Regulation by a protein-free carbohydrate-rich diet of rat pancreatic mRNAs encoding trypsin and elastase isoenzymes

The Digestive Function of Pseudoplatystoma punctifer Early Juveniles Is Differentially Modulated by Dietary Protein, Lipid and Carbohydrate Content and Their Ratios

Pseudoplatystoma punctifer is an Amazonian catfish highly appreciated for its high flesh quality, size, and commercial value. Its aquaculture is pursued to satisfy the demands of an increasing population in the region. However, knowledge of the nutritional needs during the early life stages is necessary for improving growth and reducing the incidence of cannibalism, factors that limit the success of its commercial farming. This study aimed at evaluating the influence of four diets containing different protein and lipid levels (30:15, 30:10, 45:15, or 45:10 in %) in the digestive physiology and performance of early juveniles. The results showed that the dietary protein:lipid as well as carbohydrate levels and ratios influenced differently the whole-body proximate composition, the digestive physiology and development, and hence growth and survival. The 45:15 diet promoted the best growth, survival, and the most rapid development of the digestive system, as shown at histological (higher number of hepatocytes, goblet cells in the anterior intestine and enterocytes in all intestinal portions, and longer folds in the posterior intestine), molecular (highest amylase, lipoprotein lipase, phospholipase, trypsinogen, and pepsinogen gene expression), and biochemical (highest lipase and pepsin activities and higher alkaline phosphatase:leucine alanine peptidase activity ratio) levels. Lipids were favored over carbohydrates as source of energy, with lipids promoting a protein-sparing effect at adequate energy:protein ratio. Carbohydrate content higher than 25% was excessive for this species, leading to unbalanced lipid metabolism and fat deposition in the liver.

Adaptive response of the rat pancreas to dietary substrates: parallel regulation of trypsinogen and pancreatic secretory trypsin inhibitor

Chronic pancreatitis has been associated with malnutrition in alcoholic patients and malnourished juveniles. The composition of the diet, especially the protein content, regulates the synthesis of secretory proteins in the rat pancreas. Adaptive responses of the pancreas have shown that anionic proteases (e.g., trypsinogen) are upregulated during protein deprivation. We hypothesize that the (cationic) pancreatic secretory trypsin inhibitor (PSTI) is down-regulated after a protein-deficient diet. Low PSTI levels might cause a lack of protection from prematurely activated trypsin and therefore enhance the risk for pancreatic inflammation. Over a period of 1 month, rats were fed one of four isocaloric diets with a casein content varying from 0 to 82%. PSTI and trypsinogen mRNA remained fairly constant, irrespective of the diet composition. Trypsinogen and elastase secreted into pancreatic juice were upregulated after a protein-deficient diet relative to a control diet. Contrary to our hypothesis, PSTI was also upregulated. Parallel secretion of trypsinogen and PSTI appears to ensure protection against premature activation even under extreme dietary conditions.

Stability of the mRNA encoding some pancreatic hydrolases is modulated by dietary protein intake in the rat

Wistar rats fed on either a high-protein or a protein-free diet were examined to determine their pancreatic hydrolase mRNA stabilities in comparison with those of control animals receiving a standard diet. Actinomycin D was used to inhibit transcription and, after isolating the pancreatic RNA, the specific messengers were quantified by performing dot-blot hybridization with cDNA probes. In the rats fed on a high-protein diet, only the half-lives of anionic trypsinogen I and elastase I (EC 3.4.21.36) were affected. Interestingly, when rats were fed on the protein-free diet, most of the hydrolase mRNA half-lives showed changes, except that corresponding to lipase. In these rats, the half-life values of the mRNA coding for anionic trypsinogen I, chymotrypsinogen and procarboxypeptidase B increased, in sharp contrast with those of the amylase and elastase I mRNA, which decreased. These results strongly suggest that the mechanism whereby the biosynthesis of pancreatic hydrolases is regulated, depending on the presence or absence of proteins in the diet, is not unique and provide evidence that the stability of mRNA encoding most, if not all, the hydrolases in pancreatic cells is modulated by the dietary protein content.

Dietary modulation of the mRNA stability of trypsin isozymes and the two forms of secretory trypsin inhibitor in the rat pancreas

The stability of the mRNAs encoding pancreatic trypsin isozymes, namely the cationic form and the two anionic forms I and II, as well as that of the secretory trypsin inhibitors I and II, were studied in rats fed on either a high-protein diet, or a protein-free diet compared with a standard diet for a 10-day period. Either immediately or 3 h and 6 h after injecting the transcription inhibitor, actinomycin D, the mRNA levels were quantified by performing dot-blot hybridization with specific oligonucleotide probes. Under high-protein dietary conditions, the stability of the mRNAs coding for anionic trypsin II and cationic trypsin showed no change, whereas that of anionic trypsin I and the two forms of secretory trypsin inhibitor were affected. The mRNA half-life of anionic trypsin I and trypsin inhibitor II increased, in sharp contrast with that of trypsin inhibitor I, which decreased. When rats were fed on a protein-free diet, the stabilities of both anionic trypsin forms and trypsin inhibitor I increased, whereas that of trypsin inhibitor II decreased and that of cationic trypsin remained unchanged. The present results show the existence of differences in the mechanisms whereby gene expression of trypsin isozymes and secretory trypsin inhibitors is regulated, although they are synthesized in parallel in the pancreatic acinar cell and stored in zymogen granules before being secreted into the intestinal lumen.

Isolation and nucleotide sequence of cDNA clone for bovine pancreatic anionic trypsinogen. Structural identity within the trypsin family

A cDNA clone encoding an anionic form of bovine trypsinogen was isolated from a pancreatic cDNA library. The corresponding 855-nucleotide mRNA contains a short 5' noncoding region of 8 nucleotides and a long 3' noncoding region of 56 nucleotides in addition to a poly(A) tail of at least 50 nucleotides. The deduced amino acid sequence for the anionic pretrypsinogen (247 residues) includes the N-terminal 15-amino-acid signal peptide followed by an 8-amino-acid activation peptide. The zymogen (232 residues) contains an additional C-terminal serine, compared with the amino acid sequence of bovine cationic trypsinogen. The identity between the anionic and cationic forms of bovine trypsinogen (65%) is lower than that existing between the anionic protein and other mammalian anionic trypsinogens (73-85%), suggesting that trypsin gene duplication in mammals occurred prior to the evolutionary events responsible for the species divergence. Bovine pancreatic anionic trypsin possesses all the key amino acids characteristic of the serine protease family.