No abstract

Antisense transcription is a widespread phenomenon in the mammalian genome and is believed to play a role in regulating gene expression. However, the exact functional significance of antisense transcription is largely unknown. Here, we show that natural antisense (AS) RNA is an important modulator of interferon-α1 (IFN-α1) mRNA levels. A ~4-kb, spliced IFN-α1 AS RNA targets a single-stranded region within a conserved secondary structure element of the IFN-α1 mRNA, an element which was previously reported to function as the nuclear export element. Following infection of human Namalwa lymphocytes with Sendai virus or infection of guinea pig 104C1 fetal fibroblasts with influenza virus A/PR/8/34, expression of IFN-α1 AS RNA becomes elevated. This elevated expression results in increased IFN-α1 mRNA stability because of the cytoplasmic (but not nuclear) interaction of the AS RNA with the mRNA at the single-stranded region. This results in increased IFN-α protein production. The silencing of IFN-α1 AS RNA by sense oligonucleotides or over-expression of antisense oligoribonucleotides, which were both designed from the target region, confirmed the critical role of the AS RNA in the post-transcriptional regulation of IFN-α1 mRNA levels. This AS RNA stabilization effect is caused by the prevention of the microRNA (miRNA)-induced destabilization of IFN-α1 mRNA due to masking of the miR-1270 binding site. This discovery not only reveals a regulatory pathway for controlling IFN-α1 gene expression during the host innate immune response against virus infection but also suggests a reason for the large number of overlapping complementary transcripts with previously unknown function.Electronic supplementary materialThe online version of this article (doi:10.1007/s00018-012-1216-x) contains supplementary material, which is available to authorized users.

Background: A hallmark of HIV-1 gene expression is that unspliced genomic RNA, which also acts as mRNA for the expression of Gag/Pol, is exported to the cytoplasm. Rev directs this transport through the nuclear export signal (NES).

IL-1β-stimulated rat hepatocytes are a newly identified source of TNF-α in the liver. TNF-α mRNA and asRNA are expressed in rats and humans, and the TNF-α asRNA reduces the stability of the TNF-α mRNA. Hepatocytes and TNF-α asRNA may be therapeutic targets to regulate levels of TNF-α mRNA.

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-α1 (IFN-α1) 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-α1 gene. This inhibitory effect, resulting from nuclear retention of IFN-α1 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-α1 mRNA was arrested upon inhibition of CRM1. This finding was further supported by overexpression of ΔCAN, a defective form of the nucleoporin Nup214/CAN that inhibits CRM1 in a dominant-negative manner, which resulted in the effective inhibition of IFN-α1 gene expression. Subsequent RNA fluorescence in situ hybridisation and immunocytochemistry demonstrated that the IFN-α1 mRNA was colocalised with CRM1, but not with TAP, in the nucleus. These results therefore imply that the nuclear export of IFN-α1 mRNA is mediated by CRM1. However, truncation of the 3′ UTR did not negatively affect the nuclear export of IFN-α1 mRNA that lacked the ARE, unexpectedly indicating that this CRM1-dependent mRNA export may not be mediated via the ARE.