Abstract 3490: Targeting E3 ligases in liver cancer stem cells with disruption of TGF-β signaling

Abstract

Abstract Introduction: Hepatocellular Cancer (HCC) remains the 4th common cause of cancer deaths worldwide. Because it is often diagnosed at an advanced stage in a fragile cirrhotic patient, effective therapies whether they are chemotherapy or biologically based are disappointingly few, with a narrow therapeutic index. Recent studies support a key role for TGF-β signaling in suppressing these tumors. We have previously found that deletion of Smad3/4 adaptor β2SP results in a dramatic and spontaneous formation of liver (HCC) with exon 15 mutations in 11% of human HCC. β2SP+/− and β2SP+/−/Smad3+/− mice develop visceromegaly and multiple cancers (70% of mice), including HCC spontaneously presenting a strong model of the hereditary human cancer syndrome, Beckwith-Wiedemann (BWS) as well as the stages of human HCC. High levels of E3 ligases KEAP1 and PRAJA are observed in human HCCs and in tumors of these mutant mice. Our hypothesis is that disruption of the TGF-β tumor suppressor pathway (through β2SP, Smad3 and Smad4) leads to a proliferative potential in cells that then acquire secondary events such as activation of E3 ligases (KEAP 1, PRAJA and others), presenting these as potentially powerful new targets for HCC. Methods & Results: (1) β2SP+/− and β2SP+/−/Smad3+/− mice are ideal models for a human stem cell disorder (BWS) and HCC progression. (2) Inhibition of PRAJA in the developing Zebrafish embryo leads to high levels of apoptosis. (3) PRAJA is a key E3 Ligase that modulates β2SP and Smad3. (4) PRAJA levels markedly over- expressed in human HCC cell lines and tumor tissues and promotes HCC cell growth. (5) KEAP1 levels are raised in β2SP knockout tissues and human HCCs (6) Knocking-down KEAP1 inhibits HCC tumor cell growth. (7) Seven tumor suppressors are discovered to be potential targets of KEAP1 in HCC. (8) Triterpenoid derivates (Oleanolic Acid/RTA 402/RTA 405) inhibit KEAP1/Nrf2 pathway, as well as PRAJA and inhibit HCC tumor cell growth. Conclusions: Our studies demonstrate that E3 Ligases KEAP1 and PRAJA are dramatically raised in human and mouse hepatocellular cancers when TGF-β signaling is inactivated, inhibiting KEAP1 or PRAJA reduces HCC growth. Determining the synergistic therapeutic effects by targeting both KEAP1 and PRAJA will provide us important insights into development of new therapeutic targets for HCC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3490. doi:1538-7445.AM2012-3490