All TLR2−/− mice developed HCC at the end of the sixth month, but only 63% of WT mice developed such lesions at this time. However, 100% of WT or TLR2−/− mice developed liver tumors at the end of the eighth month after DEN injection (Fig. 1B). The number of HCC tumor nodules was also increased in the TLR2−/− mice Epigenetics inhibitor (Fig. 1C). Although the TLR2−/− mice displayed

no difference in a very early hepatic injury (Fig. S1E,F), they showed persistent elevated serum levels of ALT (Fig. 1E) as compared to WT mice after DEN injection. Thus, TLR2−/− mice with HCC had shorter mean survival times than WT mice (Fig. 1F). Collectively, the data indicate that knockout of TLR2 increases the susceptibility to the DEN-induced hepatocarcinogenesis

high throughput screening compounds and progression. DEN is a typical chemical carcinogen and forms adducts with DNA after liver metabolization by cytochrome P450 2E1. These adducts may cause liver injury, DNA mutation, and tumorigenesis.19 No significant difference in cytochrome P450 2E1 activity was detected between TLR2−/− and WT mice (data not shown), indicating that the elevated HCC development in TLR2−/− livers did not simply result from changes in DEN metabolites. Further, the TLR2−/− mice exhibited enhanced accumulation of ROS in their liver tissues (Fig. 2A), which was sustained from the early to the late phase of HCC progression (Fig. 2B). Additionally, we found that TLR2−/− livers showed an accumulation of oxidative stress-associated products, including protein carbonyl and 8-OHdG-linked proteins (Fig. S2A) and

the activation of lipid peroxides (LPO) (Fig. S2B). Generally, cellular stress and oxidative damage should induce programmed cell death in the liver through apoptosis and autophagy. However, TLR2−/− mice displayed a persistent decrease in cell death (Fig. 2C,D) compared to WT mice. Indeed, TLR2−/− mice exhibited a decrease in autophagy-associated cell death as marked by Lamp1 and TUNEL double staining (6.0 ± 1.7 versus 1.5 ± 0.3, P < 0.05) (Fig. 3A,B) as well as a decrease in apoptotic cell death as evidenced by suppressed cleavage of caspase-3 (Fig. 3C,D). These results indicate that TLR2 deficiency protects liver cells from oxidative stress-induced death. Cellular senescence is regarded ADP ribosylation factor as a physiological barrier against carcinogenesis and tumor progression.20-22 Senescence can be induced by many stimuli, such as dysfunctional telomeres and other sources of DNA damage.21 As a typical biomarker of senescence, SA β-galactosidase (β-gal) staining was increased in WT livers but not in TLR2−/− livers after DEN treatment (Fig. 4A,B). Despite ROS accumulation (Fig. 2A) and DNA damage (Fig. S2A) in the liver tissue, γ-H2A.X (phosphorylated histone H2A.X), a typical biomarker of DNA damage repair, was suppressed in TLR2−/− livers (Fig.