Abstract 4443: Targeting E3 ligase PJA1 via TGF-β pathway in hepatocellular carcinoma

Background: RING-finger E3 ligases are instrumental in the regulation of inflammatory cascades, apoptosis, and cancer. However, their roles are relatively unknown for specific pathways such as TGF-β/Smad signaling that when disrupted, drive hepatocellular carcinoma (HCC). We, therefore, analyzed the TCGA database for the TGF-β pathway associated 22 E3 ligases, and identified mRNA alterations in 55% of tumors, most prominently for UCHL5 (16.4%), PJA (12.7%), WWP2 (11.8%), SKP2 (9.1%), SMURF1 and SMURF2 (8.2 and 9.1%, respectively), ITCH (6.4%) and KEAP1 (6.4%). We previously uncovered increased PJA1 expression with loss of TGF-β substrates Smad3 and β2SP in TGF-β deficient mice (β2SP+/-/Smad3+/- mice), which spontaneously develop characteristics of a human stem cell syndrome, and HCC by 12-15 months. Here, we report PJA1 oncogenic function in HCC progression. HCC. Methods & Results: (1) Analyses of primary HCC datasets (91 cases) reveal increased PJA1 correlates with decreased levels of TGF-β/Smad3 and their regulated genes including Smad8 and TGFBR3. (2) To better understand the implications of PJA1-mediated deregulation of Smad3-promoted transcription in HCC, we compared transcriptomes of HepG2 cells silenced by PJA1 shRNA or treated with TGF-β: 1,584 genes including c-FOS were co-up-regulated and 1,279 genes including TERT were co-down-regulated. PJA1 expression in HCC was negatively associated with c-FOS and SERPINE1 expression. (3) PJA1 interacts with the Smad3 MH2 and Linker domains and multiple domains of β2SP to promote ubiquitin-mediated phosphor-Smad3 degradation in a TGF-β dependent manner, resulting in decreased TGF-β target gene activity. (4) We found that PJA1 expression in HCC is negatively associated with c-FOS and SERPINE1 expression. (5) Overexpression of a RING-domain-deleted PJA1 mutant reduced the proliferation rate in HCC cells and PJA1 knockdown significantly decreased anchorage-independent growth and tumorigenicity of HCC cells. (6) Our results also demonstrate that PJA1 promotes liver cancer stem cell formation in Smad3+/- mice. Conclusions: This study demonstrates that loss of expression of β2SP and Smad3 through PJA1 could play an important role in HCC progression and that PJA1 is a potential novel therapeutic target for this lethal disease. Citation Format: Kazufumi Ohshiro, Jian Chen, Wilma Jogunoori, Chu-Xia Deng, Bibhuti Mishra, Shulin Li, Lopa Mishra. Targeting E3 ligase PJA1 via TGF-β pathway in hepatocellular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4443.