FGF21 enhances GLUT4 translocation in primary brown adipocytes and the hunt for new hepatokines

Fibroblast growth factor 21 (FGF21) is a liver-derived hormone that regulates several key factors influencing bodyweight – including energy expenditure, food intake, and sucrose/sweet-taste preference. FGF21’s tissue targets are conferred by β-Klotho (KLB), it’s obligate co-receptor thar forms a complex with FGFR1c to initiate downstream signaling. KLB is found in several regions of the brain, adipose tissue, the pancreas, and the liver. Acute administration of FGF21 to lean and obese rodents increases insulin sensitivity, quantified by glucose uptake, by driving glucose to brown adipose tissue (BAT). Despite the numerous FGF21 analogues and mimetics that have made it to clinical trials, little is still known about the downstream signaling that is elicited when FGF21 is applied to BAT. FGF21, like other FGFs, activates MAPK signaling via MEK -> ERK1/2, a kinase with a plethora of targets. To explore how FGF21 signaling increases insulin sensitivity, we employed phosphoproteomics to analyze FGF21 treated primary brown adipocytes (PBAs) to probe it’s downstream signaling targets at acute timepoints (5 and 15 minutes). We found several proteins involved in glucose transporter type 4 (GLUT4) translocation were phosphorylated in response to FGF21 treatment, most of which overlap with insulin treated PBAs. Ribosomal Kinase 2 (RSK2) was identified as a possible mediator of the phosphorylation of GLUT4 machinery in FGF21-treated samples, due to a lack of complete Protein Kinase B (AKT) activation in the absence of insulin and known phosphorylation activity against Akt substrate of 160 kDa (AS160). To confirm our findings, we utilized a mouse model that allowed us to kinetically read GLUT4 translocation in PBAs to investigate FGF21’s effects on insulin-stimulated GLUT4 translocation.