In Vivo Effects of Cyclooxygenase-2 Deletion on Cellular Signaling in Hepatocellular Carcinoma Xenografts in Nude Mice

AIM: It is known that cyclooxygenase-2 (COX-2) overexpression is associated with growth of hepatocellular carcinoma (HCC) cells, but its in vivo mechanisms remain to be determined. METHODS: In the present study, the parental HuH7, a human HCC cell line, and COX-2 deleted HuH7 cells were inoculated to nude mice to assess the in vivo effects of COX-2 deletion (C2D) on cellular signaling in HCC xenografts. RESULTS: We found that C2D significantly inhibited Ki-67 and increased peroxisome proliferator-activated receptor-gamma (PPAR-gamma) expression, and reduced plasma prostaglandin E2 and a-fetoprotein levels in association with reduced Rb phosphorylation and cyclin D1/CDK4 complex, and increased p21/CDK4 complex, indicating COX-2 overexpression promotes G1-S transition of the cell cycle in human HCC xenografts via a series of complicated signaling. C2D also resulted in significantly reduced phosphorylation of Akt, indicating a potential role of PTEN/PI3K/Akt pathway in COX-2-mediated alteration of HCC growth. We also demonstrated that C2D significantly inhibited histone deacetylase-2 (HDAC-2) expression, but increased the acetylation levels of histone-3 and histone-4, indicating that altered histone acetylation by histone deacetylases may be involved in COX-2-mediated HCC cell proliferation. In contrast to previous in vitro reports, we found that C2D did not significantly affect apoptosis and formation of activated caspase-3 and 9 in HCC xenografts. CONCLUSION: Our results revealed that C2D results in a series of in vivo alteration of cellular signaling in HCC xenografts that demonstrated important pathogenic mechanisms of COX-2 overexpression in HCC growth and provided valuable information on searching novel approaches to HCC chemoprevention.