Rapamycin Inhibits Liver Growth during Refeeding in Rats via Control of Ribosomal Protein Translation but Not Cap-Dependent Translation Initiation

Abstract

We examined the role of the mammalian target of rapamycin (mTOR) in hepatic cell growth. To dissociate cell growth from cell proliferation, we employed an in vivo model of nonproliferative liver growth in rats, refeeding after 48 h of food deprivation. Starvation resulted in a decrease in liver mass, liver protein, and cell size, all of which were largely restored after 24 h of refeeding. Administration of the mTOR inhibitor, rapamycin, before the refeeding period partially inhibited the restoration of liver protein content. Refeeding was also associated with an increase in ribosomal protein S6 phosphorylation and phosphorylation of the eukaryotic initiation factor (eIF) 4E binding protein 1 (4E-BP1). 4E-BP1 phosphorylation was accompanied by a decrease in the abundance of the complex containing 4E-BP1 with eIF4E. These changes were prevented by rapamycin administration. However, association of eIF4E and eIF4G and eIF2alpha phosphorylation, both of which are stimulated by refeeding, were insensitive to rapamycin. The functional importance of these observations was confirmed by polysome fractionation, which showed that translation initiation of 5' oligopyrimidine tract-containing mRNAs, which encode ribosomal proteins, was inhibited by rapamycin, whereas translation of signal transducer and activator of transcription 1 (STAT1), a cap-dependent mRNA, was unaffected. The abundance of ribosomal proteins paralleled total protein content during refeeding in both control and rapamycin-injected rats. We conclude that accretion of liver protein during refeeding is dependent on mTOR-mediated activation of the translation of ribosomal proteins but not dependent on mTOR-mediated activation of cap-dependent translation initiation.

Polypyrimidine Tract-Binding Protein Is Involved in Regulation of Albumin Synthesis in Response to Food Intake

“…This increase might induce activation of the mTOR signaling pathway. As in the previous study (22), phosphorylation of S6 was increased in fasted rats compared with fed rats (data not shown). However, the level of PTB in the cytoplasmic fraction is increased in fasted rats.…”

“…The mRNAs encoding most ribosomal proteins contain a 5 ¢ -TOP sequence ( 23 ). Rapamycin treatment before a refeeding for 24 h clearly inhibited recruitment of ribosomal protein mRNA into polysomes ( 22 ). On the other hand, the results of microarray analysis showed that 78 different mRNAs had increased polysome association after feeding in the liver (21).…”

Role of Glutamine Depletion in Directing Tissue-specific Nutrient Stress Responses to L-Asparaginase

“…1S). These findings are consistent with the work of Anand and Gruppuso (55), which show that formation of the eIF4G⅐eIF4E complex during refeeding is rapamycin-resistant; i.e. is not dependent on signaling via mTOR.…”

Regulation of fetal liver growth in a model of diet restriction in the pregnant rat

“…B: hierarchy of fold-differences between the two groups. An inflection point calculated using these data was a log2 ratio of 1, corresponding to a fold difference of 2. regulator of translation initiation that is responsive to amino acid restriction (3,59). To assess these two signaling pathways, we analyzed fetal liver homogenates from control and IUGR fetuses by phospho-specific Western immunoblotting.…”