• 2018-07
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  • 2020-02
  • Several RNA BPs like the ELAV protein family member HuR


    Several RNA-BPs, like the ELAV protein family member HuR, tristetraprolin (TTP) or the KH-type splicing regulatory protein (KSRP) have been shown to interact with these AREs and thereby lead to stabilization or destabilization of the mRNA [5]. Control of nucleocytoplasmic mRNA export is also very important in the post-transcriptional gene regulation. RNA transport from the nucleus to the Trimidox occurs via nuclear pore complexes consisting of about 30 proteins termed nucleoporins [6]. The key factor mediating poly(A)+ mRNA export is a heterodimer of TAP and its binding partner Nxt (or p15). Several cofactors, like Aly and UAP56 are needed to recruit the mRNA to the nuclear pore. However, in the last years, evidence emerged that some mRNAs, especially those containing AREs in their 3′-UTRs, are exported from the nucleus by a different mechanism involving the nuclear export receptor CRM1 [7], [8]. CRM1 (or exportin-1) belongs to the family of karyopherin receptors and was first identified as an important factor mediating nuclear export of the HIV-1 mRNA [9]. HIV and other retroviruses encode for adapter proteins (Rev proteins), that recruit CRM1 to incompletely spliced retroviral mRNA. Amongst others, nuclear export of cellular mRNAs by CRM1 has been shown to be part of the post-transcriptional regulation of the IFN-α-, COX-2-, the murine iNOS and the HLA-A gene [7], [10], [11], [12]. Furthermore, Schutz et al. identified a whole set of mRNAs in activated T-cells that are exported via CRM1 [13]. However, the exact mechanism by which this transport occurs is not yet clear and a unifying principle is hard to establish. To date, no general RNA sequence element involved in CRM1-mediated nuclear export could be identified. While nuclear export of the c-fos mRNA seems to involve the 3′-UTR [14], nucleocytoplasmic transport of the CD83 mRNA was shown to depend on a cis-active element in its coding region [15]. Moreover, there is more than one adapter protein mediating CRM1-dependent mRNA transport. Adapter proteins described to be involved in this mRNA export pathway include HuR [14], [16], Staufen [17] and the eukaryotic translation initiation factor eIF4E [18]. Beside its long-established role in translation, eIF4E was recently shown to participate in further post-transcriptional functions as nuclear export and mRNA decay pathways [19]. Culjkovic et al. presented a list of mRNAs that are targets for eIF4E in the nucleus and may be transported in an eIF4E-dependent manner. These findings include for example the cyclin D1 mRNA, which seems to be exported via a not yet clarified mechanism involving eIF4E and a newly identified structural element in the cyclin D1 mRNA 3′-UTR [20]. However, the adapter proteins for the CRM1-mediated nucleocytoplasmic transport of many other mRNAs are still unknown. The inducible isoform of the nitric oxide synthase (iNOS) is an enzyme involved in many physiologic and pathophysiologic pathways like wound healing, inflammation, and cancer [21], [22]. The regulation of human iNOS expression depends on transcriptional and post-transcriptional mechanisms [22], [23]. The iNOS mRNA 3′-UTR contains five AREs displaying destabilizing properties [24]. In the last years we identified a complex network of RNA binding proteins that interact with these AREs and modulate iNOS mRNA stability. The most important destabilizing factors in this network are the KH-type splicing regulatory protein (KSRP) [25] and the four isoforms of the AU-binding factor 1 (AUF-1, also known as hnRNP D) whereas HuR and the polypyrimidine-tract binding protein (PTB) stabilize the human iNOS mRNA [24], [26]. In contrast to its common destabilizing function, TTP also leads to stabilization of the iNOS mRNA without binding to it [27].
    Material and methods
    Discussion This study demonstrates that, beside modulation of mRNA stability, control of nucleocytoplasmic mRNA transport is another important facet in the post-transcriptional mechanisms involved in human iNOS expression. While the bulk of poly(A)+ mRNAs is exported from the nucleus via the complex of TAP/Nxt-proteins, there are some exceptions that are transported from the nucleus to the cytoplasm in a CRM1-dependent manner. Most but not all mRNAs exported in a CRM1-dependent fashion contain AU-rich elements (AREs) in their 3′-UTRs [10].