We next tested whether uremic toxin has a direct effect on the
expression of irisin in skeletal muscle cells. Indoxyl sulfate, which is
a protein-bound uremic toxin, increases significantly during
kidney function deterioration.[20] Recent studies showed that
the levels of indoxyl sulfate are associated with kidney disease
progression and mortality in CKD patients. [21,22] We treated
human skeletal muscle cells with indoxyl sulfate at different
concentrations (0, 50, 250 and 500 mM) for 24 h. In these cells,
FNDC5 protein levels decreased in a dose-dependent manner
after 24 h treatment of indoxyl sulfate. (Fig. 2A) The PGC1-a
protein levels were not altered by indoxyl sulfate. (Fig. 2B) The
irisin released into the culture medium of human skeletal muscle