Pepsinogen synthesis and secretion in isolated gastric glands

Pentagastrin increases pepsin secretion without increasing its fractional synthetic rate

We studied the effects of increasing doses of pentagastrin on gastric secretion of pepsin and on incorporation of L-[1-13C]leucine into gastric aspirate protein as an index of pepsin synthesis. Pentagastrin (0.25-4.0 micrograms.kg-1.h-1) significantly increased pepsin output from basal 76 mg/h to < or = 181 mg/h but did not significantly alter incorporation of L-[1-13C]leucine from the basal fractional synthetic rate of 3.63 +/- 0.05%/h. In four subjects in whom infusion of tracer leucine was continued for > 1 day, aspiration of pepsin between 24 and 27 h demonstrated that plateau 13C labeling of leucine in pepsin had been attained, but at a value that was only 48% of the 13C labeling of plasma alpha-ketoisocaproic acid (alpha-KIC) [0.730 +/- 0.02 (SE) vs. 1.520 +/- 0.14 atoms %excess]. This suggests that actual rates of pepsin synthesis were approximately double those calculated on the basis of alpha-KIC labeling. The results are consistent with an interpretation that increasing doses of pentagastrin cause increased secretion of pepsinogen by recruitment of gastric chief cells, each synthesizing pepsinogen at an unaltered rate. Plateau 13C enrichment of alpha-KIC may not be a valid surrogate for plateau 13C leucine enrichment when fractional synthetic rates of some secreted proteins are calculated.

Effects of cholinergic, histaminergic, and peptidergic stimulation on pepsinogen secretion by isolated human peptic cells

BACKGROUND

Whereas pepsin secretion by the human stomach has been extensively investigated, the characteristics of the peptide cell cannot be fully understood from in vivo studies. We therefore studied isolated human peptic cells to test directly cellular responses to different agents.

METHODS

Eight endoscopic biopsy specimens yielded 10(6) cells, > 90% pure and > 95% viable. Secreted pepsinogen was measured with a sensitive hemoglobin digestion method at pH 2.

RESULTS

Pepsinogen secretion was concentration-dependently stimulated by acetylcholine (ACh) (EC50 = 0.3 microM), cholecystokinin (CCK)-8 (2 nM), histamine (2 microM), and gastrin-I (30 nM) but not by bombesin or pentagastrin. ACh stimulation was inhibited 40 times more potently by atropine (IC50 = 12 nM) than by pirenzepine (IC50 = 0.5 microM). Histamine was inhibited by 10(-4) cimetidine. CCK-8 stimulation was inhibited 80% by the CCK-A-selective antagonist L364,718 (IC50 = 12 nM) but not by the CCK-B-selective antagonist L365,260.

CONCLUSION

Isolated human peptic cells from endoscopic biopsy specimens secrete pepsinogen in response to ACh > CCK-8 > histamine > gastrin-I. The human peptic cell muscarinic-cholinergic receptor is not of the M1 subtype, and the CCK-8 response is predominantly mediated by a CCK-A receptor subtype.

Gastric chief cells: receptors and signal-transduction mechanisms

Elucidation of receptors and mediators regulating gastric pepsinogen secretion has lagged behind understanding of the factors that control acid secretion. During the past decade, as a consequence of the development of in vitro models for studying the control of pepsinogen secretion at the cellular level, much information about chief cell receptors and signal-transduction mechanisms has been obtained, including the identification and characterization of receptors for secretin, vasoactive intestinal polypeptide, cholinergic agonists, gastrin, cholecystokinin, peptide YY, and cholera toxin. Moreover, these cell preparations have permitted secretagogue-induced changes in chief-cell calcium concentration, protein kinase C distribution, and phosphoinositide and cyclic nucleotide content to be measured and related to changes in pepsinogen secretion. This article reviews these advances, discusses areas of uncertainty and controversy, and indicates areas for future investigation.

Effect of hydrochloric acid and prostaglandins on pepsinogen synthesis and secretion in canine gastric chief cell monolayer cultures

The actions of hydrochloric acid, natural PGE1, PGE2 and a synthetic commercial PGE1 preparation, Enisoprost (Searle) on pepsinogen synthesis and secretion were studied in canine chief cell monolayer cultures. Hydrochloric acid, applied directly to the apical surface of chief cells using culture plate inserts (Millipore) had no effect on secretion, nor did it affect the action of any secretagogue in the basolateral medium. All prostaglandins tested showed significant stimulation of pepsinogen secretion. Basal secretion of pepsinogen after 90 min was 9.4 (1.3)% to total initial monolayer content. At 10(-6) M, PGE1 stimulated secretion was 26.1 (3.8)%; PGE2 27.9 (4)% and Enisoprost 28.8 (4.2)% of initial pepsinogen content. Stimulations by all tested prostaglandins were additive with carbachol (10(-4) M) and CCK (10(-9) M), but not with VIP (10(-6) M), dbcAMP (10(-3) M) or forskolin (10(-6) M) responses. All three prostaglandins stimulated pepsinogen synthesis as measured by 14C labelled amino acid incorporation into pepsinogen. Time course experiments were similar to those for forskolin and showed shorter time delays between stimulus and increased synthesis rate than carbachol but longer than dbcAMP. Stimulated pepsinogen secretion was inhibited by high pepsin concentrations (greater than 800 micrograms/ml) in the medium. The inhibited abolished simultaneous carbachol induced stimulation of synthesis but prostaglandin or forskolin stimulation only after two hours. Combined with the shorter response time, as compared with carbachol, these data support our previous findings that potent stimulators of cAMP production can stimulate pepsinogen synthesis directly by stimulation of mRNA synthesis, independently from an increased secretion. The additivity of effects with carbachol or CCK and similarity with forskolin stimulated synthesis supports the suggestion that the actions of prostaglandins are mediated by cAMP.

Control of pepsinogen synthesis and secretion in primary monolayer cultures of canine gastric chief cells

We have studied pepsinogen synthesis and secretion in primary monolayer cultures of canine gastric chief cells. Monolayers were formed after approximately 48 hr. Pepsinogen synthesis was studied by adding 14C-labeled amino acids to the culture medium. Basal secretion of de novo synthesized pepsinogen after 4 hr was 4 +/- 1.2% of total newly synthesized pepsinogen. Basal secretion of stored pepsinogen after 90 min was 8 +/- 1.4% of total pepsinogen content. Carbachol, dbcAMP, forskolin, VIP, CCK-8, and the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate all stimulated secretion of de novo synthesized pepsinogen and preformed pepsinogen. Only additive interactions were found. dbcAMP caused a peak outburst of stored pepsinogen in the first 10 min. Carbachol stimulation was time dependent. Stimulation of de novo synthesized pepsinogen secretion was time dependent for both carbachol and dbcAMP. dbcAMP caused an immediate 10-fold increase in pepsinogen synthesis, but carbachol did so only after a lag time of 30 min. This was identical to the time necessary for the appearance of labeled pepsinogen in the medium. Addition of atropine after 2 hr resulted in a return to basal synthesis. Stimulated pepsinogen synthesis was always observed concomitant with stimulated pepsinogen secretion. These results show that most external stimuli for pepsinogen synthesis are dependent upon prior depletion of pepsinogen stores, which then triggers synthesis, while stimulation of cAMP production stimulates pepsinogen synthesis more directly.

Differential expression of pepsinogen isozymogens in a patient with Barrett esophagus

The pepsinogen A (PGA) isozymogens in the gastric mucosa and Barrett epithelium of a female patient with Barrett esophagus were studied on different occasions during a 3-year period by electrophoretic analysis of in vivo steady-state pepsinogen in biopsies by activity staining in combination with variant specific monoclonal antibodies and of de novo synthesized pepsinogen by autoradiography. In Barrett epithelium only one (Pg3) or two (Pg3 and Pg5) primary PGA gene products were detected, whereas in gastric mucosal biopsies three (Pg3, Pg4 and Pg5) primary gene products were demonstrated on all occasions. These differences strongly suggest differential expression/activation of individual gene numbers in the PGA gene cluster in Barrett esophagus and are in line with the preneoplastic nature of this condition. The mechanism behind this deregulation is currently under investigation by cell biology and molecular genetic techniques.

Discrepancies between gastric mucosal and urinary pepsinogen A patterns and in vitro synthesis and secretion of human pepsinogen

The relationship between electrophoretic pepsinogen A (PGA) patterns from urine and gastric mucosa was studied in healthy volunteers and in patients with various gastric disorders. Discrepancies between urinary and gastric PGA patterns were found in 63.3% of the individuals. In 9% of the subjects with these discrepancies, the phenotype class in urine was different from that in gastric mucosa. The differences were found in all diagnostic groups. The highest frequency of differences was found in patients with gastric ulcer. The differences were not related to the serum PGA level. More than 80% of the differences were caused by a lower relative intensity of pepsinogen A fraction 5 (Pg5) in urine than in gastric mucosa. The possible origin of differences in PGA isozymogen patterns was studied by organ culture of gastric biopsies. In vitro synthesis and secretion of pepsinogens were studied by electrophoresis and autoradiography. The synthesis rate of PGA in biopsies of 1-2 mm diameter was 40-100 ng/hr. Posttranslational modification of PGA isozymogens was demonstrated. Pg2 and part of Pg4 probably are secondary products of Pg3 and Pg5, respectively. In some individuals the secretion rate of Pg3 was low compared to the other isozymogens. The conversion of Pg3 into Pg2 and the differential secretion of the isozymogens may explain some of the discrepancies between gastric and urinary PGA patterns.

Pepsinogen synthesis in monolayer culture of human and rabbit gastric mucosal cells

We have established monolayer cultures of human and rabbit gastric mucosal cells and of isolated rabbit gastric chief cells. These cultures were capable of de novo pepsinogen synthesis and secretion, demonstrated by electrophoresis and subsequent autoradiography of cell lysates and growth medium after culture in the presence of 14C-labelled amino acids. Cultures could be maintained for 1 week without overgrowth by fibroblasts.

The influence of omeprazole on the synthesis and secretion of pepsinogen in isolated rabbit gastric glands

Regulation mechanisms of pepsinogen (EC 3.4.23.) synthesis and secretion were studied by following newly synthesized [14C]-labeled pepsinogen during culture of isolated rabbit gastric glands. Omeprazole, a substituted benzimidazole, while almost completely abolishing acid production at 10(-4) M, strongly stimulated secretion of preformed and newly synthesized pepsinogen. Although the pepsinogen synthesis at this concentration of omeprazole was reduced to about 55% of the control rate, a two-fold absolute increase of total secreted pepsinogen was found. This increase was not due to a non specific leakage through disruption of chief cell membranes, as no increase of lactate dehydrogenase in the culture medium could be demonstrated. The stimulated secretion was influenced neither by 10(-3) M cimetidine, 10(-3) sodium thiocyanate nor 10(-4) M atropine. No additivity was found between the carbachol (10(-4) M) or dibutyryl cyclic AMP (10(-3) M) and the omeprazole induced pepsinogen secretion.