Hormonal stimulation in the exocrine pancreas results in coordinate and anticoordinate regulation of protein synthesis.

Abstract

24-h intravenous caerulein infusion studies in the rat were combined with in vitro amino acid incorporation studies followed by high- resolution separation of proteins by two-dimensional isoelectric focusing and SDS gel electrophoresis to study the extent to which persistent changes in the biosynthesis of exocrine pancreatic proteins are regulated by cholecystokinin-like peptides. Beginning in the third hour of optimal hormone infusion at 0.25 microgram kg-1 h-1, changes were observed in the synthetic rates of 12 proteins, which progressed over the course of the 24-h study. Based on coordinate response patterns, exocrine proteins could be classified into four distinct groups. Group I (trypsinogen forms 1 and 2) showed progressive increases in synthetic rates reaching a combined 4.3-fold increase over control levels. Group II (amylase forms 1 and 2) showed progressive decreases in synthesis to levels 7.1- and 14.3-fold lower than control levels, respectively. Group III proteins (ribonuclease, chymotrypsinogen forms 1 and 2, procarboxypeptidase forms A and B, and proelastase 1) showed moderate increases in synthesis, 1.4-2.8-fold, and group IV proteins (trypsinogen 3, lipase, proelastase 2, and unidentified proteins 1-4) did not show changes in synthesis with hormone stimulation. Regulation of protein synthesis in response to caerulein infusion was specific for individual isoenzymic forms in the case of both trypsinogen and proelastase. The ratio of biosynthetic rates of trypsinogen forms 1 + 2 to amylase forms 1 + 2 increased from a control value of 0.56 to 24.4 after 24 h of hormonal stimulation (43.5-fold increase). Biosynthetic rates for an unidentified protein (P23) with an Mr = 23,000 and isoelectric point of 6.2 increased 14.2- fold, and the ratio of synthesis of P23 to amylase 2 increased 200-fold during caerulein infusion. During hormone stimulation the anticoordinate response in the synthesis of pancreatic glycosidases (decreased synthesis) and serine protease zymogens (increased synthesis) explain previous observations that showed little change in rates of total protein synthesis under similar conditions.

Multiple-level caerulein control of the gene expression of secretory proteins in the rat pancreas

“…It thus emerges that the relative levels of individual translation products corresponding to the above-mentioned four groups of proteins (and hence those of the mRNAs) were not correlated with the relative amounts of enzymes biosynthesized in vivo. It should be emphasized here that under the same experimental conditions, Schick et al [14] observed a 2.7-fold decrease and a 1.3-fold increase in the rate of amylase and serine protease synthesis, respectively, when caerulein was continuously infused for 6 h. These results therefore point to the existence of a translational control exerted by caerulein on pancreatic cnzyme biosynthesis during the first 6 h of enzyme stimulation.…”

“…Although infusion of caerulein has been found to induce the synthesis of a new protein, namely protein P-23 [14] (which appeared on a twodimensional gel as a discrete spot between trypsinogens I + 2 and trypsinogen 3 of molecular mass 23 kDa and isoelectric point 6), no translational product corresponding to the mRNA of protein P-23 could be detected on our autoradiography, even after 24 h of caerulein infusion. This observation is also in favour of a translational effect of the hormone on protein P-23 biosynthesis.…”

“…However, any variation in the levels of mRNAs in total RNA extracts cannot be ruled out, since the mRNA population accounts for less than 3% of total RNA. Changes in individual mRNAs might be directly related to the already reported decrease observed in the rate of amylase synthesis in amino acid incorporation studies in vitro after a 3-h period of caerulein infusion [14].…”

“…The most important effects were observed for amylase and trypsinogen. Biosynthesis of amylase forms 1 and 2 fell progressively to as low as one seventh and one fourteenth of the control levels, respectively, whereas the rates of synthesis of trypsinogen forms 3 and 2 showed a combined fourfold increase over control levels [14].…”

Nutritional Regulation of Pancreatic and Biliary Secretions

“…In the rat pancreas, twenty-one individual exocrine proteins have been identified by twodimensional electrophoresis : four forms of procarboxypeptidase, three forms of trypsinogen, two or three forms of amylase, two forms each of chymotrypsinogen and proelastase, one form each of lipase and RNAse (EC 3.1 .27.5), and four unidentified glycoproteins (Poort & Poort, 1981;Schick et al 1984~). In the rat, it has been demonstrated that, following treatment (5 d-3 weeks) with diets containing normal (220 g/kg)-high (820 g/kg) protein levels at the expense of carbohydrate, amylase and the majority of proteases are synthesized in direct proportion to amounts of their corresponding nutritional substrates in the diet.…”