The DNA repair enzyme apurinic/apyrimidinic endonuclease 1 (APE1) plays a central role in base excision repair and functions as a reductive activator of various transcription factors. Multiple other functionalities have been ascribed to APEI in addition to these major functions. A recent study showed that APE1 knockdown upregulated the expression of a set of genes related to extracellular matrix (ECM) production, indicating an additional novel biological role for this enzyme. Based on this finding, we have investigated the effect of APE1 downregulation on ECM-related gene expression and its biological consequences. Endogenous APEI expression was downregulated in human cervical carcinoma HeLa cells and human lung carcinoma A549 cells using siRNA. When the expression of six ECM-related genes (TGFB1, LAMC1, FN1, COL1A1, COL3A1, and COL4A1) was evaluated, we found that APE1 knockdown upregulated the expression of TGFB1 in both cell lines. APE1 downregulation promoted actin rearrangement, inducing F-actin accumulation in HeLa cells and the dissipation of stress fibers in A549 cells. We also discovered that APE1 knockdown enhanced cellular motility in A549 cells, which was suppressed by the inhibition of transforming growth factor (TGF)-fil signaling. These results suggested that APE1 controls the organization of actin cytoskeleton through the regulation of TGF-131 expression, providing novel insights into the biological significance of APE1. (C) 2015 Elsevier Inc. All rights reserved.