Triiodothyronine (T3) response in liver of FGF21 knockout mice

Thyroid hormone (TH) influences metabolic pathways by binding to specific receptors (TRs), which are conditional transcription factors. T3 works through TRs to induce fibroblast growth factor (FGF) 21, a peptide hormone that is usually induced in fasting and influences lipid and carbohydrate metabolism via local hepatic and systemic endocrine effects. While administered TH and FGF21 display overlapping actions, including reductions in serum lipids, current models suggest that these hormones act independently in vivo. Here, we examined mechanisms of TH regulation of FGF21 expression and tested the possibility that FGF21 is required for induction of hepatic TH-responsive genes. We confirm that active TH (T3) and the TRβ selective thyromimetic GC-1 increase FGF21 transcript and peptide levels in mouse liver and that this effect requires TRβ. T3 also induces FGF21 in cultured hepatocytes and this effect involves direct actions of TRβ1, which binds a TRE within intron 2 of FGF21. Gene expression profiles in wild type and FGF21 knockout mice are highly similar indicating that FGF21 is dispensable for the majority of hepatic T3 gene responses. A small subset of genes displays diminished T3 response in the absence of FGF21. However, most of these are not obviously involved in T3-dependent hepatic lipid and carbohydrate metabolic processes. Accordingly, T3-dependent effects upon serum lipids are maintained in the FGF21-/- background. Our findings suggest that T3 regulates genes involved in classical hepatic metabolic responses independently of FGF21. Total RNA was obtained from mouse liver of WT and FGF21-/- treated with DMSO or T3