Adenylate-uridylate-rich elements (AU-rich elements; AREs) are found in the 3' untranslated region (UTR) of many messenger RNAs (mRNAs) that code for proto-oncogenes, nuclear transcription factors, and cytokines. AREs are one of the most common determinants of RNA stability in mammalian cells. [1] The function of AREs was originally discovered by Shaw and Kamen in 1986. [2]
AREs are defined as a region with frequent adenine and uridine bases in a mRNA. They usually target the mRNA for rapid degradation. [3] [2] ARE-directed mRNA degradation is influenced by many exogenous factors, including phorbol esters, calcium ionophores, cytokines, and transcription inhibitors. These observations suggest that AREs play a critical role in the regulation of gene transcription during cell growth and differentiation, and the immune response. [1] As evidence of its critical role, deletion of the AREs from the 3'UTR in either the TNF gene or GM-CSF gene in mice leads to over expression of each respective gene product, causing dramatic disease phenotypes. [4] [5]
AREs have been divided into three classes with different sequences. The best characterised adenylate uridylate (AU)-rich Elements have a core sequence of AUUUA within U-rich sequences (for example WWWU(AUUUA)UUUW where W is A or U). This lies within a 50–150 base sequence, repeats of the core AUUUA element are often required for function.
A number of different proteins (e.g. HuA, HuB, HuC, HuD, HuR) bind to these elements and stabilise the mRNA while others (AUF1, TTP, BRF1, TIA-1, TIAR, and KSRP) destabilise the mRNA, miRNAs may also bind to some of them. [6] For example, the human microRNA, miR16, contains an UAAAUAUU sequence that is complementary to the ARE sequence and appears to be required for ARE-mRNA turnover. [7] HuD (also called ELAVL4) binds to AREs and increases the half-life of ARE-bearing mRNAs in neurons during brain development and plasticity. [8]
AREsite—a database for ARE containing genes—has recently been developed with the aim to provide detailed bioinformatic characterization of AU-rich elements. [9]
No real ARE consensus sequence has been determined yet, and these categories are based neither on the same biological functions, nor on the homologous proteins. [3]
AREs are recognized by RNA binding proteins such as tristetraprolin (TTP), AUF1, and Hu Antigen R (HuR). [10] Although the exact mechanism is not very well understood, recent publications have attempted to propose the action of some of these proteins. AUF1, also known as hnRNP D, binds AREs through RNA recognition motifs (RRMs). AUF1 is also known to interact with the translation initiation factor eIF4G and with poly(A)-binding protein, indicating that AUF1 senses the translational status of mRNA and decays accordingly through the excision of the poly(A) tail. [10]
TTP's (ZFP36's) expression is rapidly induced by insulin. [11] Immunoprecipitation experiments have shown that TTP co-precipitates with an exosome, suggesting that it helps recruit exosomes to the mRNA containing AREs. [12] Alternatively, HuR proteins have a stabilizing effect—their binding to AREs increases the half-life of mRNAs. Similar to other RNA-binding proteins, this class of proteins contain three RRMs, two of which are specific to ARE elements. [13] A likely mechanism for HuR action relies on the idea that these proteins compete with other proteins that normally have a destabilizing effect on mRNAs. [14] HuRs are involved in genotoxic response—they accumulate in the cytoplasm in response to UV exposure and stabilize mRNAs that encode proteins involved in DNA repair.
Problems with mRNA stability have been identified in viral genomes, cancer cells, and various diseases. Research shows that many of these problems arise because of faulty ARE function. Deficiency of the ZFP36 family show that ZFP36 ARE binding proteins are critical regulators of T cell homeostasis and autoimmunity. [15] Some of these problems have been listed below: [10]
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, and ultimately affect a phenotype. These products are often proteins, but in non-protein-coding genes such as transfer RNA (tRNA) and small nuclear RNA (snRNA), the product is a functional non-coding RNA. The process of gene expression is used by all known life—eukaryotes, prokaryotes, and utilized by viruses—to generate the macromolecular machinery for life.
In molecular genetics, the three prime untranslated region (3′-UTR) is the section of messenger RNA (mRNA) that immediately follows the translation termination codon. The 3′-UTR often contains regulatory regions that post-transcriptionally influence gene expression.
Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products. Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources. Virtually any step of gene expression can be modulated, from transcriptional initiation, to RNA processing, and to the post-translational modification of a protein. Often, one gene regulator controls another, and so on, in a gene regulatory network.
Polyadenylation is the addition of a poly(A) tail to an RNA transcript, typically a messenger RNA (mRNA). The poly(A) tail consists of multiple adenosine monophosphates; in other words, it is a stretch of RNA that has only adenine bases. In eukaryotes, polyadenylation is part of the process that produces mature mRNA for translation. In many bacteria, the poly(A) tail promotes degradation of the mRNA. It, therefore, forms part of the larger process of gene expression.
The TATA-binding protein (TBP) is a general transcription factor that binds to a DNA sequence called the TATA box. This DNA sequence is found about 30 base pairs upstream of the transcription start site in some eukaryotic gene promoters.
Cleavage and polyadenylation specificity factor (CPSF) is involved in the cleavage of the 3' signaling region from a newly synthesized pre-messenger RNA (pre-mRNA) molecule in the process of gene transcription. In eukaryotes, messenger RNA precursors (pre-mRNA) are transcribed in the nucleus from DNA by the enzyme, RNA polymerase II. The pre-mRNA must undergo post-transcriptional modifications, forming mature RNA (mRNA), before they can be transported into the cytoplasm for translation into proteins. The post-transcriptional modifications are: the addition of a 5' m7G cap, splicing of intronic sequences, and 3' cleavage and polyadenylation.
The miR-29 microRNA precursor, or pre-miRNA, is a small RNA molecule in the shape of a stem-loop or hairpin. Each arm of the hairpin can be processed into one member of a closely related family of short non-coding RNAs that are involved in regulating gene expression. The processed, or "mature" products of the precursor molecule are known as microRNA (miRNA), and have been predicted or confirmed in a wide range of species.
Poly(rC)-binding protein 2 is a protein that in humans is encoded by the PCBP2 gene.
ELAV-like protein 1 or HuR is a protein that in humans is encoded by the ELAVL1 gene.
Tristetraprolin (TTP), also known as zinc finger protein 36 homolog (ZFP36), is a protein that in humans, mice and rats is encoded by the ZFP36 gene. It is a member of the TIS11 family, along with butyrate response factors 1 and 2.
Heterogeneous nuclear ribonucleoprotein D0 (HNRNPD) also known as AU-rich element RNA-binding protein 1 (AUF1) is a protein that in humans is encoded by the HNRNPD gene. Alternative splicing of this gene results in four transcript variants.
Scaffold attachment factor B, also known as SAFB, is a gene with homologs that have been studied in humans and mice.
Nucleolysin TIAR is a protein that in humans is encoded by the TIAL1 gene.
CUG triplet repeat, RNA binding protein 1, also known as CUGBP1, is a protein which in humans is encoded by the CUGBP1 gene.
HuD otherwise known as ELAV-like protein 4 is a protein that in humans is encoded by the ELAVL4 gene.
Post-transcriptional regulation is the control of gene expression at the RNA level. It occurs once the RNA polymerase has been attached to the gene's promoter and is synthesizing the nucleotide sequence. Therefore, as the name indicates, it occurs between the transcription phase and the translation phase of gene expression. These controls are critical for the regulation of many genes across human tissues. It also plays a big role in cell physiology, being implicated in pathologies such as cancer and neurodegenerative diseases.
HSURs are viral small regulatory RNAs. They are found in Herpesvirus saimiri which is responsible for aggressive T-cell leukemias in primates. They are nuclear RNAs which bind host proteins to form small nuclear ribonucleoproteins (snRNPs). The RNAs are 114–143 nucleotides in length and the HSUR family has been subdivided into HSURs numbered 1 to 7. The function of HSURs has not yet been identified; they do not affect transcription so are thought to act post-transcriptionally, potentially influencing the stability of host mRNAs.
In genetics, gene isoforms are mRNAs that are produced from the same locus but are different in their transcription start sites (TSSs), protein coding DNA sequences (CDSs) and/or untranslated regions (UTRs), potentially altering gene function.
Brain cytoplasmic 200 long-noncoding RNA is a 200 nucleotide RNA transcript found predominantly in the brain with a primary function of regulating translation by inhibiting its initiation. As a long non-coding RNA, it belongs to a family of RNA transcripts that are not translated into protein (ncRNAs). Of these ncRNAs, lncRNAs are transcripts of 200 nucleotides or longer and are almost three times more prevalent than protein-coding genes. Nevertheless, only a few of the almost 60,000 lncRNAs have been characterized, and little is known about their diverse functions. BC200 is one lncRNA that has given insight into their specific role in translation regulation, and implications in various forms of cancer as well as Alzheimer's disease.