FGF21 and metabolic dysfunction-associated steatotic liver disease: A key co-receptor in the FGF21-KLB-FGFR signaling pathway; mediates the binding of FGF21 to FGFR to activate intracellular signaling; regulates energy homeostasis and metabolic processes (especially in liver and adipose tissue); involved in the cross-regulatory network with non-alcoholic fatty liver disease (NAFLD) pathological processes