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.