The effect of activated protein C on experimental acute necrotizing pancreatitis

Acute pancreatitis is a local inflammatory process that leads to a systemic inflammatory response in the majority of the cases [1‐3]. Severe and life-threatening complications requiring intensive care occur in about 25% of patients with acute necrotizing pancreatitis (ANP) [4]. While the intra-acinar premature activation of digestive enzymes is central to pathophysiological mechanisms of injury, acinar cell apoptosis, increase in oxidative stress, microcirculatory derangements, and release of cytokines contribute to progression of injury and development of extrapancreatic complications [1‐5]. Severe acute pancreatitis is usually a result of glandular necrosis [6]. Nuclear factor-κB (NF-κB), a transcription factor that is associated with immediate early gene activation, plays a critical role in the development of necrosis. Although the exact mechanism of NF-κB activation is unknown, once stimulated it leads to production of several inflammatory cytokines, including tumor necrosis factor (TNF)-α [7]. This cytokine is known to increase the severity of pancreatitis by further increasing cytokine production, enhancing pancreatic leukocyte sequestration and accelerating acinar cell apoptosis, ultimately leading to a systemic inflammatory response [8, 9]. It has been demonstrated that inhibition of NF-κB activation reduces acinar cell damage and decreases the severity of pancreatitis [10]. Recently, anti-TNF-α treatment in experimental pancreatitis was reported to be of benefit, especially when administered early [11]. However, its effect on established necrotizing pancreatitis is not known.

The protein C pathway serves as a major system for controlling thrombosis, limiting inflammatory responses, and potentially decreasing endothelial cell apoptosis in response to inflammatory cytokines [12]. Recombinant human activated protein C (APC) is known to inhibit coagulation and inflammation, and to promote fibrinolysis in patients with severe sepsis [13]. Binding of APC to the endothelial cell protein C receptor results in a number of actions, including increased activity of APC itself and inhibition of both NF-κB and apoptosis [14].

Edema progresses to necrosis in about 20% of patients with acute pancreatitis [15]. The pancreas is infected in 40–70% of patients with necrotizing pancreatitis, and the mortality rate may be up to 40% when the necrotic tissue becomes superinfected [16]. The most important cause of death in necrotizing pancreatitis is secondary infections, which generally result from translocation of enteric bacteria from the intestine via mainly lymphatic, hematogenous, or transmural routes [17]. On the other hand, prophylactic antibiotic therapy was not found to decrease mortality in controlled clinical trials [18]. Although selective gut decontamination and, to some extent, enteral nutrition were shown to decrease infectious complications [19, 20], no specific agent that can strengthen the gut barrier or inhibit translocation of micro-organisms from the gut lumen has yet been identified.

Our aim in the present study was to investigate the effects of recombinant human APC on the progression of experimental ANP. Considering its significant role in inflammatory responses, we hypothesized that APC may alter the degree of local inflammation, development of necrosis and bacterial contamination, and thus the severity of acute pancreatitis.

The experiment was approved by the Institutional Animal Use and Care Committee of the Gülhane Medical Academy and was performed in accordance with the US National Institutes of Health guidelines for the care and handling of animals.

Rats with ANP had extensive parenchyma and fat necrosis, and polymorphonuclear leukocyte infiltration on histologic examination. The total histopathologic score was significantly reduced in group III (10.31 ± 0.47) compared with group II (14.00 ± 0.52; P < 0.001). Although there were marked improvements in pancreatic tissue edema, inflammatory infiltration, fat necrosis, acinar necrosis scores, and perivascular inflammation in APC-treated group III compared with saline-treated group II, there was no significant difference in hemorrhage scores between the two groups (Fig. 1). Histopathologic findings in the groups are summarized in Table 1.

Acute pancreatitis represents a severe form of inflammation that often leads to severe damage to the gland. Progression from edematous to necrotizing pancreatitis – a process that usually determines the patients' prognosis – is mediated by NF-κB [7]. In the present study, plasma IL-6 and TNF-α levels, together with amylase, were significantly increased after induction of ANP. Stimulation of production of either acute phase proteins and adhesion molecules or several inflammatory cytokines, including TNF-α, IL-1β and IL-6, occurs after NF-κB activation in acute pancreatitis [7]. However, we observed that amylase, and plasma IL-6 and TNF-α levels were all significantly decreased in APC-treated animals.

APC has been shown to inhibit production of TNF-α by decreasing activation of NF-κB [22]. In contrast to many immunomodulatory agents previously tested clinically, recombinant human APC was found to be significantly beneficial in the PROWESS (Recombinant Human Activated Protein C Worldwide Evaluation in Severe Sepsis) study and was approved by the US Food and Drug Administration for use in patients with severe sepsis and septic shock [23, 24]. A significant decrease in protein C concentrations was found during the initial phase of experimental ANP [25]. Furthermore, drotrecogin alfa (activated) treatment was recently reported to improve progression of severe sepsis after ANP in two cases [26]. Based on these data and those presented above, APC replacement may interrupt, at least partly, the pathophysiological cascade of inflammation and related events during ANP.

We found significant improvements in pancreatic histology after treatment with recombinant APC. Edema, acinar cell necrosis, fat necrosis, and perivascular inflammation, which occur in almost all inflammatory processes in any organ, resolved in pancreatic tissues from animals treated with APC. However, although we know that the decrease in APC occurs during the initial period of pancreatitis, we only studied its effects in established ANP because we believe that an experimental model should simulate the situation in humans. Indeed, clinically, only a small number of patients with acute pancreatitis present during the early stages of disease. Therefore, the results of the present study are relevant to clinical necrotizing pancreatitis in humans. The concept of administering APC to patients with the disease immediately after the diagnosis is established is rational and should be the focus of research. Nevertheless, more experimental and clinical evidence is needed if we are to evaluate the value of such prophylactic use of APC.

Little is known about effects on the coagulation system in ANP. Because the degree of hemorrhage affects the extent of local and systemic complications in ANP, maintenance of a normal coagulation system in the pancreatic microcirculation in order to prevent thrombosis or bleeding is a desirable objective. Protein C is a critical participant in normal coagulation mechanisms. One interesting finding in the present study was the similarity in hemorrhage scores between groups II and III. In comparison with control animals, APC neither decreased nor increased the incidence of hemorrhagic fields in tissue samples. This not only may refect the anticoagulant effect of APC but also suggests that the coagulation system in pancreas remains intact, even with the organ in a necrotic state.

Evaluation of bacterial translocation after APC treatment was another aim of the study. We found lesser MLN and pancreatic bacterial contamination in APC-administered rats than in control animals. The impact of superinfection of the pancreas is summarized above. The decreased contamination rates may reflect APC-related improvements in gut mucosa. Contamination of necrotic tissues occurs primarily because of translocation of enteric micro-organisms [27]. Our study does not indicate any direct effect of APC on intestinal mucosa, although many factors have been reported to underlie bacterial translocation [27], including intestinal mucosal injury, cecal bacterial overgrowth, decreased gut motility, and compromised host immune functions. Failure of the gut to act as a barrier against bacterial translocation as a result of nitric oxide (NO)-dependent mechanisms [28] has been accepted as one of the most potent origins of sepsis and subsequent organ failure after pancreatitis [29], and inhibition of inducible NO synthase was shown to decrease the incidence of bacterial translocation [30]. Isobe and coworkers [31] found an inhibitory effect of APC on inducible NO synthase induction by decreasing TNF-α production in rats with endotoxin-induced hypotension; a similar action of APC in the impaired intestinal mucosa of the rats with ANP might have been at work in the present study. Future studies addressing the association between APC and NO-dependent damage in the intestine following ANP induction will help to identify a possible second role of APC in the pathogenesis of the disease.

APC improved pancreatic histology and decreased the incidence of bacterial translocation from the intestine in rats with experimental ANP. APC and its reduction appear to play an important role in the pathogenesis of this life-threatening disease. Therefore, the effects of replacement of this mediator in ANP should be a focus of future investigations.