High phosphate levels promote muscle atrophy via myostatin expression in differentiated L6 myotubes

Abstract

Background: Sarcopenia is the age-induced, progressive loss of skeletal muscle mass and function. This phenomenon is observed in patients with chronic kidney disease (CKD). However, the intracellular mechanism underlying the progressive sarcopenia in CKD has not been completely elucidated. Although hyperphosphatemia contributes to cellular senescence, it is unclear whether this condition induces skeletal muscle atrophy. The aim of this study was to determine the effect of hyperphosphatemia on skeletal muscle. Methods: Differentiated rat myoblast cells (L6) were exposed to normal (0.9 mM; CON), medium (2.5 mM), and high (3.8mM; HPi) phosphate conditions for 10 days. We measured the protein levels of myosin heavy chain (MHC) and myostatin and determined the ratio of phosphorylated p70S6K and cleaved caspase-3 by western blot. The expression levels of the myogenic transcriptional regulators MyoD and myogenin were measured by qPCR. Results: The levels of MHC were gradually downregulated depending on the phosphate concentration. Myostatin in HPi was about 20 times higher than in CON (P<0.001). In myotubes cultured in HPi, protein synthesis was significantly lower, and degradation was significantly higher than in CON (P<0.01). The mRNA expression of MyoD in HPi was significantly lower than in CON. Conclusions: This study showed that hyperphosphatemia strongly induced muscle atrophy with the accumulation of myostatin. This mechanism might include the downregulation of protein synthesis, upregulation of proteolysis, and attenuated muscle regeneration.

Association of Serum Phosphate with Low Handgrip Strength in Patients with Advanced Chronic Kidney Disease

“…In line with our observations, growing evidence supports a correlation between higher serum Pi and lower muscle strength. Our present and previous in vitro studies have demonstrated that rodent muscle cells cultured in a high-Pi-containing medium exhibit muscle atrophy characterized by the accumulation of myostatin, accelerated senescence with integrin-linked kinase overexpression, reduced proliferative capacity, activation of autophagy, reductions in mitochondrial membrane potential, and activation of Nrf2 signaling [ 35 , 36 , 37 , 38 ]. Furthermore, increased Pi can reduce Ca 2+ release from the sarcoplasmic reticulum, which may lead to reduced cross-bridge activity and impaired muscle contraction [ 39 , 40 ].…”

Skeletal Muscle Injury in Chronic Kidney Disease—From Histologic Changes to Molecular Mechanisms and to Novel Therapies

“…In differentiated C2C12 myotubes, phosphate induces oxidative stress and impairs mitochondrial function [255]. Furthermore, in L6 myotubes phosphate decreases the expression levels of myosin heavy chain, increases myostatin expression, and elevates caspase-3 activity in a dose-dependent manner, indicating that phosphate downregulates protein synthesis and upregulates protein degradation and induces atrophy and cell death [273]. Furthermore, phosphate induces atrophy in L6 myotubes by activating autophagy [274].…”