Abstract 5437: Astrocyte Elevated Gene-1 (AEG-1) interacts with retinoid X receptor (RXR) and abrogates its function.
Authors: Srivastava Jyoti , Robertson Chadia L. , Gredler Rachel , Siddiq Ayesha , Rajasekaran Devaraja , Santhekadur Prasanna K. , Fisher Paul B. , Sarkar Devanand 
Published: August 2014

Abstract
The oncogene Astrocyte Elevated Gene-1 (AEG-1), also known as Metadherin (MTDH), is overexpressed in a diverse array of cancers, including hepatocellular carcinoma (HCC), and plays an important role in promoting tumor development and progression. Protein-protein interaction predominantly mediates the tumor promoting effects of AEG-1. We now describe screening of a human liver cDNA library by yeast two hybrid assay resulting in identification of Retinoid X Receptor (RXR) as an AEG-1-interacting protein. Site-directed mutagenesis assays identified an LXXLL motif in the N-terminus of AEG-1 mediating interaction with RXR. We employed human HCC cell lines stably overexpressing AEG-1 or with knockdown of AEG-1 and mouse hepatocytes isolated from a transgenic mouse with hepatocyte-specific expression of AEG-1 (Alb/AEG-1) and from AEG-1 knockout (AEG-1 KO) mouse for our studies. Luciferase reporter assays in these cells demonstrate that AEG-1 profoundly inhibits RXR-mediated transcriptional regulation of its heterodimer partner, such as Retinoic Acid Receptor (RAR), Thyroid Hormone Receptor (TR), Liver X Receptor (LXR) and Peroxisome Proliferator-Activated Receptor (PPAR), in the presence or absence of their corresponding ligands. In this study, we focus on retinoic acid (RA) and thyroid hormone (T3) function. AEG-1 markedly inhibits expression of RA- and T3-induced genes. Overexpressed AEG-1 in cancer cells and in transgenic hepatocytes accumulates in the cytoplasm and inhibits RXR function by trapping RXR in the cytoplasm precluding its nuclear translocation. Additionally, AEG-1 induces phosphorylation of RXR, via ERK and p38 MAPK pathways. Phosphorylated RXR has low transcriptional activity, functions as dominant negative inhibitor of RXR and might facilitate hepatocarcinogenesis by AEG-1. Indeed, chromatin immunoprecipitation (ChIP) assay using RA-target genes, RARB and HOXA1, shows decreased DNA binding of RXR, even upon ligand-treatment. Overexpression of AEG-1 significantly protects human HCC cells, mouse hepatocytes and human promyelocytic leukemia cells from the killing effects of retinoic acid and its synthetic analogs. By inhibiting the expression of T3-regulated gene type I deiodinase (DIO1), AEG-1 abrogates peripheral conversion of inactive T4 to active T3 which is reflected by significantly low T3 level in the circulation of Alb/AEG-1 mice compared to WT mice. Since AEG-1 is upregulated in a wide-variety of cancers it might play an important role in regulating a diverse array of RXR-dependent signaling pathways that are critical for regulating the oncogenic process. Synthetic retinoids are being evaluated as chemopreventive agents for HCC as well as a therapeutic modality for PML. AEG-1 expression level might determine the sensitivity of the patients to these retinoids. Additionally AEG-1 might play a key role in ‘low T3 syndrome’ associated with cancer.
Citation Format: Jyoti Srivastava, Chadia L. Robertson, Rachel Gredler, Ayesha Siddiq, Devaraja Rajasekaran, Prasanna K. Santhekadur, Paul B. Fisher, Devanand Sarkar. Astrocyte Elevated Gene-1 (AEG-1) interacts with retinoid X receptor (RXR) and abrogates its function. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5437. doi:10.1158/1538-7445.AM2013-5437