Modulation of mRNA turnover and translation reflect key paradigms of post-transcriptional gene regulation in physiology and pathology of higher eukaryotes. Besides mutations in the genome, tumor-related gene expression profiles frequently rely on alterations in post-transcriptional control mechanisms. Together with the non-coding RNAs, particularly the micro RNAs which either affect transcript stability or the level of the encoded protein, RNA binding proteins (RBPs) have a critical impact on various mRNA modalities (Keene and Tenenbaum, 2002; Keene, 2007). The ubiquitously expressed turnover and translation regulatory-RNA binding protein (TTR-RBP) human antigen R (HuR), a member of the embryonic lethal abnormal vision (ELAV) like protein family is one of the best characterized RBP in this scenario. In addition to mRNA stability and translation, HuR can affect additional RNA features namely splicing and intracellular transport of mRNA with the latter one being highly relevant for spatial control of protein synthesis (for review see: Tekotte and Davis, 2002; Martin and Ephrussi, 2009). Members of the Hu protein family, target many short-lived mRNAs preferentially those bearing destabilizing AU-rich elements (AREs) in their 3′or 5′untranslated regions (UTRs) which have also been summarized as post-transcriptional “Hu-mRNA regulons” (Ma et al., 1996; Myer et al., 1997; Pascale and Govoni, 2012). A large variety of these mRNAs encode proteins which implement hallmarks of cancer including proliferation, cell survival, immune evasion, angiogenesis, and metastasis (for review see: Mazan-Mamczarz et al., 2008; Abdelmohsen and Gorospe, 2010). Accordingly, an upregulation of HuR was observed in many tumors (López de Silanes et al., 2005; Abdelmohsen and Gorospe, 2010; Wang et al., 2013). It is for this reason that, HuR is considered a promising target of novel anticancer therapies. Nevertheless, keeping in mind that HuR is indispensable and necessary for normal life and development (Ghosh et al., 2009), approaches aiming at the total knockdown of HuR expression have to be considered with caution. Alternative strategies which would allow for only a temporal intervention with some specific HuR activies are therefore certainly much more desirable.

Similar to many other RBPs, HuR is a typical shuttling protein which translocates from the nucleus to the cytoplasm and backward (“HuR shuttling”). Pathologically, elevated cytoplasmic HuR levels in patients correlate with an increase in tumor size, higher invasion and poor outcome as convincingly demonstrated in ovarian (Erkinheimo et al., 2003), breast (Heinonen et al., 2005), and colorectal cancer, respectively (Denkert et al., 2006). Structurally, the bidirectional transport of HuR is related to the HuR nucleocytoplasmic shuttling sequence (HNS) residing within the hinge region of the protein. Transport of HuR across the nuclear envelope requires an interaction of the HNS with components of the nucleo-cytoplasmic transport machinery, including transportins 1 and 2 (Rebane et al., 2004), importin-1α and the export route via the chromosome region maintenance 1 (CRM1) nuclear pore receptor also known as exportin 1 (Siddiqui and Borden, 2012). Importantly, in most cell types the shuttling of HuR is not a constitutive process but transiently induced by activation of different signaling devices (Kim et al., 2008; Doller et al., 2010). Accordingly, redistribution of HuR is a direct target of specific post-translational modifications including methylation (Li et al., 2002), and protein kinase dependent phosphorylation (Abdelmohsen et al., 2007; Doller et al., 2007, 2008). Importantly, these protein kinases, e.g., checkpoint kinase 2 (Chk2) and enzymes of the protein kinase C family have also been implicated in the regulation of tumor-related processes (for a review see: Doller et al., 2008).

In addition to HuR’s nucleo-cytoplasmic transit, the specific cytoplasmic HuR localization, e.g., to the translation apparatus, the exosomes or the processing (P)-bodies for storage of mRNA is an important determinant for the destiny of the HuR-bound target mRNAs. Directed mRNA transport to a distinct cytoplasmic destination is furthermore highly relevant for spatial and temporal control of protein synthesis (Hovland et al., 1996; Tekotte and Davis, 2002; Martin and Ephrussi, 2009). Previous data demonstrated that the ubiquitously expressed HuR (Cheng et al., 2013; Doller et al., 2013) and its neuronal relative HuD (Fujiwara et al., 2011) can utilize either the actin- or microtubule-dependent cytoskeleton for transport of mRNA cargo. Interestingly, the nuclear export of HuR seems closely linked to its cytoplasmic trafficking since pharmacological disruption of either of both cytoskeletal transport systems prevents accumulation of HuR protein in the cytoplasm (Cheng et al., 2013; Doller et al., 2013). Targeting of both transport systems therefore represents a valuable approach to intervene with post-transcriptional ELAV/Hu mRNA regulons.

A growing list of chemically diverse compounds perturbing the dynamics of microtubules have proven clinical efficacy in the treatment of various human cancers. The main therapeutic effect of currently used microtubule inhibitors seems to arise from cell cycle arrest due to inhibition of the mitotic spindle typically followed by an induction of apoptosis due to mitotic catastrophe (Jordan et al., 1996; Huszar et al., 2009). Microtubule destabilizing drugs as the vinca alkaloids vinblastine and vincristine from the Madagascar periwinkle Catharanthus roseus or, colchicine, an alkaloid derived from the autumn crocus Colchicum autumnale prevent the polymerization of microtubules (Huszar et al., 2009). In contrast, microtubule stabilizers such as paclitaxel or the hemisynthetic taxol docetaxel prevent the depolymerization of tubulin (Jordan and Wilson, 1998). Despite of their antagonistic effects on tubulin polymerization, all of these compounds are potent inhibitors of mitosis (Jordan and Wilson, 1998). In addition to affecting mitosis, microtubules are important for cell motility and for transport of organelles, vesicles and are relevant for long-distance transport of protein and mRNA (for review see: Holt and Bullock, 2009; Blower, 2013). The microtubule-RNA contact needed for RNA transport is mediated by formation of a ternary complex between the RBP, a microtubule-associated protein (MAP) and the zipcode which in most cases resides in the 3′UTR of the mRNA (Antic and Keene, 1998). Importantly, transport of an mRNA does not exclusively occur via one distinct cytoskeletal route but can switch between actin-and microtubule-directed transport system as demonstrated for β-actin mRNA (Martin and Ephrussi, 2009). A prominent example for microtubule-dependent mRNA transport is HIF-1α. Searching for novel pharmacological inhibitors for HIF1α, the administration of the indoline sulfonamide MPT0B098 a novel small-molecule inhibitor of microtubule polymerization to lung adenocarcinoma A549 cells efficiently impaired HIF1α-triggered gene expression (Cheng et al., 2013). These sulfonamide-based compounds were synthesized in order to overcome various modes of resistance and to achieve an improved pharmacological profile when compared to clinically established microtubule inhibitors (Nien et al., 2010). Mechanistically, MPT0B098 binds directly to the colchicine-binding moiety of tubulin. The destabilization of HIF1α mRNA by MPT0B098 was shown to be mainly due to inhibition of nucleo-cytoplasmic HuR shuttling (Cheng et al., 2013). Since HIF1α itself targets a large variety of genes, MPT0B098 interferes with the post-transcriptional expression of many tumor-related genes involved in tumor growth, cell metabolism, cell survival, and other functions. Furthermore, MPT0B098 has antiangiogenic properties as it affects the growth and density of microvessels in tumor specimens (Cheng et al., 2013). The pharmacological benefit of MPT0B098 is furthermore highlighted by strong antiproliferative effects even on multidrug-resistant human cancer cells (Nien et al., 2010). Similar to other microtubule-targeting drugs, MPT0B098 can induce cell growth arrest by avoiding the transition of cells from G2 to the M phase (Cheng et al., 2013). In contrast to MPT0B098, the microtubule stabilizing taxene docetaxel was found to increase HuR-mediated stabilization of COX-2 mRNA in the mammary epithelial cell line 184B5/HER mainly through and activation of the p38 mitogen activated (MAP) kinase and PKC mediated signaling mainly due to the induction of stress (Subbaramaiah et al., 2000, 2003). Notably, the stimulatory effect on COX-2 mRNA stability by taxanes demonstrated in this cell line is mainly relying on an increased HuR binding to the 3′UTR of the COX-2 mRNA. Since HuR phosphorylation by p38 MAPK and PKC can activate HuR-dependent stabilization on target mRNA (Lafarga et al., 2009; Doller et al., 2010) the net effect on HuR mRNA regulons by taxanes may critically depend on the extent of drug-induced activation of these stress kinases in a given cell type. In another study, microtubule perturbation by taxol or vinblastine inhibit HIF1α expression through the polysomal release of HIF1α mRNA although the authors did not formally demonstrate a direct correlation of the disrupted HIF1α mRNA transport with HuR (Carbonaro et al., 2011). Instead, a functional role of the microtubule-dependent cytoskeleton in the control of protein translation by HuR was highlighted by a report from Antic and Keene (1998). These authors demonstrated a critical role of microtubule-associated ELAV proteins in targeting mRNP particles from RNPs to the translation apparatus. Mechanistically, the interaction between ELAV proteins and microtubules is thought to be indirect and mediated, e.g., by MAPs. In case of HuD, the light chain of MAP1B (LC1) is linking the RBP-bound mRNA to microtubules, a process which is critical for the long-distance transport of ARE-containing mRNAs in neurons (Fujiwara et al., 2011). Given the plethora of tumor related genes targeted by HuR, it is plausible that in addition to interfering with tumor cell mitosis, an inhibition of post-transcriptional HuR regulons may essentially contribute to the potent antitumorigenic activity of clinically established microtubule inhibitors. In addition to the clinically established microtubule targeting drugs, microtubule-associated motor proteins, namely the kinesins have gained increased attention as novel targets for cytoskeleton-interfering therapies (Huszar et al., 2009). To date, more than 14 different kinesin protein families with an overall of 600 kinesins have been identified. Many of them are not only involved in mitosis regulation, but are highly relevant for microtubule dependent intracellular transport as well. Currently, there is pharmacological effort to test synthetic kinesin inhibitors in phase I and phase II trials in order to overcome side effects and drug resistance often observed with conventional microtubule inhibitors. It would be interesting to investigate some of the most promising kinesin inhibitors for their potential suppressive effect on HuR shuttling and related RNA trafficking.

The actin-based cytoskeleton is involved in different cellular processes including cell shapening, cell motility, cell adherence, endocytosis and exocytosis, and intracellular transport of cargo. Therefore, it represents another attractive target for cancer chemotherapy (Jordan and Wilson, 1998). However, due to the lack of specificity for different types of actin, the use of most actin-targeting inhibitors tested so far have proven detrimental “off target” effects (Fenteany and Zhu, 2003). Nevertheless, compounds which preferentially act on cytoskeletal actin while having only week effects on cardiac, muscle, or smooth muscle actin represent attractive candidates for novel cancer therapy.

The list of genes regulated by the multifunctional RBP HuR is steadily growing and accordingly, its critical impact in carcinogenesis was validated for almost every type of human tumor. Therefore, HuR is considered a promising therapeutic target for treatment of human malignant diseases. Since the constitutively increased transport to the cytoplasm constitutes a major principle of post-transcriptional mRNA regulons by HuR, interfering with its subcellular redistribution reflects a valuable approach for targeting tumorrelevant HuR-mRNA regulons. Although cytoskeleton-dependent transport of Hu proteins was documented only by a couple of publications, our knowledge from directed mRNA transport implicates a stimulus- and cargo-specific transport of both Hu proteins along different cytoskeletal tracking systems critical for most of their cytoplasmic functions. In addition, current pharmacological antitumor concepts aiming at the inhibition of cytoskeletal dynamics should be reconsidered since some of the beneficial antitumor activities of these drugs may derive from inhibition of HuR. Besides the well-established microtubule inhibitors, the actomyosin dependent cytoskeleton represents another valuable target for anti-HuR strategies. The question whether HuR trafficking in tumor cells may generally depend on both types of transport systems, or is rather cell-type specific is an intriguing issue which warrants future investigation. Together with the inexhaustible source of natural actin inhibitors a large number of synthetic inhibitors of myosin and kinesin motors is principally available for targeting post-transcriptional mRNA regulons by HuR and related RBPs.

WE wrote the review, but all coauthors made substantial and intellectual contribution to the work as they had a major impact on the overall concept and furthermore approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.