30TLR4-mutant mice also strongly displayed less liver fibrosis upon bile duct ligation, indicating that the LPS-TLR4 pathway plays an important role in hepatic fibrogenesis25 Similarly, we found ablation of TLR4 reduced the generation of inflammatory cytokines
in DEN-induced liver early damage and cancer formation later on. Production of these cytokines depends on LPS/TLR4 in hematopoietic-derived Kupffer cells, as depletion of Kupffer cells14 or antibiotics treatment to reduce LPS levels prior to DEN treatment inhibited the induction of the inflammatory mediators. In agreement, inhibition of TLR4 activation in myeloid cells, exerted through transplantation of TLR4−/− bone marrow, inhibited inflammatory responses following DEN-induced
hepatic insult. Because mature livers have extremely low rates of cell turnover, DEN-exposed Selumetinib hepatocytes do not yield genetically transformed progeny in the absence of hepatomitogens. TNFα and IL-6 were identified as the major Kupffer cell-produced factors that enhance the growth of surviving DEN-initiated hepatocytes.14 In light of the compensatory proliferation that promotes chemical hepatocarcinogenesis was significantly reduced in Kupffer cell-depleted mice,14 in chimeric mice containing TLR4−/− bone marrow and in antibiotics treated mice, it is reasonable that activation of TLR4 signaling by LPS in Kupffer cells is essential for driving expression of these proliferation-stimulating cytokines. Consistently, ablation of Myd88 led to a reduced incidence BMS-907351 in vivo of HCC in response to treatment with DEN.31 Therefore, we concluded Gemcitabine clinical trial that LPS engagement of TLR4 in myeloid cells, specifically Kupffer cells, in the liver of mice subjected to DEN treatment produces paracrine-acting, tumor-promoting cytokines that not only cause inflammation but also stimulate the proliferation of adjacent premalignant hepatocytes. Remarkably, TLR4 stimulates both liver cell proliferation and survival, which explains the profound
tumor-suppressive phenotype observed in TLR4−/− mice. Although TLR4 ablation did not result in spontaneous chronic liver pathology, these animals had increased sensitivity to disease in a model of DEN-induced liver injury. By contrast, mice pretreated with LPS were protected against DEN-induced acute liver injury. Evading apoptosis is generally considered as a classic cellular mechanism contributing to cancer.32 Our results demonstrate that TLR4 activation is a survival signal allowing tumor cells to escape apoptosis; thus, inhibition of endotoxin accumulation has anti-oncogenic effects. Therefore, the increased epithelial apoptosis during tumor promotion and the decreased inflammatory compensatory proliferation may eventually halt liver tumor progression in TLR4−/− mice. Because NF-κB is a major downstream signaling component of TLR4 signaling, similar observations were also made in mice with deletion of IKKβ in hepatocytes.