Prognostic Significance of iNOS in Hepatocellular Carcinoma

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Epidemiological research indicates a contribution of chronic inflammatory liver diseases to development of hepatocellular carcinoma (HCC). Mounting evidence shows a carcinogenic role of NO• produced by inflammatory or cancer cells. The calcium-independent inducible isoform, iNOS, produces under inflammatory stimulation large amounts of NO• through the conversion of l-arginine to l-citrulline. NO• and NO•-derived oxidants can oxidize biomolecules, causing DNA damage. NO• interferes with the oxidative metabolism at different levels. It upregulates the AMP-activated protein kinase, thus switching off the ATP-consuming pathways such as lipogenesis or gluconeogenesis, while switching on the ATP-producing pathways such as fatty acid and glucose oxidation. Moreover, increase in mitochondrial NO• above physiological levels affects the oxygen-binding site of cytochrome c oxidase and induces pyruvate dehydrogenase kinase-1, an inhibitor of pyruvate dehydrogenase complex, thus inhibiting electron transport and oxygen consumption. NO• may also interact with different signaling pathways in hepatocarcinogenesis, including COX2, inhibitor of κB kinase (IKK)/nuclear factor B (NF-κB), and RAS/extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling. NO• stimulates COX-2 activity, and COX-2 inhibitors block NO• production in HCC cells. Furthermore, NO• and its derivatives may influence prostaglandin production by inducing lipid peroxidation and arachidonic acid release from cell membranes. Stimulation of EP2 receptor by PGE2 induces the association of the α subunit of the regulator G protein signaling and AXIN. This leads to the inactivation of glycogen synthase kinase-3β (GSK-3β) with consequent nuclear accumulation of β-catenin and increase in its transcriptional targets, c-Myc, c-Jun, and cyclin D1. PGE2 can also stimulate cell growth through the activation of several tyrosine kinase receptors, including EGFR and the PI3K/Akt pathways. Recent research on the interplay between iNOS and IKK/NF-κB and RAS/ERK pathways in HCC showed that these interactions are highest in the highly aggressive preneoplastic and neoplastic liver lesions of genetically susceptible rats and c-Myc/Tgf-α transgenic mice. The determination of iNOS expression in human HCC showed highest values in a subtype with poorer prognosis. Interestingly, iNOS levels are directly correlated with genomic instability, proliferation rate, and microvessel density of human HCC and inversely correlated with apoptosis and patients’ survival. These observations suggest that iNOS upregulation and changes in iNOS/NF-κB and iNOS/H-RAS/ERK cross talks are prognostic markers for HCC. Moreover, the block of iNOS signaling by a specific inhibitor such as aminoguanidine leads to a consistent decrease in HCC growth in c-Myc/TGF-α transgenic mice, decrease in growth and increase in apoptosis in human HCC cell lines, suggesting that the key components of iNOS signaling could represent therapeutic targets.