Involvement of molecular chaperones in the aberrant aggregation of secretory proteins in pancreatic acinar cells

Antichaperone activity and heme degradation effect of methyl tert-butyl ether (MTBE) on normal and diabetic hemoglobins

Because of the extensive use of methyl tert-butyl ether (MTBE) as an additive to increase the octane quality of gasoline, the environmental pollution by this compound has increased in recent decades. Environmental release of MTBE may lead to its entry to the blood stream through inhalation or drinking of contaminated water, and its interactions with biological molecules such as proteins. The present study was proposed to comparatively investigate the interactions of MTBE with hemoglobin (Hb) from diabetic and nondiabetic individuals using various spectroscopic methods including UV-visible, fluorescence, chemiluminescence, and circular dichroism. These results demonstrated the effects of MTBE on heme degradation of Hb and the reaction of these degradation products with water generating reactive oxygen species. Interaction of Hb with MTBE enhanced its aggregation rate and decreased lag time, indicating the antichaperone activity of MTBE upon interaction with Hb. Furthermore, the diabetic Hb showed more severe effects of MTBE, including heme degradation, reactive oxygen species production, unfolding, and antichaperone behavior than the nondiabetic Hb. The results from molecular docking suggested that the special interaction site of MTBE in the vicinity of Hb heme group is responsible for heme degradation.

Heat shock proteins and autophagy in rats with cerulein-induced acute pancreatitis

Background/Aims

Heat shock proteins (HSPs) protect rats from cerulein-induced acute pancreatitis (AP) by preventing the subcellular redistribution of cathepsin B and the activation of trypsinogen. Autophagy plays a critical role in the secretion of digestive enzymes and triggering of cerulein-induced AP via the colocalization of trypsinogen and lysosomes. Therefore, using a rat cerulein-induced AP model, we investigated whether HSPs prevent AP by regulating autophagy.

Methods

Twelve hours after fed standard laboratory chow and water, the experimental groups (cerulein, water-immersion [WI]-cerulein and heat-shock [HS]-cerulein) and the control groups (control, WI, and HS) received one intraperitoneal injection of cerulein (50 µg/kg) or saline, respectively. All of the rats were sacrificed at 6 hours after injection. The severity of the AP was assessed based on the serum amylase level and the histological and electron microscopy findings. Western blotting was also performed for HSP60/70 and LC3B-II.

Results

WI and HS induced HSP60 and HSP70, respectively. The induced HSP60/70 effectively prevented the development of cerulein-induced AP. Autophagy developed in the rats with cerulein-induced AP and was documented by the expression of LC3-II and electron microscopy findings. The WI-stressed rats and HS-treated rats did not develop cerulein-induced autophagy.

Conclusions

HSPs exert protective effects against cerulein-induced AP in rats by inhibiting autophagy.

Alteration of chaperonin60 and pancreatic enzyme in pancreatic acinar cell under pathological condition

AIM

To investigate the changes of chaperonin60 (Cpn60) and pancreatic enzymes in pancreatic acinar cells, and to explore their roles in the development of experimental diabetes and acute pancreatitis (AP).

METHODS

Two different pathological models were replicated in Sprague-Dawley rats: streptozotocin-induced diabetes and sodium deoxycholate-induced AP. The contents of Cpn60 and pancreatic enzymes in different compartments of the acinar cells were measured by quantitative immunocytochemistry.

RESULTS

The levels of Cpn60 significantly increased in diabetes, but decreased in AP, especially in the zymogen granules of the pancreatic acinar cells. The elevation of Cpn60 was accompanied with the increased levels of pancreatic lipase and chymotrypsinogen in diabetes. However, a decreased Cpn60 level was accompanied by high levels of lipase and chymotrypsinogen in AP. The amylase level was markedly reduced in both the pathological conditions.

CONCLUSION

The equilibrium between Cpn60 and pancreatic enzymes in the acinar cells breaks in AP, and Cpn60 content decreases, suggesting an insufficient chaperone capacity. This may promote the aggregation and autoactivation of the premature enzymes in the pancreatic acinar cells and play roles in the development of AP.

Expression differences in mitochondrial and secretory chaperonin 60 (Cpn60) in pancreatic acinar cells

In pancreatic acinar cells, chaperonin Cpn60 is present in all the cellular compartments involved in protein secretion as well as in mitochondria. To better understand the role Cpn60 plays in pancreatic secretion, we have evaluated its changes under experimental conditions known to alter pancreatic secretion. Quantitative protein A-gold immunocytochemistry was used to reveal Cpn60 in pancreatic acinar cells. Cpn60 immunolabelings in cellular compartments involved in secretion were found to decrease in acute pancreatitis as well as upon stimulation of secretion and in starvation conditions. A major increase in Cpn60 was recorded in diabetic condition. This was normalized by insulin treatment. Although in certain situations changes in secretory enzymes and in Cpn60 correlate well, in others, nonparallel secretion seemed to take place. In contrast, expression of mitochondrial Cpn60 in acinar cells appeared to remain stable in all conditions except starvation, where its levels decreased. Expression of Cpn60 in the secretory pathway and in mitochondria thus appears to behave differently, and Cpn60 in the secretory pathway must be important for quality control and integrity of secretion.

Expression differences in mitochondrial and secretory chaperonin 60 (Cpn60) in pancreatic acinar cells

In pancreatic acinar cells, chaperonin Cpn60 is present in all the cellular compartments involved in protein secretion as well as in mitochondria. To better understand the role Cpn60 plays in pancreatic secretion, we have evaluated its changes under experimental conditions known to alter pancreatic secretion. Quantitative protein A-gold immunocytochemistry was used to reveal Cpn60 in pancreatic acinar cells. Cpn60 immunolabelings in cellular compartments involved in secretion were found to decrease in acute pancreatitis as well as upon stimulation of secretion and in starvation conditions. A major increase in Cpn60 was recorded in diabetic condition. This was normalized by insulin treatment. Although in certain situations changes in secretory enzymes and in Cpn60 correlate well, in others, nonparallel secretion seemed to take place. In contrast, expression of mitochondrial Cpn60 in acinar cells appeared to remain stable in all conditions except starvation, where its levels decreased. Expression of Cpn60 in the secretory pathway and in mitochondria thus appears to behave differently, and Cpn60 in the secretory pathway must be important for quality control and integrity of secretion.

Chaperone and antichaperone activities of trigger factor

Reduced denatured lysozyme tends to aggregate at neutral pH and competition between productive folding and aggregation substantially reduces the efficiency of refolding. Trigger factor, a folding catalyst and chaperone can, depending on the concentration of trigger factor and the solution conditions, cause either a substantial increase (chaperone activity) or a substantial decrease (antichaperone activity) in the recovery of native lysozyme as compared with spontaneous refolding. When trigger factor is working as a chaperone, the reactivation rates of lysozyme are decelerated and aggregation decreases with increasing trigger factor concentrations. Under conditions where antichaperone activity of trigger factor dominates, the reactivation rates of lysozyme are accelerated and aggregation is increased. Trigger factor and lysozyme were both released from the aggregates on re-solubilization with urea indicating that trigger factor participates directly in aggregate formation and is incorporated into the aggregates. The apparently dual effect of trigger factor toward refolding of lysozyme is a consequence of the peptide binding ability and may be important in regulation of protein biosynthesis.

Water immersion stress induces heat shock protein 60 expression and protects against pancreatitis in rats

BACKGROUND & AIMS

Heat shock proteins (Hsps), induced by cell stress, are known to protect against cellular injury. Recent studies have indicated that Hsp60 expression, induced by exposure to water immersion stress, protects against pancreatitis induced by administration of supramaximal doses of cerulein in rats. However, the mechanisms responsible for this protection are not known.

METHODS

Rats were water-immersed for 3-12 hours. Pancreatitis was induced by cerulein administration.

RESULTS

The results confirm that prior induction of Hsp60 expression by water-immersion stress significantly ameliorates the severity of cerulein-induced pancreatitis as judged by the markedly reduced degree of hyperamylasemia, pancreatic edema, and acinar cell necrosis. Water immersion also prevents the subcellular redistribution of cathepsin B from a lysosome-enriched fraction to a heavier, zymogen granule-enriched fraction that is known to occur in this model of pancreatitis. Intra-acinar cell activation of trypsinogen that occurs shortly after exposure to a supramaximally stimulating dose of cerulein both in vivo and in vitro is prevented by prior water-immersion stress and Hsp60 expression. The protection against pancreatitis that follows water-immersion stress is not caused by alterations of cholecystokinin receptors, because water immersion does not alter the typical biphasic amylase secretory response to stimulation with cerulein.

CONCLUSIONS

Water-immersion stress induces Hsp60 expression, ameliorates cerulein-induced pancreatitis, and prevents intra-acinar cell activation of trypsinogen. We suggest that Hsp60 protects against cerulein-induced pancreatitis by preventing trypsinogen activation within acinar cells.

Colocalization of chaperone Cpn60, proinsulin and convertase PC1 within immature secretory granules of insulin-secreting cells suggests a role for Cpn60 in insulin processing

Many of the mechanisms that control insulin processing and packaging by interaction with different elements along the secretory pathway remain poorly understood. We have investigated the possibility that Cpn60, a member of the heat shock protein family, may be present in rat insulin-secreting cells, participating in the proinsulin-insulin maturation process. Immunofluorescence and high resolution immunocytochemical studies revealed the presence of the Cpn60 protein all along the insulin secretory pathway, being particularly abundant over the proinsulin-containing immature secretory granules. Double-labeling experiments showed associations between Cpn60 and proinsulin, as well as between Cpn60 and PC1 convertase, with a preferential binding to proinsulin. These findings paralleled those of coimmunoprecipitation studies showing the Cpn60 chaperone and the mature form of the PC1 convertase in proinsulin immunoprecipitates, as well as the PC1 in Cpn60 immunoprecipitates from total islet cell extracts. In vitro binding of Cpn60 to proinsulin, insulin and glucagon was also documented. Cpn60, significantly abundant in proinsulin-containing secretory granules where conversion of proinsulin to insulin takes place, and the colocalization of the chaperone with proinsulin and PC1 convertase suggest that the Cpn60 protein may play a role directing precise molecular interactions during insulin processing and/or packaging.

Roles of molecular chaperones in pancreatic secretion and their involvement in intestinal absorption

This review focuses on the contribution of molecular chaperones in the secretory process of digestive enzymes and their interaction with enterocytes. By using biochemistry and immunocytochemistry, we have shown that Grp94, Cpn10, Cpn60, and protein disulfide isomerase (PDI) are present all along the rough endoplasmic reticulum-Golgi-granule secretory pathway of the pancreatic acinar cells and are secreted into the acinar lumen. Two other molecular chaperones, Grp78 and the Hsp70, appear to be restricted to the rough endoplasmic reticulum and the trans-Golgi apparatus, respectively. We have found that chaperones can be associated with pancreatic enzymes along the secretory pathway. Indeed, double immunogold and immunocoprecipitation revealed an association between Cpn60 and the colipase-dependent lipase (CDL) and between Grp94 and the bile salt-dependent lipase (BSDL). These complexes are secreted into the acinar lumen and diverted to the duodenal lumen. These findings led us to investigate these enzyme-chaperone complexes in intestinal tissue. Grp94, Cpn60, and PDI are present on microvilli and on the endosomal compartment of enterocytes. Furthermore, we have shown that the Grp94-BSDL complexes are internalized by enterocytes through classical endocytosis. Upon dissociation of the BSDL-Grp94 complex in the late endosome, BSDL is transferred to the basolateral membrane. We propose that Grp94 interacts with specific receptors and/or could force the associated protein to adopt a specific conformation that allows its binding to corresponding membrane receptors and its internalization by enterocytes. These two hypotheses need not to be exclusive. The existence of such a pancreatic secretion-intestinal absorption link speaks in favor of a coordinated functional connection between these two entities, through molecular chaperones, in order to optimize intestinal activities.

Mechanism of the antichaperone activity of protein disulfide isomerase: facilitated assembly of large, insoluble aggregates of denatured lysozyme and PDI

Protein disulfide isomerase (PDI), a folding catalyst and chaperone can, under certain conditions, facilitate the misfolding and aggregation of its substrates. This behavior, termed antichaperone activity [Puig, A., and Gilbert, H. F., (1994) J. Biol. Chem. 269, 25889] may provide a common mechanism for aggregate formation in the cell, both as a normal consequence of cell function or as a consequence of disease. When diluted from the denaturant, reduced, denatured lysozyme (10-50 microM) remains soluble, although it does aggregate to form an ensemble of species with an average sedimentation coefficient of 23 +/- 5 S (approximately 600 +/- 100 kDa). When low concentrations of PDI (1-5 microM) are present, the majority (80 +/- 8%) of lysozyme molecules precipitate in large, insoluble aggregates, together with 87 +/- 12% of the PDI. PDI-facilitated aggregation occurs even when disulfide formation is precluded by the presence of dithiothreitol (10 mM). Maximal lysozyme-PDI precipitation occurs at a constant lysozyme/PDI ratio of 10:1 over a range of lysozyme concentrations (10-50 microM). Concomitant resolubilization of PDI and lysozyme from these aggregates by increasing concentrations of urea suggests that PDI is an integral component of the mixed aggregate. PDI induces lysozyme aggregation by noncovalently cross-linking 23 S lysozyme species to form aggregates that become so large (approximately 38,000 S) that they are cleared from the analytical ultracentrifuge even at low speed (1500 rpm). The rate of insoluble aggregate formation increases with increasing PDI concentration (although a threshold PDI concentration is observed). However, increasing lysozyme concentration slows the rate of aggregation, presumably by depleting PDI from solution. A simple mechanism is proposed that accounts for these unusual aggregation kinetics as well as the switch between antichaperone and chaperone behavior observed at higher concentrations of PDI.