Retinol-binding protein 4 mRNA translation in hepatocytes is enhanced by activation of mTORC1.

Increased expression of the peptide hormone retinol-binding protein 4 (RBP4) has been implicated in the development of insulin resistance, type 2 diabetes, and visual dysfunction. Prior investigations of the mechanisms that influence RBP4 synthesis have focused solely on changes in mRNA abundance. Yet, the production of many secreted proteins is controlled at the level of mRNA translation, as it allows for a rapid and reversible change in expression. Herein, we evaluated mRNA translation using sucrose density gradient centrifugation. In the liver of fasted rodents, mRNA translation was low. In response to refeeding, mRNA translation was enhanced and RBP4 levels in serum were increased. In H4IIE cells, refreshing culture medium promoted mRNA translation and expression of the protein. mRNA abundance was not increased by either experimental manipulation. Enhanced mRNA translation was associated with activation of the kinase mechanistic target of rapamycin in complex 1 (mTORC1) and enhanced phosphorylation of the translational repressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1). In H4IIE cells, expression of a 4E-BP1 variant that is unable to be phosphorylated by mTORC1 or suppression of mTORC1 with rapamycin attenuated activity of a luciferase reporter encoding the mRNA 5'-untranslated region (UTR). Purine substitutions to disrupt a terminal oligopyrimidine (TOP)-like sequence in the 5'-UTR prevented the suppressive effect of rapamycin on reporter activity. Rapamycin also prevented upregulation of mRNA translation in the liver and reduced serum levels of RBP4 in response to feeding. Overall, the findings support a model in which nutrient-induced activation of mTORC1 upregulates mRNA translation to promote RBP4 synthesis. RBP4 plays a critical role in metabolic disease, yet relatively little is known about the mechanisms that regulate its production. Herein, we provide evidence for translational control of RBP4 synthesis. We demonstrate that activation of the nutrient-sensitive kinase mTORC1 promotes hepatic mRNA translation. The findings support the possibility that targeting mRNA translation represents an alternative to current therapeutic interventions that lower serum RBP4 concentration by promoting urinary excretion of the protein.