Translational Control of FGF21 Mrna Expression is Responsive to Nutritional Stress

FGF21 is an endocrine hormone associated with diseases such as non‐alcoholic fatty liver disease (NAFLD), Type‐2‐Diabetes, and obesity. Remarkably, in studies where obese rodents were treated with FGF21, not only was obesity attenuated, but complications such as hepatic steatosis were largely resolved. Because the majority of circulating FGF21 is produced by the liver, understanding the molecular mechanisms that regulate hepatic expression is critical. In part, FGF21 expression is regulated through changes in FGF21 gene transcription. However, recent studies show that production of FGF21 by hepatocytes is stimulated by glucagon treatment independent of changes in FGF21 mRNA expression, suggesting translational control of the FGF21 mRNA. However, the mechanism(s) involved in such regulation are as yet unexplored. Thus, the aim of the present study was to explore translational control of FGF21 mRNA in a rodent model of obesity. Moreover, because translation of many mRNAs is regulated through modulation of the protein kinase activity of mTOR in complex 1 (mTORC1), we hypothesized that translational control of FGF21 mRNA is in part due to an attenuation of mTORC1 activity.Two studies have been completed thus far. In the first study, obesity‐prone male Sprague Dawley rats were fasted overnight. The next morning one‐half of the rats were randomly re‐fed for 1 hr, and livers were removed and homogenized. Homogenates were centrifuged through sucrose density gradients, and the gradients were fractionated while the absorbance at 254 nm was measured. RNA was isolated from each fraction, and analyzed by qRT‐PCR and RNA‐Seq analyses. In the second study, obesity‐prone rats were fed a high‐fat diet (HFD; 42% calories from fat) or a low‐fat control diet (5.2% fat) ad libitum, for 2 weeks. Rats were weighed every other day, blood glucose concentrations were measured 16‐h prior to sacrifice following a 5‐h fast, and livers were subjected to polysome profile analysis.Results from the first study show that re‐feeding resulted in a dramatic activation of mTORC1 in the liver as assessed by increased phosphorylation of its direct targets, 4E‐BP1 and p70S6K1. There was also a shift in distribution of mRNAs from smaller (i.e. fewer ribosomes attached) to heavier (i.e. more ribosomes attached) polysomes in response to refeeding. Importantly, although expression of the FGF21 mRNA was upregulated in response to refeeding, there was a shift in the polysome distribution of the FGF21 mRNA from heavy to light polysomes, indicating that refeeding led to downregulation of FGF21 mRNA translation. In the second study, there was no effect of diet on FGF21 mRNA expression, however the proportion of FGF21 mRNA in heavy compared to light polysomes was greater in rats consuming the Western compared to the control diet. Furthermore, body weight and fasting blood glucose concentrations were the same in rats fed a Western compared to a control diet, suggesting that the rats had not yet developed insulin resistance.The results show that the FGF21 mRNA is translationally controlled in response to fasting and refeeding in association with changes in mTORC1 activity. Moreover, short‐term consumption of a Western diet enhances translation of the FGF21 mRNA without altering expression of its mRNA. Studies are in progress to explore possible regulatory mechanisms controlling FGF21 mRNA translation, e.g. identification of regulatory elements in the 5′‐UTR of the mRNA.Support or Funding Information(Supported by NIH grant DK13499)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.