HuR regulates p21 mRNA stabilization by UV light

Expression of the cyclin-dependent kinase inhibitor p21 is highly induced by many stresses, including exposure to short-wavelength UV light (UVC), which increases p21 mRNA stability. Investigation into the mechanisms underlying this stabilization process revealed that proteins present in cytoplasmic lysates of human RKO colorectal carcinoma cells formed complexes with p21 mRNA that were inducible by treatment with UVC and other stress agents. The ubiquitous Elav-type RNA-binding protein HuR was identified within the p21 mRNA-protein complexes, as antibodies recognizing HuR supershifted these complexes and revealed HuR-immunoreactive proteins complexing with p21 mRNA on Western blots. Lowering of endogenous HuR levels through expression of antisense HuR decreased p21 RNA-protein complexes, greatly reduced the UVC inducibility and half-life of p21 mRNA, and prevented UVC-mediated induction of luciferase activity in p21 3' untranslated region-containing reporter constructs. Our findings indicate that HuR plays a major role in regulating stress-induced p21 expression by enhancing p21 mRNA stability and that these effects are coupled to HuR's elevated presence in the cytoplasm.     

Nuclear export of NF90 is required for interleukin-2 mRNA stabilization

In response to T cell activation signals, the half-life of interleukin-2 (IL-2) mRNA is greatly extended. The cis elements mediating IL-2 mRNA stabilization are located in its 5' and 3' untranslated regions (UTR). The 3'UTR also contains AU-rich elements (AREs) that mediate rapid IL-2 mRNA degradation in the cytoplasm of nonstimulated T cells. NF90, a previously described RNA binding protein, binds to a subregion of the 3'UTR that contains several AREs and slows down the degradation of IL-2 mRNA. In nonstimulated cells, NF90 is mostly nuclear, but T cell activation results in its accumulation in the cytoplasm. The nuclear export of NF90 is required for IL-2 mRNA stabilization.     

The oncogenic RNA-binding protein Musashi1 is regulated by HuR via mRNA translation and stability in glioblastoma cells

Musashi1 (Msi1) is an evolutionarily conserved RNA-binding protein (RBP) that has profound implications in cellular processes such as stem cell maintenance, nervous system development, and tumorigenesis. Msi1 is highly expressed in many cancers, including glioblastoma, whereas in normal tissues, its expression is restricted to stem cells. Unfortunately, the factors that modulate Msi1 expression and trigger high levels in tumors are largely unknown. The Msi1 mRNA has a long 3' untranslated region (UTR) containing several AU- and U-rich sequences. This type of sequence motif is often targeted by HuR, another important RBP known to be highly expressed in tumor tissue such as glioblastoma and to regulate a variety of cancer-related genes. In this report, we show an interaction between HuR and the Msi1 3'-UTR, resulting in a positive regulation of Msi1 expression. We show that HuR increased MSI1 mRNA stability and promoted its translation. We also present evidence that expression of HuR and Msi1 correlate positively in clinical glioblastoma samples. Finally, we show that inhibition of cell proliferation, increased apoptosis, and changes in cell-cycle profile as a result of silencing HuR are partially rescued when Msi1 is ectopically expressed. In summary, our results suggest that HuR is an important regulator of Msi1 in glioblastoma and that this regulation has important biological consequences during gliomagenesis.     

Competition between RNA-binding proteins CELF1 and HuR modulates MYC translation and intestinal epithelium renewal

The mammalian intestinal epithelium is one of the most rapidly self-renewing tissues in the body, and its integrity is preserved through strict regulation. The RNA-binding protein (RBP) ELAV-like family member 1 (CELF1), also referred to as CUG-binding protein 1 (CUGBP1), regulates the stability and translation of target mRNAs and is implicated in many aspects of cellular physiology. We show that CELF1 competes with the RBP HuR to modulate MYC translation and regulates intestinal epithelial homeostasis. Growth inhibition of the small intestinal mucosa by fasting in mice was associated with increased CELF1/Myc mRNA association and decreased MYC expression. At the molecular level, CELF1 was found to bind the 3′-untranslated region (UTR) of Myc mRNA and repressed MYC translation without affecting total Myc mRNA levels. HuR interacted with the same Myc 3′-UTR element, and increasing the levels of HuR decreased CELF1 binding to Myc mRNA. In contrast, increasing the concentrations of CELF1 inhibited formation of the [HuR/Myc mRNA] complex. Depletion of cellular polyamines also increased CELF1 and enhanced CELF1 association with Myc mRNA, thus suppressing MYC translation. Moreover, ectopic CELF1 overexpression caused G1-phase growth arrest, whereas CELF1 silencing promoted cell proliferation. These results indicate that CELF1 represses MYC translation by decreasing Myc mRNA association with HuR and provide new insight into the molecular functions of RBPs in the regulation of intestinal mucosal growth.