep• sin Bl, can also activate trypsinogen. Trypsin can produce many harmful effects. It can act on several substrates to produce many substances responsible for the local and systemic effects of acute pancreatitis. Trypsin causes proteolytic destruction of the pancreatic paren• chyma. It converts kallikreinogen to kallikrein [15] thereby producing, indirectly, bradykinin. It also activates the clotting and complement systems via activation of the Hageman factor. These factors contribute to local inflammation, thrombo• sis, tissue damage, and hemorrhage and systemic manifestations of acute pan• creatitis.

Chymotrypsin has damaging effects similar to those of trypsin.

Mesotrypsin Most recently, Rinderknecht et al. [19] have reported the characterization of a novel form of trypsinogen in pancreatic juice, which they call mesotrypsinogen, because it migrates electrophoretically between the anionic and cationic forms of trypsinogen. It is, both biochemically and immunologically, distinct from these trypsinogens, but, like them, it is activated by and appears to be a serine . Mesotrypsinogen levels in pancreatic juice are less than 10% of the trypsinogen level, but the specific activity of meso trypsin, on a molar basis, Role of Pancreatic Enzymes in Acute Pancreatitis 71 is three times greater than that of trypsin. A very striking observation is the al• most total inability of biological trypsin inhibitors to reduce mesotrypsin activity. Mesotrypsinogen can activate trypysinogen even in the presence of pancreatic trypsin inhibitor. These findings suggest that mesotrypsin might be capable of causing intrapancreatic digestive enzyme activation during the early stages of pancreatitis. Furthermore, if mesotrypsin were released into the circulation, it could be expected to retain enzymatic activity even in the presence of various serum trypsin inhibitors including alpha-I-antitrypsin and alpha-2-macroglobin. Damage done to extra pancreatic tissue either by meso trypsin itself or by sub• stances produced as a result of the proteolytic activity of meso trypsin on circulat• ing substrates, therefore, might explain the multi organ abnormality sometimes seen in acute pancreatitis. What is the Physiological Function of Mesotrypsin? Obviously, these new and very exciting findings need further investigation and confirmation. Rinderknecht et al. [19] note that mesotrypsin can degrade trypsinogen to inactive products without liberating free trypsin. Thus, the physiological function of meso trypsin may ac• tually be that of removing trypsinogen from environments which favor its prema• ture and possibly pathological activation and, thus, preventing rather than caus• ing, intra pancreatic digestive enzyme activation. A number of critical issues re• main unexplored. For example, what is the pancreatic content of mesotrypsi• nogen both in health and disease? What are the serum levels of mesotrypsinogen and meso trypsin during health and disease?

Elastase Also activated by trypsin, mainly dissolves the elastic fibers of blood vessels, and its action is strongly implicated in human necrotizing pancreatitis associated with hemorrhage.

Kallikrein Many of the local and systemic features of acute pancreatitis can be attributed to the actions of two low-molecular weight vasoactive , bradykinin and kallidin. These kinins are cleaved from kinogens, alpha-2-globulins present in plasma and lymph, by two proteolytic enzymes, trypsin and kallikrein. The latter enzyme is present as inactive kallikreinogen in pancreas, salivary glands, and plasma, and it is activated to kallikrein by , by kallikrein itself, and by trypsin. The generation of kinins is, in turn, inhibited by circulating alpha-I-anti• trypsin and by aprotinin, a found in bovine and . The kinins, once liberated, are rapidly inactivated by a variety of local pancreatic and systemic factors. They have a broad spectrum of biological activity, which in• cludes vasodilatation and increase in vascular permeability (both promoting shock), pain, and leukocyte accumulation. 72 M. V. Singer et al.

Lipolytic enzymes

Lipase Lipase induces necrosis in severe pancreatitis. Its role in initiation of the dis• ease in man is not well understood and there is evidence that lipase assumes a principal importance only after the chain reaction has already begun. Fat necrosis occurs due to the combined actions of bile , phospholipase A2, and lipase. Adipocyte plasma membranes are disrupted by phospholipase A2 in the presence of bile acids, allowing access of lipase to stored triglycerides. Fatty acids are re• leased and combined with Ca2+ to form insoluble soaps. The mechanism of fat necrosis which may occur at distant sites is less certain [14].

Phospholipase A Phospholipase A is an enzyme of considerable interest with regard to the patho• genesis of acute pancreatitis, since its reaction products, lysolecithin and lysoce• phalin, are strong cytotoxic substances. Its substrates, lecithin and cephalin, are the main constituents of cellular membranes and, moreover, lecithin is an essential component of bile. Bile acids are activators of phospholipase A. Thus, if bile reflux occurs and phospholipase A is activated within the pancreas, it could destroy pancreatic cells. Detailed reviews of the role of phospholipase A in acute pancreatitis have been made by Creutzfeldt and Schmidt [5] and Nevalainen [14]. The enzyme is synthesized by the pancreatic acinar cells, liberated to the pan• creatic juice, and secreted in the duodenum for digestive purposes. It is synthe• sized and secreted under normal circumstances in enzymatically inactive form as pro phospholipase A, which is activated by trypsin. Phospholipase A is inhibited by, e.g., zinc, ethylenediaminetetraacetate (EDTA), and many other substances e.g., drugs [14, 22]. The pathogenetic role of phospholipase A in acute pancreati• tis is supported by the observations that it and lysolecithin, when injected into the pancreatic duct of experimental animals, cause histologically similar changes - namely necrosis - in the gland as seen in cases of human acute pan• creatitis. Increased phospholipase A and lysolecithin contents are found in pan• creatic tissue in acute pancreatitis. Phopholipase A is elevated in the serum ofpa• tients with acute pancreatitis [14]. Phospholipase A might cause injury not only within the pancreas but also in various other vital organs, e.g., decrease in arterial blood pressure. It seems pos• sible that the adult respiratory distress syndrome in acute pancreatitis is caused by the action of phospholipase A on the pulmonary surfactant. The mental con• fusions seen in severe acute pancreatitis might be due to demyelination of the gray and white matter of the brain caused by phospholipase A. Clotting disorders might occur and phospholipase A might be involved in the formation of the myo• cardial depressant factor, which has been reported to contribute to the decreased cardiac output in acute pancreatitis. Role of Pancreatic Enzymes in Acute Pancreatitis 73

Conclusion

There is experimental evidence that both proteolytic and lipolytic enzymes are in• volved in the process of "autodigestion," which leads to pancreatic necrosis dur• ing acute pancreatitis. Nevertheless, a number of pathogenetic mechanisms ofhu• man acute pancreatitis remain incompletely understood.

References

I. Adler G, Kern HF (1984) Fine structural and biochemical studies in human acute pancreati• tis. In: Gyr KE, Singer MV, Sarles H (eds) Pancreatitis - concepts and classification. El• sevier, Amsterdam 2. Adler G, Arnold R, Kern HF (1984) Supramaximal hormonal or neural stimulation - does it occur in humans? In: Gyr KE, Singer MV, Sarles H (eds) Pancreatitis-concepts and clas• sification. Elsevier, Amsterdam 3. Allan BJ, Tournut R, White TT (1973) Intraductal activation of human pancreatic zymogens. N Engl J Med 288:266 4. Becker V (1980) Akute Pankreatitis - Morphologie, Pathogenese, Prognose. Chirurg 51:357-363 5. Creutzfeldt W, Schmidt H (1970) Aetiology and pathogenesis of pancreatitis (current con• cepts). Scand J Gastroenterol (Suppl) 6:47-62 6. Geokas MC, Rinderknecht H (1978) Free proteolytic enzymes in pancreatic juice of patients with acute pancreatitis. Am J Dig Dis 19:591-598 7. Gilliland L, Steer ML (1980) Effects of ethionine on digestive enzyme synthesis and dis• charge by mouse pancreas. Am J Physio1239:G418--G426 8. Hendricks JC, DiMagno EP, Go VLV, Dozois RR (1980) Reflux of duodenal contents into the pancreatic duct of dogs. J Lab Clin Med 96:912-921 9. Hermon-Taylor J (1977) An etiological and therapeutic review of acute pancreatitis. Br J Hosp Med 18:546-552 10. Kern HF, Adler G, Scheele GA (1984) The concept of flow and compartmentation in under• standing the pathobiology of pancreatitis. In: Gyr KE, Singer MV, Sarles H (eds) Pancreati• tis - concepts and classification. Elsevier, Amsterdam 11. Koike H, Steer ML, Meldolesi J (1982) Pancreatic effects of ethionine: blockade of exocy• tosis and appearance of crinophagy precede cellular necrosis. Am J Physiol 242:G297- G307 12. Lampel M, Kern H (1977) Acute interstitial pancreatitis in the rat induced by excessive doses of a pancreatic secretagogue. Virchows Arch Pathol Anat HistoI373:97-117 13. McCutcheon AD (1968) A fresh approach to pathogenesis of pancreatitis. Gut 9:296-310 14. Nevalainen TJ (1980) The role ofphosphoJipase A in acute pancreatitis. Scand J Gastroen• teroI15:641-650 15. Ofstad E (1970) Formation and destruction of plasma kinins during experimental acute hem• orrhagic pancreatitis in dogs. Scand J Gastroenterol 5 SuppI5:1--44 16. Owyang C, Dozois RR, DiMagno EP et al. (1977) Relationships between fasting and post• prandial pancreaticoduodenal pressures, pancreatic secretion and duodenal volume flow in the dog. Gastroenterology 73: 1046 17. Ranson JH (1984) Acute pancreatitis: pathogenesis, outcome and treatment. In: Creutzfeldt W (ed) The exocrine pancreas. Clin Gastroenterol13:843-864 18. Rao KN, Tuma J, Lombardi B (1976) Acute hemorrhagic pancreatitis in mice. Intraparen• chymal activation of zymogens, and other enzyme changes in pancreas and serum. Gastro• enterology 70:720-726 19. Rinderknecht H, Renner IG, Abramson SB, Carmack C (1984) Mesotrypsin: a new inhibi• tor-resistant protease in human pancreatic tissue and fluid. Gastroenterology 86:681-692 74 M. V. Singer et a1.: Role of Pancreatic Enzymes in Acute Pancreatitis

20. Schmidt H, Creutzfeldt W (1976) Etiology and pathogenesis of pancreatitis. In: Bockus HL (ed) Gastroenterology, 3rd edn. Saunders, London 21. Steer ML, Meldolesi J (1984) Experimental acute pancreatitis. Relevance of models to clini• cal disease. In: Gyr KE, Singer MV, Sarles H (ed) Pancreatitis - concepts and classification. Elsevier, Amsterdam 22. Tykkii HT, Vaittinen EJ, Mahlberg KL, Railo JE, Pantzar PJ, Sarna S, TaUberg T (1985) A randomized double-blind study using CaNa2 EDTA, a phospholipase A2 inhibitor in the management of human acute pancreatitis. Scand J GastroenteroI20:5-12