Planning your every move: the role of ß-actin and its post-transcriptional regulation in cell motility.

Cell motility is a tightly regulated process that involves the polymerization of actin subunits. The formation of actin filaments is controlled through a variety of protein factors that accelerate or perturb the polymerization process. As is the case for most biological events, cell movement is also controlled at the level of gene expression. Growing research explains how the ß-actin isoform of actin is particularly regulated through post-transcriptional events. This includes the discovery of multiple sites in the 3' untranslated region of ß-actin mRNA to which RNA-binding proteins can associate. The control such proteins have on ß-actin expression, and as a result, cell migration, continues to develop, and presents a thorough process that involves guiding an mRNA out of the nucleus, to a specific cytosolic destination, and then controlling the translation and decay of this message. In this review we will provide an overview on the recent progress regarding the mechanisms by which actin polymerization modulates cell movement and invasion and we will discuss the importance of post-transcriptional regulatory events in ß-actin mediated effects on these processes.

Turnover of AU-rich-containing mRNAs during stress: a matter of survival.

Cells undergo various adaptive measures in response to stress. Among these are specific changes in the posttranscriptional regulation of various genes. In particular, the turnover of mRNA is modified to either increase or decrease the abundance of certain target messages. Some of the best-studied mRNAs that are affected by stress are those that contain adenine/uridine-rich elements (AREs) in their 3'-untranslated regions. ARE-containing mRNAs are involved in many important cellular processes and are normally labile, but in response to stress they are differentially regulated through the concerted efforts of ARE-binding proteins (AUBPs) such as HuR, AUF1, tristetraprolin, BRF1, and KSRP, along with microRNA-mediated effects. Additionally, the fate of ARE-containing mRNAs is modified by inducing their localization to stress granules or mRNA processing bodies. Coordination of these various mechanisms controls the turnover of ARE-containing mRNAs, and thereby enables proper responses to cellular stress. In this review, we discuss how AUBPs regulate their target mRNAs in response to stress, along with the involvement of cytoplasmic granules in this process.

Transportin 2 regulates apoptosis through the RNA-binding protein HuR.

In response to severe stress, apoptotic cell death is engaged. Apoptosis is a well orchestrated process that involves the activation and implication of many factors. In this study, we identified a role for the nuclear trafficking factor TRN2 (transportin 2) in cell death. TRN2 is normally responsible for the nuclear import of the RNA-binding protein HuR. During apoptosis, however, HuR accumulates in the cytoplasm. This is due to the caspase-mediated cleavage of the cytoplasmic fraction of HuR. One of the cleavage fragments generated by this processing of HuR interacts with TRN2 and thus blocks the re-import of HuR into the nucleus. This concentrates HuR in the cytoplasm, advancing apoptosis. Therefore, increasing or decreasing the levels of TRN2 has an inverse consequential effect on cell death, demonstrating for the first time the role of a nucleocytoplasmic transport factor in apoptosis.

HuR and myogenesis: being in the right place at the right time.

The process of muscle cell differentiation into myotubes, termed myogenesis, depends on a complex coordination of myogenic factors, many of which are regulated post-transcriptionally. HuR, an mRNA-binding protein, is responsible for regulating the expression of several such myogenic factors by stabilizing their mRNAs. The critical role for HuR in myogenesis also involves the nucleocytoplasmic shuttling ability of this protein. Indeed, in order to perform its stabilizing functions, HuR must accumulate in the cytoplasm. This requires its dissociation from the import factor Transportin 2 (TRN2) which is actually caused by the cleavage of a portion of cytoplasmic HuR. In this review, we describe the roles of HuR during myogenesis, and the mechanisms regulating its cytoplasmic accumulation. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Protein kinase RNA/FADD/caspase-8 pathway mediates the proapoptotic activity of the RNA-binding protein human antigen R (HuR).

The RNA-binding protein human antigen R (HuR) has been implicated in apoptosis in multiple ways. Several studies have shown that in response to a variety of stresses HuR promotes the expression of proapoptotic mRNAs, whereas others reported its regulatory effect on antiapoptotic messages. We recently showed that in response to severe stress, HuR is cleaved to generate two cleavage products (CPs), HuR-CP1 (24 kDa) and HuR-CP2 (8 kDa), by which it promotes apoptotic cell death. Here, we show that this cleavage event is dependent on protein kinase RNA (PKR). Surprisingly, although in response to the apoptotic inducer staurosporine PKR itself is not phosphorylated, PKR triggers the cleavage of HuR via its downstream effector FADD that in turn activates the caspase-8/caspase-3 pathway. This effect, however, does not require the phosphorylation of the eukaryotic translation initiation factor 2alpha. Additionally, we observed that these HuR-CPs are sufficient to trigger cell death in the absence of activation of the PKR pathway. Therefore, our results support a model whereby in response to lethal stress, PKR, without being phosphorylated, activates the FADD/caspase-8/caspase-3 pathway to trigger HuR cleavage, and the HuR-CPs are then capable of promoting apoptosis.

Decoding ARE-mediated decay: is microRNA part of the equation?

Messenger ribonucleic acids (mRNAs) containing adenine/uridine-rich elements (AREs) in their 3' untranslated region are particularly labile, allowing for the regulation of expression for growth factors, oncoproteins, and cytokines. The regulators, effectors, and location of ARE-mediated decay (AMD) have been investigated by many groups in recent years, and several links have been found between AMD and microRNA-mediated decay. We highlight these similarities, along with recent advances in the field of AMD, and also mention how there is still much left unknown surrounding this specialized mode of mRNA decay.

Caspase-mediated cleavage of HuR in the cytoplasm contributes to pp32/PHAP-I regulation of apoptosis.

The RNA-binding protein HuR affects cell fate by regulating the stability and/or the translation of messenger RNAs that encode cell stress response proteins. In this study, we delineate a novel regulatory mechanism by which HuR contributes to stress-induced cell death. Upon lethal stress, HuR translocates into the cytoplasm by a mechanism involving its association with the apoptosome activator pp32/PHAP-I. Depleting the expression of pp32/PHAP-I by RNA interference reduces both HuR cytoplasmic accumulation and the efficiency of caspase activation. In the cytoplasm, HuR undergoes caspase-mediated cleavage at aspartate 226. This cleavage activity is significantly reduced in the absence of pp32/PHAP-I. Substituting aspartate 226 with an alanine creates a noncleavable isoform of HuR that, when overexpressed, maintains its association with pp32/PHAP-I and delays the apoptotic response. Thus, we propose a model in which HuR association with pp32/PHAP-I and its caspase-mediated cleavage constitutes a regulatory step that contributes to an amplified apoptotic response.

The RNA-binding protein HuR promotes cell migration and cell invasion by stabilizing the beta-actin mRNA in a U-rich-element-dependent manner.

A high expression level of the beta-actin protein is required for important biological mechanisms, such as maintaining cell shape, growth, and motility. Although the elevated cellular level of the beta-actin protein is directly linked to the long half-life of its mRNA, the molecular mechanisms responsible for this effect are unknown. Here we show that the RNA-binding protein HuR stabilizes the beta-actin mRNA by associating with a uridine-rich element within its 3' untranslated region. Using RNA interference to knock down the expression of HuR in HeLa cells, we demonstrate that HuR plays an important role in the stabilization but not in the nuclear/cytoplasmic distribution of the beta-actin mRNA. HuR depletion in HeLa cells alters key beta-actin-based cytoskeleton functions, such as cell adhesion, migration, and invasion, and these defects correlate with a loss of the actin stress fiber network. Together our data establish that the posttranscriptional event involving HuR-mediated beta-actin mRNA stabilization could be a part of the regulatory mechanisms responsible for maintaining cell integrity, which is a prerequisite for avoiding transformation and tumor formation.