CRM1-dependent, but not ARE-mediated, nuclear export of<i>IFN-α1</i>mRNA

Abstract

While the bulk of cellular mRNA is known to be exported by the TAP pathway, export of specific subsets of cellular mRNAs may rely on chromosome region maintenance 1 (CRM1). One line of evidence supporting this hypothesis comes from the study of mRNAs of certain early response genes (ERGs) containing the adenylate uridylate-rich element (ARE) in their 3' untranslated regions (3' UTRs). It was reported that HuR-mediated nuclear export of these mRNAs was CRM1-dependent under certain stress conditions. To further examine potential CRM1 pathways for other cellular mRNAs under stress conditions, the nuclear export of human interferon-alpha1 (IFN-alpha1) mRNA, an ERG mRNA induced upon viral infection, was studied. Overproduction of human immunodeficiency virus type 1 Rev protein reduced the expression level of the co-transfected IFN-alpha1 gene. This inhibitory effect, resulting from nuclear retention of IFN-alpha1 mRNA, was reversed when rev had a point mutation that made its nuclear export signal unable to associate with CRM1. Leptomycin B sensitivity experiments revealed that the cytoplasmic expression of IFN-alpha1 mRNA was arrested upon inhibition of CRM1. This finding was further supported by overexpression of DeltaCAN, a defective form of the nucleoporin Nup214/CAN that inhibits CRM1 in a dominant-negative manner, which resulted in the effective inhibition of IFN-alpha1 gene expression. Subsequent RNA fluorescence in situ hybridisation and immunocytochemistry demonstrated that the IFN-alpha1 mRNA was colocalised with CRM1, but not with TAP, in the nucleus. These results therefore imply that the nuclear export of IFN-alpha1 mRNA is mediated by CRM1. However, truncation of the 3' UTR did not negatively affect the nuclear export of IFN-alpha1 mRNA that lacked the ARE, unexpectedly indicating that this CRM1-dependent mRNA export may not be mediated via the ARE.