To investigate the possibility that Tg adipose is somehow defective and resistant to FGF21, a single injection of 1 mg/kg recombinant human FGF21 (rhuFGF21) was administered in naïve WT and Tg mice, and FGF21-induced signaling was investigated in WAT and liver tissues (Figure 3). We chose rhuFGF21 as it is more potent than mouse FGF21, and anticipated to yield clear signaling responses [15]. Immunoblot analyses revealed expected ERK signaling in WT WAT with rhuFGF21, but a diminished phospho-ERK response in Tg WAT (Figure 3A). We further assessed the signaling of rhuFGF21 by analyzing Fibroblast growth factor receptor substrate 2 (FRS2) phosphorylation (Figure 3B). Injection of rhuFGF21 led to a clear induction of FRS2 phosphorylation, apparent as a larger 85 and smaller 75 kD band (due to multiple phosphorylation sites). In unstimulated Tg WAT, we observed FRS2 phosphorylation with a unique triple-banding pattern (approx. 80–85 kDa), which was unchanged in response to rhuFGF21, suggesting basal activation of the pathway, proximal to ERK activation. We next interrogated the FRS2 adaptor Src homology domain 2 containing protein tyrosine phosphatase (SHP2) (Figure 3C). In
