Role of cathepsin B in intracellular trypsinogen activation and the onset of acute pancreatitis

Acute pancreatitis

The pathophysiologic concept of acute pancreatitis focuses on early events inside acinar cells such as activation of trypsin. Despite significant progress in the understanding of molecular events in experimental pancreatitis, knowledge of these mechanisms has not yet been translated into therapeutic strategies useful in humans. A single morphologic or laboratory marker reliably predicting the individual course of acute pancreatitis still awaits discovery. The clinical outcome of acute pancreatitis is dependent on the presence of necrosis and systemic complications. The extent of the systemic inflammatory response to local pancreatitis seems to be the rate-limiting step. Several mechanisms shed light on the interconnectivity between intra-acinar protease activation and immunoregulatory processes. Deciphering these cell biologic connections has already had an impact on the clinical management of patients and the identification of prognostic markers in human disease. However, further investigations of pancreatic pathophysiology are needed to initiate novel therapeutic strategies.

Autophagic cell death and its execution by lysosomal cathepsins

In the last decade, the molecular mechanisms of apoptosis, a major type of active cell death (type I cell death) have largely been clarified in mammalian cells. Particularly, the caspase family of proteinases has been shown to play crucial roles in the execution of apoptosis. Differing from apoptosis, type II cell death is known to be associated with autophagosomes/autolysosomes and appear in the developing nervous system (CLARKE, 1990). We have previously shown that delayed neuronal death occurring in the CA1 pyramidal layer of the gerbil hippocampus after brief forebrain ischemia is apoptotic in nature and autophagosomes/autolysosomes abundantly appear in the neurons before DNA fragmentation. To further understand the roles of autophagosomes/autolysosomes in active cell death, we examined the apoptosis of PC12 cells using morphological and biochemical techniques. PC12 cells are known to undergo apoptosis when cultured in the absence of serum. In such an environment, the mitochondrial pathway of apoptosis is activated; cytochrome c is released from mitochondria, and caspase-9/caspase-3 are activated. We have first examined morphological features of PC12 cells during the apoptotic process following serum deprivation, and found that autophagy is induced from the early stage of the process in the cells before typical nuclear changes. When autophagy is inhibited in the cells by 3-methyladenine, an autophagy inhibitor, they are largely protected from apoptosis. In relation to the induction of autophagy in PC12 cells following serum deprivation, immunoreactivity, protein amounts, and the proteolytic activity of lysosomal proteinases, particularly cathepsins B and D, are all greatly altered; those of cathepsin B drastically decrease in the cells from the early stage of serum-deprived cultures, whereas those of cathepsin D increase. Moreover, PC12 cells overexpressing cathepsin D undergo apoptosis more rapidly in serum-deprived cultures than wild-type cells, whereas those overexpressing cathepsin B increase the viability. These lines of evidence suggest that autophagy is involved in PC12 cell death following serum deprivation, this type of cell death being regulated by lysosomal proteinases, cathepsins B and D, downstream autophagy.

Towards specific functions of lysosomal cysteine peptidases: phenotypes of mice deficient for cathepsin B or cathepsin L

The lysosomal cysteine peptidases cathepsin B and cathepsin L are abundant and ubiquitously expressed members of the papain family, and both enzymes contribute to the terminal degradation of proteins in the lysosome. However, there is accumulating evidence for specific functions of lysosomal proteases in health and disease. The generation of 'knock out' mouse strains that are deficient in lysosomal proteases provides a valuable tool for evaluation of existing hypotheses and gaining new insights into the in vivo functions of these proteases. In this minireview, we summarise and discuss the findings obtained by analysis of mice that are devoid of cathepsin B or cathepsin L. In brief, cathepsin L appears to be critically involved in epidermal homeostasis, regulation of the hair cycle, and MHC class II-mediated antigen presentation in cortical epithelial cells of the thymus. Cathepsin B plays a major role in pathological trypsinogen activation in the early course of experimental pancreatitis and contributes significantly to TNF-alpha induced hepatocyte apoptosis.

Are genetically modified mice useful for the understanding of acute pancreatitis?

Treatment of patients with acute pancreatitis has greatly improved due to a better understanding of the pathophysiology of the disease. This pathophysiology includes the activation and release of pancreatic enzymes in the interstitium, the autodigestion of the pancreas, and a multiple organ dysfunction after their release into the systemic circulation. Moreover, significant evidence exists that synthesis and release of proinflammatory cytokines and chemokines are also responsible for the local injury and systemic dispersion of the inflammation. The use of knockout mice devoid of active pro- or anti-inflammatory mediators allows examination of the effects of a specific cytokine without any drawbacks induced by pharmacological manipulations. The results obtained from these genetically modified mice show that numerous mediators have a major role in the pathophysiology of acute pancreatitis. They also clearly demonstrate that a single genetic deletion cannot completely prevent the occurrence of pancreatic or distant organ injury. However, the fact that the immune system is characterized by redundancies of ligands and receptors complicates the full understanding of each report. The utility of such experimental models might have limitations, and a full extrapolation of experimental data from genetically modified mice to humans must be done with caution.

Gain-of-function mutations associated with hereditary pancreatitis enhance autoactivation of human cationic trypsinogen

Hereditary pancreatitis (HP), an autosomal dominant disorder, has been associated with mutations in the cationic trypsinogen gene. Here we demonstrate that the two most frequent HP mutations, Arg117 --> His and Asn21 --> Ile, significantly enhance autoactivation of human cationic trypsinogen in vitro, in a manner that correlates with the severity of clinical symptoms in HP. In addition, mutation Arg117 --> His inhibits autocatalytic inactivation of trypsin, while mutation Asn21 --> Ile has no such effect. The findings strongly argue that increased trypsinogen activation in the pancreas is the common initiating step in both forms of HP, whereas trypsin stabilization might also contribute to HP associated with the Arg117 --> His mutation.