Glucagon-dependent suppression of mTORC1 is associated with upregulation of hepatic FGF21 mRNA translation

Fibroblast growth factor 21 (FGF21) is a peptide hormone that acts to enhance insulin sensitivity and reverse many of the metabolic defects associated with consumption of a high-fat diet. Recent studies show that the liver is the primary source of FGF21 in the blood and that hepatic FGF21 expression is upregulated by glucagon. Interestingly, glucagon acts to upregulate FGF21 production by primary cultures of rat hepatocytes and H4IIE and HepG2 hepatocarcinoma cells independent of changes in FGF21 mRNA abundance, suggesting that FGF21 protein expression is regulated posttranscriptionally. Based on these observations, the goal of the present study was to assess whether or not FGF21 mRNA is translationally regulated. The results show that FGF21 mRNA translation and secretion of the hormone are significantly upregulated in H4IIE cells exposed to 25 nM glucagon, independent of changes in FGF21 mRNA abundance. Furthermore, the glucagon-induced upregulation of FGF21 mRNA translation is associated with suppressed activity of the mechanistic target of rapamycin in complex 1 (mTORC1). Similarly, the results show that rapamycin-induced suppression of mTORC1 leads to upregulation of FGF21 mRNA translation with no change in FGF21 mRNA abundance. In contrast, activation of mTORC1 by refreshing the culture medium leads to downregulation of FGF21 mRNA translation. Notably, refeeding fasted rats also leads to downregulation of FGF21 mRNA translation concomitantly with activation of mTORC1 in the liver. Overall, the findings support a model in which glucagon acts to upregulate FGF21 production by hepatocytes through suppression of mTORC1 and subsequent upregulation of FGF21 mRNA translation.

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Retinol binding protein 4 (Rbp4) mRNA translation in hepatocytes is enhanced by activation of mTORC1

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00494.2020 ◽

2020 ◽

Author(s):

Jaclyn E. Welles ◽

Allyson L Toro ◽

Siddharth Sunilkumar ◽

Shaunaci A Stevens ◽

Carson J Purnell ◽

...

Keyword(s):

Binding Protein ◽

Gradient Centrifugation ◽

Mrna Translation ◽

Mrna Abundance ◽

Retinol Binding Protein ◽

Luciferase Reporter ◽

Sucrose Density Gradient Centrifugation ◽

Retinol Binding Protein 4 ◽

Translational Repressor ◽

H4iie Cells

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 Rbp4 mRNA translation using sucrose density gradient centrifugation. In the liver of fasted rodents, Rbp4 mRNA translation was low. In response to re-feeding, Rbp4 mRNA translation was enhanced and RBP4 levels in serum were increased. In H4IIE cells, refreshing culture medium promoted Rbp4 mRNA translation and expression of the protein. Rbp4 mRNA abundance was not increased by either experimental manipulation. Enhanced Rbp4 mRNA translation was associated with activation of the kinase mTORC1 and enhanced phosphorylation of the translational repressor 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 Rbp4 mRNA 5′-untranslated region (UTR). Purine substitutions to disrupt a terminal oligopyrimidine (TOP)-like sequence in the Rbp4 5′-UTRprevented the suppressive effect of rapamycin on reporter activity. Rapamycin also prevented upregulation of Rbp4 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 up regulates Rbp4 mRNA translation to promote RBP4 synthesis.

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