The Role Of Gut-Derived Endotoxin In Chemically Induced Hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is a prevalent form of liver cancer that occurs two to four times more frequently in males than in females. The most prominent factors associated with HCC include chronic hepatitis B and C viral infection and other chronic necroinflammatory liver diseases, such as those caused by alcohol consumption or hepatic metabolic disorders. Chronic liver disease leads to continual injury and a wound-healing response that causes a torrent of problems, including advanced hepatic fibrosis or cirrhosis. A high level of plasmatic endotoxin or lipopolysaccharide (LPS), a cell-wall component of Gram-negative bacteria, is a common finding in the portal and systemic circulation of cirrhotic patients. This accumulation is likely due to changes in the intestinal mucosal permeability and increased bacterial translocation, coupled with a deficient clarification activity of the hepatic reticuloendothelial system. High levels of endotoxin may be responsible for the pathogenesis of the chronic inflammatory alterations that characterize cirrhosis and, hence, the major complications that arise in this disease.Toll-like receptor 4 (TLR4) is a pattern recognition receptor that recognizes endotoxin and signals through adaptor molecules myeloid differentiation factor 88 (MyD88) and Toll/IL-1 receptor domain-containing adaptor-inducing interferon-β(TRIF) to activate transcription factors nuclear factor NF-κB, activator protein 1 (AP-1), and interferon regulatory factors (IRFs), that initiate innate immunity. The liver is well equipped to respond to endotoxin because TLR4 is present on both parenchymal cells (hepatocytes) and nonparenchymal cells, which include Kupffer cells, sinusoidal endothelial cells, stellate cells, and hepatic dendritic cells. Both cell populations possess intact TLR4 signaling pathways. Kupffer cells are the best-characterized target of endotoxin in the liver, where they have a crucial role in causing hepatocellular damage by producing pro-inflammatory cytokines (e.g., TNF-a and IL-6) and affect hepatic sinusoids to increase vascular permeability. Although hepatocytes also express low levels of the TLR4 receptor, they are only weakly responsive to LPS and may serve to uptake and remove endotoxin from the portal and systemic circulation. However, the effects of endotoxin in vivo on hepatic function and tumorigenesis are not well defined. A causal link between inflammation and cancer is widely accepted. Robust clinical and epidemiologic data support the role of inflammation, induced by chronic hepatitis B or C viral infections and alcohol abuse, as a key player in HCC development. However, the exact molecular mechanisms and gatekeepers accounting for cellular transformation remain elusive. Given the important role of NF-κB signaling in mediating inflammatory signals, attention has been focused on its role in mediating the link between inflammation and the development of liver tumors. Inhibiting NF-κB obstructs later stages of tumor progression in Mdr2-deficient mice, which develop HCC in the context of chronic bile duct inflammation. By contrast, mice lacking the NF-κB upstream activator IκB kinaseβ(IKKβ) specifically in hepatocytes exhibit a marked increase in chemically induced hepatocarcinogenesis, suggesting that NF-κB has a protective function against HCC development. Interestingly, compared with the deletion of IKKβonly in hepatocytes, the additional deletion in Kupffer cells results in a remarkable decrease in tumor load. These apparently contradictory conclusions may reflect the distinct roles for inflammatory signals in epithelial cells and inflammatory cells during HCC formation.Here we found that the circulating levels of LPS were elevated in experimental models of carcinogen-induced hepatocarcinogenesis. Reduction of LPS using antibiotics regimen in rats or genetic ablation of its receptor TLR4 in mice prevented excessive tumor growth and multiplicity. Additional investigation revealed that TLR4 ablation sensitizes the liver to carcinogen-induced toxicity via blocking NF-κB activation and sensitizing the liver to reactive oxygen species (ROS)-induced liver injury, but lessens inflammation-mediated compensatory proliferation. Reconstitution of TLR4-expressing myeloid cells in TLR4-deficient mice restored DEN-induced hepatic inflammation and proliferation, indicating a paracrine mechanism of LPS in tumor promotion. By contrast, disruption of the mucosal barrier allowing microbial translocation or in vivo LPS pre-challenge protects hepatocytes against carcinogen or death-receptor-induced liver injury. The survival effects of LPS on normal or malignant hepatocytes are largely mediated by the transcription factor NF-κB, whose inactivation reversed the LPS-mediated hepatic protection. Thus, sustained LPS accumulation represents a pathological mediator of inflammation-associated hepatocellular carcinoma (HCC) and manipulation of the gut flora to prevent pathogenic bacterial translocation and endotoxin absorption may favorably influence liver function in cirrhotic patients who are at risk of developing HCC.