Abstract 2784: Targeting TGF-β regulated E3 ligases: a novel therapeutic approach for primary liver cancer

Background: Hepatocellular cancer (HCC) is the 3rd leading cause of cancer death in the world, that is difficult to treat due to late diagnosis and few viable targeted therapeutics. A large number of studies have demonstrated that the TGF-β pathway plays a fundamental role in the biology of the GI tract, and as such, several components of the pathway are commonly targeted in various GI cancers. TGF-β signaling is regulated by the ubiquitin-proteasome pathway, in which E3 ubiquitin ligases recognize and target proteins for degradation by the proteasome. Numerous E3 ubiquitin ligases have been identified as negative regulators of different components of the TGF-β pathway. More recently, Praja and Keap1 have been identified as E3 ligases that ubiquitinates β2-spectrin (β2SP) in a TGF-β-dependent manner. Accordingly, Praja E3 ligase activity regulates TGF-β signaling by controlling β2SP abundance through ubiquitin-mediated degradation. Hypothesis: Specific E3 ligases, which disrupt TGF-β/β2SP/Smad3 tumor suppressor pathway lead to uncontrolled activation of chromatin and tumor formation. Therefore, small molecule inhibitors that specifically target these E3 ligases could present a novel therapeutic approach for liver cancer. Materials & Methods: Broad genome datasets including 110 cases of HCC patients were retrieved from The Cancer Genome Atlas (TCGA) and analyzed. 23 HCC and normal liver tissue specimens were immunostained with anti-Praja1 antibody. Whole mount in situ hybridization histochemistry (ISHH) assay was used to determine knock down Praja1in zebrafish embryos. A small chemical compound library was screened for E3 ligase inhibitors targeting Praja1. Results: (1) Broad genome analysis reveals that E3 ubiquitin ligases regulating the TGF-β pathway are altered in GI cancers mainly by increase in mRNA and that they negatively correlate with patient survival. (2) Praja1 expression is dramatically raised in human HCCs with loss of TGF-β signaling. (3) Praja1 inhibits TGF-β/Smad3 tumor suppressor function and increases c-Myc activities. (4) Inhibition of Praja1 leads to apoptosis and suppresses its oncogenic activities. (5) Small chemical inhibitors suppress Praja1 activities and restores TGF-β tumor suppressor function in HCC cells. (6) Small chemical inhibitors induce apoptosis and inhibit HCC cell growth and tumorigenesis. Conclusions: E3 ubiquitin ligases that modulate the TGF-β pathway are frequently increased in HCC. Praja1 presents a therapeutic tar