Two distinct adaptive responses in the synthesis of exocrine pancreatic enzymes to inverse changes in protein and carbohydrate in the diet

RUMINANT NUTRITION SYMPOSIUM: Effects of postruminal flows of protein and amino acids on small intestinal starch digestion in beef cattle

Many nutritionists adopt feeding strategies designed to increase ruminal starch fermentation because ruminal capacity for starch degradation often exceeds amounts of starch able to be digested in the small intestine of cattle. However, increases in fermentable energy supply are positively correlated with increased instances of metabolic disorders and reductions in DMI, and energy derived by cattle subsequent to fermentation is less than that derived when glucose is intestinally absorbed. Small intestinal starch digestion (SISD) appears to be limited by α-glycohydrolase secretions and a precise understanding of digestion of carbohydrates in the small intestine remains equivocal. Interestingly, small intestinal α-glycohydrolase secretions are responsive to luminal appearance of milk-specific protein (i.e., casein) in the small intestine of cattle, and SISD is increased by greater postruminal flows of individual AA (i.e., Glu). Greater flows of casein and Glu appear to augment SISD, but by apparently different mechanisms. Greater small intestinal absorption of glucose has been associated with increased omental fat accretion even though SISD can increase NE from starch by more than 42% compared to ruminal starch degradation. Nonetheless, in vitro data suggest that greater glucogenicity of diets can allow for greater intramuscular fat accretion, and if greater small intestinal absorption of glucose does not mitigate hepatic gluconeogenesis then increases in SISD may provide opportunity to increase synthesis of intramuscular fat. If duodenal metabolizable AA flow can be altered to allow for improved SISD in cattle, then diet modification may allow for large improvements in feed efficiency and beef quality. Few data are available on direct effects of increases in SISD in response to greater casein or metabolizable Glu flow. An improved understanding of effects of increased SISD in response to greater postruminal flow of Glu and casein on improvements in NE and fates of luminally assimilated glucose could allow for increased efficiency of energy use from corn and improvements in conversion of corn grain to beef. New knowledge related to effects of greater postruminal flow of Glu and casein on starch utilization by cattle will allow nutritionists to more correctly match dietary nutrients to cattle requirements, thereby allowing large improvements in nutrient utilization and efficiency of gain among cattle fed starch-based diets.

Systematic review of diet in the pathogenesis of acute pancreatitis: a tale of too much or too little?

BACKGROUND/AIM

The role of diet as the cause of acute pancreatitis (AP) has been suggested. The aim of the current review was to determine if there exists sufficient evidence linking nutrition, or the lack of it, to the pathogenesis of AP.

PATIENTS AND METHODS

A systematic search of the scientific literature was carried out using Embase, PubMed, MEDLINE, and the Cochrane Central Register of Controlled Trials for the years 1965 - 2011 to obtain access to studies involving dietary factors and the pathogenesis of AP.

RESULTS

A total of 17 studies were identified describing diet and AP. These included 12 human and 5 animal studies. 8 reports were found to link malnutrition and/or refeeding to the pathogenesis of AP. Two studies found an increased consumption of fats and proteins in patients with alcohol-related AP while 1 study noted a lesser intake of carbohydrate in patients. However, none of these differences attained statistical significance. A recent prospective case-control study found a significantly higher risk for AP amongst patients eating par-boiled rice and fresh water fish.

CONCLUSIONS

Evidence from literature does not appear to support the role of diet as a single bolus meal as a cause for AP. Prolonged consumption of diets rich in proteins and fats may work synergistically with gallstones / alcohol to trigger an attack of AP indicating a possible role of diet as a cofactor in the development of AP possibly by lowering the threshold needed by these other agents to lead to the attack of AP.

CCK-independent mTORC1 activation during dietary protein-induced exocrine pancreas growth

Dietary protein can stimulate pancreatic growth in the absence of CCK release, but there is little data on the regulation of CCK-independent growth. To identify mechanisms whereby protein stimulates pancreatic growth in the absence of CCK release, C57BL/6 control and CCK-null male mice were fed normal-protein (14% casein) or high-protein (75% casein) chow for 7 days. The weight of the pancreas increased by 32% in C57BL/6 mice and 26% in CCK-null mice fed high-protein chow. Changes in pancreatic weight in control mice were due to both cell hypertrophy and hyperplasia since there was an increase in protein-to-DNA ratio, total DNA content, and DNA synthesis. In CCK-null mice pancreatic growth was almost entirely due to hypertrophy with both protein-to-DNA ratio and cell size increasing without significant increases in DNA content or DNA synthesis. ERK, calcineurin, and mammalian target of rapamycin complex 1 (mTORC1) are activated in models of CCK-induced growth, but there were no differences in ERK or calcineurin activation between fasted and fed CCK-null mice. In contrast, mTORC1 activation was increased after feeding and the duration of activation was prolonged in mice fed high-protein chow compared with normal-protein chow. Changes in pancreatic weight and RNA content were completely inhibited, and changes in protein content were partially abated, when the mTORC1 inhibitor rapamycin was administered during high-protein chow feeding. Prolonged mTORC1 activation is thus required for dietary protein-induced pancreatic growth in the absence of CCK.

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

The levels of mRNAs coding for trypsin and elastase isoenzymic forms were determined in the pancreatic tissue of rats fed a high-carbohydrate protein-free diet for a 0-5-day period. No change in the amounts of mRNAs coding for the two isoelastases was observed, although previous results showed that the biosynthesis of anionic elastase was markedly increased, whereas the biosynthesis of cationic elastase decreased, suggesting the existence of a translational-control mechanism in response to nutritional substrates. In contrast, the levels of mRNAs specific for the three isotrypsins were significantly enhanced, possibly as a result of transcriptional regulation and/or a change in messenger stability. In combination with earlier observations of an overall decrease in cationic trypsin biosynthesis during the same nutritional manipulation, these results suggest that formation of this enzyme is also subject to translational control.

Time-course of changes in pancreatic size and enzyme composition in rats during starvation

The effect of starvation for 3, 5, or 7 d on body weight, fat stores, pancreatic weight, and enzyme composition was studied in 300 g rats and was compared with a 3-d fast in 200 g rats. In the 300 g animals, fasting led to a gradual hypotrophy of the pancreas with a marked, continuous decrease in amylase content. Pancreatic lipase, trypsinogen, chymotrypsinogen, proelastase, and secretory trypsin inhibitor contents increased temporarily, but by d 7, they declined to about the initial values. This decline in enzyme levels coincided with the exhaustion of fat stores. The decrease in amylase content could be related to decreases in circulating insulin levels, whereas the temporary increase in lipase content may be owing to changes in plasma free fatty acid concentrations. In 200 g rats, starvation for 3 d led to exhaustion of fat stores that was accompanied by greater losses of pancreatic weight, protein, and amylase contents. In addition, the levels of trypsinogen and chymotrypsinogen decreased and lipase was unchanged. These findings indicate that during starvation, changes in pancreatic secretory enzymes are time-dependent and vary with the age, body weight, and/or adipose tissue mass of the rats.

Changes in individual rates of pancreatic enzyme and isoenzyme biosynthesis in the obese Zucker rat

Both alterations of enzyme content and a markedly decreased secretory response to selected physiological stimuli have been demonstrated previously in the pancreas of the obese Zucker rat. The purpose of the present investigation was to determine the degree to which alterations of enzyme content could be attributed to changes in enzyme biosynthesis. Amylase content of obese rats was decreased by 50%, whereas lipase and trypsinogens were significantly increased. However, the decrease in amylase content was less than might have been predicted from the rate of amylase biosynthesis (80% decrease), and the increases in content of trypsinogen(s) and lipase were greater than would have been predicted from alterations in the absolute rates of biosynthesis. In view of the rapid turnover of pancreatic enzymes under normal conditions, it seems probable that a markedly decreased secretory response to various stimuli leads to an increased content of some enzymes in the pancreas of the obese rat. Ciglitazone treatment, which decreases insulin resistance in obese animals and leads to normalization of glucose metabolism in their pancreatic tissue, restored the enzyme-synthesis rates towards normal, showing that the abnormalities of enzyme synthesis were linked to the insulin resistance rather than to the obese genotype itself. Lipid inclusion bodies were found in acinar cells of obese rats. These bodies have previously been described in acinar cells of starved animals, which, in common with the acinar tissue of the obese Zucker rat, have decreased glucose metabolism.

The rat elastase I regulatory element is an enhancer that directs correct cell specificity and developmental onset of expression in transgenic mice

A total of 134 base pairs of the 5' flanking sequence of the elastase I gene is sufficient and necessary to direct expression of the passive human growth hormone gene (hGH) to the exocrine pancreas. We demonstrate that this elastase I regulatory region contains a transcriptional enhancer which directs acinar cell-specific expression in transgenic animals. The elastase I enhancer specifies correct expression of the linked hGH gene in an orientation- and position-independent manner and can activate a heterologous promoter. The enhancer also directs the appropriate temporal activation of the hGH gene in the developing pancreas. Transcription is initiated correctly for the elastase I or hGH promoter, and the transcripts are correctly processed regardless of the enhancer position within or outside the fusion gene. The elastase I enhancer generates coincident DNase I-hypersensitive sites in pancreatic chromatin when moved 3 kilobases upstream or within the first intron of the hGH gene and when associated with the hGH promoter.

The rat elastase I regulatory element is an enhancer that directs correct cell specificity and developmental onset of expression in transgenic mice

A total of 134 base pairs of the 5' flanking sequence of the elastase I gene is sufficient and necessary to direct expression of the passive human growth hormone gene (hGH) to the exocrine pancreas. We demonstrate that this elastase I regulatory region contains a transcriptional enhancer which directs acinar cell-specific expression in transgenic animals. The elastase I enhancer specifies correct expression of the linked hGH gene in an orientation- and position-independent manner and can activate a heterologous promoter. The enhancer also directs the appropriate temporal activation of the hGH gene in the developing pancreas. Transcription is initiated correctly for the elastase I or hGH promoter, and the transcripts are correctly processed regardless of the enhancer position within or outside the fusion gene. The elastase I enhancer generates coincident DNase I-hypersensitive sites in pancreatic chromatin when moved 3 kilobases upstream or within the first intron of the hGH gene and when associated with the hGH promoter.

Stimulation of pancreatic secretory process in the rat by low-molecular weight proteinase inhibitor. II. Regulation of total protein and individual enzyme biosynthesis

Oral application of a single dose of a new synthetic proteinase inhibitor Camostate (Foy-305) in male Wistar rats was carried out together with studies of in vitro amino acid incorporation followed by separation of proteins by two-dimensional gel electrophoresis. The aim of this experiment was to analyze changes produced by the inhibitor in total protein and individual enzyme biosynthesis. Administration of 100 mg/kg Foy-305 resulted in significant inhibition of total pancreatic protein synthesis, without changes in fractional rates for individual enzymes. 50 mg/kg Foy-305 induced a 10-fold elevation of cholecystokinin (CCK) levels in serum; this persisted for 3 h and led to a significant increase in the total rate of protein synthesis with peak values at 6 and 9 h (78% and 84% above control levels, respectively), returning to control by 15 h. Changes in fractional rates of synthesis occurred with a latency of 6 h and were restricted to amylase and the anionic form of trypsinogen and chymotrypsinogen. Amylase biosynthesis decreased by about 40% from control levels at 9 h to return to control levels by 15 h. Increased synthesis of trypsinogen and chymotrypsinogen was observed; this was also phasic. The results show similar enzyme-specific regulation as previously described for exogenous CCK stimulation and for the adaptation of the pancreas to diets enriched in protein. They demonstrate the effectiveness of pulsatory endogenous hormone release in the regulation of protein synthesis.

Insulin resistance is accompanied by impairment of amylase-gene expression in the exocrine pancreas of the obese Zucker rat

Insulin plays a major role in the control of pancreatic amylase biosynthesis. In this study we determined glucose metabolism by pancreatic acini as well as the pancreatic content of both amylase protein and amylase mRNA during development of insulin resistance in the obese Zucker rat. At age 4 weeks there were no abnormalities detected in the above parameters, although the obese animals were already hyperinsulinaemic. At 6 weeks glucose metabolism was decreased by 50% in acini from obese rats, whereas pancreatic amylase-gene expression was only slightly impaired. At 22 weeks glucose metabolism was decreased by 50%, amylase content by 55% and amylase mRNA by 60% in acinar tissue of obese rats. As expected, hyperinsulinaemia increased markedly with age. Thus development of severe insulin resistance was associated with impairment of amylase-gene expression. To decrease insulin resistance, one group of adult obese rats was treated with Ciglitazone for 4 weeks. A lowered plasma insulin concentration without alteration of food intake was taken as evidence of decreased insulin resistance. This was associated with normalization of glucose metabolism and a marked increase of both amylase content of pancreatic tissue and amylase mRNA. In conclusion, both the increase of insulin resistance with age and its partial reversal by Ciglitazone treatment appear to modulate pancreatic amylase-gene expression in the obese Zucker rat.