Impaired autophagy and organellar dysfunction in pancreatitis

Abstract

Recent findings from our group, obtained on experimental in vivo and ex vivo models of pancreatitis, reveal that this disease causes a profound dysfunction of key cellular organelles, lysosomes and mitochondria. We found that autophagy, the main cellular degradative, lysosome-driven process, is activated but also impaired in acute pancreatitis because of its' inefficient progression/resolution (flux) resulting from defective function of lysosomes. One mechanism underlying the lysosomal dysfunction in pancreatitis is abnormal processing (maturation) and activation of cathepsins, major lysosomal hydrolases; another is a decrease in pancreatic levels of key lysosomal membrane proteins LAMP-1 and LAMP-2. Our data indicate that lysosomal dysfunction plays an important initiating role in pancreatitis pathobiology. The impaired autophagy mediates vacuole accumulation in acinar cells; furthermore, the abnormal maturation and activation of cathepsins leads to increase in intra-acinar trypsin, the hallmark of pancreatitis; and LAMP-2 deficiency causes inflammation and acinar cell necrosis. Thus, the autophagic and lysosomal dysfunctions mediate key pathologic responses of pancreatitis. On the other hand, we showed that pancreatitis causes acinar cell mitochondria depolarization, mediated by the permeability transition pore (PTP). Genetic (via deletion of cyclophilin D) inactivation of PTP prevents mitochondrial depolarization and greatly ameliorates the pathologic responses of pancreatitis. Further, our data suggest that mitochondrial damage, by stimulating autophagy, increases the demand for efficient lysosomal degradation and therefore aggravates the pathologic consequences of lysosomal dysfunction. Thus, the combined autophagic, lysosomal and mitochondrial dysfunctions are key to the pathogenesis of pancreatitis.

Figure 1

Lysosomal dysfunction leads to autophagy impairment and other pathologic responses of pancreatitis.

Figure 2

In pancreatitis, defective cathepsin processing/activation, resulting in impaired autophagy, causes accumulation of large autophagic vacuoles and active trypsin in acinar cells. (A). Electron micrograph showing abnormally large autophagic vacuoles in pancreas of a rat with cerulein pancreatitis. Boxed area demonstrates various types of autophagic vacuoles containing intact or partially degraded cargo (e.g., zymogen granules). (B). Schematic illustrating the hypothesis that in pancreatitis, the pathologic, intra-acinar trypsin accumulation results from an imbalance between the activities of CatB, which converts trypsinogen to trypsin, and CatL, which degrades both trypsin and trypsinogen. The stimulatory and inhibitory effects of pancreatitis on these cathepsins are shown by (+) and (−) symbols, respectively.

Figure 3

Permeability transition pore (PTP) opening, leading to loss of the mitochondrial membrane potential ΔΨm, is a common event in models of pancreatitis. (A). Schematic of PTP. OM, the outer mitochondrial membrane; IM, the inner mitochondrial membrane; VDAC, voltage-dependent anion channel; ANT, adenine nucleotide translocase: CypD, cyclophilin D. (B). PTP opening in experimental pancreatitis is mediated by different mechanisms: through an increase in cytosolic Ca²⁺ in the cerulein and bile acid models (Ref. 21); a decrease in ATP-synthase activity in arginine-induced pancreatitis (Ref. 46); and a decrease in NAD level in alcoholic pancreatitis (Ref. 24).

Figure 4

Schematic illustrating the hypothesis that mitochondrial and lysosomal dysfunctions in pancreatitis potentiate each other, promoting impaired autophagy.