Abstract B34: PI3K/p110β modulates tumor suppressor gene NR0B2 expression in human liver cancer cells
Authors: Tu Yanjie , Li Jean , Zhang Lisa 
Published: December 2020

Background: NR0B2 (nuclear receptor subfamily 0 group B member 2), also called SHP (small heterodimer partner), is an orphan nuclear receptor with a unique feature of lacking a conventional zinc-finger DNA binding domain. It acts as a transcriptional repressor by binding to other nuclear receptors and regulates various metabolic pathways. NR0B2 gene expression was significantly reduced in many human malignancies, including liver cancers, and overexpression of NR0B2 protein suppresses liver cancer development, indicating it is a tumor suppressor. Previous reports showed that NR0B2 gene expression is upregulated by several transcription factors, including FXR; however, it is not clear if and how cellular signaling pathways are involved in transcription factors-regulated NR0B2 gene expression in human cancer cells. Phosphatidylinositol 3-kinases (PI3Ks) are a group of major cellular signaling molecules that regulate multiple cellular signal pathways and their genetic abnormalities are found in most of human cancers. In liver cancers, animal study with genetic ablation of PTEN, the negative PI3K regulator, resulted in 60% of tumor development in the liver, suggesting an important role of PI3K-PTEN pathway in liver cancers.

Methods: To investigate if PI3K pathway is involved in the regulation of NR0B2 expression in human liver cancer HepG2 cells, multiple PI3K inhibitors were used to suppress PI3K activity. NR0B2 gene expression was assessed using real-time RT-PCR assay, Western blot and luciferase reporter driven by NR0B2 gene promoter (2200-LUC).

Results: At the mRNA level, NR0B2 gene expression is significantly elevated after being treated by PI3K inhibitor LY294002, along with JNK inhibitor SP600125. In contrast, MAPK/ERK inhibitors PD98059 and U0126 drastically suppressed NR0B2 gene expression. With the NR0B2 promoter-driven luciferase reporter (2200-LUC), luciferase assay results revealed that MAPK/ERK kinase inhibitors U0126 and PD184161 abolished the basal and FXR agonist GW4064 -stimulated luciferase activities, in agreement with the results at the mRNA levels. Interestingly, PI3K inhibitor BKM120/LY294002 and PI3K/p110β inhibitor BL140 drastically enhanced the basal luciferase activity of 2200-LUC reporter but had no effect on GW4064-stimulated 2200-LUC activities. We also determined that GW4064-stimualted NR0B2 protein expression was abolished by U0126 and PD184161 but not by PI3K inhibitors. In contrast, PI3K inhibitors increased NR0B2 protein levels at a time- and concentration-dependent manner.

Conclusion: These data suggest that NR0B2 gene expression is regulated by two major intracellular signal pathways, PI3K and MAPK. While MAPK/ERK signal pathway is involved in FXR-mediated NR0B2 expression, PI3K pathway suppresses NR0B2 expression that might be involved in liver cancer development or progression.

Citation Format: Yanjie Tu, Jean Li, Lisa Zhang. PI3K/p110β modulates tumor suppressor gene NR0B2 expression in human liver cancer cells [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr B34.