Fibroblast growth factor 21/extracellular signal-regulated kinase 1/2 pathway as a key link mediates the effects of low-protein diet on skeletal muscle fiber type transformation in pigs.

Low-protein (LP) diets play a pivotal role in inducing skeletal muscle remodeling toward an oxidative phenotype in pigs, the underlying targets and mechanisms remain uncertain. This study aimed to investigate whether fibroblast growth factor 21 (FGF21), an endocrine signal associated with low protein intake, mediates the effect of LP diets on myofiber type transition in pigs. Twenty-four healthy castrated weaned pigs (Landrace × Yorkshire) with an initial body weight (BW) of 7.25 ± 0.12 kg were assigned and fed one of two dietary treatments: a normal-protein (NP group) diet containing 19.78% crude protein (CP) or an LP diet containing 16.91% CP (LP group), over a 28-day trial. The LP diet was supplemented with crystalline amino acids to achieve an equal content of limiting amino acids. Additionally, half of the pigs in each dietary group were randomly administered an extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor (0.5 mg/kg BW, i.p.) weekly, while the remaining pigs received an equivalent dose of the vehicle (dimethyl sulfoxide). After blood sampling, all pigs were euthanized to collect liver and longissimus dorsi (LD) muscle samples. In vitro, we also explored the FGF21-induced myofiber conversion in porcine skeletal muscle satellite cells (PSCs), which were transfected with overexpression plasmid vector and ERK1/2 inhibitor. Results demonstrated that FGF21 was robustly increased by LP diets (P < 0.01), evidenced by elevated mRNA and protein expression levels of FGF21 in liver and LD muscle, as well as increased serum FGF21 concentration. Furthermore, pigs fed the LP diet exhibited a higher proportion of oxidative myofibers (P < 0.001), increased expression level of slow myosin heavy chain (MyHC) (P < 0.05) and myoglobin (P = 0.080), and enhanced succinate dehydrogenase activity (P < 0.01) in LD muscle. These effects were largely attenuated by ERK1/2 inhibitor administration. The activation of ERK1/2 and mTOR complex 1 (mTORC1) in the LD muscle of pigs fed LP diets was also suppressed by ERK1/2 inhibitor treatment (P < 0.05). In vitro, FGF21 overexpression increased slow MyHC protein expression, ERK1/2 phosphorylation and mTORC1 activity in PSCs (P < 0.05). Furthermore, inhibition of the ERK1/2 signaling significantly eliminated the FGF21-induced enhancement of oxidative myofiber expression and mTORC1 activity in PSCs (P < 0.05). Collectively, our findings suggest that the FGF21-ERK1/2 pathway serves as a key link mediator of LP diet-induced myofiber specification in pigs, and provide new insights into the understanding of the metabolic benefits of LP diets in pigs.