FGF21 adenovirus

Among the 22 fibroblast growth factors (FGFs), FGF21 has now emerged as a key metabolic regulator. However, the mechanism by which FGF21 mediates its metabolic effects itself remains largely unknown. Here, researchers show that FGF21 inhibits mammalian target of rapamycin complex 1 (mTORC1) and improves insulin sensitivity and glycogen storage in a hepatocyte-autonomous manner. Administration of FGF21 in mice inhibits mTORC1 in the liver, and mice deficient in FGF21 exhibit marked insulin-stimulated mTORC1 activation and exacerbated hepatic insulin resistance (IR). FGF21 inhibits insulin- or nutrient-stimulated mTORC1 activation, thereby enhancing Akt phosphorylation in HepG2 cells under normal and IR conditions. TSC1 deficiency abolishes FGF21-mediated mTORC1 inhibition and enhancement of insulin signaling and glycogen synthesis. Surprisingly, hepatic βKlotho knockdown or hepatic mTORC1/ribosomal protein S6 kinase 1 hyperactivation abolished the hepatic insulin-sensitizing and glycemic-control effects of FGF21 in diet-induced insulin-resistant mice. Furthermore, FGF21 ameliorates diet-induced methionine- and choline-deficient steatohepatitis. These findings suggest that mTORC1 inhibition by FGF21 has therapeutic potential for the treatment of IR and type 2 diabetes.

To investigate whether FGF21 is the driving force behind the suppression of hepatic mTORC1 and the improvement of insulin sensitivity in vitro, the researchers rigorously evaluated the effects of FGF21 overexpression in multiple hepatocytes. Adenovirus-mediated overexpression of FGF21 (Ad-FGF21) potently inhibited insulin-stimulated mTORC1 activity compared to Ad-GFP (Figure 1A). Consistent with the in vivo results, FGF21 inhibited insulin-stimulated IRS-1 phosphorylation, thereby inducing phosphorylation of Akt and GSK-3b. The effects of FGF21 on mTORC1 activity and insulin signaling were confirmed in nutrient- and amino acid-stimulated HepG2 cells (Figure 1B), palmitic acid/BSA (bovine serum albumin)-induced insulin-resistant HepG2 cells (Figure 1C), and human Huh7 hepatocytes. Notably, in cells expressing Ad-FGF21, medium levels of FGF21 were approximately 4,000 pg/mL (Figure 1D), suggesting that adenoviral-delivered FGF21 expression may inhibit mTORC1 through hormone stimulation. To further test the functional impact of FGF21-mTORC1 on insulin sensitivity, glycogen synthesis activity was measured in primary hepatocytes. Hepatic glycogen synthesis was induced by insulin and synergistically induced by co-treatment with FGF21 (Figure 1E). Similarly, similar effects were shown using acute and low-dose inhibition of mTORC1 activity. Interestingly, co-treatment with FGF21 and rapamycin did not cause additional stimulation of glycogen synthesis compared with FGF21 or rapamycin alone, suggesting that FGF21 and rapamycin may promote glycogen synthesis through a mechanism similar to that of inhibiting mTORC1 in hepatocytes.

Figure 1. FGF21 stimulates hepatic insulin action and glycogen synthesis by repressing mTORC1 activation in hepatocytes. (Gong Q I, et al., 2016)