['Increased phosphorylation (activation) of signal transducer and activator of transcription 3 (Stat3) on tyrosine 705 leads to renal fibrosis. Indoxyl sulfate, a uremic toxin, induces renal fibrosis through expression of transforming growth factor-β(1) (TGF-β(1)) in proximal tubular cells. The present study aimed to determine whether Stat3 is involved in indoxyl sulfate-induced dysfunction of proximal tubular cells.', 'Localization of phosphorylated Stat3 in the kidneys of normal, subtotally nephrectomized, and AST-120-treated subtotally nephrectomized rats was examined by immunohistochemistry. The effect of indoxyl sulfate on phosphorylation of Stat3 and the role of Stat3 on indoxyl sulfate-induced cellular effects were examined using human proximal tubular cells (HK-2 cells).', 'Subtotally nephrectomized rats showed increased immunostaining of phosphorylated Stat3 in the renal tubules compared with normal rats. Administration of AST-120, which reduces serum level of indoxyl sulfate, to subtotally nephrectomized rats reduced the immunostaining of phosphorylated Stat3 in the renal tubules. Indoxyl sulfate induced phosphorylation of Stat3 in HK-2 cells. Stat3 small interfering RNA suppressed indoxyl sulfate-induced expression of an inflammation marker gene (monocyte chemotactic protein-1), fibrosis marker genes (TGF-β(1), α-smooth muscle actin) and a subunit of nuclear factor-ĸB (p65), and attenuated a cellular senescence marker, senescence-associated β-galactosidase activity.'] Does stat3 contribute to indoxyl sulfate-induced inflammatory and fibrotic gene expression and cellular senescence?