Review article
NEDDylation in liver cancer: The regulation of the RNA binding protein Hu antigen R

https://doi.org/10.1016/j.pan.2015.03.006Get rights and content

Abstract

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide and the third leading cause of cancer death. The current view of cancer progression and malignancy supports the notion that cancer cells must undergo through a post-translational modification (PTM) regulation and a metabolic switch or reprogramming in order to progress in an unfriendly environment. NEDDylation is a post-translational modification of the proteins involved in several processes such as cell growth, viability and development. A ground-breaking knowledge on a new critical aspect of HCC research has been to identify that NEDDylation plays a role in HCC by regulating the liver oncogenic driver Hu antigen R (HuR). HuR is a RNA-binding protein that stabilizes target mRNAs involved in cell dedifferentiation, proliferation, and survival, all well-established hallmarks of cancer. And importantly, HuR levels were found to be highly representative in liver and colon cancer.

These findings open a completely new area of research, exploring the impact that NEDDylation plays in liver diseases and paving the way for novel therapeutical approaches.

Section snippets

The RNA-binding protein HuR: regulation, functions and implications in liver cancer

The mammalian Hu/elav family of RNA-binding proteins (RBPs) includes the neuronal members HuB, HuC and HuD, and the more ubiquitous member HuR. Whereas the neuronal Hu has been related with neuronal development, neuronal plasticity and memory [1], [2], HuR (also known as ELAVL1) was first described to stabilize ARE-containing mRNAs [3]. After this, it has been also shown to modulate the translation, both enhancing and inhibiting it [4], [5].

HuR protein contains three RNA recognition motifs

Stabilization of target mRNAs

Some of the HuR-stabilized target mRNAs include p21, c-fos, VEGF, the MAPK phosphatase (MKP)-1, iNOS, granulocyte/macrophage colony-stimulating factor (GMCSF), sirtuin 1 (SIRT1), TNFα, B-cell lymphoma-2 (Bcl2), myeloid cell leukemia-1 (Mcl1), cyclooxygenase-2 (COX-2), γ-glutamylcysteine synthetase heavy subunit (γ-GCSh), urokinase-type plasminogen activator (uPA) and its receptor (uPAR), p53, interleukin (IL)-3, and cyclins A2, B1, E1, and D1 [11], [12].

The exact mechanisms by which HuR is able

Regulation of HuR function

According with the involvement of HuR in many important biological processes, its function is tightly regulated at many levels: abundance and integrity of the protein, subcellular localization and post-translational modification. The levels of HuR protein are regulated in many ways. Among them, we can find:

Regulation of HuR localization

Although HuR is predominantly nuclear, the best characterized HuR functions take place in the cytoplasm. Together with the HuR HNS domain, several transport machinery components are also necessary, including transportin 1, transportin 2 and CRM1 (chromosome region maintenance 1) [35], [36]. The phosphorylation of HuR by Cdk1, AMPK, PKC and p38 also influences HuR transport.

Together with the ubiquitination, HuR is also posttranslationally modified by several kinases and by methylation. These

HuR and cancer traits

Hu/elav family has been involved in most malignancies: breast, colon, stomach, kidney, pancreas, esophagus, prostate, skin, lung, etc. [44]. Indeed, numerous HuR-regulated mRNAs have been identified to directly contribute to the acquisition of the cancer traits, including enhanced ability to proliferate, enhanced cell survival, elevation of local angiogenesis, evasion of immune recognition, and invasion and metastasis (Fig. 2) [12].

Implication of HuR in specific cancer types

Taking into account the regulation of the cancer traits targeted by HuR, numerous studies have examined the levels of this RBP in individual cancer types.

HuR was found misregulated in breast, pancreatic, ovarian, oral, lung, gastric, pharyngeal and nervous system cancers.

HuR was identified as an important prognostic factor in breast, pancreatic and ovarian cancers. In colon cancer tissues, the expression and localization of HuR was associated with advanced tumor stage [11], [12].

The role of HuR

NEDDylation: protein stabilization and post-translational modification

The ubiquitin-like (UBL) molecule NEDD8 (neural precursor cell-expressed developmentally downregulated-8) is the protein involved in the NEDDylation, a post-translational modification of the proteins involved in several processes such as cell growth, viability and development. NEDD8 was identified in 1992 and the analysis of the sequence found a 60% amino acid identity with Ub [47], [48]. NEDD8 appears as mainly nuclear, and is highly conserved in most eukaryotes and expressed in most tissues,

NEDD8 conjugating cascade

Similar to Ub, NEDD8 attaches covalently to a lysine in the target protein. The NEDD8 conjugation cascade involves E1, E2, E3 and deNEDDylating enzymes. NEDD8 is first synthesized as a precursor processed at the Gly76 residue by NEDP1 (NEDD8 protease 1) and UCH-L3 (Ubiquitin carboxyl-terminal hydrolase L3) deNEDDylases. After this, the C-terminal glycine of NEDD8 is activated by the E1 NEDD8-activating enzyme (NAE), composed of APP-BP1 (APP binding protein 1) and Uba3 (ubiquitin-like modifier

NEDD8 substrates

Until recently, the only known substrates for the NEDDylation were the members of the cullin family of proteins [49]. Cullins interact with RING finger proteins to enable the recruitment of E2 enzymes [58]. These cullin-RING ligase complexes (CRLs) target numerous substrates for ubiquitination, which is increased by NEDDylation, and thus have an impact on cellular processes such as cell growth, development, signal transduction, transcriptional control, genomic integrity and tumor suppression

NEDD8 relation with cancer

As commented above, the NEDDylation of many substrates (cullins, p53, Mdm2, p73, BCA3, L11 ribosomal protein, EGFR) is related with tumorigenesis [51]. Many of the substrates of the CRL have been involved in cancer; the misregulation of the CRLs NEDDylation modifies the ubiquitination of their substrates, thus producing impairment in DNA replication, cell cycle progression, stress responses, etc.

Recently, it was generated an inhibitor of the NAE, MLN4924, that blocks substrate NEDDylation. This

NEDDylation in liver cancer

Taking in consideration the role of HuR in liver tumorigenesis and its tightly regulated levels, we provided new mechanisms involving the post-translational modification NEDD8 that explains the overexpression of HuR in HCC and colon cancer. There is a strong and positive correlation between the levels of HuR and Mdm2 in a cohort of human HCC and metastatic colon cancer to the liver. This correlation was corroborated by the findings that, according with the transformation status of the cells,

Financial support

This work is supported ETORTEK-2012 (to M.L.M.-C), Sanidad del Gobierno Vasco 2013 (to M.L.M.-C), FIS PI11/01588 (to M.L.M.-C), CIBERehd is funded by the Instituto de Salud Carlos III.

Author contribution

David Fernández-Ramos. Acquisition of data; analysis and interpretation of data. Critical revision of the manuscript.

María L Martínez-Chantar. Study concept and design; analysis and interpretation of data; study supervision; drafting of the manuscript; obtained funding.

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