Messenger RNA decay is an essential step in gene expression to set mRNA abundance in the cytoplasm. The binding of proteins and/or noncoding RNAs to specific recognition sequences or secondary structures within mRNAs dictates mRNA decay rates by recruiting specific enzyme complexes that perform the destruction processes. Often, the cell coordinates the degradation or stabilization of functional subsets of mRNAs encoding proteins collectively required for a biological process. As well, extrinsic or intrinsic stimuli activate signal transduction pathways that modify the mRNA decay machinery with consequent effects on decay rates and mRNA abundance. This review is an update to our 2001 Gene review on mRNA stability in mammalian cells, and we survey the enormous progress made over the past decade.

The degradation of some proto-oncogene and lymphokine mRNAs is controlled in part by an AU-rich element (ARE) in the 3' untranslated region. It was shown previously (G. Brewer, Mol. Cell. Biol. 11:2460-2466) that two polypeptides (37 and 40 kDa) copurified with fractions of a 130,000 x g postribosomal supernatant (S130) from K562 cells that selectively accelerated degradation of c-myc mRNA in a cell-free decay system. These polypeptides bound specifically to the c-myc and granulocyte-macrophage colony-stimulating factor 3' UTRs, suggesting they are in part responsible for selective mRNA degradation. In the present work, we have purified the RNA-binding component of this mRNA degradation activity, which we refer to as AUFl. Using antisera specific for these polypeptides, we demonstrate that the 37-and 40-kDa polypeptides are immunologically cross-reactive and that both polypeptides are phosphorylated and can be found in a complex(s) with other polypeptides. Immunologically related polypeptides are found in both the nucleus and the cytoplasm. The antibodies were also used to clone a cDNA for the 37-kDa polypeptide. This cDNA contains an open reading frame predicted to produce a protein with several features, including two RNA recognition motifs and domains that potentially mediate protein-protein interactions. These results provide further support for a role of this protein in mediating ARE-directed mRNA degradation.The c-myc gene is important for the control of cellular growth, differentiation, and transformation (reviewed in references 17 and 41). It belongs to the class of immediateearly genes whose expression is required to drive cells from Go to G1 following stimulation of quiescent cells by growth factors. However, the c-myc gene is not unique in terms of having an essential role in cellular growth processes. It has been known for decades that specific and timely changes in the expression of multiple genes are required for proper embryonic development and cell maturation (20). Expression of genes such as c-myc seems to be regulated not only at the levels of transcription, attenuation, nuclear processing, and translation but also at the level of mRNA turnover (reviewed in reference 41). Indeed, direct half-life measurements indicated that c-myc mRNA has a half-life of 15 to 40 min (19). These and other studies (41) demonstrated that the control of c-myc mRNA turnover might be an important means of regulating both the level and the timing of c-myc expression.Many proto-oncogene mRNAs are very unstable. The rapid turnover of c-myc mRNA is controlled by sequences in the 3' untranslated region (3'UTR) or by coding region sequences (reviewed in references 33 and 60). A common feature of many labile mRNAs, such as those for c-myc, c-fos, and granulocyte-macrophage colony-stimulating factor (GM-CSF), is the presence of an AU-rich element (ARE) in the 3'UTR which is one cis-acting element responsible for their rapid degradation (reviewed in reference 3, 48, and 60). It AUUUA (70). This might be important because...

Cytokine and proto-oncogene messenger RNAs (mRNAs) are rapidly degraded through AU-rich elements in the 3' untranslated region. Rapid decay involves AU-rich binding protein AUF1, which complexes with heat shock proteins hsc70-hsp70, translation initiation factor eIF4G, and poly(A) binding protein. AU-rich mRNA decay is associated with displacement of eIF4G from AUF1, ubiquitination of AUF1, and degradation of AUF1 by proteasomes. Induction of hsp70 by heat shock, down-regulation of the ubiquitin-proteasome network, or inactivation of ubiquitinating enzyme E1 all result in hsp70 sequestration of AUF1 in the perinucleus-nucleus, and all three processes block decay of AU-rich mRNAs and AUF1 protein. These results link the rapid degradation of cytokine mRNAs to the ubiquitin-proteasome pathway.

A tetracycline-regulated reporter system was used to investigate the regulation of cyclooxygenase 2 (Cox-2) mRNA stability by the mitogen-activated protein kinase (MAPK) p38 signaling cascade. The stable ␤-globin mRNA was rendered unstable by insertion of the 2,500-nucleotide Cox-2 3 untranslated region (3 UTR). The chimeric transcript was stabilized by a constitutively active form of MAPK kinase 6, an activator of p38. This stabilization was blocked by SB203580, an inhibitor of p38, and by two different dominant negative forms of MAPK-activated protein kinase 2 (MAPKAPK-2), a kinase lying downstream of p38. Constitutively active MAPKAPK-2 was also able to stabilize chimeric ␤-globin-Cox-2 transcripts. The MAPKAPK-2 substrate hsp27 may be involved in stabilization, as ␤-globin-Cox-2 transcripts were partially stabilized by phosphomimetic mutant forms of hsp27. A short (123-nucleotide) fragment of the Cox-2 3 UTR was necessary and sufficient for the regulation of mRNA stability by the p38 cascade and interacted with a HeLa protein immunologically related to AU-rich element/poly(U) binding factor 1.

Transient expression of some proto-oncogenes, cytokines, and transcription factors occurs as a cellular response to growth factors, 12-O-tetradecanoylphorbol-13-acetate, antigen stimulation, or inflammation. Expression of these genes is mediated in part by the rapid turnover of their mRNAs. A+U-rich elements in the 3' untranslated regions of these mRNAs serve as one recognition signal targeting the mRNAs for rapid degradation. I report the identification of a cytosolic factor that both binds to the proto-oncogene c-myc A+U-rich element and specffically destabilizes c-myc mRNA in a cell-free mRNA decay system which reconstitutes mRNA decay processes found in cells. Proteinase K treatment of the factor abolishes its c-myc mRNA degradation activity without affecting its RNA-binding capacity. Thus, RNA substrate binding and degradation appear to be separable functions. These findings should aid in understanding how the cell selectively targets mRNAs for rapid turnover.mRNA stability plays an important role in regulating gene expression (9, 10, 34). The rapid turnover of an mRNA ensures that it is maintained at relatively low steady-state levels and that changes in the turnover rate can affect its steady-state level over a short period of time. This type of regulation allows transient alterations in the expression of some cytokines, transcription factors, and the c-myc and c-fos proto-oncogenes in response to growth factors (7, 37, 46), 12-O-tetradecanoylphorbol-13-acetate (23, 37), antigen stimulation (24), or inflammation (45). A common feature of the mRNAs encoded by all of these genes is an A+U-rich element (ARE) in the 3' untranslated region (3'UTR) (14). These AREs serve as one signal targeting the mRNAs for rapid turnover (18,39,46), although sequences in the c-myc 5'UTR (18,32,33) and the c-fos coding region (19,40) can, in some instances, influence mRNA turnover.Although much is known about the cis-acting elements controlling mRNA stability, the trans-acting factors remain largely undefined. The development of cell-free mRNA decay systems makes possible the identification, purification, and characterization of these trans-acting factors (5,31,35,41). Ross and colleagues developed a system that is based on earlier in vitro translation systems and includes polysomes from the erythroleukemia cell line K562, magnesium, potassium, ATP, GTP, an ATP-regenerating system, and a 130,000 x g postribosomal supernatant (S130) (13,35 (nt) from the 3' end, while the second product lacked 12 nt from the 3' end. In subsequent steps, the mRNA was degraded in a 3'-to-5' direction. The same degradation pathway appeared in cells during degradation of H4 histone mRNA (36). In addition, Brewer and Ross (11,12) found that c-myc mRNA was degraded in vitro by a sequential pathway, in which the A+U-rich sequences of the 3'UTR were cleaved only after most or all of the poly(A) was removed. The remainder of the mRNA was degraded in subsequent steps. This pathway has been observed in vivo for c-myc mRNA (42). Thus, the in vitro mRNA de...