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The Role Of Pge2 Biosynthesis And Metabolism In Liver Injury And Liver Cancer

PGE2 plays an important role in liver inflammation and carcinogenesis. Its metabolism is regulated by a cascade of reactions catalyzed by enzymes including COX-1/2, mPGES-1/2, 15-PGDH. Among these regulators, mPGES-1 is a cytokine-inducible enzyme mainly responsible for catalyzing terminal synthesis of PGE2, 15-PGDH catalyzes the oxidation of PGE2 to 15-keto-PGE2. In this context, we exogenously expressed mPGES-1 or 15-PGDH genes in mice hepatocytes to constitute a physiological condition ideal for evaluating PGE2 and its metabolites function in liver pathogenesis. In the first part, we developed transgenic mice with targeted expression of mPGES-1 in the liver and assessed the response of the transgenic mice to Fas-induced hepatocyte apoptosis and acute liver injury. Compared to wild type mice, the mPGES-1 Tg mice showed less liver hemorrhage, lower serum alanine transaminase and aspartate transaminase levels, less hepatic necrosis/apoptosis, and lower levels of caspase activation after intraperitoneal injection of the anti-Fas antibody Jo2. Western blotting analyses revealed increased expression and activation of the serine/threonine kinase Akt and associated anti-apoptotic molecules in the liver tissues of Jo2-treated mPGES-1 Tg mice. Pretreatment with the mPGES-1 inhibitor (MF63) or the Akt inhibitor (Akt inhibitor V) restored the susceptibility of the mPGES-1 Tg mice to Fas-induced liver injury. Our findings provide novel evidence that mPGES-1 prevents Fas-induced liver injury through activation of Akt and related signaling. This finding is consistent with previous reports of the anti-apoptotic and pro-proliferative role of PGE2. Our results suggest that induction of mPGES-1 or treatment with PGE2 may represent a potential therapeutic strategy for the prevention and treatment of Fas-associated liver injuries. In the second part, we generated transgenic mice with targeted expression of 15-PGDH in the liver and the animals were subjected to LPS/GalN-induced acute liver inflammation and injury. Compared to the wild type mice, the 15-PGDH Tg mice showed lower levels of alanine aminotransferase and aspartate aminotransferase, less liver tissue damage, less hepatic apoptosis/necrosis, less macrophage activation, and lower inflammatory cytokine production. In Kupffer cell cultures, treatment with 15-keto-PGE2 or the conditioned medium (CM) from 15-PGDH Tg hepatocyes inhibited LPS-induced cytokine production. Both 15-keto-PGE2 and the CM from15-PGDH Tg hepatocyes also up-regulated the expression of PPAR-γ downstream genes in Kupffer cells. In cultured hepatocytes, 15-keto-PGE2 treatment or 15-PGDH overexpression did not influence TNF-α-induced hepatocyte apoptosis. These findings suggest that 15-PGDH protects against LPS/GalN-induced liver injury and the effect is mediated via 15-keto-PGE2, which activates PPAR-γ in Kupffer cells and thus inhibits their ability to produce inflammatory cytokines. Accordingly, we observed that the PPAR-γ antagonist, GW9662, reversed the effect of 15-keto-PGE2 in Kupffer cell in vitro and restored the susceptibility of 15-PGDH Tg mice to LPS/GalN-induced acute liver injury in vivo. Our findings not only support the pro-inflammatory role of PGE2, but also reveal a novel anti-inflammatory role of 15-keto-PGE2. The data suggest that induction of 15-PGDH expression or utilization of a 15-keto-PGE2 analog may be therapeutic for treatment of endotoxin-associated liver inflammation/injury. Consistent with a pro-carcinogenic role for PGE2, overexpression mPGES-1 enhances growth of either HCC or cholangiocarcinoma cells, while overexpression 15-PGDH inhibits tumor cell growth in vitro. In the third part, we use a pharmacological method to induce 15-PGDH in cholangiocarcinoma tumor cells to inhibit PGE2 production. Our results indicated that treatment of human cholangiocarcinoma cells (CCLP1 and TFK-1) with ω-3 PUFA (DHA) or transfection of these cells with the Fat-1 gene (encoding Caenorhabditis elegans desaturase which converts ω-6 PUFA to ω-3 PUFA) significantly increased 15-PGDH protein level in cholangiocarcinoma cell lines. Human cholangiocarcinoma cells treated with DHA or transfected with a Fat-1 expression vector showed reduction of miRNA26a and miRNA26b (both miRNAs target 15-PGDH mRNA thus inhibiting 15-PGDH translation). Consistent with these findings, we observed that overexpression of miR26a or miR26b decreased 15-PGDH protein, reversed ω-3 PUFA-induced accumulation of 15-PGDH protein, and prevented ω-3 PUFA-induced inhibition of cholangiocarcinoma cell growth. Knockdown of 15-PGDH also attenuated ω-3 PUFA-induced inhibition of tumor cell growth. We observed that ω-3 PUFA suppressed miRNA26a and miRNA26b by inhibiting c-myc, a transcription factor that co-regulates a gene cluster comprised of miR-26a/b and carboxy-terminal domain RNA polymerase II polypeptide A small phosphatases (CTDSPs). Accordingly, overexpression of c-myc enhanced the expression of miRNA26a/b and prevented ω-3 PUFA-induced inhibition of tumor cell growth. Taken together, our results support a pro-tumorigenic role for PGE2, and suggest induction of 15-PGDH as potential way for the prevention and treatment of human cholangiocarcinoma. / 1 / LU YAO

  1. tulane:45958
  2. local: td005616
Identiferoai:union.ndltd.org:TULANE/oai:http://digitallibrary.tulane.edu/:tulane_45958
Date January 2015
ContributorsYao, Lu (author), Wu, Tong (Thesis advisor), School of Medicine Pathology and Laboratory Medicine (Degree granting institution)
Source SetsTulane University
LanguageEnglish
Detected LanguageEnglish
TypeText
Formatelectronic
RightsNo embargo

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