We investigated the signaling pathways underlying nano-TiO2-induced apoptosis in cultured human lymphocytes. Nano-TiO2 increased the proportion of sub-G1 cells, activated caspase-9 and caspase-3, and induced caspase-3-mediated PARP cleavage. Nano-TiO2 also induced loss of mitochondrial membrane potential, which suggests that nano-TiO2 induces apoptosis via a mitochondrial pathway. A time-sequence analysis of the induction of apoptosis by nano-TiO2 revealed that nano-TiO2 triggered apoptosis through caspase-8/Bid activation. We also observed that inhibition of caspase-8 by z-IETD-fmk suppressed the caspase-8/Bid activation, caspase-3-mediated PARP cleavage, and apoptosis. Nano-TiO2 activated two MAPKs, p38 and JNK. In addition, the selective p38 inhibitor SB203580 and selective JNK inhibitor SP600125 suppressed nano-TiO2-induced apoptosis and caspase-8 activation to moderate and significant extents, respectively. Knockdown of protein levels of JNK1 and p38 using an RNA interference technique also suppressed caspase-8 activation. Our results suggest that nano-TiO2-induced apoptosis is mediated by the p38/JNK pathway and the caspase-8-dependent Bid pathway in human lymphocytes. (C) 2009 Elsevier Inc. All rights reserved.