Feeding activates protein synthesis in mouse pancreas at the translational level without increase in mRNA

Abstract

To determine the mechanism of meal-regulated synthesis of pancreatic digestive enzymes, we studied the effect of fasting and refeeding on pancreatic protein synthesis, relative mRNA levels of digestive enzymes, and activation of the translational machinery. With the use of the flooding dose technique with L-[3H]phenylalanine, morning protein synthesis in the pancreas of Institute for Cancer Research mice fed ad libitum was 7.9 +/- 0.3 nmol phenylalanine.10 min(-1).mg protein(-1). Prior fasting for 18 h reduced total protein synthesis to 70 +/- 1.4% of this value. Refeeding for 2 h, during which the mice consumed 29% of their daily food intake, increased protein synthesis to 117.3 +/- 4.9% of the control level. Pancreatic mRNA levels of amylase, lipases, trypsins, chymotrypsin, elastases, as well as those for several housekeeping genes tested were not significantly changed after refeeding compared with fasted mice. By contrast, the major translational control pathway involving Akt, mTOR, and S6K was strongly regulated by fasting and refeeding. Fasting for 18 h decreased phosphorylation of ribosomal protein S6 to almost undetectable levels, and refeeding highly increased it. The most highly phosphorylated form of the eIF4E binding protein (4E-BP1) made up the 14.6% of total 4E-BP1 in normally fed animals, was only 2.8% after fasting, and was increased to 21.4% after refeeding. This was correlated with an increase in the formation of the eIF4E-eIF4G complex after refeeding. By contrast, feeding did not affect eIF2B activity. Thus food intake stimulates pancreatic protein synthesis and translational effectors without increasing digestive enzyme mRNA levels.

Fasting vs dietary restriction in cellular protection and cancer treatment: from model organisms to patients

“…In mice, a shortterm starvation (24-72 h) decreases IGF-I production by 70% and causes an 11-fold increase in its inhibitory partner IGF-binding protein 1 (IGFBP-1) (Tannenbaum et al, 1979;Frystyk et al, 1999;Lee et al, 2010). In response to fasting, mice also show reduced protein synthesis, reduced AKT activity via TRB3, a mammalian homolog of Drosophila tribbles (Du et al, 2003), reduced mTOR/S6K, increased 4E-BP1 activity (Sans et al, 2004) and increases FOXO-1, -3, -4 (Imae et al, 2003).…”

The mTOR Signaling Pathway and Regulation of Pancreatic Function

“…The importance of the TORC1 pathway in the exocrine pancreatic response to feeding is shown by the activation of the downstream components when mice fasted overnight are refed (35). In this study, protein synthesis was also increased with feeding without a change in mRNA levels for digestive enzymes, indicating the importance of translational control primarily by the TORC1 pathway in synthesis of new digestive enzymes after secretion.…”

Digestive Enzymatic Responses of Chickens Feed-restricted and Refed as Affected by Age

“…The decrease in pancreatic enzyme activity in younger animals is supported by the report by Palo et al (1995) on chickens that were feed-restricted for 7 d, which could be attributed to a decrease in protein synthesis, given that fasting inhibited pancreatic protein synthesis in mice (Sans et al, 2004). Moreover, protein restriction inhibited the response of acinar cells to cholecystokinin (Prost and Belleville, 1991), which plays a role in stimulating enzyme secretion in the pancreas.…”

“…Pancreatic cells were not evaluated in this study, but it is possible that there was no atrophy, given that after 3 d of refeeding, the pancreas weight was similar to the control in younger chickens and higher than the control in older animals. The recovery can be explained by an increase in pancreatic protein synthesis as shown by Sans et al (2004). However, these authors did not show a related increase in the mRNA levels of the digestive enzymes, which was most likely due to the shorter refeeding time (2 h of refeeding).…”

“…In their study, Crozier et al (2009) verified that after 4 d of receiving a normoproteic diet, protein-restricted mice showed a reversal of the decreased concentration of amylase and chymotrypsin. The restoration of enzyme activity could occur because pancreatic cell death did not occur after feed restriction, and instead, only cell atrophy or a decrease in protein synthesis in the pancreas occurred (Sans et al, 2004). Pancreatic cells were not evaluated in this study, but it is possible that there was no atrophy, given that after 3 d of refeeding, the pancreas weight was similar to the control in younger chickens and higher than the control in older animals.…”