Abstract 4447: Targeting SET oncoprotein reactivates the tumor-suppressor PP2A and shows synergy with sorafenib in hepatocellular carcinoma
Authors: Hung Man-Hsin , Shiau Chung-Wai , Shin Chih-Ting , Yu Hui-Chuan , Tai Wei-Tien , Liu Chun-Yu , Wang Cheng-Yi , Chen Kuen-Feng 
Published: August 2015

Background:

Hepatocellular carcinoma (HCC) is the fifth major cancer in the world. Sorafenib is the first and only approved targeted therapy for patients with advanced HCC that brought only a 3-month additive survival benefit to patient. Thus, there is an unmet need for developing effective treatment against HCC. Protein phosphatase 2A (PP2A) is an important tumor suppressor whose functions are suppressed by the presence of its cancerous inhibitor, such as SET/I2PP2A and cancerous inhibitor of PP2A (CIP2A). Previously, we’ve shown that enhancing PP2A activity by inhibiting CIP2A promotes apoptosis of HCC (Oncogene 2010, MCT 2011). Here, we report the anti-HCC potential of a new approach to reactivate PP2A by interrupting the binding between SET and PP2A.

Method:

Using the quinzoline backbone, we designed and developed a novel PP2A activator, PA. HCC cell lines were treated with PA alone or in combination with sorafenib, and apoptosis, signal transduction and phosphatse activity were analyzed. Combination index (CI) was used to evaluate the synergy of combining PA and sorafenib. Small interference RNA was applied to silence gene; co-immunoprecipitation (co-IP) and proximity ligation assay (PLA) were used to determine the association of SET and PP2A within tumor cells. In vivo anti-HCC effects of PA and combination therapy of PA and sorafenib were tested in xenografted nude mice.

Results:

PA induced apoptosis, in association with reactivation of PP2A and downregulation of p-AKT, in a dose-dependent manner in HCC cell lines (PLC5, Hep3B, SK-Hep1, Huh7). Overexpression of Akt and knockdown of PP2A abolished the anti-HCC effects of PA. Using co-IP and PLA, we found that PA interrupts the binding of SET and PP2A within HCC cells in a dose-dependent and time-dependent manner. These data indicated that inhibition of SET-associated PP2A inactivation mediated the downregulation of p-Akt and the cytotoxic effects of PA against HCC. In addition, adding PA to sorafenib significantly induced more potent cell death than sorafenib alone in all HCC cell lines tested. CI suggested good synergy between PA and sorafenib under the combination ratio of 1:5. Importantly, the anti-tumor effects of PA and PA combining with sorafenib were validated in PLC5 xenografted tumors.

Conclusion:

In the current study, we showed that interfering the binding of SET to PP2A induce potent cellular death through PP2A-dependent p-Akt downregulation. This novel approach demonstrates great potency of PP2A enhancer and its synergy with sorafenib for the treatment of HCC.

Citation Format: Man-Hsin Hung, Chung-Wai Shiau, Chih-Ting Shin, Hui-Chuan Yu, Wei-Tien Tai, Chun-Yu Liu, Cheng-Yi Wang, Kuen-Feng Chen. Targeting SET oncoprotein reactivates the tumor-suppressor PP2A and shows synergy with sorafenib in hepatocellular carcinoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4447. doi:10.1158/1538-7445.AM2015-4447