AEG-1 Regulates Retinoid X Receptor and Inhibits Retinoid Signaling

Abstract

Retinoid X Receptor (RXR) regulates key cellular responses such as cell growth and development, and this regulation is frequently perturbed in various malignancies, including Hepatocellular Carcinoma (HCC). However, the molecule(s) that physically govern this deregulation are mostly unknown. Here, we identified RXR as an interacting partner of Astrocyte Elevated Gene-1 (AEG-1)/Metadherin (MTDH), an oncogene upregulated in all cancers. Upon interaction, AEG-1 profoundly inhibited RXR/Retinoic Acid Receptor (RAR)-mediated transcriptional activation. Consequently, AEG-1 markedly protected HCC and acute myeloid leukemia (AML) cells from retinoid- and rexinoid-induced cell death. In non-tumorigenic cells and primary hepatocytes, AEG-1/RXR co-localizes in the nucleus where AEG-1 interferes with recruitment of transcriptional co-activators to RXR preventing transcription of target genes. In tumor cells and AEG-1 transgenic hepatocytes, overexpressed AEG-1 entraps RXR in cytoplasm, precluding its nuclear translocation. Additionally, ERK, activated by AEG-1, phosphorylates RXR which leads to its functional inactivation and attenuation of ligand-dependent transactivation. In nude mice models, combination of all-trans retinoic acid (ATRA) and AEG-1 knockdown synergistically inhibited growth of human HCC xenografts. The present study establishes AEG-1 as a novel homeostatic regulator of RXR and RXR/RAR that might contribute to hepatocarcinogenesis. Targeting AEG-1 could sensitize HCC and AML patients to retinoid- and rexinoid-based therapeutics.

Astrocyte Elevated Gene-1 (AEG-1) Regulates Lipid Homeostasis

“…Co-activators, such as SRC-1, interact with nuclear receptors via an LXXLL motif (28). We documented that AEG-1 contains an LXXLL motif, through which it interacts with RXR that prevents co-activator recruitment and inhibits RXR function even in the presence of ligand (25). We now document augmented LXR and PPAR␣ activity in enterocytes of AEG-1KO mice that hinders cholesterol and fatty acid absorption that may be one mechanism that leads to a lean phenotype.…”

“…Generation of AEG-1KO Mouse-The AEG-1KO mouse was generated in a C57BL/6:129/Sv background (25,29). We have back-crossed the line to C57BL/6 for 10 generations and obtained similar results, which are described in this paper, for both the WT and AEG-1KO mice on the C57BL/6 background as on the C57BL/6:129/Sv background.…”

“…24 h later, the animals were euthanized, and livers were immediately removed. A ChIP assay was performed using a SimpleChIP Plus kit (Cell Signaling Technology, Danvers, MA) according to the manufacturer's instructions (25). The antibodies used were rabbit anti-LXR␣ (Santa Cruz Biotechnology, Inc.), anti-acetylated histone H3 (AHH3), and anti-SRC-1 (Cell Signaling).…”

“…Longer-We generated an AEG-1KO mouse in which the promoter region, exon 1, and part of intron 1 of the AEG-1 gene was deleted using a Cre-loxP system (25,29). AEG-1KO mice were viable and fertile, although the litter size was smaller than expected.…”

“…We previously documented that AEG-1 interacts with retinoid X receptor (RXR) (25), a master regulator of cell growth metabolism and development mediating the action of hormones, vitamins, and lipids (26,27). RXR heterodimerizes with one-third of the 48 human nuclear receptor superfamily members, including LXR and PPAR␣ (27).…”