Molecular model for the binary complex of uropepsin and pepstatin

The three-dimensional structure of human uropepsin complexed with pepstatin has been modelled using human pepsin as a template. Uropepsin is an aspartic proteinase from the urine, produced in the form of pepsinogen A in the gastric mucosa. The structure is bilobal, consisting of two predominantly beta-sheet lobes which, as observed in other aspartic proteinases, are related by a pseudo twofold axis. A structural comparison between binary complexes of pepsin:pepstatin and uropepsin:pepstatin is discussed.     

Isolation and characterization of a pepsin C zymogen produced by human breast tissues.

An aspartic proteinase present in cyst fluid from women with gross cystic breast disease was purified by a procedure involving affinity chromatography on pepstatin-agarose and size-exclusion high performance liquid chromatography. The amino-terminal sequence of the purified breast proteinase was identical to that corresponding to gastric pepsinogen C. Additional data on cleavage specificity, pH optimum, and immunological properties supported the close relationship between both molecules. Northern blot analysis and polymerase chain reaction amplification studies performed on RNAs obtained from normal and pathological breast tissues demonstrated that the protein is produced by mammary carcinomas and cysts, but not by the normal resting mammary gland. Immunohistochemical staining of paraffin-embedded tissue sections confirmed the existence of a subset of tumors that have the ability to synthesize and secrete this pepsin zymogen. On the basis of these results, we suggest that pepsinogen C expression by human mammary epithelium may be involved in the development of breast diseases, being also of potential interest as a biochemical marker of the hormonal imbalance underlying these pathologies.     

A comparative study of two kinds of cathepsin D-type proteinases from rat gastric mucosa

The localization of cathepsin D-like acid proteinase in the rat stomach and other tissues was studied, and its biochemical properties were compared with those of rat gastric cathepsin D (EC 3.4.23.5). Cathepsin D-like acid proteinase existed overwhelmingly in the mucosal layer and was hardly detected in the gastric juice. Its subcellular distribution profile was very similar to that of acid phosphatase, but not to that of pepsinogen. This proteinase-like enzyme activity was also found in rat splenic extract. These results strongly suggest that the proteinase is a lysosomal enzyme. In addition, cathepsin D-like acid proteinase demonstrated an in vitro transition of molecular species during storage at -30 degrees C. Although this molecular change was distinctive in ion-exchange column chromatography and susceptibility to some enzyme inhibitors, it was not accompanied by a significant decrease in molecular weight. To compare cathepsin D-like acid proteinase with ordinary cathepsin D, gastric cathepsin D was newly purified to apparent homogeneity in polyacrylamide gel electrophoresis. Its biochemical properties demonstrate that this is a true cathepsin D in rat gastric mucosa. Moreover, this cathepsin D activity was not abolished by treatment with antiserum specific to cathepsin D-like acid proteinase or pepsinogen. From these results, we can conclude that the proteinase is a lysosomal acid proteinase different from newly purified gastric cathepsin D.     

Properties of soluble fusions between mammalian aspartic proteinases and bacterial maltose-binding protein

The mammalian aspartic proteinases procathepsin D and pepsinogen form insoluble inclusion bodies when expressed in bacteria. They become soluble but nonnative when synthesized as fusions to the carboxy terminus of E. coli maltose-binding protein (MBP). Since these nonnative states of the two aspartic proteinases showed no tendency to form insoluble aggregates, their biophysical properties were analyzed. The MBP portions were properly folded as shown by binding to amylose, but the aspartic proteinase moieties failed to bind pepstatin and lacked enzymatic activity, indicating that they were not correctly folded. When treated with proteinase K, only the MBP portion of the fusions was resistant to proteolysis. The fusion between MBP and cathepsin D had increased hydrophobic surface exposure compared to the two unfused partners, as determined by bis-ANS binding. Ultracentrifugal sedimentation analysis of MBP–procathepsin D and MBP–pepsinogen revealed species with very large and heterogeneous sedimentation values. Refolding of the fusions from 8 M urea generated proteins no larger than dimers. Refolded MBP–pepsinogen was proteolytically active, while only a few percent of renatured MBP–procathepsin D was obtained. The results suggest that MBP–aspartic proteinase fusions can provide a source of soluble but nonnative folding states of the mammalian polypeptides in the absence of aggregation.     

The high-resolution crystal structure of porcine pepsinogen.

The structure of porcine pepsinogen at pH 6.1 has been refined to an R-factor of 0.173 for data extending to 1.65 A. The final model contains 180 solvent molecules and lacks density for residues 157-161. The structure of this aspartic proteinase zymogen possesses many of the characteristics of pepsin, the mature enzyme. The secondary structure of the zymogen consists predominantly of beta-sheet, with an approximate 2-fold axis of symmetry. The activation peptide packs into the active site cleft, and the N-terminus (1P-9P) occupies the position of the mature N-terminus (1-9). Thus changes upon activation include excision of the activation peptide and proper relocation of the mature N-terminus. The activation peptide or residues of the displaced mature N-terminus make specific interactions with the substrate binding subsites. The active site of pepsinogen is intact; thus the lack of activity of pepsinogen is not due to a deformation of the active site. Nine ion pairs in pepsinogen may be important in the advent of activation and involve the activation peptide or regions of the mature N-terminus which are relocated in the mature enzyme. The activation peptide-pepsin junction, 44P-1, is characterized by high thermal parameters and weak density, indicating a flexible structure which would be accessible to cleavage. Pepsinogen is an appropriate model for the structures of other zymogens in the aspartic proteinase family.     

Immunocytochemical localization of rabbit gastric lipase and pepsinogen

Lipase and pepsin activities were determined in rabbit gastric biopsy specimens. Lipase activity was found to be restricted to a small part of the fundic mucosa, near the cardia, whereas pepsin activity spread over about two thirds of the total fundic area, overlapping that of lipase. The cells producing these two enzymes were labeled by immunofluorescence using polyclonal antibodies against rabbit gastric lipase (RGL) or antibodies against rabbit pepsinogen. The immunocytochemical localization showed unequivocally that RGL and pepsinogen, which were both present in the cardial area, were in fact located in different gastric cells. The cells producing pepsinogen were in the lower base of the gastric fundic glands, whereas the cells producing RGL were in the upper base of the same glands. The cells producing pepsinogen and RGL showed no significant morphological differences. In the part of the fundic area, where only pepsin activity was detected, cells producing pepsinogen covered both the lower and the upper base of the gastric glands. No chief cells were observed in the antral mucosa. RGL and pepsinogen could represent useful gastric enzyme markers for cellular differentiation studies.     

Cholinergic agonist-induced pepsinogen secretion from murine gastric chief cells is mediated by M1 and M3 muscarinic receptors

Muscarinic cholinergic mechanisms play a key role in stimulating gastric pepsinogen secretion. Studies using antagonists suggested that the M3 receptor subtype (M3R) plays a prominent role in mediating pepsinogen secretion, but in situ hybridization indicated expression of M1 receptor (M1R) in rat chief cells. We used mice that were deficient in either the M1 (M1R-/-) or M3 (M3R-/-) receptor or that lacked both receptors (M(1/3)R-/-) to determine the role of M1R and M3R in mediating cholinergic agonist-induced pepsinogen secretion. Pepsinogen secretion from murine gastric glands was determined by adapting methods used for rabbit and rat stomach. In wild-type (WT) mice, maximal concentrations of carbachol and CCK caused a 3.0- and 2.5-fold increase in pepsinogen secretion, respectively. Maximal carbachol-induced secretion from M1R-/- mouse gastric glands was decreased by 25%. In contrast, there was only a slight decrease in carbachol potency and no change in efficacy when comparing M3R-/- with WT glands. To explore the possibility that both M1R and M3R are involved in carbachol-mediated pepsinogen secretion, we examined secretion from glands prepared from M(1/3)R-/- double-knockout mice. Strikingly, carbachol-induced pepsinogen secretion was nearly abolished in glands from M(1/3)R-/- mice, whereas CCK-induced secretion was not altered. In situ hybridization for murine M1R and M3R mRNA in gastric mucosa from WT mice revealed abundant signals for both receptor subtypes in the cytoplasm of chief cells. These data clearly indicate that, in gastric chief cells, a mixture of M1 and M3 receptors mediates cholinergic stimulation of pepsinogen secretion and that no other muscarinic receptor subtypes are involved in this activity. The development of a murine secretory model facilitates use of transgenic mice to investigate the regulation of pepsinogen secretion.     

The isolation and properties of a non-pepsin proteinase from human gastric mucosa.

1. A non-pepsin proteinase, proteinase 2, was successfully isolated free from pepsinogen (by repetitive chromatography on DEAE- and CM-celluloses) from the gastric mucosa of a patient with a duodenal ulcer and the uninvaded mucosa of a patient with a gastric adenocarcinoma. 2. Proteinases 1a and 1b, found in gastric adenocarcinoma, were not found in the gastic mucosa of these patients. 3. Proteinase 2 was shown to have an asymmetrical broad pH-activity curve with a maximum over the pH range 3.0-3.7. 4. Proteolytic activity of proteinase 2 was inhibited by pepstatin; the concentration of pepstatin giving 50% inhibition is of the order of 3nm. 5. Inhibition of proteolytic activity by carbenoxolone and related triterpenoids indicated that at pH 4.0 proteinase 2 possesses structural characteristics relating it to the pepsins and at pH 7.4 to the pepsinogens. 6. The sites of cleavage of the B-chain of oxidized insulin for proteinase 2 at pH 1.7 and pH 3.5 were shown to be similar to those previously established for human pepsin 3 and for the cathepsin E of rabbit bone marrow. 7. The non-pepsin proteinase 2 (cathepsin) of human gastric mucosa has properties more similar to cathepsin E than to the cathepsins D.     

Difference in effects of pirenzepine and atropine on carbachol-induced pepsinogen secretion from isolated gastric glands

The effect of pirenzepine on carbamylcholine (carbachol)-stimulated pepsinogen secretion was compared with that of atropine in the isolated guinea pig gastric glands. Pirenzepine and atropine caused a dose dependent inhibition of carbachol-stimulated pepsinogen secretion. Moreover, pirenzepine as well as atropine produced a rightward shift in the dose response curve of carbachol-stimulated pepsinogen secretion but did not alter the maximum increase in pepsinogen secretion. Results therefore demonstrate that pirenzepine acts as a specific receptor antagonist in the interaction of carbachol with its receptor on gastric chief cells. However, pirenzepine was 50 times less potent than atropine in inhibiting pepsinogen secretion. Half maximal inhibitory concentration of pirenzepine was 2 X 10(-5) M when a maximally effective concentration of carbachol was used, while that of atropine was 4 X 10(-7) M. Results, therefore, suggest that muscarinic receptor on gastric chief cells to which pirenzepine binds may be an intermediate affinity type.     

Pepsinogen C expression in tumors of extragastric origin

We have examined by immunohistochemistry the ability of human carcinomas of various origin to produce pepsinogen C, an aspartyl proteinase mainly involved in the digestion of proteins in the stomach and recently found to be associated with breast carcinomas. Of the 268 tumors analyzed 80 (29.8%) showed positive staining for pepsinogen C. These positive tumors included 12 gastric (38.7% of the 31 examined cases), nine pancreatic (42.8%), two renal (20%), 12 prostatic (40%), three bladder (27.3%), 14 endometrial (29.7%) and 18 ovarian (40%) carcinomas. We also detected 10 melanomas (50%) that were positive for pepsinogen C. By contrast, immunohistochemical staining for the proteinase was not detected in colorectal, cervical, lung and basal cell skin carcinomas. These results demonstrate that pepsinogen C, a proteolytic enzyme of highly restricted expression in human tissues, can also be expressed by a wide variety of human carcinomas. In addition, and similar to pepsinogen C expression in breast carcinomas, the production of this enzyme by different human tumors might be related to putative hormonal alterations associated with the development and progression of these tumors.     

Changes with Development in the Expression of Cathepsin E in the Fetal Rat Stomach

The changes with development in the expression of cathepsin E in the fetal rat stomach were examined immunochemically and immunohistochemically. The activity of acid proteinase in fetal gastric extracts increased dramatically during late gestational stages, rising from 0.017 units per mg of protein on day 15 of gestation to 0.591 units per mg of protein on day 21 of gestation. Electrophoretic analysis, combined with immunological tests, showed that the increase was due exclusively to increases in the activity of the monomeric and dimeric forms of cathepsin E, while SDS-PAGE-immunoblot analysis revealed that both forms are present as a 43-kDa proenzyme. Immunohistochemically, cathepsin E was localized in the cytoplasm of all proliferating epithelial cells of pars glandularis on day 16 of gestation or later. As revealed by conventional histological methods, surface mucous cells and parietal cells appeared for the first time in specimens on day 19 of gestation, and all of these cells were immunopositive for cathepsin E. The present study further indicated that cathepsin E is the predominant aspartic proteinase in the stomach of young rats, until pepsinogen C appears. Based on these results, possible roles of gastric cathepsin E are discussed.     

Inhibition of pepsinogen secretion in isolated gastric glands by amphotericin B is secretagogue specific

Pepsinogen secretion from isolated gastric glands, stimulated by 8-bromoadenosine 3',5'-cyclic monophosphate (8BrcAMP), forskolin, or cholecystokinin octapeptide, was inhibited by the presence of amphotericin B in the incubation medium. However, amphotericin had no effect, or only a slight effect (less than 10% inhibition), on pepsinogen secretion stimulated by crude secretin. Incubation of glands with either of the mitochondrial inhibitors, rotenone or carbonyl cyanide m-chlorophenylhydrazone, reduced pepsinogen secretory responses both to 8BrcAMP and to crude secretin. This suggests that amphotericin inhibition, which is secretagogue specific, was not the result of a general metabolic inhibition. Amphotericin caused an increase in sodium and chloride content and a decrease in potassium content of glands. Experiments in which the medium content of either sodium, potassium, or chloride was varied, suggested that part of the amphotericin inhibition could be attributed to a rise in intracellular chloride content. Results did not support the involvement of changes in intracellular sodium or potassium content in the inhibitory mechanism of amphotericin. It was concluded that amphotericin caused a rapid and secretagogue-specific inhibition of pepsinogen secretion in isolated gastric glands, and that the mechanism of inhibition may, to some extent, involve changes in intracellular chloride content.     

Combined effect of phorbol ester and, A23187 or dibutyryl cyclic AMP on pepsinogen secretion from isolated gastric glands

In isolated guinea pig gastric glands, pepsinogen secretion was stimulated by the phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate (TPA) in a dose dependent manner. Calcium-deprivation from the medium resulted in the decrease in TPA-induced pepsinogen secretion. The combination of 0.4 microM Ca2+ionophore A23187 and TPA stimulated pepsinogen secretion slightly higher than the calculated additive value for each agent. This synergistic effect of the agents supports a role of calcium-activated, phospholipid-dependent protein Kinase (protein Kinase C) in gastric pepsinogen secretion. Furthermore, pepsinogen secretion was also stimulated by dibutyryl cyclic AMP (dbc AMP) and dbc AMP slightly enhanced TPA-induced pepsinogen secretion. Results suggest that gastric chief cells possess at least two different secretory pathways for pepsinogen which are probably dependent on protein kinase C and cyclic AMP, respectively.     

Novel ways to prevent proteolysis - prophytepsin and proplasmepsin II

The recently determined crystal structures of two aspartic proteinase zymogens, prophytepsin from barley and proplasmepsin II from the malarial parasite Plasmodium falciparum, have provided new insights into zymogen inactivation. Prophytepsin shows a variation of the mechanism of inhibition used by the well-known gastric aspartic proteinase zymogens, whereas proplasmepsin II uses a completely new mode of inactivation.     

Novel ways to prevent proteolysis — prophytepsin and proplasmepsin II

The recently determined crystal structures of two aspartic proteinase zymogens, prophytepsin from barley and proplasmepsin II from the malarial parasite Plasmodium falciparum, have provided new insights into zymogen inactivation. Prophytepsin shows a variation of the mechanism of inhibition used by the well-known gastric aspartic proteinase zymogens, whereas proplasmepsin II uses a completely new mode of inactivation.     

Electron microscopy of the oxynticopeptic cells of the gastric glands and the intestinal glands of the caecum of the guineapig.

Histomorphology of the gastric and intestinal glands was investigated in 19 sexually mature, adult guineapigs by light and transmission electron microscopy. Gastric glands exhibited the cytological characteristics of oxynticopeptic cells capable of both hydrochloric acid (HCl) and pepsinogen secretion. In the literature, occurrence of oxynticopeptic cells in the proventriculus of the domestic fowl (Toner, 1963; Bell & Freeman, 1971) and in the gastric glands of frogs has been reported (Sedar, 1961; Patt & Patt, 1969; Forte & Forte, 1970). It has been claimed by other investigators (Herriot et al., 1938; Long, 1967) that simultaneous secretion of HCl and pepsinogen by a single, not completely differentiated 'pure' cell type, was highly effective for rapid conversion of the zymogen to active enzyme. Under the light microscope with haematoxylin and eosin stain, the protein secreting activity of gastric glands in guineapigs was masked by the HCl secreting activity, thus morphologically resembling the oxyntic cells. Therefore, different cell types, for example protein-secreting peptic cells and the acid-secreting oxyntic cells, could not be distinguished on the basis of their morphology and staining affinity. For histochemical evaluation of the sections with stains-all method, most cells in the gastric glands responded by a positive reaction to protein. Further, protein containing cells were seen in the intestinal glands of the guineapig caecum. The function of this cell type was correlated with caecotrophic food habits of this species.     

Regulation of progastricsin mRNA levels in sea bass (Dicentrarchus labrax) in response to fluctuations in food availability

In this study the sea bass (Dicentrarchus labrax) pepsinogen C gene was isolated. The nucleotide sequences of all exons are presented. The organization of the gene is compatible with that of other aspartic proteinases. The predicted 388-residue amino acid (aa) sequence of sea bass pepsinogen C consists of a signal sequence of 16 amino acid residues, an activation peptide of 43 residues, and the mature pepsin of 329 residues containing the two characteristic active-site aspartic acids. We also analyzed fasting-induced changes in the expression of progastricsin mRNA, using real-time RT-PCR absolute quantification. Progastricsin mRNA copy number was downregulated under conditions of negative energy balance, such as starvation, and upregulated during positive energy balance, such as refeeding. These findings offer new information about the sea bass progastricsin gene and support a role of this gastric digestive enzyme in the regulation of food intake in sea bass.